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Nevera, Crater Mountain - Main Zone, High Grade Zone |
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Papua New Guinea |
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Au Cu
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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.
Available as Full Text for direct download or on request. |
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The Nevera or Crater Mountain gold deposits, including the Main and High Grade zones, are located 65 km SW of Goroka, 280 km ESE of Porgera and 240 km WNW of Hidden Valley in Papua New Guinea (#Location: 6° 30' 7"S, 145° 5' 11"E).
The Nevera deposits are associated with the Plio-Pleistocene or Holocene (?) Crater Mountain Volcanic Complex, one of a series of strato-volcanoes situated in the PNG thrust belt along the leading edge of the Australian Continental Plate. The volcanic complex includes two large craters, each of over one kilometre in diameter, and ~30 small vents as well as hot springs and associated sub-volcanic intrusives spread over more than 200 km2. This complex formed after two separate phases of volcanism, and are underlain by marine sediments of the Chim Formation. Mineralisation and alteration is widespread in the volcanic sequences and the Cretaceous Chim Formation which comprises bioturbated and laminated grey calcareous siltstone and mudstone with local sandy horizons.
The Nevera deposits are distributed over a zone that is ~3.5 km long in a north-south direction, and >2.5 km wide. This area is draped over a prominent high north-south ridge, which projects northwards at ~2300 m asl from the main east-west Crater Mountain range and terminates near the northern edge of the mineralised zone. The epithermal gold mineralisation is located towards the northern end of the system.
The Nevera deposits are part of a complex epithermal-porphyry system. Drill testing and petrology suggest a deep primary high temperature porphyry copper-gold mineralising event is associated with tonalite porphyries intruding basaltic andesites, younger more evolved dacites and feldspar-quartz porphyries of the Crater Mountain volcanic/intrusive pile and bleached and indurated shales of the Chim Formation. The tonalite porphyries and volcanic country rock have undergone extensive prograde potassic metasomatism which accompanied the early porphyry copper-gold mineralisation, as well as strong thermal metamorphism of the country rock. The porphyry copper-gold mineralisation is widely distributed and occurs as multiple stages of B- and C-type porphyry style quartz veining within which abundant hypersaline, gas and liquid-rich aqueous fluid inclusions occur in granoblastic quartz.
This phase was followed by an episode of intense retrograde epithermal phyllic/silicic alteration which overprints the earlier porphyry copper-gold mineralogy and textures, and in places has associated carbonate+base metal sulphide±gold mineralisation. The strong phyllic overprint decreases downward. The phyllic alteration overprint in places may have been strong enough to remobilise/redistribute prograde porphyry-related copper and gold.
Anhydrite is locally a major component of the epithermal retrograde phyllic alteration as well as the primary porphyry style veining and related prograde potassic metasomatic alteration, and occurs deep in the system as intergrowths with preserved secondary biotite and K-feldspar. The occurrence of anhydrite rather than alunite in the retrograde hydrothermal overprint is taken to indicate the near neutral pH of the invading hydrothermal fluids reacting with the wallrock.
Chalcopyrite and bornite are the principal copper minerals contained within prograde potassic metasomatism/hornfels replacement/recrystallisation assemblages, and that contained within paragenetically associated porphyry style quartz veining. The chalcopyrite and bornite stability persists with retrograde phyllic alteration overprinting, whereas minor amounts of chalcopyrite and more abundant tennantite/tetrahedrite comprise the copper mineralogy present within base metal-carbonate fracture-fill and breccia cement assemblages.
The Main Zone gold mineralisation is relatively shallow lying 150 to 300 m below the northern end of the ridge, and comprises low-sulphidation epithermal carbonate-base metal sulphide-gold mineralisation covering an area in excess of 600 x 400 m and is 150m thick. It is largely hosted in a "mixing zone" located at the base of the Crater Mountain volcanics, extending a short distance down into the underlying Mesozoic Chim Formation shales. The base of the Main Zone is only marked by a reduction in Au assay values and not by visual reduction in mineralisation, which continues for hundreds of metres through strongly bleached and indurated Chim Formation shales with abundant brittle fracturing and breccia commonly displaying multi-generational quartz-pyrite and quartz-carbonate-base metal sulphide veining. In this lower zone there are sporadic short (~2m) sections with medium and rarely high gold grades associated with prominent quartz-carbonate-pyrite-base metal sulphide veins..
A JORC compliant inferred resource of 24 Mt @ 1.0 g/t Au has been defined in the Main Zone (Crater Gold release to the ASX, 2013) using a 0.5 g/t Au cut-off. This includes 9.4 Mt @ 1.46 g/t Au using a 1.0 g/t Au cut-off.
The High Grade Zone comprises high grade, high-sulphidation epithermal quartz-pyrite-gold mineralisation, extending from surface to several hundred metres depth, possibly in excess of 500 m. Local artisanal miners produced an estimated 0.5 tonnes of gold from a small area of shallow workings (maximum 50 m depth) in the base of a steep mineralised spur between 2005 to 2012. A small underground starter operation, aimed at mining up to 0.25 Mt of ore at a grade of up to 30 g/t Au was commenced in 2015 to provide cash for further exploration.
This is a preliminary summary and is based on, and paraphrases, reports available from the Crater Gold Mining website including a report by Coote (2012) of Applied Petrologic Services & Research.
The most recent source geological information used to prepare this decription was dated: 2013.
Record last updated: 23/12/2015
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.
Crater Mountain
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Perrier N, Ambrosi J P, Colin F and Gilkes R J, 2006 - Biogeochemistry of a regolith: The New Caledonian Koniambo ultramafic massif: in J. of Geochemical Exploration v88 pp 54-58
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Yang K, Whitbourn L, Mason P and Jon Huntington J, 2013 - Mapping the Chemical Composition of Nickel Laterites with Reflectance Spectroscopy at Koniambo, New Caledonia: in Econ. Geol. v.108 pp. 1285-1299
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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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