Eoin H. Macdonald ME, FIEAust CP Eng has an international career spanning 65 years as a consulting mining engineer and is highly regarded for his previously published work on alluvial mining technology. As Australian Special Advisor to CCOP (Committee for Co-ordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas) in the ESCAP (Economic and Social Commission for Asia and the Pacific) programme of the United Nations Development Programme, he cooperated with Special Advisors from the USA, UK, France, Germany, the Netherlands, Japan, Canada and Australia in the search for and appraisal of surficial and nearshore deposits globally along the continental shelves. This resulted in a bi-annual series of international training courses on gold prospecting and evaluation with the author as Director of Studies. He has advised on alluvial gold projects associated with exploration, mining, treatment and evaluation worldwide, including Australia, New Zealand, South Korea, Papua New Guinea, and extensively throughout the Americas, Africa, the Indian sub-continent and South East Asia.
Geology of gold ore deposits
Gold, because of its high density and attraction to iron, is believed to have been mainly concentrated in the Earth’s solid and compacted Fe-Ni core during the Earth’s accretionary stage along with Sn, Mo, and other highly siderophile minerals. These minerals are also present in Fe-sulphides in the surrounding mafic-ultramafic rock, the uppermost part of which is in a semi-molten state. During partial melting of the mantle, gold derived from the sulphides is contributed to the magmatic fluids and vapours that circulate to the surface through rifts in the crustal rocks in either extentional or compressional tectonic regimes. Tectonic regimes (cratons, ocean basins, divergent margins, convergent basins and transform boundaries) are related to one another through those earth processes encompassed by plate tectonic theory.
In the plate tectonic model, intensive research on the nature of continental and oceanic plate interactions and the associated geological processes that control the genesis of ores has made possible an understanding of many unresolved geological features and processes. The theory provides logical explanations for many aspects of Earth’s geology and history including the opening and closing of ocean basins, origins of mountain ranges, geological structures, distribution of mineral resources, and palaeoclimates. Important deposit types are distinguished according to geological setting, host rock type, associated minerals and depths of emplacement. They comprise volcanic hosted massive sulphides (VHMS), mesothermal ore bodies, intrusion related porphyry and non-porphyry deposits, and epithermal deposits of both low and high sulphidation styles. Residual and detrital deposits are developed wherever the unroofing of a sufficiently large primary gold orebody contributes gold to the regolith under stable conditions of weathering erosion and deposition. This may be done directly, e.g. Palaeozoic to present or in stages, e.g. Palaeozoic to Mesozoic to present. Some authors also apply the same tectonic principles to explanations of certain aspects of Precambrian geology, particularly the genesis of greenstone belts, but this is a contentious issue.
Key geological features of gold ore geology discussed in this chapter are crustal evolution, tectonic elements of plate boundaries, hydrothermal gold systems, source rocks, provenance and the time rate of unroofing of orebodies.
2.1 Crustal evolution
No direct evidence exists of crustal evolution earlier than about 4.57 billion years ago but it is generally believed that while the Earth was still in a molten state gravitational forces acted to concentrate the densest material towards the centre and lighter material closer to the surface. The geological timescale (see Table 5.2) describes the timing and relationship between events that have occurred during Precambrian times in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy. Highly siderophile elements such as the platinum-group elements and Au were thus effectively concentrated into the Fe-Ni core during Earth’s accretionary stage (Solomon and Shen, 1997). At this time the core was probably enclosed in a partly molten ‘magma ocean’ over the surface of which thin platelets of simatic and lesser sialic material were moved about and subducted under the influence of the hot convecting mantle fluids (Lowe and Ernst, 1992). When the surface layer cooled and solidified, a thin film of crustal rock formed around the Earth. Large cracks developed because of thermal stress breaking crustal material into individual rigid plates of ‘lithosphere’ of continental size, which moved as independent units across the hot plastic part of the upper mantle (asthenosphere). Depressions in the crust formed as natural basins filled with water rising up through fissures in the crust and from the Earth’s early atmosphere to accumulate as oceans.
Various explanations are given for the observed spatial variations of the ore-forming elements in the mantle and crust and particularly of mantle heterogeneity generated during the Earth’s accretionary stage of enriched mantle resources. One possibility is the addition of a small amount (< 1 mass%) of oxidised accretional veneer from the core after its formation. The introduction of highly siderophile elements into the Earth’s mantle might then account for the heterogeneous nature of distributions of Au and platinum group elements in the mantle (Kimura et al., 1974; McDonough and Sun, 1995). Another speculation is that core-mantle interaction and rising of the boundary layer might have introduced important amounts of Au and Pt into the mantle through underplating of mantle plume and subducted lithosphere. Solomon and Shen (1997) suggest that while the evolution and final solidification of a circum-global magma ocean in the upper mantle might have resulted in large-scale mantle heterogeneity, these effects could have been counterbalanced by vigorous mantle convection. Available geophysical and geochemical observations point to a pyrolitic type mantle without major differences between the upper and lower mantle (McDonough and Sun, 1995).
2.1.1 Magma-forming processes
The molten or partly molten rock materials making up magma have varying compositions, temperatures, crystal contents, volatile contents and thereby varying rheological properties (McBirney and Murase, 1984). In their simplest form magmas are produced at mid-ocean ridges when hot mantle material rises from the asthenosphere to fill the gaps between diverging plates. The process begins with a mantle convection cell rising to the surface bringing with it ultramafic parent magma. The parent magma fractionally melts as it approaches the surface creating a mafic melt, which forms the oceanic crust while leaving an ultramafic residue behind in the mantle. Magma may reside for some time in high-level volcanic magma chambers, which are periodically replenished and tapped, and continuously fractionated to provide the mixing of individual magmas or development of composition zonation (Cas and Wright, 1995). Magmas filling the rift between spreading plates either solidify as vertical sheeted dykes or spill out at the seafloor to form pillow lavas.
Heat energy from the interior of the Earth rises to the surface due to the action of convection cells within the asthenosphere. The hot plastic rock cools and is turned over slowly at the base of tectonic plates carrying continents, and moves parallel to the Earth’s surface at about 10 cm/year before descending back into the mantle at subduction zones to be reheated. In the plate tectonic model, the Earth’s crust is broken into seven major and numerous minor lithospheric plates, which continuously jostle against one another as they move as independent, rigid units across the partly molten asthenosphere. The direction and rate of movement of any one plate is influenced by its size and shape and by the size, shape and motion of the surrounding plates. New oceanic crust is in process of formation by the upwelling of basaltic material at extentional plate margins (e.g., mid-ocean ridges, back-arc basins) constructive plate margins, while older crust is being consumed at convergent margins where subducting plates sink back into the asthenosphere. Figure 2.1 is a conceptual cross-section of the seafloor hydrothermal system showing the driving force of seafloor spreading when a plume of hot magma rises under the ocean rift forcing the plates to move apart, and the involvement of divergent and convergent plate settings.
Composition and mineralogical characteristics of erupted magma are the end result of a complex history of processes causing chemical and physical change. Widely different geological histories include the degree of partial melting of the source rocks and other melting events and the nature and extent of the sedimentary cover. Additional factors are the amount of contamination from the wall rock and subducting slabs, periodic replenishment of fresh magma, and tapping and fractionating of magmas in a succession of magma chambers as they rise to the surface. Figure 2.2 is a schematic representation of the principal components of magma genesis, fluid flow and metallogenesis in convective plate settings where oceanic crust is subducted beneath continental lithosphere.
Influence of sediment on magma characteristics
Factors that link the assimilation of various sediment types to the melt during the magma-forming process strongly influence the composition and mineralogy of the hydrothermal fluids. Wind-blown sediments of all types are deposited at slow rates over the marine environment. The greatest influence of contamination by silicic materials is along continental margins where billions of tonnes of suspended fine to coarse bedload sediment are discharged by rivers into the ocean each year; and along the ocean trenches where turbulent currents distribute great quantities of pebbles, sand, silt and clay. Chemically formed sediments are precipitated as a result of reactions of the seawater to products of both on-shore erosion and off-shore volcanic action. In warm tropical waters, extraction of calcium carbonate from seawater by marine...
Erscheint lt. Verlag | 21.2.2007 |
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Sprache | englisch |
Themenwelt | Technik ► Bergbau |
ISBN-10 | 1-84569-254-3 / 1845692543 |
ISBN-13 | 978-1-84569-254-4 / 9781845692544 |
Haben Sie eine Frage zum Produkt? |
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