Abstract

Most Ni-Cu-platinum-group element (PGE) deposits of the Sudbury Igneous Complex (1.85 Ga) have massive or disseminated magmatic-segregational characteristics. However, stringers and disseminations of chalcopyrite-rich, platinum-group mineral (PGM)-bearing, Fe-Ni-Cu sulfides at the contact of the Lindsley 4b orebody with the enclosing Murray Granite (2.4 Ga) contain quartz, K feldspar, biotite, epidote, stilpnomelane, amphibole, garnet, and other hydrous silicates, associated with PGM, sphalerite, galena, native Au, pyrite, and mackinawite. These assemblages in fracture-controlled veins and disseminations are compatible, with hydrothermal processes being responsible for the enriched grade near the contact. Fluid inclusion petrography and microthermometry reveal that hydrothermal activity took place in five stages. Early NaCl-CaCl 2 -H 2 O- type fluids were trapped in K feldspar of the metasomatically altered Murray Granite at temperatures of about 370 degrees to 410 degrees C and 270 degrees C. Later quartz intergrown with sulfides contains fluid inclusions with NaCl-KCl-CaCl 2 -H 2 O- type solutions trapped between 200 degrees to 380 degrees C and 130 degrees to 230 degrees C. SEM-EDS analyses of chlorides in inclusions reveal that these fluids were also enriched in Ba, Pb, Fe, and Mn. Later fluids trapped in secondary inclusions (200 degrees -240 degrees C) were CaCl 2 -(NaCl)-H 2 O type. Temperature calculations assume 2 kbars lithostatic pressure. All aqueous inclusions are characterized by high salinity (23-43 wt %). Low-temperature secondary inclusions contain CO 2 -CH 4 fluids. Carbonic fluids are related to metamorphism accompanying deformation during the Penokean orogeny. The higher temperature fluids were heated by the Sudbury Igneous Complex. The observations support the hypothesis that very saline hot fluids interacted with igneous minerals and primary magmatic ores and remobilized base and precious metals, and precipitated them along permeable, fractured, and brecciated zones to produce Cu-PGE-Au concentration, disseminated sulfides, and veins.

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