Introductory Chapter: Mineralogy, Geochemistry and Metallogeny of Granites

1. Introduction

Granites are usually a medium- to coarse-grained magmatic rocks composed mostly of quartz, K-feldspar and plagioclase. The granites originated from magma with a higher content of silica and alkali metal oxides, which cool slowly and solidify in different levels of the continental crust. Granites in this continental crust form bodies of highly different size and form. The biggest bodies of granites form the batholiths, which could expose over more as hundreds of square kilometres. Granites are the most significant member of a larger family of granitic rocks and/or granitoids. The individual members of granitic rocks are classified according to their content of quartz, K-feldspar and plagioclase, based on international QAPF classification that was adopted by the International Union of Geological Sciences (IUGS) [1]. The granitic rocks and/or granites usually contain as minor mineral components such as micas (biotite, muscovite, Li-micas), amphiboles and pyroxenes. For granites, granitic rocks are also significant content of accessory minerals, as well as ilmenite, magnetite, apatite, zircon, monazite and xenotime. In some types of granites, which are associated with Sn-W mineralisation, are very significant accessory mineral topaz. Granites have significant highly different textures, from equigranular to porphyritic. Some cases have individual mineral grains, mostly grains of K-feldspars larger than the rock groundmass that form phenocrysts of different size (usually up to 2–5 centimetres) (Figure 1). Granites that occur in nature are usually lighter-coloured rocks with scattered darker minerals such as biotite, amphibole and/or relative scarce pyroxene.

2. Mineralogy

Mineralogy of granites was all the time studied using polarisation microscopy. For this research are recently used highly sophisticated polarisation microscopes produced by the Carl Zeiss and Leica Microsystems companies from Germany and/or the Olympus company from Japan. All these polarisation microscopes use different number of oculars and objectives. For presentation of microphotographs, microscopes use highly sophisticated digital cameras having high resolution. For simple manipulation with these cameras, good software is provided, which could also analyse these pictures with different possibilities of qualitative and/or quantitative analyses of investigated granite samples.

Some other, also frequently used microscopic investigations of granites are using cathodoluminescence. Cathodoluminescence imaging is a very powerful tool to identify the intergrowth textures of different minerals, especially of quartz, K-feldspar and topaz (Figures 2 and 3). For the cathodoluminescence in mineralogy are used two different methods. By using of optical microscope cathodoluminescence (OM-CL), the CL spectra and colour images are obtained using a hot-cathode luminescence microscope. For the study of electron microscope cathodoluminescence (SEM-CL) are used electron microscopes and or electron microprobes equipped by different CL detectors [2, 3, 4]. The SEM-CL cathodoluminescence produced more detailed pictures of intergrowth textures, used also for detailed study of zoning of accessory minerals (zircon). For detailed microscopic study, different accessory minerals, especially zircon, are recently used through scanning electron microscopes.

3. Geochemistry

The chemical analyses of granites and their rock-forming and accessory minerals are recently very useful methods, which are sometimes distinctly more used in comparison with microscopy-oriented investigations of these rocks. For chemical analysis, main rock-forming components are recently used, predominantly the X-ray-fluorescence analysis (XRF), without decomposition of rock samples in solutions. For the analysis of rare elements, as well as for instance Ba, Sr., Rb, Zr, Y and REE are recently used two different analytical methods, instrumental neutron activation analysis (INAA) and/or inductive coupled plasma emission mass spectrometry (ICP-MS) [5]. The XRF analyses are produced usually by the majority of mineralogical and geochemical institutes and/or geological surveys in different countries in the whole world. The production of INNA analysis is more complicated and is coupled with the presence of nuclear reactors. The production of ICP-MS analysis is concentrated on relatively a small number of geochemical institutes. However, the INNA and ICP-MS analyses of different geological materials could be well obtained from two geochemical laboratories in Canada (Act Labs and ACME). Both commercial laboratories are equipped with very sophisticated laboratory equipment and could produce very good chemical analyses, which are certified by a high number of international geochemical standards.

The significant part of geochemical analyses recently represents chemical analyses of rock-forming and especially accessory minerals. For these purposes are used scanning electron microscopes equipped by energy-dispersive X-ray detectors (EDS) or more better electron microprobe analysers (EMPA), which are equipped with EDS detectors and wavelength-dispersive X-ray detectors (WDS), which are more suitable for quantitative analyses of rare elements, inclusive of REE. Recently are predominantly used microprobes produced by CAMECA in France and JEOL in Japan. The main interest is usually concentrated on detailed chemical analyses of different accessory minerals, predominantly on analyses of zircon, monazite and xenotime. By study of granites coupled with different ore mineralisation (Sn-W, U) the microprobe analyses are concentrated also on chemistry of Sn-W-Nb-Ta minerals and different uranium- and thorium-bearing minerals.

The important part of geochemistry represents interpretation and presentation of geochemical data. For these purposes are recently used predominantly two different software packages, the commercial software Minpet [6] and free available software GCDKit [7]. The software Minpet could be used only with Windows, version 7, whereas software GCDKit could be used under Windows versions 7-11. This software offers distinctly more possibilities for presentation and interpretation chemical analyses of granites, together with their statistical analyses, determination of granite melt temperatures based on analyses of Zr and REE and basic principles of geochemical modelling igneous processes.

The predominantly used geochemical classifications of granites are based on the determination of A/CNK ratio (mol. Al₂O₃/(CaO + Na₂O + K₂O). According this ratio are granites classified as peraluminous or metaluminous granites and/or as S- and I-type granites [8, 9]. Some other classifications of granites on I- and S-types are based on distribution of magnetite or ilmenite [10]. Similar classification of granitic rocks is based on modified alkali-lime index (Na₂O + K₂O – CaO) and content of SiO₂. According to this classification, the granites could be distinguished on alkalic, alkali-calcic, calc-alkalic and calcic granitic rock [11]. Some other geochemical family of granites is A-type granites (anorogenic) which are in detail classified using the determinations of Al, Ga, Nb, Ta, Y and Zr [12, 13].

4. Metallogeny

Granites are very often associated with different types of ore mineralisation, predominantly with U and Sn-W-Li bearing mineralisation. The mineralised granites are in these cases altered in different rock types, as well as aceites, episyenites and greisens. The basic internationally accepted terminology of these hydrothermally altered granitic rocks was adopted by the International Union of Geological Sciences (IUGS) [14].

The origin of aceites is coupled with uranium mineralisation originated in shear zones, which occurs in granitic rocks or in highly metamorphosed rocks (paragneisses). These rock series are usually altered in mixture of albite, chlorite and clay minerals, with different distributions of uranium minerals (uraninite, coffinite) (Figure 4). This low-temperature hydrothermal alteration is coupled with significant removal of original magmatic quartz. For describing of these altered rocks, evolved in disseminated uranium deposits in the Massif Central and Armorican Massif, France, were in the past used term episyenites [15, 16]. However, according to the recent IUGS classification for metasomatic rocks [14], the term episyenite could be abandoned. The term aceite was introduced to geosciences by Omel’yanenko [17].

The greisenisation is one of most significant hydrothermal alterations, which occurs in granite-related Sn-W ore deposits. Historically, the term ‘greisen’ has been used firstly by miners from the Krušné Hory/Erzgebirge Mts. to describe wall rocks consisting of quartz, Li-mica and topaz surrounding the Sn-W mineralisation, which occurs in this area [18]. This area hosts a number of Sn-W deposits (e.g. Cínovec/Zinnwald, Altenberg, Ehrenfriedersdorf, Krásno-Horní Slavkov) bound to greisenised granite stocks of the Variscan granite bodies. These granites represent highly fractionated, high-F, Li-mica granites of the Krušné Hory/Erzgebirge batholith [19].

5. Conclusions

The chapter discussed the composition of granites, together with used mineralogical and geochemical methods, which are recently used for description and discussion of their composition and origin. The mineralogical and geochemical methods, together with study of relation of granites to uranium and tin-tungsten ore deposits, are recently the most significant research methods used in geoscience.

Acknowledgments

The author would like to thanks to the support of the long-term conceptual development research organisation RVO: 67985891.

Conflict of interest

There is no conflict of interest.