A Geological Review of the Tampakan Copper-Gold Deposit, Southern Mindanao, Philippines

摘要

Located on the island of Mindanao in the Southern Philippines, the Tampakan copper-gold deposit represents a major high-sulfidation epithermal deposit superimposed on an underlying porphyry copper system. With a recent JORC compliant indicated and inferred resource estimate of 1104 million tonnes at 0.65 per cent copper and 0.26 g/t gold at a 0.3 per cent copper cut-off grade, Tampakan represents the largest undeveloped copper-gold deposit in Southeast Asia. Since its acquisition by Indophil Resources' affiliate Sagittarius Mines Inc in August 2002, the deposit is the subject of an ongoing preliminary feasibility study into project viability that is initially focused on a higher-grade copper zone in the north-central part of the deposit. The deposit is hosted by a sequence of subaerial andesitic flows (Tampakan Andesite Sequence) related to a highly eroded andesitic stratavolcanic complex of Pliocene age. Detailed mapping and geochronological dating has identified four volcanic cycles or centres that have been sequentially developed and eroded. This stratovolcanic complex is interpreted to lie on the northern extension of the Sangihe volcanic arc that extends from Northern Sulawesi in Indonesia. The Tampakan district is located within a complex tectonic setting dominated by WNW trending wrench faults that form part of the trans-Mindanao Cotabato Fault Zone. The Tampakan deposit is located near the intersection of one of several WNW trending splay structures with a prominent NNE trending dilational structure, one of many that transect the host volcanic complex. High-sulfidation style copper-gold mineralisation at Tampakan is broadly associated with and central to a flat-lying tabular body of pervasive silica and silica-clay alteration developed within a district- scale lithocap of advanced-argillic and argillic hydrothermal alteration that extends over an area of 90 km~2 within the volcanic complex. Deposit geology has been developed from a systematic program of diamond drilling that currently comprises 102 completed holes (38 400 m) with most holes ending in lower grade (0.2 - 0.3 per cent Cu) porphyry-copper style mineralisation at downhole depths of 350 - 500 m. Superimposed high-sulfidation mineralisation and associated alteration passes transitionally to porphyry copper style mineralisation and alteration at depth. The interface between these styles of mineralisation can be quite irregular as a result of structural control on hydrothermal fluids. At an approximate 0.3 per cent copper contour, the deposit comprises a flatlying to gently dipping, tabular body of mineralisation extending over an area of 2 km by 1.6 km and ranging in thickness from 200 - 400 m. Although there is a close spatial correlation between these mineralisation types, the earlier porphyry event has been dated at 4.25 Ma and the later high-sulfidation event at 3.2 - 3.4 Ma. High-sulfidation epithermal mineralisation and alteration is strongly influenced by the interpiay of stratigraphic and structural controls, in particular NNE-trending dilatant faults. This style of mineralisation is typically associated with silica and silica-clay (pyrophyllite-dickite-alunite) hydrothermal alteration assemblages that have obliterated or overprinted original textures and mineralogy. Zones of silica alteration often display multi-phase brecciation and vuggy porosity resulting from intense acid leaching. Sulfide mineralisation typically comprises the copper-rich species digenite, bornite, and enargite, and to a lesser extent chalcocite and covellite, and usually occurs as fractures, veins and vug-fills or as disseminations at depth. Pyrite is usually present throughout and enargite distribution appears to be preferentially associated with late-stage structures and areas of strong silicificarion. Porphyry-copper style mineralisation at depth comprises disseminated chalcopyrite, bornite and pyrite that is associated with variably developed quartz-stockwork veining. This m