Earth Sciences, Geology; Divar VMS deposit; Cyprus type; MORB; Sistan suture zone; Iran
Abstract: The Divar VMS deposit is located in the Nehbandan ophiolite complex (NOC). It is hosted in the Sistan suture zone (SSZ) marking the boundary between the Lut and Afghan continental blocks. This area is composed mainly of various ophiolitic rock units representing a tectono-sedimentary mélange, which are commonly interpreted as being the remnants of Sistan oceanic lithosphere. The major host rocks cropping out in the Divar deposit are mantle peridotites include clinopyroxene- (cpx-) rich harzburgites and depleted harzburgites with gabbronorite, cumulate gabbro, basalt and pelagic sediments. They are interpreted as being related to the rifting and subsequent Late Triassic continental break-up of the southern Neothethys. In this study petrographic observations mineral chemistry with whole rock chemistry, and rare earth element (REE) modeling were carried out on the different rocks associated with Divar deposit. The presented data led to the following conclusions: (1) The depleted harzburgites proved to be related to the residual mantle after 18–22% removal of melts. This is comparable with abyssal peridotites; (2) The cpx-rich harzburgites represent the residual mantle after the removal of 11–13% mid-ocean ridge basalt-type (MORB). Subsequently, the residual mantle was enriched in light REE (LREE) by subduction-derived fluids; (3) The gabbronorite and cumulate gabbro represents a portion of oceanic crust generated in a mid-ocean ridge setting; (4) The basalts belong to the N-MORB-type with no considerable crustal contamination. From the top to bottom, the Divar deposit is characterized a gossan zone, a thick massive ore zone, and a poorly developed stockwork zone. In Divar deposit, host rocks are indicated by serpentinized peridotites and severely altered mafic rocks that include disseminated and stockwork ore under massive lenses. These rocks showed a mineral assemblage of carbonate + quartz-jasper + chlorite + albite + epidote that demonstrate greenschist facies ocean floor metamorphism. Moreover, in the footwall rocks, sericitization and hematitization are also observed. Primary mineralization includes bedded ore, sulphide breccias, stockworks and chimney fragments. Ore mineral abundances decrease from pyrite, magnetite, to chalcopyrite, and sphalerite. Highlightly, there are two distinct mineralization phases' namely massive sulphide (stage-1) and massive magnetite (stage-2). The spatial relationships with the host rocks and textural evidence (e.g., magnetite replacing sulphides) indicate that sulphide minerals and magnetite were generated in different stages. The transition from sulphide to magnetite mineralization is interpreted as being related to the variation of H2S content in the ore fluids. The massive sulphide ore (stage 1) might have formed by primary hydrothermal fluids enriched in H2S and Fe. The second stage (massive magnetite) seem to be related to the later hydrothermal fluids enriched in Fe but poor in H2S, replacing the pre-existing sulphide minerals. The lithological features (stockwork/massive ore and gossan), mineral paragenesis and alteration patterns of the Divar deposit are all consistent with a Cyprus-type VMS system mineralization. However, magnetite formation after the early sulphide mineralization is quite different with typical Cyprus-type VMS deposits where magnetite commonly occurs in lower sections. Consequently, although the general features of Divar deposit is analogous to those of Cyprus-type deposits, but strong magnetite mineralization especially in late stages may indicate that this deposit could be considered as an exceptional Cyprus-type VMS deposit.