Mineralogical and geochemical characterization of Nonsulfide Zn-Pb mineralization at Silvermines and Galmoy (Irish Midlands).

The nonsulfide zinc–lead mineralization in the Irish Midlands represents an example of surface oxidation of primary sulfide mineralization, redeposition and preservation under glacial till. In this preliminary study, we report on the mineralogy and chemical composition of the nonsulfide zinc mineralization at Silvermines and Galmoy deposits. At Silvermines, the analyses were carried out on samples from two drill cores (drillholes 302/3 and 303/3) located in Knockanroe Townland, at the periphery of the main orebody. At Galmoy, the nonsulfides were sampled from the G-orebody between 77 and 93 m below the surface, in an area where oxidation is very strong. At both sites, the dominant mineral composition of oxidized ores is relatively simple with smithsonite– hemimorphite at Silvermines and smithsonite at Galmoy. Primary sulfides and secondary carbonates/silicates are generally associated at both localities. Whole rock chemical analysis, SEM and optical studies have revealed evidence for open space filling and replacement of both primary sulfides and Carboniferous dolomite host rock by nonsulfide minerals. Smithsonite at Silvermines shows an evolution of different forms from early encrustations and botryoidal aggregates to late “rice grain” shaped crystals and a systematic decrease of Fe content with time in the paragenetic sequence. Early smithsonite replacing dolomite host rocks can be Fe- and Mn-rich. This evolution points to slightly more reducing conditions during deposition of the early Zn carbonates, compared to late smithsonites, which coexist with Mn and Fe (oxy)hydroxides. A complex association of at least three different, often intergrown types of smithsonite aggregates were observed at Galmoy, where replacement of sulfides and vug-infilling are predominant. Early globular or crustiform Pb- and Ca-rich smithsonite is followed by tiny rounded (Fe-poor) to one-dimensional (Fe-rich) clusters of smithsonite and again by late “rice grain” shaped smithsonite. Hemimorphite is virtually absent. Thin encrustations of sulfates, such as gypsum, boyleite, rozenite, copiapite and jarosite, often coat both sulfide and nonsulfide minerals and testify to recent and/or ongoing alteration. The lack of rhombohedral smithsonite, which characterizes the upper phreatic zone in Sardinia and in Belgium, indicates the relative immaturity of the weathering profiles in Ireland, the absence of a well-developed karstic network and, in consequence, the uneconomic character of nonsulfide concentrations.