Geological, isotope geochemical and fluid inclusion constraints on the Mishu SEDEX-type Barite (Pb-Cu-Zn) system, NW Iran

Abstract

The Late Precambrian sedimentary-volcanic sequence of the Kahar Formation at Mishu mountains, NW Iran, hosts the oldest SEDEX-type barite-(Pb-Cu-Zn) mineralization of Iran. These occurrences include Aralan-Pyrbala, Almas, Shanjan, and Daryan that are hosted by organic material-bearing shales, siltstones, and fine-grained sandstones of the Kahar Formation. Based on the morphology of the mineralization and its textures, the mineralization has formed as strata-bound (stockwork zone and massive ore) and stratiform (bedded ore) ore bodies. The stockwork zone consists of an irregular network of vein-veinlets that cross-cut the sedimentary host rocks. The massive ore contains both massive barite and semi-massive sulfide minerals (galena, chalcopyrite, pyrite, and minor sphalerite and tetrahedrite-tennantite). The bedded ore consists of barite-rich layers that are concordant with the bedding of the host rocks. Barite represents the main mineralization at Mishu mountains. Microthermometry on barite minerals from various ore facies (stockwork zone, bedded ore and massive ore), demonstrates that homogenization temperatures (Th L+V) and salinities decrease from the stockwork zone (163 to 285 ℃; and 3.43 to12.40 wt% NaCl equiv.) to the bedded ore facies (142 to 229 ℃; and 3.11 to 8.78 wt% NaCl equiv). The stockwork zone is interpreted to have formed near the paleo-subsurface, and the bedded ore within seafloor sediments. Hence, this trend of the temperatures and salinities probably reflects mixing of cold seawater with hydrothermal fluids, where the seawater proportion is higher in the bedded ore than in the stockwork zone. Based on the homogenization temperatures and salinities of the mineralizing fluids, a buoyancy plume model is proposed as the mechanism for the ore precipitation. The δ34S and δ18O values of barites from the Mishu mineralization range from 34.1 to 41.3‰ and 10.2 to 12.6‰, respectively. These values indicate the influence of seawater. Based on the δ34S versus δ18O diagram, the studied samples are located in the range of cold seeps barite, a term that is commonly used in the literature to describe barite mineralization at continental margins. Sulfur and oxygen isotope composition of barite, along with the sedimentological, textural, mineralogical, and fluid inclusion characteristics, suggest that the Mishu mineralization represents a vent-proximal (Selwyn-type) SEDEX ore deposit.