The mobilization of selected trace metals from shales by aqueous solutions; effects of temperature and ionic strength

Abstract

Removal of trace metals from shales by subsurface brines is often suggested as an origin of base metals in low-temperature, strata-bound ore deposits. This investigation studied the hypothesis that brine leaching of shales can increase metal content of subsurface brines sufficiently that the brines could potentially serve as ore-forming fluids. Two aspects were studied: the effect on mobilization of changes in the brine temperature (25 degrees , 50 degrees , and 90 degrees C) and of changes in the ionic strength (0, 0.7, 2.0, and 4.0). The results of this study support the hypothesis. The following observations were made: (1) Increasing either the temperature or ionic strength of the brine increased the amount of leaching, but the trends were neither simple nor linear; a brine with an ionic strength of 10 at 120 degrees C might not be any more effective in leaching trace metals than a brine with an ionic strength of 5 at 90 degrees C. (2) Changing the salt composition of the brine changed the amount and type of mobilization. Calcium, for example, was found to be effective in leaching Pb and Zn, while K was found to be effective in mobilizing Cu.