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Kodu, Mt Bini |
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Papua New Guinea |
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Cu Au Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Kodu (previously known as Mt Bini), porphyry copper-gold deposit is located on the west side of the Owen Stanley Range, 50 km east-north-east of Port Moresby, in the Central Province of Papua New Guinea at an altitude of around 1200 m. It is adjacent to the Kokoda Track. (#Location: 9° 18'S, 147° 35'E).
The first mineralisation was discovered in 1992 during a helicopter-supported stream sediment sampling program. At a sample site on Ofi Creek, 2km downstream from Mt Bini, an anomalous pan concentrate sample (157ppm Au) and a pyritic silicified float sample (20.7g/t Au, 463g/t Ag, 0.15% Cu and 0.6% Pb) were collected. Follow-up stream sediment and ridge and spur soil sampling resulted in the delineation of a 2000 x 200 m +0.2 g/t Au anomalous zone, largely enclosing an area of 650 x 30 m with +150 ppm Cu and +18 ppm Mo. A peripheral Pb anomaly was associated with late stage, low sulphidation epithermal carbonate base-metal over an area of 1800 x 1300 m. The first drill hole encountered 235 m @ 0.31% Cu, 0.47 g/t Au from 104 m.
The Kodu deposit occurs along a mineralised trend which extends south-east from the gold deposits of the Wau District and includes the Wafi and Hidden Valley deposits. The district lies within the Owen Stanley Thrust Belt of the Eastern New Guinea Orogen. On a local scale, the deposit falls within a NNE-trending, 15 to 20 km wide extensional zone in which basement metamorphics are unconformably overlain by Pliocene basic to intermediate sub-aerial volcanic rocks and intruded by the coeval Pliocene monzonite to diorite stocks and dykes of the 10 x 4 km Bavu Basic Igneous Complex which is located to the north-west of the deposit. The Tolokuma Au-Ag vein system lies within the comparable Mt Cameron Volcanic Complex to the west.
Basement is composed of phyllites and slates of the Jurassic to Cretaceous Kagi Metamorphics, part of the Owen Stanley Complex which consist mainly of folded greenschist facies, locally comprising carbonaceous, psammitic and pelitic meta-sediments. The basement was probably metamorphosed in the Middle Miocene when the Papuan Ultramafic Belt collided with and was obducted onto the advancing Australian Plate.
In the deposit area, the Pliocene volcaniclastics are composed of tuffaceous meta-sediments with porphyritic, vesicular basalt and andesitic lava. Various calc-alkaline to shoshonitic rocks outcrop on the south side of the Bavu Complex, in the vicinity of the Kodu deposit. The main Mt Bini Porphyry is a 650 x 275 m, potassium-rich, calc-alkaline, composite intrusive stock composed of diorite to monzo-diorite to quartz-monzonite and granodiorite and has been dated at 4.42 ±0.04 Ma.
The Mt Bini stock is located at the intersection of ENE and NNE fractures, and has been divided into two phases, namely: P1, a fine grained, phenocryst rich, porphyritic quartz diorite, containing plagioclase phenocrysts in a microcrystalline groundmass of quartz, orthoclase and biotite; P2, a quartz diorite porphyry with abundant plagioclase, biotite and clinoamphibole phenocrysts in a fine grained matrix of orthoclase, plagioclase and secondary biotite, which forms the southern half of the stock, centred on Mt Bini, and the ENE trending, 350 x 45 m Sirimu Dyke which is some 350 m to the south-west. Both of these phases are cut by later dykes of similar composition but with weaker alteration and mineralisation. A barren dyke cuts mineralisation and, while the extent of hydrothermal alteration clearly declines in these younger intrusions, overall relationships are still unclear. The Track Diorite, an unaltered, relatively potassium rich and strongly magnetic diorite is located some 500 m to the north of the Mt Bini Stock.
A number of alteration types are recognised, including an early potassic episode divided into an initial phase of pervasive and fracture fill secondary biotite and a later phase of orthoclase, which is also developed as selvages to later crosscutting quartz-sulphide±magnetite veinlets. The magnetite associated with this alteration contributes to an anomalous magnetic response over the stock. Chalcopyrite dominates over bornite, mostly occurring as disseminations and on fractures, and within quartz-magnetite veins, while gold occurs as inclusions within chalcopyrite. Minor amounts of anhydrite have been recognised. This alteration is centred on the Mt Bini stock and the Sirimu Dyke. The potassic core is surrounded by propylitic alteration, with a magnetite-chlorite-tremolite-actinolite-quartz-sulphide assemblage close to the inner potassic and phyllic zones, and quartz-carbonate-epidote±magnetite±chalcopyrite veins with minor tennantite-tetrahedrite in the periphery. Extensive, retrograde post potassic, phyllic-argillic alteration mainly as a fracture controlled sericite-quartz-pyrite assemblage, is found in the outer 50 m of the stock and in adjacent country-rock phyllites to form a 1500 x 1500 m halo around and within the stock. Cherty/quartz + tourmaline alteration accompanies narrow (<2 m thick) breccias associated with phyllic alteration, in a number of zones surrounding the stock, particularly in the south-west. Intermediate argillic alteration overprints the system to a depth of 100 to 200 m below the topographic high formed by the stock.
Cu-Au mineralisation is concentrated about the margin of the stock, both within the stock and the host phyllite. The primary ore occurs as a complex series of overlapping phases. The first phase comprises fracture and disseminated sulphides and is associated with the early pervasive potassic alteration. It is predominantly composed of fine grained pyrite and chalcopyrite. Magnetite is commonly intergrown with the sulphides and is strongly correlated with the higher grade mineralisation. The second phase occurs as a quartz stockwork with pyrite, chalcopyrite, magnetite and molybdenite. Gold occurs as inclusions in chalcopyrite. This stockwork is well developed in the brittle phyllite country rock and often coincides with strong overprinting phyllic alteration. Thin veinlets of orthoclase-biotite-quartz±chlorite±carbonate containing pyrite and chalcopyrite cut the main stockwork mineralisation. Coarse grained sulphides are related to the propylitic phase. Minor tennantite-tetrahedrite is also associated with propylitisation. Galena-sphalerite veinlets found immediately adjacent to the Mt Bini intrusions and more distal galena-sphalerite-carbonate veins are also associated with propylitic alteration. Late pyrite and chalcopyrite with sericite, carbonate and chlorite veinlets cut all of the previous styles of mineralisation.
The primary mineralisation has a broad zonation of sulphides. Chalcopyrite volumetrically exceeds pyrite in the core of the Mt Bini Stock, while pyrite predominates in the periphery. Molybdenite is found in the outer sections of the deposit, often accompanied by galena and sphalerite around the periphery of the stock. Gold is richer in the earlier phases of the intrusion, while the later stages appear to be barren. The core of the stock is essentially barren, with the mineralisation having a "donut" form in the outer porphyry shell.
A leached cap is preserved in the upper tens of metres to a maximum of 70 m (below the highest point over the Mt Bini Stock). Metal levels increase towards the base of the leached cap with localised occurrences of malachite staining and oxide copper mineralisation. Supergene, fine grained, chalcocite and covellite is found locally below the leached cap.
High grade, epithermal gold mineralisation overprints the porphyry Cu-Au style as veins up to a metre thick over a strike length of 1400 m, following the ENE to NE structures, generally associated with the Sirimu Dyke immediately to the west of the Mt Bini Stock. Within this trend, individual veins strike NW. These occur as chalcedonic veins and stockworks, while silver rich crustiform and colloform banded quartz, stibnite and rhodochrosite have returned intersections of as much as 8 m @ 19 g/t Ag. Chalcedony veins and stockworks west of the the Bini stock have returned assay values over 20m of 0.56g/t Au.
Dugmore and Leaman (1998) quoted an inferred resource of 85 Mt @ 0.4% Cu, 0.6 g/t Au.
In 2007, the total indicated + inferred resource was quoted at 276 Mt @ 0.27% Cu, 0.30 g/t Au, 77 ppm Mo, 1.7 g/t Au, including an indicated resource of 105 Mt @ 0.30% Cu, 0.35 g/t Au, 68 ppm Mo, 2 g/t Ag. These figures were based on 10235 m of drilling in 30 drill holes (Frontier Resources web site, Jan 2008).
The most recent source geological information used to prepare this decription was dated: 2007.
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.
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Dugmore M A and Leaman P W, 1998 - Mount Bini copper-gold deposit: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 843-848
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Leaman P, 1996 - The Mt Bini porphyry copper-gold deposit and its tectonic setting, Papua New Guinea: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide pp 13.1 - 13.10
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