Metallization and post-mineral hypogene argillization, Lost River tin mine, Alaska

Abstract

The Lost River Sn and W deposit occurs in a buried granite pluton and in associated rhyolite dikes that intrude Paleozoic limestone. The dikes and parts of the granite were greisenized and then argillized irregularly. Metallization accompanied greisenization rather than argillization, although both processes probably were closely related in time. Fe-Zn ratios in sphalerite indicate that the ore minerals were deposited at a temperature between 425 degrees C. and 740 degrees C. This temperature is within the range of the temperatures at which topaz, a common associate of the ore minerals, has been synthesized in the laboratory. The temperature of deposition of the ore minerals is above the temperature interval in which clay minerals are stable. Thus, clay minerals could not have formed while ore was being deposited. As temperatures fell and entered the stability range of the clay minerals, argillic alteration encroached upon greisen ores and wall rocks. Reaction rims between quartz and topaz indicate that kaolinite could have formed according to the following reaction: topaz + quartz + water + limestone --> kaolinite + fluorite + carbonic acid. The clay minerals, which formed from diverse rock types, consist of kaolinite, dickite, mixed-layered chlorite-montmorillonite, and minor montmorillonite, accompanied by variable amounts of muscovite and zinnwaldite. The dickite is most common in and near late veins that cut the earlier-formed greisen. Temperature is believed to be the principal agent governing the relations between ore deposition and argillization. Similar relations are to be expected in other high-temperature deposits where abundant veining and fracturing indicate that wall rocks reached an isothermal condition above a maximum temperature of 480 degrees C. during ore deposition.