Work

Hydraulic-thermodynamic modeling of the upper Cretaceous “Mafic-Pelitic” Ermioni VMS deposit, Greece

Year: 2022

Type: article

Abstract: Numerical, geochemical, hydraulic and thermodynamic modeling is combined with fluid inclusion and fluid chemistry analyses to shed light on the mechanisms of hydrothermal circulation, alteration patterns and ore formation of the fossilized stringer ore of the Upper Cretaceous, “Mafic-Pelitic”-type VMS hydrothermal system of Ermioni (Karakasi mine). The atypical Ermioni VMS deposit, with estimated reserves of ∼ 1.0 Mt, comprises almost monomineralic and barren pyrite (≥95 vol%), followed by minor chalcopyrite and traces of sphalerite. The Karakasi VMS stringer and semi-massive to massive ores were deposited at moderate temperatures (330°-430 °C) and low-to-moderate salinity values (∼2 to ∼ 10 wt% NaCl equivalent) by a CO2-CH4-Si-Mg-Fe-bearing, slightly neutral (pH = 3.6–5.0) and reduced (logfO2 = HM-2.7 to HM-3.9 and logfS2 = PP + 1.7 to PP-2.5) stringer ore-forming fluid that was discharging at the paleo-seafloor at hydrostatic pressures of ∼ 165 bars and velocities of 8.9 to 12.4 mm/s. Simple cooling, seawater contamination up to 35 vol% and wall-rock interaction of decreasing water-to-rock ratios (from ∼ 800 down to ∼ 20) were the mechanisms that contributed to the observed alteration patterns, i.e., silica, silica-chlorite, silica-epidote (±albite-chlorite-sericite), grunerite and albite-prehnite zones. Remobilization of base metals from the footwall volcaniclastic and ultramafic rocks, combined with sulfidation during hydrothermal circulation, led to the increase of the logαFeCl+ and eventually of the logαCuCl0 values of the Karakasi stringer ore fluid and provoked the precipitation of almost monomineralic pyrite. Hydraulic-thermodynamic modeling suggests the formation of hydrothermal plumes, indicated by the 300° and 350 °C isotherms, with mainly inwards ore-fluid circulation and along the main normal faults. The collated data assist in designing new exploration targets of VMS deposits related to volcaniclastic rocks distal to known or inferred heat sources in rift-related, back-arc geotectonic settings.

Source: Ore Geology Reviews

DOI: 10.1016/j.oregeorev.2022.105039