Modern Concepts of Ore Genesis

 

Till the end of the 18th century, geologists had a static concept of the earth, i.e. they believed that it was created exactly as we see it today. Since the beginning of the 19th century, the new way of looking at the earth included the recognition that constant changes take place as geological forces modify the surface and interior of the earth. The geological cycle as deduced by Hutton postulates that the geological environment in any place undergos a cyclic change through a number of stages as outlined below:

1. Erosion and planing down of mountains - The Weathering Stage.
2. Formation of sedimentary rocks - The Sedimentary Stage.
3. Burial in the deep crust - The Plutonic Stage.
4. Folding, faulting, mountain building and igneous intrusion - The Orogenic Stage.

It must be considered that since ores are rocks, they are likely to be intimate parts of the environment in which they occur. This is supported by the evidence obtained from the examination of a wide variety of ores and the geological environment in which they occur. It is confirmed that most ore deposits indeed appear to be related fundamentally to their environment. To be more specific, ores are not merely parts of their environment, each one of them is a characteristic part of a particular type of environment. This realization leads to another important principle --the geological environment existing in any segment of the earth's crust or surface is not static -- it is constantly changing. In most cases the changes involved are progressive and systematic, that is to say that the environmental metamorphosis follows an evolutionary pattern. This systematic change is referred to as CRUSTAL EVOLUTION. Most of these changing environments are those that have certain ore deposits of their own particular characteristic kind.

Current theories of the genesis of ore deposit can be divided into internal (endogene) and external (exogene) or surface processes. It must be understood that more than one mechanism may be responsible for the formation of an ore body. Example - stockwork porphyry copper deposit at depth (epigenetic) with a syngenetic massive sulfide deposit at the surface. The Table at the end of the document summarizes the principal theories of ore genesis,

Depending upon whether an ore deposit formed at the time of and together with the enclosing rock, or was introduced into it by subsequent processes, they are classed as:

Syngenetic - A deposit formed at the same time as the rocks in which it occurs. Ex. Banded Iron Formation

Epigenetic - A deposit introduced into the host rocks at some time after they were deposited. Ex. Mississippi Valley-type Deposits

 

THEORIES OF ORE DEPOSIT GENESIS

Origin Due to Internal Processes

Magmatic Segregation

Separation of ore minerals by fractional crystallization during magmatic differentiation.

Pt-Cr deposits
Bushveld, S.A.
Titanium deposit
Tahawas, N.Y.

Liquid immiscibility. Settling out from magmas of sulfide, sulfide-oxide or oxide melts which accumulate beneath the silicates or are injected into country rocks or extruded on the surface.

Cu-Ni ores of Sudbury, Canada and the nickel extrusives of Kambalda, West Australia.

Pegmatitic Deposition

Crystallization as disseminated grains or segregations in pegmatites.

Li-bearing pegmatites of Kings Mtn. N.C.

Hydrothermal

Deposition from hot aqueous solutions of various sources.

Porphyry Cu-Mo deposits of the
W. Cordillera.

Lateral Secretion

Diffusion of ore and gangue forming materials
from the country rocks into faults and other structures.

Gold deposits of Yellowknife, B.C. and the Mother Lode, CA.

Metamorphic Processes

Pyrometasomatic (skarn) deposits formed by replacement of wall rocks adjacent to an intrusive.

W deposits at Bishop, CA. Fe deposits
Iron Mtn UT.

Initial or further concentration of ore elements by metamorphic processes.

Homestake Au Mine, Lead, South Dakota.

 

Origin Due to Surface Processes

Mechanical Accumulation

Concentration of heavy minerals into placer

Placer Au deposits of Alaska and California.

Sedimentary Precipitation

Precipitation of certain elements in sedimentary environments.

Banded Iron Fm.
of the Canadian
Shield.

Residual Processes

Leaching of soluble elements leaving concentrations of insoluble elements.

Nickel laterites
of New Caledonia
and Arkansas bauxite.

Secondary or Supergene Enrichment

Leaching of certain elements from the upper part of a mineral deposit and their reprecipitation at depth to produce higher concentrations.

The upper portion of many porphyry copper
deposits.

Volcanic Exhalative Process

Exhalations of sulfide-rich magmas at the surface, usually under marine conditions.

Mt. Isa, Aust., Sullivan and Kidd Creek,Canada, Kuroko,Japan.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


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