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 / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Magnetite-apatite lodes, in the Upper Triassic Iron Mask batholith, south-central British Columbia, are tabular bodies up to 200 m long and 6 m wide which consist of 50 to 90 percent magnetite, 10 to 40 percent apatite and variable amounts of amphibole. Lodes occur in close spatial association with alkaline "porphyry-type" copper mineralization, disseminated-magnetite rich diorite and late syenitic units. To aid in determining the genesis of these lodes 84 samples of lode magnetite and disseminated magnetite from dioritic, syenitic and picritic units of the batholith were analyzed by atomic absorption spectrophotometry for: chromium, cobalt, copper, lead, magnesium, manganese, nickel, titanium, vanadium and zinc, fourteen samples were also analyzed for major and minor oxides by electron microprobe. Minor element data indicates a magmatic-injection origin for the lodes; magnetite being concentrated by immiscibility between magnetite-apatite and an alkalic magma. With the exception of copper and lead, minor element variations in magnetite due to sampling error and analytical variations are insignificant relative to between and within rock unit variations, as revealed by analysis of variance. Disseminated magnetite from picrite has high and distinctive contents of chromium, magnesium, nickel and zinc relative to disseminated magnetite in syenite and diorite. Minor element concentrations in disseminated magnetite from syenite and diorite are very similar statistically. Lode magnetite, compared to disseminated magnetite from diorite and syenite, is markedly lower in chromium and less so in titanium and vanadium, however other elements occur in statistically similar concentrations.. Minor element concentrations in magnetite from Iron Mask lodes are statistically the same as those in magnetite from magmatic iron deposits in Kiruna, Sweden and Missouri, U.S.A. Magnetite from hydrothermal vein and metasomatic deposits has lower chromium and nickel contents than Iron Mask lode magnetite. Geochemical evidence presented here suggests that Iron Mask lodes are: 1) genetically related to the Cherry Creek syenite and Pothook diorite units of the Iron Mask batholith, and 2) magmatically emplaced based on analogy to Kiruna and Missouri ores. Experimental documentation of immiscibility between a magnetite-apatite melt and a silicate magma allows a model to be developed that describes the genesis of the Iron Mask batholith. and associated magnetite-apatite lodes. Crystal settling of plagioclase and pyroxene from the Iron Mask magma fromed the early. Pothook diorite and enriched the residual magma in iron and alkaliis. The magma also differentiated toward the experimentally determined magnetite-apatite eutectic composition-(i.e. 20 to 35 weight percent apatite in total magnetite plus apatite). When the eutectic was reached after crystallization of Pothgok diorite, (just before the Cherry Creek syenite started to crystallize) magnetite and apatite separated together from the silicate magma as an immiscible melt, and settled to the base of the magma chamber. The magnetite-apatite melt was injected into fractures to form lodes after the surrounding Cherry Creek magma had largely crystallized. Explosive emplacement of Cherry Creek breccias and associated copper mineralization resulted from a~vapor bubble formed in the final stages of. Cherry Greek syenite crystallization. The model presented shows that magnetite-apatite lodes in the Iron Mask batholith are magmatic-injection in origin. Their genesis, as well as associated porphyry-type copper mineralization, is an integral part of a differentiating alkalic intrusion. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Unknown