Nature of ore-forming fluids in the Mehdiabad world-class sub-seafloor replacement SEDEX-type Zn-Pb-Ba-(Cu-Ag) deposit, Iran; constraints from geochemistry, fluid inclusions, and O-C-Sr isotopes

Abstract

Abstract The Mehdiabad deposit is the largest accumulation of siliciclastic-carbonate hosted zinc in the world, with a total resource of 394 Mt, grading 4.2% Zn, 1.6% Pb, and 36 g/t Ag. In this study, trace element, fluid inclusions studies, and O-C-Sr isotopic analyses were carried out on the host rock and related sulfide ores in the Mehdiabad deposit, aiming to reveal the nature of ore-forming fluids. The average REE contents of the ore samples are higher than those of the host rock samples. The Cd values of sphalerite in the ores range from 0.11 to 0.63 wt%, which suggested that the Mehdiabad Zn-Pb-Ba-(Cu-Ag) mineralization was formed in a sedimentary-exhalative environment. Investigation of the fluid inclusions reveals relatively moderate temperature (105–261 °C) with scattered salinity (0.03–19.44 wt% NaCl eq.). This finding suggests that hydrothermal brine fluid mixing with seawater could be the primary mechanism that prompted ore formation. The host limestones have δ13CPDB values ranging from +1.9 to +2.2‰, and δ18OSMOW values from +1.7 to +2.7‰. Hydrothermal dolomites have δ13CPDB values from +10.6 to +11.2‰, and δ18OSMOW values from +13.4 to +15.0‰. These O-C isotopic studies suggest that CO2 in the hydrothermal fluids mainly originated from marine carbonate rocks. In the Mehdiabad deposit, δ18O values of all barite samples vary within a narrow range from 7.0 to 10.8‰, which roughly fall within that of the Early Cretaceous seawater. Sr isotope data for barite in the Mehdiabad deposit is more radiogenic than Lower Cretaceous seawater. This suggests that radiogenic Sr in the hydrothermal barites mainly originated from fluid-rock reactions involving radiogenic clastic minerals derived from Sangestan strata underlying the Taft Formation.