GEOLOGICAL SOCIETY OF THE PHILIPPINES

 

Geology and Mineralization of the Bananghilig Gold Deposit, 
Tambis Mineral District, Barobo, Surigao del Sur, Philippines

F.A. Jimenez Jr.1, Hernulfo B. Ruelo2, Ciceron A. Angeles Jr.1, 
Grace P. Galet1, Marynelle D. Boticario1, and Samuel F. Declaro1

1Philsaga Mining Corporation 
2Consultant

 

Extended Abstract

The Bananghilig Gold Deposit is characterized by hydrothermal breccia veins and associated sulphide fracture fills hosted in an elliptical-shaped diatreme breccia body measuring at least 1,000m long and 750m wide, and in the surrounding igneous wallrocks. The mineralization as well as the diatreme host is still open at depth and to the south at a newly discovered extension called B2 area beneath the younger sedimentary cover. The Bananghilig Project, currently at feasibility study stage, is operated by Philsaga Mining Corporation, which also operates the Co-O Gold Mine located 40 kilometers to the south.

The Bananghilig Gold Deposit is located in the Tambis Mineral District, a NE-SW trending, 7km long structural corridor, which is orthogonal to the SE-NW striking Lianga Fault splay of the southern segment of the Philippine Fault Zone in eastern Mindanao The district hosts several prospects exhibiting multi-mineralization gold-copper styles. It is floored by andesitic volcanic basement rocks intruded by a fertile Tertiary-age igneous package consisting of multi-phasalcalc-alkaline, high level sub-volcanic intrusive rocks, namely diorite, andesite porphyry, feldspar porphyry and dacitic stocks and dikes. This igneous package is subsequently cut by extensive bodies of phreatomagmaticdiatremes. They comprised of matrix- to clast-supported monomictic to polymictic types and contain unsorted angular to sub-rounded clasts of andesitic basement and intrusive rocks in a comminuted rock flour and magmatic crystal matrix. Clast sizes range from granule to building-sized mega-blocks. The Tambis intrusive-breccia complex is overlain by younger post-mineral limestone and basal mudstone-conglomerate beds to the south and to the east.

The gold mineralization style is silver-base metal-rich and is classified as an intermediate sulphidation epithermal system. It occurs in at least 3 NE-SW trending sub-parallel zones, namely Sorex and Garden zones, which contain the bulk of the mineral resources, and Malinao zone. It is typified by multi-episodic sulphide±silica±clay-bearing hydrothermal breccia veins and hydraulically emplaced sulphide fracture fills cutting the diatreme and the adjacent wallrocks. The hydrothermal breccia veins are sub-vertical to steeply dipping to the southeast, averaging <10 centimeters to about a meter thick, and form anastomosing pinch-and-swell patterns for several meters to tens of meters following a sub-parallel, generally NE-SW trending strike. They are normally off-set by post-mineral faults, but persist down to >500m from the surface downdip, and exhibit typical epithermal mineral growth and fracture-fill textures of hydrothermal brecciation, hydrofracturing and minor crustiform±colloform banding.

The hypogene ore mineralogy consists mainly of pyrite, minor amounts of sphalerite, chalcopyrite and galena, and trace amounts of arsenopyrite, arsenic-bearing pyrite, marcasite, arsenic-bearing marcasite, sulphosalts, eltectrum/native gold and silver±goldtellurides. There are also traces of exotic native lead, native tellurium, bornite, cubanite and chalcopyrrhotite. High gold concentrations appear to be associated with elevated silver and base-metals. There are negligible amounts of supergene oxidation. However, there is appreciable supergenely enriched gold occurring in the soils, colluvium and alluvium around the deposit which is targeted by artisanal miners.

Hydrothermal alteration consists mainly of widespread argillic (illite±silica) alteration at the surface and narrows down at depth. The said alteration overprints laterally and vertically the pre-existing regional chloritic and propylitic alteration assemblages. Silicic alteration is common in the hydrothermal breccias and sometimes in the immediate surrounding wallrocks.

Petrographical and fluid inclusion studies revealed that the gold mineralization was introduced along with alteration by upflowing silica-saturated, 220o to 320oC neutral pH, weakly saline (0.5 – 4.0 wt. % NaCl equiv.) hydrothermal waters. A blanket of late-stage barren kaolinite±calcite alteration produced by low-temperature (<200oC) slightly acid bicarbonate waters is superimposed over the higher temperature (>200oC) illite- and sulphide-rich alteration assemblage, marking the cessation of hydrothermal activities at Bananghilig. Fluid inclusion studies also indicated that the Tambis intrusive-breccia complex was moderately to deeply eroded, with the paleo-surface estimated to be at 500 to 950m above the current surface.

As of January 29, 2013, the JORC compliant indicated and inferred resources at 0.8 g/t gold cut-off are estimated at 1.08 million ounces of gold at a grade of 1.6 g/t gold.

 
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