VEIN SEQUENCE CHRONICLES CUPOLA GROWTH AND COLLAPSE: THE BUTTE EXAMPLE

The sequence of veins in the Butte, Montana porphyry copper deposit exemplify a pattern that applies similarly to magmatic-hydrothermal deposits globally. The sequence (with Butte names and typical temperatures), comprises deep early quartz-chalcopyrite-pyrite veins with biotitic alteration (EDM; 650̊C) and their distal variants, then quartz-molybdenite (550̊C), followed by pyrite-quartz with gray sericitic envelopes (//GS; 500̊C), and concluding with large, through-going, polymetallic fissure veins (Main Stage; 350̊C). Beneath the Butte vein complex lies a massive zone of now-compact breccia of re-broken quartz veins mixed with granite and quartz porphyry that are intensely altered to sericite, pyrite and quartz and local topaz. The breccia body is likely the upper part of a collapsed hydrothermal cupola where dilution of rock Ti and Zr by quartz show that quartz fills at least 40 wt% of once-open space, and mottled SEM-CL quartz texture reveals pervasive strain. Cupola vein textures show that much of the exposed collapse coincides with and follows quartz-molybdenite vein time.

Previous studies show that EDM and quartz-molybdenite veins formed in hydrofractures emanating from a lithostatically pressured cupola above the quartz porphyry (QP) magma body. It appears that partial cupola foundering formed a domical zone of pyrite-quartz//GS veins at initially hydrostatic pressures, and final collapse of the cupola extruded initially highly acidic fluids that form Main Stage veins. Independent evidence (Reed, et al., 2013) shows that a magmatic fluid becomes highly acidic as it cools and makes all veins, so it is probable that the pyrite-quartz//GS and Main Stage fluids emerged as the once-porous cupola foundered, first at ~500̊C and then further at ~350̊C, as it weakened owing to acid attack of the structurally supporting granite and QP blocks. The global occurrence of the general vein sequence exemplified by Butte, with numerous variations in details of vein composition, quantity, and temperature, can be understood in terms of the generality the process of cupola formation and eventual collapse. Depending on the volumes of collapse at various times, the quantity of fluid expelled in any given “stage” varies from one system to another, yielding variations in the quantities of veins of the various stages.