CHLORINE ENRICHMENT AND HYDROUS ALTERATION OF THE SUDBURY BRECCIA HOSTING FOOTWALL Cu Ni PGE MINERALIZATION AT THE FRASER MINE, SUDBURY, ONTARIO, CANADA

Abstract

An assemblage of hydrous, Cl-bearing alteration-induced minerals is present within 150 m of footwall-style Cu–Ni–PGE sulfide mineralization hosted by the Sudbury Breccia at the Fraser Copper zone, Fraser mine, Onaping–Levack area, Ontario. The pyroxene hornfels to hornblende hornfels contact-metamorphic aureole of the Sudbury Igneous Complex (SIC) contains less than 15 modal % hydroxysilicates. Near the mineralized areas, a diagnostic assemblage of secondary minerals of the albite–epidote hornfels facies is present in the matrix of the Sudbury Breccia; it contains 25 to 80% actinolite + chamosite as well as epidot e + sodic plagioclase (An2.8–42) + quartz ± titaniferous magnetite ± biotite ± K-feldspar ± titanite. The Cl content and Cl/(Cl + F) values of amphibole and biotite in this matrix are elevated within 150 m of mineralization, and increase toward known occurrences of sulfides. Parameters of K, Na, and A-site occupancy correlate with Cl content in actinolite in such matrix material. The value of Mg# is lower in biotite in the matrix near mineralized areas and is inversely correlated with Cl/(Cl + F) in the bioti te, reflecting Mg–Cl or Fe–F avoidance. Calculated temperatures of amphibole–plagioclase equilibration for the diagnostic hydrous assemblage are between 442 to 540 ± 75°C. The assemblage formed during a retrograde metamorphic event during or after footwall Cu–Ni–PGE mineralization, and after passage of peak conditions of contact metamorphism at the base of the SIC. The enrichment of Cl in this assemblage was the result of the increased amounts of free Cl relative to F in the equilibrating fluid , and favorable cation proportions for incorporation of Cl into the structure of hydroxysilicate minerals. The Cl-bearing assemblage accounts for a significant portion of the bulk halogen content of Sudbury Breccia matrix. It is unique to zones of Sudbury Brec cia that host mineralization, and hence may be used to identify prospective areas.