few centimeters on either side of a vein to forming a thick halo around an
orebody.
There are two main divisions of wall rock alteration: hypogene and
supergene. Hypogene alteration is caused by ascending hydrothermal
solutions and supergene alteration by descending meteoric water reacting
with previously mineralized ground. A third mechanism for the formation of
wall rock alteration is the metamorphism of sulphide orebodies.
Types of wall rock alteration
1. Advanced argillic alteration
– characterized by dickite, kaolinite,
pyrophyllite and quartz. Sericite is usually present and frequently
alunite, pyrite, tourmaline, topaz and zunyite. This is one of the most
intense forms of alteration, often present as an inner zone adjoining
many base metal vein or pipe deposits associated with acid plutonic
stocks. This alteration involves extreme leaching of bases (alkalis and
calcium) from all aluminous phases such as feldspars and micas. The
generation of advanced argillic alteration (ɫɤɨɪɨɫɬɶ ɚɪɝɢɥɥɢɬɨɜɨɣ
ɢɡɦɟɧɟɧɢɹ) can be very important in developing the high permeability
necessary for the circulation of enormous quantities of hydrothermal
fluids and vein growth.
2. Sericitization
-this is one of the most common types of alteration in
aluminium-rich rocks such as slates, granites. The dominant minerals
are sericite and quartz, pyrite often accompanying them. Muscovite is
stable over a wide pressure-temperature field and this account for its
common occurrence as an alteration mineral. If potassium is
introduced into the wall rocks then rocks low in this element, such as
diorites, can be sericitized. Wall rock alteration is progressive with
some minerals reacting and being altered more rapidly than others.
Sericitization (ɫɟɪɢɰɢɬɢɡɚɰɢɹ) may grade from into three types of
higher grade alteration and one of lower grade.
3. Intermediate argillic alteration
– the principal minerals are kaolin-
montmorillonite group minerals occurring mainly as alteration
products of plagioclase.
4. Propylitic alteration –
a complex alteration characterized by chlorite,
epidote, albite and carbonate (calcite, dolomite or ankerite).
5. Chloritization
(ɯɥɨɪɢɪɨɜɚɧɢɟ) – chlorite may be present alone or with
quartz or tourmaline in very simple assemblages. Hydrothermal
chlorites often show a change in their Fe:Mg ratio with distance from
the orebody, usually being richer in iron adjacent to the sulphides.