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Geochemistry of placer gold, Koyukuk-Chandalar mining district, Alaska
Journal of Geochemical Exploration, 31 (1989) 97-115 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
97
Geochemistry of placer gold, Koyukuk-Chandalar mining district, Alaska ELWIN L. MOSIER, JOHN B. CATHRALL, JOHN C. ANTWEILER and RICHARD B. TRIPP
U.S. Geological Survey, MS 973, Box 25046, Denver Federal Center, Denver, CO 80225-0046, U.S.A. (Received May 10, 1988; revised and accepted September 8, 1988)
ABSTRACT Mosier, E.L., Cathrall, J.B., Antweiler, J.C. and Tripp, R.B., 1989. Geochemistry of placer gold, Koyukuk-Chandalar mining district, Alaska. J. Geochem. Explor., 31: 97-115. The Koyukuk-Chandalar mining district of the Brooks Range mineral belt in north-central Alaska contains numerous placer gold deposits but few known lode gold sources. Gold grains, collected from 46 placer localities and 6 lode gold sites in the district, were analyzed for Ag and 37 trace elements utilizing direct current-arc optical emission spectroscopy. When possible, several measurements were made on each sample and averaged. Gold content was calculated by the summation of the 38 elements determined and subtracting from 100. The objectives of our study were to characterize the deposits by defining the type and number of distinct geochemical characteristics for the Au, to determine relationships of Au in placer deposits to possible lode sources (placer and lode), to identify possible primary sources of placer gold, and to study processes of placer formation. Interpretation of results emphasize that the Au grains are almost invariably ternary (Au-Ag-Cu) alloys. The average Cu content is 0.040% and the average Ag content and fineness [ ( A u / A u + A g ) × 1,000] are 10.5% and 893 parts per thousand, respectively, for the 46 placer localities. Six geochemically distinct types of placer gold can be identified in the Koyukuk-Chandalar mining district based on Ag and Cu values. One type with an average Ag content of 21.2%, an average Cu content of 0.007%, and 786 average fineness is found only in the eastern part of the district. Placer gold grains that have an average Ag content of 6.0%, an average Cu content of 0.276%, and 940 average fineness were found in the western part of the district. Four intermediate types generally occur in order across the district. Variations in the chemistry of the placer gold can be related to variable depositional environments at the primary gold sources. Placer gold geochemistry is important in determining the origin and depositional environment of the primary Au sources and could add to the knowledge of the thermal history of the southcentral Brooks Range.
INTRODUCTION The composition of native Au and its relation to gold ore genesis have rec e i v e d c o n s i d e r a b l e a t t e n t i o n . E x c e l l e n t s u m m a r i e s o f w o r k d o n e p r i o r t o 1976
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© 1989 Elsevier Science Publishers B.V.
98 were presented in Boyle (1979). The application of Au compositional data to mineral exploration and ore genesis studies using emission spectrographic analyses was described in a series of reports (Antweiler and Campbell, 1977, 1982a,b; Antweiler et al., 1977, 1980; Love et al., 1979). Because native Au is always alloyed with more or less Ag, most studies have emphasized relations between the binary system of Au and Ag, e.g., Au/Ag ratios and fineness. The important ternary system of Au, Ag, and Cu has been largely neglected in many previous studies because the amount of Cu relative to Au and Ag is small. These three elements have very similar chemical properties. Gold, Ag, and Cu are members of Group IB of the periodic system, most commonly exhibit monovalency ( + 1 ), and, as native metals, are crystallographically alike. Gold and Ag have an atomic radius of 1.44 A compared to Cu at 1.28 ~,, and as a result, most samples of native Au contain less Cu than Ag. In the laboratory, Au and Ag are continuously miscible in all proportions and form solid solutions; however, in nature the series appears to be discontinuous. No native AuAg alloy has been reported with a fineness less than 400. Copper is miscible in all proportions in the liquid state with either Au or Ag, and forms a solid solution with Au; but is has a limited range of miscibility with Ag in the solid state. Results of emission spectrographic analyses of Au from six lodes and of placer gold from several localities in the Koyukuk-Chandalar mining district, Brooks Range, Alaska, with emphasis on the Au-Ag-Cu ternary system are discussed in this report. METHOD OF STUDY Placer gold samples were collected from 46 localities in the Koyukuk-Chandalar mining districts (Fig. 1 ). Most of the mining claims in the districts that were being mined in 1982 and 1983 are included in the study, Miners very generously supplied the Au or Au concentrate or allowed panning in the deposit at most localities. At a few localities, Au was recovered by panning stream alluvium. Gold for analysis was obtained from concentrates by hand-picking in the laboratory using a binocular microscope. No other laboratory treatment was applied to the Au. Six lode gold sources were also sampled and analyzed. The Au grains studied ranged in size from less than 100 mesh (0.15 mm) to approximately 3.0 mm, but most were intermediate in size, 0.2-1 mm. Because individual native Au grains exhibit extreme variable composition, multiple analyses were made for each sample location. An analytical estimate is improved according to the ratio s/n, where s = standard deviation and n-- number of determinations. Thus, for example, the value of s is halved when n--4. A total of 460 analyses were made on the 46 placer locations and 28 analyses on the 6 lode golds. Emission spectrographic results and a sample location map for the placer gold, lode gold, and heavy-mineral concentrates have been previously reported
99
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Fig. 1. Map of lode gold (A-G) and placer gold (1-46) sample localities in the Koyukuk-Chandalar mining districts, Alaska.
(Mosier and Lewis, 1986). The placer and lode gold were analyzed by a direct current-arc, direct burn procedure described by Mosier (1975) and the concentrates were analyzed by the emission spectrographic procedure described by Grimes and Marranzino (1968). Elements reported include Au, Ag, Cu, Zn, Ga, Pb, As, Sb, Cd, Bi, In, Hg, Te, Ni, Co, Sn, Mo, Ge, Pt, Pd, Ba, St, Zr, V, Cr, Y, La, Sc, Nb, B, Ta, Be, W, Mn, Fe, Mg, Ca, Ti, and Si. Spectrographic results were obtained by visual comparison of spectra derived from the sample against spectra obtained from standards made from pure oxides, graphite, and 99.999% metallic Au. Pure A1203 was added to the standards and samples as a codistillation agent. Standard concentrations are geometrically spaced over any given order of magnitude of concentration as follows: 100, 50, 20, 10, and so forth. Samples whose concentrations are estimated to fall between those values are assigned values of 70, 30, 15, and so forth. Standard concentrations are based on a 5-mg gold sample weight. Because of the nature of native gold, it is often difficult to weigh exact 5-mg samples and in many cases there is less than 5 mg of Au available for analysis. Therefore, the reported concentration values are corrected to reflect a 5-mg sample weight by the following formula: reported concentration value = determined value × 5/sample weight
100
Because the sample weight often varies from the 5-mg weight designed for the method and because the data are computer generated, the results listed in the open-file report (Mosier and Lewis, 1986) often carry nonsignificant digits to the right of the significant digits. The analysts did not determine the values to the accuracy suggested by the extra numbers. The values shown in this report are average values for each sample location using two significant figures from the previously reported data. Preliminary reports and interpretation were made when the work was in progress (Antweiler and Cathrall, 1983; Antweiler et al., 1983, 1984, 1985; Cathrall and Antweiler, 1985). Table 1 shows the range of values, arithmetic mean, and standard deviation for fineness, Au, Ag, Cu, and dross obtained from six different placer and one lode gold sample. Dross is expressed as percent X and is the sum of all the elements determined other than Au and Ag. The analytical precision has been previously determined to be well within a factor of 2 (Mosier, 1975). As is typical of native Au, any one sample generally shows a compositional range that exceeds the analytical precision. Repetitive analyses of most samples show a range of values that tend to be close to the mean; however, it is not uncommon to have some values that are considerably different from the mean. Experience has shown that the combined accuracy and precision of the analytical method is sufficient to distinguish differences in the chemical composition of native Au originating from various source areas (Antweiler and Sutton, 1970; Antweiler and Campbell, 1982b ). By averaging a number of analyses for each sample location, the validity of the analytical observations is increased. REGIONAL GEOLOGY
The Koyukuk and Chandalar mining district lies within the Brooks Range mineral belt of north-central Alaska (Fig. 1 ). The district is about 160 km in length and lies almost entirely within the Wiseman and Chandalar 1 ° × 3 ° quadrangles. Gold has been mined in the district since the early 1890's. Most Au production has come from Quaternary fluviatile placer deposits, but, Aubearing quartz veins have also been mined. The geology of the study area is complex, being complicated by three or more episodes of metamorphism and multiple episodes of faulting, including some thrust faulting (Brosge and Reiser, 1964, 1971; Chipp, 1970; Dillon et al., 1980, 1986)° The study area comprises a central metamorphic belt of metaigneous, metavolcanic, and metasedimentary rocks of Precambrian and (or) early Paleozoic age. The rocks are predominantly schist (quartz mica, green, calcareous, and chlorite quartz) with some phyllite and quartzite. The northern half of the Wiseman quadrangle contains Paleozoic sedimentary and metasedimentary rocks, mostly Devonian conglomerate, shale, limestone, and dolomite. Cretaceous sedimentary rocks including sandstone, graywacke sandstone, conglomerate, shale, and siltstone lie along the southern border of the Wiseman
101 TABLE1
A statisticalsummary for six placer types and one lode gold, Koyukuk-Chandalar mining district, Alaska Min. value
Max. value
Arith. mean
St. dev.
# Ob.
Type 1 locality 3
Fineness Au% Ag% Cu% X%
682 67.3 15.6 0.0029 0.313
843 83.7 31.4 0.013 2.59
786 77.7 21.2 0.0086 1.11
51 5.15 5.04 0.0032 0.750
8 8 8 8 8
Type 2A locality 37
Fineness Au% Ag% Cu% X%
814 81.2 8.97 0.0093 0.297
910 90.3 18.5 0.038 0.990
880 87.4 12.0 0.020 0.659
31 3.05 3.06 0.0072 0.207
12 12 12 12 12
Type 2B locality 12
Fineness Au% Ag% Cu% X% Fineness Au% Ag% Cu% X%
852 84.1 6.19 0.0046 0.542 845 81.5 7.00 0.015 0.847
938 93.2 14.6 0.0089 1.34 929 91.8 15.0 0.053 4.05
910 90.3 8.94 0.0071 0.742 907 88.7 9.12 0.031 2.19
37 3.88 3.62 0.0015 0.284 23 2.98 2.18 0.012 1.12
7 7 7 7 7 14 14 14 14 14
Type 4 locality 22
Fineness Au% Ag% Cu% X%
846 82.7 2.63 0.015 1.31
973 95.4 15.0 0.070 4.87
930 90.5 6.76 0.037 2.70
34 3.74 3.35 0.024 1.06
10 10 10 10 10
Type 5 locality 31
Fineness Au% Ag% Cu% X%
883 87.6 1.75 0.058 0.606
982 97.6 11.6 1.15 1.87
945 93.7 5.41 0.282 0.880
38 3.73 3.77 0.389 0.447
7 7 7 7 7
Lode Gold locality D
Fineness Au% Ag% Cu% X%
740 72.5 19.5 0.0064 0.814
801 78.4 25.5 0.056 2.37
775 76.1 22.1 0.019 1.81
22 2.33 2.15 0.021 0.593
5 5 5 5 5
Type 3 locality 13
Au [Fineness = ~u-r ~ ~g × i000; X--sum of elements other than Au and Ag]
102
quandrangle. Cretaceous granitic plutons, chiefly quartz monzonite and monzonite with granite, granodiorite, and syenite, lie along the southern border of the Chandalar quandrangle. Rocks of quartz monzonite and granitic composition with Paleozoic U / P b and Pb/c~ ages and discordant Mesozoic potassium-argon dates (Grybeck et al., 1977; Dillon et al., 1979) are located in the north-central parts of the Chandalar quadrangle and south-central Wiseman quandrangle. The Precambrian and (or) lower Paleozoic basement rocks experienced pre-Mississippian metamorphism and all the rocks were regionally metamorphosed twice during Mesozoic time (Dillon et al., 1980; Dillon, 1982 ). Late Mesozoic tectonism caused widespread obduction thrusting, lateral faulting, and uplifting resulting in an accreted terrane (Tailleur, 1973; Grybeck and Nokleberg, 1979, pp. B21; Jones et al., 1984). Quaternary glacial, alluvial, and colluvial deposits containing placer gold unconformably overlie the polymetamorphic rocks (Hamilton, 1978, 1979).
RESULTS AND DISCUSSION
Analytical data Collection localities for samples are indicated on Fig. I and analytical results for fineness, Au, Ag, and Cu are presented in Table 2. The lettered sites are lode gold localities and the numbered sites are placer gold localities. Geographic names were taken from the 1:63,360 scale topographic maps of the Wiseman and Chandalar quadrangles. Gold values were determined by difference and fineness is expressed as parts per thousand [ (Au/Au + Ag) X 1,000]. The Ag and Cu content for all placer samples (460) are plotted in histograms (Fig. 2 ). The histograms show log-normal distribution.
Geochemistry of placer gold Silver and Cu values obtained for each site were averaged. The Ag content of the 46 placer gold localities ranged from 3.5 to 22.9 wt.% with an average value of 10.5 (:k-g). Copper contents for the 46 placer gold localities ranged from 0.006 to 0.282 wt.% with an average value of 0.040 (L'~). When normalized Cu is plotted against normalized Ag (Fig. 3), six distinct geochemical types of placer gold are identified in the Koyukuk-Chandalar district. Normalized values are obtained by dividing the site Ag or Cu value by the ~ or L~ value. Normalized values are used in place of real values to get the numbers in a range somewhere near 1, thereby simplifying the numbers in the graph. A value of 1 correlates with the average, 10.5% for Ag and 0.04% for Cu. The geochemical types of placer gold are described as follows:
103 150
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100
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.0018 .0026 .0038 .0056 .0083 .012
.018
.026
.038
.056
.083
.12
12
18
.18
.26
.38
.56
.83
1.2
COPPER VALUES
N U M B E R
150
m
100
--
60
--
O F S
A M P L E S
0,56
0.83
1.2
1.8
2.6
3.8
5.6
8.3
26
38
58
SILVER VALUES
Fig. 2. Histograms for silver and coppervalues for placer gold fromthe Koyukuk-Chandalar mining districts, Alaska, 460 samples. Type Type Type Type Type
1 - having high Ag content and low Cu content; 2 (A&B) - having average Ag content and low Cu content; 3 - having average Ag content and moderate but below average Cu content; 4 - having below average Ag content and above average Cu content; and 5 - having low Ag content and high Cu content.
104 TABLE2 Locality and average fineness and average Au, Ag, and Cu content of gold samples from the Koyukuk-Chandalar mining district, Alaska. (Fineness expressed as parts per thousand and element results expressed in wt. %; A - G, lode gold, 1-46 placer gold; N = number of analyses ) Site
Geographic name
N
Fineness
Au
Ag
Cu
A. B. C. D. E. F. G. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.
St Mary's Creek Little Squaw Mine Summit Mine Mikado Open Pit Mikado Mine Enevelo Mine I Sukakpak Mountain Middle Fork Big Squaw Creek Big Creek St. Mary's Creek Tobin Creek Bench at Tobin Creek Eightmile Creek Garnet Creek 2 Lake Creek Magnet Creek Gold Creek Linda Creek Sheep Creek Hammond River Gold Bottom Creek Bench at Swift Creek Swift Creek Vermont Creek Webster Gulch Thompson Pup Thompson Pup Faye Creek Archibald Creek Smith Creek Smith Creek 3 Union Gulch Mascot Creek Conglomerate Creek Jay Creek Birch Creek Spring Creek Crevice Creek Sawyer Creek E m m a Creek Clara Creek Myrtle Creek
1 7 4 8 1 0 7 1 7 10 9 11 5 6 6 3 6 24 14 14 1 3 20 15 5 1 14 1 64 12 22 1 22 7 7 7 2 7 5 3 6 10
887 857 844 775 775
88.0 84.5 83.4 74.9 76.5
11.2 14.1 15.4 23.5 22.2
0.020 0.013 0.016 0.013 0.009
912 769 840 794 803 781 895 856 942 925 914 922 904 907 912 886 925 923 881 844 858 842 912 943 949 907 934 944 964 946 934 946 887 914 904 882
86.4 76.1 82.5 78.6 79.4 77.0 88.6 85.1 93.7 92.3 90.0 90.6 89.6 88.7 89.8 86.9 90.7 89.4 87.1 79.4 84.0 83.5 89.0 92.3 93.4 90.0 91.0 91.9 94.9 93.7 92.3 93.7 87.9 91.0 89.0 87.4
8.4 22.9 15.7 20.4 19.5 21.6 10.3 14.3 5.8 7.5 8.5 7.6 9.5 9.1 8.7 11.2 7.0 7.4 11.8 14.7 13.9 15.6 8.6 5.6 5.0 9.3 6.4 5.4 3.5 5.3 6.5 5.4 11.1 8.6 9.5 11.8
0.009 0.006 0.021 0.009 0.006 0.007 0.015 0.036 0.063 0.006 0.012 0.013 0.015 0.031 0.061 0.018 0.028 0.022 0.030 0.029 0.020 0.009 0.052 0.058 0.052 0.009 0.065 0.050 0.068 0.049 0.269 0.282 0.023 0.061 0.033 0.014
105 TABLE 2 (continued) Site
Geographic name
N
Fineness
Au
Ag
Cu
36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.
Slate Creek Slate Creek Porcupine Creek TwelvemileCreek TwelvemileCreek TwelvemileCreek Tramway Bar SmalleyCreek DavisCreek GoldBench Mine Prospect Creek
6 12 6 25 2 14 5 5 4 3 20
943 880 887 885 894 907 884 894 863 899 864
93.6 87.4 88.1 84.2 88.2 89.8 85.3 88.9 85.6 89.1 85.3
5.7 12.0 11.2 11.0 10.4 9.2 11.2 10.5 13.6 10.0 13.4
0.042 0.020 0.038 0.023 0.026 0.031 0.026 0.030 0.037 0.028 0.013
1Insufficient Au for analysis was collected at this site. 2Two samples had high Ag content (40%) and were not calculated in the data. 3Five samples appeared to have solder contamination and were not calculated in the data.
Type I gold placers occur at the east end of the study area and Types 2B, 2A, 3, 4, and 5 generally occur in order proceeding west across the district (Fig. 5). The n o r t h - s o u t h string of Type 3 is all from one drainage, the K o y u k u k River draining from the north, so they are not indicating a n o r t h - s o u t h pattern. The average longitude for the placer types is shown in Fig. 4. There is some scatter Type 2 placer gold may be divided into two subgroups which are called Type 2A and Type 2B. Golds of Type 2B are similar in composition to Sukakpak Mountain lode gold (site G, Fig. 1 ) and are derived from creeks near Sukakpak Mountain. Normalized values for Ag and Cu and the ratio of normalized Cu to normalized Ag for the 46 localities are given in Table 3 and are presented by placer type. In this study, all calculations are made on the average of site values and not on the average of the total analytical observations. To empirically test the validity of the analytical data and groupings and for better clarification, the ratio of normalized Cu to normalized Ag is presented graphically in Fig. 4. W i t h the exception of Type 2B, which is unique, there is no overlap of the ratio values demonstrating the validity of the groupings. Type 2B ratios are within the T y p e 2 range of values. Other geochemical features of the placer gold types are given in Table 4. Disregarding Type 2B, there is a systematic increase in Au, fineness, Cu, and Au/Ag ratio values from Type 1 to Type 5. The Cu content of Type 2B fits between Types 1 and 2B. Conversely, with the exception of Type 2B, there is a systematic decrease in Ag, A u / C u ratio, and Ag/Cu ratio values from Type 1 to Type 5. The Ag content of Type 2B fits between Types 3 and 4. From these figures, Type 2B gold appears to be anomalous. The Au does not fit the pattern established by the chemistry of the other placer gold types. An interesting feature is that when a regional plot is made of the placer gold deposits by "type",
106
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i iiiiiiiii~iiiiiii!ii!~ .... :~iiiiiiii!~iiiiiiiiiiiiiiiiiiiii~i! T Y PE
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TYPE TYPE
1
Ag/Ag
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Fig. 3. Plot of normalized Cu versus normalized Ag showing six separate geochemical types of placer gold in the Koyukuk-Chandalar mining district, Alaska. which we attribute to the natural variation which occurs in native Au composition or possibly a mixture of two or more primary Au sources.
Geochemical interpretation We interpret the regional pattern of the placer gold types to be related to the depositional environment of the primary Au source sites. In a hydrothermal system, Ag migrates farther from the heat source and tends to be enriched nearer the surface (Shcherbina, 1956). Shcherbina concluded that Au enrichment (a high Au/Ag ratio) occurs mostly in higher temperature and deeperseated deposits and Ag enrichment (a low Au/Ag ratio) is characteristic of low-temperature ore deposits of intermediate or shallow depths. Fisher (1945)
107 TABLE3 Normalized Cu and Ag values and the ratio of normalized__Cu to normalized Ag for the placer gold localities. Koyukuk-Chandalar mining district, Alaska (Cu = 0.040 and Ag = 10.5 ). (The number in parentheses is the number of analytical observations for each site and the total analytical observations for each placer type. ) Site Type I 1 (1) 3 (10) 4 (9) 5 (11) Avg. (31) Type 2A 2 (7) 15 (3) 20 (14) 21 (1) 35 (10) 37 (12) 46 (20) Avg. (67) Type 2B 6 (5) 9 (3) 1O (6) 11 (24) 12 (14) 25 (1) Avg. (53) Type 3 7 (6) 13 ( 14 ) 16 (20) 17 (15) 18 (5) 19 (1)
1 Cu/Cu
2 Ag/Ag
Ratio 1/2
0.15 0.22 0.15 0.18 0.18
2.18 1.94 1.86 2.06 2.01
0.07 0.11 0.08 0.09 0.09
0.52 0.45 0.50 0.22 0.35 0.50 0.32 0.41
1.50 1.07 1.32 1.49 1.12 1.14 1.28 1.36
0.35 0.42 0.38 0.15 0.31 0.44 0.25 0.33
0.38 0.15 0,30 0.32 0.28 0.22 0.29
0.98 0.71 0.81 0.72 0.90 0.89 0.84
0.39 0.21 0.37 0.44 0.32 0.25 0.35
0.90 0.78 0.70 0.55 0.75 0.72
1.36 0.87 0.67 0.70 1.12 1.40
0.66 0.90 1.05 0.79 0.67 0.51
Site Type 3 (cont.) 32 (5) 34 (6) 38 (6) 39 (25) 40 (2) 41 (14) 42 (5) 43 (5) 44 (4) 45 (3) Avg. (136) Type 4 8 (6) 14 (1) 22 {64 ) 23 (12) 24 (22) 26 (22) 27 (7) 28 (7) 29 (7) 33 (3) 36 (6) Avg. {157) Type 5 30 (2) 31 (7) Avg. (9)
1 Cu/Cu
2 Ag/Ag
Ratio 1/2
0.58 0.82 0.95 0.58 0.65 0.78 0.58 0.75 0.92 0.70 0.73
1.06 0.90 1.07 1.05 0.99 0.88 1.07 1.00 1.30 0.95 1.02
0.55 0.91 0.89 0.55 0.66 0.89 0.54 0.75 0.71 0.74 0.74
1.58 1.52 1.30 1.45 1.30 1.62 1.25 1.70 1.22 1.52 1.05 1.41
0.55 0.83 0.82 0.53 0.48 0.61 0.51 0.33 0.50 0.82 0.54 0.59
2.87 1.83 1.59 2.74 2.71 2.66 5.45 5.15 2.44 1.85 1.94 2.57
6.72 7.05 6.89
0.62 0.51 0.57
10.84 13.82 12.33
f o u n d t h a t in o r e d e p o s i t s f o r m e d d e e p b e l o w t h e s u r f a c e , A u o f h i g h p u r i t y w a s d e p o s i t e d e v e n w h e n A g w a s a b u n d a n t . N e a r t h e s u r f a c e h o w e v e r , so l o n g as s u f f i c i e n t A g w a s a v a i l a b l e , a l o w - g r a d e a l l o y w a s f o r m e d w i t h a m i n i m u m f i n e n e s s s l i g h t l y b e l o w 500 a n d i n t e r m e d i a t e g r a d e s o f A u w e r e f o r m e d a t int e r m e d i a t e d e p t h s . B o y l e (1979, pp. 1 9 7 - 2 0 7 ) , d o c u m e n t s a n u m b e r o f s t u d i e s s h o w i n g i n c r e a s e d f i n e n e s s a n d ( o r ) i n c r e a s e d A u / A g r a t i o s w i t h d e p t h o f dep o s i t i o n . H o w e v e r , he c a u t i o n s t h a t few p r e c i s e s t u d i e s o f t h e v a r i a t i o n w i t h increasing depth of the Au/Ag ratios of single ore shoots or veins have been
108
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Fig. 4. Graphical presentation of the range of values and average value for the ratio of normalized Cu (Cu/Cu) to normalized Ag (Ag/Ag). The number above the bar represents the average longitude for the placer types. TABLE 4 Geochemical features of placer gold types Type 1
# of placer sites # of analytical observations Fineness Au% Ag% Cu%
Au/Ag Au/Cu Ag/Cu
Type 2A
Type 2B
Type 3
Type 4
4
7
6
16
11
31 786 77.9 21.2 0.007 3.67 11,100 3,030
67 866 85.5 13.2 0.017 6.55 5,320 818
53 914 90.2 8.4 0.013 10.8 7,200 667
136 890 87.2 10.8 0.029 8.07 3,000 370
157 937 92.2 6.20 0.057 14.9 1,600 110
Type 5
2 9 940 93.0 6.00 0.276 15.5 340 22
made. Therefore, with a few exceptions, Au with a high Au/Ag ratio can be expected to be found in higher temperature and deeper seated deposits and Au with a low Au/Ag ratio most commonly occurs in low-temperature ore deposits of intermediate or shallow depths. The chemical composition of placer gold is a geochemical signature of the Au shed from the outcrops of the lode gold sources. Placer gold does not necessarily correspond exactly to the composition of Au as deposited in the ore, primarily due to differential solution of surface Ag of Au particles in the alluvial environment. The removal of Ag from Au particles is a common phenomenon in placer golds (Desborough, 1970) and has long been recognized (McConnell, 1907 ). It is referred to as surface refining and takes place only to
109 ~8~__.
~o
150 °
WISEMAN
QUADRANGLE
CHANDALAR
3 .5
=4
4e 4
2
2A 14 % B
QUADRANGLE
~2B .4
1 ,1 1"1"2A
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67 o
67 o
BETTLES
QUADRANGLE 0
I
500
I
I
I
I
,2 Fig. 5. Regional plot of placer gold by "type."
a limited depth from the surface of the individual grains (Boyle, 1979, p. 336) and therefore causes only slightchanges in the overall Au/Ag ratio.Some investigators m a y argue that the rim of higher fineness found on many placer gold grains is the resultof A u accretion. In our study, sufficientAu was obtained from 15 of the placer gold locations to permit analyses of a -0.5-mm fraction and a + 0.5-mm fraction.Values for Ag content of the -0.5-mm fractionare plotted against Ag content of the + 0.5m m fraction (Fig. 6). Four of the sitesshow significantlyhigher Ag content in the larger size fraction and four of the sitesshow significantlyhigher Ag content in the smaller size fraction. T w o sites show a moderate increase in Ag content in the smaller size fraction and the remaining five sites are nearly equal in Ag content in the two size fractionsbut are biased to the smaller size fraction. In general, surface area varies inversely with its grain size. If the 0.5-mm Au grains were being altered by preferential chemical leaching of Ag from the outermost zone of Au particles,or Au in solution were accreted onto the grains, one would expect this fraction to have a lower Ag content. However, in our study such is not the case. This may indicate that the placer gold was not significantlyaltered from the lode gold source and that Ag and Cu content of the Koyukuk-Chandalar placer gold grains represent geochem-
110
I
20
Z
Q 0 < I 09 u3
I
i
I
~5
•
5
10
Silver in weigh! percenl
15
20
+ 35 MESH F R A C T I O N
Fig. 6. Silver values for -35-mesh fraction versus silver values for + 3 5 - m e s h gold.
fraction of placer
ical signatures inherited from their eroded primary sources. A study in South Africa of Au grains from the Witwatersrand palaeo-placer and Barberton Archaean vein deposits using electron microprobe to determine Ag and Hg content also shows no indication of Au alteration during fluvial transport or by post-depositional geological processes (von Gehlen, 1983). The change in Ag and Cu content of the placer gold from east to west in the district suggests there is a similar change in the composition of lode gold reflecting differing depositional environments. Heat sources accompanying metamorphism were apparently progressively deeper from east to west. The ternary system Au, Ag, and Cu further identifies this gradient. When normalized Cu is ratioed to normalized Ag (Table 3), the resulting value represents Cu enrichment or depletion with respect to Ag. The relationship of Ag and Cu with Au is shown by plotting this value against fineness (Fig. 7). The line of solid solution for the ternary system could be dependent on the physico-chemicai parameters at the time of ore deposition and, therefore, reflects the depth of deposition for the primary gold. The physico-chemical parameters that influence metal solubilities, and, as a corollary, ore deposition, are factors such
111
1
LINE OF SOLID SOLUTION 'FOR THE TERNARY SYSTEM
0
Placer gold
1"3 Chandalar lode gold •
Sukakpak lode gold
Cu/C~ Ag/Ag
0 0 0
0 o
0 TYPE 2B' o
900
8OO
FINENESS
Fig. 7. Relationship for the ternary system of Au, Ag, and Cu in the Koyukuk-Chandalar mining district, Alaska.
as temperature, pH, hydrogen and oxygen fugacity, chlorine and bisulfite activity, and hydrogen sulfide fugacity. These factors are complexly interrelated, and definitive answers are difficult to make quantitatively as well as difficult
112
to describe. In the ternary system for native Au, higher Cu values correlate with high fineness. Copper and Ag show an inverse relationship. This relationship has also been observed in a study using microprobe analyses for Ag and Cu content of placer gold from the Colorado mineral belt (Desborough et al., 1970). In that study, detectable Cu (0.1-0.25 wt.% ) was found principally in low Ag content Au grains, which the authors concluded must characterize a certain type of lode source. This also appears to be true in the Koyukuk-Chandalar district. At increasing depth, Cu appears to have a greater opportunity relative to Ag to form a solid solution with Au. In the hydrothermal models described by Reed and Spycker (1986) and by Buchanan (1981), base metals occupy an early paragenetic position. The base-metal horizon occurs deep, below the boiling level, and the precious-metal horizon occurs nearer the surface, above the boiling level. At the level of boiling, a mixed zone of precious- and base-metal mineralization occurs. Is it reasonable then that Au that is precipitated deeper in the hydrothermal system because of some change in the physical-chemical parameters would contain relatively lower Ag content and higher Cu content than Au precipitated nearer the surface? It is apparent that by and large, the Cu/fineness relations of placer gold in the Koyukuk-Chandalar districts can be predicted from the solid-solution curve. Chandalar lode golds also fit the curve (Fig. 7). The exception is Sukakpak Mountain lode gold and placer gold from nearby creeks, our Type 2B placer gold (Fig. 7). This Au is all either (1) depleted in Cu relative to the fineness or (2) depleted in Ag (yielding very high fineness) relative to Cu. We do not have a good explanation for why this Au's ternary relations differ. The lode gold from Sukakpak Mountain is found in a stibnite vein that occurs at the contact between marble and schist. The Chandalar lode gold is found in dilatant quartz veins in a graywacke-type host rock. It is probable that in Type 2B Au, Sb had an influence on the proportioning of Ag and/or Cu content in native Au during Au deposition. CONCLUSIONS
Geochemical studies of placer gold from the Koyukuk-Chandalar mining district reveal striking contrasts in the Ag and Cu content of placer gold that may have a direct relation to the depositional environments of the primary gold source sites. Based on Ag and Cu content, six different geochemical types of placer gold can be identified ranging from Type 1 which has high Ag content (21.2 wt.%) and low Cu content (0.007 wt.%) to Type 5 which has low Ag content (6.0 wt.% ) and high Cu content (0.276 wt.%). Type 2 placer gold is divided into two subgroups. From Type 1 to Type 5 placer gold, Au, Cu, and Au/Ag values increase systematically, and Ag, Au/Cu, and Ag/Cu values decrease systematically. A regional plot of the placer gold types shows Type 1 placers occurring in the eastern portion of the district with Types 2-5 generally
113
occurring in order westwardly across the district, suggesting a variation in the primary gold sources. The ternary system of Au, Ag, and Cu shows that higher Cu values correlate with high Au content and that Ag and Cu have an inverse relationship. We hypothesize that the three elements were selectively partitioned from the ore-forming hydrothermal solutions yielding ternary signatures dependent upon variable physical-chemical parameters at the time of Au deposition, and that the depth of the depositional site of the primary Au sources may be the most critical control. Based on the gradient that is inferred from the Au analyses, late Mesozoic tectonism of rocks now forming the Brooks Range may have not only caused the ore-forming hydrothermal solutions to have been emplaced in progressively lower thrust plates from east to west across the Koyukuk-Chandalar district but may have had an important influence on the chemistry of the resulting lode and placer golds in the district.
REFERENCES Antweiler, J.C. and Campbell, W.L., 1977. Application of gold compositional data to mineral exploration in the United States. J. Geoehem. Explor., 8: 17-29. Antweiler, J.C. and Campbell, W.L., 1982a. Gold in exploration geochemistry. In: A.L. Levinson (Editor), Precious Metals in the Northern Cordillera. Assoc. Explor. Geochem., Calgary, pp. 33-44. Antweiler, J.C. and Campbell, W.L., 1982b. Implications of the compositions of lode and placer gold. In: T.G. Theodore, W.N. Blair and J.T. Nash (Editors), Geology and Gold Mineralization of the Gold Basin-Lost Basin mining districts, Mohave County, Arizona. U.S. Geol. Surv., Prof. Pap., 1361: 245-264. Antweiler, J.C. and Cathrall, J.B., 1983. The relationship of gold in placer deposits of the Brooks Range, Alaska to primary gold sources [abstr ]. In: 34th Alaska Science Conference Abstracts. Arctic Div., Am. Assoc. Advancement Sci., p. 95. Antweiler, J.C. and Sutton, A.L., 1970. Spectrochemical analysis of native gold samples. U.S. Clearinghouse Fed. Sci. Tech. Inf., PB Rep. 194809, 32 pp. Antweiler, J.C., Love, J.D. and Campbell, W.L., 1977. Gold content of the Pass Peak Formation and other rocks in the Rocky Mountain Overthrust Belt, northwestern Wyoming. Wyoming Geol. Assoc. Guideb., 1977, pp. 731-749. Antweiler, J.C., Love, J.D., Mosier, E.L. and Campbell, W.L., 1980. Oligocene gold-bearing conglomerate, southeast margin of Wind River Mountains, Wyoming. In: Stratigraphy of Wyoming. Wyoming Geol. Assoc. Guideb., pp. 223-237. Antweiler, J.C., Watterson, J.R., Cathrall, J.B. and Mosier, E.L., 1983. Possible origin of placer gold in Koyukuk and Chandalar districts, Brooks Range, Alaska. U.S. Geol. Surv., Polar Research Symposium. Abstr Program, U.S. Geol. Surv. Circ., 911: 52-53. Antweiler, J.C., Cathrall, J.B. and Tripp, R.B., 1984. U.S. Geological Survey Alaskan Gold Project. In: Sixth Annual Conference on Alaskan Placer Mining. Mineral Industry Research Laboratory, Univ. of Alaska, Fairbanks, MIRL Rept. 69: 69-72. Antweiler, J.C., Tripp, R.B., Cathrall, J.B. and Mosier, E.L., 1985. Studies of gold in the Chandalar and Koyukuk Districts, Wiseman, and Bettles Quadrangles - A progress report. U.S. Geol. Surv., Circ., 945: 28-29. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geol. Surv. Can., Bull. 280, 584 pp.
114 Brosge, W.P. and Reiser, H.N., 1964. Geologic map and section of the Chandalar Quadrangle, Alaska. U.S. Geol. Surv., Misc. Geol. Invest. Map 1-375, 1 sheet, scale 1:250,000. Brosge, W.P. and Reiser, H.N., 1971. Preliminary bedrock geologic map of the Wiseman and eastern Survey Pass Quadrangles, Alaska. U.S. Geol. Surv. Open-File Rep. 71-479, scale 1:250,000. Buchanan, L.J., 1981. Precious metal deposits associated with volcanic environments in the southwest. In: W.R. Dickinson and W.D. Payne {Editors), Relations of Tectonics to Ore Deposits in the Southern Cordillera. Ariz. Geol. Soc. Digest, XIV: 237-260. Cathrall, J.B. and Antweiler, J.C., 1985. Progress Report on U.S. Geological Survey Alaskan Gold Project. In: J.A. Madonna {Editor), Proceedings of the Seventh Annual Conference on Alaskan Placer Mining. Alaska Prospectors Publishing, Fairbanks, Alaska, pp. 42-47. Chipp, E.R., 1970. Geology and geochemistry of the Chandalar area, Brooks Range, Alaska. Alaska Div. Mines Geol., Geol. Rep. 42, 39 pp. Desborough, G.A., 1970. Silver depletion indicated by microanalysis of gold from placer occurrences, Western United States. Econ. Geol., 65:304-311. Desborough, G.A., Raymond, W.H. and Tagman, P.J., 1970. Distribution of silver and copper in placer gold from the northeastern part of the Colorado mineral belt. Econ. Geol., 65: 937-944. Dillon, J.T., 1982. Source of lode- and placer-gold deposits of the Chandalar and Upper Koyukuk districts, Alaska. Alaska Div. Geol. Geophys. Surv., Open-File Rep. 158, 22 pp. Dillon, J.T., Pessel, G.H., Chen, J.H. and Vesch, N.C., 1979. Tectonic and economic significance of Late Devonian and Late Proterozoic U-Pb zircon ages from the Brooks Range, Alaska. Alaska Div. Geol. Geophys. Surv. Geol. Rep., 61: 36-41. Dillon, J.T., Pessel, G.H., Chen, J.H. and Vesch, N.C., 1980. Middle Paleozoic magmatism and orogenesis in the Brooks Range, Alaska. Geology, 8: 338-343. Dillon, J.T., Brosge, W.P. and Dutro, J.T., Jr., 1986. Generalized geologic map of the Wiseman Quadrangle, Alaska. U.S. Geol. Surv., Open-File Rep. 86-0219, scale 1:250,000. Fisher, N.H., 1945. The fineness of gold, with special reference to the Morobe goldfield, New Guinea. Econ. Geol., 40: 449-495, 537-563. Grimes, D.J. and Marranzino, A.P., 1968. Direct-current arc and alternating-current spark emission spectrographic field methods for the semiquantitative analysis of geologic materials. U.S. Geol. Surv. Circ. 591, 6 pp. Grybeck, D. and Nokleberg, W.J., 1979. Metalogeny of the Brooks Range, Alaska. In: K.M. Johnson and J.R. Williams {Editors), The United States Geological Survey in Alaska - Accomplishments during 1978. U.S. Geol. Surv., Circ., 804-B: B19-B22. Grybeck, D., Beikman, H.M., Brosge, W.P., Tailleur, I.L. and Mull, C.G., 1977. Geologic map of the Brooks Range, Alaska. U.S. Geol. Surv., Open-File Rep. 77-166B, sheet 1 of 2. Hamilton, T.D., 1978. Surficial geologic map of the Chandalar Quadrangle, Alaska. U.S. Geol. Surv., Misc. Field Studies Map MF-878-A, scale 1:250,000. Hamilton, T.D., 1979. Surficial geologic map of the Wiseman Quadrangle, Alaska. U.S. Geol. Surv., Misc. Field Studies Map MF-1122, scale 1:250,000. Jones, D.L., Silberling, N.J., Coney, P.J. and Plafker, G., 1984. Lithotectonic terrane map of Alaska (west of the 141st Meridian). In: N.J. Silberling and D.L. Jones {Editors), Part A of Lithotectonic Terrane Maps of the North American Cordillera. U.S. Geol. Surv., Open-File Rep. 84-523, scale 1:250,000. Love, J.D., Antweiler, J.C. and Mosier, E.L., 1979. A new look at the origin and volume of the Dickie Springs-Oregon Gulch placer gold at the south end of the Wind River Range. Thirtieth Annu. Field Conf., 1978, Wyoming Geol. Assoc. Guideb., pp. 379-381. McConnell, R.G., 1907. Report on gold values in the Klondike high level gravels. Geol. Surv. Can., Rep. No. 979, 34 pp. Also, Geol. Surv. Can., Mem., 284: 217-238. Mosier, E.L., 1975. Use of emission spectroscopy for the semiquantitative analysis of trace ele-
115 ments and silver in native gold. In: F.N. Ward (Editor), New and Refined Methods of Trace Analysis Useful in Geochemical Exploration. U.S. Geol. Surv. Bull., 1408: 97-105. Mosier, E.L. and Lewis, J.S., 1986. Analytical results, geochemical signatures, and sample locality map of lode gold, placer gold, and heavy-mineral concentrates from the Koyukuk-Chandalar mining district, Alaska. U.S. Geol. Surv., Open-File Rep. 86-345, 171 pp., I pl. Reed, M.H. and Spycker, N.F., 1986. Boiling, cooling, and oxidation in epithermal systems: A numerical modeling approach. In: B.R. Berger and P.M. Bethke (Editors), Geology and Geo= chemistry of Epithermal Systems. Reviews in Economic Geology, vol. 2 Soc. Econ. Geol., pp. 249-272. Shcherbina, V.V., 1956. Geochemical significance of quantitative Ag-Au ratios. Geochemistry, 3: pp. 301-311. Tailleur, I.L., 1973. Probable rift origin of Canada Basin, Arctic Ocean. In: M.G. Pitcher (Editor), Arctic Geology. Am. Assoc. Pet. Geol. Mere., 19: 526-535. von Gehlen, K., 1983. Silver and mercury in single gold grains from the Witwatersrand and Barberton, South Africa. Mineral. Deposita, 18: 529-534.
97
Geochemistry of placer gold, Koyukuk-Chandalar mining district, Alaska ELWIN L. MOSIER, JOHN B. CATHRALL, JOHN C. ANTWEILER and RICHARD B. TRIPP
U.S. Geological Survey, MS 973, Box 25046, Denver Federal Center, Denver, CO 80225-0046, U.S.A. (Received May 10, 1988; revised and accepted September 8, 1988)
ABSTRACT Mosier, E.L., Cathrall, J.B., Antweiler, J.C. and Tripp, R.B., 1989. Geochemistry of placer gold, Koyukuk-Chandalar mining district, Alaska. J. Geochem. Explor., 31: 97-115. The Koyukuk-Chandalar mining district of the Brooks Range mineral belt in north-central Alaska contains numerous placer gold deposits but few known lode gold sources. Gold grains, collected from 46 placer localities and 6 lode gold sites in the district, were analyzed for Ag and 37 trace elements utilizing direct current-arc optical emission spectroscopy. When possible, several measurements were made on each sample and averaged. Gold content was calculated by the summation of the 38 elements determined and subtracting from 100. The objectives of our study were to characterize the deposits by defining the type and number of distinct geochemical characteristics for the Au, to determine relationships of Au in placer deposits to possible lode sources (placer and lode), to identify possible primary sources of placer gold, and to study processes of placer formation. Interpretation of results emphasize that the Au grains are almost invariably ternary (Au-Ag-Cu) alloys. The average Cu content is 0.040% and the average Ag content and fineness [ ( A u / A u + A g ) × 1,000] are 10.5% and 893 parts per thousand, respectively, for the 46 placer localities. Six geochemically distinct types of placer gold can be identified in the Koyukuk-Chandalar mining district based on Ag and Cu values. One type with an average Ag content of 21.2%, an average Cu content of 0.007%, and 786 average fineness is found only in the eastern part of the district. Placer gold grains that have an average Ag content of 6.0%, an average Cu content of 0.276%, and 940 average fineness were found in the western part of the district. Four intermediate types generally occur in order across the district. Variations in the chemistry of the placer gold can be related to variable depositional environments at the primary gold sources. Placer gold geochemistry is important in determining the origin and depositional environment of the primary Au sources and could add to the knowledge of the thermal history of the southcentral Brooks Range.
INTRODUCTION The composition of native Au and its relation to gold ore genesis have rec e i v e d c o n s i d e r a b l e a t t e n t i o n . E x c e l l e n t s u m m a r i e s o f w o r k d o n e p r i o r t o 1976
0375-6742/89/$03.50
© 1989 Elsevier Science Publishers B.V.
98 were presented in Boyle (1979). The application of Au compositional data to mineral exploration and ore genesis studies using emission spectrographic analyses was described in a series of reports (Antweiler and Campbell, 1977, 1982a,b; Antweiler et al., 1977, 1980; Love et al., 1979). Because native Au is always alloyed with more or less Ag, most studies have emphasized relations between the binary system of Au and Ag, e.g., Au/Ag ratios and fineness. The important ternary system of Au, Ag, and Cu has been largely neglected in many previous studies because the amount of Cu relative to Au and Ag is small. These three elements have very similar chemical properties. Gold, Ag, and Cu are members of Group IB of the periodic system, most commonly exhibit monovalency ( + 1 ), and, as native metals, are crystallographically alike. Gold and Ag have an atomic radius of 1.44 A compared to Cu at 1.28 ~,, and as a result, most samples of native Au contain less Cu than Ag. In the laboratory, Au and Ag are continuously miscible in all proportions and form solid solutions; however, in nature the series appears to be discontinuous. No native AuAg alloy has been reported with a fineness less than 400. Copper is miscible in all proportions in the liquid state with either Au or Ag, and forms a solid solution with Au; but is has a limited range of miscibility with Ag in the solid state. Results of emission spectrographic analyses of Au from six lodes and of placer gold from several localities in the Koyukuk-Chandalar mining district, Brooks Range, Alaska, with emphasis on the Au-Ag-Cu ternary system are discussed in this report. METHOD OF STUDY Placer gold samples were collected from 46 localities in the Koyukuk-Chandalar mining districts (Fig. 1 ). Most of the mining claims in the districts that were being mined in 1982 and 1983 are included in the study, Miners very generously supplied the Au or Au concentrate or allowed panning in the deposit at most localities. At a few localities, Au was recovered by panning stream alluvium. Gold for analysis was obtained from concentrates by hand-picking in the laboratory using a binocular microscope. No other laboratory treatment was applied to the Au. Six lode gold sources were also sampled and analyzed. The Au grains studied ranged in size from less than 100 mesh (0.15 mm) to approximately 3.0 mm, but most were intermediate in size, 0.2-1 mm. Because individual native Au grains exhibit extreme variable composition, multiple analyses were made for each sample location. An analytical estimate is improved according to the ratio s/n, where s = standard deviation and n-- number of determinations. Thus, for example, the value of s is halved when n--4. A total of 460 analyses were made on the 46 placer locations and 28 analyses on the 6 lode golds. Emission spectrographic results and a sample location map for the placer gold, lode gold, and heavy-mineral concentrates have been previously reported
99
68"
150 o
WISEMAN
68 °
QUADRANGLE
CHANDALAR
QUADRANGLE
G
,6
2o
.,30 =29 -28
27. 26 22~..,~]p't~: 14. 13 "12
"31
EO.~I ,B
,,,;o
4
c
3- j
"8
-"" A
°32
o33 .38 "34 '35
440..39 ,42
o+3
670
67 o
~,5 "44 BETTLES
QUADRANGLE 0 I
I
I
I
I
500 I
• 46
Fig. 1. Map of lode gold (A-G) and placer gold (1-46) sample localities in the Koyukuk-Chandalar mining districts, Alaska.
(Mosier and Lewis, 1986). The placer and lode gold were analyzed by a direct current-arc, direct burn procedure described by Mosier (1975) and the concentrates were analyzed by the emission spectrographic procedure described by Grimes and Marranzino (1968). Elements reported include Au, Ag, Cu, Zn, Ga, Pb, As, Sb, Cd, Bi, In, Hg, Te, Ni, Co, Sn, Mo, Ge, Pt, Pd, Ba, St, Zr, V, Cr, Y, La, Sc, Nb, B, Ta, Be, W, Mn, Fe, Mg, Ca, Ti, and Si. Spectrographic results were obtained by visual comparison of spectra derived from the sample against spectra obtained from standards made from pure oxides, graphite, and 99.999% metallic Au. Pure A1203 was added to the standards and samples as a codistillation agent. Standard concentrations are geometrically spaced over any given order of magnitude of concentration as follows: 100, 50, 20, 10, and so forth. Samples whose concentrations are estimated to fall between those values are assigned values of 70, 30, 15, and so forth. Standard concentrations are based on a 5-mg gold sample weight. Because of the nature of native gold, it is often difficult to weigh exact 5-mg samples and in many cases there is less than 5 mg of Au available for analysis. Therefore, the reported concentration values are corrected to reflect a 5-mg sample weight by the following formula: reported concentration value = determined value × 5/sample weight
100
Because the sample weight often varies from the 5-mg weight designed for the method and because the data are computer generated, the results listed in the open-file report (Mosier and Lewis, 1986) often carry nonsignificant digits to the right of the significant digits. The analysts did not determine the values to the accuracy suggested by the extra numbers. The values shown in this report are average values for each sample location using two significant figures from the previously reported data. Preliminary reports and interpretation were made when the work was in progress (Antweiler and Cathrall, 1983; Antweiler et al., 1983, 1984, 1985; Cathrall and Antweiler, 1985). Table 1 shows the range of values, arithmetic mean, and standard deviation for fineness, Au, Ag, Cu, and dross obtained from six different placer and one lode gold sample. Dross is expressed as percent X and is the sum of all the elements determined other than Au and Ag. The analytical precision has been previously determined to be well within a factor of 2 (Mosier, 1975). As is typical of native Au, any one sample generally shows a compositional range that exceeds the analytical precision. Repetitive analyses of most samples show a range of values that tend to be close to the mean; however, it is not uncommon to have some values that are considerably different from the mean. Experience has shown that the combined accuracy and precision of the analytical method is sufficient to distinguish differences in the chemical composition of native Au originating from various source areas (Antweiler and Sutton, 1970; Antweiler and Campbell, 1982b ). By averaging a number of analyses for each sample location, the validity of the analytical observations is increased. REGIONAL GEOLOGY
The Koyukuk and Chandalar mining district lies within the Brooks Range mineral belt of north-central Alaska (Fig. 1 ). The district is about 160 km in length and lies almost entirely within the Wiseman and Chandalar 1 ° × 3 ° quadrangles. Gold has been mined in the district since the early 1890's. Most Au production has come from Quaternary fluviatile placer deposits, but, Aubearing quartz veins have also been mined. The geology of the study area is complex, being complicated by three or more episodes of metamorphism and multiple episodes of faulting, including some thrust faulting (Brosge and Reiser, 1964, 1971; Chipp, 1970; Dillon et al., 1980, 1986)° The study area comprises a central metamorphic belt of metaigneous, metavolcanic, and metasedimentary rocks of Precambrian and (or) early Paleozoic age. The rocks are predominantly schist (quartz mica, green, calcareous, and chlorite quartz) with some phyllite and quartzite. The northern half of the Wiseman quadrangle contains Paleozoic sedimentary and metasedimentary rocks, mostly Devonian conglomerate, shale, limestone, and dolomite. Cretaceous sedimentary rocks including sandstone, graywacke sandstone, conglomerate, shale, and siltstone lie along the southern border of the Wiseman
101 TABLE1
A statisticalsummary for six placer types and one lode gold, Koyukuk-Chandalar mining district, Alaska Min. value
Max. value
Arith. mean
St. dev.
# Ob.
Type 1 locality 3
Fineness Au% Ag% Cu% X%
682 67.3 15.6 0.0029 0.313
843 83.7 31.4 0.013 2.59
786 77.7 21.2 0.0086 1.11
51 5.15 5.04 0.0032 0.750
8 8 8 8 8
Type 2A locality 37
Fineness Au% Ag% Cu% X%
814 81.2 8.97 0.0093 0.297
910 90.3 18.5 0.038 0.990
880 87.4 12.0 0.020 0.659
31 3.05 3.06 0.0072 0.207
12 12 12 12 12
Type 2B locality 12
Fineness Au% Ag% Cu% X% Fineness Au% Ag% Cu% X%
852 84.1 6.19 0.0046 0.542 845 81.5 7.00 0.015 0.847
938 93.2 14.6 0.0089 1.34 929 91.8 15.0 0.053 4.05
910 90.3 8.94 0.0071 0.742 907 88.7 9.12 0.031 2.19
37 3.88 3.62 0.0015 0.284 23 2.98 2.18 0.012 1.12
7 7 7 7 7 14 14 14 14 14
Type 4 locality 22
Fineness Au% Ag% Cu% X%
846 82.7 2.63 0.015 1.31
973 95.4 15.0 0.070 4.87
930 90.5 6.76 0.037 2.70
34 3.74 3.35 0.024 1.06
10 10 10 10 10
Type 5 locality 31
Fineness Au% Ag% Cu% X%
883 87.6 1.75 0.058 0.606
982 97.6 11.6 1.15 1.87
945 93.7 5.41 0.282 0.880
38 3.73 3.77 0.389 0.447
7 7 7 7 7
Lode Gold locality D
Fineness Au% Ag% Cu% X%
740 72.5 19.5 0.0064 0.814
801 78.4 25.5 0.056 2.37
775 76.1 22.1 0.019 1.81
22 2.33 2.15 0.021 0.593
5 5 5 5 5
Type 3 locality 13
Au [Fineness = ~u-r ~ ~g × i000; X--sum of elements other than Au and Ag]
102
quandrangle. Cretaceous granitic plutons, chiefly quartz monzonite and monzonite with granite, granodiorite, and syenite, lie along the southern border of the Chandalar quandrangle. Rocks of quartz monzonite and granitic composition with Paleozoic U / P b and Pb/c~ ages and discordant Mesozoic potassium-argon dates (Grybeck et al., 1977; Dillon et al., 1979) are located in the north-central parts of the Chandalar quadrangle and south-central Wiseman quandrangle. The Precambrian and (or) lower Paleozoic basement rocks experienced pre-Mississippian metamorphism and all the rocks were regionally metamorphosed twice during Mesozoic time (Dillon et al., 1980; Dillon, 1982 ). Late Mesozoic tectonism caused widespread obduction thrusting, lateral faulting, and uplifting resulting in an accreted terrane (Tailleur, 1973; Grybeck and Nokleberg, 1979, pp. B21; Jones et al., 1984). Quaternary glacial, alluvial, and colluvial deposits containing placer gold unconformably overlie the polymetamorphic rocks (Hamilton, 1978, 1979).
RESULTS AND DISCUSSION
Analytical data Collection localities for samples are indicated on Fig. I and analytical results for fineness, Au, Ag, and Cu are presented in Table 2. The lettered sites are lode gold localities and the numbered sites are placer gold localities. Geographic names were taken from the 1:63,360 scale topographic maps of the Wiseman and Chandalar quadrangles. Gold values were determined by difference and fineness is expressed as parts per thousand [ (Au/Au + Ag) X 1,000]. The Ag and Cu content for all placer samples (460) are plotted in histograms (Fig. 2 ). The histograms show log-normal distribution.
Geochemistry of placer gold Silver and Cu values obtained for each site were averaged. The Ag content of the 46 placer gold localities ranged from 3.5 to 22.9 wt.% with an average value of 10.5 (:k-g). Copper contents for the 46 placer gold localities ranged from 0.006 to 0.282 wt.% with an average value of 0.040 (L'~). When normalized Cu is plotted against normalized Ag (Fig. 3), six distinct geochemical types of placer gold are identified in the Koyukuk-Chandalar district. Normalized values are obtained by dividing the site Ag or Cu value by the ~ or L~ value. Normalized values are used in place of real values to get the numbers in a range somewhere near 1, thereby simplifying the numbers in the graph. A value of 1 correlates with the average, 10.5% for Ag and 0.04% for Cu. The geochemical types of placer gold are described as follows:
103 150
--
100
-
50
!
.0018 .0026 .0038 .0056 .0083 .012
.018
.026
.038
.056
.083
.12
12
18
.18
.26
.38
.56
.83
1.2
COPPER VALUES
N U M B E R
150
m
100
--
60
--
O F S
A M P L E S
0,56
0.83
1.2
1.8
2.6
3.8
5.6
8.3
26
38
58
SILVER VALUES
Fig. 2. Histograms for silver and coppervalues for placer gold fromthe Koyukuk-Chandalar mining districts, Alaska, 460 samples. Type Type Type Type Type
1 - having high Ag content and low Cu content; 2 (A&B) - having average Ag content and low Cu content; 3 - having average Ag content and moderate but below average Cu content; 4 - having below average Ag content and above average Cu content; and 5 - having low Ag content and high Cu content.
104 TABLE2 Locality and average fineness and average Au, Ag, and Cu content of gold samples from the Koyukuk-Chandalar mining district, Alaska. (Fineness expressed as parts per thousand and element results expressed in wt. %; A - G, lode gold, 1-46 placer gold; N = number of analyses ) Site
Geographic name
N
Fineness
Au
Ag
Cu
A. B. C. D. E. F. G. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.
St Mary's Creek Little Squaw Mine Summit Mine Mikado Open Pit Mikado Mine Enevelo Mine I Sukakpak Mountain Middle Fork Big Squaw Creek Big Creek St. Mary's Creek Tobin Creek Bench at Tobin Creek Eightmile Creek Garnet Creek 2 Lake Creek Magnet Creek Gold Creek Linda Creek Sheep Creek Hammond River Gold Bottom Creek Bench at Swift Creek Swift Creek Vermont Creek Webster Gulch Thompson Pup Thompson Pup Faye Creek Archibald Creek Smith Creek Smith Creek 3 Union Gulch Mascot Creek Conglomerate Creek Jay Creek Birch Creek Spring Creek Crevice Creek Sawyer Creek E m m a Creek Clara Creek Myrtle Creek
1 7 4 8 1 0 7 1 7 10 9 11 5 6 6 3 6 24 14 14 1 3 20 15 5 1 14 1 64 12 22 1 22 7 7 7 2 7 5 3 6 10
887 857 844 775 775
88.0 84.5 83.4 74.9 76.5
11.2 14.1 15.4 23.5 22.2
0.020 0.013 0.016 0.013 0.009
912 769 840 794 803 781 895 856 942 925 914 922 904 907 912 886 925 923 881 844 858 842 912 943 949 907 934 944 964 946 934 946 887 914 904 882
86.4 76.1 82.5 78.6 79.4 77.0 88.6 85.1 93.7 92.3 90.0 90.6 89.6 88.7 89.8 86.9 90.7 89.4 87.1 79.4 84.0 83.5 89.0 92.3 93.4 90.0 91.0 91.9 94.9 93.7 92.3 93.7 87.9 91.0 89.0 87.4
8.4 22.9 15.7 20.4 19.5 21.6 10.3 14.3 5.8 7.5 8.5 7.6 9.5 9.1 8.7 11.2 7.0 7.4 11.8 14.7 13.9 15.6 8.6 5.6 5.0 9.3 6.4 5.4 3.5 5.3 6.5 5.4 11.1 8.6 9.5 11.8
0.009 0.006 0.021 0.009 0.006 0.007 0.015 0.036 0.063 0.006 0.012 0.013 0.015 0.031 0.061 0.018 0.028 0.022 0.030 0.029 0.020 0.009 0.052 0.058 0.052 0.009 0.065 0.050 0.068 0.049 0.269 0.282 0.023 0.061 0.033 0.014
105 TABLE 2 (continued) Site
Geographic name
N
Fineness
Au
Ag
Cu
36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.
Slate Creek Slate Creek Porcupine Creek TwelvemileCreek TwelvemileCreek TwelvemileCreek Tramway Bar SmalleyCreek DavisCreek GoldBench Mine Prospect Creek
6 12 6 25 2 14 5 5 4 3 20
943 880 887 885 894 907 884 894 863 899 864
93.6 87.4 88.1 84.2 88.2 89.8 85.3 88.9 85.6 89.1 85.3
5.7 12.0 11.2 11.0 10.4 9.2 11.2 10.5 13.6 10.0 13.4
0.042 0.020 0.038 0.023 0.026 0.031 0.026 0.030 0.037 0.028 0.013
1Insufficient Au for analysis was collected at this site. 2Two samples had high Ag content (40%) and were not calculated in the data. 3Five samples appeared to have solder contamination and were not calculated in the data.
Type I gold placers occur at the east end of the study area and Types 2B, 2A, 3, 4, and 5 generally occur in order proceeding west across the district (Fig. 5). The n o r t h - s o u t h string of Type 3 is all from one drainage, the K o y u k u k River draining from the north, so they are not indicating a n o r t h - s o u t h pattern. The average longitude for the placer types is shown in Fig. 4. There is some scatter Type 2 placer gold may be divided into two subgroups which are called Type 2A and Type 2B. Golds of Type 2B are similar in composition to Sukakpak Mountain lode gold (site G, Fig. 1 ) and are derived from creeks near Sukakpak Mountain. Normalized values for Ag and Cu and the ratio of normalized Cu to normalized Ag for the 46 localities are given in Table 3 and are presented by placer type. In this study, all calculations are made on the average of site values and not on the average of the total analytical observations. To empirically test the validity of the analytical data and groupings and for better clarification, the ratio of normalized Cu to normalized Ag is presented graphically in Fig. 4. W i t h the exception of Type 2B, which is unique, there is no overlap of the ratio values demonstrating the validity of the groupings. Type 2B ratios are within the T y p e 2 range of values. Other geochemical features of the placer gold types are given in Table 4. Disregarding Type 2B, there is a systematic increase in Au, fineness, Cu, and Au/Ag ratio values from Type 1 to Type 5. The Cu content of Type 2B fits between Types 1 and 2B. Conversely, with the exception of Type 2B, there is a systematic decrease in Ag, A u / C u ratio, and Ag/Cu ratio values from Type 1 to Type 5. The Ag content of Type 2B fits between Types 3 and 4. From these figures, Type 2B gold appears to be anomalous. The Au does not fit the pattern established by the chemistry of the other placer gold types. An interesting feature is that when a regional plot is made of the placer gold deposits by "type",
106
iiiJii:~!i T Y P E
5
iiii!~il
B
i iiiiiiiii~iiiiiii!ii!~ .... :~iiiiiiii!~iiiiiiiiiiiiiiiiiiiii~i! T Y PE
4
..... 1 -
:0::
TYPE
3
TYPE TYPE
1
Ag/Ag
2
Fig. 3. Plot of normalized Cu versus normalized Ag showing six separate geochemical types of placer gold in the Koyukuk-Chandalar mining district, Alaska. which we attribute to the natural variation which occurs in native Au composition or possibly a mixture of two or more primary Au sources.
Geochemical interpretation We interpret the regional pattern of the placer gold types to be related to the depositional environment of the primary Au source sites. In a hydrothermal system, Ag migrates farther from the heat source and tends to be enriched nearer the surface (Shcherbina, 1956). Shcherbina concluded that Au enrichment (a high Au/Ag ratio) occurs mostly in higher temperature and deeperseated deposits and Ag enrichment (a low Au/Ag ratio) is characteristic of low-temperature ore deposits of intermediate or shallow depths. Fisher (1945)
107 TABLE3 Normalized Cu and Ag values and the ratio of normalized__Cu to normalized Ag for the placer gold localities. Koyukuk-Chandalar mining district, Alaska (Cu = 0.040 and Ag = 10.5 ). (The number in parentheses is the number of analytical observations for each site and the total analytical observations for each placer type. ) Site Type I 1 (1) 3 (10) 4 (9) 5 (11) Avg. (31) Type 2A 2 (7) 15 (3) 20 (14) 21 (1) 35 (10) 37 (12) 46 (20) Avg. (67) Type 2B 6 (5) 9 (3) 1O (6) 11 (24) 12 (14) 25 (1) Avg. (53) Type 3 7 (6) 13 ( 14 ) 16 (20) 17 (15) 18 (5) 19 (1)
1 Cu/Cu
2 Ag/Ag
Ratio 1/2
0.15 0.22 0.15 0.18 0.18
2.18 1.94 1.86 2.06 2.01
0.07 0.11 0.08 0.09 0.09
0.52 0.45 0.50 0.22 0.35 0.50 0.32 0.41
1.50 1.07 1.32 1.49 1.12 1.14 1.28 1.36
0.35 0.42 0.38 0.15 0.31 0.44 0.25 0.33
0.38 0.15 0,30 0.32 0.28 0.22 0.29
0.98 0.71 0.81 0.72 0.90 0.89 0.84
0.39 0.21 0.37 0.44 0.32 0.25 0.35
0.90 0.78 0.70 0.55 0.75 0.72
1.36 0.87 0.67 0.70 1.12 1.40
0.66 0.90 1.05 0.79 0.67 0.51
Site Type 3 (cont.) 32 (5) 34 (6) 38 (6) 39 (25) 40 (2) 41 (14) 42 (5) 43 (5) 44 (4) 45 (3) Avg. (136) Type 4 8 (6) 14 (1) 22 {64 ) 23 (12) 24 (22) 26 (22) 27 (7) 28 (7) 29 (7) 33 (3) 36 (6) Avg. {157) Type 5 30 (2) 31 (7) Avg. (9)
1 Cu/Cu
2 Ag/Ag
Ratio 1/2
0.58 0.82 0.95 0.58 0.65 0.78 0.58 0.75 0.92 0.70 0.73
1.06 0.90 1.07 1.05 0.99 0.88 1.07 1.00 1.30 0.95 1.02
0.55 0.91 0.89 0.55 0.66 0.89 0.54 0.75 0.71 0.74 0.74
1.58 1.52 1.30 1.45 1.30 1.62 1.25 1.70 1.22 1.52 1.05 1.41
0.55 0.83 0.82 0.53 0.48 0.61 0.51 0.33 0.50 0.82 0.54 0.59
2.87 1.83 1.59 2.74 2.71 2.66 5.45 5.15 2.44 1.85 1.94 2.57
6.72 7.05 6.89
0.62 0.51 0.57
10.84 13.82 12.33
f o u n d t h a t in o r e d e p o s i t s f o r m e d d e e p b e l o w t h e s u r f a c e , A u o f h i g h p u r i t y w a s d e p o s i t e d e v e n w h e n A g w a s a b u n d a n t . N e a r t h e s u r f a c e h o w e v e r , so l o n g as s u f f i c i e n t A g w a s a v a i l a b l e , a l o w - g r a d e a l l o y w a s f o r m e d w i t h a m i n i m u m f i n e n e s s s l i g h t l y b e l o w 500 a n d i n t e r m e d i a t e g r a d e s o f A u w e r e f o r m e d a t int e r m e d i a t e d e p t h s . B o y l e (1979, pp. 1 9 7 - 2 0 7 ) , d o c u m e n t s a n u m b e r o f s t u d i e s s h o w i n g i n c r e a s e d f i n e n e s s a n d ( o r ) i n c r e a s e d A u / A g r a t i o s w i t h d e p t h o f dep o s i t i o n . H o w e v e r , he c a u t i o n s t h a t few p r e c i s e s t u d i e s o f t h e v a r i a t i o n w i t h increasing depth of the Au/Ag ratios of single ore shoots or veins have been
108
15172 5 150.36
4
I
rr
I
1 14£76=
~.~2B
14987
~2A 1
•
15026 I I
~3
I
1148.261 I
I
0 10
J L• I0.0
01.0
.j1
CutOu Ag/Ag
Fig. 4. Graphical presentation of the range of values and average value for the ratio of normalized Cu (Cu/Cu) to normalized Ag (Ag/Ag). The number above the bar represents the average longitude for the placer types. TABLE 4 Geochemical features of placer gold types Type 1
# of placer sites # of analytical observations Fineness Au% Ag% Cu%
Au/Ag Au/Cu Ag/Cu
Type 2A
Type 2B
Type 3
Type 4
4
7
6
16
11
31 786 77.9 21.2 0.007 3.67 11,100 3,030
67 866 85.5 13.2 0.017 6.55 5,320 818
53 914 90.2 8.4 0.013 10.8 7,200 667
136 890 87.2 10.8 0.029 8.07 3,000 370
157 937 92.2 6.20 0.057 14.9 1,600 110
Type 5
2 9 940 93.0 6.00 0.276 15.5 340 22
made. Therefore, with a few exceptions, Au with a high Au/Ag ratio can be expected to be found in higher temperature and deeper seated deposits and Au with a low Au/Ag ratio most commonly occurs in low-temperature ore deposits of intermediate or shallow depths. The chemical composition of placer gold is a geochemical signature of the Au shed from the outcrops of the lode gold sources. Placer gold does not necessarily correspond exactly to the composition of Au as deposited in the ore, primarily due to differential solution of surface Ag of Au particles in the alluvial environment. The removal of Ag from Au particles is a common phenomenon in placer golds (Desborough, 1970) and has long been recognized (McConnell, 1907 ). It is referred to as surface refining and takes place only to
109 ~8~__.
~o
150 °
WISEMAN
QUADRANGLE
CHANDALAR
3 .5
=4
4e 4
2
2A 14 % B
QUADRANGLE
~2B .4
1 ,1 1"1"2A
o4
,5
,3 "4
,2A e4 e2A
67 o
67 o
BETTLES
QUADRANGLE 0
I
500
I
I
I
I
,2 Fig. 5. Regional plot of placer gold by "type."
a limited depth from the surface of the individual grains (Boyle, 1979, p. 336) and therefore causes only slightchanges in the overall Au/Ag ratio.Some investigators m a y argue that the rim of higher fineness found on many placer gold grains is the resultof A u accretion. In our study, sufficientAu was obtained from 15 of the placer gold locations to permit analyses of a -0.5-mm fraction and a + 0.5-mm fraction.Values for Ag content of the -0.5-mm fractionare plotted against Ag content of the + 0.5m m fraction (Fig. 6). Four of the sitesshow significantlyhigher Ag content in the larger size fraction and four of the sitesshow significantlyhigher Ag content in the smaller size fraction. T w o sites show a moderate increase in Ag content in the smaller size fraction and the remaining five sites are nearly equal in Ag content in the two size fractionsbut are biased to the smaller size fraction. In general, surface area varies inversely with its grain size. If the 0.5-mm Au grains were being altered by preferential chemical leaching of Ag from the outermost zone of Au particles,or Au in solution were accreted onto the grains, one would expect this fraction to have a lower Ag content. However, in our study such is not the case. This may indicate that the placer gold was not significantlyaltered from the lode gold source and that Ag and Cu content of the Koyukuk-Chandalar placer gold grains represent geochem-
110
I
20
Z
Q 0 < I 09 u3
I
i
I
~5
•
5
10
Silver in weigh! percenl
15
20
+ 35 MESH F R A C T I O N
Fig. 6. Silver values for -35-mesh fraction versus silver values for + 3 5 - m e s h gold.
fraction of placer
ical signatures inherited from their eroded primary sources. A study in South Africa of Au grains from the Witwatersrand palaeo-placer and Barberton Archaean vein deposits using electron microprobe to determine Ag and Hg content also shows no indication of Au alteration during fluvial transport or by post-depositional geological processes (von Gehlen, 1983). The change in Ag and Cu content of the placer gold from east to west in the district suggests there is a similar change in the composition of lode gold reflecting differing depositional environments. Heat sources accompanying metamorphism were apparently progressively deeper from east to west. The ternary system Au, Ag, and Cu further identifies this gradient. When normalized Cu is ratioed to normalized Ag (Table 3), the resulting value represents Cu enrichment or depletion with respect to Ag. The relationship of Ag and Cu with Au is shown by plotting this value against fineness (Fig. 7). The line of solid solution for the ternary system could be dependent on the physico-chemicai parameters at the time of ore deposition and, therefore, reflects the depth of deposition for the primary gold. The physico-chemical parameters that influence metal solubilities, and, as a corollary, ore deposition, are factors such
111
1
LINE OF SOLID SOLUTION 'FOR THE TERNARY SYSTEM
0
Placer gold
1"3 Chandalar lode gold •
Sukakpak lode gold
Cu/C~ Ag/Ag
0 0 0
0 o
0 TYPE 2B' o
900
8OO
FINENESS
Fig. 7. Relationship for the ternary system of Au, Ag, and Cu in the Koyukuk-Chandalar mining district, Alaska.
as temperature, pH, hydrogen and oxygen fugacity, chlorine and bisulfite activity, and hydrogen sulfide fugacity. These factors are complexly interrelated, and definitive answers are difficult to make quantitatively as well as difficult
112
to describe. In the ternary system for native Au, higher Cu values correlate with high fineness. Copper and Ag show an inverse relationship. This relationship has also been observed in a study using microprobe analyses for Ag and Cu content of placer gold from the Colorado mineral belt (Desborough et al., 1970). In that study, detectable Cu (0.1-0.25 wt.% ) was found principally in low Ag content Au grains, which the authors concluded must characterize a certain type of lode source. This also appears to be true in the Koyukuk-Chandalar district. At increasing depth, Cu appears to have a greater opportunity relative to Ag to form a solid solution with Au. In the hydrothermal models described by Reed and Spycker (1986) and by Buchanan (1981), base metals occupy an early paragenetic position. The base-metal horizon occurs deep, below the boiling level, and the precious-metal horizon occurs nearer the surface, above the boiling level. At the level of boiling, a mixed zone of precious- and base-metal mineralization occurs. Is it reasonable then that Au that is precipitated deeper in the hydrothermal system because of some change in the physical-chemical parameters would contain relatively lower Ag content and higher Cu content than Au precipitated nearer the surface? It is apparent that by and large, the Cu/fineness relations of placer gold in the Koyukuk-Chandalar districts can be predicted from the solid-solution curve. Chandalar lode golds also fit the curve (Fig. 7). The exception is Sukakpak Mountain lode gold and placer gold from nearby creeks, our Type 2B placer gold (Fig. 7). This Au is all either (1) depleted in Cu relative to the fineness or (2) depleted in Ag (yielding very high fineness) relative to Cu. We do not have a good explanation for why this Au's ternary relations differ. The lode gold from Sukakpak Mountain is found in a stibnite vein that occurs at the contact between marble and schist. The Chandalar lode gold is found in dilatant quartz veins in a graywacke-type host rock. It is probable that in Type 2B Au, Sb had an influence on the proportioning of Ag and/or Cu content in native Au during Au deposition. CONCLUSIONS
Geochemical studies of placer gold from the Koyukuk-Chandalar mining district reveal striking contrasts in the Ag and Cu content of placer gold that may have a direct relation to the depositional environments of the primary gold source sites. Based on Ag and Cu content, six different geochemical types of placer gold can be identified ranging from Type 1 which has high Ag content (21.2 wt.%) and low Cu content (0.007 wt.%) to Type 5 which has low Ag content (6.0 wt.% ) and high Cu content (0.276 wt.%). Type 2 placer gold is divided into two subgroups. From Type 1 to Type 5 placer gold, Au, Cu, and Au/Ag values increase systematically, and Ag, Au/Cu, and Ag/Cu values decrease systematically. A regional plot of the placer gold types shows Type 1 placers occurring in the eastern portion of the district with Types 2-5 generally
113
occurring in order westwardly across the district, suggesting a variation in the primary gold sources. The ternary system of Au, Ag, and Cu shows that higher Cu values correlate with high Au content and that Ag and Cu have an inverse relationship. We hypothesize that the three elements were selectively partitioned from the ore-forming hydrothermal solutions yielding ternary signatures dependent upon variable physical-chemical parameters at the time of Au deposition, and that the depth of the depositional site of the primary Au sources may be the most critical control. Based on the gradient that is inferred from the Au analyses, late Mesozoic tectonism of rocks now forming the Brooks Range may have not only caused the ore-forming hydrothermal solutions to have been emplaced in progressively lower thrust plates from east to west across the Koyukuk-Chandalar district but may have had an important influence on the chemistry of the resulting lode and placer golds in the district.
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