McQueen reported in 2006 in Ore Deposits that some "ore" deposits consist of crustal concentrations of useful elements that can be exploited at a profit. Like all crustal rocks, they consist of minerals formed by geological processes. There are four basic geological requirements for any ore deposit to form:
i) a source for the ore components (metals and ligands);
ii) a mechanism that either transports these components to the ore deposit site and allows the appropriate concentration or removes non-ore components to allow residual concentration;
iii) a depositional mechanism (trap) to fix the components in the ore body as ore minerals and associated gangue; and
iv) a process or geological setting that allows the ore deposit to be preserved.
The basic requirements for ore formation. The degree of element concentration during transport and deposition is a critical factor, but enriched sources can be important for reducing the required concentration factor and hence the required efficiency and probability of ore formation. Additional and essential requirements include energy (generally thermal, gravitational or deformational) to power the transport mechanism and a suitable crustal structure to focus ore-forming components and accommodate their deposition. Metals are largely derived from the mantle or crust by partial melting and fluid-related leaching. Ligands can be provided from the same sources, or from the atmosphere, hydrosphere and biosphere. Transport is mainly by mechanical or mass transfer mechanisms and by fluids. On and near the surface, biological processes can also concentrate and transport ore components or remove non-ore components.
Hydrothermal fluids are a major transport medium for many ore systems; these fluids are essentially water, with lesser and variable amounts of CO2, H2S, SO2, CH4, N2, NaCl and other salts, as well as dissolved metal complexes. They are derived from a variety of sources, including i) water-rich silicate melts, ii) circulated sea, and groundwater, iii) formational, diagenetic and metamorphic fluids. At upper crustal levels, the fluids are typically hotter than the rocks they traverse and in which they deposit their ores; they have variable pH and Eh, and they may be charged with a range of metal complexing agents including Cl- and HS-.
Deposition of ore minerals results from changes to physiochemical parameters, including temperature, pressure, pH, redox state and total concentration of ligands. These changes are associated with such processes as addition of components by contamination, phase separation, cooling across a temperature gradient, pressure decrease, fluid mixing and reaction with host rocks. A large variety of geological processes can meet the essential requirements for ore deposit formation and the concentration of ore elements is best viewed as part of the geological (and geochemical) cycle. The vast array of ore deposit types and their particular elemental compositions result from the complex interplay of favorable combinations of source, transport and depositional variables. Although ore formation is a common and intrinsic part of crustal evolution, large and super large ore deposits require the coincidence of particularly favorable combinations of processes and source parameters.
This review outlines the key primary geochemical expressions of the main metalliferous ore deposit types found in Australia. To a large extent, these expressions reject ore-forming processes and are best described in reference to the different genetic ore types. There is now a large literature on Australian ore deposits, including their geochemical features (e.g., Knight, 1975; Hughes, 1990; Berkman and Mackenzie, 1998; AGSO Journal, 1998; Solomon and Groves, 2000; Jaques et al., 2002).
Ore deposits can be classified on the basis of:
• Composition of the deposit (contained elements).
• Form of the deposit (size, shape, orientation and ore mineral distribution).
• Associated host rocks or geological structures (ore associations).
• Interpreted genesis of the deposit (processes, controls).
Geologists generally prefer genetic classification schemes that also incorporate elements of composition, form and association. From these, it is possible to construct predictive models that can be used to search for geological environments in which appropriate ore-forming processes have probably operated. Increasing knowledge of planetary evolution and the global plate tectonic system now provides a better understanding of the context of ore-forming geological environments and processes. A number of major metallogenic epochs are recognized and these are thought to relate to global geodynamic processes, including major periods of crustal break-up and convergence (e.g., Jaques et al., 2002).
A simplified genetic classification encompassing all ore deposit types is provided. This scheme highlights the broad categories of ore-forming processes and the later overprints that may affect the deposits. Deposits can also be broadly subdivided into syngenetic (formed with the enclosing rocks) and epigenetic (introduced into pre-existing rocks). Deposit form and geometry can vary greatly but, in many cases, these features also reject the nature of the ore-forming process. Thus, hydrothermal deposits show forms related to the geometry of the fluid channel ways (e.g., veins or stockworks along fractures). Syngenetic deposits are commonly stratabound (i.e., confined to a particular stratigraphic layer or unit) or stratiform (i.e., confined to the stratigraphy and internally stratified or layered). Examples of deposit types related to the major groups of ore-forming processes are given.
The author concludes that primary ore deposit expressions, particularly ore-host rock relationships and element and mineral distributions, are key guides for mineral exploration in unweathered rocks and for deep or blind ore systems. They can be more important than the prevailing theories of ore genesis, even though genetic models may help explain the expressions. Knowledge of primary dispersion and associated alteration can also assist in interpreting secondary element dispersion patterns and ore deposit expressions in the weathering zone.
Secondary dispersion patterns may be derived from weathering of primary patterns or leakage anomalies, as well as from the orebody itself. Identifying the secondary signal of primary dispersion, as distinct from secondary dispersion from the ore, can help in drill targeting. Appropriate interpretation of geochemical data from weathered host rocks can also identify the primary wall rock alteration, where there is some knowledge of the likely primary minerals.
The key primary geochemical dispersion characteristics of the major ore types are summarized.
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