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Mt Muro
Kalimantan, Indonesia
Main commodities: Au Ag

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The Mount Muro gold deposit is located 300 km to the west of Balikpapan within the Regency of Murung Raya, Province of Central Kalimantan, Republic of Indonesia, and is <100 km south of the Equator (#Location: 0° 40"S, 114°E).

Mount Muro is situated in the eastern part of the South-east Asian Sundaland cratonic plate which is composed of Mesozoic and older metamorphosed sedimentary and volcanic rocks which have been intruded by granites. In the region this plate includes a mosaic of continental blocks rifted from Australia during the Late Jurassic, and an amalgamation of several Permo-Cretaceous volcano-plutonic arcs. Late Cretaceous to Early-Tertiary continental and marine sediments were deposited along the northern shelf margin of the plate, and subsequently deformed during the Eocene when the China Sea plate was subducted westward beneath Borneo section of the Sundaland plate. The resultant subduction complex lies to the north of Mount Muro, occurring as arcuate fold belt of intensely deformed marine lithologies, distributed along the border between Kalimantan and Sarawak. To the south of Mount Muro, continental and shallow marine sedimentary rocks accumulated in the subsiding Barito basin during the Tertiary and Quaternary.

The Mount Muro mineralisation is hosted by andesitic volcanic rocks which are thought to have formed above an Oligocene-Miocene southward-dipping Bernioff zone during a brief period of subduction. These andesites form part of a 400 km long string of discontinuous volcanic centres that forms a broad, Central Kalimantan continental arc, which host the Mirah, Gunung Mas, the Kelian and Masupa Ria gold occurrences and deposits, which with Mount Muro, define the the Central Kalimantan Gold Province.

The local stratigraphy of the deposit area is composed of mid-tertiary intermediate calc-alkaline sub-aerial lavas, volcanic breccias and tuffs, with lesser siltstones and shales. These are intruded by late stage dacitic plugs, overlain by limited, fine grained basalt representing the youngest rocks in the area. To the northwest of the deposit, deformed Mesozoic shales, sandstones and carbonate rocks outcrop, while mid-Tertiary limestones, shales and sandstones with interbedded andesitic volcanic rocks of the Barito basin are mapped to the southeast.

The dominant structural directions are NE, NNW and WNW, with Mount Muro lying along a NE structural trend. Kelian also lies on this trend. NW-SE orthogonal subduction-related compression accentuated the arc-parallel NE structures and conjugate WNW and NNE structures, with mineralisation commonly found in these latter two fracture systems.

Gold and silver mineralisation occurs in hydrothermal altered fault and fracture zones which cut across the dominantly andesitic volcanic and pyroclastic host sequence, with the best grades usually found in multiphase quartz veins and breccias within these structures. Mineralised "lodes" include:  i). quartz veins that may be up to 5 m wide, commonly brecciated by faulting with crustiform, comb, cockade, colloform and chalcedonic quartz vein types which have been localy brecciated and re-sealed;  ii). altered tectonic breccias, which are dominantly cataclasites and fault gouge enclosing fragments that vary from clay to gravel-sized, usually sub-angular to rounded and comprise of wallrock, brecciated quartz veins and sulphides, suggesting movement after the deposition of veins;  iii). altered wallrocks, which are typically strongly fractured and clay-rich with stockwork veining and are found both within and enclosing the fault zones.

Where these lodes are associated with thin discontinuous sulphide-rich stringers and veins with associated quartz veins, they commonly have bonanza grades. The lodes, where porous, have been preferentially oxidised to depths of ~100 m below the surface, where they contain abundant iron and manganese oxides, in contrast to a depth of only ~25 m in the adjacent country rocks.

A number of separate vein systems make up the greater deposit, including Serujan Central, Tengkanong, Permata and Kerikal, which are more siliceous and form spines of elongated, steep-sided ridges. Saddles within the ridge-line commonly correspond to cross-cutting fault zones, while intervening valleys are usually lode parallel faults. A 1 to 10 m thick blanket of eluvial clay and scree usually overlies are flanks the lodes, and is mineralised for distances of up to 100 m from the lodes. This layer is composed of deeply weathered clays with quartz vein and andesite fragments derived from the erosion of the lodes, quartz-rich fault zones and enclosing country rocks. Its base is occupied by a layer of semi-consolidated quartz and rock-fragment scree.

Economic mineralisation comprises electrum, acanthite, argentite, stromeyerite and jalpaite. Primary gold occurs as ~680 fine electrum. Visible native wire and flake silver and specks of native gold are occasionally visible. Electrum occurs as fine, elongate and granular specks in quartz, rarely exceeding 50µm in diameter, although blebs as large as 1 mm in diameter have been observed in the high grade sections of the high grade zone at Serujan North. Electrum is also intergrown with sulphides, predominantly pyrite, but also galena, sphalerite, acanthite and occasionally tetrahedrite. Sulphides, which include pyrite, sphalerite, galena, chalcopyrite, jalpaite, stromeyerite, chalcocite, acanthite and rare covellite, occur as fine grained, disseminations in 1 to 5 mm bands in layered quartz veins, and occasionally form sulphide dominant veins that are 5 to 40 cm thick.

Pods and shoots of high to very high grade gold and silver are found in all of the lodes, generally corresponding to coarse-grained sulphide veins at lode-cross fault intersections, bifurcation of lode structures and flexure points, with local grades as high as 2000 g/t Au and 5000 g/t Ag. These zones and enclosed rocks bulk up to intervals of 5 to 20 g/t Au and 100 to 1500 g/t Ag and persist for lengths of 10 to 150 m, widths of 0.2 to 8 m and plunge to depths of 200 m. These high grade zones may be separated by intervals of 1 to 3 g/t Au, 20 to 150 g/t Ag.

Mining has been from 5 main pits at Permata (which is 1200 m long, and ~9 km from the mill); Hulubai (the 1200 m long northern extension of the eastern limb of the Permata system); Bantian (which is 2800 m long, and is ~10 km by road from the mill and 600 m to the West of the Permata-Hulubai system); and Serujan (~2 km by haul road from the mill, and is 1500 m long, composed of Central and Eastern Serujan).   Production has also come from Kerikil (~12 km from the mill, to the NE of Mt Muro, where two structures are mineralised over an 860 m strike length).   Other resources mined and outlined are Gerantung, Tengkanong, Arong Maan, Jalan and Bukit. The main deposits have been mined to depths ranging from 50 to 150 m, generally to 55 to 80 m, with a ratio of strike length to pit depth of 15:1. Unoxidised ore extends below the past mining limits.

The cumulative strike length of ore bearing structures at these systems is greater than 10 km.

In December 1994, when production commenced:
        proven+probable reserves were 8.5 Mt @ 3.9 g/t Au 86 g/t Ag.
In December 2011 reserves an resources were (Straits Resources website, 2011):
        proven reserves were: 5.937 Mt @ 2.5 g/t Au, 59 g/t Ag, and
        probable reseves were 6.82 Mt @ @ 2.5 g/t Au, 59 g/t Ag
        measured+indicated+inferred resources were 19.54 Mt @ 1.9 g/t Au, 42 g/t Ag.

To the end of 2011, over 97 t (3.125 Moz) of gold had been produced from the deposit, mostly from within 100 m of the surface.

The most recent source geological information used to prepare this decription was dated: 2001.    
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
   Selected References:
Moyle A J, Bischoff K, Alexander K R, Hoogvliet H  1996 - Mt Muro gold deposit, Indonesia: in   Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996 AMF, Adelaide    pp 7.1-7.9
Simmons S F, Browne P R L  1990 - Mineralogic, alteration and fluid-inclusion studies of epithermal gold-bearing veins at the Mt Muro prospect, Central Kalimantan (Borneo), Indonesia: in Hedenquist W J, et al., (Eds), 1994 Epithermal Gold Mineralisation of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, J. of Geochemical Exploration   v35 pp 63-103 =41 pages (copied 2 onto one)
White, N.C., Leake, M.J., McCaughey, S.N. andd Parris, B.W.,  1995 - Epithermal gold deposits of the southwest Pacific: in    J. of Geochemical Exploration   v.54, pp. 87-136.

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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