Deatils about the types of wall rock alteration and its significance.

1. Wall Rock Alteration
Introduction:
1. The country rocks that contain hydrothermal ore deposits almost invariably exhibit reaction effects caused by hot
circulating fluids, that causing changes in mineralogy that are most marked adjacent to the vein and become less
distinct further away.
2. There are two main divisions of wall rock alteration: hypogene and supergene. Ascending hydrothermal solutions
induce hypogene alteration, while descending meteoric water reacts with previously mineralized ground causes
supergene alteration. A third type mechanism giving rise to the formation of wall rock alteration is the
metamorphism of sulfide orebodies.
Alteration:
1. Alteration is defined by any change in the mineralogical composition of a rock brought about by physical or
chemical means, especially by the action of hydrothermal fluids.
2. The nature of the alteration products depends upon (1) the character of the wall rock, (2) the character of the invading
fluid, (such factors as Eh, pH, vapor pressure, anion – cation composition, the degree of hydrolysis), and (3) the
temperature and pressures condition during reaction.
Important Reactions during wall rock alteration:
Many types of reaction that happen during the time of wall rock alteration that are given below,
(a) Hydrolysis: 3KAlSi3O8 (K-feldspar) + 2H+(aq) ↔ KAl3Si3O10(OH)2 (Muscovite) + 6SiO2 (Quartz) + 2K+(aq)
(b) Hydration: 2Mg2SiO4 (Olivine) + 2H2O + 2H+ ↔ Mg3Si2O5(OH)4 (Serpentine) + Mg+2
(aq)
(c) Dehydration: Al2Si2O5(OH)4 (Kaolinite)+ 2SiO2 ↔ Al2Si4O10(OH)2 (Pyrophyllite) + H2O
(d) Silication: CaCO3 (Calcite) + SiO2 ↔ CaSiO3 (Wollastonite) + CO2 (gas)
CaMg(CO3)2
(Dolomite) + SiO2 (aq) ↔ (CaMg)Si2O (Diopside)
(e) Megnesium metasomatism: 2CaCO3 (Calcite)+ Mg+2
(aq) ↔ (Ca, Mg) (CO3)2
(Dolomite) + Ca+2
(aq)
(f) Decarbonation: (CaMg)(CO3)2
(Dolomite)+ 2SiO2 ↔ (CaMg)Si2O6 (Diopside)+ 2CO2 (gas)
Types of Wall rock Alteration:
1. Potassic Alteration: This type of alteration is characterized by formation of new K-feldsper and/ biotite with minor
sericite, chlorite and quartz. Accessory amount of magnetite/hematite also occur. It is representing the highest
temperature form of alteration (500-600⁰) associate with porphyry Cu type deposit.
[Porphyry type Cu deposits are characterized by significant presence of zones of altertered wall rock which include
zones of (a) potassic alteration, (b) phyllic alteration, (c) argillic alteration and (d) propylitic alteration.]
2. Phyllic (Sericitic) Altertaion: This type of alteration is more common in ore forming hydrothermal process and
forms over wide range of temperature by hydrolysis of feldspar, to produce white color sericite mica. This alteration
associate with porphyry Cu deposit and VMS deposit in felsic rock.

2. 3. Greisenization: This type of alteration is linked to granite formation, where latter stage high acidic fluid reacts with
K-feldspar and muscovite to produce quartz, tourmaline, topaz, and lepidolite. This type of alteration is the direct
indication of Tin and Tungsten deposition in the rock and associated with lithium deposition.
4. Propylitic Alteration: This type of alteration is also very widespread, and mainly affect plagioclase feldspar by
the formation of clay minerals (Kaolinite and montmorillonite). It is typically form below 250⁰ by the H+
metasomatism reaction.
5. Silication Alteration: Silication is the process of conversion of carbonate mineral or rocks in to silicate mineral or
rocks. This type of alteration is associate with skarn type of polymetallic deposition, when fertile, acidic magmatic
fluid infiltrate the carbonate rock.
6. Fenitization: A fenite is quartzo-feldspathic rock (rich in Na+ and K+ ion) develop at the contact between alkali
(carbonatite) igneous intrution and surrounding country rock. Fenite is comprise of alkali-feldspar, with some
aegirine, alkali-hornblende, nepheline, and apatite.
7. Chloritization: Here mafic-ultramafic minerals are altered by chlorite mineral. Very common surrounding the sea
floor massive sulfide deposition.
8. Bleaching: The absence of any distinct mineral assemblage is marked by a color change between the altered and
unaltered rock. Generally, the effect of iron oxidation (Fe).
Influence of Original Rock Types
A survey of wall rock alteration reveals some consistency in terms of the composition of the host rock. (Boyle 1970). Thus,
for example,
• The most prevalent types of alteration in acidic rocks are sericitization, argillization, silicification and pyritization.
• Intermediate and basic rocks generally show chloritization, sericitization, pyritization and propylitization.
• In carbonate rocks the principal high temperature alteration is skarnification or silication.
• Whereas normal shales; slates and schists are frequently characterized by tourmalinization, especially when hosting
tin, the tungsten deposits.