Porphyry Cu-Mo Stockwork Formation by Dynamic, Transient Hydrothermal Pulses: Mineralogic Insights from the Deposit at Butte, Montana

摘要

The systematic distribution of vein and alteration mineral assemblages in porphyry Cu deposits largely arises from changes in the temperature and pressure of fluids that traversed fractures throughout the hydrothermal system. Magmatic and hydrothermal minerals record the complex history of the fluctuating temperature and pressure regime as hydrothermal fluids transfer heat from their magmatic source to cold wall rock in response to lithostatic-to-hydrostatic pressure variations. We examine the thermal profile of the porphyry Cu-Mo deposit in Butte, Montana, by determining formation temperatures for magmatic and hydrothermal samples representing different time frames and depths within the deposit, in context with sample pressure estimates. We use three independent mineral thermobarometers: Ti in quartz, Zr in rutile, and Xnig-Ti in biotite, from which we estimate that final dike injection temperature, and hence the initial magmatic-hydrothermal fluid temperature, was ~700°C while the ambient host-rock temperature was -450° to 500°C. We find a magmatic-hydrothermal continuum represented in hydrothermal veins, ranging from -710° to <440°C. Distinct mineral generations within vein samples consistently display large temperature ranges, spanning 50° to 250°C, capturing the transient thermal condition of the ascending aqueous fluids. Mineral precipitation temperatures within veins show the same range as those in accompanying envelopes, indicating at least partly contemporaneous formation of veins and envelopes. Hydrothermal veins of a single type show no systematic relationship between temperature and depth within the deposit, although different vein types show systematic temperature ranges as a function of depth. We observe anomalous crosscutting relationships indicating that porphyry vein formation tempera-tures fluctuated significantly within a single cubic centimeter parcel of rock from one vein-forming episode to another. We suggest that the thermal profile does not mimic domical isograds based on alteration mineral zones, but rather it mimics an irregular pattern following active fractures at any given time and evolves by discrete cycles of dynamic, transitory, high-temperature hydrofracturing, fluid release, and vein formation that overprints cooler host-rock temperatures.