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Thallium in gold-silver-bearing quartz veins and associated volcanic rocks from the Como mining district, Vevada, U.S.A.
Chemical Geology, 54 (1986) 27--34 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
27
THALLIUM IN GOLD-SILVER-BEARING QUARTZ VEINS AND ASSOCIATED VOLCANIC ROCKS FROM THE COMO MINING DISTRICT, NEVADA, U.S.A. P H I L I P J. M A S S A and M O H A M M E D I K R A M U D D I N *
Geochemistry Laboratory, Department of Geology, Eastern Washington University, Cheney, WA 99004 (U.S.A.) (Accepted for publication June 18, 1985)
Abstract Massa, P.J. and Ikramuddin, M., 1986. Thallium in gold--silver-bearing quartz veins and associated volcanic rocks from the Como mining district, Nevada, U.S.A. Chem. Geol., 54 : 27--34, Ninety-two samples of Au--Ag-bearing quartz veins and associated volcanic rocks from the Como mining district, Nevada, have been analyzed for Au, Ag, T1, Rb, K, Ba and Sr, and data on K/T1, K/Rb, Ba/Tl, Tl/Sr and Rb/Sr ratios are presented. The Au and Ag concentrations along with the TI/Sr and Rb/Sr ratios increase in order from unaltered andesite to altered andesite and Au--Ag-bearing quartz veins while the K/T1, K/Rb and Ba/Tl ratios decrease. The relationship between T1, Rb and K indicates that T1 in the mineralized rocks is more enriched than Rb which, in turn, is more enriched than K. The data obtained in this study suggest that the content of T1 and the Ba/Tl and K/T1 ratios are useful exploration tools in delineating precious metal vein deposits, which are associated with volcanic rocks and which show a wide variation in concentrations of Au and Ag due to their natural inhomogeneous distribution. 1. I n t r o d u c t i o n R e c e n t studies o n the use o f T1 as a guide to mineral deposits ( I k r a m u d d i n , 1982, 1 9 8 3 a , 1 9 8 6 ; I k r a m u d d i n et al., 1 9 8 3 , 1 9 8 4 , 1 9 8 6 ) suggest t h a t h y d r o t h e r m a l l y altered r o c k s associated with various t y p e s o f ore deposits c o n t a i n high c o n c e n t r a t i o n s o f T1, low K/T1 and Ba/T1, a n d high T1/Sr ratios. In o r d e r to e x a m i n e f u r t h e r the behavior o f T1, Rb, K, Ba and Sr d u r i n g h y d r o t h e r m a l processes, a detailed s t u d y o f the A u - - A g - b e a r i n g q u a r t z veins and associated volcanic r o c k s f r o m t h e C o m o mining district, Nevada, has been u n d e r t a k e n . *Author to whom correspondence should be addressed. 0009-2541/86]$03.50
T h e m a i n p u r p o s e o f this p a p e r is t o evaluate the possibility o f utilizing the abund a n c e o f T1 and the Ba/T1 and K/T1 ratios as e x p l o r a t i o n t o o l s in search o f precious m e t a l vein deposits h o s t e d b y volcanic rocks. 2. L o c a t i o n o f t h e area, sampling a n d analytical m e t h o d s T h e C o m o mining district is l o c a t e d in t h e n o r t h e r n Pine N u t M o u n t a i n s at lat. 3 9 ° 1 0 ' N and long. 1 1 9 ° 3 0 ' W in L y o n C o u n ty, N e v a d a (Fig. 1). It covers an area o f 21 k m 2. Access to the s t u d y area is b y State H i g h w a y 50 t o D a y t o n and t h e n b y s e c o n d a r y r o a d s t o C o m o Pass. A t o t a l o f 92 r o c k samples was collected f r o m t h e C o m o mining district; 37 f r o m the
© 1986 Elsevier Science Publishers B.V.
2~
and precision of analyses were estabiishe~:~ with the help of U.S.G.S. and ..ANME international reference standards. DAr' TON
c,
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3. Geologic setting
/ ~ ( rD,4C Y.4UIN,+
,I
i N EVADA \,
\
......
39"00' i
. . . ._~_ ........
o,
~m
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8 !19°30'
I
Fig. 1. Map showing the location of the Como mining district, Nevada. quartz veins, 32 from the altered andesite adjacent to the veins, and 23 from the unaltered andesite. The chip samples from the larger quartz veins were obtained at 0.6-m intervals along traverses normal to the veins. The individual specimens used in this study averaged 3--5 kg. The quartz veins and the associated volcanic rocks were analyzed for Au, Ag, K, Rb, Ba and Sr by the methods described in Ikramuddin et al. (1983, 1986) and Massa (1984). T1 was determined by electrothermal atomic absorption spectrophotometry utilizing a recently developed precise and rapid m e t h o d (Ikramuddin, 1983b). The accuracy
The most abundant rock types cropping out in the Como mining district are andesitic flows, tufts and breccias which have been correlated with the Tertiary Kate P e ~ Formation and the underlying Atta Formation (Thompson, 1956; Thompson arid White, 1964; Whitebread, 1976; Russell, 1981). The volcanic rocks in the Como district host several Au--Ag-bearing qum~z veins that have the general strike direction of NW to NE. The quartz veins vary in width from ~ 1.5--10 m and g e n e r a l l y extend for several hundred meters. They are normally observed to contain breccia clasts of the host andesite. In the areas sampled, t,he alteration of the volcanic rocks is confined to narrow zones associated with the quartz veins. Alteration is most intense near the vein, beginning with a porous and bleached sericite-clay zone to a transitional propylitic zone and finally to fresh andesite. A detailed discussion of the geology of the Como mining district is given in Russell (1981) and Massa {1984).
TABLE I Concentrations of Au and Ag in quartz veins and associated volcanic rocks from the Como mining district, Nevada Au (ppm) average range
Ag (ppm) average range
0.002
0.001--0.006
0.127
0.192
0:005--1.81
13.5
0.210--116
5.07
0.045--51.0
72.4
1.20 --850
Unaltered andesite [ 23 ]
Altered andesite [32] Quartz veins [37 ]
[. ] = number of samples.
0.061--0.220
29 4. Results and discussion The contents of Au, Ag, T1, Rb and K in hydrothermally altered andesite are significantly higher (Table I) and Ba and Sr are lower (Table II) than those in unaltered andesite. A comparison of the abundances of lithophile elements, which have very similar geochemical characteristics, indicates that T1 in altered rocks is more enriched than Rb which, in turn, is more enriched than K (Table II; Fig. 2). For example, T1, Rb and K in altered andesite are enriched by factors of 7.2, 6.3 and 2.0, respectively, compared to unaltered andesite (Fig. 2). In the quartz veins, T1 and Rb are enriched by factors of 4.5 and 2.6, respectively, and K depleted by a factor of 1.5 (enriched by a factor of 0.67), compared to unaltered andesite (Fig. 2). The average percentage increases for these elements are: altered andesite, T1 : Rb : K = 46.5% : 40.6% : 12.9% and quartz veins, T1 : Rb : K = 57.9% : 33.5% : 8.6%; some quartz veins show extremely high percentage increases of T1. The data presented above suggest that although T1, Rb and K are concentrated and transported in hydrothermal fluids, T1 is concentrated to a greater degree than Rb and K. The relative increase of T1 in hydrothermal fluids compared to Rb and K is consistent with the geochemical characteristics of this element, which forms a more covalent and weaker bond with oxygen than Rb and K (Vlasov, 1966; De Albuquerque and Shaw, 1972; Ikramuddin et al., 1983). The geochemical data presented in Tables I and II indicate that the abundances of Au and Ag consistently increase and Ba and Sr consistently decrease in order from unaltered andesite to altered andesite and Au--Ag-bearing quartz veins. The contents of T1, Rb and K, on the other hand, do not exhibit exactly similar patterns in their distribution. These elements show a consistent increase in their concentrations from unaltered andesite to altered andesite and then decrease in Au--Ag-bearing quartz
veins. This decrease is expected due to scarcity of the minerals which could incorporate T1, Rb and K in their crystal structures. It is interesting to note, however, that the element ratios, Ba/T1, K/T1 and K/Rb, show a consistent and systematic decrease in order from unaltered andesite to altered andesite and finally Au--Agbearing quartz veins; similarly T1/Sr and Rb/Sr ratios show a consistent and systematic increase. These observations support our suggestion that the element ratios have a broader application in mineral exploration than the abundances of individual elements (Ikramuddin, 1986; Ikramuddin et al., 1986). All the element ratios mentioned above can be used to distinguish mineralized rocks (altered andesites and quartz veins) from unmineralized rocks (unaltered andesites), but Ba/T1 and K/T1 ratios appear to be more useful. The K/Rb ratios of a large number of samples in altered andesite overlap with those in unaltered andesite, whereas T1/Sr and Rb/Sr ratios show a greater scatter in their values in altered andesite compared to Ba/T1 and K/T1 ratios (Table II); high T1/Sr and Rb/Sr ratios can also result from selective leaching of Sr relative to T1 and Rb during supergene alteration (Armbrust et al., 1977; Ikramuddin et al., 1983). The contents of T1, Rb and K in Au--Ag veins and associated volcanic rocks are plotted on a triangular diagram in Fig. 3. The altered andesites and quartz veins plot away from the K apex and closer to the T1 apex. A large number of samples in the fields of altered andesite and quartz veins is seen to overlap, which is probably due to the presence of volcanic breccia clasts i:1 the veins. The majority of the background rocks (unaltered andesites) forms a tight cluster close to the K apex. These relationships further demonstrate that although T1, Rb and K are concentrated in hydrothermal fluids, T1 is comparatively more enriched than Rb which, in turn, is more enriched than K. The concentrations of T1, Rb, Ba and Sr in unaltered andesite, altered andesite
:['ABLE II A v e r a g e s a n d r a n g e s of Tl, Rb, K, Ba a n d Sr c o n t e n t s a n d K / T I ' 1 0 ~, K / R b , B a / T I . 1 0 z, T l - 1 0 4 / S r , a n d R b / S r in the ~ ~ ks f r ~ t t h e C o m o m i n i n g district, N e v a d a (all e l e m e n t s in p p m e x c e p t K, w h i c h is in w t . % )
Unaltered andesite [23] Altered andesite [32] Quartz veins [ 3 7 ]
T1
Rb
K
Ba
0.173 0,050--0.371 1.24 0.528--3.32 0.772 0.180--3.21
14.3 3.81--39.0 89.7 35.1 - ] 7 2 37.2 2.80--168
[.58 1002 ].11--2.45 786--1320 3.22 922 :1.30--5.32 82--1635 1.06 148 0,06--3.98 23-613
Sr
KITI'104
K!Rb
BaF} "i, ((}~
785 495--1047 281 29--685 65.1 16--191
13.1 3.00 --32.4 2.83 0.885--5.03 2,35 0.444--4.33
1361 340--292] 414 246--1008 290 194--343
854 24.7 - 1 7 0 ~{,45 h05--20,3 397 ],2]--15.5
[ • ] = n u m b e r of samples.
20-
32.9
15 10-
5. AI t e Fed Andes~tes 5. 10-
15, 20,
4.9 15 1
O,u e r t z Veins
51015" 2C TI
Rb
K
80
Sr
K
TI×IO q
K Rb
BO ~ x T o "z
TI x 10 q Sr
Rb SF
Fig. 2. Enrichment and depletion factors in the altered andesites and quartz veins of the C o m o mining district. Nevada. Enrichment factors are calculated by dividing the average values of elements and element ratios in the altered andesites and quartz veins by average values of elements and element ratios in the unaltered andesites. Depletion factors represent the average values of elements and element ratios in unaltered andesites divided by the average values of elements and element ratios in the altered andesites and quartz veins•
31
TI'104/Sr
Rb/Sr
2.39 0.617--10.4 69.1 10.2--406 96.1 23.1--386
0.019 0.005--0.064 0.625 0.064--3.68 0.633 0.119--1.63
and quartz veins are also plotted on a series of triangular diagrams (T1--Rb--Sr, T1--Ba-Sr and T1--Ba--Rb) in Fig. 3. It is interesting to note that in all these triangular diagrams, altered andesites and quartz veins plot near the T1 apices, whereas unaltered andesites fall near Ba and Sr apices or near the Ba--Sr boundary. These ternary relationships testify to the enrichment of T1 and depletion of Ba and Sr in hydrothermal
TlxlO000
TI x I(:X:)O
RbxlO0
K
Rb
Sr TI x I 0 0 0
TI x I000
©
/--,Ba
\ Rbx I0
Ba
Sr
Fig. 3. T e r n a r y r e l a t i o n s h i p b e t w e e n T1 a n d o t h e r e l e m e n t s (A = T 1 - - R b - - K , B = T 1 - - R b - - S r , C = T I - - B a - - R b , a n d D = T 1 - - B a - - S r ) in t h e r o c k s f r o m t h e C o m o m i n i n g d i s t r i c t , N e v a d a (A = u n a l t e r e d a n d e s i t e , • = a l t e r e d a n d e s i t e , * = q u a r t z veins).
fluids associated with mineralization at the Como mining district. The data presented in the preceding paragraphs indicate that the mineralized rocks are enriched in T1 and show a depletion in K/T1 and Ba/T1 ratios. In all the triangular diagrams given in Fig. 3, the samples plotting near the T1 apices generally contain high Au and Ag co n cen tr at i ons and those plotting near K, Sr and Ba apices have lowest Au and Ag contents. A plot of Ba/TI ratios vs. Au concentrations across a quartz vein and alteration zones in Fig. 4 also show a close association o f low Ba/T1 ratios with high Au values and vice versa. The observations made above suggest that Au and Ag are associated with T1 in h y d r o t h e r m a l fluids and t h at the high cont ent s of T1 and low values o f Ba/T1 and K/T1 ratios in the rocks may be a good indication of the presence o f Au an d /o r Ag mineralization. Samples containing the lowest concent r a t i ons of Au and Ag are always seen to have lowest contents o f T1 and highest Ba/T1 and K/T1 ratios (Tables I and II; Fig. 4). On the other hand, samples containing the highest concentrations o f Au and Ag are not always seen to have the highest c o n c e n t r a t i o n of T1 and lowest Ba/T1 and K/TI ratios. Au and Ag generally show a wide scatter in their contents in mineralized rocks, whereas T1 concentrations and Ba/T1 and K/T1 ratios exhibit least scatter. The ranges of values given in Table I indicate that the c ont e nt s o f Au and Ag vary by factors of 362 and 552 in altered andesite and by factors of 1133 and 708 in quartz veins, respectively. In contrast to this large variation in the concentrations of Au and Ag, the cont ent s o f T1 and the values of Ba/T1 and K/T1 ratios show a much less variation. The variation factors for T1, Ba/TI and K/T1 are 6.29, 5.68 and 19.3, respectively, in altered andesite and 17.8, 9.75 and 12.8, respectively, in quartz veins. The large variation in the c ont e nt s of Au and Ag can be a t t r i but ed to the natural inhomogeneous distribution of these elements
and the use of improper sampie :,..diec~o}and sample preparation techniqu~,~,, These factors are probably also respo};~blc foi the absence of significant positive c-~-relation between Au and T1 and significant ~egative' correlations between Au and Ba~TI anti between Au and K/T1. The validity of the above conclusions can be supported with the help of evidences obtained in th~s study, which are listed below: (1) Analyses of some minerahzed samples show very similar T1 contents and Ba, I1 and K/T1 ratios but variable concentrations of Au and Ag. (2) A few altered andesites having high ~tl contents and low Ba/TI and K TI ratios contain Au and Ag concentrations smlilar ~ those in the fresh unaltered andesite~ (3) Some samples of quartz veins collected within a distance of 0.6 m or less show concentrations of Au and Ag varying by a factor of 100 or more. but Ba/T1 and K/T1 ratios remain almost identical. The large variations in the concent rat m ns of precious metals can be minimized to a certain e x t e n t and representative values obtained if larger samples (> 10 kg) are collected and proper sample preparation techniques are used (jaw, cone and roller crushing of rocks_ followed by splitting and pulverizing 5o 200 mesh). The data presented above suggest that a potential mineralized area may be missed if geochemical exploration is carrmd o u t solely based on the concentrations of Au and Ag, particularly if care is not taken m utilizing proper sample collection and sample preparation techniques. In such a situation. the use of T1 and K/T1 or Ba/T1 ratios can prove to be rewarding. 5. Conclusions
The Au--Ag-bearing quartz veins and h y d r o t h e r m a l l y altered andesite are enriched in T1, Rb and K and :depleted in Ba and Sr compared to u n a l t e r e d andesite. In the mineralized rocks, Ba/ T t and KtT1
33
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PROPYLITIC
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ARGILLIC }ALTERATION
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QUARTZ
VEIN
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25 •
.
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'
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-
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it
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Fig. 4. Plot of the Au contents vs. Ba/TI.10 2 ratios in the quartz vein and argillic and propylitic zones of altered andesite, Como mining district, Nevada.
ratios are l o w e r a n d T1/Sr and R b / S r ratios are higher. T h e high c o n t e n t o f T1, low Ba/T1 and K/T1 ratios, a n d t h e r e l a t i o n s h i p b e t w e e n T1 and several o t h e r l i t h o p h i l e e l e m e n t s ( R b , K, Ba and St) can be used successfully t o d e l i n e a t e p o t e n t i a l vein d e p o s i t s h o s t e d b y volcanic rocks. T h e c o n t e n t o f T1 a n d the Ba/T1 a n d K/T1 ratios c a n p a r t i c u l a r l y p r o v e to be useful e x p l o r a t i o n guides in r o c k s w h i c h s h o w a large v a r i a t i o n in t h e c o n c e n t r a t i o n s o f p r e c i o u s m e t a l s due to t h e i r n a t u r a l inh o m o g e n e o u s d i s t r i b u t i o n . In c e r t a i n cases, e l e m e n t r a t i o s are m o r e useful i n d i c a t o r s of mineral deposits than the absolute abundances of elements.
Acknowledgements This research was p a r t i a l l y f u n d e d b y Meridian L a n d a n d Minerals Co., S p o k a n e , W a s h i n g t o n , a n d b y grants and c o n t r a c t s to t h e G e o c h e m i s t r y L a b o r a t o r y , E a s t e r n Washington University, C h e n e y , Washingt o n . We t h a n k C a n d y Oswald a n d Philip O w e n s f o r their assistance in p r e p a r i n g this paper. References Armbrust, G.A., Oyarzum, J. and Arias, J., 1977. Rubidium as a guide to ore in Chilean porphyry copper deposits. Econ. Geol., 72: 1086--1100.
De Albuquerque, C.A.R. and Shaw, D.M., 1972. Thallium, In: K.H. Wedepohl (Editor), Handbook of Geochemistry II. Springer, Berlin, Sect. 8J-B-0. Ikramuddin, M., 1982. The relation between Tl, Rb, and K in mineralized and non-mineralized rocks. Geol. Soc. Am., Abstr. Prog., I4: 520. Ikramuddin, M., 1983a. The use of Ba/T1 ratios as a guide to mineralization. Geol. Soc. Am.. Abstr. Prog., 15: 601. Ikramuddin, M., 1983b. A rapid and precise method for the determination of thallium in geological materials at the one nanogram per gram level. At. Spectrosc., 4: 101--103. Ikramuddin, M., 1986. Use of thallium ratios in mineral exploration. J Geochem. Explor. (in press). Ikramuddin, M., Asmerom, Y., Nordstrom, P.M., Kinart, K.P., Martin, W.M., Digby, S.J.M., Elder, D.D., Nijak, W.F. and Afemari~ A.A.~ 1983. Thallium: a potential guide to mineral deposits. J. Geochem. Explor., 19: 465--490. Ikramuddin, M., Besse, L. and Nordstrom, P.M., 1984. The relation between Tl, Rb, and K in the Carlin-type gold deposits. Assoc. Explor. Geochem., Abstr. Prog., p. 37. Ikramuddin, M., Besse, L. and Nordstrom, P.M., 1986. Thallium in the Carlin-type gold deposits.
Appl. Oeochem., 3 (in press} Massa, P.J., 1984. Abundance and behavior' oi' ~i. Rb, K, Ba, and Sr in gold- silver veto,,, and a~ sociated volcanic rocks from the Com(~ minin~ district, Lyon County, Nevada. M.S. The~is, Eas~ ern Washington University, Cheney, Wa:;b., .95 pp. Russell, K., 1981. Geology and ore deposits oi the Como mining district, Lyon Couaty Nevada M.S. Thesis, California S t ~ e University). Fres~.c~. Calif., 85 pp. Thompson, G.A., 1956. Geology oi; the., Virgima City quadrangle, Nevada. U.S. Geol. Stirs,, Bulk. 1042-C: 45~-77. Thompson, G.A. and White, D.E., 1 9 6 4 Regional geology of the Steamboat Springs area~ Washoe County, Nevada. U.S. Geok Surv.~ Prof. Pap. 458-A:A1 A52. Vlasov, K.A., 1966. Geochemistry and Mineralogy of Rare Elements and Genetic Types of Their Deposits, Vol. I -- Geochemistry of The Rare Elements. Israel Program for Scientific Translations, Jerusalem, 688 pp. Whitebread, D.H., 1976. Alteration and geochemistry of Tertiary volcanic rocks in parts of t h e Virginia City quadrangle, Nevada. UiS. Geok Surv., Prof. Pap. 936, 43 pp.
27
THALLIUM IN GOLD-SILVER-BEARING QUARTZ VEINS AND ASSOCIATED VOLCANIC ROCKS FROM THE COMO MINING DISTRICT, NEVADA, U.S.A. P H I L I P J. M A S S A and M O H A M M E D I K R A M U D D I N *
Geochemistry Laboratory, Department of Geology, Eastern Washington University, Cheney, WA 99004 (U.S.A.) (Accepted for publication June 18, 1985)
Abstract Massa, P.J. and Ikramuddin, M., 1986. Thallium in gold--silver-bearing quartz veins and associated volcanic rocks from the Como mining district, Nevada, U.S.A. Chem. Geol., 54 : 27--34, Ninety-two samples of Au--Ag-bearing quartz veins and associated volcanic rocks from the Como mining district, Nevada, have been analyzed for Au, Ag, T1, Rb, K, Ba and Sr, and data on K/T1, K/Rb, Ba/Tl, Tl/Sr and Rb/Sr ratios are presented. The Au and Ag concentrations along with the TI/Sr and Rb/Sr ratios increase in order from unaltered andesite to altered andesite and Au--Ag-bearing quartz veins while the K/T1, K/Rb and Ba/Tl ratios decrease. The relationship between T1, Rb and K indicates that T1 in the mineralized rocks is more enriched than Rb which, in turn, is more enriched than K. The data obtained in this study suggest that the content of T1 and the Ba/Tl and K/T1 ratios are useful exploration tools in delineating precious metal vein deposits, which are associated with volcanic rocks and which show a wide variation in concentrations of Au and Ag due to their natural inhomogeneous distribution. 1. I n t r o d u c t i o n R e c e n t studies o n the use o f T1 as a guide to mineral deposits ( I k r a m u d d i n , 1982, 1 9 8 3 a , 1 9 8 6 ; I k r a m u d d i n et al., 1 9 8 3 , 1 9 8 4 , 1 9 8 6 ) suggest t h a t h y d r o t h e r m a l l y altered r o c k s associated with various t y p e s o f ore deposits c o n t a i n high c o n c e n t r a t i o n s o f T1, low K/T1 and Ba/T1, a n d high T1/Sr ratios. In o r d e r to e x a m i n e f u r t h e r the behavior o f T1, Rb, K, Ba and Sr d u r i n g h y d r o t h e r m a l processes, a detailed s t u d y o f the A u - - A g - b e a r i n g q u a r t z veins and associated volcanic r o c k s f r o m t h e C o m o mining district, Nevada, has been u n d e r t a k e n . *Author to whom correspondence should be addressed. 0009-2541/86]$03.50
T h e m a i n p u r p o s e o f this p a p e r is t o evaluate the possibility o f utilizing the abund a n c e o f T1 and the Ba/T1 and K/T1 ratios as e x p l o r a t i o n t o o l s in search o f precious m e t a l vein deposits h o s t e d b y volcanic rocks. 2. L o c a t i o n o f t h e area, sampling a n d analytical m e t h o d s T h e C o m o mining district is l o c a t e d in t h e n o r t h e r n Pine N u t M o u n t a i n s at lat. 3 9 ° 1 0 ' N and long. 1 1 9 ° 3 0 ' W in L y o n C o u n ty, N e v a d a (Fig. 1). It covers an area o f 21 k m 2. Access to the s t u d y area is b y State H i g h w a y 50 t o D a y t o n and t h e n b y s e c o n d a r y r o a d s t o C o m o Pass. A t o t a l o f 92 r o c k samples was collected f r o m t h e C o m o mining district; 37 f r o m the
© 1986 Elsevier Science Publishers B.V.
2~
and precision of analyses were estabiishe~:~ with the help of U.S.G.S. and ..ANME international reference standards. DAr' TON
c,
cc i C~TT'
I
<(:> L
~.
3. Geologic setting
/ ~ ( rD,4C Y.4UIN,+
,I
i N EVADA \,
\
......
39"00' i
. . . ._~_ ........
o,
~m
Xg, _ _ _ 4!
8 !19°30'
I
Fig. 1. Map showing the location of the Como mining district, Nevada. quartz veins, 32 from the altered andesite adjacent to the veins, and 23 from the unaltered andesite. The chip samples from the larger quartz veins were obtained at 0.6-m intervals along traverses normal to the veins. The individual specimens used in this study averaged 3--5 kg. The quartz veins and the associated volcanic rocks were analyzed for Au, Ag, K, Rb, Ba and Sr by the methods described in Ikramuddin et al. (1983, 1986) and Massa (1984). T1 was determined by electrothermal atomic absorption spectrophotometry utilizing a recently developed precise and rapid m e t h o d (Ikramuddin, 1983b). The accuracy
The most abundant rock types cropping out in the Como mining district are andesitic flows, tufts and breccias which have been correlated with the Tertiary Kate P e ~ Formation and the underlying Atta Formation (Thompson, 1956; Thompson arid White, 1964; Whitebread, 1976; Russell, 1981). The volcanic rocks in the Como district host several Au--Ag-bearing qum~z veins that have the general strike direction of NW to NE. The quartz veins vary in width from ~ 1.5--10 m and g e n e r a l l y extend for several hundred meters. They are normally observed to contain breccia clasts of the host andesite. In the areas sampled, t,he alteration of the volcanic rocks is confined to narrow zones associated with the quartz veins. Alteration is most intense near the vein, beginning with a porous and bleached sericite-clay zone to a transitional propylitic zone and finally to fresh andesite. A detailed discussion of the geology of the Como mining district is given in Russell (1981) and Massa {1984).
TABLE I Concentrations of Au and Ag in quartz veins and associated volcanic rocks from the Como mining district, Nevada Au (ppm) average range
Ag (ppm) average range
0.002
0.001--0.006
0.127
0.192
0:005--1.81
13.5
0.210--116
5.07
0.045--51.0
72.4
1.20 --850
Unaltered andesite [ 23 ]
Altered andesite [32] Quartz veins [37 ]
[. ] = number of samples.
0.061--0.220
29 4. Results and discussion The contents of Au, Ag, T1, Rb and K in hydrothermally altered andesite are significantly higher (Table I) and Ba and Sr are lower (Table II) than those in unaltered andesite. A comparison of the abundances of lithophile elements, which have very similar geochemical characteristics, indicates that T1 in altered rocks is more enriched than Rb which, in turn, is more enriched than K (Table II; Fig. 2). For example, T1, Rb and K in altered andesite are enriched by factors of 7.2, 6.3 and 2.0, respectively, compared to unaltered andesite (Fig. 2). In the quartz veins, T1 and Rb are enriched by factors of 4.5 and 2.6, respectively, and K depleted by a factor of 1.5 (enriched by a factor of 0.67), compared to unaltered andesite (Fig. 2). The average percentage increases for these elements are: altered andesite, T1 : Rb : K = 46.5% : 40.6% : 12.9% and quartz veins, T1 : Rb : K = 57.9% : 33.5% : 8.6%; some quartz veins show extremely high percentage increases of T1. The data presented above suggest that although T1, Rb and K are concentrated and transported in hydrothermal fluids, T1 is concentrated to a greater degree than Rb and K. The relative increase of T1 in hydrothermal fluids compared to Rb and K is consistent with the geochemical characteristics of this element, which forms a more covalent and weaker bond with oxygen than Rb and K (Vlasov, 1966; De Albuquerque and Shaw, 1972; Ikramuddin et al., 1983). The geochemical data presented in Tables I and II indicate that the abundances of Au and Ag consistently increase and Ba and Sr consistently decrease in order from unaltered andesite to altered andesite and Au--Ag-bearing quartz veins. The contents of T1, Rb and K, on the other hand, do not exhibit exactly similar patterns in their distribution. These elements show a consistent increase in their concentrations from unaltered andesite to altered andesite and then decrease in Au--Ag-bearing quartz
veins. This decrease is expected due to scarcity of the minerals which could incorporate T1, Rb and K in their crystal structures. It is interesting to note, however, that the element ratios, Ba/T1, K/T1 and K/Rb, show a consistent and systematic decrease in order from unaltered andesite to altered andesite and finally Au--Agbearing quartz veins; similarly T1/Sr and Rb/Sr ratios show a consistent and systematic increase. These observations support our suggestion that the element ratios have a broader application in mineral exploration than the abundances of individual elements (Ikramuddin, 1986; Ikramuddin et al., 1986). All the element ratios mentioned above can be used to distinguish mineralized rocks (altered andesites and quartz veins) from unmineralized rocks (unaltered andesites), but Ba/T1 and K/T1 ratios appear to be more useful. The K/Rb ratios of a large number of samples in altered andesite overlap with those in unaltered andesite, whereas T1/Sr and Rb/Sr ratios show a greater scatter in their values in altered andesite compared to Ba/T1 and K/T1 ratios (Table II); high T1/Sr and Rb/Sr ratios can also result from selective leaching of Sr relative to T1 and Rb during supergene alteration (Armbrust et al., 1977; Ikramuddin et al., 1983). The contents of T1, Rb and K in Au--Ag veins and associated volcanic rocks are plotted on a triangular diagram in Fig. 3. The altered andesites and quartz veins plot away from the K apex and closer to the T1 apex. A large number of samples in the fields of altered andesite and quartz veins is seen to overlap, which is probably due to the presence of volcanic breccia clasts i:1 the veins. The majority of the background rocks (unaltered andesites) forms a tight cluster close to the K apex. These relationships further demonstrate that although T1, Rb and K are concentrated in hydrothermal fluids, T1 is comparatively more enriched than Rb which, in turn, is more enriched than K. The concentrations of T1, Rb, Ba and Sr in unaltered andesite, altered andesite
:['ABLE II A v e r a g e s a n d r a n g e s of Tl, Rb, K, Ba a n d Sr c o n t e n t s a n d K / T I ' 1 0 ~, K / R b , B a / T I . 1 0 z, T l - 1 0 4 / S r , a n d R b / S r in the ~ ~ ks f r ~ t t h e C o m o m i n i n g district, N e v a d a (all e l e m e n t s in p p m e x c e p t K, w h i c h is in w t . % )
Unaltered andesite [23] Altered andesite [32] Quartz veins [ 3 7 ]
T1
Rb
K
Ba
0.173 0,050--0.371 1.24 0.528--3.32 0.772 0.180--3.21
14.3 3.81--39.0 89.7 35.1 - ] 7 2 37.2 2.80--168
[.58 1002 ].11--2.45 786--1320 3.22 922 :1.30--5.32 82--1635 1.06 148 0,06--3.98 23-613
Sr
KITI'104
K!Rb
BaF} "i, ((}~
785 495--1047 281 29--685 65.1 16--191
13.1 3.00 --32.4 2.83 0.885--5.03 2,35 0.444--4.33
1361 340--292] 414 246--1008 290 194--343
854 24.7 - 1 7 0 ~{,45 h05--20,3 397 ],2]--15.5
[ • ] = n u m b e r of samples.
20-
32.9
15 10-
5. AI t e Fed Andes~tes 5. 10-
15, 20,
4.9 15 1
O,u e r t z Veins
51015" 2C TI
Rb
K
80
Sr
K
TI×IO q
K Rb
BO ~ x T o "z
TI x 10 q Sr
Rb SF
Fig. 2. Enrichment and depletion factors in the altered andesites and quartz veins of the C o m o mining district. Nevada. Enrichment factors are calculated by dividing the average values of elements and element ratios in the altered andesites and quartz veins by average values of elements and element ratios in the unaltered andesites. Depletion factors represent the average values of elements and element ratios in unaltered andesites divided by the average values of elements and element ratios in the altered andesites and quartz veins•
31
TI'104/Sr
Rb/Sr
2.39 0.617--10.4 69.1 10.2--406 96.1 23.1--386
0.019 0.005--0.064 0.625 0.064--3.68 0.633 0.119--1.63
and quartz veins are also plotted on a series of triangular diagrams (T1--Rb--Sr, T1--Ba-Sr and T1--Ba--Rb) in Fig. 3. It is interesting to note that in all these triangular diagrams, altered andesites and quartz veins plot near the T1 apices, whereas unaltered andesites fall near Ba and Sr apices or near the Ba--Sr boundary. These ternary relationships testify to the enrichment of T1 and depletion of Ba and Sr in hydrothermal
TlxlO000
TI x I(:X:)O
RbxlO0
K
Rb
Sr TI x I 0 0 0
TI x I000
©
/--,Ba
\ Rbx I0
Ba
Sr
Fig. 3. T e r n a r y r e l a t i o n s h i p b e t w e e n T1 a n d o t h e r e l e m e n t s (A = T 1 - - R b - - K , B = T 1 - - R b - - S r , C = T I - - B a - - R b , a n d D = T 1 - - B a - - S r ) in t h e r o c k s f r o m t h e C o m o m i n i n g d i s t r i c t , N e v a d a (A = u n a l t e r e d a n d e s i t e , • = a l t e r e d a n d e s i t e , * = q u a r t z veins).
fluids associated with mineralization at the Como mining district. The data presented in the preceding paragraphs indicate that the mineralized rocks are enriched in T1 and show a depletion in K/T1 and Ba/T1 ratios. In all the triangular diagrams given in Fig. 3, the samples plotting near the T1 apices generally contain high Au and Ag co n cen tr at i ons and those plotting near K, Sr and Ba apices have lowest Au and Ag contents. A plot of Ba/TI ratios vs. Au concentrations across a quartz vein and alteration zones in Fig. 4 also show a close association o f low Ba/T1 ratios with high Au values and vice versa. The observations made above suggest that Au and Ag are associated with T1 in h y d r o t h e r m a l fluids and t h at the high cont ent s of T1 and low values o f Ba/T1 and K/T1 ratios in the rocks may be a good indication of the presence o f Au an d /o r Ag mineralization. Samples containing the lowest concent r a t i ons of Au and Ag are always seen to have lowest contents o f T1 and highest Ba/T1 and K/T1 ratios (Tables I and II; Fig. 4). On the other hand, samples containing the highest concentrations o f Au and Ag are not always seen to have the highest c o n c e n t r a t i o n of T1 and lowest Ba/T1 and K/TI ratios. Au and Ag generally show a wide scatter in their contents in mineralized rocks, whereas T1 concentrations and Ba/T1 and K/T1 ratios exhibit least scatter. The ranges of values given in Table I indicate that the c ont e nt s o f Au and Ag vary by factors of 362 and 552 in altered andesite and by factors of 1133 and 708 in quartz veins, respectively. In contrast to this large variation in the concentrations of Au and Ag, the cont ent s o f T1 and the values of Ba/T1 and K/T1 ratios show a much less variation. The variation factors for T1, Ba/TI and K/T1 are 6.29, 5.68 and 19.3, respectively, in altered andesite and 17.8, 9.75 and 12.8, respectively, in quartz veins. The large variation in the c ont e nt s of Au and Ag can be a t t r i but ed to the natural inhomogeneous distribution of these elements
and the use of improper sampie :,..diec~o}and sample preparation techniqu~,~,, These factors are probably also respo};~blc foi the absence of significant positive c-~-relation between Au and T1 and significant ~egative' correlations between Au and Ba~TI anti between Au and K/T1. The validity of the above conclusions can be supported with the help of evidences obtained in th~s study, which are listed below: (1) Analyses of some minerahzed samples show very similar T1 contents and Ba, I1 and K/T1 ratios but variable concentrations of Au and Ag. (2) A few altered andesites having high ~tl contents and low Ba/TI and K TI ratios contain Au and Ag concentrations smlilar ~ those in the fresh unaltered andesite~ (3) Some samples of quartz veins collected within a distance of 0.6 m or less show concentrations of Au and Ag varying by a factor of 100 or more. but Ba/T1 and K/T1 ratios remain almost identical. The large variations in the concent rat m ns of precious metals can be minimized to a certain e x t e n t and representative values obtained if larger samples (> 10 kg) are collected and proper sample preparation techniques are used (jaw, cone and roller crushing of rocks_ followed by splitting and pulverizing 5o 200 mesh). The data presented above suggest that a potential mineralized area may be missed if geochemical exploration is carrmd o u t solely based on the concentrations of Au and Ag, particularly if care is not taken m utilizing proper sample collection and sample preparation techniques. In such a situation. the use of T1 and K/T1 or Ba/T1 ratios can prove to be rewarding. 5. Conclusions
The Au--Ag-bearing quartz veins and h y d r o t h e r m a l l y altered andesite are enriched in T1, Rb and K and :depleted in Ba and Sr compared to u n a l t e r e d andesite. In the mineralized rocks, Ba/ T t and KtT1
33
}
PROPYLITIC
ALTERATION
ARGILLIC }ALTERATION
I
QUARTZ
VEIN
I 700
25 •
.
/
/\
600
\ I
20
.. . . , :
..
p
: "".
BQ T[ x1-O 2 15
."
h i
..
.
..
.......
•
;
...,
]
-
n
;
i ;', ..;. , , ., i / ' /. " : ;' -; ,, ..;
10
! t
~
I; i
; #i ...e .....
~rn
._..
!
.
t t
400
,"
I
',
'
, • ,
f
Au
ppb
/
-
,/
300
200 .
: "t" I
:::
", ',
! ".
." t
:
I
/ •
,
500
t
it
I~l#
•
:i_:
"
,. .e. ~'"'"'
..e. ' " " l " " - . .
e. . . . . . . . . I . " ' "
100 "
. . .e . . . . .". . . .
'*
i I
6m
~m
Om
Fig. 4. Plot of the Au contents vs. Ba/TI.10 2 ratios in the quartz vein and argillic and propylitic zones of altered andesite, Como mining district, Nevada.
ratios are l o w e r a n d T1/Sr and R b / S r ratios are higher. T h e high c o n t e n t o f T1, low Ba/T1 and K/T1 ratios, a n d t h e r e l a t i o n s h i p b e t w e e n T1 and several o t h e r l i t h o p h i l e e l e m e n t s ( R b , K, Ba and St) can be used successfully t o d e l i n e a t e p o t e n t i a l vein d e p o s i t s h o s t e d b y volcanic rocks. T h e c o n t e n t o f T1 a n d the Ba/T1 a n d K/T1 ratios c a n p a r t i c u l a r l y p r o v e to be useful e x p l o r a t i o n guides in r o c k s w h i c h s h o w a large v a r i a t i o n in t h e c o n c e n t r a t i o n s o f p r e c i o u s m e t a l s due to t h e i r n a t u r a l inh o m o g e n e o u s d i s t r i b u t i o n . In c e r t a i n cases, e l e m e n t r a t i o s are m o r e useful i n d i c a t o r s of mineral deposits than the absolute abundances of elements.
Acknowledgements This research was p a r t i a l l y f u n d e d b y Meridian L a n d a n d Minerals Co., S p o k a n e , W a s h i n g t o n , a n d b y grants and c o n t r a c t s to t h e G e o c h e m i s t r y L a b o r a t o r y , E a s t e r n Washington University, C h e n e y , Washingt o n . We t h a n k C a n d y Oswald a n d Philip O w e n s f o r their assistance in p r e p a r i n g this paper. References Armbrust, G.A., Oyarzum, J. and Arias, J., 1977. Rubidium as a guide to ore in Chilean porphyry copper deposits. Econ. Geol., 72: 1086--1100.
De Albuquerque, C.A.R. and Shaw, D.M., 1972. Thallium, In: K.H. Wedepohl (Editor), Handbook of Geochemistry II. Springer, Berlin, Sect. 8J-B-0. Ikramuddin, M., 1982. The relation between Tl, Rb, and K in mineralized and non-mineralized rocks. Geol. Soc. Am., Abstr. Prog., I4: 520. Ikramuddin, M., 1983a. The use of Ba/T1 ratios as a guide to mineralization. Geol. Soc. Am.. Abstr. Prog., 15: 601. Ikramuddin, M., 1983b. A rapid and precise method for the determination of thallium in geological materials at the one nanogram per gram level. At. Spectrosc., 4: 101--103. Ikramuddin, M., 1986. Use of thallium ratios in mineral exploration. J Geochem. Explor. (in press). Ikramuddin, M., Asmerom, Y., Nordstrom, P.M., Kinart, K.P., Martin, W.M., Digby, S.J.M., Elder, D.D., Nijak, W.F. and Afemari~ A.A.~ 1983. Thallium: a potential guide to mineral deposits. J. Geochem. Explor., 19: 465--490. Ikramuddin, M., Besse, L. and Nordstrom, P.M., 1984. The relation between Tl, Rb, and K in the Carlin-type gold deposits. Assoc. Explor. Geochem., Abstr. Prog., p. 37. Ikramuddin, M., Besse, L. and Nordstrom, P.M., 1986. Thallium in the Carlin-type gold deposits.
Appl. Oeochem., 3 (in press} Massa, P.J., 1984. Abundance and behavior' oi' ~i. Rb, K, Ba, and Sr in gold- silver veto,,, and a~ sociated volcanic rocks from the Com(~ minin~ district, Lyon County, Nevada. M.S. The~is, Eas~ ern Washington University, Cheney, Wa:;b., .95 pp. Russell, K., 1981. Geology and ore deposits oi the Como mining district, Lyon Couaty Nevada M.S. Thesis, California S t ~ e University). Fres~.c~. Calif., 85 pp. Thompson, G.A., 1956. Geology oi; the., Virgima City quadrangle, Nevada. U.S. Geol. Stirs,, Bulk. 1042-C: 45~-77. Thompson, G.A. and White, D.E., 1 9 6 4 Regional geology of the Steamboat Springs area~ Washoe County, Nevada. U.S. Geok Surv.~ Prof. Pap. 458-A:A1 A52. Vlasov, K.A., 1966. Geochemistry and Mineralogy of Rare Elements and Genetic Types of Their Deposits, Vol. I -- Geochemistry of The Rare Elements. Israel Program for Scientific Translations, Jerusalem, 688 pp. Whitebread, D.H., 1976. Alteration and geochemistry of Tertiary volcanic rocks in parts of t h e Virginia City quadrangle, Nevada. UiS. Geok Surv., Prof. Pap. 936, 43 pp.