A presentation on Hydrothermal wall rock alteration with case studies on geophysical applications. References : https://drive.google.com/drive/folders/16VSZMPMASMNVB47JdBUa_7udBk1qvK2U?usp=sharing

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.