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Kucing Liar, Big Gossan, Ertsberg/Ertzberg |
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Irian Jaya, West Papua, Indonesia |
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Cu Au
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Super Porphyry Cu and Au
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In addition to the Grasberg porphyry deposit, the Gunung Bijih (or Ertsberg) Mining District of West Papua, Indonesia, hosts high grade gold-copper ore as sediment hosted skarns fringing both the Grasberg Igneous Complex and the adjacent, marginally older Ertsberg Diorite, 2 km to the SSE.
These skarns include the Kucing Liar deposit adjacent to and below the eastern margin of the Grasberg Igneous Complex; whilst the WNW-ESE elongated, 2.5 x 1 km Ertsberg Diorite, is fringed by the vertically stacked GBT, IOZ and DOZ of the East Ertsberg Skarn System on its northeastern margin, the Ertsberg skarn on its NW extremity, the Big Gossan on its SW corner, and the Dom orebody towards it's SE extremity.
With the exception of the original Ertsberg Skarn of 32 Mt @ 0.46 g/t Au, 2.2% Cu, all are underground mining operations.
This record is a adjunct to the more detailed description of the Grasberg, Ertsberg/Ertzberg record, which should be consulted for an outline of the stratigraphy and setting of the district and its skarn and related porphyry style mineralisation.
In the Ertsberg Mining District, the intruded sequence comprises a lower, 2000 m thick Jurassic to Cretaceous sequence of sandstones, shales and limestones. These are overlain by 1600 m of Miocene age limestones and dolomites of the New Guinea Limestone Group, commencing with the 290 m thick Paleocene Waripi Formation, which includes an upper evaporite band, and the overlying 200 m Eocene to Oligocene Faumai Formation. These are followed in turn by younger sandstones and carbonates.
Skarn style mineralisation in the Ertsberg Mining District is found in both Mesozoic and Tertiary sequences. At both Kucing Liar and Big Gossan ore is hosted by both the Palaeocene Waripi Formation and the sandstones and limestones at the top of the Jurassic-Cretaceous sequence. In the East Ertsberg skarns of GBT, IOZ and DOZ the hosts are the limestones at the top of the Jurassic to Cretaceous, the Waripi Formation and the Faumai Formation, while Dom is exclusively within the Faumai Formation. The different protoliths have produced different skarn mineralogies, which in turn also vary laterally. There are both prograde and retrograde assemblages, and quartz vein stockworks, as well as over-printing hydrothermal breccias.
The Kucing Liar deposit is hosted by the Waripi Formation of the Tertiary New Guinea Group and the Ekmai Formation of the Cretaceous Kembalangan Group. The alteration and mineralisation assemblages are different in these units and from the East and West ends of the known mineralised body.
Prograde alteration in the Waripi Formation was initiated by the formation of pyroxene and potash feldspar alteration, followed by the development of quartz stockworks. This was succeeded, in sequence, by alteration assemblages composed of pyroxene, monticellite/humite, green biotite (phlogopite), green garnet, brown biotite, red garnet, silica/pyrite/sericite, epidote, and magnetite.
Retrograde events consisted of a tremolite/actinolite phase which formed after the green garnet alteration, with actinolite altering the earlier pyroxenes and tremolite alteration of the earlier monticellite/ humite events. The latest alteration comprised the formation of quartz/anhydrite stockworks.
The temperature of formation of the prograde alteration is estimated to be ~400 to 600°C while the retrograde events were at ~200 to 400°C. The skam mineralisation is flanked in the hanging wall by bedded dolomites of the Waripi Formation, altered to coarse recrystallised white to beige marble.
The magnetite alteration event at the end of the prograde phase is characterised by the formation of massive magnetite within the calc-silicate skams of the Waripi Formation, followed by sulphide replacement which is hosted by fractures in the magnetite, which in the Waripi Formation, occurs at the SE end of the orebody. It comprises two stages of pyrite, followed by chalcopyrite and covellite, and subsequently by pyrrhotite. The fringes of the mineralisation zone are marked by galena, molybdenite and sphalerite. All of the silicates and sulphides are cut by hydrothermal breccias comprising milled fragments of the country rocks, igneous rocks, and sulphides in a rock flour matrix.
Mineralisation in the Ekmai Sandstone comprises quartz stockwork alteration followed by potash feldspar flooding of the sandstone. The temperature of formation of the potash feldspar alteration is estimated at 500 to 700°C. This was followed by an oxidising fluid which leached the earlier alteration and deposited covellite, and lesser sulphur into the pits resulting from the leaching. Progressing to the NW, the mineralisation changes with an increase in covellite at the expense of chalcopyrite. Also moving to the west, there is an increase in coarse pyrite which carries gold. The alteration and mineralisation are in some cases cut off by hydrothermal breccias and intrusive rocks that are intensely argillised. The presence of sulphides as clasts in the intrusive breccias, indicates that they formed post-ore.
The Kucing Liar deposit is localised in the hangingwall of the NW-SE trending, steeply SW dipping Idenberg#1 Fault, ~500 m SW across strike from the margin of the Grasberg Igneous Complex. It is also just over a kilometre along strike from the Ertsberg Diorite. However, mineralisation has been dated at 3.01 ±0.02 (Re:Os molybdenite; Mathur, Titley, Ruiz and Friehauf, 2005), whilst pre-magnetite phlogopite from the Kucing Liar skarn has returned an age of 3.41 ± 0.03 Ma, which is within error of the age of secondary biotite in a fine-grained Dalam intrusive rock (3.33 ±0.12 Ma; Pollard, Taylor and Peters, 2005), which is interpreted to represent alteration from the first pulse of mineralisation at Grasberg.
The most recent source geological information used to prepare this decription was dated: 1999.
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.
Ertsberg
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Selected References:
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Widodo S, Belluz N, Wiwoho N, Kusnanto B, Manning P, Edwards A, Macdonald G 1998 - Geology of the Kucing Liar Ore Body, Irian Jaya, Indonesia: in Porter T M (Ed.), Porphyry and Hydrothermal Copper and Gold Deposits - A Global Perspective PGC Publishing, Adelaide pp 49-60
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| References in PGC Publishing Books: | |
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Sugeng Widodo, Belluz N, Nur Wiwoho, Bowo Kusnanto, Manning P, Macdonald G, Edwards A, 1998 - Geology of the Kucing Liar Ore Body, Irian Jaya, Indonesia, in Porter T M, (Ed.), Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective, pp 49-60
 Abstract
Paterson J T and Cloos M, 2005 - Grasberg Porphyry Cu-Au Deposit, Papua, Indonesia: 2. Pervasive Hydrothermal Alteration, in Porter T M, (Ed), Super Porphyry Copper and Gold Deposits: A Global Perspective, v2 pp 331-355
Abstract
Paterson J T and Cloos M, 2005 - Grasberg Porphyry Cu-Au Deposit, Papua, Indonesia: 1. Magmatic History, in Porter T M, (Ed), Super Porphyry Copper and Gold Deposits: A Global Perspective, v2 pp 313-329
Abstract
Friehauf K C, Titley S R and Gibbins S L, 2005 - Porphyry-Style Mineralisation in the Ertsberg Diorite, Gunung Bijih (Ertsberg/Grasberg) District, West Papua, Indonesia, in Porter T M, (Ed), Super Porphyry Copper and Gold Deposits: A Global Perspective, v2 pp 357-366
Abstract
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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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