Elemental zonation and geochemistry of sulphide ore of the Ab-Bagh sedimentary-exhalative Zn-Pb deposit, southeastern margin of the Malayer-Esfahan metallogenic belt

The Ab-Bagh Zn-Pb deposit is located in the southeastern part of the Malayer-Esfahan metallogenic belt (MEMB). A large number of Zn-Pb deposits in the MEMB (approximately 170 deposits), are hosted by early Cretaceous carbonate rocks, and few of them are hosted by Jurassic detrital and organic matter-rich sedimentary rocks. In this study, we present the results obtained from fieldwork, ore and host rock petrographic studies, main oxides, trace and rare earth elements analysis and chemistry of sulfide minerals.

This study focused on the geochemistry of both of the two ore-bearing horizons that constitute the Ab-Bagh deposit (12 samples). Trace and rare earth elements analysis were conducted using an Agilent 7500ce ICP-MS instrument, Zn and Pb were analyzed via ICP-OES and main oxides were determined via XRF in the IGME central laboratory, Barcelona, Spain. To determine chemistry of sulfide minerals, four polished thin sections were studied using EPMA in IGME central laboratory.

Ore-bearing horizons of the Ab-Bagh deposit are hosted in the late Jurassic-early Cretaceous clastic-carbonate sequence. The northeast-trending, steeply southeast-dipping rocks have been subdivided into five units. Based on position, there are two zinc- and lead-bearing stratigraphic ore horizons. Ore horizon 1 is hosted by late Jurassic-early Cretaceous black shale and siltstone. The wedge-shaped ore body is located close to a synsedimentary fault. Petrographic studies indicate that mineralization comprises three sulfide ore facies: stockwork, bedded and massive ore facies. Ore horizon 2 occurs in early Cretaceous carbonates and comprises of massive ore facies that are concordant with host rock layering; it is also underlain by stockwork facies. Textures include framboidal, laminated, breccia, replacement, massive and vein-veinlet.Due to alternating pulses of hydrothermal fluid (zone refining process), temperature and redox condition changes, SEDEX deposits are chemically zoned. Based on geochemical studies, metal zoning patterns are well developed in the Ab-bagh SEDEX type Zn-Pb deposit. Both vertical and horizontal zonation is present in this deposit. In SEDEX deposits, moving outward from the stockwork facies, ore bearing horizons display decreasing Zn/Ba, Zn/Mn, Zn/Fe, Pb/Zn, Pb/Fe, Pb/Ag, and Cu/ZnþPb ratios, reflecting the oxidation control on mineralogy and the temperature dependent solubility of the base metals.Vertical and horizontal chemical zonation is well developed in ore horizon 1 of Ab-Bagh deposit, in a way that Pb/Ag increase from base to top and Cu/(Zn+Pb) and Pb/(Zn+Pb) increase from top to base in massive ore facies. Also, horizontally, Zn/Al2O3, S/Al2O3 and Cu decrease and Ba increases from massive to bedded ore facies. There is a significant positive correlation between Cd/Zn, Co/Zn and Ag/Pb in sulphide ores from ore horizon 1. REE content of massive sulphide ore is very low, however, REE content of bedded ore is more than the massive and stockwork ore facies. Fe and Cd content of sphalerite increases from margins to center, which reflects drop in temperature of ore-forming fluid over time. Also geochemical features of sphalerite and pyrite suggest that they are formed in anoxic environment.

The Ab-Bagh Zn-Pb deposit is hosted by Late Jurassic-Early Cretaceous black shales and Early Cretaceous limestone. The deposit consists of two horizons of Zn-Pb sulfide and oxide lenses situated at the different stratigraphic levels, each with an associated stringer vein zone. Vertical and horizontal chemical zonation is well developed in ore horizon 1 of Ab-Bagh deposit. Lateral zonation away from the center of fluid discharge is controlled mostly by zone refining in the vent complex and is typically accompanied by a decrease in the thickness of the stratiform body.