Geological Occurrence of Quartz

      Quartz is one of the most important silica minerals in nature and an important component of the lithosphere, widely found in igneous rocks, metamorphic rocks, sedimentary rocks, and hydrothermal veins. Based on different mineralization characteristics and physicochemical properties, quartz can be classified into igneous, sedimentary, metamorphic, and hydrothermal minerals (Figure 1).

Figure 1(a) Crystal; (b) Quartzite; (c) Vein quartz; (d) Granite quartz; (e) Siltstone; (f) Quartz sand (cited from Pan et al., 2022).

1. Magmatic Quartz

      Intermediate-acidic granites, diorites, and rhyolites are the main igneous rocks rich in quartz minerals. Granite, in particular, is a typical SiO2-supersaturated rock, widely distributed, and is the most abundant rock constituting the continental crust. Its chemical composition is characterized by high silicon content and low iron, magnesium, and calcium content. Mineral composition shows abundant quartz (>20%), alkali feldspar, acidic plagioclase, and other silica-alumina minerals. Quartz is anhedral, filling the spaces between feldspar grains. Granite pegmatites are shallow-facies rocks genetically related to granites, formed by recrystallization of surrounding granites or decomposition of mineral components composing the granites. Granite pegmatites typically occur as veins, are grayish-white or pale red, with coarse mineral grains, generally >5 mm in diameter. These pegmatite bodies are mostly composed of high-purity quartz, usually accompanied by dolomite, mica, or some rare minerals.

      Since magmatic quartz is mostly associated with other silicate minerals and is difficult to separate, it has a high impurity content, thus limiting its industrial applications. However, granite pegmatite quartz is formed during the slow cooling and crystallization process of high-temperature magma. Due to the high temperature of the magma and the long cooling time, impurity elements inside the quartz grains gradually migrate to the grain boundary edges or micro-inclusions. Therefore, the resulting quartz has an extremely low impurity concentration, few gas-liquid inclusions, and vein sizes can reach tens or hundreds of meters, making it a high-quality raw material for producing high-purity quartz.

2. Sedimentary Quartz

     The main quartz-rich rock types in sedimentary rocks are quartz sandstone and quartz sand. Quartz sandstone is a sandy rock containing abundant cement, formed from fully recrystallized siliceous material, with secondary enlarged rims developing around the quartz clasts, resulting in a very tight cementation. Quartz sandstone is typically light grayish-white or yellowish-white; a light brownish-red color indicates the presence of ferruginous cement. The quartz clasts are mainly rounded to sub-rounded single crystals of quartz, typically exceeding 95% content, easily sorted, predominantly medium to fine-grained, and containing small amounts of flint and feldspar. Quartz sand is a sandy quartz mineral with quartz as its main mineral component, usually formed from various rocks through long-term weathering in nature, often accompanied by growth stone, rock fragments, mica, clay minerals, etc.

3. Metamorphic Quartz

      Quartzite is a metamorphic rock with the highest SiO2 content, composed almost entirely of quartz. It typically exhibits a granular metamorphic texture with significant grain size variation, a massive structure, and a quartz content exceeding 75% in its mineral assemblage, accompanied by minor amounts of feldspar, mica, amphibole, epidote, magnetite, etc. Compared to quartz sandstone, quartzite ore is denser and harder. Quartzite can be formed from the recrystallization of cryptocrystalline siliceous rocks, or it can be a metamorphic rock formed from quartz sandstone or other siliceous rocks containing various impurities through regional metamorphism or thermal contact metamorphism. During the metamorphic recrystallization process, some impurities in the quartzite undergo elemental migration, thereby increasing the purity of the quartz. High-chemical-purity (WSiO2 > 98%) metamorphic quartzite can serve as a high-quality raw material for high-purity quartz.

      Metamorphic rocks also contain numerous quartz-bearing rocks, such as mylonite, amphibolite, gneiss, granulite, and eclogite. When the SiO2 content of quartz in these rocks is high, they all have the potential to become high-purity quartz raw material deposits. Furthermore, in metamorphic rocks, quartz is frequently captured as inclusions by other minerals. For example, during rock subduction, quartz is extensively captured as inclusions by garnet. These quartz inclusions within garnet, along with other mineral inclusions such as amphibole and rutile, form a mineral thermobarometer, which can accurately record the P-T conditions at various growth stages of garnet and invert the garnet growth process.

4. Hydrothermal vein quartz

     Hydrothermal vein quartz is formed by siliceous hydrothermal fluids secreted from underground magma filling rock fissures. Its mineral composition is singular, consisting almost entirely of dense quartz masses. Hydrothermal vein quartz is mostly white or milky white, with a granular structure and a SiO2 content typically exceeding 98%. Minerals such as growth stone, mica, fluorite, barite, tourmaline, garnet, calcite, rutile, apatite, pyrite, and hematite are often found in the quartz crystal fissures or on the crystal faces. Hydrothermal vein quartz deposits are usually irregular veins, ranging in length from tens to hundreds of meters, in width generally a few meters, and in thickness from a few meters to tens of meters.

      Hydrothermal vein quartz is mainly of magmatic hydrothermal and metamorphic hydrothermal types. Magmatic hydrothermal vein quartz, primarily composed of granitic magma, is formed through the precipitation and crystallization of SiO2 fluids under specific conditions containing numerous two-phase gas-liquid inclusions within the minerals. This type of hydrothermal vein quartz mainly exhibits H2O-rich and CO2-poor fluid inclusions, and also contains salt-bearing fluid inclusions. Metamorphic hydrothermal vein quartz is formed by the release of large amounts of water from mineral metamorphism during magmatic activity, creating metamorphic hydrothermal solutions. These ore-bearing solutions move along ductile shear zones under tectonic stress, and due to changes in P-T conditions, SiO2 becomes supersaturated and precipitates. The ore-forming fluids of this type of hydrothermal vein quartz are generally low-salinity, and it mainly exhibits CO2-rich fluid inclusions. Due to its simple composition, few associated minerals, high SiO2 content, coarse crystal grains, and ease of separation, hydrothermal vein quartz is one of the most promising raw material minerals for producing high-purity quartz.