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Rosario Bunawan district - Co-O

Mindanao, Philippines

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The Co-O gold mine is located within the Rosario-Bunawan district on the East Mindanao Ridge, between the towns of Rosario in the north and Bunawan in the south, ~7 km east of the national highway, and ~200 km north of Davao City on Mindanao Island, Southern Philippines (#Location: 8° 15'N, 126° 03'E).

Regional Setting

The Philippine archipelago represents a complex tectonic collage resulting from convergence of the Eurasian, the Philippine Sea, and the Indo-Australian plates (Rangin 1991; Pubellier et al., 2003, 2004), with Mindanao being the southernmost major island of the archipelago. It is bounded to the west by the Negros and the Cotabato trenches marking eastward-dipping subduction zones, in their waning stages. Late Miocene to Pleistocene (~5.4 to ~2.5 Ma) volcanic rocks are attributed to tectonic activity along these trenches. In contrast, the steep west-dipping seismic zone below the southern Philippine Trench, along the eastern margin of Mindanao, is interpreted to be related to the recent initiation of subduction (Cardwell et al., 1983; Pubellier et al., 1991). Hornblende andesite porphyry plugs dated at 1.4±0.5 Ma (K/Ar, UNDP, 1984) may be related to subduction processes along this trench.

The East Mindanao Ridge is defined on its western margin by the Agusan-Davao Basin that formed during regional NE-SW extension in the early to middle Miocene, governed by major NNW-SSE, north-south and ENE-WSW trending faulting (Pubellier et al., 1991). Subsequet oblique collision between the Philippine Sea and the Eurasian plates,in the middle Miocene at ~15 Ma, iniated the NNW-SSE trending sinistral Philippine Fault, a longitudinal structure that extends throughout the length of the Philippine archipelago, dislocates the sediments of the Agusan-Davao Basin and was still active to recent times (Aurelio et al., 1991; Pubellier et al., 1991; Rangin 1991). To the east of the ridge, NNW-SSE-trending normal faults were reactivated by sinistral strike-slip deformation, with several splays of the Philippine Fault cutting the East Mindanao Ridge (Pubellier et al., 1991). At the beginning of oblique collision, the far-field stress in the East Mindanao Ridge changed from an initial NW-SE compression to E-W compression in the late Miocene-Pliocene to recent times (Pubellier et al., 1991).

The Philippine Fault in Mindanao is the suture zone separating the Eurasian plate to the west (overlain by Miocene to Holocene volcanic and sedimentary rocks) and the Philippine arc to the east, comprising a basement of greenschist facies metavolcanic and metavolcaniclastic rocks, overlain by a volcanic sequence and interlayered sedimentary rocks of Eocene to Miocene age (Aurelio et al., 1991; Rangin 1991; Pubellier et al., 2003, 2004).

Metamorphosed pre-Paleocene ultramafic rocks, are confined to the northern part of the island (Pubellier et al., 1991), where they are overlain by a volcanic sequence and interlayered sedimentary rocks of Eocene to Miocene age (Aurelio et al., 1991). The latter includes massive, middle Miocene andesites and dacites (20 to17 Ma K/Ar; Bellon and Rangin 1991), capped by limestone of upper Miocene to Pliocene age (Pubellier et al., 1991), overlain in turn by conglomerates with upper Miocene carbonate clasts, greywackes, and sandstones. This clastic succession is intercalated in its upper part by andesitic lava flows dated at 2.5 to 0.25 Ma (K/Ar, Pubellier et al. 1991). Intrusive rocks of two types of are distinguished:
i). diorites, which are associated with porphyry copper mineralisation intruded in the middle Miocene; and
ii). hornblende andesite porphyry plugs, which were dated near Surigao at 13±0.6 Ma (K/Ar) and in Davao del Norte at 1.4±0.5 Ma (K/Ar) (UUNDP, 1984).

Co-O MIne

The Co-O gold mine and several small-scale operations/prospects are located within the Rosario-Bunawan district, exploiting epithermal gold mineralisation that is confined to narrow (0.2 to 4 m) low-sulphidation quartz-chalcedony-calcite veins in volcanic and volcaniclastic wall rocks. At Co-O, these veins extend over an interval of at least 600 m and have been delineated to a maximum depth of 350 m, although still open to the west east and at depth in 2009. Three major vein orientations are recognised:
i). the NNW-SSE-trending set with a sinistral strike-slip sense of deformation (the Philippine Fault trend);
ii). the ENE-WSW-trending dextral strike-slip set (Palawan trend) and associated Riedel veins; and
iii). a WNW-ESE trending conjugate set (Co-O trend).

Three structural stages are defined:
i). extensional shear or shear veins formed in all three trends during regional NW-SE compression and near vertical vein opening (D);
ii). reactivation of veins in the Philippine Fault, veins associated with the Palawan, and, to a lesser extent, the Co-O trends during E-W compression and near horizontal N-S oriented vein opening (D
). New D extensional shear or shear veins formed in the Philippine Fault, and structures associated with the Palawan and associated Riedel trends;
iii). the D
-stage block faulting, which subsequently displaced all of the auriferous veins.

The Rosario-Bunawan district is located between two Philippine Fault splays, which acted as a lateral ramp system during the oblique convergence of the Philippine Sea plate and the Eurasian plate, which resulted in a change from the compressional D
to a transpressional D regime, which was a prerequisite for the two-stage vein opening and hydrothermal mineralisation, leading to an economic gold enrichment.

The wall rocks that host the gold mineralisation are dominated by middle to late Miocene andesitic to basaltic volcanic rocks, including lava flows, subvolcanic stocks and various pyroclastic units. The middle Miocene limestones are thrust over this suuite in the northeastern part of the district. In addition, limestones form a tectonic klippe to the east of the Co-O mine, while these lithologies are locally intruded by a diorite body which has a hornfels halo several metre-wide in the volcanic rocks and the limestones. All of the lithological units are unconformably overlain by sandstones and conglomerates, which contain alluvial gold.

Most of the auriferous veins in the district are texturally and mineralogically similar, having massive centres with grey to milky quartz-chalcedony and minor calcite, barite, pyrite, chalcopyrite, and gold. The massive centres in many cases are rimmed by laminated or crustiform fabrics. Up to 20 cm wide black silica veins are spatially associated with the massive veins. Contacts with the wall rocks are marked by breccias, containing wall rock and black silica fragments up to 10 cm in diameter, cemented by quartz and chalcedony. Elevated gold grades of between 2 and 250 g/t are confined to the 0.2 to 4 m wide veins, whereas the alteration selvages are largely barren.

Four mineral assemblages can be distinguished in the auriferous veins:
i). "Black silica" comprising ~95 vol.% quartz with a very fine-grained, cross-hatched texture and very fine-grained, disseminated pyrite that is responsible for the black color (cf., Simmons and Christenson 1994; Dong et al., 1995);
ii). Quartz-chalcedony-calcite-sericite veins with variable amounts of pyrite, chalcopyrite, sphalerite, barite, rutile, and galena. In the Co-O mine these average ~ 8 g/t Au;
iii). Calcite-barite-pyrite veins with only minor quartz-chalcedony and with a texture ranging from blocky, bladed to crustiform, with mm-scale banding defined by calcite and locally, pyrite bands;
iv). Crustiform chalcedony and massive sulphide comprising chalcopyrite-pyrite-galena-sphalerite coating heterolithic, altered wall rock fragments and massive sulphide fragments that are variably sized, up to 5 cm in diameter. Locally, <0.1 mm diameter spherules comprising a chalcopyrite core and marcasite rim are observed. Trace minerals in the massive sulphide coating are chalcedony and barite.

Three alteration zones are distinguished forming halos of variable thickness surrounding all of the auriferous quartz veins symmetrically:
i). regional-scale distal alteration zone, characterized by chlorite-epidote-calcite-quartz-pyrite-illite-muscovite-mixed layer assemblages, probably representing a propylitic-style, with local traces of a chalcopyrite-sphalerite-luzonite-enargite assemblage;
ii). grey-white muscovite-rich alteration zone that forms an intermediate alteration zone and is up to several tens of metres wide, comprising an assemblage of muscovite-chlorite-calcite-quartz-pyrite±chalcopyrite-kaolinite-rutile; and
iii). 10 to 20 cm wide, pervasive, white sericite (mixed layer silicate) alteration zone that represents the alteration proximal to the auriferous veins, comprising illite-muscovite-calcite-quartz/chalcedony-pyrite±anhydrite±kaolinite±ankerite±sphalerite±enargite.

Published reserves and resources in August 2012 (Medusa Mining, 2012) to a 3 g/t Au cut-off, were:
    Indicated + inferred resources - 6.215 Mt @ 10.1 g/t Au
    Indicated + inferred resources - 1.820 Mt @ 9.7 g/t Au (included within the resource).

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Kolb J and Hagemann S,  2009 - Structural control of low-sulfidation epithermal gold mineralization in the RosarioBunawan district, East Mindanao Ridge, Philippines: in    Mineralium Deposita   v.44 pp. 795-815


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