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Wetar Island - Lerokis, Kali Kuning, Meron

Indonesia

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The Lerokis, Kali Kuning and Meron deposits, located on the north central coast of Wetar Island, Indonesia, comprise precious metal-rich volcanogenic massive base metal sulphide and barite deposits, which are hosted by Neogene volcanic rocks and minor oceanic sediments and lie within part of the Inner Banda Arc. These deposits have produced approximately 17 tonnes of gold.
(#Location: Kali Kuning - 7° 42' 34"S, 126° 19' 35"E; Lerokis - 7° 43' 27"S, 126° 17' 32"E; Meron - 7° 42' 32"S, 126° 20' 49"E).

Wetar Island is entirely composed of Neogene volcanic rocks with minor oceanic sediments. The basement of the island comprises submarine basaltic-andesites with local pillows, intruded by rhyodacite domes and overlain by dacitic lavas, tuffs and breccias, debris flows, globerigina limestones and lahar deposits.

Initial interest was in the associated gold mineralisation. The deposit comprises gold-silver mineralisation within stratabound barite sand units that occur on the flanks and locally above mound-like zones of massive sulphides. These barite bodies comprise friable barite and minor sulphides, cemented by a series of complex arsenates, oxides, hydroxides and sulphates, with gold present as <10 micron free grains. Linear and pipe-like structures comprising barite and iron-oxides beneath the barite deposits are interpreted as feeder structures to the barite mineralisation. The massive sulphides are hosted by submarine dacitic and andesitic breccias which are overlain by post-mineralisation limestone of Pliocene age. The barite bodies are up to 37 m thick and consist of barite grains in a matrix of iron-oxides, silica and clay. They formed in small basin-like depressions on the margins of felsic domes and commonly show evidence of slumping. Claystone and ferruginous chert are commonly found above the baritic sequence. The orebodies were covered by post-mineralisation chert, gypsum, Globigerina-bearing limestone, lahars, subaqueous debris flows and pyroclastics rocks.

The majority of the gold occurs as electrum associated with limonite, jarosite and goethite, while silver is predominantly associated with tetrahedrite and sulphosalts. The mineralisation has isolated high Pb values (>1 %) and enrichment in Hg (average 18 ppm), As and Sb. The underlying massive sulphide zones are oval shaped in plan and have a funnel-shape on section. They consist of dominantly pyrite-marcasite (locally arsenian), with minor chalcopyrite which are cut by late fractures infilled with covellite, chalcocite, tennantite-tetrahedrite, enargite, bornite and Fe-poor sphalerite. The sulphide units are commonly brecciated with cobble-size pyrite clasts in a pyritic matrix. At depth they grade into pyritic stockwork. Alteration occurred as two main events, namely: i). illite-diaspore-chlorite-adularia ± quartz + alunite ( at >220°C), and ii). quartz, opal, cristobalite, smectite, kaolinite, zeolites, carbonate, jarosite and alunite (at <20°C). The former event took place around 4.7 M ago (Sewell and Wheatley, 1994). This alteration extends over an area of about 25 sq. km around Lerokis and Kali Kuning and has a vertical extent of at least 250 m. Altered footwall volcanic rocks have an illite K-Ar age of 4.7±0.16 Ma and a 40Ar/39Ar age of 4.93±0.21 Ma.

The gold resource contained (van Leeuwen, 1994):
    Lerokis - 2.9 Mt @ 3.3 g/t Au, 106 g/t Ag
    Kali Kuning - 2.2 Mt @ 5.5 g/t Au, 146 g/t Ag; and
    Meron - 0.3 Mt @ 3.5 g/t Au, 110 g/t Ag.

Copper orebodies occur within coherent massive sulphide units, with lesser, generally lower grade material occurring within intensely clay-sericite pyrite altered andesitic tuffs in the footwall and lateral extent of the massive sulphide units. These sulphides immediate underlie both the Kali Kuning and Lerokis gold orebodies at Wetar.

Based on drilling, the Kali Kuning massive sulphide resource has been classified into three metallurgical sub-types - Leached, Transition zone and Primary massive sulphide. The Leached and Transition types reflect incipient in-situ leaching of the massive sulphide unit by meteoric water. The lower copper grade Transition zone ~1.5% Cu) material, contains a higher proportion of readily leachable copper minerals, than the higher grade (~2.9%) Primary massive sulphide.

Based on previous drilling data and 73 new diamond drill holes, the resources at the Kali Kuning and Lerokis deposits have estimated mineral resources of (Finders Resources website, 2009-2011):
    Measured + indicated + inferred resources at Kali Kuning - 6.3 million tonnes, grading 2.52% Cu and 0.7 g/t Au, at a 0.5% Cu cut-off.
    Measured + indicated + inferred resources at Lerokis - 2.7 million tonnes, grading 2.3% Cu and 0.7 g/t Au, at a 0.5% Cu cut-off.
    Combined proven + probable reserves - 8.168 million tonnes, grading 2.4% Cu and 0.7 g/t Au, at a 0.5% Cu cut-off grade.

Updated JORC compliant resources and reserves at November 2014 (Finders Resources website 2016) were:
Mineral resources
  Kali Kuning @ 0.4% Cu cut-off
    Leached ore - Measured - 0.2 Mt @ 0.5% Cu; Indicated - 0.03 Mt @ 0.8% Cu; Inferred - 0.02 Mt @ 1.1% Cu; TOTAL - 0.2 Mt @ 0.6% Cu.
    Transition ore - Measured - 1.1 Mt @ 1.3% Cu; Indicated - 0.3 Mt @ 1.5% Cu; Inferred - 0.1 Mt @ 1.7% Cu; TOTAL - 1.6 Mt @ 1.4% Cu.
    Hypogene ore - Measured - 4.1 Mt @ 2.8% Cu; Indicated - 0.6 Mt @ 2.6% Cu; Inferred - 0.1 Mt @ 2.1% Cu; TOTAL - 4.7 Mt @ 2.8% Cu.
    TOTAL - Measured - 5.4 Mt @ 2.4% Cu; Indicated - 1.0 Mt @ 2.1% Cu; Inferred - 0.2 Mt @ 1.7% Cu; TOTAL - 6.6 Mt @ 2.4% Cu.
  Lerokis @ 0.5% Cu cut-off
    Hypogene ore - Measured - 2.1 Mt @ 2.4% Cu; Indicated - 0.4 Mt @ 2.2% Cu; Inferred - 0.1 Mt @ 1.5% Cu; TOTAL - 2.6 Mt @ 2.3% Cu.
  Lerokis and Kali Kuning combined
    All ore types - Measured - 7.5 Mt @ 2.4% Cu; Indicated - 1.4 Mt @ 2.2% Cu; Inferred - 0.3 Mt @ 1.6% Cu; TOTAL - 9.2 Mt @ 2.4% Cu.
Ore reserves
  Kali Kuning @ 0.4% Cu cut-off
    Proved - 5.4 Mt @ 2.4% Cu; Probable - 0.9 @ 2.1% Cu; TOTAL - 6.3 Mt @ 2.4% Cu. Waste - 5.9 Mt.
  Lerokis @ 0.5% Cu cut-off
    Proved - 2.1 Mt @ 2.3% Cu; Probable - 0.4 @ 2.0% Cu; TOTAL - 2.5 Mt @ 2.3% Cu. Waste - 1.9 Mt.
  Combined
    Proved - 7.5 Mt @ 2.4% Cu; Probable - 1.4 @ 2.1% Cu; TOTAL - 8.9 Mt @ 2.4% Cu.

The most recent source geological information used to prepare this summary 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.


Kali Kuning

Lerokis

Meron

  References & Additional Information
   Selected References:
Scotney P M, Roberts S, Herrington R J, Boyce A J and Burgess R  2005 - The development of volcanic hosted massive sulfide and barite–gold orebodies on Wetar Island, Indonesia: in    Mineralium Deposita   v40 pp 76-99


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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