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Mamut |
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Sabah, Malaysia |
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Cu Au
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Super Porphyry Cu and Au
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The Mamut porphyry copper deposit is located on the flank of Mount Kinabalu, 12 km SE of its 4095 m peak, at an elevation of 1300 to 1500 m above sea level, on northern Borneo Island, in the Malaysian State of Sabah.
(#Location: 6° 1' 38"N, 116° 39' 21"E).
Copper anomalies were first detected in a Malaysian Geological Survey regional geological and geochemical program, as reported in Fitch (1958), who noted the presence of anomalous copper in the basalts and ultrabasic rocks in the Labuk Valley. From 1963, the United Nations special fund financed the Lubuk Valley Project which involved continuous stream sediment surveys led to the discovery of anomalous values in the area of where the Mamut ore deposit was located (Cooper et al., 1964; Woolf et al., 1965; Newton-Smith, 1965; Lepeltier, 1971). State government research (Kirk, 1966) led to an open tender for exploration which was awarded to the Overseas Mineral Resources Development Company of Japan. That company continued exploration, drilling, and underground development of the deposit, culminating in open pit mining and production that began in May 1975. Mining continued until closure in 1999 after producing 2.1 million tonnes of concentrates containing 520 000 tonnes of copper, 39 tonnes of gold and 255 tonnes of silver. The deposit approximately lies within the >300 ppm Cu geochemical soil sampling contour.
The deposit is developed within a quartz monzonite porphyry stock that is a satellite/apophysis of the 9 Ma (K-Ar; Jacobson 1970) Upper Miocene, K-rich Mount Kinabalu batholith that covers and area of ~40 x 20 km (Jacobson, 1970; Kosaka and Wakita, 1975, 1978). Whereas other petrographically similar K-rich quartz monzonitic stocks are found around the Mount Kinabalu batholith, major porphyry Cu mineralisation is restricted to the Mamut stock with traces of copper in the Bambangan area ~3 km to the west and minor mineralisation at the Mount Nungkok prospect which is associated with low K intrusive rocks 20 km to the NW (Bull, 1976). The district also includes multiple granodiorite intrusives, collectively referred to as the Kinabalu Magmatic Zone. The deposit lies within the up to 90 km wide, NW-SE trending, sinistral tectonic zone known as the Kinabalu Fault Zone that cuts across central Sabah and has localised multiple intrusions and extrusions of acidic and ultrabasic rocks, including the the Kinabalu Magmatic Zone. This tectonic zone has been recognised as a suture of collision between the
Kalimantan micro-continent and the Sulu arc (Hamilton, 1979).
Geology
The Mamut deposit is surrounded by sedimentary rocks correlated with the Crocker and Trusmadi formations of the Rajang Group (Kasama et al., 1970; Jacobson, 1970; Tokuyama and Yoshida, 1974). These rocks are widespread across northern Sabah, and comprise a thick flysch-type sedimentary succession. In the Mount Kinabalu area, the sedimentary rocks mainly consist of siltstone to fine-grained sandstone with some alternating turbidites, accompanied by serpentinised ultramafic rocks and igneous intrusions. The sedimentary rocks have been divided into:
• Undifferentiated sedimentary and metamorphic rocks of uncertain age, although from geological relationships, are interpreted to be stratigraphically older than the Trusmadi Formation. They comprise two recognisable members: i). the E Member, composed of inter-fingering spilite, spilitic tuff, and slate in its lower part, and rhythmic alternations of sandstone and mudstone in its upper sections; ii). the W Member, which overlies the E Member and is composed of well-sorted, graded and rhythmically bedded sandstone and mudstone with common slump structures.
• Trusmadi Formation, subdivided into the: i). the Lower or L Member, which is >400 m thick and widely developed in and around the Mamut deposit. It is composed of fine grained sandstone at its base, grading upward into massive, poorly sorted, poorly graded, coarse sandstone. It also includes layers of spilite, spilitic tuff and intercalations of mudstone. This member is an important host to ore at Mamut, particularly the fine sandstone of its lower sections which constitute ~20% of the host rock to ore. ii). the Upper or U Member, which is >500 m in thickness and consists of black shaly mudstone which contains numerous angular to sub-angular blocks of sandstone and siltstone. It also contains local intercalations of sandstone and thin layers of green tuff.
• Croker Formation, which is widely exposed in western Sabah and consists of rhythmic alternations of sandstone and shale to the SW of the Mamut area with well-developed slump structures. The formation may range from Paleocene to lower Miocene in age, and is fault juxtaposed against the Trusmadi Formation (Jacobson, 1970).
• Serpentinised peridotite, numerous bodies of which occur along the 'Kinabalu Fault' tectonic zone and in the Mamut mine area, where one body occurs within and parallel to the bedding of the lower Trusmadi Formation and has been interpreted to represent a concordant intrusion. The orientation of antigorite and talc within the serpentinite is mostly aligned parallel the bedding of the host rock, although locally, the serpentinite bodies are discordant to bedding. It has been fractured and mineralised and comprises ~30% of the ore zone. The ultramafic rocks to the SE, at Darvel Bay, have been interpreted to represent oceanic lithosphere obducted onto the Borneo microcontinent (McManus and Tate, 1986), whilst garnet lherzolite near Ranau, ~6 km south of Mamut, is interpreted to have been derived from the upper mantle, beneath the Kalimantan micro-continent (Imai and Ozawa, 1991).
Three main igneous rock types have been differentiated:
• Quartz monzonite porphyry, a north-south elongated, 1200 x 350 m stock of which forms the core to the Mamut deposit. Phenocrysts comprise 3 to 4 cm crystals of orthoclase and smaller plagioclase, hornblende, biotite and pyroxene, set in a groundmass of fine grained quartz, orthoclase, biotite, hornblende and apatite. The phenocryst-groundmass ratio is ~1:1. It has a high K2O content. Quartz monzonite porphyry hosts ~50% of the total ore.
• Granodiorite porphyry, which occurs as widespread dyke swarms. It is composed of phenocrysts of plagioclase with lesser orthoclase, quartz, biotite, and hornblende with a phenocryst to groundmass ratio of ~1:4. The granodiorite porphyry dykes are interpreted to be post-mineral and post-date the quartz monzonite porphyry and contain xenoliths of both hornfels and microdiorite.
• Microdiorite porphyry, which also occurs as widespread dyke swarms, and comprises fine-grained amphibole, biotite, plagioclase, garnet and minor orthoclase, quartz and apatite. It is locally strongly silicified and pyritic and contains chalcopyrite.
Faulting in the deposit area has been divided into two stages, the first which is related to emplacement of the porphyry and its subsequent mineralisation and a second, post-mineral stage of Quaternary regional faulting. Four pre-mineral fault or fracture systems have been recognised at Mamut. The principal faulting in the mine area is north-south, which is widely distributed. NW-SE, NE-SW and east-west faults and fracture systems are also present. The north-south and east-west sets are combined to fracture and offset large blocks in the mine as horsts and grabens, and all are mineralised, although subsequent post-ore displacement is evident where these faults cut quartz monzonite porphyry mineralised quartz veins. Fracture-controlled quartz vein mineralisation has a most common NE orientation. The Mamut deposit lies in the northern section of a large, circular, topographic anomaly evident on aerial photography, characterised
by a circular pattern of drainage and ridge pattern. Major east-west-trending faults north and south of Mamut are members of a regional Quaternary fault system in which blocks to the south of each have been progressively down-dropped.
Mineralisation and Alteration
The Mamut deposit is hosted by quartz monzonite porphyry, serpentinised peridotite and fine-grained sandstone to siltstone of the Trusmadi Formation (Kosaka and Wakita, 1975, 1978; Akiyama, 1987). The Mamut quartz monzonite porphyry is divided into an east and a west body by a north-south fault. Both are mineralised. The east body is elongated NE-SW, with a sharp boundary between ore grade mineralisation and wall rocks. It contains predominantly NE striking quartz veins that carry much of the mineralisation, with a little disseminated sulphide in the host porphyry. In contrast to the west body, which is strongly silicified, silicification is sparse in the east body, where the dominant alteration is near complete sericitisation. The east porphyry body is discontinuous at depth and hosts the smaller proportion of the total reserve (Kósaka and Wakita, 1978).
The west body represents the main bulk of quartz monzonite porphyry, and is elongated north-south, and dips steeply to the east. In contrast to the east body, where the porphyry contains abundant xenoliths of sandstone and siltstone,the west body quartz monzonite porphyry is inclusion poor, and is interpreted to represent a deeper section of the stock. Also, in contrast to the east body, the contact zone between the west quartz monzonite porphyry and contiguous wall rocks is characterised by strong silicification, and by an abundance of quartz veins, which are the principal host of copper mineralisation. High-grade copper mineralisation is found in both the veins, and as disseminations within the porphyry. A NE-striking, late syn-mineral, granodiorite porphyry dyke containing abundant microdiorite and altered shale xenoliths contains low grade Cu, but scattered zones of high-grade gold (Kósaka and Wakita, 1978).
The principal ore mineral is chalcopyrite, which occurs together with pyrite and pyrrhotite. Magnetite and hematite, although in part comprising primary magmatic rock forming minerals, are widely distributed, as is minor sphalerite, galena and molybdenite. Both Cu and Au are principally hosted as disseminated mineralisation and in the quartz stockworks (Akiyama, 1984). Supergene copper minerals include chalcocite, digenite, covellite, azurite and malachite. Although some supergene enrichment is indicated, it is only minor, having been eroded during rapid uplift.
Mamut is unusual in that it contains widely distributed pyrrhotite, mostly concentrated within the high-grade copper shell. Whilst pyrite is more abundant within the deposit as a whole, characteristic samples from the high grade shell have pyrrhotite : chalcopyrite : pyrite ratios of (6 to 8.5) : (1.5 to 2) : (1 to 2).
The distribution of sulphide minerals is zoned, with the high grade copper shell straddling the periphery of the stock, corresponding to an aureole of strong silicification, immediately internal of which, the highest gold grades are found. Silver and molybdenum are strongest just to the outside of the high-grade copper shell, whilst lead and zinc are concentrated outside of that zone again (Kósaka and Wakita, 1978).
The overlapping Pb, Zn and a late Sb mineralisation pulse are spatially controlled by the NNE-trending fractures, accompanying a phyllic alteration halo surrounding the inner advanced argillic envelope within the fractures. This fracture controlled mineralisation cuts the principal disseminated and stockwork Cu-Au orebody. Iami (2000) interpreted the paragenetic sequence of mineralising events incorporated the following four stages;
i). formation of disseminated Cu-Fe sulphides and magnetite mineralisation, principally chalcopyrite and pyrrhotite as well as magnetite and pyrite, are associated with biotite-potassic alteration that is intensely developed along the outer intrusive contact of the quartz monzonite porphyry. Rare cubanite is also found as exsolution lamellae in chalcopyrite. In the mineralised biotite zone, hornblende phenocrysts of the quartz monzonite porphyry are totally replaced by hydrothermal biotite. In contrast, within the 'barren core', inboard of the outer high grade Cu shell, magnetite predominates over Cu-Fe sulphides, and the hornblende is instead hydrothermally replaced by tremolite-actinolite and chlorite. The amount of pyrite in the disseminated biotite altered ore is subordinate to chalcopyrite and pyrrhotite, whose abundance varies widely. Serpentinised peridotite is totally altered to biotite along the intrusive contact with the quartz monzonite porphyry and has ubiquitous disseminated magnetite and Cu-Fe sulphides. Fine-grained sandstone to siltstone country rock is also strongly biotite altered with mosaic aggregates of orthoclase and recrystallised quartz particularly along the intrusive contact with the same porphyry. External to the high grade Cu-shell, the siltstone is altered to an assemblage of orthoclase biotite. .
ii). quartz veinlet stockworks, where pyrite, as well as chalcopyrite, is common in place of pyrrhotite. Pyrrhotite grains in overprinted biotite altered disseminated ore are sometimes replaced by pyrite along their margin, and are cut by pyrite stringers. Aggregated secondary biotite is often
replaced by chlorite in contact with stockwork quartz veinlets. Molybdenite occurs as stringer veinlets or associated with quartz veinlets. It seems to have formed later than the principal stockwork quartz veinlets. Au-rich electrum is found in the biotite zone in association with disseminated chalcopyrite, cubanite and pyrrhotite, and in quartz veinlet stockworks accompanied by chalcopyrite. It also occurs in the massive magnetite dissemination associated with chalcopyrite in the hydrothermally biotite altered serpentinite (Akiyama, 1984).
iii). fracture controlled Pb and Zn sulphides, and the subsequent Sb mineralisation (see the next point), are spatially controlled by north-south to NE-trending fractures that crosscut and overlap the principal Cu-Au orebody, and are composed of disseminated and veinlet stockwork mineralisation. These phases are associated with quartz and dolomite veinlets that are surrounded by the phyllic alteration
halo and an advanced argillic envelope which includes kaolinite and/or nacrite. Pyrite of these phases sometimes encloses minute stannite, tetradymite and bismuthinite crystals.
iv). overlapping fracture-controlled Sb mineralisation, as previewed above. Stibnite is found in association with gangue dolomite where Sb mineralisation occurs alone. Reaction products such as tetrahedrite and jamesonite are present when Sb mineralisation overlapped the previously formed chalcopyrite and galena, whilst spherical, framboidal-like aggregates of fine-grained pyrite are locally enclosed in chalcopyrite interstices. Some of the tetrahedrite-group minerals associated with stibnite are significantly Ag-rich. It is therefore concluded that Ag is accompanied by the overlapping Sb as the latest stage of the hydrothermal system.
According to Kósaka and Wakita (1978) the overall mineral paragenesis comprises pyrite → pyrrhotite and chalcopyrite → galena → sphalerite → stibnite, with some late chalcopyrite in the youngest fractures.
External to the main mineralisation, propylitic alteration, characterised by the assemblage epidote-chlorite-calcite occurs as veinlet alteration, although its extent is not clearly defined.
K-Ar dating of hydrothermal biotite which replaced the phenocrystic hornblende of the Mamut quartz monzonite porphyry at the ore shell near the intrusive
contact, yielded and age of 7.0 ±0.2 Ma (Imai 2000).
Resources and Reserves
Prior to mining, the total ore reserve was estimated (Kosaka and Wakita, 1978 and Akiyama 1984) to be:
178.747 Mt @ 0.476 wt.% Cu (Akiyama 1984), including
mineable ore of 83 Mt @ 0.59 wt.% Cu, 0.6 g/t Au and 4 g/t Ag (Kosaka and Wakita, 1978).
Subsequent exploration outlined and additional 36 Mt of ore (Kosaka and Wakita, 1978).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2000.
Record last updated: 26/8/2023
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.
Mamut
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Selected References:
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Imai, A., 2000 - Genesis of the Mamut Porphyry Copper Deposit, Sabah, East Malaysia: in Resource Geology v.50, pp. 1-23.
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Imai, A., 1994 - Sulfide globules associated with a felsite intrusion in the Mount Kinabalu Quartz Monzonite, Sabah, East Malaysia; Sulfide melt immiscibility in a highly silicic melt: in Econ. Geol. v.89, pp. 181-185.
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Kosaka, H. and Wakita, K., 1978 - Some Geologic Features of the Mamut Porphyry Copper Deposit, Sabah, Malaysia: in Econ. Geol. v.73, pp. 618-627.
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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