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Hidden Valley, Hamata, Kerimenge
Papua New Guinea
Main commodities: Au Ag

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The Hidden Valley low sulphidation carbonate-base metal-gold epithermal deposits are within the Morobe Goldfield, in the Morobe Province of Papua New Guinea, 210 km NNW of Port Moresby. The total production from the Goldfield to 1975, which also includes the historic Wau and Edie Creek epithermal deposits and the extensive Bulolo placer accumulations, amounted to 115 tonnes of Au, of which approximately 15% was from lodes, the remainder being from alluvial operations.

The Morobe Goldfield lies within the Highlands Fold Belt of the New Guinea Orogenic Province, situated between the overthrust Papuan Ultramafic Belt to the north-east and the Neogene sediment filled Aure Trough to the south-west. The basement in the district, the Cretaceous Owen Stanley Metamorphics (which include blue-grey graphitic slate, chloritic and sericitic schist and phyllites, with minor quartzite and marble), is intruded by the 14.3 Ma Miocene Morobe Granodiorite Batholith. These are overlain by a Pliocene sequence that includes, the Bulolo Ignimbrite which is several hundred metres thick and dips at 40 to 65°E, and the Otibanda Formation comprising 700 m of poorly sorted auriferous conglomerate and sandstone with minor reworked tuff and ignimbrite near the base, and the Pliocene (3.8 to 3.4 Ma) intrusive dacite to andesite porphyry of the Edie Porphyry which has strongly altered (silica-sericite-pyrite) the surrounding metamorphics. These are all cut by the Namie Breccia of the Wau Maar complex which is generally coarse and poorly sorted and is believed to be of subaerial phreatomagmatic origin. Some of the Edie dacite porphyries in the district are present as domes emplaced during the formation of the maar and generally found along the ring fault. All of these are overlain in part by Pleistocene fluvial and fluvioglacial sediments. A major NNW trending, 10 km long, east dipping fault zone, the Escarpment Fault cuts most rocks up to the beginning of Maar Formation. Exposures of the fault show fault gouge, silicification, pyritisation, as well as hosting intrusions of Edie Porphyry and hydrothermal breccias similar to the Namie Breccia.

All of these rocks lie within the down faulted, NNW-SSE trending Bulolo Graben/Wau Basin, between two blocks of Morobe Granite. This graben is extensional, with listric faults, and some structures that, at stages, were subjected to reverse movement. These graben faults have influenced the distribution of breccia and intrusives and are believed to be in part reactivated older structures. It would seem that the graben was active prior to, during and probably after the Pliocene magmatism and mineralisation. Primary gold occurs in a number of forms in the district. These include i). Lodes and Veins - stringers and veins of quartz with gold on the margin of the Morobe Granodiorite, mainly in the Kaindi metamorphics, ii). mesothermal gold associated with the Pliocene dacite-andesite porphyries and iii). the bulk of the gold which is epithermal and hosted by rocks related to the Wau Maar.

The extensive vertical relief in the Morobe district and the graben preserves and exposes varying styles of intrusion-related low sulphidation epithermal gold mineralisation. At the deepest levels (e.g., the Hamata deposit), massive quartz-sulphide-gold style mineralisation is associated with magnetite-K feldspar porphyry style alteration; at mid-levels (e.g., Wau and Hidden Valley) carbonate-base metal-gold mineralisation occurs, while bonanza gold-grade epithermal quartz-Au-Ag mineralisation (e.g., Edie Creek) crops out at the highest topographic level. Most deposits display overprinting relationships (e.g., Edie Creek), while the Kerimenge prospect hosts three low sulphidation mineralisation styles, variably telescoped over a 250 m vertical extent.

In the Hidden Valley project area the Morobe Granodiorite batholith (locally a coarse grained monzogranite) is flanked by fine metasediments of the Owen Stanley Metamorphics. Both are cut by narrow dykes and small stocks of Pliocene porphyry (correlated with the Edie Porphyry) ranging from hornblende-biotite to feldspar-quartz porphyries. A number of commonly argillic altered and gold anomalous breccias are known, including both hydrothermal and overprinting structural breccias.

The Hidden Valley deposit area is dominated by a series of post Miocene faults controlling the gold mineralisation, including an early north trending set and the main NW faulting. The NW trending, 30 to 35° NE dipping Hidden Valley Fault, with its associated breccia, is the most important structure in the deposit area, forming a planar structural base underlying the main mineralisation. The main Hidden Valley Kaveroi orebody is broadly tabular, up to 100 m thick, and is localised immediately above and parallel to the Hidden Valley Fault.

Alteration is variable, ranging from pervasive to intense, accompanying veins and breccias. All rocks exhibit propylitisation (chlorite-epidote-pyrite), from pervasive to discrete alteration associated with structures. Argillic alteration (kaolinite-carbonate and occasionally smectite) is associated with breccias and veins. Adularia is present in carbonate veins, while an envelope of phyllic alteration with an illite-leucoxene-quartz assemblage commonly surrounds these veins, joints and breccias.

Gold and silver mineralisation is found within veins that occur within structurally controlled stockworks within the granodiorite on the hangingwall of the Hidden Valley Fault in three vein-alteration suites, as follows: i). quartz veins with weak pyrite-chlorite-epidote propylitic alteration, which are generally barren, and are thicker and more prevalent in the metamorphics than in the granodiorite; ii). chlorite veins with hematite-quartz-carbonate-pyrite, which are mainly found as either larger composite veins or a microveins in stockworks in the granodiorite, above and below the Hidden Valley Fault, often associated with strong propylitic alteration and are only rarely gold mineralised, and iii). carbonate veins which contain the main gold ore, and were developed in three pulses, namely a). irregular carbonate-quartz-adularia veinlets which are not strongly mineralised; b). carbonate-adularia-mixed sulphides-gold as a second pulse, occurring as carbonate veining (often with collorm banding) enveloped by quartz-adularia-sulphide selvages, from which a 4.15 Ma date has been obtained from adularia, with more sulphides (pyrite, sphalerite, galena and chalcopyrite) and rare visible gold, and c). carbonate-kutnahorite-gold representing a third phase which has visible gold and tetrahedrite infilling corroded earlier sulphides. Gold correlates best with the carbonate content of these veins rather than with adularia or base metal sulphides.

The Hamata deposit is hosted within the Morobe granodiorite, which it is cut by two types of andesite porphyry dyke, characterised by feldspar and hornblende phenocrysts respectively, and by dacite porphyry (correlated with the Edie Porphyry). The Edie Porphyry occurs as a larger, unaltered body to the west, and a series of altered, smaller dykes within the ore zone. Primary gold mineralisation at Hamata is related, and proximal, to the altered Edie Porphyry intrusions. It occurs in at least three, NE-striking and 45 to 50°SE dipping, sub-parallel zones (Masi, Lower and Eastern) that are each up to 50 m thick. They are composed of low density and diffuse zones of quartz-pyrite veining within K feldspar-sericite altered granodiorite, although at the upper contacts of each zone, 3 to 4 m wide reefs of pyrite-hematite±magnetite-quartz veins are well developed, and contain the main gold ore zones. Each reef zone contains one or more of these ore zones, with strike lengths of up to 600 m, down-dip extents of 200 to 300 m and thicknesses of 1 to 10 m. Three stages of mineralisation are recognised: Stage 1 - semi-regional thin magnetite-hematite-pyrite veins with K feldspar-sericite selvages; Stage 2 - coarse pyrite-hematite-magnetite-quartz fracture fill with sericite alteration and the bulk of the gold mineralisation, occurring as non-refractory 20 to 30 µm blebs in pyrite; and Stage 3 - shear veins of quartz-pyrite-arsenopyrite-marcasite with sericite and clay alteration, and only low, refractory gold.

The Hidden Valley Mine consists of the Hidden Valley Kaveroi and Hamata open pits, which are ~6 km apart, and an ore processing facility, situated in steep, heavily forested, mountainous terrain.

At the end of 2003 the total mineral resource at Hidden Valley and the nearby Hamata and Kerimenge prospects was: 53.3 Mt @ 2.7 g/t Au, 47.1 g/t Ag for 144 tonnes (4.72 Moz) of gold, with 1730 tonnes (55.7 Moz) of silver.

Reserves and resource at Hidden Valley Kaveroi as of June 2010 (Morobe JV, Newcrest Mining website, 2012) were:
    Total Measured + Indicated + Inferred Mineral Resource - 96 Mt @ 1.65 g/t Au, 33 g/t Ag for 158 tonnes (5.1 Moz) of contained gold;
    Proved + Probable Reserves (a subset of the resources) - 55.7 Mt @ 1.82 g/t Au, 35.6 g/t Ag for 101 tonnes (3.26 Moz) of contained gold.

Reserves and resource at Hamata as of August 2011 (Morobe JV, Newcrest Mining website, 2012) were:
    Total Measured + Indicated + Inferred Mineral Resource - 6.66 Mt @ 2.4 g/t Au, for 15.9 tonnes (0.5 Moz) of contained gold;
    Proved + Probable Reserves (a subset of the resources) - 5.6 Mt @ 2.12 g/t Au for 11.9 tonnes (0.38 Moz) of contained gold.

The operation is controlled by the Morobe Mining Joint Venture (Harmony 50%, Newcrest 50%).

The most recent source geological information used to prepare this decription was dated: 2004.    
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.

  References & Additional Information
   Selected References:
Carswell J T  1990 - Wau Gold deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1763-1767
Denwer K P and Mowat B A,  1998 - Hamata 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. 833-836
Denwer K P, Leach T M and Mowat B A,  1995 - Mineralisation of the Morobe Goldfield, Morobe Province, Papua New Guinea: in Mauk J L and St George J D, (Eds.),  Pacrim 95 congress, Exploring the Rim, Auckland, New Zealand, 19-22 November 1995,  The AusIMM, Melbourne,   Proceedings volume pp. 181-185
Hutton M J, Akiro A K, Cannard C J, Syka M C  1990 - Kerimenge gold deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1769-1772
Nelson R W, Bartram J A, Christie M H  1990 - Hidden Valley Gold-Silver deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1773-1776

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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