Summary:
The Frieda River is a porphyry copper-gold deposit of island arc affinity. Mineralisation is mainly hosted by the Horse microdiorite, which intruded into older diorites and volcanics of the Frieda River Igneous Complex.
The mine area lies on the Frieda River Igneous Complex (“FRIC”), in the northern foothills of the Central Cordillera, towards the southern margin of the New Guinea Thrust Belt. The igneous complex is approximately 17 km long by 7 km wide and is considered to be the remnants of a large stratavolcano. Mineralisation comprises a cluster of copper-gold deposits, with the most significant of these being the Horse-Ivaal-Trukai, Ekwai and Koki (HITEK) porphyry deposits.
The Frieda River deposit is associated with a mid-Miocene calc-alkaline intrusive centre, with mineralisation associated primarily with hydrothermal fluids and alterations associated with dioritic intrusions. Porphyry-style copper mineralisation occurs both within the diorites and adjacent rock types. Seven intrusive centres have been identified of which the three principal ones are the Horse (southeast), Ivaal (central) and Trukai (northwest) centres; together with the adjacent Ekwai and Koki mineralisation centres to the north and northeast these form the Horse/Ivaal/Trukai, Ekwai and Koki copper-gold porphyry deposit (HITEK), which is the focus of the Frieda River development studies. The Nena deposit further to the northwest is high sulphidation epithermal in style, with complex syngenetic mineralogy with associated enargite (copper arsenic sulphide) and relatively high levels of arsenic. The Nena deposit is not included in the current development plans, but is likely to be developed as a later add-on to the main Frieda River mineralisation.
The oldest rocks forming the basement to the FRIC consist of faulted blocks of Ok Binai schist, phyllite and mudstones, and ultramafic rocks of Cretaceous to Eocene age; these rocks have undergone regional metamorphism to phyllite-greenschist facies.
The unconformably overlying FRIC comprises lavas and pyroclastic rocks of basaltic-andesitic composition, gently dipping to the southwest, and flanked by volcaniclastic and epiclastic sediments (the Wogamush Formation). Multi-phase diorite to quartz-diorite intrusions cut the sediments and volcanics.
The intrusive and volcanic units are of similar ages, from 12-17 million years. Six principal intrusive geological units have been identified; from oldest to youngest these comprise the Frieda Diorite Porphyry (Fdp) and the Koki Diorite Porphyry (Kdp) which are considered pre-mineralisation, the Horse Microdiorite (Hmd) and the Hornblende Monzonite (Hbm) which comprise the principal mineralised units, and the post mineralisation Knob Diorite (Kd) and the Flimtem Trachyandesite (Ftm). The late stage Flimtem Trachyandesite post-dates mineralisation and alteration and forms barren cross-cutting dykes which dilute the overall block grade.
The HITEK mineralisation is hosted largely in the Horse Microdiorite, with a general northwest orientation. The Hmd is considered the principal source of mineralising fluids; the Koki mineralisation is hosted largely in the Kdp and the Hbm also contains some high grade mineralisation. The sequence is cut by late Ftm northeast-trending trachyandesite dykes. Both the Kd and Ftm are interpreted as being intruded after the main phyllic alteration event.
The mineralisation is associated primarily with hydrothermal fluids and alterations associated with the dioritic intrusions. Mineralisation occurs both within the intrusive diorites and in the adjacent volcanics and volcaniclastics. The ore zone is subdivided by cross cutting fault zones.
The mineralisation zones are associated with concentric alteration assemblages, centred on the main intrusives. The core is a potassic zone associated with chalcopyrite, bornite and gold; original hornblende has been replaced by biotite-magnetite. Surrounding the potassic zone is a phyllic alteration assemblage with chalcopyrite mineralisation and where the biotite is replaced by chlorite. The outer margins of the deposit are represented by a barren silica-clay-pyrite (“QIP”) assemblage typically with high sulphur (>6% S).
Grades in the core potassic zone are typically greater than 0.4% Cu and 0.3g/t Au and reach up to 1% Cu and 0.6g/t Au.
Much of the rock is cut by fine veins of anhydrite. At depth this gives hard competent rocks but at shallower levels where the anhydrite has weathered to gypsum and has been dissolved, the cores are typically broken.
Copper mineralisation comprises mostly fine grain aggregates of chalcopyrite and bornite with supergene chalcocite. Chalcopyrite-bornite quartz vein stockworks are overprinted by quartz-chalcopyrite-pyrite veins. Mineralisation and grade continuity characteristics are good extending for some hundreds of metres. Gold is typically fine grained and associated with bornite and chalcopyrite.
There is a low level of deleterious elements with pyrite being the only significant sulphide gangue mineral (with the exception of the arsenic-rich Nena deposit to the northwest); silver, molybdenum, zinc, lead and arsenic occur in minor to trace amounts within the HITEK deposits.