The Hoyle Pond gold deposit is hosted in complexly deformed mafic-ultramafic volcanic rocks of the Hersey Lake and Central Formations (Tisdale assemblage) in the Porcupine gold camp, located approximately 15 km northeast of Timmins in the Abitibi greenstone belt. The deposit is hosted within a homoclinal sequence of south-facing stacked volcanic flows of high Mg tholeiitic basalt, basaltic komatiite and komatiite flows, and interbedded high Mg tholeiitic basalt and Fe-rich tholeiitic basalts. The bulk of the gold mineralization was emplaced at lithologic contacts along late shear zones associated with isoclinal folding and thrusting. The mineralization is characterized by micron- to centimeter-sized flakes of free gold or veinlets in quartz-carbonate (dolomite and ferroan dolomite) shear and extension vein arrays. At the mine scale a broad carbonate alteration is evident. Two alteration zones surrounding the veins were mapped at the meter scale: an inner sericite alteration zone composed of sericite (muscovite), fuchsite (Cr muscovite), quartz, arsenopyrite, pyrite, ferroan-dolomite, dolomite, and graphite plus tourmaline, and an outer zone of albite alteration consisting of albite, quartz, ferroan dolomite, and dolomite.
Geochemical analyses of 355 samples show that the REE, Zr, Al2O3, TiO2, and Y were largely immobile during alteration and mineralization. CO2, K2O, Na2O, Cr2O3, Rb, As, B, SiO2, and CaO, and locally Fe2O3, FeO, and MgO, were mobile during alteration and mineralization. The chromium enrichment is not primary and is interpreted to have been caused by remobilization from ultramafic rocks during mineralization. In addition, an intense graphite alteration, originally derived from organic matter, probably from sedimentary rocks, and now associated with mineralization, is present in zones that were porous and permeable at the time of mineralization. In common with other mesozonal orogenic gold deposits, gold was likely transported as a thio complex. The Cr enrichment in the wall rock indicates that Cr was mobile, most likely as Cr6+. Because species such as Cr6+ are transported under oxidizing conditions and carbon and Au-HS species are transported under reduced conditions, we suggest that more than one fluid was involved in the mineralization and/or alteration. Mixing between reducing and oxidizing fluids is thought to have reduced Cr6+ to Cr3+, oxidized the organic matter to form graphite, oxidized sulfur to form sulfides, and precipitated Au. This is consistent with the observed para-genesis. Reducing fluids rich in boron, arsenic, carbon, and Au may have been generated from sedimentary rocks (Porcupine assemblage?) at depth and expelled during orogenesis into syndeformation structures within overlying volcanic rocks where they mixed with oxidizing fluids containing Cr, K2O, SiO2, Na2O, and CaO.