Economic Geology - Principles and Practice

Chapter 1

Geological ore formation process systems (metallogenesis)

Synopsis

Energy flow from the Earth's interior and from the sun drives geological process systems. The concentration of ore and minerals is part of these systems, which comprise intrusive and extrusive magmatism, weathering, erosion, transport and sedimentation, followed by diagenesis and metamorphism. In this chapter, we aim to acquire an overview of these systems in respect of the principles which govern the generation of ore deposits. Finally, the inspection of the different major systems is brought together in a synthetic view of global dynamics and metallogeny (i.e. the science of ore deposit formation). This chapter lays the ground for the rest of the book.

For a long time in the past, processes associated with differentiation and cooling of magmatic bodies were thought to be the main agents of ore deposit formation. Starting with mafic melt, ore minerals can form upon cooling or metal-rich melts can segregate from the silicate liquid. Because mafic silicate minerals crystallize at higher temperature, intermediate and felsic residual melts are formed with their own suite of ore deposits. Late-stage magmatic fluids collect metals and produce hydrothermal mineralization. Lindgren (1933), Niggli (1948), Schneiderhöhn 1932, 1962), Stanton (1972), Guilbert & Park (1986) and many others developed this concept of igneous ore formation. In addition, the role of weathering, erosion and sedimentation in concentrating metals was recognized. Metamorphic processes were seen to transform previously existing ore but without appreciable mass transfer.

More recently, these earlier views (here very simplified) on ore deposit formation were fundamentally expanded (Robb (2005), Evans (1998)). First, the discovery of plate tectonics caused a revolution in understanding the dynamic interaction of the Earth's crust and mantle. Plate tectonics determine the origin and distribution of many ore deposits. Present ore-forming processes were investigated. Outstanding impulses brought the exploration of ocean floor hydrothermal venting that produces metal concentrations, which closely resemble long-known ore deposits (e.g. copper on Cyprus Island).

The application of new technologies of the geosciences (e.g. trace element analysis, microprobe, isotope geochemistry, fluid inclusions investigations, mathematical modelling and simulation) guided by old and new hypotheses, led to changes in metallogenetic thinking and to the recognition of additional ore formation systems. One example is the dehydration of sedimentary basins during diagenesis: Expelled fluids cause appreciable geochemical mass-transport and formation of numerous metallic and mineral concentrations, without involvement of igneous processes. Furthermore, the role of dissolved salt, hydrocarbons, reefs and karst cavities in diagenetic ore formation was illuminated. Long after the first hypothetical considerations, metamorphism was finally proved to cause migration of aqueous fluids that transport and precipitate metals.

The classification of ore deposits by major earth process systems is in principle quite simple. Complications arise mainly because of the extreme variability of individual deposits due to manifold combinations of different processes and factors. Therefore, some authors prefer to arrange deposits into associations and types, which are related by geological setting, paragenesis and form, but not necessarily by the same genetic process (Routhier (1963), Laznicka 1985, 1993). Other authors dispense with geological environs and concentrate mainly on processes (Robb (2005)). In this book, fundamental geological cycles (Figure 1.3) and ore-forming systems are to guide the reader through metallogeny.

The genetic terms of Table 1.1 provide the basic vocabulary of metallogeny. The non-genetic descriptors stratiform (layer-shaped) and strata-bound (restricted to certain strata) only denote shape and position of an orebody in relation to sedimentary features, not its origin. Comprehensive explanations of geological and mining terms can be found in the Dictionary of Mining (AGI (1999)) and the Glossary of Geology by Neuendorf et al. (2005). Geological time nomenclature in this book follows Walker & Geissmann (2009).

Table 1.1 Common metallogenetic terms.

Syngenetic – denotes ores and minerals that formed at the same time as their host rocks (most often applied to sedimentary rocks and ore) Epigenetic – ores were emplaced into pre-existing rocks of any origin (e.g. veins, metasomatic ore) Hypogene – ores that were formed by ascending solutions (e.g. Mississippi Valley type lead-zinc) Supergene – ore formation by descending solutions (meteoric water interacting with rocks during surficial weathering processes) Lateral secretion – concentration of metals by abstraction from surrounding rock Endogenetic – concentration caused by processes in the Earth's interior (magmatism or metamorphism) Exogenetic – concentration caused by processes at the Earth's surface (sedimentation, weathering).

1.1 Magmatic Ore Formation Systems

A very large and diverse group of ore deposits originates by various processes during formation, evolution, emplacement and crystallization of silicate melts (magmas) in the upper mantle and in the Earth's crust.

Most post-Archaean magmatic rocks can be classed according to their plate-tectonic environment. Rocks of the ophiolite association (basalt, gabbro, ultramafic rocks) are remnants of former mid-ocean ridges, back arc basins, and of early and primitive parts of immature oceanic island arcs. Mature island arcs and active continental margins are distinguished by profuse amounts of orogenic andesites and equivalent intrusive magmatic rocks. Continental collision causes melting of sialic crust and voluminous granitic magmatism. Continental rifts are associated with bimodal alkaline volcanism (basalt and rhyolite). Extensional deformation of continents and mantle melting result in emplacement of layered mafic intrusions, flood basalts and alkaline magmatic provinces. Most notable are subvolcanic ring complexes and kimberlite diatremes that transport diamond from 200 km depth to the surface.

The association of certain igneous rocks with specific metal ores was established long ago. Ultramafic rocks host ores of nickel, chromium and platinum, gabbro and norite copper, cobalt, nickel, iron, titanium and vanadium, andesite and intermediate intrusive rocks induce copper and gold ore, and granites are related to beryllium, lithium, tin and tungsten concentrations. Essentially, this distribution was understood as a result of the geochemical fate of different metals during fractional crystallization (solid-liquid fractionation) of silicate melt bodies (Goldschmidt (1958)). Meanwhile, magmatic rocks can be further differentiated according to plate-tectonic setting, source rocks, degree of partial melting, role of volatiles and many other genetic variables. Examples are the various basalt types (N- and E-MORB, intraplate, island arc: Pearce et al. (1984); Pearce (1982); Winchester & Floyd (1977)), or the S-, I- and A-granitoids. We shall see later in this chapter that some of these rock classes are related to specific ore deposits.

Impact magmas result from heat and high pressure caused by collision of extraterrestrial bodies with the Earth. Melting affects part of the crust and in rare cases even the upper mantle. Impact magmas differ chemically from other melts because whole volumes of crust are liquefied, whereas normally partial melting is the rule. In addition, the impacting body may induce geochemical anomalies, especially regarding siderophile elements (e.g. platinum, iridium, cobalt and nickel). Post-impact cooling can induce hydrothermal systems that are able to redistribute matter and provoke ore deposit formation.

In conclusion, the geodynamic environment controls the formation of ores from silicate melts in several ways. At the scale of ore-forming processes caused by single magmatic bodies, the following major genetic stages are differentiated:

• Orthomagmatic ore deposits are formed before the melt cools to complete solidification, or in other terms, in the liquid stage before solidus.
• Pegmatitic ore deposits are the result of segregation of small residual melt batches from a large crystallizing magma body approaching the solid state; fertile pegmatite melt is characterized by high amounts of volatiles and of incompatible and rare elements.
• Magmatic-hydrothermal ore deposits are...