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Kalimantan, Indonesia
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The Ruwai Polymetallic Fe-Zn-Pb-Ag ±Cu skarn altered deposit is located within the Schwaner Mountains of Kalimantan on the island of Borneo, ~410 km NW of Banjarmasin, Indonesia (#Location: 1° 32' 9"S, 111° 18' 7"E).

Mineralisation at Ruwai was discovered by a Dutch investigation program in 1918. Subsequently it was investigated by Renison Goldfields Consolidated Ltd. between 1987 and 1991, PT Tebolai Seng Perdana from 1992 to 1996, and Scorpion Schwaner Minerals, Inc. between 1996 and 1997. Since 2008, the deposit has been mined by PT Kapuas Prima Coal, Tbk, initially for iron, before concentrating on Zn-Pb-Ag from 2016.

Regional Setting

Ruwai lies towards the south western extremity of the ~1300 x 250 to 600 km Central Borneo Metallogenic Belt that extends from the northeastern tip of Borneo, where it has a NNE trend, curving to NE-SW and broadening to be aligned east-west as it crosses the west to SW coast of the island.

Borneo is part of what is now Sundaland, which includes the Malay Peninsula and the islands of Borneo, Sumatra, Java and possibly west Sulawesi. It also includes the shallow shelves of the Java Sea, the Gulf of Thailand, and sections of the South China Sea that together make up the Sunda Shelf. This amalgamated shelf and island block covers an area of ~1.8 million km2. It does not have a thick cratonic basement, but was built during the Mesozoic from a collage of Palaeozoic ro Mesozoic micro-continental blocks rifted from both Asia and Australia which collided to form a wide accretionary zone along the Asia-Pacific boundary. This south and east boundary was an active continental margin from the Triassic until early in the Late Cretaceous. From the Late Cretaceous it formed the southeastern, largely emergent, continental promontory of the Eurasian plate and was separated by deep oceans from India and Australia. During glacial periods over the last 2 m.y., it was exposed as a continuous land mass with limits that approximately follow the current 120 m isobath. As such, Borneo has a Palaeozoic to Mesozoic basement that is unconformably overlain by Cenozoic sedimentary basins deposited over and between the variably emergent islands (Hall and Nichols, 2002; Breitfeld et al., 2017).

The oldest Palaeozoic rocks known in Borneo are the Ordovician to Silurian Embuoi Complex schists in the northwest of the island, dated at 462.4 ±2.6 to 453.3 &polusmn;1.9 Ma (U-Pb zircon; Zhu et al., 2022). The area now covered by the island was subjected to prolonged tectonic and related magmatic activity throughout the Mesozoic.

Triassic arc-related magmatic activity resulted from subduction of the Palaeo-Pacific plate to the NW and west beneath the eastern Sundaland margin (Breitfeld et al., 2017; Hennig et al.), e.g., the 320 ±3 to 201 ±2 Ma granitoids intruding the Embuoi Complex (K-Ar; Williams et al., 1988); the 217 ±1 to 208 ±1 Ma Jagoi granodiorite (U-Pb; Breitfeld et al., 2017) and 233 ±3 Ma metatonalite in the Ruwai deposit area in the northwestern Schwaner Mountains in southwestern Borneo (U-Pb; Setiawan et al., 2013). At the same time, the Meso-Tethys Ocean was being suducted below Sumatra and Myanma on the western margin of Sundaland and SE Asia.

Jurassic magmatism is also represented in the southern Schwaner Mountains, including the 186.7 ±2.3 to 153.5 ±3.5 Ma Belaban granite, the 151.2 ±1.2 Ma Mentembah granite, the products of the continuing Palaeo-Pacific and Meso-Tethys subduction, the latter accommodating the separation and approach of the Southwest Borneo block from Gondwana to the SW (Davies et al., 2014; Hennig et al., 2017; Breitfeld et al., 2020). A carbonate platform known as the Ketapang Complex was developed during the Middle to Late Jurassic, accompanied by rapidly subsidence that continued into the Early Cretaceous (Morley, 1998; Basir and Uyop, 1999; Hennig et al., 2017). This complex comprises a lower sequence of Jurassic to Cretaceous (Albian to Cenomanian) fossiliferous limestone, overlain by an upper suite of clastic units that include siltstone-sandstone, crystal-lithic tuff, and graphite-bearing shale with a maximum depositional age of ~78.0 ±0.8 Ma in the Campanian of the Late Cretaceous (Li et al., 2021; Simbolon et al., 2019, and references therein).

No pre-Cretaceous mineral deposits have been found to date within the Central Borneo metallogenic belt, other than some 197.8 ±8.1 Ma (Pt-Os age) Early Jurassic age platinum group mineralisation in an accretionary complex of the Meratus Mountains in easternmost Borneo (Coggon et al., 2010).

The most active magmatic interval in the region was during the Cretaceous, when voluminous granitoids were emplaced in the northern and southern Schwaner Mountains of southwestern Borneo, e.g., the ~118.6 ±1.1 to 111.8 ±1.1 Ma Sepauk tonalite; the 101.5 ±0.6 to 96.8 ±0.6 Ma Laur granite; ~84.7 ±1.3 to 78.4 ±0.5 Ma Sukadana granite (Bretifeld et al., 2020), some of which have been metamorphosed to form the ~131.3 ±1.0 to 110.1 ±0.7 Ma Pinoh Metamorphic Group (Bretifeld et al., 2020). This magmatic activity immediately followed the Early Cretaceous accretion of the Southwest Borneo Block to Sundaland (Hall, 2012; Breitfeld et al., 2017; Hennig et al., 2017). This block is interpreted to have been rifted from Australia in the Jurassic. Late Cretaceous postcollisional magmatism followed the collision between the East Java and West Sulawesi blocks and ended the Mesozoic magmatic cycle (Davies et al., 2014; Hennig et al., 2017). This collision was followed by the docking of the East Java and West Sulawesi blocks to the south of the SW Borneo Block and then Late Cretaceous post-collisional magmatism. This ended the Mesozoic magmatic cycle (Davies et al., 2014; Hennig et al., 2017).

Only one Cretaceous mineral deposit has been found in the Central Borneo metallogenic belt, the Lamandau Fe-Cu deposit, with a reported Cretaceous intrusive age of 82.1 ±1.7 to 78.7 ±2.3 Ma (U-Pb zircon; Li et al., 2015), although this had not been confirmed by direct dating of mineralisation (Dana et al., 2023).

Cenozoic tectonic activity was accompanied by subduction-related volcanism and basin formation, strongly influenced by the 35° anticlockwise rotation of Borneo (e.g., Hall and Nichols, 2002; Hall, 2012; Advokaat et al., 2018). It has been suggested that the amalgamated Borneo Block was accreted into Sundaland during the middle Eocene (Hall et al., 2008; Advokaat et al., 2018). Paleogene magmatism commenced during the Eocene, and was characterised by calc-alkaline volcanism associated with subduction of the South China Sea to the SE and the formation of the Rajang accretionary complex that straddles the northwestern margin of the Central Borneo Metallogenic Belt (Soeria-Atmadja et al., 1999).

Only two epithermal gold prospects, Long Bigung (K-Ar age: 40.6 ±4.4 Ma) and Long Pahagai (K-Ar age: 48.6 ±1.0 Ma), are known to be associated with Eocene magmatism (Baharuddin, 2011).

Neogene magmatism, while still initially subduction-related, evolved via high-K calcalkaline during the Mio-Pliocene, to tholeiitic Plio-Pleistocene within-plate basalt (Soeria-Atmadja et al., 1999). The Neogene Sintang Suite is an important host to most epithermal gold deposits within the Central Borneo Metallogenic Belt, having close spatial and temporal associations with deposits and prospects such as Muyup, Masuparia, Kelian, and others (Thompson et al., 1994; Davies et al., 2008; Baharuddin, 2011). This suite has been dated at ~30.4 ±0.9 to 23.0 ±0.7 Ma (K-Ar; Melawi basin) and 17.9 ±0.2 to 16.4 ±0.1 Ma (K-Ar; Ketungau basin) in northwestern Kalimantan (Williams and Harahap, 1987) and more recently by to ca. 21.1 ±0.2 to 18.6 ±0.2 Ma in West Sarawak (U-Pb; Breitfeld et al., 2019). The epithermal gold mineralisation of the Kelian deposit, as dated at 20.8 ±0.5 to 20.6 ±0.2 Ma (K-Ar; sericite); 20.2 ±0.3 Ma; (U-Pb adularia; van Leuween et al., 1990; Abidin, 1996) closely corresponds to the emplacement of the Sintang Suite, namely 19.70 ±0.06 Ma (U-Pb; andesite); 19.8 ±0.1 Ma (U-Pb; quartz-phyric rhyolite); 19.5 ±0.1 Ma (U-Pb; quartz-feldspar-phyric rhyolite; Setiabudi, 2001; Davies, 2002; Davies ±, 2008). The Mamut porphyry Cu-Au deposit (K-Ar age: 6.98 ±0.30 Ma) is the youngest known mineral deposit found in Borneo (Imai, 2000).

Geology and Mineralisation

The oldest rocks exposed in the immediate Ruwai deposit area belong to the Upper Triassic to Middle Cretaceous Matan Complex, or Kuayan Formation (Hermanto et al., 1994). These mainly comprise hornfelsed felsic volcanic rocks, including crystal-lithic tuff, with intercalated siltstone beds. This complex predominantly crops out in the Southwest Gossan and Karim zones, in the southwestern and central sections respectively of the SW-NE trending zone of mineralisation. In these areas, it is intruded by the Sukadana Granite, which includes quartz-diorite, monzonite, granodiorite, diorite and diorite porphyry, and then by the Sintang intermediate to mafic intrusions, both of which are of Cretaceous age (Idrus et al., 2011; Dana et al., 2019; Simbolon et al., 2019; Widyastanto et al., 2019). These intrusions have been classified as I-type and magnetite-series granitoids (Widyastanto et al., 2019; Dana et al., 2022). The main intrusions, mostly found in the Gojo and Southwest Gossan zones at the NE and SW extremities of the mineralised interval respectively, comprise granodiorite, monzonite and quartz diorite. The more mafic intrusions include several varieties of diorite, mainly quartz diorite, diorite, diorite porphyry and microdiorite, and dolerite. Except for the Paleogene Bunga Intrusions dolerite dykes and younger diorite that intruded all other lithological units/suites, including the mineralisation, there is no field evidence of crosscutting relationships between the quartz diorite, monzonite, or granodiorite (Dana et al., 2023).

The bulk of the sedimentary sequence at Ruwai belongs to the Jurassic Ketapang Complex, the most common and thickest unit in the Ruwai area. This complex is also intruded by the same granitoids described above, and has been subdivided into two main packages, comprising i). a lower pelitic suite composed of siltstone, sandstone and carbonaceous shale, overlain by ii). carbonate rocks, which consist of limestone and marl (Dana et al., 2019; Simbolon et al., 2019). In the immediate Ruwai area, these lithologies have been subjected to contact metamorphism during granitoid emplacement to produce hornfels and marble. The Ketapang Complex is unconformably overlain by the Cretaceous bimodal Kerabai Volcanics, dated at 74.8 ±0.7 to 65.6 ±1.1 M (K-Ar; De Keyser and Rustandi, 1993).

The magmatic rocks at Ruwai were emplaced in three pulses (U-Pb zircon ages; Dana et al., 2023), namely the Early Cretaceous (~145.7 and 106.7 and 105.7 Ma; andesite-dacite), Late Cretaceous (~99.7 to 97.1 Ma; diorite-granodiorite) and late Miocene (~10.94 to 9.51 Ma; diorite-dolerite). The skarns and massive mineralisation at Ruwai are hosted by marble of the Jurassic Ketapang Complex, which was intruded by the Sukadana granitoids that belong to the Late Cretaceous magmatic event. As such, the Ruwai skarn altered mineralisation dated at ~97.0 to 94.2 Ma (see below) occurred in the Late Cretaceous, associated with Palaeo-Pacific subduction beneath Sundaland after accretion of the Southwest Borneo Block. Ruwai is the first significant occurrence of Cretaceous mineralisation recorded in the Central Borneo Metallogenic Belt.

The Ruwai skarn deposit comprises four mineralised zones, distributed over a 3.5 km interval, hosted within the from the NE to SW, specifically: i). Gojo, ii). Karim, iii). Central Gossan, and iv). Southwest Gossan. On the basis of their metal zonation and mineralogy, Gojo and Karim to the NE have been interpreted to represent the proximal skarn, with Central Gossan and Southwest Gossan forming the distal zones (Simbolon et al., 2019). These zones follow a NE-SW trending thrust fault that was the main principal control of mineralisation (Idrus et al., 2011; Setijadji et al., 2011; Simbolon et al., 2019). In the Central Gossan zone, this thrust fault occurs as a mineralised fault breccia. Shallow exposures of mineralisation have largely been subjected by supergene argillic alteration, resulting in pervasive replacement by swelling clays and moderate to intense oxidation. Beneath the weathered zone, chlorite-calcite-epidote propylitic alteration is common in the non-calcareous rocks, whilst potassic K feldspar and biotite hypogene alteration has locally affected the volcanic lithologies.

Within the carbonate hosts, the prograde skarn alteration assemblage is dominated by garnet, diopside and wollastonite, with accessory rare earth element (REE)- and high field strength element (HFSE)-bearing minerals, zircon, thorite, monazite and cerite. The latter suite are typically fine grained, 0.5 to 20 µm phases with euhedral prismatic shapes that can be found either as inclusions of single crystals, or clusters within garnet.

Retrograde phase minerals include epidote, amphibole and phyllosilicate group minerals. The most common of these are the epidote group minerals, usually epidote sensu stricto and allanite. Retrograde titanite and rutile are typically associated with epidote. The most common of the amphibole group minerals include ferroactionolite and manganese actinolite. Of the phyllosilicates, the most abundant is chlorite, as well as corrensite (interstratified chlorite and smectite), stilpnomelane and illite. Calcite, quartz, titanite, rutile, and apatite are other retrograde phases, although some calcite, quartz and apatite are also associated with the prograde phase.

The prograde-stage garnet and retrograde-stage titanite have been dated at 97.0 ±1.8 to 94.2 ±10.3 Ma and 96.0 ±2.9 to 95.0 ±2.0 Ma, respectively (U-Pb; Dana et al., 2023). These ages are similar to Re-Os ages obtained on sulphides (96.0 ±2.3 Ma) and magnetite (99.3 ±3.6 Ma).

The mineralisation at Ruwai has been subdivided into three main groups based on mineralogical assemblages:
Massive magnetite, which is commonly found in Gojo and Karim proximal skarn zones and is mostly associated with garnet skarn alteration. These magnetite bodies vary from 1 to 15 m in thickness, with grades up to 1.0 m at 53.8% Fe and 0.02% Cu (PT Kapuas Prima Coal, Tbk., unpub. data, 2020).
Massive sulphides, which extend from the western part of Karim to the southwestern part of the Southwest Gossan zones, in th edistal skarn alteration zone. It is characterised by sphalerite and galena with minor pyrrhotite and pyrite, and is the principal source of Zn-Pb-Ag mineralisation, with average grades of 14.6% Zn, 6.4% Pb, 0.5% Cu and 360.8 g/t Ag in 1.0 m of massive sulfide (PT Kapuas Prima Coal, Tbk., unpub. data, 2020).
Massive pyrrhotite, which is only found in the Central Gossan and Southwest Gossan zones, also in the distal skarn alteratio zone, and is characterised by abundant pyrrhotite with fine-grained disseminated sphalerite and trace amounts of chalcopyrite. The average grades are typically 0.05% Pb, 5% Zn and 15 g/t Ag over a 1.0 m interval, and it is a secondary lead-zinc source (Simbolon et al., 2019).

Within these sulphide bodies, four metallic mineral assemblages are recognised, namely:
i). Sulphides, which include pyrite, sphalerite, galena, pyrrhotite, chalcopyrite and marcasite. Pyrite is commonly partially to completely replaced by galena, pyrrhotite or magnetite, whilst pyrrhotite and chalcopyrite are commonly found as exsolution phases within sphalerite.
ii). Oxides, that consist of magnetite, hematite and ilmenite. Hematite typically replaces magnetite, and both are mostly found in massive magnetite bodies, although some minor disseminated magnetite occurs in the surrounding wall rocks. In contrast, ilmenite was only identified in trace amounts in massive pyrrhotite.
iii). Arsenides, which comprise arsenopyrite, cobaltite and glaucodot [(Co
0.50Fe0.50)AsS]. Minor arsenopyrite is also found in association with other sulphides. Cobaltite and glaucodot typically occur in solid solution within arsenopyrite.
iv). Bi-Ag bearing minerals represented by at least nine different species, including tetrahedrite, native bismuth, cosalite, tsumoite, bismuthinite, joseite, Bi-Te-S, Bi-Te-Ag, and Bi-Ag-S (Dana et al., 2022). Bi-bearing minerals typically occur as inclusions in galena or pyrite, although fine disseminated grains have also been observed. Tsumoite [BiTe] was identified as rare infill between pyrite grains and in association with magnetite mineralisation.

Skarn altered mineralisation typically occurs along the contact between the intrusion and siltstone and marble. Within the proximal Gojo and Karim zones, endoskarn is developed and mostly takes the form of massive garnet-epidote-magnetite with some relict igneous textures preserved (Simbolon et al., 2019). Toward the distal Central and Southwest Gossan zones, the intrusions mostly occur as a smaller dyke in which endoskarn is not well developed, although some partial replacement by garnet-epidote has occurred. Exoskarn is not continuous along the contact between marble and intrusions due to the composition of the marble, which locally contains abundant siliciclastic impurities, possibly representing a marl/calcareous siliciclastic protolith (Idrus et al., 2011; Setijadji et al., 2011).

The deposit area is cut by mainly NE to NNE trending, NW or SE dipping faults and by crosscutting NNW trending dextral strike-slip faults. The latter are locally ENE trending in the Ruwai mineralised zone. The NE to NNE trending faults are spatially and temporally related to pre- to syn-mineral skarn alteration, whereas the NNW- and ENE-trending structures may have formed post-mineral and dissected and/or transposed the mineralisation into the four ore zones (Idrus et al., 2011; Simbolon et al., 2019). A prominent NNE-trending, steeply SE-dipping, and sinistral fault with 300 to 350 m of displacement passes between the Central and Southwest Gossan zones and locally terminates the mineralisation to the west (Simbolon et al., 2019).

Based on geochemical and stable isotopic data (C-O-S), Dana et al. (2023) interpret the Ruwai skarn mineralisation to have formed from oxidised fluids at ~160 to 670°C. The mineralising fluids and metals were mostly magmatic in origin but with significant crustal input.


Mining One Pty Ltd. (Hutchin, unpub. report, 2018; quoted by Dana et al., 2023) indicates a total estimated resource of:
    14.43 Mt @ 4.94% Zn, 3.28% Pb, and 108.11 g/t Ag.

The information in this description is almost entirely drawn from Dana et al. (2023) as cited below.

The most recent source geological information used to prepare this decription was dated: 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.


  References & Additional Information
   Selected References:
Dana, C.D.P., Agangi, A., Idrus, A., Chelle-Michou, C., Lai, C.-T., Ishida, M., Guillong, M., Gonzalez-Alvarez, I., Takahashi, R., Yano, M., Mimura, K., Ohta, J., Kato, Y., Simbolon, D.R. and Xia, X.-P.,  2023 - The Age and Origin of the Ruwai Polymetallic Skarn Deposit, Indonesia: Evidence of Cretaceous Mineralization in the Central Borneo Metallogenic Belt: in    Econ. Geol.   v.118, pp. 1341-1370. doi: 10.5382/econgeo.5009.

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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