Mineralogy, geochemistry and genesis of alkaline igneous rock-related gold-silver telluride deposits of central Montana

Abstract Gold-silver telluride deposits in central Montana contain > 400 t Au and are spatially and genetically related to major faults in the Great Falls Tectonic Zone (GFTZ) and the Lewis and Clark Line. They are also related to alkaline igneous intrusive rocks, including monzonites, syenites, diorites, tinguaites, dacites, lamprophyres, and trachytes. Deposit styles include bonanza veins, carbonate replacement at igneous-carbonate contacts, breccia pipe-hosted, and structurally controlled igneous-hosted. Ore-related breccias are a common feature. The ore mineralogy is complex, and locally contains native gold/electrum, Au-Ag tellurides (calaverite, sylvanite, krennerite, petzite, empressite, stützite, and hessite), Bi-tellurides (tetradymite, tellurobismuthite), Bi sulphosalts, and rare precious-metal sulphotellurides. Alteration related to ore-stage fluids is localised primarily adjacent to veins and characterised by silicification, fluoritisation, adularia-sericite, and roscoelite±clays. Fluid inclusion studies suggest that gold telluride ores were deposited from low-temperature (130 o –270 o C), moderately saline (1–12 equiv. wt % NaCl), locally boiling, CO 2 -poor, near neutral, relatively oxidising fluids. Oxygen and hydrogen isotope studies support the concept that the deposits formed from a continuum between magmatic and meteoric fluids, whereas sulphur isotope compositions of sulphides suggest a magmatic sulphur source or sulphur that was leached from sulphides in volcaniclastic and clastic sedimentary rocks. Lead isotope compositions are permissive of a crustal source with a contribution from Palaeozoic or Proterozoic sedimentary rocks hosting the alkalic igneous rocks. Porphyry molybdenum and Carlin-like Au-Te deposits are also genetically related to the GFTZ and Lewis and Clark Line and represent end-members that form a continuum with epithermal gold-silver telluride deposits.