A Geochemical Survey of the Upper Part of Loon Creek, Custer County, Idaho Charles R. Knowles Technical Report 79.-Q Id1lho Geological Survey 1979 University of Idaho Moscow, Idaho 83844 A GEOCHEMICAL SURVEY OF THE UPPER PART OF LOON CREEK, CUSTER COUNTY, IDAHO Charles R. Knowles Idaho Geological Survey University of Idaho Technical Report 79-6 Moscow, Idaho 83843 October 1979 TABLE OF CONTENTS Page INTRODUCTION .... 1 GEOGRAPHIC SETTING. 3 PREVIOUS INVESTIGATIONS 3 STRATIGRAPHY AND LITHOLOGY. 4 The Yellowjacket Formation 6 Hoodoo Quartzite . 6 Micaceous Quartzite and Garnet Schist. 7 Dolomitic Marble . 7 Rocks of the Idaho Batholith 7 Tertiary Intrusion 8 Challis Volcanics and Related Dikes. 9 Quaternary Deposits. 9 STRUCTURAL GEOLOGY. 10 AEROMAGNETIC DATA 10 GEOCHEMISTRY. 11 METHODS OF DATA PRESENTATION. 13 DISTRIBUTION OF TRACE ELEMENTS. 15 Silver 15 Gold .. 16 Copper 17 Cadmium. 18 Molybdenum 18 Lead ... 19 ~ll Zinc . 19 INTERPRETATION AND SUMMARY OF THE GEOCHEMICAL RESULTS 20 ACKNOWLEDGE~ffiNTS. 22 BIBLIOGRAPHY AND SELECTED REFERENCES. 22 APPENDIX A. ' 37 APPENDIX B. 41 LIST OF TABLES Table lA: Statistics for Elements in 64 Samples of Stream Sediments, Upper Part of Loon Creek, Custer County, Idaho. 25 Table IB: Statistics for Log-Transformed Elements in 64 Samples of Stream Sediments, Upper Part of Loon Creek, Custer County, Idaho . .. 2S Table lC: Log Values From Table IB Transformed to Parts Per Million (or Parts Per Billion for Au). 26 Table 2A: Statistics for Elements in 141 Soil Samples, Upper Part of Loon Creek, Custer County, Idaho .... 27 Table 2B: Statistics for Log-Transformed Elements in 141 Soil Samples, Upper Part of Loon Creek, Custer County, Idaho. ~7 Table 2C: Log Values From Table 28 Transformed to Parts Per Million (or Parts Per Billion for AU). 28 Table 3; Anomalous Stream-Sediment Samples in the Upper Loon Creek Area (all Elements are in Parts Per Million Except Gold Which is in Parts Per Billion) . 29 Table 4: Anomalous Soil Samples in the Upper Loon Creek Area (all Elements are in Parts Per Million Except Gold Which is in Parts Per Billion) . .. 30 Table SA: Pearson Correlation Coefficients Between Elements in 47 Stream Sediment Samples Characterized by Rocks of the Idaho Batholith 31 iv Table 5B: Pearson Correlation Coefficients Between Elements in 11 Stream Sediment Samples From Rocks Characterized by the Yellowjacket Formation 31 Table 6A: Pearson Correlation Coefficients Between Elements in 11 Soil Samples and Rock Types of the Tertiary Intrusions 32 Table 6B: Pearson Correlation Coefficients Between Elements in 10 Soil Samples and Rock Types of the Hoodoo Quartzite 32 Table 6C: Pearson Correlation Coefficients Between Elements in 94 Soil Samples Characterized by Rocks of the Idaho Batholith _ 33 Table 6D: Pearson Correlation Coefficients Between Elements in 20 Soil Samples and Rock Types of the Yellowjacket Formation. 33 Table 7: Mean and Standard Deviation of Elements From Stream-Sediment Samples by Rock Type Which is Most Characteristic of the Drainage From Which the Sample was Collected . 34 Table 8: Mean and Standard Deviation of Elements From Soil Samples by Rock Types of the Area ..... _... 35 LIST OF FIGURES Figure 1: Location Map of Study Area, Custer County, Idaho 2 Figure 2: Map Showing Reconnaissance Geology and Sites of Stream-Sediment Samples) Custer County, Idaho. 5 Figure 3: Map Showing Sites of Soil Samples, Custer County, Idaho 12 v A GEOCHEMICAL SURVEY OF THE UPPER PART OF LOON CREEK, CUSTER COUNTY, IDAHO By Charles R. Knowles 1 INTRODUCTION Encouraged by significant mineral findings in a previous reconnaissance geochemical survey of the Warm Springs Creek-Loon Creek­ Rapid River area in Custer and Lemhi Counties (Knowles and Bennett, 1978), I decided to study by sampling and reconnaissance mapping the upper part of the Loon Creek drainage. The purpose of this study was two-fold: first, to delineate accurately the extent of the mineralized areas detected in the previous study on Loon, Pioneer, and Tango Creeks; second, to test the idea that stream sediments collected in a reconnaissance study at a density of one sample per square mile will adequately define large mineral-rich regions. Approximately 50 square miles (130 square kilometers) were sampled on a one-half mile (800 m) square grid at approximately one sample within each 160 acres. The location of the study area is shown in Figure 1. lIdaho Bureau of Mines and Geology, Moscow, Idaho 83843 r--­ 2 CUSTER~ ~ / STUDY AREA " 1 ~ Hj L \ ' c,," 1 At 1 --+--~e-t- __ t1-­ .=" v<­ . -tz '+~ ,0.. ;;;;,1 jl t-­ " MI~ l6 ~ ~- , ['i lG"'''''" ,i,,,,,,"', " t. ~, 7' " '~ \ ,.t' I 6 \'1 I, " I ( 1 I"J ",)\______ I I -1 - +-­~- ,qPea• . IJ c-t---r­ - . j I) . I .,. J4 If,d -,­ I ~I I ~.y./ ­ rf '\""''''- "?"'r" .. - I j - r, I i -r7 l-'"t­ ~ tJ i ~. ~_ ~""'''IO I ~ \..... Peak I I 1 ." -I ,£1), ,_/t- /~-t-- --~ ~~," "I ,I i I I,',I"; 11II - pr; 36 ".l,d­ \. '" )-~~ IQ'~~v ~ 1"'( , <1 I I ~. !'""~f , t, 1/ · t~'\ t-'--t-'­ :r +k' --1.1 - -AI ~ ,_~ ~4\ I ,f , I f r;(l\~~'~ -~ D ·i~,l . - --+11)­ "I f{ 1 \ " ' ' --+----.--f­ - 8:r, - --­ -­ +-­ - +---­'"' t- ~r" · -+i~~'H-r-· /. 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V • "~,:,, .• o \. ... I I r Figure 1. Location map of study area, Custer County, Idaho. I 3 GEOGRAPHIC SETTING The upper part of the Loon Creek drainage is in the Salmon River Mountains in east-central Idaho between lat 44°25' - 44°35' N., long 114°45' - 115°0' IV. Access is from U. S. Highway 93 on a dirt road that follows the Yankee Fork of the Salmon River, north along Jordan Creek, over the Jordan Creek-Mayfield Creek Summit and along Mayfield Creek to its confluence with Loon Creek. From this confluence the area can be reached only by pack trail. Pack trails follow Loon and Pioneer Creeks to their sources. The major drainage system is formed by Loon, Pioneer, and Tango Creeks. The study area boundaries follow these drainage divides to the confluence of Loon and Pioneer Creeks, an area of approximately 50 square miles (130 square kilometers). The topography in the area is extremely rugged with elevations ranging from 10, 329 feet (3, 148 m) at the peak ca11 ed "The General" and 10,012 feet (3,052 m) at Knapp Peak (Knapp Peak was formerly and locally known as Tango Peak) to 6,000 feet (1,829 m) at the confluence of Loon Creek and Pioneer Creek. PREVIOUS INVESTIGATIONS The earliest major geologic work in the area is by J. B. Umpleby and D. C. Livingston (1920). Their investigations include the mining sites near Sheep Mountain and in the Seafoam mining district. C. P. Ross discusses the various mines in the area in two reports (1927, 1930) and in "The Geo logy and Ore Dep osits of the Cas to Quadrang1e , 4 Idaho" (1934). This report contains the only existing geologic map that covers much of the adjacent area to the north of his mapped area. Ross also discusses the Lost Packer Mine, the nearest active mine 5 miles north of the study area. The northwestern and western parts proximate to the area have been mapped and sampled by Cater and others (1973) for the Idaho Primitive Area study. This work is useful, as many locations of mines and small prospects are recorded. A compilation of geologic mapping at 1:250,OOO-scale on the Challis 2° AMS quadrangle has recently been rereased by the Idaho Bureau of Mines and Geology (Rember and Bennett, 1979). Several graduate theses on the region include Cass (1973), Fenoe (1977), Sibbett (1976), Yinger (1976), and Stiles (1976), north and northeast of the region, and Crist (1978) south of the area. The reconnaissance geochemical survey of Knowles and Bennett (1978) was conducted in the area of Warm Springs Creek, Loon Creek, and Rapid River. STRATIGRAPHY AND LITHOLOGY The reconnaissance mapping done during the summer of 1978 is shown on the geologic map (Figure 2). The rocks in the area are typified outside the region by Ross (1934) and Cater and others (1973). I will use the rock units described by Cater and others (1973) which I feel are probably more accurate than those in the older work of Ross. The only discrepancy between these two works is in the description of the metasedimentary units; Ross described many of these as Ordovician in age, and Cater and others classed them as Precambrian. EXPLANAnON MAP SYMBOL Ag > 10 ppm _ S Au > 131 ppb _ G Cu > 32 ppm _ Cu Cd > 2 ppm _ Cd Mo > 22 ppm _ M Pb > 45 ppm _ L Zn > 231 ppm _ Z Quaternary depoSIts - Oal Tertiary intrUSIon - Ti Challis volcamcs & related dikes - Tcv RockS 01 the tdaho ba(holilh - KI ~ Micaceous quartzite,garnet schist· P(;qs //~ YellowJ3ck.et formation - P8y Hoodoo quartzite - : PE:h Dolomite marble - PE:m Fault Inferred fault o Sample location • Anomalous sample NORTH o! ! 1 MILE • KllOMEJEA Map showin~ reconnaissance geology and sites of stream-sedlment samples, Custer County, Idaho.