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Acoje

Luzon, Philippines

Main commodities: Ni Cr Co PGE PGM
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The Acoje massif contains primary magmatic, PGE-bearing nickel sulphide, chromitite and secondary lateritic nickel deposits.   It is part of the mafic-ultramafic Zambales ophiolite complex, near Santa Cruz, in the province of Zambales, the Philippines, approximately 270 km by road northwest of Manila on the mid-west coast of Luzon (#Location: 15° 45' 30"N, 119° 59' 30"E).

The Zambales Complex is a supra-subduction zone ophiolite, related to the juxtaposition of a Mesozoic mid-ocean ridge-type block of back-arc crust and an island arc terrane. The two terranes are apparently separated by a sinistral strike-slip fault, the Subic Bay Fault Zone which is characterised by highly sheared, allochthonous outcrops and NW-SE trending magnetic and bouguer anomalies.

The primary sulphide and chromitite mineralisation is hosted within the Acoje critical zone that comprises a variably serpentinised lower ultramafic zone and a relatively less altered upper mafic zone.

The main rock type on the western margin of the complex, representing the basal cumulate sequence, is medium to coarse grained mantle harzburgite that has gradational contacts with a transition zone dunite to the east striking north-south and extending over a length of more than 12 km. The dunite is massive and granular, and consists of olivine with disseminated chromite and magnetite. The dunite is overlain by gabbro, olivine gabbro, norite and pegmatite. Deformational structures are not pronounced, although micro-fracturing is moderate to intense.

At least two chromitite and four PGE bearing nickel sulphide zones are found in the dunite adjacent to north-south trending, steeply east dipping, 1 to 25 m thick pyroxenitic lenses towards the eastern margin of the Ultramafic Complex. The chromite and nickel zones are closely associated but occur as separate bodies, the chromitite being older. These layers have been encountered over an 8 km strike length.

The Acoje mine produced over 3.3 Mt of high quality chromium concentrate from 1935 to 1993 and was historically one of the world's largest metallurgical chromite suppliers.

The four PGM bearing nickel sulphide zones, which are each up to 15 m thick, are associated with black dunite lenses several hundred metres to the east of the thick extensive chromitite zones. The black colour is thought to be the result of fine inclusions of magnetite and graphite within the silicate groundmass. The primary mineralisation in the sulphide zones comprises pyrrhotite-pentlandite intergrowths with minor chalcopyrite associated with PGE including platinum, palladium, rhodium, ruthenium, osmium and iridium with gold, silver and troilite.

Approximately 0.54 Mt of sulphide ore were mined over widths averaging 3 m between 1971 and 1975 to produce 10360 t of nickel concentrate, containing 1.2 t of Pt, Pd and Au. These sulphide zones have an overall grade that varies from 0.2% to 0.8% Ni and averaged 2.2 g/t PGE+Au in the blocks mined.

The melts responsible for this mineralisation are characterised by rocks of high-MgO basalt to boninitic composition which are believed to represent second or third-stage melts, carrying higher PGE budgets. The formation of the mineralisation was apparently affected by olivine, chromite and sulphide crystallisation.

Two separate sulphide associations have been defined, namely:

i). a troilite (±pyrrhotite)-dominated assemblage within the mafic zone, apparently representing the original mineralogy of the magmatically precipitated sulphides throughout the cumulate sequence; and
ii). a pentlandite-dominated suite within the ultramafic zone, which is interpreted to have evolved from the primary magmatic sulphides during low-temperature re-equilibration.

The paragenetic evolution from the magmatic assemblage to the low-temperature assemblage has been interpreted to be as follows:

i). pyrrhotite, pentlandite and chalcopyrite or cubanite, with magnetite,
ii). pyrrhotite, pentlandite and/or cubanite, with magnetite,
iii). troilite or mackinawite, pentlandite and magnetite,
iv). troilite or mackinawite, pentlandite, native Cu and magnetite,
v). pentlandite, native Cu and magnetite, and
vi). pentlandite, native Cu and Fe-Ni alloy with magnetite.

Comparisons between sulphide and coexisting silicate-hydrosilicate-oxide assemblages suggest this latter alteration occurred during retrograde serpentinisation of the Acoje massif. The serpentinisation is characterised by an assemblage of antigorite, chrysotile, asbestos, chlorites and other non-aluminous magnesian silicates.

Lateritic nickel mineralisation is developed over lateritised ultramafic bedrock above the 250-m elevation on relatively flat, plateaus or sub-plateaus and on gentle ridge slopes in areas of moderate relief. The laterite zones to the east and south where the topography is steeper, are thin and uneconomic.

The Acoje nickel laterite contains two layers, namely:

i). an upper red, iron rich soil or limonite horizon up to 4 m thick containing 0.65 to 1.2% Ni, 50 to 52% FeO, 1 to 3% MgO and 0.06 to 0.12% Co, and
ii). an underlying nickel rich rock saprolite horizon which is up to 4 or more metres thick with 2 to 3.5% Ni, 9 to 15% FeO, 25 to 30% MgO and 0.01 to 0.02% Co.

A JORC compliant resource of 33.8 Mt @ 0.95 % Ni and 0.07% Co has been delineated within the upper limonite zone (Rusina Mining, 2006) within portion of the lateritised zone, while earlier estimates over this and adjacent zones have indicated a total resource of 52.9 Mt @ 1.68% Ni.

The most recent source geological information used to prepare this summary was dated: 2006.    
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

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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