Molybdenite occurs in the aplitic border phase of a Paleozoic batholith of granite and alaskite, which intrudes Cambrian quartzites and Ordovician (?) volcanics. In the Ackley City orebody, molybdenite is disseminated in altered aplite; a honeycomb of alteration was formed by the mineralizing solutions that penetrated the aplite without marked structural control. Silicification preceded muscovitization in space, but the two processes may be in part contemporaneous.The mineralization is believed to be closely related genetically to the miarolitic aplite phase in which it occurs. A hypothesis is presented for the origin of the aplite, and its close relationship to the mineralization. The latter is believed to be due either to acid fluids--gases, or hydrothermal condensates, or to a combination of acid gases and alkaline liquids. Graton's hypothesis that ore-forming fluids are alkaline liquids fails to explain the relationships. The main period of mineralization is high-temperature with intermediate-temperature late phases. The Ackley City orebody is classified as a "complex aplite."The Wylie Hill deposit is of intermediate-temperature and the Crow Cliff deposit is a pegmatitic type.

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