Applied Geochemistry: Advances in Mineral Exploration Techniques

By Athanas S. Macheyeki, Dalaly Peter Kafumu, Xiaohui Li and Feng Yuan

Applied Geochemistry: Advances in Mineral Exploration Techniques is a book targeting all levels of exploration geologists, geology students and geoscientists working in the mining industry. This reference book covers mineral exploration techniques from multiple dimensions, including the application of statistics – both principal component analysis and factor analysis - to multifractal modeling. The book explains these approaches step-by-step and gives their limitations. In addition to techniques and applications in mineral exploration, Applied Geochemistry describes mineral deposits and the theories underpinning their formation through worldwide case studies.

• Includes both conventional and nonconventional techniques for mineral exploration, including lithogeochemical methods
• Highlights the importance and applications of multifractal models, 3D - mineral prospectivity modeling
• Features case studies from mines and mineral exploration ventures around the world

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 5, 2020
• ISBN: 9780128212080

Athanas S. Macheyeki

Athanas S. Macheyeki is a geologist, currently a Full-Time Commissioner for the Mining Commission in Tanzania and the former CEO of the Tanzania Extractive Industries Transparency Initiative, former Manager of Applied Geology - Geological Survey of Tanzania, former Principal of the Mineral Resources Institute - Dodoma and former Head of Department of Geology - University of Dodoma. Prior to working for the government of Tanzania, he served as Senior Exploration geologist for Kabanga Nickel Company Ltd, Anglo American Company (Tanzania) and was one of the geologists who explored and evaluated the Buzwagi gold deposit, one of the world-class gold mines. He is the holder of BSc., MSc., and PhD in Geology and developer of lithogechemical ratios useful for Ni-Cu sulphide exploration, particularly on concealed sulphide ore bodies. He has authored/co-authored over 12 publications, most of which are in peer reviewed journals.

Book preview

Pearson.

Chapter 1

Elements of exploration geochemistry

Abstract

This chapter presents the basic components of exploration geochemistry: rocks and their types, soil, regolith, geochemical elements, and a brief introduction of mineral exploration. The chapter begins by explaining the geochemical cycle; that is, processes within the earth’s interior, the surface, and the atmosphere all that lead to the formation of mineral deposits in rocks. The genesis and origin of igneous, sedimentary, and metamorphic rocks are described. The circulation of geochemical elements in rocks, soils, regolith, and plants is described. Soil and regolith formation and geochemical elements exchange, mobility and characteristic pattern within the earth’s surface processes are also explained. Further, chemical elements of the Periodic Table in terms of their abundance, classification, and significance in mineral exploration are highlighted. Finally, mineral exploration and its stages are described with emphasis on the role of exploration geochemistry.

Keywords

Geochemical cycle; types of rocks; soil types; regolith; geochemical elements; exploration geochemistry; mineral exploration

1.1 Introduction

Geochemistry is a branch of science that deals with the content and distribution of chemical elements in minerals, ores, rocks, soils, waters, and the atmosphere. It also studies the circulation of these chemical elements in nature based on the properties of their atoms and ions (Goldschmidt, 1954). Exploration geochemistry is a branch of applied geochemistry that uses chemical elements, properties, and distribution in nature to locate and characterize economic mineral deposit(s). In order to conceptualize the distribution and circulation of chemical elements in nature, it is practical to have a framework of the geochemical cycle between the different reservoirs and earth compartments (e.g., Lasaga and Ohmoto, 2002; Eriksson et al., 2004; Condie, 2011; Brimblecombe, 2014; Petsch, 2014).

Geochemical cycle refers to the earth’s internal and external processes involving carbon, phosphorous, sulfur, water, and rock elements exchange (Fig. 1.1A and B). These processes within the earth’s interior (lithosphere, crust, mantle, and core), surface and near surface (hydrosphere, biosphere, and soils), the atmosphere (troposphere, stratosphere, mesosphere, and thermosphere), and outer space (meteorites and asteroids) greatly affect the mineral formations in the geochemical cycle. Thus the study of geochemical cycle is the study of rock chemistry, soil chemistry, water chemistry, organic chemistry (or biogeochemistry), and gas chemistry. With the geochemical cycle in mind, we can be able to appreciate geochemical processes that lead to formation of mineral deposits and the methods used to explore the minerals (exploration geochemistry). In the next subchapters, emphasis is given to the use of rock and soil chemistry in mineral exploration.

Figure 1.1 (A) The simplified geochemical cycles of carbon and sulfur, including burial and weathering of sedimentary carbonates, organic matter (OM), evaporites, and sulfides. The relative fluxes of burial and weathering of OM and sulfide minerals play a strong role in controlling the concentration of atmospheric O2. (B) Schematic illustration of the geochemical cycles for carbon and oxygen. Note that the long-term production rate of atmospheric O2 equals the burial rate of organic C in sediments, and that the atmospheric O2 is consumed by reduced volcanic gas and by rock weathering (Lasaga and Ohmoto, 2002). b, Burial, w, weathering. Source: (A) From Petsch, S.T., 2014. The global oxygen cycle. Treatise Geochem., 437–473. Available from: https://doi.org/10.1016/b978-0-08-095975-7.00811-1.

1.1.1 Rocks and their types

Rocks are made up of minerals and lithified sediments. There are three main types of rocks; igneous rocks, sedimentary rocks, and metamorphic rocks. In a simplified model of formation, they can be represented by the rock cycle diagram (Fig. 1.2).

Figure 1.2 Idealized diagram of the rock cycle.

The rock cycle is an idealized diagram that explains how rocks are formed and how they are related to each other in terms of processes that lead to their formations. The rock cycle genesis and origin include the formation and the relationships of igneous rocks, sedimentary rocks, and metamorphic rocks.

1.1.1.1 Igneous rocks

Igneous rocks can be classified into intrusive (plutonic) and extrusive (volcanic) rocks. Igneous rocks sometimes called magmatic rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either the earth’s mantle or crust. Solidification into rock occurs either below the surface (intrusive) rocks or on the surface (extrusive) rocks. Those in between are termed as intermediate (hypabyssal) rocks. Igneous (intrusive and extrusive) rocks are generally classified using grain size and mineral composition as shown in simple schematic classification that identify the main forms of igneous rocks by the variation of grain size (volume of mineral in percent) on the ordinate and mineral composition in ether increasing silica (acidic) of ferromagnesian minerals (basic) on the abscissa (Fig. 1.3).

Figure 1.3 Mineralogical classification of common magmatic rock types. Source: From Schön, J.H., 2015. Rocks—their classification and general properties. Physical Properties of Rocks. Fundamentals and Principles of Petrophysics, pp. 1–19.

1.1.1.1.1 Intrusive rocks

An intrusive rock is a rock whose source is magma and is referred to as plutonic rock. The magma must have solidified deep into the crust resulting in relatively large minerals (crystals). Classification diagram for intrusive (plutonic) rocks is shown in Fig. 1.4. This classification scheme is based on mineral content (vol.%): Q=quartz, A=alkali feldspar (including albite); P=plagioclase; F=feldspathoids; M=mafic and related minerals. Rocks with M less than 90 are named according to their positions in the QAPF diagram, the light-colored constituents being calculated to the sum 100. The following are treated: granitoids and related rocks, ultramafic and gabbroic rocks, charnockitic rocks, feldspathoidal rocks (Streckeisen, 1976).

Figure 1.4 General classification and nomenclature of plutonic rocks according to mineral content (in vol.%). Q+A+ P=100, or A+P+F=100 (Streckeisen, 1976).

Further references on igneous rocks include Cox et al. (1979), Cas and Wright (1987), Le Bas and Streckeisen (1991), Rollinson (1993), Bellieni et al. (1995), Gillespie and Styles (1999), and Schön