The geology of the Frieda River copper prospect, Papua New Guinea

Abstract

The Frieda River Prospect is located in structurally disrupted middle Myocene eugeosynclinal sediments that form part of the New Guinea Mobile Belt in the West Sepik District of Papua New Guinea. The structurally-controlled comagmatic extrusives and intrusives of the Frieda River Complex represent a remnant volcanic edifice of a near shore stratovolcano that was interstratified with marine sediments at the southern margin of a submerging trough. Epithermal copper-gold and massive sulphide and porphyry copper deposits are associated with early and late phases, respectively, of a geothermal-hydrothermal system that caused extensive district-scale advanced argillic alteration along the central axis of the Frieda Complex. Igneous activity, as indicated by K-Ar hornblende ages, extended over an interval of at least 4 m.y. On the basis of. field relations and K- Ar ages this igneous activity is subdivided into early intrusion (17.3 to 16.8 m.y.), main intrusion (16.8 to 13.1 m.y.), and late alteration (about 13.5 to 11.2 m.y.) events. Geothermal-hydrothermal activity extended from earlier than 14.0 +- 0.4 m.y. until about 11.2 +- 0.2 m.y. ago. The Nena copper-gold mineralization was emplaced prior to 13.0 +- 0.4 m.y. whereas the porphyry copper systems were developed during a cooling history of at least 2 m.y. between 13.6 +- 0.4. m.y. and 11.5 +- 0.2 m.y. ago. Igneous rocks of the Frieda River Complex are all of andesitic composition and belong to a normal K calc-alkaline suite. Their diversity in lithology and dispersion in chemistry is due to them representing cumulate-melt mistures that have recrystallyzed from hot (> 9OOC), oxidised magmas. Exposed intrusive rocks were emplaced at less than 2 km and Ftotal < 0.5 kb. Approximately .50 km2 that measures about 13 km by 4 km, has been altered over a vertical extent of 2000 m in the centra1 regions of the Complex. District-scale advanced argillic alteration progresses from deep-level sericite-pyrophyllite-diaspore assemblages upward and outward through assemblages dominatd by quartz, alunite, kaolinite­-dickite, to propylitic assemblages in marginal zones. The Nena mineralization represents a zone within this system that was anomalously rich in S, Cu, Au and As. Mainly fracture-controlled luzonite-enargite-chalcocite mineralization of the Nena deposit followed pervasive quartz-rich, inner massive pyrite and outer alunite-dominated alteration. It is estimated to have been emplaced within 200 to 400 m of the surface at temperatures between 200 to 300°C. Biotitic alteration in the porphyry copper systems is viewed as an extension of magmatic processes and was caused by an alkali-chloride-rich volatile phase that evolved from a hydrated and oxidized cumulate-melt mixture while the magma was above the solidus. Sequential breaching of a solidified carapace and influx of cooler meteoric waters caused the inner zones of vein-controlled transitional alteration and progressively outer zones of chloritic, sericitic ± andalusite and, at deep levels, peripheral propylitic alteration. At higher levels the advanced argillic zones which consist of pyrophyllite-diaspore, alunite, and kaolinite developed as pH and temperature decreased with time from a near maximum of .600°C (Eastoe, 1976) in the inner transitional zone to about 300°C in the upper and outer advanced argillic assemblages. The major controls on the development of the alteration-mineralization zonation were oxidation of H2S to produce fluids of low pH, temperature decrease, dilution effects, host-rock mineral equilibria reactions and the relation between reaction rates and duration of events. At least 1 billion tonnes of S has been added to the overall geothermal-hydrothermal system. Pb and K, and Rb and Cu have been added to district-scale advanced argillic alteration and the porphyry copper mineralization, respectively. Trace elements that are depleted in the intensely silicified and massive pyrite zones associated with district-scale are normally enriched in the outer alunitic and kaolinite-dickite alteration zones. Most trace elements in the porphyry copper systems have only been locally transferred. Major element chemistry of the various alteration assemblages are compatible with mineralogic changes.