Metamorphosed Hydrothermal Alterations of the Auriferous Lalor VMS Deposit and Relations with Ore Styles

English Abstract:

The Paleoproterozoic Lalor auriferous volcanogenic massive sulphide deposit in Snow Lake, Manitoba, is hosted in a complex volcanic succession referred to as the Lalor volcanic succession. The deposit consists of stratigraphically and structurally stacked Zn-rich, Au-rich and Cu-Au-rich ore lenses. The volcanic rocks that host the deposit were affected by intense and laterally extensive ore-related synvolcanic hydrothermal alteration. These altered rocks were subsequently subjected to syn-deformation amphibolite-grade metamorphism that resulted in the development of distinct minerals and metamorphic mineral assemblages of varying composition in response to variably altered precursors. Five distinct alteration- and metasomatism-related chemical associations (K, K-Mg-Fe, Mg-Fe, Mg-Ca and Ca) were defined based on the mineralogy (mineral assemblages) and geochemistry of the altered zones. Mapping of the host volcanic rocks and ore-related mineral assemblages and chemical associations at Lalor indicate that: 1) the bulk geochemistry of the ore-related alterations has largely been preserved despite metamorphic processes, and the primary alteration styles can be inferred, 2) the Zn-rich massive sulphide lenses are preferentially associated with the low- to high-temperature K and Mg-Ca alteration zones, 3) the Zn-rich massive sulphide lenses formed at two distinct stratigraphic positions as a result of protracted seafloor/sub-seafloor hydrothermal activity, 4) the Cu-Au-rich zones at depth are associated with transposed discordant high-temperature Mg-Fe altered rocks and presumably represent footwall feeders, and 5) the precious metals were in part locally remobilized into low-strain sites during the main deformation event and consequently are not currently spatially associated with one chemical association in particular.