Characterization of metamorphic assemblages and assessment of Cu-Pb-Ag-Au-Zn mobility at the Lalor deposit, Snow Lake, Manitoba

Abstract

The Lalor VMS deposit is located within the Snow Lake arc assemblage at the easternmost end of the Paleoproterozoic Flin Flon belt. A regional metamorphic event at 1.81 Ga, up to middle amphibolite facies (550˚C, 5kbar), recrystallized the mineral assemblages in the massive sulfide and hydrothermally altered rocks that are associated with the formation of VMS deposits in the area. The Lalor deposit differs from most VMS deposits in the area in that it contains low-sulfide Au rich zones that are proximal to but separate from the massive sulfide lenses. Gold mineralization primarily occurs in three rock types at the Lalor deposit: massive sulfides, and calc-silicates to carbonate silicates, and Fe-Mg altered rocks. Electrum is the dominant form of gold, but gold also occurs in sulfosalt phases such as aurostibite (AuSb2) and in tellurides such as petzite (Ag3AuTe2). It is often associated with hessite (Ag2Te), altaite (PbTe), chalcopyrite (CuFeS2), and galena (PbS). Gold mineralization occurs along fractures, grain boundaries, cleavage planes, and as discrete inclusions in metamorphic minerals. There are two dominant metal associations with gold: Cu-Au and Pb-Au. The Cu-Au association is more common and occurs in a variety of rock types (including massive sulfides and Fe-Mg altered rocks), whereas the Pb-Au association is restricted to calc-silicate to carbonate silicate altered rocks. In the massive sulfides, gold content and distribution are a function of primary VMS zone refining processes; however, Au has been locally remobilized during metamorphism and deformation. In the Fe-Mg altered rocks proximal to massive sulfide, the distribution and tenor of Au reflects primary zone refining and local remobilization due to metamorphism and deformation, whereas in Fe-Mg altered rocks distal to massive sulfide, gold distribution is largely a product of pre-peak to peak metamorphic remobilization of primary VMS gold via fluid-dominated transportation. In the calc-silicate to carbonate silicate rocks, which contained or contain carbonate, Au distribution reflects metamorphic remobilization. Metamorphic devolatilization of primary carbonate bearing rocks is responsible for adding components such as H2O, CO2 and S2 into a fluid phase that remobilized Au and some metals (e.g. Cu, Pb) in the deposit, and in altered rocks located proximal and distal to the massive sulfide lenses. Gold was mobilized and transported as a sulfur complex, with CO2 acting as a buffer for a low salinity fluid phase such that it could maintain an elevated gold content for transport and deposition. The result of this mobilization is responsible for the Pb-Au in calc-silicate to carbonate silicate altered rocks