Defining hydrogeology of the Gohar-Zamin open pit mine, Iran: a case study in a hard-rock aquifer

Abstract

Gol-Gohar iron ore mines, with a reserve of about 1.1 billion tons, constitute the largest iron ore mine complex in Iran. Pit number 3 (Gohar-Zamin mine) has the largest reserve of ore among the others. Pit advancement toward deeper parts of the aquifer has increased groundwater inflow into the pit. During exploratory drillings in Gohar-Zamin mine, some of the boreholes became artesian. The water outflow from these boreholes has high salinity (classified as brine) and high pressure of a gas with an unknown origin. A conceptual model of groundwater flow was established using the available evidence and information. The prepared model revealed that there is a minimum of three aquifers, two alluvial aquifers (with electrical conductivity (EC) of 4,000–37,000 μS/cm) and one hard-rock aquifer (EC of 91,800–169,000 μS/cm) in the study area, with different lithological and hydrochemical characteristics. The brine outflow from the artesian boreholes originated from the hard-rock aquifer, which is a confined and high-pressure system without active recharge, i.e. an unsustainable resource. There is no hydraulic connection between the pit mine and the playa, located 13 km north of the mine, through the hard-rock aquifer, and the brine in the hard-rock aquifer is fossil water or has a metamorphic origin. The present study shows that the groundwater in the hard-rock aquifer is entrapped between fault planes without any hydraulic connection with overlying aquifers. Further, the results suggest that numerical groundwater flow models can only be successfully applied to the alluvial aquifer.