Geophysics of the Elang Cu-Au porphyry deposit, Indonesia, and comparison with other Cu-Au porphyry systems

Abstract

Elang is a large porphyry Cu-Au deposit situated about 70 km east of Batu Hijau on Sumbawa Island, Indonesia. The deposit is associated with a series of tonalite porphyry intrusions that are hosted by andesitic volcanics. Mineralisation is associated with potassic alteration (chlorite-magnetite±biotite) that produces a discrete magnetic high of about 700 nT. Forward and inverse modelling have been used to define this zone and the results have been used to direct drilling. Very strong chargeabilities are associated with the porphyry alteration and resistivity from pole-dipole IP and airborne EM shows the extent of the alteration system. The Elang alteration system clearly shows up as a conductive zone in a relatively resistive background. There is an advanced argillic lithocap, up to 200 m thick, covering much of the deposit and is highly resistive and well defined with resistivity. Elang is typical of a number of Cu-Au porphyry systems in that magnetite is associated with mineralisation and produces a strong discrete magnetic anomaly. It has a larger potassic zone than most systems, which may be due to more than one porphyry centre. The Elang system is more pyrite rich than many, leading to very strong chargeabilities, both in the ore zone and the pyrite halo. There is a broad resistive low due to clay alteration and sulphide veining similar to some other porphyry systems