Chlorite is an alteration mineral that is a key guide in the search for porphyry-copper type deposits and epithermal-mesothermal veins, because it occurs in propylitic and chloritic alteration respectively, in addition, it is a component of low to medium grade metamorphic rocks of metamorphism.
Characteristics and physical properties of chlorite
| Chlorite | Characteristics and Properties |
|---|---|
| Class | Silicates |
| subclass | Phyllosilicates |
| Cluster | Chlorites, clays |
| Origin and geological environment | Common in propylitic alteration in porphyry copper, volcanogenic massive sulfide (VMS) and vein mesothermal environments. Low to medium grade metamorphic mineral. |
| associated rocks | Igneous rocks where ferromagnesians alter to chlorite. Low to medium grade metamorphic rocks (slates, phyllites, greenschists, quartzites) |
| Associated minerals | Epidote, albite, calcite, actinolite, sericite, clay, pyrite. |
| Color | Dark green to greenish gray |
| Brightness | Vitreous, pearly, opaque |
| Stripe | Greenish to gray-green |
| cleavage | perfect in one direction |
| Fracture | Irregular |
| Mohs hardness | 2 to 3 |
| Density or specific weight | 2.6 to 3.3 gr/cm3 |
| crystalline system | Monoclinic |
| Applications | Guide to exploration of porphyry copper, VMS and epithermal. |
Formula and chemical composition: (X,Y)4-6(Si,Al)4O10(OH,O)8
The “X” and “Y” in the formula represent ions, which can include: Fe + 2, Fe + 3, Mg + 2, Mn + 2, Ni + 2, Zn + 2, Al + 3, Li + 1, or Ti + 4. The composition and physical properties of chlorites vary as these ions substitute for each other in a solid solution.
The chlorite it belongs to the silicates, specifically to phyllosilicates and to the group of chlorites.
It is distinguished because it is usually replacing ferromagnesian minerals from all types of rocks (igneous, sedimentary and metamorphic).
The color of its stripe is characterized by being greenish to greenish gray similar to the color of the same mineral.
This mineral has a characteristic dark green to greenish gray color and has a low hardness (between 2 and 3).
In addition, its specific gravity varies between 2.6 to 3.3.
Geological setting and formation
The chlorite It is characterized for being an indicator mineral of propylitic hydrothermal alteration (chlorite+epidote and carbonates) and chloritic alteration in porphyry copper systems.
Its genesis is associated with epithermal and mesothermal environments related to mineralized veins and breccias.
In addition, it is a guide mineral that is related to volcanogenic massive sulfide deposits (VMS), where chlorite is considered to be the result of the interaction of cold seawater with hot hydrothermal fluids, related to the chalcopyrite “stringer” veins. pyrite.
In metamorphic rocks it forms in environments of low to medium grade metamorphism (metamorphic facies) related to slates, phyllites, greenschists and chloritic schists (schists).
Chlorite in thin section or under the microscope
The following describes the important optical properties that chlorite has seen in the petrographic microscope that serve for its identification.
SHAPE: Commonly occurs in sheets or plates similar to those of micas. It can totally or partially replace biotite
COLOR: Occurs as light green, colorless or yellow (depending on composition)
Pleochroism: Variable, usually in shades of green
TWINS: Five twins With composition plane {001}, difficult to recognize in thin section
EXFOLIATION: Yes it presents and it is Perfect
INDICES OF REFRACTION, nα: 1,570 – 1,670,nβ: 1,550 – 1,690,nγ: 1,570 – 1,690
RELIEF: It is characterized by having Low relief + a Medium +
BIRREFRINGENCE:0.000 to 0.020 (Low)
INTERFERENCE COLOR: Low, first order and often anomalous interference colors (ultra-blue, brown or purple)
PARAGENISIS OR MINERAL ASSOCIATION: It appears very common in metamorphic rocks (diagnosis of green schist facies). It is also frequent in different types of igneous rocks due to the alteration of ferromagnesian minerals (biotite, amphiboles, pyroxenes, garnet…)
How to identify chlorite?
It must be taken into account that the chlorite it is a very frequent alteration mineral, so its identification will be quite easy with practice.
In general, chlorite occurs as flakes or sheets, replacing ferromagnesian minerals such as biotite, which gives the rock a soapy appearance.
Use of a steel scribe is recommended to abrade buildup of chlorites, when you scrape them, run your finger over them and watch how little flakes stay on your finger.
Types of Chlorites
Alteration or secondary chlorite
In outcrops the chlorite when it occurs in a chloritic hydrothermal alteration it is grayish blue to green.
while the chlorite that occurs in propylitic alteration is dark bluish green, it is recognized because it is associated with minerals such as epidote, calcite, pyrite and hydrothermal structures such as quartz and sulfide veinlets.
Metamorphic chlorite
The chlorites that forms in metamorphic environments occurs mainly in types of regional and contact metamorphism.
When it occurs in a regional metamorphism, it appears aligned consistent with schistosity, and is a fundamental part of the foliation of the rock as a nematoblastic texture.
In contact metamorphism, chlorite crystallizes at about 400°C, it does not appear aligned and rather makes the rock become “soapy”.
It is associated with rocks such as slate, phyllite and schists (green shale and chloritic schist), in regional metamorphism.
Exploration and exploitation
The geological mapping of anomalies of chlorite It is important to find hydrothermal alterations such as the propylitic one that coincides with the external edges of possible copper-related deposits (copper porphyries).
In addition, they are important in the exploration of volcanogenic massive sulfides (VMS).
Also a small extent of propylitic and chloritic alteration can be correlated in epithermal and mesothermal type deposits in the formation of mineralized veins.
Therefore, its exploration is considered as a guide for the search for cheap minerals that are mainly related to copper (chalcopyrite, bornite, covelin, chalcocite, etc).
However, this mineral must always be related to other minerals that confirm the hydrothermal alterations of the mineral deposits.
This mineral does not have a specific profitable use, so it is not used for economic exploitation.
Uses of chlorites
- Guide to find propylitic and chloritic alteration associated with possible porphyry-type copper and epithermal deposits related to mineralized veins.
- It is a mineral with low potential for industrial use.
- It does not have physical properties that make it suitable for any particular use, and it does not contain components that make it a target for economic mining. As a result, it is not mined and processed for any specific use.
- Its primary use is as a matched component of crushed rock for building materials.