Abstract

An isolated 20-m-long, quartz latite porphyry dike with unusual sulfide-bearing vesicles is described. The dike occurs within the Bingham porphyry copper orebody and contains xenoliths of previously veined, altered, fractured, and mineralized monzonite. The dike is massive, highly porous but impermeable. It contains 0.5 percent Cu as chalcopyrite in vesicles which range in size from microscopic to 15 cm in diameter. The dike is zoned, with the brown, highly vesicular core zone and a late aplitic phase containing more K 2 O, MgO, Cu, S, and porosity and less Fe 2 O 3 , CaO, and Na 2 O than the green rock which forms the border of the dike. The dike is composed of quartz, altered plagioclase, biotite, and very large orthoclase phenocrysts in a groundmass of orthoclase, quartz, biotite, and vesicles. Crystallization of the groundmass is believed to have produced a strong permeability contrast with the highly fractured monzonite host rocks and resulted in a closed chemical system in the center of the dike. The proportions of minerals and void space in the vesicles and fluid inclusion data are used to calculate the composition of the fluid which filled the vesicles. The composition is:CuFeS 2 9% by weightSiO 2 8NaCl 21KCl 11(Na, K)F 11H 2 O 40These constituents comprised about 15 percent of the magma just before the groundmass froze. Fluid inclusion studies indicate the quartz in the vesicles crystallized from a boiling fluid at 480 degrees C. Other authors calculate the confining pressure to be less than 800 bars. The magma which formed this dike was probably subaluminous, rich in halogens, alkalis, and water and probably remained magmatic in its behavior to temperatures well below 600 degrees C. The consistency of these conclusions with published experimental results is discussed.

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