Abstract

The approximate compositions of closed basin saline waters that existed in probable source rocks of stratiform copper deposits hosted by low-energy sediments were derived by the simulated evaporation of dilute inflow waters from basalt and granite provenances. The saline water derived from dilute inflow water in the basalt provenance had a pH of 7.6 and was saturated with talc, dolomite, and gypsum. The saline water derived from dilute inflow water in the granite provenance had a pH of 7.7 and was saturated with quartz, calcite, talc, and gypsum. Both waters were 3 molal in chloride and were also saturated with hematite, fluorite, and fluorapatite. The waters probably represent likely end members in the compositional spectrum of metal-transporting waters which existed in the source rocks of the stratiform copper deposits. Copper and cobalt exhibit solubility maxima of 1,500 ppm or greater and 150 ppm, respectively, at oxygen fugacities in the range 10 (super -35) to 10 (super -55) bars. Lead solubility is similar in the two waters and is a maximum of 20 ppm at oxygen fugacities greater than 10 (super -60) bars. Zinc solubility is a maximum of 80 ppm in the basalt-derived saline water, and 20 ppm in the granite-derived saline water. Silver solubility is a maximum of 200 ppm at oxygen fugacities above 10 (super -35) bars. Barium and iron solubilities are less than 5 ppm. Abundances of copper, silver, and in some instances cobalt, in all common source rocks result in concentrations of these metals in the saline waters less than their saturation limits. Concentrations of barium, iron, lead, manganese, and zinc in the saline waters are limited by water composition rather than by abundance in source rocks. The high copper, cobalt, lead, and zinc solubilities are consistent with the hypothesis (Haynes, 1986a) that most sulfides in stratiform copper deposits hosted by low-energy clastic sediments are deposited within 50 cm of the sediment-water interface during bacterial sulfate reduction. Porosity and permeability constraints imposed by uncompacted analogs of ore-hosting sediments show that the copper contents of the metal-transporting waters must be greater than 100 ppm for formation of ore which grades 1 percent or more copper. The modeling demonstrates that the proposed waters are potentially effective metal transporters capable of forming stratiform copper ore when the prevailing oxygen fugacity is within the range 10 (super -35) to 10 (super -55) bars. The saline waters are capable of transporting at least 100 ppm copper, and therefore, are likely to produce copper ore under the porosity and permeability constraints imposed by the hypothesis.

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