Ore-genesis and conceptual model of ore target criteria of the Mamut porphyry copper deposit, Sabah Malaysia

Abstract

The Mamut porphyry copper deposit in northwestern Sabah is genetically associated with adamellite porphyry which intruded into host serpentinite and siltstone. Adamellite porphyry intrusives were possibly derived from Kinabalu batholith. Copper-gold mineralization is observed in strongly fractured zones which are spatially related to the belt of northwesterly extending ophiolite complexes.
The first exploration activities at Mamut area started in 1965 with detailed drainage geochemistry by the UN Labuk Valley Project. The Mamut deposit was assessed for its grade and lateral extent based on the results of drilling by the Geological Survey of Malaysia, followed by further detailed systematic exploration and exploitation programs by the OMRD company of Japan. The mine has been in operation for 12 years since 1975 and is currently producing 500, 000 tons of crude ore averaging 0.56% Cu and 0.6-0.7 g/t Au per month.
The second phase of exploration commenced in 1984 in anticipation of additional ore reserves in the periphery of the deposit. The exploration pgrgram was worked out on the basis of ore target criteria, which were developed from the conceptual genetic model of the Mamut porphyry copper system as follows.
(1) Obducted slabs peeled off from the oceanic plate were thrust up from the Sulu Sea onto and into the geosynclinal sediments and then block-faulted by compressional stress of subducting plate.
(2) Regionally, adamellite intrusives structurally controlled by fault system were initially emplaced to a high crustal level. At Mamut, adamellite porphyry intruded preferably along the intersections of N-S and E-W trending faults. The intrusives introduced magmatic hydrothermal fluids, which subsequently produced an initial stage of quartz-biotite alteration and copper-gold mineralization.
(3) The magmatic fluids extended outwards from the intrusives and interacted with convecting meteoric waters, giving rise to quartz-sericite-pyrite assemblage mainly in host rocks.
(4) Granodiorite porphyry dikes were emplaced from south. Lead-zinc mineralization of calcite-quartz veins was introduced in association with hydrothermal fluids of waning stage.
The above tectonic style and proposed genetic model of the deposit are consistent with the tectonic history of ophiolite in Sabah.
The M1 area, a northern continuation of the deposit, and the bottom extension of the deposit were thus selected for final drilling targets. A total of 50 drill holes carried out during the period 1984 to 1986 have shown that ore-grade mineralization underlies, to a depth not less than 400 m from the original surface, the area of already confirmed mineralization and it extends as well to the north beyond the initially proposed pit limit. The newly-confirmed ore reserves are estimated to be 37 million tons with a cut-off grade of 0.35% Cu.