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TitleCharacterization and spatial distribution of the alteration facies for the mafic to ultramafic Potter Massive Sulphide Deposit, Abitibi Greenstone Belt, Ontario
AuthorPréfontaine, S; Gibson, H L; Houlé, M G; Mercier-Langevin, P
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Programs with Abstracts vol. 34, 2011 p. 177
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20130574
File formatpdf
Lat/Long WENS-82.0000 -80.0000 49.0000 48.0000
Subjectseconomic geology; stratigraphy; igneous rocks; volcanic rocks; mineral occurrences; volcanogenic deposits; mineral deposits; gold; copper; mineralization; sulphides; sulphide deposits; Archean; alteration; facies; mafic volcanic rocks; Potter Deposit; Precambrian; Proterozoic
ProgramTargeted Geoscience Initiative (TGI-4), Gold Ore Systems
AbstractThe Potter Cu-Zn-Co VMS deposit located in the eastern part of the (2720-2710 Ma) Kidd-Munro assemblage of the Abitibi greenstone belt, is somewhat unique among Archean VMS deposits, in that it is hosted in mafic volcaniclastic units that are enclosed entirely within a dominantly ultramafic succession. The mineralization consists of eleven stacked tabular lenses of semimassive to massive sulphides mostly composed of pyrrhotite, sphalerite and chalcopyrite. The hydrothermal footprint is essentially recorded within the volcaniclastic rocks where the matrix and juvenile fragments were preferentially replaced. The alteration typically occurs as irregular stratabound zones that surround the ore lenses. Three alteration facies were recognized based on the matrix mineralogy, from proximal to distal: 1) chlorite, 2) albite-calcite, and 3) chloriteclinopyroxene. The proximal chlorite facies is characterized by an increase in chlorite abundance and locally talc, both within the matrix and the fragments, and by the first appearance of Fe-rich chlorite. The albite-calcite facies is characterized by widespread replacement of the matrix by albite, calcite and minor Mg-rich chlorite and by a partial chloritization of the fragments. The distal chlorite-clinopyroxene facies, developed within the matrix, is composed of Mg-rich chlorite, clinopyroxene and albite, the clinopyroxene being a product of metamorphism. Fragments are not chloritized in this distal alteration facies. The high permeability of the volcaniclastic rocks allowed significant ingress of heated seawater-dominated fluids, which formed the distal and transitional Mg-rich chlorite, albite, and calcite alteration facies (facies 2-3). The proximal Fe-rich chlorite alteration (facies 1), associated with a maximum water/rock ratio and peak temperature, is developed only close to the sulphide lenses due to the lack of focused fluid pathways in the footwall (e.g. discrete synvolcanic faults). Despite deformation and a patchy, diffuse, and gradational contacts between the alteration facies, the original geometry of the hydrothermal system can be illustrated along a mineralized section: the proximal alteration is developed within 10m of the ore lenses, the near-proximal to distal facies is developed between 10m and 80m away from the ore lenses, and the distal facies is present at least 35m away from the ore lenses. This study shows that the stacking of ore lenses has resulted in a complex geometry of overlapping alteration facies. Moreover, synvolcanic dykes and sills were responsible, at least in part, for the distribution of the alteration by controlling fluid pathways. Therefore, the nature of the volcaniclastic rock and the geologic environment are the main factors controlling the primary distribution and mineralogy of the alteration.