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Results of Automated Scanning Electron Microscope (SEM) Analyses of Rock and Stream Sediment Samples from the Taurus Porphyry Copper Deposit Area, Tanacross Quadrangle, Eastern Alaska

Open-File Report 2022-1046
By: , , and 
https://doi.org/10.3133/ofr20221046

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Abstract

Numerous porphyry copper-molybdenum-gold and epithermal deposits define a belt that extends from Eastern Alaska to western Yukon, Canada. An orientation study conducted near the Taurus porphyry deposit was designed to test methods that require minimal sample collection, preparation, and analytical time to determine the viability of indicator mineral studies as a reconnaissance exploration method. Bulk stream sediments and altered and mineralized rocks were sieved to the 0.105−0.25 millimeter fraction (+140, −60 mesh) and passed over a shaking table to create a moderate to heavy mineral separate that was mounted in epoxy and subsequently analyzed using automated scanning electron microscope (SEM) techniques. Seven polished thin sections of core were also analyzed. Among the advantages of automated SEM techniques compared to visual mineral identification are that thousands of grains can be rapidly identified in each sample (about 1 hour per sample) and small quantities of indicator minerals that may be missed during traditional visual analyses can be detected. Automated SEM analyses of stream sediment and rock samples show that specific minerals (chalcopyrite, bornite, and jarosite) are indicators of potential mineralized areas. Svanbergite, an aluminum sulfate phosphate mineral, was identified in mineralized rocks and in nearly all stream sediment samples (up to 9 kilometers) downstream from the Taurus and other porphyry occurrences but not epithermal occurrences. It was not identified in areas with no known mineralization and thus it is possibly one of the best indicator minerals for porphyry copper (+/- molybdenum, gold) occurrences.

Suggested Citation

Kelley, K.D., Pfaff, K., and Graham, G.E., 2022, Results of automated scanning electron microscope (SEM) analyses of rock and stream sediment samples from the Taurus porphyry copper deposit area, Tanacross quadrangle, eastern Alaska: U.S. Geological Survey Open-File Report 2022–1046, 12 p., https://doi.org/10.3133/ofr20221046.

ISSN: 2331-1258 (online)

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Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Methods
  • Results
  • Conclusions
  • References Cited
  • Appendix 1. Results of TIMA Analyses
Publication type Report
Publication Subtype USGS Numbered Series
Title Results of automated scanning electron microscope (SEM) analyses of rock and stream sediment samples from the Taurus porphyry copper deposit area, Tanacross quadrangle, eastern Alaska
Series title Open-File Report
Series number 2022-1046
DOI 10.3133/ofr20221046
Year Published 2022
Language English
Publisher U.S. Geological Survey
Publisher location Reston VA
Contributing office(s) Geology, Geophysics, and Geochemistry Science Center
Description Report: vi, 12 p.; Table
Country United States
State Alaska
Online Only (Y/N) Y
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