Abstract:

The McDougall I, McDougall II and C-shafit faults are synvolcanic structures that accommodated subsidence and acted as conduits for magmas and ascending hydrothermal fluids that were responsible for the formation of volcanogenic massive sulfide deposits within the Mine Sequence of the ~2.7Ga, Noranda Volcanic Complex. Fluid inclusions within hydrothermal quartz-chalcopyrite breccia veins in these faults are of moderate to high salinity and have minimum temperatures of entrapment (Th - homogenization temperature) <250°C, a temperature range in which chloride brines are<br/>incapable of transporting significant quantities of Cu, unless under unusually oxidizing conditions. Fluid Inclusion Assemblage (FIA) salinities and homogenization temperatures from early pre-mineralization comb-textured quartz indicate that the fluids had salinities of 5.8 - 14.0wt% NaCl equiv., and low-moderate temperatures of 125 -212°C (up to 378.9°C - for individual inclusions not part of an FIA). Phase separation caused by boiling within the conduit, resulted in inclusions with salinities of 0.5 - 3.6wt% NaCl equiv., below that expected for Archean seawater. Secondary ore-related FIAs<br/>have salinities of 4.1 -17.4 wt% NaCl equiv., minimum temperatures of entrapment with Th ranging from 138.0 to 236.7°C and salinity vs. Th trends suggestive of mixing between lower salinity and higher temperature fluids.<br/>Results from microthermometry and salt decrepitation mound analysis are consistent with fluids that are predominantly evolved seawater with compositions having been modified via fluid-rock interaction within semiconformable alteration zones in the Mine Sequence. Less common are saline fluids inclusions containing dominantly immiscible CO2 within the McDougall II fault that suggest phase separation at depth and heterogeneous entrapment, with salinities in the 0.8 - 21.3wt% NaCl equiv. range, Thco2 from 9.1 - 21.1°C and decrepitation temperatures (T<0 of 294.3 - 477.4°C. The calculated hydrostatic pressure for CO2 bearing inclusions indicates that trapping occurred of 9.2- 13.9 MPa, consistent with an open to seafloor fault system and a water depth of <400m. Volatiles from quartz and chalcopyrite separates include, in order of abundance H2O » CO2 > CH4 > N2 > COS > C2- HC's > C3- HC's, and are consistent with the volatile composition of discharging fluids from modern, back-arc hydrothermal systems. Quartz separates have 613CCH4 and 513Cco2 isotopic signatures representative of a depleted source for C02-bearing fluids in the McDougall I fault, and magmatic and seawater sources for C02-bearing fluids in the McDougall II fault. The latter data indicate that magmatic fluids were, at least episodically, incorporated into a dominantly evolved ~2.7Ga seawater hydrothermal system at Noranda.