Hydrothermal andalusite and corundum in the Elkhorn District, Montana

Andalusite and corundum occur in the core of a porphyry Cu-Mo system centred on a quartz-feldspar porphyry stock on the east edge of the Boulder batholith. The quartz-feldspar porphyry is intruded by a post-mineral(?) feldspar porphyry and is cross-cut by a major post-mineral fault. Andalusite occurs as islands surrounded by muscovite in plagioclase phenocrysts in K-feldspathized quartz-feldspar porphyry north of the fault. South of the fault, andalusite occurs as subhedral laths, or as granules, in intensely sericitized and silicified feldspar porphyry. Andalusite also occurs in metasediments and in the quartz-feldspar porphyry of the metamorphic aureole of the Boulder batholith. Corundum occurs as anhedra surrounded by fine-grained muscovite in altered quartz-feldspar porphyry north of the fault and in intensely quartz-veined quartz-feldspar porphyry south of the fault. Andalusite and corundum are considered to be hydrothermal in origin because of their localization in intensely altered rock, their textural relations to secondary feldspars and micas, and by analogy with comparable occurrences at Butte, Montana, and El Salvador, Chile. Possible mechanisms of formation are explored with the aid of phase diagrams.