Abstract

A variety of evidence suggests that the fluids forming stratiform and red-bed copper deposits derived their Cu, Ag, and other metals from adjacent red sandstones and shales. Copper and Ag are soluble in moderately concentrated chloride brines at intermediate oxidation states, but published data suggest that Cu is strongly adsorbed on goethite and hematite at pH near 7 and temperatures of 0 degrees to 100 degrees C. Also, Pb, Zn, Co, and Ni are soluble in chloride brines, but are only present in some red-bed Cu deposits. To examine variability in relative adsorption of 0.5 mg/l Ag, Co, Cu, Ni, Pb, and Zn, adsorption on goethite (195 m 2 /l) has been measured as a function of pH (5-9), Cl (super -) (0, 1M, 4M,), temperature (25 degrees C, 50 degrees C), and redox state (air, Fe (super +2) -goethite). Under oxidized conditions at 25 degrees C and 1M Cl, the order of decreasing adsorption is Cu-Pb-Zn-Co-Ni-Ag, with Cu and Pb almost completely adsorbed at pH values greater than 6. However, in 1M Cl at 25 degrees C with Eh buffered by Fe (super +2) -goethite, the order of decreasing adsorption is Pb-Zn-Co-Ni-Cu(-Ag?), and less than 50 percent of the Cu is adsorbed at pH 7. An exploratory experiment with 3M NaCl + 1M Na 2 SO 4 at 25 degrees C with about 4,000 m 2 /g of hematite as adsorbent and 75 mg/l Cu, Ag, Zn, Pb, and Co and 40 mg/l Fe (super +2) was conducted to approximate conditions in a red-bed pore fluid during diagenesis. An appreciable fraction of the Cu and Ag remained in solution at pH 7, but Pb, Zn, and Co were completely adsorbed. Based on this data, the observed range of metal associations (Cu, Cu-Ag, Cu-Co, Cu-Zn-Ag, etc.) in red-bed and stratiform copper deposits seems explainable by variations among districts in pH, Eh, temperature, major element content of pore fluid, and Fe oxide character of the diagenetic environment. Similar adsorption phenomena may account for varying metal ratios in other low-temperature ore deposits and in noneconomic metal enrichments.

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