Vertical Dispersion of Elements in Thick Transported Cover above the Thalanga Zn-Pb-Cu Deposit, Queensland, Australia: Evidence of Redox-Induced Electromigration
S. M. Hamilton, G. J. S. Govett, 2010. "Vertical Dispersion of Elements in Thick Transported Cover above the Thalanga Zn-Pb-Cu Deposit, Queensland, Australia: Evidence of Redox-Induced Electromigration", 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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Rabbit-ear anomalies in soil geochemical surveys over buried sulfide deposits are commonly reported and may be ubiquitous. A rotary air blast drilling transect over a deeply buried part of the Thalanga stratiform Zn-Pb-Cu deposit, Queensland, Australia, shows discrete, high-contrast chimney-like geochemical anomalies in ore-forming elements. The 6-m-wide massive sulfide zone, with 15 percent combined Zn, Pb, and Cu, is overlain by 50 m of flat-lying, semilithified, transported sediments of the Campaspe Formation. Total digestion analysis of Zn and Pb shows anomaly to background ratios of 100/1 and 15/1, respectively, starting in the upper weathered bed rock and extending for at least 10 m upward into the cover materials. The strongest part of the Zn and Pb responses are approximately 15 and 10 times the width of the deposit, respectively, and concentrations are highest in the outer part of the chimney and diminish toward the center. The responses in both elements show a bias toward the hanging wall in terms of position and magnitude. Although the magnitude of the anomaly diminishes with thickness of sedimentary rocks, its contrast remains high to the surface, where previously published soil geochemical responses show distinct rabbit-ear anomalies in all four of the surface transects. The surface rabbit-ear responses appear to be a direct expression of the subsurface halo anomalies.
The halo and rabbit-ear responses are proposed to result from electromigration of ore-forming cations due to redox-induced spontaneous polarization at the edges of a reduced chimney over the ore. Reduced chimneys occur where buried reduced features are actively oxidizing. A strong redox gradient at the edges of the reduced chimney induces the electrical polarization of conductive and semiconductive mineral grains in overburden with their negative poles pointing in the oxidizing redox direction, which is outward from the chimney. The electrical fields of all these microscopic dipoles are additive in series and result in a macroscopic electrical field throughout the redox gradient that is positive inward and negative outward. The magnitude of the horizontal redox gradient is strongest at surface and diminishes rapidly with depth and, therefore, dipole development is strongest at surface. Current, in the form of a cation flux, migrates inward and downward. At depth, where dipole development is weakest, the current migrates outward and upward completing the electrical circuit and inducing upward movement of ore-forming cations. Electrical dipole-forming bacteria within the redox gradient may contribute to current in a similar fashion.
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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.