PARAGENESIS AND FLUID INCLUSION STUDY OF EPITHERMAL AU-AG VEINS AT THE DRUMLUMMON MINE, MONTANA

The Drumlummon Mine, located 30 km NW of Helena, Montana, was an important producer of Au and Ag in the late 19^th and early 20^thcenturies. A set of NE- to E-striking quartz/carbonate fissure veins cut contact metamorphosed, impure dolomite of the mid-Proterozoic Helena Formation. The epithermal mineralization at Drumlummon has not been dated, but may be associated with nearby porphyry-Mo mineralization of Eocene age. Samples examined in this study were provided by mine operators at Drumlummon who recently reopened some of the historic underground workings and discovered a previously unknown, Au-Ag-rich vein.

Primary ore minerals at Drumlummon include galena, sphalerite, chalcopyrite, Ag-rich tetrahedrite, Ag-rich sulfosalts (pearceite, pyrargyrite), argentite/acanthite, and electrum (X_Au = 0.4 to 0.6). Sphalerite is pale-colored, devoid of chalcopyrite inclusions, and poor in Fe (X_FeS = 0.01 to 0.03). Pyrite is relatively uncommon. The gangue is dominated by quartz but is locally rich in calcite, dolomite, and adularia, especially in higher-grade ore shoots. Bladed intergrowths of carbonate minerals and quartz are common, suggestive of boiling. Chlorite is the dominant wallrock alteration mineral. Secondary ore minerals include stromeyerite (AgCuS) and native silver. Geometric intergrowths of stromeyerite, native silver and chalcopyrite suggest that these minerals formed by replacement of Ag-rich tetrahedrite and/or Ag-sulfosalts. Thermodynamic calculations show that the primary ore mineral assemblage formed between 180 and 340°C. This temperature range overlaps with the range in homogenization temperatures of primary fluid inclusions in quartz. Although most of the fluid inclusions had low salinity (< 3 wt% NaCl_eq), a subset were more saline, containing up to 21 wt% NaCl_eq. This suggests that magmatic fluids may have episodically enriched the hydrothermal system in dissolved solutes, including base and precious metals. Assuming hydrostatic pressure for the ore-forming fluids, which locally boiled, the depth of emplacement of the veins was less than 1 km. Secondary Ag mineralization at Drumlummon is less well-constrained, but may be related to an active geothermal system in the district that contains a reservoir of shallow groundwater that has been heated to temperatures near 100°C.