Geology, fluid inclusions, C–O–S–Pb isotopes and genesis of the Ahangaran Pb-Ag (Zn) deposit, Malayer-Esfahan Metallogenic Province, western Iran

Abstract

The Malayer-Esfahan Metallogenic Province (MEMP) of western Iran hosts many Pb-Zn deposits and occurrences, largely hosted within a sequence of clastic detrital and carbonate host rocks. A well-known example is Ahangaran which is composed of several hypogene orebodies with estimated resources of ~1.5 Mt sulfide ore grading 4 wt% Pb, 0.5 wt% Zn and 200 g/t Ag, plus a supergene iron-bearing oxyhydroxide ore with 1 Mt reserve and grading 30–35 wt% FeO. The sulfide orebodies are primarily hosted in Lower Cretaceous dolostone and sandstone along the marginal-southern part of the NW–SE trending Kolehbid Syncline. Galena and pyrite, together with chalcopyrite and sphalerite in lesser amounts, comprise the primary sulfide ores. Minor sulfosalt minerals including bournonite, tetrahedrite, freibergite, famatinite and meneghinite have also been identified by electron probe microanalysis. Three paragenetic stages are recognized: (1) pre-sulfide stage (oxide stage); (2) main sulfide stage (including three sulfide sub-stages); and (3) supergene stage. Fluid inclusion studies conducted on pre- and main-sulfide stages identified two end-member fluids: a hot (~290 °C) and saline fluid (~20 wt% NaCl eq.) and a cooler (<150 °C), lower salinity fluid (~7 wt% NaCl eq.). C–O–S isotope studies suggest that these fluids originated as an 18O-enriched, heated basinal brine that carried Pb, Ag, and Ba, and a low salinity, SO42−- and H2S-bearing fluid derived from Mesozoic seawater. Sulfide required for ore formation was generated by thermochemical sulfate reduction and the sulfur of sulfate minerals was derived directly from dissolved SO42−. The Pb isotope compositions are homogeneous with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.458 to 18.501, 15.623 to 15.708 and 38.512 to 38.715, respectively. These ratios are indicative of a continental crustal reservoir as the source of lead and presumably other metals. Taken together, our observations suggest that the Ahangaran sediment-hosted Pb-Ag (Zn) deposit is an epigenetic, structurally-controlled and strata-bound deposit with fluids and metals derived from the Paleozoic basement rocks. This model for formation of the Ahangaran deposit is different to magmatic-hydrothermal, sedimentary-exhalative or Mississippi Valley-types, and to some extent is similar to Irish-type deposits.