Wall rock alteration involves the transformation of wall rocks through hydrothermal processes, resulting in changes in mineralogy and chemistry due to hot fluid interactions. Various types of alteration include phyllic, silication, hematization, and propylitic, each characterized by distinct mineral assemblages and processes. Geophysical methods can assist in identifying key host rocks and alteration zones, providing valuable insights for mineral exploration.

1. Wall Rock Alteration
By Adithya Shashidhara Shettar

2. What is Wall-
Rock Alteration?

3. Wall Rocks
● Wall rock is the rock that constitutes the
wall of an area undergoing geologic activity.
Eg. the rock along the neck of a volcano.
For the case of hydrothermal deposits, wall
rocks are faults, fracture networks and
shear zones.

4. Wall Rock Alteration
● Hydrothermal alteration is a complex process involving mineralogical, chemical and
textural changes, resulting from the interaction of hot aqueous fluids with the rocks
through which they circulate, under evolving physico-chemical conditions.
● In essence, hydrothermal fluids chemically attack the mineral constituents of the wall
rocks, which tend to re-equilibrate by forming new mineral assemblages that are in
equilibrium with the new conditions. The process is a form of metasomatism, i.e.
exchange of chemical components between the fluids and the wall-rocks. Therefore, it is
also likely that the fluids themselves may change their composition as a result of their
interaction with the wall rocks.
● The main factors controlling alteration processes are: (1) the nature of wall rocks; (2)
composition of the fluids; (3) concentration, activity and chemical potential of the fluid
components, such as H+, CO2, O2, K+, H2S and SO2

5. What are the
types of Wall-
Rock Alteration?

6. Phyllic (or Sericitic) Alteration
● Occurs over a wide temperature range by hydrolisis of feldspars
to form sericite (fine-grained white mica) with minor associated
quartz, chlorite and pyrite.
● Associated with porphyry Cu deposits and volcanogenic massive
sulphide deposits.

7. Silication
● It is the conversion of a carbonate mineral
or rock into a silicate mineral or rock.
● Main process which accompanies
formation of polymetallic skarn deposits
which develop when an acidic, magmatic
fluid infiltrates a carbonate host rock.
● Silication is not to be confused by
silicification, which refers to the deposition
of new quartz or amorphous silica.

8. Hematization
● Alteration that is associated with oxidizing fluids often results in the formation
of minerals with a high Fe3+/Fe2+ ratio and, in particular, hematite with
associated K-feldspar, sericite, chlorite, and epidote.

9. Propylitic
● Probably the most widespread form of Alteration
● Characterized by the assemblage chlorite + epidote + calcite
● Due to presence of the green minerals chlorite and epidote this zone is usually
easily recognizable by its color.
● Associated sulfides include pyrite, copper sulfides, galena, sphalerite and a host
of complex arsenides.
● Often this zone can be quite large and is useful during mineral exploration.

10. Role of
Geophysics?

13. These maps highlighted key
host rocks in the area:
epidote-rich rocks, epidosite,
magnetite pyrite-quartz-
muscovite schist, massive
kyanite and kyanite bearing
rocks.
They have anomalous
potassium signatures that
represent the phyllic/argillic
alteration of orthogneisses.
The propylitic alteration
(epidosites, quartz-
epidosites) is recognizable
as two cyan colored areas on
the KThU image and
medium (red) on the K
image

14. The Zacarias Au-Ag-Ba deposit is
characterized by a discrete anomaly
controlled by a NE trend with low
amount of Th and U, intermediate
values of K and high gradient in the
analytical signal amplitude.

15. Thank you.