THE GOOD, THE BAD, AND THE UGLY – A MATURITY-INDEX FOR SUPERGENE NON-SULFIDE ZINC DEPOSITS

Supergene non-sulfide Zn deposits consist of high-grade Zn-carbonate, oxide, or silicate ores and contain some 11% of the world’s known Zn reserve. Compared to sulfide deposits, their attraction lies i) in their distinct scarcity or lack of Pb, S and other undesirable elements, ii) their relatively low-energy recovery by SX-EW, and iii) the generation of higher economic value on site. Tonnages range from < 1 Mt to > 200 Mt with grades of 7% to an impressive 25% Zn. Deposits of this group include Skorpion, Namibia; Mae Sod, Thailand; Lan Ping, China; Angouran and Mehdi Abad, Iran; and Franklin/Sterling Hill (?), USA (subsequently metamorphosed?). Several of the deposits contain minor sulfides, generally considered as remnants of protore. Recent investigations indicate highly complex transformation processes from sulfides to non-sulfides and, particularly, the supergene separation of Zn from other metals is still poorly understood.

The proposed maturity index incorporates both, the degree of oxidation and the fractionation of Zn from other metals. A high maturity index characterizes Zn-only deposits, hosting exclusively non-sulfide ore, formed by complete oxidation plus advanced metal fractionation (e.g. Skorpion and Sterling Hill/Franklin?). However, ‘in-situ’ oxidation of pre-existing sulfides alone is unable to produce such highly mature non-sulfide ores due to a high solubility of Zn and subsurface dispersion. Moderate maturities are the result of oxidation without or with minor metal fractionation only (e.g. Mehdi Abad). The least mature deposits would be the typical (‘dirty’) Zn-Pb massive sulfide deposits, that, nevertheless, account for approximately 90 % of the known zinc reserves world-wide. The index for non-sulfide zinc deposits reflects geochemical and mineralogical metallogenetic processes and is of both, economical and technological significance.