Abstract

The results of isotopic studies on carbonates and sulfur-bearing minerals of the Irankuh deposit occurring in Lower Cretaceous carbonate strata provide information concerning the nature of the fluids responsible for Zn-Pb-Ba mineralization and associated dolomitization. An epigenetic model for the Irankuh deposits is supported by their discordant nature, emplacement along the Irankuh fault, progressive depletion in stable carbon isotope ratios of the host dolostones as mineralization is approached, and isotopic characteristics of the ore-stage dolomites and barites. The host-rock dolostones show extensive depletion in 18 O (delta 18 O = -4.9 to -12.7ppm) due to diagenetic modification and/or recrystallization at increased temperatures. Based on isotopic and fluid inclusion data, the ore-stage dolomites in the deposits are interpreted as having formed from warm ( approximately 100 degrees C) saline, 87 Sr-enriched fluids similar to oil field brines. The overlap in delta 18 O values and 87 Sr/ 86 Sr ratios between host dolostones and sucrosic and saddle dolomites implies that these rocks have been isotopically modified and precipitated by similar fluids.The delta 34 S values of metal sulfides range from -3.6 to -9.6 per mil whereas barite values, ranging from 14.3 to 18.9 per mil, fall within the range of Cretaceous-age sulfate values. It is inferred that seawater sulfate acted as the source of the sulfides and barite mineralization. The reduced sulfur was generated by partial reduction of seawater sulfate by organic matter under hydrothermal conditions. The presence of a hydrocarbon (i.e., traces of bitumen) and saddle and fracture-filling dolomite with low delta 13 C values (ranging to -6.4ppm), in conjunction with the temperature of ore deposition, provides evidence for thermochemical sulfate reduction. Carbonates and barites show 87 Sr/ 86 Sr ratios enriched in 87 Sr which is atypical of derivation from normal seawater. The enrichment of 87 Sr is explained by extensive interaction of fluids with the underlying siliciclastic rock evolving toward more radiogenic compositions. The release of the overpressurized fluids appears to have been due to dehydration reactions and regional tectonism. The metal-bearing fluids responsible for mineralization migrated along the major Irankuh fault and mixed with sulfate-bearing fluids at the ore depositional site. Hydrologic considerations demonstrate that the proposed model is feasible.

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