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Geochemistry of magnetite and the genesis of magnetite-apatite lodes in the iron mask batholith, British Columbia Cann, Robert Michael


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.

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