| Author | Acosta-Góngora, P; Gleeson, S A; Samson, I M; Corriveau, L ; Ootes, L; Taylor, B E; Creaser, R A; Muehlenbachs, K |
| Abstract | The NICO deposit is located in the Paleoproterozoic Great Bear magmatic zone, Northwest Territories, Canada, and contains significant Co-Au-Bi mineralization. The majority of the NICO mineralization is
in the Bowl zone, hosted by the Middle unit of the Treasure Lake Group (TLG), where intensely amphibole-magnetite altered precursor rocks are interpreted to be carbonate-rich wackes and siltstones. The pervasively altered rocks are crosscut by a set
of pre-ore quartz ± calcite-amphibole-K-feldspar veins (S1). The mineralization at NICO 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, hematite (± emplectite, wittichenite). Two sets of barren quartz and quartz ±
dolomite-amphibole-K-feldspar-chalcopyrite veins (S2 and S3) post-date the mineralization. Another mineralized zone, which is interpreted as an extension of the NICO system, is the Southern Breccia zone, a 3 km long by 0.5 km wide brecciated and
albitized corridor that hosts minor U-Cu-Mo mineralization, and occurs 0.5 km SE of the Bowl Zone. Two molybdenite samples from the Bowl Zone and the Southern Breccia zones yielded Re-Os ages of 1865 ± 9 and 1877 ± 8 Ma, consistent with the
interpreted ca. 1870 Ma age of the NICO deposit.
Ore mineral sulfur isotope values (d34S) range from -3 to 11¿, with a mode at 5¿. Host TLG whole-rock d34S values range from -1 to 6¿. Magnetite from pre-ore alteration and magnetite that was
co-precipitated with early sulfarsenides in veins have oxygen isotope values (d18O) that range from -0.4 to 2¿ and -0.9 to 2¿, respectively. Pre- and post-ore quartz and calcite from the S1, S2 and S3 veins have d18O values from 9 to 17¿. The
hydrothermal fluids that precipitated the magnetite-arsenopyrite veins have calculated d18O values of 6 to 9¿, consistent with magmatic-derived waters. Similarly, vein calcite has d18O and d13C values consistent with derivation from a magmatic
source. The S in the NICO system could be magmatic, but a non-magmatic component derived from the host metasedimentary rocks cannot be ruled out.
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, was transported in saline to hyper-saline brines (LV-Bi, 2-16 wt.% NaCl equiv., 8-22 wt.% CaCl2 equiv.; LVS-Bi, >37 wt.% NaCl equiv.), with homogenization temperatures
(Th) of 137 to >350°C. If a pressure correction is applied to the LV inclusions using a minimum entrapment temperature of 271.4°C (the Bi melting point), a crystallization depth of between approximately 5 and 8 km is indicated.
The least altered
TLG lithologies contain low concentrations of Au (< 2 ppb), Co (10 ppm) and Cu (14 ppm), which are lower than the average upper continental crust, and it is therefore unlikely that the TLG was the source of these elements in the NICO deposit; this
further supports a magmatic origin for the metals. However, the Carbonate unit of the TLG contains significant amounts of S (1300 ppm) and As (25 ppm) and it is possible that this unit, now completely metasomatised, contributed some of these
elements; this is consistent with the S isotope results.
We consider the NICO deposit to have magmatic-hydrothermal origin, related to emplacement of 1870 Ma intermediate to felsic plutons at depths of > 4 km. These intrusions are interpreted to
have been the main sources of Au, Co, and Cu. Other important ore forming elements, such as As and S, as well as the Ca in the Bi-bearing brines, could have had mixed metasedimentary and hydrothermal magmatic fluid sources. |