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 ( approximately 50% by area of volcanic terranes) and are found within volcanic sequences of at least three distinct affinities. Group I, which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only approximately 10 percent by area of volcanic terranes, is composed of bimodal, tholeiitic basalt-basaltic andesite, and high silica rhyolite. The basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element (REE) contents, low light to heavy REE ratios (most with La N /Yb N = 0.8-3), and strong negative Eu anomalies. The Kamiskotia, Matagami, and Chibougamau (Lower cycle) volcanogenic massive sulfide areas, all of which are also underlain by large, synvolcanic gabbroic complexes, are associated with group I volcanic sequences. The Kidd Creek, Potter, Normetal, and Horne deposits are also included in this category. Group II, which is host to one-third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percent by area of volcanic terranes, is composed of bimodal, transitional tholeiitic to calc-alkalic andesitc and rhyolite, characterized by intermediate high field strength element contents and slightly higher REE ratios (La N /Yb N = 1-4: Noranda camp (excluding the Horne deposit) and Val d'Or camp). Group III is host to only one known deposit, the Selbaie mine, which is unusual in that much of its mineralization cuts stratigraphy. The Selbaie mine sequence contains calc-alkalic andesite-rhyolite with relatively low high field strength element and REE contents, and higher REE ratios (La N /Yb N = 3-9). The vast majority of volcanogenic massive sulfide-bearing mafic and felsic volcanic rocks in the Abitibi subprovince have La N /Yb N < 5.Barren volcanic sequences are group IV--calc-alkalic basaltic andesite to rhyodacite, with low high field strength element and relatively high REE ratios (La N /Yb N = 8-20), represented by the upper Skead Group, the Quebec Hunter Mine Group, and the Upper cycle Chibougamau rocks; and group V, mafic to felsic alkalic volcanic rocks, with high REE ratios (La N /Yb N = 12-62), represented by the Timiskaming, Opemisca, and Ridout series rocks.Group I is most similar to thickened oceanic rift suites (e.g., Galapagos spreading center, Iceland East rift), group II is similar to suites in rifled island arcs (e.g., Hokuroku district, Japan), and group III is comparable to continental arc suites (e.g., Southern Volcanic zone, central Chile). Groups IV and V are comparable to arc-related suites derived from metasomatized mantle, with variable amounts of crustal contamination (e.g., Setouchi area, Japan; Roman province, Italy).Known Ni-Cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills, represented by chill compositions with high MgO contents (20-35 wt %, anhydrous), very low incompatible element contents, and depleted signatures (La N /Sm N = 0.5-0.8; Zr/Y < 2.5; Alexo, Marbridge, and Shaw Dome deposits), in comparison to most barren komatiites in the Abitibi subprovince. Their depleted trace element signatures indicate minimal crustal assimilation, which would appear contrary to the widely held view that crustal contamination (e.g., addition of silica, sulfur, oxygen) of a mantle-derived melt is necessary for sulfur saturation and magmatic sulfide segregation. This problem may be reconciled by considering the dynamics of komatiite flows (Lesher and Arndt, 1990) where contaminated* Present address: 23 Euclid Avenue, Ottawa, Ontario, Canada KiS 2W2. komatiite liquid segregated and deposited sulfide and was subsequently swept downstream and replenished by uncontaminated magma over the deposit as the flow ebbed and cooled. 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.