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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1986
Controls of gold mineralization in the southern portion of the Hodson Mining District west Mother Lode gold belt California
David Alexander King The University of Montana
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CONTROLS OF GOLD MINERALIZATION
IN THE SOUTHERN PORTION OF THE HODSON MINING DISTRICT,
WEST MOTHER LODE GOLD BELT,
CALIFORNIA
By
David Alexander King
B. A., Montana State University, 1980
Presented in partial fulfillment of the requirements
for the degree of
Master of Science in Geology
University of Montana
1986
Approved by
\ (v Chairman, Board of Examiners
Dean, Graduate School
r ’,- / / ' L- Date UMI Number: EP38961
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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 King, David Alexander, M.S., December 1986 Geology
CONTROLS OF GOLD MINERALIZATION IN THE SOUTHERN PORTION OF THE HODSON MINING DISTRICT, W EST MOTHER LODE GOLD BELT, CALIFORNIA
Director: Ian M. Lange
Gold mineralization in the Hodson Mining district, located in the West Belt of the Mother Lode, is similar in character to the deposits of the Central Mother Lode. Gold mineralization is hosted in a major, penetrative, oblique, dextral shear zone, localized along stratigraphie discontinuities between Late Jurassic island arc sequences of metasedimentary and metavolcanic rocks. Lensoidal bodies of fault-bounded serpentinized ultramafic rocks also occur along this zone. Three stages of deformation and at least two stages of metamorphism have affected the rock packages, producing slaty cleavage (D1), penetrative, plastic deformation (D2), and brittle thrusting (D3). Regional greenschist facies metamorphism accompanied D1 deformation while more isolated shear zone alteration and metamorphism occurred during D2 deformation. Two types of gold mineralization occur: high-grade, quartz stringer and breccia zones and low-grade, stratiform horizons of carbonate-rich metavolcanic rock. The quartz stringer zones are oriented along D2 fabric, predominantly in the metasedimentary rocks, within a major shear zone, along the contact between metasedimentary and metavolcanic rocks. Carbonate-rich, gold-bearing horizons parallel stratigraphy and represent altered mafic to ultramafic(?) flows, pyroclastics and tuffaceous sediments. They contain quartz, ankerite, mariposite (chromiferous mica), pyrite and gold. Carbonate alteration preceded the development of the quartz stringer zone but is apparently not syngenetic. The gold mineralization in the Hodson Mining district is probably a result of shear zone secretion and consists of an early carbonate-gold enrichment, followed by a later redistribution and gold concentration in quartz-stringer zones during D2 deformation. Multi-element soil and rock geochemistry indicates arsenic is the best pathfinder element for gold mineralization. Arsenic shows a broader, stronger response over both the quartz-stringer zone and the stratiform, carbonate zone gold mineralization. Table of Contents
Abstract jj Table of Contents iü List of Figures iv List of Tables v List of M aps vi Acknowledgments vii Introduction 1 G eo lo g y 5 Stratigraphy 5 Structure 8 Alteration and Metamorphism 11 Geochemistry 14 Mineralization 21 Discussion 36 Syngenetic Model 36 Epigenetic Model 39 Genesis of mariposite 41 Speculation 44 Conclusion 45 References 46 Appendix A. Pétrographie Descriptions 50 Appendix B. Geochemical sampling and analytical techniques 58 Rock sampling 58 Soil sampling 58 Analytical techniques 59 Appendix C. Whole rockchemistry 60
III List of Figures
Figure 1 Generalized geologic map of the western Sierra Nevada metamorphic belt, showing location of study area. Modified from Clark (1976). Figure 2 Map of the Mother Lode gold belts and the Foothills copper- zinc belt. Figure 3 Geology map of the Hodson mining district. 6 Figure 4 Geology map of the southern Hodson mining district. 7 Figure 5 Stratigraphie section of the southern Hodson mining district. 9 Figure 6 Tectonic development in the Hodson mining district. 12 Figure 7 Stereonet plot of cleavage development in the Hodson mining 13 district. Figure 8 Diagrammatic illustration showing alteration zones surrounding 15 gold quartz veins in greenstone lithologies. (after Colvine et al., 1984). Figure 9 Magnetic map of the southern Hodson mining district. 16 Figure 10 Multi-element soil geochemistry in relation to geology. 19 Figure 11 Rock geochemistry in relation to geology. 22 Figure 12 Map of gold distribution in soil. 23 Figure 13 Map of arsenic distibution in soil. 24 Figure 14 Map showing separate metal zones. 25 Figure 15 Cross section, B-B', showing geology and geochemistry. 28 Figure 16 Cross section, C-C, showing geology and geochemistry. 29 Figure 17 Cross section, D-D', showing geology and geochemistry. 30 Figure 18 Longitudinal section, E-E', showing geology and geochemistry. 31 Figure 19 Photomicrograph of mariposite schist, showing pyrite 32 preferentially located along mariposite lineation bands. Figure 20 Photomicrograph of rotated pyrite cube in mariposite schist, 32 showing pressure shadows of feathery quartz. Arrows indicate direction of rotation. Figure 21 Structure contour map of the Hodson fault zone (volcanlc- 33 slate contact), showing dilatant zone distribution and major gold mineralization. Figure 22 Dilatant zone development as a result of fault displacement. 34 Figure 23 Dilatant zone development as a result of shearing, block 35 rotation and wrench faulting.
IV List of Tables
Table 1: Interpreted mineral paragenetic sequence. 17 Table 2: Pathfinder elements for gold mineralization (see figure 10). 20 List of Maps
Map 1: Detailed geology map of the southern portion of the Hodson mining district. Scale 1:2400 map folder
VI Acknowledgments
This thesis project was generously supported by Meridian Mineral Company.
They provided funds and released from confidentiality much of the analytical data.
I would like to thank Patrick C. Cavanaugh, who was not only instrumental in
getting the project started, but has also continued to offer assistance and
comments throughout the study. Dr. James W. Sears contributed his time and
expertise, helping me to formulate the structural interpretations of the region. I
would also like to thank the University of Montana and my advisors Dr. Ian
M. Lange, Dr. Steve D. Sheriff and Dr. Richard J. Field for providing the knowledge,
support and freedom that allowed me to pursue this thesis.
VII Introduction
Gold mineralization in the Mother Lode region of California has long been considered to be related to the development of the Sierra Nevada batholith. More recent theories propose a shear zone or metamorphogenic origin. The mineralization also contains attributes which suggest an earlier, possibly syngenetic origin. To test these hypotheses, I conducted a detailed examination of the Hodson mining district and a reconnaissance investigation of the entire Mother
Lode region.
The Mother Lode region lies in the Sierra Nevada metamorphic belt consisting of several allocthonous terranes of Paleozoic and Mesozoic eugeoclinal, ocean floor and island arc related sediments (Saleeby, 1981). The terranes are separated by major shear zones which subparallel the north-trending, steeply- dipping stratigraphy. Within and adjacent to these shear zones are ophiolite complexes, melange zones and linear belts of serpentinized ultramafic rocks. Gold
mineralization occurs in and adjacent to these shears, often associated with
ultramafic bodies (Figure 1).
Three parallel belts of gold mineralization together with smaller, less continuous belts form the Mother Lode system. Gold mineralization occurs as quartz fissure veins and as large bodies of carbonate-altered greenstone known as
"gray ore". A belt of exhalative massive sulfide deposits (Kemp, 1982), known as the Foothills Copper-zinc belt, overlaps and lies west of the gold belts in more felsic volcanic rocks. The Hodson mining district lies in the west gold belt of the
Mother Lode, 5 kilometers northwest of Copperopolis, California, and contains both gold and copper-zinc mineralization (Figure 2). WE^TTERN SIERRA NEVADA METAMORPHIC BELT. CALIFORNIA
12?* 120* EXI-l.ANAl ION Sus.inviiie
Ccno/oic scJinit-nijr> aiiJ v<»tc»nic fuCks
Gramitc rocks of thr Sierra Nevada balhuhth. chiefly of Mrso^oii ape
Mesozoic sedimenlary and vulcanic rocks in places strongly metamorphosed
Ultramafic rocks, chiefly of Mesozoic age
Paleozoic sedimentary and vulcanic rocks, in places strongly metamorphosed
C ontact
Mother Lode gold belt Fault Mel ones fault zone Dotted where concealed
^ îA C K A M E N l
East gold belt Study 1 19 ' area West gold belt Bear Mountain fault 'zonë ?
Area o f map Modesto
CALIFORNIA
25 50 m iles
25 50 k ilo m e tr e s
Figure 1 Generalized geologic map of the western Sierra Nevada metamorphic belt, showing location of study area. Modified from Clark (1976). %Ro»i‘«»oo»# \ ao''£, explanation
PLACER/:; <‘P«-\> ’'^ v; i . -/ /" t ^ G^oniric 'ocki
vShing»t Soringt I Slot®, some phyllif* and conglom*. EL D O R A D O '□te (Mariposa Formation) OiMifl» Greenstone, □mphiDante, Ohfor'te scnist Whit# Ook s\\'- AMADOR piymow'h Slate, mico scnist, quortjite. some nornfels and i mesione (Cdoveras Formation in port) : CALAVERAS MOTHER LODE % Gold belt or vein system
GOLD BELT Roiirood Tertiary rockS ore not snowm R'Ch
Mo\imorn Rontn
Wojh»ngton- • - • TUOLUMNE
\^\V\Coll'»rv»ll# \\ EAST GOLD Copper-zinc belt \ ooftd«oct ' BELT
s r ' ^ W Location of Figure 3.
MARIPOSA ' \ ‘' ’^ ^ 0 o g Town ^ Mn WEST GOLD Wt«t. 00* CooiTf*,' ♦WWW'v^V^ Cl«oringhouM BELT
Coiofodo'w
Figure 2 Map of the Mother Lode gold belts and the Foothills copper-zinc belt. Southern half of the western Sierra Nevada metamorphic belt, California. From Clark (1970). Geology
The Hodson mining district lies along the Hodson fault zone, part of the major northwest-trending Bear Mountain fault zone. The trace of the fault zone is marked by lensoidal bodies of serpentine (Figure 3). Shearing is localized along structural discontinuities between upper Jurassic slate and metavolcanic rocks.
Gold mineralization occurs within the shear zone as quartz stringer zones and as bodies of carbonate-rich metasedimentary and metavolcanic rocks. Horizons of copper-zinc mineralization also occur in this zone. The rock packages strike northwest and dip steeply to the east, and have been altered to greenschist facies by regional metamorphism. Shear zone alteration overprints regional metamorphism and is localized along the slate-volcanic contact. The southern
Hodson mining district is divided into three regions: (1) a western slate belt; (2) an eastern greenstone belt; and (3) a central zone of shearing, which includes altered protoliths from the other two belts (Figure 4).
Stratigraphy
The slate sequence is upper Jurassic in age and known regionally as the Salt
Spring Slate. It Is 3 kilometers thick and consists of black and brown fissile slate, with interfingering lenses and tongues of pyroclastic rocks and associated coarse greywacke, tuff and pebble conglomerate (Figure 5).
The volcanic sequence, regionally known as the Copper Hill Volcanics, is Copperopolis fault zon
EXPLANATION
I Copper H ill Volcanics Location of Figure 4 L "W -mafic metavolcanics. - si. cherty slate. w /' Fault zone -with thrust faults 1 Mile Salt Spring Slate. y Gold mine jCrrrj Quartz diorite(?). o Copper mine Serpentine.
Figure 3. Generalized geologic map of the Hodson mining d is tric t, showing gold mines and nearby copper mines. (WW) Wilbur Womble mine, (GK) Gold Knoll mine, (MK) Mountain King mine, (R) Royal mine, (K) Keystone, (U) Union, (E) Empire. Modified after Clark (1970) and; Taliaferro and Solari (1944). U)
CO
Jg s Jg s Jsl ub
70' J g s
Jchs N ub Jchs 5 0 3 0' 4 5 " . 4 5' CO J c h s 80' 5 0 0 0 E - WW Jgw Jsl
J c g 70' 68 Jsl 20' Jg w
Jsl 4 6 0 O E
3 CT 4 8 0 0
ICO 300 CO 200
4 0 0 8 0 0
Figure 4. Geology map of the southern Hodson Mining district See Figure 5, lithologie descriptions. WW= Wilbur Womble Mine, GK= Gold Knoll Mine. 8
upper Jurassic in age and conformably overlies the slate sequence. It consists primarily of subaqueous mafic to intermediate (with minor felsic) pyroclastic rocks with subordinate massive, amygdaloidal flows, aggregating a combined thickness of 1500 meters (Clark, L, 1970). The volcanic rocks are predominantly tholeiitic in
composition, with local calc-alkaline volcanic centers containing associated
massive sulfide deposits (Kemp, 1982). In the Hodson mining district, the volcanic
package thickens and coarsens to the northwest, indicating probable derivation from that direction.
Shear zone lithologies are altered, heavily sheared sedimentary and volcanic
rocks. The type of alteration and degree of shearing is partially dependent on the
original composition and competency of the rock type. Thus, shear zone
lithologies, while discriminated primarily by alteration assemblages, also reflect
primary stratigraphy. These associations are discussed under the alteration
section of this paper. The width of shearing varies from 30 meters to greater than
200 meters across the slate-volcanic contact. It is wider in the less competent
mafic pyroclastic rocks.
Structure
Three deformational events are recorded in the rocks of the study area;
isoclinal folding D^, shearing and thrusting (Figure 6). Isoclinal folding
accompanied greenschist facies metamorphism and is expressed by bedding plane
and axial planar cleavage (Sq and S.,, figure 7) (Eric et al., 1955; Clark, 1960).
Folding was followed by shearing producing the Hodson fault zone. Shearing West Jsl- Black and brown fissile slate, derived from siltstone (3 Km).
Jgw- Graywacke, t u f f , siliceous zones and schistose metavolcanic rocks (0-100 m eters).
Jcg- Petromict conglomerate (0-30 meters).
Increasing intercalated volcanic debris near volcanic contact. Shearing, development of penetrative Sg cleavage, pencil structures and tig h t, chevron fold s. A lteration halos of sericitization and disseminated pyrite. Gold-bearing, quartz stringer veins.
Massive, white, brecciated, gold-bearing quartz vein (0-6 meters) Sg orientation.
Jmt JmSj- Mariposite schist. Contains iron carbonate, quartz, mariposite, pyrite and variable chlorite. Enriched in gold, silver, tungsten, barium, arsenic and boron (0-15 meters). Jch» - Jchs- Chlorite schist. Pyroxene-plagioclase crystal and lith ic- crystal mafic tuff. Often sheared and locally carbonate altered (major component of Copper Hill Volcanics). Jm t t/i JmSg- Maripositeschist. Forms as carbonate alteration halos surrounding quartz veins. Low gold content, high in arsenic, antimony and boron (0-8 meters).
ub- Serpentinite and ultramafic rocks. Fault-bounded, internally sheared (0-60 meters).
Jms^- Pyritic, cherty(?) horizon. Interfingering, pyrite, (Î) mariposite, sericite. quartz schist and pyritic slate horizons. Massive su lfid e horizon. Anomalously high in copper and zinc (0-10 meters). -Jsl
Jgs- Greenstones. Undifferenciated mafic pyroclastic flows agglomerates and massive amygdaloidal basalts (ab)
(ab) (1500 meters). East
Figure 5 Stratigraphie section of the southern Hodson mining district. Mapped from trenches and stream gullies, located approximately along section B- B', Figure 4. 10
is most evident in the slates and sericite schists producing penetrative cleavage, pencil structures, chevron folds and quartz stringer zones. Multiple generations of folded, brecciated and ribboned quartz veins, indicate ductile and brittle conditions existed within the shear zone (Chace, 1949; McKinstry and Ohie, 1949). The direction of shear displacement (determined by rotated crystals, oriented crenulation cleavage, sigmoidal surfaces, rotated blocks and en echelon tension gashes) is right lateral (Simpson and Smith, 1983). Shear zone cleavage (Sg) contains two orientations; compressional shear or slip cleavage; and tensional
shear cleavage (Figure 7).
The third deformational event is represented by thrust faulting (Dg). Relative
motion, indicated by drag folds and striated, graphitic shears, suggests that over- thrusting is directed to the southwest. The total amount of displacement is not
known; however, stratigraphie offset (Figure 11) indicates at least 37 meters of
low-angle thrusting has occurred. The orientation of the thrust planes correlates
well with expected orientations developed within a shear couple. The thrusting
event probably represents a culmination of Dg forces, but with brittle dislocation
rather than ductile deformation.
In addition, east-west cross faulting is suggested by the anomalous
curvature in the thrust plane, abrupt termination of the southern ultramafic body,
and termination of certain stratigraphie horizons (Figures 4, 9 and 21). Cross
faulting is probably related to thrusting, though its orientation, style and
displacement are not known. Major serpentine bodies appear structurally
controlled; they are fault-bounded, cross-cut stratigraphy and boudin in areas of
dilatancy, controlled mainly by warped surfaces along the shear zone. 11
Alteration and Metamorphism
Two episodes of metamorphism affected the rocks in the Hodson mining district. The first, accompanied isoclinal folding (D^) as regional greenschist facies metamorphism. The intensity is uniform throughout the study area and is characterized by chloritization of original mafic minerals. The second episode of metamorphism accompanied shear zone deformation (Dg), producing localized alteration within the shear zone characterized by carbonate alteration.
Alteration assemblages reflect the chemistry of the host rock and the
proximity and intensity of shearing. Shearing and alteration intensity is greatest
along the sediment-volcanic contact, decreasing outward, forming cylindrical
alteration shells surrounding dilatant zones. Dilatancy occurs within the shear
zone due to wrench faulting and ductile shear. Alteration in slate horizons is
minimal, consisting of a slight bleaching (potassic alteration) and development of
disseminated pyrite. Alteration intensity increases in mafic volcanic rocks, often forming multiple zones with distinct mineral suites. Central quartz veins are
surrounded by a zone of ankerite, mariposite and pyrite, followed by a zone of
chlorite plus or minus pyrite and talc, and typically flanked (on the east) by
serpentine (Figure 8). Similar alteration patterns occur in Timmins, Ontario, Canada
(Colvine, et al., 1984), the southcentral Mother Lode (Evans and Bowen, 1977), and
in South Africa (Pearton, 1978). Delineation of the serpentine bodies was
facilitated using a magnetometer survey (Figure 7). The anomalously high
magnetic response is due to disseminated magnetite which formed during
serpentinization of the ultramafic rocks (Figure 9). 12
(a)
(b )
(c)
Figure 6 Tectonic development in the Hodson mining district. (a) Isoclinal folding, axial planar cleavage development (0.,); (b) shear zone, dextral strike-slip displacement with the development of gash veins (D^); and (c) west- verging thrust faulting (D^). 13
Bedding or axial planar Shear zone penetrative cleavage (S^and S^). cleavage. Compressional (S2c)-
Shear zone penetrative Thrust plane orientation. cleavage. Tensional (S2t)-
Figure 7 Stereonet plot of cleavage development in the Hodson mining district. 14
The generalized paragenetic sequence of alteration and gold mineralization is depicted in Table 1. Most of the alteration occurred during shear zone deformation (Dg), with complex, overlapping and repeated phases. Crosscutting
relationships indicate that the sequence of alteration began with widespread
carbonate development, followed by mariposite and sericite formation. Pyrite followed, with development along mariposite lineation bands. Quartz and quartz-
carbonate veining occurred during and after mica development and was
accompanied by sulfidation and contemporaneous or later gold mineralization.
Brecciation of the quartz veins, with additional?) gold mineralization, may have
occurred during the D3 thrusting event.
Geochemistry
In order to determine trace metal associations and possible pathfinder
elements for the gold mineralization, I conducted a soil and rock geochemical
investigation. An experimental "soil-line" (located along section A-A', Figures 12
and 13) containing 17 sample locations was analysed for 11 elements (As, Hg, Te,
Mn, Pb, Zn, Sb, B, F, W and Au, Table 2 and Figure 10. Analytical procedures are
listed in Appendix 2). The results indicate that:
1. in addition to gold, arsenic is the best pathfinder element for gold mineralization, followed by tungsten;
2 . arsenic also outlines mariposite schist horizons, which are not always mineralized;
3 . arsenic anomalies form a stronger, broader response, extending out and beyond gold mineralization; 15
I'AFIC METAVOLCANIC ROCKS
METASEDIMENTARY ROCKS
Figure 8 Diagrammatic illustration showing alteration zones surrounding gold quartz veins in greenstone lithologies. (after Colvine et al., 1984). o
5 2 0 0 0 5 50 0 0
5 5 0 0 0 5 5 0 0 0 5400E -
5000E - ww 5 ( 0 0 0 GK
4 6 0 0 E -
SCALE I. 4800
too 200 300
4 0 0 8 0 0
Figure 9 Magnetic map of the southern Hodson mining district.
05 17
D, D2 D3 isoclinal folding shearing th ru s ti ng greenschist facies penetrative cleavage metamorphism shear zone metamorphism
C h lo rite
Carbonate
Seri c i te
M ariposite
?
P yrite
Quartz
Gold
Serpentinization
Table 1; Interpreted mineral paragenetic sequence. 18
4. mercury delineates areas of soil contamination by mill tailings (because amalgamation plates were used to extract gold in the old mills);
5. soil geochemistry correlates well with rock geochemistry (Figures 10 and 11);
6 . the strongest gold mineralization is in the quartz stringer zone and central portion of the mariposite schist horizons.
Geochemistry maps of gold and arsenic distributions in soil outline two
major linear bands of anomalously high metal concentration, which correspond to the mineralized quartz stringer zone and mariposite schist horizons (compare
Figures 12 and 13 with Figure 4).
A comparison of elemental distributions in soils shown in Figure 10 with
bedrock lithologies, indicates four geochemical zones with unique metal
enrichments (Figure 14):
1. A copper-zinc zone, with enrichment of Cu, Pb, Hg, Te and Mn with minor Au and Ag. The copper-zinc zone exhibits a much stronger response in drill hole samples (underlined element indicates it is unique to that zone.) (Figure 13).
2. An eastern mariposite schist stratiform horizon, with massive, white quartz veins, enriched in As, B, W, Au, Hg and minor Zn and F.
3. A western, chlorite-rich, mariposite schist stratiform horizon with minor, irregular quartz pods. This zone is proximal to the quartz breccia zone, but shows enrichment in Te, As, F, B, W and Au.
4. A quartz stringer zone consisting of quartz veining, brecciation and replacement at the volcanic-sediment contact. It is enriched in Au, As, W, B, F and Te.
The variability of metal suites in the separate zones is in part related to the host rock lithologies. Tellurium and manganese concentrations increase In the volcanic rocks, while barium, boron, fluorine and zinc background concentrations are greater 19
Geology G e o lo g y
600-, 2 00 -,
I 50 -
300- I 00 -
ISO- 5 0 -
20 I 20-
0 80- 10-
0 4 0 - 100
6 0
0 0 2 - 2 0
1000 0(0
6 0 0 0 06
200 0 02
2 000-,
1500-
600-
500- 2 0 0 -
2 0 -,
w 10- (epml
4750E 5650E 5650E 4750E
Figure 10 Multi-element soil geochemistry in relation to geology. Location, grid-line 6400N, section A -A ' on soil geochemistry maps. 50 foot, spot- sample spacing. * = Detection limit. See Figure 5 for stratigraphie descriptions. 20
Table 2: Pathfinder elements for gold mineralization (see figure 10).
Arsenic (As): Excellent pathfinder element. Very strong response oyer a broader area than gold. Arsenic also delineates carbonate altered zones which are not enriched In gold. Mercury (Hg): Fair correlation, not very consistent but also picks up hydrothermally altered areas Mercury is best used to delineate contamination from mill tailings. Tellurium (Te): Fair correlation with quartz veining and carbonate altered zones. Shows strong, broad response in volcanics over the copper-zinc horizon. Manganese (Mn): Very erratic, poor pathfinder element. Fails to delineate quartz veining and carbonate altered zones. Shows highest response over copper-zinc horizon. Fair lithology indicator for volcanics. Lead (Pb): Poor pathfinder element. Very little to no response. Slight anomaly over copper-zinc zone. Zinc (Zn): Poor pathfinder element. Shows highest response over slate. Picks up eastern mariposite schist horizon and hydrothermally altered zone, but fails to resolve western quartz breccia zone. Good lithology indicator for slate. Antimony (Sb): Inconsistent. Strong response over carbonate altered gold mineralization, but no response over quartz breccia gold mineralization. Boron (B): Fair correlation with gold, but also shows anomalous high response in slates. Stronge response over carbonate altered rock, similar to arsenic. Good lithology indicator for slates. Barium (Ba): Fair correlation with gold. High values over slate and the western carbonate altered zone, but fails to outline quartz breccia mineralization. Small, broad response over copper-zinc mineralizations. Fluorine (F): Fair to good correlation with gold. Picks up both gold zones with a stronger response over the quartz breccia zone. Anomaly is broader, but peaks are not well resolved. Tungsten (W): Good correlation with gold but weak response. Gold (Au): Soil geochemical response shows excellent correlation with rock geochemistry. Strong response over quartz stringer zone and western carbonate altered zone. 21
in the slates. Other metal variation, such as the high concentration of As, Sb, and
B in the eastern mariposite schist horizon, could be the result of fluid temperature variations between shear zones.
Mineralization
Gold mineralization in the study area is controlled by both structure and
stratigraphy; structure dominates. The two main types of gold mineralization are
quartz stringer zones and stratiform horizons of gold-bearing, mariposite schist.
Copper-zinc mineralization forms stratabound horizons stratigraphically above the
gold mineralization in the mafic volcanic rocks, and forms within chlorite shear
zones, adjacent to fault-bounded serpentine bodies (see Map 1).
Structurally controlled gold mineralization, located within the Hodson fault
zone, forms high-grade (averaging 0.08 ounces of gold per ton), quartz stringer
zones. These zones consist of elongate, irregular, discontinuous clots and
boudined quartz stringer veins. The stringer zones form strikingly chaotic,
ribboned patterns with approximately equal amounts of quartz and country rock.
They occur primarily in the slate sequence adjacent to the volcanic contact, and to
a lesser degree, in the mariposite schists and tuffaceous (sericitic) slate sequences
(Figures 15, 16 and 17). The zones, typically 6 to 10 meters wide, attain widths of
over 40 meters at the Gold Knoll mine.
The quartz stringers are orientated along Sg major and minor cleavage
directions, as well as along Sg axial plane cleavage and folded in microfolds,
indicating that they developed during the oblique shearing event. The folded 22
3 01
20À Au (ppm) 04
SO-,
Ag 3 0 1 '0 4
3004 As (ppm) 1004
5545 E 5115 ni! to Geology (/) to t/) £ O £ o E D 5 -o “3 “3 -3-3-3
Figure 11 Rock geochemistry in relation to geology. Located along grid line 6400N, trench E. Samples are 10 foot average, continuous rock chips. See Figure 5 for stratigraphie descriptions. 02
5400 E .
0 02 0 2 5 0 0 0 E- WW ___ -GK
4600E
o
( iin I ' Ml r il r .iwii ,i 1 i 0 . (IJ , ( I , II 5 d ml 1 . H tMnn hi > I il . Sdrn(ili'. I ik III I VI r y M l It-i'l .iluiig liiu'b b | u i t d ,it 4IIII 11 ml III11- r v,i 1 I, / SCALE r. 4000
100 200 1. __l _ I I 4 0 0 8 0 0 F(
Figure 12 Map of gold distribution in soil.
ro CO o
100
4 0 0 5400E -
zoo 4 0 0
5 0 00E - WW GK 2 00 200 O 4 6 0 0 E -
scale
r 4800
100 200 300
4 0 0 6 0 0
Figure 13 Map of arsenic distibution in soil.
ro •p. in
5 4 0 0 E -
5 0 0 0 E - WW GK
4 6 0 0 E -
l: • 4 0 0 0
100 200 300
4 0 0 8 0 0
Figure 14 Map showing separate metal zones.
r o oi 26
and unfolded quartz stringers appear Identical, with no consistent cross-cutting relationships. Folded, brecclated and ribboned gold-bearing quartz veins Indicate quartz Influx occurred repeatedly at temperatures and pressures near the boundary
between brittle and ductile conditions.
Stratigraphically controlled gold mineralization occurs In carbonate-altered,
mariposite schist horizons. The horizons are low grade (averaging 0.02 ounces of
gold per ton), stratiform, and appear stratabound, Interflngering with slate and
serlclte schist horizons (Figures 15, 16, 17 and 18). They subparallel and are cut
by the quartz stringer zone. The horizons are llthologlcally and chemically distinct,
containing a unique suite of enriched elements (see Figure 10). It is unclear
whether these horizons represent original sedimentary accumulations or are the
result of an epigenetic mineralization process.
In thin-sectlon a typical mariposite schist contains 40% quartz, 30% ankerlte
or ferroan dolomite, 15% tuffaceous material, 10% mariposite or serlclte and 1 to
2% pyrite. Mariposite Is a distinctive, bright-green, chromlum-rlch variety of
muscovite (or phenglte), similar to fuchslte. The matrix of the schist is composed
of subangular, fine quartz sand and silt with lesser Interstitial, carbonate. Quartz is
strained, recrystallized and locally shows sedimentary sorting and grading.
Tuffaceous material consists of minor, euhedral (commonly zoned) pyroxene
(auglte?) and clasts containing unorlented, felted mats of plagloclase laths.
Crystals are thoroughly altered to fine-grained assemblages of carbonate, sericite
and chlorite. The rock Is sheared, commonly mylonitlzed, with mariposite and
pyrite development along llneatlon bands producing a marked schlstosity. The 27
bands are subparallel to compositional layering and form a penetrative cleavage.
Pyrite occurs preferentially along mariposite lineation bands (Figure 19) as sharply euhedral, 2 to 3mm cubes and pyritahedrons. Pyrite cubes are often rotated and surrounded by pressure shadows of feathery quartz (Figure 20). Coarse cubic pyrite is the dominant gold carrier with enclosed gold particles from one micron to one millimeter (Meridian Company report).
The orientation and distribution of mineralized quartz veins within the shear zone appears to be controlled by dilatancy. Movement along the shear zone is right-lateral with a lesser thrusting component, therefore, dilatant zones form as a result of strike-slip displacement and dip-slip movement (Figure 22). The result is an anastromosing network of gash veins developed by tensional dilation within the shear zone. District-wide and throughout the Mother Lode region, ore bodies (and serpentine bodies) form at spaced intervals along the shear zone. This periodicity could result from rotation of blocks within a shear couple, producing alternating regions of compression and extension (Figure 23). The spacing between zones of dilatancy would be determined by the size of the rotational blocks, which is dependent on the width of the shear zone, the intensity and depth of shearing, and the competency of the rock package.
Horizons of copper-zinc mineralization form statigraphically above the gold mineralization in the mafic volcanic rocks. They consist of finely disseminated pyrite and minor chalcopyrite and form within silicified zones, in sericite schists, beneath slate horizons and in chlorite shears adjacent to fault bounded serpentine bodies (Figure 18 and Map 1). UJ
09 08 07 06 040 D29
JSl Jsi N) 00 SCALE U2400 600. 50 Au ppm X 0 2 As a Zn ppm x 50 Figure 15. Cross section B-B", along grid line 5600N, showing geology and geochemistry. Note: gold mineral ization in shear zone along metasediment-metavolcanic contact and in highly fractured and sheared upper- plate rocks. Zinc mineralization in slate horizons and in pyritic interbeds within the undifferentiated greenstone package. See Figure 5 for lithologie descriptions. LU 017 DI6 DIS D34 1000- s w NE ro co SCALE 50 600- 200 Au ppm X 0 2 As ppm X 5 0 Figure 15. Cross section C-C, along grid line 7200N, showing geology and geochemistry. Note; gold mineralization in mariposite schist horizons and in shear zone, along the metasediment-metavolcanic contact. Arsenic anomalies show excellent correlation with gold anomalies. See Figure 5 for lithologie descri pti ons. 020 019 DIS 039 D49 D37 1000- s w NE co o SCALE 12400 600. 50 Au ppm X 0 2 As ppm X 50 Figure 17. Cross section D-D, along grid line 8000N, showing geology and geochemistry, Note: gold mineralization in mariposite schist horizons and in shear zone, along the metasediment- metavolcanic contact. See Figure 5 for lithologie descriptions. 041 0 49 DJ5 0 33 029 GK 0 47 D27 ww 1200 - NW SE Jchs ub 980- Jtl Jchi Jsl 940' El in Fl Zn As (ppm » 100) ALE 1.4800 Very low. 100 200 300 I tic undl f f e renl idled greeneluDe pdckdge. 4 0 0 8 0 0 Figure 18. Longitudinal section, showing geology and geochemistry, Section E-E*. See Figure 5 for lithologie descriptions, and Figure 4 for location. 32 Figure 19 Photomicrograph of mariposite schist, showing pyrite preferentially located along mariposite lineation bands. Field of view = 2 mm. ______ V #, y 4 ; J 3 k Figure 20 Photomicrograph of rotated pyrite cube in mariposite schist, showing pressure shadows of feathery quartz. Arrows indicate direction of rotation. Field of view - 2 mm. - 5 0 0 ------ - 7 0 0 6 0 0 b40U t 9 0 0 50' 8 0 0 4 5' 30 1 - 4 8 0 0 100 200 4 0 0 duo Figure 21. Structure contour map of the Hodson fault (volcanic-slate contact), showing distribution of major gold mineralization by dot pattern. 00 00 34 a) b) Figure 22 Dilatant zone development as a result of fault displacement. (a) Dip slip, high angle reverse motion; (b) shear zone, strile slip motion; and (c) combination of shearing and overthrusting. Figure 23 Dilatant zone development as a result of shearing, block rotation and wrench faulting. 35 Discussion Gold mineralization in the southern portion of the Hodson mining district contains both syngenetic and epigenetic attributes. Gold mineralization in secondary structures suggests an epigenetic genesis, while gold mineralization in stratigraphically-related, mariposite schist horizons, indicates possible primary, syngenetic gold mineralization with subsequent secondary concentration during deformational events. Any model or models that attempt to explain the genesis of the gold mineralization must effectively address the relationships between stratigraphically and structurally controlled gold mineralization. Ore genesis models which are applicable include exhalative-hydrothermal and shear zone secretion. Syngenetic Model An exhalative-hydrothermal model was proposed by Kemp (1982) for the formation of the Foothills copper-zinc belt, which parallels the Mother Lode gold belts. A syngenetic, exhalative model has also been suggested by other workers for the formation of the Mother Lode gold belts. In the syngenetic model gold is deposited by volcanic-related, hot spring activity on the sea floor (Rye and Rye, 1974; Karvinen, 1980). The resulting deposits are stratiform and consist of chemical, sedimentary exhalites. The deposits form in association with mafic and/or ultramafic volcanic rocks and are overlain by a sedimentary sequence. 36 37 Exhalative mineralization in the form of massive sulfide deposits are well documented in the Mother Lode region (Kemp, 1982) and occur in rocks of practically all ages (Franklin et al., 1981). Known examples of exhalative gold deposits are less common, occurring principally in Archean terranes (Karvinen, 1980; Hutchinson, 1976). Exhalative gold deposits may be present in the Sierra Nevada Metamorphic belt because the rock sequences are remarkably similar to the greenstone belts of Archean age. Support for exhalative gold mineralization in the Hodson mining district and throughout the Mother Lode gold belts includes: 1. the parallel nature of the belt of syngenetic, exhalative, volcanigenetic copper-zinc deposits 2. the formation of the gold deposits at volcanic-sedimentary rock contacts 3. the deposits contain a distinct suite of trace metals often associated with exhalative gold mineralization (gold, silver, arsenic, tungsten, boron, barium and antimony) 4. the deposits contain diagnostic alteration minerals associated with exhalative gold mineralization (ankerite, mariposite or chromiferous mica, sericite and quartz) 5. the deposits are stratiform and are often delineated by diffuse, gradational contacts (ore body contacts are typically grade cutoffs (Knopf, 1929)) 6. the deposits are often associated with conglomerate horizons, indicating proximity to faulting (hydrothermal feeder pipes are located along faults) 7. the deposits occur at spaced intervals similar to exhalative, massive sulfide deposits, possibly representing the size of hydrothermal convection cell 38 8. the deposits occur primarily along major sedimentological transitions, but also to a lesser degree as smaller deposits between major belts (indicating the continuation of hydrothermal, exhalative activity but with more sediment dilution) While these characteristics are permissive evidence for syngenetic, exhalative mineralization, there are certain aspects of the deposits which do not support the model. They include: 1. no apparent exhalative, chemical facies transition or variation along strike (i.e. absence or paucity of chert horizons) 2. the lack of (identified) crosscutting hydrothermal feeder pipes (necessary for venting hydrothermal solutions onto the sea floor) 3. stratigraphy in the study area is reversed: volcanic rocks are stratigraphically above sedimentary rocks (exhalative deposits typically form at the base of a sedimentary sequence) 4. strong spatial association of mineralization with post-depositional, fault zones (characteristic of epigenetic, not syngenetic mineralization) 5. strong spatial association of mineralization with serpentinized ultramafic bodies (most of the ultramafic bodies are structurally emplaced along the fault zones, compatible with structurally controlled gold mineralization) 6. carbonate alteration overprints regional metamorphic fabric (gold mineralization is spatially related to carbonate alteration and therefore, may also be post-regional metamorphism) Many of the above discrepancies in the exhalative model can be summarily explained. Mineralization associated with post-depositional fault zones may be coincidental, related only because tectonic forces and exhalative gold mineralization are both often localized along structural discontinuities represented by volcanic-sedimentary rock contacts. The gold deposits may be related to the ultramafic bodies because erosion and concentration of metals from these bodies 39 could form the gold-bearing, mariposite schist horizons (Schreyer, 1982; Hinse, et a!., 1986). The gold may have already been present in the rock units prior to shearing, and was merely remoblized during carbonate alteration. Furthermore, hydrothermal feeder pipes are often hard to distinguish and may only be indicated by footwall alteration assemblages. Finally, volcanicly-induced hydrothermal, alteration-mineralization may have proceeded volcanic outpourings. Epigenetic Model The close spatial relationship between gold mineralization and major shear zones suggests an epigenetic origin for the formation of the Mother Lode gold deposits. A shear zone secretion model, proposed by Zimmerman (1981) and Nesbitt et al., (1986), and largely adapted from Boyle (1959 and 1979) may have operated. In the model. Mother Lode-type gold deposits form by secretion of metamorphic or deep meteoric fluids along shear zones. Movement along these zones produces heat, causing circulation of fluids which leach metals from the surrounding rocks. The fluids then migrate to low pressure, dilatant zones produced by fault displacement. Gold and other constituents precipitate in these zones, possibly due to decreased pressure and/or chemical reduction. Shear zones are common structural characteristics of accreted continental margins. They often develop due to oblique convergence or transpression, resulting in wrench faulting with accompanied strike-slip and minor reverse motion. The shear zones subparallel stratigraphy, and are characteristically delineated by lensoidal bodies of serpentinized ultramafic rocks. The ultramafic 40 rocks are believed to be structurally emplaced along the shear zones (Misra and Keller, 1978; Snoke, et al, 1982). The shear zone secretion theory, therefore can explain many attributes of the gold mineralization in the study area, throughout the Mother Lode region, and in similar greenstone terranes. Permissive evidence includes: 1. the gold mineralization is associated with faults and major shear zones 2. primary controls of gold mineralization are along secondary structures 3. existence of active faulting during mineralization (successive periods of folded, brecciated and ribboned, gold quartz veins) 4. paucity of vertical mineral zoning (possibly resulting from deep-seated mineralization) 5. spatial association of serpentinized, ultramafic bodies to many of the deposits (because both are probably controlled by zones of dilatancy) 6. mineralization contains a distinct suite of trace metals: gold, silver, arsenic, tungsten, boron, barium and antimony (compatible with mesothermal, hydrothermal conditions) 7. mineralization contains diagnostic alteration minerals: ankerite, mariposite or chromiferous mica, sericite and quartz (which developed during shear zone activation) 8. deposits are lensoidal-shaped with diffuse mineralized contacts (compatable with deep-seated mineralization) 9. deposits occur at spaced intervals along the shear zones (possibly indicating local zones of dilatancy developed by shearing) 10. carbonate alteration clearly overprints regional metamorphic fabric (carbonate alteration accompanied gold mineralization) 11. mineralization and shearing occur primarily along major sedimentological transitions (which also represent structural discontinuities) 41 The shear zone secretion theory fails, however, to sufficiently explain the common, parallel occurrence of exhalative, massive sulfide mineralization, and the existence of interfingering, low grade, stratiform horizons of mariposite schist. A major question concerning the genesis of the gold mineralization Is its relationship to the shear zone. Is the gold mineralization related to the shear zone or is the shear zone developed at the stratigraphie transition where earlier gold mineralization occurred? The key to the problem may lie in the genesis of the gold-bearing, mariposite schist. Genesis of mariposite Throughout the Mother Lode and in other, similar greenstone terranes, gold mineralization is often closely related to, and hosted in mariposite schist, or rocks containing abundant iron carbonate and green, chromiferous mica (mariposite and fuchsite). This rock-type has been variously described as: 1. a metamorphosed alunite deposit (Schreyer, 1982) 2. chemical sediment, classified as a mixed carbonate-sulfide iron formation (Karvinen, 1980) 3. carbonate altered serpentine (Knopf, 1929; Evans and Bowen, 1977; Pearton, 1978) 4. carbonate altered mafic pyroclastic or tuffaceous sediments (this investigation) 5. carbonate altered komatiite (Fyon and Crocket, 1980; Schreyer, 1982) 6. carbonate altered, differentiated tholeiitic sill (Phillips, 1986) 7. carbonate altered basalt (Andrews and Wallace, 1983; Fyon and Crocket, 1980) 42 The large number of suggested protoliths for chrome mica-bearing rock, indicates that It can probably form from alteration of many different rock types, controlled principally by the chemistry of the enclosing rock and environment of alteration. Chromiferous mica occurs In two environments: In detrltal, metasedlmentary rocks, and more commonly. In greenstone, metavolcanic rocks (Leo, et al, 1965). In the metasedlmentary rocks the mica Is associated with detrltal chromium, concentrated In heavy metal accumulations (Leo, et al, 1965; SInha-roy and Kumar, 1984; Ramlengar et al, 1978). In metavolcanic rocks, as In the Hodson mining district, the mica is associated with carbonate alteration and gold mineralization. In the metavolcanic rocks three elements are always present: (1) mafic to ultramafic Igneous rocks; (2) greenschlst facies (or higher) metamorphism; and (3) major shear zones. The chromiferous mica occurs In or adjacent to the ultramafic rocks, suggesting that they provide the source for the chromium, either by hydrothermal alteration and leaching (Whitmore, et al., 1946), or by erosion and concentration In heavy metal accumulations. Metamorphism supplies the correct combination of temperature, pressure and fluid chemistry for development of the mica; and the major shear zones provide the necessary fluid pathways and dilatant zones for the collection and precipitation of the mineral. An Important spatial relationship Is that many of the altered ultramafic bodies hosting chromiferous mica and gold mineralization are believed to be structurally emplaced along the shear zone. The alteration post dates emplacement, which strongly discounts the posslbllty that the chrome mica and the gold are of an exhalative source. The chrome mIca-bearIng rock Is therefore, not diagnostic of 43 exhalative gold mineralization, rather, it indicates alteration of mafic and ultramafic igneous rocks related to shearing in greenschlst facies metamorphic terranes. Not all carbonate-rich, greenstone gold deposits are associated with chrome mica-bearing rock. In California, gold mineralization occurs in other rock types along the major shear zones, indicating gold mineralization is more directly related to the shear zone and not a particular host rock. These relationships lead me to believe that the gold mineralization in the Hodson mining district formed epigenetically due to the circulation of mineralizing fluids in a shear zone. In addition, in the shear zone secretion model, mineralization occurs at great depth along the shear zone (Nesbitt et al, 1986; Kerrich and Fyfe, 1981), thus necessitating large vertical uplift and erosion to expose them at the surface. In the syngenetic, exhalative gold model, however, mineralization should be readily detectably on the sea floor. No examples of active exhalative gold systems have yet been documented. While the above reasoning strongly supports shear-zone genesis for the formation of greenstone-type gold deposits, certain characteristics remain enigmatic, such as: (1) the common association of parallel belts of exhalative massive sulfide mineralization; (2) the occurrence of gold deposits only along certain shear zones and not others; and (3) the common, regional, spatial, plutonic association of many of the deposits. These relationships can be explained by their common association with accretionary tectonics which is the requisite environment for the formation of greenstone-type gold deposits. The entire length of the western margin of North American, from southern 44 California north through Alaska is composed of accreted terranes (Jones, et al., 1977; Monger and Price, 1982). Carbonate-rich, greenstone-type gold deposits occur intermittenly along its entire length: from those of the Mother Lode of California; north to the Klamath Mountains in northern California and Oregon; the Seven Devils Complex of Idaho; the Caribou, Cassiar and other districts in British Columbia; to deposits of the Juneau Gold Field in Alaska (Nesbitt, et al, 1986). These occurrences indicate that shear zone-related gold deposits are not unique to the Mother Lode region and can occur throughout accreted volcanic, greenstone terranes. Greenstone-type gold deposits are very common in rocks of Archean age. The litho-tectonic characteristics of the Archean deposits are similar to those of the Mother Lode, probably indicating the past existence of accretionary plate margins. Speculation Additional studies which may be useful in shedding further light on the genesis of greenstone-type gold deposits would be to identify the tops and bottoms of these systems, in order to define vertical alteration and chemical zoning. The higher portion of these systems may be enriched in antimony and mercury while at deeper levels tungsten concentration may increase. Stable isotope studies may be useful to "see through" metamorphic chemical changes to determine the extent of shear zone related alteration. It would also be interesting to compare the location, peridicity and structural relationships between greenstone gold deposits and spatially associated copper-zinc deposits. Conclusion Structural data from cleavage and fault plane orientations indicate three periods of deformation affected the rocks of the Hodson mining district: D^- isoclinal folding; Dg- strike slip displacement; and D3- reverse motion. Regional greenschlst facies metamorphism accompanied D., folding while shear zone metamorphism and alteration accompanied Dg strike slip motion. Gold mineralization is controlled primarily along Dg structures with minor stratigraphie control. Relationships strongly suggest that the epigenetic gold mineralization formed due to shear zone secretion, in a brittle-ductile shear zone. Successive periods of gold mineralization progressed from widespread carbonate alteration, to the development of quartz stringer zones, ending with brittle fracture and breccia zone development. 45 References Andrew, A. J., and Wallace, H., 1983, Alteration, metamorphism and structural patterns associated with Archean gold deposits: Preliminary observations in the Red Lake area, In Colvlne, A. C., ed.,in The geology of gold In Ontario, Ontario Geol. Surv., Misc. paper 110, p.141-163. Boyle, R. W., 1959, The geochemistry, origin and role of carbon dioxide, water, sulfur and boron In the Yellowknife gold deposits. Northwest Territories, Canada: Econ. Geol., v.54, p.1506-1524. Boyle, R. W., 1979, The geochemistry of gold and Its deposits: Together with a chapter on geochemical prospecting for the element: Geol. Surv. Canada, Bull.280, 584p. Chace, F. M. 1949, Origin of the Bendigo saddle reefs, with comments on the formation of ribbon quartz: Econ. Geol., v.44, p.561-597. Clark, L. D, 1970, Geology of the San Andreas 15-mlnute quadrangle, Calaveros County, California: Cal, DIv. Mines and Geol., Bull. 195, 23p. , 1976, Stratigraphy of the north half of the western Sierra Nevada metamorphic belt, California: U.S. Geol. Survey, Prof.Paper 923, 26p. Clark, W. B., 1970, Gold districts of California: Cal. DIv. Mines Geol. Bull. 193, 183p Colvlne, A. C. et al, 1984, An Integrated model for the origin of Archean lode gold deposits, Ontario Geol. Surv. Open File Report 5524, 98p. Dodge, F. C. W., KIstler, R. W., and Sllberman, M. L, 1983, Isotopic studies of the marlposlte-bearing rocks from the south-central Mother Lode, California: Cal. Geol., v.36, no.9, p.201-203. Evans, J. R., and Bowen, O. E., 1977, Geology of the southern Mother Lode Tuolumne and Mariposa Counties, California: Cal. DIv. Mines and Geol., Map sheet 36. Eric, J. H., Stromqulst, A. A., and Swinney, C. M., 1955, Geology and mineral deposits of the Angles Camp and Sonora quadrangles, Calaveros and Tuolumne Counties California: Cal. DIv. Mines and Geol. Spec. Report 41, 55p. 46 47 Franklin, J. M., Lydon, J. W. and Sangster, D. P., 1981, Volcanic-associated massive sulfide deposits: Econ. geol., 75th anniv. vol., pp.485-627. Fyon, J. A., and Crockett, J. H., 1980, Volcanic environment of carbonate alteration and stratiform gold mineralization, Timmins area, in Roberts,R.G., ed.. Genesis of Archean volcanic-hosted gold deposits: Ontario Geol. Surv. Open File Report 5293, p.66-91. Hinse, G. J., Hogg, G. M., and Robertson, D. S., 1986, On the origin of Archean vein- type gold deposits with reference to the Larder Lake "break" of Ontario and Quebec: Mineral. Deposita, v. 21, p. 216-227. Hutchinson, R. W., 1976, Volcanogenic sulfide deposits and their metailogenic significance: Econ. Geol., v.68, p.1223-1246. Jones, D. L., Silberling, N. J., and Hillhouse, John, 1977, Wrangellia- A displaced terrane in northwestern North America: Can. Jour. Earth Sci., v.l4, p. 2565-2577. Karvinen, W. O., 1980, Geology of evolution of gold deposits, Timmins area, Ontario, in Roberts,R.G., ed.. Genesis of Archean volcanic-hosted gold deposits: Ontario Geoi. Surv. Open File Report 5293, p.1-43. Kemp, W. R., 1982, Petrochemical affiliations of volcanogenic massive sulfide deposits of the foothill Cu-Zn belt. Sierra Nevada, Cal.; Ph.D Dissertation, Univ. of Nevada, Reno, 360p. Kerrich, R., and Fyfe, W. S., 1981, The gold-carbonate association: source of C02 and C02 fixation reactions in Archean lode deposits: Chemical Geol., v.33, pp.265-294. Knopf, A., 1929, The Mother Lode system of California: U.S. Geol. Surv. Prof. Paper 157, 88p. Leo, G- W., Rose, H. J., Jr., and Warr, J. J., 1965, Chromian muscouvite from the Sierra de Jacobina, Bahia, Brazil: Amer. Mineralogist, v.50, p.392-402. McKinstry, H. E., and Ohie, E. L, Jr., 1949, Ribbon structure in goid-quartz veins: Econ. Geol., v.44, p.87-109. Misra, K.C., and Keller, F. B., 1978, Ultramafic bodies in the southern Appalachians: A review: Amer. Jour, of Sci., v. 278, p. 389-418. Monger, J. W. H., Price, R. A., and Tempelman-Kluit, D. J., 1982, Tectonic accretion and the origin of the two major metamorphic and plutonic weits in the Canadian Cordilleran: Geol., v.lO, p.70-75. 48 Nesbitt B. E., Murowchick, J. B. and Muehienbachs, K., 1986, Dual origins of lode gold deposits in the Canadian Cordillera: Geology, v. 14, p. 506-509. Pearton, T. N., 1978, The geology and geochemistry of the Monarch ore body and environs, Murchison range, northeastern Transvaal, in Verwoerd,W.J., ed., Mineralization in metamorphic terranes: Geol. Soc. of South Africa, Special Publication 4, p.77-86. Phillips, G. N., 1986, Geology and Alteration in the Golden Mile, Kalgoorlie: Econ. Geol. V.81, p.779-808. Ramiengar, A. S., Devadu, G. R., Viswanatha, M. N., Chayapathi, N., and Ramakrishnan, M., 1978, Banded chromite-fuchsite quartzite in the older supracrustal sequence of Karnataka: Jour, of Geol. Soc. of India, v.19, no.12, p.577-582. Rye, D. M. and Rye, R. 0., 1974, Homestake gold mine. South Dakota: I. Stable isotope studies: Econ. Geol., v. 69, p. 293-317. Saleeby, J., 1981, Ocean floor accretion and volcanoplutonic ore evolution of the Mesozoic Sierra Nevada, in Ernst,W.G.,ed., The geotectonic development of California (Rubey Volume I): Prentice-Hall, Inc., p.132-181. Schreyer, W., 1982, Fuchsite-aluminum silicate rocks in Archean greenstone belts: Are they metamorphosed alunite deposits?: Geologische Rundschau, v.71, no.1, p.347-360. Simpson, C., and Schmid, S. M., 1983, An evaluation of criteria to deduce the sense of movement in sheared rocks: Geol. Soc. Am., Bull., v. 94, p. 1281-1288. Sinha-roy, S., and Kumar, G. R. R., 1984, Fuchsite-bearing quartzite in the Sargur equivalent rocks of North Kerala: Jour. Geol. Soc. India, v.25, p.120-122. Snoke, A. W., Sharp, W. D., W right J. E., and Saleeby. J. B., 1982, Significance of mid-Mesozoic peridotitic to dioritic intrusive compleses, Klamath Mountains- western Sierra Nevada, California: Geology, v. 10, p. 160-166. Taylor, B. E., 1981, Hydrothermal fluids in the Mother Lode gold deposits of California: EOS, v.62, p.1059. California: U.S. Geol. Surv., Geophysical Investigations, Map GP-671. Whitmore, D. R. E., Berry, L. G., and Hawley, J. E., 1946, Chrome micas: The Amer. Mineralogist, v.31, p.1-21. Zimmerman, J. E., 1983, The geology and structural evolution of a portion of the 49 Mother Lode belt Amador County, California: unpubl. M.S. Thesis, Univ. of Arizona, Tucson, A r, 139p. Appendix A Pétrographie Descriptions Many of the thin section from the Hodson mining district are clouded by alteration and weathering products. Carbonate alteration is particularly prevalent, obscuring primary textures with recrystallzed carbonate. In most instances the carbonate is an iron-rich dolomite, presumably ankerite. The discrimination between mariposite and fine sericite is not possible in thin section. Therefore, the presence and percent of mariposite in the rock was determined by hand sample identification. Rock Name: Pebble conglomerate, No. W -65. Mineral % Important Properties 70% Quartz Angular to subrounded, <0.1mm. to >2mm. (in thin section). In o/c, up to 5 cm. Strained, rotated, often w / inclusions. 25% Plagioclase As >2m m . clots of interlocking, subhedral crystals, and as subhedral crystals in matrix. Fairly fresh, equidimensional and lath-shaped. 4% Carbonate Tan rhombs, in slate clasts. 1% Sericite Fine kneedles disseminated throughout. <1% Actinolite? Lt. brn., fibrous. Acicular habit. Textues, Alteration and Comments: Conglomeratic: Variable clast size. Sheared: Smeared-out, elongated 5:1. Rotated and strained quartz. Carbonate rhombs, growing in slate. 50 51 Rock Name: Meta siltstone, No. 44, Mineral % Important Properties 90% Quartz Fine, equigranular qtz. in matrix of slate, and 30% recrystalized, coarse qtz. in veinlets and clots. 3% Carbonate Along fluid influx lineation bands; 2 directions. Also as diss. rhombs. 3% FeOx As It. brn. alteration of rhombic carb. blebs-diss., and as alt. boarders of recrystalized qtz. vnlets. Tr. Sericite Diss. fine kneedles w / carbonate. Chert(?) Gray, isotropic blebs, diss. in matrix and as gash vns. w / carbonate. Textures, Alteration and Comments Rock is sheared, with carbonate and sericite influx along "C" surfaces, semi-parallel to bedding. Qtz.-filled, gash veins orientated at 45° from "C" surfaces. Rock Name; Coarse, polymict conglomerate. No. 25 Mineral % Important Properties 60% Chert clasts Elongate clasts of chert or fine slate. Subangular. 20% Quartz Recrystalized, interlocking qtz., often rolled along zones of shearing. 10% Vole, clasts Predominantly clots of unoriented plag. laths, w / some blocky crystals. 3% Carbonate Growth along interclast, fluid migration lineation ("C" surface). Tr. Sericite W/carbonate, along foliation (and fluid migration) parallel to clast elongation. 2% FeOx Lt. brn., w/carb. Textures, Alteration and Comments Conglomeratic. Sheared and elongated 1:5. Carb. alt. only inter-clast. Clast supported matrix. Slight imbrickation. Clast size: 2 to 8 mm. length. 52 Rock Nam e: Greywacke, No. W -64, Mineral % of rock Important Properties 50% Slate clasts Composed of variable, fine to coarse qtz. grains. Stretched, similar to ripup clasts. 25% Quartz Strained, recrystalized frags, and oblate clasts. 20% Orthoclase Lt. grey to white, biaxial (-), large 2V. Subangular frags., some square, equidimensional, others oblate, anhedral. Tr. Carbonate lineation bands Tr. Sericite W/carbonate. Textures, Alteration and Comments Subangular grains, rotated, elongated, sheared. Bimodal. Rock Name: Contorted slate. No. W -52. Mineral % Important Properties 60% Quartz Lt. grey, euhedral. Occurs in hing of axial plane, in crenulationss. Also as strained blebs throughout. 30% Sericite Fibrous, It. brn. to colorless. Throughout rock. Tr. Opaques BIk. flakes, possibly graphite. 2% FeOx Lt. brn. Textures, Alteration and Comments Crenulation cleavage. Could also be named a Sericite, quartz schist. 53 Rock Nam e: Mariposit, carbonate, quartz schist, No. W -53. Mineral % Important Properties 85% Quartz Granular thoughout entire rock. Sed. clastic. Also as euhedral, recrystalized open spaces. 10% Carbonate Secondary, as smears and surflcial coatings. Cuts groundmass. 8% Muscovite Problably mariposite and sericite. With carb. Tr. Actlnollte(?) Acicular, radiating kneedles on qtz. Tr. Chlorite BIk. to dk. green. Some penene chi. Berlin blue. Textures, Alteration and Comments Granular qtz. Interlocking grains, equigranular, w / clots of larger qtz. Protollth: greywacke. Rock Name: Mariposit schist. No. W -54. Mineral % Important Properties 50% Quartz Granular, fine silt sediment. Difficult to esrimate. 30% Carbonate dolomite. Pervasively alt. rock. Granular text. 17% Muscovite Maripostie. In lineated streaks throughout rock. < 1 % Pyrite Anhedral blebs, ass. w / mariposite lineation bands. Textures, Alteration and Comments Fine grained. Relict crystals In matrix. Sheared. Name: Carbonate altered, pyrite, mariposite, qtz. schist. Protollth; Crystal tuff, or tuffwacke. 54 Rock Nam e: Mariposit schist, cut by veining. No. W -55. Mineral % Important Properties 70% Carbonate In vein, 85%, as euhedral dolomite (polysynthetic twinning). In matrix, as fine grained (clastic ?) assemblage of carb. and plag. (albite ?), carb.= 70%, plag.=30%. 15% Muscovite Mariposite. In fluid migration, lineation bands ("C" surfaces). Pyrite preferencially devel. along bands. 10% Quartz In groundmass as fine banding. 15% of vein. Later than carbonate. < 1 % Pyrite Blebs, cubes, boudins, elongated, sheared, smeared and rotated. Textures, Alteration and Comments History: A clastic vole, sed., w / early carb. alt.; shearing and the formation of mariposite bands and pyrite (Gold mineralization). Continued shearing, deformation of pyrite. Secondary carb. veining w / qtz. (reconcentration of gold). No pyrite in vein carb. Deformation, strained calcite. Name: Sheared, carbonate altered, pyrite, mariposite, quartz schist. Protolith: Tuffwacke. Rock Name: Mariposit schist. No. W -61, Mineral % Important Properties 50% Quartz Spherical and oblate blebs, possibly qtz. eyes. Strained, granular, in groundmass as sed. 45% Carbonate As blebs, similar to qtz. Also pervasive throughout entire rock, as alt. Obscurs text. 2% Muscovite Diss. thoughout. Alined, schistousity. < 1 % Pyrite Cubes and aggregates in secondary qtz. clots. Occurs along fluid migration lineations, and with carb. Textures, Alteration and Comments Sed. clastic text.; lineation of micacious minerals, Carbonate alt. Clots of recrystalized qtz. in voids. Cut by qtz.-carb. vn. 55 Rock Name: Mylonitized, cherty slate. No. 44a. Mineral % Important Properties 35% Quartz Recrystalized along low presure, "C" and "S" surfaces. Strained. 30% Matrix Slaty rx. w / alt., bik., grungy, isotropic matrix; chert (?). 10% Chert As above. 5% Carbonate Predominantly along "S" surfaces. Textures, Alteration and Comments Heavily sheared, mylonitized. Fluid influx along "C" surfaces, carb. alt. Devel. of gash vns. (ladder struct.) in more competent hor. Rock Name: Vein quartz. No. W-50. Mineral % Important Properties 65% Quartz Strained, euhedral, interlocking crystals. Carb. before qtz. 30% Carbonate As above, elongate. Tr. Muscovite Mariposite, along small shear. Textures, Alteration and Comments Coarsly crystaline, carb. before qtz. and minor carb. on qtz., euhedral, interlocking. Vein assayed 0.3 opt. Au. Name: Compond qtz.-carb. vein. 56 Rock Nam e: Carbonate alt. volcanic rock, No. W -59. Mineral % Important Properties 70% Carbonate Rock is completly carbonate altered. Equicrystaline to slightly elongate rombs and plates. 20% Quartz In groundmass w / carb., and fibrous in pressure shadows surrounding pyrite cubes. < 1 % Pyrite Cubes and blebs. Rotatated. Appears to have grown in the matrix, post carb., or contemporaneous. Tr. Sericite Also chlorite. Textures, Alteration and Comments Relict plag. and pyroxene (?) crystals. Porphyritic text. Heavily carb. alt. Name: Carbonate alt. plagioclase porphyry. Rock Name: Carbonate alt. chlorite schist. No. W-51. Mineral % Important Properties 50% Carbonate Forms matrix, obsquers text. Ragged, splotchy, w / FeOx patches. 25% Glass Chloritlzed pumic frags, (fiame). Elongation 1:5. Foliation. 10% FeOx Lt. redish-brn., cloudy patches on iron-carb. (ankerite ?). 15% Feldspar Ortho(?), as mosaic of stuby and alt. laths, in groundmass. <1% Sericite Scaly aggregates, in shadow struct, of rotated pyrite cubes < 1 % Pyrite Cubies. Rotated. Textures. Alteration and Comments Name: Carbonate alt., sheared, sericite, quartz, chlorite schist. 57 Rock Name: Mafic pyroclastic. No. 87b. Mineral % Important Properties 40% Plagioclase Unorientated phenos. Cloudy, saussaritized, corroded edges. 35% Relict Px. Brn,totally alt pyroxene crystal. Now carbonate. Large, euhedral. 10% Slate LIthic clasts. 10% Chlorite Chloritlzed matrix. Textures, Alteration and Comments Name: Carbonate alt. augite(?), plag. porphyry. Rock Name: Mafic pyroclastic, No. W -66. Mineral % Important Properties 40% Glass Isotropic groundmass, Chloritlzed. 15% Amygdules Filled w / It. blue-green, fibrous, pleochroic (yell-green to clear) zeolites or chlorite. 10% Pyroxene Euhedral, porphyritic. 15% Plagioclase Saussuritized, cloudy, glomeral-porphoritic. 10% Feldspar Orthoclase(?). Stubby and lath-shaped crystals in groundmass. Textures, Alteration and Comments Glassy, porphyritic, abund. spherilltes. No apparent shearing. Name: Chloritlzed, amygdaloidal, pyroxene, plag. pyroclastic. augite(?), plag. porphyry. Appendix B Geochemical sampling and analytical techniques Rock sampling Rock chip samples from backhoe trenches and underground workings were taken in 5 to 10 pound sample bags, as "continuous chips". Most of the trench samples and many of the underground samples were of oxidized rock. Drill holes 1 to 25 were drilled using a pneumatic, conventional rotary drill. Drill holes 26 to 51 were drilled using a reverse circulation, rotary drilling method. Holes were drilled using compressed air when possible, however, water was used in many of the holes below 100 feet, because of the high water table. Most samples were taken as average splits from 5 foot intervals. Soil sampling Standard soil sampling techniques were employed. Soil samples were taken in half pound samples, from the "B" horizon. Thick profiles of terra rosa soil form over serpentine and silicate-carbonate bodies, whereas thin soil forms over slaty rocks. 58 59 Analytical techniques The majority of the geochemical samples were analyzed by Skyling Labs in Tucson, Arizona. Standard sample preparation methods and analytical techniques were employed. Internal standards, duplicate samples, repeat runs and cross checking samples with other laboratories were used to confirm accuracy. Most samples were analyzed using graphite furnace, atomic absorption methods. Exceptions include: mercury- mercury detector; boron- semi- quantitative, emission spectroscopy; and fluorine- specific ion electrode. Samples with values greater than 6 ppm (parts per million) gold were re-analysised using a fire assay method. Overall accuracy for sample preparation and analysis is ± 10% (personal comunications. Jack Allen, geochemist. Skyline Labs). Whole rock chemistry (wt. %) No. Rock description SiOg TiOg AlgO] FegOg MnO MgO CaO KgO 2 Massive, dk. green metavolc. 54.3 0.550 15.40 8.1 0.110 5.3 10.90 0.26 1 Chi. Sch., s i. carb. alt. 40.8 1.100 16.40 10.0 0.140 3.8 11.20 1.40 11 Talcose, chi. sch. 38.1 0.060 7.50 8.9 0.230 19.8 8.20 <0.01 10 Carb. a lt., chi. sch.,tr. mar. 39.1 0.120 9.60 8.2 0.150 18.6 4.70 0.72 4 Bik, mafic sch., ultra mafic ? 40.0 0.170 5.00 10.5 0.130 28.1 4.10 <0.01 3 Ank. mar. sch., no vis. qtz. 40.3 0.073 9.80 8.4 0.120 9.0 7.70 0.70 7 Anko mar. sch., O2, G. Knoll pit 62.2 0.150 13.60 14.3 0.043 1.1 0.08 3.50 o > o 5 Diabase sill?, carb. alt. 55.2 0.081 16.10 5.1 0.069 3.3 4.8 3.30 6 Peridotite, si. serp. 44.4 0.230 15.40 7.6 0.150 9.0 17.80 0.03 8 Partially serp. ultramafic 41.3 0.007 0.29 7.9 0.091 38.2 0.24 <0.01 9 Serpentine 41.4 0.340 10.60 11.1 0.170 23.8 3.50 0.06 Appendix C. Whole rock chemistry from the southern Hodson mining district. Samples taken along section D-D', near the Gold Knoll Mine.