Two simple constraints: that the deposits formed within 1 km of the surface, and that the temperature of the ore fluids at the time of mineral deposition was 100 degrees to 150 degrees C. For base metal deposits to form as the result of the compactive expulsion of basin brines, brine expulsion must be episodic. Episodic pulses of geopressured brines moving through the near-surface sites of ore deposition, followed by much longer periods of exposure to cool surface waters, could produce the color banding of sphalerite, the cycles of sulfide precipitation and dissolution, and the distinctive local tectonic features associated with mississippi valley-type deposits. Basin features favoring the right kind of episodic dewatering are identified.--Modified journal abstract.

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