SW MONTANA TALC DEPOSITS: GROWTH ENHANCEMENT BY CRACK-SEALING PROCESSES IN BASEMENT CARBONATES UNDER THE PROTEROZOIC BELT BASIN?

Talc (Mg₃Si₄O₁₀(OH)₂) deposits in Southwest Montana are hosted in calcic to dolomitic Precambrian marbles. The minerals and textures in the marbles reflect a wide range of pressures, temperatures, and volatile components from diverse environments during Proterozoic tectonism. Talc formation within the carbonates appears to have occurred during younger multi-stage mineralization events, with some economic-sized amounts (≥ 25,000 short tons) in locations where conditions were especially conducive to talc growth.

Because hydrothermal talc formation reaction kinetics are slow, a process that increases talc production efficiency by ≥2 orders of magnitude is needed to generate copious talc and to prepare the host rocks for textures and characteristics developed in later stages (e.g. botryoidal talc in vug zones at mines). Hydraulic fracturing of marble reservoirs is possible where high fluid overpressure approaches lithostatic pressure such that marble tensile strength is repetitively reached (Secor, 1965). Swarm-type microruptures might enable hot (~190 – 250 °C) brines to constantly contact fresh reactants within carbonates via crack-seal mechanisms (cf. Cox, 2016). Constant volumetric strain-adjustments facilitated by low friction materials (e.g. talc, graphite, chlorite) may initiate cascades of porosity shifts that permit fluid-mobile products to exit and precondition unreacted carbonate or calc-silicates. Talcification likely started in the most ‘receptive’ layers of carbonate (e.g. composition, permeability, structure); talc pods, blebs, and/or stringers formed in adjacent horizons that had (or developed) access to fluids as tectonic activity and dissolution progressed. This dynamic system could have been long-lived as episodic slip on a local master fault rejuvenated the hydraulic fracturing.

A greenschist facies hydrothermal system may have arisen in response to accumulation of overlying Missoula Group equivalent sediment (Mesoproterozoic Belt Basin) to provide the necessary P/T/fluid conditions. Assuming the subsiding Lemhi subbasin extended into this portion of the Dillon Block, stable isotopes (δ¹⁸O and δD values) in selected talc samples are permissive of high fluid:rock ratios and circulating basinal fluids.