Geology and Mineralization of the Porphyry - High-Sulphidation Epithermal Cu-Au Deposit at Tampakan, Southern Mindanao, Philippines

 

 

Leonardo L. Subang¹, Alan G. Buenavista², Jethro M. Gonzales³, George P. Moreno⁴,
Virgilio T. Baratang Jr.⁵, Melvin C. Sebua⁶, and Novelwin A. Santos⁷

1. Senior Exploration Geologist - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500
2. Project Manager - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500
3. Chief Project Geologist - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500
4. Senior Project Geologist - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500
5. Senior Project Geologist - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500
6. Exploration Geologist - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500
7. Junior Project Geologist - Sagittarius Mines Incorporated,
JPM Building, Bula-Lagao Road, General Santos City 9500

Abstract

The Tampakan Cu-Au deposit is located in the province of South Cotabato, Southern Mindanao. The deposit represents an extremely telescoped porphyry – high-sulphidation epithermal Cu-Au system developed in an island arc setting. At a 0.3% Cu cut-off grade, this deposit constitutes a very large undeveloped JORC-compliant Indicated and Inferred Resource of 1100MT with an average grade of 0.7% Cu and 0.3g/t Au.

The Tampakan mineral district lies within the Cotabato Fault Zone, a trans-Mindanao deformational feature characterized by a series of NW-WNW trending left lateral strike-slip faults. The Tampakan Cu-Au deposit is localized at the intersection of these NW-WNW shears with NNE-NE trending shears and ENE trending dilational fault structures. The upper high-sulphidation Cu-Au mineralization is largely hosted in a sequence of andesitic to polymictic breccias and andesitic flows that are interpreted to represent a series of diatremes and endogenous domes. The lower porphyry Cu-Au mineralization is hosted in a polyphase dioritic complex made up of fine-coarse grained porphyritic-porphyry stocks and intrusive breccia bodies.

Paragenetically, the Tampakan Cu-Au system starts with an early porphyry style mineralization. The most widespread drill-hole intercepts of this type of mineralization is in the form of low to moderate density gray-pink quartz stockworks associated with intermediate argillic (silica-chlorite-sericite-hematite) and phyllic (silica-clay-sericite-pyrite) alteration. Copper, in almost all core exposures, is present in the form of chalcopyrite as thin bands in the quartz veins and as micro-fracture fills in the dioritic host rocks. The porphyry style mineralization is most clearly seen at the deeper portions of the system where it is not overprinted. At higher levels, this is expressed only as ghost veinlets masked by high-sulphidation overprint, characterized by silica+alunite+pyrophyllite+dickite±diaspore±sericite alteration with gypsum-anhydrite stockworking cutting porphyry-related quartz and with pyrite-bornite-digenite replacing porphyry-related chalcopyrite. To date, drilling has intersected only the top of the porphyry mineralization in Tampakan. Some drill holes show an increase in density of porphyry-related quartz stockworkings at depth with corresponding increases in copper and gold grades. In the future, it is highly possible that higher grade sections of this porphyry Cu-Au mineralization will be found by deeper drilling campaigns.

The high-sulphidation mineralization begins with an early high arsenic phase. This phase is characterized by coarse pyrite+enargite±luzonite±chalcopyrite±molybdenite associated with silica+alunite+pyrophyllite±diaspore alteration. This early high-sulphidation mineralization is most developed in a silica-alunite cap as fracture, vug and breccia matrix fills. Due to its localization in the silica-alunite cap, the morphology of this mineralization is grossly stratiform. Towards the deeper levels, below the silica-alunite cap, this high arsenic mineralization tapers into sub-vertical silica+quartz+sulphide feeder breccias.

The early high arsenic phase is followed by a middle high copper phase characterized by the deposition of pyrite+bornite+digenite+chalcopyrite±molybdenite±enargite. The mineralization of these high-copper mineral species is associated with an alteration assemblage consisting of silica+dickite+sericite±alunite. This middle phase is developed as disseminations and microfracture fills starting from the base of the silica-alunite cap where a clotted silica-clay layer begins. It continues down into a porphyritic to granoblastic silica-clay section. The gross morphology of the distribution of this copper-rich phase is tabular where hosted by diatreme breccias. It is sub-vertical and largely fault controlled where hosted by less permeable porphyry lithologies. Similar to the enargite-rich zones, the bornite/digenite-rich zones are also centered at and fed by sub-vertical silica-clay-sulfide breccia zones and/or clay-sulphide flooded fault zones.

The late phase of the high-sulphidation mineralization is characterized by high gold deposition. Sulfide mineralization is made up of fine pyrite+enargite+chalcopyrite±tetrahedrite/tennantite± molybdenite±sphalerite±galena with occasional free gold. This mineralization phase is associated with silica+clay+illite±chlorite alteration and drussy crustiform banded fine quartz and chalcedony textures. Often, the quartz drusses are lined with tiny quartz crystals and occasional blades suggestive of boiling. The extent of the deposition of the high-gold phase is limited along faults and fault-bounded hydrothermal breccia zones that are mostly trending NE-ENE. So far, these high-gold zones are mostly observed at the WNW side of the Tampakan Cu-Au system, and have seen very limited drilling evaluation.

A scoping study into project viability is currently being done on the Tampakan Cu-Au deposit by Sagittarius Mines Incorporated (SMI). In addition to the 85 holes previously drilled by Western Mining Corporation (WMC), SMI has drilled 28 more holes from July 2003 to September 2004, focusing in the further evaluation of the higher-grade core of the deposit. The current understanding on the geology and mineralization of the Tampakan Cu-Au deposit is based largely on the authors’ logging of SMI drill cores and re-logging of WMC drill cores.