MULTIPHASE MAGMATISM IN THE BISMARK STOCK; RELATION TO ZINC SKARN AND PORPHYRY ZINC ALTERATION IN THE BISMARK MINE, CHIHUAHUA, MEXICO

Field observation, petrographic description, and XRF whole rock analysis allow for the discrimination of three texturally distinct igneous phases, with minor variants, in the Bismark stock: 1) flow-banded rhyolite, 2) seriate-equigranular granitoid, and 3) porphyrytic granitoid. The granitoids can be subdivided based on whole rock chemistry, modal mineralogy, and texture.

The main phase of the stock is type 2A, which is an equigranular granite-alkali granite and contains 0.1-6 mm crystals of orthoclase, plagioclase, quartz, biotite, amphibole and accessory zircon, titanite and monazite. Type 2B is an equigranular marginal phase with similar mineralogy, though contains somewhat less plagioclase.

The porphyrytic phases have 20-60% phenocrysts that are 1-6 mm in length, crystalline groundmass (>0.1 mm), and range in composition from diorite to alkali granite. These appear similar in hand sample, however in thin section differences are apparent. Porphyries – type 3A and 3C have phenocrystic orthoclase, plagioclase and quartz ± mafic minerals and orthoclase + quartz groundmass. Porphyry – type 3B is less silicic than the other igneous rocks at Bismark, dominated by orthoclase in both the groundmass and as phenocrysts.

The multiphase nature of the Bismark stock is likely responsible for the various pulses of green garnet-dominated skarn and massive sulfide ore mined at Bismark. Multiphase skarn development and overprinting of fracture controlled alteration styles, including the sphalerite-mineralized “porphyry zinc” style, further support the origin of the Bismark skarn as having been produced by multiphase magmatism, and not, as previously thought, by a single igneous intrusion.