Ore Forming Processes
The ore-forming processes may be grouped into on the
basis of two broad categories:
1. Based on relation with country rocks
2. Based on Genesis
1. Based on relation with country rocks
(i) Syngenetic Ore Deposits:
Ore deposits which are formed as the same time as
the enclosing rock, are called “Syngenetic Ore
Deposits”.
Example: Sedimentary Ore Deposits such as Banded
Iron Formation
(ii) Epigenetic Ore Deposits:
The Ore deposits that are formed later than the
enclosing rock, are called “Epigenetic ore deposits”.
Example: Hydrothermal ore deposits
2. Based on Genesis
(a) Igneous Processes
(b) Metamorphic processes
(c) Sedimentary processes
(a)Igneous Processes:
The deposits are formed by crystallization of magmatic
melt in different environments. The principal types
are Magmatic and hydrothermal deposits.
(i) Magmatic deposits: Magma consists of a
multitude of constituents, which are in mutual
solution. As the magma approaches the earth's
surface its temperature and the external pressure
drop, with the result of crystallization and
differentiation of minerals in a definite sequence.
The formation temperature of different magmatic
deposits varies from 1500'C to 300C°. They may be
further subdivided into early magmatic and late
magmatic deposits. These deposits commonly occur
in the following forms: segregations, disseminations
and injections.
Example of Magmatic deposits: Diamond occurrence
in Panna, M.P.
(ii) Hydrothermal deposits: These deposits formed by
hydrothermal solutions.
What is hydrothermal solution?
As magma cools, main rock forming minerals first
crystallized, So that a fluid remain consist metals,
which were originally present in magma.
This fluid which is a hot water solution containg
mineralized liquids derived from an intrusive magma,
is called hydrothermal solution.
This hydrothermal solution moves in cracks and
openings and precipitate their dissolved minerals.
For Example: Gold, Silver, Copper, Lead, Zinc and
mercury, etc.
They can be further classified on the basis of
temperature of deposition (Hypothermal,
Mesothermal and Epithermal,) and mode of
formation (Cavity and Replacement type).
(b) Metamorphic processes:
Metamorphic processes profoundly alter pre-
existing mineral deposits and form new ones. The chief
agencies involved are heat, pressure, time, and various
solutions. The materials acted upon are either earlier
formed mineral deposits or rocks. Valuable nonmetallic
mineral deposits are formed.
Example: Marble deposits from metamorphism of
Limestone (Sedimentary rock).
Slate deposits from metamorphism of Shale
(Sedimentary rock).
(i) Contact metasomatism: Contact metasomatism is a
process of formation of new mineral by reaction
between the rock and the escaping high temperature
gaseous emanation with other important materials
from the intrusive magma.
(C) Sedimentary processes:
From physical or chemical weathering, the materials
are released from the source rocks.
This material transported by means of various
agencies, such as wind, water and ice and deposited in
a basin followed by diagenesis into sedimentary
formation under a suitable environment.
These deposits occur in form of beds. The important
metallic and non-metallic sedimentary deposits are
iron ore, manganese ore, limestone, etc.
The coal deposits are a distinct group of sedimentary
deposits in which biochemical changes have also taken
place duringthe process of deposition and diagenesis.
The deposits of sedimentary origin occur very
commonly in the form of layered formations of
extensive aerial extent and considerable depth.
