Date of Award
Master of Science (MS)
The Golden Chest Au-W Mine has been intermittently mined since the 1880’s. Mineralized quartz veins have been deposited into the Upper Prichard Formation of the Proterozoic Belt Supergroup. The Golden Chest is located in the Summit Mining District within the Murray Gold Belt region of the greater Coeur d’Alene (CdA) Mining District. A modern geochemical study of the Golden Chest has never been conducted at the Golden Chest.
This thesis investigates, with the collaboration of New Jersey Mining Company (NJMC), the paragenesis of ore forming minerals, the mineralogy of the deposit and the genesis of the ore forming processes. A discussion of lead isotope data compiled from literature and from NJMC records is also provided in this thesis. Ore compositions were identified using SEM-EDS and reflected and transmitted light microscopy at Montana Tech. Sulfur isotope analyses of pyrite, galena and sphalerite were performed at the University of Nevada-Reno, whereas oxygen isotope analysis of quartz and scheelite separates were performed at the University of Oregon.
Analyses of 35 gold/electrum grains gave an average of 78 ± 22 wt% Au. Gold grains were found in quartz vein material with pyrite, galena, arsenopyrite and low iron sphalerite (0.81±0.42 wt% Fe) in most samples. Pyrite is the earliest deposited sulfide and a strong correlation was found between gold and galena. Gangue mineralogy is dominated by milky quartz with lesser amounts of potassium feldspar and carbonates such as calcite, ankerite and ferroan dolomite. Several monzonite to lamprophyre dikes that previous workers have associated with mineralization were examined via SEM and showed widespread alteration of primary minerals. No gold and only minor sulfides were found associated with these intrusions. Hydrothermal alteration of the Prichard Fm. next to quartz veins tended to be weak, with chloritic, sericitic and silicic alteration assemblages.
A study of fluid inclusions in mineralized quartz and scheelite veins produced an average homogenization temperature of 195±45ºC. Low salinities of 0.3±0.26 wt% NaCl were estimated. Stable isotopes of d18O and δ34S provided independent geothermometry estimates for isotopic fractionation of quartz-scheelite and pyrite-galena respectively. The result of the analysis yielded average temperature estimates of 332ºC for quartz-scheelite and 324ºC for pyrite-galena. Combining the fluid inclusion and stable isotope data results in an estimated pressure range of 1.1 to 3.5 kbars and a depth range (assuming lithostatic conditions) of 3.8 to 12.2 km. This range in P-T conditions is consistent with vein formation in an orogenic gold system near the brittle-ductile transition zone.
Stable-isotope studies gave similar results to previous work on veins of the Couer d’Alene mining district. d18O values range from 11.6 to 14.7‰ for quartz and +6.0 to +7.0‰ for scheelite. These minerals formed from a vein-forming fluid (+7.5 ± 1.2‰) with either a magmatic or metamorphic origin that underwent extensive fluid-rock interaction. Analyses of δ34S isotopes produced results (-0.1 to 6.7‰) that cannot be used to distinguish between a sedimentary or magmatic origin of sulfur. Lead isotopes from the Golden Chest are consistent with deposition of “young” lead that is also found in the Cretaceous-Tertiary Bitterroot Lobe of the Idaho Batholith (48-66 Ma) and the nearby Gem Stocks (110-118 Ma). Golden Chest lead isotopes are dissimilar to “old” (Precambrian) lead found in the Ag-Pb-Zn deposits of CdA.
Brown, Alexander, "THE GEOLOGY AND GEOCHEMISTRY OF THE GOLDEN CHEST GOLD DEPOSIT, MURRAY, IDAHO" (2019). Graduate Theses & Non-Theses. 211.