Geological, mineralogical and geostatistical studies on Zarghat magnesite deposit, Saudi Arabia

Abstract

Zarghat magnesite deposit, located about 570 km NNE of Jeddah, occurs in the upper part of a carbonate sequence which together with an underlying andesite unit constitutes the Jibalah Group of Late Proterozoic are. The Jibalah Group represents the youngest rocks of the Precambrian sequence of the Arabian Shield. The reserves with a grade of 38-48% MgO are estimated by BRGM to be 3 million tons. The magnesite is exposed on the surface in the form of small hills, and associated with dolomite, cherty limestone, and siltstone. Structurally, these rocks make a syncline which appears to be asymmetrical structural basin. Petrographic investigations with microscope and SEM show that the magnesite is mostly microscope and SEM show that the magnesite is mostly cryptocrystalline, massive but at places sheared and fragmented. XRD studies demonstrate that the magnesite is either in the form of pure magnesite with minor traces of dolomite or impure magnesite with abundant dolomite. Dolomite veins of a later phase with variable thickness have cut across the magnesite bodies. The DTA investigation has shown that the decomposition temperature of magnesite is about 620℃. To determine the spatial distributions of variables within the magnesite ore body of the central-hill, semivariograms and cross-semivariograms functions for the several selected variables were computed. These functions revealed continuous behaviour near the origin which is common in most sedimentary deposits. The field and laboratory evidences all suggest that the magnesite has been deposited in a sedimentary shallow marine basin along with the other carbonate rocks followed by some post depositional (diagenetic) changes.