In the Little Rocky Mountains, porphyry intrudes Pre-Cambrian metamorphic rocks and overlying sedimentary rocks, with large roof pendants of Pre-Cambrian rocks projecting downward into the porphyry mass. A series of very high-angle faults radiate from the central part of the uplift. Hydrothermal solutions rising along these faults have deposited gold and other minerals, giving rise to epithermal deposits.The ore deposits, which are worked primarily for gold, are mostly of low grade. In the ore zones, quartz is the most abundant mineral although pyrite, the only sulphide noted, is common. Fluorite is locally common close to the faults. Some relatively high grade ore consists of a mixture of quartz, pyrite, fluorite, gold and sylvanite.

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