Abstract

The Precambrian complex of the Gold Butte-Bonelli Peak area is about 25 by 35 km and consists of gray porphyritic perthite-quartz-biotite granites and quartz monzonites (adamellites) that intrude Precambrian garnet-cordierite-sillimanite and hornblende gneisses, migmatites, and older granites, pyroxenites, and hornblendites. These younger granites strikingly resemble the Finnish rapakivi granites geologically, petrographically, and chemically. They are exposed over an area approximately 20 by 30 km and represent the roof of a batholith with numerous stocks, dikes, roof pendants, inclusions, and plutonic breccias. They have sharp contacts with their host rocks and have affected these rocks only slightly. No orientation of feldspar phenocrysts can be observed in most of the granite body. The crystallization of this granite has apparently been undisturbed by orogenic movements. It is a postkinematic disharmonious granite.

Potassium feldspar and quartz started to crystallize first in this granite. The main constituents are perthite, quartz, oligoclase-andesine, albite, and biotite. Accessory minerals are sphene, hornblende, zircon, apatite, allanite, and fluorite. Typical rapakivi texture with some ovoid perthite phenocrysts surrounded by oligoclase-albite mantles occurs in many places but seems to be more common near the contacts. Quartz appears in two generations, the first of which is commonly subhedral to euhedral (hexagonal bipyramids).

Small irregularly shaped miarolitic and mineralogically complex pegmatite bodies occur in the immediate vicinity of the contacts, partly in the granite and partly in the metamorphic rocks. They contain much allanite; some contain concentrations of samarskite, columbite, zircon, and other rare minerals. They commonly show well-developed zoning. No larger pegmatites associated with the granites could be observed. Gold-bearing fluorite quartz veins are numerous.

These granites weather in some areas into a sharp-edged coarse gravel, best described by the Finnish term moro. The feldspar phenocrysts and other mineral grains are partly separated by this weathering process, more so than during the weathering of regular granites. It is remarkable that the moro develops in this dry, hot desert climate in the same way as in the relatively humid and cold climate of Finland.

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