A geochemical and fluid inclusion study of the arsenopyrite-stibnite-gold mineralization, Moreton's Harbour, Notre Dame Bay, Newfoundland

Kay, Elizabeth Alexandra (1981) A geochemical and fluid inclusion study of the arsenopyrite-stibnite-gold mineralization, Moreton's Harbour, Notre Dame Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Mineralized hydrothermal veins of the Moreton's Harbour area, occurring within a thick sequence of dominantly basaltic pillow lavas and pyroclastic rocks, are concentrated within a central volcaniclastic unit and intimately associated with felsic dykes. There are about fifty veins, up to 30 cm thick, occupying fractures perpendicular to the bedding. They can be broadly classified into three types, viz. I arsenopyrite-dominated, II stibnite-dominated, and III base metal + arsenopyrite-dominated, all with quartz and calcite as the major gangue minerals. -- Type I veins are Au-rich, type III are Au + Ag-rich, and type II are Au-poor but slightly enriched in Pd. Fluid inclusion data indicate deposition of Au-rich type I veins from CO₂-rich low salinity fluids above 300ﾟC, whereas the type II Au-poor veins were deposited from relatively saline low-CO₂ fluids at temperatures below 220ﾟC. Both fluid inclusion and arsenopyrite composition data suggest pressures of 900 to 1500 bars, in agreement with lithostatic pressure indicated by the overlying volcanic pile. -- The Au mineralization is considered to have resulted from temperature decrease through 300ﾟC, below which the stability of Au-complexes declines abruptly. Carbon dioxide abundance suggests the involvement of carbonate complexes, with retrograde boiling resulting in loss of CO₂ and drop in carbonate activity, brecciation of arsenopyrite and deposition of calcite. A range of criteria suggest that the fluid was derived from felsic magma which produced the dykes and pyroclastic rocks, and hence was penecontemporaneous with the felsic volcanic activity. Concentration of veins within the central pyroclastic unit results from the fact that it occurs at a depth within the volcanic pile with suitable P-T-X conditions for deposition.

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