University of Nevada Reno Omparative Geology And
University of Nevada Reno
omparative Geology and Geochemistry With Respect to \/PPrecious Metal Mineralization of Selected California Coast Range Mercury Mining Districts
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science
by
Hilbert Nathaniel Shields
Mines Library University of Nevada - Reno Reno, Nevada 89557
May, 1982 MINES UMRAJUT I s 17 5A
The thesis of Kilbert N. Shields is approved:
Thesis Advisor
University of Nevada
Reno
May 1983 11 TABLE OF CONTENTS
Page Acknowledgement...... vii A b s t r a c t ...... viii Introduction ...... 1 Purpose of Study ...... 1 Selection of Areas of Study ...... 2 Method of S tudy ...... 4
Geology of Areas of Study ...... 7 Coast Range Regional Geology...... 7 General Geologic Setting...... 7 Mineralogy of Mercury Deposits...... 12 Sulphur B a n k ...... 14 Introduction...... 14 Regional Geology ...... 17 Stratigraphy...... 17 Mineralization...... 19 Results of Sampling...... 22
Syar Quarry-St. John's Mine Area ...... 25 Introduction ...... 25 Regional Geology ...... 27 Stratigraphy ...... 28 Mineralization ...... 30 Results of Sampling...... 31 Mt. Diablo Mine A r e a ...... 35 Introduction ...... 35 Regional Geology ...... 37 Stratigraphy...... 37 Mineralization...... 39 Results of Sampling...... 41 Guadalupe Mine A r e a ...... 44 Introduction...... 44 Regional Geology ...... 46 Stratigraphy ...... 47 Mineralization...... 49 Results of Sampling...... 50 Ill
page Ambrose Comstock Mine Area...... 54
Introduction...... 54 Regional Geology...... 56 Stratigraphy...... 57 Mineralization...... 61 Results of Sampling...... 64 New Idria Mine Area...... 69 Introduction...... 69 Regional Geology ...... 7 0 Stratigraphy...... 72 Mineralization...... 74 Results of Sampling...... 76 Rinconada Mine Area ...... 79 Introduction...... 79 Regional Geology ...... 81 Stratigraphy...... 81 Mineralization...... 83 Results of Sampling...... 83
Discussions and Interpretations...... 8(5 Geochemistry ...... 86 Introduction...... 86 Cluster Analyses on an n x m Data Matrix. . 90 Cluster Analyses on an n x n Data Matrix. -103
Silica-Carbonate Rocks ...... 106 Conclusions...... 109
Bibliography...... Ill Appendix...... 114 IV
ILLUSTRATIONS (Plates are in Pocket) Plate 1. Geologic and Sample Location Map, Sulphur Bank Mine Area, Clear Lake County, California 2. Geologic and Sample Location Map, Syar Quarry - St. John's Mine Area, Solano County, California 3. Geologic and Sample Location Map, Mt. Diablo Mine Area, Contra Costa County, California 4. Geologic and Sample Location Map, Guadalupe Mine Area, Santa Clara County, California 5. Geologic and Sample Location Map, Ambrose-Comstock Mine Area, San Benito County, California 6. Geologic and Sample Location Map, New Idria Mine Area, San Benito County, California 7. Geologic and Sample Location Map, Rinconada Mine Area, San Luis Obispo County, California
GRAPHS Page
Graph 1. Bar Graph Showing Distribution of Elements Found at Sulphur Bank Mine Area . 23 2. Bar Graph Showing Distribution of Elements Found at Syar Quarry-St. John's Mine A r e a ...... 33
3. Bar Graph Showing Distribution of Elements Found at Mt. Diablo Mine Area. . .42 4. Bar Graph Showing Distribution of Elements Found at Guadalupe Mine Area . . .51
5. Bar Graph Showing Distribution of Elements Found at Ambrose-Comstock Mine A r e a ...... 65
6. Bar Graph Showing Distribution of Elements Found at New Idria Mine Area . . .77 V
7. Bar Graph Showing Distribution of Elements Found at Rinconada Mine Area. . . 8 4
FIGURES Figure 1. Index Map of California Showing Location of Mine Areas Studied...... 3
2. Simplified Tectonic Map of California. . . .8 3. Location and Access Map of the Sulphur Bank Mine A r e a ...... 15
4. Photograph Showing the Contact Between Bleached and Boulder Zones...... 20
5. Location and Access Map of the St. John's Mine - Syar Quarry Area...... 26
6. Field Sketch Map of Geology with Sample Locations, Syar Quarry Area...... 29
7. Location and Access Map of the Mt. Diablo Mine A r e a ...... 36
8. Location and Access Map of the Guadalupe Mine A r e a ...... 45
9. Location and Access Map of the Ambrose- Comstock Mine Area and the New Idria Area..55
10. Geologic Map of the No.Stayton District. . 58 11. Photograph Showing Acicular Stibnite and Silicified Breccia at Vein Margin ...... 63
12. Geologic Map of the New Idria District . . 71
13. Location and Access Map of the Rinconada Mine A r e a ...... 80
14. Correlation Matrix and Dendrogram from a Cluster Analysis of 43 Soil Samples. . . 91
15. Correlation Matrix and Dendrogram from a Cluster Analysis of 21 Serpentinite Outcrop Samples...... 9 3
16. Correlation Matrix and Dendrogram from a Cluster Analysis of 72 Silica-Carbonate Rock Samples ...... 94 VI
17. Correlation Matrix and Dendrogram from a Cluster Analysis of 78 Sedimentary Rock Samples...... 95 18. Correlation Matrix and Dendrogram from a Cluster Analysis of 94 Igneous Rock S a m p l e s ...... 97
19. Correlation Matrix and Dendrogram from a Cluster Analysis of 47 Retort Tailing S a m p l e s ...... 9 8 20. Correlation Matrix and Dendrogram from a Cluster Analysis of All Rocks That Contained Either Au or Ag (46)...... 100 21. Correlation Matrix and Dendrogram from a Cluster Analysis of 80 Stream Sediment S a m p l e s ...... 101 22. Correlation Matrix and Dendrogram from a Cluster Analysis of 32 Knoxville Stream Sediment Samples...... 1°2 23. Correlation Matrix and Dendrogram from a Cluster Analysis of 51 Silica-Carbonate Rocks. (Samples were clustered on the basis of similarity between the elements in each sample)...... 105 24. Diagram Showing Gains and Losses by Weight of Principal Oxides During Alteration of Serpentinite to Silica-Carbonate Rock..107
TABLES
Table 1. Summary of Analytical Results ...... 87 2. The distribution of samples by rock type from each district, used to derive rock sample correlation matrices...... 89 3. Chemical analysis showing change from serpentinite to silica-carbonate rock at New Almaden...... 107 4. Elemental abundance in common rocks and rock forming minerals...... 118 ACKNOWLEDGEMENTS Vll This project evolved from work with Gulf Mineral Resources Co. (GMRC), assisting Mr. C. N. Upchurch on a gold reconnaissance exploration project in California.
The metals exploration manager of GMRC, Mr. T. Heidrick, was instrumental in securing funding for this project. I would like to acknowledge his assistance, and the help of GMRC's staff both in Denver and Reno. I would like to thank Mr. Upchurch for his many suggestions and for the stimulating and often argumentative discussions that surrounded them. Also, C. Bonds for her imaginative and creative drafting and D. Kennedy for her patience and cooperation in typing this manuscript. This study was undertaken under the chairmanship of Dr. L. C. Hsu and the supervision of Dr. L. T. Larson and Dr. H. LeMay of the University of Nevada, Reno. I would like to extend my thanks to these gentlemen and to all the other staff and fellow students who helped me during my period of residence at the above institution. I would especially like to thank W. Crone for his help with the
computer statistics. Finally, I would like to acknowledge the landowners that gave us permission to repeatedly go on their property.
Also, the -various laboratories for their help and cooperation with all the analytical work. Vlll
ABSTRACT
The geochemical relationship between mercury deposits and precious metal occurrences was studied in seven Calif ornia mercury mining districts. Four hundred thirty-four rock, stream sediment, soil and mercury retort tailings samples from seven districts were analysed for Au, Ag, Hg, Sb, As, and Te. The study confirmed that gold and silver are associated with the Coast Range mercury districts. The mineralization appears to be more strongly controlled by high angle faults rather than by rock type. Statistical studies have demonstrated that viable precious metal targets are more likely to be localized in volcanic and/or young marine sedimentary rocks (as opposed to silica-carbonate rocks). A strong element grouping between Au, Hg, Sb, and As was found to be characteristic of silica-carbonate rocks. A statistically unrelated suite was formed by Ag and Te which was characteristic of gold bearing volcanic and sedimentary marine rocks.
Those mercury districts localized about high angle faults in volcanic or marine sedimentary rocks are apparently more receptive to Au and Ag mineralization and should constitute the primary target hosts in precious metal prospecting. 1 INTRODUCTION The seventies have departed as the decade of disseminated gold mining, but Carlin, Pinson, Alligator Ridge and Jerritt Canyon will all continue to make significant impact for decades to come. Gulf Mineral Resources Co. (GMRC) is one of a number of companies now in "elephant country" looking for gold deposits in the California Coast Ranges. Preliminary geochemical sampling in old mercury mining districts high lighted a need for the company to re-evaluate these districts in terms of precious metal geology.
Purpose of Study The fundamental problem addressed in this work is how to find another gold deposit in the Coast Range Mercury Mining districts. Homestake Mining Company has found such a deposit (MacLaughlin) in Napa County, California. This led to the basic assumption that the gold deposits are genetically as well as geographically related to the mercury deposits. This study aims to evolve an understanding of the mercury deposits and to identify what if any of their geochemical characteristics can help to locate gold deposits. To do this, seven mercury districts were selected for geo chemical sampling. The samples were analysed for a suite of elements that are characteristic of high level mineral ization in hydrothermal systems. The elements are Au,Ag, Hg, Sb, As, Te, Tl, and Bi; Appendix 1 at the end of this report
is a brief discussion on the geochemistry of these elements. 2 A limited number (about 100) of analyses were performed for Tl and Bi on samples from all areas. These elements were not detected so their analyses were discontinued. The geology of these seven districts is evaluated through a literature study complemented by field studies. The interpretation of the geochemical analyses and the evaluation of the geology in light of current ore deposit theories constitute the main thrust of this work.
Selection of Areas of Study Figure 1 is an index map of California showing the location of the areas selected for study. This author has recognized three geologic settings for the larger mercury deposits: a) Those deposits that are hosted primarily in
silica-carbonate rocks. b) Those deposits that are hosted in rocks other than silica-carbonate rocks; that is in sedim
entary and/or volcanic rocks. c) Those deposits with significant mercury mineral
ization in both the silica-carbonate and the associated rocks. The Guadalupe Mine Area in Santa Clara County, the Mt. Diablo Mine Area in Contra Costa County, the Rinconada Mine Area in San Luis Obispo County, and the Comstock Mine Area in San Benito County are representative of the first category of deposits. The Ambrose Mine area of the North 3
124° 122° 120° I
1. Sulphur Banks Mine Area 2. Syar Quarry- St. Johns Mine Area 3. Mt. Diablo Mine Area 4. Guadalupe Mine Area 5. Ambrose-Comslock Mine Area 6. New Idria Mine Area 400-1 1. Rinconada Mine Area
\ SCALE i Sacram ento \
{f3 —N ------Son Francisco f* \ \ \ o 4 \ ' \ ®5 \ • Fresno 06 \
Pacific Ocean •7 • Bakersfield
— ^ •Los A n g l e s •*a O------
FIGURE I. Index map of California showing the location of the mine areas studied. 4 Stayton district, San Benito County and the Syar Quarry
Area in Solano County are representative of the second category of deposits. The Sulphur Bank Mine Area in Lake County is also representative of the second category of deposits. It was chosen for its value as an orientation study because the hot spring system associated with the deposit is still active. The St. John's Mine Area in Solano County and New Idria Mine Area in San Benito County are represen tative of the third category of deposits. (The Syar Quarry- St.John's Mine Area in Solano County are grouped together as one "Area" of study; similarly the Ambrose-Comstock Mine Area in San Benito County.) An additional factor which influenced the choice of areas was the availability of geologic literature and maps. Also an important consideration was the willingness of the property owners to grant access to their property.
Method of Study
During the summer of 1981 three months were spent with GMRC collecting geochemical samples. After sponsorship of
this project was approved an additional twenty days were spent doing field work specifically related to this project. The field work consisted of geochemical sampling of streams, soils, retort tailings and rock samples in and around mercury mineralized areas. A detailed description of how this was done can be found in Appendix 2 attached to 5 the back of this report.
The geology and field relations were the overriding dictates of where and how samples were taken. The avail able geology maps were field tested for accuracy and where necessary they were modified to fit the author's view of the existing geology. These modified maps showing the locations of the samples can be found at the back of this report (in pocket). A considerable amount of time was spent on trying to understand the local geology since it governs the inter pretation of the geochemical results. The samples were analysed for Au, Ag, Hb, Sb, As, and Te. A brief summary of the methods of analyses can be found in Appendix 3 at the back of this report. A large number of hand specimens representing the various rock types encountered were collected. Some of these were analysed by x-ray diffraction using a Norelco diffractometer. This was done mainly to establish the
sulphide mineralogies. A number of thin sections were also studied to first establish gross mineralogy and to establish paragenesis. These studies to a large extent complemented each other. The results of the geochemical analyses are first discussed within a district context. The data is presented in the form of bar charts. These are only intended to give the reader a visual impression of the elements present in each sample and also a "feel" for the relative 6 abundance of each element.
The samples are categorized by sample type (soil, stream, retort tails, and the various rock categories) and subjected to cluster analysis to develop correlation matrices and dendrograms. This work was done using the Fortran programs in Davis (1973); they were modified by W. Crone.
i 7
GEOLOGY OF AREAS OF STUDY
Coast Range Regional Geology General Geologic Setting The lithologic-tectonic framework of California developed principally during Mesozoic time when various segments of oceanic crust and island-arc were accreted to older sialic crust, resulting in the westward growth of the continent. The discrete tectonostratigraphic terrains that resulted from these events are often characterized by specific types of metallic mineral deposits in some. terrains (Albers, 1981). The westernmost group of rocks accreted to California during this time is the Franciscan assemblage, which forms much of the Coast Ranges. Ultra- mafic rocks, especially serpentinite and its alteration product/silica-carbonate rock, occupy suture zones separating various accreted terrains. The dominant characteristic of the Coast Ranges is its division into elongate topographic and lithologic strips underlain by discrete basement rocks that are separated by profound structural boundaries (see Figure 2). In the east, concealed beneath the Central Valley, is the enigmatic boundary between Sierra Nevada basement and the Coast Range Franciscan. Westward, the next major boundary is the San
Andreas fault zone, which separates Franciscan basement from the granitic-metamorphic basement of the Salinian block south of Monterey. The Nacimiento-Sur fault zone separates
Salinian rocks from more Franciscan basement to the south- 8
FIGURE 2 Simplified tectonic map of California showing major structural features and generalized location of mercury deposits within the Coast Ranges. (After Albers, 1981.) 9 west. Figure 2 is a simplified tectonic map of California after Albers (1981).
The Franciscan eugeosynclinal rocks have been described as a series, a formation, or an assemblage. Some portions have been called a melange, a tectonic unit produced by fragmenting and mixing of several rock types. Lithologically, the Franciscan is dominated by grayish green graywackes composed mainly of quartz and plagioclase feldspar in a chlorite-mica matrix. These rocks constitute some 90% of the voluminous Franciscan. They are thought to have been derived from the erosion of a volcanic highland and were then deposited in deep marine basins, usually by turbidity currents or submarine mud flows (Norris and Webb, 1975). Associated with the graywacke are locally extensive inter- bedded dark shales and occasional limestones, as well as rarer pelagic chert and shale. Within this array of sedim entary rocks are some altered submarine volcanics, now mostly greenstones,and metamorphic rocks including a blue glaucophane schist and a more common green chlorite schist (Albers, 1981). These Franciscan rocks have been intruded by ultrabasic igneous rocks, now serpentinites. In some cases the serpentinites have been injected as normal molten intrusives (Norris and Webb, 1975), but in other instances they occur in sill-like sheets that lack the thermal alteration of enclosing rocks. These plastic serpentinites have also been squeezed up through the overlying rocks as 10 plugs or diapirs (Bailey, et al, 1964). The prevailing view is that the serpentinites are altered masses derived from the upper mantle and transferred tectonically to the upper levels of the crust (Norris and Webb, 1975).
The Franciscan assemblage is bounded on the east by the coeval Great Valley sequence which consists of ophiolites at its base overlain by a thick succession of clastic sedimentary rocks. The presence of ultramafic complexes including serpentinites and dunites in the Franciscan, suggests that the sediments were deposited in a deep oceanic trench directly on mantle material or on a thin oceanic crust overlying the mantle (Norris and Webb, 1975). Albers (1981) says the ophiolite at the base of the Great Valley sequence is separated from the Franciscan assemblage by the Coast Range Thrust (approximate location is shown in Figure 2). This he claims, marks a paleosubduction zone extending most of the length of the Great Valley. Also shown in Figure 2 is the location of the silica-carbonate, mercury mineralized areas in relation to the Coast Range Thrust. Bailey, et al (1964) found that all mercury deposits in the. California Coast Range plot within 0.6 miles "tectonic thickness" of the Coast Range Thrust. Most of the mercury deposits of the Coast Ranges are associated with silica- carbonate rocks. Some of the serpentinites were probably part of the ophiolites forming the base of the Great Valley sequence. These were then carried over Franciscan assemblage 11 rocks on the Coast Range Thrust. However, some serpentinite bodies such as at New Almaden, are apparently in the form of plugs or diapirs squeezed up from below (Bailey, et al, 1964) .
Silica-carbonate rock is the term applied in the mercury mining districts of California to a rock derived from serpentinites by hydrothermal alteration. It is composed principally of silica (quartz, chalcedony, or opal)
and carbonates,primarily magnesite, with minor dolomite and/or calcite. It usually occurs as fault controlled elongate bodies within or adjacent to larger serpentinite bodies. Because it is harder and more resistant than the surrounding serpentinite it usually forms prominent out crops. These are colored tan or buff due to oxide staining, and have a characteristically pitted surface due to the carbonates being leached out. Other than the Franciscan formation, the only major
pre-Cenozoic sediments in the Coast Ranges are in the Great Valley sequence. This is an enormous thickness of miogeosynclinal late Jurassic to late Cretaceous shales, sandstones and conglomerates. The lower part of the Great Valley sequence is the late Jurassic, Knoxville formation, a dark shale (which partially hosts the McLaughlin gold
deposit). Volcanic rocks unconformably overlie or intrude the Franciscan assemblage and Great Valley sequences throughout 12 the Coast Ranges. In the Geysers (Clear Lake) Area, these are mainly lava flows and pyroclastics of the Sonoma
Volcanics. The dacites, basalts, and andesites of the Clear Lake volcanic series cover extensive areas of the Mayacamas Mountains (late Pliocene, early Pleistocene 2-4 m.y.a.).
In the Stayton District, the Quien Sabe Volcanics consist of post lower Miocene (less than 20 m.y. old) intrusions, flows and dikes of andesites, basalts, and rhyolites; some of these host Hg/Sb mineralization.
Mineralogy of Mercury Deposits The ore and gangue mineralogy of the mercury districts are fairly consistent except for local variations in relative abundance. The dominant ore minerals are cinnabar, metacinnabar and native mercury. The main sulphide gangue minerals are pyrite, marcasite and stibnite. Associated with the suplhides is silica as quartz, chalcedony and opal. Cinnabar (HgS, the red sulphide of mercury) is the most abundant mercury ore mineral in the areas studied.
Metacinnabar (HgS, the black sulphide of mercury) ranks second to cinnabar in abundance. According to Ross (1942), it was the principal ore mineral in the Mt. Diablo Mine. It was also important at the New Idria Mine (Linn, 1968).
Native mercury (Hg) is mentioned in most of the old mine reports. It contributed significantly to total production at the St. John's Mine, Vallejo District (personal commun ication, R. Azevedo, 1981). The only other known mercury 13 mineral in the study areas is the limited occurrence of tiemannite (HgSe) at New Idria (Learned, 1962).
Pyrite (FeS2) is by far the most common metallic mineral associated with mercury deposits. Geologic literature and the present investigation indicate its occurrence in 70% - 80% of all cinnabar deposits although its relative abundance varies widely from one deposit to another. Marcasite (FeS2) is also a common associate of the mercury deposits. Stibnite (Sb2S2) is found in most cinnabar deposits and all gradations of proportion of cinnabar to stibnite was encountered, especially in the
North Stayton District. In spite of the local differences between mercury districts, the gangue mineralogy is consistent. Silica (Si02 )., in the form of quartz, chalcedony and opal is the most common associate of the mercury ores. Ross (1942) says that chalcedony is probably the most abundant silica mineral in the U. S. mercury deposits; this is especially true of California deposits. The carbonate minerals are primarily magnesite with minor amounts of dolomite. Hydro carbons are widely distributed in, but are seldom abundant constituents of, the mercury deposits. Clay minerals are often found with those of the kaolinite group,most abundant as wall rock alteration products. 14
Sulphur Bank Mine Area Introduction
The historic Sulphur Bank Mine is at the end of the east arm of Clear Lake, Lake County, California, about three miles north of the town of Clear Lake Highlands (see Figure 3). It is in the bottom right- hand corner of the Clear Lake Oaks 7.5' quadrangle sheet (T. 13 N., R. 7 W.). The area is easily reached by paved roads and by serviceable mine roads within the study area. The author has not been able to locate the current owners of the mine property.
The Sulphur Bank Mine is on relatively flat ground, some 1400 feet above sea level, but only 0-40 feet above the current Clear Lake level. The area has been greatly disturbed and there is hardly any vegetation; dumps and bleached andesite extend over most of the area. Flat grasslands with scattered clumps of live oak trees lie adjacent to the mine workings and along the shores of the lake. No significant drainage has developed over the area; gulleys feed runoff either into the mine pits or directly into the lake. Since its discovery by John A. Veatch in 1856, Sulphur
Bank has been featured in the geologic literature. The three major, relatively recent treatises on this area are those of Everhart (1946), White (1955), and White and Roberson (1962). The latter paper draws heavily on the 15
FIGURE 3 Location and access map of Sulphur Banks Mine Area. 16 writings of LeConte and Rising (1882) , LeConte (1883) , Becker (1888, p:251-269), Forestner (1903), Ranson and Kellog (1939), Ross (1940), Veatch (1882), and Whitney
(1865) . (See Bibliography in White and Roberson. (1962) for abovementioned references.) The work by Everhart was part of the U.S. Geological Survey program of strategic-mineral investigations.
The Sulphur Bank Mine was first mined in 1865 for the native sulphur deposited near the surface. The mercury boom of the 187 0's soon converted the cinnabar, which was a contaminant of the native sulphur, into a valuable asset;
it was first mined in 1873. The mine was worked almost continuously until 1905, when working conditions forced the closure of underground operations. "Work in the mine in those early days was described as being extremely unpleasant, owing to heat, acrid dust, and suffocating gases. Apparently almost all the mining was performed by Chinese labor, because white men refused employment", Everhart (1946). The period between 1873 and 1885 were the boom production days, averaging 10,500 flasks per year (one flask equals 76 pounds). The mine was reopened in 1927 when open pitting commenced and production was maintained until 1947. The
last period of significant production was 1955-1957. Up to the end of 1944, Sulphur Bank had produced 126,285 flasks of mercury. 17 Regional Geology
Rocks exposed in the Sulphur Bank Mine area and surrounding country range from Jurassic to Recent. The oldest unit is Franciscan intercalated sandstones and shales. These are overlain by units of ancestral Clear Lake sediments which are in turn overlain by augitic andesite lava flows. These are all followed by modern Clear Lake sediments, locally derived, post-andesite
clastic sediments and landslide material (see Plate I in pocket).
The rocks are cut by two major sets of faults which mainly affect the Franciscan rocks. The main fault zone trends about N. 70° E. and dips from 50° SE to near vertical.
It is intersected by a set of northwest striking faults, dipping steeply northeast. These originated before the Recent epoch but some Recent recurrent movement has caused small displacements and flexures in the andesite flow.
Plate I is a composite map of the geology of the area; it is based mainly on Everhart (1946) but there are some modi fications by this author based on field observations.
Stratigraphy
Franciscan Group. The rocks of the Franciscan formation
consist of a dark green massive graywacke, which is slightly
contorted and metamorphosed; it contains intercalcated but often discontinuous units of slightly metamorphosed shales.
The graywacke consists principally of grains of quartz, 18 plagioclase, lithic fragments and degraded biotite and illite. The black shales contain a similar group of
minerals. These units commonly contain lenses and vein- lets of quartz and calcite. Where the formation crops out at the surface as in the western part of the mine area, the upper 5-10 feet is bleached containing abundant clays. A small unit of fault gouge and breccia material was mapped by Everhart (1946). This unit was not discernible in the field except possibly as a zone of more distorted shaley material containing abundant quartz veinlets. Lake Sediments. The lake sediments consist of poorly bedded conglomerate and breccia which consist of sandstone and shale derived from the Franciscan formation, in a
shaley or sandy matrix (White and Roberson, 1962). The sediments are light gray to greenish gray in the lower parts of the workings but are bleached light gray or white near the surface. According to White and Roberson (1962), the unit is 200 feet thick in the south-central part of the mine area but thins abruptly to the east, north and west. Radio-carbon age determination on carbonized wood fragments (found in the sediments) suggests it should be considered late Pleistocene, perhaps on the order of
50,000 years old. Andesite Lava Flows. An andesite lava overlies the lake
sediment and covers more than half of the map area shown in Plate I. This andesite has been subjected to exterisive 19 alteration; and three distinct horizons have been recognized. The upper or bleached zone is white and consists predom inantly of opalized andesite; it was formed by sulphuric acid attack. It is in both sharp and gradational contact
with the underlying,intermediate boulder zone. This horizon is characterized by "boulders" of relatively fresh, dark andesite and opalized gray or white rocks, surrounded by spheroidal or exfoliation shells (White and Roberson, 1962). The contact of these two units is beautifully exposed, as can be seen in Figure 4. The basal zone of the andesite shows columnar jointing with localized alteration adjacent to joints, in an otherwise fairly fresh rock. Associated with the andesite in some localities is a banded vessicular andesite or basalt breccia which is thought to represent the upper units of the flow.
Post Andesite Sediments. The andesite flow is locally overlain by sands and gravels of possible lacustrine origin. These units occur as bedded reddish-brown clasts varying in size from granules to fine clay. They include clasts of phyllitic (Franciscan) rock as well as rounded pebbles of milky quartz. These units are fairly recent depositional features; landslide debris and mine waste form an extensive blanket covering most of the southern portion of the map area.
Mineralization The ore mineral at the Sulphur Bank Mine is cinnabar; 20
Figure 4. Photograph showing the contact between the Bleached and Boulder Zone Andesites.
0 10 1______I feet
1 = Boulder Zone 2 = Bleached Zone 21 however, some metacinnabar was identified by x-ray diffraction (White and Roberson, 1962). All the ore deposits were restricted to depths near and below the inferred position of the water table prior to mining. As stated earlier, the mine was first put into production for native sulphur which was present mainly in the upper bleached zone. The cinnabar content increased with depth and the highest grade ore was found as veins and disseminated masses in the lower sections of the andesite and in the lake sediments below their contact (White, et al, 1962). The minerals associated with the cinnabar included marcasite, pyrite, metacinnabar, stibnite, montmorillonite dolomite, calcite and rarely a little opal (White, et al, 1962). Metacinnabar commonly occurs as euhedral crystals, but it has been identified as coatings on pebbles previously rounded by agitation in spring pools, in an area devoid of nearby cinnabar. White and Roberson (1962) say this is one of the indications that the active Sulphur Bank Hot Spring system continues to transport and deposit mercury. Pyrite and marcasite are found in lake sediments and in the other hydrothermally affected rocks. They probably formed by combination of sulphur supplied by the thermal waters with iron derived from local rocks. Stibnite commonly occurs as clusters of hairlike needles. It is often deposited on metacinnabar which in turn is usually deposited on pyrite, marcasite or cinnabar (White, et dl, 22 1962). Carbonate minerals are generally scarce with dolomite and some calcite found in rocks below the andesite.
Travertine and aragonite are thought to be actively precipi tating from the carbonated waters discharging on the shores of Clear Lake. Quartz is very abundant as a detrital mineral in the lake sediments and in the Franciscan rocks. Most of the vein quartz is pre-Quaternary and unrelated to
the present hydrothermal activity. The most abundant form of opal is a porous residue of the leaching of the andesite by I^SO^ in the bleached and boulder zones. Montmorillonite along with kaolinite and some alunite has replaced andesite in the boulder and basal zones (White, et al, 1962).
Hydrocarbons were not observed by this author but according to White, et al (1962), they were associated with the ore.
Results of Sampling
This area represents a well documented and active geothermal system that deposited considerable quantities of cinnabar. Samples were taken from rocks exposed in the eastern proximity of the pit, as well as from dumps and runoff (see Plate I). The relative abundance of the elements analysed for each sample is shown in the chart of Graph 1. All but one of the samples of dump material (6007 and 6008), runoff from dumps (6009) or retort tailings (6011 and 6012) contained trace amounts (at detection limit of 0.05 ppm gold). Some of these samples also have detec tion limits (0.1 ppm) values of silver. There is widespread mercury mineralization, with a range of values from ELEMENTS IN PPM R OCK OE Al ape ae lte a cl indicated at plottedscale are samples All NOTE- AE / DATE: BY: BY: a rp Soig itiuin of Distribution Showing Graph Bar n ok tem eot Tailings Retort Stream, Rock, in o : s e p y T k c o R to H. N.Shields ape ta g of cl hv number have scale off go that samples heightof vertical maximum a to f p witn n prpit bars. appropriate in written ppm of T« □ □ Hg □ st> □ □ Au □ Serpentinite t i n i t n e p r e S 4 = 3 Sedi y r a t n e im d e S = 2 = lca- e t a n o b r a -C a ilic S = 1 = ac 1982 March upu Bns ie Area Mine Banks Sulphur Ag lmn scale Element s A c i n a c l o v / s u o e n g I Au, Ag, Hg, Sb,As,Te Hg, Ag, Au, t ol Samples Soil 8t lerlk District Cl ear lake ( l" =.5 l" ppm) ( (l" (l" s 10 s (l" ppm) (l" 1.0=(l" ppm) (l" -100ppm) (l" (l"= 1.0 ppm) Lake County,Lake = California rp No.1 Graph 100 ppm) RG NO.DRWG. 2
DetectionLimit 0.05ppm
.005 ppm
5 0 ppm 0 5 .01 ppm 0.1 ppm 0 ppm 10
IE NO. FILE
23
24 0.94 to 300 ppm, and a mean sample value of 78 ppm.
Although stibnite is reported in the literature on the area only 23% of the samples contained antimony, ranging from 5.0 to 77 ppm with a mean of 41 ppm. Arsenic is present in 14% of the samples taken with values ranging from 11 to 54 ppm, with a mean of 26 ppm. Tellurium was detected (detection limit .001) in 56% of the samples at an average value of 0.115 ppm. Almost all the tellurium values occurred in samples of Franciscan assemblage sedimentary rocks. 25 Syar Quarry - St.John's Mine Area Introduction
Although there is no officially recognised Vallejo Mercury Mining District, Weaver (1949) described the
Quicksilver Deposits of the Sulphur Springs Mountain Area". The St. John's Mine was the major mercury producer of this area* The other part of this district is centered around the currently operating Syar Quarry, 2% miles southeast of the St. John's Mine. The Syar Quarry extends into ground previously mined for mercury.
The St. John's Mine is in section 33 (T.4 N., R.3 W.) of the Cordelia 7.5' quadrangle, Solano County. It is approximately two miles northeast of Vallejo (See Figure 5). The Syar roadstone and crushed rock quarry is in section 10 (T. 4 N., R.3 W.) of the Benicia 7.5' quadrangle. This readily accessible area is one mile east of Vallejo (see Figure 5). The quarry is owned and operated by Jim Syar; his access permission is needed since the old mercury mine areas can only be reached via quarry roads. The St. John's Mine - Syar Quarry is on Sulphur Springs Mountain which attains a maximum height of 957 feet. The area consists of gently rolling hills with grassy slopes and occasional clumps of live oak growth in the higher ground. The drainage in the area is dominated by streams flowing parallel to the northwesterly ridges; a small transverse stream flows west from the St. John's Mine area. FIGURE 5 Location and access map of St. Johns Mine Area— Syo r Quarry Area. 27 This mercury mining area has not been subjected to an economic geology study. The only reference found is a brief mention of the St. John's Mine in "Geology of the Coast Ranges North of the San Francisco Bay Region, California" by C. E. Weaver (1949).
The first discoveries of mercury ore in the St. John's Mine area were made in 1852; by 1880, when the St. John's
Mine was forced to close, it had produced 11,530 flasks of mercury (Weaver, 1949). Intermittent production between 1899-1908, 1914, 1916, 1923, and 1936, brought total production closer to 20,000 flasks.
Regional Geology
This area is conspicuously different from the other_ studies because it lacks large outcrops of Franciscan sedimentary rocks (see Plate II). The dominant sedimentary rocks are shales and sandstones of the undifferentiated lower Cretaceous/upper Jurassic, Knoxville and Horsetown formations. These sediments are intruded by a series of shallow Tertiary andesites, the Sulphur Springs Mountain
Andesite. In the southeast corner of the map area and especially in the Syar Quarry area, Weaver (1949) mapped a hornblendite unit adjacent to. Knoxville sediments. This appears to be incorrect since most of the Syar Quarry area is in andesite and basaltic-andesite, apparently capped by a silicified fragmental volcanic and associated tuffs (C. Upchurch, 1981, personal communication). Figure 6 is a 28 field sketch of the observed geology in the northern quarry
There are extensive outcroppings of serpentinite in the St. John s Mine vicinity. Some of these are altered
to silica-carbonate rocks and host mercury mineralization. These rocks occur in a horst-like fault block which trends N. 20 W. and are in fault contact with the Knoxville sedimentary rocks (see Plate II).
Stratigraphy
The Knoxville formation is extensive in the northern Coast Ranges; it is predominantly a dark shale with inter calated thin graywacke beds. The basal units of the Knox ville contain basaltic pillow lavas, breccia and basaltic sandstone; suggesting an affinity with some of the Franciscan. Where this rock is in contact with the Sulphur Springs andesite it shows contact metamorphism forming cherty quartz with associated small ill-formed dolomite grains. The Sulphur Springs Mountain andesite intrudes both the Franciscan serpentinites and the shales and sandstones of the Knoxville formation. The rock is fine grained and is pink to reddish-buff in color. It consists of pheno- crysts of plagioclase imbedded in a groundmass of lathshaped plagioclase microlites. The rock has been subjected to intense alteration and has chill margins developed where it contacted the sedimentary rocks. Extensive outcrops of fault bounded serpentinite 29 30 occur in a narrow northwest trending zone which extends south of the Syar Quarry (see Plate 2). The serpentinite was only accessible for study near the St. John's Mine road; here they are mostly altered to silica—carbonate rock. The serpentinites in this area are not in any way distinctive. They consist essentially of bluish green, and sheared serpentine with chrysotile asbestos veinlets.
The silica-carbonate rock is dark gray to black containing some visible cinnabar and disseminated pyrite.
In thin section most of the rock is replaced by fine grained magnesite clouded with iron oxides, or by magnesite grains embedded in cryptocrystalline chert.
Figure 6 is a field sketch of a local fragmental volcanic which was not shown on Weaver's (1949) map. The original rock was a coarse, fragmental (non-welded) volcanic with fragments of rhyolite, basalts and andesites jumbled together. The rocks have undergone intense pervasive sili- cification with dense cherty quartz replacing glass fragments. Hand specimens from sample locality 3164 (see Figure 6) are of a chertstone, a water laid vitric tuff (fine ash) that was converted to cryptocrystalline chert sometime after deposition
(C. Upchurch, 1981, personal communication). The units are pervasively and locally intensely altered to mostly kaolinite.
Mineralization The mercury ore bodies commonly occur as stockworks within areas of complex faulting and are composed of 31 irregular masses of breeciated rock cemented with cinnabar and gangue (Weaver, 1949). Wherever the Knoxville shale in in contact with the silica-carbonate rock the former is breeciated and silicified. The ore at the St. John's
Mine is in fault zones between silica-carbonate rocks and the Knoxville sediments. It occurs as disseminations or
pockets among fractures within the silica-carbonate rocks; also, as disseminations in Knoxville sediments and as carbonate replacement in fractured andesite bodies (Weaver, 1949).
A large number of samples of the silicified fragmental volcanic were taken at Syar Quarry (see cluster of samples
in the lower right-hand corner of Figure 6). Visible mercury mineralizaiton was confined to the rocks represented by samples 6191 and 6192. Here cinnabar occurred as small disseminated crystals in a dark, dense silica matrix. Rock
quarrying has consumed several mine adits (C. Upchurch, 1981, personal communication) including the mouth of the adit shown in Figure 6. This adit was apparently drifted along the fault currently exposed in the back of the adit; no mercury mineralization was observed.
Results of Sampling A total of ninety geochemical samples consisting predominantly of rock (with a limited number of soil and stream sediment) samples were taken in this district. The 32 results of the geochemical analyses are displayed in Graph
2; Plate 2 and Figure 6 are geology and sample location maps.
Only two samples containing trace amounts (0.05 ppm Au) were found in the heavily sampled silicified volcanic breccia (shown in Figure 6). Silver mineralization (0.1 - 0.4 ppm) was found in the volcanic breccia, the altered volcanics associated with it, and even in the apparently unmineralized andesites adjacent to the above mentioned rocks. At the St. John's Mine Area the shales and sandstones contained 0.1 - 0.2 ppm silver. Sample 6226 was taken some 1000 feet from any known mineralization; this shale contained detectable (0.1 ppm) silver. (This could have been caused by sample contamination or by a poor analysis.) Mercury was detected in all samples analysed, ranging in values from 0.01 to 600 ppm with a mean of 8 ppm. Arsenic was detected in 79% of the samples, with a mean assay value of 31 ppm (range from 5 to 160 ppm). Antimony was detected in 30% of the samples and it had a range of values from 1 to 200 ppm, with a mean value of 8 ppm. The higher levels of mercury mineralization were found in the rocks from the St. John's Mine Area; these were predominantly Knoxville sedimentary and silica-carbonate rocks. The mercury is associated with relatively low level antimony.
The silicified volcanics from the Syar Quarry Area were found to have less mercury mineralization than the St. 2 3 - Element scale protection Lir> 't
Au ( l “ - .5 ppm) 0.05 ppm □ 0.1 ppm Ag (l“ = 1.0 ppm) □ .01 ppm | Hg ( l" = 10 ppm) □ | Sb ( l" = 100 ppm 1.0 ppm □
| A s 0" = 100 ppm) 5.0 ppm c (l" = .5 ppm) 0.2 ppm c 1 T e
K e y to R o c k T y p e s:
1 = Silica-Carbonate 2 = Sedimentary 3 = Serpentinite 4 = Igneous/volcanic
NOTE- All samples are plotted at scale indicated to a maximum vertical height of 2 ; samples that go off scale have number of ppm written in appropriate bars.
Bar Graph Showing Distribution of
° § E S Au, Ag, Hg, Sb, As,Te in Rock,Stream, Retort Tailings & Soil Samples ~ b _ Syar Quarry-St. Johns Mine Area Vallejo District Solano County, California n » « » k ; oi o — CVJ K> 5 K O) O — O) O to ^ io to k oo o> o~Mir)
The Mt. Diablo Mine Area of this study includes the Ryne Mine as well as the Mt. Diablo Mine. The area is in section 22 (T. IN., R. 1 E.) of the Clayton 7.5' quadrangle, Contra Costa County. The Mine Area is forty miles east-northeast of San Francisco and approximately five miles southeast of the small township of Clayton (see Figure 7).
The topography of the district is dominated by Mt. Diablo, 3849 feet above sea level. The drainage consists of a circular pattern of streams draining off the higher ground about Mt. Diablo and North Peak. Dense brush and forest cover much of the north facing slopes but the south facing slopes are grass covered.
For this study the work of E. H. Pampeyan (1963), Special Report 80, California Division of Mines and Geology, is the main source of reference. However, there have been a number of geologic studies of the area that are referenced in Pampeyan (1963). The most noteworthy studies are by Whitney(1865), Turner (1891,1898), Taff
(1935), and Ross (1940). Sometime between 1863 and 1864, cinnabar was discovered on the northeast slope of the North Peak, near the current site of the Ryne Mine. The total production up until 1956 is estimated at $1.5 million (amount of Hg not known). Claims for silver were located on the northwest side of FIGURE 7 Location and access mop of Mt. Diablo Mine Area.
i 37 Mount Zion, and copper ores containing traces of gold were discovered on the slopes on Eagle Peak and Black Point (all are within the Mt. Diablo district).
Regional Geology
The Diablo Range is made up of a number of folds lying in echelon; Mt. Diablo is on the north end of the range sitting on the crest of an anticline. The rocks in the vicinity of Mt. Diablo consist of a basement of jumbled and broken igneous; sedimentary rocks are overlain by a sequence of felsic volcanics (Pampeyan, 1963).
The Franciscan basement rocks make up the main mass of Mt. Diablo and are in fault contact with the surrounding younger sedimentary rocks. The Franciscan sediments are divided into two parts by a narrow northeast trending band of serpentinite. This serpentinite has been hydrothermally altered to silica-carbonate rock which host the local mercury mineralization. Plate 3 (in pocket) is a composite geologic map of the area (after Pampeyan, 1963).
Stratigraphy Franciscan Rocks. Rocks of the Franciscan formation underlie the main peaks of Mt. Diablo and North Peak. These rocks have been stratigraphically dated as no younger than Late Jurassic. They consist of thin bedded cherts and graywackes with minor interlaminated shales. They are highly fractured and are crisscrossed by numerous veinlets 38 of quartz and carbonates. The chert horizons are grayish- red to pale green, mostly thin bedded with intercalated shales. The graywacke is an indurated, dark, poorly sorted and dirty sandstone. Typically, it is fine to medium grained and shows little evidence of bedding. It is cut by numerous veinlets of quartz and carbonate. The principal constituents of the grains are quartz, feldspar, chert and dark volcanic rock fragments.
The serpentinites found in the other areas studied are part of the Jurassic Franciscan assemblage. However, Pampeyan (1963) claims that "the serpentinite is certainly younger than the Franciscan rocks that it caps; it may be older than the Knoxville formation and the rocks of early Cretaceous age." The serpentinite is pale green to greenish black on fresh surfaces and weathers to a grayish orange.
In fresh exposures it consists of partly serpentinized peridotite blocks in a matrix of highly foliated serpentine. The serpentine is broken into countless fragments with shiny curved surfaces. The original igneous textures have been completely destroyed. The silica-carbonate rocks are composed of varying amounts of quartz, opaline silica, magnesite and other carbonates. It varies widely in texture, some of it consisting of paper thin laminae while some is dense, massive and irregularly banded. In thin section the sheared or foliated serpentine is replaced by quartz which often 39 preserves the serpentinite textures.
Lower Cretaceous Mudstone and Sandstone. These rocks are exposed in a narrow stream section in the northwest of the study area (see Plate :3. ). They consist of alternating thin beds of shaley mudstones and arkosic sandstones with distinct bedding planes.
Landslide Travertine and Recent Deposits. Landslide deposits of considerable areal extent overlie the previously described formations (see Plate 3 ). They vary from a few inches up to 35 feet or more in thickness and consist principally of poorly sorted angular fragments, ranging from clay size particles to blocks up to 30 feet in diamenter, of Franciscan rocks and serpentinite. A travertine deposit approximately
500 feet east of the Mt. Diablo Mine overlies the Quaternary landslides and alluvium. Although it is now mostly covered with waste from the mine, it originally measured 600 by 200 feet (Pampeyan, 1963). The rock is yellowish-gray, porous and layered parallel to the gentle slopes on which it lies.
A small stream flows over the travertine deposit but the source of the water is obscured by the cover of mine waste. Recent alluvial deposits cover the lower slopes and creek bottoms. They are composed of poorly sorted clay to cobble size fragments of rocks currently undergoing erosion.
Mineralization Mt. Diablo and Ryne Mines are both in a lenticular body of fault bounded silica—carbonate rock (see Plate 3_). 40 A Quaternary landslide covers the central area between the two mined bodies and the silica—carbonate rock may be continuous below this cover (Pampeyan, 1963) .
The ore in the Mt. Diablo Mine area consists of meta cinnabar and cinnabar in almost equal amounts. The best ore occurred in fractured and brecciated silica-carbonate rock along the footwall of the mineralized zone, usually associated with pods of massive iron sulphides (Pampeyan, 1963). In some ore specimens the black metacinnabar was masking the red color of cinnabar; this was probably due to the meta-stable metacinnabar reverting to cinnabar (Pampeyan, 19 63) . A number of stibnite-bearing quartz veins cut the mercury mineralized rocks. These probably represent the latest stage of sulphide mineralization. The most abundant iron sulphide present was marcasite; pyrite did occur as disseminations and late stage void fillings. The other gangue minerals were quartz and minor amounts of dolomite and calcite and scattered blebs of hydrocarbons. The Ryne Mine, 1800 feet northwest of the Mt. Diablo Mine, is probably on the same body of silica-carbonate rock.
Here cinnabar is the dominant mercury sulphide with meta cinnabar found only in minor amounts; the latter occurs both in the silica—carbonate rocks and in fractures in the
Franciscan sedimentary rocks (Pampeyan, 1963). Marcasite is the dominant iron sulphide and it is associated with quartz, calcite and dolomite. 41 Results of Sampling.
A total of sixty geochemical samples were collected in this area. They consisted predominantly of rock samples with a limited number of retort tailings, soil and stream sediment samples. The result of the analyses of these samples is shown in Graph 3. Plate .. 3_ is a geologic and sample location map (for convenience, the underground workings at the Ryne Mine are projected to the surface and the relative locations of the samples plotted on it) .
Precious metal mineralization was detected in 17% of the samples; there were nine gold values (average = 0.144 ppm) and one silver value at 0.6 ppm. Three of the gold values were obtained from silica-carbonate rock samples at the Mt. Diablo Mine. The remaining gold values were from retort tailing materials. (Silica-carbonate rocks hosted almost all the mercury ore in this district, thus we would expect it to be the major rock type represented by the end product of processed ore.) The one silver value is from the Ryne Mine underground workings,again from silica- carbonate rocks. The Mt. Diablo Mine Area is highly anomalous in antimony and arsenic. Antimony was detected in almost all samples, with values ranging from 1 to 10800 ppm and an average value of 711 ppm. Arsenic was detected in all samples at an average value of 176 ppm. The bulk of rock samples were of silica-carbonate rocks which were V E L P M A S O S NO. ELEMENTS IN PPM L L
2 «o i 2 00 «o 1 2 00 <0 • 2 00 N. 3 2 QD 00 2 2 00 o> 3 2 (T) o 2 2 o> i CM 2 a> I 2 to 1 2 O • o o 2 to O to ' o IO tO CM 1 lf> o to tO 1 o to tO M- 1 to o VO tO 1 to 1 ROCK OD 00 to 1 4 CO 0> K> O to 2 K to 1 to CM N- 1 to K to 4 K ^ K ^ K » « (O»« I 4 2 J 4 I I I I 1 4 1 1 N available - CD N- A results As o K CD ® K
0 D 0 00 00 CD 00
______; ll D etectionLimit
.5 ppm 0.05
. ppm0.1 .01 ppm 5.0 ppm5.0 0.2 ppm0.2 1.0 ppm
IE NO. FILE
L 1
often crisscrossed by numerous late quartz-dolomite veins.
Both the veins and the silica-carbonate rock contained disseminated acicular growths of stibnite; no arsenic mineral was detected.
Mercury was detected in all the samples analysed;
the high assays were from silica-carbonate rocks. The values ranged from 0.01 to 844 ppm, with an average value
of 69 ppm. Three of the Au bearing samples (3031, 3032,
and 1438) contain surprisingly low Hg values (an average of 1.03 ppm); the significance of this is not known. Tellurium only occurs in 13% of the Mt. Diablo
samples; however, the average value of 0.85 ppm is the
highest value for all the districts. 44 Guadalupe Mine Area
Introduction
The New Almaden District has yielded nearly 40% of the mercury produced in the United States. Bailey and
Everhart (1964) regarded an 80 square mile area extending across the hills containing the mines to the crest of
the Coast Ranges, as the New Almaden District. This study is restricted to the Guadalupe Mine Area in the northwesterly portion of that district. It is located in section 30,
(T. 8 S., R. 1 E.) of the Los Gatos 7.5' quadrangle, Santa Clara County. The mine area is four miles west of
Los Gatos and is accessible by paved roads (see Figure 8). The Guadalupe Mine area is currently owned by Jim Zanardi, who operates an earth-fill garbage dump at the site. The topography is dominated by the Los Capitancillos
Ridge with elevations ranging from 800 feet in the north west to 1786 feet in the southeast. The moderately rugged hills are cut by the transverse Guadalupe, Alamitos and Llagas Creeks, all of which flow into the Santa Clara Valley. Grasslands are extensive, but areas over 1700 feet are blanketed with "Chaparral", a dense, head-high intergrowth of shrubs and small trees. Despite the prominence of the New Alamaden district as a mercury producer, little has been published about the geology of the mines or the district. This study is based on the work of Bailey and Everhart (1964). Their work is 45
FIGURE 7. Locotion and access map of Guadelupe Mine Area. 46 based on the data accumulated over six years by workers of the U.S.G.S. and the California Division of Mines.
The production of mercury from the New Almaden
district, since the recognition of mercury ores in 1845, is 1,137,727 flasks, with a value of about $55 million. The ore deposits of the Guadalupe Mine area were first recognised by Josiah Belden who encountered cinnabar-painted Indians along Guadalupe Creek in 1846. Total production
from the Guadalupe Mine to the end of 1947 exceeds 112,600 flasks, placing Guadalupe Mine sixth among the California mercury mines.
Regional Geology
The rocks of the New Almaden district range in age
from late Jurassic to Recent. The oldest assemblage is the basal Franciscan group which includes graywacke, silt- stone, mafic volcanic rocks, cherts, and minor amounts of metamorphic rocks. Intrusive into the Franciscan are many
tabular bodies of serpentinite, some of which have been hydrothermally altered to silica-carbonate rocks. The Quaternary sediments consist of upper Miocene sandstones and shales with some included felsic volcanic material.
Some steep fractures trending north to northeast generally extend into the silica-carbonate bodies. These are widest close to their contact with the Franciscan assemblage rocks. Many younger faults following old shear zones indicate recurrent movement along these zones (Bailey and Everhart, 1964). Stratigraphy
Franciscan Group. The most abundant rock type in the
district is Franciscan arkosic graywacke. Characteristic features of this graywacke are its dirty appearance and its high feldspar content. The proportion of feldspar normally exceeds the amount of quartz. Grains of quartz or feldspar are generally angular but co-existing rock
fragments (usually of greenstone) are somewhat rounded.
Thin section of the graywacke reveals angular grains of sodic plagioclase, quartz and rock fragments in a fine grained matrix of the same materials.
A variety of mafic volcanic rocks and associated pyroclastics are interbedded with the sedimentary
Franciscan rocks. These rocks are highly altered and chloritized and are grouped under the general heading of "greenstones". In many of the greenstones alteration has formed minerals of the epidote group from pyroxenes. Hornblende schists and similar but less schistose amphibolites occur in the vicinity of the Guadalupe Mine (Bailey and Everhart, 1964). Their field relation ships are commonly obscured by extensive weathering. Typically, the rock is dark green or almost black, some times containing albitic, light areas elongated parallel to schistosity. The principal minerals found in thin section include hornblende, epidote, chlorite and albite 43 (Bailey and Everhart, 1964).
Outcrops of serpentinite, although not extensive in the area, are fairly important since they are locally altered to silica-carbonate rocks. Normally one finds an intensely sheared, foliated and shiny serpentinite that ranges in color from white through light to moderately deep green in peripheral areas of the serpentinite bodies. In the core areas there is a more blocky serpentinite which contains massive blocks of dark green to black unsheared serpentinite which has a pseudoporphyritic
texture. The serpentinite blocks are contained in an envelope of highly sheared shale or siltstone which is locally called "alta". The alta is similar to myIonite gouge and owes its properties to shearing and to its original lithology. The silica-carbonate rocks host most of the mercury mineralization in this district. The more silica rich
silica-carbonate rocks which contain few carbonate veinlets are usually unfavorable for ore deposition
(Bailey and Everhart 1964). A carbonate-rich variety which contains mostly dolomite with quartz-carbonate veinlets normally contains higher grade mercury mineraliza tion (Bailey and Everhart 1964). Quaternary alluvium covers the floor of Santa Clara
Valley, and narrow fingers extend for several miles up the larger tributary canyons. Terrace gravels perched on terraces as much as 100 feet above the canyon floor have 49 been mapped in Guadalupe Canyon. Most of the landslides
in the area are actually rock slides consisting chiefly
of bedrock, talus and soil.
Mineralization
The mineralogy of the mercury ore in the Guadalupe
Mine area is fairly straightforward. The ore mineral is cinnabar which occurs abundantly as fine grained to micro
crystalline aggregates replacing silica-carbonate rock.
Metacinnabar, the black tetrahedral mercuric sulphide was
not found, but is documented in the district (Bailey and Everhart, 1964). These workers also documented the presence of native mercury in vugs and fractures in the silica- carbonate rocks; it is almost invariably associated with cinnabar. Tiemannte, the gray mercuric selenide, is also reported by Bailey and Everhart (1964). A number of base metal and other sulphides are associated with the mercury ore. Pyrite is the most abundant and pervasive occurring as disseminations and
in veinlets. Minute needles of stibnite (Sb2S2) occur in banded dolomite veins which also contain quartz, pyrite, and cinnabar. "Where both stibnite and cinnabar occur in a single vein, they alternate in thin layers but are not intermingled; although both minerals were deposited in the same period of mineralization, they are not strictly contemporaneous",(Bailey and Everhart, 1964). Sphalerite was found by these same authors in a breccia vein on the 50 800' mine level/ where it formed a coating on quartz.
Associated with sphalerite in this breccia were chalco- pyrite and galena. The breccia dike is younger than the
mercury ore which it cuts; it did not contain cinnabar.
The main associated gangue minerals are dolomite,
magnesite and quartz. The dolomite occurred as narrow vein fillings associated with quartz and cinnabar, and as
later post mineral dolomite, quartz and pyrite veinlets.
Magnesite is the dominant "replacement" carbonate in the
silica-carbonate rocks. Quartz, though not so abundant as carbonates, is widely distributed as the original constituent of the silica-carbonate rock. It also occurs with cinnabar during the ore stage and to a limited extent,
'in post mineralization veins.
Results of Sampling Sixty-five geochemical samples almost evenly distributed
among rock, retort tailings, soil and stream sediments
(see Graph 4) were taken in this area. These samples were analysed for the usual suite of elements (Au, Ag, Hg, Sb,
As and Te) . The results of the analyses are displayed in
Graph 4; the location of the samples is shown on the geologic map of Plate 4 (in pocket). Twenty-two samples (representing all the different
sample categories above) contained detectable gold; the values ranged from 0.05 to 0.3 ppm with an average value of 0.105 ppm. Samples 3061, 3063, and 3065 were of silica- 51
Element scale Detection Limit
[~| Au (l" = .5ppm) 0.05 ppm
| I Ag a" = l.Oppm) 0.1 ppm
| | Hg (l" = 10ppm) .01 ppm
| | Sb ( I" = 100 ppm) 1.0 ppm
As (l"=IOOppm) 5.0 ppm
| | Te ( l" = 1.0 ppm) 0.2 ppm
Key to Rock Types:
1 = Silica- Carbonate 5 2 = Sedimentary Q. CL 3 = Serpeniinite 4 = Igneous/volcanic \-(/) z 5LU UJ _i lli NOTE: All samples are plotted at scale indicated to a maximum vertical height of Z" ; samples that go off scale have number of ppm written in appropriate bars.
Bar Graph Showing Distribution of Au, Ag, Hg, Sb, As, Te in Rock,Stream, Retort Tailings S Soil Samples Guadalupe Mine Area New Almaden District Santa Clara County, California SAMPLE C\] o — to to CVI ro ro 00
Hg (Bailey and Everhart, 1964). It was almost invariably mined with the silica-carbonate rock as mercury ore. The rock samples 3098 and 3102 were of mine dump material
(consisting of silica-carbonate rock and alta); they contained gold. The presence of gold in the alta-silica- carbonate rock is also confirmed by the gold present in the retort tailings samples. It seems as though the mineralizing fluids may have been trapped by the impermiable alta layer with consequent precipitation of gold in the alta and the rocks immediately below it. Mercury, antimony and arsenic were detected in virtually all samples; the retort tailings contained the higher values.
Mercury ranged in value from 0.06 to 280 ppm with an average value of only 22 ppm (the lowest value in the silica- carbonate mineralized areas). Antimony ranged in value from 1 to 377 ppm, with an average value of 59 ppm.
Arsenic values ranged from 5 to 350 ppm with an average of 113 ppm. The Sb, As and Hg mineralization is fairly evenly distributed throughout this sample population; there seems 52 carbonate rocks which contained a stockwork of quartz- dolomite veinlets. These stockwork veinlets were selectively sampled (3072, 3073); they did not contain gold. Additional
silica—cart)onate rock samples also did not contain detectable gold. Also all the high grade mercury ore was contained in
silica-carbonate rocks which were bounded by "alta". The alta contains gold (sample 3100); it also usually contained Hg (Bailey and Everhart, 1964). It was almost invariably mined with the silica-carbonate rock as mercury ore. The rock samples 3098 and 3102 were of mine dump material
(consisting of silica-carbonate rock and alta) ; they contained gold. The presence of gold in the alta-silica- carbonate rock is also confirmed by the gold present in the retort tailings samples. It seems as though the mineralizing fluids may have been trapped by the impermiable alta layer with consequent precipitation of gold in the alta and the rocks immediately below it. Mercury, antimony and arsenic were detected in virtually all samples; the retort tailings contained the higher values.
Mercury ranged in value from 0.06 to 280 ppm with an average value of only 22 ppm (the lowest value in the silica- carbonate mineralized areas). Antimony ranged in value from 1 to 377 ppm, with an average value of 59 ppm. Arsenic values ranged from 5 to 350 ppm with an average of 113 ppm. The Sb, As and Hg mineralization is fairly evenly distributed throughout this sample population; there seems 53 to be a positive linear relationship between the three elements.
There was no detectable silver in the samples from this district (at detection limit 0.1 ppm).
Tellurium was found in only five percent of the samples the range of values was 0.2 to 0.4 ppm with an average value of 0.26 ppm. Ambrose - Comstock Mine Area 54 Introduction
Forestner (1903) designated as the Stayton District 35 square miles of scattered mineralization in northern San Benito , southern Santa Clara, and southwestern Merced
Counties, California. The Ambrose-Comstock Mine Area in the northernmost portion of this district is the subject of this study. The North Stayton District incorporating the Ambrose-Comstock Mine Area was delineated by the boundaries of accessible properties (see Figure 10 and Plate 5) .
The North Stayton District is 13 miles northeast of Hollister in T.ll, 12 S., R. 7 E. of the Mariposa Peak 7.5' quadrangle sheet (see Figure S). The property is owned by Mr. J. Indart; his ranch house is very close to the Ambrose
Mine. The topography of the North Stayton District is dominated by steep hills and deep canyons of dissected volcanic terrains, with intervening gentle rolling hill sides. Most of the land in this area is used for cattle grazing especially in the vicinity of scattered springs. The steeper hills have small stands of live oak while the canyons are densely scrub vegetated with abundant poison oak. The drainage pattern is controlled by north-south ridges acting as drainage divides for mostly westerly flowing streams. Montaray Bay 56 Prior to the detailed geologic study by Bailey and
Meyers (194 2) , some preliminary reconnaissance mapping was done by Forestner in 1903. Bahia Guimaraes (1972) did a Ph.D. dissertation at Stanford University entitled "The Genesis of the Antimony-Mercury Deposits of the Stayton District, California".
The veins of the Stayton district were first mined in 18 7 0-18 7 5 for their antimony ore, but the more valuable cinnabar rapidly superseded it. By 1880, the Stayton,
Gypsy and several smaller mines had produced 1800 flasks of mercury and the smaller Comstock Mine in the north had produced 300 flasks. By 1940, total mercury production from the district was 2690 flasks (no post 1880 production from the Comstock Mine). No antimony production was recorded but probably only a few hundred tons were produced
(Bailey and Meyers, 1942). Mr. R. B. Knox, then owner of the Stayton and other mines, reported to Bailey and Myers that some of the antimony veins contain between $0.50 and $6.00 gold (1930's price) per ton (Bailey and Myers, 1942); no gold was ever produced.
Regional Geology The rocks of the North Stayton district are mostly
Tertiary volcanics which intrude and overlie sedimentary rocks of the Franciscan assemblage and the Great Valley sequence. The folded and faulted Franciscan rocks form the basement throughout most of the area but are only 57 exposed in the northern part of the district. Feldspathic sandstone, dark gray shales and conglomerates of the Great Valley sequence occur in the western and northwestern portion of Forestner s Stayton district. The Tertiary volcanics consist of basaltic-andesites, andesites, agglomerates, tuffs and rhyolites (see Figure 10). At the Comstock Mine (see Plate 5) serpentinites have undergone hydrothermal alteration to form mercury mineralized silica- carbonate rock. All other mercury mineralization is confined to northwesterly trending silicified fault zones within the Tertiary volcanics.
Landslides, creep soil, debris flow and alluvial deposits cover extensive areas within the district.
Stratigraphy
Franciscan Rocks. The exposed rocks in the North Stayton district range in age from Jurassic to Recent. The Franciscan formation of suspected Jurassic age is predominantly a gray, greenish-gray or buff, poorly sorted medium grained gray- wacke. The grains are mostly quartz and feldspars with smaller amounts of dark gray shale, recrystallized chert and greenstone. Fragments of andesite and dacite, both fresh and silicified, are common. Serpentinites are intruded into the sedimentary
Franciscan sequence near the Ambrose and Comstock Mines. This rock occurs in various shades of green and is very soft where unaltered; it is highly sheared and broken. Locally 5 8
Figure 10. Geological Map of the North Stayton District, (In Pocket) 59 well developed cross fibers of chrysotile asbestos occur in narrow but often continuous veinlets.
Iri the vicinity of the Comstock Mine, the serpentinite has been altered to silica-carbonate rock consisting chiefly of chalcedonic quartz and magnesite. Nearly all the silica-
carbonate rock exposed are strongly weathered with character
istically prominant and deeply pitted outcrops, stained pale tan by iron oxides. In thin sections, the intense silicifi- cation of the rock is obvious. Magnesite and quartz have replaced olivine and occur in a maze of interconnecting
pods and veinlets. Marcasite is the dominant sulphide occurring in disseminations? stibnite needles occur in isolated clusters.
Bailey and Myers (1942) separated the Tertiary igneous rocks into four basic rock groups, three of which outcrop in the area of study.
Basaltic Extrusive Rock. This rock covers about half of the study area and is differentiated into basaltic-andesites, agglomerates, tuffs, and tuff-breccia by Guimaraes (1972). The basaltic andesite is the dominant rock and is dark green to nearly black on fresh surfaces, and tan to brown on weathered surfaces. As much as 75% of the rock has an aphanitic groundmass with chalcedony, calcite or limonite filling in cavities. In thin section, the rock consists of scattered phenocrysts of plagioclase and augite, with lesser amounts of the orthopyroxenes. Some of the plagioclase 60 phenocrysts are riddled with glassy inclusions. The matrix of the rock is a swarm of plagioclase microlites with few mafites. The agglomerate units interbedded with the basaltic andesites are made up of fragments of andesitic rocks, tuffs, cherts and feldspathic sandstone in a pink to grayish- green tuffaceous and sandy matrix (Guimaraes, 1972). The tuffs are mostly nonwelded, commonly gray with graded bedding. They are probably ash fall tuffs but some units are welded, devitrified, poorly sorted and do not show layering, suggesting that flow tuffs may also be present (Guimaraes, 1972). The tuff-breccias are light gray and composed of plagioclase, orthoclase and quartz. They do not show bedding or welding and are poorly sorted (Guimaraes, 1972) .
Andesitic Intrusive Rocks. This light gray to reddish brown porphyritic igneous rock was mapped as an andesite and andesitic agglomerate unit by Guimaraes (1972). In thin section, this rock more resembles a dacite porphyry, with prominent subhedral phenocrysts of plagioclase feldspar, with some smaller phenocrysts of hornblende and biotite.
The plagioclase is fresh but it is corroded in certain zones to kaolin and calcite. The matrix of the rock consists of tiny randomly splayed plagioclase laths embedded in quartz. Rhyolitic Intrusive Rock. This rock outcrops as two intrusive plugs in the northeastern part of the district, an 61 area not accessible to this worker. According to Bailey
and Myers (1942), the rock is light colored, in places white,
and contains small phenocrysts of quartz and plagioclase. Guimaraes (1972) claims it also contains phenocrysts of potassium feldspar, with minor biotite in the aphanitic
groundmass. The rhyolite is the youngest of the volcanic rocks and from the shape of the eastern mass, its emplace ment was apparently controlled by pre-existing fractures.
Mineralization
Two basically different types of mercury mineralization are represented by the Comstock and Ambrose Mines. The Comstock mineralization is in an elongate body of silica- carbonate rock (see Plate 5). The Ambrose Mine is a north extension of the suite of antimony-mercury mineralized
"silicified fault zones" or quartz veins found further south (see Figure 10) . The vein system is hosted by an intrusive
Tertiary andesitic/dacitic body but with an inferred extension in Franciscan graywacke (see Plate 5). The silica-carbonate rocks of the Comstock Mine area occur as an elongate body parallel to the fault bounded western perimeter of a small body of serpentinite. That the antimony-mercury mineralized quartz veins constituted high grade mercury ores is fairly unique to both this study and Coast Range mercury deposits. These narrow (2-4 feet wide) veins are localized in high angle faults in Tertiary volcanic rocks. The vein mineralogy 62 consist of a sugar texture" bull quartz hosting pods of massive and acicular stibnite. A silicified fault breccia usually borders these veins; it contains acicular stibnite which is very often oxidized to a massive stockwork of stibiconite and other antimony oxides (see Figure 11). in the vein and dump materials examined cinnabar was seen as disseminations of small euhedral crystals; some cinnabar
"paint" was observed. Guimaraes (1972) points out a regional zonation of stibnite dominated veins in the west and cinnabar veins in the east (see Figure 10) . Pyrite and marcasite were both abundant in the dump materials; in the vein these sulphides were pervasive.
Guimaraes (1972) did extensive research on the nature and extent of the alteration associated with the quartz veins in (Forestner's) Stayton District. He recognizes a silicified zone adjacent to the fault plane that consists of quartz, K-feldspar and montmorillonite. An argillized zone is found between the silicified zone and the country rock. It is composed of montmorillonite, quartz, iron sulphides and locally kaolinite and altered plagioclase. Guimaraes (1972) compared the major element chemistry of the country rocks and their alteration products. This suggested that the alteration zones are enriched in silica, potash, water and total sulphides. A limited number of samples were taken south of the Ambrose-Comstock Mine area (see Plate 5) . Here it was found Figure 11. Photograph showing acicular stibnite and silicified breccia at vein margin. 64 that the mineralization was hosted in fault breccias pre sumably formed during movement along fault planes in the volcanics. With the passage of hydrothermal solutions the rock fragments were altered to a mixture of silica and clay bearing minerals. The resulting indurated fault—breccia constitutes the main ore bearing rock in the Central Stayton
Area. This is especially true where these silicified fault zones coalesce and form high grade deposits as at the Stayton Mine.
Results of Sampling
A total of eighty rock, stream and soil geochemical samples were taken, mainly in the Ambrose and Comstock Mine
Areas. A limited number of samples were taken in the southern portion of the North Stayton district (see Figure 10) . The results of the analyses are shown in Graph 5 and the sample locations are shown on Plate 5 (in pocket). The best precious metal mineralization within all districts studied was encountered in this area. Twenty-one percent of the samples contained detectable gold, with values ranging from 0.05 to 4.5 ppm and averaging 0.556 ppm. Silver was found in 30% of the samples, with values ranging from 0.1 to 45 ppm and at average value of 3.9 3 ppm. The precious metal mineralization was virtually restricted to the quartz veins and the alteration envelope developed around them (also the dump material from mined veins). NO. ELEM ENTS IN PPM O vo K VO 4 O VO D C K 4 o vo O CO 4 O VO CD 4 - CVJ CD O vo 4 CD O VO 4 O VO VO CD 4 VO VO CD O 4 O K 00 VO 4 CD 00 O VO 4 o D C 00 vo 4 o vo _ CT) 4 . D C O VO C 4 VJ D C O VO ro 4 D C O VO 4 I— — ROCK o o to ro to■o vo to CD D C — O D ID ID 4 4 10(0(0(0 m m 5 m m vo m — N- Kp K ^ ( ^^ (O o o o cvi 0 ( cvj 0(0 r^ 10(0(0(0*0(0(0 OO OO OOO O O I O 0 O CO K VO *0 O IO K O DC T )( T )O ) O ) I— — STREAM O VO VO to fO (O lO M-«0
O vo VO ro VO IO VO to ^ o vo vo K. ro K to CD N. TAILINGS V d id Id 1C CM o (o to 1CId vo ro ro vo to K vo to ro CD vo to SOIL ro O vo f O VO VO Vf o 0 t. 00 tv. . K v0 vo o o o vo vo ro vo to vo to to ro vo OE Al ape areplotted samples atNOTE: All scale indicated Key BY: DATE: DATE: a rp Soig itiuin of Distribution Showing Graph Bar n okSra,Rtr Tailings Retort Rock,Stream, in a Bnt B SantaClara Counties B Benito San o k Types: ck o R to □ □ □ □ □ o maximum vertical a height to of C ape ta g ofsae have offscale go that samples of written ppm appropriate in bars. H.N.Shields = epentinite Serp = 3 =2 I / c ic n lca o s/v u o e n Ig = 4 = Slllco-Corbonote = 1 Ambrose-ComstockMine Area 1 March1982 Te 1 1 | 1 Hg lmn scaleElement u A g A b S ei entary Sedim Au, Ag, Hg, Sb,As,Te Hg, Ag, Au, i ol Samples Soil 8i " I ( 100ppm) 0 0 1 = " l ( (/"= (/"= " l ( " l ( ( l" - .5 ppm) .5 - l" ( Stayton District Stayton = = = California 0pm) ppm 10 . p ) ppm 1.0 5 m) pm p .5 ppm) 0 0 1 rp No. 5 Graph drwg . no number ; 2 . II 1 0 0 . 0 ( 0.05 0.1 ppm .01 5.0 0.2 ppm 0.2 1.0 oe sor some ppm ppm ppm ppm ppm FILE
r o f
6 no 5
.
66 Samples 6078, 6079, 6059, 6060, 6061, and 6062 (see Plate 5) were all taken from quartz veins; they do include clay material. Samples 6081 and 6082 were from a waste dump immediately adjacent to the mine workings. Samples 6091 and 6092 were taken in the central area of the district (see
Figure.10) across a 40' wide highly silicified andesite breccia. (This silicified zone is part of a much larger suite of such zones which were in an inaccessible area.) Those two samples contained very good (see Graph 5) gold and especially silver values. Samples 6093 and 6094 were taken in dump materials in the same area as samples 6091 and 6092. As can be seen in the graph, these samples represent almost ore grade materials (the rock was a silicified volcanic). An attempt to locate the possible source of the dump material failed. More than half of the stream sediment samples contained detectable amounts of either gold or silver. These streams were almost invariably found to be draining areas with at least one "vein" structure. There were no gold or silver values detected in the mercury mineralized silica-carbonate rocks at the Comstock Mine. Mercury was detected in all the samples of this area, ranging in value from 0.04 to 600 ppm with an average value of 33 ppm. The mercury values from the rock samples form two distinct groups. The high mercury values are associated with the silica-carbonate rocks of the Comstock Mine and the 67 silicified fractures associated with the Mariposa Mine. (The latter area is classified by Guiamareas (1972) as a "mercury type" silicified fault zone.) The low mercury values are associated with the quartz veins of the Ambrose Mine (and the other "antimony-type" silicified fault zones).
Antimony and arsenic occur in about fifty percent of the samples; they have an apparent close positive correlation. The higher antimony values are associated with the quartz of the Ambrose Mine. This is expected since large acicular clusters of stibnite are abundant in the quartz. No arsenic mineral was detected but it is suspected that arsenic is chemically associated with the stibnite.
Tellurium occurs in 58% of the samples with values ranging from 0.001 to 2.0, with an average of 0.42 ppm. Although it is widely distributed throughout the sample population, it is most persistent in the soil samples developed over the serpentinites near to the Comstock
Mine (see Plate 5). The Ambrose-Comstock Mine area is fairly unique in this study, in both its geology and mineralization. It is the only area studied where silica-carbonate rocks are geo graphically associated with volcanic rocks. The volcanics contain zones of fault breccia which are potentially receptive to mineralizing fluids. (Compare these with similarly ground prepared silica-carbonate rocks.) It is obvious that the fault zones within the volcanics were favored 68 over the silica-carbonate rocks for the precipitation of Au, As, Sb and As; mercury mineralization favored the silica- carbonate rocks. New Idria Mine Area Introduction
The New Idria Mining District, in San Benito County, is only 140 miles southeast of San Francisco and 125 miles northwest of Bakersfield; yet it is one of the most remote areas of California (see Figure 9) . It is in section 29 (T. 17 S., R. 12 E.) of the Idria 7.5' quadrangle sheet. f Idria, a company owned town, had a population of 4 00; it is now an eerie modern "ghost town", since the company closed its operation in 1962.
The "Quicksilver Deposits of the New Idria District,
San Benito and Fresno Counties"(Eckels and Meyers, 1946) is the most comprehensive descriptive study of the district. In 1957, R. G. Coleman did a Ph.D. dissertation on the
"Mineralogy and Petrology of the New Idria District". Other publications include a paper by R. K. Linn in the 1968
Graton Sales Volume II, and also an article by R. K. Linn and W. F. Dietrich (1961) in the U. S. Bureau of Mines
I.C. 80-3. The New Idria was the second largest in all time production, from 1910 to 1912, of all the North American mercury mines. The bulk of the production came from the
New Idria Mine with lesser amounts from the San Carlos-
Molino zone, the Sulphur Springs-Creek zone, and the
Aurora Mine. The New Idria district is in a moderately rugged 70 section of the Diablo range. The flanks of the range are rugged and marked by sharp sandstone ridges cut by deep steep walled canyons. A central uplifted area of serpen- tinite has less relief, but contains the headwaters of the San Benito River. Much of the central area was once covered by dense stands of pine and cedar; these have largely been removed for mining purposes. Consequently, large areas are now barren of heavy vegetation and have only a brush cover.
Regional Geology
The major structural features of the region consist of a series of northwest trending en-echelon folds invol ving sedimentary rocks ranging in age from Jurassic to Quaternary. Striking across the sediments in their western exposures is a strong system of faults that trends roughly parallel and subsidiary to the San Andreas fault farther west (see Plate 6 ). These are accompanied by swarms of related smaller faults of diverse attitude. A conformable sequence of Cretaceous and Tertiary sediments comprises most of the region. These beds are underlain unconformably by a basement of Franciscan sand stone intruded by ultramafic rocks. The ultramafics, which are primarily serpentinites, occur as a prominent elongate oval body bordered by sedimentary rocks. This contact is dotted with mercury mines (see Figure 12). The core of Franciscan rocks (including the serpentinite)were forcibly NEW 1 D R I A DISTRICT SAN BENITO AND FRESNO COUNTIES CALI FORN I A EXPLANATION
SCALE IN MILES *Q L S ^ | Londilldl
I 3adlmanli,undivldad(8hgMI)r ton a on do it • holt, londilont and c on giant 1 ra 1 1 ) r~?=,r*J]'MOftne fermilien (Oi^enla ihila with L I I I I T J I • n i • t . o I lanJiimn)
Ponoca for mo I ion ( 0 r o y ahala ond bio«n mu»»ly»,conCf»llongrji i o n u 1 1 u n • I
Sa r pa n11na (Allirtd UllroPo»lc,inlruiiva roc a
Fronoltcan group ( Mo * 1 1 v a , or a o t>c aandaiu — * 1— I minor »hola,cnari ond graomlono)
Sllico*corbonola rock (All*rad tar panlint,of'a contain minor cinnabar dapoiin)
MINT
CtMl, FI C Y 0 I a a , W B M y a r a , A f II i u 41 . Suivujad In 1940-41 ______
Figure 12: Geology of New Idria District (from R. K, Linn Ore Deposits of the United States, 1933—1964; Part IX, Chapter 8) 72 intruded into the younger sediments along the axis of the
anticline (Eckels and Meyers, 1946). In the northeast,
the core has overthrust sedimentary beds along an irregular fault, the New Idria Thrust Fault (see Plate 6 ) .
Stratigraphy
The exposed rocks in the New Idria district range
in age from Jurassic to Recent. The Franciscan formation of Jurassic age consists of massive sandstones and graywackes with intercalated shales. These are unconformably overlain by the Upper Cretaceous Panoche shales. The Moreno shale of Upper Cretaceous age conformably overlies the Panoche and it is overlain by several thousand feet of Tertiary marine sediments.
Franciscan Rocks. The Franciscan rocks consist mostly of sandstone occurring as a discontinuous ring in and around
the oval body of the serpentinite. It is essentially an arkosic sandstone containing abundant feldspar and quartz with some mafic minerals and interstitial clay. In the field it can be recognised by its massive greenish-gray to brown outcrops, often containing small quartz, calcite and/ or dolomite veinlets. There are considerable local variations due to differing degrees of induration and low grade meta morphism. The large intrusive New Idria serpentinite is the focus of all mercury mineralization in the district. It displays a familiar bluish-green color in fresh outcrops 73 but is often stained reddish-brown to black by iron oxides. Most of the serpentinite is highly sheared and although massive blocks are not uncommon, the soft and sheared variety is dominant. The serpentinite has
randomly orientated crysotile asbestos fibres occurring as mat-like growths in veinlets.
Silica-carbonate rock, the "quicksilver rock" of
California, is widely distributed in the New Idria district.
It consists mainly of chalcedony, quartz, opal and a
granular carbonate which is usually magnesite or dolomite.
The weathered rock forms prominent and distinctive brown
outcrops which have a porous texture due to the solution
and removal of carbonates.
The upper Cretaceous Panoche formation is found bordering the elevated plug of Franciscan sedimentary and ultramafic rocks. It is a marine formation consisting
of about equal parts of silty-shale and concretionary sandstone, and reaches a maximum thickness of about 20,000
feet. The sandstone is a massive thick bedded unit which
forms prominent outcrops. The shale is more of a massive siltstone but with distinct stratification displayed by
alternate layers of siltstone, fine sandstone and clays.
The Panoche is the most important host rock for the mercury
deposits; most of the New Idria Mine production comes
from altered shales of this formation. Mineralization 74
The mercury deposits in the New Idria district are hosted mainly in fault bounded sedimentary rocks in the periphery of the serpentinite plug. Minor deposits also occur in isolated bodies of silica-carbonate rock. All the mercury deposits are found in rocks that have been affected by some form of alteration. Hydrothermally altered sediments of the Panoche formation are the most widespread of the altered rocks. The shale horizon has been altered from a light to dark gray color to almost black. Induration mainly through extensive silicification has rendered the rock very hard and brittle. The sandstone horizons have been bleached white and in areas of intense alteration feldspar grains have been kaolinized; this rock was also hardened by silicification.
It is not certain whether the exotic suite of minerals found in the New Idria district is related to the fact that most of the mercury mineralization is in Cretaceous marine sediments. The main mercury minerals consist of cinnabar, metacinnabar, and native mercury; tiemannite
(HgSe) was reported by Learned.(1962).Pyrite was an abundant constituent of the Panoche shale; it occurred as disseminations and in veinlets. Marcasite was present in lesser amounts in the shales and the silica-carbonate rocks; in the latter it was associated with stibnite. Learned (1962) reported the presence of chalcopyrite, sphalerite, 75 millerite (NiS)and gypsum in a breccia unit encountered at the 800 foot level in the New Idria Mine.
The mineralization at the Aurora Mine (see Plate 6 ) ; is restricted to clusters of cinnabar disseminated in quartz.
A relevant study at this point is to take a look at the sequence of faulting as ascertained by and stated in Jones (1972) :
Episode:
1. ) West-northwest trending thrust fault (New Idria Thrust).
2. ) Northwest-trending tear faults.
3. ) West-northwest trending thrust faults- renewed movement on the New Idria Thrust Fault.
4. ) Steep east-northeast trending normal and reverse
faults. "The sequence of faulting above is substantiated but it seems likely that they were all formed during the same period of movement and that they overlapped in time" (Linn, 1968). This period of faulting is contemporaneous with mineralization localized along the hanging wall of the New
Idria Thrust. It separates the Panoche sediments on the footwall from Franciscan formation on the hanging wall side. The mercury mineralizaton are localized along the
New Idria Thrust. The Panoche sediments, which have been subjected to similar ground preparation as the silica- carbonate rocks, have been much more extensively mineral- 76 ized. The New Idria is the only area studied where there
is silica-carbonate rock in the proximity to substantially
mineralized sedimentary rocks.
Results of Sampling
A total of forty retort tailings, stream sediments
and rock geochemical samples were taken in this area. The
result of analyses of these elements is shown in Graph 6;
the sample locations are shown in Plate 6 (in pocket). Precious metal mineralization was found in fifty-two
percent of the samples from all three sample categories.
Gold was found in 10% of the samples, all values occurring at detection limit (0.05 ppm). The gold mineralization was
restricted to pyritized, mercury mineralized Panoche shales and sandstones. (No gold was found in the silica-carbonate rocks.) Some 50% of the samples contained detectable silver, with values ranging from 0.1 to 0.2 and an average value of 0.14 ppm. Again all the silver values were contained in the Panoche shales and sandstones; the silica-
carbonate rocks did not contain any silver mineralization.
The New Idria is characterized by low Sb, As, and Te;
its mercury values are comparable to the other areas of study. Fifty percent of the samples contained detectable
Sb, ranging from 1-59 ppm with an average of 7 ppm. Arsenic was detected in 43% of the samples, at average value of
3.5 ppm. Mercury occurs in all the samples ranging in value between 0.04 to 1500 with an average value of 123 ppm. ELEMENTS IN PPM 78
Although it occurs in all rock types, the higher values are found in the sedimentary rocks. Tellurium occurs in
45% of the samples with an average value of 0.31 ppm. The Te distribution is similar to that of mercury: it predominates in the sedimentary rocks.
In spite of the low levels of Au - Ag mineralization
(mostly at detection limits) New Idria is an interesting prospect. The study of sequence of mineralization (page 75) suggest contemporaneous faulting and mineralization.
The faulting brecciated both the silica-carbonate rocks and the Panoche sediments; they were both subjected to extensive silicification. The Panoche sediments, which have been subjected to similar ground preparation processes as the silica-carbonate rocks are more extensively mineralized. 79 Rinconada Mine Area Introduction
The Rinconada Mine Area is located about 11 miles northeast of Santa Margarita, California. It lies in
the southwest sector (T.30 S., R.14 E.) of the Santa
Margarita Lake 7.5' quadrangle (see Figure 13). The
Rinconada Mine is accessible from Santa Margarita via Pozo Road; the rest of the area can be reached by good, county maintained gravel roads.
This part of the Coast Ranges consists of moderately
rugged hills on which a humid climate has contributed to
the development of a fairly deep weathered zone. Although
the higher ground supports thick stands of live oak and
several species of pine, the dominant vegetation is dense growths of manzanita, madrona and several species of sage.
The most authoritative work in this district was commissioned by the Strategic Minerals Investigation and
undertaken by Eckel, Yates and Granger (1941). In 1970, Western Minerals Exploration Company's geological engineer,
Mr. R. H. Neudeck, completed a geologic report on the
Rinconada Mine, a copy of which was obtained by this worker from the current property owner. The Rinconada Mine was one of the larger producers of mercury in San Luis Obispo County; an estimated 3000 flasks was produced from the mine since 1872. It is believed
that significant production from the mine ceased in 1935
(Eckels, et al, 1940) . There are a number of open pit FIGURE 13 Location and access map of Rinconoda Mine Area. 81 sites, and the largets of these was the Mercury Belle; no production was recorded.
Regional Geology
The Rinconada district is bordered on the east by the Nacimiento Fault and the west by the East Huasna
Fault (see Plate 7). These roughly paralelling faults, with an approximate strike of N.45°E., enclose a block of Franciscan assemblage which has been thrust over Cretaceous marine shales, sandstones and conglomerates. The Franciscan assemblage includes melanges of serpentinites and silica- carbonate rocks (see Plate 7).
Stratigraphy
Franciscan Formation. The Franciscan is dominated by dark shales with some interbedded sandstones and siltstones, and intrusive serpentinites. The dark gray to black shale is gradational into the intercalated sandstones because the bedding structures have been obliterated by crumbling and metamorphism. They are sheared and highly contorted, and this often serves to distinguish them from the younger
Cretaceous shales. The sandstone is medium to coarse grained, containing abundant detrital feldspar and relatively scarce quartz. It is usually massive and shows little evidence of bedding. Several large areas of serpentinite are within the Franciscan sedimentary sequence. The serpentinite is usually dark colored with various shades of green. According 82 to Eckles, et al, (1940) this unit occurs as "remnants of a warped sheet capping Franciscan" sedimentary rocks and forming the tops of moderately high hills. At the Rinconada Mine, the serpentinite is altered to silica-carbonate rocks containing chalcedony and quartz in an intimate intergrowth with granular carbonates. The carbonates are essentially magnesite and dolomite with some ankerite. This silica- carbonate rock again is the host of the mercury and associated mineralization.
Cretaceous Rocks. The Cretaceous sequence in this area is presumed to be several thousand feet thick. It is sub divided into a lower marine shales and sandstone unit and an upper sandstone and conglomerates unit.
The lower Cretaceous consists largely of inter- laminated brown to gray-black shales and locally prominent thin interbedded sandstones. The shales resemble those of the Franciscan formation, but are less contorted and sheared.
The upper Cretaceous is characterized by a gray to yellowish, medium to coarse grained sandstone. It contains numerous grains of feldspar interspersed with the predominant grains of quartz. Tertiary Rocks. A sequence of Tertiary sedimentary rocks, most of which are probably Miocene, unconformably overlie Cretaceous and older sedimentary rocks. They consist of shales, limestones and sandstones with intercalated conglomerates. The shales are mostly dark gray to black due 83 to a high content of carbonaceous matter. The sandstone is light buff to cream, fine to coarse grained and consists principally of quartz sand and some feldspar grains.
Results of Sampling
A total of seventy-seven rock (some underground), stream and retort tailing samples were taken in this area.
The result of the analyses of these samples is shown in Graph 7; their locations are shown on the geologic map of Plate ” 7 (in pocket) .
It was very surprising not to find any detectable gold (detection limit 0.05 ppm) in the samples, especially since its presence was reported by at least two reliable sources (Eckel, et al, 1940, and J. Whiteford, 1981, personal communication). Silver was found in 12% of the samples; all but one of the values were at detection limit (0.1 ppm). (This places obvious limitations on the weight that can be put on them in interpretations.) The silver values were found to cluster in one area (see Plate 7 ) where silica-carbonate rock, gouge of Franciscan shales and sandstones and Cretaceous marine shales and sandstones co-exist. These rocks do not show any alteration and/or mineralization. Four detection limits values of Ag were found in the streams draining the Rinconada Mine tailings dumps. The Rinconada Mine Area is characterized by low levels of Sb, As and Te; its Hg values are at a comparable level ELEMENTS IN PPM 1 NOTE' All samples are plotted indicated samples at All scale NOTE' e t Roc Types: ck o R to Key DATE: DATE: BY: a rp Soig itiuin of Distribution Showing Graph Bar in Rock,Stream, Retort Tailings Retort Rock,Stream, in H.N.Shields □ □ □ □ □ □ o mxmmvria hih o 2 ; maximum avertical heightto 2 of ape ta o f sae ae number have scale off that gosamples of writtenppm in appropriate bars. 3 = I vol ic n a lc o /v s u o e n Ig = 4 Sedimentary = 2 - lca- e t a n o b r a -C a ilic S - 1 I March 1982 Ele Au epentinite Serp Ag Sb Hg Te As San Luis Obispo Luis County, San Au, Ag, Hg, Sb, As,Te Sb, Hg, Ag, Au, icnd Mn Area Mine Rinconada Si Samples Soil & et cl DetectionLimit scale ment Rinconada District Rinconada (l" = (l" = " / ( (l" = (l" = 0" (l" = (l" (l“ = California 1.0 ppm) rp No.7 Graph IOO IOO ppm) IOOppm) 10 ppm) .5 ppm) 1.0ppm) RG NO.DRWG. II (O.OI ppm for
.5 ppm 0.05
0.1 ppm .01 ppm stream samples) 5.0 ppm 0.2 ppm
1.0 ppm
IE NO. FILE
85 with the other areas of study. There is only one sample with detectable(1 ppm) Sb and four with As values averaging
1.2 ppm. Mercury occurs in all the samples with values ranging from 0.04 to 1600, with an average of 58 ppm. The higher mercury values were from underground samples of silica-carbonate rocks. This area has the highest percentile of samples containing Te (66%) of all the areas studied. The values range from 0.001 to 0.8 with an average of 0.15 ppm. The Te mineralization in this district is apparently independent of rock type. DISCUSSIONS AND INTERPRETATIONS
Geochemistry Introduction.
The analytical results at the end of each district study were meant to give the reader a "feel" for the dis tribution and relative abundance of the elements. Table 1 gives a summary of the analytical results. In this section correlation matrices and dendrograms provide a quantitative comparative study of various sample categories. It is hoped that this study will help to identify geochemical characteristics and elemental associations. The samples from all districts were assembled together to represent one geochemical survey area with distinctions made for the various types of samples. The lithogeochemical samples are classified by rock type into silica-carbonate, serpentinite, sedimentary and igneous/volcanic rocks. Although the sedimentary rock group consists essentially of
Franciscan graywacke, it does include the Tertiary sandstones and shales found at New Idria and Rinconada. Similarly the igneous/volcanic suite of rocks includes the assorted volcanics, intrusives and miscellaneous vein material from Stayton, the lava flows from Sulphur Bank and the silicified volcanics and pillow lavas at Syar Quarry. The combination of these essentially different rocks is not desirable, but with the limited number available for statistical treatment it is perhaps the most practical way. Table 1: Summary of Analytical Results.
0 w V Percentage of R a n g e o f Averoge Value V 0. Sample Containing Values In PPH I n P P H f . S 0 ei DISTRICT V. Au aR Hr SD As T e Au Ag Hg S b As T e A u A g Hg S b As Te JULPIIUR .05 .1 .44 1 5 .001 9 A M K S 22 32 16 1 0 0 23 14 56 t o t o t o t o t o to . 0 0 5 0 . 1 5 78 41 26 . 1 1 5 .05 .2 3 0 0 77 54 .47
.05 .1 .04 1 5 .001 M l t . L J O 90 2 26 1 0 0 3 0 7 9 11 .05 .23 8 8 31 .22 t o to t o t o t o t o .05 .8 6 0 0 2 0 0 1 6 0 .6
.05 .1 .01 1 5 .2 rrr. 60 IS 2 1 0 0 96 1 0 0 13 t o t o t o to to to .144 .6 69 7 1 1 1 7 6 .05 )I ABl.O .2 .6 844 l o e o o 1 8 0 0 3. 1
.05 .06 1 5 .2 1IADU- 6S 39 0 1 0 0 BB 92 5 t o 0 to t o to t o .105 0 22 59 11 3 .26 .4 u j p r .3 2 0 0 3 7 7 3 5 0
.05 .1 .04 1 5 .001 3 . 9 315! 1 5 6 .42 HORHI 8 0 21 39 1 0 0 59 59 50 to t o to to to t o . 5 5 6 33 iT A Y T O M 4 . 5 4 5 6 0 0 8 5 0 0 7 6 0 0 2 . 0
.05 .1 .04 1 5 .2 irw 4 0 10 50 1 0 0 50 4 3 45 to t o to to t o to 0 .19 1 2 3 7 3 . 5 .31 I PH I A .05 .2 1 5 0 0 59 19 .6
.1 .04 5 .001 .15 RIKCON- 77 0 12 1 0 0 1 5 66 0 to t o 1 to t o .023 .11 58 1 1.2 ADA .4 1 6 0 0 4 0 .8
Averages for Au, Ag, Te were calculated based on the number of mineralized samples. Averages for Hg, Sb, As 18 based on all samples.
CO
i m m m ** i 88 A greater statistical problem than the grouping together of dissimilar material is perhaps the bias intro duced into the clusters by large blocks of samples from a few districts. The magnitude of this problem can be seen on Table 2. This author is of the opinion that the geo chemical associations are not invalidated by the above limitations. The igneous/volcanic rocks do not represent the area of study since 91% of these samples come from the Stayton and Vallejo districts only.
A cluster analysis compares a number of variables or samples and arranges them into a dendrogram according to similarity levels of clustering. The different routines of cluster analysis permit the use of a number of parameters to classify variables into distinct groups. In this study cluster analyses are accomplished using n x m and n x n data matrices, where n is the number of observations or samples, and m is the number of elemental variables. The data were logorith- mically (base 10) transformed in accordance with Ahrens (1954):
"the concentration of an element in geochemical analyses is CiV, J ,1 lognormallv distributed in specific rock types." The data were also standardized into a unitless form by subtracting the mean of the data set from each observation and dividing the results by the standard deviation. For the standardized n x m data, a m x m product moment correlation matrix between elements using r—coeficient as the similarity measure, was computed. The cluster analyses were performed using the
FORTRAN program CLUSTER in Davis (1973). The program performs 89
Table 2: The distribution of samples by rock type from
each district, used to derive rock sample
correlation matrices.
NUMBER OF SAMPLES OF EACH ROCK TYPE
DISTRICTS Silica- Igneous Carbonate Sedimentary Serpentinites Volcanic
Sulphur Bank 0 9 0 6
Vallejo 3 18 0 6
Mt. Diablo 24 6 2 2 Guadalupe 11 4 3 0
Stayton 4 2 2 25
New Idria 5 20 4 0 Rinconada 25 19 10 0 90 the cluster analyses by the weighted pair group method of clustering with arithmetic averages. It prints the results of the clustering in the form of a dendrogram. (A dendrogram is a "tree diagram" that indicates the degree of mutual similarity between single elements and groups of elements" Davis, 1973.)
Cluster Analyses On An n x m Data Matrix.
A cluster analysis on an n x m data matrix compares individual elements from samples within a specific (sample) rock group with the same elements from other samples in that group. In other words, the Au from serpentinite sample
1387 is correlated with the Au from serpentinite sample
1388, 1389, and so on; similarly Ag, Hg, Sb, As and Te.
This type of analyses should highlight the elemental associations that might exist in a sample group.
Soil Samples. The strongest correlation found in the soil samples was between Sb and As (see Figure 14). They form a significant (higher than the significance level determined for that sample population) cluster with Hg. Thus the suite Hg, Sb and As forms a significant cluster. This suite appears to be associated with Au and Ag but below the level of signi ficance. The suite Au—Ag-Hg-Sb-As has a low level negative correlation with Te. This is because the samples were taken from soils developed over silica—carbonate rocks which have little or no Te but relatively high Au, Ag, Hg, Sb, and As. 91
Figure 14: Correlation matrix and dendrogram from a
cluster analysis of 43 soil samples from
all districts studied. CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability= 0.264 r-coefficient
Au A£ Hg Sb As Te Au 1.000
Ag .1943 1.000 (Values above level of significance are underlined.) Hg . 2999 -.0683 1.000 Sb .2061 -.0536 .4805 1.000
As .2116 -.0539 .6582 . 7481 1.000
Te -.1475 .0112 -.7043 -.6897 -.9269 1.000
DENDROGRAM:
-.2203 .0666 .2544 .5693 .2544 .2591 0848 .0895 .2639 .4382 .6125 .1868 +----+— -+---- 1----- 1-----f----+---- f----+----+---- f----+---- h -- — ------Au
T------Hg Sb --- As ------Ag ______— — ------— --- Te +----+-- ■+■------H------1------1------1------1------1------1------1------1------.1719 .0024 .1767 .3510 .5253 .6996
Values along x-axis are similarities 92
On the other hand, the samples taken from soils developed over serpentinites have relatively high Te.
Serpentinites. The best correlation developed in the serpentinite rocks is between Sb and As (see Figure 15). They form a significant cluster with Hg. There is also a good correlation between Au and Ag. Gold and silver form a cluster with Te but below the significance level. The two suites Hg-Sb-As and Au-Ag-Te appear to have no linear relationship with each other.
Silica-Carbonate Rocks. The first order correlation in this group of rocks is between Sb and As (see Figure 16). They, in turn, form a significant cluster with Hg and Au. Silver and Te form a cluster below the level of significance. As a group Ag and Te have no linear relationship to the suite
Au-Hg-Sb-As. The behavior of Te in silica-carbonate rocks can be related to its behavior in the soil and serpentinite samples.
The serpentinites and the soil developed above them are relatively rich in Te but the silica-carbonate rocks also developed from serpentinites are devoid of Te. This means that there is a net loss of Te during the replacement process or during the sulphide invasion.
Sedimentary Rocks. The strongest correlation found in this group of rocks is between Sb and As (see Figure 17). They form a significant cluster with Au. There is also a good correlation between Ag and Te. The two groups Sb—As-Au and 93
Figure 15: Correlation matrix and dendrogram from a
cluster analysis of 21 serpentinite rock
samples from all districts studied. CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability = 0.340 r-coefficient
Au Sb As Te
Au 1.000
Ag .7969 1.000 (Volues above level of significance
Hg .3126 .0136 1.000
Sb .5238 .1601 .7406 1.000
As .4644 .0976 .8309 .9021 1.000
Te .1570 .3748 -.5632 0.2776 -.3859 1.000 ]
DENDROGRAM: .7969 -.1085 .2659 .7857 .9021 .1489 .0330 .2149 .3968 .5787 . 76D6 .9426 +--- +----+---- b--- b— f---- +----+ -+-- -f---- 4-j----b---- !------Au ------Ag
Te Hg Sb ---- As + ------1------h ------+■------f------+ ------+ ------b — ■+----H---- + ■+---- -.0580 .1240 .3059 .4878 .6697 . .8516
Values along x-axis are similarities 94
Figure 16: Correlation matrix and dendrogram from a
cluster analysis of 72 silica-carbonate rock samples from all districts studied.
CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability = 0.235 r-coefficient
Au Ag Hg Sb As Te Au 1.000
Ag .0674 1.000 (Values above level of significance
Hg .2697 -.3563 1.000 Sb .1962 -.2420 .2796 1.000
As -.1471 .4046 .6065 1.000
Te -.1988 .2293 -.2093 -.7138 -.4329 1.000
DENDROGRAM: .2972
Values along x-axis are similarities 95
Figure 17: Correlation matrix and dendrogram from a
cluster analysis of 78 sedimentary rock
samples from all distr:' -its studied.
CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability= 0.214 r-coefficient
Au A£ Hg Sb As Te
Au 1.000
Ag .2352 1.000 (Values above level of significance are underlined.) Hg .1507 .0707 1.000
Sb . 3253 .0707 .4802 1.000 As .0366 .1332 .1185 .2104 1.000
Te .0615 .201 -.1553 -.2535 -.5232 1.000
DENDROGRAM: .2501 -.1372 1005 .2380 .4802
Values along x-axis are similarities 96
Ag-Te, however, do not have a linear relationship. That
the Au and Ag clusters are unrelated may be due to the fact that Au is present in only 9% of the samples compared with 39% for Ag.
Igneous/Volcanic Rocks. The primary correlation in this suite of rocks is between Sb and Ag (see Figure 18). They form a high level association with Te. This suite forms a
significant cluster with As, but has no linear relationship with Au or Hg. The lack of correlation with Au is probably due to the distortion introduced by the samples from Vallejo which are virtually devoid (3%) of Au. The mercury assays in igneous rocks are an order of magnitude lower than the values in the other rock categories. This explains its absence in the association. It is fairly important to note that there is good positive correlation of Te with Ag, Sb, and As. In the Franciscan assemblage rocks, the correlation is between Au,
Hg, Sb, and As, with Te and Ag being insignificant.
Retort Tailing Samples. This category of samples represents an aggregate of rock types that hosted exploitable mercury ore. The strongest correlation is again between Sb and As
(see Figure 19). They form a significant cluster with Au. There is also a significant correlation between Ag and Hg.
The two groups, however, have no apparent linear relationship. The fact that the Au and Ag clusters are unrelated may again 97
Figure 18: Correlation matrix and dendrogram from a
cluster analysis of 94 volcanic rock samples from all districts studied.
CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability = 0.235 r-coefficient
Au Ag Hg Sb As Te
Au 1.000
Ag .4715 1.000 (Volues obove level of significance
Hg .2097 .0015 1.000
Sb .1455 .4918 -.0163 1.000 1.000 As .1046 .2711 -.1267 . 3975 Te .2239 . 4463 .0106 .4872 . 2228 1.000
DENDROGRAM: .4918
Values along x-axis are similarities 98
Figure 19: Correlation matrix and dendrogram from a
cluster analysis of 47 retort tailing samples
from all districts studied. CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability - 0.264 r-coefficient
Au ha . Hg Sb As Te Au 1.000
Ag -.1558 1.000 (Values above level of significance are underlined.) Hg .0517 .2938 1.000
Sb .6011 -.0667 .2813 1.000
As .6081 0.0954 . 2687 .9853 1.000
Te .0756 -.1071 -.0382 .0232 -.232 1.000
DENDROGRAM: .0950 -0.25] .2938 .6046 .9853 .0655 .1164 .2982 .4801 .6620 .8438 1.0257 + ------1------+ ------»------+ ------+ ------+ ------f ------+ ------+ ------f------+ ------f ------Au
Sb As Te Ag ------Hg +■------1------H------H------H------1------1------H------1------1------1------1------+ .0254 .2073 .3892 .5710 .7529 .9348
Values along x-axis are similarities 99 be due to the presence of only 9% Ag in the sample population compared with 33% Au.
Gold/Silver Bearing Rocks. In order to approximate precious metal mineralized rocks, all the rock samples containing Au or Ag were grouped into one data set. The best correlation was again found between Sb and As. The Sb-As suite is closely associated with Au and to a lesser extent with Hg (see Figure
20). A second insignificant cluster was formed between Te and
Ag. The Sb-As-Au-Hg and Te-Ag suites are strongly negatively correlated. This clustering can roughly be translated into a district characterization by using the sample results as shown on the bar charts. The suite Ag-Hg-Sb-As characterizes the Guadalupe and Mt. Diablo Mine areas and the suite Ag-Te characterizes the Vallejo, North Stayton and New Idria Mine areas (the Sulphur Bank and Rinconada districts cannot be easily put into either category).
Stream Sediment Samples. The primary correlation developed in this sample group is again between Sb and As; they form a significant cluster with Hg, Ag, and Au (see Figure 21). Tellurium has no linear relationship with the above suite. Thirty-two stream sediment samples were taken from streams draining the area in and around the McLaughlin gold deposit near Knoxville in Napa County, California. The correlation matrix and dendrogram from these "Knoxville" sediment samples is shown in Figure 22. These samples repre sent a restricted area of fixed geology, compared to the 100
Figure 20: Correlation matrix and dendrogram from a
cluster analysis of rock samples containing either Au or Ag (46) from all districts studied. CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability = 0.235 r-coefficient
Au Ag Hg Sb As Te
Au 1.000
Ag -.7272 1.000 (Values above level of significance
«g .3194 -.4509 1.000
Sb .4093 -.5011 .2225 1.000
As .4310 -.5332 . 3201 .7747 1.000 Te -.1044 .2739 ,3276 -.1578 .,2429 1.000
DENDROGRAM: .2954
Values along x-axis are similarities 101
Figure 21: Correlation matrix and dendrogram from a
cluster analysis of 80 stream sediment
samples from all districts studied.
CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability = 0.200 r-coefficient
Au Ag Hg Sb As Te
Au 1.000
Ag .2766 1.000 (Values above level of significance
Hg .2488 .4259 1.000
Sb .2079 .3825 .3220 1.000
As .3396 .3415 .4182 .6712 1.000
Te -.0012 .1413 -.0931 . 0974 -.1426 1.000
DENDROGRAM:
-.0002 .2682 .3660 .4259 .6712 -0.27|L .0938 .2146 .3355 .4563 .5772 ; .6980 +-- — +----+----b -f---- 1---- -+---- 1----- 1----- 1-----1 H---- 1------Au Ag
Hg Sb As
Te
+ ------f------b ------b ------b — f----1----- +---- b — +--- 1------b--- * .0333 .1542 .2750 .3959 .5168 .6376
Values along x-axis are similarities 102
Figure 22: Correlation matrix and dendrogram from a
cluster analysis of 32 "Knoxville" stream sediment samples.
CORRELATION MATRIX:
Level of Significance Similarity measure is at 95% Probability = 0.0306 r-coefficient
Au Ag Hg Sb As Te
Au 1.000
Ag .6295 1.000 (Volues above level of significance
Hg -.1470 -.4318 1.000
Sb .1984 -.0846 0.942 1.000
As .0124 -.3506 .3959 .7779 1.000
Te -.0230 .1785 -.3783 -.0026 -.0520 1.000
DENDROGRAM: -.1877 .0777 .2450 .6295 .7778 .2264' -.0526 -.1212 .2951 .4689 .16427 .8165 + ----- h---- + ---- + ----- 1------f-----+■-----+ ----- f----- 1------h----- f ----- + ------Au Ag
Te
Hg
--- - As +---- t-----+----+----+----+----+----+----H---- +----+----+--- * -.1395 .0343 .2082 .3820 .5558 .7296
Values along x-axis are similarities 103 other stream sediment samples which represent a variety of areas of differing geology. However, it is still instructive to compare the two correlation matrices. The strongest correlation found in both sample sets is between Sb and As. In the "Knoxville" samples there is also a strong primary correlation between Au and Ag. In the general stream sediment samples this primary correlation does not exist between Au and Ag. Here Au and Ag are correlated with Hg,
Sb and As. These elements form one significantly clustered suite which is not related to Te. In the Knoxville samples there are two distinct primary clusters: Au-Ag, and Hg-Sb-As; these two clusters are apparently unrelated. Tellurium shows a closer affinity to the cluster Au-Ag but there is no signi ficant correlation between them.
It was hoped to combine the sample results from all districts and all rock types into one population and then perform multi-elemental n x m cluster analyses on them.
However, this was not possible since the design of the computer programs limited the number of samples that the computer was capable of handling.
Cluster Analyses On An n x n Data Matrix. A cluster analysis on an n x n data matrix compares an individual sample with the other samples from the same sample (rock) group. The samples are compared on the basis of the proportional relationship that exists between all the elements found in any one sample. This type of analysis should 104
indicate the level of similarity between samples from a district and also the level of similarity between districts.
Silica-Carbonate Rocks. Cluster analysis on an n x n data matrix was performed on all the sample categories discussed above. However, the pattern found was essentially the same and thus only the dendrogram for silica-carbonate rocks is presented (see Figure 23). The dendrogram shows that each district is peculiar enough to form a distinct cluster, as in Rinconada, Mt. Diablo and Guadalupe (see Figure 23). How ever, there are enough similarities between the districts
that inter-district grouping will occur, as at the top of
the diagram. These findings are intuitively obvious since we are dealing with gross mercury mineralization; however,
nature is not uniform and one should expect local variations within the larger framework. Figure 23:Dendrogram from cluster analysis of 51 silica-carbonate rock samples. Samples were
clustered on the basis of similarity between the elements in each sample. Sample # -.2173 ,0113 .2440 ,4744 ,7053 ,9.159 -.3327 -.1020 ,12114 ,3593 .5899 ,8204 1.0312/ Levels of + ------4 ------4------4------4------4------4------4------4 ------4 ------4 ------4 ------, similarity. -- '*1 ,926r] — "Rmconada 14j ,8319 — Rinconada
18 .1990 — New Idria
3 .92921 — Rinconada _ Rinconada 15 .9981
32 .0368 Vallejo
2 .9974
7 . 9960
12 .9840
4 .9985
5 .9997
8 .9932 11 1.0000
13 .9232 * ■■■— Rinconada
4 .9900
9 .9998
10 . 9888
1— . J 7 ,7 0 6 $ _ — New Idria
16 . 9 0 8 6 ”! I— * — New Idria 1____ 31 .6995 1 — Mt. Diablo 19 .9742“ ] 1 New Idria 1 20
21 . 93 l"Z
22 . 9930 ■North Stayton HZ 23 • 5566 44 . 951T - 1
-----x 4 5 1.0000
— 4 7 .9999
—» 51 ,9985 _ 49 .9938 Mt. Diablo
46 .9885
48 .9839
50 . 1936
27 ,9999
28 .9893 Guadalupe
30 .6285
29 .96737 3 ] — Guadalupe ■ c 36 30£3_j — Mt. Diablo 3 3 .9797 ■ d 41 .8541 37 . 9939 r C 30 .9 6 6 4 39 . 9674
40 .9919 c 43 .9791 42 . J 130
24 .997^____ -
— 25 .9896 26 .7540 Guadalupe
— 34 .9650
35 ------4------4 ------4 ------4 ------| ------4------4------4------1------4------1 -- I 3327 -,1020 ,1286 .3593 .3899 ,8206 1.0512 -.2173 .0133 .2440 ,4744 .7053 ,9359 105 106
Silica-Carbonate Rocks
Silica-carbonate rocks host the mercury deposits in six of the seven areas studied. These rocks are formed by the hydrothermal alteration of serpentinites; their genesis implies the existence at one time of a hydrothermal system.
This raises the potential for mineralization during the silica-carbonate formation phase or at any time after that phase by reactivating an existing system.
The conversion of serpentinite to silica-carbonate rock is chiefly a replacement process, wherein the textures of the serpentinite are retained during its replacement by a mixture of quartz and magnesite. The process is not a simple molecular replacement since a limited amount of migration of the constituents is indicated by the arrangement of the replacing minerals (Bailey and Everhart, 1964).
An analysis of serpentinite and its derivative silica- carbonate rock from the New Almaden Mine is given in Table 3 (after Bailey and Everhart, 1964) . A comparison of the chemical analyses show that the change is principally one of dehydration and carbonitization. This is shown diagrammatically in Figure 24, again after
Bailey and Everhart (1964). The silica-carbonate rocks are enclosed by high angle faults, and in larger bodies, are preferentially altered along their margins. It would not be unreasonable to assume that these faults acted as conduits for the fluids that formed the silica-carbonate rocks. In the areas studied, 107
TABLE 3 - Analysis of x o d i lion. the New Aln.eden Kinc showing change lion. stsrjx=n finite to silica- Ccsrhontie rocX (after Bailey and Myers, 1964).
Massive Sili ca-Carbonate Serpentini te rock Si02 3 6 . 0 31.22 a i2o3 3.04 .94 Fe2°3 3.66 2.22 FeO 3.72 2 . SB MnO . 09 2.58 HgO 36.99 2B .78 CaO None . 04 Wa20 None None k2o None None «204 1 S. 00 .59 C°2 .s< 33.16 None None P2°S s .14 . 02 .38 . 24 Cr2°3 n . d . n . d. S03 Total 100.01 99.87
Figure 2+ •' Diagram showing gains and losses by weight of principal oxides in hydrothermal alter ation of a unit volume of serpentinite to silica-carbonate rock, assuming volume for volume replacement. (After Bailey and Myers, 1964)
SlNPtNllNl SU IC A C A H B O N A 1 I fcOCK Sp C =.7 4b Sp *• = .?«? 108 it does not seem that there was any mineralization accom panying the silica-carbonate formation event.
The brittle silica-carbonate rocks were later subjected to brecciation by recurrent movement along their bounding faults. This served as the ground preparation for the later deposition of quartz, dolomite, pyrite, marcasite, stibnite, metacinnabar and cinnabar. Gold and silver, presumably accompanying this sulphide phase, were detected in only 10% of the silica-carbonate rocks sampled. At New Idria the Tertiary sedimentary rocks in fault contact with the silica- carbonate rocks were also brecciated and indurated by silicification. These rocks are extensively pyritized, have the same suite of minerals as the silica-carbonate rocks and were the hosts to the more productive mercury mines. These younger sedimentary rocks were unfortunately not extensively sampled. However, 75% of the samples from those rocks contain precious metal. At the Stayton district, silicified fault zones in volcanics host the main mercury deposits. These fractures are on the same trend (NNW) as adjacent fracture zones hosting silica—carbonate rocks; both have a similar mineral suite. When sampled for precious metals the silica-carbonate rocks were barren but 64% of the samples from the silicified fault zone contain precious metals. There is the suggestion (albeit tenuous) that the silica-carbonate rocks may not be as suitable as the younger sedimentary or fractured volcanic rocks as hosts for precious metal deposits. 109
Conclusions
This study has confirmed that precious metal mineral ization is associated with the Coast Range mercury districts. Although the mode of occurrence of gold is not yet known, high angle faults probably acted as conduits for ore bearing fluids. The fluids were rich in C02 and Si02- Tha paragenetic sequence of minerals found in the mercury districts is well established. Fine grained silica is associated with cinnabar, pyrite, marcasite, stibnite, dolomite and calcite. Stibnite is usually deposited after cinnabar but the iron sulphides are coeval with all miner alizing phases. The presence of hydrocarbons has been noted in almost all the districts. Radtke and Scheiner
(1970) in their "Carlin Type" studies have suggested that hydrocarbons or other carbonaceous material critically govern the deposition of gold. Although the mercury districts do have "Carlin Type"characteristics, the absence of thallium is a noteworthy difference.
The geological evaluation of the districts has led to the tenuous suggestion that those mercury deposits not hosted in silica-carbonate rocks may make better gold prospects.
There is evidence at the North Stayton district that the mineralization in the volcanics is preferentially enriched in gold and silver. The younger (than Franciscan) marine sedimentary rocks at New Idria are also apparently more receptive than the silica-carbonate rocks to precious metal mineralization. 110 The grouping of elements that emerged from the statis tical study complement the geological deductions. A Au-Hg- Sb-As suite was found to be characteristic of mineralization in silica-carbonate rocks. These silica-carbonate rock areas include Guadalupe and Mt. Diablo where the prospects of economic precious metal mineralizations are not high.
A weak Au-Ag-Te suite was found to be characteristic of areas of volcanic and young marine sedimentary rocks to which the North Stayton and New Idria districts belong.
The geology of these areas is favorable for economic precious metal deposits. Mercury mineralization seems to be controlled by the presence of high angle faults. Antimony and arsenic have a strong positive correlation regardless of the type of data manipulation. This is not unusual in view of the geochemical characteristics of these elements. The geological and geochemical evidence clearly suggest that there is a good potential for gold deposits associated with the Coast Range mercury districts. Also, those mercury districts localized about high angle faults in volcanic or sedimentary marine rocks should be the primary precious metal exploration targets. Ill
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Jones, R. B., 1972, Geochemistry as a Guide in Underground Exploration for Mercury Ore at the New Idria Mine, San Benito County, California: Unpublished M.Sc. Thesis, University of Nevada, Reno. Learned, R. E. , 1962, Paragensis of Mercury Ore Deposits: Unpublished M.Sc. Thesis, University of California of Los Angeles. ______^1966, The Solubilities of Quartz, Quartz- Cinnabar and Cinnabar-Stibniue in Sodium Sulphide Solutions, and Their Implications for Ore Genesis: Unpublished Ph.D. Dissertation, University of California, Riverside.
I nn, R.K., 1968, New Idria Mining District: Graton Sales, Vol. 2, ed. J.P. Ridge, p.1623-1649. ______, and Dietrich, W. F., 1961, Mining and Furnacing Mercury Ore at the New Idria M i j. . e , San Beni :o County, California: U.S. Bureau of Mines, I.C.8033, 1-14.
Moiseyev, A. N., 1971, A Non-Magmatic Source for Mercury Ore Deposits: Econ mic Geology, Vol. 66, No. 4, p. 591-601. Neudeck, R. H. , 1970, Geologic Report on Rinconada Mine, San Luis Obispo County, California: Unpublished.
Norris, R. M. , and Webb, R. V'., 1976, Geology of Califo-'nir - J. Wiley and Sons, Inc., San Francisco. Pampeyan, E. H., 1963, Geology and Mineral Deposits of Mount Diablo, Contra Costa County, California: California Division of Mines and Geology, Special Report 80. Radtke, A.S., and Scheiner, B. J., 1970, Carlin Gold Deposit, Nevada: The Role of Carbonaceous Materials in Gold Deposition: Economic Geology, V. 65, No. 2, p. 87-102.
Ross, C. P., 1942, Some Concepts on the Geology of Quick silver Deposits in the United States: Economic Geology, Vol. 37, No. 6, p. 439-465. Weaver, C. E., 1949, Geology of the Coast Ranges Immediately North of the San Francisco Bay Region, California: Geological Society of /unerica Mem. 35. 113
White, D., 1955, Thermal Springs and Epithermal Ore Deposits: Economic Geology, Fiftieth Anniversary Volume, p. 99-154. , and Roberson, C. E. , 1962, Sulphur Bank, California, A-Major Hot-Spring Quicksilver Deposit; in Petrologic- Studies - A Volume in Honor of A. F. Buddington: New York, Geological Sociaty of America, p. 397-428. 114
APPENDIX 1
Geochemistry of Elements Analysed
The ultimate aim of this research project is to identify an efficient and economical geochemical tool to aid in the exploration for gold mineralization in the mercury mining districts of California. Along with gold and silver, the primary target minerals, Hg, Sb, As, Te, Bi and Tl were also sought to assess their value as path finder elements in this geologic environment. The suite chosen is typical of elements normally found in the upper levels of epithermal precious metal systems. Table 4 is an estimate of average elemental abundance in some common rocks and rock forming minerals. The data is taken from the Handbook of Geochemistry (1980). Only those analyses of rocks from western United States were used to compile the table. Gold Gold, atomic number 79, belongs to group lb of the periodic table. It is strongly siderophillic and somewhat chalcophillic; the latter characteristic is limited to the formation of several gold tellurides. Gold occurs in nature mainly as the metal and as various alloys especially with silver. In fact, native gold often contains some 10% - 15% by weight of silver. Except for the varieties of native gold and the gold telluride compounds, all other minerals contain only small amounts of gold.
Some general geochemical trends of gold include the fact that the range of gold values for the majority of igneous rock is approximately from 0.5 to 5 ppb (see Table 4); gold content generally decreases from mafic to felsic types. Generally, gold distribution in soil horizons is highly variable but work by Aripova and Talispov (1966) shows that the highest gold contents are formed at depths of thirty to forty inches. Where the metal is in close association with secondary iron minerals, the upper soil horizon is generally impoverished. When gold ores are subject to aspects of the weathering cycle most of the gold minerals tend to be concentrated mechan ically but can be transported for great distances. The oxidation of sulphides is essential for supergene enrich ment of gold. When gold is so dispersed dissolution in ferric sulphate-sulphuric acid solution is the most probable mode of transfer.
Silver Silver, atomic number 47, belongs to the same group lb 115
as gold in the periodic table. It is most commonly found in nature as a sulphide or sulphosalt, or as a minor element in a sulphide, or rarely as the native element. Boyle (1968) finds silver to be present in nearly all silicate minerals but seldom exceeding 600 ppb (see Table 4). Although there are a number of silver compounds, the most common silver minerals are acanthite, pyrargyrite, proustite, tetrahedrite-tennantite, chloragyrite, argentojarosite and native silver.
Silver appears very frequently associated with deposits of gold and lead, but may also be associated with a wide range of chalcophile elements, including copper, zinc and bismuth. Like gold, it does not form simple ions and is a rare and widely dispersed element because of its physical and chemical characteristics. Because of its atomic configuration and properties, silver is more closely associated with gold than with any other element. The Ag:Au ratios in ore deposits have been well studied. These characteristics of silver make it a good pathfinder element for gold.
Mercury Mercury, atomic number 80, belongs to group lib of the periodic table. It is a chalcophile element that is normally associated with Sb, Se, Ag, Zn, and Pb in sulphide deposits. It has relatively high mobility which is only limited by its absorption to solid organic matter. Mercury occurs in nature chiefly as nearly pure sulphide (HgS) cinnabar, and in lesser amounts as the black sulphide metacinnabar and as liquid native quicksilver. Mercury is also found in a number of minerals in which it is not an essential constituent. The small differences between the atomic radii of mercury, copper, lead, silver, gold, zinc and cadmium, make.- it possible for mercury to enter the structures of various minerals containing these elements. Mercury is very common as a mineral deposit in the upper surface levels of geothermal systems. In these systems a geochemical association of mercury with antimony, arsenic and gold has been recognised. Most epithermal ore systems are fossilized geothermal systems. The abundance data on mercury is shown in Table 4.
Antimony Antimony, atomic number 51, occurs in group Va of the periodic table. It is a chalcophile element that occurs associated with Au, Ag, Hg, and As, in complex precious metal deposits and with Pb and Zn in some base metal deposits. From a crystallochemical point of view 116 antimony is closely related to arsenic and bismuth and is completely miscible with arsenic. Antimony can form alloys with silver (of up to 11% Sb) without causing any structural changes. Antimony forms numerous minerals including metallic antimony, alloys and antimonides, sulphides and sulphosalts. The more common minerals include stibnite, stibiconite, pyrargyrite, tetrahedrite and cervantite.
Antimony is very commonly found associated with mercury in modern geothermal systems as well as in many epithermal precious metal districts. This association is the main reason why the element is a good gold mineraliza tion pathfinder element. The abundance data on antimony is also shown in Table 4.
Arsenic
Arsenic, atomic number 33, occurs in the same group Va of the periodic table as antimony. It is a chalcophile element that shows an especially strong geochemical affinity with gold in practically all types of gold deposits. Minerals of which arsenic is a major constituent include the element arsenides, sulphides/sulphosalts, and numerous arsenates. Arsenopyrite is the most abundant ore mineral, but realgar, orpiment and enargite are commonly occurring arsenic bearing minerals. Arsenic and antimony are hexagonal isostructural compounds with a complete solid solution series between them. It is an extremely volatile element which is very common as a discharge from active metal bearing geothermal systems, and can form extensive dispersion halos around some precious metal ore deposits. Table 4 also shows the abundance data on arsenic.
Tellurium Tellurium, atomic number 52, occurs in group Va of the periodic table. It is a chalcophile element which is normally associated with As, Sb, Se, Ag, Au in epithermal precious metal deposits. The most important telluride minerals are the gold, silver and gold-silver tellurides. Tellurium can substitute to a limited extent for sulphur in sulphides, and in sulphosalts it has similar properties to antimony. In the natural environment, tellurium has few lithophile tendencies; it is mainly restricted to sulphides and low-temperature supergene minerals. In the oxidation zone of ore deposits tellurium shows little tendency for migration even at rather high oxidation potentials. Becuase of this stability, tellurides accumulate in placer deposits together with gold and other heavy minerals. As a result of its wide dispersion in hypogene miner alizing processes and its relative immobility in the supergene zone, tellurium can be a useful pathfinder element. Also if found in the presence of bismuth associated with epithermal precious metal mineralization, the ratio Te:Bi can indicate depth to which the system is exposed (T. Heidrick, person, 1981). The abundance data on tellurium is also shown in Table 3.
Bismuth
Bismuth, atomic number 83, belongs to the group Va elements of the periodic table. Although Bi is a distinctly chalcophile element, it exhibits some litho- phile tendencies. The bismuth minerals are fairly rare and are mainly found in hydrothermal ore deposits where Bi usually occurs in combination with either sulphur or other metals such as Au, Se, and Te. The geochemistry of bismuth is dominated by its presence in sulphides; it shows an especial preference for galena, in which there is an indication that the bismuth content tended to increase with an increase in temperature (Schosel, 1955, in Handbook of Geochemistry). The sulphide minerals of Bi are found widely distributed in hydrothermal deposits, in pyrometasomatic deposits, alongside granite contacts with limestones and in other pneumatolytic deposits. A limited number of analyses were performed for this element for the reasons implied above, and also to ascertain Te:Bi rations. Because of its total absence from the mercury districts this element is not discussed any further in this report.
Thallium Thallium, atomic number 81, occurs in group Illb of the periodic chart. Thallium minerals are of very rare occurrence in nature, and are only formed during epithermal stages of hydrothermal processes or under supergene conditions. This element forms characteristic dispersion halos around some gold deposits (such as Carlin and Round Mountain, Nevada ). For this reason a limited number of analyses for the element were performed. Also other studies on the behavior of thallium in ore deposition shows thallium enrichment is usually accompanied by a concentration of volatiles such as arsenic, antimony and silver (C.A.R. DeAlburquerque, D.M. Shaw, Handbook of Geochemistry, 1980). Like bismuth, no thallium responses were found in our sample areas and its limited analyses was discontinued. It also does not occur further in any data compilation or subsequent discussion in this work. TABLE 4 An estimate of average elemental abundance in the more common rock materials and some common rock forming minerals. ELEMENTS IN PPB
MATERIAL Au Ag Sb As Hg Te Bi Tl Igneous Rocks: Granite 2 50 200 1000 39 2000 Granodiorite 3 50 200 21 2000 Andesite 70 180 2800 4 370 Basalt 4 100 383 1000 7 370 Gabbro 5 100 144 1200 160 Peridotlte^ Serpentinite 7 50 100 1000 4 160 Sedimentary Rocks: Sandstone 3 250 1000 1200 2000 Siltstone 3 680 Shale 3 190 2600 12000 2000 Carbonates 2 150 300 1100 Metamorphic Rocks: Hornfels 9 700 Argillite & Slate 1 500 2000 450 Schists 2 600 3100 600 340 Quartzite 5 220 1500 2200 Carbonate Rocks 2 150 Common Rock Forming Minerals: Quartz 1 ’-50 100 Feldsapr 2 -50 103 400 100 Biotite 5 660 5000 Hornblende 3 850 7000 800 Magnetite/Ilmenite 7 210 600 400 118 119
APPENDIX 2 Sampling Technique
For the stream sediment sampling, the main drainage of the areas of interest were marked on the field map (usually 7.5' quadrangle sheets) prior to going into the area. Samples were collected from tributaries draining directly from areas of known mineralization and then fanning away from these to pick up unknown mineralizations. Samples were also collected far enough away from the mineralization to give "background" values. There were a number of occasions when it was not possible to get the ideal sites; then a compromise area was usually selected to best represent that area. Once there the sample itself was taken preferentially from granules and finer sized material. The samples were always taken to represent current stream flow material. Usually 10 to 30 spot grabs of the stream bed material over a distance of 25 to 300 feet were taken. The samples usually weighed between 2-10 pounds. This material was labeled, and bagged and then sent directly to the lab for analysis.
The lithogeochemical samples were by far the larger group taken, sampling fresh, altered or weathered vein/ rock material. An attempt was always made to get a channel or semicontinuous chip channel across the strike of the body being sampled. At each site usually 104 pounds of material was collected and a brief description of the rock type made in the sample ticket book.
The material herein referred to as retort tails usually consists of fine, red dust with some coarser material and was obviously, by its appearance and proximity to the furnaces, subjected to some form of metallurgical treatments. Where these sites were identified a channel sample was taken perpendicular to the layering, usually of 5-10 pounds of material. There were a number of sites where mined material was heaped, these represented possible ore stockpiles and/or mine waste. At these localities known rock types were preferentially selected and composited into rock samples. It is fairly obvious this came from the workings in the immediate vicinity. The exact source of these materials is not known, however. This can sometimes be very frustrating as in sample 6093 and 6094 from the Ambrose-Comstock Mine area. Both samples are of gold ore grade material yet their exact source and nature of occurrence is unknown. The final category of samples taken were soil samples. A limited number of these were taken from four to 10 inches below the present soil surface, normally at spot locations. 1 2 0
In all cases, care was exercised not to sample soil that was subjected to any recent movement. These samples usually averaged two pounds and a note was always made of the underlying rock type. 1 2 1 APPENDIX 3
Method of Analysis The majority of geochemical analyses were performed by Barringer Resources, Inc., and Hunter Mining Laboratory, Inc., both of Reno, Nevada. All analyses for tellurium were performed by Bondar-Clegg, Inc., Canada, and Skyline Labs, Inc., of Colorado.
Both Reno laboratories utilized a similar method of sample preparation and analyses. Initially stream sediment samples were sieved and a split taken for pulping to -80 mesh. A split of the pulps was used for trace analyses by Atomic Absorption (AA). Later, with the stream sediment samples it was suggested it would be better to reduce the bulk sample by quartering and then pulp all size fractions prior to taking a split for analysis. The rock samples were split down to suitable volume by quartering the pulp prior to splitting for analysis. The soil and tailings were treated in a similar manner to the rock samples. All reject material and any remaining pulp is in storage at GMRC's Reno office. The following is a brief summary of the analytical technique by element used by Hunter Mining Laboratory:
Gold and Silver A 10.0 gram pulp sample is weighed into a 250 ml beaker. Aaua-Regia is then added into the beaker and placed on a hot plate where the sample is digested for 1% - 2 hours at approximately 200°F. The digested pulp in the beaker is heated until a point just before dryness. The gold and silver is then brought back into solution using (6M)HC1 and an acetate buffer. Silver is run directly by Atomic Absorption (AA) while the gold is extracted from an aliauot of the 6M HCl + Acetate buffer solution; the recovered solution is then analyzed by AA for gold.
Mercury A 2 gram pulp sample is treated with concentrated sulphuric and nitric acids for digestion on a hot plate set at medium heat. Potassium permanganate and hydroxylamine hydrochlorite (as a reducing agnet) are added to the solution of the digested pulp; ZnCl2 is then added and Hg determined using flameless AA.
Arsenic and Antimony A 1.0 gram pulp sample is weighed into a shaker tube 1 2 2
and 6M HCl solution saturated with a bromine solution is added; this mixture is shaken, filtered and then analysed by AA. Bismuth A 5.0 gram pulp sample is weighed into a pyrex tube, concentrated perchloric acid is then added and ths solution is digested (on a hot plate) for six hours. The cooled solution is brought up to a volume and analysed by AA.
Tellurium The tellurium analyses were performed by two labora tories with vastly different detection limits; unfortunately there has been no response to date from these laboratories as to the nature of their analytical technique.
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