(III) Main rock types

The minerals in greisen are mainly composed of quartz and lepidolite, other minor minerals are topaz, fluorite and tourmaline, and some also contain apatite, solar garnet and some metal-bearing minerals, such as wolframite, scheelite, cassiterite, molybdenite, pyrite, galena and sphalerite, which can form ore-bearing greisen. The main rock types are greisen, quartz greisen, mica greisen, topaz greisen and ore-bearing greisen. The content of the above minerals in rocks changes greatly, resulting in more rock types.

1. greisen

The main minerals in greisen are quartz and lepidolite, in which the quartz content is mostly above 40%, but often around 60%, and the lepidolite content is mostly between 25% and 40%. Topaz, fluorite, tourmaline and apatite in rocks are generally less than 5%. It has medium-coarse grained (or medium-fine grained) flaky granular metamorphic structure and massive structure (photo 7- 19).

Some plagioclase and potash feldspar (mainly microcline) in the original granite remain in the greisen, which has a metamorphic semi-automorphic structure. According to the content of metasomatic altered minerals in rocks and the degree of preservation of original rock characteristics (Table 7- 1), it can be divided into weak greisenized granite (Figure 7- 17) and greisenized granite (Figure 7-6544).

2. Ying Shi Yun Ying Yan.

If the time content of greisen is > 2/3 and < 90 (85)%, it can be called Shi Ying greisen in time and greisen in time (photo 7-20). Rock has flaky granular metamorphic structure and massive structure. When the chronological content is more than 90 (85)%, almost all the minerals in the rock are single mineral rocks composed of chronological, which is essentially a high-temperature silicified quartzite called chronological rock.

In terms of minerals and mineral contents, this kind of time-rich dolomite and time-mineralized rocks are very similar to muscovite quartzite and quartzite formed by regional metamorphism. Their differences are as follows: ① The geological occurrences of the two rocks are different, and the time-rich dolomite and time-mineralized rocks are closely related to granite intrusive rocks, and often coexist with other types of dolomite and time-dependent granites; However, regional metamorphic quartzite and muscovite quartzite have no genetic relationship with granite, and often coexist with felsic metamorphic rocks such as mica schist. ② There are often volatile minerals such as topaz, fluorite, tourmaline, etc., and various kinds of metasomatic texture often appear in the rocks. Special attention should be paid to the structural characteristics of mica and feldspar minerals replaced by light mica, timely, topaz and fluorite. The above characteristics cannot appear in muscovite quartzite and quartzite formed by regional metamorphism. ③ Chronological greisen and chronomineralized rocks are mostly irregular, veinlets and reticulated veins, sometimes closely related to greisen-related deposits. However, muscovite quartzite and quartzite are mostly layered and often have a gradual transition relationship with their surrounding rocks.

3. Mica greisen

When the content of lepidolite in greisen is > 2/3 and < 90 (85)%, it is called muscovite or mica-rich greisen (photo 7-2 1) and has a granular flaky crystal structure. When the content of mica is more than 90 (85)%, a light mica rock almost composed of mica is formed (Photo 7-22), and its structure is flaky metamorphic structure. Muscovite and sericite often coexist in rocks, and mica is unevenly distributed in rocks, but is produced as a block aggregate. In mica aggregates, there are often irregular feldspar residues or fluorite, topaz and other minerals, forming fluorite topaz mica rock (photo 7-24) and albite muscovite rock (photo 7-23). Mica greisen often contains metal minerals, which is related to the fact that the rock is often distributed on the top of granite, where tungsten mineralization and tin mineralization are concentrated.

4. Huangshi Yingyan

Topaz often appears in various greisen rocks. When the topaz content in the rock is more than 5%, it can be named. Such as topaz yunying rock, topaz yingshi yunying rock, topaz mica yunying rock and fluorite muscovite topaz (photo 7-25). In rare cases, when the topaz content in the rock is more than 90 (85)%, it can be called topaz rock. Topaz is mostly white, light gray, light yellow and grayish brown. It is a common granular crystal, sometimes columnar crystal with a group of cleavage, and its hardness is greater than its age. Microscopically, topaz is mostly colorless, protruding in the middle, and the interference color is first-class yellow. Both parallel cylinder and cleavage are parallel extinction, parallel cylinder is positive ductility, parallel cleavage is fast light direction, and biaxial crystal is positive light. These characteristics can be distinguished from chronotropic and feldspar in rocks. Topaz greisen has granular (or flaky granular) metamorphic structure and massive structure.

5. fluorite greisen

A small amount of fluorite is often seen in various types of greisen. When the content of fluorite is more than 5%, fluorite can participate in the naming of rocks, which are called fluorite greisen, fluorite quartz greisen and fluorite mica greisen. If the fluorite content in the rock is less than 5%, the word "containing" can be added before the above rock. Among them, fluorite mica muscovite is more common, and the rocks are granular flaky metamorphic structures and massive structures. Fluorite is mostly granular, colorless, light green, purple or blue in color, with small hardness and complete cleavage, so the section of fluorite in thin slices often has two sets of rhombic cleavage or three sets of cleavage intersecting into a triangle. The flake is colorless, sometimes in the form of spots with different shades, patches of purple, with negative middle-high protrusions, which are completely dim under the orthogonal mirror, and are isotropic minerals. To sum up, fluorite is different from other minerals in greisen by its unique color, cleavage, negative mid-high protrusion and isotropic body.

6. Dianyunyingyan

When the content of tourmaline in greisen is more than 5%, tourmaline can participate in the naming of rocks. If the content of tourmaline is less than 5%, use the word "containing" before tourmaline, such as tourmaline Yunying and charged Yunying. Tourmaline is generally not contained in greisen formed by granite protolith, but it is often contained in greisen dikes of surrounding rocks such as argillaceous rocks and phyllite invaded by granite. The above characteristics reflect that boron in tourmaline mainly comes from the surrounding rocks of sedimentary rocks. Tourmaline is a columnar crystal with longitudinal stripes on the cylinder, spherical triangle or hexagon in cross section, and cleavage is not developed, and cracks are often seen near the column. Tourmaline varies greatly in color, including colorless, green, blue, yellow, brown, black and pink. Its color is related to its composition. Iron is dark, and lithium is light. Under the microscope, tourmaline is a central protruding tourmaline, and its cylinder is the lightest when polarized in parallel, and the darkest when polarized in parallel, and its absorption is just the opposite to that of biotite. The color of thin tourmaline is often banded with different shades, and the interference colors are mostly second-order, parallel extinction, negative ductility and uniaxial negative light. Electromica quartz is often distributed on both sides of veins such as tungsten and tin.

7. Ore-bearing greisen

All kinds of metal minerals in the above-mentioned greisen are often called ore-bearing greisen, and the most common ones are wolframite, scheelite, cassiterite, molybdenite, galena and sphalerite. Most of these metal minerals are embedded or distributed in rocks, and sometimes they can constitute ores with industrial value. When naming rocks, metal minerals can be placed before the names of various greisens, such as greisen with wolframite, greisen with amphibole, mica greisen with cassiterite, greisen with cassiterite and molybdenite. Among them, mica muscovite contains metal minerals, which is the most common. This is because mica muscovite is often distributed on the top of granite. In addition, the porosity of mica muscovite is large, which is beneficial to the filling of these metal minerals. These ore-bearing greisens are important signs for finding such metal deposits.