Abstract

Epithermal gold-silver deposits in the Toodoggone River area are hosted by Early Jurassic high K trachyandesite and trachydacite volcanic rocks that erupted during two major cycles spanning roughly 7 m.y. Volcanism and related plutonic activity is intimately associated with the structural development of an elongate, volcano-tectonic depression in which extensional fracture zones confine the epithermal deposits. Younger sedimentary rocks overlay and protected the deposits from erosion.Gold and silver in the Toodoggone district occur most commonly in quartz veins associated with adularia, sericite, and calcite. Locally gold barite veins cut volcanic rocks that are pervasively replaced by varying proportions of fine-grained silica, dickite, and natroalunite.Fluid inclusions in quartz from most precious metal vein deposits are typical of epithermal environments. They contain low-temperature, dilute hydrothermal fluids of predominantly meteoric origin (T h < 200 degrees -300 degrees C and approximately 3 equiv wt % NaCl). Some deposits, notably at the Baker mine, exhibit overprinting by higher temperature, high-salinity, magmatically derived fluids (T h > 300 degrees C and >30 equiv wt % NaCl).Light stable isotope analyses show marked 18 O enrichment in the volcanic rocks, hydrothermally altered host rocks, and mineralized vein quartz (with adularia or sericite) of about 8 per mil. The isotopic composition of the hydrothermal fluid is calculated to be about delta 18 O = 0 per mil. These hydrothermal fluids are isotopically evolved mixtures derived from meteoric and magmatic waters with possible metamorphic input. Whole-rock analyses of mainly fine grained quartz-dickite mineral mixtures document the largest 18 O shifts with values up to 15.7 per mil. This is interpreted to be due to lower temperatures and/or the presence of significant quantities of magmatic fluids in these altered zones.K-Ar age determinations on alunite from several zones of intensely acid-leached rocks suggest that an early mineralizing episode apparently occurred around 190 Ma or earlier during a depositional hiatus between the two major volcanic cycles. This coincides with the uplift and cooling of several plutons. K-Ar dates on adularia and sericite from precious metal-bearing quartz veins, some of which are from volcanic rocks of the upper cycle, suggest that there was also a younger mineralizing event. Recent argon step-heating dates indicate that the major gold-silver deposits with adularia-sericite range from 189.7 + or - 2.6 to 186.7 + or - 1.7 Ma (Clark and Williams-Jones, 1991). This closely follows the latest stages of Toodoggone volcanism and fault block subsidence.A depth zoning model for mineralization illustrates the two types of epithermal regimes: adularia-sericite and acid sulfate. Gold barite in the extensive, advanced argillic and argillicly altered zones at the Al deposit represent high-level, near-surface acid sulfate alteration formed on the flanks of a stratovolcano. The advanced argillic altered zone at Silver Pond, next to the major Lawyers adularia-sericite deposit, represents a downfaulted acid sulfate outflow (i.e., the upper portion of the Lawyers hydrothermal system). The Lawyers deposit and most of the other occurrences are adularia-sericite-type deposits formed at slight depth. These deposits occur in grabens or half-grabens in an extensional fault regime that created the central Toodoggone depression, nested within the broader volcanic belt.

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