Wallrock alteration and geochemistry of the Randsburg mining district, Kern and San Bernardino Counties, California

Item Type text; Thesis-Reproduction (electronic)

Authors Wiggins, Martin Robert, 1951-

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.

Download date 06/10/2021 05:36:46

Link to Item http://hdl.handle.net/10150/558201 WALLROCK ALTERATION AND GEOCHEMISTRY

OF THE RANDSBURG MINING DISTRICT,

KERN AND SAN BERNARDINO COUNTIES, CALIFORNIA

by

Martin Robert Wiggins

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCES

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 9 2 2

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED i Z j * d r

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below: 3

ACKNOWLEDGEMENTS

Among the many people who contributed in various ways to this thesis I would like to extend special recognition to the members of my committee: John Guilbert, my thesis director, for his inspiration, his constant willingness to talk shop, and for introducing me to my field area; David Hendricks for his patient instruction in X-ray diffraction techniques for clay minerals and for letting me have the run of his laboratory; and Joaquin Ruiz for pinch-hitting at the last moment when I needed a substitute committee member.

In addition I would like to acknowledge a great debt to

Jim Briscoe of JAB A Inc., who not only shared his knowledge of the Randsburg district with me but also graciously allowed me access to his files, had his office staff make base maps for me, and let me stay in JABA's mobile exploration headquarters while I was doing my field work. His probing questions were of great use in focusing my work in the district.

A special thanks is due Warren Hinks, of Westland

Minerals, who funded my research. It is safe to say that the thesis would never have been started without his financial support.

I would also like to thank Casey Danielson of Echo Bay

Inc. who generously shared Randsburg district drill core with me, thereby affording me a three-dimensional look at the district. DEDICATION

To my wife and partner, Cyndy Wiggins, without whose support and understanding this thesis could never have been completed. 5

TABLE OF CONTENTS

I. LIST OF FIGURES oooooooooo

II. LIST OF TABLES oooooooooooooooooooooooooooooo x] VO

III. ABSTRACT..», 0606 0000000 000000000000 10

IV. INTRODUCTION...... OOO OOOOOOOO 0 00 000000000 oil

Loc&t 00 ...... oo...... 0000060000000 o o e e o 1 1

Previous Work...... 0000000000000 o o o o o 1 1

Statement of the Problem... 0000 000000000 o e e o o 1 4

V. DISTRICT DESCRIPTION...... 00 00000000000 o o e c o 1 6

Physiography 000000. 0000000 0000016

General Geology.. 000000 o o o o o 1 6 Rock Types... oooooooooo o o o o o 2 2

Johannesburg Gneiss.... 0000000000000 o o o o o 2 2 Atolia Quartz Monzonite o o o o o 2 3

Rand Schist...... 0000000. 00000 o o o © o 2 5 Hypabyssal Intrusives ...... 27

Structure and Tectonics.... 0000000 ©oo©©29 Ore Deposit Types.... Tungsten ...... o o o o o 3 1 Gold. Silver...... 32 Mineralizing Episodes...... o o o o o 3 3 VI. METHODS OF STUDY___ ...... o o o o o 3 5 Clay Mineral Structure..... Illite-Sericite-Hydromica...... X-ray Diffractometry...... o o o o o 4 2 Sample Preparation...... e o o o o 4 3 Mineral Identification.... o o o e o 4 5 Quantitative Alteration.... e o o o o 4 9 Geochemical Analysis...... o o o © ©50 Geostatistical Analysis.... o o o O o 5 0 Petrographic Analysis...... 52 Ultraviolet Light Examination ...... 00000 5 2 VII. MINERALIZATION, ALTERATION, AND GEOCHEMISTRY. ______53 Mineralization...... 00000 5 3 01C G 2j O n 6S 00000000 0 OO OOOO OOOOOOO O O OO 00 00000 5 3 Tungsten Episode Veins...... o o o o o 5 3 Gold Episode Veins ...... 00000 5 3 Silver Episode Veins...... 56 Clay Mineral Descriptions»000000000 o o o o o 5 8 Tungsten Episode Veins...... 58 Gold Episode Veins»000000000000 o o o o o 5 8 ■ Silver Episode Veins 00000000000 o o o o o 6 0 Intersection of Different Vein Types ____©60 Vein Scale Alteration ...... ____©60 Tungsten Episode Veins ...... o ... .60 Gold Episode Veins»»» ...... 0000064 Silver Episode Veins*« 0000069 6

TABLE OF CONTENTS - Continued

District Scale Alteration .74 Intermediate Argillie Alteration.. .74

Silicification opoooooo oooooooo .74

Phyllic Alteration. o o o o o o o oooooooo .74

Propylitic Alteration...... o o o o o o o .74 Vein Scale Geochemical Zoning...... 76 Tungsten Episode Veins...... 76

Gold Episode Veins...... oooooooo .81 Silver Episode Veins...... 81

VIII. INTERPRETATION. oooooooo oooooooo .83

Hydrothermal AlterationLooo o oooooooo .83

Tungsten Episode Veins oooooooooooo . .83 Gold Episode Veins...... 85 Silver Episode Veins...... 86 Supergene Alteration...... 86 Geochemical Zoning...... 87 Quantitative Alteration...... 88 IX. SUMMARY AND CONCLUSIONS ...... 94 Age Relationships Among Mineralizing Episodes..94 Vein Set Characteristics...... 94 Tungsten Episode Veins...... 94 Gold Episode Veins...... 96 Silver Episode Veins...... 97 Study Techniques ...... 98 , Clay Mineral X-ray Diffraction. . .98 Geochemistry ...... 98 Quantitative Alteration...... 99 Petrographic Analysis...... 99 Geostatistical Analysis...... 99 Ultraviolet Light Examination.. .100

Conclusions.. oooooooooooo© .100 APPENDIX A: CLAY SAMPLE XRD RESULTS BY VEIN TYPE..102 APPENDIX B: ELEMENTARY STATISTICS ON RAW GEOCHEMICAL DATA...... 106 APPENDIX C: ANALYTICAL RESULTS...... 107 REFERENCES...... @^[email protected] 7

LIST OF FIGURES

FIGURE 1, Study Area Location ...... 12

FIGURE 2, Tectonic Setting of the Rand Mountains ...... 1717

FIGURE 3a, Physiographic Setting...... 18

FIGURE 3b, Panorama of Randsburg district...... 19

FIGURE 4, Geology of the Eastern Rand Mountains...... 21

FIGURE 5, Distribution of the Pelona, Rand, and Orocopia Schists...... o...... 28

FIGURE 6 , Sample Locations...... o...... 36

FIGURE 7, Phyllosilicate Structure...... 39

FIGURE 8 , Phyllosilicate Structure.... .40

FIGURE 9, Tungsten Vein ...... 54

I'lOiTJ^^E 10, Gold Vein...... o...... 55

FI CURE 11, S i lx^ er XZ e i n...... 57

FIGURE 12, Phy111c Alteration...... 61

FIGURE 13, Argil lie Alteration...... 62

FIGURE 14, Clay Mineral Assemblages and Alteration Zones...... 6

FIGURE 15, Argillic Alteration...... 67

FIGURE 16, Silicification...... 68

FIGURE 17, Phyllic Alteration...... 70 ) FIGURE 18, Phyllic Alteration...... 71

FIGURE 19, Propylitic Alteration...... 72

FIGURE 20, Propylitic Alteration...... 73

FIGURE 21, District Scale Alteration Patterns...... 75 8

LIST OF FIGURES - Continued

FIGURE 22, Log Values for Tungsten Vein, Wallrock, and Background Element Abundance Levels...... 78

FIGURE 23, Log Values for Gold Vein, Wallrock, and Background Element Abundance Levels...... 79

FIGURE 24, Log Values for Silver Vein, Wallrock, and Background Element Abundance Levels...... 80 9

LIST OF TABLES

TABLE 1, Classification of Phyllosilicates Related to Clay Minerals ...... 37

TABLE 2, d-Spacing for Clay Mineral Identification...... 44

TABLE 3, Effects of Diagnostic Treatments on d-Spacing...47

TABLE 4, Quantitative Estimation of Clay Fractions...... 48

TABLE 5, Quantitative Alteration...... 51

TABLE 6 , Relative Clay Abundances in Randsburg...... 59 District Veins

TABLE 7, Randsburg Mining District Vein and Alteration Envelope Widths...... 65

TABLE 8 , Maximum Values for Randsburg Mining District Veins, Greenschist, and Wallrocks...... 77

TABLE 9a, Tabulation of Quantitative Alteration - Tungsten Episode Vein Sets - Randsburg Mining District, California...... 89

TABLE 9b, Tabulation of Quantitative Alteration - Tungsten Episode Vein Sets - Randsburg Mining District, California....0.0.000...... 90

TABLE 9c, Tabulation of Quantitative Alteration - Tungsten Episode Vein Sets - Randsburg Mining District, California...... 91

TABLE 1 0 , Summary of Data By Vein Sets ...... 95 10

ABSTRACT

Mineralization in the Randsburg Mining District is characterized by three mineralization-alteration episodes, tungsten, gold, and silver, each with its own distinct set clay mineral assemblages and wallrock alteration. The tungsten episode clay fraction is characterized by a suite of clays composed of kaolinite, illite, smectite, mixed-layered illite-smectite, chlorite, and mixed-layered smectite- chlorite. Gold episode clay fractions are distinguished by the dominance of kaolinite and illite to the exclusion of other clays. Silver episode veins contain clay fractions dominated by kaolinite, with lesser illite, smectite, and mixed-layered illite-smectite. These veins are distinguished by the presence of smectite or mixed-layered illite-smectite and the absence of chlorite. Wallrock alteration patterns are complex, reflecting episodic, multistage mineralization.

District-scale quantitative alteration mapping proved to be a potentially useful exploration technique in the district. 11

lETRODUCTIOH

Location

The Randsburg Mining District lies in Kern and San

Bernardino counties in the northwestern Mojave Desert 240 kilometers NNE of Los Angeles, California, along U.S. Highway

395 (Figure 1). Three small towns within the district,

Randsburg, Johannesburg, and Red Mountain, together have a population of 600 people. The nearest large town is

Ridgecrest, 29 kilometers north along Highway 395.

Previous Work

Although the present-day Randsburg Mining District was first prospected in the 1860s and major lode gold discovered in 1885, little mention of the district occurred until

Hershey,s(1902a) brief description of schistose rocks in the

Rand Mountains. The first comprehensive examination of the district was by Hess(1909). His report on gold mining in the

Randsburg quadrangle broadly outlined the geology of the district and summarized its mining history. An unpublished masters thesis by Hillis(1924) dealt with the geology of the

Randsburg district but was superceded by Hulin(1925), who comprehensively described and named major formations in the district, drafted the first general geological map, and described in detail the district's tungsten, gold, and silver deposits.

Descriptions of the geology of specific gold, silver. 12

FIGURE 1

STUDY AREA LOCATION

[O LA

LAS VEGAS

RIDGECREST

BAKERSFIELD IDSBURG

BARSTOW

HW Y 345

SAN BERNARDINO

LOS ANGELES BLY TH E ) 13

and tungsten deposits are in Storms(1909), Hess(1909),

Dolbear(1910), Nevius(1916), Hulin(1925), Lemmon and

Dorr(1940), and Gardner(1954). Vanderburg(1931),

Prommel(1937), and Frolli(1940) deal with mining and milling

in the district. Randsburg Mining District production

statistics are in Brown(1913-14), Bradley(1915-16),

Tucker(1920,1924,1929), Sampson and Tucker(1933), Horton and

Gaylord(1934), Merrill and Gaylord(1938,1939), Wright,et

al,(1953), Troxel and Morton(1962), Clark(1970), and Fife and

Brown(1980).

The regional geology of the Mojave Desert was first described by Thompson(1929), and papers by Jahns(1954) ,

Dibblee(1967), and DeCourten(1979) all place the Randsburg district within the Mojave's general geologic framework.

Regional structural and tectonic relationships that have

influenced the Mojave Desert region are in papers by

Hewitt(1955), Dibblee(1967), Ehlig(1968), Garfunkel(1974), and

Dillon and Haxel(1978). Considered with detailed structural

studies of the Rand Schist (Vargo,1972 and Postlethwaite,1983) and a structural study of the entire Randsburg Mining District

(Morehouse,1988), these papers are extremely important in understanding the timing and localization of mineralization in the district. 14

Statement of the Problem

Recently, Morehouse (1988) demonstrated the existence at

Randsburg of three discrete vein sets, each with different orientations, ages, precious metal profiles, and alteration assemblages. The alteration assemblages hold great promise in evaluation of the district. When the alteration minerals and assemblages are understood, zoning patterns, mineralization histories and Overprint effects can be more readily worked out. One of the recognized characteristics of many epithermal gold mining districts is the significant amounts of clay associated with gold mineralization as hydrothermal alteration. Clays have been described as alteration products in a number of mines in the Randsburg district. The purpose of this study is to utilize newly available structural information from Morehouse(1988) to assemble a picture of the wallrock alteration and geochemical zoning characteristics of each discrete kinematic and mineralizing episode, with particular emphasis on the identities and distributions of clay minerals and the variations of these minerals in time and space. Predictable patterns of clay alteration assemblages might be explorationally significant. From a production viewpoint, detailed knowledge of clay mineralogy and clay mineral distribution at Randsburg should have a salutary effect on explosives consumption, hauling costs, and heap leaching or milling operations. 15

This study began with field work in the fall of 1988.

Sample profiles were collected of alteration assemblages across each vein set in various locations in the district, in outcrop, in mine openings where possible, and in drill core.

In addition, samples were collected along two lines of traverse across the district. Samples were returned to the

University of Arizona for X-ray diffraction analysis, geochemical assay, staining tests, ultraviolet fluorescence tests, and petrographic analysis. Data were plotted on district maps and utilized for determining wallrock alteration and geochmical zoning characteristics of each mineralizing episode, for determining overprint and intersection effects, and for assembling a picture of the distribution of various alteration clay mineral assemblages.

Results indicate that alteration clay mineral assemblages are distinctive enough to allow identification of each discrete kinematic and mineralizing episode, that at the vein- scale, only arsenic, antimony, gold, and silver are zonally distributed, that arsenic may serve as a pathfinder element for gold, and that mineralization in the district may have been truncated and moved in an unspecified direction from the district. 16

DISTRICT DESCRIPTION

s Physiography

The Randsburg Mining District is located in the Mojave

Desert, a physiographic province of Southeastern California

(Figure 2 ). This region is a fault block, bounded to the north and northwest by the Garlock Fault and to the southwest by the San Andreas Fault (Hewett,1954). These two master

shear zones tectonically sever the Mojave block from the

Sierra Nevadas and the Basin and Range province and from the

California coastal ranges respectively.

Locally, the Randsburg district is situated along the eastern edge of the Rand Mountains, the dominant topographic

feature in the district (Figures 3a and 3b) . This northeasterly-trending range is thirty-two kilometers long and five kilometers wide, with a distinctly asymmetric profile that contrasts a steep and rugged northwestern slope with a gentle, minimally dissected southeastern slope. Government

Peak (1145 m) is the point of maximum relief, and from its heights the physiographic features that bound the district are apparent. To the north are the Cantil Valley and the El Paso

Mountains; to the east are the Lava Mountains; to the south is a gentle slope leading to the Kramer Hills; and to the west

lies the balance of the Rand Mountains.

General Geology

The dominant lithology in the district is a unit of the 17

FIG UR E 2

34"

33" Foults (exposed, conceded) I# # ! Pre-Terliory rocks

TECTONIC SETTING OF THE

RAND MOUNTAINS (RM)

STUDY AREA □

F R O M Garfunkel (1974) 18

FIGURE 3a

1X1 PHYSIOGRAPHIC SETTING

STUDY AREA' Q

20 KILOMETERS

FROM Dibblee (1967) RANDSBURG MINING DISTRICT

LOOKING NW

FIGURE 3b 20

Rand-Orocopia-Pelona Schist, the Rand Schist, a series of fairly flat-lying, low-grade schists that have been post- metamorphically warped into a broadly folded anticline plunging gently to the west, subparallel to the Garlock Fault

(Dibblee,1967? Vargo,1972; Morehouse,1988). The schists are exposed as a window of autochthonous lower plate rocks beneath the Rand Thrust (Figure 4). Allochthonous upper plate rocks exposed to the northeast, outside the study area, and in the far southeast corner of the district form two distinct units

(Vargo,1972; Postlethwaite,1983). The northeastern upper plate rocks are a mixed package of steeply dipping gneiss, amphibolite, marble, and quartzite known as the Johannesburg

Gneiss and thought to include Paleozoic through Triassic age rocks with a Precambrian provenance (Morehouse,1988). The southeastern exposure of upper plate rock is composed solely of batholithic Atolia Quartz Monzonite, which has been tectonically juxtaposed atop the older Rand Schist.

Dibblee(1967) considered the Atolia Quartz Monzonite to be

Cretaceous in age and Silver(1984) obtained a U/Pb date of 85 ma. The core of the Randsburg Mining District is intruded by a stock along the axis of the schist antiform. It is wedge- shaped in plan view, approximately six and one half kilometers long and one kilometer wide at its base, tapering to the west.

Hulin(1925) believed it to be a portion of the Atolia Quartz

Monzonite, but recent work (Vargo,1972; Morehouse,1988) has 21

GEOLOGY OF THE EASTERN RAND MOUNTAINS

O 2 KILOMETERS l ^ o l Fanglomerate

g i g Rhyolitic breccia

| ^ | Rhyolitic felsite

FIGURE Atolia quartz monzonite

fe g -j Rand schist

Johannesburg gneiss After Dibblee, 1967 3 Marble unit of PMJg 22 demonstrated that its composition is far too variable to be part of the same body. A later series of diabase dikes and rhyolite-latite dikes, plugs, and extrasives radiates outward from an extrusive core centered about two small hills in the northeast edge of the district.

Rock Types

Johannesburg Gneiss

The Johannesburg Gneiss crops out at only two localities in the Randsburg area (Figure 4). Its northernmost occurrence lies outside the main study area 4.5 kilometers north of

Johannesburg. In aspect, it is a mass three kilometers long and one kilometer wide that structurally overlies the younger

Rand Schist. Its other occurrence is as 10 meter to 10 centimeter xenoliths in the Randsburg Stock in the core of the district (Hulin,1925? Vargo/1972; Morehouse,1988). The

Johannesburg Gneiss is a heterogeneous rock unit consisting of three major lithologies; in decreasing order of abundance they are, banded amphibolite to quartzo-feldspathic gneiss, massive marble up to 7m thick, and minor massive to coarsely banded quartzite. Although described by Hess(1909) and named by

Hulin(1925), the Johannesburg Gneiss has not been studied in detail. There is agreement on a marine sedimentary protolith on the basis of carbonates with probable sandstones. The derivation of the present-day marble and quartzite layers is straightforward but the parentage of the amphibole-bearing 23 gneisses is more obscure. They have variously been thought to be ferruginous shales (Hulin,1925), meta-diorites

(Miller,1946), and an intercalated sequence of ferruginous shales, limestones, and sandstones invaded by dioritic intrusives and subsequently metamorphosed (Vargo,1972). The precise age of the Johannesburg Gneiss is unknown.

Hulin(1925) assigned it a Precambrian age based on met amorphic grade. Vargo(1972) indicated a miogeoclinal or platformal depositional setting, which would be more typical of Paleozoic time. Recently, Morehouse(1988) suggested, on the basis of preliminary Rb/Sr isotope work, that the Johannesburg Gneiss includes Paleozoic through Triassic age rocks. The

Johannesburg Gneiss records at least two episodes of metamorphism. The older episode was progressive and probably coincided with regional and contact metamorphism caused by the diachronous intrusion of the Great California Batholith. The younger episode resulted in mylonites coincident with overthrusting of the. Rand Schist at 70-80 ma (Vargo,1972;

Morehouse,1988).

Atolia Quartz Monzonite

The Atolia Quartz Monzonite was first described by

Hulin( 1925) who noted three areas that are all part of the same batholithic mass. Only two are within the study area.

The larger of the two is found in the southeast corner of the district (Figure 4) and is a structurally featureless, uniform 24 holocrystalline, medium-to coarse-grained mass composed of subequal amounts of quartz, orthoclase, and plagioclase with

10-20 percent biotite and hornblende. Hulin(1925) thought it might lie partially in fault contact with the Rand Schist and subsequent work by Vargo(1972), Postlethwaite(1983), and

Morehouse(1988) confirmed that the Atolia Quartz Monzonite occurs structurally above the Rand Schist all along the southern border of the Rand Mountains. The other area is 2 kilometers south of Randsburg where a stock six and one half kilometers E-W and one kilometer N-S at its widest, intrudes the Rand Schist and tapers westward. Its average composition is that of a quartz monzonite but it varies from diorite to granite and from coarse- to fine-grained. It contains xenoliths of Rand Schist of varying sizes and two large xenoliths of Johannesburg Gneiss. Hulin(1925) and Vargo(1972) considered the "Randsburg Stock" to be a part of the Atolia

Quartz Monzonite that intruded the Rand Schist.

Morehouse(1988) argued, on the basis of determination of episodic quartz monzonite intrusion together with a broad sequence of finer-grained porphyries and felsites, that the stock is an intrusive complex that documents a series of intrusive events along the Rand Schist anticlinorium ranging from the mid-Cretaceous well into Tertiary time. Initiation of intrusive activity is marked by a U/Pb date of 85 ma on

Atolia Quartz Monzonite (Silver,1984). Duration of intrusive 25 activity is not radiometrically constrained on its younger end but is interpreted by Morehouse(1988) to have extended to at least 32 ma based on fault relationships. Dibblee(1967) assigned all of the quartz monzonite bodies that crop out over the western Mojave Desert to the same intrusive event, the emplacement of the Great California Batholith, He found them to be Cretaceous based on lead-alpha determination in zircons.

Rand Schist

The Rand Schist (Figure 4) occupies the core of the Rand

Mountains and constitutes the dominant lithology of the

Randsburg district. It is composed of three main rock types: gray, quartz-albite ± muscovite schist (85-90%); a dark green, albite-epidote-actinolite-chlorite schist (10-15%) occurring as both a concordant, well-foliated unit and in massive crosscutting units; and quartzites, found as discontinuous lenses and beds 2-25 centimeters thick intercalated with green and gray schist (Vargo,1972; Postlethwaite,1983).

The gray schist has been interpreted to be a metagraywacke the protolith of which was fine-grained marine elastics that, judging by its 1.5 km thickness and its continuity, most likely accumulated in a deep basin

(Hulin,1925; Vargo,1972; Postlethwaite,1983). That the grayschist had a continental provenance has been determined by

Bennett and DePaolo(1982) and Haxel and Dillon(1978). There is general agreement that the well-foliated greenschist must 26 have been derived from a protolith of basaltic flows and tuffs interbedded or mixed with the more dominant clastic sediments. The mafic mineral assemblages and relict textures

(Postlethwaite,1983) characteristic of the greenschists support this view, and Bennett and DePaolo(1982) investigated the greenschists and found them to be similar to MORBs. The more massive crosscutting greenschist units were interpreted by Hulin(1925) and Postlethwaite(1983) as shallow intrusives on the basis of lack of relict textures, crosscutting relationships, and small extent of areal exposure.

Metamorphism of the Rand Schist is thought by Ehlig(1968) to have coincided with thrusting along the Rand Thrust at 70-80 ma; metamorphic grade is inverted, with increasing metamorphic grade toward the thrust surface (Graham and England,1976;

Postlethwaite,1983). Recently, Silver and Nourse(1986) have disputed Ehlig's contention that metamorphism is related to thrusting and have identified a stack of four distinct tectonic packets separated by regionally extensive low-angle faults ("Rand thrusts") and constrained to the time period between the latest granite at about 80 ma and the onset of

Miocene sedimentation and volcanism 18-25 ma ago. Hulin(1925) thought the Rand Schist to be Precambrian in age, while later workers have favored a late Cretaceous or early Tertiary age.

Morehouse(1988) obtained a "crude" Rb/Sr isochron suggesting that the Rand Schist protolith accumulated at 160 + 1.6 ma and 27 a K/Ar age of 70.8 ± 1.6 ma on metamorphic mariposite for a minimum cooling age of metamorphism for the schist. The Rand

Schist is generally correlated with a number of similar schist bodies collectively known as the Pe1ona-Orocopia-Rand Schist

(Figure 5). All of the Pelona, Orocopia, and Rand Schists underlie major thrust faults presumed to be segments of a single, formerly continuous structure known as the Vincent-

Chocolate Mountain thrust (Ehlig,1968; Haxel and Dillon,1978;

Jacobson,1983a).

Hypabyssal Intrusives

Hulin(1925) was first to note the presence of widespread shallow intrusive rocks in the district as pipes, dikes, and sills. He remarked that the largest of the intrusives appeared to be a pipe forming a major portion of a large hill southeast of Johannesburg, from which dikes seemed to fan outward for a radius of five kilometers. Hulin described these intrusives as rhyolitic to latitic, generally porphyritic, light in color, with feldspars and quartz as characteristic phenocrysts. Dike intersections imply several episodes of injection, cooling, and fracture. He also noted a similar series of diabase dikes that appeared to be younger than the rhyolite—latite dikes. Vargo(1972) described the rhyolite-latite dikes as ranging from one to thirty meters wide and from three to hundreds of meters long and noted that one dike set appeared to be oriented generally E-W while the 28

FIGURE 5

RAND MOUNTAINS

SAN GABRIEL MOUNTAINS

ANGELES ARIZONA

Vincent-Chocolate Mtn-Rand Thrust System 100 km | Pelona-Orocopia-Rand S c h is t

DISTRIBUTION OF THE PELONA,

RANDt AND OROCOPIA SCHISTS

FROM Jacobson, 1983 29 other dike set appeared to be oriented N-S. A detailed study of both the rhyolite-latite and the diabase intrusives by

Morehouse(1988) disclosed a wealth of information concerning the geologic and structural evolution of the district. He documented a rhyolitic intrusive-extrusive event centered about two hills southeast of.Johannesburg that he referred to as the "Silicap" and "Brecciacap" respectively. The westernmost hill, the "Silicap,'V is almost certainly the large intrusive with radial dikes that Hulin(1925) described.

Together, the two hills make up what Morehouse referred to as the "Osdick Rhyolitic Complex,” an intricate body of plugs, dikes, and pyroclastics emplaced during a succession of hypabyssal to extrusive events that radiate out from the complex. Three major sets of dike clusters related to the complex were identified by Morehouse. He also identified a series of diabase dikes fanning outward from the complex and used radiometric dating and cross-cutting relationships between the rhyolite-latite and the diabase dikes to define and constrain movement on younger (<32 ma) faults in the district.

Structure and Tectonics

In broad outline, the Randsburg Mining district sits athwart the juncture of several regional structural trends

(Figure 2). It lies at the northern edge of the Mojave block, which is bounded by the northwest-trending, right-lateral San 30

Andreas Fault and the west-trending, left-lateral Garlock

Fault. Interaction between these two master shear zones has progressively rotated the Mojave block throughout the latter half of the Cenozoic. The principal structural event in the district was the thrusting of allochthonous Johannesburg

Gneiss and Atolia Quartz Monzonite over the autochthonous sediments and volcanics of the Rand Schist protolith. The objections of Silver and Nourse(1986) not withstanding, it is likely that metamorphosis coincident with thrusting resulted in formation of the Rand Schist and was accompanied by development of a thin section of mylonites in the thrust zone

(Vargo,1972; Postlethwaite,1983). Detailed structural and macroscopic alteration mapping along with radiometric age dating allowed Morehouse(1988) to define a sequence of four discretely superimposed structural and mineralizing episodes between 32 ma and 10 ma. He identified left-lateral movement on the Garlock Fault as the predominant structural influence on the Randsburg district during this time. Northward passage of the Mendocino Fracture Zone (Glazner and Supplee,1982) at about 20 ma appears to correlate with a period of mid-Tertiary detachment fault episodes in southern California and may explain the many flat faults common throughout the district.

Finally, detailed investigations by Morehouse indicate that the Rand Schist anticline and its axial intrusive complex

(”Randsburg Stock") have acted as an elongate diapir which 31 served to focus structural events and mineralization near its axis.

Ore Deposit Types

In the first comprehensive description of the ores of the

Randsburg district, Hulin(1925) recognized three groups of mineral deposits, each formed during a period of mineralization distinct from the others, and each characterized by emplacement of a valuable metal. These were, in decreasing age, tungsten; gold, and silver mineralization, each of which appeared to be confined to distinctly separate areas, with some overlap between gold and tungsten mineralization and between gold and silver mineralization.

Tungsten

Tungsten production from the district was described by

Hulin as having come mainly from a vein system that cut through the Atolia Quartz Monzonite near the town of Atolia,

just south of the southeastern corner of the district.

Tungsten ores occurred largely as open space fillings with little or no wall rock replacement and were mostly scheelite with a gangue of quartz and calcite. The scheelite veins strike generally N80W and dip steeply north. Minor production also occurred from placers in the district.

Gold

Hulin(1925) described gold ores of the Randsburg district as either open space fillings or as "impregnations" developed 32 along a suite of northwest striking veins that dipped northeast at low to moderate angles. The veins occurred in schist or quartz monzonite or as stockworks of quartz stringers characteristically formed in quartz monzonite.

Quartz, arsenopyrite , and pyrite were common gangue minerals with minor to moderate scheelite and minor galena and talc.

Iron oxides were abundant along fractures in oxidized portions of the deposits. Gold was observed by Hulin to occur mostly in the free state and practically all gold ore was oxidized.

Calcite veinlets and blebs were distinctly later than gold veins. Current gold mining operations in the district exploit micron-size disseminated gold.

Silver

Silver deposits occurred in only a limited area of the

Randsburg district near the present town of Red Mountain.

Silver ore was found in two vein systems, the first of which struck roughly N40E and dipped southeast at moderate to high angles. A later set strikes almost due north and dips both east and west at high angles. Hulin observed miargyrite, stylotypite (silver-rich bournonite), pyrargyrite, and proustite among primary silver ores, with very fine-grained bluish-gray quartz and some calcite, chalcedony, and opal as gangue minerals. He also asserted that veins of both sets dip steeply to the east or southeast near the surface but flatten at depth. Deposits were found only in the Rand Schist but 33

localization was probably structural rather than due to host rock (Hulin,1925).

Mineralizing Episodes

Morehouse(1988) employed detailed structural mapping, macroscopic alteration mapping, and radiometric dating to clarify relationships of structural and mineralization episodes in the district. He was able to define four discrete, superimposed structural episodes from 32 ma to 10 ma, three of which were economically mineralized. The oldest episode in the district emplaced scheelite in E-W to N-E striking, north-dipping structures related to establishment of a transtensive regime along the Garlock Fault. A K/Ar date on sericite with scheelite mineralization enabled Morehouse to establish initiation of this episode at 32.3 ± 8 ma. The second mineralizing/structural episode occurred from about 20 ma to 17 ma and was related to northward migration of the

Mendocino Fracture Zone. Northward movement of the Mendocino

Fracture Zone "perturbed" the dominant left-lateral stress regime established by the Garlock Fault and resulted in emplacement of gold mineralization into a suite of back-to- back extensional faults striking northwestward and dipping northeastward at low to moderate angles. The extensional regime established during this time period may be related to detachment fault formation which would in turn explain

R u l i n g (1925) observation that flat faults are a common 34

occurrence in the district. During the period from 17 ma to

15 ma, a swarm of east-west trending pyrite-bearing rhyolite

dikes were emplaced around a complex volcanic ediface centered

along the axis of the Rand Schist anti form. Structural

activity was related to reactivation by the Garlock Fault of

its left-lateral stress regime after passage of the Mendocino

Fracture Zone. The final episode of structural and mineralizing activity occurred from about 15 ma to less than

11 ma and was characterized by multistage development of an en

echelon normal left-lateral fault system striking northeast, dipping at moderate to high angles southeast, and linked by tensional faults striking north-south and dipping both east and west (Morehouse,1988). This structural episode, which emplaced silver, was related to continued episodic left-

lateral movement on the Garlock Fault. In the balance of this paper, clay mineral and alteration mineral assemblages characteristic of the three mineralizing episodes delineated above will be described. It should be understood that the terms "tungsten vein episode" ,"gold vein episode", and

"silver vein episode" refer to the mineralizing episodes defined by Morehouse(1988) and that these labels define veins

in which tungsten or gold or silver was the most significant metal but by no means the only metal extracted. 35

METHODS OF STUDY

To provide a quantifiable description of alteration at both the district and individual vein scale, 209 separate samples were collected from precious metal and tungsten veins in the district and along two district traverses (Figure 6).

Representative samples were analyzed geochemically. X-rayed to identify clay minerals present, and petrographically examined to determine alteration and textures. Field occurrences were assessed with quantitative alteration mapping techniques.

Individual methods are summarized below.

Clay Mineral Structure

Because a major aspect of the study is concerned with analysis of clay minerals in the Randsburg district and because clay mineral structures are not familiar in detail to most geologists, a brief introduction to the more important aspects of clay minerals and their structures is provided here. The following discussion was derived from Grim(1968),

Carroll(1970), Klein and Hurlbut(1977) and Eslinger and

Pevear(1988) to whom the interested reader is referred for a more comprehensive discussion. A useful classification of phyllosilicate clay minerals is reproduced as Table 1 and summarizes present thought concerning clay types.

As a group, clay minerals are predominantly phyllosilicates, characterized by silica tetrahedra (Figure

7a) linked to form sheets composed of six-membered rings of

Classification of Phyllosilicatcs Related to Clay Minerals TABLE 1 L a y e r In te rla y e r G ro u p S u b g ro u p Species (Examples) T v p e M a te ria l 1:1 N o n e o r Serpentine-kaolin S e rp e n tin e Chrysotile, lizardite, berthicrine, M 2C ) o n ly ( x - 0 ) K a o lin kaolinite, dickite, nacrite, halloysite

2:1 N o n e Talc-pyrophyllite T a lc Talc, willemseite ( x - 0 ) Pyrophyllite Pyrophyllite ,

I1 y d ra te d S m e c tite S a p o n itc Saponitc, hectorite, sauconite, stevensite, etc. exchangeable (x - 0.2-0.6) Montmorillonite Montm orillonite, beidellite, nontronite cations

H y d ra te d Vcrmiculite Trioctahedral vcrmiculite Trioctahedral vcrmiculite exchangeable (x - 0 ,6 -0 .(>) D(octahedral vcrmiculite Dioctahcdral vcrmiculite cations

Nonhydrated T r u e m ica Trioctahedral true mica Phlogopite, biotite, lepidolite, annite. cations (x -0.5-1.0) Dioctahcdral true mica Muscovite, illite, glaucontie, pargonile, celadonite

Nonhydrated Brittle mica Trioctahedral brittle mica C lin to n ite cations (x - 2 .0 ) Dioctahcdral brittle mica M a rg a r itc

C h lo rite Trioctahedral chlorite Clinochlore, chamosite, nimite, pennantite H y d ro x id e (x - variable) Dioctahcdral chlorite D o n b a s s ite 2:1 mixed-layer Variable None N o n e Hydrobiolite, rectorite, corrensite, (re g u la r) aliettite, tosudite, kulkeite

M o d u la te d None No group name No subgroup name Antigoritc, greenalite

'1:1 la y e r ( x - 0 )

M o d u la te d H y d ra te d Sepiolite-palygorskite S e p io lite Sepiolite, laughlinite 2:1 la y e r exchangeable (x - variable) Palygorskite Palygorskite cations No group name V a ria b le (x - variable) No subgroup name Minnesotaite, stilpnamelane, zussmanite

FROM Eslinger & Pevear,1988 co 38 tetrahedra each of which shares three of four oxygens (Figure

7b). Aluminum (Al+3) may substitute for silicon (Si*4), but does so relatively rarely and requires a coupled substitution.

Tetrahedral sheets always exist in combination with an octahedral sheet composed of cations surrounded by six neighboring oxygens or hydroxyls (Figure 7c). Aluminum (Al+3) is the most common cation in octahedra but may be substituted for by 3/2 magnesium (Mg*2) and ferrous (Fe*2) or ferric (Fe*3) iron. Octahedral sheets, unlike tetrahedral sheets, can exist by themselves as the hydroxide minerals brucite. Mg(OH)2, and gibbsite, A1(OH)3 but technically these are not clay minerals.

Nonetheless, the chemistry and geometry of these two minerals form the basis for separation of phyllosilicates into the two major groups, dioctahedral and trioctahedral. In dioctahedral phyllosilicates such as pyrophyllite, only two out of three N6 cation positions are filled with M*3. Trioctahedral phyllosilicates such as talc have three of three cation positions filled with M*2. In combination with one or more tetrahedral layers, octahedral layers of either gibbsite or brucite may be thought of as the building blocks of clay mineral structures (Figure 8). There are two basic layer types. Bonding together a single tetrahedral layer by substituting its unshared apical oxygens for hydroxyls in an octahedral layer results in the simplest phyllosilicate clay mineral structure, the 1:1 layer. If the octahedral layer 39

F IG U R E 7

O X Y G E N IO N SILICON ION

ALUMINUM, MAGNESIUM, ETC, IONS

STRUCTURAL FRAMEWORK OF CLAY MINERALS

F R O M Grim, 1968 40

F IG U R E 0

Trioctahcdral Oioctahedral

•• Brucltc m G-bbsite

m •• m

Anllgtxite . Kaolinitc

Phlogoplle Muscovite

I iI Interlayer cation

t I °\

O • Oeygro o - M/tko.y*

• ■ klv«Twx*#m

SCHEMATIC DEVELOPMENT OF

SOME PHYLLOSILICATE STRUCTURES

FROM Klein and Hurlbut, 1985 41 is trioctahedral, the result is antigorite (Mg+2) or chamosite

(Fe+2)? an aluminous dioctahedral layer yields kaolinite

(Figure 8). The other major layer type is the 2 ;1 layer, a composite of two tetrahedral sheets and one octahedral sheet linked together. Like the 1:1 layer, the 2:1 layer may be either dioctahedral or trioctahedral. Individual 1:1 or 2:1 layers can be neutral or bear a negative charge arising from isomorphous substitution of cations in the tetrahedral or octahedral sheets. Such a charge is balanced by cations in the space between layers (Figure 8). Identical layers may be stacked in different ways, producing various clay polytypes

(polymorphs) and layers of different type and composition or charge may be stacked in either ordered or random fashion to produce mixed-layered clays. It is a key feature of clay minerals that they sensitively reflect, in composition and structure, many aspects of the environment in which they form, and they are thus useful in studies such as the one proposed here, to distinguish between hydrothermal events.

Xllite-Sericite-Hydromica

The term "illite” was introduced by Grim,et al(1937) as a general name for clay-size minerals of the mica group that are commonly found in argillaceous sediments. Modern usage (Sroden and Eberl,1984) of the term "illite" refers to a nonexpanding dioctahedral aluminous potassium mica-like mineral that occurs in the clay-size fraction. The term 42

’’sericite*' is a term used to indicate highly birefringent, fine-grained, dioctahedral micaceous material, normally of hydrothermal origin, as seen under the petrographic microscope.

The mineral or group name illite represents the dominantly potassic dioctahedral aluminous mica-like fraction of clay-size materials known as sericite or hydromica in studies of hydrothermal alteration, soil mica or illite in soils, and illite in sedimentary rocks. The material encompasses a wide range of compositional solid solutions

(Velde,1977).

X-ray Diffractometry

The great power and utility of X-ray powder diffraction analysis of clay minerals lies in the fundamental nature of clay minerals themselves, which consist of characteristic types of layer structures and interlayer materials repetitively overlying one another and yielding a series of reflections, termed basal reflections, that are a measure of the distance between basal planes ((001),(002), (00L) of tetrahedral/octahedral units (the "d-spacing") in the clays.

D-spacing is diagnostic for the major clay mineral groups.

Table 2 summarizes d-spacings and peak intensities for representative clay minerals and suggests four basic groups sorted according to d-spacing for first order basal reflection: 43

(1) 1:1 minerals such as kaolinite and serpentine with

a first order basal reflection near 7 angstroms;

(2) non-swelling 2:1 minerals such as pyrophyllite,

talc, lepidolite, muscovite, and illite, with first

order basal reflections near 10 angstroms;

(3) swelling 2:1 minerals such as smectite and

vermiculite with a first order basal reflection

near 14 angstroms and chlorite, which is non­

swelling but has a similar 14 angstrom d-spacing;

(4) sepiolite and palygorskite, 2:1 minerals with first

order basal reflections near 12 and 10.5 angstroms

respectively.

Sample Preparation

Selected vein and wall rock samples were crushed in stages to minus 125 microns (-120 mesh), homogenized, split, and pulverized in a rotary mill to minus 62 microns (-230 mesh). After pulverization a 50 g sample was weighed out. To disaggregate floccules, 25 ml of sodium hexametaphosphate dispersant was added to each sample along with sufficient distilled water to dilute the suspension to 100 ml containing about 3-5 g of clay. Each sample was homogenized for five minutes with an ultrasonic probe and then transferred to a large flask, diluted to 1200 ml with distilled water, and allowed to sediment for an appropriate time as determined by

Stokes' Law. A pipette was used to extract the coarser TABLE 2 Spacings in A, and Intensities for Preliminary Identification of Clay Minerals Intensity of (00L) Minerals d (001 L = 1 2 3 4 5 d (060) Kaolinitc group 7.15-7.20 100 90 15 10 4 1.489 Mg-scrpentine 7.25-7.35 100 100 20 20 1.536-1.540 Fc-serpcntinc 7.04 100 100 5 1.555 Bcrthicrinc Pyrophylllitc 9.20 80 30 100 5 10 1.493 Talc 9.35 1.527 Smectites (dioctahcdral) variable 1.49-1.50 (trioctahcdral) variable 1.52-1.54 Vcrmiculitc 14.3 100 10 15 30 40 1.541 Muscovite 10.0-10.05 >100 55 >100 20 75 1.499 Phlogopiite 10.0-10.05 >100 20 >100 30 65 1,538 Biotitc 10.0 100 20 90 10 10 1.53 Ccladonitc 9.95 50 70 10 1.51 Glauconite 9.95 100 60 20 1.511 Paragonitc 9.62 30 20 100 30 1.481 Chlorites 14.15-14.35 70 100 50 80 30 1,549 (magnesian) Chlorite (iron-rich) 14.10-14.25 20 100 20 50 10 1.56

Scpiolitc d(110) = 12.1-12,3,1 = 100;d(131) = 4.30,1 = 25-40 Palygorskile d(110) = 10.4-10.5,1 = 100;d(121) = 4.25,1 = 10-30 FROM Eslinger & Pevear.l 988

J 45 substrate from the clay fraction (<2u diameter) by withdrawal of 25 ml of the suspension from a predetermined depth (10 cm).

This resultant clay suspension was successively saturated with

Mg+2 and K+ by addition of magnesium and potassium chloride solutions to modify basal spacings and make rapid identification possible (Theissen and Harward, 1962). Smear- on-glass slide mounts were made to produce basal-plane oriented samples for X-ray analysis.

X-ray dif fractometry was performed in the X-Ray

Laboratory of the University of Arizona Geosciences Department utilizing a Siemens D-500 X-ray machine generating copper k alpha radiation, with a beam and detector slit of 1.0 and 0.15 respectively, a 960 volt scintillator, and a graphite monochrometer. Standard operational setting of 30 kilovolts at 40 milliamps, a scan rate of 2° theta per minute, a chart speed of 2 cm per minute, 1-second time constant, and a measuring range of 1 X 103 impulses per second were used.

Samples were scanned from 4° to 50° two theta after air drying, from 4° to 30° two theta after heating to 180° C, and from 4° to 14° two theta after the various treatments suggested by

Whittig and Allardice(1986) and described below and in Table

3.

Mineral Identification

Identification of clay minerals by X-ray powder diffraction is not perfectly straightforward because many 46 basal reflections overlap such as the (00L) reflections of chlorite and kaolinite, and because some clay minerals, such

as smectite, have variable basal spacings depending on degree of hydration. Various treatments must be performed on clay samples to modify basal spacings and yield unambiguous

identification of clay mineral groups.

In this study, preferentially oriented samples were subjected to X-ray diffraction after Mg+2 saturation, air drying and after ethylene glycol solvation in order to discriminate samples containing expandable clay minerals.

Heating samples to 180° C allowed for later quantitative estimates of relative clay mineral abundances. Heating Mg+2- saturated samples to 350° C gave an indication as to whether or not palygorskite and sepiolite might be present, and heating Mg+2-saturated samples to 550° C served to distinguish chlorite (002) from kaolinite (001). Saturation with K+ aided in identification of smectite by collapsing first order peaks to near 12 A, thus differentiating it from vermiculite which collapses to near 10 A with K+ treatment. Heating K+-saturated clay to 550° C helped to differentiate chlorite and kaolinite by intensifying the 14.2 A peak of chlorite. A complete listing of diagnostic treatments is given in Table 3.

Quantitative estimates of clay mineral abundances at

Randsburg (Table 4) were obtained using the method of

Griffin(l971). In the present study, no attempt was made at TABLE 3 Effect of Some Diagnostic Treatments on Spacing for First Low Angle Reflection of Clay Minerals (spacings in A are approximate)

CsCl- Ethylene Hydrazine- Reflection Mineral (50% R.H.) Glycol 350 C 550 C DMSO Disappears @ Imogolite 20-12 A 20-12A 19A D - 300-450 C Kaolinite 7 7 7 D 11 550-550 C Dickite 7 7 7 7 or D 11 550-650 C Nacrite 7 7 7 7 or D 11 550-650 C Halloysite(7A) 7 7 7 D 11 450-520 C Halloysitc(lQA) 11 10 7 D 11 450-520 C Serpentine 7 7 7 7 or D 7 575-700 C dale 9.4 9.4 9.4 9.4 9.4 850-1000 C Pyrophyllite 9.2 9.2 9.2 9.2 9.2 600-850 C Mica (illite) 10 10 10 10 10 800-1000C Smectite (Mg, Ca) 15 17 10 10 18 700-1000 C Smectite (K) 12 12-17 10 10 18 700-1000 C Vermiculitc (Mg, Ca) 14 14 10 10 18 700-1000 C Vermiculite (K) 10 10-14 10 10 18 700-1000 C Chlorite (Mg-rich) 14 14 14 14 14 800 C Chlorite (Fe-rich) 14 14 14 14 14 650 C Palygorskite 10.5 10.5 10.5+9.2 9.2 10.5 700 C Sepiolitc 12.2 12.2 12.2+10. 9.4 12.2 700 C D = peak disappears FROM Eslinger & Pevear,1988

-j 48

TA B LE 4 QUANTITATIVE ESTIMATION OF CLAY FRACTIONS

Sample# Vein Type %Kaolinite %Chlorite %lllite %Smectite RMD-9-1 Tungsten 53.5 0 39.8 6.7 RMD-9-2 Tungsten 18 31.5 24.3 26.2 RMD-9-3 Tungsten 14.1 11.2 47.1 27.6 RMD-14-1 Tungsten 48 0 50.7 1.3 RMD-14-2 Tungsten 41.7 0 53.9 4.4 RMD-15-1 Tungsten 0.8 0 7.1 92.1 RMD-15-2 Tungsten 50.8 0 45.2 4 RMD-34-1 Tungsten 57.9 10.4 23.6 8.1 ( RMD-34-2 Tungsten 21.8 23.2 51.6 3.4 RMD-34-3 Tungsten 11.5 28 52.1 8.4 RMD-7-1 Gold 7.6 0 89.5 2.9 RMD-7-2 Gold 2.4 0.6 2.7 94.3 RMD-7-3 Gold 10.2 39.2 49.3 1.3 MRD-12-1 Gold 54.8 0 45.2 0 RMD-12-2 Gold 28.7 0 66.2 5.1 RMD-12-3 Gold 5.9 0 94.1 0 RMD-13-1 Gold 0.3 0 99.7 0 RMD-13-2 Gold 0 0 100 0 RMD-13-3 Gold 4.8 0 95.2 0 RMD-10-1 Silver 81,6 0 15.9 2.5 RMD-10-2 Silver 70 0 26.1 2.9 RMD-19-1 Silver 76.2 0 0 23.8 RMD-19-2 Silver 0 0 0 100 RMD-19-3 Silver 10.3 0 NANA RMD-20-1 Silver 7.8 0 90.4 1.8 RMD-20-2 Silver 0.5 0 69.9 29.6 RMD-20-3 Silver 0 0 81.2 18.8 RMD-20-4 Silver 9.2 0 77.8 13 R.D-20-5 Silver 0 0 49 51 RMD-20-6 Silver 26.1 0 0 73.9 RMD-20-7 Silver 14 0 NA NA RMD-45-2 Au/W 17.8 0 78.9 3.3 RMD-45-3 Au/W 6.8 0 NA NA RMD-45-4 Au/W 1.8 8.1 87.2 2.9 RMD-16-1 Au/Ag 2.1 0 72.5 25.4 RMD-16-2 Au/Ag 0 0 90.7 9:3 RMD-51-1 Ag/W 66.1 0 29.6 4.3 RMD-51-2 Ag/W 0 0 90.7 9.3 49 clay polytype determination, nor were mixed-layered clays identified as to sub-type (random, ordered, etc.) since such determinations were beyond the scope of interest of this work.

Quantitative Alteration

Alteration data may be presented in a number of different ways, including ternary plots, activity-activity diagrams, or

Eh-pH plots (Rose and Burt,1979). Because of its direct applicability to mapping and computer modeling, the method of

Guilbert(1986) was chosen to gather and present alteration data. This technique involves the discrimination of intensive from extensive parameters and evaluation of their pervasiveness. Intensive parameters are those of pressure, temperature, and composition, which are seen (in like manner to metamorphic petrology) to establish the mineralogic characteristics and thereby the alteration mineralogy of the rock. Following the usage of Guilbert(1986), these assemblages are designated as potassic, propylitic, phyllic, argillie, and silicification. Extensive parameters are anologous to heat content, reaction progress, and time and are a measure of the degree to which susceptible minerals in a rock are altered, as estimated by volume percent on a scale of

1 to 10. Pervasiveness. also estimated by volume percent on a scale of 1 to 10, is a measure of the array of alteration effects with reference to veinlet control versus uniform distribution. A complete listing of terms used in the 50 quantitative alteration scheme appears as Table 5. A summary of quantitative alteration data by vein type is presented in

Tables 9a, 9b, and 9c.

Geochemical Analysis

In order to evaluate the geochemistry of altered wall rock flanking the various mineralized veins in the district, a representative suite of thirty-seven samples was chosen and submitted to Skyline Labs, Tucson, Arizona for geochemical analysis. Samples were examined (AA) for gold, silver, arsenic, antimony, mercury, copper, lead , zinc, molybdenum, and tungsten. Analytical results together with detection levels are listed in the Appendix.

Geostatistical Analysis

Data derived from the geochemical analysis performed by

Skyline Labs was evaluated utilizing a computerized geostatistical program developed by Davis(1986). The program evaluates data input as matrices and calculates such elementary statistics as mean, standard deviation, variance, and correlation coefficients. Inspection of raw analytical data revealed that molybdenum was at the lower limit of detection. For this reason, molybdenum was eliminated from statistical calculations. Analytical results for the remaining nine element suite were used to calculate the mean, variance, and standard deviation for each element and correlation coefficients were calculated for gold, silver, and 51

TABLE 5

Quantitative Alteration

A) KEY TO ASSEMBLAGES

K = potassic (biotite, orthoclase, ± sericite, anhydrite, apatite, calcite, scheelite) p = propylitic (epidote, chlorite, Mg-Fe-Ca carbonates + albite/orthoclase)

S = phyllic (quartz, sericite, ± pyrite, chlorite) A = argillic (kaolinite, montmorilIonite, K-spar metastable)

Q = silicification L = limonitization

ANK = ankerite

B) PARAMETERS

A) INTENSIVE - represented by the letter corresponding to the mineral assemblage(s) present (see A above); represents pressure, temperature, composition B) EXTENSIVE - heat content/reaction progress/time defines degree to which susceptible minerals in rocks are altered, as estimated by volume percent on a scale of 1-10

C) PERVASIVENESS - represents the array of alteration in the rock; estimated by shift from veinlet control to even distribution on a scale of 1-10

AFTER Giiilbert(1986) 52 tungsten with reference to each of the remaining elements analyzed for (APPENDIX B). No simple relationship was found to exist for any of the various element pairs.

Petrographic Analysis

A total of 122 thin sections representative of district vein types, rock types, alteration suites, and diamond drill core were examined for primary mineralogy and petrographic texture. In addition, alteration mineralogy was determined and quantified (Guilbert,1986). Twenty-six thin sections were stained with sodium cobaltinitrite to determine the presence of potassium feldspar and twenty-seven thin sections made from skeletonized core provided by Casey Danielson of Echo Bay Inc. were used to derive a generalized picture of variation in mineralogy, texture, and alteration with depth (see Figure 6 for drill hole locations).

Ultraviolet Light Examination

At the suggestion of James Briscoe of JAB A Inc., all samples, including diamond drill core, were examined for scheelite using a hand-held ultraviolet lamp in an effort to see if gold mineralization was in any way correlated megascopically with tungsten mineralization. No useful relationship was noted. 53

E3IHERALIZATIOM „ ALTERATIOM „ AM) GEOCHEMISTRY

Mineralization

Ore Zones

Access to underground is poor in the Randsburg District due to unsafe ground conditions around old mine workings. For this reason, description of mineralization in veins is based upon historical records, limited mine exposures, examination of dump material, and for gold veins, drill core information generously provided by Echo Bay Ltd.

Tungsten Episode Veins

Tungsten ore zones consist of a series of narrow (30 cm), steeply dipping veins cutting Atolia Quartz Monzonite host rock (Figure 9). Mineralization is mostly scheelite, with a gangue of quartz and calcite and occurs largely as open space fillings with little or ho wallrock replacement. Most veins were worked only to depths of 30 to 60 meters.

Gold Episode Veins

Mineralization in gold episode veins occurs as open space fillings, as fracture coatings, or in stockworks of quartz stringers. Quartz, arsenopyrite, and pyrite are common gangue minerals with minor to moderate scheelite and minor galena and talc. Iron oxides are abundant along fractures. Late stage

Mg, Fe, and Ca carbonate veinlets crosscut earlier mineralization. Ore zones are hosted entirely within Rand

Schist (Figure 10). Accessible underground workings are 54 55 56

oxidized with no primary sulfides. A look at deeper levels of the district (surface to 95 meters) is provided by core from

an eight hole drilling program conducted in 1987 by Echo Bay

Ltd. Within the oxidized zone (surface to 44 meters) quartz,

Mg, Fe, and Ca carbonates, limonite after pyrite, and scorodite after arsenopyrite are common gangue minerals. Gold mineralization is associated with moderate to strong FeOx.

Within the mixed zone (24 to 47 meters) fracture-controlled sulfides and oxides appear. Pyrite and arsenopyrite average

1 % by volume and are 25-30 % oxidized. Other gangue minerals

include quartz, Mg, Fe, and Ca carbonates, and limonite.

Within the sulfide zone (below from 36 to 47 meters) mineralization generally includes 3% to 5% pyrite plus arsenopyrite, quartz, CaC03, FeC03, limonite, and trace to minor scheelite. Gold in the ore zones is associated with pyrite and arsenopyrite, quartz, and limonite and is reported to occur in the free state.

Silver Episode Veins

Silver ores occur in two intersecting steeply dipping vein systems hosted in Rand Schist. Ore zones consist of 3 to

9 meter-wide veins whose boundaries are gradational with the wall rock (Figure 11) Silica in various forms (quartz, chalcedony, opal) is the dominant gangue mineral.

Argentiferous vein mineralization occurs as miargyrite, pyrargyrite, and proustite with very fine-grained bluish-gray 57

FIGURE 11

SILVER VEIN 58 quartz and some calcite, chalcedony, and opal as gangue minerals. Inclusions of angular schist fragments are ) abundant.

Clay Mineral Descriptions

Tungsten Episode Veins

Tungsten episode veins are moderately pervasively argillized (A-5-10) and show the universal presence of kaolinite and illite that is characteristic of the district and thus not a distinguishing feature of such veins. What highlights clays from these veins is their apparent heterogeneity (APPENDIX A). Kaolinite, chlorite, smectite, mixed-layered smectite/illite, and mixed-layered smectite/chlorite can all be found in several tungsten episode veins. If host rock lithology is considered, chlorite is a constituent only of clays coming from veins hosted in Atolia

Quartz Monzonite. Tungsten episode veins hosted in Rand

Schist produce smectite, mixed-layered smectite/illite, or kaolinite (Table 6).

Gold Episode Veins

Gold episode veins are strongly pervasively argillized

(A-8-10). Three out of four samples X-rayed show only kaolinite and illite as their clay fraction (APPENDIX A), a characteristic of gold episode veins. Chlorite, smectite, mixed-layered smectite/illite and mixed-layered smectite/chlorite are also present but they are far less 59

TABLE 6 RELATIVE CLAY ABUNDANCES IN RANDSBURG DISTRICT VEINS

HOST ROCK VEIN TYPE KAOL ILL CHL SMEC ML ML I/S S/C Krs Tungsten + ++ ND AA ND Kaqm Tungsten ++ + A A AA

Krs G o l d Var ++ —— - A —— . Krs Silver ++ ND -— ND

Krs Au—W a ++ ND - ND ND Krs Au-Ag ND ++ ND A - ND Krs Ag-W ND ND AA ND

KEY: Krs = Rand Schist Kaqm = Atolia Quartz Monzonite

++ = very abundant (>50%) + = abundant (20% - 50%)

A = some (5% - 20%) - = minor (1% - 5%) — = trace (< 1%) Var = extremely variable ND = none detected 60 common than kaolinite and illite (Table 6).

Silver Episode Veins

Silver Episode veins are strongly pervasively argillized

(A-10-10). Distinguishing features appear to be the presence of smectite or mixed-layered smectite/illite, and the absence of chlorite (Table 6)= Silver episode veins also display the presence of kaolinite and illite characteristic of the entire

Randsburg district.

Intersection of Different Vein Types

Areas where different mineralization episodes intersect are overprinted with the clay fraction characteristic of the younger episode (APPENDIX A).

Vein Scale Alteration

Tungsten Episode Veins

Regardless of wallrock type, alteration is predominantly weakly pervasively propylitic (P-2-10). In Rand Schist, propylitic alteration is characterized by variably sericitized and/or argillized but still recognizable albite porphyroblasts, chlorite, and minor epidote (Figures 12 and

13). Propylitic alteration merges gradationally with the general greenstone metamorphic grade characteristic of the

Rand Schist. In thin section greenschist metamorphic grade is arbitrarily distinguished from propylitic alteration by the presence of actinolite and/or >5% chlorite. In Atolia quartz monzonite, propylitic alteration is characterized by variably 61

O.f m m 62

F IG U R E 13

O.H m m 63

Figure Description of Photomicrographs

(12) Phyllic alteration in gold vein hosted in Rand Schist.

Note large sericitized albite porphyroblast drapped with

muscovite bundles in lower right of picture. (35%)

(13) Argillie alteration in gold vein hosted in Rand Schist.

Upper left corner of photo shows rounded quartz grain

surrounded by low birefringence clay. Note altered and

indistinct albite porphyroblasts at center of photo.

(35%)

(15) Argillic alteration in silver vein hosted in Rand Schist.

Albite porphyroblasts are indistinct and display specks

of low birefringence clay material. (80%)

(16) Silicification in a silver vein hosted in Rand Schist.

Note outline of large albite porphyroblast in center of

photo, now being replaced by quartz. (35%)

(17) Phyllic alteration in a silver vein hosted in Rand

Schist. Note late carbonate veinlet crossing through

sericitized feldspar at upper right of photo. (35%)

(18) Same as photo (E) but (80%)

(19) Plane polarized view of propylitically altered Rand

Schist from wallrock of tungsten vein. (80%)

(20) Same photo as (G) with crossed polarizers. (80%) 64 chloritized biotite, variably argillized feldspars, and patches and veinlets of late-stage magnesium, iron, and calcium carbonates = Propylitization in quartz monzonite wallrock is often succeeded outboard by zones of texturally later weak to moderate silicification (Q-2-10 to Q-5-10) characterized by the presence of major quartz and minor feldspar and chlorite (Figure 14). Ranges for vein and alteration envelope widths of all RMD vein types appear below in Table 7.

Gold Episode Veins

Alteration within 5 cm of the centers of gold episode veins is predominantly weakly to strongly pervasively phyllic

(S-2-10 to S-10-5), characterized by the dominance of quartz- sericite ±pyrite as an alteration assemblage (Figure 14).

Wallrock may be only mildly sericitized (Figure 15) or susceptible minerals may be completely altered, leaving only minor grains of quartz floating in a matrix of sericite.

Kaolinite and smectite are generally below 5%. Phyllic alteration grades outward into a more argillized zone (Figure

16) characterized by subequal proportions of kaolinite and sericite, with many veins carrying minor to major (1-90%) mixed-layered smectite/chlorite as well. This argillized zone grades outward into a weakly pervasive propylitized zone (P-1-

10) marked by the presence of chlorite, carbonates of magnesium, iron, and Calcium, and minor epidote, as well TABLE 7: RANDSBURG MINING DISTRICT

VEIN AND ALTERATION ENVELOPE WIDTHS

VEIN TYPE VEIN WIDTH ALT. ENVELOPE WIDTH TUNGSTEN 10 - 100 CM 15- 60 CM GOLD 50 - 120 CM 30 - 45 CM SILVER 3 - 4 M 3.5 - 4.5 M

O)

CLAY MINERAL ASSEMBLAGES AND ALTERATION ZONES

Tungsten Veins / / // / // / f / o / fresh wallrock N T /

C/y////7// / // /77)

ML smecr/ilh qtz < 2 > co

Gold Veins § / / -v° / / / / / fresh / / / / / / / greenschist / / / kaol

ser (7 7 7 // ////X z J-JJ.j / / ) chi C / / / / / / / 7) ML smec/chl //// j

Silver Veins

fresh greenschist

kaol (2ZZZZ3=o

ser cr / ? / / / 7/777////3 S m e C £ r / / / / 7D qtz < = 67

FIGURE 15

1___ !___ I o . a 68

o.V- m m 69 as iron oxides after pyrite and arsenopyrite (?). As with tungsten episode veins, there is no sharp demarcation between the propylitic zone and the host rock Rand Schist,

Silver Episode Veins

The most distinctive feature of silver episode veins is their highly argillized/sericitized/silicified nature and the complexity of alteration patterns (Figures 17 and 18). In the most comprehensivley sampled silver episode vein, alteration is strongly pervasively argillic (A-10-10) within 20 cm of vein center, passing outward into a 40 cm zone of strongly pervasive silicification (Q-10-10) and succeeded by a 3 meter wide zone of moderately pervasive (S-5-10) phyllic alteration

(Figure 14). Another silver episode vein is overwhelmingly argillized, while yet a third silver episode vein reverses the first pattern, displaying a more phyllic vein center (Figures

19 and 20), grading outward into an argillized zone. This pattern suggests multistage, episodic pulses of mineralization. Most vein centers are composed of a clay matrix in which scattered, rounded, polycrystalline quartz grains float. Ferromagnesian minerals and feldspars are entirely absent. Argillic alteration grades typically into a zone of subequal sericite and smectite that yields outward to a highly sericitized zone. Siliceous veinlets cut through the argillic matrix at both the microscopic and macroscopic scale.

Phyllic alteration yields sharply to propylitically altered 70 71

FIGURE 18

L j O, f m m 72

FIGURE 19

i____i___ i 0,SL mm 73 74

Rand schist host rock.

District-Scale Alteration

Intermediate Argillie Alteration

Occurrence of intermediate argillie alteration as the dominant alteration assemblage is confined to the extreme eastern edge of the district and corresponds roughly to the area of most extensive silver mineralization (Figure 21).

Argillization within this zone is confined to widely-scattered veins. The zone delimited in Figure 21 reflects the outer edge of any reported argillization.

Silicification

Silicification is confined to the eastern half of the district with the most extensive zone corresponding almost exactly to distribution of silver mineralization (Figure 21).

Phyllic Alteration

Generally speaking, the entire Randsburg district displays low-level sericitization but the highest levels are confined to the eastern edge of the district with minor zones of strong sericitization along the western edge (Figure 21).

Propylitic Alteration

Although the metasedimentary and metavolcanic host rocks of the district are themselves mildly propylitized

(greenschists), there is a pronounced centering of strong propylitization along the east-central edge of the Randsburg district (Figure 21). FIGURE t.1

ALTERATION BOUNDARIES

~>l

Vein-Scale Geochemical Zoning

Table 8 shows element abundance levels for veins and wallrock in the Randsburg district and compares them with general background levels for greenschists (Taylor and McLennon, 1985 Table 2.15). By inspection it is apparent that data ranges are extreme. To smooth the data, element abundance ranges were grouped and displayed logarithmically in Figures 22, 23, and 24. Element abundances and distributions for individual vein sets are described below. In general, arsenic, antimony, gold, silver, and tungsten are elevated significantly near vein centers, decreasing rapidly outward, while copper, lead, zinc, and molybdenum remain within background ranges.

Tungsten Episode Veins Antimony, copper, lead, zinc, and molybdenum in vein centers and wallrock of tungsten episode veins fall into normal background ranges for greenschist (Figure 22). Within vein centers, only gold, silver, arsenic, and tungsten are elevated significantly above background. Gold values range up to 400 times background level, silver up to 24 times background, and tungsten up to 32 times background level.

Arsenic is extremely elevated, with values as much as 9 million times normal background levels. Gold, silver, arsenic, and tungsten are similarly elevated in wallrock of tungsten episode veins but those elements diminish rapidly TABLE 8

M A X I M U M VALUES (ppm) LOR HM D VEINS AN U GREENSCHIST AND WALLROCKS _____

AU AGAS SBHGCU PB ZN MO w VEIN

TUNGST 0.8 1.2 14000 10 0.05 16 12 85 2 64

GOLD 0.65 1.5 13000 25 2.6 40 8 70 4 40

SILVER 0.05 6.2 1500 145 0.16 130 14 44 2 82

GRNSCH 0.0018 0.05 0.0015 5.5 0.01 25 20 71 1.5 2

VVALLR

L U N G S T 0.15 0.65 3550 31 0.09 48 16 115 2 60

GOLD 0.3 0.8 6100 22 0.48 120 16 125 2 18

SILVER 1.8 3.5 870 70 0.16 120 14 16 6 325 FIGURE 22 Log Values for Tungsten Vein, Wall rock And Background Element Abundance Levels

1 0 0 0 0 0 |

1 0 0 0 0 m

1000 c o 100|!

CD 10|' CL (/) ■c 1! 03 CL 0.1!’

0.01

0.001 Au Ag As Sb Cu Pb Zn Mo W

VEINS WALLROCK BACKGRND IUE 23 FIGURE

Parts Per Million And Background Element Abundance Levels Abundance Element Background And Log Values For Gold Veins, Wallrock, Veins, Gold For Values Log VEINS u g s b u b n o W Mo Zn Pb Cu Sb As Ag Au ALOK BACKGRND H m WALLROCK FIGURE 24 Log Values For Silver Veins, Wallrock, And Background Element Abundance Levels

c o

CD CL CO t r 03 CL

0.011!

0.001 i i r r i i i i Au Ag As Sb Cu Pb Zn Mo W

H H VEINS H i WALLROCK g g BACKGRND 00 o 81 outward (within 50 cm) to near detection limits, except for arsenic, which is reduced by a factor of 10.

Gold Episode Veins

For gold episode veins and wallrock antimony, copper, lead, zinc, molybdenum, and tungsten occur within normal background ranges for greenschist (Figure 23). Only gold, silver, and arsenic are elevated significantly. Gold values in veins are up to 360 times background. Silver values in veins are 30 times background levels, while arsenic values in veins are as much as 86 million times normal background levels. Values for gold, silver, and arsenic are similarly elevated in wallrock. Gold and arsenic are distinctly zoned, with high levels near vein centers decreasing by a factor of

10 within 20 to 50 cm outboard.

Silver Episode Veins

Silver episode veins and wallrock show elevated levels of gold, silver, arsenic, antimony, and tungsten. Copper, lead, zinc and molybdenum fall within normal background ranges for greenschist (Figure 24). Gold values in veins are up to 27 times background. Silver values are as much as 124 times background levels, while arsenic values are as much as 1 million times background. Antimony occurs up to 26 times background level and tungsten is 41 times background. Values for gold, silver, arsenic, antimony, and tungsten are similarly elevated in wallrock. Silver, arsenic, and antimony 82 are distinctly zoned, with highest values near vein centers, diminishing to near background levels within 1 to 2 meters outboard of center. Tungsten values do not follow this pattern. Anomalous tungsten values are distributed throughout the altered wallrock. 83

INTERPRETATION

Hydrothermal Alteration

Rose and Burt(1979) noted that hydrothermal alteration mineral assemblages necessarily reflect pertinent conditions of temperature and pressure, wallrock composition, chemistry of the hydrothermal fluid, energy, and time available for equilibration. The expected zonal pattern for alteration in felsic wallrock is for susceptible minerals such as feldspars and muscovite to be replaced by sericite in the zone nearest the introduced hydrothermal fluids, by kaolinite or expandable clay minerals further away, and by minerals characteristic of propylitic alteration even further away from vein centers

(Meyer & Hemley, 1967? Velde,1977; Guilbert & Park, 1986).

Tungsten Episode Veins

Regardless of wallrock type, alteration nearest the center of tungsten episode veins is predominantly argillic, grading outward into a zone of propylitic alteration, which, in Atolia quartz monzonite may be followed by a weakly to moderately silicified zone. That there is no zone of phyllic alteration (sericite) near the centers of tungsten episode veins is probably indicative of a low K+/H+ activity ratio

(Guilbert and Park, 1986, Figure 5-3) and of temperature conditions and the amount of time available for wallrock equilbration. The absence of pyrophyllite in the presence of kaolinite and free silica implies that temperatures at the 84 centers of tungsten episode veins were less than 300° to 350°

C. (Guilbert and Park, 1986, Figure 5-3). The possibility that temperatures may have been below 200° C at the centers of tungsten episode veins is raised by the presence of smectite, stable only at temperatures less than 120° C (Velde, 1977), as the dominant clay mineral near some vein centers. Clays are not truly reliable indicators of temperature, alone however, since both hydrothermal fluid chemistry and equilibration time affect alteration assemblages formed. Stability of feldspars and micas in hydrous environments is controlled by hydrolysis, in which potassium is progressively stripped from silicate framework minerals according to the following reactions (Rose and Burt,1979):

(1) 3KAlSi308 + 2H+ KAl3Si30lo(0H)2 + 6Si02 + 2K+

(K-feldspar) (sericite) (quartz)

(2) 2KAl3Si30lo(0H)2 + 2H+ +3H20 3Al2Si205(0H) 4 + 2K+

(sericite) (kaolinite)

It is possible that sericite was indeed the first formed alteration product but that continued hydrolysis over time progressively stripped potassium from vein centers and formed kaolinite. Tungsten veins hosted in Rand Schist display a 85 vein center with subequal kaolinite and sericite supporting the view that the absence of a phyllic or pyrophyllite altered zone is probably due to equilibration of relatively low temperature hydrothermal fluids over time. Fluid inclusion studies would be needed to more precisely determine hydrothermal temperatures. Argillic alteration grades outward into a zone of propylitic alteration characterized by the presence of epidote and chlorite and by lack of appreciable potash metasomatism as shown by the presence of relatively unaltered K-feldspars and micas. The zone of silicification occasionally developed outboard of propylitic alteration in some tungsten veins was not well studied. It may be the end result of mild to moderate conversion of potassium feldspar to sericite as per equation (1) or it may be a product of late stage ankeritization of sericite in schist and resultant excess Si02 production.

Gold Episode Veins

Alteration near the centers of gold episode veins is phyllic, grading outward into argillic that yields in turn to propylitic alteration. The presence of a phyllically altered zone may indicate that gold mineralizing fluids were of slightly higher temperatures than those emplacing tungsten. It is possible that there was a rapid decrease in temperature or in aK+ gradient outward as indicated by the presence of low- temperature, mixed-layered smectite/chlorite in the wallrock 86 outboard from the phyllic zone.

Silver Episode Veins

Alteration in silver veins is more complex than in gold or tungsten veins. A number of possible explanations suggest themselves. The multistage, episodic nature of silver mineralization probably resulted in reactivation of earlier conduits as new pulses of mineralizing fluids swept through the system. Alteration envelopes overlapped, particularly as tensional faults linking older northeast- striking faults, were mineralized. Multiple episodes of silicification are characteristic of silver veins and it may be that one or more of these episodes walled off the central portions of some veins, forcing mineralizing fluids outboard into wallrock and overprinting existing alteration, a process common to many epithermal systems (Silberman and Berger,1985).

Supergene Alteration

Several lines of evidence suggest that not all observed alteration effects are hydrothermal. A 1987 diamond drilling program by Echo Bay Ltd. defined a zone of oxidation from surface to about 44 meters in the district (Figure 6). Within this zone all primary sulfides have been converted to limonite and the host rock is C03-leached and partially clay altered

(kaolinized K-feldspars). A mixed zone below the oxide zone shows fracture-controlled oxides and sulfides. At deeper levels a sulfide zone is present with 3-5% sulfides, primarily 87 pyrite and arsenopyrite» The Randsburg district as a whole is colored an earthy reddish brown to orange yellow from the various iron oxides present at the surface. X-ray powder diffraction (XRD) analyses performed for this study indicate that kaolinite is a major component of the clay fraction of all three mineralized vein sets. Taken together these three lines of evidence suggest at least some supergene attack of hypogene near-surface sulfides, leaving behind iron oxides and generating sulfuric acid to strip potassium from host rocks and convert K-feldspar and mica to kaolinite. That supergene processes are responsible for only a limited amount of alteration is indicated by the observation that kaolinites are are not generally well-ordered but have broad XRD peaks (Meyer and Hemley,1967), by the presence of mixed-layered clays, and by the observation that it is possible to distinguish among mineralizing episodes ; on the basis of clay mineral assemblages.

Geochemical Zoning

Compared with normal background ranges for greenschist, wallrock associated with the district's three mineralizing episodes is distinctly anomalous for precious metals and pathfinder elements such as arsenic and antimony, is often anomalous for tungsten, and is within normal background ranges for base metals. Values for gold, silver, and arsenic are very high (400, 24, and 9 million times background level, 88 respectively) near vein centers and diminish to near detection levels within 50 cm outboard. Tungsten follows this pattern in tungsten and gold episode veins but is anomalous throughout alteration envelopes of silver episode veins. This odd distribution pattern may be due to the postulated multi-stage, episodic pulses of silver mineralization. Elevated gold and silver values near vein centers would be an expected result of mineralizing episodes. Elevated arsenic values near vein centers are probably directly attributable to the presence of arsenopyrite. Elevated antimony values near vein centers of silver episode veins are probably due to the presence of stibnite.

Quantitative Alteration

Of potential significance is the application of quantitative alteration techniques (GuiIbert,1986) to samples collected for geochemical and petrographic evaluation. Tables

9a, 9b, and 9c are tabulations of quantitative alteration noted in samples of vein and wallrock collected from the three distinct vein sets found in the district. Tabulations from these tables were first sorted by intensive parameter (see

Table 5), then divided by extensive parameter into high, medium, and low groups, and finally, plotted on district maps in order to derive general boundaries for various alteration zones. Pervasiveness was ignored in these plots since, by inspection, it was apparent that alteration in the majority of 89

TABLE 9a TABULATION OF QUANTITATIVE ALTERATION - TUNGSTEN EPISODE VEIN SETS - RANDSBURG MINING DISTRICT, CALIFORNIA

SAMPLE#TYPE FAR HW HW VEIN FW FAR FW RMD-9-3 TUNGSTEN Q-2-10 RMD-9-2 TUNGSTEN P-2-10 RMD-9-1 TUNGSTEN A-7-10 RMD-14-2 TUNGSTEN L-5-10 RMD-14-1 TUNGSTEN A-5-10 RMD-15-2 TUNGSTEN L-5-10 RMD-15-1 TUNGSTEN A-10-1 RMD-21-1 TUNGSTEN Q-3-10 RMD-21-3 TUNGSTEN A-6-10 RMD-21-2 TUNGSTEN A -10-8 RMD-34-3 TUNGSTEN 0-5-10 RMD-34-2 TUNGSTEN A-2-10 RMD-34-1 TUNGSTEN A-5-10 RMD-35-1 TUNGSTEN L-6-10 RMD-35-2 TUNGSTEN Q-2-10 RMD-35-3 TUNGSTEN L-6-10 RMD-36-1 TUNGSTEN L-10-10 RMD-36-2 TUNGSTEN L-5-10 RMD-36-3 TUNGSTEN CHL-8-1 RMD-36-4 TUNGSTEN CHL-10- RMD-36-5 TUNGSTEN CHL-3-1 RMD-39-1 TUNGSTEN L-4-10 RMD-39-2 TUNGSTEN L-5-10 RMD-39-3 TUNGSTEN CHL-2-1 RMD-40-1 TUNGSTEN P-2-10 RMD-40-2 TUNGSTEN A-2-10 RMD-40-3 TUNGSTEN P-3-10 RMD-40-4 TUNGSTEN 0-10-1 RMD-41-1 TUNGSTEN P-3-10 RMD-41-2 TUNGSTEN A-9-10 RMDr41-3 TUNGSTEN P-4-10 RMD-41-4 TUNGSTEN P-1-10 RMD-41-5 TUNGSTEN P-1-10 RMD-42-1 TUNGSTEN P-1-10 RMD-42-2 TUNGSTEN P-3-10 RMD-42-3 TUNGSTEN Q-2-10 RMD-43-1 TUNGSTEN P-4-10 RMD-43-2 TUNGSTEN P-4-10 RMD-43-3 TUNGSTEN A-4-10 90

TABULATION OF QUANTITATIVE ALTERATION - GOLD 1 A EPISODE VEIN SETS- RANDSBURG MINING DISTRICT, CALIFORNIA

RMD-7-2 GOLD Q-2-10 RMD-7-1 GOLD S-3-10 RMD-7-3 GOLD L-l-10 RMD-11-2 GOLD L-5-10 RMD-11-1GOLD S-6-10 RMD-12-3 GOLD S-10-5 RMD-12-2 GOLD L-3-10 RMD-12-1 GOLD S-10-5 RMD-13-3 GOLD Q-7-10 RMD-13-2 GOLD S-5-10 RMD-13-1 GOLD CHL-8-1 RMD-17-3 GOLD L-8-10 RMD-17-2 GOLD CHL 7-1 RMD-17-1 GOLD A-8-10 RMD-18-1 GOLD ANK-5-1 ■ RMD-18-2 GOLD A-10-10 RMD-25-1 GOLD CHL-9-1 RMD-25-2 GOLD L-9-1 RMD-25-3 GOLD N/A RMD-25-4 GOLD L-4-10 - RMD-33-4 GOLD L-8-10 RMD-33-3 GOLD S-10-1 RMD-33-2 GOLD A-9-10 RMD-33-1 GOLD A-10-1 RMD-38-1 GOLD P-6-10 RMD-38-2 GOLD Q-9-10 RMD-38-3 GOLD GRAPHI RMD-44-1 GOLD P-1-10 RMD-44-2 GOLD A-7-10 RMD-44-3 GOLD P-1-10 RMD-47-1 GOLD P-4-10 RMD-47-2 GOLD P-7-10 RMD-47-3 GOLD GRAPHP RMD-48-1 GOLD A-10-10 RMD-48-2 GOLD S-8-10 RMD-48-3 GOLD A -10-5 RMD-48-4 GOLD S-2-10 RMD-49-1 GOLD P-5-10 RMD-49-2 GOLD Q-7-10 RMD-49-3 GOLD P-1-10 91

TABLE 9c

TABULATION OF QUANTITATIVE ALTERATION - SILVER EPISODE VEIN SETS - RANDSBURG MINING DISTRICT, CALIFORNIA

RM D-10-2 SILVER L-5-10 RMD-10-1 SILVER A-6-10 RMD-19-3 SILVER S-5-10 RMD-19-2 SILVER A-10-10 RMD-19-1 SILVER A-10-10 RMD-20-1 SILVER S-8-10 RM D-20-2 SILVER S-5-10 RMD-20-3 S ILVER S-6-10 RMD-20-4 SILVER 0-10-10 RMD-20-5 SILVER A-10-10 RMD-20-6 SILVER 0-10-1 RM D-20-7 SILVER A-10-10 92 samples was either evenly distributed (a value of 10) or confined to veinlets (a value of 1). Superimposing general boundaries (Figure 21) yields a distinctly asymmetric pattern, with zones telescoping against one another at the eastern edge of the district. There are several plausible explanations for this effect. One possiblity is that the asymmetry is simply a function of sample density. Comparing sample locations

(Figure 6) to zones outlined in Figure 21, it is apparent that the telescoping of alteration zones along the district's eastern edge coincides with the most comprehensively sampled line of district traverse along which 30% of all samples lie.

However, in other areas of the district where sample densities are almost as great the same telescoping effect does not occur. Another possibility is that telescoping of alteration zones is due to sample area edge effects. That is, zones may butt up against one another simply because there are few sample locations east of the lines of district traverse. This idea can probably be discounted because no other district edges show similar telescoping. It may be that the telescoping of alteration zones near the eastern edge of the district is due entirely to alteration effects that occurred during silver vein mineralization. Alteration during emplacement of silver was strongly argi11ic/phy1lie and silicic, the very zones that are seen to telescope into one another near the district's eastern edge. Still another 93 explanation is that the three vein types are episodic and are focused in the same area, creating the telescoping effect outlined. A final explanation, and by far the most intriguing for the explorationist, is that mineralization/alteration was originally symmetrical and has been truncated and translated in some unspecified direction or is buried at a deeper level.

Hulin(1925) noted a flat fault that truncated all silver mineralization in the district. As described by him, the flat fault occurred in along the district's eastern edge. Such truncation and translation would certainly explain the dramatic lack of recognized mineral resources east of highway

395. 94

SUfflARY AMD CONCLUSIONS

This section summarizes characteristics of the three

distinct mineralized vein sets found in the district (Table

10), evaluates various techniques used to define those characteristics, and sets forth conclusions drawn as a result of this study,

Age Relationships Among Mineralizing Episodes

In general, tungsten mineralization is older than gold mineralization, which in turn is older than silver mineralization = Morehouse(1988) defined three discrete structural-minera1izing episodes in the Randsburg district.

The oldest episode emplaced tungsten mineralization into E-W to N-E striking, north-dipping structures, was initiated at

32.3+ 8 ma (K/Ar date) and continued episodically until 20 ma when an extensional regime emplaced gold mineralization into a suite of back-to-back faults striking northwestward and dipping at low to moderate angles northeastward.

Potassium/Argon dates constrain gold mineralization to the period from 20 ma to 17.6 ma. Silver mineralization was initiated at 11.6+ 0.3 ma (K/Ar date on sericite) and may have continued until 7.49+ 0.2 ma.

Veins Set Characteristics

Tungsten Episode Veins

Tungsten episode veins hosted in Atolia quartz monzonite display kaolinite, illite, smectite, and mixed-layered illite- 95

SUMMARY OF DATA BY VEIN SETS

TUNGSTEN EPISODE VEINS T A B L E 10

Clay Abundance Ranges KAOLILLCHL SMEC ML I S MLS-C 20-50% 20-50% 0-20% 5-20% 5-20% 0-20%

Gcochcmieal Ranges (ppm) LOCATIONAU AGAS SB HG CU PD ZN W VEIN <.05-.80 <.05-1.2 390-1400 3.7-10 .04-.05 14-16 10-12 38-85 <2-64 VVALLRX <.05-. 15 .05-.65 95-3550 3.1-31 .01-.09 - 14-48 ■8-16 75-115 18-60

Quantitative Alteration V E IN CTR A-5-10 moderately pervasive argillizalion W A L L R O C P-2-10 weakly pervasive propylitization (5-20 cm outboard) W A L L R O C Q-2-10 weakly pervasive silitification (15-60 cm outboard)

GOLD EPISODE VEINS

KAOL ILL CHL SMEC ML I-S MLS-C VAR >50% <1% 1-5% 5-20% <1%

Geochemical Ranges (ppm') LOCATION AU AG AS SB HG CU PB ZN W 2-40 VEIN .1-.65 .35-1.50 3200-13000 11-25 .01-2.6 4-40 6-8 6-70 WALLRX <.05-.30 .05-.80 55-6100 1.7-22 <.01-.48 16-120 8-16 12-125 <2-18

VEIN CTR A-8-10 strongly pervasively argillized W ALL ROCK S-2-10 weak pervasive phyllic alteration (15-25 cm outboard) W ALL ROCK P-1-10 weakly pervasively propyl it ized (30-45 cm outboard)

SILVER EPISODE VEINS

KAOL ILL CHL SMEC M L l-S "MLS-C > 50 % 5-20% ND 1-5% <1% ND

Geochemical Ranges (pjim ) CU PB ZN W LOCATION AU AG AS SB HG 8-130 4-14 6-44 32-82 VEIN <.05 ^ .65-6.2 60-1500 6.5-145 .04-.16 4-14 <2-16 <2-325 WALLRX <.05-1.8 .15-3.5 50-870 4.9-70 .02-. 16 10-120

Quantitative Alteration VEIN CTR A-10-10 strongly pervasively argillized (0.2 - 0.6 meters outboard) W A L L ROC 0-10-10 strongly pervasively siliciiied (1-3 meters outboard) W A L L ROC] S-5-10 moderately pervasive phyllic alteration 96 smectite or illite-chlorite in their clay fraction. Veins hosted in Rand schist contain kaolinite, illite, smectite, and mixed-layered illite-smectite but no chlorite. The relative abundance of clays in tungsten episode veins is kaolinite >

illite » > smectite > mixed-layered illite-smectite > chlorite

> mixed-layered smectite-chlorite. The heterogeneity of the clay fraction of tungsten episode veins distinguishes them from other mineralizing episodes. Precious metals, arsenic, and tungsten are all anomalous in tungsten episode veins.

Gold values are 27 to 444 times background; silver values are up to 24 times background; arsenic ranges from 2.6x10s to 9x10s times background levels; and tungsten values are up to 30 times background levels. Antimony, copper, lead, zinc, and molybdenum values are within normal background ranges.

Wallrock from tungsten episode veins is moderately pervasively argillized near (1-5 cm) vein centers, weakly pervasively propylitized within 5-20 cm of vein centers, and displays occasional zones of weakly pervasive silicification within 15-

60 cm of vein centers.

Gold Episode Veins

The clay fraction of gold episode veins is dominated by kaolinite and illite. The dominance of these two clays to the exclusion of other clays is a distinguishing characteristic of gold episode veins. Lesser constituents include chlorite, smectite, mixed-layered smectite-illite and mixed-layered 97 smectite-chlorite. The relative abundance of clays in tungsten episode veins is: illite > kaolinite » > mixed­ layered illite-smectite > smectite > chlorite > mixed-layered chlorite-smectite. Precious metals, arsenic, and tungsten are all anomalous in gold episode veins. Gold values are 55 to

361 times background; silver values are up to 30 times background; arsenic ranges from 2x10* to 8x10* times background; and tungsten values are up to 20 times background levels. Antimony, copper, lead, zinc, and molybdenum values are all within normal background ranges.

Wallrock from gold episode veins is strongly pervasively argillized near (1-15 cm) vein centers, weakly pervasively phyllically altered within 15-25 cm, and weakly pervasively propylitized within 30-45 cm of vein centers.

Silver Episode Veins

Silver episode veins contain clay fractions dominated by kaolinite, with lesser illite, smectite, and mixed-layered illite-smectite. These veins are distinguished by the presence of smectite or mixed-layered illite-smectite and the absence of chlorite. The relative abundance of clays in silver episode veins is: kaolinite » illite > smectite > mixed-layered illite-smectite. Precious metals, pathfinder elements (As, Sb), and tungsten are all anomalous in silver episode veins. Gold and tungsten are only weakly elevated (3x and 4x background) while silver is 13 to 124 times background 98 and arsenic ranges from 40,000 to 120,000 times background level. Copper, lead, zinc, and molybdenum occur within normal background ranges. Wallrock alteration patterns from silver episode veins are complex and imply multistage, episodic periods of mineralization. Argillization, sericitization, and silicif ication are all moderate to intense but vary in location depending on the vein being sampled. The bleached yellow-white color imparted by intense argillization and sericitization and the silicified ribs exposed in weathered outcrops serve to visually distinguish silver episode veins from other mineralizing episodes.

Study Techniques

Clay Mineral X-ray Diffraction

Clay mineral assemblages are distinctive enough in the

Randsburg district to allow identification of each mineralizing episode. However, X-ray determination of clay type is a cumbersome, time-consuming process, ill-suited to exploration needs. As part of a feasibility or mine planning study this technique would be a valuable aid in determination of the probable effect of clay type and content on such variables as explosives consumption, hauling costs, heap leach characteristics, and milling characteristics,

Geochemistry

Geochemical analysis of vein and wallrock samples appears to be a much more efficient technique than clay mineral 99 determination for identifying mineralization at Randsburg.

However, by itself, geochemical analysis is not precise enough to distinguish among the three mineralizing episodes in the district. Insufficient funding was available to comprehensively sample veins, wallrock, and outcrops in the district and it is therefore not possible to apply vein scale geochemical zonal patterns to the district as a whole.

Quantitative Alteration

Quantitative alteration studies proved to be a potentially useful exploration technique in the district. The utility of quantitative alteration studies lies in their ease, speed, and inexpensiveness of application. In this aspect quantitative alteration greatly resembles leached capping studies done for porphyry copper exploration. When exploration budgets limit the number of geochemical analyses that can be done, quantitative alteration studies might still identify areas worthy of more extensive examination.

Petrographic Analysis

Petrography was very valuable in deriving a generalized picture of variations in mineralogy, texture, and alteration at both vein and district scale.

Geostatistical Analysis

No simple relationships were discovered to exist for any of the various elements that formed part of the suite analyzed for. This may be a function of the limited sample population 100 or it may reflect the true state of affairs in the district.

It certainly reflects the old truism that "gold is where you find it".

Ultraviolet Light Examination

Although tungsten is anomalous in all three mineralized vein sets, it proved megascopically impossible to correlate scheelite with gold mineralization by means of a hand-held ultraviolet lamp. This was true of both core and outcrop specimens.

Conclusions

(1) Mineralization in the Randsburg Mining District during

the latter half of the Cenozoic is characterized by

three superimposed mineralization-alteration episodes,

each with its own distinct set of wallrock alteration

clay mineral assemblages.

(2) Clay mineral assemblages are distinctive enough to allow

identification of each of the mineralizing episodes.

However, given the time-consuming nature of clay mineral

identification, geochemical analysis is a more time and

cost-effective exploration method.

(3) Element abundance levels for wallrock affected by the

various mineralizing episodes are within normal ranges

for greenschist with the exception of arsenic,

antimony, gold, silver, and tungsten that are elevated

far above background levels near respective vein centers 101

and then diminish rapidly outward.

(4) Results of a district-scale quantitative alteration

study raise the distinct possibility that originally

symmetrical mineralization has been truncated and moved

in an unspecified direction or buried at a deeper level

east of the eastern margin of the study area.

(5) At the vein-scale, high arsenic values are associated

with high gold values and arsenic may therefore serve as

a path-finder element for gold. 102

APPENDIX A: CLAY SAMPLE XRD RESULTS BY VEIN TYPE

GOLD VEINS

SAMPLE # FLEEKER # RESULTS

RMD-7-1 16 kaolinite, mica, ML illite/smectite

RMD-7-2 17 kaolinite, mica, ML smectite/chlorite

RMD-7-3 18 kaolinite, mica, ML illite/smectite, discrete chlorite

RMD-11-1 24 kaolinite, mica

RMD-11-2 25 kaolinite, mica, ML illite/smectite

RMD-12-1 26 kaolinite, mica

RMD-12-2 27 kaolinite, mica, and minor discrete smectite

RMD-12-3 28 kaolinite, mica

HMD-13-1 29 kaolinite, mica

RMD-13-2 30 mica

RMD-13-3 31 kaolinite, mica

RMD-48-1 61 kaolinite, mica, ML chlorite/smectite

RMD-48-2 62 kaolinite, mica

RMD-48-3 63 kaolinite, mica, ML chlorite/smectite, discrete chlorite

RMD-48-4 64 kaolinite, mica 103

SILVER VEIBTS

SAMPLE # FLEEKER # RESULTS

RMD-10-1 7 kaolinite, mica, discrete smectite

RMD-10-2 8 kaolinite, mica, discrete smectite

RMD-19-1 9 kaolinite, discrete smectite

RMD-19-2 10 discrete smectite

RMD-19-3 11 kaolinite, mica, ML illite/smectite

RMD-20-1 1 kaolinite, mica, ML illite/smectite

RMD-20-2 2 mica, ML illite/smectite

RMD-20-3 12 mica, ML illite/smectite

RMD-20-4 13 kaolinite, mica, ML smectite/illite

RMD-20-5 14 mica, ML illite/smectite

RMD-20-6 3 kaolinite, discrete smectite

RMD-20—7 15 kaolinite, mica, ML smectite/illite

RMD-53 46 kaolinite, mica, ML illite/smectite

TUMGSTEM VEINS

SAMPLE # FLEEKER # RESULTS

RMD-9-1 54 kaolinite, mica, discrete smectite

RMD-9-2 55 kaolinite, mica, ML smectite/chlorite

RMD-9-3 56 kaolinite, mica, ML smectite/chlorite

RMD-14-1 57 kaolinite, mica, ML smectite/i Hi t e 104

TUWGSTEM ¥EIMS -Continued

SAMPLE # FLEEKER fi RESULTS

RMD-14-2 58 kaolinite, mica, ML smectite/illite

RMD-15-1 59 kaolinite, mica, ML smectite/illite

RMD-15-2 60 kaolinite, mica, ML smectite/illite

RMD-21-1 68 kaolinite, mica, ML smectite/chlorite

RMD-21-2 69 kaolinite, mica

RMD-21-3 70 kaolinite, mica

RMD-22-1 71 kaolinite, mica

RMD-22-2 72 kaolinite, mica, discrete smectite, discrete chlorite

RMD-34-1 73 kaolinite, mica, ML smectite/chlorite

RMD-34-2 74 kaolinite, mica, ML smectite/chlorite

RMD-34-3 75 kaolinite, mica, discrete chlorite

GOLD/TUNGSTEN VEIN IMTERSECTIOMS

SAMPLE # FLEEKER # RESULTS

RMD-8 101 kaolinite, mica

RMD—45—1 102 kaolinite, mica, discrete smectite

RMD-45-2 103 kaolinite, mica, discrete smectite

RMD-45-3 104 kaolinite, mica, discrete smectite

RMD-45-4 105 kaolinite, mica, discrete smectite

RMD-46-1 106 kaolinite, mica, discrete smectite 105

GOLD/TlMGSTEN VEIH IMTERSECTIOMS - Continued

SAMPLE # FLEEKER # RESULTS

RMD-46-2 107 kaolinite, mica, discrete smectite

RMD-46-3 108 kaolinite, mica, ML chlorite/smectite

RMD-46-4 109 kaolinite, mica, ML smectite/chlorite

GOLD/SILVER VEIN INTERSECTIONS

SAMPLE # FLEEKER # RESULTS

RMD-16-1 98 kaolinite, mica, ML smectite/illite

RMD-16-2 99 kaolinite , mica, discrete smectite

RMD-16-3 100 mica, ML illite/smectite

SILVER/TUNGSTEN VEIN INTERSECTIONS

SAMPLE # FLEEKER # RESULTS

RMD-51-1 110 kaolinite, mica, ML , smectite/illite

RMD—51—2 111 kaolinite, mica

RMD-51-3 112 kaolinite, mica, discrete smectite

RMD-52-1 113 kaolinite, mica

RMD—52—2 114 kaolinite, mica, ML smectite/illite

RMD-52-3 115 only qtz peaks show in this sample

RMD-52-4 116 kaolinite, mica, and minor ML illite/smectite 106

APPENDIX B: ELEMENTARY STATISTICS ON RAW GEOCHEMICAL DATA

Coef. of ariable Mean Variance Std Dev Variation

An 0.32 0.15 0.38 1.23

Ag 0.77 1.21 1.10 1.43

As 2713 14542686 3813 1.41

Sb 22.06 710.29 26.65 1.21

Hg 0.15 0.18 0.43 2.85

Cu 33.51 1062.48 32.60 0.97

Pb 11.57 29.81 5.46 0.47

Zn 49.89 1341.82 36.63 0.73

W 37.51 3056.48 55.29 1.47

NOTE: raw data in ppm 107

APPENDIX C: ANALYTICAL RESULTS

SAMPLE NUMBER Au Ag As Sb Cu (ppm) (ppm) (ppm) (ppm) (ppm) RMD-7-1 0.55 0.35 3200 25 40 RMD-7-2 <.05 0.05 55 5.5 16 RMD-7-3 <.05 0.05 115 1.7 20 RMD-12-1 0.55 0.75 5650 11 4 RMD-12-2 0.25 0.4 950 6 38 RMD-12-3 0.3 0.8 1600 4.3 34 RMD-16-1 0.7 0.85 2950 44 14 RMD-16-2 0.65 0.2 2200 45 30 RMD-16-3 1.2 0.9 2750 41 12 RMD-19-1 <.05 0.15 860 23 120 RMD-19-2 0.5 0.5 450 41 18 RMD-19-3 <.05 0.65 95 11 8 RMD-20-1 1.8 0.85 50 4.9 10 RMD-20-2 0.6 0.75 120 7.5 10 RMD-20-3 <.05 0.5 870 49 105 RMD-20-4 <.05 3.5 480 70 42 RMD-20-5 <.05 6.2 350 19 24 RMD-20-6 <.05 1.3 1500 145 130 RMD-20-7 <.05 0.7 60 6.5 22 RMD-21-1 0.15 0.45 1200 4.6 48 RMD—21—2 0.1 0.65 3550 31 26 RMD-21-3 0.8 1.2 14000 10 16 RMD-34-1 <.05 <.05 390 3.7 14 RMD—34—2 <.05 0.05 300 3.1 14 RMD-34-3 <.05 0.05 95 4.4 20 RMD—45—1 0.1 0.2 800 27 18 RMD-45-2 0.05 0.4 610 3 20 RMD—45—3 0.15 0.25 650 2.2 10 RMD—45—4 0.05 0.3 1050 14 32 RMD—48—1 <.05 0.3 830 4.3 32 RMD-48-2 0.65 1.5 6800 14 22 RMD-48-3 0.1 1.0 13000 18 30 RMD-48-4 0.1 0.45 6100 22 120

DETECTION LIMIT 0.005 0.005 0.2 0.1 2 A P PEN D IX C: Continued

SAMPLE NUMBER Pb Zn Mo W (ppm) (ppm) (ppm) (ppm)

RMD-7-1 8 70 <2 40 RMD-7-2 12 60 <2 <2 RMD-7-3 12 60 <2 <2 RMD-12-1 6 6 <2 18 RMD-12-2 8 20 <2 <2 RMD-12-3 10 12 <2 18 RMD-16-1 16 20 <2 40 RMD-16-2 14 65 <2 58 RMD-16-3 18 22 <2 46 RMD-19-1 8 16 <2 325 RMD-19-2 4 10 <2 105 RMD-19-3 14 8 <2 32 RMD-20-1 12 2 <2 12 RMD-20-2 14 <2 <2 <2 RMD-20-3 10 <2 <2 56 RMD-20-4 4 4 <2 16 RMD-20—5 4 6 <2 64 RMD-20-6 6 44 <2 68 RMD-20-7 10 14 <2 82 RMD-21-1 16 75 <2 18 RMD-21-2 14 115 <2 <2 RMD-21-3 12 38 <2 <2 RMD-34-1 10 85 <2 64 RMD-34-2 8 90 <2 52 RMD-34-3 10 75 <2 60 RMD—45—1 34 95 . <2 20 RMD-45—2 14 36 <2 <2 RMD-45-3 14 60 <2 10 RMD-45—4 16 90 <2 14 RMD-48-1 16 95 <2 <2 RMD-48—2 6 50 <2 <2 RMD-48-3 6 70 <2 18 RMD-48—4 10 125 <2 10

DETECTION LIMIT 2 2 2 10 109

REFERENCES

Albers, J.P., 1981, A 1itho-tectonic framework for metallo- genic provinces of California: Economic Geology, V. 76, p.765-790.

Bailey, S.W., ed., 1984, Micas: Reviews in Mineralogy, V. 13, Mineralogical Society of America, 584 pp.

Bailey, S.W., ed., 1988, Hydrous phyllosilicates (exclusive of micas): Reviews in Mineralogy, V. 19, Mineralogical Society of America, 725 pp.

Bennett., V., and DePaolo, D.J., 1982, Tectonic implications of Nd isotopes in the Pelona, Rand, Orocopia, and Catalina schists, southern California: GSA Abstracts with Programs, V.14, p.442.

Bradley, W.W., 1915-16, Fifteenth report of the state mineralogist: California State Mining Bureau, p .830-839.

Brown, G.C., 1915, Mines and mineral resources of Kern County, California: California State Mining Bureau, Report 24 of the State Mineralogist, p.471-523.

Burchfiel, B.C., and Davis, G.A.,1981, Mojave Desert and environs: in, Ernst, W.G., ed., The geotectonic development of California (Rubey Volume 1): Prentice-Hall, Inc., p.217-152.

Carroll, D., 1970, Clay minerals: A guide to their x-ray identification: GSA Special Paper 126, 80pp.

Cheeney, R.F., 1983, Statistical methods in geology:

Clark, W., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, 186 pp.

Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: USGS Bulletin 1693, 379 pp.

Davis, J.C., 1986, Statistics and data analysis in geology: 2nd ed., John Wiley and Sons, 646 pp.

DeCourten, F.L., 1979, Rock units in the Mojave Desert province: California Geology, California Division of Mines and Geology, November, p.235-247. 110

Dibblee, T.W.,Jr., 1967, Areal geology of the western Mojave Desert, California; USGS Professional Paper 522, 152pp.

Dolbear, S.H., 1910, The occurrence of tungsten in the Rand district: Engineering and Mining Journal, V.90, p. 904- 905.

Ehlig, P.L., 1968, Causes of distribution of Pelona, Rand, and Orocopia schists along the San Andreas and Garlock faults: in, Dickinson, W.R., and Grantz, A., eds., Geologic problems of the San Andreas fault system: Stanford University Publications in Geological Science, V.ll, p.294-306.

Eslinger, E ., and Pevear, D., 1988, Clay minerals for petroleum geologists and engineers: SEPM Short Course No. 22, 361 pp.

Fife, D.L., and Brown, A.R., eds., 1980., Geology and mineral wealth of the California desert: South Coast Geological Society, Anta Ana, California

Frolli, A.W., 1940, Open-pit mining and milling methods and costs at the Yellow Aster Mine: USBM Information Circular 7096, 46pp.

Gardner, D.L., 1954, Gold and silver mining districts in the Mojave Desert region of southern California: California Division of Mines Bulletin 170, V.l, p.51-58.

Garfunkel, Z., 1974, Model for the late Cenozoic tectonic history of the Mojave desert, California, and for its relation to adjacent regions: GSA Bulletin, V.85, p.1931-1944, December.

Glazner, A.F., and Supplee, J.A., 1982, Migration of Tertiary vclcanism in the southwestern United States and subduction of the Mendocino fracture zone: Earth and Planetary Science Letters, V.60, p.429-436.

Glazner, A.F., and Bartley, J.M., 1984, Timing and tectonic setting of Tertiary low-angle normal faulting and associated magmatism in the southwestern United States: Tectonics, V.3, No.3, p.385-396.

Glazner, A.F., et al, 1989, Magnitude and significance of Miocene crustal extension in the central Mojave Desert, California: Geology, V.17, No.l, p.50-53, January. I l l

Goode11, P.C., and Petersen, V., 1974, Julcani mining district, Peru: a study of metals ratios: Economic Geology, V.51, p.415-426.

Graham, C.M., and England, P.C., 1976, Thermal regimes and regional metamorphism in the vicinity of overthrust faults: an example of shear heating and inverted metamorphic zonation from southern California: Earth and Planetary Science Letters, V.31, p.142-152.

Griffin, G.M., 1971, Interpretation of x-ray diffraction data: in, Carver, R.E., ed., Procedures in sedimentary petrology: John Wiley and Sons, p. 541-557.

Grim, R.E., 1968, Clay mineralogy: McGraw-Hill Inc., 384 pp.

Guilbert, J.M., 1986, in, Guilbert, J.M., and Park, C.F., 1986, The geology of ore deposits: W.H. Freeman and Company, 985p.

Haxel, G., and Dillon, J., 1978, The Pelona-Orocopia schist and Vincent-Chocolate Mountain thrust system, southern Californi:, in, Howell, D.G., and McDougall, K.A., eds., Mesozoic Paleogeography of the western United States:, SEPM Pacific Section: Pacific Coast Paleogeography Symposium 2, p = 453-469.

Hershey, O.H., 1902a, Some crystalline rocks of southern California: American Geologist, V.29, p.273-290.

Hess, F.L., 1909, Gold mining in the Randsburg quardrangle, California: USGS Bulletin 430-A, p.23-47.

Hewitt, D.F., 1955, Structural features of the Mojave Desert region: GSA Special Paper 62, p.377-390.

Hillis, D.M., 1924, The Randsburg mining district: unpublished M.A. thesis, Stanford University.

Horton, F.W., and Gaylord, H.M., 1934, Gold, silver, copper, lead, and zinc in California: Statistical appendix to Minerals Yearbook: 1934, USBM, p.194-197.

Hulin, C.D., 1925, Geology and ore deposits of the Randsburg quadrangle, California: California Mining Bureau Bulletin 95, 152pp. 112

Jacobson, C.E., 1983, Structural geology of the Pelona and Vincent thrust, San Gabriel Mountains, California; GSA Bulletin, V. 94, p.753-767

Jahns, R.H., ed., 1954, Geology of southern California; California Division of Mines Bulletin 170

Johnston, W.D.Jr.,1940, The gold-quartz veins of Grass Valley, California; USGS Professional Paper 194, 101pp.

Kistler, R.W., and Peterman, Z.E., 1978, Reconstruction of crustal blocks of California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks; USGS Professional Paper 1071,

Kistler, R.W., et al, 1983, Isotopic studies of mariposite- bearing rocks from the south-central Mother Lode, California; California Geology, September, p.201-203.

Klein, C., and Hurlbut, C.S., Jr., 1985, Manual of mineralogy (after James D. Dana); 20th ed., John Wiley and Sons, 596pp.

Knopf, A., 1929, The Mother Lode system of California; USGS Professional Paper 157, 88pp.

Koschmann, A.H., and Bergendahl, M.H., 1968, Principle gold-producing districts of the United States;- USGS Professional Paper 610, 283pp.

Lemmon, D.M., and Dorr, J.V., 1940, Tungsten deposits of the Atolia district, San Bernardino and Kern Counties, California; USGS Bulletin 922-H, p.205-245.

Merrill, C.W., and Gaylord, H.M., 1938, Gold, silver, copper, lead, and zinc in California; Minerals Yearbook, 1938, p.230-236.

Merrill, C.W., and Gaylord, H.M., 1939, Gold, silver, copper, lead, and zinc in California; Mine Report; USBM Yearbook, 1939, p.247-264.

Miller, W.J., 1946, Crystalline rocks of southern California; GSA Bulletin, V. 57, No.5, p.457-540.

Morehouse, J.A., 1984, Structural geology of the Randsburg mining district, Mojave Desert, California; paper presented at the 12th annual Geoscience Daze, University of Arizona, Tucson, 29 pp. 113

Morehouse, J.A., 1988, A synopsis of the geologic and structural history of the Randsburg Mining District, California: unpublished MS thesis, University of Arizona, Tucson, Arizona, 59pp.

Nevius, J.N., 1916, Notes on the Randsbtirg tungsten district, California: Mining and Engineering World, July, p.7-8.

Postlethwaite, C.E., 1983, The structural geology of the western Rand Mountains, northwestern Mojave Desert, California: unpublished MS thesis, Iowa State University, 92pp.

Promitiel, H.W.C., 1937, Sampling and testing of a gold- scheelite placer in the Mojave Desert: USBM Information Circular 6960, 18pp.

Rose, A.W., and Burt, D.M., 1979, Hydrothermal alteration: in, Barnes, H.W., ed., Geochemistry of hydrothermal ore deposits: 2nd ed., John Wiley and Sons, New York, p.

Sampson, R.J., and Tucker, W.B., 1933, Gold resources of Kern County, California: California Journal of Mines and Geology, Report 26 of the State Mineralogist, p .272-339.

Silberman, M.L., and Berger, B.R., 1985, Relationship of trace-element patterns to alteration and morphology in epithermal precious-metal deposits: in, Berger, B.R., and Bethke, P.M., eds., Geology and geochemistry of epithermal systems: Reviews in Economic Geology, V. 2, Society of Economic Geologists, p .203-227.

Silver, L.T., and Nourse, J.A., 1986, The Rand Mountains "thrust" complex in comparison with the Vincent Thrust- Pelona schist relationship: [abst], GSA Cordilleran Section, 82nd meeting, Abstracts with programs, 18(2), p .185.

Sroden, J., and Eberl, D.D., 1984, Illite: in, Bailey, S.W., ed., Reviews in Mineralogy, V.13, Mineralogical Society of America, p.495-544.

Storms, W.H., 1909, Geology of the Yellow Aster mine: Engineering and Mining Journal, V.87, p.1277-1280. 114

Taylor, S.R., and McLennan, S.M., 1985, The continental crust: its composition and evolution: Blackwell Scientific Publications, 312p pp.

Theissen, A. A., and Harward, M„ E», 1962, A paste method for preparation of slides for clay mineral identification by X-ray diffraction: Soil Sci Soc Am Proc., 26: p. 90-91.

Thompson, D.G., 1929, The Mojave Desert region, California, a geographic, geologic, and hydrologic reconnaissance: USGS Water Supply Paper 578, 759pp.

Troxel, B.W., and Morton, P.K., 1962, Mines and mineral resources of Kern County, California: California Division of Mines and Geology, County Report 1, 370pp.

Tucker, W.B., 1920, Seventeenth report of the state mineralogist: California State Mining Bureau, p.361.

Tucker, W.B., 1924, Mines and mineral resources of Kern County: Gold: California State Mining Bureau, Report 20 of the State Mineralogist, p.185-199.

Tucker, W.B., 1929, Mining in California: Kern County: California State Mining Bureau, Report 25 of the State Mineralogist, p.20-75.

Vanderburg, W.O., 1931, methods and costs of concentrating tungsten ores at Atolia, San Bernardino County, California: USBM Information Circular 6532, 12pp.

Vargo, J.M., 1972, Structural geology of a portion of the eastern Rand Mountains, Kern and San Bernardino Counties, California: unpublished MS thesis, Los Angeles, California, University of Southern California, 117pp.

Velde, B . , 1977, Clays and clay minerals in natural and synthetic systems, Elsevier, 218pp.

Whittig, L. D . , and Allardice, W. R . , 1986, X-ray diffraction techniques: in, Klute, A., ed., Methods of soil analysis, Part 1: Physical and mineralogical methods; Agronomy No. 9, Am Soc of Agronomy, p. 331- 362. 115

Wilkins, J., et al, 1986, Mineralization related to detachment faults: a model: in, Beatty, B., and Wilkinson, P.A=K., eds., Frontiers in geology and ore deposits of Arizona and the southwest: Arizona Geological Society Digest, V.16, p.108-117-

Wright, L.A., et al, 1953, Mines and mineral deposits of San Bernardino County, California: California Journal of Mines and Geology, V.49, No. 1-2, p.49-257.