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Miwah |
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Sumatra, Indonesia |
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Au Ag
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Super Porphyry Cu and Au
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The Miwah high-sulphidation epithermal gold deposit is located within the Regency of Pidie, Province of Nanggroe Aceh Darussalam, Indonesia, approximately 130 km southeast of the provincial capital Banda Aceh.
Sumatra forms the southwestern margin of Sundaland, an extension of the Eurasian continental plate, and comprises accreted exotic fragments of continental plates and volcanic arcs derived from the breakup of Gondwana during the Late Palaeozoic and Mesozoic. On-going pulses of subduction in the Mesozoic, Tertiary and Quaternary resulted in the development of magmatic arcs along the length of Sumatra. The Australia-Indian plate is currently being obliquely subducted beneath Sumatra producing a northwesterly oriented fore-arc basin (between the subduction trench to the SW and the west coast of Sumatra), several generations of magmatic arc, the Sumatra Fault System (SFS - 50 to 100 km inland of, and parallel to the SW coast) and a back arc basin (over much of the remainder of the island to to the NE of the fault system). Mineral deposits distributed along the length of Sumatra are associated with these different ages of magmatic arc.
The Permo-Carboniferous Sundaland continental basement rocks, interpreted to underlie much of Sumatra, are mostly exposed in northern- and western-central-Sumatra, and sporadically
and less abundantly in southern Sumatra. In the region surrounding Miwah in northern-Sumatra, these basement rocks comprise argillites, sandstone, quartzite and limestone of the Tapanuli and Peusangan Groups. Basement is overlain by subaerial to oceanic arc assemblages of the Woyla Group, which were either accreted to, or thrust onto, the proto Sundaland southern margin during the Mesozoic. Following periods of up-lift and partial erosion of the basement rocks, overlying sedimentary rocks were deposited from the Tertiary onward. In the vicinity of Miwah, east of the SFS, the Woyla Group is overlain by Tertiary volcanics and volcanogenic sediments, ranging in age from Eocene to Pliocene.
The geology of Sumatra is dominated by predominantly NW-SE aligned arc-parallel structural lineaments that reflect the trend of the dextral SFS, which has been active for much of the Tertiary. Arc-normal, or east-west to NE-SW faults are less common as these tend to be tension release structures related to trans-tensional forces which built-up in the rocks adjacent to the SFS.
Mineral occurrences within Sumatra appear to be associated with at least three magmatic arcs:
(i) the Neogene (Miocene-Pliocene) Sunda-Banda arc along the western coastal range of Sumatra, which comprises basaltic-andesitic lava flows and small shallow intrusions. Mineralisation within this arc includes the Martabe high sulphidation gold deposit, the Krueh, Lebong Tandai and other polymetallic deposits hosted in volcano-plutonic centres;
(ii) the middle to late Cretaceous Sumatra-Meratus arc in the centre, where tin deposits predoinate, with minor copper and gold occurrences in the Singarak cluster and in areas north of Tembang (formerly Rawas); and
(iii) the arcuate Neogene Aceh arc, which is only found in northeastern Sumatra, hosts the Miwah high-sulphidation epithermal gold mineralisation and other epithermal and porphyry copper-gold occurrences at Butung, Tangse, Pisang Mas, Sable, Woyla, Abong, Takengon and Barisan.
The Miwah deposit is hosted within a sequence of shallow dipping Plio-Pleistocene andesitic to dacitic lavas, tuffs and agglomerates of the Leuping Volcanics, located on the southern flank of the Sague Volcanic Centre. The lowest exposed unit is a monotonous andesite, overlain by the main host to gold mineralisation, an intertonguing andesite and dacitic to andesitic agglomerate. Immediately north of Miwah, the Leuping Volcanics are intruded by an 800 m diameter body of unaltered Pliocene biotite-hornblende rhyodacite, that is faulted and flow banded, displays autobrecciation near its margins, and has extensively fractured and altered the adjacent volcanic sequence. A thin, recent, unaltered subaerial tephra blanket of poorly consolidated quartz feldspar hornblende ash, which is related to the nearby active Peut Sague volcano, is unconformably draped over palaeosols and part of the sub-cropping mineralisation.
The Miwah gold deposit is adjacent to a 70°-trending arc normal fault splay of the SFS. Intense and pervasive hydrothermal alteration has obliterated most rock textures making rock identification difficult in the deposit area, although ghost remnants of the original phreatic breccia are occasionally visible within the silica alteration zone. At Miwah Bluff (which forms the western part of the prominent, resistant altered and mineralised ridge), the principal host rocks are phreatomagmatic diatreme breccia and/or possible volcanic breccias, while at Block M that occurs on the north-eastern end of the same altered ridge, mineralisation occurs within gently dipping andesite tuff, volcanic breccia and lava which have been intruded by narrow hydrothermal breccias, andesite and dacite flow domes, and late-stage, thin, strongly magnetic hornblende-andesite porphyry dykes.
The andesitic volcanic rocks, domes, dykes and diatremes have been subjected to extensive advanced argillic and argillic alteration, zoned from: (i) central vuggy to dense quartz-rutile-pyrite, passing outwards and down into (ii) quartz-alunite, and (iii) marginal zones of quartz-kaolinite, and a broad halo of (iv) low temperature illite-smectite, to (iv) peripheral kaolinite-smectite-illite-pyrite assemblages. This alteration overprints earlier propylitic and locally, phyllic assemblages.
The central quartz and quartz-alunite alteration occur as: (i) restricted zones within inferred NNW trending structures which parallel the principal fault direction and outcrop along the eastern margins of the prospect; (ii) lesser NNE trending structures outcropping as thin ridges, parallel to another of the key faults; (iii) broad zones within the diatreme breccias, possibly reflecting the high permeability of the breccia matrix; and (iv) shallow, up to >100 m thick north to northeast dipping ledges, hosted by volcanic rocks (Corbett and Leach, 1997).
The early quartz and quartz-alunite alteration produced brittle host rocks that form semi-continuous, structurally and lithologically controlled, shallow dipping (10 to 15°), broadly tabular and laterally extensive body of massive, residual vuggy silica-sulphide alteration that generally carries >1 g/t Au and forms a resistant ENE-trending whale-back ridge. This ridge, that can be traced for over 1000 m along strike, has an average width of 300 m and a vertical extent of up to 200 m. Similar epithermal gold mineralisation, is also found at South Miwah Bluff, 300 m to the SSW of Miwah Bluff, where sub-cropping blocks of residual vuggy silica-sulphide alteration have been mapped over an area of approximately 650 by 300 m, and at Sipopok, 1.5 km to the NE.
The mineralisation occurs in two styles: (i) a series of stacked tabular bodies (lodes), the largest of which is 1200 m by 300 to 400 m wide, and ~200 m thick, averaging 1.28 g Au/t gold, with higher grade sections of >4 g/t Au, although the deeper tabular zones are thinner and lower grade. At least 8 lodes are known at the main Miwah deposit, down to a depth of 300 m below the main ridge. The upper lodes are separated by thin (1 to a few metres) barren zones, while those at greater depth may be separated by intervals of as much as 30 m; and (ii) vertical diatreme breccia feeder zones that underlie and cut through the tabular body and have gold contents that are significantly higher, often around 30 g/t.
These brittle hosts were subsequently fractured and brecciated during mineralisation which progresses from early pyrite-rich quartz veins, to later breccia zones and veins of brassy pyrite, overgrown by copper sulphide minerals, dominated by luzonite at shallow levels in the south, and enargite at deeper levels to the north. Primary gold mineralisation is commonly associated with disseminated fine-grained pyrite and minor arsenopyrite, mostly in vuggy silica and silica-alunite alteration facies. Higher gold grades are preferentially associated with zones that have undergone multiple hydrothermal events, e.g., reactivated breccias, veins, fractures and superimposed alteration effects, while peripheral argillic altered rocks generally contain <5% pyrite and are poorly gold mineralised.
Copper mineralisation is commonly associated with the vuggy silica and advanced argillic alteration facies, and occurs as tennantite/tetrahedrite and cubanite, and also as fine-grained crystalline enargite and luzonite, while hypogene covellite and rare chalcocite are locally found at depth, with tennantite in more distal locations to the east. The copper sulphides are intergrown with quartz and banded chalcedony, and with alunite locally at depth, while native sulphur commonly occurs as open cavitiy and fracture fill. Although copper values are often anomalous within the gold zone, they rarely exceed 0.1% Cu. Gold, copper and arsenic are closely correlated, possibly associated with enargite/luzonite, while gold is inferred to have been locally deposited with early arsenopyrite or arsenical pyrite. Cu:Au ratios in drill hole assay data increase with depth and to the north.
Late-stage chalcedonic and banded crustiform quartz veinlets locally occur within the vuggy silica, suggesting a superimposed low-sulphidation mineralising event. Moderate oxidation within the residual vuggy silica zone is widespread and present to around 60 m depth within Miwah Bluff and Block M. Beneath this depth, oxidation is more restricted to fractures and breccia zones.
Resource estimates in 2011(Wong et al., 2011) at a 0.2 g/t Au cut-off, were:
Oxide ore - 2.0 Mt @ 0.5 g/t Au, 1.33 g/t Ag,
Mixed ore - 76.2 Mt @ 0.98 g/t Au, 2.75 g/t Ag,
Primary ore - 25.7 Mt @ 0.94 g/t Au, 2.55 g/t Ag,
TOTAL at a 0.2 g/t Au cut-off - 103.9 Mt @ 0.94 g/t Au, 2.68 g/t Ag,
TOTAL at a 0.6 g/t Au cut-off - 61.1 Mt @ 1.31 g/t Au, 3.36 g/t Ag.
This summary is closely based on sections of Taylor, 2011 available on the East Asia Minerals website, and Wong et al., 2011.
The most recent source geological information used to prepare this decription was dated: 2011.
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.
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Selected References:
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Wong H, Setyawan M P H and Taylor I, 2011 - Geology and discovery history of the Miwah gold deposit, Aceh, Sumatra, Indonesia: in NewGenGold 2011, Conference Proceedings, Perth WA, Louthean Media, Perth pp. 191-199
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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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