Introduction to Ore-Forming Processes

Laurence Robb

• Publisher: Wiley-Blackwell
• Year: 2020
• ISBN:
  9781119232384
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

A comprehensive account of ore-forming processes, revised and updated

The revised second edition of Introduction to Ore-Forming Processes offers a guide to the multiplicity of geological processes that result in the formation of mineral deposits.The second edition has been updated to reflect the most recent developments in the study of metallogeny and earth system science.

This second edition contains new information about global tectonic processes and crustal evolution that continues to influence the practice of economic geology and maintains the supply of natural resources in a responsible and sustainable way. The replenishment of depleted natural resources is becoming more difficult and environmentally challenging. There is also a change in the demand for mineral commodities and the concern around the non-sustainable supply of 'critical metals' is now an important consideration for planners of the future. The book puts the focus on the responsible custodianship of natural resources and the continuing need for all earth scientists to understand metallogeny and the resource cycle. This new edition:

• Provides an updated guide to the processes involved in the formation of mineral deposits
• Offers an overview of magmatic, hydrothermal and sedimentary ore-forming processes
• Covers the entire range of mineral deposit types, including the fossil fuels and supergene ores
• Relates metallogeny to global tectonics by examining the distribution of mineral deposits in space and time
• Contains examples of world famous ore deposits that help to provide context and relevance to the process-oriented descriptions of ore genesis

Written for students and professionals alike, Introduction to Ore-Forming Processes offers a revised second edition that puts the focus on the fact that mineral deposits are simply one of the many natural wonders of geological process and evolution.

Part I
Igneous Processes

1
Igneous Ore‐Forming Processes

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Topics

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Metallogeny of oceanic and continental crust

Fundamental magma types and their metal endowment

The relative fertility of magmas and the “inheritance factor”

1. “late‐veneer” hypothesis
2. diamonds and kimberlite/lamproite
3. metal concentrations in metasomatized mantle
4. S‐ and I‐type granites

Partial melting and crystal fractionation as ore‐forming 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

1. sulfide solubility
2. sulfide–silicate partition coefficients
3. the R factor
4. PGE clusters and hiatus models

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Case Studies

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1. Box 1.1 Diamondiferous Kimberlites and Lamproites: The Orapa Diamond Mine, Botswana and the Argyle Diamond Mine, Western Australia
2. Box 1.2 Carbonatites and Alkaline Intrusions as Sources of Critical Metals
3. Box 1.3 Partial Melting and Concentration of Incompatible Elements: The Rössing Uranium Deposit
4. Box 1.4 Boundary Layer Differentiation in Granites and Incompatible Element Concentration: The Zaaiplaats Tin Deposit, Bushveld Complex
5. Box 1.5 Crystal Fractionation and Formation of Monomineralic Chromitite Layers: The UG1 Chromitite Seam, Bushveld Complex
6. Box 1.6 Addition of External Sulfur and Sulfide Immiscibility: The Komatiite‐Hosted Ni–Cu Deposits at Kambalda, Western Australia
7. Box 1.7 New Magma Injection and Magma Mixing: The Merensky Reef, Bushveld Complex
8. Box 1.8 Magma Contamination and Sulfide Immiscibility: The Sudbury Ni–Cu Deposits

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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 separates 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 I of this book.

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).

Mineralization hosted in	Area (km^2)	Value of sales, 1971 (10^6 US$)	% of total area	% of total value	Unit value (US$ km^−2)
Granites	163 100	1 973	13.3	3.4	12 000
Mafic layered complexes	36 400	7 288	3.0	12.5	200 200
Total (igneous)	199 500	9 261	16.3	15.9	46 400
Sedimentary rocks	1 023 900	49 137	83.7	84.1	47 900

1.2 Magmas and Metallogeny

It is well known that magmatic ore deposits vary as a function of the composition of the igneous host rock, and that the variable metal endowments in magmas are inherited from the source rocks that were melted to form them. It is widely recognized that many of the chalcophile and siderophile elements (such as Ni, Co, Pt, Pd, and Au), for example, are more likely to be associated with mafic rock types, whereas concentrations of lithophile elements (such as Li, Sn, Zr, U, and W) are typically associated 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 highest 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 mines, in the Witwatersrand Basin of South Africa, extend to over 4000 m deep and this places an effective limit on ore body exploitation, at least in terms of safety and economic viability. 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's 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 ocean floor turbidity currents and 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 metamorphosed oceanic lithosphere can be observed in ophiolite complexes which represent segments of the ocean crust (usually back‐arc bas...