THE BUTTE HYDROTHERMAL SYSTEM: ONE MAGMATIC FLUID YIELDED ALL VEIN TYPES

The Butte (Montana) porphyry Cu-Mo system consists of two kilometer-scale mineralization “domes” that straddle a quartz porphyry dike swarm. Mineralization is zoned in widely overlapping shells of centimeter-scale, stockwork veinlets, from an interior of quartz (qz) and qz-molybdenite veins lacking alteration or having thin K-feldspar alteration envelopes, outward to qz-sulfide veins bordered by biotite (bi), andalusite, plagioclase, K-feldspar (kf) and sericite (ser), then outward to assemblages of ser-kf-chlorite, then outermost assemblages of kf-ser-chl-epidote+/-bi or of qz-py veins with intense ser-py alteration. A third mineral center of pervasive, intense qz-ser-pyrite alteration on py-qz veins that cut those listed above, lies between the domes, and extends to great depth. H and O isotopic analyses of ser and bi show that magmatic fluids produced alteration in all three centers.

Geochemical calculations (program CHILLER) of hydrothermal reactions in the Butte system at 100 MPa and 200° to 600°C show that a single initial fluid composition reacted with the Butte host granite is capable of producing the entire sequence of alteration mineral assemblages as temperature decreases from near-magmatic to 200°C, and that zoned alteration assemblages on single veins reflect decreasing effective water/rock ratio from the fracture outward. The single-fluid concept is supported by fluid inclusion analyses showing that fluids of similar bulk composition (~4 wt% NaCl equiv and 5 mol% CO₂) dominate both early quartz veins and later py-qz veins. Qz veins with thin or no alteration envelopes are inferred to form where pressure-drop precipitated quartz, sealing the fracture before significant wall rock reaction could proceed. Other vein types are inferred to form from cooling (and unmxing into vapor and brine) of this fluid combined with wall rock reaction.

The well established progression of porphyry Cu-Mo alteration from early, high-T potassic assemblages to later, moderate-T sericitic is inferred to be an inevitable consequence of staged release of stored fluid that cools through time becoming more acidic, as opposed to sequential release of a magmatically evolved fluid. A variation on such stored fluid possibly also yields latest veins of Cu, As, Zn, and Pb sulfides.