Geochemistry of volcanic rocks associated with Cu-Zn and Ni-Cu deposits in the Abitibi Subprovince

Volcanogenic massive sulfide (VHMS) deposits in the Abitibi subprovince are preferentially associated with volcanic successions containing >150 m thicknesses of felsic volcanic rocks and are found within volcanic sequences of at least three distinct affinities. Group I is composed of bimodal, tholeiitic basalt-basaltic andesite, and high silica rhyolite with high high field strength element and heavy rare earth element (REE) contents, low light to heavy REE ratios, and strong negative Eu anomalies. Group II is composed of bimodal, transitional tholeiitic to calc-alkalic andesite and rhyolite, characterized by intermediate high field strength element contents and slightly higher REE ratios. Group III contains calc-alkalic andesite-rhyolite with relatively low high field strength element and REE contents, and higher REE ratios. Group I is most similar to thickened oceanic rift suites, group II is similar to suites in rifted island arcs and group III is comparable to continental arc suites. Known Ni-Cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills. Their depleted trace element signatures indicate minimal crustal assimilation. When considered together with geologic features such as evidence for thermal erosion in the footwall (Langmuir deposits) and the presence of sulfidic footwall rocks (Shaw Dome deposits), the distinctive, depleted geochemical signature of mineralized komatiites is useful in exploration for Ni-Cu deposits. -from Authors