Exploration, Discovery, and Geology of the Ntaka Hill Nickel Sulfide Deposits: A Potential New Proterozoic Nickel District in the Nachingwea Area, Mozambique Belt, Tanzania
P. A. Tirschmann, C. S. MacDougall, G. R. Katchen, 2010. "Exploration, Discovery, and Geology of the Ntaka Hill Nickel Sulfide Deposits: A Potential New Proterozoic Nickel District in the Nachingwea Area, Mozambique Belt, Tanzania", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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The Ntaka Hill nickel sulfide deposits are hosted in the peridotitic to pyroxenitic Ntaka ultramafic intrusion located in the Nachingwea area and are the first significant occurrence of nickel sulfides in the Tanzania portion of the Late Proterozoic Mozambique belt. High-grade nickel sulfide mineralization was first discovered at Ntaka Hill in 2006. Six near-surface sulfide deposits have since been delineated containing a measured and indicated mineral resource of 1.8 million tonnes (Mt) @ 1.82 percent Ni and 0.31 percent Cu. The recent discovery history can be traced back to the presence of a historic surface copper oxide malachite showing. Subsequent soil sampling defined a large coincident Ni-Cu anomaly that provided the impetus for airborne and ground geophysical surveys and ultimately diamond drilling leading to the initial discovery. Further ground electromagnetic surveys were successful in defining additional moderate to high conductance anomalies resulting, upon drill testing, in the discovery of five additional nickel sulfide zones.
The Ntaka intrusion is postulated to have formed from a relatively primitive, high MgO magma, dominated by the crystallization and accumulation of olivine and pyroxene. The intrusion is characterized by high MgO contents, low CaO, Al2O3, Cu, PGE, and incompatible element contents, and relatively flat chondrite-normalized REE profiles lacking Eu anomalies. The geologic setting is similar to that of the Early Proterozoic Thompson Nickel belt in Canada. There supracrustal rocks formed on a continental margin platform and were intruded by ultramafic sills that interacted with the sulfidic metasedimentary rocks to produce the resulting nickel deposits.
The Ntaka Hill sulfide zones occur in separate south-plunging lenses but may represent remnants of a dismembered original basal sulfide zone. The zones consist of magmatic, remobilized, and graphite-bearing mineralization with variable nickel grades of as much as 17 percent. Mineralization consists of disseminated, net-textured, and massive magmatic sulfides, as well as remobilized semimassive and massive sulfide veins and stringers composed of pyrrhotite, pentlandite, pyrite, chalcopyrite, and violarite. Pentlandite is the main nickel-bearing sulfide mineral occurring as coarse grains and eyes that are as large as 5 cm in diameter. Pyrrhotite-rich, nickel-poor, graphite-bearing, disseminated to massive sulfide mineralization occurs at several locations within the Ntaka intrusion and is thought to have formed by assimilation of graphitic metasedimentary rocks.
The Ntaka intrusion possesses a number of elements critical to the formation of nickel sulfide deposits and good potential exists to discover additional nickel sulfide deposits, both in the Ntaka Hill area and regionally. Exploration challenges in this underexplored belt include a complex deformation history, an abundance of graphitic metasedimentary rocks, and a paucity of outcrop.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.