GEOCHEMISTRY OF VOLCANIC-ROCKS ASSOCIATED WITH CU-ZN AND NI-CU DEPOSITS IN THE ABITIBI SUBPROVINCE

Volcanogenic massive sulfide (VHMS) deposits in the Abitibi subprovinc e 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 le ast 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 eart h 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, M atagami, and Chibougamau (Lower cycle) volcanogenic massive sulfide ar eas, all of which are also underlain by large, synvolcanic gabbroic co mplexes, are associated with group I volcanic sequences. The Kidd Cree k, Potter, Normetal, and Horne deposits are also included in this cate gory. Group II, which is host to one-third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percen t by area of volcanic terranes, is composed of bimodal, transitional t holeiitic to calc-alkalic andesite and rhyolite, characterized by inte rmediate 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 S elbaie 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 cont ents, and higher REE ratios (La(N)/Yb(N) = 3-9). The vast majority of volcanogenic massive sulfide-bearing mafic and felsic volcanic rocks i n the Abitibi subprovince have La(N)/Yb(N) < 5. Barren volcanic sequen ces are group IV-calc-alkalic basaltic andesite to rhyodacite, with lo w high field strength element and relatively high REE ratios (Ls(N)/Yb (N) = 8-20), represented by the upper Skead Group, the Quebec Hunter M ine Group, and the Upper cycle Chibougamau rocks; and group V, mafic t o felsic alkalic volcanic rocks, with high REE ratios (La(N)/Yb(N) = 1 2-62), represented by the Timiskaming, Opemisca, and Ridout series roc ks. Group I is most similar to thickened oceanic rift suites (e.g., Ga lapagos spreading center, Iceland East rift), group II is similar to s uites in rifted island arcs (e.g., Hokuroku district, Japan), and grou p III is comparable to continental arc suites (e.g., Southern Volcanic zone, central Chile). Groups IV and V are comparable to arc-related s uites derived from metasomatized mantle, with variable amounts of crus tal 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 compositio ns with high MgO contents (20-35 wt %, anhydrous), very low incompatib le element contents, and depleted signatures (La(N)/Sm(N) = 0.5-0.8; Z r/Y < 2.5; Alexo, Marbridge, and Shaw Dome deposits), in comparison to most barren komatiites in the Abitibi subprovince. Their depleted tra ce element signatures indicate minimal crustal assimilation, which wou ld 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. Thi s problem may be reconciled by considering the dynamics of komatiite f lows (Lesher and Arndt, 1990) where contaminated komatiite liquid segr egated and deposited sulfide and was subsequently swept down-stream an d replenished by uncontaminated magma over the deposit as the flow ebb ed 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 dist inctive, depleted geochemical signature of mineralized komatiites is u seful in exploration for Ni-Cu deposits.