ENVIRONMENTAL SIGNIFICANCE OF CADMIUM AND OTHER TRACE-ELEMENT CONCENTRATIONS IN SPHALERITE FROM MINERAL DEPOSITS

Weathering of mineral deposits and their wastes results in the release of acid and metals into the environment. Sources of metals such as Fe, Cu, Pb, and Zn are major sulfide ore minerals (for example, pyrite, pyrrhotite, chalcopyrite, galena, sphalerite), which have occurrences that vary according to deposit type. Environmentally significant metals such as Cd, Hg, and Mn do not commonly form their own sulfide minerals, and instead, occur as solid solution in sphalerite. Cadmium and Mn concentrations of natural sphalerite are known to reach 5 wt. %; Hg concentrations are known to reach 0.5 wt. %. This study examines sphalerite trace-element concentrations using electron probe microanalysis. The spectrum of sphalerite-bearing deposit types includes massive sulfide deposits [Kuroko-(Mineral District, VA), Besshi- (Ely, VT), Noranda- (Bald Mountain, ME), and Sedimentary exhalative- (Rajasthan, India; Sullivan, BC) types], Mississippi Valley-type deposits (Central Tennessee), and actively forming seafloor massive sulfides.

Preliminary results show that the Fe concentrations range from 0.1 to 13 wt. %, Cd concentrations range from 0.1 to 0.8 wt. %, and Mn concentrations reach 0.2 wt. %. Some sphalerites may contain detectable amounts of Co, Hg, and Ni. Data indicate that sphalerite from massive sulfide deposits has higher Fe concentrations (up to 13 wt. %) than from Mississippi Valley-type deposits (< 1 wt. %); sphalerites from Sedimentary-exhalative and Besshi-type deposits tend to have higher Cd concentrations than those from Kuroko-type deposits. The Ely sample (Besshi-type) has the highest analyzed Mn concentrations.

Potentially leachable trace elements are present in sphalerites in significant quantities. Also, elevated Fe concentrations in some sphalerites enhance acid generation. Drainage chemistry, particularly in terms of relative proportions of Zn, Cd, and Mn, should reflect sphalerite trace-element chemistry in acidic systems where the main source of these elements is sphalerite. In alkaline systems, such as waters draining Mississippi Valley-type deposits, drainage chemistry may not reflect mineral chemistry due to precipitation of secondary hydroxides and related sorption reactions. Future efforts will compare the sphalerite and drainage chemistries for a wider range of deposit types.