Anhydrite-pyrite-magnetite-pyroxene-type deposits in volcanic basins of a Mesozoic continent, Yangtze River Valley, China

Anhydrite-pyrite-magnetite-pyroxene type deposits occur in volcanic basins of the Mesozoic continental crust, Yangtze River valley, China. These types of mineral deposits consist of large magnetite ore bodies, pyrite ores, and an anhydrite layer. Thedeposits have mainly two types of alteration zoning, melanocratic alteration and leucocratic alteration. Magnetite ores occur in association with melanocratic alteration. Pyrite ores and the anhydrite layer occur in leucocratic altered rocks. Investigations indicate that isopleths of the major-and trace-element compositions in minerals are all parallel to the boundaries of alteration zones. The spatial distribution of various mineral solid solutions and polymorphs has been documented in the alteration profile, from the lower level upward stratigraphically and from the inner part outward (proximal to distal). These compositional variations indicate the presence of a temperature and a concentration gradient in the ore-forming hydrothermal system during ore formation. The temperature for separating two sets of ores and alteration zones was at 300–350‰C, near the critical point of water. Intersecting and crossing the H

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O critical curve changes the property of hydrothermal fluids resulting of precipitation of magnetite ores, which is coincident with the transition from melanocratic to leucocratic alteration types.