controls of gold mineralization in the southern portion of
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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1986
Controls of gold mineralization in the southern portion of the Controls of gold mineralization in the southern portion of the
Hodson Mining District west Mother Lode gold belt California Hodson Mining District west Mother Lode gold belt California
David Alexander King The University of Montana
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UNIVERSITY OE. MONTANAJL 5 #
CONTROLS OF GOLD MINERALIZATION
IN THE SOUTHERN PORTION OF THE HODSON MINING DISTRICT,
W EST MOTHER LODE GOLD BELT,
CALIFORNIA
By
David A lexander King
B. A., M ontana State University, 1980
Presented in partial fu lfillm ent of the requirem ents
for the degree of
M aster of Science in G eology
University of M ontana
1986
Approved by
\(v
Chairm an, Board of Examiners
Dean, G raduate School
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King, David Alexander, M.S., December 1986 Geology
CONTROLS OF GOLD MINERALIZATIONIN THE SOUTHERN PORTION OF THE HODSON M INING DISTRICT,W EST MOTHER LODE GOLD BELT,CALIFORNIA
Director: Ian M. Lange
Gold m ineralization in the Hodson Mining district, located in the W est Belt of the M o th er Lode, is sim ilar in character to the deposits of the Central M other Lode. Gold m ineralization is hosted in a m ajor, penetrative, oblique, dextral shear zone, localized along stratigraphie d iscontinuities betw een Late Jurassic island arc sequences of m etased im entary and m etavolcanic rocks. Lensoidal bodies of fau lt-b o u n d ed serpentin ized u ltram afic rocks also occur along this zone.
Three stages of deform ation and at least tw o stages of m etam orph ism have affected the rock packages, producing slaty c leavage (D1), penetrative, plastic deform ation (D2), and brittle thrusting (D3). Regional g reenschist facies m etam orph ism accom panied D1 deform ation w hile m ore isolated shear zone alteration and m etam orphism occurred during D2 deform ation . Tw o types of gold m ineralization occur: h igh -g rade , quartz s tringer andbreccia zones and lo w -g rad e , stratiform horizons of carb o n ate -rich m etavolcan ic rock. The quartz stringer zones are oriented along D2 fabric, predom inantly in the m etased im entary rocks, w ith in a m ajor shear zone, along the contact betw een m etasedim entary and m etavolcanic rocks. C arb o n ate -rich , g o ld -b earin g horizons parallel s tratigraphy and represent altered m afic to ultram afic(?) flows, pyroclastics and tu ffaceous sedim ents. They contain quartz, ankerite, m ariposite (chrom iferous mica), pyrite and gold. C arbonate alteration preceded the developm ent of the quartz stringer zone but is apparently not syngenetic. The gold m ineralization in the Hodson M ining district is probably a result of shear zone secretion and consists of an early ca rb o n a te -g o ld enrichm ent, fo llow ed by a later red istribution and gold concentration in q u artz -s trin g er zones during D2
deform ation .M u lti-e le m e n t soil and rock geochem istry indicates arsenic is the best
pathfinder e lem en t fo r gold m ineralization. A rsenic shows a broader, stronger response over both the q u artz -s trin g er zone and the stratiform , carbonate zone gold m ineralization.
Table of Contents
A bstract jjTable of C ontents iüList of Figures ivList of Tables v
List of Maps vi
A cknow ledgm ents vii
In troduction 1
G eo logy 5
Stratigraphy 5Structure 8A lteration and M etam orphism 11G eochem istry 14M ineralization 21
Discussion 36
Syngenetic M odel 36Epigenetic M odel 39Genesis of m ariposite 41Speculation 44
Conclusion 45
References 46
Appendix A. Pétrographie Descriptions 50
A ppendix B. G eochem ical sam pling and analytical techniques 58
Rock sam pling 58Soil sam pling 58A nalytical techniques 59
A ppendix C. W hole rock chem istry 60
III
List of Figures
F ig u re 1 G eneralized geo log ic m ap of the w estern Sierra Nevada m etam orph ic belt, show ing location of study area. M odified from Clark (1976).
F ig u re 2 M ap of the M other Lode gold belts and the Foothills c o p p e r- zinc belt.
F ig u re 3 G eology m ap of the Hodson m ining district.F ig u re 4 G eology map of the southern Hodson mining district.F ig u re 5 S tratigraphie section of the southern Hodson mining district. F ig u re 6 Tecton ic developm ent in the Hodson m ining district.F ig u re 7 S tereonet plot of c leavage developm ent in the Hodson m ining
district.F ig u re 8 D iagram m atic illustration showing alteration zones surrounding
gold quartz veins in g reenstone lithologies. (after Colvine et al., 1984).
F ig u re 9 M agnetic m ap of the southern Hodson mining district.F ig u re 10 M u lti-e le m e n t soil geochem istry in relation to geology.F ig u re 11 Rock geochem istry in relation to geology.F ig u re 12 Map of gold d istribution in soil.F ig u re 13 Map of arsenic distibution in soil.F ig u re 14 Map show ing separate m etal zones.F ig u re 15 Cross section, B-B', show ing geology and geochem istry.F ig u re 16 Cross section, C -C , show ing geo logy and geochem istry.F ig u re 17 Cross section, D -D ', show ing geo logy and geochem istry.F ig u re 18 Longitudinal section, E-E', show ing geology and geochem istry. F ig u re 19 Photom icrograph of m ariposite schist, showing pyrite
preferentia lly located along m ariposite lineation bands.F ig u re 20 Photom icrograph of ro tated pyrite cube in m ariposite schist,
showing pressure shadow s of feathery quartz. Arrow s indicate direction o f rotation.
F ig u re 21 S tructure contour map of the Hodson fau lt zone (vo lcan lc - slate contact), show ing d ilatant zone distribution and m ajor gold m ineralization.
F ig u re 22 D ilatant zone deve lopm ent as a result of fault d isplacem ent. F ig u re 23 D ilatant zone developm ent as a result of shearing, block
rotation and w rench faulting.
679
1213
15
161922232425 2829303132
32
33
3435
IV
List of Tables
T a b le 1: In terpreted m ineral paragenetic sequence. 17T a b le 2: Pathfinder e lem ents fo r gold m ineralization (see figure 10). 20
List of Maps
M ap 1: Detailed geo logy m ap of the southern portiono f the Hodson m ining district. Scale 1:2400 map fo lder
VI
Acknowledgments
This thesis pro ject was generously supported by M erid ian M ineral Com pany.
They provided funds and released from confidentia lity much of the analytical data.
I w ould like to thank Patrick C. Cavanaugh, w ho was not only instrum ental in
getting the pro ject started, but has also continued to o ffe r assistance and
com m ents th ro u g h o u t the study. Dr. Jam es W. Sears contributed his tim e and
expertise, helping m e to fo rm u la te the structural in terpretations of the region. I
w ould also like to thank the U niversity of M ontana and m y advisors Dr. Ian
M. Lange, Dr. S teve D. Sheriff and Dr. Richard J. Field for providing the know ledge,
support and freedom that a llow ed me to pursue this thesis.
VII
Introduction
Gold m ineralization in the M other Lode region of California has long been
considered to be related to the developm ent of the Sierra Nevada batholith. M ore
recent th eo ries propose a shear zone or m etam orphogenic origin. The
m ineralization also contains attributes w hich suggest an earlier, possibly
syngenetic origin. To tes t these hypotheses, I conducted a detailed exam ination of
the Hodson m ining district and a reconnaissance investigation of the entire M other
Lode region.
The M o th er Lode region lies in the Sierra Nevada m etam orphic belt
consisting of several allocthonous terranes of Paleozoic and M esozoic eugeoclinal,
ocean flo o r and island arc related sedim ents (Saleeby, 1981). The terranes are
separated by m ajor shear zones w hich subparallel the north -tren d in g , s te e p ly -
dipping stratigraphy. W ithin and ad jacent to these shear zones are ophiolite
com plexes, m elange zones and linear belts of serpentin ized u ltram afic rocks. Gold
m ineralization occurs in and ad jacent to these shears, often associated w ith
ultram afic bodies (Figure 1).
Three parallel belts of gold m ineralization to g e th er w ith sm aller, less
continuous belts form the M other Lode system . Gold m ineralization occurs as
quartz fissure veins and as large bodies of carb o n a te -a lte re d greenstone known as
"gray ore". A belt of exhalative m assive sulfide deposits (Kemp, 1982), known as
the Foothills C o p p er-z in c belt, overlaps and lies w est of the gold belts in m ore
fe ls ic vo lcanic rocks. The Hodson m ining d istrict lies in the w est gold belt of the
M o th er Lode, 5 k ilom eters northw est o f Copperopolis, California, and contains both
gold and co p p er-z in c m ineralization (Figure 2).
WE^TTERN SIERRA NEVADA METAMORPHIC BELT. CALIFORNIA
12?* 120*E X I - l . A N A l I O N
C c n o / o i c scJ i n i t -n i j r> ai iJ v<»tc»nic fuCks
G ram itc rocks o f th r Sierra Nevada balhu hth . ch iefly o f M rso^oii ape
Mesozoic sedim enlary and vulcanic rocks in places strongly m etam orphosed
U ltram afic rocks, ch iefly o f Mesozoic age
Paleozoic sedim entary and vulcanic rocks, in places strongly m etam orphosed
Sus.inviiie
Contact
East gold belt 119 '
Area o f map
Modesto
C A L IF O R N IA
Mother Lode gold belt Mel ones fa u lt zone
^ îA C K A M E N l
Study area
West gold belt Bear Mountain fa u lt 'zonë ?
FaultD o tted where concealed
25 50 m i le s
25 50 k i l o m e t r e s
F ig u re 1 G eneralized geo log ic m ap of the w estern Sierra Nevada m etam orph ic belt, showing location of study area. M odified from Clark
(1976).
%Ro»i‘«»oo»#\ ao''£,
PLACER/:; < ‘P « -\> ’ ' v;i . -/ /" t
IvShing»tSoringt
EL D O R A D OOiMifl»
Whit# Ook s\\'- AMADORpiymow'h
MOTHER LODE %
GOLD BELT
: C A L A V E R A S
RoiiroodR'Ch
Mo\imorn Rontn
Wojh»ngton- • - • TU O LU M N E
\^ \V \C o ll'» rv » ll# \ \
EAST GOLD BELTCopper-z inc b e l t \ ooftd«oct '
s r ' ^ WLocation of Figure 3.
' \ ‘' ’^ ^ 0 o g Town ^ Mn CooiTf*,' ♦WWW'v^V^M A R IP O S A
WEST GOLD
BELTWt«t. 00*
Cl«oringhouM
Coiofodo'w
e x p l a n a t i o n
Goniric 'ocki
Sl o t® , s om e p h y l l i f * and c o n g l o m * .
' □ t e ( M a r i p o s a F o r m a t i o n )
Greenstone, □mphiDante,Ohfor'te scnist
S l a te , m ic o sc n i s t , q u o r t j i t e . some
n o r n f e l s an d i m e s i o n e ( C d o v e r a s F o r m a t i o n in po r t )
Gold be l t o r ve in sy s te m
T e r t ia r y rockS o re no t snowm
F ig u re 2 Map of the M o th er Lode gold belts and the Foothills co p p er-z in cbelt.
Southern half of the w estern Sierra Nevada m etam orph ic belt, California. From Clark (1970).
Geology
The Hodson m ining d istrict lies along the Hodson fau lt zone, part of the
m ajor n o rth w e s t-tre n d in g Bear M ountain fau lt zone. The trace of the fault zone is
m arked by lensoidal bodies of serpentine (Figure 3). Shearing is localized along
structural d iscontinuities betw een upper Jurassic slate and m etavolcanic rocks.
Gold m ineralization occurs w ith in the shear zone as quartz stringer zones and as
bodies of ca rb o n ate -rich m etased im entary and m etavolcanic rocks. Horizons of
c o p p er-z in c m ineralization also occur in this zone. The rock packages strike
northw est and dip steep ly to the east, and have been altered to greenschist facies
by regional m etam orphism . Shear zone alteration overprints regional
m etam orph ism and is localized along the s la te -vo lcan ic contact. The southern
Hodson m ining d istrict is divided into th ree regions: (1) a w estern slate belt; (2) an
eastern greenstone belt; and (3) a central zone of shearing, w hich includes altered
proto liths from the o ther tw o belts (Figure 4).
Stratigraphy
The slate sequence is upper Jurassic in age and known regionally as the Salt
Spring Slate. It Is 3 k ilom eters th ick and consists of black and brown fissile slate,
w ith in terfingering lenses and tongues of pyroclastic rocks and associated coarse
greyw acke, tu ff and pebble cong lom erate (Figure 5).
The volcanic sequence, reg ionally known as the C opper Hill Volcanics, is
C o p p e ro p o lis fa u lt zon
EXPLANATION
I Copper H i l l Volcanics L "W -mafic metavolcanics.Location of Figure 4
1 Mile- s i . cherty s la te .
Salt Spring Slate.
jC r r r j Quartz d io r i te (? ) .
Serpentine.
w / ' Fault zone-with th rus t fa u l ts
y Gold mine
o Copper mine
Figure 3. Generalized geologic map of the Hodson mining d is t r ic 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
Jslub
70' J g s
JchsubN
Jchs5 0
30 ' 4 5 " .45 ' J c h s 80 'CO
5 0 0 0 E -WW Jgw Js l
68J c g 70' 20'JslJg w
Js l 4 6 0 O E
3CT
</>4 8 0 0
200 300IC OCO
4 0 0 8 0 0
Figure 4. Geology map of the southern Hodson Mining d is t r ic t WW= Wilbur Womble Mine, GK= Gold Knoll Mine.
See Figure 5, lith o lo g ie descriptions.
8
upper Jurassic in age and conform ably overlies the slate sequence. It consists
prim arily o f subaqueous m afic to in term ed ia te (w ith m inor felsic) pyroclastic rocks
w ith subordinate m assive, am ygdalo idal flow s, aggregating a com bined thickness
of 1500 m eters (Clark, L , 1970). The volcanic rocks are predom inantly tho le iitic in
com position, w ith local ca lc -a lka lin e volcanic centers containing associated
m assive sulfide deposits (Kemp, 1982). In the Hodson mining district, the volcanic
package th ickens and coarsens to the northw est, indicating probable derivation
from th at direction.
Shear zone litho log ies are altered, heavily sheared sedim entary and volcanic
rocks. The type of a lteration and degree of shearing is partially dependent on the
original com position and com petency of the rock type. Thus, shear zone
lithologies, w hile d iscrim inated prim arily by alteration assem blages, also reflect
prim ary stratigraphy. These associations are discussed under the alteration
section of this paper. The w idth of shearing varies from 30 m eters to g reater than
200 m eters across the s la te -vo lcan ic contact. It is w ider in the less com petent
m afic pyroclastic rocks.
Structure
Three defo rm ationa l events are recorded in the rocks of the study area;
isoclinal fo ld ing D^, shearing and thrusting (Figure 6 ). Isoclinal fo ld ing
accom panied g reensch ist facies m etam orphism and is expressed by bedding plane
and axial planar c leavage (Sq and S.,, figure 7) (Eric et al., 1955; Clark, 1960).
Folding was fo llo w ed by shearing producing the Hodson fault zone. Shearing
West
Jmt
Jch» -
t/iJm t
-Jsl
(ab)East
J s l- Black and brown f is s i le s la te , derived from s ilts ton e (3 Km).
Jgw- Graywacke, t u f f , s iliceous zones and schistose metavolcanic
rocks (0-100 m eters).
Jcg- Petromict conglomerate (0-30 meters).
Increasing in terca la ted volcanic debris near volcanic contact.
Shearing, development of penetrative Sg cleavage, pencil structures and t ig h t , chevron fo lds. A lteration halos of s e ric it iz a tio n and disseminated p y r ite . Gold-bearing, quartz
string er veins.
Massive, w hite , brecciated, gold-bearing quartz vein (0-6 meters) Sg o rien ta tio n .
JmSj- Mariposite schist. Contains iron carbonate, quartz, m ariposite, pyrite and variable c h lo rite . Enriched in gold, s ilv e r , tungsten, barium, arsenic and boron (0-15 meters).
Jchs- Chlorite schist. Pyroxene-plagioclase crystal and l i th ic -
crystal mafic tu f f . Often sheared and lo c a lly carbonate altered
(major component o f Copper H il l Volcanics).
JmSg- Maripositeschist. Forms as carbonate a lte ra tio n halos
surrounding quartz veins. Low gold content, high in arsenic, antimony and boron (0-8 meters).
ub- Serpentin ite and u ltram afic rocks. Fault-bounded, in te rn a lly
sheared (0-60 meters).
Jms^- P y r it ic , cherty(?) horizon. In te rfin g e rin g , p y r ite ,
( Î ) m ariposite, s e r ic i te . quartz schist and p y r it ic s la te
horizons. Massive su lfide horizon. Anomalously high in
copper and zinc (0-10 meters).
Jgs- Greenstones. Undifferenciated mafic pyroclastic flows
agglomerates and massive amygdaloidal basalts (ab)
(1500 meters).
F ig u re 5 S tratigraphie section of the southern Hodson mining district. M apped from trenches and stream gullies, located approxim ately along section B- B', Figure 4.
10
is m ost ev ident in the slates and seric ite schists producing penetrative cleavage,
pencil structures, chevron fo lds and quartz stringer zones. M ultip le generations of
fo lded, brecciated and ribboned quartz veins, indicate ductile and brittle conditions
existed w ith in the shear zone (Chace, 1949; McKinstry and Ohie, 1949). The
direction o f shear d isp lacem ent (determ ined by rotated crystals, oriented
crenulation cleavage, sigm oidal surfaces, rotated blocks and en echelon tension
gashes) is right lateral (Sim pson and Smith, 1983). Shear zone cleavage (Sg)
contains tw o orientations; com pressional shear or slip cleavage; and tensional
shear c leavage (Figure 7).
The th ird deform ational event is represented by thrust faulting (Dg). Relative
m otion, indicated by drag fo lds and striated, graphitic shears, suggests th at o v e r-
thrusting is d irected to the southw est. The total am ount of d isp lacem ent is not
known; how ever, stratigraphie o ffset (Figure 11) indicates at least 37 m eters of
lo w -a n g le thrusting has occurred. The orientation of the thrust planes corre lates
w ell w ith expected orientations developed w ithin a shear couple. The thrusting
event probably represents a culm ination of Dg forces, but w ith brittle d islocation
rather than ductile deform ation .
In addition, e a s t-w e s t cross fau lting is suggested by the anom alous
curvature in the thrust plane, abrupt term ination of the southern ultram afic body,
and term ination of certain stratigraphie horizons (Figures 4, 9 and 21). Cross
faulting is probably re lated to thrusting, though its orientation, style and
disp lacem ent are not known. M ajo r serpentine bodies appear structurally
controlled; they are fa u lt-b o u n d ed , c ro s s -c u t stratigraphy and boudin in areas of
dilatancy, contro lled m ainly by w arped surfaces along the shear zone.
11
Alteration and Metamorphism
Tw o episodes of m etam orph ism affected the rocks in the Hodson m ining
district. The first, accom panied isoclinal fo ld ing (D^) as regional greenschist facies
m etam orph ism . The in tensity is uniform throughout the study area and is
characterized by ch loritization of original m afic m inerals. The second episode of
m etam orph ism accom panied shear zone deform ation (Dg), producing localized
alteration w ith in the shear zone characterized by carbonate alteration.
A lteration assem blages reflect the chem istry of the host rock and the
proxim ity and in tensity of shearing. Shearing and alteration intensity is g reatest
along the sed im en t-vo lcan ic contact, decreasing outw ard, form ing cylindrical
alteration shells surrounding d ilatant zones. D ilatancy occurs w ith in the shear
zone due to w rench faulting and ductile shear. A lteration in slate horizons is
m inim al, consisting of a slight bleaching (potassic a lteration) and developm ent of
dissem inated pyrite. A lteration intensity increases in m afic volcanic rocks, often
form ing m ultip le zones w ith distinct m ineral suites. Central quartz veins are
surrounded by a zone of ankerite, m ariposite and pyrite, fo llow ed by a zone of
chlorite plus or m inus pyrite and talc, and typically flanked (on the east) by
serpentine (Figure 8). S im ilar a lteration patterns occur in Tim m ins, Ontario, Canada
(Colvine, et al., 1984), the southcentral M other Lode (Evans and Bowen, 1977), and
in South Africa (Pearton, 1978). Delineation of the serpentine bodies was
fac ilita ted using a m ag n e to m ete r survey (Figure 7). The anom alously high
m agnetic response is due to d issem inated m agnetite which form ed during
serpentin ization of the u ltram afic rocks (Figure 9).
(a)
12
( b )
(c)
F ig u re 6 Tecton ic deve lopm ent in the Hodson m ining district.(a) Isoclinal fo lding, axial p lanar c leavage deve lopm ent (0.,); (b) shear zone, dextrals trike -s lip d isp lacem ent w ith the deve lopm ent of gash veins (D^); and (c) w e s t- verg ing th rust fau lting (D^).
13
Bedding or axial planar cleavage (S^and S^).
Shear zone penetrative cleavage. Compressional(S2c)-
Shear zone penetrative cleavage. Tensional ( S 2 t ) -
Thrust plane orientation.
F ig u re 7 S tereo n et plot of c leavage developm ent in the Hodson m ining district.
14
The generalized paragenetic sequence of alteration and gold mineralization is
depicted in Table 1. M ost of the alteration occurred during shear zone
deform ation (Dg), w ith complex, overlapping and repeated phases. Crosscutting
relationships indicate that the sequence of alteration began with widespread
carbonate developm ent, fo llowed by m ariposite and sericite formation. Pyrite
fo llowed, w ith deve lopm ent along mariposite lineation bands. Quartz and q u artz -
carbonate veining occurred during and after mica developm ent and was
accom panied by sulfidation and contem poraneous or later gold mineralization.
Brecciation of the quartz veins, with additional?) gold mineralization, may have
occurred during the D 3 thrusting event.
Geochemistry
In order to determ ine trace metal associations and possible pathfinder
elem ents 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 elem ents (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 elem ent for gold mineralization, fo llow ed by tungsten;
2 . arsenic also outlines mariposite schist horizons, which are not always
mineralized;
3 . arsenic anom alies form a stronger, broader response, extending out and beyond gold mineralization;
15
I'AFIC METAVOLCANIC ROCKS
METASEDIMENTARY ROCKS
F ig u re 8 D iagram m atic illustration showing alteration zones surrounding gold quartz veins in greenstone lithologies. (after
Colvine et al., 1984).
o
5 2 0 0 05 5 0 0 0
5 5 0 0 0 5 5 0 0 0 5 4 0 0 E -
5 0 0 0E -5(000ww
GK
4 6 0 0 E -
S C A L E
I. 4 8 0 0
200 300too
4 0 0 8 0 0
Figure 9 Magnetic map of the southern Hodson mining district.
05
17
D, D2 D3is o c lin a l fo ld in g greenschist fac ies
metamorphism
shearing penetra tive cleavage
shear zone metamorphism
th ru s ti ng
C h lo rite
Carbonate
Seri c i te
M ariposite
?
P yrite
Quartz
Gold
Serpentin ization
T a b le 1; Interpreted mineral paragenetic sequence.
18
4. m ercury delineates areas of soil contamination by mill tailings (because am algam ation plates w ere used to extract gold in the old mills);
5. soil geochem istry correlates well with rock geochem istry (Figures 10 and 11);
6 . the strongest gold mineralization is in the quartz stringer zone and central portion of the mariposite schist horizons.
Geochem istry m aps of gold and arsenic distributions in soil outline tw o
m ajor linear bands of anom alously high metal concentration, which correspond to
the mineralized quartz stringer zone and mariposite schist horizons (compare
Figures 12 and 13 w ith Figure 4).
A comparison of e lem ental distributions in soils shown in Figure 10 with
bedrock lithologies, indicates four geochem ical zones with unique metal
enrichm ents (Figure 14):
1. A copp er-z in c zone, with enrichm ent of Cu, Pb, Hg, Te and Mn with m inor Au and Ag. The copper-z inc zone exhibits a much stronger response in drill hole samples (underlined e lem ent 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, ch lorite -r ich , mariposite schist stratiform horizon with minor, irregular quartz pods. This zone is proximal to the quartz breccia zone, but shows enrichm ent in Te, As, F, B, W and Au.
4. A quartz stringer zone consisting of quartz veining, brecciation and replacem ent at the vo lcan ic -sed im en t contact. It is enriched in Au, As,W, B, F and Te.
The variability of m etal suites in the separate zones is in part related to the host
rock lithologies. Tellurium and m anganese concentrations increase In the volcanic
rocks, while barium, boron, fluorine and zinc background concentrations are greater
19
Geology
600-,
300-
ISO-
I 20-
0 80-
0 4 0 -
0 0 2 -
1000
60 0
200
600-
20 0 -
w(epml
4750E5650E
G e o lo g y
2 0 0 -,
I 50-
I 0 0 -
5 0 -
20
10 -
100
6 0
2 0
0(0
0 06
0 02
2 000-,
1500-
500-
2 0 -,
10-
5 6 5 0 E 4750E
Figure 10 M u lt i -e le m e n t soil geochem istry in relation to geology. Location, g r id - l ine 6400N, section A -A ' on soil geochem istry maps. 50 foot, spot- sample spacing. * = Detection limit. See Figure 5 for stratigraphie descriptions.
20
T a b le 2: Pathfinder e lem ents for gold mineralization (see figure 10).
Arsenic (As):
Mercury (Hg):
Tellurium (Te):
Manganese (M n ):
Lead (Pb):
Zinc (Zn):
Antim ony (Sb):
Boron (B):
Barium (Ba):
Fluorine (F):
Tungsten (W ): Gold (Au):
Excellent pathfinder element. Very strong response oyer a broader area than gold. Arsenic also delineates carbonatealtered zones which are not enriched In gold.Fair correlation, not very consistent but also picks up hydrotherm ally altered areas Mercury is best used to delineate contam ination from mill tailings.Fair correlation with quartz veining and carbonate altered zones. Shows strong, broad response in volcanics over the copper-z inc horizon.Very erratic, poor pathfinder element. Fails to delineate quartz veining and carbonate altered zones. Shows highest response over co pp er-z in c horizon. Fair lithology indicator for volcanics. Poor pathfinder element. Very little to no response. Slightanom aly over copper-z inc zone.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.Inconsistent. Strong response over carbonate altered gold mineralization, but no response over quartz breccia gold mineralization.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.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-z inc mineralizations.Fair to good correlation with gold. Picks up both gold zones with a stronger response over the quartz breccia zone. Anom aly is broader, but peaks are not well resolved.Good correlation with gold but w eak response.Soil geochem ical response shows excellent correlation w ith rock geochemistry. Strong response over quartz stringer zone and w estern carbonate altered zone.
21
in the slates. O ther metal variation, such as the high concentration of As, Sb, and
B in the eastern mariposite schist horizon, could be the result of fluid tem perature
variations betw een shear zones.
Mineralization
Gold mineralization in the study area is controlled by both structure and
stratigraphy; structure dominates. The tw o main types of gold mineralization are
quartz stringer zones and stratiform horizons of gold-bearing, mariposite schist.
C o pper-z inc m ineralization forms stratabound horizons stratigraphically above the
gold mineralization in the mafic volcanic rocks, and forms within chlorite shear
zones, adjacent to fau lt-bounded serpentine bodies (see Map 1).
Structurally controlled gold mineralization, located within the Hodson fault
zone, form s h ig h -g rad e (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 am ounts 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 m eters 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
Au(ppm)
3 01
2 0 À
0 4
Ag
SO-,
3 0 1
'0 4
As(ppm)
3004
1004
51155545 E
Geologyt/)
D 5 -o
(/)£“3
toO“3
ni!to
£ o E- 3 - 3 - 3
F ig u re 11 Rock geochem istry 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
0 02
W W ___
0 2
-GK
o
5400 E .
5 0 0 0 E-
4 6 0 0 E
( 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 Ml It-i'l .iluiig liiu'b b|uitd ,it4IIII 11 m l III11- r v,i 1 I, / S C A L E
r. 4 0 0 0
1001. I
4 0 0
200 __l_ I
8 0 0 F(
Figure 12 Map of gold distribution in soil.
roCO
o
100
4 0 05 4 0 0 E -
zoo 4 0 0
5 0 0 0 E -WW
GK2 00 200
O4 6 0 0 E -
s c a l e
r 4 8 0 0
300100 200
4 0 0 6 0 0
Figure 13 Map of arsenic distibution in soil.
ro•p.
in
5400E -
W W 5 0 0 0 E -GK
46 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 ooi
26
and unfolded quartz stringers appear Identical, with no consistent cross-cu tt ing
relationships. Folded, brecclated and ribboned go ld -bearing quartz veins Indicate
quartz Influx occurred repeatedly at tem peratures and pressures near the boundary
betw een brittle and ductile conditions.
Stratigraphically controlled gold mineralization occurs In carbonate-a ltered,
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 elem ents (see Figure 10). It is unclear
w hether these horizons represent original sedimentary accumulations or are the
result of an epigenetic mineralization process.
In th in -sec tlo n 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 , ch rom lum -rlch variety of
m uscovite (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 (com m only zoned) pyroxene
(auglte?) and clasts containing unorlented, felted mats of plagloclase laths.
Crystals are thoroughly altered to f in e -g ra ined assemblages of carbonate, sericite
and chlorite. The rock Is sheared, com m only mylonitlzed, with mariposite and
pyrite d eve lop m en t 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 3m m cubes and pyritahedrons. Pyrite cubes are often rotated and
surrounded by pressure shadows of feathery quartz (Figure 20). Coarse cubic
pyrite is the dom inant gold carrier with enclosed gold particles from one micron to
one m illim eter (Meridian C om pany report).
The orientation and distribution of mineralized quartz veins within the shear
zone appears to be controlled by dilatancy. M ovem ent along the shear zone is
righ t- la tera l with a lesser thrusting component, therefore, dilatant zones form as a
result of strike-s lip d isplacem ent and dip-slip m ovem ent (Figure 22). The result is
an anastromosing network of gash veins developed by tensional dilation within the
shear zone. D is tr ic t-w ide and throughout the M other 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 determ ined 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 com petency of the rock package.
Horizons of co pp er-z in c mineralization form statigraphically above the gold
m ineralization in the mafic volcanic rocks. They consist of finely disseminated
pyrite and m inor 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).
N)00
UJ
D2904007 060809
JSl 1000-<Jchs
Jsi
S C A L E
U 2 4 0 0600.
50
Au ppm X 0 2 As a Z n ppm x 5 0
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 pyrit ic interbeds within the undifferentiated greenstone package. See Figure 5 for l i thologie descriptions.
LU
DI6 D34DIS017
1000-
s w N E
roco S C A L E
600-50
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 l i thologie descri pti ons.
D37D49039DIS020 019
1000-
s w N E
coo S C A L E
1 2 4 0 0600.
50
Au ppm X 0 2 As ppm X 5 0
Figure 17. Cross section D-D, along grid line 8000N, showing geology and geochemistry, mineralization in mariposite schist horizons and in shear zone, along the metasediment- contact. See Figure 5 for l i thologie descriptions.
Note: gold metavolcanic
ww GK D27DJ5 033 029041 049 047 1200 -
NW SE
Jchs
ub 9 8 0 -
Jtl Jch iJsl
9 4 0 ' El in Fl
Zn As(ppm » 100)
A L E
1 .4800Very low.
200 300100I tic undl f f e renl idled greeneluDe pdckdge.
4 00 8 0 0
Figure 18. Longitudinal section, showing geology and geochemistry, for l i thologie descriptions, and Figure 4 for location.
Section E-E*. See Figure 5
32
F ig ure 19 Photomicrograph of mariposite schist, showing pyrite preferentially located along mariposite lineation bands. Field of
v iew = 2 mm.
________
V #, y
4 ; J 3 k
F ig u re 20 Photomicrograph of rotated pyrite cube in mariposite schist, showing pressure shadows of feathery quartz. Arrows indicate
direction of rotation. Field of v iew - 2 mm.
- 5 0 0 --------
- 7 0 0
6 0 0
b 4 0 U t9 0 0
50'8 0 04 5'
30
1 - 4 8 0 0
100 200
4 00 duo
Figure 21. Structure contour map of the Hodson fau lt (volcanic-slate contact), showing distr ibution of major gold mineralization by dot pattern.
0000
34
a)
b)
F ig u re 22 Dilatant zone deve lopm ent as a result of fault displacement.(a) Dip slip, high angle reverse motion; (b) shear zone, strile slip motion; and (c) com bination of shearing and overthrusting.
Figure 23 Dilatant zone development as a result o f 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 -re lated, mariposite schist horizons, indicates possible primary,
syngenetic gold mineralization with subsequent secondary concentration during
deform ational events. Any model or models that attem pt to explain the genesis of
the gold m ineralization must effectively address the relationships betw een
stratigraphically and structurally controlled gold mineralization. Ore genesis
m odels which are applicable include exhalative-hydrotherm al and shear zone
secretion.
Syngenetic Model
An exhalative -hydrotherm al m odel was proposed by Kemp (1982) for the
form ation of the Foothills co pp er-z inc belt, which parallels the M other Lode gold
belts. A syngenetic, exhalative m odel has also been suggested by other workers
for the form ation of the M other Lode gold belts. In the syngenetic model gold is
deposited by vo lcan ic-re la ted , hot spring activity on the sea floor (Rye and Rye,
1974; Karvinen, 1980). The resulting deposits are stratiform and consist of
chemical, sed im entary exhalites. The deposits form in association with mafic
a n d /o r ultramafic volcanic rocks and are overlain by a sedimentary sequence.
36
37
Exhalative m ineralization in the form of massive sulfide deposits are well
docum ented in the M other Lode region (Kemp, 1982) and occur in rocks of
practically all ages (Franklin et al., 1981). Known examples of exhalative gold
deposits are less com m on, occurring principally in Archean terranes (Karvinen,
1980; Hutchinson, 1976).
Exhalative gold deposits m ay be present in the Sierra Nevada M etam orphic
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 M other Lode gold belts includes:
1. the parallel nature of the belt of syngenetic, exhalative, volcanigenetic co pp er-z in c deposits
2 . the form ation of the gold deposits at vo lcan ic-sedim entary 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 conglom erate 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 hydrotherm al convection cell
3 8
8 . the deposits occur primarily along m ajor sedimentological transitions, but also to a lesser degree as smaller deposits between m ajor belts (indicating the continuation of hydrothermal, exhalative activity but with m ore sedim ent dilution)
W hile 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 fo rm 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 ultram afic bodies (m ost of the ultramafic bodies are structurally em placed along the fault zones, compatib le with structurally controlled gold mineralization)
6 . carbonate alteration overprints regional m etam orphic fabric (gold mineralization is spatially related to carbonate alteration and therefore, m ay also be post-reg iona l m etam orphism )
M any 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
m ineralization are both often localized along structural discontinuities represented
by vo lcan ic -sed im en tary rock contacts. The gold deposits may be related to the
ultramafic bodies because erosion and concentration of metals from these bodies
3 9
could form the g o ld -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 m erely remoblized during carbonate alteration. Furthermore,
hydrotherm al feeder pipes are often hard to distinguish and may only be indicated
by foo tw all alteration assemblages. Finally, vo lcanic ly-induced hydrothermal,
a ltera tion -m inera liza tion may have proceeded volcanic outpourings.
Epigenetic Model
The close spatial relationship betw een gold mineralization and major shear
zones suggests an epigenetic origin for the form ation of the M other Lode gold
deposits. A shear zone secretion model, proposed by Z im m erm an (1981) and
Nesbitt et al., (1986), and largely adapted from Boyle (1959 and 1979) m ay have
operated. In the model. M other Lode-type gold deposits form by secretion of
m etam orphic or deep m eteoric fluids along shear zones. M ovem ent 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 /o r chemical reduction.
Shear zones are com m on 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 em placed 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 m ineralization in the study area, throughout the M other 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-sea ted mineralization)
5. spatial association of serpentinized, ultramafic bodies to many of the deposits (because both are probably controlled by zones of dilatancy)
6 . m ineralization contains a distinct suite of trace metals: gold, silver, arsenic, tungsten, boron, barium and antimony (compatible with m esotherm al, hydrotherm al conditions)
7 . m ineralization contains diagnostic alteration minerals: ankerite, mariposite or chrom iferous mica, sericite and quartz (which developed during shear zone activation)
8 . deposits are lensoidal-shaped with diffuse mineralized contacts (com patable w ith d e e p -s e a te d 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 m etam orphic fabric (carbonate alteration accom panied gold mineralization)
11. mineralization and shearing occur primarily along m ajor sedim entological transitions (which also represent structural discontinuities)
41
The shear zone secretion theory fails, however, to sufficiently explain the com m on,
parallel occurrence of exhalative, massive sulfide mineralization, and the existence
of interfingering, low grade, stratiform horizons of mariposite schist.
A m ajor 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
go ld -b earin g , m ariposite schist.
Genesis of mariposite
Throughout the M other 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 ro ck -type has been variously described as:
1. a m etam orphosed alunite deposit (Schreyer, 1982)
2 . chem ical sediment, classified as a mixed carbonate-sulfide iron form ation (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 num ber of suggested protoliths for chrom e m ica-bearing rock, indicates
that It can probably form from alteration of many different rock types, controlled
principally by the chem istry of the enclosing rock and environm ent of alteration.
Chrom iferous mica occurs In tw o environments: In detrltal, m etasedlm entary
rocks, and m ore com m only. In greenstone, metavolcanic rocks (Leo, et al, 1965). In
the m etased lm entary rocks the mica Is associated with detrltal chromium,
concentrated In heavy metal accumulations (Leo, et al, 1965; S Inha -roy and Kumar,
1984; Ram lengar et al, 1978). In m etavolcanic rocks, as In the Hodson mining
district, the mica is associated with carbonate alteration and gold mineralization.
In the m etavolcanic rocks three elem ents are always present: (1) mafic to
ultramafic Igneous rocks; (2) greenschlst facies (or higher) metamorphism; and (3)
m ajor shear zones. The chromiferous mica occurs In or adjacent to the ultramafic
rocks, suggesting that they provide the source for the chromium, either by
hydrotherm al alteration and leaching (Whitmore, et al., 1946), or by erosion and
concentration In heavy metal accumulations. M etamorphism supplies the correct
com bination of tem perature, pressure and fluid chemistry for developm ent 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 Im portant spatial relationship Is that many of the altered ultramafic bodies
hosting chrom iferous mica and gold mineralization are believed to be structurally
em placed along the shear zone. The alteration post dates em placem ent, which
strongly discounts the posslbllty that the chrom e mica and the gold are of an
exhalative source. The chrom e m Ica -b ear In g 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 m etam orphic terranes.
Not all carbonate -r ich , greenstone gold deposits are associated with chrome
m ica -b ear in g rock. In California, gold mineralization occurs in other rock types
along the m ajor 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 docum ented.
W hile the above reasoning strongly supports shear-zon e genesis for the
form ation of g reen s to n e -typ e gold deposits, certain characteristics remain
enigmatic, such as: ( 1) the com m on 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 com m on, regional, spatial, plutonic
association of m any of the deposits. These relationships can be explained by their
com m on association with accretionary tectonics which is the requisite environment
fo r the form ation of g reen s to n e -typ e gold deposits.
The entire length of the western margin of North American, from southern
44
California north through Alaska is com posed of accreted terranes (Jones, et al.,
1977; M o n g er and Price, 1982). C arbonate-r ich , g reen sto ne-type gold deposits
occur in term itten ly along its entire length: from those of the M other 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 zo n e-re la ted gold deposits are not unique
to the M oth er Lode region and can occur throughout accreted volcanic, greenstone
terranes. G reen s to n e -typ e gold deposits are very com m on in rocks of Archean
age. The litho -tec to n ic characteristics of the Archean deposits are similar to those
of the M oth er 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 g reen s to n e -typ e gold deposits would be to identify the tops and
bottom s of these systems, in order to define vertical alteration and chemical
zoning. The higher portion of these systems may be enriched in antimony and
m ercury w hile at deeper levels tungsten concentration may increase. Stable
isotope studies may be useful to "see through" m etam orphic chemical changes to
determ ine the extent of shear zone related alteration. It would also be interesting
to com pare the location, peridicity and structural relationships between greenstone
gold deposits and spatially associated cop per-z inc deposits.
Conclusion
Structural data from cleavage and fault plane orientations indicate three
periods of deform ation affected the rocks of the Hodson mining district: D^-
isoclinal folding; Dg- strike slip displacement; and D3- reverse motion. Regional
greenschlst facies m etam orphism accompanied D., folding while shear zone
m etam orph ism 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
fo rm ed due to shear zone secretion, in a britt le -ductile shear zone. Successive
periods of gold m ineralization progressed from widespread carbonate alteration, to
the deve lopm ent of quartz stringer zones, ending with brittle fracture and breccia
zone developm ent.
45
References
Andrew, A. J., and Wallace, H., 1983, Alteration, m etam orphism and structural patterns associated w ith Archean gold deposits: Preliminary observations in the Red Lake area, In Colvlne, A. C., ed., i n The geology of gold In Ontario, Ontario Geol. Surv., Misc. paper 110, p .141-163 .
Boyle, R. W., 1959, The geochem istry, origin and role of carbon dioxide, water, sulfur and boron In the Yellowknife gold deposits. Northwest Territories, Canada: Econ. Geol., v.54, p.1506 -1 5 2 4 .
Boyle, R. W., 1979, The geochem istry of gold and Its deposits: Together with achapter on geochem ical prospecting for the element: Geol. Surv. Canada,Bull.280, 584p.
Chace, F. M. 1949, Origin of the Bendigo saddle reefs, with com m ents on the form ation of ribbon quartz: Econ. Geol., v.44, p.561 -597 .
Clark, L. D, 1970, Geology of the San Andreas 15-m ln u te quadrangle, Calaveros County, California: Cal, DIv. Mines and Geol., Bull. 195, 23p.
, 1976, Stratigraphy of the north half of the western Sierra Nevadam etam orphic 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 m arlpos lte -bearing rocks from the south -centra l Mother Lode, California: Cal. Geol., v.36, no.9, p.20 1 -2 0 3 .
Evans, J. R., and Bowen, O. E., 1977, Geology of the southern M other Lode Tuo lum ne and Mariposa Counties, California: Cal. DIv. Mines and Geol., Map
sheet 36.
Eric, J. H., S tromqulst, A. A., and Swinney, C. M., 1955, Geology and mineral deposits of the Angles Camp and Sonora quadrangles, Calaveros and Tuolum ne 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 environm ent of carbonate alteration and stratiform gold mineralization, T immins area, in Roberts,R.G., ed.. Genesis of Archean vo lcan ic -hosted gold deposits: Ontario Geol. Surv. Open File Report 5293, p.6 6 -9 1 .
Hinse, G. J., Hogg, G. M., and Robertson, D. S., 1986, On the origin of Archean ve in - 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.6 8 , p .1223-1246 .
Jones, D. L., Silberling, N. J., and Hillhouse, John, 1977, W rangell ia - A displaced terrane in northwestern North America: Can. Jour. Earth Sci., v . l4 , p. 2 5 6 5 -2 5 7 7 .
Karvinen, W. O., 1980, Geology of evolution of gold deposits, Timmins area, Ontario, i n 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 C u-Zn belt. Sierra Nevada, Cal.; Ph.D Dissertation, Univ. of Nevada, Reno, 360p.
Kerrich, R., and Fyfe, W. S., 1981, The go ld -carbonate association: source of C 02and C 0 2 fixation reactions in Archean lode deposits: Chemical Geol., v.33,pp .265-294 .
Knopf, A., 1929, The M other Lode system of California: U.S. Geol. Surv. Prof. Paper
157, 8 8 p.
Leo, G- W., Rose, H. J., Jr., and W arr, J. J., 1965, Chromian m uscouvite from the Sierra de Jacobina, Bahia, Brazil: Amer. Mineralogist, v.50, p.392 -4 0 2 .
McKinstry, H. E., and Ohie, E. L, Jr., 1949, Ribbon structure in go id -quartz veins: Econ. Geol., v.44, p.8 7 -1 0 9 .
Misra, K.C., and Keller, F. B., 1978, Ultramafic bodies in the southern Appalachians: A review: Am er. Jour, of Sci., v. 278, p. 389 -4 1 8 .
M onger, J. W. H., Price, R. A., and Tem pelm an-K lu it , D. J., 1982, Tectonic accretion and the origin of the tw o m ajor m etam orphic and plutonic weits in the Canadian Cordilleran: Geol., v.lO, p.70 -7 5 .
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 geochem istry of the Monarch ore body and environs, Murchison range, northeastern Transvaal, in Verwoerd,W.J., ed., M ineralization in m etam orphic terranes: Geol. Soc. of South Africa, Special Publication 4, p.77 -8 6 .
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 chrom ite -fuchs ite quartzite in the older supracrustal sequence of Karnataka: Jour, of Geol. Soc. of India, v.19, no.12, p.5 7 7 -5 8 2 .
Rye, D. M. and Rye, R. 0., 1974, Hom estake gold mine. South Dakota: I. Stableisotope 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 developm ent of California (Rubey Volum e I): Prentice-Hall, Inc., p .132-181.
Schreyer, W., 1982, Fuchsite -a lum inum silicate rocks in Archean greenstone belts: Are they m etam orphosed 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 m o vem en t in sheared rocks: Geol. Soc. Am., Bull., v. 94, p. 1281-1288.
S inha-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 r ig h t J. E., and Saleeby. J. B., 1982, Significance of m id -M eso zo ic peridotitic to dioritic intrusive compleses, Klamath M ountains- w estern Sierra Nevada, California: Geology, v. 10, p. 160-166.
Taylor, B. E., 1981, Hydrotherm al fluids in the M other Lode gold deposits of California: EOS, v.62, p .1059. California: U.S. Geol. Surv., Geophysical Investigations, Map G P -671 .
W hitm ore , D. R. E., Berry, L. G., and Hawley, J. E., 1946, Chrom e micas: The Amer. Mineralogist, v.31, p .1 -21.
Z im m erm an, J. E., 1983, The geology and structural evolution of a portion of the
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M oth er Lode b e l t A m ador County, California: unpubl. M.S. Thesis, Univ. of Arizona, Tucson, A r , 139p.
Appendix A
Pétrographie Descriptions
M any of the thin section from the Hodson mining district are clouded by
alteration and w eathering products. Carbonate alteration is particularly prevalent,
obscuring primary textures with recrystallzed carbonate. In most instances the
carbonate is an iron -r ich dolom ite, presumably ankerite. The discrimination
betw een mariposite and fine sericite is not possible in thin section. Therefore, the
presence and percent of m ariposite in the rock was determined by hand sample
identification.
Rock Name:
Mineral %
Pebble conglom erate , No. W -6 5 .
Im portant Properties
70% Quartz
25% Plagioclase
4% Carbonate 1% Sericite < 1 % Actinolite?
Angular to subrounded, < 0 .1 m m . to > 2 m m . (in thin section). In o/c , up to 5 cm. Strained, rotated, often w / inclusions.As > 2 m m . clots of interlocking, subhedral crystals, and as subhedral crystals in matrix. Fairly fresh, equidimensional and la th -shaped .Tan rhombs, in slate clasts.Fine kneedles disseminated throughout.Lt. brn., fibrous. Acicular habit.
Textues, A lteration and Comm ents:Conglom eratic: Variable clast size.Sheared: S m eared -o u t , e longated 5:1. Rotated and strained quartz. Carbonate rhombs, grow ing in slate.
50
51
Rock Name: Meta siltstone, No. 44,
M ineral % Im portant Properties
90% Quartz
3% Carbonate
3% FeOx
Tr. Sericite Chert(?)
Fine, equigranular qtz. in matrix of slate, and 30% recrystalized, coarse qtz. in veinlets and clots.Along fluid influx lineation bands; 2 directions. Also as diss. rhombs.As It. brn. alteration of rhombic carb. blebs-diss., and as alt. boarders of recrystalized qtz. vnlets.Diss. fine kneedles w / carbonate.Gray, isotropic blebs, diss. in matrix and as gash vns. w / carbonate.
Textures, A lteration and C om m entsRock is sheared, w ith carbonate and sericite influx along "C" surfaces, semi-paralle l to bedding. Q tz .-f i l led , gash veins orientated at 4 5 ° from "C" surfaces.
Rock Name;
Mineral %
Coarse, polymict conglom erate. No. 25
Im portant Properties
60% Chert clasts 20% Quartz
10% Vole, clasts
3% Carbonate Tr. Sericite
2% FeOx
Elongate clasts of chert or fine slate. Subangular.Recrystalized, interlocking qtz., often rolled along zones of shearing.Predominantly clots of unoriented plag. laths, w / some blocky crystals.G rowth along interclast, fluid migration lineation ("C" surface). W /carbonate , along foliation (and fluid migration) parallel to clast elongation.Lt. brn., w /carb .
Textures, A lteration and Com m ents C onglom eratic . Sheared and elongated 1:5. Carb. alt. only inter-clast. Clast supported matrix. Slight imbrickation. Clast size: 2 to 8 mm. length.
52
Rock N a m e : Greywacke, No. W -6 4 ,
M ineral % of rock Im portant Properties
50% Slate clasts
25% Quartz 20% Orthoclase
Tr. Carbonate Tr. Sericite
Com posed of variable, fine to coarse qtz. grains. Stretched, similar to ripup clasts.Strained, recrystalized frags, and oblate clasts.Lt. grey to white, biaxial (-) , large 2V. Subangular frags., some square, equidimensional, others oblate, anhedral. l ineation bands W /carbonate .
Textures, A lteration and Com m entsSubangular grains, rotated, elongated, sheared. Bimodal.
Rock Name:
Mineral %
C ontorted slate. No. W -5 2 .
Im portant Properties
60% Quartz
30% Sericite Tr. Opaques 2% FeOx
Lt. grey, euhedral. Occurs in hing of axial plane, increnulationss. Also as strained blebs throughout.Fibrous, It. brn. to colorless. Throughout rock.BIk. flakes, possibly graphite.Lt. brn.
Textures, A lteration and Com m ents Crenulation cleavage. Could also be named a Sericite, quartz schist.
53
Rock N a m e :
M ineral %
Mariposit, carbonate, quartz schist, No. W -5 3 .
Im portant Properties
85% Quartz
10% Carbonate 8% Muscovite Tr. Actlnollte(?) Tr. Chlorite
Granular thoughout entire rock. Sed. clastic. Also as euhedral, recrystalized open spaces.Secondary, as smears and surflcial coatings. Cuts groundmass. Problably mariposite and sericite. With carb.Acicular, radiating kneedles on qtz.BIk. to dk. green. Som e penene chi. Berlin blue.
Textures, A lteration and C om m entsGranular qtz. Interlocking grains, equigranular, w / clots of larger qtz. Protollth: greywacke.
Rock Name:
Mineral %
Mariposit schist. No. W -5 4 .
Im portant Properties
50% Quartz 30% Carbonate 17% M uscovite < 1 % Pyrite
Granular, fine silt sediment. Difficult to esrimate. dolomite. Pervasively alt. rock. Granular text. Maripostie. In lineated streaks throughout rock. Anhedral blebs, ass. w / mariposite lineation bands.
Textures, Alteration and Com m ents Fine grained. Relict crystals In matrix. Sheared. Name: Carbonate altered, pyrite, mariposite, qtz. schist.Protollth; Crystal tuff, or tuffwacke.
54
Rock N a m e :
Mineral %
Mariposit schist, cut by veining. No. W -55 .
Im portant Properties
70% Carbonate
15% M uscovite
10% Quartz
< 1 % Pyrite
In vein, 85%, as euhedral dolomite (polysynthetic twinning). In matrix, as fine grained (clastic ?) assemblage of carb. and plag. (albite ?), carb.= 70%, plag.=30%.Mariposite. In fluid migration, lineation bands ("C" surfaces). Pyrite preferencially devel. along bands.In groundm ass as fine banding. 15% of vein. Later than carbonate.Blebs, cubes, boudins, elongated, sheared, smeared and rotated.
Textures, A lteration and Com m entsHistory: 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. Deform ation, strained calcite.Name: Sheared, carbonate altered, pyrite, mariposite, quartz schist.Protolith: Tuffwacke.
Rock Name:
Mineral %
Mariposit schist. No. W -61 ,
Im portant Properties
50% Quartz
45% Carbonate
2% M uscovite < 1 % Pyrite
Spherical and oblate blebs, possibly qtz. eyes. Strained, granular, in groundmass as sed.As blebs, similar to qtz. Also pervasive throughout entire rock, as alt. Obscurs text.Diss. thoughout. Alined, schistousity.Cubes and aggregates in secondary qtz. clots. Occurs along fluid migration lineations, and with carb.
Textures, Alteration and Com m ents Sed. clastic text.; lineation of micacious minerals, recrystalized qtz. in voids. Cut by qtz.-carb . vn.
Carbonate alt. Clots of
55
Rock Name: Mylonitized, cherty slate. No. 44a.
M ineral % Im portant Properties
35% Quartz 30% Matrix 10% Chert 5% Carbonate
Recrystalized along low presure, "C" and "S" surfaces. Strained. Slaty rx. w / alt., bik., grungy, isotropic matrix; chert (?).As above.Predominantly along "S" surfaces.
Textures, A lteration and C om m entsHeavily sheared, mylonitized. Fluid influx along "C" surfaces, carb. alt. Devel. of gash vns. (ladder struct.) in more com petent hor.
Rock Name: Vein quartz. No. W -5 0 .
Mineral % Im portant Properties
65% Quartz 30% Carbonate Tr. M uscovite
Strained, euhedral, interlocking crystals. Carb. before qtz. As above, elongate.Mariposite, along small shear.
Textures, Alteration and C om m ents 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 N a m e :
Mineral %
Carbonate alt. volcanic rock, No. W -59 .
Im portant Properties
70% Carbonate
20% Quartz
< 1 % Pyrite
Tr. Sericite
Rock is com pletly carbonate altered. Equicrystaline to slightly elongate rombs and plates.In groundm ass w / carb., and fibrous in pressure shadows surrounding pyrite cubes.Cubes and blebs. Rotatated. Appears to have grown in the matrix, post carb., or contemporaneous.Also chlorite.
Textures, A lteration and Com m entsRelict plag. and pyroxene (?) crystals. Porphyritic text. Heavily carb. alt. Name: Carbonate alt. plagioclase porphyry.
Rock Name:
Mineral %
Carbonate alt. chlorite schist. No. W -51 .
Im portant Properties
50% Carbonate
25% Glass 10% FeOx 15% Feldspar < 1 % Sericite < 1 % Pyrite
Forms matrix, obsquers text. Ragged, splotchy, w / FeOx patches.Chloritlzed pumic frags, (fiame). Elongation 1:5. Foliation.Lt. redish-brn., cloudy patches on iron-carb. (ankerite ?).Ortho(?), as mosaic of stuby and alt. laths, in groundmass.Scaly aggregates, in shadow struct, of rotated pyrite cubes Cubies. Rotated.
Textures. Alteration and Com m ents Name: Carbonate alt., sheared, sericite, quartz, chlorite schist.
57
Rock Name: Mafic pyroclastic. No. 87b.
Mineral % Im portant Properties
40% Plagioclase 35% Relict Px.
10% Slate 10% Chlorite
Unorientated phenos. Cloudy, saussaritized, corroded edges.Brn,tota lly alt pyroxene crystal. Now carbonate. Large, euhedral.LIthic clasts.Chloritlzed matrix.
Textures, A lteration and Com m entsName: Carbonate alt. augite(?), plag. porphyry.
Rock Name: Mafic pyroclastic, No. W - 6 6 .
Mineral % Im portant Properties
40% Glass 15% A m ygdules
10% Pyroxene 15% Plagioclase 10% Feldspar
Isotropic groundmass, Chloritlzed.Filled w / It. b lue-green , fibrous, pleochroic (ye ll-green to clear) zeolites or chlorite.Euhedral, porphyritic.Saussuritized, cloudy, g lomeral-porphoritic .Orthoclase(?). Stubby and lath-shaped crystals in groundmass.
Textures, Alteration and Com m ents Glassy, porphyritic, abund. spherilltes. No apparent shearing. Name: Chloritlzed, amygdaloidal, pyroxene, plag. pyroclastic. porphyry.
augite(?), plag.
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 m any of the underground samples were of oxidized rock.
Drill holes 1 to 25 w ere drilled using a pneumatic, conventional rotary drill.
Drill holes 26 to 51 w ere drilled using a reverse circulation, rotary drilling method.
Holes w ere drilled using compressed air when possible, however, water was used
in m any of the holes below 100 feet, because of the high water table. Most
samples w ere 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 s il ica te -carbonate bodies, whereas thin soil forms over slaty
rocks.
58
59
Analytical techniques
The m ajority of the geochem ical samples were analyzed by Skyling Labs in
Tucson, Arizona. Standard sample preparation methods and analytical techniques
w ere employed. Internal standards, duplicate samples, repeat runs and cross
checking samples with other laboratories w ere used to confirm accuracy.
Most samples w ere analyzed using graphite furnace, atomic absorption
methods. Exceptions include: m ercu ry - mercury detector; bo ro n - sem i-
quantitative, emission spectroscopy; and f luor ine- specific ion electrode. Samples
with values greater than 6 ppm (parts per million) gold were re-analysised using a
fire assay m ethod. Overall accuracy for sample preparation and analysis is ± 10%
(personal comunications. Jack Allen, geochemist. Skyline Labs).
o >o
No. Rock description SiOg TiOg
Whole rock chemistry (wt.
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. a l t . 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 l t . , chi. s c h . , t r . mar. 39.1 0.120 9.60 8.2 0.150 18.6 4.70 0.72
4 Bik, mafic sch., u l tra 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
5 Diabase s i l l ? , carb. a l t . 55.2 0.081 16.10 5.1 0.069 3.3 4.8 3.30
6 Peridotite , s i . serp. 44.4 0.230 15.40 7.6 0.150 9.0 17.80 0.03
8 Par t ia l ly 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 d is t r ic t . Samples taken along section D-D', near the Gold Knoll Mine.