Minerals of the Ag–Bi–Cu–Pb–S system from the Amensif carbonate-replacement deposit (western High Atlas, Morocco)

Abstract

The Amensif Cu–Pb–Zn–Ag-(Au) deposit is located on the northern flank of the western High Atlas Mountains, Morocco. This carbonate-replacement deposit occurs predominantly in Lower Cambrian carbonates along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly massive replacements of carbonate strata, although sulphides also occur in veins. Silicification, chloritization, local skarn formation and sulphidation are the most important hydrothermal alteration features observed.The mineralogy is dominated by base metal sulphides with subordinate sulphosalts of Ag, Bi, Sb, Pb, and Au. The ore consists of chalcopyrite, pyrite, galena, sphalerite, arsenopyrite, tetrahedrite, tennantite, and Bi–Ag–Sb–Cu–Pb–Te sulphosalts (matildite, schirmerite, native bismuth, bismuthinite, freibergite, hedleyite, and krupkaite), anglesite, covellite, malachite, and azurite. Silver commonly occurs as Ag–Bi–Sb–Pb sulphosalts intimately associated within galena. SEM analyses confirm the occurrence of invisible gold within sulphides. Although SEM analysis of auriferous sulphides indicates the presence of gold in sufficient quantities to explain the bulk gold concentrations; native gold has not been detected in our polished sections. Gangue minerals include predominantly chlorite, epidote, tremolite, calcite, Mn-dolomite, saddle dolomite, quartz, sericite, with minor andradite and vesuvianite. The presence of a bismuth association at the Amensif deposit is typical, and was effective in scavenging gold and silver. Lead isotope compositions of galena sampled from two regions in western High Atlas (Amensif and Tighardine) show a wide range in 206Pb/204Pb (18.053–18.324), 207Pb/204Pb (15.534–15.577) and 208Pb/204Pb (37.780–37.986). The Pb isotope signature suggests that Pb–Cu–Zn minerals were leached from the older reservoir of the Cambro–Ordovician volcano-sedimentary rocks during the Permian granite event. The Amensif deposit is a typical example of a distal skarn, and is compatible with a model for polymetallic carbonate-replacement type mineralization.