Date of Award
Master of Science (MS)
Brian St. Clair
The Hog Heaven mining district in northwestern Montana is unique in that it is a high-sulfidation epithermal system containing high Ag-Pb-Zn relative to Au-Cu, with a very high Ag to Au ratio (2,330:1). The deposits are hosted within the Cenozoic Hog Heaven volcanic field (HHVF), a 30 to 36 Ma suite that consists predominantly of rhyodacite flow-dome complexes and pyroclastic rocks. The HHVF is underlain by shallow-dipping siliclastic sediments of the Mesoproterozoic Belt Supergroup. These sediments are known to host important SEDEX (e.g., Sullivan) and red-bed copper (e.g., Spar Lake, Rock Creek, Montanore) deposits rich in Ag-Pb-Zn-Cu-Ba. The HHVF erupted through and deposited on the Belt strata during a period of Oligocene extension.
Outcrops and drill core samples from Hog Heaven show alteration patterns characteristic of volcanic-hosted, high-sulfidation epithermal deposits. Vuggy quartz transitions laterally into quartz-alunite alteration where large sanidine phenocrysts (up to 4 cm) have been replaced by fine-grained, pink alunite, and/or argillic alteration that is marked by an abundance of white kaolinite-dickite clay. Marginal parts of the deposit are weakly altered to illite-montmorillonite. Reflected light microscopy and SEM-EDX identified three mineralizing stages, consisting of Stage 1 enargite-pyrite-alunite-APS-bismuthinite, Stage 2 Ag-Pb-Sb-Bi sulfosalts, and Stage 3 sphalerite-galena-barite. Retrograde reactions created complex intergrowths of the Ag-Pb-Sb-Bi sulfosalts. Galena, pyrite and marcasite are prevalent throughout the deposit and in each stage of mineralization.
Sulfide minerals at Hog Heaven have a d34S range of 0.4 to 5.8 ‰. Alunite and barite have d34S in the range of 19.6 to 37.1 ‰, with barite being isotopically heaviest. Alunite and dickite δ18O range from +5.5 to +10.2 ‰, with the exception of two outlier alunite samples with -3.9 and +1.6 ‰. The δD for the alunites and dickites range from -37 to -78 ‰ with the clays having a much lighter isotopic composition. Biotite has an average δ18O of +6‰. Sanidine has a δ18O of +6.2 ‰. Temperatures obtained from S-isotope geothermometry range from 226ºC to 304ºC, with an outlier of 459ºC. These results overlap with measured homogenization temperatures for fluid inclusions in barite (168 to 252ºC, average of 209ºC). Salinities of fluid inclusions in barite range from 0 to 3.69 wt% NaCl with an average of 1.86 wt% NaCl. Assuming the fluids were trapped near the hydrostatic boiling curve for dilute NaCl solutions, the Th values correspond to a depth of trapping of about 0.5 km. This is consistent with the epithermal nature of the Hog Heaven deposits.
Based on the combined S-O-H-isotope results, sulfate and clays at Hog Heaven formed by both disproportionation of magmatic SO2 and mixing with heated and evaporated meteoric water. The bulk sulfur in the primary ore fluid at Hog Heaven was isotopically heavy, probably near +8 ‰. This heavy sulfur may have been derived by magmatic and/or hydrothermal assimilation of sulfide and/or sulfate minerals in stratabound mineralization in the underlying Belt formations, which also helps to explain the high amounts of Ag-Pb-Zn-Ba. Overall, the geology, mineralogy, and metal ratios at Hog Heaven closely resemble the Julcani district, Peru. Like many high-sulfidation epithermal deposits, there is a possibility of an undiscovered porphyry system at depth.
Kallio, Ian, "MINERALOGY, FLUID INCLUSION, AND STABLE ISOTOPE STUDIES OF THE HOG HEAVEN MINING DISTRICT, FLATHEAD COUNTY, MONTANA" (2020). Graduate Theses & Non-Theses. 240.