Basin evolution and stratigraphic correlation of sedimentary-exhalative Zn–Pb deposits of the Early Cambrian Zarigan–Chahmir Basin, Central Iran

Abstract

The Zarigan–Chahmir basin is placed in the southern part of a crustal domain known as the Central Iranian microcontinent, at the northwestern margin of Gondwana. This basin hosts abundant mineral deposits, particularly of the iron oxide–apatite (IOA), Fe–Mn exhalative, and Zn–Pb sedimentary-exhalative (SEDEX) types. The evolution of this basin is governed by the Proto-Tethys oceanic crust subduction beneath the Central Iranian microcontinent and by the resulting continental arc and back-arc. This evolution followed two major stages of rifting: (I) Stage I or syn-rift phase, related to intra-basin extension, is indicated by coarse-grained detrital sedimentary rocks and bimodal volcanism (basis of the Early Cambrian Volcano-Sedimentary Sequence; ECVSS), which filled half-graben systems. During this stage, tuff-hosted stratiform, exhalative Fe–Mn deposits along with Kiruna-type IOA deposits formed. The former deposits (e.g., Narigan) are related to early submarine rhyolithic volcanism of the rift-phase sequence, whereas the latter (e.g., Esfordi, Choghart, Chadormalu, Chahgaz) are connected to hydrothermal activity directly linked to the arc calk-alkaline magmatism. (II) Stage II or sag-phase involved the deposition of calcareous shales, siltstones and carbonates (upper part of the ECVSS). Sedimentation during this phase was controlled by basin subsidence and by the reactivation of the half-graben faults. SEDEX deposits are hosted within a carbonaceous, black siltstone unit of the sag-phase sequence, which was deposited during a period of rapid basin subsidence and under anoxic conditions. The location of synsedimentary faults and the prevalence of poorly-oxygenated bottom water conditions were key factors controlling SEDEX mineralization processes in the basin. A high geothermal gradient caused by maximum syn-rift magmatism and sub-crustal lithospheric thinning, affecting primarily the center, western and northwestern basin, may have driven a basin-scale hydrothermal fluid circulation. This finding explains the occurrence of larger, well correlable SEDEX deposits in the northwestern and central parts of the Zarigan–Chahmir basin.