MINERALOGICAL AND GEOCHEMICAL CONTROLS ON THE RELEASE OF TRACE ELEMENTS FROM SLAG PRODUCED BY BASE- AND PRECIOUS-METAL SMELTING AT ABANDONED MINE SITES

Slag, a common mine waste, was collected from the Ely and Elizabeth mines in the Vermont copper belt, from the Copper Basin mining district at Ducktown, Tennessee and from the Clayton silver mine in the Bayhorse mining district of Idaho to assess its reactivity and potential to release toxic trace elements into the environment.

Trace metals in slag from Vermont and Tennessee are dominated by Cu (1,600 to 14,000 mg/kg) and Zn (1,800 to 8,100 mg/kg), whereas the Clayton mine slag is dominated by Zn (16,000 mg/kg), Cu (8,400 mg/kg), Pb (3,400 mg/kg), As (580 mg/kg), Sn (260 mg/kg), and Ag (180 mg/kg).  On the basis of mineralogy, the slags can be divided into types: (1) fayalite-rich slag from Cu-smelting are dominated by fayalite (50 to 85 vol. %), Fe-rich spinels (5 to 25 vol. %) containing Al, Ti, Zn and traces of Cr, and disseminated Fe and Fe-Cu sulfides (< 5 vol. %) within a glassy matrix (10 to 100 vol. %) with up to 0.7 wt. % ZnO and 0.2 wt. % CuO,  (2) slag from the Clayton mine is dominated by kirschsteinite, CaFeSiO₄, (40 to 50 vol. %) and pyroxene (40 to 50 vol. %) with Fe-rich spinels (5 to 10 vol. %) containing Al, Ti, and Zn.  Also present are complex metallic blebs, containing Pb, Sb, Fe, As, Cu, and Zn metal, and Fe-Zn and Fe-Cu sulfides (< 5 vol. %),  (3) pellet slag from Ducktown consists almost entirely of glass containing up to 1.6 wt. % ZnO and 0.5 wt. % CuO, with minor disseminated Fe and Fe-Cu sulfides (< 5 vol. %).

Twenty-four hour leach studies (solution to sample mass ratio of 20:1) were conducted on <2 mm splits using deionized water and a solution that approximates eastern U.S. precipitation (pH=4.2 ± 0.1).  Leachates from most slags were found to contain elevated concentrations of Cu and Zn (up to 2,000 mg/L and 420 mg/L, respectively), well in excess of the acute toxicity guidelines for aquatic life.  For the Idaho slag, the concentration of Pb in the leachate (11,000 mg/L) also was in excess of the acute toxicity guideline. Geochemical modeling of the leachate chemistry suggests that leachates from the Vermont and Tennessee slags are saturated with amorphous silica and Al hydroxide whereas leachates from the Idaho slag are saturated with amorphous silica, several Pb phases and possibly Cu oxides.  The combined petrographic and leachate studies suggest that potentially toxic levels of metals can be leached from slags under laboratory conditions.