Publication Type:Book Chapter
Source:Targeted Geoscience Initiative 4: Contributions to the understanding of Precambrian lode gold deposits and implications for exploration, Geological Survey of Canada, Volume Open File 7852, p.157-175 (2015)
Pyrite efficiently incorporates many key metals during progressive precipitation and thus records the chemical evolution of fluids from which it was deposited. To reveal this information, a new LA-ICP-MS mapping procedure has been developed to allow generation of 2-D trace element concentration maps of minerals in petrographic section. The technique has been applied to pyrite-/pyrrhotite-bearing Au ores from three major gold deposits.<br/>Canadian Malartic is a low-grade bulk-tonnage deposit, which is located immediately south of the Larder Lake-Cadillac Fault Zone. It is hosted mainly by clastic metasedimentary rocks of the Pontiac Group, as well as by porphyritic quartz monzodiorite and granodiorite. Textural evidence and elemental mapping have revealed five types of pyrite. The pre-mineralization pyrite (py1) is likely diagenetic pyrite, with high Co, As, and Se, and low Ni, Te, Sb, Bi, and Pb. Gold-bearing pyrite 2, 3 and 4 (py2-py4) has covariant Co and Ni, high Au, Ag, Te, Bi, and Pb, and generally contains abundant potassic inclusions. Postmineralization pyrite (py5) has high Co and Ni but is low in other metals. Pyrite from the ore zones distributed along the Sladen fault zone shows evidence of post-precipitation metal enrichment in fractures associated with Ca metasomatism. Though the elemental maps cannot unequivocally discriminate the deposit type, pyrite chemistry is consistent with a two-stage model comprising early syn-pyrite Au mineralization associated with potassium alteration and a later post-pyrite upgrading associated with Ca metasomatism. The Côté Gold deposit is hosted by the Chester intrusive complex, a high-level, multi-phase synvolcanic intrusion composed of tonalite and diorite. Nickel, As, Sb, and Pb are generally relatively depleted in the cores and enriched in the rims of the pyrite grains, whereas Te and Ag are relatively enriched in the cores and depleted in the rims. Arsenopyrite grains have high Co, Ni, Se, Sb, and Te, but low Ag, Pb, and Bi. Gold is depleted both in pyrite and arsenopyrite, except locally in fractures, but is enriched in silicate minerals, possibly as nanoparticles, suggesting that gold mineralization is related to a hydrothermal event post-dating pyrite precipitation.<br/>The Musselwhite deposit is a banded iron formation-hosted lode gold deposit in a sequence of volcanic and sedimentary rocks that include carbonaceous argillite locally hosting diagenetic pyrite nodules. Cobalt, Cu, Ni, Se, As, Ag, Sb, Te, Au, Tl, Pb, and Bi are enriched in the nodules but are depleted in recrystallized pyrite and pyrrhotite. Element and Pb-isotope maps are consistent with a model whereby at least some of the ore gold was derived from fluids that garnered Au liberated via metamorphic recrystallization of auriferous diagenetic pyrite and, probably, carbonaceous material in local argillite. The elemental maps show that gold has multiple sources. The Musselwhite deposit provides evidence for local pyritic carbonaceous sedimentary rocks as one of the possible sources of Au. The Canadian Malartic and Côté deposits have a strong association with magmatic hydrothermal activity, although additional sources of Au may also have been important.