In search of the lost zinc: A lesson from the Jabali (Yemen) nonsulfide zinc deposit

摘要

The Jabali nonsulfide zinc deposit, located northeast of Sana'a (Yemen) contains a geological resource of 12.6 million tonnes of ore grading 8.9% zinc, 1.2% lead and 68 g/t silver, with a projected recovery of ca. 80% zinc. The primary sulfide deposit shows features of both Mississippi Valley and Carbonate Replacement types, and is believed to have been formed by circulating hydrothermal fluids, either associated with Mesozoic rifting, or generated from Tertiary igneous activity, developed in the area during the Red Sea crustal extension. An extension of this phenomenon should have also triggered the late uplift, which favored the oxidation of sulfides. Ore deposition has been accompanied by several dolomitization phases, some of which have been considered strictly hydrothermal.A complete quantitative (Rietveld) mineralogical and geochemical study of mineralized full-length core samples, carried out with the aim of possibly increasing zinc recovery, shows a discrepancy between the zinc grades recorded in the chemical assays, and those calculated from the sum of the ore minerals occurring in the same samples. The difference between the assayed and calculated zinc amounts in various parts of the deposit is due to the presence of Zn-rich dolomite phases (up to 20% Zn in the lattice), as well as of Mg-smithsonite (up to 12% Mg), both phases replacive of the earlier dolomites in the weathering environment. The Zn-enriched dolomite phases could be the "missing link" between pure dolomite and smithsonite. Zinc occurring in dolomite cannot be processed economically with today's methods. Analysis of the total zinc amount contained in Zn-dolomite, when compared with the zinc occurring in the processable ore minerals shows that there is a significant proportion of unrecoverable zinc. This explains why at Jabali the projected metallurgical recovery of around 80% is unlikely to be improved upon, due to the trapped zinc within the "supergene" dolomite phases. The extensive development of the Zn-dolomite bodies, which occur throughout the whole mining area, may be highly significant for the evaluation of nonsulfide zinc ores at Jabali and for the exploration philosophy of the region.The possible occurrence of Zn-dolomite has to be kept in mind when exploring for supergene Zn-nonsulfides in other mining districts where the ore is also dolomite-hosted, which may feature a significant non-recoverable phase.