Porphyry Cu-Mo stockwork formation by dynamic, transient hydrothermal pulses; mineralogic insights from the deposit at Butte, Montana

• Published in: Economic geology and the bulletin of the Society of Economic Geologists Vol. 108; no. 6; pp. 1347 - 1377
• Main Authors: Mercer, Celestine N, Reed, Mark H
• Format: Journal Article
• Language: English
• Published: Economic Geology Publishing Company 01-09-2013
• Subjects: alteration; biotite; Butte mining district; Butte Montana; cathodoluminescence; chemical composition; cooling; copper ores; diffusion; Economic geology; electron probe data; framework silicates; host rocks; metal ores; metasomatism; mica group; mineral assemblages; mineral composition; mineral deposits, genesis; mineralization; molybdenum ores; Montana; ore-forming fluids; oxides; P-T conditions; physicochemical properties; porphyry copper; porphyry molybdenum; quartz; recrystallization; rutile; SEM data; sheet silicates; silica minerals; silicates; Silver Bow County Montana; stockwork deposits; temperature; textures; trace elements; United States; veins
• ISSN: 0361-0128; 1554-0774
• DOI: 10.2113/econgeo.108.6.1347

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 XMg-Ti in biotite, from which we estimate that final dike injection temperature, and hence the initial magmatic-hydrothermal fluid temperature, was approximately 700°C while the ambient host-rock temperature was approximately 450° to 500°C. We find a magmatic-hydrothermal continuum represented in hydrothermal veins, ranging from approximately 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 temperatures 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.