The Geophysics of the Anjing Hitam Deposit: From mapping shales to a major discovery

Abstract

The Anjing Hitam sedex lead/zinc/silver deposit currently estimated to contain 10 million tonnes at approximately 25% Pb/Zn combined is one of the most significant new basemetal discoveries in the world in the last ten years. The deposit still open at its deeper southern extremity is located at Sopokomil Northern Sumatra, Indonesia some 250 kilometres SW from the city of Medan in an area of high mountainous relief and only recently recognised as having the potential to host significant basemetal deposits. In July 1999 some two years after the commencement of the Sopokomil exploration programme a major ground EM survey was commissioned using the UTEM system. Prior to the EM survey and over a period of three years the prospective shale and carbonate horizons had been drilled at sixteen different locations and failed to identify a ``substantial' deposit in the area. During the UTEM survey with every line of data containing at least two to three anomalies it became obvious that a number of innovative interpretation procedures would need to be introduced to recognise: (and compensate for) topographical effects; the geometry of the conductive source; structural disruptions to the shale unit and identification of localised conductivity increases within the shale unit whose response may be dominated by local current gathering effects. Introduction of these new interpretation techniques resulted in a number of changes to the initial EM programme and identification of a number of targets. The first drillhole of the new drilling programme targeted at one of the EM/geological targets and what is now recognised as the Anjing Hitam deposit resulted in an intersection of 17 meters at 18.8% Zn and 12.4% Pb. Subsequent DHEM surveys confirmed that the massive sulphides which characterise the deposit and which is concealed beneath the topography, are the cause of the initial UTEM response attributed to the orebody. The initial ground EM response however is now known to have been largely dominated by a current gathering response. Other geological/geophysical targets are currently pursued on the property, with two of the three drilled targets associated with massive sulphides hosting sedex lead/zinc mineralisation.