A COMPARATIVE MINERALOGICAL AND GEOCHEMICAL STUDY OF SULFIDE MINE TAILINGS AT 2 SITES IN NEW-MEXICO, USA

A comparative study of sulfide mine tailings from two sites near Silve r City in southwest New Mexico has shown the need for environmental mo nitoring in a geological context. The Cyprus-Pinos Altos and Cleveland deposits consist of Cu and Zn skarn mineralization in the Pinos Altos Mountains of New Mexico. Primary ore minerals in both deposits includ e chalcopyrite, sphalerite, and galena. The Cyprus-Pinos Altos Mine ce ased operation in 1995 and the Cleveland Mill closed in 1950. The depo sits have similar mineralogical characteristics; however, the tailings are different in terms of age, degree of oxidation and method of disp osal. The Cyprus-Pinos Altos tailings (CPAT) are stored in a lined, be rmed impoundment. They are dominantly water-saturated and exhibit no s econdary-phase formation. The grains are not cemented and show no evid ence of primary-mineral dissolution. The geochemical data show a predo minantly primary signature. The tailings pond water is neutral to slig htly alkaline (pH from 7 to 8.3), partly as a result of processing met hods. The Cleveland mill tailings (CMT) were deposited in a valley at the headwaters of an ephemeral stream. They are highly oxidized and di fferentially cemented. They have undergone numerous wet/dry cycles res ulting in extensive oxidation. Secondary minerals predominate, and con sist mainly of jarosite, goethite, hematite, and Fe-oxyhydroxides and -oxyhydroxysulfates. The pH of the stream draining the CMT is as high as 2.15. Maximum metal contents in the stream immediately downstream f rom the tailings are 5305 ppm Zn, 454 ppm Cu, 1.16 ppm Ph, 17.5 ppm Cd , 1.4 ppm As, and 0.01 ppm Hg.