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Gosowong, Kencana, Toguraci

Indonesia

Main commodities: Au Ag
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Gosowong, Kencana, 1 km south of Gosowong and Toguraci, 2 km to the WSW of Gosowong, are low sulphidation epithermal vein deposits which are part of the Gosowong Goldfield on the north 'arm' of Halmahera Island, in the North Maluku province, East Indonesia. They are located approximately 1° north of the equator and 2400 km northeast of the national capital, Jakarta.

The Gosowong Goldfield lies within a Neogene magmatic arc in moderate to steep, rain-forested and poorly outcropping, terrain.   Exploration and mining operations are carried out within the PT Nusa Halmahera Minerals Contract of Work, jointly owned by Newcrest Mining Ltd (82.5%, operator) and PT Aneka Tambang Tbk. (17.5%).

A collision zone between the opposing Sangihe and Halmahera volcanic arcs is located in the Maluku Sea to the west of Halmahera, with eastward subduction of the Maluku Sea plate beneath the Halmahera and the Philippine Sea plates having continued since the Paleogene to produce four superimposed volcanic arcs in west Halmahera. The four resulting volcano-sedimentary formations are the Paleogene(?) Bacan Formation, the Upper Miocene(?) Gosowong Formation, the Pliocene Kayasa Formation, and the Quaternary Volcanic Formation which remains active to the present. Each of the formations is separated by major regional angular unconformities.

Epithermal gold mineralisation found within the district to date is restricted to the Miocene(?) bedded andesite flows and volcaniclastics of the Gosowong Formation which are unconformably overlain by a similar suite of rocks belonging to the Pliocene Kayasa Formation, through which the Gosowong Formation is exposed as an inlier.

The Gosowong and Kayasa volcanic sequences were locally intruded by late Pliocene andesite porphyry and quartz diorite. Quaternary pyroclastic flows and airfall deposits unconformably overlie the Gosowong Formation to the east and north.

Two styles of mineralisation have been recognised to date within the Gosowong Goldfields, namely  i). low-grade copper-gold porphyry and  ii). high-grade gold-silver epithermal veining.   Four sub-economic porphyry systems, Bora, Tobobo, Ngoali, and Matat and a number of epithermal vein deposits including Gosowong, Toguraci, Ruwait, Gosowong North, Dongak, Tobobo, Langsat and Gosowong Extended (Kencana) have been recognised. The epithermal mineralisation is multi-phase, with an early quartz-adularia episode having been dated at approx. 2.8 Ma (by 40 Ar/39 Ar) at Gosowong. All discoveries to date are contained within a 10 x 4 km north-south trending mineralised corridor.

Epithermal gold mineralisation is associated with quartz-adularia-illite-pyrite alteration within the epithermal vein zone, which is in turn enveloped by an illite-quartz-pyrite set within a widespread chlorite-epidote-albite-calcite-pyrite zone.

At Toguraci, this alteration over prints the earlier alteration of the Bora porphyry copper event, comprising biotite-magnetite ±chlorite and chlorite ±epidote ±calcite ±pyrite.

Most of the mineralisation within the goldfield is hosted within subvertical, north-south striking, faults which are controlled by both lithology (preferentially hosted in diorite), and by cross-cutting NE faults (which also carry some mineralisation).

Mineralised veins display typical epithermal textures, including quartz vein breccias with clasts of both wall rocks and earlier quartz veins, demonstrating a multiphase history. Chlorite and black-sulphide bands and rims up to 0.5 cm thick (commonly accompanied by fine grained visible gold), overprint an earlier quartz-adularia event and define bonanza gold grade zones and shoots. This quartz-chlorite-sulphide phase, although volumetrically minor, contributes significantly to the overall gold inventory. Early low-grade vuggy quartz veins and stockwork, and late stage cross-cutting carbonate veinlets are also common. Within individual shoots, high gold grades are usual, although at Toguraci they are small with sharp boundaries to the adjacent low-grade mineralisation. Generally vein widths range between 2 to 6 m and are set in a wider, earlier, sub-grade stockwork zone. Broad intervals of low-grade stockwork without significant high-grade overprinting gold are also known.

The resource at Kencana in mid 2004 was 1.7 Mt @ 41 g/t Au for 70 tonnes of contained gold.
Gosowong is currently exhausted, although the pre-mine resource was stated as 1 Mt @ 24 g/t Au for 24 tonnes of gold in 1999.
The resource at Toguraci has been quoted at 0.414 Mt @ 27 g/t for 11 tonnes of gold.

The most recent source geological information used to prepare this summary was dated: 2004.    
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:
Clark, L.V. and Gemmell, J.B.,  2018 - Vein Stratigraphy, Mineralogy, and Metal Zonation of the Kencana Low-Sulfidation Epithermal Au-Ag Deposit, Gosowong Goldfield, Halmahera Island, Indonesia: in    Econ. Geol.   v.113, pp. 209-236.
Gemmell J B,  2007 - Hydrothermal Alteration Associated with the Gosowong Epithermal Au-Ag Deposit, Halmahera, Indonesia: Mineralogy, Geochemistry, and Exploration Implications: in    Econ. Geol.   v102 pp 893-922
Olberg, DJ, Rayner, J, Langmead, RP and Coote, JAR  1999 - Geology of the Gosowong Epithermal Gold Deposit, Halmahera, Indonesia: in   Pacrim 99, International Congress on Earth Science, Exploration and Mining Around the Pacific Rim, Proc. Pacrim 99 Conference, Bali, Indonesia AusIMM, Melbourne    pp 179-185
Richards T H and Basuki Dwi Priyono M D  2004 - Discovery of the Toguraci epithermal Au-Ag deposits, Gosowong Goldfield, Halmahera Island, east Indonesia: in   Hi Tech and World Competitive Mineral Success Stories Around the Pacific Rim, Proc. Pacrim 2004 Conference, Adelaide, 19-22 September, 2004, AusIMM, Melbourne,    pp. 359-366.


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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