Introduction to Ore-Forming Processes

Laurence Robb

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118687574
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Geologia e scienze della terra

Introduction to Ore-Forming Processes is the first senior undergraduate – postgraduate textbook to focus specifically on the multiplicity of geological processes that result in the formation of mineral deposits.

• Opens with an overview of magmatic ore-forming processes
  
• Moves systematically through hydrothermal and sedimentary metallogenic environments, covering as it does the entire gamut of mineral deposit types, including the fossil fuels and supergene ores
  
• The final chapter relates metallogeny to global tectonics by examining the distribution of mineral deposits in space and time
  
• Boxed examples of world famous ore deposits are featured throughout providing context and relevance to the process-oriented descriptions of ore genesis
  
• Brings the discipline of economic geology back into the realm of conventional mainstream earth science by emphasizing the fact that mineral deposits are simply one of the many natural wonders of geological process and evolution.

Artwork from the book is available to instructors at www.blackwellpublishing.com/robb.

PART ONE

Igneous Processes

CHAPTER ONE

Igneous ore-forming processes

TOPICS

METALLOGENY OF OCEANIC AND CONTINENTAL CRUST
FUNDAMENTAL MAGMA TYPES AND THEIR METAL ENDOWMENT
THE RELATIVE FERTILITY OF MAGMAS AND THE “INHERITANCE FACTOR”

“late-veneer” hypothesis

diamonds and kimberlite/lamproite

metal concentrations in metasomatized mantle

S- and I-type granites

PARTIAL MELTING AND CRYSTAL FRACTIONATION AS OREFORMING PROCESSES

TRACE ELEMENT DISTRIBUTION DURING PARTIAL MELTING

TRACE ELEMENT DISTRIBUTION DURING FRACTIONAL CRYSTALLIZATION

MONOMINERALIC CHROMITITE LAYERS

LIQUID IMMISCIBILITY AS AN ORE-FORMING PROCESS

SPECIAL EMPHASIS ON MINERALIZATION PROCESSES IN LAYERED MAFIC INTRUSIONS

sulfide solubility

sulfide–silicate partition coefficients

the R factor

PGE clusters and hiatus models

1.1 INTRODUCTION

Igneous rocks host a large number of different ore deposit types. Both mafic and felsic rocks are linked to mineral deposits, examples of which range from the chromite ores resulting from crystal fractionation of mafic magmas to tin deposits associated with certain types of granites. The processes described in this chapter relate to properties that are intrinsic to the magma itself and can be linked genetically to its cooling and solidification. Discussion of related processes, whereby an aqueous fluid phase forms or “exsolves” from the magma as it crystallizes, is placed in Chapter 2. The topics discussed under the banners of igneous and magmatic–hydrothermal ore-forming processes are intimately linked and form Part 1 of this book.

CASE STUDIES

Box 1.1 Diamondiferous kimberlites and lamproites: the Orapa (Botswana) and the Argyle (Western Australia) diamond mines

Box 1.2 Partial melting and concentration of incompatible elements: the Rössing uranium deposit

Box 1.3 Boundary layer differentiation in granites and incompatible element concentration: the Zaaiplaats tin deposit, Bushveld Complex

Box 1.4 Crystal fractionation and formation of monomineralic chromitite layers: the UG1 chromitite seam, Bushveld Complex

Box 1.5 Silicate–sulfide immiscibility: the komatiite hosted Ni–Cu deposits at Kambalda, Western Australia

Box 1.6 New magma injection and magma mixing: the Merensky Reef, Bushveld Complex

Box 1.7 Magma contamination and sulfide immiscibility: the Sudbury Ni–Cu deposits

A measure of the economic importance of ore deposits hosted in igneous rocks can be obtained from a compilation of mineral production data as a function of host rock type. A country like South Africa, for example, is underlain dominantly by sedimentary rocks and these undoubtedly host many of the valuable mineral resources (especially if the fossil fuels are taken into consideration). Nevertheless, the value of ores hosted in igneous rocks per unit area of outcrop can be comparable with that for sedimentary rocks, as indicated in Table 1.1. Although South Africa is characterized by a rather special endowment of mineral wealth related to the huge Bushveld Complex, the importance of igneous-hosted ore deposits is nevertheless apparent.

Table 1.1 A comparison of the value of mineral production from igneous and sedimentary rocks in South Africa

Source: after Pretorius (1976).

1.2 MAGMAS AND METALLOGENY

It is well known that different igneous rocks host ore deposits with different metal associations, and that this must be related somehow to the environments in which magmas are generated and the resulting compositional characteristics they inherit from their various settings. It is widely recognized, for example, that many of the chalcophile and siderophile elements (such as Ni, Co, Pt, Pd, and Au) are more likely to be associated with mafic rock types, whereas concentrations of many lithophile elements (such as Li, Sn, Zr, U, and W) are typically found in association with felsic or alkaline rock types. This has implications for understanding ore genesis and, consequently, some of the factors related to these differences are discussed below.

1.2.1 Crustal architecture and mineral wealth

Although the greatest concentrations of siderophile and chalcophile elements almost certainly reside in the mantle and core of the Earth, these are generally inaccessible due to their very great depths. In fact, most of the world’s economically exploitable mineral wealth effectively lies on the surface or just below the surface of the Earth. The world’s deepest mine, the Western Deep Levels gold mine near Johannesburg, South Africa, extends to just over 4000 m depth and this places an effective limit on ore body exploitation, at least in terms of present technologies. Nevertheless, many mineral commodities are formed much deeper in the crust than 4 km, with some even being derived from the mantle. Diamonds, for example, are hosted in kimberlite magmas that have been brought to exploitable depths by a variety of igneous or tectonic mechanisms. Understanding ore genesis processes, therefore, requires a knowledge of lithospheric (i.e. crust and upper mantle) architecture, and also of the origin and nature of the igneous rocks in this section of the Earth.

The oceanic crust, which covers some two-thirds of the Earth surface, is thin (less than 10 km) and, compared to the continents, has a composition and structure that is relatively simple and consistent over its entire extent. The upper layer, on average only 0.4 km thick (Kearey and Vine, 1996), comprises a combination of terrigenous and pelagic sediments that are distributed mainly by turbidity currents. They are often highly reduced and metal charged. This is underlain by a layer, typically 1–2.5 km thick, that is both extrusive and intrusive in character and dominantly basaltic in composition. The basalts are, in turn, underlain by the main body of oceanic crust that is plutonic in character and formed by crystallization and fractionation of basaltic magma. This cumulate assemblage comprises mainly gabbro, pyroxenite, and peridotite. Sections of tectonized and meta morphosed oceanic lithosphere can be observed in ophiolite complexes which represent segments of the ocean crust (usually back-arc basins) that have been thrust or obducted onto continental margins during continent–ocean collision.

Figure 1.1 Oceanic crustal architecture showing the main types of ore deposits characteristic of this environment. Only chromite and related deposits (Cr–Ni–Pt) are related to igneous ore-forming processes; VMS (Cu, Co, Zn) and sediment-hosted deposits (Mn, Co, Ni) are discussed in Chapters 3 and 5 respectively.

The types of ore deposits that one might expect to find associated with ophiolitic rocks are shown in Figure 1.1. They include the category of podiform chromite deposits that are related to crystal fractionation of mid-ocean ridge basalt (MORB), and also have potential for Ni and Pt group element (PGE) mineralization. Accumulations of manganese in nodules on the sea floor, metal-rich concentrations in pelagic muds, and exhalative volcanogenic massive sulfide (VMS) Cu–Zn deposits also occur in this tectonic setting, but are not directly related to igneous processes and are discussed elsewhere (Chapters 3 and 5).

The continental crust differs markedly from its oceanic counterpart. It is typically 35–40 km thick, but thins to around 20 km under rift zones and thickens to 80 km or more beneath young mountain belts. Historically, the continental crust was thought to comprise an upper zone made up largely of granite (and its sedimentary derivatives) and a lower, more mafic zone, with the two layers separated by the Conrad discontinuity (which marks a change in seismic velocities, and, therefore crustal density). More recent geophysical and geological studies clearly indicate that crustal architecture is more complex and reflects a long-lived tectonic and magmatic history, extending back in some cases over 3800 million years (Figure 1.2).

The continents have been progressively constructed throughout geological time by a variety of magmatic, sedimentary, and orogenic processes taking place along active plate margins and, to a lesser extent, within the continents themselves. In addition, continental land masses have repeatedly broken apart and reamalgamated throughout geological history. These episodes, known as Wilson cycles, have rearranged the configuration of continental fragments several times in the geological past. In the early Proterozoic, for example, it is conceivable that segments of southern Africa and western Australia might have been part of the same continent. The significance of these cycles, and the pattern of crustal evolution with time, to global metallogeny is discussed in more detail in Chapter 6.

Figure 1.2 Continental crustal architecture showing the main types of igneous-related ore deposits characteristic of this environment.

The upper crust, which in some continental sections is defined as extending to the Conrad discontinuity at some 6 km depth, is made up of felsic to intermediate compositions (granite to diorite) together with the sedimentary detritus derived from the weathering and erosion of this material. Archean continental fragments (greater than 2500 Myr old) also contain a significant component of greenstone belt material, representing preserved fragments of ancient oceanic crust. The lower crust, between the Conrad and Mohorovicic discontinuities, is variable in composition but is typically made up of hotter, and usually more dense, material. This is because temperatures and pressures in the crust increase with depth at average rates of some 25 °C km^–1and 30 MPa km^–1 respectively (Kearey and Vine, 1996). The lower crust is not necessarily compositionally different from the upper crust, but exists at higher metamorphic grades. It is also likely to be more anhydrous and residual, in the sense that magma now present at higher levels was extracted from the lower crust, leaving a residue of modified material. Some of the lower crust may be more mafic in composition, comprising material such as amphibolite, gabbro, and anorthosite.

Most of the world’s known ore deposits are, of course, hosted in rocks of the continental crust, and the full range is not shown in Figure 1.2. Some of the more important igneous rock-related deposit types are shown and these include diamondiferous kimberlites, anorthosite-hosted Ti deposits, the Cr–V–Pt–Cu–Ni assemblage of ores in continental layered mafic suites, and the Sn–W–F–Nb–REE–P–U family of lithophile ores related to granites and alkaline intrusions.

1.2.2 Magma types and metal contents

Although their rheological properties are different, the outer two layers of the Earth, the more rigid lithosphere and the ductile asthenosphere, are largely solid. Zones within these layers that are anomalous in terms of pressure or temperature do, however, form and can cause localized melting of the rocks present. The nature of the rock undergoing melting and the extent to which it is melted are the main factors that control the composition of the magma that is formed. The magma composition, in turn, dictates the nature of metal concentrations that are likely to form in the rocks that solidify from that magma.

Although it is theoretically possible to form an almost infinite range of magma compositions (from ultramafic to highly alkaline), for ease of discussion this section is subdivided into four parts, each representing what is considered to be a fundamental magma type – these are basalt, andesite, rhyolite, and alkaline magmas, the latter including kimberlite.

Basalt

Basalts form in almost every tectonic environment, but the majority of basaltic magma production takes place along the mid-ocean ridges, and in response to hot-spot related plumes, to form oceanic crust. In addition, basalts are formed together with a variety of more felsic magmas, along island arcs and orogenic continental margins. Basaltic magma may also intrude or extrude continental crust, either along well defined fractures or rifts (such as continental flood basalt provinces, or the Great Dyke of Zimbabwe) or in response to intraplate hot-spot activity (which might have been responsible for the formation of the Bushveld Complex of South Africa).

Basalt forms by partial melting of mantle material, much of which can generally be described as peridotitic in composition. Certain mantle rocks, such as lherzolite (a peridotite which contains clinopyroxene and either garnet or spinel), have been shown experimentally to produce basaltic liquids on melting, whereas others, like alpine-type peridotite (comprising mainly olivine and orthopyroxene), are too refractory to yield basaltic liquids and may indeed represent the residues left behind after basaltic magma has already been extracted from the mantle. Likewise, oceanic crust made up of hydrated (serpentinized) basalt and drawn down into a subduction zone is also a potential source rock for island arc and continental margin type magmatism. Komatiites, which are ultramafic basalt magmas (with >18% MgO) mainly restricted to Archean greenstone belts, have a controversial origin but are generally believed to represent high degrees of partial melting of mantle during the high heat-flow conditions that prevailed in the early stages of crust formation prior to 2500 Ma.

Ore deposits associated with mafic igneous rocks typically comprise a distinctive (mainly siderophile and chalcophile) metal assemblage of, among others, Ni, Co, Cr, V, Cu, Pt, and Au. Examination of Table 1.2 shows that this list corresponds to those elements that are intrinsically enriched in basaltic magmas. Figure 1.3 illustrates the relative abundances of these metals in three fundamental magma types and the significantly higher concentrations in basalt by comparison with andesite and rhyolite. The enhanced concentration of these metals in each case is related to the fact that the source materials from which the basalt formed must likewise have been enriched in those constituents. In addition, enhanced abundances also reflect the chemical affinity that these metals have for the major elements that characterize a basaltic magma (Mg and Fe) and dictate its mineral composition (olivine and the pyroxenes).

The chemical affinity that one element has for another is related to their atomic properties as reflected by their relative positions in the periodic table (see Figure 4, Introduction). The alkali earth elements (i.e. K, Na, Rb, Cs, etc.), for example, are all very similar to one another, but have properties that are quite different to the transition metals (such as Fe, Co, Ni, Pt, Pd). In addition, minor or trace elements, which occur in such low abundances in magmas that they cannot form a discrete mineral phase, are present by virtue of their ability either to substitute for another chemically similar element in a mineral lattice or to occupy a defect site in a crystal lattice. This behavior is referred to as diadochy or substitution and explains much, but not all, of the trace element behavior in rocks. Substitution of a trace element for a major element in a crystal takes place if their ionic radii and charges are similar. Typically radii should be within 15% of one another and charges should differ by no more than one unit provided the charge difference can be compensated by another substitution. Bond strength and type also effects diadochy and it preferentially occurs in crystals where ionic bonding dom...

Table of contents

• Cover
• Contents
• Title Page
• Copyright
• Preface
• INTRODUCTION: MINERAL RESOURCES
• PART 1: IGNEOUS PROCESSES
• PART 2: HYDROTHERMAL PROCESSES
• PART 3: SEDIMENTARY/SURFICIAL PROCESSES
• PART 4: GLOBAL TECTONICS AND METALLOGENY
• References
• Index