GEOLOGICAL AND EXPLORATION CHARACTERISTICS OF PRECIOUS-METAL MINERALIZATION AT THE VOLCANO MINE AND VICINITY, PELONCILLO MOUNTAINS, HIDALGO COUNTY, NEW MEXICO.
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YOUNG, THOMAS HENRY
GEOLOGICAL AND EXPLORATION CHARACTERISTICS OF PRECIOUS-METAL MINERALIZATION AT THE VOLCANO MINE AND VICINITY, PELONCILLO MOUNTAINS, HIDALGO COUNTY, NEW MEXICO
rhe University of Arizona M.S. 1982
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University Microfilms Internationa!
GEOLOGICAL AND EXPLORATION CHARACTERISTICS OF
PRECIOUS-METAL MINERALIZATION AT THE VOLCANO
MINE AND VICINITY, PELONCILLO MOUNTAINS,
HIDALGO COUNTY, NEW MEXICO
by
Thomas Henry Young
A Thesis Submitted to the Faculty of the
DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCES WITH A MAJOR IN GEOLOGICAL ENGINEERING
In the Graduate College
THE UNIVERSITY OF ARIZONA
198 2 STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of require ments for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotations from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate School when in his judg ment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED : ,
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
W. C. Peters Professor of Mining and Geological Engineering PREFACE
In view of recent rises in precious-metal prices, Continental
Materials Corporation initiated an exploration and evaluation program in the Kimball mining district of southwestern New Mexico in October 1980.
Because of the lack of a comprehensive geological study that would provide an understanding of the mineralization in a regional context, they decided to fund two master's theses to obtain a better understand ing of the exploration characteristics of the precious-metal lodes. My goals were to identify the characteristics of mineralization in the
Volcano mine area. M. Stephen Enders, also of the Department of
Mining and Geological Engineering of The University of Arizona, inves tigated an adjacent area to the south in the vicinity of the Beck mine.
The interpretations and conclusions made in this study, as in any Investigation, are based on the availability of information gathered for the study or reported by various sources. A relatively large area was examined, and refinements in the interpretations and conclusions are expected as more detailed work is accomplished. The ideas and conclusions presented in this thesis are based on my interpretations and do not necessarily reflect those of Continental Materials Corporation.
Many individuals have offered assistance in this study and de serve special recognition. One group whose labors must not be over looked are the prospectors and fortune seekers who years ago toiled with persistence under harsh conditions, leaving a wealth of exposed bedrock in their adits, shafts, drifts, and prospects. My hat comes off
iii iv to those who by either luck or skill unearthed a means of living and to those who were not as fortunate in their diggings; they all were will ing to take a chance.
I am especially grateful to Continental Materials Corporation for logistical and financial support in this Project. Dr. Henry (Hank) T. Eyrich provided the initial opportunity for the study, and Mr. Robert
J. Pulfrey provided field assistance, supplied me with much of the assay data presented in the study, spent countless hours discussing geological relationships, and offered assistance in some of the sub surface mapping and sampling responsibilities.
The current operators at the Volcano mine, Messrs. Dolan
Campbell, Jack Hales, and Jim Williams, went out of their way to pro vide access and to provide assistance in underground mapping and sam pling at the Volcano mine. They also related historic information concerning mining in the district. I have great respect for these gentlemen; they used sound judgment and were willing to assume the risks involved in reopening an abandoned mine. I trust that they have been justly rewarded for their efforts.
Mr. Don McGhee, the owner of the Sixty-six, Coyle, and Wyman claims, kindly granted access and provided me with production and his toric information of the mining district. His son Chuck aided in the clearing and sampling of several of the abandoned shafts on the properties.
I am especially indebted to Mr. M. Stephen Enders, who assisted in much of the underground mapping. We spent countless hours discussing the geologic relationships and mineralization in the V Kimball mining district, and without his work, the current understand ing of the nature of the ore deposits and geology would be far below its present level.
I would also like to thank the members of my thesis committee,
Drs. William C. Peters and Charles E. Glass of the Department of
Mining and Geological Engineering and Dr. Arend Meijer of the Depart ment of Geosciences for their careful review of the manuscript and assistance during the preparation of this thesis. Dr. Peters, my thesis advisor, provided assistance both in the field and in the office. Dr.
Glass offered valuable suggestion concerning the interpretation of remote-sensing information. Dr. Meijer assisted me in understanding some of the implications of the volcanic sequence and was especially helpful in the analysis and interpretation of trace-element distributions in the volcanic suite.
Mr. Mark K. Reagan of The University of Arizona assisted me in the operation of the x-ray fluorescence equipment and in interpre tation of the data. Mr. Mike Williams, also of The University of
Arizona, was especially helpful with his assistance in obtaining x-ray diffraction patterns. Dr. Muhammad Shafiqullah of the Laboratory of
Isotope Geochemistry, The University of Arizona, kindly provided unpublished isotopic age information on several of the volcanic rocks of the area.
I am very grateful to Dr. Don H. Richter of the U.S. Geolog ical Survey, Denver, for his careful review of the manuscript, his assistance in interpretation of field relationships, and his many written and verbal discussions concerning the geology and ore deposits of the vi area. Dr. Harald Drewes, also of the U.S. Geological Survey and Dr.
Wolfgang E. Elston of the University of New Mexico provided many helpful comments about the geology of the area.
The thesis was typed by H. R. Hauck, who showed unusual patience in accepting all of my last-minute additions and deletions. Her editorial comments were also gratefully appreciated.
I especially wish to thank my wife Bonda for her encouragement, support, and assistance throughout this project. TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS x
LIST OF TABLES xiii
ABSTRACT xiv
1. INTRODUCTION 1
Location and Access 1 Physiography, Climate, and Vegetation 3 Previous Geological Studies 4 History of Mining and Production 6 Goals 12 Procedures 12
2. GEOLOGICAL SETTING 14
Volcanic Stratigraphy 15 Lower Andesite 15 Dacite Tuff and Tuff Breccia 16 Andesite Breccia 17 Steins Mountain Rhyodacite Welded Tuff 17 Lower Clastic Rocks 21 Upper Andesite 22 Rhyolite Complexes 24 Rhyolite Tuff and Tuff Breccia 25 Older Rhyolite Domes 25 Rhyolite Porphyry Dome-flow Complexes 26 Dacite Porphyry of Steins Peak 30 Late Rhyolite Dike 31 Olivine Basalt 31 Surficial Deposits 32
3. STRUCTURAL AND VOLCANIC EVOLUTION 33
Precaldera Sequence 36 Caldera Sequence 37 Regional Tumescence and Ring Fracture Generation . . 37 Ash-flow Volcanism and Caldera Collapse 38 Caldera Fill 40 Andesite Volcanism 41 Sedimentation and Pyroclastic Volcanism 41
vii viii
TABLE OF CONTENTS—Continued
Page
Resurgent Voleanistn 42 Postresurgence Structural Disruptions 43 Postcaldera Events 49 Regional Uplift 49 Basin-and-Range Movetnen.t 50 Basaltic Intrusion 51 Erosion 51 Geophysical Evidence for the Caldera Model 51 Petrological Implications of Trace-element Geochemistry . 56
4. ORE DEPOSITS AND IMPORTANT PROSPECTS 66
Volcano Vein 67 Volcano Mine 68 Wall-rock Geology 69 Mineralization in the Volcano Mine 71 Ore Mineralogy 78 Structural Control of Ore Deposits 82 Guess Workings 85 Wyman Property. 88 North Wyman Shaft 89 South Wyman Shaft 89 Wyman Adit 92 Coyle Mine 96 Sixty-six Mine 97 Southern Workings 99 El Oro Veia 99 Northern Workings 100 Kimball Adit 100 El Oro Adit 101 El Oro Values Workings 105 East Vein 108 Central Prospects 109 Northern Prospects 109 Miscellaneous Workings 110 Princess Pat Claims Ill 6625 N, 5000 E Adit Ill 5860 N, 3400 E Adit Ill 3475 N, 5375 E Adit 113
5. BASE- AND PRECIOUS-METAL ZONING 114
6. EXPLORATION CHARACTERISTICS 125
Characteristics of Epithermal Precious-metal Mineralization 125 ix
TABLE OF CONTENTS—Continued
Page
Exploration Targets 133 Radial Fracture Set 133 Bulk-tonnage Mining Targets 134 Selective Mining Targets 136 Northern Vein Targets 139 Porphyry Molybdenum Targets 145
7. CONCLUSIONS 156
Regional Geological Setting 156 Control of Mineralization 157 Exploration Guidelines 158 Exploration Targets 159
APPENDIX: GEOCKEMICAL RESULTS 162
REFERENCES 182 LIST OF ILLUSTRATIONS
Figure Page
1. Location map showing thesis area and major prospects and mines 2
2. Geological map of a portion of the north-central Peloncillo Mountains, Hidalgo County, New Mexico in pocket
3. Cross section A-A' in pocket
4. Geologic map of the Volcano mine area, Kimball mining district, Hidalgo County, New Mexico .... in pocket
5. Cross section B-B' in pocket
6. Cross section C-C in pocket
7. Coarse lahar of lower clastic unit (Tic, fig. 2) 22
8. East vein and north-central rhyolitic plug 29
9. Generalized geologic map of caldera complex 35
10. Structural evolution of ring and radial fracture systems and reactivation of fracture system during late stages in formation of caldera complex 48
11. Aeromagnetic map of caldera complex 52
12. Sample location map for Rb, Sr, Y, Nb, and Zr trace-element analyses 57
13. Yttrium and zirconium contents in caldera-related volcanic rocks 63
14. Geologic map. Volcano mine, 100 level in pocket
15. Geologic map. Volcano mine, 150 and 200 levels. ... in pocket
16. Geologic map. Volcano mine, 300 level in pocket
17. Cross section of subsurface development at the Volcano mine in pocket
X xi
LIST OF ILLUSTRATIONS—Contin^
Figure Page
18. Sample location and silver assay map, Volcano mine, 100 level in pocket
19. Sample location and silver assay map, Volcano mine, 150 and 200 levels in pocket
20. Sample location and silver assay map. Volcano mine, 300 level in pocket
21. Photomicrograph of polished section showing sulfide mineralization at the Volcano mine 81
22. Vein contour diagram, Volcano mine in pocket
23. Dilatation along a sinuous fault plane subjected to limited strike-slip movement 84
24. Geologic map of Guess adit 87
25. Sample location and silver assay map. Volcano mine area in pocket
26. Geologic map of North Wyman shaft, 90 level 90
27. Sample location and silver assay map. North Wyman shaft, 90 level 91
28. Geologic map of South Wyman shaft, 70 level 93
29. Sample location and silver assay map. South Wyman shaft, 70 level 94
30. Geologic and sample location map of Wyman adit 95
31. Geologic map, Coyle adit in pocket
32. Sample location and silver assay map, Coyle adit in pocket
33. Geologic map of Sixty-six adit 98
34. Geologic map of Kimball adit 102
35. Geologic map of El Oro adit 103
36. Sample location and silver assay map. El Oro adit 104 xii
LIST OF ILLUSTRATIONS—Continued
Figure Page
37. Geologic map of El Oro Values adit 106
38. Sample location and silver assay map, El Oro Values adit 107
39. Geologic and sample location map of adit at 6625 N, 5000 E 112
40. Regional distribution of silver in altered rocks and veins. 115
41. Regional distribution of gold in altered rocks and veins. 116
42. Regional distribution of copper in altered rocks and veins 117
43. Regional distribution of lead in altered rocks and veins 118
44. Regional distribution of zinc in altered rocks and veins 119
45. Volcano vein, looking northwest toward the Sixty-six and Coyle workings 132
46. Spatial distribution of magmatism and stockwork molybdenum deposition in the western United States between 35 and 25 m.y. B.P 147
47. Niobium content of unaltered and weakly altered felsic igneous rocks associated with alkali-calcic and high- potassium calc-alkalic stockwork molybdenum deposits and Volcano mine area rhyolites = 148
48. Rubidium content of unaltered and weakly altered felsie igneous rocks associated with alkali-calcic and high- potassium calc-alkalic stockwork molybdenum deposits and Volcano mine area rhyolites 148
49. Quartz-rhyolite breccia at 6720 N, 4650 E 151 LIST OF TABLFR
Table Page
1. Estimated and reported production from mines in the Volcano mine area 10
2. Whole-rock analysis of a sample of Steins Mountain rhyodacite 20
3. Trace-element concentrations of caldera series rocks .... 58
4. Statistical analysis of gold and silver distribution in the Volcano mine 74
5. Statistical analysis of assay and geochemical results from regional surface samples 122
xiii ABSTRACT
Silver and minor gold have been mined intermittently since the
1880s from epithermal quartz vein deposits at the Volcano mine and vicinity in the Kimball mining district of southwestern New Mexico.
Lodes occur in quartz veins following the radial faults that define an apical graben near the center of a resurgent caldera. The caldera sequence covers a thick andesitic pile. Better mineralized vein intervals occur where the vein divides ash-flow tuff and porphyritic andesite. Anomalous copper, lead, zinc, and manganese concentrations were detected in veins distributed radially from the precious-metal lodes. At the Volcano mine, cerargyrite and argentite(?) contribute most silver values and gold occur as native metal. Minor sphalerite, galena, chalcopyrite, mottramite, pyrite, covellite, copper oxide minerals, and pyrolusite contribute to the gangue. Several brecciated zones, believed to be breccia pipes, occur in the northern rhyolitic dome-flow complex.
xiv CHAPTER 1
INTRODUCTION
Although the mines of the Kimball mining district. Hidalgo
County, New Mexico, have been active since the 1880s, little has been done to determine the geological or exploration characteristics of the precious-metal lodes. In this thesis, the regional geological setting and the physical and chemical features that characterize mineralization in the vicinity of the Volcano mine will be discussed. The discovery of new zones of mineralization is the ultimate goal.
Location and Access
The thesis area, shown on figure 1, encompasses approximately
52 km 2 centered around the Volcano mine in the north-central Peloncillo
Mountains of Hidalgo County in southwestern New Mexico. The area is bounded by lat 32°16'30" and 32°20'28" N., the Arizona-New Mexico border, and long 108°58'1" W. Parts or all of sees. 2 through 11, 14 through 23, and 26 through 30, T. 23 A., R. 21 W., NMPB & M are included. Portions of the U.S. Geological Survey San Simon, Arizona-
New Mexico 15-minute quadrangle and the Mondel, New Mexico 7i-minute quadrangle cover the area.
The nearest towns are Lordsbnrg, New Mexico, approximately
32 km to the east, and San Simon, Arizona, about 24 km to the west.
The ghost town of Steins (pronounced Stens), New Mexico, active during the construction of the Southern Pacific railroad and during the
1 2
ARIZONA NEW MEX:CO rI
40 Flagstaff^ Albuquerque
i iPhoenlx I 25
Tucson^tj^ Laa Cruces
200 Km
THESI! Ipuess \ EAST ARE \ \EL ORO VEIN a VOLCANO UVEIN \ 4VEIN \
Volcano Kimball
• "l s Beck Mine V/yman
1-10 Quarry Sterns San Simon? Ariz. I Lordsburg Coyle El Oro N.Mex. ARIZ.|N. MEX. Sixty-Six El Oro tl Values 5 Km 500 m
Figure 1. Location map showing thesis area and major prospects and mines operation of a now-abandoned railroad station, is about 9 km south of
Volcano mine. Ruins of a stage coach station can be found near the northern boundary of the thesis area.
Several mines and prospects with underground development occur within the thesis area. These include the Volcano, Guess, Wyman
Colye, Sixty-six, El Oro, El Oro Values, and Kimball properties (fig.
1)= Only the Volcano and Sixty-six mines have been recently active.
In October 1981, a small leach plant was being operated at Beck mine, located approximately 1 km south of the thesis area (fig. 1).
Access to the area is by graded gravel roads from Interstate
Highway 10 at Steins Pass, New Mexico, and San Simon, Arizona. From the northeast, the gravel roads connect with U.S. Highway 70 between
Lordsburg, New Mexico, and Duncan, Arizona. Numerous dirt roads have been constructed for mining and ranching purposes, and most roads can normally be traveled in a vehicle with truck clearance, but four-wheel drive capability is necessary for travel on some roads and during periods of inclement weather.
Physiography, Climate, and Vegetation
The narrow, north-trending Peloncillo Mountains, typical of the ranges of the southern Basin and Range province of the United States, is bounded on the east by the Animas Valley and on the west by the
San Simon Valley. These features represent graben and horst struc tures created during a period of extensional tectonism in the western orogenic margins of the United States. Witllin the thesis area, east-trending valleys cut the range at its south and north boundaries and through the center. The highest point is Steins Peak with an elevation of 1,728 m; the lowest point, near the southeastern corner of the study area, has an elevation of
1,289 m. The general topography is marked by moderately and gently sloping hills, but steep cliffs and active talus slopes are also encountered.
The Volcano mine area is in the high Sonoran desert environ ment of southwestern New Mexico. Creosote bush, tar bush, mesquite, prickly pear cactus, and ocotillo are typical vegetation varieties. The temperature during a typical day in July ranges from a low near 18°C to a high of around 36°C. Normal daily temperatures during January range from a low of -2°C to a high near 13°C.
Water is scarce in the area. A normal rainfall of only about 28 cm per year has been recorded for the region (Houghton, 1974).
Summer runoff is caught in several drainages where ranchers have con structed earthen and concrete dams. A few windmills deliver water to livestock tanks near the north and western limits of the thesis area.
Workings in the Volcano mine, which extend to a depth of about 150 m, show no signs of flooding. At the Beck mine, located south of the thesis area (fig. 1), water is encountered at a depth of about 60 m.
Previous Geological Studies
Antisell (1856) offered the first geological description of the central Peloncillo Mountains. His work was completed as part of a reconnaissance study conducted to assess the best route for a railroad linking the settlements along the Mississippi River to the West Coast of
the United States. He described the geological characteristics of a vast
area, including western and southern California, southern Arizona, and
southern New Mexico, Although he (1856, p. 152) stated that "the
[Peloncillo] Hills are [geologically] unimportant," he did recognize the
major stratigraphic sequence in the range, pointing out that sediments
are capped by volcanics and intruded by numerous "dykes" and
porphyries. He also remarked that the Peloncillo range bears a striking
resemblance to the Pyramid Mountains located to the east.
Later, Gilbert (1875) studied parts of the Peloncillo range in an investigation of the geology of southeastern Arizona and southwestern
New Mexico. Portions of the area south of the thesis area were mapped
by Gillerman (1958), Armstrong (1978), Drewes and Thorman (1980),
and Enders (1981).
The only published studies of the geology of the Volcano mine
are by Graton (1910) and Elston (1963). Graton made a brief visit to
the Kimball mining district in 1905. His analysis is included in a com
prehensive volume (Lindgren, Graton, and Gordon, 1910) describing the
ore deposits of New Mexico. In an open-file report of the New Mexico
Bureau of Mines and Mineral Resources, Elston (1963) presented a
sketch map and offered a brief description of the volcanic sequence that
hosts mineralization in the Kimball mining district. From this recon
naissance study, he recognized several geological features that led him
to later suggest that the area may host a segment of a resurgent caul
dron (Elston, 1981, personal commun.; Elston and others, 1976; Deal
and others, 1978). 6
History of Mining and Production
All mineral production from the thesis area has come from silver ores, which contain minor gold credits, taken from a number of epither- mal vein deposits in the central portion of the thesis area. The Vol cano mine had the largest production in the Kimball mining district.
The following record of mining activity at properties in the thesis area was gathered from information in U.S. Government publications, a pri vate consultant's report, and Graton's (1910) account and from conver sations with mine operators in the district and observations made during this study. Production figures, expressed in metric tons (t), were:
1875-1905—Graton (1910, p. 329) visited the Volcano mine briefly in
1905 and reported that "no work has been done in a number of years."
He also stated that prospecting was said to have begun around 1875 and that the first actual mining began in about 1883. He did not, however, specify which of the properties were developed. He reported that the
Volcano mine had been worked through a 60-m-deep shaft and that the ore was treated using a 10-stamp mill with pans. He found that most ore had been taken from above the 100 level from stopes 0.6 to 3.0 m wide and that the ore was generally oxidized with silver occurring in films of greenish silver chloride. He (1910, p. 324) added that the
Federal Group of the National Gold Mine, located at the workings now designated as the El Oro and El Oro Values properties (fig. 1), was but "little developed in 1905."
Pror^uotion records for this early period of mining are unclear.
Graton (1910, p. 329) stated that "several hundred thousand dollars 7 worth of silver" were said to have been taken from the lodes at the
Volcano mine.
1909—An unspecified amount of "siliceous silver-gold ore" was shipped from the Volcano mine (Henderson, 1911, p. 438).
1919—"Several cars" of ore were shipped from the Volcano mine
(Henderson, 1922, p. 711).
1921—Two hundred ninety-four tons of ore were shipped by the
Volcano Mines Company (Fisher, 1921).
1922—Several hundred tons were shipped from the Volcano mine
(Henderson, 1925).
1923—"Several cars" of ore were shipped from the Volcano mine
(Henderson, 1927, p. 604).
1934 and 1935—Four hundred ninety-five tons of ore were shipped to Asarco's smelter in El Paso from the Sixty-six mine by the Life In surance Development Company (Elston, 1963; Henderson, 1935).
1942—The Diamond Mining Company shipped 1,918 tons of ore from the Volcano mine to the smelter in Miami, Arizona (Henderson, 1943).
1943—R. C. Williams of the Diamond Mining Company (?) operated the Volcano group and shipped 1,482 tons of ore to the smelter in
Miami, Arizona (Henderson and Mote, 1945; McGhee, 1982, personal commun.). 1947—A "small quantity" of ore was shipped from the dump at the
Volcano mine by Robert Stolz (Martin, 1949, p. 1487; and McGhee,
1982, personal commun.).
1979-1981—Until recently, the Volcano mine was operated by Jack
Hales, Dolan Campbell, and Jim Williams of Duncan, Arizona. Jack
Hales has been familiar with the area since the 1930s when his father worked at the Sixty-six mine. For some time he was aware that mate rial on the dump at the Volcano mine contained appreciable amounts of silver and that significant ore reserves remained underground. In
1979, when economic conditions in the silver market improved, Hales, with partners Campbell and Williams, leased the Volcano mine from the
Stolz family of Boulder, Colorado. In that same year they shipped nearly two-thirds of the dump, about 2,700 tons, to Asarco's El Paso,
Texas, smelter. In late 1979 and 1980, they cleared the shaft, built a headframe, and installed a hoist and compressor. Drifts on the 100 and
200 levels were extended, and by December 1981 they had shipped
5,201 tons of ore irom stopes developed above the two levels. The operation was indefinitely halted at the end of December 1981 pending improvements in the silver market.
The Sixty-six, Coyle, and Wyman properties are owneo by Don
McGhee of Lordsburg, New Mexico. At about the same time that activity began at the Volcano mine, Mr. McGee and his son Chuck began driving an adit toward the vein at the Sixty-six mine. The adit was stopped before an intersection with the major vein structure was made. They also shipped about 200 tons of material from the dump at the Coyle property and about 400 tons from the dump at the Wyman property. In 1977 Homestake Mining Company drilled one exploration hole at the Coyle property, and in 1980 Continental Materials Corpora tion Corporation of Tucson, Arizona, began an exploration drilling program in the area. The exploration efforts of Continental and Home- stake provide the only drill information in the thesis area.
Because records of mine production are incomplete, especially in the early years of mining, it is difficult to determine the total production from the mines in the thesis area. The production records discussed indicate that nearly 13,000 tons of ore were taken from the
Volcano mine after Graton visited the Kimball mining district in 1905.
Unfortunately, some records do not clearly indicate that the ores actually came from the Volcano mine. It is possible that some of the records show the combined production of the Wyman, Coyle, or Sixty- six mines with that from the Volcano mine, especially during times when these properties may have had the same owner.
Table 1 shows both the estimated and reported production from the mines within the thesis area. Estimates of total materials extracted were made using underground measurements or stopes and drifts. At the Volcano mine, including stopes, drifts, and shafts, it is estimated that approximately 15,879 tons of material (including gangue and ore) were removed since mining began at the property. At the Wyman property, located immediately south of the Volcano mine and along the same structure, an estimated 550 tons of ore were removed from a stope developed 26 m below the surface. The workings of the Coyle property, located immediately south of the Wyman, were not entered. 10
Table 1. Estimated and reported production from mines in the Volcano mine area
Estimated Material Estimatedj^ Reported Extracted' Production Production Property (metric ton) (metric ton) (metric ton)
Volcano mine
Pre-1979 stopes 3,690 5,60r 4,851 Drifts and shafts 4,884
Stopes, 1979 through 1980 7,305 5,201 5,201
Dump — 2,700 2,700
Total 15,879 13,502 12,752
Sixty-six mine 495 495
Coyle mine
Underground 100
Dump 200 200
Wyman mine 700 550 400
Total, all mine 14,847
a. Obtained from underground measurements made during this study.
b. Derived from both production reports and underground measurements.
c. Obtained from various sources listed in text.
d. Includes an estimated 750 tons from pre-1905 development. 11
The estimated production of about 300 tons was based on the size of the
workings and includes material from the dumps shipped by McGhee. It is evident that some ore was extracted from the working's because a few
remnants of haulage facilities used to transport ore material to a lower elevation for loading' can still be found. The shaft at the Sixty-six
mine is now caved. The estimated production of 495 tons is based on
the production records. Total production from all mines since mining began in the 1880s is estimated at less than 15,000 tons.
Only a few records for ore grade are available. Fisher (1921)
stated that the reported average ore grade for the 294 tons shipped in
1921 from the Volcano mine was 1,324 g/t silver and 3.3 g/t gold.
Elston (1963) obtained a shipment report from the Sixty-six mine for
1934 and 1935 that showed an average grade of 789 g/t silver and 4.0
g/t gold for 495 tons. The average grade reported for shipments made in 1942 and 1943 was 511 g/t silver and 1.4 g/t gold.
During the recent mining operations at the Volcano mine, wider
stopes were developed. The shipped ore consisted largely of primary-
grade ore. Production records indicate that ore from newly developed
stopes contained an average of about 340 g/t silver in a i-ange betweeu
141.4 and 1,896.8 g/t and an average of 1.14 g/t gold, which ranged from 0 to 7.89 g/t. Early mining operations undoubtedly handled ore of
much higher grade, selectively taking vein intervals where secondary enrichment was strongest. 12 Goals
Continental Materials Corporation of Tucson, Arizona, became interested in the potential of properties in the Kimball mining district during the recent surge in precious-metal prices. Because available
geological surveys in the region were limited to a sketch map prepared by Elston (1963), the corporation decided to fund two theses in the district, each covering approximately 52 km2, as part of their explor ation effort. Enders (1981) examined the area surrounding the Beck mine; in this thesis the Volcano mine area is discussed.
The goals of this thesis were: (1) to determine the regional geological setting of the Volcano mine area; (2) to identify structural, geochemical, and lithological characteristics of the mineral deposits; (3) to develop exploration guidelines for ore search in the district; and (4) to delineate new exploration targets. The ultimate aim is to locate addi tional ore reserves in the district.
Procedures
A geological map at a scale of 1:12,000, covering the entire thesis area, was made to gain an understanding of the regional geolog ical setting. In the vicinity of the precious-metal lodes, geological mapping, which covers 4.5 km2, was conducted at a scale of 1:4,800.
All accessible underground workings, including over 1,000 m of adits and drifts, were mapped at a scale of 1:240. A total of 65 days were spent in the field to coniplete the mapping and sampling.
Two hundred thirty-three chip-channel and composite chip samples of vein material were collected at both surface and underground 13 exposures to determine the base- and precious-metal distribution in the hydrothermal systems. Gold and silver assay results from 167 samples collected by Robert Pulfrey of Continental Materials Corporation and from 57 samples collected by J. D. Fisher (1921) for a consultant's re port on the Volcano mine were also used in the analysis of precious- metal distribution in the hydrothermal system. Thirteen rocks were se lected for trace-element analysis for Rb, Sr, Y, Zr, and Nb to aid in correlating volcanic units and to suggest trends or changes in the mag- matic evolution of the volcanic system.
Over 30 thin sections and one polished section were made of samples selected from over 250 hand samples to identify petrological features of the volcanic units, to determine alteration habits of the hydrothermal system, and to aid in determining vein mineralogy and ore paragenesis. X-ray diffraction methods were used to identify some of the vein minerals of the Volcano mine. The metric system of measurement was used throughout this thesis. Assay values for gold and silver have been converted to grams
per metric ton (g/t), (1 g/t = 1 part per million (ppm) = 0.0292 troy ounces per short ton). Geochemical values for other elements are re
ported as either parts per million (ppm) or as weight percentages.
To reference locations, a grid system using north and east coordinates with distances measured in meters has been established.
The location of the main shaft at the Volcano mine near the center of the thesis area has been arbitrarily designated as 5000 N, 5000 E. CHAPTER 2
GEOLOGICAL SETTING
The felsic and intermediate volcanic rocks hosting mineralization at the Volcano mine is part of the extensivee volcanic terrane that covers much of the central Peloncillo Mountains. Elston (1978) remarked that in Oligocene and early Miocene time a huge volcanic province, covering over a million km^, extended over southwestern New
Mexico, portions of southwestern Colorado, the Great Basin of Utah,
Nevada, and eastern California, and parts of West Texas, Arizona, and northern Mexico. The volcanic sequence is gently tilted to the north exposing Mesozoic and Paleozoic sedimentary strata and Proterozoic ig neous and metamorphic rocks to the south. Portions of this sequence have been described or mapped by Gillerman (1958), Armstrong and others (1978), and Drewes and Thorman (1980).
Geological maps of areas to north, east, and west of the thesis area were not available. Members of the U.S. Geological Survey are currently finishing a geological map of the San Simon, Arizona-New
Mexico quadrangle, which will include the areas north and west of the
Volcano mine area. The strata east of the mine area are covered by
Quaternary alluvial units. Enders (1981) recently completed mapping an area immediately south of the thesis area as part of an M.S. thesis at
The University of Arizona. Elston (1963) made a sketch map of
14 15 portions of the thesis area as part of a study of the ore deposits of
Hidalgo County.
The most comprehensive regional study of th<» sequence south of the thesis area was made by Gillerman (1958). He observed that the thick andesite pile and related tuffs and breccias are much more than
1,500 m thick. The upper portions of this andesite sequence are exposed in the thesis area.
For my study, a l:12,000-scale geological map of the area sur rounding the Volcano mine (fig. 2, in pocket), which covers approximately 52 km^, was prepared to show the relationships of the regional geology and structure to the ore deposits. The volcanic sequence is shown in cross section on figure 3 (in pocket).
Details of the geology of the mine area in the central part of the thesis area are shown on a l:4,800-scale map (fig. 4, in pocket).
Cross sections across the mine area are shown on figures 5 and 6 (in pocket).
Volcanic Stratigraphy
Lower Andesite
The oldest rocks exposed in the map area consist of andesitic flows, tuffs, and breccias (Tla, fig. 2), which crop out in valleys near the southwestern, western, and northern parts of the mapped area.
These correlate with the thick sequence that Gillerman (1958) described to the south of Interstate Highway 10. The strata dip from 20 to 40 degrees toward the north and northwest and show great variation in texture and composition throughout the more than 1,500-m thickness 16 exposed in the thesis area. The flows are commonly reddish brown, gray, or purple, weakly porphyritic, and commonly propylitically altered. Epidote and calcite are the most common secondary minerals.
Gillerman (1958) provided a detailed description of both the petro- graphic and surficlal characteristics of the andesite sequence exposed south of the map area, Phenocrysts constitute 10 to 30 percent of the typical rock. Andesine (An^g) makes up 75 percent of the phenocrysts, and pyroxene makes up most of the remaining phenocrysts. Biotite is present in amounts of up to 2 to 3 percent.
Fine-grained magnetite occurs in amounts ranging from zero to 5 percent.
Several episodes of intermediate to acid volcanism are recorded within this thick sequence. South of the thesis area, a series of flows, breccias, and tuffs of rhyolitic composition form a prominent landform designated as Quarry Peak. Gillerman (1958) remarked that this se quence appears to rest with a slight angular unconformity on the underlying andesites; he thus considered the complex as a separate unit, which is younger than the underlying andesites. Enders (1981), however, located several areas where the Quarry Peak rhyolite complex is enveloped between andesite flows. He believes that the Quarry Peak rhyolite complex was emplaced during the volcanic episode that occurred within the longer, continuing eruption of the andesitic magmas.
Dacite Tuff and Tuff Breccia
Near the southwest corner of the mapped area (fig. 2), a len ticular rhyodacite-dacite series of pyroclastic rocks (Tdt, fig. 2) rests 17
upon or was intercalated with the older andesites. The unit comprises
predominantly angular, argillically altered dacite and rhyodacite frag
ments, which are 1 to 2 cm in diameter and set in a pumice-rieh
groundmass. Locally, the tuff contains breccia fragments of hornblende dacite, which exceed 5 cm in diameter. The coarse breccia varieties form conspicuous rounded knobs near 2650 N, 3200 E. Because coarse breccia fragments compose part of this tuff i/nit, a local, unidentified source is suspected. To tha west, a densely welded hornblende-biotite tuff showing a moderately eutaxitic texture is poorly exposed in drainage bottoms.
Andesite Breccia
Lenses of tuff breccia (Tab, fig. 2), up to 15 m thick, are intercalated with the andesitic flows toward the top of the older andesite sequence. These lenses are exposed along the western bound- daries of the western part of the mapped area and have been mapped separately. The breccia consists of angular andesite fragments, 1 to 2 cm wide, and rare rhyodacite and rhyolite clasts. Many clasts are argillically and propylitically altered. Sorting is sometimes apparent, but a random distribution of the fragment sizes is typical. A thin section of a sample of the breccia shows clasts of pilotaxitic andesite in a pumice-rich groundmass, which composes about 20 percent of the volume.
Steins Mountain Rhyodacite Welded Tuff
The Steins Mountain rhyodacite welded tuff (Tsm, fig. 2) covers much of the south-central mapped area. It was deposited onto 18
an irregularly eroded surface of the andesite, tuff, and breccia se quence previously described. The andesite sequence was tilted 10 to 20 degrees north before the eruption of the ash-flow sheet. Prior to the episode of ash-flow volcanism, a paleosurface, which was elevated to the
northwest and west, was developed. The ensuing ash-flow£ filled the
topographic depressions and lapped up against the andesite walls to the
west and northwest.
The bulk of the welded tuff sequence consists of a single cool ing unit, which exceeds 500 m in thickness. At the base a tuff brec cia, 0.5 to 10 m thick, containing andesite fragmentsis extensively exposed. Locally, a basal vitrophyre is present.
In the densely welded central zone, phenocrysts of quartz, sanidine, and plagioclase (An^Q make up 30 to 40 percent of the rock, and angular lithic andesite fragm.ents compose 10 to 15 percent.
Half of the phenocrysts consist of strongly embayed glassy to white quartz crystals, generally 1 to 2 mm in diameter. Plagioclase and sani dine crystals, 1 to 2 mm in diameter, make up about 40 percent of the
phenocrysts and occur in nearly equal amounts. The typical sanidine
phenocryst is moderately altered to sericite and clays, whereas the plagioclase, showing some degradation to sericite and clay minerals, is
relatively fresh.
Biotite flakes, less than 1 mm in diameter, occur throughout the
groundmass. They usually make up less than 3 percent of the volume.
The matrix consists of an aphanitic arrangement of clays, zeolite
minerals, and flattened, devitrified pumice, giving the rock a strong eutaxitic texture. 19
A whole-rock analysis of a sample collected by Robert Pulfrey
of Continental Materials Corporation from the upper portions of the
densely welded zone is given in table 2. Compositions of typical
rhyolites and rhyodacites reported by Nockolds (1954) are also ^ven
for comparison. The classification of rhyodacite was based on the Si02
concentration. The relatively high KgO and low CaO content, however,
falls closer to the composition of a rhyolite. Perhaps the rock should
be more properly designated a high-potassium rhyodacite.
The upper 120 m consists of buff, punky, partially welded,
argillically altered tuff. This rock forms the footwall and ore host to
most of the mineralization at the Volcano mine. Richter (1981, personal
commun.) observed a similar 15-m-thick tuff sequence in the Rooster-
comb area located in Arizona approximately 7.5 km west of the Volcano
mine. At the Roostercomb locality, however, the tuff is sandwiched
between two andesite flows. If the two tuffs are correlative, a separate
cooling unit in the Steins Mountain welded tuff is probably indicated
where only the late ash-flow pulse reached the Roostercomb locality. It
is also possible that the tuff at the Roostercomb area was derived from
a separate and possibly unrelated volcanic pulse.
In thin section, feldspars, presumably sanidine, which make
make up 15 to 20 percent of the rock in the upper zone or cooling unit,
are completely altered to clays. Equal amounts of corroded phenocrysts of cristobalite and strongly embayed quartz phenocrysts are preserved.
The groundmass consists of clay minerals and volcanic glass with a
weakly eutaxitic texture. Modal quartz and cristobalite account for 20
Table 2. Whole-rock analysis of a sample of Steins Mountain rhyodacite
Composition Steins Mountain % (or ppm) Rhyodacite Rhyolite^ Rhyodacite
SiOg 68.3 73.6 66.3
AI2O3 14.96 13.4 15.4 MgO 0.96 0.3 1.6
NagO '3.56 3.0 4.1
K2O 5.44 5.4 3.0
^®2®3 0.08 1.2 2.1 FeO 1.70 0.8 2.2
TiOg <0.01 0.2 0.7 0.06 0.1 0.2
MnO 0.04 0.3 0.1
SrO <0.01 — —
RbgO ^198 — —
F ^811 — —
Li20 0.13 — —
S 0.22 — —
H2O — 0.8 0.7
L.O.I. 2.7 — —
a. Average composition of typical rhyolites and rhyodacites after Nockols (1954)
b. Reported in parts per million. 21 approximately 20 percent of the rock. Finely disseminated pyrite is nearly ubiquitous in this upper, fumarolically altered zone.
Lower Clastic Rocks
Bedded tuffaceous sandstone and mudstone, air-fall tuff, and mudflow breccia (Tic, fig. 2) are irregularly distributed above the nonwelded zone of the Steins Mountain ash-flow tuff. Tuffaceous sandstone is the most voluminous, followed by air-fall tuff and mudflow deposits. The thickness of this unit ranges from a few meters to over
70 m, thinning to the north and east. Outcrops are generally masked by debris from the overlying, more resistant volcanic rocks. Repre sentative sections of this sequence can be best observed in drainage floors near 5250 N, 4050 E and 4950 N, 3275 E west of the Volcano mine.
The lahar (mudflow) deposits are poorly bedded but are locally well sorted. Clasts of propylitically altered andesite predominate, with a minor contribution of siliceous volcanic fragments. The larger cobble- size clasts are generally sub rounded, whereas the smaller pebble-size fragments exhibit variable degrees of angularity. The matrix, which contains about 20 percent, randomly oriented plagioclase laths, 0.3 cm wide, accounts for approximately 40 percent of the rock volume. The rock consists of brown to gray, fine-grained mud and andesite lava or ash that cement the andesite fragments. Propylitic alteration selectively affected either the clasts or the groundmass. This textural feature is shown in figure 7. It is evident that many of the clasts were propylit ically altered before they were redeposited in the lahar. The lahar Figure 7. Coarse lahar of lower clast unit (Tic, fig. 2)
Individual clasts show differential propylitic alteration. 23
deposits are restricted to a zone in the western map area of figxire 2
near 5250 N, 4050 E.
Tuffaceous sandstone and mudstone deposits are exposed
beneath the lahar deposits and south of the Sixty-six and El Oro Values
mines near 3500 N, 5300 E, They are generally well sorted and some
times exhibit cross bedding. Appreciable amounts of pyroclastic materi
al are contained in these beds, and a division between sedimentary and
pyroclastic members is not always sharp.
Notable exposures of pyroclastic-dominated tuff occur in the
west map area of figure 2 below and intercalated with the lahar deposits
near 4950 N, 3275 E, Some of the air-fall tuff beds contain ejecta up
to 20 cm wide.
Upper Andesite
The upper andesite (Tua, fig. 2) forms the hanging wall of the
Volcano mine vein system. The 250-m-thick unit consists of a series of
flows that stretch across the center of the map area shown on figure 2 in a broad belt forming slopes and eroded hills. The characteristic
rock is a medium-gray, pcrphyritic andesite containing 25 to 35 percent
plagioclase (An^Q_gQ) phenocrysts and up to 5 percent hornblende
phenocrysts in a pilotaxitic groundmass. The g::oundmass is composed
of plagioclase microlites, pyroxene, and magnetite.
Although some flows are massive, most contain from 10 to 50
percent coarse andesite breccia fragments. Rare rhyolite and rhyoda-
cite fragments are also present. 24
Propylitic alteration is widespread in this unit, being strongest near the Volcano mine vein system. Flows showing a high lithic content are more readily affected than are the dense flows. Epidote, chlorite, clay minerals, and calcite are common alteration minerals.
The upper andesite unit is generally more porphyritic than the typical andesite of the older series. This distinction is helpful in dis tinguishing the two andesite series in remote outcrops. The contact between the lower and upper andesite in the northwest area of the thesis is not clear because of the strong argillic and propylitic alter ation and poor exposure.
Rhyolite Complexes
The rhyolite complexes, exposed in the northern and central thesis area (fig. 2) were emplaced as a series of flows, domes, dikes, and pyronlr^tics. Similarities in texture and mineralogy, disruption by faulting, and the effects of deuteric and hydrothermal alteration complicate attempts to make accurate distinctions between eruptive sequences. In this study, only major variations in the rhyolitic sequence that persist over wide areas will be considered.
The rhyolites observed in the thesis area have been divided into six units on figure 2. Further classification has been made in the
Volcano mine area on figure 4.
Rhyolite Tuff and Tuff Breccia. Pyroclastic rocks of rhyolitic composition (Trt, fig. 2) cover the porphyritic andesite sequence.
Typical exposures can be observed at 8100 N, 5950 E and 5815 N 6175
E. The tuff typically consists of bedded fine-grained pumice, but sections containing a significant contribution of coarse, 1- to 20-cm- diameter breccia fragments are common. Breccia fragments are most often of rhyolitic composition, but rare andesitic fragments can also be observed. At 4240 N, 5660 E, the tuff grades upward from a fine grained bedded tuff to a coarse tuff breccia composed almost entirely of fragments of strongly flow-banded rhyolite.
Older Rhyolite Domes. In the far northern part of the thesis area, domes of strongly flow-banded rhyolite (Tor, fig. 2) are partially exposed. They form a small knob at 8700 N, 6275 E and cover the northern slope of Steins Peak near 8600 N, 2700 E. Some of the poorly exposed rhyolitic rocks found in the valley near 8050 N, 3875 E may also warrant this classification but have been included with the younger group of rhyolitic dome-flow complexes that constitute the major pulse of rhyolitic volcanism in the thesis area. Several outcrops, however, show flow-banding characteristics similar to the those of the older group. In general, however, the rocks of this valley exhibit a more massive textural habit characteristics of the younger group.
At the Steins Peak exposures, the older rhyolite can be ob served between talus-covered slopes, drainage bottoms, and exposures of the lower andesite sequence. A border vitrophyre is exposed at
8600 N 2325 E. Elsewhere, the boundary zone is covered. The vitrophyre consists of reddish-brown and gray volcanic glass with 1 to
2 percent biotite and 5 to 10 percent fine-grained sanidine phenocrysts.
A weak to moderate foliation in the matrix can usually be detected. Beyond the vitrophyre, the rhyolite exhibits a strong flow- banded texture where the predominantly white groundmass is crossed by parallel lenses of gray quartz. Biotite phenocrysts make up 1 to 3 percent of the rock, whereas sanidine phenocrysts make up 15 to 20 percent. The orientation of the foliation of the rhyolites at the Steins
Peak locality is quite variable, but dips are generally to the south.
Similar to the domes on the north flank of Steins Peak, the domes at 8700 N, 6275 E consists of strongly flow-banded rhyolite. The steeply dipping foliation roughly defines an oval-shaped dome. The groundmass shows tight flow banding where the white matrix is crossed by thin, light-gray quartz bands. Occasional embayed quartz and cor roded sanidine cry§.tals occur as phenocrysts, which reach 0.3 cm in diameter. Very fine grained biotite flakes are disseminated throughout the groundmass and make up less than 1 percent of the volume.
Pvhyolite Porphyry Dome-flow Complexes. Flows and domes of biotite rhyolite cover much of the northern thesis area. A division in map units has been made to separate those members that are clearly flows (Trpf, fig. 2) from those with affinities to extrusive-intrusive centers (Trpd, fig. 2). The division between the two map units is not sharp, and the contacts shown on figure 2 are approximate.
To the northeast, the rhyolites clearly exhibit flow character istics. They are generally marked by a basal, 10- to 20-m thick vitophyre that grades into flow-banded biotite rhyolite. The foliation is generally flat lying, but local contortions are common. The rock tex tures vary both laterally and vertically from the suspected extrusive 27 centers (Trpd, fig. 2); flow foliation is strong to barely detectable, lithophysae sometimes account for up to 30 percent of the volume, and the color varies between buff white to light gray to reddish brown.
The characteristic rhyolitic flow shows moderate flow-banding textures, is buff to reddish gray, and is nearly void of lithophysae. Ten to 20 percent sanidine and 2 to 3 percent biotite phenocrysts are distributed in the aphanitic groundmass. The feldspar phenocrysts are deuterically altered to clays, and where they are subjected to surface conditions are either leached of all feldspar components or are partially filled with remnant clay masses.
Four thin sections from samples of the dome-flow sequence were studied. All show remnants of the subhedral, elongate feldspar pheno crysts that are completely replaced by both sericite and clay minerals.
Biotite, which is arranged roughly parallel to the foliation, is commonly converted in part to hematite. The quartz-rich aphanitic groundmass is divided by clay minerals and fine-grained feldspars. These minerals are often segregated into bands that define the foliation. Occasional embayed quartz phenocrysts, about 1 mm wide, were observed.
The typical rhyolites included in the dome-flow complex classifi cation (Trpd, fig. 2) are similar in texture and identical in mineralogy to their flow equivalents described earlier. The major difference is in foliation characteristics. The dome-flow complex rhyolites exhibit a flow banding significantly more contorted than can be found in the rhyolite flows that were extruded to the north and west, and unlike the distal flow members, the foliation often shows steep dips. In some exposures, foliation textures are absent. 28
In the south-central map area, two rhyolite dome-flow complexes intrude the porphyritic upper andesite sequence (Tua, fig. 2) and the
Steins mountain ash-flow tuff (Tsm, fig. 2) near 4650 N, 5275 E and
3700 N, 5500 E. Texturally and mineralogically, they are similar to the rhyolites of the northern dome-flow complexes and have been mapped as equivalent units on figure 2. However, because of their spatial rela tionship to the mineralization, they warrant separate descriptions.
The rhyolite (fig. 8) centering between the Volcano mine and the El Oro mine was apparently the product of a single intrusive pulse.
It consists of a central zone of moderately to weakly flow-banded rhj'^o- iite, which is surrounded by a 3- to 8-m-wide intrusive breccia zone.
The dome-flow complex immediately south of the El Oro Values and Sixty-six mines was emplaced as a result of several volcanic pulses.
The cross section shown on figure 5 illustrates the nature of the dome-flow sequence as it intrudes the volcanic series. The volcanic neck, or dome center, for this system is believed to be situated near
3880 N, 5240 E. Here, the flow-banding textures become particularly contorted and the rhyolitic textures show the greatest variations. From this volcanic neck, the dome "mushroomed" (fig. 5) toward the south and intruded the volcanic strata in a near-lateral fashion, crossing the
Steins Mountain ash-flow tuff-tuffaceous sediment-porphyritic andesite sequence. Its intrusive base is marked by a vitrophyre, which is ex posed at 3060 N, 5400 E and 3690 N, 5320 E. Near 3820 N, 5260 E, an approximately 10-m-thick air-fall tuff is intercalated with the dome-flow complex. The tuff dips to the north at about 30 degrees. It is possible that this tuff is equivalent to the tuff that caps the porphyritic 29
Figure 8. East vein and north-central rhyolitic plug
Photograph taken looking southeast. Rhyolitic plug (Trpd, fig. 2), which is flanked by border breccia, intrudes porphyritic andesite (Tua, fig. 2). East vein separates porphyritic andesite (Tua, fig. 2) from strongly silicified Steins Mountain ash-flow tuff (Tsmp, fig. 2), forming a prominent ridge. 30 andesite (Trt, fig. 2) at 4240 N, 5600 E, which has been previously described. Undoubtedly, the tuff was not oripnally deposited at this site but was transported during subsequent eruptions in the dome-flow sequence.
Dacite Porphyry of Steins Peak. The intrusive relationship between the rhyolitic dome-flow complexes and the dacite porphyry of
Steins Peak (Tdp, fig. 2) is not apparent. Talus obscures the contact between these two units. Both units have therefore been represented in the explanation of the geological map shown on figure 2 as coeval.
It is suspected, however, that the Steins Peak dacite intrudes the rhyolitic sequence.
The dacitic rocks that cap Steins Peak, the highest point in the thesis area at an elevation of 1,788 m, are among the freshest of the shallow intrusive or extrusive rocks found in the thesis area. Deuteric alteration has not obliterated the feldspars as it has in the rhyolitic rocks. Weak flow banding was found in outcrops on the southeast slope near 8000 N, 2850 E, but these textures are rare elsewhere.
The aphanitic gray groundmass holds 30 percent phenocrysts of biotite, potassium feldspar, and plagioclase. The dacite classification was based entirel;'^ on the phenocryst concentrations, and a whole-rock analysis may indicate that the rock deserves a more felsic classification.
Plagioclase (An^^) makes up to 60 percent of the phenocrysts. It forms subhedral laths that show oscillatory and normal zoning. Sericite com monly replaces the cores of these feldspars. Orthoclase occurs as sub hedral, 2- to 4-mm-wide grains that account for about 20 percent of the 31 phenocrysts. Quartz crystals, which form 10 percent of the pheno- crysts, are embayed and exhibit textures indicative of strain; some crystals are commonly broken and show undulatory extinction. Biotite accounts for about 10 percent of the phenocrysts. Tiny magnetite grains, favoring the rims, are included in the bioMte crystals. The groundmass consists of a very fine grained matrix of quartz, feldspar, biotite, and magnetite.
Late Rhyolite Dike. A felsic rhyolite dike (Tlr, fig 2) cuts the volcanic pile near the west boundary of the thesis area. It shows a weak, nearly vertical flow foliation that follows the trend of the dike.
A thin section shows that the rock consists of 75 percent very fine grained quartz and 25 percent feldspar grains about 1 mm in diameter.
The feldspar grains along with lesser amounts of quartz are roughly segregated into parallel bands that define the foliation in the dike.
Very fine grained blebs of hematite, believed to be alteration products of biotite, are distributed throughout the matrix.
Olivine Basalt
Two basaltic plugs and numerous basaltic dikes (Tob, fig. 2) intrude the older volcanic sequence. Their emplacement marks the final volcanic episode observed in the thesis area. The plugs are con spicuously circular in outcrop and measure from 210 to 400 m in diameter.
The rock is massive to moderately vesicular, containing from 10 to 30 percent fine to coarse phenocrysts of olivine, which are partially altcred to iddingsite. The groundmass contains a mixture of 32 fine-grained clinopyroxene, magnetite, and intergranular plagioelase
(An^g) arranged in random orientations.
Fine-grained olivine basaltic dikes are irregularly arranged around the plugs in a widely spaced radial pattern. Individual dikes can rarely be traced for more than 100 m along strike. Only the more persistent dikes have been shown on the geological map (fig. 2).
Surficial Deposits
Quaternary units, which include gravel terrace deposits (Qgt), talus debris deposits (Qtd), and alluvial units (Qal), were mapped on figure 2 only where sufficiently thick and widespread to cover the bed rock geology. CHAPTER 3
STRUCTURAL AND VOLCANIC EVOLUTION
The geological sequence hosting mineralization at the Volcano mine exhibits many of the characteristics of a resurgent caldera com
plex. The model proposed by Smith and Bailey (1968), with minor
modifications by Lipman (1978), serves as an excellent guide to under standing the structural and volcanic sequence observed in the Volcano
mine area. This model will be used in the following description of the structural and volcanic events.
Elston and others (1976) were the first to suggest that areas of the north-central Peloncillo Mountains have aspects of a cauldron com
plex. After reviewing Elston's (1963) earlier reconnaissance study in
the area, Elston and others (1976, p. 98) concluded that "the gold- silver veins of the Kimball district are associated with rhyolitic domes in
what appears to be an east-west segment of a ring fracture zone." De tailed mapping completed for this thesis and by Enders (1981) has con firmed their hypothesis by delineating the northern, eastern, and southern boundaries of a resurgent caldera. A U.S. Geological Survey map of the area in Arizona west of the Volcano mine is currently near completion, and the western boundary of the proposed caldera should be delineated upon its release.
Southwestern New Mexico is the site of one of the major clus ters of mid-Tertiary caldera complexes in the western United States,
33 Elston (1978) has compiled a list of some 28 caldera complexes, dated between 39 and 19 m.y. B.P., that are known or suspected in southwestern New Mexico. Deal and others (1978) offered descriptions of nine caldera complexes located in Hidalgo County.
Differences in opinion exist concerning the role calderas play in the formation of ore deposits. Elston (1976, 1978) reported that almost all mining districts in southwestern New Mexico of known or suspected mid-tertiary age lie in the ring fracture zones or on the central domes of resurgent calderas. McKee (1979), however, found only that 2 of the 31 collapsed calderas identified in Nevada are sites of major mining districts. Rytuba (1981) has recently presented a summary and analy sis of the relationship between calderas and ore deposits in the western
United States.
A l:62,500-scale geological map (fig. 9) has been prepared to show the proposed caldera boundaries that were defined in work com pleted for this thesis, in mapping by Enders (1981) and the U.S.
Geological Survey (Richter, 1981, personal commun.), from aerial photo graphic interpretation, in limited field study outside the boundaries of the thesis area to the west, and from interpretation of airborne mag netic data. Discussions of the eruptive history of the caldera complex will be restricted to features observed within the boundaries of the thesis area. Because the intent of this study was to explain only the mineralization in the Volcano mine area and geological mapping, laboratory analyses, and determination of age relationships are still being conducted by members of the U.S. Geological Survey and others within the proposed caldera boundaries, the discussion of the caldera ?'•' '"' J*''.. ^' *• I •'—• •,, < -jgt' - .1:^ "•:•• "rl..-—"-' ^ 'A
• ^ (}• I Roastercaniti
•l^t.••k " • '• ' U
5000 E
Figure 9. Generalized geologic map of caldera complex
35
EXPLANATION (see text for description of map units)
ll'jvlum A Basalt
Trd - Rhyolitic Dikes Trf - Rhyolitic Flows Tr - Rhyolitic Dome-Flow Complexes
Tu Upper Andesite Porphyry
Steins Mountain Ash-Flow Tuff
5000 N Tq - Quarry Peak Rhyolite Complex (see Gillerman (1958) and Enders (1981) for description) Tl - Lower Andesltic Flows and Tuffs
Fault, ball on downthrown side
Contact, approximately located
Strike and dip of foliation inferred limits of major subsidence Source! of Geologic Mapping N m Figure 2, i this study Airphoto interpretation
0 N 1 Enders (1S81) 5000 E
3 km
1 : 62,500
)lex
36 complex in a regional context presented here should be considered preliminary. The analysis is offered, however, to place the observed structural and volcanic features in the Volcano mine area in their proper regional geological context and to explain the generation of the gold and silver mineralization. A caldera model is best suited for this analysis.
Precaldera Sequence
The northerly dipping Tertiary volcanic strata that host the caldera sequence cover a basement of Precambrian crystalline rocks and
Paleozoic and Cretaceous sedimentary rocks. The pre-Cenozoic rocks are exposed approximately 6 km south of Steins Pass on Interstate
Highway 10 (fig. 1). Descriptions of these rocks are offered by
Drewes and Thorman (1980), Armstrong and others (1978), and Giller- man (1958),
Following a period of quiescence and structural deformation, great volumes of andesitic flows, pyroclastics, and related epiclastics were deposited on the Precambrian, Paleozoic, and Mesozoic rocks pos sibly beginning as early as Late Cretaceous time and extending into early Tertiary time.
Marvin and others (1978) reported a fission-track date of
44.7±2.7 B.P for the older andesite sequence at a location approximately
13 south-southwest of the Volcano mine. Damon (1981, personal com- mun.) suggested that the date may have been reset by a thermal event and that the andesites could be a part of the Hidalgo volcanic series with reported ages ranging from 55 to 70 m.y. that covered large portions of southern New Mexico (Thorman and Drewes, 1978; Marvin and others, 1978).
At least one stage of intermediate to siliceous volcanism, the
Quarry Peak complex described by Gillerman (1958), is represented in the older andesite sequence. Within the boundaries of my study area, dacitic to rhyodacitic pyroclastic rocks and welded tuff (Tdp, fig. 2) are intercalated in the lower andesite sequence (Tla, fig. 2) near 2650
N, 3200 E.
Following the eruption of the andesitic lavas, a period of quiescence prevailed during which a marked erosional surface was de veloped in the thick andesite pile. During this period, the strata were tilted 10°-20'^ N.
Caldera Sequence
The caldera sequence intrudes and partially covers the tilted and strongly eroded andesitic pile. The paieosurface of the andesitic pile in the west and northwest map area (fig. 2) were elevated at the time of ash-flow eruption. The ash flows filled shallow areas but left elevated regions uncovered.
The description of the formation of the caldera series has been divided into seven stages. Special emphasis is given to those event associated with mineralization.
Regional Tumescence and Ring Fracture Generation
Regional doming and initial formation of concentric ring frac tures mark the initial stage of caldera formation according to the model 38 proposed by Smith and Bailey (1968). Because of incomplete expo sures, lack of geological mapping in the surrounding areas, and sub sequent disruption by Basin-and-Range movement, this stage was not identified in the area covered by this thesis.
Ash-flow Volcanism and Caldera Collapse
Ash-flow volcanism represents the first major caldera-related volcanic event recognized in the thesis area. The apparent lack of multiple cooling intervals in the tuff sequence suggests that the eruptions occurred rather suddenly, presumably emanating from the ring fracture zone. North of the Volcano mine vein system, evidence for collapse preceding or accompanying the ash-flow eruption was not observed. Younger volcanic units cover the ring fracture zone, obscuring the structural relationships. Enders (1981), however, believed that a megabreccia zone, similar to those described by Lipman
(1976) from the San Juan Mountains of Colorado, developed at the southern margins of the caldera and that it was probably emplaced before the ash-flow eruptions ceased.
Because of regional structural disruptions and incomplete expo sures, it is difficult to decipher the collapse history of the caldera and to calculate the volume of material erupted during the ash-flow stage.
For the Platoro caldera of the San Juan Mountains, Colorado, where, as in the Steins Mountain area, only fragmental evidence concerning the original distribution of the ash-flow tuff sequence exists. Lipman
(1975) devised a highly simplified model to estimate the volume of erupted magma. He then estimated the total vertical collapse by 39
applying Smith's (1960) proposal that the volume of collapse should
approximate the volume of magma erupted. Exposures in the Kimball
mining district offer even less evidence of the original extent of the
ash-flow sheet than did those described by Lipman in Colorado, making estimates tenuous.
The model proposed by Lipman is useful, however, to derive an order of magnitude estimate of the volume of erupted magma during ash-flow volcanism and the subsequent collapse for the caldera de scribed here. In his model, Lipman used an inverted cone to represent the portion of the erupted ash-flow magma deposited within the caldera boundaries. In the Steins area, the caldera measures approximately 10 km in diameter and the ash-flow sequence has a thickness of at least
500 m. A cone of these dimensions would yield a volume of about 13.1 km3 of compacted ash. It is difficult to determine the lateral distribu tion of the ash that was erupted from the caldera, but if at least as much material flowed beyond the caldera walls as was retained within, more than 26 km® of magma were erupted from the chamber during the ash-flow eruptions. Application of Smith's (1960) reasoning would indi cate a vertical collapse of the caldera of about 330 m.
It should be emphasized that the above calculations are based on a variety of assumptions that have a capacity for significant vari ation and that they are offered only as initial figures to gain a rough approximation of the magnitude of the eruptions. As more information is gathered, refinements will be possible. Smith (1979) considered ash- flow eruptions in the range of 1 to 100 km® as products of small 40
epicontinental ring structures and volumes ranging from 100 to 1,000
km' as derived from large epicontinental ring structures.
Overflow of the ash-flow sheet beyond the walls of the caldera
can be observed north of the proposed caldera in the SW? sec. 28 and
SEj sec. 29, T. 22 S., R. 21 W. (Richter, 1981, personal commun.).
Gillerman (1958), Drewes and Thorman (1980), and Enders (1981)
described occurrences covering 5 to 8 km^ south of the caldera boun
daries, The thickest section occurs at Steins Mountain, approximately 3
km south of the thesis area, where a section measuring over 200 m
thick is exposed. The top of the sequence at this location has been
removed by erosion.
An isotopic age of 33.7 m.y. was obtained from a Steins Moun
tain ash-flow tuff sample collected near 3500 N, 7000 E, and an isotopic
age of 35.3 m.y. was determined for a sample reported to be of Steins
Mountain rhyodacite collected from a locality north of the caldera wall
and beyond the boundaries of the thesis area (Richter and Shafiqullah,
1982).
Caldera Fill
Volcanism continued, but on a much smaller scale, after the ex
trusion of the Steins Mountain ash-flow tuff sequence. Pyroclastic
material covered and preserved much of the nonwelded top, or upper cooling unit, of the ash-flow sheet. The air-fall tuffs are intercalated
with lenticular sandstones, mudstones, and lahars that partially filled the depression left after the collapse of the caldera. 41
Andesite Volcanism
Andesitic flows and flow breccias (Tua, fig. 2), reaching a thickness of about 250 m, were extruded from the magma chamber, which was nearly depleted by siliceous magma. The appearance of andesitic magmas following the eruption of rhyodacitic magmas suggests that a compositional gap may have existed in the underlying magma chamber. Petrological implications of the magma-chamber geochemistry will be discussed in a later section in this chapter.
Sedimentation and Pyroclastic Volcanism
At some time following the andesitic stage, tuffaceous sand stones and lesser amounts of mudstone and air-fall tuffs filled lower lying areas within and immediately beyond the caldera walls. At this time, resurgent doming, centering near the Sixty-six and Volcano mines, was initiated as magma again accumulated in the underlying chamber. A suggestion of this episode of deformation is preserved in the foliation of the Steins Mountain ash-flow tuff. The foliation defined by the flattened pumice fragments dips predominantly northeast in the northeast quarter of the map area of figure 2 and northwest in the northwest quarter of the map area. Another consequence of the re gional doming is the development of a radial fracture set that now hosts mineralization in the Volcano mine vein system. Although additional disruptions along this set are evident, the initial development of the set is believed to have occurred at this stage in the development of the caldera. Movement along these fracture during this period was prob ably minor. 42
Resurgent Volcanism
The emplacement of the rhyolitic dome-flow complexes in the northern half of the thesis area and near the Volcano, Sixty-six, and
El Oro Values mines (figs. 2 and 4) shows a complex history of devel opment. Additional detailed mapping, petrographic work, trace- and major-element geochemical analyses, and possibly geophysical studies will be necessary to understand the details of the volcanic and struc tural maze created by these eruptions. The rhyolites now cover the ring fracture zone and obscure early structural relationships.
A rhyodacite sample from north of Doubtful Canyon, which is located at the northern boundary of the thesis area, has been dated by
K-Ar on biotite at 33.0 ±1.1 m.y. B.P. (Marvin and others, 1978). A fission-track age from zircon of 28.7 ± 1.4 m.y is given for the same sample, and Marvin and others stated that the discordance is not understood. It is probable that later thermal events have reset the zircons, making fission-track dates inaccurate in the area. Although this location is not within the boundaries of my study and was not visited, it is reasonable to suspect that the sample was collected from a flow that emanated from the ring fracture zone during this period of resurgent volcanism. A fission-track study on zircon from a vitrophyre from within the thesis area at 5350 N, 8850 E yielded an age of 31.4 ±
1.4 m.y. (Marvin and others, 1978). It is difficult to consider the date as valid because such an age discrepancy was found in the sample collected to the north. If the age is correct, a gap in time of about 2 million years marks the interval between the ash-flow eruptions and the period of resurgent volcanism. 43
A voluminous resurgent volcanic episode was not noted along
the southern caldera ring fracture system (Enders, 1981). Enders
(1981, p. 21) remarked, however, that small rhyolitic bodies "may in
trude" portions of the ring fracture zone at the southern margins.
Rhyolite dikes and dome-flow sequences cover much of the
Roostercomb area, the proposed western boundary of the caldera, which
is located west of the thesis area (fig. 9).
Postresurgence Structural Disruptions
The structural response to the extrusion of large volumes of
rhyolitic magmas during the episode of resurgent volcanism has been a
reactivation of faults along the ring fracture zone and renewed activity
along the radial fracture set.
The north-trending fractures that host mineralization in all
mines and prospects shown on figure 4 are part of the radial fracture
system developed during the episode of resurgent doming previously described. The centrally located rhyolitic dome-flow complexes define
the apex of the resurgent dome from which these fractures emanated.
As shown in figures 4 and 6, the veins hosting mineralization at the
Volcano mine and the north-trending vein between 6250 N, 5650 E and
2800 N, 5475 E define an apical graben in the caldera sequence. Pres ent erosion exposes the porphyritic andesite (Tun) in fault nontaot with
the Steins Mountain ash-flow tuff in the central map area (figs. 2 and
4). The faults along the radial fracture system exhibit a marked de crease in displacement to the north and south of this central zone.
They are difficult to trace beyond 6500 N north of the Volcano mine. 44
South of the mine, they are lost in the alluvium-filled valley near the southern margins of the thesis area.
Near the northern boundaries of the map area shown on figure
4, detailed geological mapping shows that the radial fracture set dis places an east-trending fault that separates rhyolite from andesite.
This radial fracture set is believed to have been initiated during the beginning stages of doming prior to resurgent volcanism and should therefore predate the east-trending fault. To account for this apparent discrepancy in the structural evolution, multiple periods of displacement involving both fracture sets is offered as a resolution. If the radial fractures were initiated prior to the resurgent eruptions as a response to the radial stresses resulting from regional doming as the magma chamber was rejuvenated, as Smith and Bailey (1968) have suggested in their analysis of resurgent calderas, the displacement along these faults in the Volcano mine area during resurgent doming was probably minor.
A conclusion that takes into account that the formation of the Peloncillo arch caused postcaldera tilting of 10° to 20° N. is that the doming that caused deformation in the Steins Mountain ash-flow tuff created radial dips of 5 to 10 degrees in the foliation of the Steins Mountain ash-flow tuff. Compared to doming in some of the other resurgent calderas as at Creede, Colorado, from where radial dips of 25 to 45 degrees have been reported (Steven and Ratt6, 1965), the expected fault response along the radial fractures in the Volcano mine area to the formation of the resurgent dome from the accumulation of magma would be small.
It is suspected, but not absolutely clear, that high-angle move ment along the radial fracture set predates final movement along the 45 ring fracture faults. Because major movement along the radial set was largely restricted to the central apical region of the caldera where re surgent doming was greatest, displacement along these faults was prob ably minimal at their extension in the ring fracture zone.
The first major response to evacuation of portions of the magma chamber following resurgent volcanism was the formation of the apical graben structure near the cast-central portion of the proposed caldera.
Following this stage, normal high-angle faults juxtaposed blocks of the dome-flow complex rhyolites into fault contact with other members of the rhyolite complexes, the porphyritic andesite unit (Tua, fig 2), and the lower andesite series (Tla, fig. 2).
Some evidence suggests that a late rhyolitic pulse centered near
3850 N, 5275 E south of the Sixty-six and El Oro Values mines post dates the graben-forming stage of disruption along the radial fracture set. At this locality, as shown on figure 4, the vein of the El Oro system dwindles and finally pinches out within the dome-flow complex.
Silicifieation and cover by talus and scree obseure the r-elaiionship, and it is unclear whether the vein was cut off by a late magmatic pulse or simply was initiated here. The vein cuts the rhyolitic plug to the north at 4680 N, 5280 E. South of the central rhyolite dome-flow com plex, as shown on figure 4, one strong and another weaker fracture can be traced several meters to the east. The west vein is aligned with the Volcano mine vein system exposed north of the complex at the
Sixty-six mine and probably represents a continuation of this system, but the relationship is obscured by talus and scree on the saddle near
3820 N, 5210 E that divides the two zones. Although the weaker vein 46 to the east is believed to be a split or subsidiary structure of the Vol cano vein system, the possibility that this vein represents a continu ation of the El Oro vein system cannot be dismissed. From under ground and surface exposures, however, it can be shown that the
Volcano mine vein system commonly consists of a series of two or more parallel veins.
The observation that the displacement along the radial fracture system decreases considerably with distance from the centrally located rhyolites strongly suggests that the major movements along these faults were related to the emplacement of these rhyolites.
At underground exposures in the Volcano, Guess, Wyman,
Coyle, Kimball, and El Oro workings, the rakes on slickensides plunge
15° to 20° S., indicating that near strike-slip movement was the final episode of displacement along the radial fracture set. Although this episode was of great importance in creating dilatant zones that allowed open space for the transport of the ore-forming fluids and for the deposition of the precious metals in the veins, the lateral displacements were minor in comparison to the normal high-angle graben-forming movements that occurred earlier.
The El Oro fault system occurs in the center of the apical graben structure, and major high-angle displacement along the plane cannot be detected. It is believed that this fault was not active during the initial episodes of resurgent doming or during the stage of high- angle fault movement but was created during the final episodes of disruption marked by near strike-slip movement. Late rhyolitic mag- matic activitj?^ in the central domes could have triggered this episode. 47
Because the nature of the movement along the radial fault strv ^tures was complex, it is difficult to estimate the lateral and ver tical displacement of each of these episodes. This task is further complicated by the observation that high-angle normal displacement along these faults decreases significantly laterally from the central dome area. The strike-slip phase affected zones along the fault plane extending further from the central dome area than the lateral limits of high-angle normal disruptions. Fault contacts between the moat rhyo- lites and the porphyritic andesite near 5760 N, 4480 E show a displace ment of about 25 m along the radial Volcano mine vein. At least 90 m of porphyritic andesite are exposed in the valley that contains the El
Oro and Sixty-six mines, indicating that the vertical displacement of the graben block on the west bounding fault exceeds 90 m.
Postsubsidence deformation that results in the formation of an apical graben is a common and logical conclusion to an episode of resur gent volcanism in a caldera sequence. Figure 10 summarizes the probable structural and volcanic events related to the resurgent stage in the formation of the caldera in the study area. First, resurgent doming caused radial stresses in the overlying volcanic strata (fig.
10a). Initial relief was accomplished by minor fault movement along these radially distributed zones of weakness. When the magma chamber was again tapped during resurgent volcanism, the first structural re sponse was apical subsidence of the central graben along the radial fracture system (fig. 10b). Major high-angle normal faults along the ring fracture zone followed as a result of the emplacement of the moat rhyolite sequence (fig. 10c). Late magmatic activity in the Figure 10. Structural evolution of ring and radial fracture systems and reactivation of fracture system during late stages in formation of caldera complex
a. Radial fractures showing little displacement are formed dur ing initial stages of resurgent doming. Structural weaknesses may have been developed as early as the period of regional tumescence.
b. An apical graben is formed as a response to the release of rhyolitic magmas (Trpd and Trdf, fig. 2) along the ring fracture zone to the north and west and near the center of the caldera complex.
c. Major subsidence within the ring fracture zone followed the final release of rhyolitic magmas (Trpd and Trpf, fig. 2).
d. Late strike-slip movement and extension of the radial frac ture system marks the final structural episode in the formation of the caldera complex. Mineralization followed this episode. 48
4- /
/
- /
/ /
N 3 km — 6— Volcano Mine - 6000 N, 5000 E 1 : 62,500 '
( Figure 10. Structural evolution of ring and, radial fracture systems 49 east-central apical region of the caldera complex caused additional structural disruptions resulting in strike-slip movement along the radial fracture set (fig. lOd). The El Oro mine vein system was probably created during this late event. Others faults that experienced activity during this episode are believed to have been preexisting structures.
Hydrothermal fluids permeated the structures and deposited base and precious metals at some time after the strike-slip episode; possibly during or shortly after the formation of the Peloncillo arch, described in the next section.
Postcaldera Events
Following the eruption of the caldera sequence and related structural disruptions, the volcanic strata of the thesis area were sub jected to region uplift, Basin-and-Range block faulting, intrusion of basaltic lavas, and continuing erosion.
Regional Uplift
Gillerman (1958) has shown that the central Peloncillo range was uplifted during Tertiary time to form the Peloncillo arch. At the crest of the arch, centered near Granite Gap approximately 23 km south of the thesis area where U.S. Highway 80 crosses the Peloncillo Mountains, erosion has exposed the Precambrian and Paleozoic sections. North of
Granite Gap, the overlying Tertiary (and Cretaceous?) volcanic rocks generally dip north; to the south, a general southerly dip prevails.
Gillerman also described a series of northwest-trending faults dividing the arch that are intruded by numerous dacite and latite dikes. A sample from a latite dike collected south of Steins Pass (fig. 1) shows a 50
K-Ar age of 27.0 ± 0.8 m.y. on biotite and 26.1 ± 0.8 m.y. on potas sium feldspar (Hoggart, Silverman, and Todd, 1977).
A confinement of the age of the uplift between 26 and 34
B.P. can be made based on consideration that the caldera sequence with a preliminary age of around 34 m.y. was tilted during the uplift in the formation of the Peloncillo arch and that the dikes intrude zones of weakness created during the formation of the arch. Nine dates showing a range from 29.8 to 32.5 m.y. B.P. mark the intrusive events that re sulted in the emplacement of the granitic rocks of the Granite Gap area
(Hoggart and others, 1977). If it can be assumed that the formation of the Peloncillo arch was related to the magmatic doming related to em placement of the Granite Gap plutons, the age of the uplift can be further narrowed.
Basin-and-Range Movement
Basin-and-Range faulting has created the north-northwest trend in the topographic expression of the Peloncillo range. Christensen and
Lipman's (1972) proposal of a 30 m.y. age for the onset of block fault ing and crustal extension for this region agrees with the geological relationships observed in the thesis area.
Minor left-lateral slippage along east-trending faults (fig. 4) that never exceed 15 m in displacement marks the final structural dis ruption in the mine area. This event, which occurred after the ore fluids deposited precious metals in the radial vein sets, may have been a tensional product of Basin-and-Range faulting. 51
Basaltic Intrusions
A date of 6.1 ± 0.2 B.P. (Marvin and others, 1978) was obtained from the southern basaltic plug at 6100 N, 3940 E (fig. 2).
The emplacement of the plugs and the widely spaced radially distributed
basaltic dikes mark the end of volcanism in the Volcano mine area.
Erosion
The present topographic expression of the Volcano mine area
has been largely controlled by differential erosion of the volcanic strata. Partially welded to nonwelded tuffs and andesite are generally exposed in valleys, whereas rhyolite and densely welded tuff define uplands. Structural features do not usually define pronounced landforms except where silicification is intense or a fault brings together units of different erosional characteristics. Structural features have, however, acted as conduits for the eruption off competent rhyolitic or rhyodacitic magmas and have therefore indirectly had a major influence in the control of the topographic expression of the volcanic sequence.
Geophysical Evidence for the Caldera Model
The only available geophysical study covering IJie thesis area is an aeromagnetic survey covering the southern half of the Silver City,
New Mexico A MS quadrangle that was recently completed as an open-file report by the U.S. Geological Survey (1980). Figure 11 was prepared using the aeromagnetic data and the geological information shown on figure 2. An attempt at a detailed interpretation of the depths from which the magnetic responses were derived was not made, and all hifefr«> fltaera
0 E 5000E
Figure 11. Aeromag-netic map of caldera complex. — Adapted from U.S. Geological Sur
52
OOOON
5000N
contour interval 10 gammaa
5000E 10000E
— Adapted from U.S. Geological Survey (1980)
53 discussions that follow are based on a generalized interpretation of the geophysical data.
The most striking feature of the geophysical map is the arcuate alignment of a series of magnetic highs in the north and west portions of the area shown on figure 11. This trend is less well defined in the southeast quarter where it continues passing directly southwest of the
Beck mine. In the southeast quadrant of the area, the arcuate anomaly is cut by a series of nearly east-trending highs.
Another interpretation of the anomalous arcuate contours can be made by considering the area within the semicircle as an area of anomalously low magnetic response set within a region of an overall strong but irregular magnetic susceptibility. An interpretation using this logic would be that the magnetic highs surrounding this area are not necessarily anomalous but are typical of those in the surrounding area, which show an irregular high contrast in magnetic susceptibility.
The thick volcanic pile in the surrounding terrane, not shown on figure
11, does produce a magnetic pattern containing irregularly scattered highs and lows. The close spacing of the magnetic contours in the north map area of figure 11 in the area between the high response of the arcuate anomaly and the interior lows is indicative of near-surface contrasts in magnetic susceptibility (Breiner, 1980). This inter pretation is in agreement with the magnetic habit of the rocks observed at the surface. Andesites containing moderate amounts of magnetite occur at or near the surface along the arcuate trend of high magnetic response. Rhyolites showing a low magnetite content are the predom inant rock types found in the area south of the arcuate trend. If 54 subsidence occurred along the ring fracture zone, the strata of the caldera series with a general magnetic susceptibility lower than that of the surrounding andesites are separated from the lower andesite unit by fault surfaces that define the ring fractures. The boundaries of the ring fracture zone v;here the greatest subsidence has occurred would therefore be defined by the break in slope of the contours from the zone of high magnetic susceptibility to the zone of low magnetic suscep tibility. This interpretation agrees with the geological relationships observed within the thesis area. The separation between the dome-flow complex rhyolites (Trpd, fig. 2) from the rhyolite flows (Trpf, fig 2) that are believed to have crossed the caldera margins is coincident with this geophysical boundary.
The Roostercomb area in Arizona in the western map area (fig.
11) present a problem in this interpretation. It derives its name from the irregular topographic expression of a nearly vertical rhyolitic dike that protrudes from the volcanic sequence of andesites and rhyolites.
It is logical to suspect that the dike was formed as part of the resurgent stage of moat rhyolite volcanism during the formation of the caldera, similar to the events described for the northern thesis area.
The northern rhyolite domes lie within the arcuate zone of high mag netic susceptibility. In the Roostercomb area, the prominent dike does not lie within this arcuate trend but, although not shown as a magnetic peak, is aligned with the trend of magnetic highs. A possible explana tion for this apparent discrepancy is that the rhyolitic dike that defines the Roostercomb structure did not intrude the fault structure where major subsidence had occurred but rather that it intruded a concentric 55 ring fracture that was situated west of the zone of major ring fracture subsidence. In Smith and Bailey's (1968) model, a series of concentric ring fractures are created as a response to regional doming during the initial stage in the formation of the caldera complex. It is possible that the ring fracture zone at the Roostercomb locality offered less resistance to the intruding magmas than did the ring fracture along which maximum subsidence occurred.
The centrally located rhyolitic dome-flow complexes near the
Volcano and Sixty-six mines are represented as geophysical lows on figure 11. Other areas showing low magnetic susceptibility occur near
4200 N, 3000 E and 5100 N, 1000 E. It is possible that rhyolitic intru- sives with low relative magnetic susceptibilities are buried at shallow depths at these localities.
Because the caldera sequence was tilted to the north during the formation of the Peloncillo arch (Gillerman, 1958), erosion has left only remnants of the volcanic strata of the caldera sequence in the southern boundaries of the proposed caldera. Exposures of the older andesite sequence are common in this area, and a generally higher and more irregular magnetic response, similar to that of the regional habit, occurs along the southern margins of the caldera. The suspected ring fracture zone is still, however, discernible in places, where it is marked by diffuse changes in the magnetic susceptibility of the rocks.
To the east the thick alluvial cover prohibits attempts to inter pret the magnetic responses. 56
Petrological Implications of Trace-element Geochemistry
Pearce and Norry (1979) have studied the variations of tita nium, zirconium, yttrium, and niobrium in igneous rocks and offered suggestions concerning their distribution during fractional crystalliza tion of mafic, intermediate, and felsic magmas. Because these elements have a high field strength (charge/radius), they are usually not mobilized when the rock is subjected to aqueous fluids and are there fore useful for studying altered or weathered rocks. Because fresh samples from the surface in the Volcano mine area are rare, due largely to surface weathering and deuteric alteration, trace-element abundances of yttrium, zirconium, and niobrium were determined by the x-ray fluorescence method for 13 samples to aid correlating individual flows, plugs, and tuffs, and to identify general magmatic trends in the evolution of the caldera sequence. An understanding of the magmatic evolution of the sequence is important in a search for porphyry-type mineralization and in determinations of relative ages of precious-metal mineralization. These aspects of the study will be discussed in chapter
6, which deals with exploration targets. The locations at which these samples were collected are shown on figure 12, and the abundances of the trace elements detected in the study are listed on table 3. The sample set includes two samples of rhyodacite ash-flow tuff, one sample of porphyritic andesite, nine samples of rhyolitic rocks of the resurgent volcanic pulse, and one sample of dacite from Steins Peak.
Because of the limited number of samples analyzed and the lack of whole-rock analyses on the volcanic sequence, it is difficult to make 57
1 I T Rniitifyot HatveQ* 4000 E eooo E-) •e4i414 • ' " Oi(l Sum* Station ' "-V I •• '•^U'as; i ,1 ] • •• ^6 11 l«n« P^Kl .. 5 , ,yj ; - A '•- t r/ / , '. V / '' •• I •' • • 400 r^,/ / i
J ^ _ 'f
- ^
-e<*OjO:N ^ ' a'S . , , / •403 X«>/8 "TKJ'V ^- 40S9 —V--L— — 5,4,0;» ' - G/c A. "K V""-
% ; Ir '401. ' . - , 18 16
• 406
4000 N • • "• I-- • / ,'_^ a/-J; « S : -• -I
2 Km
1: 40,000
Figure 12. Sample location map for Rb, Sr, Y, Nb, and Zr trace-element analysis 58
Table 3. Trace-element concentrations of caldera series rock
Trace Element, ppm^
Sample Rock Type Rb Sr Nb Y Zr
401 Porphyritic andesite 87 335 8 41 308
402 Rhyolite 444 108 16 51 269
403 Rhyolite 669 51 19 57 233
4o4 Rhyodactic welded tuff 168 157 15 42 305
405 Rhyolite 764 37 20 95 235
406 Rhyolite 438 36 21 61 226
407 Rhyolite 200 16 20 49 159
408 Dacite 188 197 16 45 302
409 Rhyolite 382 35 24 43 273
410 Rhyolite 748 33 19 67 177
411 Rhyodacitic welded tuff 246 67 24 47 104
412 Rhyolite 6 25 8 194 184
413 Rhyolite 252 44 22 70 226
a. Determinations made using x-ray fluorescence. 59 firm conclusions about the evolution of the magma g-eochemistry throughout the caldera sequence. Therefore, only general observations will be made in the following paragraphs.
To account for interferences in the determination of Y, Nb, and
Zr concentrations by x-ray fluorescence analysis, it is necessary to determine the concentrations of Rb and Sr. It has been shown that Sr is mobile under surface conditions, and the wide scatter of Sr values found in the sample set (table 3) of this study reflects this phenome non. For this reason, Sr cannot be considered f. useful tool for cor relating units or marking changes in the magma geochemistry. Implica tions of the Rb concentrations will be discussed in chapter 6 in the section dealing with the potential for porphyry molybdenum mineraliza tion.
With the exception of samples 401 and 412, Nb values range between 15 and 24 ppm. When factors of instrumental error are applied, most values show some overlap. Sample 401, an andesite, con tains, as expected, the lowest Nb concentration. Sample 412 shows an equally low concentration. This sample was collected from a strongly silicified and argillically altered rhyolite, and it is likely that Nb was partially leached by hydrothermal fluids. The Rb, Sr, and Y concen trations for this sample also show unusual values. Pearce and Norry
(1979) cautioned that fluids containing complexing agents with high activities such as F may displace some of the trace elements under consideration.
The oldest rocks of the caldera sequence are rhyodacites of the
Steins Mountain ash-flovv tuff. Two welded tuff samples were selected for trace-element analysis. Sample 404 was collected from the upper
portion of the densely welded zone, whereas sample 411 came from a
slumped block of the megabreccia zone near the Beck mine at the south
ern caldera boundary south of the thesis area (fig. 9). Enders (1981)
believed that the welded tuff at this locality represents an early erup
tive phase that was extruded before the major pulse of ash-flow vol-
canism. The disparity between the trace-element concentrations in the
two samples suggests that these two tuff sequences were not derived
from the same magmatic pulse. A correlation between these tuffs may
be apparent in a trace-element analysis of samples throughout the sec
tion of the Steins Mountain ash-flow tuff.
There is some debate concerning the relative age of the
porphyritic andesite (Tua, fig. 2) in reference to the caldera. Richter
(1981, personal commun.) described a tuff sequence in the Roostercomb
area (fig. 9) west of the thesis area that is similar to the upper zones of the Steins Mountain ash-flow tuff found in the thesis area. At the
Roostercomb locality, however, the tuff is sandwiched between andesitic
flow units that appear to be members of the lower andesite sequence
(Tla, fig. 2). If the tuffs at the Roostercomb locality and the tuff in
the thesis area are equivalent and the andesitic flow that caps the tuff
at the Roostercomb locality is a member of the lower andesite unit (Tla, fig. 2), the so-called upper zone of the Steins Mountain ash-flow tuff
(Tsmp, fig. 2) and the porphyritic andesite (Tua, fig. 2) may be part of the lower andesite sequence (Tla, fig. 2) and be older than the
Steins Mountain welded tuff. This would be possible if prior to the eruption of the Steins Mountain ash-flow tuff sequence, deep 61
erosion of the andesite pile (Tla, fig. 2) left the nonwelded tuff in a
position on a southerly dipping slope where the Steins Mountain ash-
flow tuff lapped up against the nonwelded tuff. Present erosion could
expose the sequence with the nonwelded tuff appearing to overlie the
densely welded ash-flows. Field relationships between the upper, par
tially welded and nonwelded zone of the Steins Mountain ash-flov/ tuff
and the densely welded zone are not clear. Considering the partial exposures and the distribution of both the welded and nonwelded tuff sequences in the area shown on figure 2, the argument that the se quence progresses upward from the ash-flow tuff, pyroclastics, tuff- aceous sedimentary rocks through porphyritic andesite is most easily defended. The similarities between Y, Zr, and Nb concentrations found in samples 401, porphyritic andesite, and 404, Steins Mountain ash-flow tuff, supports this argument and suggests that the andesite and ash- flow were derived from the same magma.
Smith (1979) described a similar volcanic sequence at Crater
Lake, Oregon, where andesitic flows and tuffs cap a rhyodacitic ash-flow tuff. The Crater Lake system is only about 6,600 years old and offers an excellent opportunity to study fresh exposures in a cal dera sequence. The volume of erupted magma and the size of the
Crater Lake caldera is similar to that of the caldera described here.
Using the data presented by Williams (1942) and Williams and Goles
(1969), Smith makes the argument that a compositional gap in a single magma chamber must have existed at Crater Lake. He explained the fractional crystallization and differentiation left a compositionally zoned magma chamber with rhyolitic and rhyodacitic rns-gmcis u.t the top of the chamber, which were removed during the initial magmatic pulses. As no rocks of intermediate composition are known in the sequence, he argued that the andesitic magmas that cover the ash-flow tuffs must have coexisted with the more siliceous magmas in the chamber and were separated by a sharp interface. He added that similar systems with smaller magma-chamber volumes tend to exhibit an even stronger compo sitional contrast, whereas larger systems show more gradual changes.
He further added that lavas, in general, become more mafic with time in eruptions at caldera complexes. Many investigators, including Ratt6 and Steven (1967), Steven and Lipman (1976), Christiansen and others
(1977), Elston and others (1976), and Bailey, Dalrymple, and Lanphere
(1976) described larger caldera systems in the western United States where a change toward more mafic compositions of magmas is noted as increasing depths of the chamber are tapped.
The resurgent rhyolites in the thesis area offer special prob lems in interpretation. Individual flows and domes commonly show wide textural variations that often make correlation difficult. Structure dis ruption and sometimes poor exposure compound this problem. IVIost of the area covered by the moat rhyolites was not mapped in detail and trace-element geochemistry has been used to aid in verifying some geo logical conclusions made during the field study.
Of the element studied, Y and Zr both show a relatively wide variation in the sample suite and offer the best geochemical tools for correlating units and observing trends in the magmatic evolution. In the Y-Zr plot shown on figure 13 most samples of rhyolitic composition fall near a negatively sloping trend. Samples 405 and 412 deviate 63
Rock Types
o Rhyolite 200- X Rhyodacitic 412 Ash-Flow Tuff o • Oaelte 190- • Andeslts
406 o 80 ppm Y -f
80-
413 70- o o 410 406 ^ o 60- Ov, 403 402 407 vo 60- 411 o X 408 409 ^ ^ 404 40- • 401
H 1 1 1 1 1 1 1 1 1 1 1- 100 120 140 160 180 200 220 240 260 280 300 320
ppm Zr Figure 13. Yttrium and zirconium contents in caldera-related volcanic rocks 64 significantly from this trend. As mentioned earlier, hydrothermal fluids may have created changes in the trace-element concentrations in these samples. Both were collected near vein systems where hydrothermal al teration was noted in some surrounding rocks. The deviation of sample
411, a welded tuff from a slump block of the megabreccia zone south of the thesis area, from the trend suggests that it was either derived from a separate magma or that its parent magma was unusually influenced by the assimilation of wall rocks. Sample 407 was collected from a rhyolite dike that cuts the entire caldera sequence. Its deviation from the trend suggests that the dike was not emplaced during the main resur gent pulse but during a late eruptive phase from the addition of new magma to the chamber or from a foreign source. The relatively low Zr and Y concentrations in the sample are not indicative of a late differ entiate that was ejected during a late caldera-related phase.
A problem is posed by the unusually high Zr content in the porphyritic andesite (sample 401) and the Steins Mountain welded tuff
(sample 404) relative to the resurgent rhyolite groups that defines the linear trend (samples 402, 403, 408, 409, and 413). Considering the stratigraphic position of these units, which were erupted during a rela tively early period in the evolution of the caldera, their position in the trend indicates that the differentiation during which Y was enriched and Zr was depleted progressed. In a comparative study of the frac tionation trends if 24 volcanic suites, Pearce and Norry (1979) did not describe a trend of differentiation that follows this pattern. Inter preting experimental data, they did, however, find that fractionation of biotite and zircon would produce a trend of this tj^pe. During the 65 period of quiescence in the eruption of the caldera series between the episode of andesite volcanistn that delivered some of the caldera fill and the episode of resurgent volcanism, the addition of magma to the par tially depleted chamber further complicates the analysis of the geochem- ical evolution of the caldera. Such conditions as changes in the pattern of plate subduction, partial melting, and assimilation of wall rock can all influence primary magma chemistry at a deeper source. McBirney
(1979) supported the contention that significant magmatic changes between these two episodes are likely. He argued that the magmatic evolution in most eruptive systems is controlled by more than simple fractional crystallization. He explained that a deep-seated system is responsible for long-term variations between suites of rocks erupted in successive volcanic episodes, whereas variations at shallow levels result from fractionation and collection of a buoyant liquid under the roof of a high-level reservoir. O'Hara and Mathews (1981) pointed out the diffi culties in understanding the geochemical evolution of a system if the chamber is periodically replenished, periodically tapped, and con tinuously fractionated. They listed some 38 variables or conditions that
influence the geochemical signature or magmatic trends at a vol canic center. CHAPTER 4
ORE DEPOSITS AND IMPORTANT PROSPECTS
Most ore deposits and important prospects that occur within the boundaries of the thesis area (fig. 2) have been developed within the radial fracture system described in the previous chapter. All produc tion has come from silver mineralization with lesser amounts of gold.
Lead, zinc, copper, fluorine, and manganese occur in small quantities with the silver lodes but have never contributed to the value of the ore. One prospect in the northwest thesis area, the Princess Pat claim, shows strong manganese mineralization.
Three veins in the radial fracture system account for most of the development and are the sites of most of the prospecting in the thesis area and will be described separately. Because of current or past property and claim divisions, several mines or workings have been developed at different locations along the same vein and have been named separately. The most productive of these veins, referred to in this study as the Volcano vein, has been developed at the Guess, Vol cano, Wyman, Coyle, and Sixty-six properties and at an unnamed shaft at 3000 N, 5185 E (fig. 4). Drifts, shafts, and shallow prospect pits evacuated on the central vein include the Kimball, El Oro, and El Oro
Values workings and several unnamed shafts and pits located near 5860
N, 5080 E. The third vein, which extends from about 6250 N, 5650 E to 2800 N, 5475 E and is referred to here as the East vein, has been 67 explored through two shafts sunk near 5900 N, 5800 E and shallow prospect pits between 4800 N, 5720 E and 4080 N, 5700 E.
Development along other veins within the thesis area consists of numerous prospect pits, shallow shafts, and short drifts and adits.
Volcano Vein
The Volcano vein occupies the west fault zone, which defines the apical graben described in the previous chapter. As shown on figure 2, the vein can be traced along strike for a distance of over
3,500 m from 3000 N, 5200 E to 6700 N, 4250 E. If the poorly exposed vein at 1750 N, 4800 E can be connected to this system, as suspected, the total length of the vein system can be extended to nearly 5,000 m.
The fault plane normally dips 60° to 80° E. and is sinuous, ranging in strike from N. 40° VV. to N. 15° E. Strongly dilated zones are gener ally marked by brecciated and silicified wall rock, which is cemented by white quartz. Calcite may constitutes the major gangue where one or more of the walls is composed of andesite.
Silicification of the wall rocks is the most common alteration feature along the vein. It is especially well developed adjacent to strongly dilated zones where the footwall consists of partially welded to nonivelded Steins Mountain rhyodacite. Silicification extends to approxi mately 50 m from the vein at some localities. Andesite wall rocks generally show intense argillic alteration. The clay mineralogy of this alteration assemblage has not been determined. Calcite veinlets com monly riddle the argillically altered zones. Zones of moderately argil- lized andesite extends from 1 to over 30 m from the vein. 68
Vein intervals showing higher silver grades are generally asso
ciated with greater veins widths.
Volcano Mine
The Volcano mine straddles the Volcano vein at perhaps its widest part and has been the largest producer of previous metals in the
Kimball mining district. Because of the accessibility at the surface and through underground workings and the economic importance of the property, the Volcano mine was examined in the greatest detail.
The workings on four levels, the 100, 150, 200, and 300 levels
(29, 45, 45, and 87 m, respectively) were mapped at a scale of 1:240
(figs. 14, 15, and 16, in pocket). A fifth level, reported to be at 150 m (500 level), could not be visited as access through a winze was blocked by track and lagging used in the most recent mining operation.
One mining partner. Jack Hales, reported that when the mine was reopened in 1979, the level was accessible by ladder and that it was developed only as s'lort crosscuts into the hanging wall and footwall.
Developmental work along the vein at this level was not reported.
Figure 17 (in pocket) shows the mine workings as of September
1981 in a cross section projected to a perpendicular plane. Most early stopes were developed above the 100 level, where secondarily enriched ore was commonly encountered. During the most recent mining venture on the property, stopes were Hijv'^'oped at the north end of the mine above the 100 and 200 levels.
The vein system at thvi . mine is marked by one or more parallel veins or vein splits, whit., rist of strongly silicified wall- rock breccia and quartz. The vein is generally best developed in the
Steins Mountain ash-flow tuff footwall where the brittle nature of the rock made it more susceptible to fracturing, which left ample space for the ore fluids that followed to deposit base and precious metals.
Wall-rock Geology. The footwall of the Volcano mine consists of partially welded to nonwelded Steins Mountain ash-flow tuff. The rock is strongly silicified and forms a conspicuous ridge along the Volcano vein. In thin section, strongly embayed quartz eyes remain as the only phenocrysts. Cristobalite fills voids in the weakly eutaxitic ground- mass. Feldspar ghosts are occupied by clay minerals, and the original texture of the ground mass is obscured by secondary, quartz and clay minerals. At the 300 level, the rock shows stronger welding is less affected by hydrothermal alteration. Silicification and argillic alteration of the groundmass and feldspar phenocrysts are still moderately strong.
Biotite flakes are generally replaced by hematite, which gives the rock a reddish-brown color. Weak to moderate iron oxide staining is ubiq uitous at all levels. Silicification is controlled by the density of fractures in the footwall. On the surface, the zone of silicification reaches a width of about 50 m. On the 100 level, silicification is moderate throughout the entire 33-m length of the crosscut driven into the footwall at 5044 N, 4987 E (fig. 14). This is the longest crosscut in the mine. Here, four quartz veins, separated by strongly silicified, sheeted, and brecciated wall rock, give an overall vein width of 12 m.
The vein zone grades to the west into a zone of moderately silicified and fractured wall rock. Another strongly silicified zone is exposed 70
near the western face of this crosscut. Here, thin parallel quartz
veins control the silicification of a 1-m-wide zone. Similar alteration
habits were observed in the crosscut driven into the footwall at all levels. None, however, offers the same advantages in regard to lateral exposure as the crosscut on the 100 level.
A thin, 2 to 10 cm, gouge zone, consisting of brown or
greenish-gray mud, generally separates the footwall from the hanging
wall. The hanging wall is composed of strongly argillized andesite por
phyry (Tua, fig. 2). Veins are generally not well developed in the andesite but, where present, are composed predominantly of calcite with
equal or less amounts of quartz. Both manganoan calcite and white cal cite are common. An exceptional quartz vein in the hanging-wall
andesite is exposed in a crosscut on the 300 level at 5058 N, 4989 E
(fig. 16). In the stopes near the shaft above the 100 level, it is difficult to distinguish a sharp division between the hanging-wall and
footwall vein sets. Calcite accounts for 20 to 50 percent of the vein
material here, which may indicate that the stope was developed in a
relatively wide vein on the east side of the fault plane. If this is the case, it v/ould mark the only area where significant mineralization was observed in the hanging wall. Because the andesite is generally a poor
host to mineralization in the Volcano mine, crosscuts were driven into the east wall rock at only two locations on the 100 level and at only one location on the 300 level. Intense argillic alteration persists throughout the length of these crosscuts. 71
Mineralization in the Volcano Mine. The strongest vein inter vals are generally best developed adjacent to the west wall of the thin gouge zone that separates the andesite from the ash-ilow tuff. Multiple parallel veins, which pinch and swell along the sinuous fault plane, are common. Individual quartz veins sometimes reach 3.4 m in width. On the 300 level, as shown on figure 16, numerous vein splits to the south-southwest are common. The splits are usually narrow and pinch at a distance of several meters from the vein. A similar split, exposed at the surface, intersects the main vein structure at 5165 N, 4910 E and extends for about 150 m. Although present, crustiform or comb textures, common in many epithermal systems, are not typical features of the veins of the Volcano mine. Massive white quartz veins, which cement strongly silicified wall-rock breccia, predominates.
Eighty-seven channel samples were collected in the Volcano mine to determine the distribution of precious metals, both in the veins and in the wall rocks. These results have been compiled in the appendix.
Sample locations with the color-coded assay results for silver are plotted on figures 18, 19, and 20 (in pocket), which can be used as overlays with the geological level maps on figures 14, 15, and 16. In addition, silver assay values from 51 channel samples collected by
Fisher (1921) for a private consultant's report have been included on the geochemical overlays and are listed in the appendix. Fisher plotted his sample locations on roughly surveyed level maps, so the position of his samples shown on the assay maps in this thesis should be consid ered as approximate. Unfortunately, because all of his gold assay values contained a less than 1.70 g/t detection limit, a comparison of gold distribution in the vein sj'^stem is not possible using his sample values. The detection limit for gold for the assays of samples collected for this study is 0.03 g/t. All statistical calculations considering both gold and silver distribution have been made using only the values for samples collected for this study.
To determine the distribution of precious metals in the vein system, a separation was made between those samples collected at each level and those sample that included predominantly vein material, andesite wall rock adjacent to the vein. Steins Mountain ash-flow tuff wall rock adjacent to the vein, and Steins Mountain ash-flow tuff wall rock collected some distance from the vein.
Considerable judgment was necessary in the classification of these samples. Samples included in the major vein category included those collected across predominantly strong quartz-calcite vein material or quartz-cemented breccia fragments. Because of the intense silicifica- tion, it was often difficult to determine the boundary between the major vein and the welded tuff wall rocks. Material from zones marked by sheeting or breccia that did not contain a major contribution of quartz was not considered major-vein material. These zones, where identified, are shown on the geological level maps and sometimes contain significant assay values. Other investigators may have included these zones in a classification of vein material. If one end of the channel sample of pre dominantly wall-rock material included the boundary between the major quartz vein material and the wall rock, the sample was considered a wall-rock sample. The classification of "distal sample" was given to those wall-rock samples that do not include the vein border zones. 73
Narrow veins, veinlets, and splits from the major vein sometimes crossed the sample interval at these distal locations. Samples from these zones generally showed higher assay values than did samples that were not as well fractured. Samples included in the distal wall-rock classification were available only from the Steins Mountain welded tuff where sufficiently long crosscuts have been driven.
The purpose of this classification scheme was to identify the distribution patterns of precious-metal mineralization in the vein system.
Features such as maintaining a minable width or including wall-rock material lying between two veins that would be included in an analysis of the ore potential were not considered. For such a study, additional sampling directed toward obtaining samples at more regular intervals should be undertaken. A less strict definition of vein material that would include strongly sheeted or brecciated zones as well as massive quartz should also be be considered. From the type of analysis offered here, however, initial estimates of the feasibility of bulk tonnage mining versus selective mining are possible. A distinct division between the assay values of the vein material and the values expected from the inclusion of the distally located reserves necessary in a bulk tonnage mining venture is clear.
A statistical comparison of the assay values obtained from the underground sampling is presented in table 4. Mean gold and silver values, standard deviations, and correlation coefficient were determined for the assay results in each of the described categories. All sample types were included in a comparative analysis of each level and of the entire mine. 74
Table 4. Statistical analysis of gold and silver distribution in Volcano mine
Gold Silver Number Mine of Average Average Level Sample Type® Samples g/t s g/t s r
100 vein 11 155.7 134.4 0.44 0.74 0.42
100 Tua, hanging wall 12 46.1 31.4 0.37 0.46 -0.18
100 Tsm, footwall 8 59.1 40.5 0.34 0.38 0.81
100 Tsm, distal 21 49.3 63.8 0.38 0.59 0.44
150 vein 2 119.1 117.6 0.09 0.12 —
200 vein 12 122.3 109.7 0.26 0.29 0.22
200 Tua, hanging wall 2 15.4 2.4 0.2 0.02 —
200 Tsm, footwall 3 25.7 22.5 tr tr —
300 vein 9 173.9 224.7 0.31 0.38 -0.22
300 Tua, hanging wall 2 7.4 9.0 tr tr —
300 Tsm, footwall 3 58.9 40.3 0.32 0.21 0.32
300 Tsm, distal 2 8.6 7.3 0.03 0.05 —
All vein 34 146.6 150.5 0.32 0.49 0.15
All Tua, hanging wall 16 37.5 31.2 0.29 0.41 0.02
All Tsm, footwall 14 51.9 37.6 0.26 0.32 0.75
All Tsm, distal 23 45.8 62.0 0.35 0.57 0.46
All samples 87 84.9 112.0 0.31 0.47 0.19
a. Vein = sample including only strong quartz or quartz-cemented breccia fragments; Tua, hanging wall = sample of andesite porphyry collected from hanging wall adjacent to vein; Tsm, footwall = sample of Steins Mountain welded tuff collected adjacent to vein; Tsm, distal = sample of Steins Mountain welded tuff not adjacent to vein, may include subsidiary veinlets or brecciated zones. All 87 samples are chip- channel. For sample location and assay values, see figures 18-20 and appendix. 75
As expected, the quartz vein material yielded the highest gold and silver values for each level. A relatively high standard deviations for silver and gold assay values are characteristic of vein material and shows that the precious metals are not evenly distributed throughout the vein but are concentrated in ore shoots. In general, the wider vein zones produced the higher assay values for silver. This feature is shown on the sample assay overlay maps of figures 18-20. It cannot be shown that silver mineralization decreased with depth. The highest average assay value from vein material, 173.9 g/t, was obtained for samples collected from the 300 level. Two samples, 130 and 132, which contained 382.2 and 690/8 g/t silver, respectively, strongly biased the average assay value from this level and contributed to the high standard deviation of 224.7 g/t obtained for this sample set.
Gold and silver were not distributed together in the vein samples as indicated by the low correlation coefficient of 0.154 (± 1 = perfect correlation; 0.000 = no correlation). The correlation between gold and silver for all samples is not much better (r = 0.186). High gold values do appear to correlate with high silver values in the Steins
Mountain ash-flow tuff wall rock (r = 0.750), The average gold-silver ratios for vein material is 1:458. If only wall-rock and distal wall-rock samples are included this ratio rises to 1:146, suggesting that gold is more widely distributed in the Volcano mine vein system than is silver.
Another explanation for the increase may be that if background values for gold and silver are detectable in almost all rocks associated with mineralization in the Volcano mine, the higher ratio may reflect the relative enrichment of silver in the vein system where gold values 76 remain relatively constant throughout as a high background. The problem with this explanation is shown in the high standard deviations for gold values found in nearly all categories listed on table 4. It can be generalized, however, that gold is more widely distributed than sil ver. The average gold values, considering the four sample-type categories, are relatively more constant than average silver values.
Average assay values for gold range from a low of 0.26 g/t for samples from the Steins Mountain ash-flow tuff wall rocks to a high of 0.35 g/t in the distally located wall-rock samples. Generally, thin quartz veins were included in the channel samples that yielded the highest gold assay values in the distal sample set, suggesting that gold was prob ably more mobile than silver in the hydrothermal system. Samples from the 100 level were more enriched in gold than were samples collected at lower levels.
The Steins Mountain ash-flow tuff wall rocks adjacent to the major vein generally contained higher silver assay values than did other wall rocks. Silver values in the andesite wall rocks were for the most part low. An exception was found in samples from the 100 level where an average of 46.1 g/t silver was obtained from samples consisting pre dominantly of andesite. Several of these samples, however, were col lected from strongly sheeted and fractured andesite enveloped by major vein material (figs. 14 and 18). Cerargyrite, a secondary silver min eral, was identified as a fracture coating in some of these samples. It is suspected that the high values are related more to the structural irregularities in the vein that were conducive to secondary enrichment 77 than to a primary depositional factor such as depth in the system or wall-rock geochemistry.
Six samples from the underground survey were selected for analysis for copper, lead, zinc, and molybdenum. The results are listed in the appendix. Because of the small number of analyses, it is difficult to suggest any statistically significant base-metal trends in the mine. The samples do not indicate any major changes in base-metal content with depth. Copper, lead, zinc, and molybdenum values averaged 150, 650, 480, and <10 ppm, respectively. Discrepancies were found, however, in laboratory results for molybdenum in other samples collected for this study, and only a few of the samples were submitted to another laboratory for verification (appendix). For this reason, it is difficult to consider the molybdenum, distribution in the
Volcano mine vein system.
Four channel samples were collected across the Volcano mine vein at the surface near 5120 N, 4940 E to determine the surface expression of base- and precious-metal mineralization. The results of the analyses of samples 226, 227, 228, and 229 are listed in the appen dix. Sample 226 was collected across vein material overlying stoped workings believed to have produced high-grade ore from earlier work ings. Sample 227 includes strongly silicified and fractured Steins
Mountain ash-flow tuff, sample 228 includes vein material located north of the current underground working, and sample 229 includes silicified footwall material located next to the vein at the intervals sampled at site 228. Only trace amounts of gold and up to 41.1 g/t silver were found in the surface vein samples, and trace gold and up to 15.4 g/t silver 78 were found in the surface wall-rock samples. Averages of copper, lead, and zinc values dropped to 74, 1., and <10 ppm, respectively.
The apparent surface leaching of gold, silver, copper, lead, and zinc is important in interpretations of surface sample results for vein systems not exposed by underground workings. Antimony, arsenic, and mer cury values of 27, 1, and 0.167 ppm, respectively, were obtained from sample 226.
Fluorine analyses were made on three samples (132, 152, and
176) that showed strong silver mineralization on each of the three major levels. The fluorine concentrations detected in the samples ranged from
4.05 to 9.46 percent. Because no fluorine minerals were identified in hand specimens from the veins of the Volcano mine, it is suspected that laboratory errors may be responsible for these anomalous concentrations
If the analyses were correct, however, a strong correlation with silver content (r = 0.938) is indicated. Unfortunately, no samples from the surface were included in the fluorine analyses.
Ore Mineralogy. Minerals containing copper, lead, zinc, gold, and silver occur as very finely disseminated grains in the Volcano mine making macroscopic ore analysis difficult. Most of the older stopes were developed above the 100 level where secondary enrichment has added to the values of the ore. Because of its value, little ore of this type remains for examination. Sample 100 is a grab sample of very high grade secondarily enriched ore from a stope above the 100 level.
Cerargyrrite (AgCl) was identified as the major supergene silver mineral. It occurs as a greenish waxlike coating on fracture surfaces 79 within the major vein structure and among brecciated zones of the wall rocks of both andesite and Steins Mountain welded tuff. Silver chloride was rarely observed in workings on the 100 level and in stopes between the 100 and 200 levels.
Very fine grained native gold was observed in a pan concen trate from a crushed split of sample 100. To determine if tellurides may also hold some of the gold or silver values, tellurium analyses were performed on two samples (100 and 132) from secondary and primary ore zones. No tellurium was detected, suggesting that gold is dis tributed in the vein system only as native metal.
Primary-grade sulfide mineralization constituted most of the ore extracted during the recent mining operation at the Volcano mine. Un fortunately, the phase contributing the primary silver values was not positively identified. A very fine grained gray-black mineral, sus pected to be argentite, is finely disseminated throughout the quartz vein material. Attempts at obtaining an x-ray diffraction pattern and to grind a polished section across the unknown phase to make a positive identification failed. Argentite, argentiferous galena, and native silver are the most reasonable guesses for the mineral(s) contributing the silver values to the ore.
Antiuiony and arscnic analyses '.vere made on fl few samples from the 100, 200, and 300 levels to determine if sulfosalts constitute one of the ore minerals in the Volcano mine vein. The results of these anal yses are listed in the appendix. In all three samples, not more than 1 ppm arsenic and 36 ppm antimony were detected. The source of the weakly anomalous antimony values is not known, but it could indicate 80 the presence of rare sulfosalt minerals. A anonymous source reported that he had identified "feather ore" (one of several Pb-Fe-Sb-S or
Pb-Sb-S minerals, including jamesonite, zinkenite, boulangerite, and meneghinite, that sometimes form with a featherlike appearance) from a sample from the dump at the Volcano mine. Feather ore was also iden tified in this study in hand specimen (sample 220) at a shallow prospect pit exposing a narrow quartz vein northeast of the Volcano mine at 6375
N, 7075 E.
Galena, sphalerite, covellite(?), chalcopyrite, copper oxide minerals, mottramite, and possibly feather ore contribute to the base- metal values found in the samples collected in the Volcano mine. Inter- growths of galena (PbS) and sphalerite ((Zn,Fe)S) are found replacing pyrite in the sulfide ore zones. The possibility that the galena may be argentiferous was mentioned earlier. No galena or sphalerite crystals or crystal aggregates measuring over 2 mm in width were found.
Figure 21, a photomicrograph of a polished section, shows replacement of pyrite by sphalerite. Very fine grained needles of covellite(?)
(CuS), barely visible on the photomicrograph, and an unidentified phase form a rim at the boundary of the sphalerite grain. X-ray dif fraction patterns from a single crystal mounts in a Gandolfi camera show that small amounts of galena occur as intergrowths in sphalerite in the vein system.
Chalcopyrite (CuFeSg) is an infrequent phase found among pyrite in the ore zones. Bluish copper oxides, believed to be chryso- colla ((Cu,Al)2H2Si20g(OH)^-H20), were rarely found on the 300 level and in a stope above the 100 level. 81
Figure 21. Photomicrograph of polished section showing sulfide mineralization at the Volcano mine
Sample consists of vein material collected from stope above the 200 level. Sphalerite (sp) envelops corroded pyrite (py) in quartz (qt). X-ray diffraction patterns from similar occurrences in the Volcano mine show that sphalerite commonly contains intergrowths of galena (not evident in photomicrograph). Unidentified mineral (um, silver phase (?)) showing strong reflectance replaces sphalerite at rim. Thin needlelike covellite crystals (cv) are barely visible as intergrowths in unidentified mineral. Uncrossed polars. 82
Mottramite ((Cu,Zn)Pb(VO^)(OH)), the copper end-member of a solid-solution series with descloizite, was identified by X-ray diffraction methods. It occurs sparingly on the 100, 200, and 300 levels as a greenish-yellow, very finely crystalline coating on fracture surfaces and as a filling of small vugs in quartz veins.
The source of the anomalous concentrations of UgOg in samples
100 and 132 (70 and 100 ppm respectively) has not been identified.
Pyrite (FeSg) is common on all levels but rarely exceeds 1 percent by volume in the quartz veins. It is generally very fine grained and forms subhedral crystals in quartz vein material, often giving the quartz a grayish tint. Limonite after pyrite is commonly disseminated in the Steins Mountain ash-flow tuff wall rocks. Its oc currence is not restricted to the mine area; it is commonly distributed throughout the nonwelded and partially welded zones of the ash-flow sheet at nearly all exposures west of the Volcano mine.
As mentioned earlier, fluorine quantities of 9.46, 4.06, and 4.05 percent were found in samples 132, 152, and 176, respectively. Be cause no fluorine-bearing minerals were observed in macroscopic analy sis of the samples, it is suspected that a laboratory error is responsible for the anomalously high results. If the analyses were correct, clear or white, fine-grained fluorite was probably overlooked.
Pyrolusite (MnOg) is common as a coating on fractures and in vugs in the vein material and in the wall-rock voids. Additional man ganese occurs in manganoan calcite. An association between silver min eralization and the black manganoan calcite was not detected. Structural Control of Ore Deposition. Near strike-slip movement was mentioned earlier as the final direction of movement along the fault that now hosts mineralization at the Volcano mine. To identify the flexures in the plane of the vein that were conducive to the formation of dilatant zones in the Volcano mine, a vein contour diagram was prepared (fig. 22, in pocket). The method of vein contouring as described by Conolly (1936) involves contouring the perpendicular distance of the fault plane against an inclined reference plane passed nearly parallel to it. Clifton, Buchanan, and Durning (1980) have shown that this technique can be effective if strike-slip movement had a major influence on open-space requirements for vein filling. In the
Oatman mining district in northwest Arizona, where, as in the Volcano mine vein system, mineralization is hosted in a radial vein system in a volcanic setting, Clifton and others developed vein contour diagrams from surface, drill, and underground mine data and were able to predict buried dilatant zones along the fault structure that led to discoveries of additional ore reserves.
The expected effects of limited strike-slip movement along a sinuous fault surface produced by the structural events described in the previous chapter are illustrated on figure 23. The model suggests that nearly vertical ore shoots would be expected with this type of movement. The vein contour diagram of the Volcano mine (fig. 22) shows that the fault plane has a shape similar to that of a spherical cap. Most of the ore has been taken from stopes developed in an area on the upper right-hand quadrant of the diagram where the contours are especially steep in relationship to the reference plane. The steep FOOTWALL BLOCK - STEINS MOUNTAIN ASH-FLOW TUFF (Ttm)
Vein splits developed In footwall Pinch zones
Dllatant zoneo
HANQINQ-WALL BLOCK - ANDE8ITE PORPHYRY (tuar
Figure 23. Dilatation along a sinuous fault plane subject to limited strike-slip movement At the Volcano vein, strike-slip movement followed an episode of high-ansle normal-fault activity. Modified from Clifton, Buchanan, and Burning (1980), 85 interval is separated by relatively flat intervals near the center of the diagram and in the upper right-hand corner of the diagram. Consider ation of the 15- to 20-degree rakes on the slickensides observed in the gouge zones in the Volcano mine suggests that right- lateral slip at the gentle southerly inclination could have caused the dilatation in the fault zone in this region. Evidence for right-lateral slip along the Volcano mine fault plane was observed in surface exposures north of the Vol cano mine near 5760 N, 4480 E (fig. 4) where the fault contact between andesite porphyry and rhyolite is right laterally displaced by the
Volcano fault. Structure in the stopes developed around the shaft above the 100 level are more difficult to explain. An unidentified vein
•flexure probably occurs in this region. Due to moderate cover and surface development above this interval, adequate surface control on the position of the fault plane was not obtained. Also, because of access problems, a detailed survey of the upper stopes that may have aided in obtaining better definition of the fracture plane was not made.
Guess Workings
Workings at the Guess property consist of a short adit and several shafts and numerous prospect pits along the Volcano vein sys tem immediately north of the Volcano mine (fig. 4). These working have also been referred to as the Silver and Saddle workings by other investigators, but the property is general^ referred to as the Guess workings by local mine developers. The late John Guess, who resided about 1 km south of the thesis area, had owned the claims since the 1930s. 86
On the southern part of the property, both the hanging wall and footwall of the vein are in porphyritic andesite (Tua, fig. 2), which shows strong argillic alteration within a few meters of the vein.
Widespread propylitic alteration persists irregularly for up to several hundred meters from the vein. It is especially evident on the west side of the vein.
Immediately north of the natural-gas pipeline (fig. 4), the Vol cano vein is approximately 1 to 2 m wide and dips between 70° and 85°
E. Two shafts were sunk about 10 m apart at this location on the vein to an estimated depth of 15 to 25 m. The vein material on the dump consists predominantly of quartz, with ealeite contributing 10 to 25 per cent of the volume. Rare fine-grained pyrite casts were identified in the weakly iron-stained vein material. Honey-brown crystalline jarosite, forming 1- and 2-mm blades, was identified in a few of the voids in the vein material. Moderately strong silver mireralization (247 g/t silver and 0.07 g/t gold) was detected in sample 239, which includes a com posite of chips of vein material from the dump.
Between the shafts and the Guess adit to the north, the vein, which is exposed in several shallow prospects, is generally narrow. At a distance of about 500 m north of the two shafts, the vein widens and was explored by a deep pit and an adit at 5690 N, 4530 E. The adit was mapped at a scale of 1:240 and is shown on figure 24. The workings consist of a 46-m-long adit along the Volcano vein and an
18-m-long crosscut driven into the footwall. At its widest part, the vein exceeds 2 m in width. The vein consists predominantly of roughly banded calcite separated by irregular bands or lenses of gypsum and 87
190 (27.3 g/t Ag)
181 (5.1 g/t A 0 to 10 cm widegypsum seam splits quartz-calcite vein where dashed sheeted; strong arglllic alteration
5725 N sjiyi 92(18.9 g/t Ag)
88
hanging wall and footwati consist of argilllcally altered andesite porphyry (Tua)
189-\ (22.3 g/t Ag)
0 to 30 cm wide gypsum seam splits quartz-calcite vein where dashed
188 \ (317.2 g/t Ag 5700 N
SCALE maters Ii59
1 : 240 GUESS ADIT
Figure 24. Geologic map of Guess adit 88 brecciated quartz. The locations of three channel samples collected across the vein and one sample from the footwall and one from the hanging wall are also shown on figure 24. Moderately strong silver mineralization was detected in vein sample 188, which assayed 317.1 g/t silver and 0.75 g/t gold (appendix). Low silver and moderate gold contents were detected in the other two vein samples. Moderate silver
(56.6 g/t) was detected in the sample from the hanging wall, and low silver (5.1 g/t) was found in the footwall sample.
North of the Guess adit, several 10- to 20-m-deep shafts and a few prospect pits expose the Volcano vein, which dips between 60° and
80° E. None of the workings along this interval was entered. Near
5980 N, 4400 E, north of the point where both walls consist of rhyolite, the vein pinches to a point where it can no longer be traced. It can again be followed from 6200 N, 4400 E, where calcite fills the dilated fault zone.
Assay values for gold and silver drop in these northern exten sions. As shown on figure 25 (in pocket) and in the appendix, an assay value of 89.2 g/t was obtained for a dump sample (sample 338) from the shaft at 5730 N, 4500 E, immediately north of the Guess adit, and an assay value of 5.1 g/t silver was obtained for sample 339 from the dump of a shaft located to the north at 5850 N, 4450 E.
VVyman Property
Two shafts, one adit, and several shallow prospect pits expose the Volcano vein along the portion of the vein referred to as the Wyman property. North Wyman Shaft. The north Wyman shaft is the only working on the Wyman property that showed significant gold and silver production. The shaft was cleared and entered by a hand-operated windlass. Figure 26 shows in plan view the 90 (26.5-m) level that was developed near the bottom of the shaft. As mentioned in the introduc tory chapter, an estimated 550 tons of gold and silver ore were taken from stopes at this level. The only other development in the shaft con sists of crosscuts into both wall at a level 12 m below the surface.
This level was not entered. Silver assays of vein material, shown on figure 27 and list in the appendix, range from 89.1 g/t to 365 g/t and average 225.3 g/t. Gold values from vein samples range from trace amounts to 1.85 g/t, averaging 0.58 g/t.
Both walls consist of partially welded, strongly silicified Steins
Mountain ash-flow tuff. Although the crosscut into the hanging wall was not mapped, it is suspected that to the east, within a few meters, a fault contact between welded tuff and porphyritic andesite would be encountered, possibly with accompanying gold and silver mineralization.
As described in a previous section, parallel vein sets are common in the
Volcano mine, which is located immediately north of the Wyman property.
South Wyman Shaft. The south Wyman shaft follows the Volcano vein at a dip of 80 degrees to a depth of 22 m. Short drifts to the north and south along the vein are partially blocked by muck. The only other development in the working consists of a crosscut into the silicified Steins Mountain ash-flow tuff footwall. A level map and 90
60S0 E
'.•topad 4.S m b«low level crostcut at bottom of stope
strong quartz vein
stoped 2 m below level
stoped 6 m above level IX vein - strong quartz with sllicified ! wall rock breccia 4775 N 26 both wall rocks consist of sllicified Steins Mountain ash-flow tuff (Tsmp)
% vein pinches SCALE 10 meters
1 : 240 NORTH WYMAN SHAFT
Figure 26. Geological map of North Wyman shaft, 90 level 91
EXPLANATION S050 E ISlTchannsI sample location showing sample number, orientation of cttannel. and sample Interval. 394 Silver Assay Color/Symbol Code (from crosscut at bottom of stops) silver - S't ^•9 > 850.0 SB® 200.1 - 8S0.0 fESQ 100.1 - 200.0 60.1 - 100.0 M * Geochemlcal and assay values are listed in the appendix. 4775 N 388 (muck sample) 0 meters 1 : 240 Figure 27. Sample location and silver assay map, North Wyman shaft, 90 level Figure 26 is the corresponding geologic map. Samples by R. J. Pulfrey, Continental Materials Corporation, and T. H. Young, 1981. 92 accompanying' assay map are shown on figures 28 and 29. The shaft was entered by a hand-operated windlass. Four veins are exposed in the underground workings. The major vein, which defines the fault contact between porphyritic andesite and Steins Mountain welded tuff showed assay values of 577.6 and 257.1 g/t silver and 4.01 and 0.17 g/t gold over a 2.6-m width. Nearly parallel subsidiary veins cut the Steins Mountain ash-flow tuff to the west. Assays show that silver mineralization is present but is not as strong in these veins as in the major vein (fig. 29). The footwall is composed of strongly silicified and fractured Steins Mountain welded tuff. Argillically altered andesite makes up the hanging-wall vein. VVyman Adit. The 30-m-long Wyman adit, whose portal is located betvjeen the north and south Wyman shafts, was driven on the hanging-wall side of the slickensided gouge zone that separates Steins Mountain ash-flow tuff from porphyritic andesite (Tua, fig. 2). As shown on the geological map of the workings (fig. 30), the Volcano vein is exposed only in the crosscut that extends into the footwall. Assay values for two samples from the vein, listed in the appendix, show moderate to weak silver mineralization, averaging 81.4 g/t; gold averages 0.22 g/t. The vein zone exposed in the crosscut is 3.7 m wide. It con sists of zones of silicified ash-flow tuff breccia fragments, which are cemented by quartz and separated by seams of silicified intact wall rock and thin quartz veins. N 5128 E i Strongly silicifisd ash-flow tuff 4626 N (^backfilled recciated wall rock, i strong Fe oxides narrow quartz vein atiCiig quartz vein I moderately siliclfied and argillically altered Steins Mountain ash-flow tuff (Tsnop) leached and argillically altered andesite • porphyry (Tua) backfilled SCALE S 10 meters 1 : 240 SOUTH WYMAN SHAFT Geotogy by T.H. Young, June, 1881 Fipure 28. Geologic map of South Wyman shaft, 70 level 94 8126 E 4825 N- >^378(muck sampla) 10 meter* EXPLANATiCn k.i«Al58 Approx. chahnel sample location showing sample number, orientation of channel, and sample interval. -4600 N (see figure 27 for silver assay color/symbol code) Figure 29. Sample location and silver assay man, South VVvman shaft, 70 level Figure 28 is the corresponding geologic map. Samples by R. J. Pulfrey, Continental Materials Corporation, and T. H. Young, 1981. 95 6050 E 5075 E 24 4750 N west wall of adit sllckenslded gouge 2 to 5 cm thick — easi wail OT aaii argillically altered / andesite porphyry 194 (77.2 g/t Ag): 193 (85.7 o/t Ag'),:^ siliclfied, partially / welded Steins Mountain rhyodaclte ash-fkjw tuff vein - brecciated andi,\ siliclfied ash-flow tuff with quartz and calcite SCALE 4725 N 10 meters 1 : 240 WYMAN ADIT Geolog^byTJH^Toung^lajr^lS^ Figure 30. Geologic and sample location map of VVyman adit 96 As in all workings along the "Volcano vein, the rakes on the slickensides in the gouge zone are approximately 20 degrees toward the south. Coyle Mine Several shafts and prospects pits wer'; dug along the Volcano vein from 4400 N, 5160 E on the east flank of a silicified ridge immedi ately south the Wyman part of the vein (fig. 4). Strong silicification occurs in the upper partial welded and nonwelded zone of the Steins Mountain ash-flow tuff. None of the shafts was entered, but from sur face observations, limited stoping is suspected in some workings. Assays of vein material from the dumps of shafts showed silver contents as high as 1,383.3 g/t (sample 254) with gold reaching a maximum of 3.26 g/t (fig. 25 and appendix). An adit intersects the vein on the Coyle part of the Volcano vein 88 m from the portal, which is located at 4659 N, 5232 E. Figure 31 (in pocket) shows a plan view of the adit and drift. No stopes were developed in the workings. Chip-channel samples of vein material collected from these workings, shown on figure 32 (in pocket) and listed in the appendix, showed moderate silver mineralization along the vein, ranging from a high of 172.1 g/t to a low of 33.6 f/t. Gold values range from 0.24 g/t to 0.62 g/t in the vein zone. Similar to the distribution observed in the Volcano mine, the gold-silver ratio is significantly higher in the samples consisting predominantly of wall-rock material Llian in those samples consisting of nearly all vein material. The average silver and gold content of the sampled vein material is 75.0 97 g/t silver and 0.6 g/t gold. An average of 7.27 g/t silver and 0.34 g/t gold was detected in wall-rock material. From these averages, a gold-silver ratio of 1:125 is indicated in the samples containing predominantly vein material and 1:21 for those consisting of wall-rock material. The vein consists predominantly of quartz- and calcite-cemented breccia fragments of Steins Mountain ash-flow tuff with lesser fragments of argillically altered and silicified andesite porphyry. Sixty-six Mine The Sixty-six mine has been the second most productive mine in the thesis area. The shaft was sunk near the bottom of a wash and is now caved. An adit was recently driven toward the vein north of the caved shaft. The adit was stoped just short of an intersection with the major vein structure. The walls of the adit consist of intensely argil- lized porphyritic andesite cut by numerous thin calcite and quartz vein- lets. A plan map of the adit is shown on figure 33. No samples were collected in the workings. To the north, several shallow shafts were sunk into the vein, which dips between 57° and 73° E. As stated in the first chapter, an average grade of 788.5 g/t silver and 3.99 g/t gold were reported from a shipment of 495 tons in 1934 and 1935. Samples of vein material from the dumps and prospect pits located north of the Sixty-six shaft show strong silver mineraliza tion, with a maximum of 846.1 g/t silver and 3.50 g/t gold in sample 313 (fig. 25 and appendix). 98 6150 E bicachad, arglliically altered andesite porphyry composes both walls of adit N i II brecciated zone with minor quartz II SCALE 5 10 meters 240 SIXTY - SIX ADIT Geology by T.H. Young, July, 1881 Figrure 33. Geologic map of Sixty-six adit 99 Southern Workings Two shafts and several prospect pits were dug on the southern extension of the Volcano vein between 3760 N, 5230 E and 3000 N, 5170 E. Neither shaft was entered during the study. The northern shaft exposes an ill-defined structure. Vein material from the dump con sists of calcite- and quartz-cemented rhyolite and tuff breccia frag ments. The shaft at the southern location exposes a strong quartz vein containing about 5 percent very fine grained pyrite. A sample of vein material from the dump assayed 51.4 g/t silver and 0.86 g/t gold. El Pro Vein The El Oro vein can be traced for over 2,200 m from the south ern rhyolitic dome-flow complex near 3860 N, 5330 E to the northern moat rhyolites at 5980 N, 4980 E. The sinuous vein contains strongly dilatant zones where quartz and calcite fill open space. Splits from the vein to the northwest and west are common near the valley floor be tween the El Oro and El Oro Values mine (fig. 4). Reference points in the andesites or rhyolites that could be used to determine the displace ment along the fault plane were not identified. Note, however, that the disruption of the andesite-rhyolite contacts along the fault plane near 4680 N, 5250 E and 5860 and 5080 E is minor and displacement along the fault is probably not greater than a few tens of meters, but these con tacts are nearly vertical, making it difficult to make reliable estimates. As mentioned in the previous chapter, it is probable that this fault may not have been active during the early stages of resurgence, as were the other faults that host mineralization in the thesis area. 100 Pyrite is much more abundant along the El Oro vein than in the Volcano vein or the East vein. In several prospects pits near the El Oro Values dump, fine-grained pyrite occurs in the vein in quantities of up to 5 percent. Rare amethystine quartz was observed in the vein with very fine grained pyrite at 4200 N, 5300 E. Manganese oxide commonly coats breccia fragments and fracture surfaces. As in most vein intervals in the thesis area, manganese is more abundant where both wall rocks consist of rhyolite. Northern Workings Several shafts and prospect pits were dug near 5860 N, 5080 E toward the northern limits of the vein. None of the shafts was entered. In the north, the vein divides rhyolite on the east from porphyritic andesite on the west. The vein is 0.5 to 1.5 m wide and dips from 65° to 80° E. It is composed of 70 to 80 percent quartz, with the remainder consisting of calcite segregated into irregular lenses. As shown on figure 25 and listed in the appendix, vein material collected from several dumps at the prospects and shafts in this area show moderate silver mineralization. Sample 213 with the highest value (236.5 g/t silver and 3.77 g/t gold) was collected from the dump of a 3-m-deep prospect pit where a 1.5-m-wide quartz vein cuts porphyritic andesite. Kimball Adit The Kimball adit exposes about 17 m of the El Oro vein north of the central rhyolite plug at 5000 N, 5395 E. A plan map of the 101 workings is shown on Figure 34. The vein consists predominantly of intensely argillized andesite breccia fragments cemented by calcite and quartz. A thin gouge zone, dipping 55° to 70° E, is developed in the footwall side of the vein. Rakes on the surface plunge about 26° S. Both wall rocks consist of porphyritic andesite. Stopes were not devel oped in the workings; no samples of the vein material were collected. El Oro Adit The El Oro adit intersects the vein between the two centrally located rhyolitic dome-flow complexes. Figure 35 shows a plan view of the workings. Porphyritic andesite (Tua, fig. 2) is the country rock on both walls. Within 1 to 2 m of the vein, argillic alteration obscures the original porphyritic texture of the rock. Propylitic alteration, con sisting predominantly of chloritization, is intense around calcite veinlets, which cut the wall rocks. Elsewhere, propylitization is widespread but irregularly distributed. The vein, which consists predominantly of quartz-cemented breccia fragments, ranges in width from less than 0.5 m to almost 2 m. The vein dips from 50° to 60° E. As shown on figure 35, disseminated fine-grained pyrite, which is partially oxidized, constitutes up to 5 percent of the volume of the vein near the end of the workings. Bluish copper oxide associated with zones of disseminated pyrite coats vein material and altered wall rock. Here gold contributes more to the values of the ore than does silver. Average values for samples 412, 413, 414, and 415, taken from the vein where disseminated pyrite and copper oxides were observed (fig. 36), were 4.1 g/t gold and 57.4 *400 E wall rocks - argHMcally altered porphyrittc andesite —5000 N vein consists of brecciated waH rock with quartz and caicHe cement arglllic alteratioi^ moderate in crosscut SCALE 10 meters 1 : 240 KIIMBALL ADIT Figure 34. Geologic map of Kimball adit fine-grained disseminated pyrlte end Cu oxides, intense argiiiic alteration disseminated fine-grained pyrite 78 89 and weaic Cu oxides in hanging wall 76 both wall rocks composed of andesite porphyry (Tua), locally altered argillically and/or propylitics^lly V vein - predominat brecclated and si wall rock EL ORO ADIT Geology by T.H. Fig. 35. Geologic map of El Oro adit 103 I pyrite inging wall SCALE 10 meters 61 78 vein - predominately weak to moderate chloritization and brecclated end silicified 85 weak argillic alteration in wall rock 63 andesite porphyry . EL ORO ADIT Geology by T.H. Young. May. 1981 416 41T 413 415 414 412 419 420 422 411 09 410 408 407 406 405 404 403 02 401 400 397 Figure 36. Sample location and silver assay map, El Oro adit Figure 35 is the corresponding geologic map. Samples by R. J. Pulfrey, Continental Materials Corporation (1981) 104 EXPLANATION 1158 Approximate channel sample location showing sample number, orientation of channel, and sample interval. Csee figure 27 for silver assay ccior/symbol code# SCALE 10 meters 1 ; 240 403 401 400 399 396 397 398 395 % Oro adit i by R. J. Pulfrey, 105 g/t, giving a gold-silver ratio of about 1:14. Assay values for ail samples collected in the working averaged 0.04 g/t gold and 0.8 g/t silver, giving a gold-silver ratio of 1:20. No gold or silver minerals were observed in the vein material. Graton (1910), however, reported that finely disseminated free gold was identified in an oxidized portion of the vein at an unidentified locality somewhere near the El Oro workings. El Oro Values Workings The El Oro Values workings consist of a shaft and an adit driven through the fault contact between porphyritic andesite and the southern rhyolitic dome-flow complex. The shaft was not entered, but the adit was mapped in detail and is shown on figure 37. From the portal, the adit was driven along the vein that sepa rates porphyritic andesite on both walls. A thin fault at 4023 N, 5356 E shows right-lateral displacement bringing flow-banded rhyolite into contact with the porphyritic andesite. The adit continues to the west through the rhyolite and intersects an intrusive contact (?) between andesite and rhyolite at 3998 N, 5325 E. The vein is encountered again at 3998 N, 5320 E where both walls are composed of porphyritic ande site. At the surface, above this vein segment, the vein is enveloped by rhyolite. The vein consists predominantly of brecciated wall-rock frag ments cemented by quartz and minor calcite. It is stronger in the interval exposed near the portal. In the portal segment, as shown on figure 38 and in the appendix, silver assays averaged 170.2 g/t and 6325 E both walls argllllcaSly altered porphyrltlc andesito, brecclated ^ and Fe stained (Tua) • 4025 N contorted, flow-banded rhyolite t (Trpd) Fe oxides > in fractures SCALE 10 meters '54 '76 8 a vein - quartz and calcite cemented argillically altered andesite and tuff breccia fine-grained rhyolTte (?) / airfall tuff (Trf) sheeted zone -.'iis'.y.. (TO.) Interbedded airfall tuff and •rgilllcally altered porphyrltic andesite (Tua-TrlJ 5350 E EL ORO VAI Geology by Figure 37. "Geologic map of El Oro Values adit 106 )26 E both walls arollllcally altered vein - quartz cemented porphyritic andesite, brecclated breccia, strong Fe oxides and Fe stained (Tua) ^ * depth of shaft unknown vein material on dump j contains 2 - 5% finely' disseminated pyrite contorted, flow-banded rhyolite (Trpd) Fe oxides in fractures calcit illy alt brec ff (Tr 4000 N siisstsd rone y altered . ic andesite (Tuai luff sred e (Tu«-Trt) 5350 E EL ORO VALUES ADIT Geology by T.H. Young, May, 1981 7. Geologic map of El Oro Values adit 5325 E 6350 E 23 424 42 42? SCALE 10 meters 1 : 240 432 431 430 429 EXI 158 Approximi showing sampl channel, and t (aee figure 27 color/symbol Figure 38. Sample location and silver assay map, El Oro Values adit Figure 37 is the corresponding geologic map. Samples by R. J. Pulfrey, Continental Materials Corporation, 1981. 107 6350 E 23 424 426 42 427 4026 N SCALE 10 meters 240 4000 N EXPLANATION 158 Approximate channel sample iccstion •howing sample number, orientation of channel, and sample Interval. (see figure 27 for silver assay color/symbol code) Sample location and silver assay map, El Oro Values adit the corresponding geolo^c map. Samples by R. J. Pulfrey, Corporation, 1981. 108 gold assays averaged 4.77 g/t. Lower values were detected in the samples from the fault separated segment of the vein to the west where an average silver assay value of 45.2 g/t and gold of 2.25 g/t were reported. Very fine grained, gray metallic minerals, suspected of be ing argentite, were found in quartz vein material at the end of the adit. A small bead of silver was obtained after the sample was heated with a torch. East Vein Development along the East vein consists of two shafts sunk near the northern limits of the vein system and numerous prospect pits dug at various intervals along the vein, particularly between 4080 N, 5700 E and 4800 N, 5720 E. The vein is composed predominantly of quartz, but it contains significant calcite where both wall rocks consist of andesite. The East vein was traced from about 2800 N, 5475 E to 6250 N, 5650 E. South of 4000 N, 5700 E, talus from the southern rhyolitic dome-flow complex masks exposures of the vein. Although bedrock is no longer exposed, float derived from two shallow prospect pits at 3656 N, 5725 E shows that the underlying rock consists of tuffaceous sand stone and porphyritic andesite. Within 10 m to the east, Steins Moun tain ash-flow tuff is exposed, suggesting that the fault structure lies to the east between the prospect pits and the ash-flow tuff exposures. To the south, a vein exposed at 3040 N, 5600 E is believed to be an extension of this fault system, giving the structure an overall traceable length of about 3,300 m. 109 Central Prospects Near 4400 N, 5640 E, the wall rock on the east side of the vein consists of partially welded and nonwelded Steins Mountain ash-flow tuff. Strong silicification of the permeable welded tuff leaves a con spicuous ridge, similar to that observed at the Volcano mine. The wall rock on the west side of the vein consists of argillically altered porphyritic andesite (Tua, fig. 2) and silicified tuff breccia (Ttb, fig. 2). At this point, the vein is crustified, consisting of thin, repeating bands of drusy, light-green quartz. Rare amesthystine quartz was found in the vein interval near 4270 N, 5690 E. Moderately high silver assays were obtained from the vein mate rial that showed crustiform or comb textures. Elsewhere, the values were generally low. Assay data for the vein system is shown on figure 25 and listed in the appendix. The best values were found in a sample of chips of vein material from a dump at a prospect pit at 4460 N, 5650 E, where 411.4 g/t silver and 3.53 g/t gold were obtained (sample 322). An average for all samples collected from the central portion of the East vein was 0.55 g/t gold and 43.6 g/t silver. Northern Prospects Two shafts and several prospects, located near 5900 N, 5800 E, constitute the northern workings along the East vein. As shown on figure 4, several veins split to the west and southwest from the main fault structure here. They cannot, however, be traced beyond 20 m from their intersection with the major vein trend. 110 The northernmost shaft, at 5980 N, 5775 E, was sunk where the vein separates rhyolitic flows from strongly argillized porphyritic andesite. To the south, at 5890 N, 5860 E, a shaft has been sunk at the intersection of the major vein and a southv/est-trending vein split. The wall rocks in this working are composed of andesite, which shows strong argillic alteration adjacent to the veins. Neither of the shafts was explored during this study. As shown on figure 25, silver and gold assays from vein mate rial collected from the dumps of the shafts and across the vein in the prospect pits were moderately low, averaging 13.3 g/t silver and trace amounts of gold. The highest value was for sample 207, collected from the dump at the northernmost shaft, which showed 30.9 g/t silver and 0.07 g/t gold. Miscellaneous Workings Numerous prospect pits have been dug within the thesis area by prospectors searching for mineralization that might rival that found along the Volcano vein. Vein material or altered wall rock has been sampled from many of these prospects and from vein intervals not exposed by early diggings. The assay or geochemical results from this study are listed in the appendix. A discussion of base- and precious- metal zoning patterns will follow in the next chapter. A few of the more important prospects that are not develop ments along the radial fracture system are described in the following subsections. New targets suggested for future investigations that were identified during this study will be discussed in Chapter 6. Ill Princess Pat Claims A shallow prospect at 6675 N, 2620 E exposes a 0.5- to 1.5-m- wide vein that follow a late rhyolite dike (Tlr, fig 2) and is enclosed by it. The vein consists of silicified rhyolite breccia fragments cemented by quartz and manganese oxide. An assay from a chip-channel sample (233) collected across the vein showed only trace amounts of gold and silver. However, 11.50 percent manganese and 1,300 ppm zinc were detected in the sample. The vein could not be traced along strike beyond about 15 m to the north or south from the prospect. 6625 N, 5000 E Adit A northeast-trending quartz vein was examined in a 30-m-long adit near 6625 N, 5000 E. A plan map of the workings, including sample locations and assay information are shown on figure 39. The vein, which dips 55° to 65° SE. and ranges 0.5 to 1.5 m in width, is composed almost entirely of quartz. Strong manganese oxides coat breccia fragments and fill some voids in the vein material. Assays from two samples (217 and 218) of the vein material show a low silver content, averaging about 7.4 g/t. Only trace amounts of gold were detected in the vein material. 5860 N, 3400 E Adit An adit was drive 8 m into intensely altered andesite porphyry at 5860 N, 3400 E. Argillic alteration leaves the normally dark- colored andesite white and chalky with little of its original texture preserved. Thin limonitic quartz and calcite veinlets form a moderately dense stock- 112 5000 E 6625 N backfilled AQ) strong Mn oxides In wall rock breccia 56 both wall rocks consist of silicified and argillically altered flow-banded rhyollte vein - strong quartz with - Mn oxides, minor calcite 6600 N SCALE 10 meters 1 ; 240 6625 N. 5000 E ADIT Geology by T.H. Young, June. 1981 Fig"ure 39. Geologic and sample location map of adit at 6625 N, 5000 E 113 work in the vicinity of the adit. Sample 195, a composite of chips from the stockwork zone within the adit, showed only trace amounts of precious metals. 3475 N, 5375 E Adit An 8- to 10-m-long adit driven through the southern rhyolite dome-flow complex at 3475 N, 5375 E exposes the slightly argillized flow-banded rhyolite. No structures were identified in the workings. CHAPTER 5 BASE- AND PRECIOUS-METAL ZONING Limited analyses for copper, lead, zinc, and molybdenum were made on selected samples of vein material collected from surface or near-surface exposures throughout the thesis area. The districtwide assay and geochemical results of base- and precious-metal concentra tions are tabulated in the appendix. Figures 40-44 illustrate the re gional distribution of base- and precious-metal values obtained from vein material. Because of the high density of samples, sample locations for gold and silver and assay results for silver in the central mine area were not included on the smaller scale assay maps of figures 40 and 41 but are shown on the l;4,800-sc8le map (fig. 25). All molybdenum analyses were reported as less than 10 ppm (the detection limit of the laboratory). As discussed earlier, discrepancies were found in some sample results for molybdenum and it is not clear if the final results reported in the appendix are valid. Only a few samples were submitted to another laboratory for a check on the analyses (appendix). Within the thesis area, high values for gold and silver were generally restricted to the central mine area shown on figure 4. Be cause of leaching at surface vein exposures, the problems involved in the interpretation of results of surface sampling were discussed in the previous chapter in the section where the mineralization at the Volcano mine was discussed. The silver assay data presented on figure 25, 114 = I Karwi *> Ct i S-cit.- r. ••'„ LI. F^9K. •-§ •000 N- . ' y fs '>i - iff ^'n ./• >34 < 24» ®24t ^u«t® 237 ( -#46© V ,@203 ®"'. ... • >£. o ''•'iiR221 •^'• 2 . ^ , . ^ „©208 ' 212© ' ^ ®200 ^2^ ®280 16 . I • j^- : . - ^ i r e)242 - E-fiV H»r?r ^• . %, e 4000 H— fc-. • 2000 N- 4000 E •000 E •000 E Figure 40. Regional distribution of silver in altered rocks and veins Samples by R. J. Pulfrey, Continental Materials Corporation, and T. H. Young, 1 115 Silver Coior/Symbo! Code < ft99 ^ •000 N - •ft >990.0 200.1 - seo.o ^'n 100.1 • 200.0 •0.1 - 100.0 4$ ^^u-t<&237 9 € f®.1 - so.o , -«.. <2C.1 o i»2© ..«• 5?^ 02 to 'f «000 M- i.r*« .. a lion N f 25) i S>242 4000 It — |»»r (_ SCALE Km 2000 N- 1 : 40.000 •00^0 e • ^ •000 E t ± tion of silver in altered rocks and veins mtinental Materials Corporation, and T. H. Young, 1981. 1 I Rarifii* 4 ®0!d Si*** Stati'j. i ! ' -/fits • • ' ^ I I ^'C, - ' •000 I i - ^ ^ •—. ^ J. f34- :^24§ < (237 ^ ' ,€)»42 ^46fJ " 203 - ©233 €24-^'°® «.• i'.' •5 ^©221 832 @206 ©231 ©200 ©230 ©204 ©219 6000 N- .(tee mine area K rf >? 202 Minplemple locaUoi^loc .•/ST' map, figur iV ^ ' (I- ) • ©242 4 •• C 4C^ ' 4000 H- _ '**•' • "^^^^757" 19- - a: 1^37 •9 2000 N- •000 E Figure 41. Regional distribution of gold in altered rocks and veins Samples by R. J. Pulfrey, Continental Materials Corporation, and T. H. Young, l! 116 HZEzecsnTraBBcr^ai 1 f. Gold Coi'^r/Symbol Code •000 «/t ' •( ' .-."J >8.00 • - |)24« -• "i 1.01 - 3.00 a 0.S1 - 1.00 1249 >^^a:®237 ,®»42 9 0.1« - 0.60 •*v 0.03 - 0.16 <0.03 .©221 r' )S 252® @^2® ©21# ^ eooo M- > are* it pP- locatton N ure 25) II > • D242 4000 H- •337 SCALE Km 2000 N- 1 : 40.000 •000 E - •000 E * ' I ibution of gold in altered rocks and veins , Continental Materials Corporation, and T. H. Young, 1981. •r=r ' O'-l .iH« -'ati jo > -,<• Peak; 5 32^ •OQO N- s^' 023r 234* 0240! J 9 0^237 ^ 2460?b 24»;. 3'^ %2A^ 23t 4) 029f ©aaa • < i •^'XD ^ - J •coo N- ./ I 239 9 229 22«- 2«7 22 . o -J) 299^''" -•o >•• • . 1 _ -d :•'' Ar- T-®243 - . ^ . .. t (f_', _ 4000 N- ^ ; ;• • •.— -? - 'V > 3 • -^-^- :3r V 2CS ^ 2i I ' J * ';•* ••.;•»'( ^ •U'-- '• ' / •4 > • . ^ "j: C . • ^ • -'• r • I _W-.-2L. I r • '/'.\\ i ! - ; ' ,J ) I 200a N- 400^6; i •000 e- 8000 C I iiri i t' ' Fip-ure 42. Refdonal distribution of copper in altered rocks and veins Samples by T. H. Younp-. 117 Copper Color/Symbol Code |23aL. 8000 N o >18« o 78- 180 • 81 - 78 "• •*:»' . J # ^237 ^ J (» 31-80 •" - i _- '3 ^ € 10-30 0 < 10 9 •• •3^ ••• ^ • V. • / "• 'S- <• i - 't eooo N- ./ N F , o ,1 t (§243' 4000 N ^-0 3 KM 200a N- 1 : 40.00e tOOO C- 8000 E • i,7 L. I of copper in altered rocks and veins I ' V ' ' ' ' ' " Kaneh" ' )• ! •Oia >u>jp»-Suti.>ri rr^,-- ,' ; : •O«OM- ^ ' V .- 9i»A2» ^ ^ - —-f^W • / _ > -©Wj-^'ik . ' '•®»4«-i • i -%• _X - i--' - '- - • I . - ^ > V.. .-.-• , - , „ -, 'V -r... 0230 ^ ••:' • -.' '• - i crWJ.-n y ' - • ' 80(H) N- - - r ^:^:- 4000 N- tOOO N— 4000 E •000 c BOO.O E i Figure 43. Regional distribution of lead in altered rocks and veins Samples collected by T. H. Young, 1981. 118 T-^ Lead Color/Symbol Code D33fi eo«o N-' • >soo t t r J 0 ML - 600 9 101 - 200 •1 - 100 FI Y «T 23T M- d € 10 - SO hf: . ^ • « 0 < 10 '' •V , • J <*-•-• - «oao N- IIN 4 > ®243 » 4000 N- 40 II . *s»^ SCALE KM »000 M- 1 : 40.000 •000 E JBOOOJ^ ion of lead in altered rocks and veins ung, 1981. T-^ t -'Arf« .lutiun SJ 9 F^aki / If 8000 N €>23r " '234^ ^ -M—.. ^24aa ^24» 246 • V •2M ©2*% K •^" • '^©.3. .»• 3000 N - 4.... — M. ' 230 229 22* 13 2 22« 28i' ©243 4otto rr- 3)40 19- *-»j 2000 4000 G •000 E saoQt Figure 44. Regional distribution of zinc in altered rocks and veins Samples by T. H. Young, 1981. 119 on •.'/ • Zinc Color/Symbol Cod« • 4 t 8000 N- Vtm y, O >soo • > # 301 - 800 % 201 - 300 >24» ^ @^237 ^ 9 101-200 • jt • D 10-100 0 <10 * ^ V • ! ecoo N - N ' Ir' " 22» i22« ll t': ©243 •i 40d0 R- 240 1 ^ 1 '2 SCALE Km 2000 N- 1 : 40^ 6000 e 8000E U L-J_ ibution of zinc in altered rocks and veins 1981. 120 which shows the distribution of silver from surface sample locations in the central thesis area, include data from both samples collected at surface vein exposures and samples collected from mine and prospect dumps that include vein material from subsurface exposures. Most samples throughout the thesis area, however, showed anomalous silver concentrations for volcanic rocks assaying over .'? = 4 g/t. Exceptionally high silver concentrations in samples located beyond the central zone were found in samples 230, 234, and 246. These samples were collected from narrow quartz veins within or near the northwestern exposures of rhyolitic dome-flow complexes. Samples 254 and 246, assaying 58.3 and 204.0 g/t silver, respectivelj', were col lected from a vein set that may represent a distal extension of the Volcano vein system. Gold was normally detected only in trace amounts in samples collected beyond the central mine area. Copper analyses from surface vein samples showed concentra tions of up to 174 ppm (sample 249). Samples collected from veins crossing moat rhyolites often contained somewhat more copper than those collected from central caldera localities. As shown on figure 42, values were widely dispersed within the moat rhyolite vein systems, and it is difficult to suggest general trends for copper distribution from the sample population. Copper analyses from surface samples (226-229) at the Volcano mine, showing copper concentrations of 63, 76, 82, and 75 ppm, respectively, are only slightly more anomalous than those for the rest of the samples. Lead is concentrated in the veins that cross the moat rhyolite dome-flow complexes north of the Volcano mine (fig. 43). The highest 121 value, 1,100 ppm, was detected in sample 249, which also showed the highest copper concentration. Surface samples collected at the Volcano mine (samples 226-228), show the effects of surface leaching in the area. Lead was detected in concentrations ranging from less than 10 to 21 ppm in the surface samples, whereas lead ranged from 400 to 1,000 ppm at underground vein exposures. Zinc, like lead, is concentrated distally from the Volcano mine area in the moat rhyolite dome-flow complexes (fig. 44). The highest values, 1,500 ppm, was found in sample 233, which also contained strong manganese mineralization. Values of less than 10 ppm were re ported for all samples collected at the surface at the Volcano mine (samples 226-228). Concentrations of zinc detected in underground samples at the Volcano mine ranged from 200 to 1,000 ppm. To compare the relative distribution of copper, lead, zinc, gold, and silver in surface and near-surface samples within the thesis area, correlation coefficients have been calculated and are listed on table 5 for the 19 surface samples for which analyses for all five of these elements were made. Average geochemical or assay values and standard deviation from the mean are also listed for each element. A very poor correlation, with coefficients between -0.1 and 0.1, was found between copper-zinc, copper-gold, lead-zinc, and zinc-gold. The concentrations of copper-silver, lead-gold, and lead-silver showed a weakly positive correlation, whereas zinc-silver showed a weak nega tive correlation. Copper-lead, copper-silver, and lead-silver showed a moderate correlation. The high standard deviations calculated. 122 Table 5. Statistical analysis of assay and geochemical results from regional surface samples. — Sample locations shown on figures 40-44; assay and geochemical values listed in appendix. Correlation Coefficient Average s Element Cu Pb Zn Ag Au ppm ppm Cu 1.000 0.667 -0.094 0.295 -0.050 63.2 41.9 Pb 0.667 1.000 0.036 0.404 0.270 162.8 315.5 Zn -0.094 0.036 1.000 -0.175 0.064 168.6 315.0 Ag 0.295 0.404 -0.175 1.000 0.406 0.95 1.46 Au -0.050 0.270 0.064 0.406 1.000 0.001 0.001 123 especially for lead, zinc, and silver, showed the influence of a few highly anomalous values. Considering the apparent surficial leaching of copper, lead, and zinc from the vein material at the Volcano mine, the subsurface impli cations of geochemical values of base- and precious-metal concentrations in surface or near-surface samples should be considered carefully. One factor that might retard surface leaching is a decrease in the perme ability of the wall rocks. The Voleano mine samples were collected from vein material whose wall rocks were composed partially welded to non- welded ash-flow tuff and argillically altered andesite. In contrast, most anomalous base-metal values were obtained from samples where com petent rhyolite makes up one or both of the wall rocks. The degree to which a vein is exposed to weathering may be an important influence on the trace-element concentration of the sample. Simple base-metal zoning as a function of thermodynamic equi libria in the hydrothermal system may explain the geochemical distribu tions shown on figures 40-44, where copper, lead, and zinc appear to be distributed beyond a zone marked by gold and silver mineralization. A detailed study of the paragenesis of the ore minerals in the Volcano mine was not made and one can only speculate from the limited base-metal analyses in the Volcano mine that silver and gold mineral ization was associated with the delivery of copper, lead, and zinc to the vein structures. It is possible, however, that base and precious metals were not delivered synchronously to the vein structures. The distal fractures may have been more permeable than the centrally' located 124 fractures at the time of the hydrothermal episode that delivered base metals. Still another possibility that could have contributed to the base- and precious-metal zoning patterns is related to the stratigraphic posi tions of the veins sets. As discussed earlier, the formation of the Peloncillo arch caused the caldera sequence to be tilted from 5° to 15° N. Present exposures of the caldera sequence show a stratigraphic progression from south to north (fig. 3). Although vertical zoning has not been established in the Volcano mine vein, it is possible that minor lead and zinc mineralization persisted above precious-metal horizons, leaving precious-metal deposits exposed in the Volcano mine area and veins showing moderate base-metal values without a precious-metal complement in the northern exposures. In most epithermal systems, the opposite zoning pattern has been described in which lead and zinc generally are enriched below the precious-metal horizon (Buchanan, 1981). The logic of these arguments will be expanded in the next chapter in a discussion concerning the potential for mineralization at depth at the northern moat rhyolite fractures. CHAPTER 6 EXPLORATION CHARACTERISTICS All past production from the mineral deposits within the thesis area has come from shallow workings along epithermal precious-metal veins where stopes were generally developed in high-grade secondarily enriched gold-silver ores. Although most "bonanza"-grade ores at sur face exposures have been exploited, the recent rise in precious-metal prices and the possibility for bulk mining of the lodes make the primary ores an attractive exploration target. Characteristics of Epithermal Precious-metal Mineralization Precious-metal mineralization along the Volcano vein system ex hibits most of the characteristics that Lindgren (1933) and more recently Buchanan (1981) have defined as typical of epithermal depos its. An exploration model for the area should encompass the limitations and characteristics of this type of deposit with special emphasis on their relationship to the geological information gathered at the surface and in underground exposures during this study. The data presented in the previous chapters reveal several important characteristics of the mineral deposits that occur within the thesis area. Many of these ideas have already been discussed in detail but have been repeated below in a list of the important exploration characteristics of precious-metal mineralization in the Volcano mine area. 125 126 1. It has been shown that the brittle Steins Mountain ash-flow tuff is the preferred host to mineralization in the vein systems of the Vol cano mine area. The vein is generally best developed where one wall is composed of andesite and one wall of partially welded to nonwelded tuff. This characteristic deviates from the generally accepted notion that andesite is the most common host in epithermal systems (Silberman, McKee, and Stewart, 1976). Although andesites sometimes host signifi cant mineralization, the veins that cross andesites are generally composed of calcite that contains only minor precious-metal values. It is believed that this characteristic is related more to the physical nature of fracturing in the system than to wall-rock equilib rium in regards to the hydrothermal fluids. It was mentioned in Chapters 3 and 4 that a late strike-slip episode of faulting probably preceded the introduction of the hydrothermal fluids. Some evidence from underground exposures suggests that the andesites had undergone argillic alteration prior to the final deposition of precious metals. In a few places, breccia zones contain clasts of both argillically altered and relatively nonaltered andesite fragments along the fault plane. During strike-slip faulting, the argillized andesite walls may have reacted by forming gouge and a fine-grained breccia matrix, which tended to fill open space created during fault movement. On the other hand, the Steins Mountain ash-flow tuff, which may have been hardened by the introduction of silica from earlier fluids, reacted competently to strike- slip faulting, leaving the necessary open space for the transport of the mineralizing hydrothermal fluids that followed. The expected effects of 127 strike-slip movement following normal high-angle fault movement was described in Chapter 4 and is displayed on figure 23. The fact that the vein material in andesite locally shows zones of relatively high grade mineralization (samples 212 and 213 and samples fr m the El Oro and El Oro Values mines) precludes a conclusion that wall-rock geochemistry is the major factor controlling mineralization in the Volcano mine area. At the Beck mine, located south of the thesis area (fig. 1), mineralization is strongest in andesite wall rocks (Enders, 1981). 2. Mineralization along the radial fracture system appears to be restricted to a zone within 1,200 m of the centrally located rhyolite plugs. Although the vein systems persisted beyond this radius, anom alous precious-metal values in vein material drop significantly. Several factors may explain this habit. First, the role of the central plugs should be considered, at least as a heat source to drive the hydrother- mal system. Buchanan (1981) found that boiling in the hydrothermal fluid is an important, if not controlling, factor in the deposition of precious metals from a hydrothermal system. Boiling is controlled by both the temperature, chemistry, and hydrostatic pressure of the solu tion. If the heat source is confined to a centrally located shallow intrusive body and the fluids are driven laterally as well as upward from the cooling plug, thermal gradients would diminish away from the plug to the point where boiling may no longer be possible. Chemical changes such as mixing with other meteoric waters during the lateral migration of these fluids is also a possibility and could affect the deposition of precious metals. 128 It is generally accepted that epithermal deposits form in con- vecting water cells where water of largely meteoric origin circulates deeply in a volcanic pile, becomes heated, and dissolves metals (White, Muffler, and Truesdell, 1971). Bethke and Rye (1979), however, found that some of the water in the Creede, Colorado, system was of magmatic origin. White (1981) admitted that some magmatic water is indicated in a few ore systems, but in most systems the magmatic contribution is less than 10 percent, below the limits of isotopic detection. Although it is not clear that magmatic fluids are an important ingredient for epi thermal mineralization, it can be reasoned that their influence would diminish as the fluids traveled away from the source, and if they are important, a distal limit to deposition of metals could be established. 3. Unfortunately, neither the top nor the bottom of the mineralized interval is exposed at any of the recognized ore systems in the thesis area. At the Volcano mine, however, some general observations con cerning the vertical habit of the vein can be made. As mentioned in the Chapter 4, it cannot be argued that a significant vertical change in the primary ore grades or base-metal distributions can be found; how ever, the character of the vein does vary vertically. At the surface and on the 100 level, the vein is more complex, containing cymoids, horsetails, and numerous bifurcations (fig. 14). At the 300 level, the character changes. The vein is nearly vertical and occupies a better defined structure, consisting generally of a single, but still irregularly bending veins. Narrow vein splits into the west wall rock are common on this level (fig. 16). Also appearing on this level are infrequent, 0.1- to 0.5-m-wide openings on the hanging wall, or andesite side, of 129 the fault plane. These cavities are lined with fine-grained white calcite crystals resembling those formed by downward percolating ground water. Two reasons can be offered to explain these open-space features. First, the existence of openings, or so-called "water courses," in the absence of vein material at Creede, Colorado, signaled the bottom of the ore intervals (Steven and Rattd, 1965). It is in ferred that conditions in the hydrothermal system at Creede precludes the deposition of vein material as well as precious metals at this ele vation. It is difficult to use this logic in the Volcano mine, however, where the "water courses," or open spaces, are developed only in the calcite-rich andesite wall rocks. Strong quartz veins, sometimes showing very high silver assays (samples 130 and 132), are developed in the adjacent ash-flow tuff wall rocks. No evidence of flooding in the mine was found on any level, indicating that the ground-water table has been well below the 300 level for some time. At some time in the dis tant past, however, ground-water inevitably permeated this elevation. Because the ground water was slightly acidic, possibly from contact with oxidizing pyrite from adjacent partially welded and nonwelded tuff to the west (Krauskopf, 1967), some of the calcite making up the vein in the andesite wall rocks could have been dissolved by ground water. As the ground-water table was eventually lowered, calcite could have been deposited on the walls by precipitation from downward-percolating ground water. At the Beck mine, located south of the thesis area (fig. 1), the lower limit to nre is exposed in the underground workings. Ore-grade material below about 60 m is uncommon (Enders, 1981). Below the ore 130 intervals, the veins give way to muddy breccias containing angular wall-rock fragments and breccias that are weakly cemented by quartz and calcite. Coarse pyrite and infrequent coarse sphalerite and galena crystals are scattered throughout the breccias and in the argillically altered and chloritized andesite wall rocks. This zone appears to be similar to the horizons that Steven and Ratt6 (1965) described in their analysis of the Amethyst vein at Creede, Colorado, where these zones characterized vein intervals below ore horizons. 4. Significant precious-metal mineralization appears to be restricted to the radial vein set that radiates from the central resurgent rhyolite domes. Several factors may contribute to this phenomenon, some of which are related to the characteristics that were discussed earlier in this section. First, because the radial fracture set may have been opened by the same magmatic activity that drove the hydrothermal system delivering precious metals to the lodes of the Volcano mine vein system, the radial fractures could have provided the only adequately permeable structures for the mineralizing fluids during the hydrothermal pulse that delivered precious metals. Second, as mentioned earlier, the veins to the north of the Volcano mine represent an interval that was at a higher elevation during the delivery of the precious metals to the radial fracture set. It is possible that in these northern veins the interval above the ore horizon is exposed and that precious metals may have been deposited below the present surface. Several of these veins show sufficient width and anomalous base-metal contents to warrant closer examination. The contention that mineralization is not exclusive to the radial fracture set is supported by the observation that 131 mineralization at the Beck mine occurs in a set that is coincident with the southern ring fracture zone. This idea will be considered in detail in the following section concerning exploration potential of the northern veins. 5. It has been demonstrated that geochemical analyses of samples of vein material collected at the surface at the Volcano mine does not necessarily indicate the magnitude of mineralization at depth. It has been found that vein material collected at the surface has been nearly completely leached of base- and precious-metal values at the Volcano mine (samples 226-229). Anomalous mercury (0.167 ppm) and antimony (27 ppm) were, however, detected in one of these samples. Because mercury and antimony analyses were not made on surface samples from nonproductive vein intervals, the significance of these anomalies is not known. 6. Silicification of the wall rocks, reaching a width of up to 50 m, is particularly intense along the vein where mineralization is especially strong. At the Volcano mine, the Coyle mine, and near 4200 N, 5700 E along the East vein, the normally incompetent partially welded tuff and andesite form conspicuous ridges as a result of this alteration. The photographs of figures 8 and 45 display this topographic feature at the Coyle mine and along the East vein. Andesite wall rocks are commonly "bleached" or nearly complete ly altered to clay minerals within a few meters of the vein. Calcite veinlets commonly cut intervals where argillic alteration is strongest. Widespread propylitic alteration is common throughout the dis trict. Several episodes of propylitic alteration are recorded in the 132 Figure 45. Volcano vein, looking northwest toward the Sixty- six and Coyle workings Steins Mountain p.sh-flow tuff (Tsm) forms the foot wall and por- phyritic andesite (Tua) forms the hanging wall. Partially welded ash- flow tuff is strongly silicified at the Coyle workings and forms a prominent ridge. The dump from the caved workings of the Sixty-six shaft is shown in the foreground. The Sixty-six adit and several shal low shafts have been driven above these workings. The Coyle work ings along the silicified ridge above the Sixty-six developments consists of small shafts and prospects pits. 133 rocks of the thesis area, and it is difficult in some areas to distinguish specifically which propylitic alteration phase, if any, was related to the hydrothermal event that delivered precious-metal mineralization to the veins. 7. The wider veins generally contain better silver mineralization that do narrower veins. In Chapter 4, a vein contour, or "Conolly" diagram (fig. 22), was used to show special structural characteristics of the vein where dilated intervals along the fault plane host nearly ver tical ore shoots. This method could also be expanded to include the entire length of the vein system to predict hidden or buried dilatant zones along the radial fault structures. Exploration Targets Exploration targets in the thesis area can be divided into three groups: (1) an extension of the known mineralization along the cen trally located fracture system, (2) precious-metal mineralization at depth in the fractures that cut the moat rhyolite complexes north of the Volcano mine, and (3) possible porphyry molybdenum mineralization at considerable depth. Radial Fracture Set In most districts with a history of past mining, the best place to search for ore is where ore has already been found. In the Volcano mine area, this involves a reexamination of the radial fracture set that emanates from the centrally located rhyolitic domes and plugs. Explora tion should be directed toward determining both the economic feasibility of selective underground mining and the merits of bulk-tonnage mining. 134 For either type, zones along the radial fracture set that show maximum dilatation have been proved to host the best mineralization. Bulk- tonnage mining targets will be considered first. Much of what is impor tant in locating zones where bulk-tonnage mining methods may be applicable also apply to a search for selective mining targets. Bulk-tonnage Mining Targets. Consideration of bulk-tonnage mining requires that the zone of silicified wall rocks adjacent to the vein be of sufficient width and grade and that the ore be amenable to milling. Because it has been shown that geochemical analyses from samples collected at the surface do not necessarily indicate the degree of mineralization at depth, it is necessary to first recognize areas where sufficient mining widths might be realized before more costly measures are undertaken to determine subsurface grades. Additional geochemical analyses to gain a better understanding of the geochemical signature of both productive and nonproductive vein intervals may be helpful in predicting significant mineralization at depth. From the very limited sampling at the surface of the Volcano mine, antimony, mercury, and fluorine may prove to be useful pathfinder elements. As discussed earlier, vein contour diagrams could be useful in predicting additional zones where vein splits or wide fracture zones are likely to occur. Shallow drilling, however, will be the ultimate test of grade in zones where bulk-tonnage mining methods are considered. Several zones along each of the major radial vein sets show suf ficient width to consider close examination. The upper partially welded and nonwelded portions of the Steins Mountain ash-flow tuff offers the 135 most promising widths along the Volcano and East veins. A zone of strongly silicified wall rock reaching up to 50 m in width extends along the Volcano vein from the Coyle property near 4260 N, 5130 E (fig. 4) to the north end of the Volcano mine at 5400 M, 4800 E. The poorly welded ash-flow tuff on the East vein is strongly silicified at a width of approximately 25 m near 4500 N, 5690 E. Two intervals along the El Oro vein contain strongly fractured wall rocks, which contain relatively wide vein zones at the surfacc; these vein zones deserve consideration for bulk-tonnage mining. At 4500 N, 5200 E, the El Oro vein and the Volcano vein are separated by only about 30 m. Several vein splits to the south and south-southwest deviate from the El Oro vein here, and the vein itself splits into several parallel veins (fig. 4). The enclosing andesite wall rocks are affected by silicification and form a prominent ridge. At 4140 N, 5340 E, a vein, 0.5 to 1 m wide, splits from the El Oro vein to the north. Numerous narrow east-trending veins and veinlets connect the split to the major vein. The major quartz vein here contains relatively intense pyrite and rare amesthystine quartz but poor gold and silver assays values at the surface. Where possible, the wall rocks at the Volcano mine were sampled to gain an understanding of the distribution of mineralization in the vein system. The results were discussed in Chapter 4 in the section describing mineralization at the Volcano mine. The best indication of the grade potential in the distal wall rocks comes from samples collected in the 33-m crosscut into the Steins Mountain ash-flow tuff wall rocks to the west on the 100 level (figs 14 and 18). Here, the average 136 assay value of a 27-m interval that included only the rocks beyond the vein (samples 103-120) showed 38.7 g/t silver and 0.34 g/t gold, whereas samples 101, 102, 121, 122, and 123 from a 7.6-m-wide interval, which covered the vein zone adjacent to the crosscut, assayed 75.1 g/t silver and 0.67 g/t gold. It should be cautioned, however, that at the surface, other vein intervals along the Volcano vein to the north (5150 N, 4920 E) showed wider zones of silicification and may show stronger subsurface gold and silver values than the interval sampled in the cross cut. Selective Mining Targets. All past production from the northern part of the Kimball mining district has come from selective underground mining of relatively high grade intervals along epithermal quartz veins. Areas amenable to selective underground mining include those where sufficient tonnage and grade compensates for the relatively high cost of development and mining in a subsurface operation. The information gathered at the Volcano mine, presented in Chapter 4, pro vides guidelines to the potential grade and width of target areas. Although it is likely that some intervals of secondarily enriched ore remain in the vein systems, it is unlikely that new discoveries would show significant intervals of greater width and higher grade than those observed at the Volcano mine. The use of vein contouring in the search for dilatant zones along the fault plane that suggest intervals of strong mineralization and sufficient mining width at depth have already been discussed. These 137 methods are also recommended in an examination of the potential for selective mining along the radial fractures. Of the three major radial veins described in this study, the Volcano vein shows the greatest width at the surface and offers the highest potential for future development. The other two veins to the east, the El Oro vein and the East vein, also show intervals where suf ficient mining width could be realized and should not be overlooked. Strong gold and silver assay values were found in several samples from dumps, shallow prospect pits, and across veins along the El Oro and East veins (fig. 25). The true depth potential of the ore shoots could be tested only by deep drilling, as nowhere are the bottoms of the ore shoots exposed in the underground workings. An estimate of the potential depth of the mineralized zones, however, can be made by examining the reported vertical ore intervals in similar systems. A definite elevation marks the top and bottom of ore shoots in most districts. In a study of 60 epi- thermal districts, Buchanan (1981) found that precious-metal zones have a restricted vertical interval of up to 1,000 m, but typical uneroded deposits average close to 350 m. He cited boiling of the hydrothermal fluids as the major control. Drummond and Ohmoto (1979) found that boiling causes profound changes in the hydrothermal fluid. Major loss of COg and usually lesser amounts of HgS are partitioned into the vapor phase and aid in the precipitation of base and precious metals. They indicated that most base metals in a hydrothermal system precipitate after only 5 percent of the mass of the solution is lost to the vapor phase but about 20 percent of the solution must vaporize before the 138 bulk of the silver precipitates. Application of their data to the vertical habit of epithermal mineralization would indicate that as the hydrother- mal fluids rise the expected vertical zoning pattern would leave a lower base-metal zone followed upward by prominence of silver mineralization. This vertical zoning pattern has been described in many epithermal deposits, including the Beck mine located south of the thesis area (Enders, 1981). It is reasonable to suspect that the hydrothermal system that delivered precious metals to the Volcano mine system may also be related to the system responsible for mineralization at the Beck mine. If a certain elevation marks the lower ore interval throughout the district, as Buchanan (1981) has indicated is typical in many, but not all, epithermal districts, it may be possible to predict the depth of mineralization at the Volcano mine by using the lower limit of ore estab lished at the Beck mine. This task is complicated by the postmineral structural disruptions that shifted the volcanic strata between the two mines. If only the disruptions caused by the formation of the Peloncillo arch, described in Chapter 3, are considered, a tilting of the caldera sequence of 10° to 15° N. must be taken into account. Because the Beck mine lies nearly 4,000 m south of the Volcano mine at an elevation 60 m below that of the Volcano mine and mineralization at the Beck mine is rare below 60 m, a projected lower ore interval for the Volcano mine is between about 850 and 1,200 m below the present surface. This estimate suggests that an unusually deep ore interval was developed in the Kimball mining district. It is more reasonable to assume either that a constant elevation to the lower ore limit was not 139 maintained in the district, that postore faulting has redistributed these elevations between the two mines, or that mineralization occurred during or following the formation of the Peloncillo arch. According to Buchanan's (1981) descriptions of typical epither- mal systems, the habit of the vein at the surface at the Volcano mine, marked by numerous horsetails, bifurcations, and cymoids, represents a horizon near the top of the vertical ore interval. From his estimates of an average vertical ore interval of 350 m for epithermal systems, it is reasonable to suspect that ore shoots at the Volcano mine that show primary grades of mineralization may extend to a depth of about 300 m belov; the present surface. It should be emphasized that any estimate at this time of the depth of mineralization at the Volcano mine must be based on variety of assumptions that are difficult to establish with certainty. The estimates have been offered only to make suggestions as to what limits are pos sible for an initial evaluation. Deep drilling must be the final test. Northern Vein Targets Although anomalous gold and silver values were generally not detected in the veins cutting the moat rhyolites north of the Volcano mine, an examination of their potential for mineralization at depth is warranted because of their sometimes wide intervals and persistence along strike. It must first be established, however, if their exposures represent the upper unmineralized horizon of a zoned precious-metal system or if they are part of a hydrothermal system that was not of the 140 same nature as the one that delivered precious metals to the veins in the Volcano mine system and are barren of precious metals at depth. Mineralization at the Beck mine, where mineralization is not hosted in a radial set but in a complex fracture system near the ring fracture zone at the southern boundary of the proposed caldera, can be considered as evidence that mineralization in the district is not re stricted to the radial fracture system. In the Volcano mine area, the Beck mine area, and the northern vein set, late structural disruptions from the emplacement of rhyolitic magmas were noted and may have caused the development of the open fissures that provided an avenue for the movement of the hydrothermal fluids. In the area of the northern veins, the disruptions from the emplacement of the dome-flow complexes offer a possible mechanism to create open space for access by mineralizing hydrothermal fluids. Enders (1981) believed that some intrusive rhyolitic material is included in the sequence composed predominantly of ash-flow tuff that covers the ring fracture zone south of the Beck mine (fig. 9). Erosion has re moved any evidence that these "feeders" actually reached the surface and formed dome-flow complexes. The radial fracture set that hosts mineralization at the Volcano mine radiates from two centrally located rhyolitic domes or plugs. It is difficult to compare the nature of the northern veins and the radial fracture set because they are enclosed by different wall rocks. The rhyolites that normally enclose the northern veins reacted brittly. Veins commonly occur in very wide quartz-filled breccia zones, which in places rapidly narrow along strike. The widths of the veins 141 in the ash-flow tuff and andesites of the radial fracture system show a somewhat more regular habit, although intervals where the veins pinch and swell along strike are common. One characteristic of the northern veins that implies a possibil ity of mineralization at depth is the anomalous concentration of base metals detected in some of the surface samples. Figures 40-44, which display the base- and precious-metal zoning patterns in the thesis area, have been discussed in the previous chapter. Although other explanations concerning this zoning pattern have also been presented, it is possible that weak base-metal mineralization persists vertically above a precious-metal interval. Weissberg (1969) found that a major loss of COg and a lesser loss of HgS to the vapor phase caused by boil- ing of a hydrothermal fluid result in a rise in the activity of S 2- and HS that allows the formation of strong thio complexes with Au, Sb, and Hg. These complexes are stable to near the paleosurface where the higher fugacity of oxygen causes precipitation of the metals. Although other explanations could be offered, the formation of these complexes could explain the deposition of base metals above a precious-metal inter val in the northern veins. Supporting this argument is the identifica tion of "feather ore" minerals (one of several Sb-Pb-S minerals, the most common of which is jamesonite) from a prospect pit at 6325 N, 7075 E, which exposes a vein cutting the northern rhyolite complexes (sample 220). The identification of this phase marks the only place where base-metal minerals were identified in hand specimen of vein material from the northern rhyolite complexes. The identification of 142 "feather ore" minerals from vein material on the dump at the Volcano mine by an anonymous exploration geologist was mentioned earlier. Several areas were identified in the field study where particu larly wide vein zones were recognized that deserve special attention. 1. Along the radial fracture set, the parallel faults that define a northeast-trending fault zone near 6000 N, 8000 E form a particularly wide zone. 2. A generally narrow fault trending northeast forms a 75-m-wide moderately fractured zone at 6500 N, 7100 E. From a shallow prospect pit 50 m to the south, minor "feather ore" minerals, discussed earlier, were identified in a strong quartz vein (sample 220). 3. At 8000 N, 5500 E, the strong northeast-trending fault zone is particularly wide, spanning about 20 m (sample 235). 4. To the north, rhodochrosite occurs in the quartz veins that split the argillically altered rhyolite flows. Sample 236 was collected from this fault zone at 7975 N, 5450 E where veinlets form a moderate stock work zone in the rhyolites. 5. Sample 249 includes a composite of chips from a very strong breccia zone at 7900 N, 4950 E where iron-stained quartz cements angular rhyolite fragments. Strongly anomalous Cu, Pb, Zn, and F (174, 1,100, and 324 ppm and 6.34 percent, respectively) and detect able silver (5.1 g/t) were found in the sample. 6. At 6720 N, 4650 E, sample 206 was collected at a circular breccia-stock work zone measuring about 25 m in diameter. A fault zone was not traced to this breccia. 143 7. Sample 248 at 7075 N, 3925 E is from an east-west elongated 10-m-wide breccia zone where white and iron-stained quartz cements silicified and argillically altered rhyolite breccia fragments. 8. Two to 3 percent very fine grained pyrite occurs in sample 231 from the intensely altered zone at the fault contact between andesite and rhyolite at 6250 N, 3550 E. 9. Sample 195 is a composite of chips from a zone of intensely argillized bleached andesite from where a very short adit has been driven at 5860 N, 3400 E. No definite vein cuts the sequence. The andesite is broken by numerous quartz and calcite veinlets suggestive of the upper portion of a typical epithermal system as described by Buchanan (1981). Reasoning similar to that used earlier to determine the depth of the precious-metal ore interval at the Volcano mine by projecting the paleoelevation of the Beck mine ore zone can also be applied to the northern set. If mineralization at the Volcano mine represents a level near the top of the precious-metal interval, as suspected, the paleo elevation of the surface of the Volcano mine can be projected beneath the northern vein sets. For example, sample 249 was collected from a wide breccia zone located about 2,000 m north of the Volcano mine and at an elevation approximately 50 m higher. The paleoelevation of the Volcano mine before the formation of the Peloncillo arch would project to a depth 400 to 600 m below the surface at the northern site, assuming a 10° to 15° N. tilting during the formation of the arch. As stated be fore, when making estimates in this manner, it is necessary to make a variety of assumptions that are often difficult to support. In addition, 144 detailed mapping and detailed structural analysis would be helpful to verify some of these estimates. Fluid-inclusion studies of material from some of these northern veins could prove to be helpful to determine if and where precious- metal ore horizons occur at depth. The application of fluid-inclusions to the study of temperature in ore deposits has been recently reviewed by Roedder (1979). First, the deposition temperature in the ore zone at the Volcano mine should be determined. Buchanan (1981) found that precious metals are deposited in most systems near 240°C but a range of 200°C to 300°C has been reported for various districts. Depositional temperatures from fluid-inclusion data from the northern veins could then be compared to the depositional temperatures determined at the Volcano mine to determine the approximate vertical interval in the hydrothermal system. Buchanan (1981) believed that the temperatures of a hydrothermal fluid drops at a fairly steady rate as the fluid migrates upward. He stated that the temperature control is aided by the existence of a large thermal reservoir contained by wall rocks, which will prevent any major temperature drop of the hydrothermal fluid, even when boiling occurs. Clifton and others (1980) demon strated that in an evaluation of mineralization of an epithermal system in the Oatman mining district of northwest Arizona, a roughly linear rela tionship between depth to the top of the ore shoot and the fluid- inclusion homogenization temperatures can be shown. 145 Porphyry Molybdenum Targets Although little has been done to identify specific targets, the possibility that a porphyry molybdenum deposit is hosted in the rhyo- litic sequences of the caldera series should be considered. Noting some similar systems, major porphyry molybdenum deposits at Pine Grove, Utah, and Questa, New Mexico, are associated with shallow rhyolitic intrusive bodies in a volcanic pile containing weided ash-fiow tuff se quences (Abbot and Williams, 1981; Carpenter, 1968). Recent studies by Mutschler and others (1981), Sillitoe (1980), White and others (1981), and Westra and Keith (1981) have shown that, with the excep tion of the Quartz Hill deposit in southeastern Alaska (Hudson and others, 1980) and Mount Tolman, Washington (Westra and Keith, 1981), major "stockwork" molybdenite deposits of the western Cordillera of North America are associated with high silica, alkali-rich rhyolitic diapirs with a specific geochemical signature in some of the major- and trace-element abundances. Bookstrom (1981) and Sillitoe (1980) argued that the special geochemical signature of these deposits may be a logical product of differentiation from a magma produced during a stage of transition in the plate tectonic setting. Bookstrom suggested that during a relatively atectonic period when a change of steepening sub- duction to back-arc rifting was noted (Coney and Reynolds, 1977), magmas were developed from partial melting of the overlying mantle wedge above the slab and emplaced diapirically at shallow levels. Dif ferentiates of high-silica, alkali-rich rhyolite magmas, strongly enriched in volatiles and incompatible trace elements then developed, v^hich drove the ensuing hydrothermal pulse. 146 As shown on figure 46, the Volcano mine area was situated in a zone marked by high-potassium calc-alkalic magmatism between 35 and 25 m.y. ago, according to Westra and Keith's (1981) analysis of mag- matic activity in the western United States. Close examination of the magma geochemistry of the rhyolitic sequence in the Volcano mine area may indicate that the suite lies within or in transition with the nearby alkali-calcic zone to the east. The important molybdenum deposits at Urad-Henderson and Climax, Colorado, lie within the alkali-calcic zone. Mount Hope, Nevada, lies within the high-potassium, calc-alkalic zone of this time period. No whole-rock analyses were obtained for samples of the rhyo litic members of the caldera sequence in the thesis area, so it is dif ficult to compare major-element concentrations to those reported in other molybdenum sequences. The trace-element abundances of Rb and Nb detected in the samples of the rhyolite suites (table 3) does show af finities to those found in productive molybdenum systems. Westra and Keith (1981) reported Nb ranges of 14 to 271 ppm and P.b ranges of 150 to 1,000 ppm for unaltered rhyolitic and granitic rocks associated with molybdenum deposits of the alkali-calcic type and a range in Nb of 0 to 40 ppm and Rb of 175 to 550 ppm for igneous rocks associated with molybdenum deposits of the high-potassium calcic-alkali series. Figures 47 and 48 clearly show that trace-element abundances of Nb and Rb obtained from the rhyolites of the thesis area are contained within these value ranges. Niobium concentrations at the low end of the alkali- calcic range and near the center of the high-potassium calc-alkalic range suggests that the high-potassium calc-alkalic system may be the 147 /a^Nogal Peak •Volcano MIn*J/ ./ V 'jp /Th||«e Rljvera Stock \f -iV—'Cave Peak SCALE 500 Km Figure 46. Spatial distribution of magmatism and stockwork molybdenum deposition in the western United States between 35 and 25 m.y. B.P. — Modified from Westra and Keith (1981). 148 ALKALI- CALCIC 1(7) HIGH K CALC- ALKALIC VOLCANO MINE AREA RHYOLITEd I I I I I I I I I I I I I I 20 40 60 80 100 120 140 160 180 200 220 240 260 280 ppm Nb Fignre 47. Niobium content of unaltered and weakly altered felsic igneous rocks associated with alkali-calcic and high-potassium calc-alkalic stockwork molybdenum deposits and Volcano mine area rhyo- lites. Numbers in parentheses indicate number of deposits from which analyses were used. Modified from Westra and Keith (1981) ALKALI- CALCIC (6) HIGH K __ ALKALIC VOLCANO _- MINE AREA RHYOLITES 1 f- 200 400 600 800 1000 ppm Rb Figure 48. Rubidium content of unaltered and weakly altered felsic igneous rocks associated with alkali-calcic and high-potassium calc-alkalic stockwork molybdenum deposits and Volcano mine area rhyolites. Numbers in parentheses indicate number of deposits from which analji-ses were used. Modified from Westra and Keith (1981). 149 proper classification for the rhyolitic sequence of the thesis area, but Rb values are closer to the alkali-calcic range, suggesting that the magma series in the thesis area may be on the "transition type" in VVestra and Keith's (1981) classification scheme. They included the molybdenum deposits at Questa, New Mexico, Mount Hope, Nevada, and Glacier Gulch, British Columbia, in this classification. Deposits at Climax, Urad- Henderson, and Mount Emmons, Colorado, and Pine Grove, Utah, have been included in the high-potassium calcic-alkali system. It should be stressed that Nb and Rb abundances alone cannot be considered conclusive factors defining the magma series according to Westra and Keith's (1981) classification scheme. The discussion pre sented above is offered only to suggest that the potential for molyb denum mineralization cannot be ruled out based on trace-element comparisons with other productive systems and that if a search for molybdenum is Initiated, systems associated with rocks of similar magma series can be used to devise an exploration model. Additional detailed mapping and geochemical surveys will be necessary to further define the magma series. Some geochemical characteristics of the hydrothermal system show conflicting evidence to support porphyry molybdenum mineraliza tion in the volcanic sequence of the thesis area. First, enrichment of Cu, Pb, Zn, U, Mn, and F observed in some of the veins (appendix) has also been noted in other molybdenum districts of the so-called Climax-type deposits (White and others, 1981; Westra and Keith, 1981; and Mutschler and others, 1981). Apparently lacking, however, is an 150 enrichment of Sn and W. Unfortunately, very limited analyses for these elements were made, and additional sampling may identify zones where these elements are enriched. The problems in the laboratory analyses for IVIo during this study have already been discussed, and it is unclear what the actual distribution of molybdenum in the hydrothermal system is. Additional analyses may indicate a different distribution of values than is reported here. The highest value obtained, 14 ppm from sample 132 from the 300 level at the Volcano mine, indicates that, although anomalous, the molybdenum concentrations may not be sufficiently high to indicate strong enrichment, at least at the exposed level of the hydrothermal system. During the field study, several areas were identified that show structural disruptions that may suggest porphyry-type molybdenum min eralization at depth. Quartz-filled breccis at 7900 N, 4950 E, 6720 N, 4650 E, and 7075 N, 3925 E are conspicuously strong and, with the ex ception of the breccia at 7900 N, 4950 E, cannot be traced to a major fault zone. At least some of these are believed to represent upper levels of breccia pipes. These zones were described in the previous section in a discussion of the possibilities that they represent the upper levels of a mineralized vein system. A description of more favorable breccias in terms of molybdenum mineralization is repeated below. The breccia at 6720 N, 4650 E is confined to a circular area with a diameter of approximately 35 m. Quartz cements angular clast of argillically altered iron-stained rhyolite fragments for which some rotation is evident. Figure 49 shows the strong quartz encapsulation of Figure 49. Quartz-rhyolite breccia at 6720 N, 4650 E 152 the rhyolite breccia fragments, which appear to have undergone rotation. At 7075 N, 3965 E the breccia zone is elongated to the east- northeast. The affected zone is approximately 50 m long and 20 m wide. It is texturally similar to the breccia at 6720 N, 4650 E. Sample 248 shows anomalous concentrations of Pb at 204 ppm, Zn at 200 ppm, and Ag at 5.1 g/t. At 7900 N, 4950 E, the breccia zone is bounded by a major fault on its north side, near the intersection of two faults. It forms a circular exposure about 70 m in diameter. Of the three zones described in this section, the breccia here shows the most intense alteration tex tures. The rhyolite fragments are strongly silicified, stained to a dark reddish brown, and intensely argillically altered. Anomalous Cu at 174 ppm, Pb at 1,100 ppm, Zn at 324 ppm, F at 4.34 percent (?), and Ag at 5.1 g/t was detected in sample 249. To determine whether some of these zones represent breccia pipes, it is recommend that, first, detailed mapping in the vicinity of the breccias and the moat rhyolite complex of the north thesis area be completed. Such a study may indicate that the zones in question are simply dilatant zones in a sinuous fault plane where breccia fragments and quartz fill the voids. Second, additional geochemical analyses and petrographic work could be helpful to identify possible alteration halos or base-metal trends in the breccias, suggesting the true nature of these structural-geoeherfiical features. The rhyolite immediately south of the El Oro Values and Sixty- six mines (3975 N, 5225 E) is strongly fractured and silicified. Al 153 though the hydrothermal system that caused this alteration is believed to be related to the generation of the precious-metal veins, these veins should be examined as possible high-level expressions of a buried molybdenum system, if an examination of the molybdenum potential in the area is considered. The aeromagnetic map shown on figure 11 and discussed in Chapter 3 shows several areas near the center of the proposed caldera where a low magnetic response was obtained. This could be an expres sion of shallow buried rhyolitic intrusive bodies similar to those exposed near the zones of precious-metal mineralization where a similar aeromag netic low was noted. Gravity studies would be a more diagnostic geo physical tool to confirm the existence of buried rhyolitic intrusive bodies. An important consideration in the evaluation of the molybdenum potential in the thesis area is the depth to the expected mineralized zones. Geological evidence suggests that the present exposures of the moat rhyolites in the northern thesis area probably represents an interval that is within the top 500 m of the caldera sequence. Mutschler and others (1981) have found in an analysis of six major por phyry molybdenum systems that the estimated depth below the original surface at the time of mineralization to the top of the ore interval ranges from 600 m at Urad-Fenderson, Colorado, to 3,650 m at Climax, Colorado. The average depth they reported is about 1,500 m. Most of the ideas discussed above were drawn from only a few observations. The intent is not to identify specific targets in a molybdenum exploration model but to present evidence suggesting that 154 molybdenum mineralization is possible in the area and, if economic con ditions in the molybdenum market are considered favorable, relatively inexpensive exploration techniques could be used to gain a better understanding of the potential. The recommended procedures for further investigations are listed below. 1. Detailed mapping should be conducted in the moat rhyolite complexes to aid in the determination of whether the breccia zones de scribed earlier are breccia pipes or simply expressions of strongly dilated zones in a sinuous fault structure. 2. Geoehernical analyses for trace and major elements in the rhyo lite sequence should be made to help identify trends of differentiation in the magma geochemistry. The data could also be used for compari son with other productive systems. Cited earlier, Westra and Keith (1981), Mutschler and others (1981), and White and others (1981) offered excellent reviews and compilations of the geochemical characteristics of documented molybdenum systems. 3. Additional trace-element analyses of vein material should be made as a tool in understanding the possible hydrothermal dispersion of elements as an expression of a mineralized system at depth. 4. Gravity and ground magnetometer studies should be conducted to aid in the identification of the shape, depth, or existence of buried rhyolitic bodies. 5. Resistivity and induced-polarizaton studies or both might be useful in the identification of buried pyritic zones near intervals of molybdenum mineralization and to estimate the depth to the possible 155 mineralized interval. Considering the expected depths to the top of a sj'steiT!, discussed earlier, it may be difficult to interpret an electrical response derived from the use of these geophysical methods. 6. Finally, diamond drilling will be the ultimate test of molybdenum mineralization at depth. CHAPTER 7 CONCLUSIONS During this study attempts were made (1) to determine the re gional geological setting for mineralization at the Volcano mine; (2) to identify structural, geochemical, and lithological controls of precious- metal mineralization in the district; (3) to define exploration guidelines for ore search in the Volcano mine area; and (4) to delineate new exploration targets. Most of these goals have, at least in part, been met, and conclusions from this study can be discussed in terms of these four major goals. Regional Geological Setting Precious-metal deposits in the vicinity of the Volcano mine are hosted in a thick volcanic pile that was extruded during a series of magmatic pulses that began before 45 m.y. B.P. and ended around 6 m.y. B.P. Within the thesis area, portions of a small resurgent caldera have been recognized. The volcanic stratigraphy is dominated by caldera-related rocks, which include rhyodacitic ash-flow tuff, andesite, tuffaceous sedimentary rocks, air-fall tuff, and rhyolitic dome-flow se quences. The caldera sequence, which has been tentatively dated at about 34 m.y. B.P., transects and covers a thick series of andesite flowsj^,.Small basaltic plugs and narrow basaltic dikes, dated at about 6 m.y. B.P., intrude the caldera sequence. 156 157 Controls of Mineralization Precious-metal mineralization in the thesis area is largely con tained in quartz and quartz-calcite veins that radiate from a domed area near the center of the proposed caldera. The best mineralization occurs in faults associated with an apical graben near the east-central portion of the resurgent dome or define it. Several rhyolitic dome-flow complexes intrude this zone. Movement along the radial fracture system progressed as a complex series of events dominated by an early episode of high-angle normal movement with displacements in excess of 90 m. This episode was followed by near strike-slip movement where displacements between 5 and 20 m were recorded. Precious metals were deposited hydrothermally as open-space filling along the fault planes. The brittle siliceous upper zones of the ash-flow tuff sequence are generally the best hosts to mineralization, although significant zones of moderate to strong precious-metal mineralization have also been identified in andesite wall rocks. With a few exceptions, vein intervals showing moderate to strong silver and gold mineralization are restricted to a radial distance of about 1,200 m from the centrally located rhyoUte dome-flow complexes. The widest vein intervals generally contain the highest silver grades. Gold, which generally occurs in much lower quantities, shows a more erratic distribution. Vertically in the vein system, silver grades are generally highest in intervals between 10 and 30 m of the surface where secon dary enrichment has resulted in the deposition on thin films of greenish cerargyrite along fracture planes. Silver grades, averaging over 1,000 g/t, are commonly encountered in zones of strong secondary 158 enrichment. Primary grades, ranging between about 200 and 400 g/t extend at least to 87 m below the surface at the Volcano mine. The primary mineral phase that contributed silver to the ores was not iden tified during this study, but argentite, or possibly argentiferous galena or native silver, is the suspected primary silver mineral. Base-metal minerals at the Volcano mine include galena, sphalerite, chalcopyrite, coveilite, copper oxide minerals, mottramite, and possibly "feather ore" minerals. Exploration Guidelines During this study, geological mapping has been useful in iden tifying suitable zones where precious-metal mineralization is possible. Additional detailed mapping and structural analysis, including vein con touring, in the district may further delineate exploration targets. Because of the effects of surface leaching, geochemical sampling of surface vein material has not proved indicative of mineralization at depth. Limited analyses show that copper, lead, zinc, gold, and silver have been nearly completely removed from the surface outcrops of vein material above intervals of high-grade mineralization at the Volcano mine. Anomalous mercury and antimony values were detected in these samples, but because similar analyses were not made for samples at unproductive vein locations, it is not known whether these anomalies s-f-e exclusive of intervals of silver mineralization at depth. Highly anomalous fluorine and moderately anomalous uranium concentrations (discussed earlier, the high fluorine values are suspicious and labora tory errors may be responsible for the apparent anomalous 159 detected in subsurface sampling of vein material in the Volcano mine may also prove to be reliable pathfinder elements of precious-metal mineralization. These elements were not analyzed in surface vein samples. Numerous working's expose vein intervals below the surface and provide valuable information concerning- ore grades and vein structures. Fluid-inclusion studies could be useful to determine the likeli hood and depth of precious-metal mineralization in the northern vein systems. Only preliminary steps have been taken in this study to deter mine the potential of porphyry molybdenum mineralization in the rhyo- litic sequences in the thesis area. Detailed mapping of the rhyolitic rocks, additional trace-element studies of vein material, including Cu, Pb, Zn, Sn, WOg, U, and Mo, trace- and major-element analyses of the rhyolites for comparison to documented systems, detailed gravity and ground magnetometer studies to determine the depth and shape of buried rhyolitic bodies, and resistivity and induced-polarization studies for the identification of buried pyritic zones have been suggested as possible methods to determine the potential of a buried porphyry molyb denum system in the district. Deep drilling will be the ultimate test of molybdenum mineral ization in the area. Exploration Targets Before a search for additional zones of base- and precious-metal mineralization is initiated in the thesis area, a preliminary examination 160 of the economic constraints involved with the types of targets sought must be made to determine whether profitability is possible if a dis covery is made= Potential exists for relatively high-grade silver-gold vein mineralization that could be selectively mined by underground methods, relatively low-grade zones of silver-gold mineralization con tained in both vein material and scattered into the wall rocks by thin veinlets that could be mined by surface bulk-tonnage mining methods, and porphyry molybdenum mineralization that occurs in relatively deeply buried rhyolitic intrusive bodies. Targets involving selective mining of high-grade vein intervals by underground methods and targets containing relatively low-grade mineralization amenable to bulk-tonnage mining methods occur along the radial fracture system in the central thesis area. Vein intervals along the Volcano vein between the Guess workings and the Sixty-six mine offer the highest potential for additional discoveries. Bulk-tonnage potential is best where silicification has altered wide zones in the partially welded and nonwelded ash-flow tuff sequence, particularly be tween the Guess workings (buried) and the Coyle property. Restricted vein intervals along the East and El Oro veins show potential for addi tional discoveries for both selective and bulk-tonnage targets. Some of the veins that cut the northern moat rhyolitic sequence may contain significant precious-metal mineralization at depth. Addi tional detailed mapping and geochemical investigations will be necessary to further define the potential of these veins. Preliminary geochemical and structural analyses indicate that porphyry-type molybdenum mineralization is possible in the resurgent 161 rhyolite sequence in the northern moat rhyolites and in the centrally located resurgent rhyolite domes. It is estimated that the depth of a possible ore zone would be in excess of 500 m. Evidence that suggests that molybdenum mineralization is possible includes the suspicion that several northern quartz-cemented fracture zones represent breccia pipes, anomalous base-metal and fluorine concentrations in some of the northern vein sets or breccia pipes, and high concentrations of rubidium and moderate niobium concentrations in some of the rhyolitic rocks. APPENDIX GEOCHEMICAL RESULTS Results of analyses are reported for samples collected at surface and subsurface exposures. Samples 10 through 57 were collected by J. D. Fisher (1921) during a private consultant's investigation. Samples 100 through 249 were collected by T. H. Young in 1981. R. J. Pulfrey of Continental Materials Corporation supplied samples locations and assay results for samples 250 through 339, 351 through 370, and 395 through 422 collected in 1980 and 1981. Samples 371 through 394 were collected by T. H. Young and R. J. Pulfrey, 1981. Sample types included chip-channel (cc), grab (gr), composite of chips randomly selected from outcrops of altered or fractured rock (cp), and composit of chips of vein material from dumps (dp). Types of materials sampled are categorized by the predominant constituents of the sample and include: footwall (fw), vein (vn), hanging wall (hw), muck (mk), brecciated rock when not directly attributed to a vein structure (bx), and altered country rock (cr). Width of sample interval is given for chip channel samples. Widths for many samples (250 through 339) collected by R. J. Pulfrey were not available (na) for this study. 162 Sample Other, ppm (or wt%) Ag Au ~ No. Type Source Width, m g/t g/t Cu Pb Zn Mo" As Sb Te UgOg WgOg Sn F Volcnno Mine 10 cc fw 2.2 27.4 <1. 11 cc fw 0.5 61.7 <1. 12 cc fw 1.5 82.3 <1. 13 cc vn 3.7 394.3 <1. 14 cc vn 3.7 764.7 <1. 15 cc vn 2.1 216.0 <1. 16 cc vn 3.0 257.2 <1. 17 cc vn 3.7 171.4 <1. 18 cc vn 1.1 109.7 <1. 19 cc vn 1.8 305.2 <1. 20 cc vn 4.6 54.9 <1. 21 cc vn 2.0 171.4 <1. 22 cc vn 2.7 946.4 <1. 23 cc vn 1.7 92.6 <1. 24 cc vn 1.4 126.9 <1. 25 cc vn 1.3 92.6 <1. 26 cc vn 1.8 140.6 <1. 27 cc vn 1.7 504.1 <1. 28 cc vn 2.3 123.4 <1. 29 cc vn 2.4 250.3 <1. 30 cc vn 2.4 123.4 <1. Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo° As Sb Hg Te W^Og Sn F Volcano Mine 31 cc vn 2.4 99. 4 <1. 32 cc hw 2.1 30. 9 <1. 33 cc hw 2.7 96. 0 <1. 34 cc vn 1.5 442. 3 <1. 35 cc vn 1.5 171. 4 <1. 36 cc fw-vn 1.5 126. 9 <1. 37 cc fw 1.5 24. 0 <1. 38 cc vn 2.5 109. 7 <1. 39 cc vn 2.9 89. 2 <1. 40 cc vn 2.8 188. 6 <1. 41 cc vn 1.8 109. 7 <1. 42 cc fw-vn 3.8 102. 9 <1. 43 cc fw 4.3 89 2 <1. 44 cc fw-vn 1.6 44. 6 <1. 45 cc fw-vn 1.6 65. 2 <1. 46 cc fw-vn 1.8 44. 1 <1. 47 cc fw-vn 1.8 41. 1 <1. 48 cc fw-vn 1.8 65. 2 <1. 49 nc fw-vn 1.8 58 3 <1. 50 cc vn 2.1 92 6 <1. a> Sample Other, ppm (or wt%) Ag Au No. 1'ype Source Width, m g/t g/t Cu Pb Zn Mo** As Sb Hg Te UgOg ^3^8 Volcano Mine 51 cc vn 3.4 140.6 <1.7 52 CO vn 3.7 106.3 <1.7 53 cc vn 1.8 308.6 <1.7 54 cc vn 2.4 178.3 <1.7 55 cc vn 2.0 192.0 <1.7 56 cc vn 0.6 48.0 <1.7 57 cc vn 0.8 109.7 <1.7 100 gr vn - 24,224,2 204.71 1.65% 2,800 1,200 <10 5 26 0.067 0 70 101 cc fw-vn 1.5 30.9 0.17 102 cc fw--vn 1.5 30.9 tr 103 cc fw-vn 1.5 32.6 tr 104 cc fw 1.5 29.1 tr lOl cc fw 1.5 18.9 0.48 106 cc fw 1.5 58.3 tr 107 cc fw 1.5 51.4 2.33 108 cc fw 1.5 51.4 0.82 109 cc fw 1.5 5.1 tr 110 cc fw 1.5 10.3 tr 111 cc fw 1.5 8.6 0.27 m cc fw 1.5 30.9 tr 113 cc fw 1.5 37.7 0.69 Sample Other ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo® As Sb Hg Te UgOg WgOg Sn Volcano Mine 114 CO fw 1.5 42.9 0.41 115 cc fw 1.5 5.1 tr 116 CO fw 1.5 5.1 tr 117 cc fw 1.5 6.9 tr 118 cc fw 1.5 246.9 0.69 119 cc fw 1.5 5.1 tr 120 cc fw 1.5 51.4 0.48 121 cc fw-vn 1.5 72.0 tr 122 cc vn 1.5 180.0 2.47 123 cc vn 1.5 61.7 0.69 124 cc fw-vn i.r 202.3 tr 125 cc vn 1.2 36.0 0.17 126 cc vn 0.8 147.4 0.38 127 cc vn 0.2 13.7 0.31 128 cc vn 1.9 34.3 0.14 129 cc fw 1.7 1.7 tr 130 cc vn 1.3 382.3 0.03 100 500 300 <10 131 cc vn 1.4 42.9 tr 132 cc vn 1.8 690.0 0.24 200 700 400 <10 1 36 0.109 0 100 ^9.46% 133 cc vn 1.6 46.3 0.21 200 600 700 <10 134 cc hw 1.2 <1.7 tr Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo® As Sb Hg Te UgOg WgOg Sn F Volcano Mine 135 cc hw-vn 1.0 77.2 0.27 136 CO fw 1.5 99.4 0.27 137 cc fw 1.5 3.4 tr 138 cc fw 1.5 18.9 0.14 139 cc fw 1.5 39.4 tr 140 cc fw-vn 1.5 58.3 0.55 141 cc fw 2.1 13.7 0.07 142 cc vn 2.0 30.9 1.20 143 cc fw-vn 2.3 65.2 0.58 144 cc vn 0.6 27.4 tr 145 cc hw-vn 0.9 101.2 0.48 146 cc vn 1.5 70.3 tr 147 cc hw-vn 1.5 58.3 0.41 148 cc vn 1.8 135.4 0.38 149 cc fw 1.5 6.9 tr 150 cc fw-vn 1.8 13.7 0.31 151 cc hw-vn 1.0 27.4 0.14 152 cc vn 2.8 457.8 0.99 200 700 1000 <10 1 34 0.151 ^4.06% 153 cc vn 0.7 34.3 0.27 154 cc hw 1.6 17.1 1.61 155 cc fw 1.9 121.7 0.79 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te UgOg WgOg Sn F Volcano Mine 156 CO fw 2.0 207.5 1.47 157 CO fw-vn 1.7 82.3 0.86 158 CC fw-vn 1.9 99.4 0.72 159 CO hw-vn 1.1 39.4 tr 160 CC vn 1.9 68.6 tr 161 CC fw 1.3 84.0 0.03 162 CC vn 1.9 90.9 tr 163 CC hw-vn 1.7 111.4 tr 164 CC fw-vn 1.4 346.3 0.41 165 CC fw 1.7 24.0 tr 166 CC fw-vn 0.7 149.2 0.14 167 CC hw 1.2 34.3 0.34 168 CC hw 1.2 17.1 0.48 1G9 CC fw 1.3 65.2 tr 170 CC vn 1.1 24.0 0.07 171 CC fw 1.9 27.4 0.21 172 CC vn 1.3 159.4 0.34 173 CC fw-vn 1.8 18.9 0.17 174 CC fw-vn 2.0 238.3 0.27 175 CC fw-vn 2.0 65.2 0.07 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te UgOg WgOg Sn Volcano Mine 176 cc vn 1.5 330.9 tr 100 1000 300 <10 <1 30 0.039 ^4.05% 177 CO vn 1.8 303.5 0.79 178 cc hw 0.7 13.7 0.10 179 cc fw-vn 1.7 97.7 tr 180 cc hw 1.9 13.7 tr 181 cc vn 1.1 190.3 0.21 182 cc fw-vn 1.5 56.6 tr 183 cc vn 0.9 77.2 0.10 184 cc hw 1.1 17.1 0.14 185 cc vn 1.6 84.0 0.86 100 400 200 <10 186 cc fw 2.1 29.1 tr 187 cc fw 1,0 46.3 tr Guess Adit 188 cc vn 1.3 317.2 0.75 189 cc hw 1.1 22.3 2.30 190 cc vn 2.1 27.3 1.03 191 cc fw 1.3 5.1 tr 192 cc vn 1.0 18.9 0.07 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo° As Sb Hg Te UgOg ^3*^8 ^ North Wyman Shaft 385 cc fw 1.2 92.6 0.55 386 cc fw-vn 0.4 61.7 0.45 387 cc fw-vn 1.2 89.2 0.41 388 cp mk - 130.3 tr 389 cc vn 1.1 116.6 1.85 390 00 vn 1.0 252.2 0.14 391 cc vn 0.9 301.8 0.51 392 cc vn 1.2 354.9 tr 393 00 vn 1.4 365.2 0.62 394 CO fw 1.6 51.4 tr South Wyman Shaft 371 cc fw 1.3 37.7 0.27 372 cc hw 0.3 121.7 2.19 373 cc vn 1.2 257.2 0.17 374 cc fw-vn 1.8 577.8 4.00 375 cc fw-vn 1.8 37.7 4.10 376 cc fw-vn 1.8 144.0 0.58 377 00 fw 3.7 25.7 36.0 378 cp mk - 155.3 0.69 379 CO vn 0.7 109.7 0.31 380 CO fw 0.7 61.7 6.86 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te ^3^0 Sn F South Wyman Shaft 381 cc fw 2.7 145.7 0.59 382 CO fw 1.0 34.3 0.41 383 cc vn 0.6 1.7 0.21 384 cc vn 0.6 61.7 0.55 Wyman Adit 193 cc fw-vn 1.8 85.7 tr 194 cc fw-vn 1.8 77.2 0.45 Coyle Adit 225 cc vn 0.8 41.1 tr 351 cc hw 3.1 <1.7 0.34 352 cc hw 3.1 <1.7 0.27 353 CO hw 3.1 <1.7 0.27 354 cc hw 3.1 <1.7 0.27 355 cc hw 3.1 <1.7 0.21 356 cc hw 3.1 2.7 0.62 357 cc hw 3.1 2.1 0.34 358 cc hw 3.1 19.5 0.51 359 cc hw 3.1 15.1 0.44 360 cc hw-vn 3.1 49.0 0.51 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te UgOg ^3^3 ^ Coyle Adit 361 CO vn 3.0 86.4 0.34 362 cc fw-vn 3.0 64.8 1.71 363 cc fw 1.8 35.3 0.24 364 cc fw 1.2 66.2 0.75 365 cc vn 0.9 172.1 0.48 366 cc vn 0.9 145.0 0.31 367 cc vn 0.9 66.2 0.69 368 cc vn 0.9 ' 89.8 0.82 369 cc vn 0.9 33.6 0.24 370 cc vn 0.9 36.7 0.31 El Oro Adit 395 cc hw-vn 0.8 15.1 0.17 396 cc vn 1.8 12.7 0.34 397 cc vn 0.9 12.7 0.34 398 cc vn 1.8 9.6 0.17 399 cc vn 2.1 19.9 0.51 400 cc vn 2.1 72.7 1.27 401 cc vn 0.8 93.3 1.0 402 cc vn 0.9 9.6 0.34 403 cc vn 0.9 58.0 2.13 404 cc vn 1.1 34.6 0.82 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo® As Sb Hg Te El Oro Adit 405 cc vn 0.9 16.1 0.79 406 cc vn 1.1 5.8 0.41 407 cc vn 0.9 24.7 1.44 408 cc vn 1.1 6.2 0.86 409 cc vn 0.9 43.2 2.74 410 cc vn 0.9 11.0 1.03 411 cc vn 1.1 37.0 2.13 412 cc vn 0.9 64.8 4.01 413 cc hw 00 75.4 5.35 414 cc vn 0.9 1.7 5.66 415 cc vn 1.1 87.8 1.37 416 cc fw-vn 1.8 23.7 1.27 417 cc vn 1.1 15.1 0.34 418 cc vn 1.1 18.2 0.24 419 cc fw-vn 0.6 5.1 0.21 420 cc fw 1.8 <1.7 0.14 421 cc fw 1.2 7.2 0.14 422 cc fw 1.2 <1.7 1.10 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te U^Og ^3^3 ^ El Ore Values Adit 423 CO vn 0.9 349.8 13.13 424 cc vn 0.8 170.8 1.37 425 cc vn 0.6 162.5 1.58 426 cc vn 1.1 208.1 3.36 427 cc vn 0.3 36.3 0.41 428 cc vn 1.1 93.6 8.78 429 cc vn 1.8 5.5 0.27 430 cc vn 0.9 82.3 3.22 421 cc vn 0.9 46.6 3.09 422 cc vn 1.1 46.3 2.40 6625 N , 5000 E Adit 217 cc vn 1.1 13.7 tr 218 cc vn 0.8 <1.7 tr 5860 N , 3400 E Adit 195 dp vn - <1.7 tr Regional Surface 196 dp vn - 58.3 0.17 197 dp vn - <1.7 tr Sample Other, ppm (or wt?i) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te ^ Regional Surface 198 cp bx - <1.7 tr 199 cc vn 0.6 3.4 tr 200 dp vn - <1.7 tr 201 cc 2.0 <1.7 tr 202 cc vn 7.6 <1.7 tr 203 dp vn - <1.7 tr 204 cc vn 0.9 <1.7 tr 205 cc vn 0.6 <1.7 tr 206 cp bx - <1.7 tr 207 dp vn - 30.9 0.07 208 dp vn - 15.4 tr 209 cc vn 1.2 92.6 1.10 210 dp vn - 92.6 1.78 211 cc vn 0.9 29.1 tr 212 dp vn - 114.9 2.67 213 dp vn - 236.6 3.77 214 dp vn - <1.7 tr 215 cc vn 0.9 <1.7 tr 216 cc fw 0.6 5.1 tr 219 dp vn - <1.7 tr 220 cc vn 0.6 <1.7 tr Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo® As Sb Hg Te UgOg W^Og Sn F Regional Surface 221 cp bx - 6.9 tr 222 dp vn - 5.1 tr 223 cc vn 0.6 10.3 tr 224 cc vn 1.7 17.1 tr 226 cc vn 1.8 41.1 tr 63 <10 <10 <10 1 27 0.167% 227 cc fw 6.1 10.3 tr 76 21 <10 <10 228 cc vn 2.4 39.4 tr 82 <10 <10 <10 229 cc fw 1.5 15.4 tr 75 <10 <10 <10 230 dp vn - 24.0 tr 124 31 1,200 231 dp vn - 5.1 tr <10 20 10 <10 1,100 232 cp bx - 5.1 tr 21 <10 <10 <10 Mn 233 cc vn 0.8 <1.7 tr 43 <10 1300 11.50% 234 cc vn 0.3 58.3 0.07 62 600 <10 <10 235 cp vn - 5.1 tr 37 10 200 <10 236 cp bx - 8.6 tr 55 32 400 <10 237 cc vn 3.1 6.9 tr 46 21 <10 <10 238 dp vn - 776.7 0.17 53 11 10 <10 239 dp vn - 241.7 0.07 42 10 12 <10 240 dp vn - 36.0 0.07 39 265 10 <10 241 cc vn 0.2 29.1 tr 242 cc vn 0.2 8.6 tr Sample Other, ppm (or wt%) Ag Au No. Type Source Width. m g/t g/t Cu Pb Zn Mo® As Sb Hg Te UgOg VVgOg Sn F Regional Surface 243 CO vn 0.2 116.6 tr 44 <10 <10 <10 244 op vn 0.6 25.7 0.07 81 <10 13 <10 245 CO vn 1.1 6.9 0.14 <10 <10 500 <10 246 cc vn 0.3 204.0 O.IQ 110 800 200 <10 247 cc vn 1.1 5.1 tr 248 cp bx - 5.4 tr 33 204 200 <10 249 cp bx - 5.1 tr 174 1100 324 <10 tr <0.10 '^4.34% 250 cc vn na 276.4 0.55 251 dp vn - 262.7 0.24 252 cc vn na 81.3 0.55 253 dp vn - 488.6 0.79 254 cc vn na 1383.6 3.26 255 cc vn na 294.9 0.51 256 cc hw na 324.7 0.93 257 dp vn - 313.4 1.61 258 cc hw na 101.2 0.89 259 cc vn na 42.5 1.27 260 dp fw - 220.1 1.23 261 cc fw na 1.7 0.48 262 cc vn na 846.3 3.50 263 cc vn na 207.8 0.55 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo® As Sb Hg Te U30g W^Og Sn F Regional Surface 264 cc vn na 28,1 0.34 265 cc hw na 203.0 0.48 266 dp vn - 113.5 1.70 267 dp vn - 220.5 1.70 268 dp vn - 5.5 0.75 269 cp cr - <1.7 0.34 270 cp cr - <1.7 0.03 271 cp cr - <1.7 0.34 272 cp cr - 1.7 0.03 273 cc vn na 2.4 0.03 274 cc vn na 7.5 13.9 275 cc vn na 240.0 7.10 276 cc vn na 33.3 1.47 277 cc vn na 35.7 1.17 278 cp cr - <1.7 0.03 279 cc vn na 2.4 1.92 280 cp cr - 33.3 1.17 281 cp cr - 7.2 0.45 282 cp cr - 19.2 0.62 283 cp cr - 5.5 0.03 284 cp cr - 17.5 0.34 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te U30g ^3^3 ^ Regional Surface 285 cp cr - 12.3 0.03 286 op cr - 29.8 1.03 287 cc vn na 277.7 5.38 288 cc vn na 65.2 1.82 289 cc vn na 15.4 0.69 290 cc vn na 805.8 6.34 291 cc vn na 3.4 0.07 292 cc vn na 64.1 2.74 293 cc vn na 42.5 4.39 294 cc vn na 59.0 2.30 295 cc vn na 12.7 1.37 296 cc vn na 28.5 0.24 297 cc vn na 77.2 2.26 298 cc vn na 94.9 2.74 299 cc vn na 90.2 13.82 300 cc vn na 129.3 1.75 301 cc vn na 38.4 0.55 302 cc vn na 46.3 0.24 303 cc vn na 38.4 0.34 304 cc vn na 240.7 13.9 305 cc vn na 59.0 0.72 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te UgOg WgOg Sn F Regional Surface 306 cc vn na 19.9 0.58 307 CO vn na 358.3 1.17 308 dp vn - 9.9 0.38 309 cc vn na 6.2 0.27 310 cc vn na 9.3 0.34 311 dp vn - 15.8 tr 312 cc vn na 10.6 0.96 313 cc vn na 41.8 0.69 314 cc vn na 6.5 0.03 315 cc vn na 10.3 0.45 316 cc vn na 3.4 0.03 317 cc vn na 3.8 0.03 318 cc vn na 2.1 0.65 319 dp vn - 33.9 0.34 320 cc vn na 136.1 0.93 321 dp vn - 217.1 0.69 322 dp vn - 411.5 3.53 323 cc vn na 11.3 0.27 324 cc hw na 19.5 0.27 325 cc fw na 14.7 0.34 Sample Other, ppm (or wt%) Ag Au No. Type Source Width, m g/t g/t Cu Pb Zn Mo As Sb Hg Te U^Og VVgOg Sn F Regional Surface 326 cc vn na 19.2 0.34 327 cc vn na 17.5 0.48 328 cc fw na 9.3 0.14 329 cc hw na 35.3 1.03 330 cc vn na 7.2 0.21 331 cc vn na 2.4 0.31 332 dp vn - 53.5 1.17 333 cc vn na 11.7 0.31 334 cc vn na 21,9 0.41 335 cc fw na 15.8 0.34 336 cc vn na <1.7 0.34 337 dp vn - 51.4 0.86 338 dp vn - 89.2 0.14 339 dp vn - 5.1 0.10 a. Discrepancies were found in some of the analyses for molybdenum. Results reported are those from second run at laboratory. Three samples were selected for reanalysis at another laboratory, which reported the following concen trations of molybdenum: sample 132, 19 ppm; sample 226, 14 ppm; and sample 230, 4 ppm. b. No fluorine minerals were detected in an examination of these samples and laboratory error is a possible explan ation for the anomalously high values reported. REFERENCES Abbot, J. T., and Williams, S. A., 1981, The Pine Grove molybdenum system, Wah Wah Mountains, Beaver County, Utah [abs.], ^ Program, 101st AIME Meeting, February 1982: p. 21. Antisell, T., 1856, Part 2. Geological report: Reports of explora tions and surveys to ascertain the most practicable and econom ical route for a railroad from the Mississippi River to the Pacific Ocean: 33rd U.S. Congress, 2nd Session, U.S. Senate Ex. Doc. 78 and House Ex. Doc. 91, p. 152-154. Armstrong, A. K., Silberman, M. L., Todd, V. R., Hoggatt, W. C., and Carten, R. B., 1978, Geology of central Peloncillo Moun tains, Hidalgo County, New Mexico: Socorro, New Mexico Bureau of Mines and Mineral Resources Circular 158, 19 p. Bailey, R. A., Dalrymple, G. B., and Lanphere, M. A., 1976, Vol- canism, structure, and geochronology of Long Valley caldera, Mono County, California: Journal of Geophysical Research, v. 81, p. 725-744. Bethke, P. M., and Rye, R. O., 1979, Environment of ore deposition in the Creede mining district, San Juan Mountains, Colorado. Part IV. Source of fluids from oxygen, hydrogen, and carbon isotope studies: Economic Geology, v. 74, p. 1832-1851. Bookstrom, A. A., 1981, Tectonic setting and generation of Rocky Mountain porphyry molybdenum deposits, in Dickinson, W. R., and Payne, W. D., eds., Relations of tectonics to ore deposits in the southern Cordillera: Tucson, Arizona Geological Society Digest, v. 14, p. 215-226. Breiner, S., 1980, Magnetics, applications for portable magnetom eters, in Van Blaricom, R., ed., Practical geophysics for the exploration geologist: Spokane, Washington, Northwest Mining Association, p. 205-237. Buchanan, L. J., 1981, Precious metal deposits associated with vol canic environments in the Southwest, m Dickinson, W. R., and Payne, W. D., eds., Relations of tectonics to ore deposits in the southern Cordillera: Tucson, Arizona Geological Society Digest, V. 14, p. 237-262. 100 183 Carpenter, R. H., 1968, Geology and ore deposits of the Questa molybdenum mine area, Taos County, New Mexico, in Ridge, J. D., ed.. Ore deposits of the United States, 1933'n'967, Vol. 2: New York, AIME, p. 1328-1350. Christiansen, R. L., and Lipman, P. W., 1972, Cenozoic volcanism and plate tectonic evolution of the western United States; II, Late Cenozoic, m A discussion on volcanism and the structure of the Earth: Philosophical Transactions of the Roya3 Society [London], series A, v. 217, p. 249-284. Christiansen, R. L., Lipman, P. W., Carr, W. J., Byers, F. M., Orkild, P. P., and Sargent, K. A., 1977, Timber Mountain-Oasis Valley caldera complex of southern Nevada: Geological Society of America Bull., v. 88, p. 943-959. Clifton, C. G., Buchanan, L. J., and Burning, W. P., 1980, Ex ploration procedure and controls of mineralization in the Oatman mining district, Oatman, Arizona: New York, AIME Preprint 80-143, 17 p. Coney, P. J., and Reynolds, S. J., 1977, Cordilleran Benioff zones: Nature, v. 270, p. 403-406. Conolly, H. J., 1936, A contour method of revealing some ore struc tures: Economic Geology, v. 31, p. 259-271. Damon, P. E., 1981, Personal communication: Professor of geology, Department of Geosciences, University of Arizona, Tucson. Deal, E. G., Elston, W. E., Erb, E. E., Peterson, S. L., Reiter, D. E., Damon, P. E., and Shafiqullah, M., 1978, Cenozoic volcanic geology of the Basin and Range province in Hidalgo County, southwestern New Mexico, m Callender, J. F., Wilt, J. C., and Clemons, R. E., eds.. Land of Cochise, New Mexico Geological Society guidebook, 29th Field Conference: Socorro, p. 219-230. Drewes, H., and Thorman, C. H., 1980, Geological map of the Steins quadrangle and the adjacent part of the Vanar quad rangle, Hidalgo County, New Mexico: U.S. Geological Survey Map 1-1220. Drummond, S. E., and Ohmoto, H., 1979, Effects of boiling on mineral solubilities in hydrothermal solutions tabs.]: Geological Society of America Abstracts with Programs, v. 11, p. 416. Elston, W. 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E., 1976, Control of mineralization by mid-Tertiary volcanic centers, southwestern New Mexico, in Elston, W. E., and Northrop, S. A., eds., Cenozoic volcanism in southwestern New Mexico: Socorro, New Mexico Geological Society Spec. Pub. 5, p. 93-102. Enders, M. S., 1981, The geology, mineralization, and exploration characteristics of the Beck mine and vicinity, Kimball mining district, Hidalgo County, New Mexico, and Cochise County, Arizona: Unpublished M.S. thesis, University of Arizona, Tucson, 109 p. Fisher, J. D., 1921, Assay map of the Volcano mine in 1921: Tucson, Arizona, Continental Materials Corporation, private file. Gilbert, G. K., 1875, Report on the geology of portions of New Mexico and Arizona, explored and surveyed in 1873, m Wheeler, G. M., Report upon geographical and geological exploration and surveys west of the one hundredth meridian. U.S. Army Engineer Department: Washington, Government Printing Office, p. 513. 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Land of Cochise, New Mexico Geological Society guidebook, 29th Field Conference: Socorro, p. 215-218. 188 U.S. Geological Survey, 1980, Aeromagnetic map of the south half of the Silver City 1® by 2° quadrangle. New Mexico-Arizona: U.S. Geological Survey Open-File Report, OF-80-1128, 4 sheets, scale 1:62,500. Weissberg, B. G., 1969, Gold-silver ore-grade precipitates from New Zealand thermal waters: Economic Geology, v. 64, p. 95-108. Westra, G., and Keith, S. B., 1981, Classification and genesis of stockwork molybdenum deposits: Economic Geology, v. 76, p. 844-873. White, D. E., 1981, Active geothermal systems and hydrothermal ore deposits, in Skinner, B. J., ed.. Seventy-fifth anniversary volume: Economic Geology, p. 392-423. White, D. E., Muffler, L. J., and Truesdell, A. H., 1971, Vapor dominated hydrothermal systems compared with hot-water sys tems: Economic Geology, v. 66, p. 75-97. White, W. H., Bookstrom, A. A., Kamilli, R. J., Ganster, M. W., Smith, R. P., Ranta, D. E., and Steininger, R. C., 1981, Character and origin of Climax-type molybdenum deposits, m Skinner, B. J., ed.. Seventy-fifth anniversary volume: Economic Geology, p. 270-316. Williams, H., 1942, The geology of Crater Lake National Park, Oregon, with a reconnaissance of the Cascade range southward to Mount Shasta: Carnegie Institute of Washington Pub. 540, 162 p. Williams, H., and Goles, G., 1969, Volume of the Mazama ash-fall and origin of Crater Lake caldera, m Dole, H. M., ed., Andesite Conference guidebook: Salem, Oregon State Depart ment of Geology and Mineral Industries Bull. 62, p. 37-41. N 0009 N 00 0 Z N 0008 O II I (i) ldJi4 / / f—^ » (A)B|i- N 0006 3 OOOi' 3 OOOS A 5000 E 6000 E Trpf {,1) k) Tla(?) Qak i I 7000 E 8000 E 9000 E ~^387 m • Jl 433J v V / CORRELATION OF MAP UNITS \ i •7 9000 N Age, in m.y. Qal Holocene QU) Holocene and Qgt ] Pleistocene 6.1 Pliocene 3 1.4 (reset?) 33.0 8000 N Oligocene 33.7 35.3 'J Eocene to TEf 44.7 Upper CictaCcOuS (Of \ (reset?) DESCRIPTION OF MAP UNITS •7 000 N Qal ALLUVIUM (HOLOCENE) - Gravel, sand, and silt detritus. Qtd TALUS DEBRIS (HOLOCENE) - Angular, unsorted fragments mantling f Qgt GRAVEL TERRACE (HOLOCENE AND PLEISTOCENE) - Older alluviutrT 11 OLIVINE BASALT (PLIOCENE) - Includes narrow dikes and small, circi a I A sample from the southern plug gave a K-Ar date of 6.li0.2 m.y j: i and Mehnert, 1 978) II / li r LATE RHYOLITE DIKE (OLIGOCENE) - Fine grained with weak foliatio \y (ti i !i Trpf RHYOLITE PORPHYRY FLOW (OLIGOCENE) - Flows derived from vent contains vitrophyric base grading upward to reddish-gray, flow ba \ • 6000 N are generally less than 30 degrees. Contact with Trpd is/'^radatio exposure 2 km north of map area gave a K-Ar date of/33.ail.1 m of 28.7+ 1.4 iD.y., a fission-track date of 31.4+1.4 m.y^ was obtain V / CORRELATION OF MAP UNiTS Age. in m.y. Qal ^ Qtd , Holocene •••.v.' > QUATERNARY Holocene and Qgt Pleistocene 6,1 Pliocene 3 1.4 (reset?) 33.0 > TERTIARY • Oligocene 33.7 35.3 Eocene to TERTIARY AND 44.7 Upper Cretaceous (OR) CRETACEOUS (reset?) DESCRIPTION OF MAP UNITS Qal ALLUVIUM (HOLOCENE) - Gravel, sand, and silt detritus. Qtd TALUS DEBRIS (HOLOCENE) - Angular, unsorted fragments mantling slopes. r Qgt GRAVEL TERRACE (HOLOCENE AND PLEISTOCENE) - Older alluvium capping stream terraces. OLIVINE BASALT (PLIOCENE) - Includes narrow dikes and small, circular plugs. A sample from the southern plug gave a K-Ar date of 6.li:0.2 m.y. (Marvin, Naeser, and Mehnert, 1978) LATE RHYOLITE DIKE (OLIGOCENE) - Fine grained with weak foliation. Trpf RHYOLITE PORPHYRY FLOW (OLIGOCENE) - Flows derived from vents in Trpd. Typicalty contains yitrophyric base grading upward to reddish-gray, flow banded rock. Dips of foliation are generally less than 30 degrees. Contact with Trpd l^\3radational. A sample from an 6000 N 5000 N Tsmp 4000 N 3000 N SJi ^smp contains vitrophyric base grading upward to reddish-gray, flov 11 / •6000 N JL- ' I are generally less than 30 degrees. Contact with Trpd iv\3rad£ Ml exposure 2 km north of map area gave a K-Ar date of/33.Oil.j iT\ of 28.7±1.4 m.y., a fission-track date of 31.4+1.4 m.y^ was obt| vitrophyre at 5375 N, 8900 E (Marvin, Naeser, and Mehner), 1! RHYOLITE PORPHYRY DOME-FLOW COMPLEX (OLIGOCENE) - Inclj close to vents. When unit clearly shows flow characteristics, tl has been given. Tdp DACITE PORPHYRY OF STEINS PEAK (OLIGOCENE) - Shallow intrusl mass of Steins Peak, the highest point in map area at an elevatf relationships with Trpd are uncertain. RHYOLITE TUFF AND TUFF BRECCIA (OLIGOCENE) - Bedded, fine t| airfall deposits with minor tuffaceous sediments. •5000 N OLDER RHYOLITE DOMES (OLIGOCENE) - Domes of strongly flow bj UPPER ANDESITE PORPHYRY (OLIGOCENE) - Flows and flow breccj Nei LOWER CLASTIC ROCKS (OLIGOCENE) - Tuffaceous sandstone, muc STEINS MOUNTAIN ASH-FLOW TUFF (OLIGOCENE) - Crystal-lithic contains a basal vitrophyre. A K-Ar date on a sample from apprc area (figure 2) gives an age of 35.3 m.y. and a sample from 35| area, gives an age of 33.7 m.y. (Richter and Shafiqullah, 1981)J Partially welded to nonwelded zone - Fumoralically altered, partis zone of Tsm. LOWER ANDESITE (EOCENE TO UPPER CRETACEOUS) - Flows, brej • 4000 N in northern map area, rock could be Tua. A fission-track date o| a sample collected approximately 8 km south of map area (Mat: This date may have been reset. Tab ANDESITE BRECCIA (EOCENE TO UPPER CRETACEOUS) Lenticula j tuff breccia intercalated with Tla. • DACITE TUFF AND TUFF BRECCIA (EOCENE TO UPPER CRETACEOl tuff breccia. Densely welded dacite ash-flow tuff in western el 2 DESCRIPTION OF MAP SYMBOLS C> £,*7 BRECCIA C- O ••• INTENSELY SILICIFIED WALL ROCK (figure 4) 3000 N .X °o° VITROPHYRIC ZONE N 78 CONTACT - Showing dip. Dashed where approximate; queried wt 21 / 8 1^^ FAULT - Showing dip and plunge of slickensides. Ball on downti \ c approximate; dotted where concealed; queried whe/k uncertj fault structure. y \ P? RHVflLITi PfiPjPMVRV HUW contains vitrophyric base grading upward to reddish-gray, flow banded rock. Dips of foliation are generally less than 30 degrees. Contact with Trpd iv'^radational. A sample from an exposure 2 km north of map area gave a K-Ar date of/^3.Qi1.1 m.y. and a fission-track date of 28.7±1.4 m.y., a fission-track date of 31.4+1.4 m.y\ was obtained from a sample of vitrophyre at 5375 N, 8900 E (Marvin, Naeser, and Mehner), 1978). RHYOLITE PORPHYRY DOME-FLOW COMPLEX (OLIGOCENE) - Includes domes and flows close to vents. When unit clearly shows flow characteristics, the designation of Tfpf has been given. DACITE PORPHYRY OF STEINS PEAK (OLIGOCENE) - Shallow intrusive unit. Composes central mass of Steins Peak, the highest point in map area at an elevation of 1,788 m. Intrusive relationships with Trpd are uncertain. RHYOLITE TUFF AND TUFF BRECCIA (OLIGOCENE) - Bedded, fine to very coarse grained airfall deposits with minor tuffaceous sediments. OLDER RHYOLITE DOMES (OLIGOCENE) - Domes of strongly flow banded rock. UPPER ANDESITE PORPHYRY (OLIGOCENE) - Flows and flow breccia. LOWER CLASTIC ROCKS (OLIGOCENE) - Tuffaceous sandstone, mudstone, lahar, and airfall deposits. STEINS MOUNTAIN ASH-FLOW TUFF (OLIGOCENE) - Crystal-lithic welded ash-flow tuff. Locally contains a basal vitrophyre. A K-Ar date on a sample from approximately 3 km north of map area (figure 2) gives an age of 35.3 m.y. and a sample from 3500 N, 7500 E, within the map area, gives an age of 33.7 m.y. (Richter and Shafiqullah, 1981). Partially welded to nonwelded zone - Fumoralically altered, partially welded to nonwelded upper zone of Tsm. LOWER ANDESITE (EOCENE TO UPPER CRETACEOUS) - Flows, breccias, and tuffs. Where queried (?) in northern map area, rock could be Tua. A fission-track date of 44.712.7 m.y. was obtained from a sample collected approximately 8 km south of map area (Marvin, Naeser, and Mehnert, 1978). This date may have been reset. ANDESITE BRECCIA (EOCENE TO UPPER CRETACEOUS) - Lenticular deposits of medium to coarse tuff breccia intercalated with TIa. DACITE TUFF AND TUFF BRECCIA (EOCENE TO UPPER CRETACEOUS) - Medium to coarse grained tuff breccia. Densely welded dacito ash-flow tuff in western exposures. Intercalated with TIa. DESCRIPTION OF MAP SYMBOLS t> BRECCIA ^ t> INTENSELY SILICIFIED WALL ROCK (figure 4) °o° VITROPHYRIC ZONE 78 CONTACT - Showing dip. Dashed where approximate; queried where uncertain. 2 1 81^^ FAULT - Showing dip and plunge of slickensides. Ball on downthrown side. Dashed where approximate! dotted where concealed; queried whe/k uncertain. Red where vein occupies / ^ fault structure. / 4000 N 3000 N 2 0 00 N Tsm 3000 E 4000 E ARIZONA NEW MEXICO 5000 E 6000 E GEOLOGIC MAP OF A PORTION OF THE NORTH- PELONCILLO MOUNTAINS, HIDALGO COUNTY, NE Figure 2 432/ \ \\ X \ Q X' rap Tank'- Qal 7000 E 8000 E 9 00 0 E )RTH-CENTRAL Geology by: T.H. Young, 1981 Y, NEW MEXICO Partially welded to nonwelded zone - Fumoralically altered, par zone of Tsm. LOWER ANDESITE (EOCENE TO UPPER CRETACEOUS) - Flows, in northern map area, rock could be Tua. A fission-track date a sample collected approximately 8 km south of map area (N This date may have been reset. ANDESITE BRECCIA (EOCENE TO UPPER CRETACEOUS) - Lenticu Tab tuff breccia intercalated with TIa. DACITE TUFF AND TUFF BRECCIA (EOCENE TO UPPER CRETACEi tuff breccia. Densely welded dacite ash-flow tuff in western DESCRIPTION OF MAP SYMBOLS BRECCIA ^ £> INTENSELY SILICIFIED WALL ROCK (figure 4) VITROPHYRIC ZONE 7 8 CONTACT - Showing dip. Dashed where approximate; queriec ^ 21 FAULT - Showing dip and plunge of slickensides. Ball on do\ approximate; dotted where concealed; queried wher&unc fault structure. / STRIKE AND DIP OF BEDS Map L( 28 STRIKE AND DIP OF FOLIATION 8-^ INCLINED JOINTING INCLINED SHAFT B VERTICAL SHAFT, OPEN - CAVED ADIT Fig X SHALLOW PROSPECT PIT TRENCH DUMP A A' CROSS SECTION (see figures 3, 5, and 6) I I SCALE 200 400 1000 meters 1 : 12,000 tially welded to nonwelded zone - Fumoralically altered, partially welded to nonwelded upper one of Tsm. ER ANDESITE (EOCENE TO UPPER CRETACEOUS) - Flows, breccias, and tuffs. Where queried (?) n northern map area, rock could be Tua. A fission-track date of 44.7±2.7 m.y. was obtained from i sample collected approximately 8 km south of map area (Marvin, Naeser, and Mehnert, 1978). This date may have been reset. ;SITE BRECCIA (EOCENE TO UPPER CRETACEOUS) - Lenticular deposits of medium to coarse jff breccia intercalated with TIa. TE TUFF AND TUFF BRECCIA (EOCENE TO UPPER CRETACEOUS) - Medium to coarse grained uff breccia. Densely welded dacito ash-flow tuff in western exposures. Intercalated with TIa. DESCRIPTION OF MAP SYMBOLS 3RECCIA NTENSELY SILICIFIED WALL ROCK (figure 4) 'ITROPHYRIC ZONE ONTACT - Showing dip. Dashed where approximate; queried where uncertain. AULT - Showing dip and plunge of slickensides. Ball on downthrown side. Dashed where approx'imatei dotted where concealed; queried where uncertain. Red where vein occupies fault structure. / \ >" I STRIKE AND DIP OF BEDS Map Location Guide 5TRIKE AND DIP OF FOLIATION NCLINED JOINTING NCLINED SHAFT Figure 2 /ERTICAL SHAFT, OPEN - CAVED ftDlT Figure SHALLOW PROSPECT PIT TRENCH DUMP CROSS SECTION (see figures 3, 5, and 6) SCALE 200 400 600 800 1000 meters 1 : 12,000 T H.Young, M.S. Thesis, 1982 Department of Mininy dOu Qcclcgica! Engineerlnp The University of Arizona \ \ 6000 N • \ EXPLANATION (See figure 2 for description and correlation of units and description of symbols) Qal ALLUVIUM 4713 ; Qtri: ^ TALUS DEBRIS Qgt GRAVEL TERRACE Trpf RHYOLITE PORPHYRY FLOW RHYOLITE PORPHYRY DOME-FLOW COMPLEX 5000 N RHYOLITE TUFF AND TUFF BRECCIA \ 50 0 0 E 6000 E 6000 N *4714 5000 N RHYOLITE TUFF AND TUFF BRECCIA UPPER ANDESITE PORPHYRY LOWER CLASTIC ROCKS STEINS MOUNTAIN ASH-FLOW TUFF Partially welded to nonwelded zone Map Location Guide Figure 2 Figure 4 4000 N IN FBSilE I O ro^ y alUiBs 4000 N SCALE 4600: 5000 E GEOLOGIC MAP OF THE VOLCANO MINE KIMBALL MINING DISTRICT, HIDALGO COUNTY, Figure 4 3000 N Geology by: T.H. Young, 1981 gooO E LCANO MINE AREA lO COUNTY, NEW MEXICO T.H. Young, M.S. Thesis. 1982 Department of Mining and Geological Engineering The University of Arizona A 2409N, 4562E 1600 m — 800 — 600 — Geology b B 2989N. 5493E 1600 m 5000N Volcano Vein by T.H. Young , 1981 (Explana 5000E Rustler Draw Silll "•'SK.j-Ti; • -•'•Si •J- • . .., • ^ ..»NA»' J Trpdf?) I 200 400 DSS SECTION A-A' Figure 3 SCALE 1:1 (Explanation - see figure 2) B' 5125N. 5225E 4337N. 4884E 1600 m 1600 m El Oro Vein —1500 1500 f 1400 1400 I \ V A' 8845N. 5360E 1600 m 1400 — 1200 — 1000 — 800 \K' — 600 - ' •• •;•• ;••• ; -ii 400 400 600 800 1000 meters SCALE 1:12,000 4230N. 5 337N. 4884E Dm — Volcano Vein El Oro Vein 1000 - 800 — 600' Geology by T.H. Yo by T.H. Young , 1981 CROSS S Fi (Explanatii ^^.v;',£lU' Tsmp 0 100 2 CROSS SECTION B-B' Figure 5 SCALE (Explanation - see figures 2 and 4) 200 400 60 SS SECTION A-A' Figure 3 SCALE 1:12,C (Explanation - see figure 2) B' 5125N. 5225E 4337N. 4884E 1600 m 1600 m — El Oro Vein 1500 1 500 1400 1 400 —I 1300 1 300 — 1200 1200 —I Geology 200 300 400 meters SCALE 1 :4800 1000 400 600 800 1000 meters SCALE 1:12.000 c . 4884E 4230N. 58841 Volcano Vein Ji El Oro East Vein Vein Geology by T.H. Young, 1981 100 200 meter! CROSS SECTION C-C SCALE 1:4800 Figure 6 (Explanation - see figures 2 and 4) T.H. Young - "M.S. The Department of Mining and Geo! The University of Ai 1000 00 meters 1 c 4230N. 5884E - 1600 m 1 500 El Oro East Vein 1400 •1300 1200 100 200 meters :ROSS SECTION C-C SCALE 1:4800 Figure 6 (Explanation - see figures 2 and 4) T.H. Young - 'M.S. Thesis, 1982 Department of Mining and Geological Engineering The University of Arizona v intensely brecciated wallrock j^with quartz and calcite cement *B7 J \ i.:r \ I TUA^\ stoped obove \ 39 1-24 V \N AgCI coating, argillically ^altered and silicified breccia fragments if 79 Tua bifurcated vein, separated '\ by orgillicolly altered, shaft intersect__s stopes VS' \ brecciated ondesite above and below \ \ \ \ \ \ \ \ \ \ \ Tsmp^ \ \. to lower stopes •'«£v \ Stoped above stoped above- Tsmp Tsmr V / 5100 N cAT J argillically licified breccia 50 75 N I vein, separated colly altered, t andesite to lower stopes . V 5050 N i.m'm \ Tsmp thin quartz veins Tsmp split in strongly to moderately welded tuff mottram! coats some fractuT 4950 E 4975 E Tsmp Figure 14 GEOLOGIC MAP I VOLCANO MINE 100 LEVEL 2% pyi (limonite Kimball Mining District in footw Hidalgo County, New Mexico Geology by T.H. Young September. 198? SCALE 0 5 10 i5 20 25 meters !•• 240 EXPLANATION WALL ROCKS Steins Mountain Welded Tuff= Partially to nonwelded ash-flow tuff, genei^lly orgilii altered and strongly silicified. Contains 0 to 5% fine groined, ditsemii^ti^ pyrite altered to limonite. ^ split in main fault (?) mottramit^^ coats some fractur «86 T ua 5025 N Tsmp 2% pyrite I(r\;r«86> (limonite) diss.' in footwall T ua stopeAk^, \fe\ abovo V 82 •M [ft ^ I* 5000 N 5000 E "9, 5025 E % i / Zi 25 meters I :'Si L ^ ^'S ' W I stoped ^ "Tsmp above \ Hi] N r7l V 'I \ \ gelded gsh-flow tuff, generally argillically \ \ r/e fin« grained, disseminotB'^ pyrite \ / \ V'i 4975 N >1 :h inocrysts in pilotaxitic groundmass. Tsmp »66 V. I»»VI9 )k SCALE 10 20 25 meters 1=240 EXPLANATION WALL ROCKS Steins Mountain Welded Tuff= Portiolly to nonwelded ash-flow tuff, generqily altered and strongly silicified. Contains 0 to 5% fine grained, dissemir ^ altered to limonite. ^ Andesite Porphyry' 15-30% piagioclase phenocrysts in pilotaxitic groundr Argillic alteration^moderate to intense. VEIN STRUCTURES Quartz Vein' Quartz filling open space. Some vein material contains stroj silicified wall rock breccia fragments. Calcite may compose up to 30%. grained pyrite common. Sheeted Wail Rock' Thin, parallel fractures filled with quartz or calcite. Brecciated Wall Rock' strongly silicified and argillicaliy altered, fragment A 4 A cemented by quartz or calcite. Fault Plane showing dip and plunge of slickensides. / 26 82 MINE SYMBOLS §2 Inclined shaft showing dip. Inclined raise or winze, chevrons pointing down. [ C I Chute [ M I Manway Horizontal survey point marking location of haulage shaft at surface 5000 T 5000 E 5025 E 25 meters stoped above' sided ash-flow tuff, generqily argillically fins grained, dissemiivQfi^d pyrite 4975 N ocrysts in pilotaxitic groundmoss. vein material contains strongly may compote up to 30%. 1% fine lied with quartz or calcite. argillically altered, fragments les. 4950 N jloge shaft at surface 5000 N, 5000E. caved T.H. Young, M.S. Thesis. 1982 Dept. of Mining and Geological Engineering, Univ. of Arizona / Tsmp 5100N quartz flooded breccia stoped abov haulage strong orgillic •Iteration 5075N 4950 E stoped above blocky breccia, strong silicifi 5025 !1_N with calcite^ Tsmp conn \}> V^A 5050N 4975 E strong argillic \ + Figure 16 GEOLOGIC MAP VOLCANO MINE 150 AND 200 LEVEL KIMBALL MINING DISTRICT Hidalgo County, New Mexico Geology by T. H. Young September, 1981 oV k SCALE blocky breccia, 10 15 20 25 meters " strong silicification J with calcite stringers I •240 connects stopes ^,^/6bove and below \ strong argillic alteration 150 LEVEL 5000N 5000 E 5025 E strong silicification EXPLANATION WALL ROCKS Steins Mountoin Welded Tuff= Partially to nonwelded ash-flow tuff, generally orgillica altered and strongly silicified. Contains 0 to 5% fine grained, disseminatvd pyrite oltered to limonite. Andesite Porphyry* 15-30% plagioclase phenocrysts in pilotaxitic groundmass. Tiia Arfiltic olteration moderate to intense. VEW STRUCTURES Quartz Vein* Ouortz filling open space. Some vein moterial contains strongly silicified well rock breccia fragments. Calcite may compose op to 30%. 1% f grained pyrite common. Sheeted Wall Reck* Thin, porallel fractures filled with quartz or calcite. • A A • Breccioted WoH Reek* strongly silicified and argillically altered, fragments A 4 4 cemented quartz or calcite. Fault Plane showing dip and plunge of slickensides. 82 MINE SYMBOLS Inclined shaft showing dip. »H» raise or winze, chevrons pointing down. I C I Chute Manway Horizontal survey point marking location of haulage shaft at surface 5000N, SOOJ A / \ 200 LEVEL 5025 N ash-flow tuff, generally orgillically grained, disteminated pyrite B in pilotaxitic groundmass. material contoins strongly iompose up to 30%. i% fine moderate silicificatio yith quartz or calcite. licolly altered, fragments strong argillic alteration calcite veinlets 5000 N 5025 E 5000 E Tsmp stiaft ot surface 5000 N, S009£. T.H. Young, M.S. Thesis, 1982 Dept. of Mining and Geological Engineering, Univ. of Arizona V Figure 16 Tua 1 SCJa orgiNir. Y^alterotion GEOLOGIC MAP i viu- ; winze to VOLCANO MINE <»'\TrI/ meter level 300 LEVEL wt jvao . IXA . Kimball Mining District . Midalgc County, New Mexico w - Geology by T.H. Young Tua September, 198 1 1^7 Tsmp strong q ^V * A'1 "Isve '1, w SCALE 83*' 20 25 meters oxides, 0 10 15 rare chalcopyrite h240 5050 N Tsmp 4975 E vein qu( strongly sil brec EXPLANATION WALL ROCKS Steins Mountain Welded Tuff= Portiolly to nonwelded osh-flow tuff, generally orgillically altered and strongly silicified. Contains 0 to 5% fine grained, disseminated pyrite altered to limonite. Andesite Porphyry* I5~30% plagioclase phenocrysts in pilotaxitic groundmass. Tua Argillic alteration moderate to intense. VEIN STRUCTURES k ^^'-strong argillic ^Iteration covered winze to 150 meter level 5075 N Tsmp strong quartz vein ^ \ I ^ ^ A I developed in andesite '\Vv A ,• minor Cu oxides, rare chalcopyrife^. ^ stoped at^l Tsmp 4975 E vein quartz and strongly siliclfied breccia mottramite coa fractures in quart:^; vein Bd ash-flow tuff, generally orgillically ine grained, disteminatvd pyrite 'ysts in pilotaxitic groundmass. 5025 N 4975 E vein quart] strongly silicif breccia EXPLANATION WALL ROCKS Steins Mountain Welded Tuff= Portially to nonwelded ash-flow tuff, generally orgillicaliy altered and strongly silicif led. Contains 0 to 5% fine grained, disseminatvd pyrite altered to limonite. Andesite Porphyry I5~30% plagioclose phenocrysts in pilotaxitic groundmass. Tua j Argillic alteration moderate to intense. VEIN STRUCTURES Quartz Vein' Quartz filling open space. Scrr.e vein materiol contains strongly silicified wall rock breccia fragments. Calcite may compose up to 30%. 1% fine grained pyrite common. Sheeted Wall Rock- Thin, parallel fractures filled with quartz or calcite. 14^ ^ A| Brecciated Wall Rock= Strongly silicified and argillicoliy altered, fragments I ^ ^ I cemented by quartz or calcite. Fault Plane showing dip and plunge of slickensides. 82*^ MINE SYMBOLS Inclined shaft showing dip. inclined raise or winze, chevrons pointing down [ C I Chute I M I Manway Horizontal survey point marking location of haulage shaft at surfoce 5000 N, 5000E. Tsmp rs E vein quartz strongly silicified breccia mottramlte fractures in 3sh-flow tuff, generally argillically grained, disseminatBd pyrite 8 in photaxitic groundmass. 5025 N stoped above material contains strongly open space lined compose up to 30%. 1% fine drusy fine-grained calcite with quartz or calcite. illicQliy altered, fragments 84 quartz - calcite breccia Tua vertical shaft bottoms^^ at 8 foot sump 5000 N 5000 E strong « silicifi Mn oxides moderate ^ shaft at surface 5000 N, 5000E. T.H. Young. M.S. Thesis, 1982 Dept. of Mining and Geological Engineering, Univ. of Arizona SOUTH-SOUTHEAST 5000 N , 5000 E EXPLANATION man way 66600O lagging v<3 j Figure 17 CROSS SECTION OF SUBSURFACE DEVELOPMENT AT THE VOLCANO MINE KIMBALL MINING DISTRICT.HIDALGO COUNTY. NEW MEXICO NORTH-NORTHWEST SURFACE 0 '' '^ V ''Ci , ti 'obo OOUBOOBOdb o6 & 60 060" 't covered by lagging and track shaft to 150 m (not visited) EXPLANATION man way C)oo o o o lagging fill \^ Figure 17 XROSS SECTION OF SUBSURFACE DEVELOPMENT AT THE VOLCANO MINE KIMBALL MINING DISTRICT,HIDALGO COUNTY. NEW MEXICO Workings projected to a vertical plane-, looking west-southwest. Data by: T.H. Young and Volcano Mine operators D. Campbell, J. Hales, and J. Williams Current to; October, 1981 o6Adfli lagooBflol covered by lagging and track V- shaft to 150 m (not visited) short cross-cuts developed into hanging wall and foot wall T.H. Young. M.S. Thesis, 1982 Dept. of Mining and Geological Engineering, Univ. of Arizona 6000 N ope below 5075 N 5050 N Figure 1b SAMPLE LOCATION AND SILVER ASSAY MAP VOLCANO MINE 100 LEVEL Kimball Mining District, Hidalgo County, New Mexico Samples by T.H. Young. June. 1981 and J.D. Fisher, December. 1921 EXPLANATION * 158 Channel sample location showing sample number, orientation of channel and sample interval. Silver Assay Color/Symbol Code silver - g/t > 350.0 200.1 - 350.0 100.1 - 200.0 50.1 - 100.0 zTTai 172 170 5025 N 1 2 160 1 59 V Mexico >mber, 192 1 5000 N 5000 E 162 1 63 number a I. 49 75 N 1 64 1 65 VOLCANO MINE 100 LEVEL Kimball Mining District, Hidalgo County, New Mexico 5000 Samples by T.H. Young. June, 1981 and J.D. Fisher, December. 1921 EXPLANATION 158 Clnannel sample location showing sample number, orientation of channel and sample interval. Silver Assay Color/Symbol Code ' silver - g/t > 350.0 200.1 - 350.0 100.1 - 200.0 50.1 - 100.0 25.1 - 50.0 0 - 25.0 Geochemtcal and assay values are listed in the appendix. Samples 10 - 57 by J.D. Fisher (1921). Samples 101 - 187 by T.H. Young (1981). SCALE 10 "IS 20 25 meters 1 : 240 4950 E 4975 E 159 1 6 1 ity, New Mexico sher, December, 1921 5000 N 5000 E 162 3 sample number. pie interval. 4975 N 164 165 ed in the appendix. er (1921). 166 ung (1981). 25 meters 167 4950 N 5025 E 168 T.H. Young, M.S. Thesis, 1982 n Mit / 5 100 N 17 6. L ffom stope 17? )10 m above level Kimbal Sample 4950 E 5050 N \/ 1 T Figure 19 SAMPLE LOCATION AND tope SILVER ASSAY MAP ibove level* VOLCANO MINE 150 AND 200 LEVELS Kimball Mining District, Hidalgo County, New Mexico Samples by T.H. Young, June, 1981 and J.D. Fisher,December, 1921 179 N 55 54 1 26 53 i 52 50 ,48 47 46 I f 45 44 or>n I c\/CI 150 LEVEL 50C0 % 5000 E 5025 E 1 24 EXPLANATION •Hr 158 Channel sample location showing sample number, orientation of channel, and sample interval. Silver Assay Color/Symbol Code silver - g/t > 350.0 ^/ / /• W/j 200.1 - 350.0 100.1 - 200.0 50.1 - 100.0 25.1 - 50.0 lifatswa;''! 0 - 25.0 Geochemical and assay values are listed in the appendix. Samples 10 - 57 by J.D. Fisher (1921). Samples 101 - 187 by T.H. Young (1981).. Scale 10 15 20 25 meters 1 : 240 44 200 LEVEL 127 5026 N mple number. 129 fiterval. 43 i42 183 182 184 d In the appendix. (1921). ng ( 1981). . 5000 M 5000 E 185 5025 E 186 25 meters 187 T.H. Young, M.S. Thesis, 1982 Dept. of Mining and Geological Engineering. Univ. of Arizona Figure 20 SAIVIPLE LOCATION AND SILVERI ASSAY MAP VOLCANO MINE 300 LEVEL Kimball Mining Di strict, Hidalgo County, New Mexico Samples by T.H. Young, June, 1981 and J.D. Fisher. December, 1921 EXPLANATION 158^ Channel sample location showing sample number, orientation of channel, and sample interval. Silver Assay Color/Symbol Code silver - g/t > 350.0 200.1 - 350.0 100.1 - 200.0 50.1 - 100.0 / 25.1 - 50.0 / 0 - 25.0 180 143 5075 N 142 Nj AND ^AP 134 135 136 13 7' 138 "A 139 140 nty, New Mexico\ i^i 5050 N er, December, 1921 1 33 ig sample number, nple interval. 5025 N 56 &ILVCRAv5JMI Mm t VOLCANO MINE 300 LEVEL Kimball Mining District, Hidalgo County, New Mexico\^''»i Samples by T.H. Young, June, 1981 and J.D. Fisher, December. 1921 EXPLANATION 15 8^ Channel sample location showing sample number, orientation of channel, and sample interval. Silver Assay Color/Symbol Code silver - g/t > 350.0 200.1 - 350.0 1 00.1 - 200.0 50.1 - 100.0 /' \ 25.1 - 50.0 0 - 25.0 ^Geochemical and assay values are listed in the appendix. Samples 10 ~ 57 by J.D. Fisher (1921). Samples 101 - 187 by T.H.Young (1981). SCALE 0 5 10 15 20 23 meters 1 : 240 4950 t 4975 E fdL IVI/~%I K \ 138 "A 139 140 ounty, New MexicoX^'^'' 5050 N isher, December, 1921 \ wing sample number. 133 sample interval. 5025 N 57 56 ! listed in the appendix. Fisher (1921). 132 .Young (1981). 13 1 130 20 29 meters 5000 N 4975 E 5000 E 1 8 1 T.H. Young, M.S. Thesis, 1982 V SOUTH-SOUTHEAST 5000N, 5000E Figure 22 VEIN CONTOUR DIAGRAM VOLCANO MINE KIMBALL MINING DISTRICT HIDALGO COUNTY. NEW MEXICO NORTH-NORTHW'E OOON, 5000E ''''•v.... NORTH-NORTHWEST •O / /// Figure 22 VEIN CONTOUR DIAGRAM VOLCANO MINE KIMBALL MINING DISTRICT HIDALGO COUNTY, NEW MEXICO Subsurface workings projected to a vertical plane (for detail see figure 17), looking west-southwest. Horizontal distance (meters) from imaginary inclined reference plane (N24W, 84E) to hanging wall of Volcano vein. Reference plane positioned on footwall side of vein. Method of vein contouring is described by Conolly (1936). SCALE iO meters 1 : 480 wm T.H. Young, M.S, Thesis, 1982 Dept. of Mining and Geological Engineering, Univ. of / ^ T.H. Young, M.S. Thesis. 1,982 Dept. of Mining and Geoiogicai Engineering, Univ. of Arizona 6000 N Figure 25 SAMPLE LOCATION AND SILVER ASSAY MAP VOLCANO MINE AREA KIMBALL MINING DISTRICT HIDALGO COUNTY, NEW MEXICO Samples by R.J. Pulfrey and T.H. Young, 1981 EXPLANATION d226 Sample location showing sample number and type of material sampled? d - composite of vein material from dump V - chip channel across vein f - foot wall h - hanging wad Silver Assay Color/Symbol Code silver - g /1 • > 350.0 200.1 - 350.0 • 100.1 - 200.0 50.1 - 100.0 m 5000 N 25.1 - 50.0 m 0 - 25.0 V V 5000 E I / \ \ N \ • \ d3 39 / d209 d210 d3-38 v201 d239 f229 ^•^v228 Av22S idj^ ) N. l/'\ d238 N 0009 frtSA f S3A >3150 / i.l/ /02P X" \ \ / 03P^^.^\ N 0009 \] 1_^T 3 0009 onn 500 meters 1 : 4800 4000 N L /•> n v250 \ d2^1 vies v255 d257 v196 h265 v264 d268^ \ c270 C271 c272 -v273 y/27' /\ I \ /\ / \ /'} 5000 E T d311 v312 v313 m dsod v307 ' /akv314' v315 V306> ®v304| V241 «> h324 h329 V323 fa?8 *|327- 'i viS4 vBOO d257 v330 / 33t' •• d294 h26S V291 v264 ^332 ^^^334 / ' v208 V336 (jPv287 _h)d24^0 C269 •4000 N .y276, \ c270, c271- c272- c2i <®i ®c280 v27$ / / ^ / / ^y / 6000 E T.H. Young. M.S. Thesis. 1982 Dept. of Mining and Geological Engineering, Univ. of Arizona footwall brecciated vein consists predominately and silicified of quartz-filled strongly brecciated Steins Mountain ash-flow tuff breccia 57 arg'illically altered andesite porphyry breccia, locally filled with quartz and calcite 58 ^4, brecciated ash-flow andesite porphyry - strongly bleached tuff and gouge quartz filled footwall breccia calcite with occasiona 67 andesite porphyry argillically altered. strongest where fractures are closely spaced EXPLANATION WALL ROCKS Steins Mountain Welded Tuff: Welded ash-flow tuff, generally argillically alterecj and strongly silicified. Upper Andesite Porphyry: 15 - 30% plagioclase phenocrysts ^ pilotaxitic groundmass. Argillic alteration, moderate to intense. N 4725 i Figure 31 GEOLOGIC MAP COYLE ADIT Kimball Mining District Hidalgo County, New Mexico Geology by T.H. Young June, 1981 SCALE 4700 N occasional quartz fills fractures 10 15 20 meters 1 : 240 thin calcite veinlets fill fractures that follow blocky joints brecciated ash-flow andesite porphyry - strongly bleacne tuff and gouge quartz filled footwall breccia calcite with occasiona andesite porphyry argillically altered, strongest where fractures are closely spaced EXPLANATION WALL ROCKS Steins Mountain Welded Tuff: Welded ash flow tuff, generally argillically altered and strongly silicified. /\ :Tua- Upper Andesite Porphyry: 15 - 30% plagioclase phenocrysts it/ \ pilotaxitic g r o u n dm a s s. Argillic a Iteration, moderate to intense. VEIN STRUCTURES Quartz Vein: Quartz filled breccia Sheeted Waii Rock; Thin, parallel fractures filled with quartz or calcite. Fracture plane showing dip 60 5150 E 5175 E Geology by T.H. Young June, 19 8 1 SCALE 4700 with occasional quartz fills fractures •| u 20 rneters €•6 thin calcite veinlets fill fractures that follow blocky joints 467 y s t s tense. luartz or 5200 E 5225 E T.H. Young, M.S. Thesis, 1982 Dept. of Mining and Geological Engineering, Univ. of Ar June, 1981 SCALE s fractures 4700 N' 1 0 15 20 meters 1 240 thin calcite veinlets fill fractures that follow blocky joints 5225 E 5200 E I T.H. Young, M.S. Thesis. 1982 Dept. of Mining and Geological Engineering. Univ. of Arizona 370 369 368 367 3 66 364 363 365 3 6 2 225 360 359 356 3 55 354 35 EXPLANATION 158*Approximate channel sample location showing sample number, orientation of channel, and sample interval. Silver Assay Color/Symbol Code silver - g/t 1*^ >350.0 200.1 - 350.0 100.1 - 200.0 50.1 - 100.0 V N Figure 32 SAMPLE LOCATION AND 4725 N SILVER ASSAY MAP COYLE ADIT KIMBALL MINING DISTRICT HIDALGO COUNTY, NEW MEXICO I Samples by R.J. Pulfrey (Continental Materials Corp.), 1981 SCALE 10 15 20 meters 4700 N 4675 N