Alcott Di porphyry copper-gold deposit is located in the remote Xingshan, the westernmost part of Papua New Guinea. The deposit is concentrated in Fubilan Mountain, under which is a set of Pliocene multi-stage adamellite porphyry, which intrudes into late Cretaceous to Miocene sedimentary rocks (including a thick limestone section).
Most of the mineralization of endogenetic porphyry exists in potassium silicate altered porphyry as veinlets and disseminated particles, and the average grade of copper is 0.2% ~ 0.4%. Calcium silicate skarn containing magnetite and sulfide, with copper content as high as 2.5%, occurs in limestone surrounding rocks. Chalcopyrite is the main endogenous copper mineral. Supergene profiles are usually well developed in tropical areas. The supergene profile above the porphyry copper deposit consists of an iron cap with a thickness of 15 ~ 200 m, and a chalcocite-rich layer with a thickness of 50 ~ 300 m is below the iron cap. The gold content in the iron cap ranges from 1× 10-6 to more than 3× 10-6, and the copper content in the enriched layer exceeds 1%. Before production, the gold reserves were 30 million tons, all of which were iron hat-shaped, with an average of Au 3×10-6 (the cutoff grade of Au was 1.5× 10-6), that is, 90 tons of gold; The sulfide ore reserves are 355 million tons, with a grade of 0.67% copper, 0.6 1× 10-6 gold and 0.01%molybdenum (cut-off grade: 0.4% copper), with an average of1.25.
Fig. 8- 18 schematic diagram of distribution of surface mineralized altered rocks in orcutt porphyry copper-gold deposit.
(quoted from R.H.Sillitoe, 1995)
A sketch showing the location of the first batch of massive chalcopyrite-bearing magnetite transformation samples discovered by kennecott geologists in western Papua New Guinea in 1968. Shi Zhuan comes from the downhill of Fubilan Mountain, which is an exposed skarn ore body, where Okut Erform Cu-Au deposit is located.
2. Exploration and discovery
There is no record of mining or mineral exploration in western Papua New Guinea. Government inspectors recorded the copper mineralization in this area for the first time in 1963 and 1967, but it was not published later. During the period of 1968, kennecott Exploration Co., Ltd. started the geochemical exploration plan of porphyry copper mine water system in West Papua with the support of helicopters. The first evidence of orcutt's biporphyrin series is the discovery of magnetite containing chalcopyrite in rivers. Shi Zhuan was first discovered about 4km south of alcott Di, and then about 4km east of alcott Di (Figure 8- 18). Subsequently, abnormal river sediments containing copper were also found in the catchment basin containing dolomite. The source of tracer stone transformation is the outcrop of massive magnetite and sulfide in calcium silicate skarn, which leads to the discovery of blue-rich porphyry strains in the middle section of 1968. At the beginning of 1969, 16 core hole was drilled to check the copper mineralization of skarn, but the result was quite disappointing. By the end of 1968, the short vegetation on Fubilan Mountain and the iron caps exposed by cutting on both sides made people interested in Fubilan Mountain rocks, although the copper value on the surface was less than 500× 10-6. 1969, the helicopter route passed the highest point of the mountain, where porphyrite mixed with limonite timely veinlets was exposed. Draw rock strain map and make drilling decision. In the middle of 1969, the first borehole was drilled, and extremely rich copper mineralization was found 60 meters below the iron cap, with a copper grade of more than 3%. At 1970 ~ 197 1, an important drilling plan was started. During the implementation of this plan, 32,860m cores were recovered from orcutt porphyry copper mine. Kennecott Company designed a development drilling plan, but the plan failed to be implemented due to the breakdown of negotiations with the government on mining rights and tax system. 1975 kennecott company withdrew from the orcutt Di project.
After kennecott withdrew, several companies joined the discussion with the government, and Cui Aikobuca Mining Company proposed to produce gold only from iron hats. It is worth pointing out that the price of gold has been rising for the first three years. Cui Aikobuka persuaded BHP Billiton to participate in this project, which started in the middle of 1975. BHP Billiton's negotiations with the government ended on 1976. Cui Aikboca withdrew, and BHP Billiton's subsidiary Amoco (37.5% shares) and Kufuor Exploration Company (a German group, 25% shares) formed an international consortium. During the negotiation, the 4000m core drilling plan was completed on behalf of the government, thus increasing the ore reserves. From the end of 1976 to the beginning of 1978, the international consortium completed 86 development boreholes in the iron cap rich in gold and the underlying sulfide zone, with a total footage of 24,400 meters. In order to collect gossan ore, porphyry ore and skarn ore for metallurgical test and grade control, an adit of about 800 meters and two shafts (with a total length of 56 meters) were drilled.
198 1 Established Okotedi Mining Co., Ltd. (partner of international consortium, 20% owned by PNG government); 1984 started mine development in the middle period, and the first gold production project was put into production, including a 22,000 t/d open pit mine and a cyanide pulp concentrator. After the transition period, at 1988, sulfide resources used for open-pit mining were treated by flotation instead of gold production. Okotedi Mining Co., Ltd. is currently affiliated to BHP Billiton (52%), Intermit Mining Company (18%) and Papua New Guinea Government (30%).
3. Summary
After several months of bold "grass-roots exploration plan", evidence of copper mineralization was obtained near alcott Di. The porphyry copper-gold deposit was discovered in less than two years. The discovery of porphyry copper deposit system is attributed to the thorough tracing of weathering and turning stones encountered along the water system during geochemical sampling. Political reasons, falling copper prices and, to some extent, steep and remote geographical location have all brought difficulties to exploration and development. All these delayed the mine production by 16 years.