Fracturing and its effects on molybdenum mineralization at Questa, New Mexico
Item Type text; Dissertation-Reproduction (electronic)
Authors Rehrig, William Allen, 1936-
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/565196 FRACTURING AND ITS EFFECTS ON MOLYBDENUM
MINERALIZATION AT QUESTA, NEW MEXICO
by
William Allen Rehrig
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF GEOLOGY
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 6 9 THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my direction by William Allen R e h r i g ______entitled FRACTURING AND ITS EFFECTS ON MOLYBDENUM MINERALIZATION
AT QUESTAj NEW MEXICO______be accepted as fulfilling the dissertation requirement of the degree of Doctor of P h i l o s o p h y ______
Dissertation D irector^ Date
After inspection of the final copy of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:*
9 / y x f
t i t , 0!
This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination. PLEASE NOTE: Several pages contain colored illustrations. Filmed in the best possible way. UNIVERSITY MICROFILMS STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to bor rowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or re production of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the in terests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED: ACKNOWLEDGMENTS
Special appreciation is due the staff cf the Department of
Geology, The University of Arizona, for their assistance and sugges tions during the course of study. Dr. Spencer R. Tit ley visited the writer in the field and provided invaluable help in reading the manu
sc rip t.
Dr. Robert Carpenter, consulting geologist and professor at the Colorado School of Mines, directed the initial six months of field work. His knowledge and experience in the area proved most helpful throughout the course of the project. Helpful conversations were held with M essrs. John Schilling, Fred Graybeal, Donald Bryant, Charles
Robinson, and Barry McMahon concerning various aspects of the work.
Permission to publish this work was granted by the Molybdenum
Corporation of America. The operational staff at the Questa mine, es pecially the Department of Geology, is thanked for aid in drafting, repro
duction, access, transportation, and for geologic information.
Financial assistance for the use of the computer during the
study was made available from funds provided The University of Arizona
by the American Metals Climax Corporation. In addition, the Department
of Geology, The University of Arizona, provided funds for student help
and computer time. Mr. Richard Call, engineering geologist, helped
solve initial problems related to the computer processing of field data.
Finally, thanks are extended to Dr. Paul Damon of the Labora
tory of Isotope Geochemistry at The University of Arizona for permission ill to quote data on radiogenic dates from three samples collected in the area of interest. The New Mexico Bureau of Mines supplied the cost of one additional potassium-argon date.
In addition to the sources of technical and scientific aid credited above, the writer wishes to acknowledge his wife, without whose help and patience the dissertation would not have been easily ended. Her many hours of work in figure preparation greatly hastened the termination date of the project. TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... vii
LIST OF TABLES...... x ii
ABSTRACT...... x iii
INTRODUCTION...... 1
Purpose of the Study . . L o c a tio n ......
H is to ry ...... H-* CO CO General Method of Study
GENERAL G E O L O G Y ...... 6
Sangre do Cristo M ountains ...... 6 Red R iver-C abresto C reek A rea...... 10 General Geology ...... 10 S tr u c tu r e ...... 12
MINE GEOLOGY...... 15
Rock T y p e s ...... 15 A ltera tio n ...... 18 Age R e la tio n s h ip s ...... 18 Fracturing and Faulting ...... 21 Stockwork A s p e c ts ...... 21 F a u l t s ...... 24 "Total" Fractures (Statistical Analysis)...... 26 Summary S ta te m en t...... 46 M ineralization ...... 49 G eneral C h a r a c te r ...... 49 M ajor V e in s...... 50 "Total" M ineralized S tr u c tu r e ...... 52 The Effects of More Recent D a t a ...... 59 Structure Analysis in Areas of Different Molybdenum Grade ...... 60 Ore C o n t r o l s ...... 62 Summary S ta te m en t...... 67
v vi
TABLE OF CONTENTS--(Continued)
Page
GENESIS OF FRACTURE FORMATION ...... 69
Fracturing in the Mine A p l i t e ...... 69 Fracture E le m e n ts ...... 69 Interrelation of Fracture E lem ents ...... 86 Relation of Fracture Elements to Mineralization and A lteration ...... 102 Sequence of Development of Fracture Elements...... 103 Summary S ta te m en t...... 109 Regional F a c to rs ...... I l l The Red River Structural Z o n e ...... I l l The N ortheast Structural Z o n e ...... 113 Precam brian B a s e m e n t ...... 117 Structure in O ther P lu to n s ...... 121 Summary S ta te m en t...... 131 The Origin and Mechanics of Fracture Form ation ...... 133 Local Fracturing ...... 134 Regional Fracturing ...... 143
SUMMATION OF D A T A ...... 168
CONCLUSIONS ...... 176
REFERENCES 188 LIST OF ILLUSTRATIONS
Figure Page
1. Location Map of Ouesta Mine A re a ...... 3
2. Tectonic Map ...... : ...... 7
3. Regional Cross Sections of the Sangre de C risto M o u n ta in s...... 8
4 . Geology of the Red River-Cabresto Creek Area Overlay—Structure of the Red River-Cabresto C reek A re a ...... 11
5. Geologic Map - Questa Molybdenum Deposit Overlay-Major Veins and Mineralized Fractures . . 16 6. Equal-area Net ...... 28
7. Construction of the Equal-area Net Plot, Lower H e m isp h ere ...... 29
8. Frequency Diagram for Road and Bench-cut Orientations in Mine Aplite of Open-pit A rea ...... 32
9. Equal-area Nets (Lower Hemisphere) Used in Testing for Bias due to Unequal Distribution of Readout Directions in Aplite of the Open-pit A rea ...... 34 10. Equal-area Nets Used in Testing for Bias due to Unequal Distribution of Readout Directions in Andesite of the Open-pit Area ...... 35
11. Equal-area Net for "Total" Fractures in Andesite and Aplite of the O pen-pit Area (1 9 6 4 )...... 42
12. Geographic Zones and Molybdenum Grade Areas Used in Fracture A n a l y s i s ...... 44
13. Equal-area Net Analysis of Faults and Fractures in the Deposit and Outlying A reas ...... in pocket
14. Equal-area Net Plots for Different-sized Fractures .... 47 15. "Total" Mineralized Structure ...... 53 vii viii
LIST OF ILLUSTRATIONS— (Continued)
Figure Page
16. Comparison of "Total" Fractures to "Total" Mineralized Fractures for the Open-pit Area (1964). . 54
17. Percent Plots: *.P‘^ ral.;z?d ^2?“!?.!- x iqq # total fractures for each 1% Counting C i r c l e ...... 57
18 . Fracture Gouge Geochemistry of the Open-pit Area .... 64
19 . Structure Contour Map Mine Aplite ...... in pocket
20 . Composite Structure of the Mine Aplite Body .... in pocket
21. Contact-conformable Fractures in Mine Aplite in Open Pit on Southwest Slopes of Highline R idge ...... 72
22. Contact-conformable Fractures in the Northwest Corner of the Open P i t ...... 73
23 . Mine Aplite; Contact-conformable Fracturing Looking NNW from Highway 38 just West of Mill . . 74
24. Contact-conformable Fractures in Mine Aplite South of Red R iv er...... 75
25 . Low-angle Contact-conformable Fractures Dipping East (right) ...... 75 26. Contact-conformable Fracturing in Log Cabin Aplite about Two Miles East of Q uesta ...... 76
27 . View Looking Northeast at Contact-conformable Fractures in Log C abin S t o c k ...... 77
28. Sheeting in the Mine A plite ...... 79
29. Mine Aplite; Sheeting in the North-central Part of the Stock above Highline R idge ...... 80
30. Mine Aplite; Northerly Striking Fracture Cleavage in Cross Section ...... 83
31. Mine Aplite; Fracture Cleavage Intensely Developed Next to Low-angle, West-dipping Fractures .... 84 ix
LIST OF ILLUSTRATIONS--(Continued)
Figure Page
32. Precambrian Granite; High-angle Sheeting in Cross Section near Columbine Creek- Red River Ju n c tio n ...... 88
33. Fracture Cleavage Joints in Precambrian Granite; Same Locality as Figure 3 2 ...... 89
34. "Fracture Sets in the Mine Stock South of Highway 38 . . . 90
35. Close-up Photograph of Figure 34 Showing NNW, Near-vertical Jointing (in sunlight) with Sheeting C h a r a c te r is tic s ...... 91
36. Sheeting (ENE) Showing Lateral Grooves and Striations . . 93
37. Lateral Movement Striations on ENE Sheeting ...... 93
38. Interrelations of Fracture Elements in the Columbine A p l i t e ...... 94
39. Near-horizontal Slickensides on East-striking Sheeting in Mine Aplite, North-central Part of Stock ...... 95
•40. Log Cabin Stock; Plan View of Fracture Cleavage D eveloped Next to ENE S h e e t i n g ...... 95
41. Local Concentration of Fracture Cleavage Adjacent to Low-angle, Contact-conformable Fracture which Shows Normal Fault Movement ...... 96 42. Fracture Relationships in Mine Aplite; Main Haulage Road at Highline Ridge ...... 97
43. Relationship of Fracture Elements in Mine Aplite .... 99
44. Characteristic Structure of the Mine Aplite and Its Relation to Alteration and Mineralization in the Pit A r e a ...... 100
45. Equal-area Nets and Percent Plot for Structure of the M ine P lu to n ...... 101
46. Relationships of Fracture Elements to Mineralization in Mine Aplite of the Open-pit A rea ...... 104
47. Equal-area Nets for Different Phases of Mineralization . . 106 48. Field Sketches Showing Relations of Fracture Cleavage . . 108 X
LIST OF ILLUSTRATIONS— (Continued)
Figure Page
49. Dikes and Elongate Intrusions...... in pocket
50. Map of the Northeast Structure Zone Between the Mine Aplite and Log Cabin G ranite ...... 114
51. Regional Fracture Analysis of Area Between Mine Aplite and Log C abin G r a n i t e ...... 116
52. Structure Diagrams: Intrusions of the Red River R e g i o n ...... in pocket
53. Sheeting in Log C abin G ra n ite ...... 123
54. Fracture Relationships in the Log Cabin G ranite ...... 124
55 . Schematic Block Diagram of the Mine Aplite Showing Fracture Elem ents ...... 135
56. Primary Structure Elements in the Strehlen Massif, Germ any ...... 138
57. Regional Northeast-striking Jointing Cutting the Grass Valley Pluton and Country Rocks, G rass V alley, C a lif o r n ia ...... 145
58. Regional N. 65° E. Joints Cutting the Caribou Pluton, Klamath M ountains, Ca lifo rn ia ...... 147 59. Zones of Fine Fissuring Cutting Across 100 Square Kilometers in Eastern R u ssia ...... 148
60. Experimentally, Produced Extension Fracturing ...... 150
61. Compression Applied to Two Pairs of Sides of a Cube with Extension Fractures Formed P arallel to the Free F a c e ...... 151
62. Possible Effects of Tension and Extension on Rock .... 153
63. Mohr Envelope Criteria for Extension Fracture Sets with Small Dihedral Angle ...... 158
64. "Zigzag" Microjoint Sets in Basement Rocks of the Middle Rocky Mountains ...... 161
65. The Effects of Uplift and East-west Spreading of Rock M a s s ...... 163 xi
LIST OF ILLUSTRATIONS— (Continued)
Figure Page 66. Systematic Cross Joints in Zones, Replacing Each Other en Echelon, Together with Non-systematic, Short, Curviplanar Jo in ts ...... 166
67. Extension and Westward Drift Affecting North America . . 167 LIST OF TABLES
Table Page
1. Analytical Results on Age Determinations from the Red River Region, New M ex ico ...... 20
2. Fracture Distribution for Different Grade Areas (% Mo) and G eographic Zones Outward from Center of Outcropping Ore D ep o sit ...... 45
x ii ABSTRACT
Precambrian rock of the Sangre de Cristo uplift is sharply cut off on the west by the downfaulted Rio Grande rift zone and is crowded against folded and reverse faulted sedimentary rocks on the east. With in this north-trending uplift, the Questa molybdenum deposit is localized along a major cross structure, the Red River Structural Zone, which strikes ENE to E. Along this zone of crustal weakness, volcanism, plutonism, hydrothermal activity, ENE to E fracturing, and dike and vein emplacement have been concentrated.
The Questa deposit occurs in the western corner of the Mine aplite, the central of three Miocene granitic stocks which have been intruded along the Red River Structural Zone. Stockwork structure is common through the deposit especially in mineralized andesite; how ever, its development does not mask certain systematic fracture sets.
In the deposit, fracture sets in the Mine aplite and, to a lesser extent in the andesite host rocks, largely simulate fracture sets in the "barren" part of the pluton. Two, near-vertical, orthogonal joint sets are recog nized: an ENE-E set called sheeting and a N-NNW set defined as frac ture cleavage. In addition, a low-angle fracture system called contact conformable fracturing exists, which parallels the attitude of the pluton contact and in the deposit dips west, southwest, and south. Another low-angle fracture set of NNE-NE strike and NW dip is also recognized.
The largest veins in the deposit strike NE to E and dip nearly vertical. The mineralization of. minor veining, which by far predominates x iii xiv in the deposit, is emplaced into the NE-E sheeting and the contact- conformable fracture set. Fracture cleavage is barren of mineralization.
Although the greatest number of mineralized structures are west-dipping contact-conformable fractures, two directions of high- angle fracturing, N. 80° E + 10° and N. 65° E. + 10°, are preferred for mineral access and fissure filling. These fracture directions repre sent the sheeting element and they parallel the trend of dike emplace ment within and surrounding the pluton. The preferred directions of mineralization and dikes correlate with ENE and E trends of geochemical anomalies, underground ore trends, and two ridges on the west contact surface of the stock which are spatially related to molybdenum minerali zation. Collectively these data indicate a fundamental structural ore control which formed deeply penetrating, "open" channelways for the
initial tapping of hydrothermal fluids. The low-angle fracturing acted to disseminate mineralization laterally through the rock at a higher level.
Mineralized NE-E sheeting and barren N-NW fracture cleavage occur in other aplite stocks and through other rock types along the Red
River Structural Zone. Contact-conformable fracturing is also common in the other plutons. The orthogonal sheeting and fracture cleavage jointing represent regional extension fracturing due to uplift and crustal
stretching. Contact fractures can be correlated with "Lager" joints and are probably caused by release of compressional intrusive pressure nor mal to the roof of the stock.
Regional analysis of extension fracturing, dikes, and veins
suggests northerly directed crustal extension and stretching during
Laramide deformation followed by westerly extension in mid-Tertiary XV time. The Red River Structural Zone by mid-Tertiary time already existed as a major anisotropic feature in the crust, and during deformation strongly reoriented the direction of regional extensional structure to parallel the nearly east strike of this zone of weakness. This mecha nism controlled the emplacement of magmatic and hydrothermal fluids into the E-NE extension fractures which were formed along the structural zone. At a later time, intensified east-west extension of rock mass in the Red River area caused N-NNW fracture cleavage jointing to cut across the zone and developed translational movements on preexisting fractu res. INTRODUCTION
Purpose of the Study
The geology of the Red River region. New Mexico, which includes the Questa molybdenum mine area has been described in pre vious studies by Larsen and Ross (1920), Vanderwilt (1938), Schilling
(1956 and 1965), Carpenter (1960 and 1968), Gustafson, Bryant, and
Evans (1966), Rippere and Lisenbee (1965) and Ishihara (1967).
McKinlay (1956 and 1957) and Clark (1966a) mapped the surrounding region.
Of this work, only that of Vanderwilt (1938) and Schilling (1956 and 1965) included more than a cursory discussion of structure at the mine. This discussion dealt mainly with high-grade veins ex posed in early underground workings. Regarding the new, low-grade, open-pit ore body, most investigators (i.e ., Gustafson et al„ 1966;
Schilling, 1965) have avoided many details concerned with structural control. Gustafson et al. have been satisfied to call the ore body a typical stockwork deposit which lacks any specific structural controls.
Only Ishihara (1967) and Carpenter (1968) have described specific frac ture and vein systems in the open-pit deposit.
By means of comprehensive detailed mapping and the statistical treatment of structural data, the results presented in this discussion show that definitive fault and fracture elements have determined the locus for the "disseminated" mineralization. Certain fracture orienta tions were preferred over others for mineral emplacement. 1 2
An objective of this dissertation is to delineate the fracture elements which have controlled ore mineralization at the Questa deposit.
However, the primary purpose is to show that this fracturing is the result of specific regional stresses combined with local, intrusion generated stress. The structural elements of the ore body will be placed
in their proper perspective in the tectonic framework of regional and
local stresses. In addition, much new information on the structure of
stock-sized intrusive bodies will be presented.
Location
The Ouesta molybdenum deposit is located in north central New
Mexico in the Sangre de Cristo Mountains as shown in Figure 1. The mill
and mine offices are situated approximately six miles east of Questa, a
small town which lies at the western front of the mountains. The deposit
occurs northwest of the mill in Sulfur Gulch, a north tributary to Red
River. Access to the mine area is by paved highways, N.M. #3 north
from Santa Fe to Questa and N.M. #38 eastward from Questa to the m ine.
H istory
The presence of molybdenite was established in 1916 by James
Fay, a prospector owning several claims along Sulfur Gulch. During the
First World War the Western Molybdenum Company took over the claims
but sold out to the R & S Molybdenum Company of Denver in 1918 with
out doing much development work. Production began in the spring of 1919.
The newly formed Molybdenum Corporation of America bought the mine in NEW MEXICO
N. MEX. O uesto
Taos
Ties Ritos
Sonfo Fe
RED RIVER APLITE
Questa Open r-. RED Pit Mine__ APLITE RIVER
Ouesto WAR-4-67
LOG C A B IR . GRANITE V
APPROX LOCATION AGE SAMPLE A OF S. ISHIARA LOCATION AGE SAMPLES (This Report) SCALE IN MILES Figure I. LOCATION MAP OF QUESTA MINE AREA 4
1920 and has retained ownership to the present day. A mill was con structed in 1923.
In the early days of mining, the ore body was worked as an extensive underground operation. The workings were linked to Red River in 1941 by a mile-long haulage tunnel which entered the area of veins from an adit near the highway about a mile southeast of Sulfur Gulch.
Between 1925 and 1945, the annual production was greater than 500,000 lbs of MoSg (Ishihara, 1967, p. 3). The total production to 1956 was
18.095.000 lbs of MoS2 (Schilling, 1956, p. 8).
During 1963 and 1964, surface drilling was carried out to ex plore for a large-tonnage, low-grade ore body. The necessary grade and volume were found, and initial stripping began in late 1964. The first ore was sent through the new crushers and mill early in 1966. Work is presently going on to expand the pit, which in 1966 was supplying over
10.000 tons per day to the mill (Gustafson et al., 196 6, p. 51).
General Method of Study
Familiarity with the Questa deposit was gained by the writer during the summers of 1964 and 1965, when initial excavation created excellent exposure of the ore body and its margins. Detailed 1:600
scale mapping of the road cut and bench exposure was carried out for
six months. Additional days of pit mapping continued intermittently through two succeeding summers as new exposures became available through mining. As the data accumulated, a distinct fracture and vein pattern
became recognized in the aplitic granite stock and andesite of the ore
body. Four months of additional field work during 1966 and 1967 were 5 utilized in mapping rocks marginal to the deposit in order to trace struc tural trends outward from the pit into the regional framework. In 1967, several nearby igneous plutons were sampled for structure^ in order to provide comparisons with the productive stock.
Large quantities of structural information were amassed during the field program. In addition to normal map representation of the data, a major part of the study evolved around equal-area net plotting of structural features such as veins, dikes, faults, and joints. This sta tistical treatment proved useful in discovering subtle structural controls of the overall mineralization. A comprehensive discussion of the equal- area net and the sources of error inherent in statistical fracture analysis is included later in this paper.
Radiogenic age dating was performed on samples from several plutons in order to relate the stresses causing fracture to geologic time.
Four potassium-argon determinations were made: three at the Laboratory of Isotope Geochemistry, The University of Arizona, and one by Geo- chron Inc. of Cambridge, M assachusetts. These are in addition to two dates published by Ishihara (1967, p. 5).
1. Throughout this dissertation, the term "structure" will be used to denote joints, faults, veinlets, veins, dikes, or any combina tion of these features. The term "fracture" will be used in a general sense to refer to any failure surface in the rock regardless of its dimen sions or the movement along it. "Breaks" and "breakage" will be used synonomously with "fractures and fracturing." GENERAL GEOLOGY
Sanqre de Cristo Mountains
The Sangre de Cristo range consists of a strongly elevated, arcuate sliver of Precambrian rock which is slightly convex to the east.
Figure 2 shows the tectonic position of this mountain block at the south end of the Rocky Mountain uplift. The Great Plains border the Sangre de
Cristos on the east and the Rio Grande trough abruptly truncates the range on the west.
Two cross sections are included (Figure 3) to illustrate the
east-west geology across the range. They show the following features,
which have north-south continuity along the axis.
1. The westernmost part of the range adjoining the large frontal
faults of the Rio Grande valley consists of denuded Precam
brian basement rocks.
2. Farther east, sedimentary Paleozoic cover forms a synclinorium,
the eastern margin of which terminates in a zone of eastward
directed thrust and reverse faults and folds.
3. East of this disturbed zone. Paleozoic and Mesozoic sedimen
tary rocks are gently tilted eastward away from the range. No
strong folding or faulting apparently exists in this area.
The cross sections depict an uplift which represents a faulted
asymmetrical synclinorium. Baltz, Read, and Wanek (1959) described the
range as part of a central uplift which was originally located over the area
now occupied by the Rio Grande trough. The uplift is considered to be 6 EXPLANATION
Tertiory - Quoternory Volcanic rock*
Tertiory- Quaternary Sediment*
Tertiory intrusive bodies
Pre&ombrlon basement PUEBLO rocks
Tertiory dikes
Normal fault SAN JUAN SAN LUIS Thrust or VOLCANIC reverse fault FIELD BASIN
TRINIDAD RATON - BASIN ----
Questo Deposit
JUAN BASIN
TAOS
JEMEZ VOLCANIC x FIELD
Scale U 2,500,000 50 miles ALBUOUEROUI
Figure 2. TECTONIC MAP
From Tectonic Mop of the United States , 1961 1
a
COLORADO NEW MEXICO ° Rofon
Questo Mine Qui.to / Red , * - /X 'R River TAOS range
CIMARRON RANGE RIO GRANDE MORENO r 12,000 o Toos valley DEPRESSION RATON Keg Tpc BASIN Kd TO Tdp . - 9,000
*2- 6,000 Ocote A‘ Wagon Mound Mora
Santo Fe
30 Miles
INDEX MAP
Pecos - Picuris Fault Zone interpretation of Miller, et ol, 1963
SANGRE DE CRISTO Rio Grande UPLIFT Picuris RATON BASIN PK dm Canadian I TQb River r 10,000 T - 8,000 6,000 u ill 4,000 « / - i 2.000 i - 0 •-2.000 -4, ooo B
Figure 3 Regional cross sections of the Sangre de Cnsto Mountains (Top section after Clark, 1966b Bottom section after Baltz, et ol , 1959) 9
Laromide in age. The axial portion of the uplift apparently became rifted and structurally depressed, forming the Rio Grande trough.
From north to south, the Sangre de Gris to uplift appears to be a southward plunging element with Precambrian rock dipping southward beneath Paleozoic cover at about the latitude of Santa Fe. The intensity of both folding and faulting along the eastward margin of the uplift steadily decreases southward (Baltz et a l., 1959) .
The area of the present mountains and the Las Vegas basin was once part of the Rowe-Mora geosyncline which accumulated sediments from Pennsylvanian to Cretaceous time. The trough portion of the geo syncline was elevated and strongly eroded during the Laramide orogeny to form the initial uplift. Some 6,000 to 7,000 feet of structural relief were created.
In many respects the Sangre de Cristo uplift is similar to other ranges in the south and central Rocky Mountains. The Wet Mountains and the Front Range are examples. Some common characteristics of these uplifts are: (1) a large and abrupt elevation of the present Pre cambrian surface, (2) important zones of reverse faulting or monoclinal flexuring along their margins, (3) a relative absence of intense folding of Paleozoic sedimentary rocks within the uplift block, and (4) a small amount of deformation in adjacent basins.
One other aspect common to these ranges is the disagreement among geologists as to the mechanics of the uplift. Lateral compression and simple vertical uplift are independently advocated. 10
Red R iver-C abresto Creek Area
General Geology
Cabresto Creek and Red River are the two major, east-west drainages which bound the molybdenum deposit on the north and south, respectively. Their unusually close spacing results in a noticeable break in the north-south continuity of the Sangre de Cristo mountain front. This erosional break is most likely controlled by the concentra tion of EW fractures in the area.
The geology of the area encompassed by Red River and Cabresto
Creek is shown in Figure 4. The Precambrian rocks consist largely of gneissic granites, amphibolite gneisses and schists along Red River, and of metaquartzites and graphite-bearing, muscovite, albite gneiss - along Cabresto Creek.
A thick sequence of Tertiary volcanic rocks rests on the eroded
Precambrian surface. In a few places along the lower north slopes of Red
River canyon, a section of pebble conglomerates and fine-grained, clas tic sedimentary rocks lies between the volcanic rocks and basement rocks. The sedimentary section has previously been considered as
Pennsylvanian-Permian in age (Sangre de Cristo formation) (Schilling,
1956, p. 13); however, recent work indicates that the section is partly tuffaceous and may belong to the Tertiary volcanic series (Clark,
1966a).
The volcanic rocks have been described (Ishihara, 1967) as a three-layer section of porphyritic andesite (1,500 to 1,700 ft) overlain by porphyritic quartz latite (0 to 1,500 ft) and capped by tuffaceous rhyolite (1,500 ft). Extensive study has subsequently revealed that the j L
\ ll A I.. V
m g
' 1 2 i
> \ slide \ \ block , > \ ) f % . m v \ 4
, \ / a % j ..... x) ( " < r \ \ 1 % '' X •I V k ! . . V x % - % \ i > " EXPLANATION i/i «e Pcrphynfic intrusive andesite Lotites ':W Quartz monzonite dikes □ Andesites m Quartz porphyry series □ Precombnon Granite porphyry Geologic contact (Dashed where uncertain) □ 5000 ioooo *eef Apiitic granite Fault □ Approx t " = i mil e Do Cite porphyry to rhyolite Approximate position of open pit SCALE □ ♦low structured o Figure 4. GEOLOGY OF THE RED RIVER-CAB RES TO CREEK AREA (After Rippere and Lisenbee,l965) V
EXPLANATION ft Mid - Terticry v ‘f ,sive body ^ ‘v1'd - Tertiary .ji-
Appro * l" = I mile SCALE Figure 4A STRUCTURE OF THE RED RiVER-CABRESTO CREEK AREA 12 implied stratified nature of these rocks is overly simplified and possibly erroneous. The latite unit is missing in many places and where present may be intrusive. The other two units also have common intrusive equiv alents associated with them*.
Clear examples are found west of Sulfur Gulch where dikes of the flow-banded rhyolite cut the latite. Flow layering is steeply dipping to vertical through much of the rhyodacite. This attitude cannot be sat
isfactorily explained by an extrusive flow or by faulting. Thus, evidence
is accumulating that the volcanic unit originally mapped as extrusive
rhyolite is a shallow intrusive rock with near-vertical flow structure.
A complex suite of other intrusive rocks is found in the Red
River-Cabresto Creek area. This suite includes granite porphyry, quartz
porphyry, aplitic granite, quartz monzonite, and porphyritic andesite.
All these rocks cut the volcanic sequence.
The most important of the above sequence of intrusions are
three stocks of aplitic granite which occur along a N. 75° E. line from
Questa to Red River. They are named from west to east: the Log Cabin
granite, the Mine aplite, and the Red River aplite. The Questa molyb
denum ore body occurs in the Mine aplite and adjacent volcanic rock.
Molybdenite also occurs in scattered veins in the Log Cabin granite.
Structure
The Red River-Cabresto Creek region is complexly fractured.
Fault and fracture trends of N to NNW and ENE to E strike, however,
can be singled out as most significant (Schilling, 1956, p. 34).
The continous larger faults of the area are represented by the
N to NNW set (Figure 4A). These faults are, for the most part, 13 high-angle normal faults. The foremost example of this fault system is the major frontal fault(s) which forms the western edge of the Sangre de
Cristo range. Normal displacements along the N to NNW structures have resulted in vertical movements of considerable magnitude. Such is the case for the strongly elevated block which includes the northern part of the Mine aplite. To the east of this block, several NNW-trending faults are repeatedly downdropped on the east side forming a downstep ping of the volcanic rocks toward the east.
According to Rippere and Lisenbee (1965), extensive hydrother mal activity, evidenced by silica and pyrite, has been channeled along the NNW faults and is a factor contributing to the erosional "cirques" or "badlands" which characterize the topography in the Red River region.
Fracturing in the E to ENE direction is a finer, more pervasive breakage that is found cutting all rock types. Schilling (1956, p. 34)
refers to faults of this system as having affected small displacements
with a multitude of parallel fractures showing no detectable movement at
all. One can see from Figure 4A that many of the dikes and elongate in
trusions trend parallel to this ENE direction. In fact, the major struc
tural weakness along which Red River and Cabresto Creek have etched
their canyons also corresponds to this E to ENE trend.
Considerable difference of opinion exists regarding the explan
ation for the ENE topographic rift with its intense parallel fracturing.
The concept of a down-faulted graben has been frequently expressed.
Schilling (1956) first proposed the idea chiefly on the basis of several
thousand feet of structural relief of the Pennsylvanian-Permian(?) sec
tion and base of the volcanic section. Both McKinley (1957) and Clark (1966a) mention, and apparently concur with, this explanation.
Bryant (oral communication) and others at the mine see no compelling evidence for an EW graben. No continuous boundary faults have been mapped. There is no EW linearity separating Precambrian rock from volcanic rocks. Furthermore, in the vicinity of the mill and mine offices, there is evidence for a normal depositional contact be tween basal andesite breccia and a Precambrian erosion surface. This author, therefore, believes that although the area in question from
Questa to Red River is a major zone of structural weakness, there is little evidence favoring its being a major graben. The difference in ele vation of the volcanic sequence may be due instead to filling of a pre vious topographic depression. This is not to say, however, that this depression did not have some earlier structural origin, such as an intense zone of fracturing . MINE GEOLOGY
The geology of the open-pit mine area is illustrated in Figure 5.
Contacts follow the topography as it was in 1964 prior to major excava tion. Features such as Highline Ridge, Sulfur Gulch, and Blind Gulch, although largely destroyed by mining operations, will still be retained for reference in this paper.
This section is mainly descriptive and will present the basic field data. Rocks in the mine will be briefly described. A full discussion of structure and mineralization will follow, ending with the definition of ore controls.
Rock Types
Ore mineralization at the Questa mine occurs in aplitic granite of the Mine aplite stock and fine-grained andesite (Figure 5). In addition to these rocks, the geologic map shows a complexly faulted dike system and intrusive plug of granite porphyry. Precambrian quartz, feldspar gneiss, and amphibolite separate the stock from andesite in the extreme southeast corner of the mapped area. To the west of and topographically above the pit, the quartz latite and flow-foliated dacite to rhyolite crop out. These units occur outside the limits mapped on Figure 5.
Ishihara (1967, p. 24) describes the greater part of the Mine aplite as a slightly porphyritic aplite. ^ Phenocrysts consist of euhedral
1. Recent petrographic work by Questa geologists coupled with Ishihara*s data shows that the mine aplite is a silica-rich, mafic- deficient quartz monzonite. The original rock name will, however, be used in this dissertation.
15 - f N 25,000 f - E 55,800 E LG C A - QET MLBEU DEPOSIT MOLYBDENUM QUESTA - MAP GEOLOGIC i iA * Aplite riAAf* ole Cl (sie material) slide ( Cql. Boulder rnt prhr dk system dike porphyry Granite cl I = ft in 500 Scale opytc pie o pii granite aplitic to aplite slightly= Aplite porphyntic Mine Ouesto Fine groined biofife bearing bearing biofife groined Fine neie Icue some (Includes andesite Intrusive, flow layered, andesite (°) (°) andesite layered, flow Intrusive, rcmnn• apioie and •• amphibolite Precambnon neie porphyry) andesite uf ( ) Tuffisite tuff rnt gneiss granite Fault Fault EXPLANATION rci o pbl dike pebble or breccia r major or fracture 36 36
EXPLANATION
Boulder Cql. ( slide material )
Granite porphyry dike system
Aplite breccia or pebble dike + N Ouesta Mine Aplite = slightly □ porphyntic aplite to oplitic granite Intrusive, flow layered, andesite (?) tuff ( Tuffisite ) ' '"T )(9roinecl biotite bearing anaesite (Includes some andesite porphyry) Precambnon : amphibolite and granite gneiss
Fault or moior fracture
Scale I in = 500 ft.
-h N 25,000 *.
\
!
A MiNBSeL OSIE -MAET-UOUESTA MOLYBDENUM DEPOSIT 17 to subhedral plagioclase, anhedral potash feldspar, anhedral quartz, and occasional flakes of biotite. The rock contains fine specks of apatite, zircon, and opaque minerals. In places, a chilled border phase is present which includes more quartz and feldspar and less mafic min e ra ls .
Gustafson etal. (1966, p. 52) note that in the ore body the intrusion has been subjected to recrystallization and shows signs of textural change. Biotite increases and the amount of albite is reduced.
Quartz grains are enlarged along with the development of thin quartz v e in le ts .
The intruded andesites are fine-grained porphyritic rocks with phenocrysts of andesine. The composition of the matrix is variable, consising of either fine-grained orthoclase and biotite or glass frag
ments. The mafic content of the andesite is very low. Locally, in addi
tion to the fine-grained andesite, flow-layered, tuffaceous andesite,
andesite porphyry, and breccia are present. The entire andesite sequence
is in places propylitized; however, areas of silicification and pyritiza-
tion are far more common. Near the aplite, and associated with molyb
denum mineralization, seriticization and secondary biotite become
important alteration products.
Cutting the aplite on Highline Ridge are several aplite breccia
or pebble dikes. The dikes are composed of well-rounded pebbles and
cobbles of aplite and lesser amounts of angular andesite fragments en
closed in a fragmental matrix of quartz, feldspar, and granite material.
Molybdenite coats rounded cobbles and is found in the matrix material;
however, mineralized fragments are also included in the rock. Thus, it 18 appears that the emplacement of the dikes was contemporaneous with mineralization. The strike direction of the dikes is parallel to that of many of the large veins in the same area.
One occurrence of post-aplite dikes was mapped by the author near the head of Sulfur Gulch. This is a 1 to 3 foot wide aphanitic ande site (?) dike. Schilling (1956) describes dikes of quartz monzonite to monzonite intruding the Mine aplite underground in mine workings.
Rippere and Lisenbee (1965) mapped quartz monzonite dikes cutting the
Red River sto ck .
A lteration
A prominent feature of the Red River region includes some large areas of what appears to be extreme hydrothermal alteration of the vol canic sequence. These areas are readily seen as prominent, yellow- orange scars on the mountain slopes where erosion in soft "altered" rock has been converted in varying degrees to "limonite," gypsum, and clays with native sulfur developed locally. A few feet below the surface, the volcanic rocks are relatively fresh except for considerable addition of silica and pyrite. It has been concluded that the clay alteration and rock decomposition are supergene phenomena: a result of attack by the sulfuric acid which was generated by oxidation of the pyrite in the rocks
(Gustafson et a l., 1966, p. 55).
Age R elationships
From the earliest reports by Larsen and Ross (1920) and
Vanderwilt (1938), the Questa mine aplite stock was referred to as
Tertiary in age. There was, however, no specific evidence for this as 19 no fossils had been found within the intruded rocks. Schilling (1956, p. 25) first assigned a late Tertiary age to the volcanic sequence on the basis of close similarities with the Miocene volcanic rocks of the
San Juan Mountains, Colorado. In addition, the Pliocene-Pleistocene basalts which cover the San Juan volcanic rocks also cap the andesites, latites, and rhyolites in the Costilla Quadrangle north of Cabresto C reek.
Recently, several radiogenic age determinations have been
made on the aplitic granite intrusions which invade the volcanic series.
The Laboratory of Isotope Geochemistry, The University of Arizona, using
the potassium-argon method, dated unaltered samples from the Mine and
Log Cabin granites. A vein of biotite-molybdenite from the mineralized
Mine aplite was also dated. Samples used for the age determinations
are located on the location map of the Questa mine area (Figure 1). The
granites have an average cooling age of 22.4 m .y. and the vein material
23.5 m .y .
Ishihara (1967, p. 5) reports two additional dates from ore
veins in the Mine and Log Cabin plutons. The particulars for all radio
genic samples are tabulated in Table 1. There is excellent agreement
between all dates clustered about 22.5 m.y. This date establishes an
early Miocene age for stock emplacement and mineralization.
The scattered outcrops of arkosic sandstone, shale, and con
glomerate described by Schilling (1965, p. 20) as Pennsylvanian-Permian
are reported by Clark (1966a, p. 59) to be tuffaceous and possibly Ter
tiary in age. 20
TABLE 1 .—Analytical results on age determinations from the Red River region, New Mexico
40Ar Sample No. K ^ A r rad. a tm o s. Apparent & Location M ineral % x lO 'lO m /g % Age m .y .
* WAR-1-67 biotite 6.15 2.45 44.3 22.3 + 0.7 Mine Aplite
*WAR-2-67 biotite 7.99 3.35 51.4 23.5 + 0.8 Vein, Mine Aplite
*WAR-4-67 biotite 7.73 3.11 57.1 22.5 + 0.9 Log Cabin Stock
**Rio Hondo hornblende 0.560 0.00206 ppm 32.6 50.9 + 2.8 Stock
*Laughlin, Rehrig, and Mauger, in preparation.
**Isochron Inc., Cambridge, M assachusetts, written communi cation, 1968. Note: Ishihara (1967, p. 5) reports two additional potassium- argon dates for the Questa region. Biotite from molybdenum veins at the Log Cabin prospect (Log Cabin granite) and from the Z-tunnel in the Mine aplite gave ages of 23 + 3 m.y. and 21 + 3 m.y. respectively. 21 Regionally metamorphosed rocks, including the Cabresto meta quartzite, are assigned to the Precambrian. The quartzite correlates with other Precambrian quartzites in the Rocky Mountains and is over- lain unconformably by late Paleozoic sedimentary rocks (Schilling,
1956). Diabase dikes which intrude the metamorphic complex but which are not themselves metamorphosed are believed to be late Precambrian in age (Clark, 1966a).
Fracturing and Faulting
Stockwork Aspects
Stockwork is the commonly used term to describe fracturing at the Questa deposit. The word defines a body of rock so interpenetrated by small veins that the whole must be mined together (AGI, 1960). With this meaning has come the concept of a maze of small, nonsystematic fractures having random orientation. Schilling (1965) apparently implies this meaning to describe the bulk mineralization at Questa. Gustafson et al. (1966) used the word stockwork to describe the deposit. They say
(p. 54): "Neither through-going faults nor preferred orientation of min
eralized fractures have been recognized. Sheeting parallel to the surface of the stock is usually prominent but not preferentially mineralized. The remaining fractures are short, discontinuous, without preferential orien
tation and exhibit little or no offset."
A similar opinion is often expressed by geologists studying
porphyry deposits in the western United States. Perhaps the stockwork
concept has become so closely associated with low-grade, large-
tonnage deposits that it has discouraged effective attempts to analyze 22 fracture controls of "disseminated" mineralization. It may partly be for this reason that Anderson (1968, p. 1180) concludes that the mecha nism of fracturing in the porphyry copper deposits is as unexplained now as it was thirty years ago.
There is little proof that porphyry-type deposits have com pletely random fracturing. The author knows of no study that has pre sented field evidence to support the thesis of fracturing and veining without preferential orientation in porphyry-type deposits.
Admittedly, the job of a structural analysis in a porphyry deposit is not easy. The existence of an ore body of this type usually is dependent upon complex fracturing. As the rock is made up of an intricate network of fracture's and veinlets spaced just inches apart, an additional deterrent to structural study is present. Under these condi tions, bench exposures in open-pit mines give a confused and compli cated picture of fracturing. From exposures of this sort, a simple fracture pattern is not readily recognized until after a thorough mapping is carried out, one which includes the systematic recording of hundreds of fracture attitudes.
The key then to an effective structural study in the complex geologic setting of porphyry-type deposits such as Questa is the con sistent notation of a large number of veins and fractures of all sizes and dimensions. This would include such features as veins two feet wide as well as small veinlets only fractions of an inch in width.
At Questa, it was found that the major structure (veins and faults) of larger size readily established the existence of a systematic pattern (see Figures 5 and 5A). This pattern stands out against a 23 background of diffuse and more numerous data consisting of small frac tures and veinlets which required statistical methods to resolve.
The results of the statistical analysis of this background data show that a great deal of the veinlet and fracture orientation in the deposit is systematic and cannot be regarded as random stockwork breakage. This is especially true for the intrusive rock of the Mine aplite stock. Some fractures were recorded for practically every orien tation; however, a relatively large number of measurements were found which grouped closely around certain specific orientations. These orientations of high fracture density define the systematic fracture sets.
The same results apply to other structure such as veinlets.
It should be emphasized at this point that the statistical anal ysis does not include fine structure such as joints, cracks, or the hair line mineralized fractures which are present in the deposit. It also does not take in singly occurring minor fractures without parallel counterparts.
Although much of the jointing is quite systematic, the nature of the rest of the minor fracturing is questionable. It may be substantially haphazard in orientation.
Certain factors may have contributed to the complexing of struc ture in the deposit. These factors include: (1) variations in attitude of • the individual members in any given fracture, vein, or dike set; (2) changes in the original attitudes of structure due to later block-fault movements, tilt, or drag; (3) divergent structural trends resulting from locally changing stress fields such as those generated by intrusive push or withdrawal; (4) changes of the regional stress field through time; (5) 24 possible shrinkage failure due to crystallization and cooling of the sto c k .
These structural complexities are best exemplified in the andesite host rocks which were intruded by the Mine stock. In these andesites, fracturing is more haphazard and close spaced than it is in the intrusion. Pre-aplite fracturing, contraction failure, and local intru
sion-generated stresses were probably responsible for most of the
superimposed complications in the fracture pattern of these rocks.
Faults
During pit mapping, thousands of fracture attitudes were re corded which ranged from tiny hairline cracks to major zones several
feet in width. Many of the fractures, especially those with the greater widths, were mapped for some distance along their strikes. These more continuous features are clearly distinguishable from the multitude of
discontinuous, small fractures, which are typical of the deposit. The
major fractures generally contain varying widths of gouge but do not
always show slickensides. However, for the purpose of clarity in the
ensuing discussion, these major fractures will arbitrarily be termed
faults or structures.
The geologic map (Figure 5) shows the pattern of these faults
in the Ouesta deposit. Predominant trends are seen in different areas
and are classified as follows:
Northeast: Faults of this direction form a broad band traversing
Highline Ridge and project weakly through volcanic rocks to the south west of the Mine intrusion. These structures are the widest and most
prominent in the pit. Approximately 60 percent of them dip steeper than 25
60°, primarily in a NW direction. Generally, these faults are more numerous in the Mine aplite than they are in the overlying volcanic ro c k s.
East-west: Faults with this strike predominate in a diffuse zone on lower Highline Ridge and up the middle part of Sulfur Gulch. They have characteristics similar to the structures of NE strike. The faults are traceable into the volcanic rocks south and west of the Mine aplite, but as in the case of the NE fault set, they are best expressed
in the aplite. About 65 percent of the EW structures have high-angle
( >60°) dips. An important observation is that the NE and E directions
of faulting parallel most of the aplite and aplite breccia dikes in the a re a .
North-south: Northerly striking faults appear to predominate in the andesite west of the Mine aplite stock. The faults have high and
low-angle dips and consistently dip to the west. Structures of this set
also exist in the intrusive body, but they are mostly low-angle faults of lesser size.
Northwest: Although not readily apparent from Figure 5, two
limited areas are known where frequent NW faults exist. One of these
areas is along and adjacent to the aplite-andesite contact on Highline
Ridge; the other area is next to the south contact of the stock and up ward from there into the andesite. Most NW-striking faults are low-
angle (dip < 60°) and appear to roughly conform in attitude to the con
tact surface of the Mine aplite stock.
The nature of the zones of predominant faulting is of consider
able importance to the present study and is dealt with at length during 26 the final part of the dissertation. The dominant faulting of the Mine aplite in the area of the deposit appears to form a continuous, fanning pattern striking from NE across upper Highline Ridge to E along Sulfur
Gulch (Figure 5). In addition, the characteristics of the faults are simi lar over the entire range of strike variation.
Nevertheless, the NE and E-striking structures do intersect and overlap somewhat on the southern slopes of Highline Ridge. Infor mation on regional fracturing (p. 87) also suggests that the NE and E directions of faulting are really representative of two separate fracture s e t s .
The areas where N and NW fault trends are most frequent are largely confined to the volcanic rocks. Here again, there appears to be a gradational merging of structure of these two trends, yet there are instances where two separate N and NW fault trends can be distinguished.
"Total" Fractures (Statistical Analysis)
More than three thousand fracture attitudes were recorded dur ing the course of pit mapping. Many of these were relatively small, discontinuous features which, because of their great numbers, made structural analysis impractical from maps alone. Furthermore, as noted previously, some degree of haphazardly oriented breakage additionally confused the picture. In order to handle these problems properly, struc tural data were treated statistically by means of equal-area net plotting.
The initial work was done by hand, a time-consuming task for such large numbers of readings. In 1967, a basic computer program (Spencer and
Clabaugh, 1967) was adapted for use on The University of Arizona com puter. This immeasurably increased the rate of data processing and 27 allowed far greater use of the technique on the Questa project. The determination of preferred structure and mineral trends was largely
made possible by this technique.
The Equal-area projection. The use and description of the
equal-area projection for the plotting and visualization of structural
data are adequately explained in standard textbooks (Badgley, 1965;
Turner and W eiss, 1963). In addition, several articles (Rodgers, 1952;
Friedman, 1964) treat the subject in some detail. Parts of the following
description are taken from an unpublished report by Mr. Barry McMahon,
Colorado School of Mines, 1967. The method is somewhat difficult to
visualize and has not been well understood by laymen or even some
geologists. The difficulty is partially overcome by use of a net gradu
ated directly in terms of strike and dip (Figure 6).
The orientations of planes in space are plotted in terms of
their poles on the equal-area net. The net represents a plan view of a
graduated lower hemisphere. The poles represent the projections of
lines normal to the planes.
As illustrated in Figure 7, the pole of a horizontal plane would
plot at the center of the net. A vertical plane would plot on the circum
ference. Graduations on the net emphasize that points representing
planes of equal strike but varying dip form lines radiating outward from
the center of the projection. Points representing planes of equal dip but
varying strike form concentric circles.
After initial plotting of poles, the statistical density is evalu
ated by counting over a prescribed grid the number of points which fall
in a small circle, the area of which is 1% of the entire projection. The 28
Figure 6 EQUAL-AREA NET Graduated for poles of planar elomonts (tower hemisphere) 29
Pole plotted on horizontal projection of equal - oreo net
Poles plotted on horizontal projection of equol-oreo net
* Projection of HORIZONTAL /"PLANE /p o le to net
-4—Projection POLE to net
Intersection of pole with hemisphere
Intersection of pole on hemisphere
Fracture poles plotted on net
Figure 7. Construction of the Equal-area Net Plot, Lower Hem isphere
(A) The plotting of a horizontal plane. (B) The plotting of a north-striking, vertical plane and a N. 45° W ., 45°S. plane. (C) The net with poles of planes plotted and overlain by counting grid. The standard counting circle whose area is 1% of the entire net is shown. This circle is centered at points on the gride and the pole points are counted. (D) Final overlay of pole density and contours. Numbers recorded are either number of poles or the percent of poles per 1% counting circle to the total number of poles. 30 counting circle is centered on each intersection of the orthogonal grid.
At this point the net consists of about 320 gridded numbers which can either be the number of counted points or a percentage of counted points to toal points. These values are then contoured, and the area between contours colored to facilitate interpretation. Spatial patterns of anoma lous density are observed directly as shown by the example in Figure
7D .
The equal-area net method is greatly superior to strike rose diagrams, as the latter cannot show simultaneous variation in both dip and strike.
Procedure and sources of error. The following procedure was
used for the structural analysis of the Ouesta ore body and adjacent
m argins.
1. Detailed, 1:600 mapping was done for all available exposure.
For the most part, this exposure included drill site road cuts
and cuts of benches.
2. Faults and fractures were classified and recorded on the basis
of width and type of filling, i.e ., alteration, quartz, molyb denite, etc.
3. Data were taken from the maps and notebooks and tabulated.
Ultimately, IBM data cards were punched.
4. Equal-area nets were used for plotting and analysis.
An early criticism of the plan was that unequal directions of .
sampling would introduce bias errors. For example, if most of the road
cuts were trending NW, the geologist would record proportionally few
NW structures which were nearly parallel to the line of mapping. The 31 greatest frequency of recordings, assuming equal density of fractures of all strikes, would be for those intersecting the line of traverse at nearly right angles. Terzaghi (1965) elaborates on this possible source of error and introduces a geometric correction factor to be used in joint
su rv e y s.
The problem has several related factors other than the dis cussed geometric one. One factor is the size, frequency, and spacing
of the planar elements being recorded. Another factor is the evenness
of the sampling surface. The author can demonstrate many examples in the pit where, because of the uneven surface of blasted bench cut and the close spacing of fracture elements, the predominant structure was
recorded nearly parallel to the direction of the cut.
Despite these exceptions, it was decided to test the structural
data of the deposit for this type of bias. The method used was to com
pare possibly biased structure taken from the total distribution of cut
exposure with structure from a balanced length of cuts for various direc
tions which should be free from bias. The steps taken were as follows:
1. All road or bench cuts us'ed in mapping were divided into 50-
foot increments.
2. These increments were plotted as strikes of vertical planes oh
a rose diagram which, for the aplite, is included as Figure 8.
This diagram shows the unbalance in cut trend in favor of
northwest-striking exposure. It substantiates the possibility
that geometrical bias may have been introduced into the overall
data. The predominant sampling of structure in the northwest
direction could, as described, favor recording more 32
Figure 8. Frequency Diagram for Road and Bench-cut Orientations in Mine Aplite of Open-pit Area
Numbers refer to the total number of 50-ft length increments of roadcuts which strike in the corresponding 10° strike segments 33
northeast-striking fractures than those striking in other direc
tions. Northwest-striking structure would be minimal.
3. To take the test further, equal road cut lengths (about 1,500
ft for the aplite) were selected for each 15° to 20° segment
of strike direction, and the fractures from each of these
balanced cut lengths were summed on an equal-area plot. The
balancing effort should have distributed the road cuts such
that any geometrical bias would be equally present for all
directions and in the final structural plot would cancel out.
For example, an excessive amount of NS-striking fracturing
recorded on EW-trending cuts would be counteracted by exces
sive sampling of EW-striking fractures taken from the same
length of cut exposure of NS trend. Likewise, an overabun
dance of NW-striking fractures for NE-trending cuts would be
countered by NE fractures recorded on an equal length of NW
c u ts .
4. An equal-area plot was made for fracturing contained in all
road and bench cuts with their unequal distribution of direc
tions. It was compared with the fracture plot for equally
weighted cuts. If bias were introduced into the former plot,
it would show up as a noticeable difference in the two equal-
area projections.
Figures 9 and 10 show the results for the aplite and andesite of the deposit. Although the contours differ in absolute value, nearly identical contour maxima for fractures are reproduced between plots for weighted and total road and bench cuts. The lack of significant 34
3565 APLITE number of measured TOTAL CUTS fractures
40 .273 50 .0156 60 .000158 69 .000001 Test for randomness (see text p. 34 )
60 /
APLITE EQUAL-WEIGHTED CUTS
.0 3 7 4 .0 00144 000001
Figure 9. Equal-area Nets (Lower Hemisphere) used in Testing for Bias due to Unequal Distribution of Roadcut Directions in Aplite of the Open-pit Area. Contours are in number of fractures per 1% of net. 35
number of measured fractures ANDESITE TOTAL CUTS 3076
.068 .00103 .000003 .000001 Test for randomness (See text, p. 39)
ANDESITE EQUAL-WEIGHTED CUTS
.00273 .000001
Figure 10. Equal-area Nets Used in Testing for Bias due to Unequal Distribution of Roadcut Directions in Andesite of the Open-pit *.rea Contours are in number of fractures per 1% of net 36 differences in the equal-area plots indicates that geometrical bias due to unbalanced directions of fracture sampling (road and bench cuts) has not influenced the data taken at the Ouesta deposit and that the correc tion factor suggested by Terzaghi (1965) need not be applied.
Care should be used, however, in attaching too much geologic significance to the absolute value of contour maxima. Furthermore, for small areas of analysis, where cuts are strongly unidirectional, inter pretation of the equal-area plots should be carried out with caution.
Another problem inherent in statistical fracture analysis per tains to the method of sampling. Obviously not all fractures can be measured over an extensive area. How is it determined which ones shall be chosen for recording? The procedure adopted for the present study within the molybdenum deposit is outlined below.
Fractures were recorded in the field on the basis of size. The following classification was used:
Class 1: Complex shear zones of multiple, close-spaced faults; greater than 5 ft in width.
Class 2: Complex shear zones, as above; 2 to 5 ft in width.
Class 3: Fractures containing gouge or crushed rock material; 1 inch to 2 ft in width.
Class 4: Fractures less than 1 inch in width but not including joints or lesser sized cracks.
Class 4 fractures are joint-like in character but are best dis tinguished from jointing by these critera: (1) a somewhat longer dimen
sion than joints, (2) single fractures which do not appear to belong to a
set, (3) noticeable widths (less than 1 inch) of gouge or any signs of movement such as striations or slickensides. 37
Nevertheless, there is some overlap between joints and Class
4 fractures, and undoubtedly many joint measurements were recorded as
Class 4 fractures. In fact as mapping progressed, it became common to record "Class 4 + multiple J" to signify one especially prominent joint like fracture with perhaps 1/2 inch of slickenside gouge which was parallel to and part of a prominent joint set.
In most cases, fracture lengths are roughly proportional to widths. The wide fractures are also the long fractures. The long breaks defined on page 24 as faults usually fall into Class 1, 2, or the wider
Class 3 fractures. All Class 1, 2, and 3 structures can be considered as major fractures to distinguish them from the multitude of small joints, fissures, and cracks which are found through the deposit. Every occur rence of a major fracture was noted during mapping.
The remainder of the fracturing consists of a pervasive network
of joints, fissures, and small cracks which collectively may be called
fine fractures. This kind of fracturing includes joints of all types as well
as minute, irregular cracks which do not exceed a foot in the maximum
exposed dimension.
Because of the relatively small size and large numbers of these
fine fractures, field recording had to be done selectively. Measurements were taken wherever several parallel fractures of this kind were recog
nized. Individual fine fractures were usually not recorded. The mapping
procedure was to carefully inspect the rock at frequent intervals for
repetitive orientations of fine fracturing. When these orientations were
found, a measurement was taken. 38
During pit mapping, fine fracturing was recorded on maps as joint symbols. These readings were not used in statistical analysis because of the excessive amount of compilation which would be required.
The presence of these fine fractures is partially reflected by the Class 4 fractures which were incorporated into equal-area plotting.
Regional fracture sampling was done using exposures along streams and natural outcrops. In spite of the fact that random sampling was an objective of the field work, certain forms of potential bias were recognized. The effects of the bias were minimized wherever possible. These effects are:
1. In outcrops of low relief, low-angle to subhorizontal fractures
were missed.
2. A tendency exists to select the next fracture for measurement
approximately parallel to the last one recorded.
3. Structure parallel to smooth outcrop faces is more easily missed
than that at sharp angles, although it has been demonstrated in
this study that this factor is relatively unimportant in the case of irregular exposures.
4. Fractures are best expressed when dipping away from the ob
server on the cut or outcrop face. These fractures may be pref
erentially measured.
Some of the bias effects were reduced by trying to integrate as many variations in outcrop as possible during sampling so that a maxi mum of structure of all orientations would be visible for inspection and measurement. Outcrops of moderate relief were usually selected. Tra verses were made in circular or irregular fashion wherever possible in 39 order to reduce the possibility of exposing an overabundance of more favorably visible fractures of certain attitudes (i.e ., such as those dipping away from the recorder or with strikes approximately normal to the trend of the outcrop).
All breaks larger than fine fractures were continuously recorded.
Every 25 to 50 feet, the outcrop was carefully inspected for evidence of fine fracturing. Any occurrence of multiple orientations of such fine fracturing was recorded at these locations.
In structural diagrams such as the equal-area net, one is faced with determining the statistical significance of maxima on the diagram, that is, the degree of departure from randomness of the contour maxima.
Mac Mahon (oral communication) has generated 6% maxima on equal-area nets using 50 random numbers taken from a phone directory. As the num ber of random points plotted increased, it was found that these maxima decreased in magnitude.
Pincus (1953) fully discussed the statistical treatment of orien tational data. Flinn (1958) examined several tests of significance applied to petrofabric diagrams and found them generally unsuitable. He con cluded that diagrams should be compared with artificially prepared ran dom diagrams and differences attributed to significant rock fabric.
Friedman (1964, p. 468-469) successfully utilized a Poisson exponential binomial limit to test for significance. The probability of obtaining at least a given number of points in any 1% counting circle of an equal- area projection is given by the equation:
X = «* -n/ k P = e (N/k)* X! 40 where P is the probability, N is the total number of points, and x equals the number of points per 1% counting circle. The letter k relates to the size of the counting circle and is 100 for a counter whose area is 1% of the net. Using the example of Friedman (p. 469), when 100 points are used, the probabilities of finding at least x points in any 1% counting area are as follows, if randomness is assumed: 0 points, 1.00; 1 point,
0.63; 2 points, 0.26; 4 points, 0.02; 6 points 0.0006; and 7 points,
0.0001. This is to say that for 100 points, the chances of obtaining a
6 point (6%) contour concentration from a random population are 6 in
10,000, and for a 7% contour, 1 in 10,000.
This test will be used throughout the ensuing statistical treat ment of fracturing, and the probabilities will be listed on most of the equal-area nets. The letter P will stand for probability and x will equal the number of points per 1% counting circle. It will be seen that even though the percentage contours are as low as 2% and 3%, as is the case
for large numbers of points, they nevertheless depart strongly from ran domness and are interpreted as having geologic significance.
Results. Figures 9 and 10 show the distribution of fracture
orientation for the andesite and aplite of the mine area. The outstanding
maximum for both rock types strikes northerly and dips <60° west. There
is also a weak correlatable maximum present for fractures striking NNW with near-vertical dip.
Differences between the fracturing in andesite and aplite are
as follows:
1. The near-vertical E to ENE-striking maximum for the aplite is
absent in the volcanic rocks. Instead, the andesite records a 41 high frequency of steep fractures with WNW strike. These were
traced to the area south of the aplite contact and are part of
the broadly fanning NE to roughly E set expressed in the major
fractures (faults) of Figure 5.
2. A NE-striking maximum of low-angle NW dip is found in the
aplite which is not represented in the andesite. This maximum
reflects the fact that structure with NE strike is more frequently
developed in the Mine aplite stock than it is in the volcanic cover rocks.
3. Finally, the andesite shows a small maximum for nearly NS-
trending, near-vertical fracturing which is not repeated in the fracturing of the stock.
The equal-area plot for "total" fractures in andesite and aplite of the deposit is shown in Figure 11. Fine fractures (see page 3 7) are excluded from this net, and this accounts for the differences between Figures 9 and 10 and Figure 11. The major low-angle maximum for fractures with north strike is present. It is exceedingly nonrandom as indicated by the probability
figures. A vast preponderance of "total" major fractures (see page 37) strike from about N. 35° W to N. 40° E and dip from 40° to 60° west. These fractures, especially those of northwest strike are roughly con
formable in attitude to the west-dipping aplite intrusive contact, and
they are believed to be related to it in origin. The weak maxima for E and ENE-striking fractures is a reflec tion of fine fracturing which is included in the plots of Figures 9 and 10.
These maxima together with one for NNW, near-vertical fractures 42
Its* for rondonrmess (See text, p 39 )
_X_ 30 6620 40 .0956 N .0019 .000006 3200 .000001 (number of measured fractures )
Figure 11. Eaunl-area Net for "Total" Fracture = in Andesite and Aplite of the Open-pit Area (1964)
Contours are in number of fractures per 1% counting circle. Fine fractures (joints) are not included in the net. 43 represent jointing and shattering in the rocks. This breakage is even more intensely developed than are the low-angle, north-striking major fra c tu re s.
Following the 1965 field season, an equal-area net analysis was performed for the pit and margins in order to compare the fracturing in mineralized rock with that in barren rock.
Extensive development drilling permitted a rough contouring of molybdenum values for the area of the deposit. Contours were selected so as to divide the area into irregularly concentric molybdenum grade areas of <0.03%, 0.03 to 0.06%, 0.06 to 0.09%, and >0.09% Mo. The molybdenum values are based on the average grade of approximately the first one hundred feet of drill hole data. The various grade areas were further subdivided into geographic zones centered on the >0.09% grade area. Figure 12 shows the distribution of grade areas and geographic zones with respect to the andesite-aplite contact.
Equal-area net plots were constructed for the fracture data of each grade area in each zone. This resulted in a complete breakdown in fracturing for different areas of the deposit and for rock of differing degrees of molybdenum mineralization. Figure 13 (in pocket) and Table
2 summarize the results of the survey and further record a basic distinc tion between fracturing in volcanic and plutonic rock.
The volcanic rock is dominated by high-angle, north to north west-striking fractures and faults. In addition to this pattern are the many fractures which strike northerly and have low-angle dips to the west. These structures are most commonly found along the western mar gin of the Mine aplite stock. 44
ZONE
NW ZONE NE ZONE
<0.03%
ZONE ZONE
0.03 -0.0 6 % SW ZONE < 0.03% Contours outlining molybdenum grade areas >0.09% SE ZONE
Approx. Scale ZONE I inch - 550 feet
Figure 12. Geographic Zones and Molybdenum Grade Areas Used in Fracture Analysis
Grade areas are controlled by development drill assay. Table 2 and Figure 13 present the results of the analysis. 45 TABLE 2 .—Fracture distribution for different grade areas (% Mo) and geographic zones outward from center of outcropping ore deposit
NE NW NS EW 150E-75°E 150W -75°W 15°W -150E 750E-75°W
HIGH-GRADE AREA > 0.09% Mo 51.6%* 18.6%* 15.5%* 14.3%* WEST ZONE 0.06-0.09% Area 38.0 16.0 24.4 21.6 0.03-0.06% " 36.2 26.7 31.4 5.7 < 0.03% " 35 .0 30.0 32.5 2.0 NORTHWEST ZONE 0.06-0.09% Area 38.0 16.0 24.4 21.6 0.03-0.06% " 36.2 26.7 31.4 5 .7 C 0.03% " 35.0 30.5 32.5 2.0 NORTH ZONE 0.06-0.09% Area 60.7 16.7 11.3 11.3 • 0.03-0.06% " 68.0 12.0 16.0 4.0 < 0 .0 3 % " 3 5 .4 24.5 33.7 6 .4 NORTHEAST ZONE 0.06-0.09% Area 56.5 28.2 6.5 8 .7 0.03-0.06% " 5 3 .4 25.0 13.3 8.3 < 0 .0 3 % " 39.2 3 1 .4 20.3 9 .1 EAST ZONE 0.06-0.09% Area 42.9 18.1 20.0 19.0 0.03-0.06% " 34.3 22.9 16.8 26.0 < 0 .0 3 % " 20.3 36.1 21.8 21.8 SOUTHEAST ZONE 0.06-0.09% Area 47.7 17.9 14.7 19.7 0.03-0.06% " 42.6 23.4 13.0 21.0 < 0 .0 3 % " 18.8 41.5 26.5 13.2 SOUTH ZONE 0.06-0.09% Area 34.6 20.7 32.6 12.1 0.03-0.06% " 24.7 33.2 21.7 20.4 < 0 .0 3 % " 18.8 41.5 32.2 13.2 SOUTHWEST ZONE 0.06-0.09% Area 37.4 3 5 .7 21.4 5.5 0.03-0.06% " 35.1 26.8 22.4 5.7 < 0 .0 3 % " 26.2 3 7 .2 29.4 7 .2 * Percentages refer to the proportion of fractures within the strike zone to the total fractures of all attitudes mapped in each area. 46 Conversely, fracturing in the stock, which is best expressed at a deeper level, most frequently strikes northeast to east and dips nearly vertical. These fractures are recognized in the volcanic rocks
and are much more subordinate there. High-angle fractures striking from
approximately N to NNW in the aplite are more typically represented by
fine fractures than by the major fracturing.
Figure 14 shows the equal-area plots for "total" major fractures,
excluding joints, which are broken down into the various size classes
which were described on page 36. There is a progressive strike change
in the fracture concentration from about N. 50° E. for Class 1 fractures
to northerly for Class 4 fractures. This evidence suggests that the large
faults and fault zones in the deposit strike northeast while smaller, more
numerous fractures strike northerly and dip rather shallowly to the west.
The weak nearly NS and EW maxima which appear on the Class 4 plot
are reflections of a nearly orthogonal, high-angle joint system which
pervades the rocks of the Mine aplite stock. The jointing and shattering
is intensely developed in the aplite within and outside of the ore body.
If jointing were included in the Class 4 plot, the magnitude of these ' maxima would become greatly accentuated.
Summary Statement
The following are the salient points concerning faulting and
fracturing in the Questa deposit.
1. The most continuous fractures (faults) form a converging pattern
centered at the site of the outcropping ore body. Although the
fracture pattern from map presentation appear to form a contin
uous fan of breakage, there is evidence that separable trends 47
TOTAL NUMBER OF FRACTURES
CLASS I (2-5 feet) (>5 feet )
1403
(l inch — 2 feet) (
Figure 14. Equal-area Net Plots for Different-sized Fractures
Contours are in number of fractures per 1% counting circle. Note the progressive swing to a NE strike for the larger structures. 48 of major fracturing exist which can be divided into the following
sets: NE, E, N, and NW. East and northeast faults are predom
inant in the aplite; north and northwest-striking structures dominate in the volcanic rocks.
2. Faults trending NE, and to a lesser extent E, constitute the
largest, most through-going structures. They have predominant
ly high-angle dips and are found most frequently in the Mine sto ck .
3. Considering "total" fracturing (excluding joints) for the deposit,
a major concentration of major fracturing strikes northerly
(+30°) and dips from 40° to 60° westerly.^ A more pervasive
and intense fracturing, however, consists of jointing and other
fine fracturing which form a nearly orthogonal system of two
near-vertical sets striking roughly NS and EW. These joints
and shattering are best expressed in the stock.
4. In volcanic rocks marginal to the ore body, north to northwest-
striking structure is most common. Low-angle breakage with
this strike appears to be related to the contact zone of the
sto c k .
5. Expressed at a deeper level, usually in aplite and closely asso
ciated with mineralized rock, NE to E high-angle fracturing is
1. Recent mining in the open pit has exposed an abundance of low-angle fractures which were not visible to the writer during his work in the deposit. These fractures, if included in the plot of Figure 11, would tend to expand the major northerly striking, west-dipping maximum toward a more northwesterly and westerly strike and south component of dip. (See page 60 for further explanation.) 49 most important. The N to NW fracture trends in aplite are
represented by fine fracturing consisting of pervasive small
joints and cracks.
Mineralization
General Character
Ore mineralization at Questa is restricted to molybdenum in the form of the sulfide, molybdenite. Some oxide molybdenum was present near the ground surface as the minerals ferrimolybdite and ilsemannite, but this was not milled.
The molybdenite occurs both in large, high-grade veins and as thin coatings on a multitude of small fractures. Actual disseminated molybdenite as fine flakes within the rock mass occurs sparingly and does not contribute much to the overall grade. The major veins initially mined were typically fissure and cavity fillings and ranged from an inch to over seven feet in width. Vanderwilt (1938) and Schilling (1956) des cribed these structures in detail.
Molybdenite occurs primarily in direct association with quartz, either as irregular aggregates in the vein or as coatings on lenticular quartz masses. Along thin, vertical veinlets (coating joint surfaces) the molybdenum is found as films of metallic, finely crystalline molybdenite
occurring with gray silica, pyrite, and flaky sericite.
Other gang tie minerals found in wins with the molybdenite are
fluorite and calcite (usually intergrown together), biotite, sericite, and kaolinite. Gypsum is often pervasively deposited in thin veinlets in
shattered andesites. The gypsum is a late hydrothermal product deposited 50 on molybdenite and may not be related to sulfide mineralization. Super gene gypsum is also found near the ground surface in oxidized sulfide v e in s.
The generalized sequence of mineralization, modified after
Schilling (1956, p. 58), is as follows:
1. Initial silica was introduced as barren quartz veins.
2. The first molybdenite appeared along with quartz, biotite, and
some potash feldspar.
3. Pyrite, together with trace amounts of chalcopyrite, sphalerite,
and galena joined the mineral sequence along with additional
molybdenite.
4. Fluorite and calcite followed, often filling fissures in earlier
quartz-molybdenite veins.
5. The final sulfide pulse appears independent of earlier minerali
zation as it has no accompanying gangue mineralization. Thin
films and coatings of molybdenite, often showing slickensides,
cut all previous veins. This type of mineralization has been
called molybdenite "paint" (Schilling, 1956, p. 55).
6. White hydrothermal gypsum, or anhydrite, was deposited in
thin veinlets through altered volcanic rocks. Some time may
have elapsed between the last sulfide deposition and this min
eralization.
Major Veins Vanderwilt (1938, p. 633) grouped the Questa veins into two categories: mineralized fractures approximately parallel to the stock
contact, and a series of thin, east-trending veinlets with steep dip. 51 Schilling (1956, p. 44) recognized intense, low-angle fracturing paral leling the southern contact of the intrusion as being the most important control of the productive structures. He refers to these together with other veins in the deposit as open-fissure fillings. In addition to these contact associated veins, Ishihara (1967, p. 44-47) described minerali zation controlled by joints and local faults.
Major veins mapped by the present writer are shown in red on the overlay sheet. Figure 5A. These structures constitute mineralization in Class 1, 2, and the larger Class 3 fractures. They are an expression of mineralization in the faults of Figure 5. The large, continuous veins showing greatest widths of mineral filling of gangue and sulfide strike
NE and ENE. These fissures, as they are called by Schilling (1956, p.
44), usually have high-angle dips. They are confined mostly to the intrusion, but a few are found well up into the andesite along the south west extension of the belt of NE fracturing defined earlier (page 24).
The northerly (NNE to NNW) striking major veins for the most part dip less than 60° to the west and show normal slip movements.
They are well developed in the andesite west of the stock but are not infrequent through the pluton. Veins with NNW to about NW strike and low-angle dip are especially common along the southwest side of High- line Ridge in andesite and aplite.
Generally the low-angle mineralized faults appear to be related to the presence of the Mine aplite stock and its western contact. The present work shows that the contact related fractures and veins are not just narrowly confined to the contact area but extend upward well into the volcanic rock and downward into the intrusive body. 52
"Total" Mineralized Structure
A certain amount of stockwork mineralization is recognized at
Questa, especially within shattered volcanic rocks adjacent to the pluton. A pervasive network of fractures of all orientations is mineralized and accounts for the existence of the ore body. However, in spite of the complexities inherent in this type of fracture mineralization, statistical analysis indicates that fractures in certain directions were preferentially mineralized. This relationship suggests some sort of subtle ore control
Figure 15 is a map of the deposit showing all fractures contain ing molybdenum mineralization which were mapped during the present study. Both the major veins (Class 1, 2, and 3) and tiny veinlets (Class
4) are included. Two distinct groupings of veins can be differentiated.
One group includes high-angle (<60° dip) veins which strike predominantly in the northeast and east directions. The trend of this group is even well expressed in the overlying andesite. This veining correlates with the major fault veins of similar attitude which were des cribed on page 51. The other group of mineralized fractures consists of a myriad of small veins which generally strike northerly and dip.less than 60° west and are present in the aplite as well as the andesite.
Equal-area net analysis of "total" mineralized fracturing in the deposit confirms the findings of the mapped data. In Figure 16 the equal-area plot for "total" mineralized fractures applies to 1964 data collected in the deposit in andesite and aplite. It represents Class 1 through Class 4 fractures but does not include fine fracturing (joints).
The net for "total" fractures, also excluding most joints and smaller 53
T o o d Molybdenum Corp of America to Coordinate system LU
- N 26,600 Q ^ _____/ 600 Map Numce's
EXPLANATION
—■ — ■ - M ne Aplite contact
H igh - angle veins ( > 60° dip )
Low - angle veins ( < 60°dip )
Questmnnb'e molybdenite mineralization
MOLYBDENITE MINERALIZATION Class 4 — . veins ( < I inch )
Cass 3 — veins (I in. — 2ft.)
-N 25,000 8 Class I and 2 veins (>2 ft)
tie- > T..V 500 0 500 1000 feet SCALE I INCH : 500 FEET
-N 23,400 Figure 15 ' TOTAL MINERALIZED STRU ' T'JRE 0 (excluding joints) 54
_x_ p_ 30 .6620 3200 40 .0956 (number of 50 .0019 measured fractures) 60 .000006 63 .000001
Test for randomness (See text, p 39 ) X _P_ 10 .1793 15 .0065 20 00005
"total" mineralized fractures Figure 16. Comparison of "Total" Fractures to "Total" Mineralized Fractures for the Open-pit Area (1964) Contours are in number of fractures per 1% of net. Fine fractures (joints) are not included in these plots. 55 fractures, is included in Figure 16 in order to compare mineralized frac tures with "total" fractures.
Two of the maxima for mineralized fractures correspond closely to the two groupings of veins distinguished from maps of the deposit; namely, NE to ENE-striking, near-vertical veins and northerly striking
(NNW to NNE) veins with dips of 40° to 60° west. In this case, where
fine fractures are excluded, the steep NE veins are subordinate in fre quency to the low-angle, northerly trending veins. The two groups of
orientations correlate with the maximum concentrations of "total" fault
ing and fracturing in the area. It is recalled that predominant fracture
sets, with the exception of N to NNW, high-angle fine fracturing, are
conformable to the two vein groups. The similarity between equal-area
net plots for "total" mineralized fractures and "total" fractures (Figure
16) well illustrates this correspondence. For the most part, the major
fracture sets contain the most mineralization.
There is, however, a statistical problem involved in evaluating
the predominant mineralized structure as an ore control. The maxima on
the equal-area net for "total" mineralized fractures may only represent
a statistical reproduction of the same concentration of "total" fractures
that was mapped.
A hypothetical case can be set up which will illustrate the
problem. It will be assumed first that a specific distribution of fractures
and veins are measured for a simplified deposit. In addition, no preferen
tial structural control exists for this hypothetical deposit. In this case,
all fractures, regardless of orientation, should have an equal probability
of becoming mineralized, and the proportion of mineralized to total 56 fractures for all attitudes should be approximately constant.
Under these conditions, if an unequal number of structures of different attitudes is mapped, for example, 500 fractures with north
strike, 300 striking NE, and only 50 fractures of NW strike, a vein dis tribution directly proportional to the unequal sample distribution for total structures measured will be produced. The percentage of mineral ized to total fractures for any orientation is assumed constant. If an arbitrary 10% is used as this percentage, 50 veins striking north, 30
striking NE, and 5 veins striking NW will have been recorded. There
fore two strong maxima will show up on the equal-area net for miner
alized fractures which will have little geologic meaning. The maxima will not indicate ore control because, as assumed, all fractures are
equally mineralized. A similar argument may apply to the data of the
Ouesta deposit (Figure 16). The diagram for mineralized fractures shows
how the major concentrations of veins and fractures are oriented, but
it may not imply preferential ore control.
To eliminate the effects of unequal fracture sampling for dif
ferent attitudes and in order to look for a more fundamental control of
mineralization, a percent plot was prepared (Figure 17). The theory
behind the use of this diagram is described as follows. As discussed
above, if no preferential control on mineral deposition in fractures were
present, then fractures of all orientations should have received equal
chances of becoming mineralized and an approximately equal percentage
of them would be expected to be veins. The ratio of mineralized to total
structure should be roughly constant. The percent plot compares this
ratio for every 1% counting circle of the equal-area net. Any marked 57
40%
3 0 %
Areo of unrelioble percents due to smoll number of total fractures measured
PIT AREA (1964): SULFIDE FILLED FRACTURES (excluding joints)
Poor reliability due to smaller numbers of total fractures
PIT AREA (1964): GANGUE > 0.09% MO AREA FILLED FRACTURES (center of surface (excluding joints) orebody ) Gongut and sulfide filled fractures
# mineralized fractures Figure 17. Percent Plots: x 100 # total fractures for each 1% Counting Circle i
58 variations from uniformity of the resulting ratios over the net implies preferential mineral filling in fractures for certain attitudes and may be regarded as a fundamental ore control.
Figure 17 is a percent plot for the fractures and sulfide veins used in preparing Figure 16. The contour configuration for the percent plot is quite different from that of the equal-area nets. An unusually high frequency of mineralization occurs for fractures striking north easterly. For steeply dipping fractures there are two especially favor able directions of sulfide veining, ENE (~N. 65° E.) and practically
EW (~-N. 80o-85° E.). For low-angle structure dipping less than 60°, a broad maximum is present for preferential veining in fractures striking from NNE to ENE.
Figure 17C is the percent plot for structure mapped in aplite which is mineralized with greater than 0.09% molybdenum. Although
differing in detail, the general NE to E directions of preferred minerali
zation are repeated. In this case, however, the addition of a probable maximum for north-striking, low-angle breakage should be noted.
The percent plot for fractures with gangue fillings of quartz,
fluorite, and calcite shows lower percent values because fewer of the
"total" fractures were solely gangue filled. Nevertheless, the plot veri
fies the preference of this type of mineralization for high-angle structure
striking NE to ENE.
Some caution is warranted in interpreting the percent plot dia
grams. The degree of confidence in the percent values increases in
proportion to the number of total structures which are mapped for any
particular orientation. If only a few fractures are seen for a specific 59 orientation, it is difficult to assign a statistically precise value to the percentage of those that are mineralized. In other words, the degree of possible error in the percent values greatly increases for small numbers of total fractures (barren and mineralized) which are recorded.
For example, suppose for one counting circle of the equal-area net only 10 total fractures were recorded and for another counting circle
100 were noted. If the mineralization on just two of the fractures in such a 1% counting circle were missed in the mapping, a 2% error would result for the percent value (mineralized to total fractures) for the 100 fracture counting circle whereas a 20% error would be introduced for the percent value of the 10 fracture counting circle.
All diagrams of Figure 17 are characterized by a relatively small number of total fractures striking NNE to NW and dipping shallowly to the east. Therefore, the possible error in percent values plotting in this area of the net is large. This should be kept in mind during interpre tation. Generally, the values in this area are exceedingly low with a few, well-scattered, exceptionally high percentages. Although large errors are possible for this area of the net, evidence from the outcrop tends to support the statistical suggestion that these fractures dipping east at low-angles are poorly mineralized.
The Effects of More Recent Data
During initial pit mapping, there were certain areas of poor exposure in the deposit which prohibited adequate structure compila tion. Recent work in new pit exposure created since the author’s mapping indicates that significant amounts of veining of certain attitudes were not mapped originally and were not included in the statistical analysis. 60
One such vein orientation strikes roughly easterly and dips less than 60° to the south. Veins in this group occur predominantly along the southern margin of the Mine aplite in the Sulfur Gulch area.
Vanderwilt (1938, p. 621) and Schilling (1956, p. 44) describe these structures as being nearly conformable to the aplite intrusive contact.
The veins were once of considerable importance to high-grade under ground mining.
Another vein attitude underestimated in the present work has become well exposed by subsequent mining on the southwest slopes of
Highline Ridge. The veins strike NNW to NW and have 40° to 60° dips to the southwest which nearly conforms to the attitude of the aplite contact in this area.
If these structures had been available for statistical analysis
(equal-area nets and percent plots), some modifications would have resulted in the diagrams. For the nets of "total" fractures and "total" mineralized fractures, maxima would probably have spread to include the attitudes of easterly strike, south-dip and northwest-strike, south west-dip. Percent plots might also have changed somewhat to emphasize this frequently mineralized structure. The conclusions resulting from the integration of old and this new data are summarized near the end of this section.
Structure Analysis in Areas of Different Molybdenum Grade
As an additional check on the validity of preferred mineraliza tion in NE to E fracture trends, fracturing in barren rock was compared with that in mineralized rock of varying molybdenum grade. Equal-area 61 plots were constructed for different grade areas in various geographic zones outward from the center of the deposit. Reference is directed to
Figure 12, the map of the zoning and grade areas which were used in the analysis, and Figure 13 (in pocket), the equal-area net map.
The data representing the relative proportion of fractures striking NE, NW, N, and E for grade areas and zones are summarized in Table 2. These data together with results from equal-area nets allow the following generalized conclusions.
1. The proportion of N. 45° E. (+ 30°) striking fractures to "total"
fractures appears to decrease away from the center of the out
cropping ore body. In other words, higher grade areas of molyb
denum are characterized by a higher frequency of NE and to a
lesser extent E trending fractures than breakage trending N and
NW.
2. The decrease in the proportion of NE fractures away from miner
alized rock corresponds to an increase in the abundance of N
and NW-striking structure. The increase is especially notice
able in the contrast between aplite and andesite. However, the
same increase away from mineralized rock within the aplite for
NE, E, and SE zones suggests an influence other than the al- •
ready described inherent distinctions between structure in the
stock and volcanic country rocks.
3. Equal-area nets indicate that there is a tendency for high-angle
fracturing to increase in barren or poorly mineralized rock. This
distinction is most apparent between the aplite stock and the andesite. Similarly, the intensity of low-angle fractures 62
increases noticeably in areas of better mineralization. This
correlation is also evidenced in pit exposures where the degree
of mineralization varies locally. The only exception to this
relationship appears in the west and southwest zones where
the predominant fracturing is low-angle in both barren and
mineralized rock.
Ore Controls
The preceding discussion has presented cumulative evidence that the NE to E direction of fracturing has been preferred for mineral filling.
This concept applies especially to high-angle structure, and two direc
tions, ENE and E, are distinguished. The attitude of low-angle fracturing which is most mineralized exhibits more scatter in attitude than does its
high-angle counterpart. The evaluation of all information (i.e ., statis
tical data for 1964, 1965, and recent data taken from inspection of new
pit exposure) indicates that much of the low-angle breakage is asso
ciated with, and roughly conformable in attitude to, the contact surface
of the Mine aplite stock. Low-angle NNE to ENE-striking, northwest
dipping fractures are possible exceptions to this association.
All of the northwest, west, southwest, and locally south-dipping
low-angle structure contributed significantly in preparing the rock for ore
mineralization. The combination of this type of breakage with the NE to
E-striking, high-angle fractures has controlled formation of the ore body.
Several features found in the area of the deposit appear to re
late to the high-angle vein and fracture system. One is the strike of
dikes which parallels the trend of the high-angle veins. A swarm of
aplite, granite porphyry and aplite-pebble dikes has been intruded into 63 the same ENE and roughly E-trending set of faults and fractures which are preferentially filled by mineralization.
The geochemical distribution of molybdenum in fractures also reflects the preferred NE to E direction for mineralization. Fracture gouge and filling were widely sampled over the deposit and its margins during pit mapping. A map of molybdenum values (Figure 18) delineates a linear N. 65° E. trending anomaly which cuts across the aplite and projects upward through the andesite toward the southwest. For some reason fractures along this zone were unusually susceptible to the flow (?) and deposition of mineral fluids. This concept may account for the elongate anomaly and the abundance of ENE veins along the same area. The favorability of the zone for mineralization may present evi dence for a deep-seated fault or fracture zone of ENE trend which aug mented the flow of mineral fluids from depth.
A third feature which may be related to the high-angle, ENE-E vein system occurs west and southwest of the pit. In that area, the contact surface of the aplite intrusion forms two elongate arches or ridges which trend roughly ENE and E and plunge gently westward
(Figure 19, in pocket). Molybdenum mineralization is spatially related to the ridges (Carpenter, 1968, p. 1337). This is evidenced by ore
trends on the 7700 ft level (Figure 19) and the geochemical distribution
of molybdenum in fractures (Figure 18).
The author feels that these ridges reflect fundamental struc
tural control either of original intrusion or subsequent metasomatic
flooding of the granitic pluton by quartz and feldspar (Carpenter, 1968).
Such a control might have developed by through-going, pre-intrusion 64
Seale I '= 500'
Figure 18 FRACTURE GOUGE GEOCHEMISTRY OF THE Lt E N - Pi r AREA Values in parts per million in molybdenum 65 faults or fissure zones which allowed intrusion or alteration to advance higher along these permeable zones to form the ridges. The same east and northeast-striking weaknesses may have become reactivated to act as primary channelways for mineral fluids which would account for the higher grade of mineralization in ore ridge areas.
Thus, the combination of an intense high-angle vein system, dikes, molybdenum.distribution in fractures, ore trends, and contact ridges all trending from ~N . 60° E. to EW establishes evidence for a basic structural control of the igneous and hydrothermal fluids. Regional information presented in this dissertation shows that these same direc tions are important throughout the Questa-Red River area.
Low-angle fracturing which contains much of the mineralization is often related to local areas of strong mineralization. As mineraliza tion increases along road and bench cuts, the intensity of low-angle breakage also increases which suggests a connection between this type of fracturing and the degree of mineral development.
The best example of low-angle fracturing acting as a local con trol of mineralization occurs along the northeast slope of Highline Ridge.
In this area, molybdenite mineralization diminishes abruptly. Detailed
structural work across the transition zone from ore to barren rock has revealed that well-developed ENE to E-striking high-angle joints cut indiscriminately through mineralized and barren rock alike. The joints are filled with varying amounts of quartz, pyrite, sericite, and molyb denite on Highline Ridge west of Blind Gulch, but they are either barren
or only lightly silicified east of Blind Gulch (Figure 5). 66 One difference in structure is, however, recognized between the two areas. To the west in mineralized rock, N to NE-striking low- angle fractures of west dip are strongly developed and mineralized .
This type of structure is practically absent in barren rock adjacent to
Blind Gulch on the east. A similar development of prominent low-angle structure is repeated in mineralized exposures higher on Highline Ridge, and it appears that this low-angle fracturing has been the cause for maintaining mineralization. With the absence of this fracture type, mineralization did not persist.
Much of the low-angle fracturing in the Questa deposit is be lieved to have been caused by the stresses related to emplacement of the Mine aplite stock (Schilling, 1956, p. 63). The NNE to NE-striking set, however, may be associated with the zone of NE faulting which traverses Highline Ridge rather than with the intrusive contact. Statis-. % tical data (percent plots) indicate that the NE low-angle set was pre ferred for mineralization over other low-angle fractures; however, this is still open to question. Subsequent to statistical analysis, mining has exposed an abundance of low-angle fracturing which is associated with the aplite contact and which may be as well mineralized as the NE set.
In summary, low-angle fracturing which envelops the west con tact area of the Mine stock is interpreted as the secondary localizing control of mineral deposition. The NE to E high-angle system of fractur ing represents the primary, deeply penetrating part of the plumbing sys tem. The low-angle fracturing acted to disperse mineral fluids laterally through the rock from the centralized NE to E channelways which may have originally tapped hydrothermal solutions at appreciable depths. z
67 Summary Statement
1. Northeast and east-striking veins are greatest in width, con
tinuity and, probably, depth penetration, but not in number.
The low-angle fractures, a major part of which strike northerly
(+30°), contain the.greatest quantity of veins.
2. Percent plots, used to emphasize preferentially mineralized
fractures, indicate that the highest density of breakage (the
low-angle system) is not necessarily the one most often min
eralized. For high-angle structure, both gangue and sulfide
mineralization conspicuously prefers to fill fractures of NE to E strike rather than those of other orientations.
Where mineralized, the low-angle fracturing is more vari
able in attitude than is the high-angle structure. Percent plots
indicate that northeasterly striking structure dipping NW is
overall the best mineralized of the low-angle fracturing; locally,
however, low-angle breakage related to the aplite contact con
tains much mineralization. This contact fracturing strikes NW
and W and dips SW and S respectively.
3 . The predominant direction of fracturing in the areas of higher
grade molybdenum is northeast. Away from the center of the
ore body, there is a progressive decrease in NE breakage and
an increase in N to NW fracturing with a corresponding increase
in the proportion of high-angle dips .
4. Consideration of all data defines two principal directions of
steeply dipping structure which are preferred for mineralization. 68
One is approximately E ( N. 85° E .) and the other ENE
( N. 60o-70° E.).
5. Geochemical analyses outline an approximately N. 65° E.-
trending zone of high molybdenum values which extends up
into andesites to the southwest of the pit. This zone corre
lates with one of the predominant directions of high-angle
veining in the subjacent rocks.
6. High-angle veining. geochemical anomaly, principal dike
directions, and underground ore trends are closely associated
with two ridges formed on the intrusive surface of the aplite.
These ENE and E-trending ridges probably reflect early fracture
control of intrusion. This fracture control later was important
in tapping and concentrating mineral fluids upward from depth
along "open" fracture channelways.
7. Ore emplacement was controlled initially by access along the
primary, steeply dipping and probably deep-penetrating ENE
to E fissures. A secondary control, largely responsible for dis
persing mineralizing fluids laterally through a large volume of
rock, was the pervasive, low-angle, westerly dipping structure
This structure also largely served as the localizing control for
mineral deposition.
8. Permeability resulting from the extensive vertical and lateral
fracturing permitted mineral solutions to pervasively penetrate
even minute fractures through the rock. This permeability was
especially important in extending ore mineralization upward
into shattered volcanic rocks . GENESIS OF FRACTURE FORMATION
In the preceding section, fractures and their relation to min eralization in the Questa deposit were described. A major part of this mineralized breakage constitutes an integral part of structure in the
Mine stock. Therefore, a key to the understanding of the origin of the
fracturing in the deposit is offered by the structure of the entire pluton.
Stock breakage may result in part from local stresses related to igneous
emplacement. In addition, regional tectonic movements probably accom
panied plutonic and hydrothermal activity and exerted their own effects on structure.
Fracturing in the Mine Aplite
Fracture Elements
In spite of the shattering and intense jointing in the Mine
aplite stock, three kinds of fractures can be distinguished because of
certain defferences in orientation and physical character. These three types will be referred to as fracture elements.
Contact-conformable fracturing. Figure 20 (in pocket) is a
composite map of fracture elements in the Mine aplite stock. Low-angle
faults and fractures adjacent to the east edge of the stock dip eastward.
Those along the western contact dip to the west. In both areas, the
fracturing conforms in dip direction to the contact surface of the pluton.
Across the southern half of the intrusive body, from west to east, this
low-angle fabric can be continuously traced from contact to contact.
69 70 The moderate west dip gradually lessens, becomes nearly horizontal, and then reverses itself to the east as the eastern contact is approached.
A crude arch of sheeted fracturing is thus defined which probably paral leled the now eroded roof of the pluton. Breakage of this type is here defined as contact-conformable fracturing. It has been frequently des cribed in the previous section in terms of low-angle structural ore con trol. The relative absence of this fracture element along the north and south contacts of the stock should be noted. In these areas, the frac tures should be roughly easterly striking. Although there is a distinct wrapping around of some low-angle, south-dipping fractures in the south where the stock necks down, little evidence is found for similar fractur ing of north dip at the north end of the exposed stock. In this north end of the stock, normal faulting (Figure 20) may have strongly elevated the area. Because of this elevation, rock containing contact-conformable fractures may have become eliminated by erosion of the upper portion of the pluton.
The area of the east contact of the Mine aplite north of the pri mary crusher (Figure 20) also shows little evidence of contact-conform able breakage. Along this part of the contact, the aplite appears to be sharply truncated by north-trending normal faults. It is felt, therefore, ' that the northeastern edge of the Mine aplite stock with its contact fractures has been downfaulted to the east and is not exposed.
Contact-conformable fracturing is locally found in abundance along the southern slopes of the middle part of Sulfur Gulch associated with the Mine aplite contact, which in that area has an easterly striking, south-dipping attitude. As a whole, however, the relative lack of 71 easterly striking contact breakage may ultimately be attributed to the fact that the pluton as it now crops out does not expose sufficient north or south-dipping contact surface. This may be a result of the north- south elongation of the pluton. This elongation together with the south drainage would expose more northerly striking contact and contact breakage than that striking easterly.
There are many other areas in the pluton where the contact
strikes northerly or northwesterly, and contact-conformable breakage is exceptionally well developed and exposed. In the open-pit on the south west slope of Highline Ridge, mining has uncovered an abundance of the element with NNW to NW strike and southerly dip (Figure 21). Figure 22
shows the contact fracturing near the stock contact.
Two areas of intense contact-conformable structure occur along the east contact of the Mine stock just west of the mill facilities and on the south side of Red River. Figures 23, 24, and 25 show the relation ships in this area.
The most impressive occurrence of contact-conformable break
age occurs in the Log Cabin stock next to the highway about two miles
east of Questa. Here, the granite-andesite contact has a gentle northerly dip. Parallel joints and faults of the contact breakage are so intensely developed that the outcrop has a thinly stratified appearance (Figures
26 and 27). An observation which bears on the origin of the contact frac
turing is its extension from the stock contact some distance into the
country rocks. More important is the fact that the fracturing persists
only about a thousand feet way from the stock. It must, therefore, be 72
Figure 21. Contact-conformable Fractures in Mine Aplite in Open- pit on Southwest Slopes of Highline Ridge
Fault surfaces dipping toward camera are NW-striking, SW-dipping contact-conformable fractures. 73
Figure 22. Contact-conformable Fractures in the Northwest Corner of the Open-pit
Pervasive low-angle fractures seen in cross section dipping toward vehicle are NW-striking contact-conformable breaks near the aplite- andesite contact. Figure 23. Mine Aplite; Contact-conformable Fracturing Looking NNW from Highway 38 just West of Mill
Contact fractures dip east near east contact of the stock. Note the irregular, layered character and slight variations in dip of the fra c tu re s. 75
Figure 24. Contact-conformable Fractures in Mine Aplite South of Red River
Note the two distinct sets.
Figure 25. Low-angle Contact-conformable Fractures Dipping East (right)
Mine aplite south of Red River. Figure 26. Contact-conformable Fracturing in Log Cabin Aplite about Two Miles East of Questa
Highly sheeted, close-spaced contact fractures strike ENE and dip north conformable to attitude of aplite-andesite contact which lies just off the photograph to the left. Figure 27. View Looking Northeast at Contact-conformable Frac tures in Log C abin Stock
The fractures strike ENE and dip about 40° N. Note the intensely layered nature of this low-angle fracture element. 78 a structural element directly related to the intrusion.
Contact-conformable fracturing usually occurs as an irregular type of low-angle sheeted fracturing. In the deposit, fractures of all sizes (Classes 1 to 4) are found which belong to this fracture element; however. Class 4 fractures are by far the most typical. Individual frac tures vary in strike and dip and often end against each other in small angles. The spacing between individual fractures is irregular. In many places it is evident that two or even three distinct low-angle fracture sets are superimposed, each varying slightly in strike (Figure 24).
Present for the most part on Highline Ridge is an abundance of
NNE-striking, west-dipping, low-angle breakage. Its influence is also noted on equal-area nets for the deposit (Figure 16). Although similar to contact breakage in appearance and importance to mineralization, its orientation departs widely from the NNW to NW strike of the stock con tact in this area. The NNE-striking fracture set may be controlled by the strong NE-trending zone of major fractures in which it occurs and which traverses Highline Ridge.
There are places, notably in the south-southwest part of the pluton, where the contact-conformable fractures become close spaced and regular in spacing, giving a blocky appearance to the rock. Indi vidual breaks are straight and planar.
Sheeting. A regularly spaced, near-vertical jointing persists
through the entire Mine aplite. Figures 28 and 29 are photographs show
ing the character of the jointing. In this type of fracturing, individual
joints are long, straight, planar features with smooth, even surfaces. 79
Figure 28. Sheeting in the Mine Aplite
Exposure on east side of Spring Gulch north of primary crusher. View looking east, parallel to the strike of vertical sheeting. 80
Figure 29. Mine Aplite; Sheeting in the North-central Part of the Stock above Highline Ridge
View looking west nearly parallel to the ENE strike of vertical sh e e tin g . 81 The closeness and uniformity in spacing of these joints together with their general parallelism is particularly impressive.
This jointing is similar in character to that which Hodgson
(1961) calls systematic cross joints and which he recognized in sedi mentary rocks of the Colorado Plateau. Regional jointing has been des cribed by Johnston and Cloos (1934) and Davis (1963) which closely resembles cross joints and the systematic high-angle jointing at Questa.
The regional jointing is often referred to as a near-vertical sheeted jointing (Davis, 1963). The term sheeting is adopted in this paper
for this kind of fracturing at Questa. It should not be confused with the
flat-lying sheeting which develops near and conformable to the topo
graphic surface as a result of erosion (Billings, 1954, p . 121-123).
The term should also be distinguished from Schilling's use of sheeting
(1956, p. 36) which was used to designate contact-conformable frac
turing.
The distribution of high-angle sheeting in the Mine aplite stock
is shown in Figure 20. The strike varies from east to about N. 60° E.
Generally, sheeting of different directions is rarely superimposed in the
same area. In certain areas, the persistence in strike of the sheeting is
suddenly interrupted. An outstanding example of this is described below.
Through the south-central part of the aplite, in the vicinity of
the primary crusher, an ill-defined but prominent major structure breaks
the continuity of the sheeting. This structure sharply separates NE-
trending sheeting from that striking W to WNW. The major structure also
corresponds to a WNW line which terminates the occurrence of 82 east-dipping contact-conformable fracturing found to the south along the east margin of the stock.
No major fault was mapped along this line; however, a zone of nearly west-trending minor faults with north dip is felt to indicate the presence of the major structure. The minor faults show indications of movement in a lateral direction.
On the south side of the zone of faults, sheeting gradually changes from about N. 60° E. to N. 45° E. next to the major structure.
This swing in the strike of the sheeting is suggestive of drag due to possible lateral movement along this major structure. Because the pos tulated major fault zone intersects the aplite-andesite contact where it is covered by talus and fill, any displacement observations of the con tact were unattainable.
The occurrence of the sheeting element is not restricted just to the intrusive body. It is found in the adjacent volcanic rocks as well as throughout the Red River-Cabresto area cutting all rock types. Details concerning its regional significance will follow in the next section.
Fracture cleavage. A third fracture element pervades the stock and is easily recognized because it consists of a particularly fine breakage which often resembles shingling or cleavage more than it does ordinary jointing (Figures 30 and 31). Aplite is often completely shat tered by countless parting surfaces spaced only fractions of an inch apart. When the rock is struck with a hammer, it shatters or cleaves in
shingle-like fashion along these surfaces. Such breaks are traceable down to microscopic proportions. 83
Figure 30. Mine Aplite; Northerly Striking Fracture Cleavage in Cross Section
Exposure in area of east contact of the stock next to the mine access road. Irregular, curviplanar nature of this element is well shown. Smooth faces normal to fracture cleavage are sheeting joints striking approximately east. 84
Figure 31. Mine Aplite; Fracture Cleavage Intensely Developed Next to Low-angle, West-dipping Fractures
Note that fracture cleavage shows no signs of hydrothermal altera tion. The low-angle fractures, however, are significantly argillized. Exposure located in north-central part of stock, above Highline Ridge. 85
Fine fractures of this kind are typically curviplanar, uneven, short, and discontinuous (Figure 30). Individual fractures vary somewhat in strike and dip, yet collectively the fracturing maintains an approxi mately constant direction. In places, two sets are observed with a 10° to 20° deviation in strike angle.
This kind of minute rock breakage has been recognized else where by several authors and called "shearing" (Graybeal, oral com munication) , "microjointing" (Wise, 1964), and "fracture cleavage"
Beal et a l., 1964). The fracture element is similar to that which
Hodgson (1961) termed non-systematic jointing although his study was restricted to sedimentary rocks. Fracture cleavage is used to designate the element in this paper.
The fracture cleavage grades into wider spaced jointing in many places. This jointing is similarly noted for its discontinuity and irregularly curviplanar surfaces.
For the most part, fracture cleavage and related jointing main tain a general NNW to N strike through the pluton with variations in a few localities (Figure 20). The dip is not consistent and although usually steep, will vary from 40° in either direction to 90°. Fracture cleavage and jointing with essentially the same orientation occur over • wide areas throughout the Red River-Cabresto region. It is definitely a regional element.
Finally, it is important to note that equal-area nets for the ore body and Mine aplite stock (see Figures 11 and 45, page 101) show no maxima for NNW to N-striking, near-vertical fracturing which is repre
sented by fracture cleavage. Because of its small size, the cleavage 86 was not compiled in these diagrams. If fracture cleavage were properly weighted and included as distinct fracture readings, a pronounced frac ture maximum would result on the statistical diagrams for structure of northerly strike and high-angle dip. This maximum together with that for
ENE-E sheeting would define the two high-angle joint sets which charac terize the Mine stock.
Interrelation of Fracture Elements
Fracture cleavage and sheeting usually form roughly orthogonal
fracture sets, and this relationship is maintained over large areas. It is
not uncommon, however, to find local deviations from this relationship
of up to about 30°. Evidence at several localities indicates that when
the strike of the sheeting changes the strike of fracture cleavage will
also change so as to maintain the orthogonal relationship. The best example of this relationship occurs in the area south
west of the primary crusher site where sheeting appears to be dragged
from N. 60° E. to N. 45° E. against a possible WNW major fault zone
(Figure 20). Fracture cleavage in this area has shifted 15°.to 20° in
strike along with the change in the strike of the sheeting. One possible
explanation for the consistent angular difference between fracture sets
involves post-fracture rotation of the rock mass. Another is simply a
change in direction of fracturing due to a changing stress field. There
are arguments for both ideas and a definitive explanation is not justified
at this time.
Details concerned with strike changes in the sheeting from NE
to about E are important in determining whether there is a gradational
fanning of the sheeting or whether two separate sets exist. Detailed 87 field observations indicate that the change in sheeting from one direc
tion of strike to the other takes place in the following manner.
A zone of transition appears to exist through which the change
occurs. In this zone, sheeting of one strike is found to gradually dimin
ish in frequency, while that of the new direction gradually becomes more
abundant. The new direction of sheeting first appears as occasional, scattered joints cutting obliquely across the well-developed sheeting of the original attitude. Subtly, the number of the joints of the new
strike increases until they predominate. At this point, sheeting of the original direction has degenerated to only rare sporadically spaced individual fractures which eventually disappear.
It is apparent from this manner of change that the ENE and the approximately E directions of sheeting are not found together in equal intensity. Also, there are distinct sets developed in different ^ areas rather than one set which gradually changes strike direction. Summarizing, the strike change in sheeting is represented by replacement of the existing set by the other set over a relatively short distance. The two sets trend N. 65° E. (+ 5°) and east (+ 10°). Areas where each set exists form domains which are sometimes remarkably linear in direction parallel to the strike of the sheeting.
The high-angle sheeting and the orthogonal jointing of the fracture cleavage type differ markedly in character as the photographs of Figures 32 and 33 illustrate. Yet there are local areas where northerly fracture cleavage jointing will look like sheeting (Figures 34 and 35), or rarely easterly striking sheeting will become very close spaced and irregular and will resemble fracture cleavage. Occurrences of this Figure 32. Precambrian Granite; High-angle Sheeting in Cross Section near Columbine Creek-Red River Junction
Note the straight, parallel, regular-spaced nature of the element. The sheeting strikes NE. Figure 33. Fracture Cleavage Joints in Precambrian Granite; Same Locality as Figure 32
This jointing strikes NNW and is of the fracture cleavage type. Note the different characteristics of this element from the sheeting. 90
Figure 34. Fracture Sets in the Mine Stock South of Highway 38
Low-angle fractures dipping east (right) are contact-conformable fractures. High-angle joint faces in sunlight are NNW-striking joints which here resemble sheeting rather than fracture cleavage. 91
Figure 35. Close-up Photograph of Figure 34 Showing NNW, Near- vertical Jointing (in sunlight) with Sheeting Characteristics
't 92
are relatively rare and while not detracting from the regionally applic
able distinction of easterly trending sheeting and northerly trending
fracture cleavage do suggest that overall mechanism of formation for
both sets may be similar.
A relatively large proportion of the sheeting shows weak
slickensides, grooves, or striations indicating small amounts of lateral movement (Figures 36 through 39). Evidence for movement is most often concentrated locally in isolated areas or along zones. In these areas,
N to NW-trending fracture cleavage becomes very intensely developed and close spaced (Figures 31 and 40). Such a relationship is found in cuts along the primary mine access road near the east contact of the
stock and on the lower end of Highline Ridge. Outside the Mine intru
sion, the relationship is well illustrated in the Columbine aplite just west of Sulfur Gulch (Figure 38) and along the north contact of the Log
Cabin stock (Figure 40). In the disturbed zones, fracture cleavage
sometimes shows slight amounts of drag against sheeted joints along
which lateral movements have occurred (Figure 40). There is thus good
evidence for a genetic relationship between roughly east-west move
ments on joints of the sheeting and the development of intense fracture
c le a v a g e .
A further correspondence may be drawn between fracture cleav
age and low-angle faults. Cleavage is often intensified immediately
adjacent to the low-angle faults usually in the footwalls (Figures 31,
41, and 42). This relationship holds for easterly and westerly dipping
low-angle structures of variable north strike. The intensification of
cleavage is also found adjacent to low-angle faults which strike 93
Figure 36. Sheeting (ENE) Showing Lateral Grooves and Striations
Mine aplite south of Highway 38. Curved breaks in cross section are NNW fracture cleavage joints.
Figure 37. Lateral Movement Striations on ENE Sheeting
Mine aplite south of Highway 38. View looking north. 94
Figure 38. Interrelations of Fracture Elements in the Columbine Aplite
Exposure north of Columbine Campgrounds in Red River Canyon. Near-horizontal slickensides on east-striking sheeting. Note the jagged, open, tensional nature of the north-trending fracture cleavage normal to the striated face. 95
Figure 39. Near-horizontal Slickensides on East-striking Sheeting in Mine Aplite, North-central Part of Stock
Figure 40. Log Cabin Stock; Plan View of Fracture Cleavage Developed Next to ENE Sheeting
Outcrops north of Highway 38, two miles east of Questa. Sheeting shows horizontal slickensides. Note slight drag effects on fracture c le a v a g e . 96
Figure 41. Local Concentration of Fracture Cleavage Adjacent to Low-angle, Contact-conformable Fracture which Shows Normal Fault Movement
Outcrops in Mine aplite south of Highway 38 in Red River Canyon. Contact breaks strike NNE and dip east. Fracture cleavage strikes n ortherly. i Figure 42. Fracture Relationships in Mine Aplite; Main Haulage Road at Highline Ridge
Long, low-angle, west-dipping breaks are argillized and often gouge filled. They show strong molybdenite mineralization. Fracture faces in sunlight facing camera showing gray-blue color (molybdenite) represent E-striking sheeting. Joints of the sheeting are cut and dis placed by the low-angle breaks. Intense N-NNW-striking fracture cleavage pervades the entire exposure, but is often concentrated adja cent to the low-angle faults in the footwalls. Most of the cleavage is barren of alteration and mineralization. 98
east-westerly (Figure 43). The most significant feature, however, is
that regardless of the attitude of the low-angle fault, the strike of the
fracture cleavage remains about the same (N to NNW).
Fracture cleavage is not as intensely developed in areas where
movement on either low-angle fractures or on sheeting is absent. In these areas, fracture cleavage is represented by wider spaced jointing which, however, still preserves the non-systematic, curviplanar charac teristics of the finer cleavage.
Reference is directed to Figure 44. This block diagram illus
trates the physical character of the three fracture elements, contact-
conformable fracturing, sheeting, and fracture cleavage.
The equal-area net for "total" fractures in the pluton (Figure 45)
shows maxima corresponding to low-angle, contact-conformable fractur
ing. Breakage associated with both east and west contacts is represented
Some of the NE-striking, west-dipping structures may be related to the
NE fracture zone crossing Highline Ridge. The double maximum^ for
near-vertical structure corresponds to sheeting and reinforces the con
cept of two distinct sheeting sets. Fracture cleavage is not adequately
shown because for much of the stock, especially in the area of the
deposit, cleavage was not compiled as a normal fracture element due to
its small scale.
1. The term double maximum is used in this report to identify two concentrations of fracture poles which are separated by less than about 30° strike angle. These pole maxima on the equal-area net depict two closely grouped fracture sets which may be uniquely related as discussed on page 157. Figure 43. Relationship of Fracture Elements in Mine Aplite
Open-pit exposure on the southwest slope of Highline Ridge. Barren fracture cleavage of northerly trend developed next to east trending mineralized fault of north dip. This relationship suggests lateral, approximately east-west movement of rock mass after min eralization. 100 101
3790 Number of measured fractures
TOTAL FRACTURES (excluding fracture cleavage) Poor reliability due to smaller
VEINS AND FILLED PERCENT PLOT fractures
Figure 45. Equal-area Nets and Percent Plot for Structure of the Mine Pluton
The percent plot represents the proportion of mineralized (gangue and sulfide) to total fractures for each 1% counting circle of the net. 102
Relation of Fracture Elements to Mineralization and Alteration
Statistical diagrams for total structure in the Mine stock
(Figure 45) provides ample evidence for preferred mineral filling in cer tain directions. A prominent maximum on the percent plot exists for high-angle structure striking ENE. This maximum is represented by sheeting in the pluton and includes mineral-coated joints together with larger veins and dikes which parallel the sheeting.
Sheeting typically carries hydrothermal mineral coatings con sisting of various combinations of quartz, pyrite, sericite, and molyb denite (Figure 44). In molybdenum-bearing rock, each of hundreds of these closely spaced breaks may be coated or filled, thus adding appre ciably to the overall mineralization. Coatings are thin, on the average no greater than 0.1 inch. Larger fault or fissure veins of metallic and gangue mineralization commonly parallel the mineralized sheeting.
Sheeting is characteristically free of gouge and argillic alteration prod ucts. Molybdenum on the joints occurs as shiny films or flakes of molybdenite.
The result of hydrothermal activity in low-angle (largely con tact-conformable) fracturing is quite different from that in the sheeting.
Low-angle veins are most often gouge filled. Gouge consists of sericite, kaolin, and chloritic clay products (Figure 31) . Near favorably miner alized rock, the gouge often is medium to dark gray and apparently is a mixture of clay products and small amounts of pulverized molybdenite.
With increasing amounts of molybdenite, the gouge darkens in color and
is called molybdenite "mud" (Schilling, 1956, p. 55). Molybdenite
"paint" (page 50) also occurs abundantly in low-angle structures 103 strongly painting the surface of the main fracture and minute fractures in the gouge, hanging wall, and footwall next to the main structure.
Partially crushed fragments of pyrite and molybdenite are commonly found mixed with the gouge.
Relatively few high-angle, northerly trending mineralized frac tures were mapped in the aplite. Where present, these veins usually occur as thin quartz veinlets (some with molybdenite) or as sparse, tiny fractures coated with molybdenum paint adjacent to low-angle faults. The quartz veins are usually frozen into the rock, and in the strict sense are not fracture coatings as in the case of the sheeting.
Throughout the stock, however, and developed in varying degree, is widespread fracture cleavage and irregular jointing of N to NNW strike which is entirely devoid of mineral filling or alteration (Figures 43 and
46). This set of fractures constitutes the barren element in the deposit.
In summary, attention is directed to Figure 46, which ideally
shows the typical relationships between the three fracture elements.
Molybdenum mineralization is seen coating contact-conformable frac turing and sheeting but not fracture cleavage. The photograph also
clearly demonstrates the relatively simple and systematic fracture pat
tern which is exposed in aplite of the ore deposit.
Sequence of Development of the "Fracture Elements
The "total" mineralized structure in the Mine aplite can be
broken down into categories which may roughly correspond to the para-
genetic sequence of mineralization from early to late. The categories
are: (1) barren quartz veins, (2) quartz-molybdenite-pyrite-fluorite
veins in various combinations, and (3) molybdenite veins as paint. 104
Figure 46. Relationships of Fracture Elements to Mineralization in Mine Aplite of the Open-pit Area
Southwest benches of Highline Ridge. Northerly view of west dipping, contact-conformable fractures and ENE sheeting (high-angle faces toward camera) showing bluish-gray molybdenite mineralization. Fracture cleavage (high-angle jointing in shade) is conspicuously b arren . 105 These three vein types are plotted on equal-area nets (Figure
47). A distinct pattern develops from the plots which shows the follow ing features: (1) barren quartz (whether early or not) is found primarily in vertical ENE to E-striking fractures; (2) molybdenum "paint," the latest sulfide mineralization, is most often found in low-angle, north-striking structure; and (3) quartz-molybdenite-pyrite-fluorite mineralization, representing the intermediate stages of mineralization, occurs primarily in the high-angle breakage but also appears to a lesser degree in low- angle fracturing.
One possible conclusion to be drawn from these findings, al though not the only one, could be that easterly sheeting and fracturing were the initial elements of failure in the pluton and that as time passed, low-angle breakage evolved, becoming progressively more permeable to mineral solutions.
There are criticisms of this idea, the chief being that later fault movement along earlier low-angle veins subsequently destroyed evidence of first mineralization. Evidence supporting this criticism includes: (1) crushed pieces of pyrite-molybdenite in fault gouge; (2) recognition of at least some low-angle quartz and sulfide veins, and
(3) quartz filling some of the contact breakage in the Log Cabin pluton.
Apparently a certain degree of the low-angle contact fracturing was early
Nevertheless, if the high amount of low-angle contact breakage which is now seen in the deposit existed throughout the period of mineralization,
it is difficult to understand why it did not become pervasively mineral ized until very late in the sequence. 106
10
Number of measured fractures
BARREN QUARTZ VEINS
( early ?)
QUARTZ- MOLY- PYRITE MOLYBDENUM VEINS FLUORITE VEINS m o lymmuo and paint ( intermediate ) ( lo ti )
Figure 47. Equal-area Nets for Different Phases of Mineralization
Contours are in percent. 107 It is even more difficult to imagine that the great "quantity of fine fracturing of the fracture cleavage element existed before hydro- thermal activity yet remained impermeable to mineralization, especially when most of the fracture cleavage lies in contact with low and high- angle mineralized channelways. Furthermore, barren fracture cleavage intricately cuts vein filling and slightly displaces low-angle fractures as illustrated in Figure 48. It is concluded, therefore, that although north-trending fractures did exist before mineralization, the vast major ity of north fractures developed after the close of hydrothermal activity.
A possible sequence of fracture formation in the Mine aplite stock is diagrammed below: Sheeting ------—-~ Contact-conformable fracturing ^ - ■...
Mineralization
Fracture cleavage ------— early late
In conclusion; a feature of considerable importance has been described in this section which deserves further emphasis. Fracture cleavage, which essentially post-dates both contact-conformable break age and sheeting, has been shown to have a genetic relationship to movements along these two structure types (page 92). This implies that the movements occurred after formation of the original structures and as such need not be related to the initial genesis of the sheeting or contact fracturing. The importance Of this concept will be brought out in a sub sequent section on origin and mechanics of fracturing. 108
Figure 48. Field Sketches Showing Relations of Fracture Cleavage
(A) Plan view of fracture cleavage cutting an ENE-striking quartz vein in the Mine aplite. (B) Cross section looking north which shows late fracture cleavage cutting and displacing a low-angle, argillized fau lt. Summary Statement
The following are the significant features of fracturing in the
Mine aplite.
1. A roughly conformable sheath of low-angle, irregular breakage
envelops the stock along the east and west-dipping contact
surfaces. High-angle normal faults have dropped much of this
element from view along the northeast part of the pluton. The
element is called contact-conformable fracturing, and its
occurrence in the margins of and locally adjacent to the intru
sion indicates a possible genetic relationship of this element
to the emplacement of the stock.
2. Highly regular and persistent near-vertical sheeting cuts dis
cordantly across the entire pluton striking from about N. 60° E.
to due east. The sheeting is not restricted to the aplite stock
but cuts rock types throughout the Red River region.
3. Changes in strike of the sheeting from approximately N. 60° E.
to east occur so subtly as to appear completely gradational;
however, two distinct sets, separated by about 15° to 30° of
strike angle are present. One set takes over in frequency at
the expense of the other through a relatively narrow zone of
transition.
4. A particularly fine, pervasive shattering and close-spaced,
irregular jointing intensely breaks the stock in a N to NNW
direction. This type of fine fracturing, called fracture cleavage.
shows notable small-scale variations in orientation due to its .110 irregular and curviplanar nature. It is, along with the sheet
ing, regional in occurrence.
5. Systematic sheeting and fracture cleavage are nearly perpen
dicular in strike and constitute a simple, orthogonal fracture
system .
6. Sheeting commonly shows evidence indicating small lateral or
strike-slip movements. Where this occurs, fracture cleavage
becomes especially intense and finely developed. The same
intensity of fracture cleavage is noted adjacent to low-angle
faults. There appears, therefore, to be a genetic relationship
between movements on these high and low-angle fractures and
the formation of intense fracture cleavage. It has been shown
that the fracture cleavage and, presumably, the movements
formed late and after the original formation of the sheeting and
low-angle breakage (at least the contact-conformable fractures).
Therefore, these movements are'not necessarily related in
origin to the sheeting or low-angle contact breakage.
7. Contact-conformable fracturing and sheeting are mineralized;
the vast majority of fracture cleavage is not. Low-angle breaks
have suffered fault movements and are typically filled with
clay gouge or show evidence of having served as channelways
for argillic alteration. Sheeting most often shows mineral coat
ings or fillings without argillic alteration. The reason for the
distinction in alteration is still not clear.
8. The probable order of development of fracture elements from
early to late is: sheeting, contact-conformable fracturing. I l l and fracture cleavage. There is, however, some overlap in
this sequence.
Regional Factors
The Red River Structural Zone
Regional dike trends were compiled over four quadrangles sur rounding the Questa deposit (Figure 49, in pocket). The most detailed presentation of dikes is Clark's (1966a) work in the Eagle Nest Quad rangle southeast of the ore body. Dikes in the other three quadrangles
(McKinley, 1956, 1957) are less common. It is not known whether this is a real geologic difference or a result of less detailed mapping.
Several dike rocks are recognized throughout the region. They are andesite, latite, rhyolite, and monzonite porphyry, from oldest to youngest. Some of these rocks are considered correlative with the mid-
Tertiary volcanic series. In the Red River area, where supplementary data have been added to the map (Rippere and Lisenbee, 1965), granite porphyry, quartz porphyry, and late andesite porphyry elongate intru sions are also included. Dikes and veins of Tertiary age have been grouped together on the map regardless of composition to facilitate vis ualization. Dikes of the various compositions do not appear to form
separable orientations which would distinguish them.
The general trend of the mid-Tertiary (?) dike emplacement over most of the map is northerly with appreciable variations toward a north west strike south of Lobo Peak. A noteworthy exception to this northly
trend occurs, however, in the Red-River-Cabresto area from Questa to
about six miles east of Red River village. In a restricted zone, varying from one to two and one-half miles in width, mid-Tertiary (?) dikes and elongate intrusive bodies suddenly turn from their regional north trend to a ENE direction. North of Cabresto Creek the dikes and intrusions appear to again resume their regional northerly trend.
Figure 49, therefore, presents evidence for a ENE to E-trend ing crustal disturbance of major proportions which disrupts and influences the regional continuity of Tertiary dike formation in this part of the
Sangre de Cristo Mountains. In addition to the dike data, the following geologic features, some of which have already been described, are listed. These features are felt to be manifestations of the same struc tural weakness that has influenced dike emplacement.
1. The three mid-Tertiary granite stocks occur along the EW zone
of disturbance. A host of other intrusive rocks crop out along
the zone and are not found outside its lim its.
2. Many of the mineralized veins emplaced along the Red River
area also trend approximately parallel to the axis of the zone.
3. A high degree of ENE to E-striking fracturing is concentrated
along the zone (refer to Figure 51, page 116).
4. In the Mine pluton, the direction of preferred igneous and
hydrothermal access and fracture filling, represented by min
eralized jointing, veins, and dikes, also generally strikes
parallel to the direction of this major zone.
Schilling (1956, p. 33) was one of the first workers in the
area to define a major zone of structural weakness through the Questa-
Red River area. He emphasizes its importance by saying: 113
The Quests molybdenum mine is located in an east-west down-faulted zone several miles wide, extending across the range, in which a number of structural features combine to form a structural pattern quite different from that found in the rest of the range. This zone apparently has served to localize the soda granite intrusives and the hydrothermal pipes (altered areas); both of which are found almost entirely within the zone. This zone also controls the drainage pattern.
Schilling (oral communication) has also pointed out numerous volcanic centers which extend from the mountain front out into the Rio
Grande valley in a westerly direction. Their presence may reflect the continuation of the major feature away from the Sangre de Cristo range.
The major easterly trending zone constitutes, in the opinion of
the author, a major zone of crustal weakness. It has controlled parallel
alignments of jointing and faulting and has acted to tap and localize
igneous and hydrothermal activity. In this sense it could be likened to
the lineament zones of the Southwest as defined and described by
Schmitt (1966) and Mayo (1958). It will be referred to hereafter as the
Red River Structural Zone.
The Northeast Structural Zone
Inspection of the dike map (Figure 49) reveals in the Red River-
Cabresto area a secondary alignment ( N. 60° E.) of dikes slightly
skewed in direction from the N. 75o-80° E. trend of the Red River
Structural Zone. This alignment is readily traced from the Log Cabin
granite to the Mine aplite and with reservation might be projected farther,
as northeast of the mine several NE-trending dikes lie along the line
(Figure 50).
The evidence for this NE zone is strengthened by a consistent
NE parallelism in the direction of regional sheeting which occurs along AQlji e' ‘o uiietoa fatrn (otd hr inferred where (Dotted) fracturing ‘ror unidirectional q f Be!' rdmnn nrhat hg-n e breakage high-ang le northeast, Predominant rdmnn nrhsuh hg-nl breakage high-angle north-south, Predominant breakage high east-west, -angle Predominant rdmnn nrhet high northwest, - a breakage nPredominant gle ieto n qc* ly qjcr*c n#Direction e n ad en rns (sulfide) trends vein and Veins ra o otrpig molybdenite dikeTertiary outcropping of Areas EXPLANATION iue 0 MAP OFTHE NORTHEAST STRUCTURAL 50 ZONE BETWEENFigure MINE THE P IE ANDAPLITE LOG CABIN GRANITE / / ooc SCALE • I INCH=2000FT OOO 1500 2000 feet 2000 1500 OOO 114 115 it (Figure 51). This sheeting changes from the easterly Red River trend to a more northeasterly trend as it crosses the NE zone. The change in the direction of sheeting was first noted on a continuous exposure of the
Log Cabin pluton along the south side of Red River canyon. The change to a NE-striking sheeting is also seen through the volcanic rocks be tween the Mine aplite and the Log Cabin granite.
Mineralized fissures and quartz veins found along the zone are generally of NE trend. Practically all the outcropping occurrences of molybdenite in the region are found along it (Figure 50). Examples are the Log Cabin prospect, veins along the west wall of Bear Canyon, the
"cirque" south of Goat Hill, BJB prospects in Goat Hill Gulch, veins in upper Goat Hill Gulch, and the Ouesta ore body. This NE trend of struc ture projects through the pit in the vicinity of Highline Ridge and may explain the broad band of NE faults, fractures, veins, and dikes which was described in the pit geology.
Other zones and areas of northeasterly sheeting are found cut ting across the Red River Structural Zone (Figure 50). The zone to the east of the Northeast Structural Zone is marked by a strong influx of NE- striking quartz veinlets in the Log Cabin stock and includes the numerous
NE-trending aplite dikes which appear to connect the Columbine aplite with the Mine pluton. These areas of northeasterly structure lack the linear continuity of the Northeast Structural Zone. Their genesis is dis cussed along with the mechanics of fracturing in the next section.
Evidence thus indicates that at least one and possibly two lines of crustal weakness underlie surface rocks of the Red River-Questa region. Structural and hydrothermal processes contributing to the 116
Figure 51. REGIONAL FRACTURE ANALYSIS OF AREA BETWEEN MINE APLITE AND LOG CABIN GRANITE 117 formation of the Questa ore body are believed to be largely influenced by the presence of these structural zones.
Precambrian Basement
Precambrian rocks are poorly exposed in proximity to the
Questa mine. Exposures of Precambrian rock are more frequent along the lower slopes of Red River Canyon, and it was in these areas that
structure data for basement rocks were collected.
The main objective in studying these rocks was to see if the
Red River Structural Zone is reflected in the basement. If the zone
existed prior to Tertiary volcanism and intrusion, a knowledge of the
structural pattern of basement rock along the zone might help bracket
its age. Another purpose of the study was to determine if preexisting
structural features in the basement rocks had significantly influenced
structural development in Tertiary rocks.
To help satisfy the objective of the Precambrian study, struc ture in basement rocks of the Red River Structural Zone was compared
with Precambrian structure outside the zone. Figure 52G and H (in
pocket) illustrate the results .
Figure 52G, a plot for structure in Precambrian gneissic granite
of the Red River zone shows a relatively simple pattern of two high-
angle fracture sets. The north-striking set is well defined. The contour
configuration for the easterly striking set is more diffused with a sug
gestion of a double maximum for ENE (almost east) and WNW-striking
fractures. The low-angle maximum for structure of NNE to NE strike and
NW dip corresponds to extensive contact-conformable fracturing which 118 was measured in a Precambrian sample site adjacent to the south-
southwest portion of the Mine aplite (Figure 51).
Figure 52H summarizes structure in gneissic granite southwest
of Flag Mountain in Lama Canyon (see Figure 49 for location). The struc
ture in this area may possibly be typical of basement rock outside the
Red River Structural Zone. The maxima on the equal-area net are similar
in several respects to Figure 52G. A correlative maximum for fractures
of north strike is seen along with a double maximum for approximately
east and west-northwest-striking fractures. The only differences be
tween the maxima in the two diagrams are: (1) the north-striking maxi
mum for Figure 52H may be a double maximum taking in the concentra
tion of NNE-striking fractures and (2) the W and WNW-striking fracture
sets (double maximum) are steeply north-dipping in Figure 52H rather
than nearly vertical. Structure in the Lama Canyon area includes a
northwest-striking fracture set which does not appear in the Red River Canyon area.
From the equal-area net data (Figures 51 and 52), it is seen
that high-angle fracturing in Precambrian rock of Red River Canyon cor responds closely to high-angle fracturing in Tertiary rocks of the same
area. At many structural sample sites along Red River Canyon, Precam
brian granite is found with two near-vertical, orthogonal fracture sets
which are similar in character to the two high-angle sets in mid-Tertiary
intrusive rocks. The approximately east-west set resembles sheeting
(Figure 32) and has common indications of lateral movement as does the
sheeting in the Tertiary rocks. The northerly striking fracture set is of
the fracture cleavage type (Figure 33), but joints are generally larger or 119
"coarser" than typical cleavage. Sheeting in the basement rocks approx imately parallels the direction of steeply dipping to vertical foliation; however, in detail the fracture set often cuts obliquely across the plane of foliation.
Fracturing in Precambrian granite was inspected in the Little
Latir Creek area about 15 miles north of the Red River zone. Fractures there are present as sets striking roughly east and northerly. There are complications to the north set with a probable double maximum present; however, generally the same two directions of orthogonal breakage exist
in the Latir Creek area that are present in the Red River-Cabresto Creek area. Furthermore, the distinction between the sheeting and fracture cleavage type of jointing is preserved in the Latir Creek area. One im portant distinction is apparent between the two locations, however. The intensity of the easterly striking fracture set (sheeting) is much less at
Latir Creek than in the Red River Structural Zone. Fractures of this set
tend to be widely spaced and subordinate in intensity to the pervasive,
northerly striking fracture set.
In all areas which were studied, fractures in Precambrian
granite are substantially more consistent and systematic than they are
in the Precambrian metasedimentary rocks of the Sangre de Cristo range;
The complex fracturing found in schists and gneisses is believed due to
stresses involved in folding together with the complications of anisot-
ropism in these layered rocks.
It is concluded from the preceding data that, although other
fracture sets may exist regionally, an orthogonal, roughly easterly and
and northerly trending fracture system is common to basement granite 120 both within and outside the Red River Structural Zone. The easterly trending set, however, is much more intense within the structural zone than it is regionally. Outside the zone, the northerly fracture set usually predominates in intensity. A high-angle WNW direction of fracturing appears strongly in Precambrian granite within and outside the structural zo n e.
Foliation trends in the Precambrian of the Red River-Cabresto
Creek area vary from NE to E (see Figure 4A). Northeast strikes appear to predominate and the foliation often trends across the easterly striking sheeting at slight angles. There is no obvious variation in the regional foliation in passing through the Red River Structural Zone. In the Latir
Lakh area north of Cabresto Creek, McKinley (1956, p. 21) described mylonite zones trending N. 60° E. which roughly parallel and cross cut the strike of the regional foliation. McKinley postulates that the mylo nite s probably formed along shear faults in Precambrian time.
Few Precambrian dikes have been mapped within the Red River
Structural Zone (Figure 49). A long pegmatite of ENE trend was noted by
Clark (1966a), and the author has recorded a few roughly east and north erly striking amphibolite dikes which are probably Precambrian in age.
Elsewhere in the range, Precambrian pegmatites and quartz veins trend mainly northeasterly (Figure 49). In summary, the Red River Structural Zone is recorded in base ment granite of the zone by intense easterly striking fracturing which persists regionally but is not as intense elsewhere as it is within the structural zone. The age of this fracturing is uncertain as it could have developed at any time since the Precambrian. The small amount of 121 information for Precambrian dikes suggests than an easterly zone of weakness did exist in Precambrian time, but quite a bit of additional work is needed in order to confirm this.
The approximately north-south fracture set of Figure 50G is regional and cuts across the structural zone without disturbance. Much of this fracturing appears to correlate with the fracture cleavage in atti tude and appearance so that a relatively recent origin is indicated. A
WNW joint set is also regional in basement granite and probably is of
Precambrian age as younger rocks do not share it.
Unfortunately, the age of the Red River Structural Zone is still in question. With certainty, however, it can be said that it did exist as a crustal dislocation prior to mid-Tertiary igneous activity and pos
sibly dates back to the Precambrian originally.
Structure in Other Plutons
Fracture studies were carried out in several plutons surrounding
the Mine aplite (Figures 49 and 52). Three of these, the Log Cabin gran
ite, the Red River aplite, and the Columbine aplite, are mid-Tertiary
plutons occurring along the Red River Structural Zone. The fourth, the
Rio Hondo granite, is a Laramide stock (Table 1) found outside the zone
(Figure 49).
Log Cabin stock. This', the westernmost of the Questa-Red
River mid-Tertiary stocks, is a pluton of crescent shape with long, tap
ering southeast extension. The intrusion consists of two phases. The
predominant type (approximately 90 percent of the stock) is a coarsely
porphyritic granite. An aplite phase is present along the margins. 122 Fracturing in the pluton can be broken into high and low-angle
sets (Figure 52B). High-angle maxima I and II are orthogonal. Maximum
I represents the sheeting element in the stock and varies from about
N. 60° E. to N. 80° E. in strike with a suggestion of a two-peak double maximum. Figure 53 shows this sheeting exposed on cliffs south of Goat
Hill on the south wall of Red River Canyon. Maximum II also shows pos
sible double maximum character and fractures for this maximum resemble the fracture cleavage joints of the Mine aplite. Much fine fracture cleav age not included in the diagram was measured with the same orientation.
Low-angle fracturing here is similar to contact-conformable breakage as defined in the Mine aplite. Maximum III represents this element along the northern contact where it is excellently exposed (Fig ures 26 and 27). Contact breakage is present elsewhere around the mar gins of the intrusion, but it was not as thoroughly measured as in the sample sites along the north contact.
On cliffs overlooking Highway 38, the area of the north contact exemplifies the same relationships between fracture cleavage, contact fracturing, and sheeting as are present in the Mine aplite. Widely spaced (five to ten feet), high-angle sheeting (N. 70o-80° E.) shows smooth, even surfaces with common evidence of strike-slip movements.
Between the sheeted joints, NNW-striking fracture cleavage is well developed. This cleavage is especially intense adjacent to the sheeting surface where it shows in places indications of drag against the sheeted joints (Figure 40). Contact-conformable fracturing is also intensely developed and individual fractures often have slipped, displacing
Slightly the ENE joints of the sheeting (Figure 54). Figure 53. Sheeting in Log C abin G ranite
Cliffs forming south side of Red River Canyon. View looking west showing prominent NE to E-striking regional sheeting in the pluton. This fracturing corresponds to maximum I on equal-area net of Figure 52B. Figure 54. Fracture Relationships in the Log Cabin Granite
Cliff outcrops just north of Highway 38 near north contact of stock. South-dipping (right), high-angle sheeting are cut by contact-conformable fractures of ENE strike and north dip. Both sheeting and contact fractures often show indications of lateral movement. Fracture cleavage is especial ly intense in this area. 125 Elsewhere in the pluton, fracture cleavage often gives way to a parallel sheeting of NNW strike which is almost indistinguishable from the ENE-trending sheeting. This relationship is seen very well at locations along the western cliffs of Bear Canyon. At many other places in the stock, it was not unusual to continue to find indications of lateral movements on fractures, which were mostly restricted to the ENE to E- trending set.
In Figure 52B, attention is directed to the small contour maxima
IV and V. These pole concentrations are generated by fractures found along the northwest wall of the entrance to Bear Canyon where the cliff trends northeast. Joints of these orientations are found only at this one locality. The maxima are nearly perpendicular in strike. The NNE frac tures are irregular, barren of hydrothermal products, and resemble frac ture cleavage joints. Joints of the NW set are filled with quartz, traces of sulfide, show evidences of alteration and have the appearance of sheeting. If both sets are rotated about 40° counterclockwise on the equal-area net, maximum IV would coincide in strike with maximum I, and maxima V and II would correlate by the same rotation.
The writer suggests that the area of unusually oriented fractures is part of a large slide block (Figure 4) located to the northwest of the mouth of Bear Canyon. The block must have rotated as a single unit, thus conserving the angular relationships between sheeting and fracture cleav ag e.
Fracturing in the Log Cabin pluton, presuming that fracture
maxima IV and V have rotated, can be resolved into NE to E-trending
sheeting, N to NNW fracture cleavage and jointing, and contact 126 conformable fracturing. The similarity of these elements with those of the Mine aplite should be noted.
Columbine apllte. This intrusion forms a small stock-like mass midway between the Mine and Log Cabin stocks. Many dikes and dike like prongs extend outward from the mass in a northeast direction forming a discontinuous bridge with the Mine aplite stock. The main Columbine stock appears to be a WNW elongate body on its eastern end, and this shape conforms to the foliation in Precambrian rock adjacent. The folia tion appears to swing to a northeast direction, however, farther from the stock to the north.
Fracturing is extremely systematic within the aplite (Figure 52D).
Two mutually perpendicular fracture sets (I and II) of near-vertical dip are present. Both sets have contour configurations suggesting double
maxima. Maximum II is typical of fracture cleavage. Maximum I is W to
WNW-striking and represents sheeting as was described in the Mine
aplite. Relationships here between these two elements are similar to
those described along the north contact of the Log Cabin granite. Sheet
ing shows consistent strike-slip movements (Figure 38) and the fracture
cleavage is intensely developed on a very small scale.
The low-angle, north-dipping maximum III is strong, but its
origin is not known. It might represent contact-conformable fracturing
except that the elongate aplite body appears almost dike-like at the
measuring site and thus should not have a gently north-dipping contact.
Fracture sets in the Columbine aplite are found well developed
in the adjacent Precambrian country rocks with one slight difference.
Fracture cleavage is significantly coarser and of a joint-like nature (Figure 33). Sheeting of easterly strike maintains its direction even when the strike of the foliation changes from WNW to NE.
Red River aplite. This stock occupies the eastern part of the
Red River Structural Zone north of the town of Red River. According to
Ishihara (1967), the stock has the greatest variety of intrusive phases.
A marginal phase occupies the northwest corner of the pluton and is rhyolitic. The major phase is a leucocratic, fine-grained aplite which,
along the margins of the stock, contains many xenoliths (up to several
feet in diameter) of quartz latite. A medium-grained porphyritic aplite
and granite make up the core of the pluton.
Fracturing is unusually complex in the stock and appears to
differ in orientation from south to north in the pluton. To the south,
along lower Mallette Creek, sheeting trends N. 650-75° W ., an anoma
lous strike for this element regionally. The fracture cleavage jointing
trends northerly to N. 20° W. In addition, a scattered but through-going
N. 15o-30° E. joint set is present.
In the north-central part of the body, a new direction of sheet
ing (N. 50o-60° E.) becomes predominant. Over a fairly extensive zone
both the NE and WNW sheeting sets are superimposed. Both sets have
been flooded in places by hydrothermal fluids (quartz and feldspar). The
coexistence of the two directions of sheeting is unusual for the region.
The N to N. 20° W. fracture cleavage persists in this area.
In the northernmost part of the aplite, sheeting appears to vary
in strike between NE and E, the directions most typical for other stocks
to the west. In the rhyolite phase of the stock in this area, fracturing
is abnormally intense and close spaced. 128 Two factors may have influenced the complexity of breakage in this stock. One is the presence of a strong NW to WNW structural trend which appears to control the sudden change in the course of the river
southeast of the town of Red River (Figure 4). Clark (1966a) maps NW- trending faulting in this area which has truncated the south end of the pluton. The other factor is the probable shallow depth of the intrusion.
The quantity of inclusions and the extreme fine-grained nature of most
of the stock suggest that the intrusion was emplaced very near the sur
face, possibly even having reached it. Both features, either a NW zone
of weakness or very little confining pressure could have contributed to
the complex fracturing.
The Rio Hondo granite. Several stocks of coarsely porphyritic
granite outcrop in the Taos Range. The largest of these intrusive bodies
is located from Rio Hondo Canyon northward toward Cristobal Creek and
into the Columbine drainage (Figure 49). The same rock occurs in patches north of Cabresto Creek along the front of the range.
The granite has been dated at 50.9 + 2.8 m .y. using the K-Ar
method on hornblende (Table 1). This agrees with indirect field evidence
(McKinley, 1957; Clark, 1966a) which implies that the granite is older
and distinct from the mid-Tertiary intrusions of the Red River valley.
The outcrop pattern of the Rio Hondo intrusions suggests elon
gation in a NNW to NW direction. In the Rio Hondo area at Italiana
Canyon (Figure 49) the rock is weakly flow-foliated and contains a par
allel alignment of dioritic xenoliths. The strike of foliation and inclu
sions is northerly, varying 15° on either side. 129 Fracture surveys were carried out in the Rio Hondo area and in
Columbine Canyon (Figures 52E and 52F). Equal-area net maxima for fractures in both localities are similar and differ significantly from the fracture patterns of mid-Tertiary plutons in the Red River Structural Zone.
Maxima I and II are the strongest shown on the nets with the exception that maximum II is weaker in the Columbine area. Fracturing representing both these maxima (especially the NNE fractures) is present in the wide rhyolite dikes which are thought to be mid-Tertiary in age and which intrude the granite. Maximum II is nearly parallel to the direc tion of the dike swarm in the Rio Hondo area. If the dikes are mid-Ter tiary in age, the structural evidence favors a maximum mid-Tertiary age for the two fracture systems.
The weaker maxima III and IV assume the two principal direc tions of fracture in the Red River zone. Maximum III, a probable double maximum, is represented by widely and irregularly spaced, long frac tures lined with epidote and chlorite. Evidence of lateral movements was noted on several of these breaks.
Fracture cleavage was not frequently recognized in the Rio
Hondo granite, although a joint set (maximum IV) is present which has a similar N-NNW strike. Fracturing of all sets, except set III, varied
in appearance from sheeting to very coarse fracture-cleavage type jo in tin g .
An interpretation of this fracture pattern is extremely tenuous without more data. However, tentatively, it appears that maxima I and
II and maxima III and IV may form two separate orthogonal fracture sys
tems . Aplite-pegmatite dikes which cut the stock strike ENE to E 130 parallel to the epldote-filled fractures of maximum III. If the epidote and aplites represent late-stage igneous and hydrothermal activity in the Laramide pluton, then the ENE-E direction of fracture was active, and forming during the Laramide movements. The fracture set might even constitute a primary extension jointing formed normal to northerly flow structure (cross joints; Balk, 1937). The high frequency of jointing for sets I and II may be mid-Tertiary or later in age because the joints cut mid-Tertiary dikes. This does not, however, rule out the possibility that the stresses causing sets I and II could be older and could have continued through mid-Tertiary time.
Regardless of the details, the important conclusion is that the intense ENE to E-striklng sheeting of the Red River Structural Zone is not present to that extent through the Laramide pluton. This indicates that the ENE to E system is concentrated along the Questa-Red River area because of the previously discussed (page 112) zone of crustal weakness which underlies that area. It also confirms a similar conclu sion which was reached on the basis of regional dike orientations and Precambrian structure (page 120).
Directions of fracture filling. On Figure 52 the equal-area nets for fracture fillings (veins and dikes) and the respective percent plots are shown. For the three mid-Tertiary plutons, the directions of preferred filling are generally duplicated. In all three cases, these trends are NE and nearly E (ENE). This demonstrates that these direc tions, which were of primary importance in the Questa ore body, are
found elsewhere throughout the Red River .Structural Zone. A unique and
specific stress environment must be postulated which allows such 131 selective emplacement of igneous and hydrothermal fluids into the rock over such a large area.
Fracture filling in the Rio Hondo intrusion consists mostly of thin epidote veinlets with some chlorite and quartz which, if related to hydrothermal activity in the stock, would be Laramide in age. These veins together with the Laramide (?) aplite-pegmatite dikes generally strike ENE in the Rio Hondo area. The veinlets assume a N to NNE trend in the Columbine Creek area.
In Precambrian granite, the recorded fracture fillings consist of quartz and sulfide veins. Small dikes are also included. The age of the veins and dikes is questionable, and some Precambrian as well as
Tertiary features may be present. Unfortunately, the possible age varia tion does not help in assigning the dike formation to a particular period of time. Nevertheless, the simplicity of the directions of fracture filling in this old rock is significant, and it is in these same two directions that Tertiary dike and vein emplacement has been controlled both along the Red River Structural Zone and through the surrounding region of the
Sangre de Cristo uplift.
Summary Statement
1. High-angle fracturing in all of the intrusions studied including
Precambrian granite is resolved into distinct, relatively simple
sets. Two orthogonal fracture sets are characteristic for each
pluton.
2. For the four mid-Tertiary aplitic granite plutons of the Red River
Structural Zone, sheeting is recognized trending ENE to E. The
sheeting is commonly represented on equal-area plots as a 132
double maximum with small included angle. Perpendicular to
this sheeting, a N to NNW direction of fracture cleavage or
cleavage-typo jointing is common to all four plutons. In
places, the fracture cleavage joint set may also constitute a double maximum.
3. Sheeting is mineralized; fracture cleavage is not. Joints of
the sheeting element in the mid-Tertiary stocks are frequently
found with indications of horizontal to subhorizontal move
ments. As in the case of the Mine aplite where movement
along the sheeting is found, the fracture cleavage becomes
especially intense and well developed.
4. The Red River aplite stock is somewhat more complicated in
structure than are the other plutons. It has a distinct WNW-
striking sheeting which exists in the southern half of the pluton.
Furthermore, equal intensities of sheeting with NE and WNW
strike coexist in the same rock over a considerable area. This
coincidence of orientation and intensity is not found elsewhere
in the Red River reg io n .
5. Contact-conformable fracturing is recognized in the Log Cabin
granite and is very well developed where the contact is be lieved to have a gentle dip.
6. The Laramide Rio Hondo granite does not exhibit the intense
east-westerly breakage typical of the Red River area.
7. High-angle fracture filling represented by veins and dikes
strikes NE to E for the mid-Tertiary intrusions along the Red
River Structural Zone. Veins and dikes strike predominantly 133 easterly and northerly in the Rio Hondo stock, and they are
generally NNW and ENE to E striking in Precambrian granite.
The Origin and Mechanics of Fracture Formation
During the development of the science of granite tectonics, there has been little attention given to the structure of stocks. Most of the work has been done on batholithic-sized bodies. This work includes studies by Hans Cloos (1921, 1922a, 1922b, 1922c, 1928, 1936), Ernst
Cloos (1936), Balk (1937), and Mayo (1935, 1937). A few additional reports are available in the literature on specific stocks. They include articles by Allen (1966), Chapman (1954), Davis (1963), and Hutchinson
(1956). Balk (1937) briefly mentions stocks in his memoir; however, his
untimely death prevented a more comprehensive treatment.
The references listed above have dealt with two general types
of plutonic bodies.
1. Primary flow-structured plutons . These intrusions show indica
tions of the movement of viscous magma. They contain typical
primary fracture patterns as discussed by Balk (1937). Plutons
of this type are partially discordant, partially concordant, and
have effected moderate deformation in bordering country rocks.
They are plutons of the mesozone according to Badgley (1965,
p . 335).
2. Non-structural plutons without prominent flow structure. This
type of intrusion is generally discordant and is often found
associated with volcanic rocks. The plutons may or may not
show country rock disturbance, and they are typically epizonal
(Badgley, 1965, p. 335) in their depth of emplacement. The mid-Tertiary aplitic plutons of the Red River area corres pond to the second group of intrusions, unfortunately the group which ■ is least represented in the literature. This inadequacy in study is prob ably owing to the difficulty of joint interpretation without the presence • or recognition of primary flow structure.
The following section of this paper will outline a theory for the origin of fracturing in the Red River stocks which may prove applicable to this kind of pluton elsewhere. The origin of local stock-related fracturing will be treated first followed by the mechanics of regional fracturing. Figure 55, a block diagram of the Mine stock, is included at the beginning of this discussion as a reference for the fracture ele ments present in the stock.
Local Fracturing
Contact-conformable fractures are felt to be spatially related to the mid-Tertiary plutons along the Red River Structural Zone and are developed by some mechanism of intrusive emplacement. Schilling (1956, p. 63) suggests that contact-conformable fracturing is the result of a stress couple of upward intrusion. He states:
As the soda granite was intruded upward, the force of intrusion formed a couple with the intruded rocks resisting the intrusion. As the outer layer of soda granite became solid, it helped the intruded rocks resist the upward movement of the still-fluid core of the stock. The stress caused by this couple was re leased by fracturing in the solidified granite and intruded rock.
Vanderwtlt (1938, p. 633) explains the contact breakage as ten
sion jointing which is "an expression of contraction due to cooling of
the magma." Ishihara (1967, p. 46) interprets this structure as marginal
fissure-type joints, in the terminology of Balk (1937). Marginal fissures, 135 136 however, dip into the Intrusion instead of outward, so the term cannot be applied to the contact breakage at Questa.
The best theory for the origin of the contact-associated frac tures must resolve these critical characteristics of this fracture element.
1. The fracturing occurs in the uppermost part of the intrusion
within a few hundred feet of the intrusive contact. The breakage
is also found in the country rocks fading out about 1,000 feet
away from the stock.
2. The fractures are approximately conformable to the attitude of
the intrusive contact. The fractures and contact surface nor
mally dip less than about 40°.
3. The fractures can dip in any direction depending only on the
attitude of the contact surface.
4. The contact fracturing envelops the arch of the pluton and on
the crest has approximately horizontal attitude.
5. Breakage consists of faults and a large quantity of joints which
show no movement.
6. The latest movements along contact-conformable faults appears
to be dip-slip for structures which strike northerly and is often
lateral for easterly striking structures. Where these movements
are concentrated, fracture cleavage is intensely developed.
Much of this fault movement is postmineral in age.
7. During mineralization, some of these contact-conformable
fractures were "open" and received gangue and sulfide mineral
ization . 137 8. Contact fracturing is evidently not well developed where the
contact has a steep dip ('^50o-60°).
Balk (1937) described two flat-lying fracture types found in plutonic rocks (Figure 56), one of which may be correlatable with con tact-conformable fracturing. Much of these data evolved from the original work by H. Cloos (1922b, 1928). The two low-angle fracture types are:
1. Primary flat-lying joints. These structures, called "Lager" by
German geologists, were thought to be conformable to layers of
flow foliation in the intrusion. Balk (1937, p. 39) states, how
ever, that although the low dip of flow layers facilitates the
development of the Lager, it need not be present and the joint
ing is not always conformable to the foliation.
Pegmatites, aplites, and quartz veins often fill the Lager
fractures. The joints may be coated with chlorite, muscovite,
or pyrite. They form most readily along the flatter or apex por
tions of bosses or along the surfaces of flat igneous sheets or la c o lith s.
2. Stretching surfaces (flat-lying normal faults). These are fault
structures which allow the intrusion to expand upward, laterally,
or in both directions by continued push from the still liquid in
trusive core. The strike of the faults may vary, but the trend of
movements is parallel to the direction of stretching (flow lines).
Dips are seldom greater than 45°. The structures are not often
filled with dikes or vein material but do show thin veneers of
crushed rock. 138
Figure 56. Primary Structure Elements in the Strehlen M assif, Germany
(f) Flat-lying normal faults; (c) cross joints; (1) longitudinal joints; (pfj) primary flat-lying joints; (apl-c) aplites on cross ' joints; (apl-p) aplites on primary flat-lying joints; (h) hardway planes (dots and dashes); (r) rift planes (rows of dashes). (From Balk, 1937). 139
In the Zobten massif, Cloos (1922c) found these structures cut by the quartz-filled Q joints, and as such, to constitute the oldest fracture element in the pluton. The stretching surfaces are large in size, continuous, and absolutely planar.
In some massifs, the faults are found near the contacts; in others, they appear not to be related to this rock boundary. They are best developed in the flatter portions of the intrusion. Evidently, the breaks can dip either inward toward the center of the intrusion or out ward, away from it.
In the light of the description of these two low-angle fracture types, the present author correlates the initial contact-conformable jointing in the Red River-Ouesta stocks with the primary flat-lying joints. A comparison of Figure 55 and Figure 56 shows the analogy. The fact that contact-conformable fractures at Questa often show irregular traces and dip slightly steeper than the contact is believed by this writer to be due to the complicating factors of normal fault movements after the jointing had formed (see page 184).
Contact-conformable fracturing does not correlate with stretch ing surfaces for the following reasons.
1. Contact breakage is so abundant that it forms an essentially
continuous sheath around the pluton rather than scattered,
individual faults.
2. Much of the contact fracturing is represented by jointing and
is not faulted.
3. The fractures are related to the contact and are roughly conform
able to it in dip. The fractures do not dip into the pluton. 140
4. Individual contact-conformable fractures generally are not
straight planar features. Their irregular nature resembles exfol
iation in appearance which is a release fracture and not a
sh ear.
5. Along the crest of the pluton, the attitudes of contact-conform
able fractures are approximately horizontal rather than dipping
(Figures 55 and 56).
Balk (1937, p. 39) suggested that volume decrease in the in truded mass might be a possible cause for primary flat-lying joints. Such a cause would agree with Vanderwilt's (1938) interpretation of contact- conformable fracturing. The following discussion develops a theory for contact-conformable fracturing based on the present research. The theory is somewhat related to the shrinkage concept mentioned above.
Slobodskoy (1966), using data from some late Tertiary plutons in Russia which have arched approximately 3 km of flat-lying sediments, has estimated the amount of intrusive pressure on the roof rocks of the plutons. This pressure estimate is defined as P^otal = ^1 + ^2 where is the lithostatic load and Pg'is the breaking load. Pressures in excess of 1000 kg/cm^ were calculated for the intrusions. This figure for pres sure represents a lower limit.
The Russian estimate of intrusive pressure represents a pioneer ing effort toward clarifying a basic problem in pluton tectonics and is included in this discussion to impress the reader that considerable com pressive stress can be generated by magmatic intrusion. In most cases, the question of intrusive stress is rendered unsolvable because of the 141 inability to reconstruct the cover over the pluton or to evaluate the effects of marginal deformation.
At Questa, these difficulties exist but do not eliminate a rough approximation of possible minimum intrusive pressure. Ishihara (1967, p. 61) feels that quartz porphyry dikes which cut the Mine aplite are vent material for the rhyolite-welded tuffs which overlie the andesite- latite volcanic section through the Red River area. If so, the andesite- latite section represents the minimum thickness over the Mine stock at the time of intrusion. Ishihara estimates this cover at 1 to 1.3 km, or if some rhyolite was extruded before stock emplacement, the total over burden thickness would approach 2 km.
Although bedding attitudes for the volcanic rocks adjacent to the pluton are not known, there are some lines of evidence which sug gest that partially forceful intrusion of the Mine stock caused consider able NS arching of the surrounding terrain. This evidence includes a northerly trending swarm of aplite dikes in Precambrian rock just west
of the pluton, the absence of the rhyolite-latite section surrounding the pluton, and extreme fracture deformation in the adjacent country rock.
If the estimate of approximately 6,600 feet of overburden during intrusion and evidence for forceful intrusive arching are valid, then the • upward pressure generated by intrusion of the stock must have slightly
exceeded the lithostatic load or 6600 psi. Although this value is sub
stantially less than the 1000 kg/cm^ 17,000 psi) calculated for the
Russian example, this possible pressure at Questa is believed sufficient
to have strongly compressed wall rocks and marginal portions of the
Mine aplite during intrusion. 142
If the mechanics of intrusion were partially forceful, the upper portion of the stock probably cooled and crystallized while much of the compressional stress was acting. This stress would be directed approxi mately normal to the contact surface in the relatively flat-lying roof part of the stock. Appreciable elastic strain energy (Emery, 1964) may have become locked into the crystallizing hood of the stock due to the pres
sure. Under these conditions, CT^, the greatest principal stress, would act vertically on a portion of rock at the contact over the pluton.
After the stock had solidified, intrusive pressure probably
abated by means of several possible mechanisms: (1 ) overall shrinkage due to the liquid-solid conversion (5%-10% volume decrease; Ramberg,
1967, p. 77), (2) .magma withdrawal, (3) loss of mechanical and heat
energy, and (4) loss of volatiles. With the diminishing of intrusive pres
sure together with some erosional unloading, the original local stress
component, O'^ , which was roughly vertical, converted to CTg with (T^
changing to a laterally directed position. The stress differential
((Tl - (Tg), aided by the stored elastic strain energy could have exceeded
the strength of the rock in tension and an exfoliation of sorts might have-
developed nearly parallel to the stock's contact surface. This is not un
like the same fracture phenomenon caused by erosional unloading, where
a similar sheeting develops. Failure of this sort is called release frac
ture (Billings, 1954, p. 118). It develops as a result of expansion upon
relief of differential stress. As such, this kind of failure is fundamentally
an extension type of fracture as defined by Griggs and Handin (1960,
p . 48) and has been experim entally produced (Borg and H andin, 1966). 143 In the Mine aplite, fracturing proceeded best where the stock contact was shallow dipping. This attitude was closest to being parallel to the laterally directed O'The intensity of failure diminished away from the contact interface both upward and into the stock where intrusive and load stresses had been less. Fracturing continued until the stress was relieved and until could no longer be maintained horizontally due to the offsetting influence of regional uplift.
In conclusion, it should be said that stretching surfaces (flat- lying normal faults) may also be present in the Mine aplite. There are many examples of northerly trending normal faults dipping both east and west across the stock which would qualify. Faulting of this type should be expected in lieu of the possible northerly arching (page 141) which accompanied intrusion. The faulting would have allowed the intrusion and wall rocks to expand to the east and west in a direction normal to the axis of the intrusive arch.
Regional Fracturing
Both sheeting and fracture cleavage in the Mine aplite are re garded as regional fracture elements with respect to the intrusion. This interpretation is based primarily on the occurrence of these elements throughout the Red River-Questa area cutting all rock types and extending far beyond the limits of the stocks .
Earlier workers in the Red River area generally agreed on this point. Schilling (1956, p. 35,36, 63) frequently referred to a pervasive east to northeast-striking, high-angle fracture set that cut the stock and was found throughout the Taos Range. Vanderwilt (1938, p. 621) defined the important system of east-west sheeting and parallel dikes found east 144 and west of the stock as regional. Neither of the authors attempted to describe the mechanics of this breakage or go into the nature of the regional stresses which may have caused it.
In his treatment of the structural elements of igneous rocks,
Balk (1937, p . 100) emphasized that in certain regions plutons exist which have locally developed cross joints (tension joints perpendicular to flow lines) and a superimposed regional system of jointing. Regional jointing may intersect cross jointing at any angle and will cut across the pluton without deviation. Furthermore, the regional joints extend far beyond the limits of the pluton. From these relationships, Balk inferred that the movements which caused the joints must have been quite independent of the plutons and that the earth's crust in this region was "stretched and lengthened in uniform directions."
Structure in a granodiorite satellite body of the Sierra Nevada batholith in the Grass Valley Quadrangle, California, is cited as an example of regional fracturing (Johnston and Cloos, 1934). A uniformly distributed, northeast-striking, near-vertical jointing cuts across pluton and wall rocks alike (Figure 57) and controls the emplacement of dikes and some quartz mineralization. Mapping in the middle Sierra Nevada
Mountains by E. Cloos revealed this same system of jo in tin g over an area of 7,500 square miles. The joints cut through all rock types and are later than intrusive-generated joints (primary joints) in the intru sions. This prompted the authors (Johnston and Cloos, 1934) to adopt a regional origin for these joints although they are vague regarding any d e ta ils . 145
VvV-V-aV
s o u . >rr° ‘-0-0 scocrttr
Figure 57. Regional Northeast-striking Jointing Cutting the Grass Valley Pluton and Country Rocks, Grass Valley, California. — (From Johnston and Cloos, 1934.)
I 146 Hutchinson (1956) describes both primary and regional joints in the Enchanted Rock batholith of Texas. Near-vertical regional joints consistently strike NE and NW. They are combined with a system of radial cross joints which are oriented perpendicular to the flow struc ture in the pluton. The regional set is younger than the cross jointing.
In a 133 m .y.-old trondhjemite pluton in the southern Klamath
Mountains of California, a well-developed system of primary cross joints and aplite-pegmatite dikes orient themselves radially around the dome of the pluton (Davis, 1963). However, a better developed and more consistently oriented planar jointing is of secondary origin and trends N. 65° E. + 10° with high dip. Joints are spaced typically one to two
feet apart and are usually filled by quartz and late hornblendic dikes.
Figure 58 shows a close similarity of this jointing with sheeting at
Questa. Here again the origin of these fractures is not explained; how
ever, the joints probably correlate with the regional set mapped by
Johnston and Cloos to the south.
Russian researchers have recently done a great deal of work on
regional structure. Tom son (1964) discovered systematic zones of fine
fissuring which control ore deposition in districts of far eastern Russia.
The zones are found to persist over a mapped area of 100 square kilo
meters and trend in NW to EW directions (Figure 59). They cut indis
criminately across differently oriented folds and through different rock
types. The fissure zones are explained as reflections of regional frac
tures in the crystalline basement rocks.
Preliminary studies by the author and T. L. Heidrick show that
jointing of a regional nature exists through the Basin and Range province L ~
Figure 58. Regional N. 65° E. Joints Cutting the Caribou Pluton, Klamath Mountains, California
Note the similarity between these regional joints and sheeting at Questa. Photograph reproduced by permission of the Geological Society of America from Davis, 1963. m m # # * - ') _ i * a f
granitoids ~ presumably a Jurassic body | j - Triassic scdirents
j , ' ^ \ - fractures [" ^ - Leaver Cretaceous sediments - Permian sediments'
■ dikcs of ^ 9^ t i c rocks [ g _ zones of fissurirg
[^$3 ’ andesites, quartz porphyries, porphyrites
Figure 59. Zones of Fine Fissuring Cutting Across 100 Square Kilometers in Eastern Russia.
Dashed blocks indicate ore districts. (From Tomson, 1964.) 149 of Arizona. This jointing is best exposed in Laramide-age stocks which are distributed throughout the province. One joint set trends ENE to EW and closely resembles the sheeting fracture element at Quests. m e set has exerted a fundamental control on the introduction of late-stage, residual magmatic and hydrothermal fluids into the stocks. This high- angle sheeted jointing has previously been noted at several localities
(Kuhn, 1941; Richard and Courtright, 1954; and Peterson, 1962) but has never been related to the same regional fracture set.
Thus, the evidence for regional fracturing over wide areas is well documented. Furthermore, regional fracturing is usually present as a uniformly oriented, high-angle sheeting, not unlike that described at .
Ouesta. For an explanation of its origin it is necessary to start with the
subject of fundamental rock mechanics.
The experimentally produced fracturing shown in Figure 60
(Griggs, 1936; and Muller and Pacher, 1965) is similar to both sheeting
and fracture cleavage. Figure 60A especially resembles sheeting. The
type of failure induced in these examples developed in rock specimens
subjected to triaxial compression. Fractures were oriented parallel to
the axis of greatest stress.
In triaxial testing with the least principal stress (ffg) at atmos
pheric pressure, failure surfaces develop which are parallel to the com
pressive load and normal to the tensile load, or the free face. When two
pairs of faces of a cubical specimen are subjected to equal compression,
fractures develop parallel to the two free faces (Foppl, 1900) (Figure 61).
These are the conditions which formed the fractures shown in Figure 60A. 150
<*;
A. 0 5cm Figure 60. Experimentally Produced Extension Fracturing (A) Parallel sheeted fractures breaking toward the face free from compressional stress. (From Is/ltiller and Pacher, 1965.) (B) Extension fractures produced by longitudinal compression and 100 atmospheres confining pressure. Note relation of wedge splitting to extension frac tures. (From Griggs # 1936.) COMPRESSION COMPRESSION COMPRESSION EXTENSION FRACTURES COMPRESSION Figure 61. Compression Applied to Two Pairs of Sides of a Cube with Extension Fractures Formed Parallel to the Free Face. 152 This kind of failure is defined by Griggs and Hand in (1960, p . 48) as extension fracture. It is distinguished from shear fracture (faults) by the fact that movement of the rock mass is normal to the walls of the failure surface rather than parallel to them. The most important point resulting from experimental studies of rock mechanics and illustrated by Figure GOB is that extension frac turing can occur when G'g is compressional (Griggs and Handin, 1960). In this case, failure surfaces are also oriented parallel to 0 *3 . In other words, pure tension is not necessary in order to form extension frac tures. It is of considerable help in explaining how such fractures can form at depth where, because of overburden pressure, tension should not e x is t. Another major problem facing a tensional interpretation of joints, especially densely sheeted breakage such as that described at Questa, is the propagation of the tensile stress across the fracture inter faces. Figure 62 illustrates the problem. In the first case. A, the rock fails in pure tension but additional stress will only tend to separate the two sides of the fracture. In nature, multiple, close-spaced fracturing (i.e. sheeting) over large areas would be difficult to produce under ten- • sion because the stress would tend to dissipate itself as fracturing progressed. For the conditions in Figure 62B, however, extension failure (indistinguishable in orientation from tension fracture) would continue to break the rock. As long as the stress difference ( - (J3 ) was main tained greater than the limiting tensile strain of the rock, each newly broken block would continue to expand toward CTg and progressively more 153 A. TEN SION B. I t i I ! 1 0*1 1 1 T V " ■ T • 1 r ^ r [ f j - ! : 1 ’I • 1 *4 1* -r ?- lx , ! ^ h j i i j Kir T2 -J 1— • A -ii ; & L y -A X 1 A 1 1 i O", COMPRESSION COMPRESSION COMPRESSION Figure 62. Possible effects of Tension »nd Extent on on Roj# (A) . (1) Tension fracture forms when strength of rock is overcome. (2-3) With further application of tension, rock will not transmit the stress across the fracture. Fracture will only tend to widen. (B) . (1) Rock fails when '!i - 3 3 exceeds the tensile strength of the rock. (2-3) The two halves of the block continue to expand toward t 3 as J 1 is maintained. Fracture will proceed as long as ) \ - 3 is greater than the strength of the rock. 154 tensile failure would take place. Ideally, this condition could repeat it self until very close-spaced breakage were produced. Because 0 * 3 is still compressive, the joints would remain closed. If a high pressure fluid has access to the fracturing however, the breaks could open and become filled, a process which is akin to vein or dike formation. The healing of newly formed extension fractures by hydrothermal or mag matic fluid could aid in producing the close-spacing of parallel failure su rfa ce s. Anderson (1951) first put the process of dike emplacement on a sound mechanical basis. As dikes are not found in conjugate sets (separated by ~ 60°) and do not show horizontal movements, it is un likely that the dikes are contained in shear faults. Anderson goes on to explain that dike-filled fractures cannot be wholly tensional due to the impossibility of exerting tension beyond a certain depth limit. Field evidence shows, however, that walls of fractures which contain dikes have moved apart by dilation. Therefore, intrusion of dikes requires ex tension in a direction normal to the plane of the dike by an amount equal to its thickness. For extensive dike swarms a sizeable amount of cumu lative extension is required. Extension fracturing in a compressive stress field with the presence of a high-pressure fluid appears to be the best explanation of diking. Anderson has concluded that by a process called wedge splitting, dikes should propagate in planes perpendicular to the minimum pressure ( 0 *3 ) in the rock. This direction is parallel to that for extension fracture as described earlier. Griggs and Handin (1960, p. 351) state that dikes (and veins) are filled extension fractures normal to the least principal 155 stress. The filling and separation occur as soon as the fluid pressure surpasses in magnitude the least principal stress. Brace (1964, p. 58) calls these features intrusion fractures and has reproduced them exper imentally in a totally compressive stress field. Dikes should normally change direction only when the orienta tion of (73 changes. However, as suggested by Harms (1966), it is possible that any preexisting anisotropism such as fractures, foliation, or fault zones may exert a more important control on dike orientation than the direction of Cg. Regional jointing is herein interpreted as the result of exten sion fracturing in the earth's crust. Parallel veining and dike injection in the presence of magmatic and hydrothermal fluids are similarly ex plained. The generalized stress situation necessary for the formation of widespread extension fracturing could result from regional compres sion, vertical uplift, or a combination of the two processes. In the Sangre de Cristo Mountains, the belt of folding and thrust faulting which borders the range on the east and the thrust or reverse faults within the range (Figure 3) have been interpreted as the effects of regional compression (Baltz et a l., 1959). Other sources (Miller, Montgomery, and Sutherland, 1963; W isser, 1957) believe that vertical forces without compression have formed the range and that folds and thrust faults are secondary features caused by lateral spread ing of the uplift or as the result of gravity "slump folding" of its flanks. An impressive argument for vertical uplift of the southern Front Range in Colorado (Figure 2) has been presented by Harms (1966). Attention has been called to the similarities between such Rocky Mountain ranges 156 as the Front Range, the Wet Mountains, and the Sangre de Cristo Range (page 9 ), so that a common origin is suggested for each of them. Harms's theory of vertical uplift may therefore also apply to the Sangre de Cristo Mountains. Evidence from the present research also does not support region al compression as a basic cause for Laramide uplift. Therefore, a tec tonic model based primarily on vertical forces will be used to develop regional stresses and extension fracturing. The following steps in this development are modified after a discussion on joint origin by Price (1966, p. 133-135). A block of rock on the crest of an uplift will experience a ver tically directed overburden stress as well as lateral stresses. Presuming no previous tectonic compression, the initial stress conditions should approximate the lithostatic confined state and all directions of stress would be about equal to the lithostatic load. Upon uplift, however, lateral and vertical stresses will begin to decay as a result of erosional decrease in gravitational load. Because of the lateral stretching involved in uplift, lateral stresses decrease more readily than does the gravita tional load, so that the vertical stress becomes the greatest principal stress (CTp. The lateral stresses also will be expected to decay at different rates depending upon many variables, the most important of which is the geometry of the uplift. Normally for an elongate uplift, lateral stress will resolve into 0 * 3 aligned normal to the long axis of the uplift. How ever, regardless of whether 0 * 3 becomes normal or parallel to the axis of the uplift, extension fracture will begin oriented perpendicular to CT3 \ 157 when the lateral stress difference (CTg " 0*3 ) exceeds about 250 to 1000 psi, the approximate long-term tensile strength of competent rocks (Price, 1966, p. 135). Dikes and veins will also assume an orientation normal to CTg except where some anisotropic feature such as a major fault zone in basement rock may exert more control on the direction of extension failure than does the regional stress. As extension fracturing proceeds in the rock, "tensile" stresses may become relieved such that lateral stresses 0 * 3 and (T2 can inter change. Further uplift will then cause a second set of fractures to de velop which are oriented roughly normal to the first set. This general sequence may have been operative in the Red River region to account for the preponderance of orthogonal fracturing which exists there. Details are described in the next section. If sheeting as described in the Red River area is an extension failure phenomenon, two characteristics of this fracturing require explan ation. The first characteristic is the two-peak double maximum of small included angle which appears on equal-area nets. The second property is the scattered but widespread evidence of lateral movement on the joints of the sheeting. With regard to the first feature, Muehlberger (1961) outlines a theoretical means of producing two sets of extension fractures with small included angle. Figure 63A shows the Leon (1934, p. 318-321) modification of the Mohr failure envelope with one stress axis (0*3 ) in tension. A critical stress circle will fit the envelope such that it is tangent to the envelope at O on the normal stress axis. For this 158 0-0 Figure 63. Mohr Envelope Criteria for Extension Fracture Sets with Small Dihedral Angle I (A). Leon Modification of Mohr Envelope. The failure envelope intersects the (Taxis at right angles. A critical stress circle is possible which is tangent at point o, and the predicted failure plane is normal to O' 3 and in the plane of O'1 0*2 (tension or extension joint). (B). Three Successive Steps of Increasing Stress Difference with , Corresponding Conjugate Fracture Sets of Increasing Dihedral Angle. ) For a limited range of stress differences, fractures of small dihedral angle can form. These are essentially extension fractures. (Figure 63 i from Muehlberger, 1961.) 159 condition, a single plane of failure is predicted with CC, the angle be tw een O'i and the fracture, equal to 90°. As shown, this failure plane is a typical tension or extension fracture. It was pointed out earlier, however, that these fractures can also be produced in a total compres sive stress field so that the failure envelope could lie entirely within the compressional side of the vertical axis. This change would not affect the rest of the theory. Muehlberger proceeds to outline a series of progressively in creasing stress differences which at failure would each have conjugate fractures of increasing dihedral angle (Figure 63B). For a limited range of stress differences there should be a pair of failure plane formed with small dihedral angle. A transition appears to exist between typical ex tension and shear failures. This transition is evidenced in the laboratory by wedge splitting (Griggs, 1936, p. 553), which is probably represen tative of the small dihedral angle fractures (see Figure GOB). Using criteria such as plumose markings, the feather-like markings on joint surfaces which are reported to occur on shear frac tures (Parker, 1942; Roberts, 1961), Muehlberger concludes that the conjugate fracturing of small included angle is really an extension frac ture phenomenon which lacks any evidence of a shear origin. Roberts (1961) reports additional experimental confirmation for this conclusion. Briefly reviewing then, under a certain range of critical stress conditions, extension fracturing, such as exists at Questa, may form in two sets with small dihedral angle. The required stresses are apparently found at shallow depth and low confining pressures. 160 An alternative mechanism for producing the same type of frac turing has been proposed by Wise (1964, p. 289) from microjoint data of the central Rocky Mountains. Slight "zigzags" in strike of micro joints (Figure 64) result in equal-area net double maximum spaced 15° to 20° apart. In this case, the two fracture sets are interpreted as "extension fracturing alternating between two pre-existing weakness directions." This explanation only pushes the question to an earlier unre solved cause. The present author considering the evidence in his thesis area suggests another possibility. If by coincidence, the direction of least principal stress (CTg) diverges slightly from some strong element of anisotropism (shear zones, fracture zones, foliation, etc.) in the rocks or in the Precambrian basement, two sets of extension fracture may result, one normal to (Tg and the other parallel to the element of anisotropism or direction of easiest failure. This mechanism is proposed for explaining the northeast-strik ing sheeting along the Northeast Structural Zone and in other scattered zones which cut obliquely across the predominant easterly striking frac turing of the Red River Structural Zone. The presence of major mylonite shear zones of N. 60° E. strike (page 120)in the Precambrian basement rock of the area may have locally exerted a stronger control on the direction of extension fracture than did the Red River Structural Zone and the regional stress. Tertiary rocks overlying these basement shear zones would take up a trend of fracturing (sheeting) which would be parallel to the direction of the major weakness below, in this case about N. 60° E. 161 Figure 64. "Zigzag" Microjoint Sets in Basement Rocks of the Middle Rocky Mountains The origin believed to be due to two preexisting weakness direc tions in the rock. (From W ise, 1964.) 162 In the Red River region and elsewhere in the Sangre de Cristo range it is, nevertheless, probable that Muehlberger's small dihedral angle fracturing is present in the rocks. It may even have been accen tuated by the presence of the anisotropic elements. The Muehlberger theory is particularly applicable to explain the northerly striking double maximum fracturing in the region. Small dihedral angle fracturing appears in many areas but has not been specifically reported as such in joint studies of these areas. Besides the several studies cited as evidence for the phenomenon (Wise, 1964; Duschatko, 1953; and Spencer, 1959), this author believes the following studies present valid data for small dihedral angle fracturing: (1) jointing in Precambrian igneous and metamorphic rocks around Central City, Colorado (Harrison and Moench, 1961; Harrison and W ells, 1955), (2) fractures in the region of the Shakhtamin molybdenum deposit of USSR (Botnikov, 1964), and (3) fracture sets in Laramide stocks of Arizona (preliminary research by the present author). The second characteristic of sheeting at Questa which is not explained by extension is the evidence for lateral movements on a great number of the NE-E striking joints. Curiously, lateral movements are also indicated on easterly (+20°) striking low-angle fractures as well. In the pit, several instances were noted of dip-slip movement on low-angle northerly striking shears which apparently caused lateral slickensides on E trending sheeting in the hanging wall (Figure 65B). An observation of even greater significance is that slippage on low-angle northerly striking faults and movements on sheeting relate directly to most of the fine fracture cleavage developed in aplitic granite of the region. 163 Direction o f ------tectonic movement / " 7 Z STRETCHING Figure 65. The Effects of Uplift and East-west Spreading of Rock Mass (A) . Uplift and East-west Spreading of Rock Mass. (1) Dip-slip movement on northerly striking structures. (2) Lateral movement on east-westerly striking fractures. (3) Formation of north-south to north- northwest trending fracture cleavage and jointing by crustal arching and movements along other structures. (B) . Lateral Slickensides along East-west Sheeting by Dip-slip Movement on Northerly Striking Normal Fault. Field sketch from open pit area. 164 Evidently, lateral movements on approximately east-striking fractures, dip-slip movement on northerly striking structures, and N-NNW striking fracture cleavage (and jointing) are all related to the same fundamental cause. These relationships put stringent conditions on the stress set up and can best be satisfied by postulating significant east-west expan sion of the total rock mass of the area (Figure 65A). Any previously existing breakage (sheeting) trending roughly EW would become compressed and produce lateral slickensides by this east-west rock extension. A pulling apart or extension of the region toward the east or west would also cause dip-slip movement to occur on northerly trending structures aligned about normal to the extension. Movements on fractures striking intermediate between north and east would tend to be oblique. Movements on preexisting fractures would tear open or shatter to produce fracture cleavage in the rocks adjacent to the sliding surfaces. This local shattering would be in addition to the major part of the northerly trending jointing which formed as a direct result of the fundamental east-west crustal stretching. This mechanism of profound EW crustal extension has strongly influenced the Sangre de Cristo region and has largely occurred after mineralization and igneous activity. The reason why northerly jointing ' generally differs in character from E-striking sheeting (Figures 32 and 33) is not entirely clear but may related to less confining pressure* or the fact that the rock already had built into it a pervasive NE to E frac ture system. Hodgson (1961) has noted an identical distinction between 1. By the time the late N-NNW jointing and cleavage occurred, uplift and erosion may have significantly reduced the lithostatic load. 165 systematic and cross jointing (Figure 6 6 ) in sedimentary rocks of the Colorado Plateau. Price (1966, p. 135) maintains that the irregular, short, non-systematic joints (fracture cleavage type) are the relatively late-forming second set of joints which result when CTg and inter change. As such, these joints would form under less confining pressure, assuming erosion were to continue to reduce the lithostatic load as up lift progressed. The fundamental mechanism for the strong EW extension is not clearly understood. It may simply be a result of continued NS arching of the region which has been operating throughout middle Tertiary time establishing the northerly trend of dike swarms and fracturing. However, there may be a general time correlation between fracture cleavage and normal fault movements on large northerly trending structures, the chief example of which are the Rio Grande rift faults. Recent discussion by Eardley (1962) and Cook (1965) calls for major EW translation of the Cordillera of the United States by means of subcrustal convection. The Rio Grande graben is thought to be an im mense rift zone related to the exten sio n and horizontal movement of the crust in this region (Figure 67). The lateral EW drag caused by convec tion and also the inherent uplift could be recorded in the extensive regional fracture cleavage and jointing in the Taos Range adjacent to a Rio Grande rift zone. 166 JOINT ZONE CROSS JOINTS Figure 6 6 . Systematic Cross Joints in Zones, Replacing Each Other on Echelon, Together with Non-systematic, Short, Curviplanar Joints Note similarities with sheeting and fracture cleavage. (From Hodgson, 1961.) 167 Figure 67. Extension and Westward Drift Affecting North America Small arrows represent apparent vectors of movement. Large arrows represent apparent resultant direction of movement. Rift in New Mexico is the Rio Grande zone. (From Cook, 1965 after Eardley, 1962.) SUMMATION OF DATA The following outline reviews the most important concepts and observations resulting from the structural evaluation of the Questa molybdenum deposit and surrounding region. These concepts will be general and constitute a preparation for final interpretative conclusions which will follow in the last section of the dissertation. For details, the reader Is referred to more comprehensive summary statements which conclude several of the previous sections. 1. Precambrian rocks of the Sangre de Cristo uplift are found ter minated by a downfaulted rift zone on the west and crowded eastward against folded and reverse faulted sediments along its eastern edge. Some 6,000 to 7,000 feet of vertical uplift is indicated. 2. Within the north-trending uplift, the Questa ore body is local ized in a major cross structure (Red River Structural Zone) of ENE-E direction. Along this one to two mile wide feature, extensive intrusive and extrusive volcanic phenomena, a plltic granite plutonism, and hydrothermal activity have been con centrated. The dominant structure is pervasive NE to E frac turing and dike-vein emplacement. Major northerly striking normal faults and parallel fracture cleavage cut across the z o n e . 3. The Questa molybdenum ore body occupies the western corner of the Mine aplite, the central pluton of three early Miocene 168 169 granite stocks intruded into the Red River Structural Zone. Mineralization extends upward into shattered andesites adja cent to the intrusive contact. Precambrian basement borders ' the stock and volcanic rocks to the north and south. 4. Stockwork breakage is common through the deposit. However, its development Is not intense enough to mask the following systematic fracture sets: (a) NE-E striking, high-angle, (b) NE-NW-strlklng, low-angle with west dip, and (c) northerly trending high-angle; a and b are most common in the pluton, c and partly bare characteristic of the volcanic host rocks. Excluding jointing, the northeast to east-striking major fractures are the most continuous and largest fractures which were mapped, but the low-angle, west-dipping fracturing con stitutes the majority of smaller structures. This pattern does not include the high degree of N-NNW-trending, near-vertical fracture cleavage, a fine jointing and microjointing which in tensely cleaves all rocks in the area. 5. The more continuous veins parallel the major fractures forming a pattern converging towards the center of the open-pit ore body. The largest and most continuous filled-fissures are high- angle structures striking NE to E. Conversely, the most abun dant mineralization is emplaced into the northerly striking, west-dipping fracture pattern which largely subparallels the attitude of the Intrusive contact. Depending on location, sig nificant strike variations in the low-angle, mineralized struc ture exists. Examples are the strong development of productive 170 NW-striking contact-conformable fracturing on Highline Ridge and the local dominance of the nearly E strike of this miner alized element south of Sulfur Gulch in the area of the old underground workings. 6. Data from pit area and the entire Mine aplite stock support the concept of two directions of preferred mineral access and fis sure filling. These directions strike N. 80° E. (+ 10°) and N. 65° E. (+ 10°) and are steeply dipping. They parallel the trend of dikes within the pluton and through the Red River Structural Zone. 7. The development of NE and to some extent E-trending fracturing Is strongest in areas of highest grade of molybdenum. 8. The geochemical distribution of molybdenum in fractures delin eates through the pluton and into andesite a northeast zone of possible primary mineral access. 9. Preferred directions of mineralization and dikes, geochemical distribution of molybdenum in fractures, and underground ore trends appear to relate to two ridges on the west contact of the Mine pluton. These ridges strike approximately E and NE. Mineralization is apparently better developed in their proximity. 10. Along with the regional influence of NE to E structure, there is a dependence of mineralization on the degree of low-angle frac turing. The overall grade of molybdenum increases locally where low, westerly dipping fracturing becomes predominant. 11. The two prerequisite fracture elements for development of the ore body were NE-E-trending sheeting and fractures and low 171 angle, contact-conformable fracturing. The former element served as deeply penetrating, open channelways for mineral fluids. The low-angle fabric acted to disseminate mineraliza tion laterally through the rock. 12. Three highly systematic fracture elements are recognized in the Mine aplite as well as in the other aplitic granite intru sions of the Red River Structural Zone. They are: a. Contact-conformable fracturing. This type of breakage largely constitutes the low-angle locus for mineral depo sition referred to above. It forms an envelope of irregularly layered breakage conforming td the flatter roof of the intru sion and extends for some distance into the pluton and upward into the host rocks. b. Sheeting. This high-angle element forms a regularly repetitive NE to E-striking jointing which probably evolved early in the sequence of stock fracturing and commonly con tains coatings of hydrothermal material. It parallels the principal directions of fracture filling in the Red River Structural Zone, is regional in development, but is much more intense along the structural zone than it is regionally. c. Fracture cleavage. This pervasive fracturing trends N- NNW perpendicular to sheeting and is also regional in extent, cutting through rocks of all ages and types. The element is found as irregular, curviplanar, discontinuous microjointing and jointing with noticeable variation in the strike and dip of individual fractures. The fracturing, in 172 its extreme development, is associated with dip-slip movements on northerly striking, low-angle structures and lateral movements on sheeting. Of special importance is its occurrence adjacent to low-angle easterly trending fractures which also show lateral movements. Fracture cleavage is largely more recent than igneous and hydro- thermal activity along the Red River Structural Zone. 13. Sheeting, fracture cleavage, and to some extent contact-con formable fracturing tend to occur statistically on equal-area plots as double maximum with small included angle (10o-20°). This seems to be a feature common to rock failure in the stock and elsewhere in the region. 14. The direction of regional mid-Tertiary dike swarms in the Sangre de Cristo range is northerly except along the Red River Struc tural Zone where the dikes suddenly turn to an ENE to E direc tion . 15. Crossing the structural zone at an acute angle are secondary northeast (N. 60° E. + 10°) trending zones which control the parallel strike of dikes, veins, and high-angle sheeting. The longest of these zones (Northeast Structural Zone) cuts through the deposit and can be traced as far to the southwest as the Log C abin g ran ite. 16. Precambrian basement rocks of gneissic granite in the Red River Structural Zone duplicate the fracture pattern found in Tertiary rocks within the zone. The two predominant trends, ENE to E and N to NNW, do exist regionally outside the zone but may be 173 accompanied by other fracture sets, and notably the ENE-E set does not here attain the intensity that it does within the struc tural zone. Foliation is generally unaffected by the presence • of the Red River zone and tends to cut NE-ENE obliquely across it. Some Precambrian dikes parallel to the structural zone have been recognized, suggesting that a major direction of weakness existed through the area in Precambrian time. 17. Analysis of other plutons along the Red River zone distinguishes mutually perpendicular sets of high-angle fracturing trending ENE to E and N to NNW. Each set often exhibits a close-spaced double maximum on the equal-area nets. The E set is sheeted and mineralized, the N set is of the fracture-cleavage type and is barren. The sheeting element in all stocks frequently shows indications of horizontal to subhorizontal movements. 18. The Laramide Rio Hondo intrusion, located outside the Red River zone does not show the intense east-westerly breakage typical of the Red River area, although the direction is nevertheless present. Of special importance is that lateral displacements of small magnitude are also recognized on east-westerly structures in this granite. 19. Fracture filling represented by veins and dikes is NE to E- trending for mid-Tertiary intrusions in the Red River Structural Zone. It is roughly E and N for the Rio Hondo stock and for Pre cambrian granite. 20. Contact-conformable fracturing is similar in most respects to primary flat-lying joints or "Lager." A release fracture origin 174 similar to that causing exfoliation is suggested by several researchers, including the present writer. 2 1 . Consistent directions of fracturing over wide areas of the earth's crust has been commonly recognized but not well ex plained . Work in experimental and theoretical rock mechanics recognizes a sheeted breakage defined as extension fracturing, which closely resembles high-angle sheeting and fracture cleavage type jointing. This type of rock failure is continu ously repetitive and can form even under triaxial compression. The fracture planes are oriented perpendicular to cfg, the least principal stress. The direction of dike and vein emplacement is theoretically and experimentally shown to coincide with the orientation of the extension fracturing. 22. Muehlberger's concept of two sets of extension fracture with small dihedral angle forming under a low critical range of stresses explains at least some double-maximum jointing at Questa. An additional possibility applicable especially to ENE and E-striking maxima is the presence of major basement ani- sotropism (i.e. shear zones, foliation) which is not oriented normal to 0 *3 * This could result in two sets of extension fail ures: a set normal to 0 * 3 and a set controlled by the basement w e a k n e ss. 23. Price's (1966, p. 133-135) theory for jointing postulates two mutually perpendicular joint sets as extension fractures due to vertical uplift. One forms early thereby relieving the tensile stress and causing the orthogonal set to subsequently develop. 175 24. Late east-west extension of the rock mass of the elevated Sangre de Cristo crustal block is necessary to explain fracture cleavage and its relationships with movements on other frac-‘ tu r e s . CONCLUSIONS The genesis of the Sangre de Cristo Mountains, together with other Laramide uplifts of the Rocky Mountains, relates closely to tec tonics of vertical uplift and phenomena such as crustal stretching, dike and vein formation, and extension fracture. These processes have strongly contributed to the evolution of the Questa molybdenum deposit. Earlier tectonic history left an anisotropic crustal framework which in fluenced this Laramide deformation. The concluding section of this paper will center around the historical development of deformation and its effects on regional tec tonics and local structure at the ore deposit. Some notes regarding general geologic history are adopted from Clark (1966b, p. 64). The discussion on historical development will end with some general con clusions regarding the overall study. The orogeny terminating the early Precambrian produced the fundamental structural "grain" of the region. This was dominated by E to NE-striking foliation, NE-trending mylonite zones, and fracturing. Many of the fractures were related to the folds either as cross joints (~N strike) or longitudinal joints f'-'NE-E strike). Less important diag onal fracturing was also present. The strongest anisotropism, that of roughly NE-E and N-NNW directions is strongly reflected through all succeeding deformational periods. Pegmatite dikes and quartz veins are generally parallel to the direction of foliation which suggests that they are later than folding. 176 177 possibly the result of postorogenic uplift. Late Precambrian diabase dikes trend WNW and would call for a new direction of regional "ten sion^)" or extension which was not inherited from the older orogeny. • This WNW direction of Precambrian-filled fractures is well documented in the central Wet Mountains to the north of the Taos range in Colorado (Christman, 1959). The age of the WNW fracture maxima in basement rocks of the dissertation area (Figure 52G, H) may also be late Precam brian. It is not known whether the Red River Structural Zone had devel oped by the end of Precambrian time. Some weak evidence (a few dikes) parallel to the zone infers its existence at that time. The positive area resulting from the Precambrian deformation was stable well into Paleozoic times. By Devonian(?) time the area had eroded sufficiently to permit marine transgression, which continued intermittently throughout the Paleozoic era. During the Pennsylvanian period the upwarps of the ancestral Rocky Mountains developed. The area of the present Sangre de Cristo uplift and Las Vegas sub-basin was part of a depressed area (Rowe-Mora geosyncline) which received sedi ments between the ancestral San Luis and Sierra Grande uplifts. During the Mesozoic era, sedimentation continued in the basin. The sedimentary rocks were largely continental types. In Cretaceous time, major marine on lap resulted in appreciable thicknesses of shale and limestone. The withdrawal of these seas at the end of the period introduced the Laramide orogeny, the second period of major tectonic deformation and uplift. The mechanics of this Laramide uplift are still being argued between those investigators advocating lateral compression and those 178 who favor vertical tectonics. The author supports the latter view. Uplift, arching, crustal stretching, and concomittant intrusive activity are pri mary features of the deformation. Folding and thrusting are felt to be secondary features caused by lateral spreading or gravity sliding on the flanks of the uplift. The role of lateral compression is therefore minimized in this interpretation. Instead, extension caused by crustal stretching is believed chiefly responsible for the structure as it is found in the up lifted areas. For some time the geologic similarities between the various ranges of the southern Rockies have been appreciated. These include such ranges as the Front Range, the Wet Mountains, and the Sangre de Cristos. The process of uplift is probably basically similar for all three ranges, and a regional stress system might be deduced from combined structural features. Harrison and Wells (1955) have studied at length the vein and fracture pattern in the Central City area west of Denver, Colorado. They recognize a consistent regional system of orthogonal joint sets in base ment rock trending ENE and NNW. The NNW set is parallel to the axis of the range; the E-NE set which is transverse to the range, is called by these workers cross jointing.* The two sets have no relation to Pre- cambrian folding, and from this they concluded that both sets are a consequence of Laramide uplifting of the Front Range. Badgley (1965) recognizes this regional pattern elsewhere in the range. 1. This is a third meaning for the term cross jointing. The term in this case signifies joints which trend across the long axis of the mountain range. 179 The superposition of Laramide veining in the Central City area is shown by Sims and Barton (1962, Fig. 2, p. 375). Veining strikes NE to E and forms a complex, closely spaced network which is essentially parallel to the cross joints referred to by Harrison and Moench (1961). The longitudinal fracturing in the Central City region is barren and ap pears to postdate mineral and igneous activity. Fracture data from the Roberts Tunnel site some 25 miles WSW of Central City (Warner and Robinson, 1967) were accumulated for both Precambrian and Laramide rocks. In Precambrian rock, jointing is com plex and a cumulative equal-area plot gave virtually an isotropic distri bution of poles, although in an earlier report Wahlstrom and Hornback (1962) do show various maxima. However, jointing in the Laramide Montezuma stock is systematic (Robinson, written communication). Two perpendicular maxima of NS to N. 30° W. and N. 60o-80° E. strike and near-vertical dip are present. These fracture sets probably correlate with the regional longitudinal and cross joints farther east in the Cen tral City region. A relatively young age for the regional fracture system is thus confirmed and it is probably associated with the Laramide defor m ation. Situated south of the Wet Mountains and east of the Sangre de Cristo range occurs the extensive dike swarms of Spanish Peaks, Colo rado. This igneous activity is believed to be late Laramide in age (Johnson, 1968). Most of the dikes, especially the more continuous ones, strike ENE. As described previously, this orientation would sug gest regional extension in a direction normal to the ENE dike swarm. Ode (1957) by means of mechanical analysis shows evidence that a 180 regional stress field coupled with radial point source stresses produced the dike distribution in the Spanish Peaks area. He suggests EW-directed compression for the regional stress; however, less intense extension stresses of CC, oriented approximately EW and 0^ oriented NS would result in the same distribution of regional stress. The present author interprets the diking as injection into extension fracturing accompanying Laramide uplift. Boyer (1962) notes that in Precambrian rocks of the Wet Moun tains, two of the strongest high-angle joint sets trend N. 45o-50° W. and N. 45o-50° E ., both longitudinal and across the NW elongation of the range. Dikes of andesite, rhyolite, quartz latite, etc. are similar in composition to mid-Tertiary dikes in the Sangre de Cristo Mountains and may be of the same age. The trend of these dikes is longitudinal to the uplift as it is in the Sangre de Cristo range. The examples above collectively support the concept of regional NE to E extension fracturing at right angles to the elongate mountain up lifts. This fracturing was actively forming during the Laramide deforma tion and was being filled by igneous and hydrothermal material where available. At some later time, possible middle Tertiary, extension frac turing and dike emplacement developed in the orthogonal (NNW-NW) d irectio n . This regional deformation corresponds quite well with the geol ogy in the southern Sangre de Cristo Mountains. The following features are correlatable. 1. NE to E fractures (cross joints) are pervasive through basement rocks. The foremost example of this direction is the Red River Structural Zone. 181 2. N to NNW fracture cleavage and jointing is intensely developed trending longitudinal to the mountain uplift. 3. Mid-Tertiary dike emplacement is also roughly longitudinal and trends north to northwest. 4. In the Rio Hondo intrusion, possible Laramide-age aplite-peg- matite dikes trend ENE. Probable mid-Tertiary rhyolite and andesite dikes are oriented NNW to NW. The chief inconsistency in the N-NW trend of mid-Tertiary dikes in this part of the Sangre de Cristo range occurs along the Red River Structural Zone, where the dikes strike ENE. To explain this inconsis tency it is necessary to postulate the following tectonic process. In the southern part of the Sangre de Cristo range, as in other mountain uplifts of the middle and southern Rockies, Laramide regional stresses were causing an extensive system of NE to E-trending cross fracturing to develop through the rocks of the N-NW elongated uplifts. This north-south directed crustal stretching was accommodated in places by strong dike and vein emplacement into these fractures. The preexisting direction of Precambrian structural weakness in the basement rocks in the region may have aided in the propagation of this cross fracturing. The Red River Structural Zone probably developed as a major feature at this . time. Guided perhaps by an initial Precambrian direction of weakness, the zone evolved during the Laramide deformation as a zone of fine frac turing (closely concentrated extension sheeting). It is likened to the long zones of fine Assuring which have been mapped by Tomson (1964) in the USSR (Figure 56). 182 As arching and the formation of extension jointing continued, the stress in the rocks may have become relieved with an interchange of the principal stress components 0*2 and O'3 (Price, 1966, p. 135). This may have occurred during middle Tertiary time. The direction of G 3 was now reoriented about 90° to a near EW orientation and a new northerly direction of extension fracture and dike emplacement began to dominate throughout the region of the initial Laramide mountain uplifts. * In the Red River area, however, mid-Tertiary structural features continued to develop with ENE to E trend. Melting and intrusion were concentrated along the major zone of crustal weakness which had largely been developed some 30 to 40 m .y. earlier during Laramide time. Dikes and veins were also locked into this zone of weakness. North to north west-trending dikes and veins, in response to the regional stress field, could not effectively cross the structural zone but were forced to turn parallel to it. Actually, the presence of the stress field is weakly ex pressed in the structural zone by intermittent, minor dikes of the regional mid-Tertiary northerly trend (Figure 47). This case of the disturbing influence of a major zone of weak ness has been reported by Tom son (1964, p. 1997) for a region in eastern Russia. A belt of NNE-striking tin veins intersects a major E-striking zone of fine fissuring. The ore veins appear to completely refract in passing through the fissure zone where their strike becomes nearly EW, parallel to the fissure zone. The original NNE strike is regained as the veins emerge from the fissure zone on the other side. Individual veins 1. An alternate cause for the change in the direction of maxi mum crustal extension is westward directed, subcrustal convection (see page 165). 183 are segmented into a multitude of fine-fissure fillings within the major zo n e . After the end of igneous and hydrothermal activity along the Red River Structural Zone, the continued EW expansion of the uplifted mountain block (or EW drag from subcrustal convection) caused the development of the extensive fracture cleavage and associated northerly trending jointing through the rocks within the Red River zone. In the area of the Questa mine, the Mine aplite stock was sub mitted to many of the mid-Tertiary stresses which have been discussed above. Specifically, these stresses consisted of the strong local com pression of the overburden load acting downward and the intrusive pres sure pushing upward normal to the contact, and regional extensional stresses, which within the Red River Structural Zone were oriented with (f 2 approximately EW, CT3 NS, and vertical. From the time of crystallization of the stock, extensional stresses fractured the Mine aplite stock along E and NE directions. Early through-going faults or zones of fracture probably controlled the ascent of magma or metasomatic fluids and resulted in the E and NE- trending ridges developed on the west and southwest flanks of the pluton. Reactivation of fracturing along these same zones and creation of addi tional sheeting by continued uplift initially tapped mineral fluids. During intrusive expansion, some low-angle stretching surfaces (normal faults) were probably produced and became filled with quartz and initial molyb denum mineralization. As intrusive pressure abated, contact-conformable release frac turing began to develop along the flatter margins of the stock. This 184 became important in providing lateral permeability for hydrothermal fluids, and mineralization, especially that of later stages, was trans lated up along the contact and into overlying andesites toward the crest of the pluton. Regional extension toward the north and south was still operative as evidenced by the formation of the ENE-E-striking aplite breccia dikes. Additional sheeting may also have developed. At some time during the multi-staged period of mineralization, collapse-structure and stockwork breakage began to evolve. In particu lar, volcanic rocks were affected. This may have been caused by several factors, singly or in combination: intrusive withdrawal, renewed mag matic push, or regional east-west extension. Low-angle contact-conform able joints of northerly strike were beginning to slip in normal fault movement. At about this time the final stage of sulfide mineralization (molybdenum "paint") was pervasively introduced taking advantage of the low-angle breakage for deposition. Intensely broken andesite was particularly susceptible to this mineralization. It is seen from the above description that if either NE-E exten sion fracturing or low-angle (largely contact controlled) breakage were not developed, the orebody would probably not have developed. The availability of more than one fracture element to hydrothermal fluids was essential. Major east-west extension of rock mass continued and formed fracture cleavage, the most recent fracture element of the area. Fracture cleavage and related jointing appear to be synchronous with normal block movements on northerly trending, high-angle faults, the chief example being the rifting forming the Rio Grande depression. 185 It is suggested that these last events may be related to west ward convection beneath the crust as postulated by Eardley (1962) and Cook (1965). However, similar results might be expected from a con tinuation of earlier mid-Tertiary regional extension toward the east-west, a result of uplift. Accentuated, late NS arching of the uplift could have thrown near-surface rocks into actual tension, caused the Basin and Range type faulting, accelerated east-west stretching, and formed the fracture cleavage. Finally, it is conceivable that subcrustal convection is the original cause for both Laramide and mid-Tertiary tectonism. Laramide arching could have resulted from convective upwelling. The 90 degree shift in the direction of extension fracturing, diking, and veining from Laramide to mid-Tertiary time might reflect convective roll over and pronounced horizontal movements. Uplift would,however, continue to be a feature of the convection cell (Figure 62A). This could be an alterna tive or perhaps an auxiliary mechanism to Price's concept of strain relief and interchange of lateral stress axes. In terms of the original purpose of this paper and after consid ering the total data and its interpretation, these final conclusions are p re se n te d . 1. Statistical treatment of large amounts of structural data is an effective method of studying subtle ore controls in complex, low-grade, porphyry-type ore bodies. 2. Much of what appears to be a stockwork of intense, haphazardly oriented breakage can be resolved into a definite fracture pat tern after systematic and detailed structure mapping. 186 3. By means of equal-area net analysis and percent plots, the fracture sets of this pattern can be related to mineralization. The most abundantly mineralized fracture element may not be the one most preferred or favorable to mineralization and the percent plot is an effective tool for discerning this. The plot is especially statistically representative if a fairly large num ber of fractures of all orientations are compiled. 4. Preferred mineralization directions, so defined, together with directions of dike emplacement and geochemical anomalies have provided a key to defining directions of primary ore con trol. The recognition of this control can prove useful in focus ing exploration in more potentially productive directions or a re a s . 5. The main principle of the primary ore control is that mineral fluids which are concentrating under high pressures will migrate upward when they are tapped by the first consistent fracturing which occurs in the newly consolidated intrusion. At Questa, this fracturing is of the extension type which resulted from regional dilation of the crust due to uplift. This satisfies the first condition for the creation of an ore body, that of deeply penetrating primary channelways. 6 . The second condition is the availability of a permeable reser voir for the mineral fluids or a site of deposition. This condition is satisfied at the Questa deposit by the intense development of contact-conformable fracturing and other low-angle breaks. 187 7. Structure in Precambrian basement rocks has had an important influence on later deformation primarily due to directional weaknesses. Basement anisotropism, such as foliation, zones of intense fracturing, or wide mylonitic shear zones,can partially control the direction of extension fracturing. Thus, the orienta tion of dikes, veins, and extension failure may not always be normal to (Tg. 8 . The fracture elements of stocks may be a result of both regional and local stresses. 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E ., and Hornback, V. Q ., 1962, Geology of the Harold D. Roberts tunnel, Colorado: West portal to station 46B + 49: Geol. Soc. America Bull., v. 73, no. 12, p. 1477-1498. 194 Warner, L. A., and Robinson, C. W ., 1967, Geology of the Harold D. Roberts tunnel, Colorado: Station 46B + 49 to east portal: Geol. Soc. America Bull., v. 78, p. 87-120. Wise, D. U ., 1964, Microjoints in basement rocks of the middle Rocky Mountains of Montana and Wyoming: Geol. Soc. America Bull., v. 75, p. 287-305. W isser, E. H., 1957, Deformation in the Cordilleran region of western United States: Colo. School Mines Quart., v. 52, no. 3, p . 5 3 -7 3 . ce 5 T COST IIX A 105*15 1:48 0 0 0 36*30 105*30 105*15' EXPLANATION DATA COMPILED FROM' ^ Mid - Tertiary dikes and elongate intrusives Mid - Tertiary "aplite" plutons Clark, 1966. Eagle Nest Quadrangle m • • • • Mid - Tertiary (?) quartz veins Laramide Rio Hondo granite McKinlay, 1956. Costilla & Latir Peak Quadrangles McKinlay, 1957. Questa Quadrangle Younger (?) Precambrian diabase dikes Approximate locality of fracture, dike, vein survey for Moly. Corp. America, 1964 -1967. Geology of Red O equal -area net analysis River Canyon Older Precambrian dikes (pegmatite) Direction of principal extension or axis of RIO HONDO GRANITE (RIO HONDO VALLEY) RIO HONDO GRANITE (COLUMBINE CRK AREA) PRECAMBRIAN GRANITE (RED RIVER CANYON) PRECAMBRIAN GRANITE (LAMA CANYON) z-A 4 PRECAMBRIAN GRANITE Figure 52 : (TOTAL) STRUCTURAL DIAGRAMS CONTINUED > 30 r a J LU LU O o o E ‘=inai\ a o O m o in N o oJ m ID ID X < A • tfV> ' 30,0 00 N - I V / \ A, , If 2 7,5 00 STOCK CONTACT INFERRED FROM EARLY WORK 2 5 ,0 0 0 N j 2 2 ,5 0 0 N \ I 1 / / 4 - v l \ * t X • h I & V Figure 20 COMPOSITE STRUCTURE \ OF THE MINE ' APLITE BODY SCALE I m = 500 ft EXPLANATION High angle sheeting (> 6 0 ° dip) High angle fracturing ( Each symbol represents approximately four measured fractures Low angle fracturing (< 60° dip ) Each symbol represents approximately four measured fractures Fracture cleavage Major fault T T LU LU € e\ r\°l\ o v ° l^ o O o o o o O o o o o \ n in O in o in o (XI in N o <\f in in in m ID <£> ID + A 30,0 0 0 N — + 2 7,500 N + 25,000 N + 22,500 N + 20,000 N EXPLa-iU TION Figure 19 intrusive contact STRUCTURE CONTOUR MAP x hashed /vhere covered Questioned where interpreted MINE APLITE M a jc r fault SHrv/ING REL ATICT' OF MINERALIZATION, Cashed where covered CONTACT SURFACE A.'D MAJOR FAULTS Questioned where i nter pre*ed C = downthrown U = upthrow n Contour interval = 100 feet Coordinate system of Molybdenum Corp of America // Large veins in the deposit K- Contours on intrusive contact Western part of pluton - centre lied b v drill rerr r i~ uni 'i-, Jur gr(ur..J ;..oppi..g bur I Or r I5rf feet i - j V Dushed where projected above the ground surface r r doubtful X 6500 Scale Questioned where especially interpretive M FILLED FRACTURES PERCENT PLOTS PLUTON TOTAL FRACTURES NO. FILLED FRACTURES v , (VEINS AND DIKES) NO. TOTAL FRACTURES x ,00/ 3790 (NO. OF ATTITUDES) MINE APLITE block sdme maximum LOG CABIN APLITE RED RIVER APLITE FIGURE DIAGRAMSSTRUCTURE INTRUSIONS OF THE RED RIVER REGION EQUAL AREA NETS; CONTOURS IN PERCENT EXPLANATION COLUMBINE APLITE EQUAL AREA NETS PERCENT PLOTS