BULK COMPOSITIONAL CONTROLS ON MINERAL ASSEMBLAGES IN METAMORPHOSED ORE DEPOSITS: AN EXAMPLE FROM THE LARONDE-PENNA AU-RICH VHMS DEPOSIT, QUEBEC, CANADA
Current work is targeting the LaRonde-Penna Au-rich massive sulfide deposit, which occurs in the late Archean Abitibi greenstone belt, Quebec, Canada. The deposit is hosted in felsic metavolcanic rocks (predominantly rhyodacite to rhyolite), which are now intensely deformed into a steeply-dipping homoclinal sequence, and is interpreted to have been metamorphosed at ~400-500ºC and at pressures <9 kbars. The now-metamorphosed altered footwall to the deposit shows mineralogical zoning with proximity to the deposit, and also along strike. The focus here is the footwall transition in the “20North (20N) lens” (massive chalcopyrite, galena, sphalerite, and pyrite), which hosts garnet-bearing assemblages that are absent in weakly- and un-altered rocks. The abundance and grain-size of Mn-bearing garnet increases with proximity to the ore lens, and the distribution of garnet has been linked to elevated Mn in the alteration halo.
P-T and T-X pseudosections generated in the phase equilibria modeling software THERMOCALC are being used to characterize bulk compositional controls on the appearance of key assemblages in the deposit footwall. In addition, data are being used to robustly relate assemblage domains in metamorphosed deposits to alteration halo compositions from unmetamorphosed deposits with the ultimate goal of developing predictive tools for mineral exploration of similar deposit types. Outcomes from this research are also being used place tighter P-T constraints on the evolution of the eastern Abitibi greenstone belt