GEOLOGICAL SOCIETY OF THE PHILIPPINES

The Manat Epithermal Gold-Silver-Lead-Zinc Project in SE Mindanao

 

by

 

Chris Middleton, Director of Exploration – Indophil Resources NL

 

Abstract

 

The Manat gold-silver-lead-zinc Project located in Comval Province SE Mindanao is a joint venture exploration project between Southern Exploration Corporation, a wholly owned subsidiary of Indophil Resources NL of Australia and Alsons Development and Investment Corporation (Aldevinco), an affiliate of the Alcantara Group. The project area is located approximately 60km northeast of Davao in the northern part of the Masara mineral district, a well-endowed section of the eastern Mindanao Volcanic Arc.  The district hosts several significant epithermal gold and porphyry copper-gold deposits including King King, Amacan (North Davao), Sabena, Hijo and Masara. The King King porphyry copper-gold deposit is one of the largest in the Philippines with a published resource of 398million tonnes at 0.34% copper and 0.52g/t gold.

 

Previous systematic exploration within the Project area conducted by WMC Exploration in joint venture with Aldevinco, included multi-element geochemical stream sediment and soil sampling surveys, geological mapping and a broad spaced induced polarisation survey. This exploration identified several prospect areas for gold and copper and culminated in a limited 6-hole diamond drilling program at three separate prospects; Taglayag, Katungbuan and Magas. A single drillhole at the Magas Prospect (MNTD6) intersected 24m at 4.53g/t gold and 51m at 2.93g/t gold from downhole depths of 76m and 193m respectively. WMCE withdrew from the project in May 1998 to focus on their Tampakan Copper Project in Southern Mindanao.

 

Indophil was successful in acquiring an interest in the Project in November 1998. An initial emphasis on geological mapping, re-interpretation of soil geochemical data and re-logging of existing drillholes within the Project area provided an important framework for ongoing exploration and confirmed the high prospectivity of the Magas Prospect, which has been the subject of a focussed exploration effort by Indophil since. Geological mapping established that most of the Project area is underlain by quartz diorite of probable Upper Miocene age intruding remnants of an older andesitic volcanic basement. Prominent NW and NNW tending structures observable in digital terrain data that transect the project area are considered to be major secondary splay structures associated with the Philippine Fault Zone. The Magas prospect is associated with one of 3 such sub-parallel mineralised structures within the Project area with the others (Manat River and Panuraon) requiring systematic grid based exploration and subsequent drilling.

 

The Magas Prospect is centred on a sub-circular feature termed the Magas Intrusive Complex located in the southern part of the Project area with approximate dimensions of 2.5km north south and at least 1.5km east west. The Complex is obscured by younger post mineralisation volcanic cover to the east. The Magas Intrusive Complex is interpreted to be an intrusion related diatreme comprising coherent bodies of andesite porphyry, monomict andesite porphyry breccias and polymict breccias. Fragments and megalithic blocks of “basement “ diorite and andesite become more prevalent towards the margin of the Complex. Breccias contain mineralised vein fragments and are also cut by vein and stockwork development. The Complex is spatially associated with widespread hydrothermal alteration including advanced argillic alteration near the core of the system.

 

Detailed mapping and channel sampling by Indophil at Magas has defined a NNW trending and structurally controlled mineralised vein and stockwork zone termed the Magas Vein Zone (MVZ) previously intersected by WMCE drillhole MNTD6.  The MVZ has been the focus of a diamond drilling program by Indophil comprising 16 holes (MNTD7 to 22) for a total of 2,830m with holes drill initially at 50m intervals along strike and later stepouts to the north at approximate 100m intervals. Excluding drillhole MNTD14, which is a major 420m stepout along trend to the southeast, the MVZ has been defined by drilling over a strike length of 1000m and a vertical range of at least 350m taking into account local topographic variations. 

 

This drilling has produced encouraging results with all holes intersecting significant gold-silver-lead-zinc vein and stockwork style epithermal mineralisation hosted by altered intrusive and volcanic rocks.  These altered host rocks form a wide (up to 80m) low-grade halo (usually 0.2g/t to 1.0g/t gold) to mineralised zones.  Higher grade shoots of mineralisation appear to be located at the intersection of the MVZ and sets of northwest trending splay veins, although the current drilling pattern is not sufficiently dense to define the limits of such shoots with any confidence. Drilling of the MVZ was considered to be of sufficient density to estimate an inferred mineral resource comprising 3.066 million tonnes at 2.76g/t gold, 17.8g/t silver, 0.60% lead and 1.20% zinc potentially amenable to open pit development and based on a 1g/t gold cut-off grade. Given that the MVZ is open along strike and at depth, this resource estimate is regarded as extremely conservative.

 

Geological mapping, rock chip and channel sampling, grid-based soil sampling and a recent induced polarisation survey suggest that the structure hosting the MVZ can be traced discontinuously to the Katungbuan Prospect located approximately 2km NNW of the northernmost limit of drilling at Magas.  Post-mineralisation cover may explain part of this discontinuity. Drilling by WMCE at Katungbuan intersected narrow but significant intervals of epithermal veining and associated alteration. 

 

In addition to the MVZ epithermal mineralisation, the SSE stepout hole MNTD14 intersected pervasive porphyry copper-gold style mineralisation over a 227m downhole interval assaying 0.19% copper and 0.30g/t gold. The epithermal mineralisation event is thought to be temporally associated but probably telescoped on this porphyry system. 

 

The Indophil-Aldevinco joint venture now plans to undertake a detailed delineation and resource definition diamond drilling program focussed on the MVZ to arrive at a measured and indicated resource on which mine scoping studies will be based. Concurrent community, environmental and metallurgical programs are planned.

 
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