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The contact metasomatic magnetite deposits of southwestern British Columbia Sangster, Donald Frederick

Abstract

Ore zones, skarn, host rocks, and associated intrusions of 12 magnetite deposits were studied in both field and laboratory to determine their mineralogical and geochemical characteristics, origin of the iron, and factors controlling emplacement of iron-bearing minerals. This study seeks a better understanding of the origin and mode of occurrence of contact metasomatic magnetite deposits which in turn may provide better guides to their exploration and evaluation. Local folds and faults are important factors in the explacement of magnetite in volcanic rocks and limestone of the Vancouver group. Adjacent stocks are of intermediate composition. Post-ore leucodiorite dykes are common in many orebodies . The author proposes that the process by which skarn is formed be called skarnification i.e. the replacement by, conversion into, or introduction of skarn. The term would include all processes by which skarn may be formed such as contact metamorphism, contact metasomatism, or regional metamorphism. Skarn in the coastal British Columbia region is composed mainly of garnet (andradite-grossularite), pyroxene (diopside-hedenbergite), epidote, and magnetite. Conformity to Gibbs Phase Rule and the non-appearance of incompatible phases is strong evidence that equilibrium was attained during skarnification. Magnetite is the major metallic mineral, but chalcopyrite, pyrite, pyrrhotite, and arsenopyrite are locally abundant. The temperature of intrusion is estimated to be in the range 800-900°C and stability relations of coexisting minerals indicate a temperature of 700- 550°C daring skarnification. The pyrite-pyrrhotite geothermometer applied to eight specimens shows that ore deposition, took place within the temperature range 400-550°C. The composition of arsenopyrite coexisting with pyrite and pyrrbotite in one orebody indicates a confining pressure of 2600± 1,000 bars during ore formation. The immediate source of iron in these deposits is believed to be nearby intrusions. The ultimate source, however, is very probably underlying volcanic rocks which have been assimilated, in part, by an advancing pluton. Iron is considered to have been derived from plutons adjacent to the orebodies and to have been carried to the sites of deposition as aqueous supercritical solutions of iron chloride. Magnetite was precipitated from the ore-forming fluid by an increase in pH brought about by reaction with limestone. Changes in the chemical and physical nature of the ore-forming fluid during ore deposition are discussed in terms of temperature, density, pH, partial pressures of oxygen and sulphur, and composition. Hydrothermal processes operative in formation of the deposits were solvate opposition, metasomatism, and cavity filling

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