Mineral Deposits & Their Modes of Formations
Almost all Earth materials are used by humans for something. We require metals for making machines, sands and gravels for making roads and buildings, sand for making computer chips, limestone and gypsum for making concrete, clays for making ceramics, gold, silver, copper and aluminum for making electric circuits, and diamonds and corundum (sapphire, ruby, emerald) for abrasives and jewelry.
We define mineral deposits as natural accumulations of minerals in the earth crust, in form of one or several mineral bodies which can be extracted at the present time or in an immediate future. Economic concentrations of metals or other mineral commodities are known as ore. Mineral deposits (metallic, non-metallic and combustible materials) constitute major raw materials for industrial development today, and the demand for these raw materials is ever increasing.
The processes of formation of mineral deposits are grouped into four main types:
- Magmatic Mineral Deposits
- Hydrothermal Mineral Deposits
- Sedimentary Mineral Deposits
- Metamorphic Mineral Deposits
1. Magmatic Mineral Deposits:
Deposits of minerals formed deep in the earth’s crust during the solidification and crystallization of basic or alkaline magma containing high concentrations of valuable minerals. These deposits vary in shape and occur in igneous rocks that are related to them in origin. They are also called as Magmatic Segregations, Magmatic Injections, or Igneous Syngenetic Deposits.
Types:
EARLY MAGMATIC DEPOSITS
- Ore minerals crystallizes earlier than that of rock silicates
- Also known as orthotectic & orthomagnetic
- Forms simultaneously with the host rocks
- Result of: Simple crystallization without concentration
- Segregation of early formed crystals
- Injection of material concentrated elsewhere by differentiation
Example: Carbonatite Deposits: Deposit contains fluorocarbons, rare earth ore minerals. The niobium ore minerals and copper sulfide ore minerals.
LATE MAGMATIC DEPOSITS
- Ore minerals crystallize later than that of rock silicates
- Formed towards the close of magmatic period
- Always associated with mafic igneous rocks
- Result of: Variation of crystallization differentiation
- Gravitative accumulation of heavy residual liquids
- Liquid separation of sulfide deposits
- Example: Ni-Cu/ Co Deposits, Chromite Deposits etc
Model for magmatic deposit formation
2. Hydrothermal Mineral Deposits
Hydrothermal mineral deposits are those in which hot, mineral laden water (hydrothermal solution) serves as a concentrating, transporting, and depositing agent. They are the most numerous of all classes of deposit. The solutions are thought to arise in most cases from the action of deeply circulating water heated by magma. Other sources of heating that may be involved include energy released by radioactive decay or by faulting of the Earth's crust.
Hydrothermal deposits are never formed from pure water because pure water is a poor solvent of most ore minerals. Rather, they are formed by hot brines, making it more appropriate to refer to them as products of hydrothermal solutions. Brines, and especially sodium-calcium chloride brines, are effective solvents of many sulfides and oxide ore minerals, and they are even capable of dissolving and transporting native metals such as gold and silver.
The magma chamber is the heat source for the hydrothermal solutions, which contain both magmatic and meteoric matter
Examples:
- Vein or Lode type Deposits: contains Poly-metallic veins of high-grade Copper, silver, gold, Lead, antimony etc
Gold in Quartz vein
- Porphyry Deposits: Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye. Famous for Copper, Gold and Molybdenum.
Porphyry associated deposits
- Skarn Deposit: Skarns are most often formed at the contact zone between intrusions of granitic magma bodies and carbonate sedimentary rocks such as limestone and dolostone. Skarn deposits host iron, copper, zinc, tin, tungsten, or molybdenum. skarn deposits include Pine Creek Mine (tungsten), Inyo County, California, USA.
Skarn Model
- Volcanogenic Massive Sulphide Deposits (VMS): A type of metal sulfide ore deposit, mainly copper-zinc-Lead which are associated with and created by volcanic-associated hydrothermal events in submarine environments.
Fig. Cross section schematically illustrating the characteristic features of volcanogenic massive sulfide (VMS) deposits
Hydrothermal deposits are categorized according to the depth and temperature at which they formed. hypothermal, mesothermal and epithermal.
- Hypothermal Deposits
Hypothermal deposits are formed at great depths and high pressures and temperatures. Temperatures may range from 300° to 500° Celsius during the formation of such deposits. Metals which may be extracted from hypothermal deposits consist of copper (Cu), molybdenum (Mo), tin (Sn), tungsten (W), gold (Au), and lead (Pb).
- Mesothermal deposit
Mesothermal deposits form at intermediate depths, temperatures, and pressures. Temperatures may range from 200° to 300° Celsius during the formation of such deposits. Metals which are mined consist of copper (Cu), zinc (Zn), silver (Ag), gold (Au), and lead (Pb).
- Epithermal deposits
Epithermal deposits form at shallow depths under relatively low temperatures and pressures. Temperatures during formation may range from 50° to 200° Celsius. Metals which are mined from epithermal deposits include silver (Ag), gold (Au), and mercury (Hg).
3. Metamorphic Deposits
Lateral secretion: Ore deposits formed by lateral secretion are formed by metamorphic reactions during shearing, which liberate mineral constituents such as quartz, sulfides, gold, carbonates, and oxides from deforming rocks, and focus these constituents into zones of reduced pressure or dilation such as faults. This may occur without much hydrothermal fluid flow, and this is typical of podiform chromite deposits.
Metamorphic processes also control many physical processes which form the source of hydrothermal fluids, outlined above.
4. Sedimentary Deposits
Mineral deposits formed during the accumulation of sediment on the bottom of rivers and other bodies of water. According to their place of formation they are divided into river, swamp, lake, sea, and ocean deposits. Depending on the type of sediment accumulation, sedimentary deposits may be mechanical, chemical, biochemical, or volcanic-sedimentary. Sedimentary deposits are conformable with respect to the sedimentary rocks enclosing them. They usually occupy a strictly defined stratigraphic position and occur in the form of layers or flat lenses.
Types of Sedimentary deposits:
1- Allochthonous: Transported or detrital origin. These include Placer type deposits. placer deposit or place is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. Minerals that form placer deposits have high specific gravity, are chemically resistant to weathering, and are durable, as a result, these minerals are concentrated in residual, beach, and stream deposits, thereby forming workable ore deposits
Examples of Placer minerals (also known as heavy minerals):
Cassiterite, Rutile, Gold, ilmenite, chromite, diamond, garnet, Pt, ruby, sapphire, and zircon.
Placer Deposits
2- Autochthonous: Are of three types:
- Biogenic deposits as coal and oil shale, and
- Residual deposits as Bauxites and laterites.
- Chemically precipitated which include the Precambrian Banded Iron Formations, the sedimentary manganese deposits, and SEDEX deposits.
SEDEX Deposits: Sedimentary exhalative deposits (SedEx deposits) are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir (usually the ocean), resulting in the precipitation of stratiform ore.
These vents discharge minerals from the Earth’s interior, forming chimneys that rise to several meters above the seabed.
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About the Author
This article is the third in the "Mineral Monday Series". It is written by Exploration Geologist & Geophysicist, Azeem Khel. By profession, Azeem researchers and facilitates the processes of mineral extraction. Azeem is a member of Destination Gold's Facebook community, and has agreed to share his knowledge, work and, experience in the "Mineral Mondays Series".