Variolitic basalts: Relations to Archean epigenetic gold deposits in the Abitibi Greenstone Belt.

Abstract

Variolites are mafic to intermediate volcanic rocks containing centimetre-sized, spherical domains, termed varioles. Varioles are dominantly composed of plagioclase spherulites. Variolitic volcanic rocks host ore in many epigenetic Au deposits within Archean greenstone belts. The geochemistry and alteration of variolitic rocks were studied within the Archean Abitibi Greenstone Belt in Harker Township, Ontario and in the Dome Mine area, Timmins, Ontario. The study demonstrates that there is a connection between the anomalously high Fe/Mg ratio of the variolites and disseminated sulphide-Au mineralization. Variolites occur dominantly in the upper, more evolved parts of Fe tholeiitic volcanic sequences. They commonly have a more differentiated composition than typical Fe tholeiitic MORB rocks. They have elevated concentrations of incompatible elements, silica, iron, titanium and phosphorous, and lower than expected concentrations of compatible elements such as vanadium and magnesium. In addition, their Fe/Mg ratio is anomalously high, commonly greater than 3.0. Varioles tend to be more concentrated in intermediate-acidic flows, which are characterized by flow banding, extensive development of hyaloclastite, and brittle fracture. Disequilibrium crystal habits, including plagioclase spherulites, branching amphiboles (after pyroxene), and dendritic oxides, are common in variolites and are related to diffusion limited growth conditions. These conditions were likely caused, in part, by the relatively silica-rich nature of the lavas and undercooling. The differentiation of the variolitic suites is interpreted to be due to fractional crystallization. The compositional range of the suite is similar to other strongly differentiated Archean tholeiitic rocks, such as the Golden Mile Dolerite in Kalgoorlie, Australia. The variolitic suites are also similar to modern evolved, oceanic suites developed in areas of thicker oceanic crust. The variolitic suites are interpreted to result from injection of tholeiitic magma along faults to high levels in the crust where lower temperatures and pressures promoted rapid and extensive differentiation. Alteration studies of variolites associated with Au mineralization revealed that the alteration mineralogy is partly related to the host rock composition. The upper, more evolved flows of the variolitic suites tend to stabilize a complex mineral assemblage, including albite, Fe-Ti oxides, and pyrite whereas the lesser evolved flows stabilize a simpler assemblage, generally dominated by carbonate minerals. In both areas, mineralized zones have significant addition of CO2, S, and Au and depletion of H2O. Hydrothermal alteration in the Harker Lake area is characterized by oxidation of the host rock with addition of Na2O and Sr and depletion of Zn, MgO, MnO and, to a minor extent, HREE, whereas alteration at the Dome Mine is characterized by reduction of the host rock, addition of K2O, Ba, CaO, B, and LREE, and depletion of Na2O, Sr, and HREE. These results reflect differences between the two areas in the size and intensity of the mineralizing events, the influence of structural styles, and the local rock types. In variolites, Au is deposited by destabilization of the Au-bisulphide complex largely due to removal of sulphur from solution by reaction with iron in the host rock to form pyrite. Bohlke (1988) has demonstrated that the Fe/Mg ratio of the host rock plays a key role in determining whether Fe-Mg(-Ca) carbonates or pyrite will be formed in the alteration zone. Pyrite will tend to form in host rocks which have a high Fe/Mg ratio. This effect is enhanced when the auriferous hydrothermal fluids have previously equilibrated with high magnesian rocks. The anomalously high Fe/Mg ratio of variolitic rocks (> 2.0) makes them ideal chemical traps for sulphide-Au mineralization. Additionally, their tendency for brittle fracture enables the fluids to affect a greater volume of rock, enhancing the potential for mineralization. Variolites may be useful in the exploration for epigenetic Au deposits. As a consequence of their composition, variolites have excellent potential to host disseminated sulphide-Au mineralization in association with shear zones or faults, which have provided the pathways for auriferous fluids.