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Paper No. 9-3
Presentation Time: 8:00 AM-6:00 PM
GEOCHEMICAL INVESTIGATION OF THE HISTORIC HECLA AG-PB MINING DISTRICT, PIONEER MOUNTAINS, MONTANA
The Hecla mining district in southwestern Montana produced 0.6 M oz Ag, 40 M kg Pb, and lesser amounts of Cu, Zn and Au, mainly in the late 19th century. In this study, we reexamine the district to determine the sources of metals and the nature of mineralizing fluids. The carbonate-hosted ore bodies display structures (bedding parallel ore shoots) and mineral assemblages (sphalerite, galena, pyrite, tetrahedrite, quartz) that are consistent with an early MVT-type origin, a late Cretaceous magmatic-hydrothermal replacement origin, or some combination thereof. Reflected light microscopy, SEM-EDS, Raman micro-spectroscopy, and electron microprobe analysis document the paragenesis of hypogene and supergene mineralization. This is supplemented with data on fluid inclusions in quartz, S-isotopes of galena, sphalerite, and pyrite, and C- and O-isotopes of dolomite in veins and altered wallrock (Cambrian dolomite). Fluid inclusion data point to mineralizing fluids that were locally rich in CO2 (three phase inclusions), with salinities between 2 to 5 wt.% NaCl equivalent, and local evidence of boiling. Many inclusions contain dawsonite (NaAlCO3(OH)2) daughter minerals. Stable isotope geothermometry is in close agreement with the fluid inclusion data, and suggests that mineralization occurred at peak temperatures in the 300-350C range. This evidence, along with close proximity to late Cretaceous plutons, suggests that the Hecla orebodies are of magmatic-hydrothermal origin, a conclusion that is in agreement with early workers who described the economic geology of the Hecla District. Supergene processes have since modified the primary sulfide mineralogy, resulting in an array of secondary metal sulfides, carbonates, and sulfates. In particular, supergene coatings of acanthite on pre-existing sulfide minerals may have greatly increased the value of the ore, especially in the early mining boom of the late 1800’s. As the mines progressed deeper into the mountain the Ag enrichment played out, and by the early 1900’s large-scale mining had effectively ceased. Nonetheless, we suggest the Hecla district has moderate potential for discovery of additional reserves of Pb-Zn-Ag ore, particularly where known ore-bearing zones are terminated by faults.