GEOCHEMICAL PROCESSES CONTROLLING THE RELATIVE ABUNDANCE OF COPPER, ZINC, AND CADMIUM IN WEATHERING PRODUCTS OF VOLCANOGENIC MASSIVE SULFIDE DEPOSITS

The fate and transport of copper (Cu), zinc (Zn), and cadmium (Cd) during weathering of sulfide-bearing deposits is of importance because these metals are potentially toxic to aquatic life in areas affected by acid mine drainage (AMD). A variety of geochemical processes affect the release of these metals during weathering of primary sulfide minerals [most commonly chalcopyrite, CuFeS₂, and sphalerite, (Zn,Fe,Cd)S] and their sequestration in secondary minerals including efflorescent sulfate salts, hydrous Fe oxides, and metal sulfides. Studies of AMD from volcanogenic massive sulfide deposits in California (CA) provide the basis for a comparison of geochemical processes affecting Cu, Zn, and Cd and their distribution in weathering products. At Iron Mountain, soluble Fe-Cu-Zn sulfate salts that form during dry periods are flushed during wet periods in both surface exposures and underground mine workings. Experimental studies using mine water at or near saturation with melanterite [(Fe,Zn,Cu,Mg)SO₄.7H₂O] showed that Cu partitions preferentially into the solid relative to Zn, resulting in higher values of Zn/Cu in residual AMD solutions during dry periods. Several kilometers downstream of Iron Mountain, where AMD is neutralized by mixing with dilute water in the Spring Creek Arm of Keswick Reservoir, deposits of hydrous Fe oxide up to 7 m thick have accumulated. Copper is preferentially sorbed to hydrous Fe oxides relative to Zn, resulting in relatively high values of Zn/Cu in pH ~6 pore water relative to the original AMD (pH 2 to 5). At the Penn Mine, microbial sulfate reduction (SR) in flooded underground mine workings causes formation of H₂S that combines with dissolved metals to form secondary sulfide minerals. The relative solubility of secondary sulfide minerals in these conditions (CuS < CdS < ZnS) causes values of Zn/Cu and Zn/Cd to become elevated in the corresponding AMD relative to other AMD at Penn Mine that is unaffected by SR. The use of AMD trace-element chemistry to infer geochemical processes must be done with caution because multiple processes can result in similar changes, e.g. elevated Zn/Cu in residual solutions. However, such inferences and associated geochemical modeling can benefit from determination of secondary mineral chemistry and knowledge of conditions of formation.