Genesis of the Paleoproterozoic NICO iron oxide-cobalt-gold-bismuth deposit, Northwest Territories, Canada: Evidence from isotope geochemistry and fluid inclusions

The NICO deposit is located in the southern portion of the Paleoproterozoic Great Bear magmatic zone, Northwest Territories, Canada. The majority of the mineralization lies within the Bowl Zone, hosted by the Treasure Lake Group (TLG), where heavily altered precursor rocks are interpreted to be carbonate-rich wackes and siltstones. These rocks are crosscut by a set of pre-ore quartz +/- calcite + amphibole + K-feldspar veins (S1). The Co-Au-Bi (+/- W-Cu-Mo) mineralization at NICO is contained in stratabound ore lenses within the Bowl Zone, and comprises a prograde assemblage of Co-rich arsenopyrite (arsenopyrite I) and loellingite, cobaltite, pyrite, actinolite, ferrohornblende, biotite and rare scheelite (+/- molybdenite) along with minor magnetite and amphibole. Retrograde assemblages resulted from re-crystallization of the Co-bearing phases to form arsenopyrite II and III, along with precipitation of marcasite, pyrite, hastingsite, native Bi-Au (+/- Te) and minor bismuthinite and magnetite. The latest stage of retrograde mineralization comprises chalcopyrite, hastingsite, chlorite, and hematite (+/- emplectite). Two sets of barren quartz dolomite + amphibole + K-feldspar + chalcopyrite veins (S2 and S3) post-date the mineralization. The Southern Breccia zone hosts minor U-Cu-Mo mineralization and is interpreted to be the deepest portion of the NICO system. Two molybdenite samples from the Bowl Zone and the Southern Breccia yielded Re-Os ages of 1865 +/- 9 and 1877 +/- 8 Ma, respectively, consistent with the interpreted ca. 1870 Ma age of the NICO deposit. This age is also synchronous with the onset of magmatism in the Great Bear magmatic arc (ca. 1875-1850 Ma). Ore mineral delta(34)s values (3.3-6.7%0, sulfides) indicate that crustal sulfur was assimilated by upwelling felsic magmatic melts. The delta(80) values of the fluids precipitating magnetite and Co-rich arsenopyrite (6 and 8%.), and from pre- and syn-mineralization magnetite (-0.8 to 1.5%.) support a magmatic-hydrothermal origin of the fluids. Two out of three calcite samples from pre-(S1) and post-ore (53) veins have also delta C-13 values consistent with a magmatic origin (-5.5 to -3.6%0). However, one calcite sample from the S3 veins has a value that indicates a reduced sulfur source (-15.6%0). This value is similar to those of the much younger (<1843 Ma) giant quartz veins cutting the GBMZ rocks. The quartz 8180 values suggest that S1 (12.7%) formed at higher temperatures than S2 (13.2-19.14%0) and S3 (9.4-17.1%0), or that in the latter two generations of veins, O-18 was enriched during fluid/rock equilibration. Secondary trails of native Bi in S1 vein quartz are associated with liquid-vapor (LV) and liquid-vapor-halite (LVS) inclusions, which indicates that Bi, and possibly Au, were transported in saline to hyper-saline brines (LV-Bi, 2-16 wt.% NaC1 equiv., 8-22 wt.% CaCl2 equiv.; LVS-Bi, > 37 wt.% NaCl-equiv.), with homogenization temperatures of 137-216 degrees C and 192 to >350 degrees C for LV-Bi (ThL+V -> L) and LVS-Bi (ThL+s -> L), respectively. The presence of calcium-rich fluids might indicate extensive equilibration of those solutions with the host TLG rocks. If a pressure correction is applied to the LV inclusions using a minimum entrapment temperature of 271.4 degrees C (the Bi melting point), a minimum crystallization depth of between approximately 5 and 8 km is indicated. Trace element analyses carried out in this study and compiled from Acosta-Gongora et al. (2014) show that the least altered metasedimentary TLG rocks contain up to six times more As (Carbonate unit, 30.5 ppm) than the average upper continental crust. Conversely, concentrations of Au (<2 ppb), Co (10 ppm) and Cu (12 ppm) are lower than the crustal values. As such, it is possible that the TLG was a source of As, but is a less likely source of Au, Co and Cu for the NICO deposit; this further supports a magmatic-hydrothermal origin for the metals. Nonetheless, the plurikilometer alteration halo of the NICO system indicates that large amounts of elements could have been leached from the TLG, and potentially some were incorporated to the system. However, detailed studies on structural geology, geochemical modeling, and mass balance calculations need to be carried out to consider if such a scenario is feasible. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.