Papua New Guinea

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The Yandera porphyry copper - molybdenum - gold deposit is located 100 km southwest of Madang in Simbu Province of Papua New Guinea, and approximately 69 km by road from the provincial capital of Kundiawa and is located at between 1600 to 2600 m elevation.

The mineralised Yandera porphyry system lies within the core of a 15 to 13 Ma early Miocene granitic pluton, the 50 x 20 km Bismarck Intrusive Complex which is located within a zone of fault deformation on the southern margin of the major northwest to southeast trending Ramu fault zone that runs along the northern margin of the highlands of Papua New Guinea. The intrusive complex, which comprises tonalite, granodiorite and quartz monzodiorite, was intruded into the strongly folded and faulted Goroka Formation metamorphics (35 to 20 Ma metamorphic age) of the Highland Fold Belt. The Ramu fault system, which comprises a zone of convergence accompanied over time by translational, normal and reverse fault movements has produced flower style fault structures on both sides of the faulted complex. The Bismarck intrusive complex on the south-eastern edge of the fault complex, bounded to the east by up-thrust marine sediments and the Ramu ophiolite complex. The northwest to southeast elongation of the Bismark pluton and the Yandera porphyry complex mimics the general trend of the Ramu fault to the east of the Yandera deposit.

Subsequent to the emplacement of the Bismarck Intrusive Complex in the Mid Miocene, the ranges to the south of the Ramu Fault in the region surrounding Yandera were subjected to uplift of as much as 4.5 km, accompanied by 3 km of denudation between 8 and 5 Ma in the Late Miocene to expose the batholithic levels of the Complex.

The porphyry copper system at Yandera is associated with a north-west trending, faulted body of porphyritic rocks that intrude the main Bismark pluton, and occur as three separate porphyritic intrusive events (the Older, Intermediate and Younger phases). Mineralisation is associated with these younger porphyries which comprise dioritic and dacitic porphyries intruding the tonalite, granodiorite and quartz monzodiorite of the Bismark Intrusive complex. They parallel the general NW-SE regional trend and are cut by SW to NE dislocations and later intrusives. The main porphyries have associated breccia zones, which are coincident with the mineralising event, while younger SW-NE trending leucocratic quartz diorite porphyry dykes are un-mineralised.

These three phases of porphyritic intrusion, comprise the Older porphyry dated at around 12.5 Ma, approximately 1 m.y. after the main complex. It is the most voluminous and is composed of tonalite, quartz-diorite, with subordinate dacite and andesite and does have some associated mineralisation. It was emplaced conditions of dextral deformation and compression from the north into NW trending shear fractures. The Intermediate porphyry forms NE and NNE trending dykes cutting the Older porphyry and the Bismarck Complex, and consists of both tonalite and dacite which are mineralogically and texturally identical to similar lithologies in the Older porphyry. The Younger porphyry is entirely composed of porphyritic dacite, which is identical in mineralogy and texture to the dacites in the other two porphyries, except for a higher proportion of finely disseminated groundmass magnetite. The Intermediate and Younger porphyries have been dated at 8 to 7 or 6.6 Ma, emplaced during or immediately after the uplift of the Bismarck Complex.

Analysis of quartz vein orientations suggests that the major mineralisation event at Yandera was initiated by a change in stress. Compression from the NE and rapid uplift were active during the forceful emplacement of the NE–NNE-trending Intermediate porphyries. The mineralisation is associated with aplitic quartz monzonite porphyry emplaced into the centre of the pre-existing NW structural grain, and has been dated at 6.6Ma.

The >0.2% Cu shell defines a NW-SE elongated, 3 x 1.5 km annular shape surrounding a low grade to barren quartz-zone core. This annulus comprises the Dimbi zone to the NE, which passes laterally into the arcuate Imbruminda zone to the north, NW and SW, and the Gremi zone to the south which connects the previous two zones to complete the annular structure. The Omora zone to the south is NNW-SSE oriented extending southwards from the Gremi zone to form the tail of an overall "Q-shape". In general, the >0.2% Cu shell of the annulus is 400 to 600 m wide. Higher grade >0.1 g/t Au is largely co-extensive with the Cu mineralisation on the SW limb of the annulus in the Imbruminda and Gremi zones, while the >100 ppm Mo mineralisation overlaps the gold and copper mineralisation to the south in the Gremi and Omora zones, and the southern tip of the Dimbi zone (Marengo Mining, 2012).

Mineralisation is associated with zones of strongly fractured rocks spatially associated with the belt of these three younger intrusive phases and occur in a number of separate, NW-SE aligned centres of mineralisation, some 3 to 5 km from the NE margin of the Bismark Intrusive Complex. The chief of the mineralised centres comprise the Imbruminda, Gremi, Dimbi and Omora zones which occur within a volume of many different porphyritic rocks. The mineralised events appear to have occurred over an extended time frame, such that it almost all lithologies are mineralised and in many instances multiple mineralisation events are locally over printed such that it is not unusual to note multiple alteration events and four or more mineralisation styles with cross cutting relationships at the same location.

The deposits generally has a low (around 1.5%) total sulphide assemblage, with both pre- and post-ore veining having been recognised. Field relationships suggest the following paragenesis of vein alteration types (Titley, et al., 1978): quartz, biotite, feldspar, quartz-sericite-clay-chlorite, and late stage clay-chlorite-zeolite. This veining appears to be asymmetrically distributed around a barren core of closely spaced, apparently barren quartz veining. This central core grades out into a superimposed quartz-K silicate vein system of quartz, quartz-biotite, biotite-feldspar and feldspar veins. Potassic alteration is best developed in the areas of aplitic quartz monzonite plugs as fracture halos, veinlet and pervasive quartz, orthoclase, biotite, epidote and magnetite as well as albite than orthoclase and displays a spatial relationship to fracture-controlled pyrite–chalcopyrite mineralisation.

The K silcate zone passes out into an envelope of propylitic (chloritic alteration of primary hornblende and biotite with scattered epidote, carbonate and clay alteration) and is overprinted by a younger, pervasive, structurally controlled phyllic quartz-sericite-kaolin, with no apparent pyrite halo (occurring as selvages to veins that both overprints, and forms a wide halo to, the earlier prograde potassic alteration). Potassic alteration is also developed outside the main mineralised zone but only along structures over widths of up to 50 m, enveloped by a halo of propylitic alteration.

The porphyry Cu-Mo-Au mineralisation is strongly fracture controlled. An early phase of fracture hosted and disseminated mineralisation with grades of up to 0.3% Cu is characterised by pyrite-chalcopyrite in association with potassic alteration (biotite>orthoclase). Younger, structurally controlled mineralisation occurs as 1 to 2 mm veinlets of chalcopyrite with lesser bornite, pyrite and magnetite with selvages of biotite-chlorite-epidote, or as 20 to 100 mm veins of pyrite-chalcopyrite with quartz, chlorite, epidote and carbonate gangue with grades ranging from 0.4 to 1% Cu. The best mineralisation occurs where the two styles are coincident in areas of greatest fracture intensity. Molybdenite is less abundant than copper sulphides and more erratic. The highest pyrite is associated with retrograde quartz-sericite-clay-chlorite-pyrite alteration peripheral to and overprinting thin fracture/veins of copper sulphides. There is a broad pyritic halo to the entire deposit. Sphalerite occurs as an accessory in the fracture/veins with chalcopyrite but galena is rare.

The hypogene mineralisation is predominantly composed of pyrite, chalcopyrite and bornite, with oxide and mixed zones containing minor malachite, chrysocolla and some chalcocite. Native copper is occasionally noted at Omora. Molybdenum mineralisation occurs as molybdenite fracture coatings. Gold and silver are present throughout the system in relatively minor quantities, although significant late precious metal concentrations appear to be localised within structural zones. The mineralisation, like the alteration exhibits a concentric zoning. There is a general correlation between better copper and molybdenum grades and stronger sericite-chlorite alteration, while a correlation is also noted between strong copper and areas of intense potassic alteration and stronger molybdenum grades with weaker potassic zones. The weathering profile varies with topography but is generally shallow, with supergene mineralisation only being weakly developed (Godfrey, 2007).

Between 1973 and 1976, the Broken Hill Proprietary Company Limited, in joint venture with Kennecott and Triako, defined a non-JORC inferred resource of: 338 Mt at 0.42% Cu, 0.10 g/t Au, 0.018% Mo (Watmuff, 1978).

A subsequent resource estimate for the Yandera Central Porphyry resource was quoted by Godfrey of Golder & Associates, 2007 as:
    Indicated Resource of 163 Mt @ 0.49% Copper Equivalent (at a 0.3% Copper equivalent cut-off); and
    Inferred Resource of 497 Mt @ 0.48% Copper Equivalent (at a 0.3% Copper equivalent cut-off).

Marengo Mining quoted a larger, but lower grade resource in Oct, 2008 at a 0.2% Cu equiv. cut-off of:
    Indicated Mineral Resource: 527 Mt @ 0.38% Cu equiv. (=0.2793% Cu, 0.0194% Mo),
    Inferred Mineral Resource: 766 Mt @ 0.33% Cu equiv. (=0.2488% Cu, 0.0082% Mo),
-or- at a 0.3% Cu equiv. cut-off:
    Indicated Mineral Resource: 314.5 Mt @ 0.48% Cu equiv. (=0.3413% Cu, 0.0135% Mo),
    Inferred Mineral Resource: 351.9 Mt @ 0.43% Cu equiv. (=0.3275% Cu, 0.0106% Mo).

Marengo Mining quoted a revised JORC and Canadian NI 43-101 compliant resource at 30 May 2012 at a 0.25% Cu cut-off of:
    Measured Resource of 248 Mt @ 0.48% Cu,
    Indicated Resource of 114 Mt @ 0.42% Cu,
    Inferred Resource of 218 Mt @ 0.37% Cu.
Contained within sections of this Cu resource are:
    Measured + Indicated Resource of 199 Mt @ 0.17 g/t Au,
    Measured + Indicated Resource of 532 Mt @ 0.01% Mo.

Marengo Mining quoted a revised JORC and Canadian NI 43-101 compliant resource at 31 April 2015 of:
    Measured + Indicated Resource of 630 Mt @ 0.33% Cu, 0.01% Mo, 0.07 g/t Au, 0.41% Cu eq.
    Inferred Resource of 117 Mt @ 0.30% Cu, 0.005% Mo, 0.05 g/t Au, 0.34% Cu eq.

Era Resources quoted a revised JORC and Canadian NI 43-101 compliant resource at 19 December 2016 of:
    Measured + indicated resource of 728.6 Mt @ 0.33% Cu, 0.01% Mo, 0.1 g/t Au, 0.39% Cu eq.
    Inferred resource of 230.6 Mt @ 0.29% Cu, 0.00% Mo, 0.04 g/t Au, 0.32% Cu eq.
  including an oxide resource of
    Measured + indicated resource of 63.75 Mt @ 0.34% Cu, 0.01% Mo, 0.12 g/t Au, 0.38% Cu eq.

The most recent source geological information used to prepare this summary was dated: 2012.    
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:
Titley S R, Fleming A W and Neale T I,  1978 - Tectonic evolution of the porphyry copper system at Yandera, Papua New Guinea: in    Econ. Geol.   v73 pp 810-828
Watmuff G,  1978 - Geology and alteration-mineralization zoning in the central portion of the Yandera porphyry copper prospect, Papua New Guinea: in    Econ. Geol.   v73 pp 829-856

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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