road to lebong

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Road to Lebong, Bengkulu Maret 24, 2010 Posted by julianusginting in Exploration , Geologi . Tags: Bengkulu , Blog , Emas , Lebong , Pertambangan , Tailing 8 comments Menunaikan tugas sebagai geologist…tentunya harus siap untuk travelling ke daerah. Namanya juga perintis alias pencari (pencari sumber mineral tentunya). Tepatnya kemaren (Selasa, 23 Maret 2010) tim saya yang terdiri dari 3 orang harus berangkat ke Lebong, sebuah Kabupaten di Bengkulu dengan ibukota MuaraAman, untuk melakukan pemetaan topografi, drilling atau pemboran sekalian sosialisasi rencana kerja. fuih…Hari ini lumayan cape..padahal baru jalan-jalan mengecek lokasi tailing (limbah sisa penambangan Belanda), kudu disempetin neh nulis di blog walaupun tulisannya jadi kacau…tapi setidaknya posting sesuatu khususnya masalah dan tentunya kondisi geologi di daerah Lebong ini. Jadi mohon maaf neh bagi teman-teman blogger yang akan berkunjung ke blog saya, soalnya mulai hari ini saya tentunya gakan bisa nulis setiap hari tapi setidaknya ada update dari blog ini. Ok, demikian dulu aja deh, sekarang mau kerjaa lagi… kerja….hehehe… Cerita Explorasi Freeport (2) Februari 24, 2010 Posted by julianusginting in Exploration , Geologi . Tags: Cerita , Eksplorasi , Freeport , Geologi , Hutan , Jalur Magma , Papua , Pegunungan 2 comments Suatu aktifitas pertambangan selalu diawali dengan aktivitas eksplorasi suatu wilayah sasaran. Untuk wilayah tujuan eksplorasi mineral seperti Provinsi Papua, data geologi yang tersedia sangat minim dan sarana infrastruktur jalan belum tersedia. Hal ini merupakan tantangan tersendiri yang harus dipecahkan sejak perencanaan suatu kegiatan eksplorasi dilakukan. Perencanaan yang

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Page 1: Road to Lebong

Road to Lebong,   Bengkulu Maret 24, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Bengkulu, Blog, Emas, Lebong, Pertambangan, Tailing8 comments

Menunaikan tugas sebagai geologist…tentunya harus siap untuk travelling ke daerah. Namanya juga perintis alias pencari (pencari sumber mineral tentunya). Tepatnya kemaren  (Selasa, 23 Maret 2010) tim saya yang terdiri dari 3 orang harus berangkat ke Lebong, sebuah Kabupaten di Bengkulu dengan ibukota MuaraAman, untuk melakukan pemetaan topografi, drilling atau pemboran sekalian sosialisasi rencana kerja.

fuih…Hari ini lumayan cape..padahal baru jalan-jalan mengecek lokasi tailing (limbah sisa penambangan Belanda), kudu disempetin neh nulis di blog walaupun tulisannya jadi kacau…tapi setidaknya posting sesuatu khususnya masalah dan tentunya kondisi geologi di daerah Lebong ini. Jadi mohon maaf neh bagi teman-teman blogger yang akan berkunjung ke blog saya, soalnya mulai hari ini saya tentunya gakan bisa nulis setiap hari tapi setidaknya ada update dari blog ini.

Ok, demikian dulu aja deh, sekarang mau kerjaa lagi…kerja….hehehe…

Cerita Explorasi Freeport   (2) Februari 24, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Cerita, Eksplorasi, Freeport, Geologi, Hutan, Jalur Magma, Papua, Pegunungan2 comments

Suatu aktifitas pertambangan selalu diawali dengan aktivitas eksplorasi suatu wilayah sasaran. Untuk wilayah tujuan eksplorasi mineral seperti Provinsi Papua, data geologi yang tersedia sangat minim dan  sarana infrastruktur jalan belum tersedia. Hal ini merupakan tantangan tersendiri yang harus dipecahkan sejak perencanaan suatu kegiatan eksplorasi dilakukan. Perencanaan yang matang mulai daripemillihan sasaran, ada tidaknya data dasar, pengetahuan geologi yang dimiliki oleh tim eksplorasi, metode eksplorasi, dukungan dana serta kesungguhan investor menjadi kunci kesuksesan suatu kegiatan eksplorasi mineral di Propinsi Papua. PT Freeport Indonesia, yang merupakan anak perusahaan Freeport-McMoran Copper and Gold Inc. (FCX), saat ini terus mempelopori kegiatan ,eksplorasi di Propinsi Papua. Adanya berbagai tantangan seperti kondisi alam yang sangat ekstrim dan tidak adanya data geologi yang bisa dijadikan acuan menyebabkan potensi kesuksesan untuk menemukan tambang baru sangat kecil (1%). Artinya,kegiatan ini sangat mahal dan beresiko tinggi. Namun demikiandengan kesungguhan dan kemauan yang besar demi membantu pemerintah untuk mewujudkan pusat-pusat kegiatan ekonomi yang baru di Propinsi Papua, Freeport tetap gigih untuk trus aktif melakukan kegiatan eksplorasi mineral dengan tujuan utama menemukan cebakan-cebakan mineral bijih ekonomis (kelas dunia) yang baru.

Kegiatan eksplorasi modern dan aktif di Papua pada wilayah Kontrak Karya (KK) PT. Freeport Indonesia dimulai sejak 1988 hingga sekarang. Mencari cadangan mineral bijih baru untuk

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menggantikan  cadangan yang sudah ditambang merupakan tahap  yang sangat penting untuk menjamin kelanjutan produksi tambang selanjutnya. Berhasilnya kegiatan eksplorasi dalam menemukan cebakan sehingga menjadi cadangan baru akan menentukan atau menjamin masa depan bagi suatu usaha industri pertambangan.

Wilayah eksplorasi aktif di Propinsi Papua hingga tahun 2007, khususnya yang dikelola oleh Freeport-Mcmoran Copper  and Gold Inc. (FCX), meliputi wilayah blok A dan B Kontrak Karya (KK) PT.FI, Kontrak Karya PT.Irja Eastern Minerals (Eastern Mineral) dan Kontrak Karya PT. Nabire Bakti Mining (NBM). Kegiatan eksplorasi di wilayah Kontrak Karya area A saat ini dipusatkan pada kegiatan pengeboran eksplorasi dan deliniasi. Uji pengeboran deliniasi bertujuan untuk menambah cadangan terbukti, sedangkan pengeboran eksplorasi bertujuan untuk mencari potensi cebakan mineral tembaga, emas,serta mineral bijih  ekonomis lainnya seperti molibdenum tipe porfiri dan skarn di permukaan maupun bawah permukaan. Di wilayah-wilayah kontrak karya tersebut, kegiatannya dipusatkan pada kegiatan pengeboran eksplorasi di lokasi yang menunjukkan adanya anomali geokimia tembaga dan emas guna melacak potensi cebakan tipe porfiri dan skarn.

Cerita Eksplorasi Freeport   (1) Februari 23, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Cerita, Eksplorasi, Freeport, Geologi, Hutan, Jalur Magma, Lempeng Australia, Lempeng Samudera Pasifik, Papua, Pegunungan3 comments

Eksplorasi adalah suatu kegiatan aktif yang dilakukan atau sekelompok orang yang sifatnya mencari sesuatu. Mencari suatu cebakan mineral bijih dibawah permukaan bumi Papua bener-bener merupakan suatu tantangan tersendiri bagi yang melakukannya. Tantangan utama yang dihadapi adalah  minimnya data geologi ditambah dengan tantangan alam dan cuaca. Selain alam pegunungan yang terjal dan cuaca yang unik seperti yang terdapat didaerah pegunungan, alam Papua juga memiliki hutan asli dengan jenis yang beragam. Mulai dengan hutan bakau, dan hutan tropis yang lebat didataran rendah, hingga hutan kabut, dan sulbapina (rumput) didataran tinggi. Sungai-sungai yang lebar, datar dan berawa dijumpai didaerah dataran rendah, sedangkan sungai-sungai yang sempit dan terjal dengan aliran air yang deras, banyak dijumpai didaerah dataran tinggi atau pegunungan. Disamping bentang alam yang indah ini, Papua juga memiliki keragaman suku dan budaya yang sangat khas dan menakjubkan.

Daerah sasaran eksplorasi tersebar didaerah dataran rendah dan dataran tinggi atau pegunungan. Daerah pegunungan menempati hampir sebagian besar di bagian tengah pulau Papua, dengan ketinggian yang beragam antara 1.800 hingga 4.000 meter diatas permukaan air laut. Puncak tertingginya mencapai 4.717 meter yang dikenal sebagai Puncak Gunung Idenberg (Ngga Pilimsit).

Daerah pegunungan ini membentang dari barat hingga ke timur. Secara geologi, rangkaian pegunungan ini mulai terbentuk kurang lebih 20 tahun yang lalu akibat dari proses tumbukan dua lempeng tektonik yakni lempeng Samudera Pasifik yang bergerak relatif ke selatan dan Lempeng Australia yang bergerak ke utara sehingga terbentuklah Pulau New Guinea. Aktivitas tektonik ini

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menerus sampai sekarang dan menghasilkan beberapa jalur magma yang saat ini sudah tidak aktif antara lain Hitadipa (6 juta tahun) dan Eastberg (3-4 juta tahun lalu) (Quarless van Ufford,1996). Pada jalur magmatik inilah para pakar geologi eksplorasi  mencurahkan perhatiannya untuk melakukan kegiatan eksplorasi karena potensi keterdapatan cebakan magmatis sangat tinggi atau menjanjikan…(continue.. )

Alterasi Fluida Khlorida   Netral Februari 8, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Alterasi, Epidote, Fluida, Illite, Mineral, Muscovite, Prophylitic, Sericite, XRDadd a comment

Alterasi Propyllitic, Illite, dan Argillic merupakan hasil alterasi dari fluida-fluida hidrotermal panas dengan pH mendekati netral.

Alterasi ini ditandai dengan adanya lempung Smectite (Montmorillonite) ataupun lempung campuran Illite+Smectite atau Smectite+Illite. Urutan Smectite, Illite+Smectite, dan Smectite+Illite berturut-turut menunjukkan suhu yang meningkat, yaitu <180°C, 180°-200°C, dan 200°-230°C. Mineral smectite atu montmorillonite jarang ada di dalam sistem epitermal yang termineralisasikan, illite+smectite tidak umum dijumpai dalam sistem epitermal yang termineralisasikan, dan smectite+illite sering terjadi ke arah Alterasi Illite dalam sistem epitermal yang termineralisasikan.

Dalam sistem epitermal yang termineralisasikan, umumnya lempung-lempung alterasi ini kurang penting diperhatikan dalam eksplorasi mineral. Lempung-lempung ini kebanyakan terbentuk di atas atau di samping zone dimana minerali-sasi logam berharga terjadi. Meskipun demikian, alterasi ini yang kandungan mineral illite-nya dominan, masih dapat berguna untuk mengarahkan eksplorasi pada zone alterasi lainnya lebih menarik.

Luas alterasi ini meliputi puluhan hingga ratusan km2 di sekitar suatu Sistem Epitermal atau Sistem Porfiri. Alterasi ini paling teramati dalam batuan andesitik untuk sistem epitermal atau batuan basaltik untuk sistem porfiri, dan dicirikan oleh adanya mineral-mineral berikut: Chlorite, Chlorite+Epidote, atau Chlorite+Epidote+Actinolite yang berurutan ke arah suhu yang lebih tinggi. Adanya epidote menunjukkan suhu di atas 240°C. Mineral-mineral yang umum adalah quartz, illite, calcite, dan kandungan pyrite hingga 1%, serta sedikit magnetite. Alterasi Prophyllitic menempati vein-vein epitermal pada level yang lebih dalam.

Batuan Prophyllitic cirinya berwarna hijau, hijau-kelabu, atau hijau kuning (jika kaya epidote), dan tanpa sekistositas atau foliasi. Batuan ini sulit dibedakan dengan batuan sekis hijau (greenschist), yaitu fasies batuan hasil metamofisme regional yang juga dapat tanpa foliasi dalam batuan-batuan basa masif.

Alterasi ini merupakan jenis alterasi yang paling umum di bagian tengah sistem epitermal. Alterasi ini mengelilingi stockwork vein quartz dan terjadi sebagai tubuh-tubuh yang berbentuk baji (wedge-shape bodies) yang dapat mencapai 100 meter lebarnya, dan menipis ke arah bawah.

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Di lapangan, Alterasi Illitic ini terlihat sebagai lempung atau lempung silika berwarna kelabu hingga kelabu hijau, kekerasannya berkisar lunak (seperti keju) hingga keras (seperti bahan bangunan). Kandungan pyrite bervariasi, dan permukaannya terlapukan, terlihat khas ternodai ion ( ion-stained ) . Alterasi ini berangsur ke arah luar (ke arah Alterasi Prophyllitic).

Pada hasil XRD (X-Ray Diffraction), kumpulan mineral alterasi ini adalah Illite-Quartz, terkadang bersama dengan Chlorite atau Pyrite. Alterasi Illitic merupakan mineral Mica atau Hydro-mica berukuran lempung yang mengandung Potassium (K) dengan struktur kristal yang sama seperti mineral sericite berbutiran kasar.

Mineral Sericite adalah mineral Muscovite yang kristalin pada pengamatan mikroskopis. Mineral illite dan sericite biasanya sulit dibedakan satu sama lain pada XRD. Sericite merupakan mineral yang khas pada zone Alterasi Phyllic pada Sistem Epithermal Porfiri Tembaga ( Phorpyry Copper ) dengan batuan penerima basaltik. Untuk mempelajari alterasi perlu diketahui bahwa, para ahli geologi USA menyebutkan Illite sebagai Sericite dan Alterasi Illitic sebagai Alterasi Sericitic atau Alterasi Phyllic.

Alterasi Illitic kadangkala terpotong oleh veinlet-veinlet quartz mengandung emas, dan kadangkala oleh rekahan-rekahan seperti rambut (hairline) atau stringers quartz-pyrite, dan rekahan-rekahan ini jika mengalami pelapukan menjadi rekahan-rekahan atau kekar-kekar limonitik yang berwarna jingga. Jika dapat dijumpai stringers, maka batuan dapat mengandung hingga 4 g/ton emas, dan bulk-nya dapat ditambang.

Alterasi Illitic kemungkinan terjadi dari hasil pendinginan fluida hidrotermal dengan pH mendekati netral ketika fluida bergerak perlahan-lahan ke arah luar menjauhi vein-vein tanpa pendidihan ( boiling ) , dan mengakibatkan penguraian (dissossiation) gas H2S dalam larutan dan turun secara mendadaknya pH hingga membentuk asam lemah. Meskipun demikian, masih belum diperoleh penjelasan mengenai alterasi ini yang dapat diterima secara luas oleh para ahli.

Alterasi ini khususnya berkembang dalam batuan andesitik dan dasitik diban-dingkan dalam batuan basic. Alasan kenyataan ini tidak atau belum diketahui. Zone-zone Alterasi Illitic yang besar merupakan indikasi yang baik untuk emas dalam Sistem Epitermal, dan menunjukkan bahwa zone-zone tersebut bukan Zone-zone Alterasi Phyllic dalam Sistem Porfiri. Perbedaan di antara kedua-nya memerlukan pemetaan dan survei geokimia terhadap daerah-daerah yang bersebelahan. Dalam batuan alterasi phyllic yang umum bersifat lebih Silicic (secara fisik lebih keras) dan lebih banyak mengandung mineral pyrit (pyritic) dari pada alterasi illitic. Dalam eksplorasi, diambil conto alterasi illitic, serta lebar dan kelimpahan (kerapatan) vein, veinlet, atau stringers yang terekam dalam conto batuan.

Prospek Geologi   Kemabu Februari 8, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Geologi, Group Kembelangan, Kemabu, Metasomatisme, Prospek, Sesaradd a comment

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Prospek kemabu terletak di sebelah selatan dari zona sesar derewo yang berarah timur-barat sebelah utara sesar Domain. Domain tersebut sebagian besar terdiri atas kelompok sedimen Kembelangan Mesozoic-Cenozoic dan terdiri dari beberapa sesar naik. Campuran Group sedimen Kembelangan  adalah intrusi alkaline sampai intermediate yang mengubah bentuk batu gamping dan sedimen kapur menyerupai stuktur kubah. Intrusi tersebut berhubungan dengan struktur regional, dengan kompleks intrusi Kemabu muncul di persimpangan satu trend utama WNW/ESE, dipping sesar naik ke utara (D1) dan sesar mendatar utama berarah ENE/SSW (D1). Intrusi tersebut juga kontak sesar dengan sedimen Kembelangan ke utara, sepanjang barat laut trend sesar mendatar sinistral.

Area prospek didasari oleh sedimen Group Kembelangan yang berisikan 1 unit serpih, batu lempeng dan lumpur  kapur(calcareous mudstone) dari Mudstone Paniai. Hal ini didasari oleh interbedded batupasir, calcareous siltstone dan batugamping dari batugamping Pogapa. Group sedimen Kembelangan terlipatkan kedalam satu rangkaian trend pasangan antiklin-sinklin yang berarah WNW/ESE, plunging moderately kearah baratlaut dan tenggara.

Satu tambahan nyata dari intrusi batholiths pada kedalaman membentuk intrusi kompleks Kemabu. Intrusi tersebut mendekati luas areal 15-20 km persegi dan membujur tampak kasar bentuknya seperti telur. Proses intrusi  telah menghasilkan alterasi metasomatisme dari litologi karbonat sekitarnya. Penelitian petrologi menunjukkan bahwa Kompleks Kambu terdiri atas porfiri diorite hornblende dan biotit dasar-intermediat yang kaya volcanic, tuff yang berhubungan dengan breksi dan intrusi/ekstrusi breksi, syienit dan skarn. Volkanik dan breksi yang manapun, baik intrusi dangkal ataupun extrusive. Penanggalan K-Ar menunjukkan Miosen bagian atas(5.7-6.8Ma), pengecualian basalt andesit di area Mandoga yang beusia 16.5 Ma(pertengahan Miosen). Pemetaan lapangan dan observasi dari inti bor menunjukkan intrusi terbentuk di alam, dengan satu tahap porfiri diorite lebih awal dilanjutkan oleh  porfiri feldspar belakangan dan phase syienit. Kemungkinan porfiri feldspar adalah sebuah bentuk fraksinasi dari fase intrusi syenite yang lebih besar. Satu bentuk dianostik intrusi syenite adalah kehadiran Kristal K-feldspar bentuk tabular pnajangnya lebih dari 5cm. pengamatan petrologi dan inti bor menunjukkan phyllic tersebar luas dan kumpulan argilik (peristiwa lebih awal)disekitar satu inti potassic yang dicetak oleh propilitik yang belakangan dan phase skarn.

Cebakan intrusi kompleks Kemabu adalah satu rangkaian halus-kasar piroklastik dan unit epiklastik(yang mungkin berusia awal pliosen). Intrusi kompleks ini terdiri dari aglomerat, lithic sampai Kristal tuff dan intrusi subvolkanik, dengan timbunan konglomerat polimik, batupasir tuffan serta endapan tuff/sedimen lacustrin. Klastik dalam litologi lebih kasar  berisi kedua intrusi dan batuan sedimen diatas intrusi kompleks serta sedimen kelompok Kembelangan.

Secara local kelompok sedimen Kembelangan adalah terlipat kuat sebagai hasil dari tekanan kompleks Kemabu. Dari hasil pemetaan mengindikasikan struktur dominan utara-selatan dengan hubungan anggotanya barat laut dan timur laut.

Copper and How it   Occurs Februari 8, 2010

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Posted by julianusginting in Exploration, Geologi, Mineral. Tags: Copper, Deposits, Iron Oxide, Mineral, Porphyry, Sediment Hosted, Skarn, Volcanogenicadd a comment

Copper is a metallic element, number 29 on the Periodic Table and known for its incredible electrical conductivity. Copper is brownish in color and is available in many forms including bars, foil, sheet, granules, plates, powder, shot, turnings, wire, insulated wire, mesh, “evaporation slugs”, and rods.

Copper is primarily found in minerals associated with sulfur or in the oxidized products of these minerals. Copper easily combines with a number of other elements and ions to form a wide variety of copper minerals and ore deposits. Copper ore deposits typically contain less than 1% copper in the form of sulphide minerals. Approximately 85 % of the world’s copper mining supply comes in the form of sulphide mineral ores and the remaining 15% comes in the form of oxide mineral ores. The following table shows the most common copper mineral ores:

Mineral Composition Wt % Copper

Colour

Native copper

Cu 100.0 Copper Red

Cuprite Cu2O 88.8 RedChalcocite Cu2S 79.9 Dark greyCovellite CuS 66.4 Indigo blueBornite Cu5FeS4 63.3 Golden brown to copper

redMalachite CuCO3Cu(OH)4 57.5 Bright GreenAzurite 2CuCO3Cu(OH)2 55.3 BlueAntlerite Cu3SO4(OH)4 53.7 GreenChrysocolla CuSiO32H2O 36.2 Bluish green, sky blue,

turquoiseChalcopyrite CuFeS2 34.6 Golden Yellow

While commercially exploited deposits of copper ores are found in many parts of the world, the most concentrated copper deposits are located in the western cordillera of the Americas, mainly in Canada, the United States, Peru and Chile. In Africa, large deposits are found in Zambia and the Democratic Republic of the Congo and in Southeast Asia, large deposits are found in the Philippines, Papua New Guinea and Indonesia.

Copper deposits comprise both copper mineral “ore” and ‘gangue’ or host rock that has to be separated from the ore. Sulphide copper ores originate from sulphur-bearing magmas, which have separated into metal sulphides and siliceous melts. The most common geological formations related to copper deposit are as follows:

Porphyry deposits : Porphyry deposits are the world’s most important source of copper, accounting for about 50% to 60% of world copper production. These deposits are associated with intrusive rocks that are uplifting from the earth’s crust and the copper

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bearing sulphides usually occur as disseminations along hairline fractures as well as within larger veins, which often form stockworks.

Sediment hosted stratiform deposits (“SSC”) : SSC copper deposits are a large, diverse class of deposits that include some of the richest and largest copper deposits in the world. They are found primarily in the African Copperbelt . These deposits formed during a time when sediments deposited in arid areas. A variety of processes were involved in different districts but metals were characteristically deposited at re- oxidation boundaries where oxic, evaporite-derived brines containing metals extracted from iron rich aquifers encountered reducing conditions.

Iron Oxide Copper gold deposits (“IOCG”) : IOCG deposits have significant amounts of copper, gold and uranium and a few large deposits have been discovered in Australia. IOCG deposits are spectrum of sulphide-deficient low ore bodies of hydrothermal origin with polymetallic ( copper and gold) enrichments resulting from proximity to continental fault zones. The deposits are characterized by more than 20% iron oxides.

Skarn deposits : Skarns occur in the proximity of porphyry copper deposits where there has been a secondary geologic event associated with temperature and pressure. A Skarn is a fine grained metamorphic rock that is usually variably in colour and is usually polymetallic often containing zinc, gold or iron but is particularly important as a host of tin, molybdenum or copper. It usually forms by thermal metamorphism and metasomatism in the contact zone of magmatic intrusions like granites with carbonate-rich rocks such as limestone or dolostone.

Volcanogenic Massive Sulpide deposits (“VMS”) : VMS deposits are concentrations of base metal (copper, zinc, lead) and sometimes precious metal (gold, silver) sulphide minerals that occur in both ancient and modern submarine volcanic and volcanic-associated sedimentary environments. They are generally formed by the exhalation of hot, metal rich fluids onto the seafloor and have strong connections with the modern day “black smoker” deposits formed at spreading underwater ridges and often form in blocks. They are usually smaller in size and are common throughout the world.

Indonesia Mineral Deposits : Volcanogenic Massive Sulphide Au   Deposits Februari 8, 2010

Posted by julianusginting in Exploration, Geologi, Mineral. Tags: Au, Deposit, Hidrothermal, Indonesia, Massive Sulphide, Mineral, Volcanogenicadd a comment

The Lerokis and Kali Kuning Au—Ag—barite deposits in Wetar island, part of the Banda arc, are considered by Sewell and Wheatley ( 1994) to be generated at and immediately beneath the seafloor and therefore to be assignable to the Kuroko-type, volcanogenic massive sulphide ( VMS) class. The stratabound bodies of Au- and Ag-bearing, ferruginous barite sand, claystone and siltstone at Lerokis and Kali Kuning are interpreted as exhalative, whereas the barite-rich

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veins and stockworks elsewhere in the eastern Banda arc and, possibly, also the stratabound replacement body at Binebase, Sangihe island ( Swift and Alwan, 1990; Carlile and Mitchell, 1994) are epigenetic but thought to be generated in proximity to the seafloor. However, occurrence of both massive pyrite—marcasite containing zones enriched in enargite and tennantite beneath the barite-rich ore at Lerokis and Kali Kuning ( Sewell and Wheatley, 1994) and alunite-bearing alteration assemblages suggest an affiliation with high-sulphidation epithermal deposits generated in a subaerial environment (Carlile and Mitchell, 1994).

Enrichment of the Lerokis and Kali Kuning mineralization in the epithermal element suite, particularly Au, Ag, As, Sb and Hg ( Sewell and Wheatley, 1994), is reminiscent of modern VMS deposits which accumulated ( and are accumulating) under relatively shallow-water conditions ( say, < 1500 m) in western Pacific and other island arcs (Herzig and Hannington, 1992; Herzig et al., 1994) as well as of possible ancient analogues like the Eskay Creek precious- and base-metal deposit in British Columbia, Canada (Britton et al., 1990; Sillitoe, 1993). The most obvious modern analogue for these probable VMS deposits of high-sulphidation affiliation in Indonesia is the mineralization reported by Minniti and Bonavia ( 1984) on Palinuro Seamount in the Tyrrhenian arc, Italy. The volcanic-exhalative mineralization is present beneath about 600 m of seawater and contains abundant barite, is enriched in Au (up to 7 ppm) and possesses Cu (up to 1%) as enargite, tennantite and chalcopyrite (Tufar, 1992).

The high-sulphidation characteristics of the Lerokis and Kali Kuning deposits and the possible modern analogue on Palinuro Seamount contrast with the low-sulphidation alteration and mineralization assemblages associated with most Phanerozoic VMS deposits and suggest that, as with epithermal deposits, two discrete categories of VMS deposits ( highsulphidation / acid-sulphate and low-sulphidation / adularia-sericite) may be distinguished.

It should be cautioned, however, that the presence of massive, pyritic sulphides displaying colloform texture and abundant barite are insufficient by themselves to denote a VMS environment because both features are commonplace in high-sulphidation deposits of unam-biguously subaerial origin. For example, the Tambo deposit, in the El Indio Au belt of northern Chile, includes barite-cemented hydrothermal breccias in which the barite and Au are associated intimately ( Siddeley and Araneda, 1986).

Indonesia Mineral Deposits : Low-sulphidation epithermal Au   deposits Februari 8, 2010

Posted by julianusginting in Exploration, Geologi, Mineral. Tags: Au, Deposit, Epithermal, Indonesia, Low Sulphidation, Mineral, Mineralizationadd a comment

Low-sulphidation /adularia-sericite epithermal Au–( Ag) deposits are widespread in Indonesia, and those of vein type in Sumatra–West Java and Central and East Kalimantan predominate. Lebong Donok in Sumatra is a medium-sized (41.5 tonnes Au) bonanza Au deposit with textural and mineralogical similarities to the much larger Hishikari deposit in Kyushu, Japan ( Van Leeuwen, 1994). The recently discovered Gunung Pongkor deposit, West Java, is substantially

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larger ( 102 tonnes Au), but also comprises classical sulphide-and base metal-poor, low-sulphidation veins (Basuki et al., 1994 ). The low sulphide content ( < 1 vol.%) contrasts with those of the base metal-rich epithermal veins that are more common elsewhere in West Java (Marcoux and Mil6si, 1994) and in most other parts of Indonesia. Vein breccias characterize many of the low-sulphidation districts, most spectacularly at Lebong Tandai, Sumatra (Jobson et al., 1994) and Cirotan, West Java (Marcoux et al., 1993) .

The only low-sulphidation epithermal deposit which approaches giant status ( defined here as > 200 tonnes Au) is Kelian in East Kalimantan. However, this bulk-mined deposit is related to intrusive rocks, is rich in base metals and yields fluid inclusion temperatures

( up to 330°C) and salinities ( > 10 wt.% NaC1 equiv.) somewhat higher than typical for epithermal deposits ( Van Leeuwen et al., 1990). Consequently, a deeper level of formation, at least 900 m based on fluid inclusion geobarometry, was proposed by Van Leeuwen et al. ( 1990). Based on examination of recent mine exposures, this writer interprets the sedimentary rock-charged Muddy breccia at Kelian as a series of diatremes related to felsic plug-domes, and perceives similarities with the diatreme-hosted Au deposit at Montana Tunnels, Montana, U.S.A., also interpreted to have formed in the deep-epithermal environment (Sillitoe et al., 1985). Both Kelian and Montana Tunnels are rich in Zn, Pb and manganoan carbonates but lack appreciable quartz.

Most of the low-sulphidation epithermal deposits and prospects in Indonesia are associated, at least spatially, with andesitic—dacitic volcanic rocks, which contrast with the felsic dome complex hosting the disseminated Au mineralization at Gunung Pani in western North Sulawesi (Kavalieris et al., 1990). The dome complex is part of a late Miocene—Pliocene felsic volcanic suite which is broadly coeval with the belt of intrusive rocks containing the Malala Mo deposit ( see above; Kavalieris et al., 1992). The presence of both epithermal Au and porphyry Mo mineralization in association with the same magmatic suite may suggest that the dome-hosted Au was concentrated in the shallow parts of a concealed porphyry Mo system (T. van Leeuwen, written commun., 1992) if it is accepted that certain epithermal precious-metal deposits, some containing Mo, represent the tops of porphyry Mo systems ( Sillitoe, 1992).

Furthermore, the enrichment of W ( as wolframite), Sn ( as cassiterite and Te-canfieldite ) and Ag ( including uytenbogaardite) in the low-sulphidation epithermal vein Au—Ag deposit at Cirotan, West Java ( Marcoux et al., 1993), is attributed by Marcoux and Milesi ( 1994) to its association with Pliocene dacitic magmatism shown, using Pb-isotopic data, to be of crustal origin. Similar Sn and Ag enrichment, as stannite, canfieldite and Ag sulphosalts, is also documented for the Mangani Au—Ag vein in Sumatra ( Kieft and Oen, 1974). Such lithophile-element enrichment in precious-metal deposits related to magmatism of crustal parentage recalls the epithermal, Ag-rich ( but Au-poor) tops to the Sn- and base metal-bearing vein and stockwork systems of the Bolivian Sn—Ag belt ( Sillitoe, 1992).

Regolith-landform based exploration   models Januari 20, 2010

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Posted by julianusginting in Exploration, Geologi. Tags: Clay, Deposit, Exploration Issue, Gold, Landform, Nickel, Regolith, Volcanicadd a comment

General

Due to the common aspects of their formation and evolution, lateritic landscapes and regoliths are broadly similar across a range of different climatic zones. This genetic link extends to chemical and physical weathering processes and allows comparison across regions through the use of some generalized dispersion and exploration models (Butt and Zeegers 1992). These models are based on the degree of preservation from erosion of the pre-existing lateritic profile, which determines the nature of the uppermost residual horizon, and the presence of transported overburden. Having established the model type, some broad characteristics of the geochemical expression of mineralization can be predicted and can be used to design sampling strategies. Many such predictions will be valid across quite diverse climatic environments and for regions in which there is little or no existing information. More detailed aspects of dispersion, however, are specific to particular regions and commodities, and their description requires models based on appropriate orientation and case studies.

Gold deposits

During lateritic weathering of lode deposits, there is little dispersion of gold except in the upper horizons (Freys sinet and Itard, 1997). Lateral detrital and chemical dispersion has occurred in the lateritic residuum, commonly with leaching and depletion in the top 1 to 3 m and enrichment below. This provides a very broad, multi-element, near-surface exploration target. However, if this material has been eroded, the target in saprolite is similar in dimensions to that in bedrock, which has an immediate impact on selecting appropriate sampling density. This gold distribution is found not only in the present savannas, but in both more humid and more arid regions. In rainforest environments, leaching is stronger, decreasing the surface expression, but giving widespread drainage anomalies. Conversely, in arid regions, gold enrichment in lateritic residuum continues to the surface, increasing its accessibility to surface sampling. Even where buried, lateritic residuum is a valuable sample medium, but accurate logging is necessary to distinguish it, in drill cuttings, from detrital ferruginous gravels. In all environments, analysis for appropriate pathfinder elements (e.g., As, Sb, W, Cu, Bi), can assist in the definition and prioritization of anomalies for further exploration.

In arid regions with acid, saline groundwaters, the enrichment in lateritic residuum is preserved. However, strong leaching and depletion of gold may occur in the top 10 to 40 m of saprolite, but with absolute, supergene enrichment below. The latter may form an important resource, but commonly shows no greater dispersion target than the underlying lode and primary alteration zone (Gray et al., 2001). If the lateritic residuum has been eroded, or never formed, the depleted zone outcrops at surface or subcrops immediately beneath transported overburden, hence except where there are pedogenic carbonates, pathfinder elements will give a better expression of mineralization than gold. Pedogenic carbonates will concentrate any gold, even where there has been considerable leaching and depletion, and are the preferred sample medium in residual soils or where the cover is less than 10 m thick. The concentrations and contrasts are low, targets

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small and usually gold only. These variations in surface expression demonstrate the importance of regolith-landform mapping, accurate logging and knowledge of the weathering history and its impacts on element dispersion for effective exploration.

Volcanic-hosted base metal deposits

Few volcanic-hosted base metal sulphide (VHMS) deposits are known in deeply weathered shield areas of S. America, W. Africa or Australia, despite their apparent prospectivity through comparison with equivalent terrane in Canada. Most of the few discoveries (and of other massive sulphide deposits, including nickel sulphides), have been made by gossan search or, latterly, by improved electromagnetic and other geophysical techniques. Most gossan discoveries have been in erosional terrain, in arid environments; this is true for Australia, where deep weathering is widespread, and other areas such as in southern Africa and the Arabian Shield, where deep weathering is absent or vestigial. There is a poor record of discovery in arid areas with intact lateritic regoliths, in depositional areas and in the humid tropics in general. Gossan profiles are commonly described as being formed of iron oxides with variable, and commonly low, base metal contents in the upper portion, underlain by secondary enrichment zones, including native metals, carbonates and sulphates, with supergene sulphides as a transition to the primary sulphide ore. These descriptions are dominantly from arid environments, and these lower zones may have formed under the contemporary climate. Observation

also indicates that gossans thin and become attenuated towards the surface. The assumption that lateritic weathering of base metal sulphides always forms gossans at surface is thus questioned, as too must be the effectiveness of many geochemical techniques that are directed at gossan search.

Lateritic weathering of VHMS deposits results in the extensive leaching of copper and zinc, leaving a gossan formed by iron oxides principally derived from chalcopyrite, pyrite and pyrrhotite. It is possible that high in the regolith, the iron oxides forming the gossan themselves dissolve, hence the near-surface expression of the deposit may be represented by a range of less mobile elements hosted by resistant minerals. In lateritic residuum at Golden Grove in Western Australia, for example, the dispersion halo is extensive, but defined by Bi, As, Sb and Sn, in addition to Cu, Zn and Au (Smith and Perdrix, 1983). There is little evidence for a widespread dispersion halo in saprolite of any of the ore elements, so that where the profile is truncated, in the absence of significant gossan, there may be little trace of the mineralization. The situation will be more complex if there is burial by transported overburden. Because VHMS deposits are major sources of labile metals, specific targeting by selective extraction analysis is a possibility in depositional areas, but is likely to be successful only if the depth to sulphides is shallow and the sulphides are actively weathering.

Interest in exploring for VHMS deposits in deeply weathered shield areas has recently increased, after a lull over much of the past two decades. Because of the uncertainties of their surface expression, very careful attention needs to be paid to geochemical procedures. Tailoring procedures to the regolith setting and understanding the potential signature has proved effective to guide sampling strategies for gold exploration and a similar approach is recommended for base metals.

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Nickel deposits

Exploration for nickel sulphides has similar issues to those for VHMS deposits related to gossan formation and preservation in lateritic environments, with the added complexity that they occur in nickel-rich host rocks. Many early discoveries of massive nickel sulphides in Western Australia were through the discovery of gossans, by ‘ironstone’ sampling or soil surveys, and the majority of these were in erosional terrain. At the original discovery sites at Kambalda, for example, although the sulphides are weathered to 150 m or more, some of the wall-rocks are essentially fresh at surface. Elsewhere, some gossans are very attenuated near the surface (e.g., Redross) and, at Harmony, it is possible that the gossan may have entirely dissolved (N.W. Brand, personal communication, 2000). Petrographic examination for boxwork textures after sulphides, multi-element geochemical analysis and statistical techniques have been widely used to discriminate gossans from other ironstones.

It is also important to understand the nature of nickel enrichment in lateritic regoliths, which may form deposits in their own right. The genesis of nickel laterites depends upon a variety of geological, geomorphological and climatic factors (Table 2). There are three types of deposit, based on the ore mineralogy (Brand et al., 1998):

Oxide deposits, mean grades 1.0-1.6% Ni: dominated by Fe oxyhydroxides, principally goethite, forming the mid- to upper saprolite and extending to the pedolith. Manganese oxides may host reserves of Co. Deposits developed over dunites (adcumulates) may contain abundant secondary silica. There is an oxide component to all deposits, but because this requires a different metallurgical process, it is commonly either discarded or stockpiled when silicates are the principal resource.

Hydrous Mg silicate deposits, mean grades 1.8-2.5% Ni: dominated by hydrous Mg-Ni silicates (e.g., “garnierite”; nickeloan serpentine and talc;) in the lower saprolite. The majority of producing nickel laterites are of this type.

Clay silicate deposits. mean grades 1.0-1.5% Ni: dominated by Ni-rich smectites such as nontronite and saponite, commonly in the mid to upper saprolite and pedolith.

The high nickel content of the weathered host rocks presents considerable difficulties in the recognition of the signature of sulphides, whether at surface or deep in the regolith. Lateritic nickel enrichments may follow steeply-dipping structural elements such as shears, potentially emulating the

distribution expected from the oxidation of sulphide-rich rocks. Commonly, high concentrations of copper and/or platinum group elements (PGE) can confirm a sulphide signature, although over disseminated deposits (e.g., Mt. Keith), the nickel, copper and PGE enrichment may occur in different units of the regolith profile (Brand and Butt, 2001). Similarly, although dispersion into weathered wall-rocks and surface media such as soil, lateritic residuum and transported overburden is possible, distinguishing between sulphide and silicate sources is commonly very difficult

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Exploration   issues Januari 19, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Cause, Characteristic, Environment, Exploration Issue, Geophysical, Mineral, Problemadd a comment

The complex regolith environment presents both challenges and opportunities for mineral exploration (Table 1). The morphological, petrophysical and compositional characteristics of the residual regoliths are commonly profoundly different from those of the rocks, including ore deposits, from which they have been derived. These affect geological, geochemical and geophysical exploration procedures and constrain their use. The presence of transported overburden and basin sediments exacerbates these problems, especially for geochemical procedures based on surface or near-surface

sampling. Geophysical survey methods, particularly airborne techniques, are commonly applied to “see through” this cover, but are hindered by interfering regolith-related responses. Conversely, because much of the regolith is residual, and has the potential to provide secondary dispersion targets that are broader than the primary mineralization itself. In addition, deep weathering also leads to the formation of many important secondary and supergene ores, notably bauxite, some iron ores, nickel-cobalt laterites, niobium-rare earth-phosphate deposits, lateritic and supergene gold, supergene copper, and industrial and building materials.

The formation of lateritic regoliths, their modification under changed climatic settings and the effects of erosion during and after these events result in significant regolith-landform control on sample media. Knowledge of the distribution and properties of regolith components is essential for successful exploration in regolith-dominated terrains, whether for deposits concealed by the regolith or for those hosted by it. Regolith-landform mapping is an essential first step, followed by characterization of the regolith materials themselves. From a geochemical perspective, regolith-landform maps can be interpreted in terms of models that describe the geochemical pathways followed by ore-related elements as they disperse during weathering and, therefore, not only indicate the most appropriate sample media but assist in data interpretation (Bradshaw, 1975, Butt and Smith, 1980; Butt and Zeegers, 1992). From a geophysical perspective, these maps, particularly at district to prospect scales, may indicate the applicability of specific procedures and provide a basis for distinguishing between regolith- and basement-related responses. Accordingly, there is increasing emphasis on regolithlandform mapping, using a combination of remote sensing procedures, followed by field checking, including drilling. Satellite imagery and conventional aerial photography remain the basic approaches to mapping, but are routinely supplemented by multispectral Landsat and airborne data for mineralogical information and airborne radiometric surveys for geochemical data (e.g., to indicate provenance). However, whilst these procedures indicate the nature and distribution of surface materials, they are poor indicators of the underlying regolith, especially in areas of transported overburden and basin cover. New electromagnetic systems, in particular, can map spatial variability of physical properties and conductivity structure in the third dimension, yielding information including total thickness of transported and residual regolith, presence of palaeochannels and an outline stratigraphy.

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Integration of these data can rapidly deliver inventories of surface materials, interpretable in terms of weathering styles and geomorphological processes, but require field inspection prior to providing definitive exploration guides. Geochemical methods have been developed for relict and erosional terrains in many deeply weathered environments, but surface techniques appear to be generally ineffective in most depositional regimes. Preliminary field work should include drilling or deep pitting – ideally in all regolith landform regimes but essential where there is transported cover – to provide a basis for selecting sample media and interpreting data. Accurate logging, however, may not be easy even in good field exposure and can be very difficult from drill cuttings. Unfortunately, this task is commonly delegated to some of the most inexperienced geologists, yet correct identification of significant regolith profile units and boundaries is of considerable importance. As an example, ferruginous lateritic residuum is an important sample medium, yet may be readily confused with sedimentary units that have different relationships with bedrock and mineralization and little or no value for sampling. Similarly, despite claims to the contrary, there are very few instances where significant geochemical anomalies in transported overburden can be successfully related to underlying mineralization, even using special analytical techniques. Exceptions include gold anomalies in calcrete, which may penetrate 5-10 m of cover (Lintern, 2002), and some base metal anomalies in basin sediments (e.g., at Osborne, Queensland), although these may have formed during diagenesis rather than weathering (Lawrance, 1999). Accordingly, recognition of the presence and thickness of transported units of any type is crucial. It is expected that the development of instrumental procedures for routine logging will be of great value in overcoming many of these problems.

Table 1: Some exploration problems and opportunities in deeply weathered terrain

Problem                                                                                              Cause Difficulty in recognizing parent lithology Mineralogical, chemical and morphological

changeVariable regolith thickness; soils derived from many different parent materials

Differential weathering and partial erosion

Subtle surface expression Strong leachingSpurious secondary enrichment and numerous ‘false positive’ anomalies

Mobility        and    re-concentration       duringweathering, erosion and deposition

Complex geochemical signatures Superimposition of multiple weathering eventsMasking by transported overburden and basin sediments, themselves possibly weathered

Ineffective chemical dispersion during post-depositional weathering and diagenesis

Attenuation     or   masking    of   geophysicalresponses

Increased distance between sensor and target Highly conductive surface layers

Development         of     false      geophysicalanomalies/responses:

Magnetics

Electromagnetics, induced polarization Radiometrics

Remote    sensing  (e.g.,   spectral imaging);

Concentration of magnetite, maghemite

Low resistivity and marked resistivity contrasts Presence        of     transported       anomalies;disequilibrium due to chemical mobility

Surface response – cannot penetrate transported cover

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radiometricsLow contrast seismic and gravity response Zonation, transitional contacts, density contrastOpportunities CausePresence of supergene ore deposits Residual or absolute accumulations, e.g.,

bauxite, N-Co laterite; Au, Nb, P, U deposits; Fe and Mn ores. Industrial minerals.

Largely residual regolith as sample medium. Tectonic stability; long exposure; little erosionWidespread geochemical anomalies in a variety of regolith horizons and materials, e.g., soil, lateritic residuum, lag, pedogenic carbonates

Physical and chemical dispersion over long periods

Regolith Distribution and   Characteristics Januari 19, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Characteristic, Mesozoic, Paleozoic, Regolith, Tertiaryadd a comment

Large areas of the world, especially the largely tropical to sub-tropical zone between latitudes 40º

north and south, are characterized by a thick regolith cover. Much of this regolith is residual and consists of intensely weathered bedrock, but there may also be an overlying component of transported material, itself weathered to varying degrees. The regolith is most extensive in continental regions of low to moderate relief, such as the Precambrian shields, and adjacent and overlying Phanerozoic sedimentary basins, of South America, Africa, India, south east Asia and Australia. Remnants are present in some areas of stronger relief, perhaps most significantly in parts of the circum-Pacific belt, where ophiolitic rocks have weathered to form high grade nickel laterites. Commonly, such regolith is absent from tectonically active and mountainous areas. Thick residual regolith is also generally absent from very arid terrains in the tropics and sub-tropics, such as the Sahara and Arabian deserts, although transported materials, including fluvial deposits and dune sands, are widespread. Nevertheless, isolated occurrences of strongly weathered regolith are recorded from these desert regions, either exposed or buried beneath the younger sediments, indicating that it was once more widespread. There is also increasing recognition of the presence of similar regolith, mainly as thick saprolite, in North America and Europe.

Much of the residual regolith has broadly lateritic characteristics, with a thick, clay-rich saprolite, generally with an overlying iron and /or aluminium-enriched horizon, although the latter may be only patchily developed or have been removed by later erosion. Lateritic regoliths are considered to have been formed under seasonally humid tropical to sub-tropical climates, although there is evidence that similar weathering may occur under cooler conditions. The age of formation varies from Palaeozoic to present day, determined by palaeomagnetic, isotopic and stratigraphic techniques. Commonly, several validated ages may be obtained from a single profile, site or district, indicating multiple stages in development. There is considerable evidence for widespread late Mesozoic to early Tertiary deep weathering extending from equatorial regions to the present high latitudes. These dates have been obtained in S. America, Africa and

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Australia; the buried bauxites and nickel laterites of eastern Europe and the Balkans are Mesozoic and there is interpreted Tertiary tropical weathering in N. Ireland. (The age of other high latitude saprolite is less well constrained, with weathered Palaeozoic and older rocks buried by Mesozoic (e.g., south Urals), Oligocene (e.g., north Urals) or Pleistocene (e.g., northern USA) deposits. Subsequently, the land surface has been subjected to a variety of conditions, ranging from humid to arid and tropical to sub-arctic, consequences of climatic change and continental drift, resulting in the physical, mineralogical and/or chemical modification of the pre-existing regolith. In places, the earlier regolith has been almost fully preserved; elsewhere, it has been partially or wholly eroded, or buried by sediments that themselves may then be weathered. The outcomes are landscapes that consist of mosaics of different regolith-landform associations

Low Sulphidation Gold-Silver-Copper   Deposits Januari 19, 2010

Posted by julianusginting in Exploration, Geologi. Tags: Copper, Deposit, Epithermal, Gold, Hidrothermal, Low Sulphidation, Silveradd a comment

Low sulphidation epithermal gold deposits are derived from reduced, near neutral pH, dilute fluids developed by the entrainment of magmatic components within deep circulating groundwaters, and are characterised by sulphur species reduced to H2S (Corbett and Leach, 1998 and references therein). Hydrothermal fluids become progressively more diluted by the incorporation of increased quantities of ground waters during migration further from the intrusion heat (and magmatic component) source, to higher crustal levels. The classification developed mainly using SW Pacific examples (Corbett and Leach, 1998), and expanded upon here, describes a series of deposit styles as end points within a continuum, where many individual ore systems (deposits) may contain several of the deposit styles developed during ore fluid evolution, or by telescoping, and repeated mineralisation. Telescoped systems generally display later formed mineralisation typical of higher crustal levels overprinting deeper earlier formed mineralisation. The majority of ore deposition is promoted by fluid cooling aided by rock reaction, and mixing of rising ore-bearing fluids with groundwaters. Contrasting groundwater types and varying crustal levels contribute towards

mineralogical differences used to categorise the styles of low sulphidation epithermal gold deposit styles (Corbett and Leach, 1998).

The third order in the classification of low sulphidation epithermal gold deposits is expanded from the terminology used to distinguish SW Pacific examples (Corbett and Leach, 1998), to account for mineralogies which result from varying associations with magmatic source rocks and input of meteoric geothermal waters, termed arc and rift low sulphidation.

The Arc Low Sulphidation gold deposits display strong field associations with intrusive rocks and are catagorised below on the basis of varying ore (pyrite, sphalerite, galena, chalcopyrite, arsenopyrite), gangue (quartz, carbonate, clay) and wall rock (clay, chlorite) mineralogies, which essentially relate to formation at increasingly shallow crustal levels as: quartz-sulphide gold ±

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copper, polymetallic gold-silver [a new class], carbonate-base metal gold, epithermal quartz gold-silver. The Rift Low   Sulphidation deposits comprise the adularia-sericite epithermal gold-silver ores in the classification of Corbett and Leach (1998), and typically occur as veins with gangue mineralogies (chalcedony, adularia, quartz pseudomorphing platy carbonate) deposited from circulating dilute (meteoric-dominated) geothermal waters, within dilatant structures, typically confined to rifts within magmatic arcs or back arc environments.

A locally recognised transition between rift and arc low sulphidation, may more rarely continue further as a transition between arc low sulphidation and high sulphidation, broadly speaking as the ore fluid displays an increased dominance of the intrusion component rather than derivation from meteroricdominated geothermal fluids, which form the rift low sulphidation (adularia-sericite) gold deposits. Note the term epithermal is not incorporated in the names of the deeper arc low sulphidation styles (quartz- sulphide gold ± copper, polymetallic gold-silver, and carbonate-base metal gold), which in some instances form at greater depths than would be expected for epithermal gold deposits