Ore Petrography, Geochemistry, and Genesis of Epithermal Silver-Gold Veins on Florida Mountain, Silver City District, Idaho

Abstract

Geochemical and petrographic studies were conducted on the Black Jack-Trade Dollar vein, a high-grade, low-sulfidation epithermal Ag-Au deposit hosted in mid-Miocene volcanic rocks of the Owyhee Mountains in southwestern Idaho. Petrographic studies of high-grade ore samples were conducted in both reflected and transmitted light. These petrographic studies revealed ore textures that indicate boiling and colloidal transport as important ore deposition mechanisms. Geochemical analysis indicated high levels of Au, Ag, Se, S, and Cu in the ores. Further analysis of the ores with an electron microprobe showed the presence of a compositional continuum of silver sulfo-selenides ranging from naumannite (Ag2Se) to acanthite (Ag2S). Additionally, the microprobe analyses revealed an unidentified Ag-Au-Se-S phase with an estimated formula of Ag3AuSeS. Stable isotope studies of ore-stage chalcopyrite grains showed that δ65Cu measurements range from -2 to 1‰ and δ34S measurements range from -1 to 1‰. Both Cu and S isotopes indicate a magmatic source for the metals. Textural, geochemical, and isotopic evidence suggests that the high-grade ores were formed when Au- and Ag-rich hydrothermal fluids were evolved from a magmatic source. These hydrothermal fluids apparently deposited their metals in the shallow vein through a combination of colloidal transport, cooling, and boiling.