Martabe - Purnama, Ramba Joring, Uluala, Barani, Horas,
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The Martabe group of gold deposits, including Purnama Pit 1 (Purnama Dalam Zone), Purnama Timur, Barani (Pelangi), Horas, Horas Barat, Ramba Joring (Baskara), Tor Uluala (Kejora), Tor Uluala Barat, Uluala Hulu Extension and Uluala Hulu, define a more than 7 km long, by ~1km wide mineralised corridor near the town of Sibolga in the North Sumatra Province of Indonesia, on the west coast of Sumatra Island, some 200 km south of Medan (#Location: 1° 31' 21"S, 99° 4' 3" E).
The main Pit 1 exploits the Purnama Dalam Zone and adjoining Purnama Timur deposit over a 1 km NNW-SSE trend. The Barani Pit is just over 1 km to the SE, while the Horas and Horas Barat deposits are just under 2 km SSE of the Barat Pit. The Ramba Joring Pit is just over 1 km NNE of the Main Pit 1. The Tor Uluala, Tor Uluala Barat, Uluala Extension and Uluala Hulu deposits are ~2 km north, ~2.2 km NNW, 2.5 km NNE and 3.5 km NNE of the main Pit 1.
These deposits comprise high sulphidation epithermal mineralisation within Tertiary volcanics and sediments proximal to a splay of the regional scale, NNW- to NW-trending, Great Sumatra Fault Complex. Wrench tectonics imposed by the oblique subduction of the Indo-Australian plate to the SW below the Eurasian plate of which Sumatra is part, resulted in episodic fault activity, pulses of high level magmatism, multistage phreatomagmatic breccias, flow dome complexes, hydrothermal alteration and gold mineralisation. Local structure reflects the regional dextral strike slip tectonics as NW to NNW fault sets, and related NE trending en echelon conjugate extensional faults to the east which together have promoted and localised fluid flow to produce zones of intense silicification and veining. A third, east-west set of contractional faults with a reverse component is also evident.
The geology of the Martabe district is divided into two domains by the NW to NNW trending Purnama Fault. To the SW are little disturbed and weakly altered mudstones, siltstones, sandstones and basaltic to andesitic lavas. In contrast, to the NE a comparable sequence of lithologies, with the addition of volcanic breccias, have been subjected to a complex series of multiphase magmatic, phreatomagmatic and hydrothermal influences.
The oldest rocks in the Martabe area are Palaeozoic meta-sedimentary rocks of the Tapanuli Group. The eastern part of the area is dominated by Triassic granites (correlated with the Sibolga granite batholith to the northwest), which have both intrusive and fault contacts with the older rocks.
The oldest of the overlying Tertiary units is the Barus Formation, which mostly comprises sediments, mainly conglomerates and sandstones, with minor siltstones and shales, that underlie much of the Martabe area. The Barus Formation sediments are overlain by the Miocene age Angkola Volcanics, a series of basaltic and andesitic lavas and breccias. The porphyritic andesite and volcanic breccia are significant host rocks to mineralisation.
A Late Tertiary dacitic dome complex, was emplaced along the faulted western margin of the granites. This complex comprises an early series of fault-controlled phreatomagmatic intrusive breccias followed by an intrusive dacitic flow dome complex that includes a dacitic porphyry, which is also a significant host rock, and a hornblende andesite, which is a late-stage intrusive and is generally unmineralised (except at Ramba Joring). Multiple diatreme facies overprinting each other are regarded as being coeval with the early dacitic intrusives in the north, while a second phase of multistage phreatomagmatic breccias have partially destroyed sedimentary sequences and define a concentric pattern around the late porphyritic hornblende andesite intrusive body.
Overlying all of these rocks is the Quaternary Toba Tuff, which forms a prominent plateau to the north of Martabe but is restricted to small erosional remnants in the Martabe area.
Mineralisation is dominantly hosted in steeply west dipping to subvertical breccia units and structural zones within the andesite host sequence that are presumed to be conduits for the hydrothermal fluids. The high sulphidation epithermal mineralisation followed extreme acid sulphate leaching of the wall rocks. This early stage leaching, which was focussed on the major structures, produced an advanced argillic assemblage with vuggy to massive silica, surrounded by a silica/dickite/alunite advanced argillic alteration envelope, grading outwards into silica-illite and peripheral kaolinite-illite argillic zones. The silicification (or removal of other minerals to leave silica) produced a vuggy, porous, brittle rock subject to fracturing which subsequently channelled mineralising fluids such that gold grades are directly proportional to the percentage of silica.
Individual deposits are composed of multiple mineralised zones, e.g., at Tor Uluala, mineralised zones are distributed over a strike length of over 1.2 kilometres, many of which outcrop at surface. This mineralisation is open at depth and is dominantly hosted in steeply west dipping to subvertical breccia units within an andesite/dacite host sequence.
Mineralisation within the corridor comprises:
i) an early weak (0.1 to 0.5 g/t Au) silica-pyrite phase immediately following the acid sulphate leaching;
ii) a low sulphidation phase of colloform banded chalcedonic silica veins with quartz and bladed carbonate 'boiling' textures with low (0.1 to 1 g/t Au);
iii) the main high sulphidation stage enargite/luzonite assemblage with associated covellite, native sulphur, pyrite, bismuthinite barite and marcasite in fractures and vugs that are mostly oxidised. In this latter stage, where un-oxidised, gold occurs as free grains and in enargite, while silver is in enargite/luzonite and as proustite/pyrargyrite inclusions in bismuthinite. The best gold is late in the hydrothermal event in late stage fracturing and crackle type brecciation of the silicified rocks marginal to the late clay-altered diatreme breccia;
iv) oxidation which is highly irregular and controlled by fracturing, has affected most of the fracture controlled mineralisation but does not seem to have resulted in any supergene upgrading. Minor to strong oxidation extending to depths greater than 100 m in places at Tor Uluala, although fresh sulphides can also be observed close to surface. While oxidation has not resulted in supergene upgrading, it has improved the metallurgical recovery of the refractory primary sulphide mineralisation.
At the end of 2003 the quoted resource at the Purnama deposit alone, amounted to 51.9 Mt @ 1.9 g/t Au for 98 tonnes (3.15 Moz) of contained gold (Newmont, 2004).
Resources (predominantly oxides) and reserves (all oxides), in Sept 2011 at a 0.5 g/t Au cut-off (G-Resources, 2011) were:
Proved + probable reserve at Pit 1, Barani and Ramba Joring - 45.5 Mt @ 2.1 g/t Au, 23.0 g/t Ag
Measured + indicated + inferred resources at Purnama, Purnama Timur, Barani, Ramba Joring, Tor Uluala and Uluala Hulu -
165.2 Mt @ 1.4 g/t Au, 13.6 g/t Ag (for 232 t, or 7.46 Moz of contained Au).
Measured+indicated+inferred resources (dominantly oxides) by deposit, in August 2012, at a 0.5 g/t Au cut-off (G-Resources, 2012) were:
Purnama - 82.04 Mt @ 1.7 g/t Au, 22 g/t Ag;
Purnama Timur - 4.97 Mt @ 1.5 g/t Au, 13.2 g/t Ag;
Ramba Joring - 38.35 Mt @ 1.0 g/t Au, 4.1 g/t Ag;
Barani - 16.9 Mt @ 1.2 g/t Au, 3.2 g/t Ag;
Tor Uluala - 31.5 Mt @ 0.9 g/t Au, 7.7 g/t Ag (inferred resources only);
Horas - 15.7 Mt @ 0.8 g/t Au, 1.7 g/t Ag (inferred resources only);
Uluala Hulu - 1.26 Mt @ 2.0 g/t Au, 24.0 g/t Ag (inferred resources only);
TOTAL - 190.72 Mt @ 1.31 g/t Au, 12.5 g/t Ag, for 250 t (8.05 Moz) of contained gold;
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 20/9/2012
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 to this deposit in the PGC Literature Collection:
Bronto Sutopo, Jones M L, Levet B K 2003 - The Martabe gold discovery: A high sulphidation epithermal gold-silver deposit, North Sumatra, Indonesia: in NewGenGold 2003, Conference Proceedings, Perth WA, Louthean Media, Perth pp 147-158|
Levet B K, Jones M L and Sutopo B 2003 - The Purnama gold deposit in the Martabe District of North Sumatra, Indonesia: in SMEDG-AIG Symposium, Friday 10 October, 2003, Asian Update on Mineral Exploration and Development - Put a Tiger in Your Tenement AIG Journal 8p|
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