Isabelle Harris, PhD
I. Harris1, E. Marsh2, E. Holley3
1Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado, United States of America
2U.S. Geological Survey, Denver, Colorado, United States of America
3Department of Mining Engineering, Colorado School of Mines, Golden, Colorado, United States of America
Placer deposits have historically been an important source of gold, yet the lode sources of these deposits often remain enigmatic. Thus, work to develop and implement methodologies to better define placer mineralization sources has academic and economic implications. Combining gold particle inclusion data with gold alloys, isotopes, and morphological characteristics creates the most comprehensive picture of placer gold source signatures. Gold grains from three locations in Flat, Alaska are analyzed to elucidate the sources of the placer gold. Flat is a historic gold mining town in the Kuskokwim Mountains consisting of placer deposits on creeks that drain multiple sources of mineralization, including district mineral occurrences that are the target of modern exploration-and potential undefined regional sources. The placers and local lode sources exhibit signatures of a multitude of mineralization types including orogenic gold, epithermal, polymetallic veins, reduced intrusion, porphyry Cu-Mo-Au, granite Sn-W, and magmatic REE mineral systems. This study aims to fingerprint the signatures of placer gold at Flat through the integration of Electron Microprobe Analysis (EPMA), LA-ICP-MS, and SEM techniques. EPMA has commonly been utilized to determine the major alloy content in gold particles, but this method alone has limitations, which led to a new direction, centering around inclusions within the gold particles. Usage of LA-ICP-MS offers a lower detection limit than EPMA, advantageous for analyzing a broader suite of trace elements in the alloy than available by EMPA. LA-ICP-MS/MS analysis allows for analysis of Pb isotopes with the utilization of ammonia in a collision cell to reduce the interference of Hg. The inclusions are first visually identified, counted, and compositionally analyzed under SEM. Mineral intergrowths, morphology, and other qualities such as rims are also noted in the host grains. Once this information is gathered, the inclusion signatures are plotted into two broad categories: metals and nonmetals. Preliminary EPMA results show variations in Au-Ag alloy content across the drainages with an increase in Ag content from Happy Creek to the Schoolhouse site, and finally Flat Creek. The average median across sites is 13.19 wt% Ag and total range is 0.22-50.09 wt% Ag. This study aims to further refine the mineralization style of the local sources, discover enigmatic distal sources, and contribute to the development of utilizing gold grains in deposit type characterization, specifically its application to placer-source relations.