Ladolam Gold Deposit, Lihir Island, Papua New Guinea: Gold Mineralization Hosted by Alkaline Rocks
Daniel Müller, Peter M. Herzig, Jan C. Scholten, Steve Hunt, 2002. "Ladolam Gold Deposit, Lihir Island, Papua New Guinea: Gold Mineralization Hosted by Alkaline Rocks", Integrated Methods for Discovery: Global Exploration in the Twenty-First Century, Richard J. Goldfarb, Richard L. Nielsen
Download citation file:
The exceptionally large gold resource at Ladolam (>1,300 metric tons (Mt) Au), Lihir Island, resulted from the transition of an early-stage, low-grade porphyry gold system to a low-sulfidation epithermal gold event. The original porphyry stage is indicated by remnant hydrothermal breccia clasts of strongly biotitemagnetite altered monzodiorite with disseminated pyrite ± chalcopyrite and poorly developed pyrite ± quartz stockwork veins. The breccia matrix is strongly mineralized with disseminated auriferous pyrite. The breccias are cut by late-stage epithermal quartz-chalcedony-illite-adularia-pyrite veins that locally contain bonanza gold grades of up to 120 g/t. Isotope data suggest a magmatic source of sulfur in the gold-bearing fluids at Ladolam.
Ladolam is hosted by alkaline rocks that range from porphyritic trachybasalts, trachyandesites, and latites to rare phonolites and olivine-clinopyroxene cumulates that are cut locally by monzodiorite stocks. Oxygen barometry on olivine-spinel-clinopyroxene phases in these rocks indicates very high oxygen fugacities (fO2), 1.4 to 4.8 log units above that of the fayalite-magnetite-quartz (FMQ) buffer. Gold-copper mineralization is commonly associated with alkaline rocks with high fO2 values and abundant primary magnetite contents; this study provides direct determination of the fO2 of such rocks. High fO2 of parental melts delays the early crystallization of magmatic sulfides, in which metals, such as gold and copper, preferentially partition and are thus depleted in the melt during fractionation.
The geochemical signature of the host rocks at Ladolam is typical of primitive to relatively evolved compositions. Their high K2O content (up to 4.7 wt %), high average K2O/Na2O ratios (0.8), and high average Ce/Yb ratios (14) are typical of high K igneous rocks transitional to shoshonites. The rocks formed by decompression melting along extensional structures either related to back-arc rifting in the Manus basin or to a flexure in the subducting Solomon plate. However, their high large-ion lithophile element (LILE) and very low high-field strength element (HFSE) contents are typical of potassic igneous rocks from oceanic (island)-arc settings. This unusual composition has probably been derived by partial melting of subduction-modified lithospheric mantle, as developed in a stalled subduction zone.
Mica phenocrysts in the rocks have unusually high halogen concentrations. Igneous phlogopites contain high fluorine (up to 5.6 wt %) and elevated chlorine (<0.08 wt %) contents. Hydrothermal biotites from rocks that display potassic alteration have low fluorine (<0.08 wt %) but very high chlorine concentrations (up to 0.15 wt %). This data makes the Ladolam rocks comparable to those from the Grasberg copper-gold deposit, Indonesia.
Figures & Tables
Integrated Methods for Discovery: Global Exploration in the Twenty-First Century
The dominant forces that affect the mining industry in today's economy are globalization, entrepreneurship, and unprecedented minerals availability. The current malaise and diminished economic importance of the base and precious metal mining industry in the world economy are the result of ongoing, longterm trends. Shifting market forces within a changed geopolitical landscape have resulted in excess supplies and reduced demands for high-unit-value mineral products and for geologists and engineers in the exploration and mining industries. Although these changes are still evolving, professionals and the mining companies that employ them must realize that these changes are irreversible.
Mining has five dominant characteristics: (1) it is essential to society, (2) it is effective in keeping society supplied with abundant, low-cost mineral products, (3) historically, it has been wealth destructive for investors, (4) it has been subsidized, and (5) it is shaped by social and political forces. What is different now is that instead of being subsidized, the industry is being handicapped by governments and abandoned by investors. The industry has reacted to a marketing problem with an inappropriate strategy of cost reduction and increased production. The increased supply has resulted in decreased prices. Mining companies are earning low or negative rates of return.
Remedies for the larger mining companies and the high-unit-value metals businesses include: Exemplary behavior to regain the trust of the public, governments and investors; Continued consolidation to become sector leaders; Profit through value-added vertical integration and direct marketing to consumers Development of innovative consumer-based financing mechanisms, especially for exploration ventures Individuals and small groups that have been displaced from the mainstream must become entrepreneurs. They must engineer their own survival by shifting careers or by finding, developing, and capitalizing on exploration and production opportunities that are unrecognized by, or are too small for, the major companies.
The mining industry is here to stay. It is too necessary to society to be abandoned; however, the future size and shape of the industry will probably be unrecognizable to most of us. It will be leaner and more opportunistic. It will be characterized by entrepreneurial corporations of all sizes that dominate niches and sectors and that anticipate and profit from the changing needs of society