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UPSC MainsGEOLOGY-PAPER-II201515 Marks
Q23.

Explain the process of formation of magmatic sulphide deposit. What is the characteristic mineral assemblage of magmatic sulphide deposit?

How to Approach

This question requires a detailed understanding of ore genesis, specifically focusing on magmatic sulphide deposits. The answer should begin by explaining the processes involved in their formation, including magma source, sulphide saturation, and concentration mechanisms. Subsequently, it should detail the typical mineral assemblage found in these deposits, highlighting the economically important minerals. A structured approach, dividing the answer into formation processes and mineralogy, will be most effective. Diagrams can be used to illustrate the processes.

Model Answer

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Introduction

Magmatic sulphide deposits are a significant source of critical metals like nickel, copper, platinum group elements (PGEs), and cobalt, essential for modern technologies. These deposits form through the concentration of sulphide minerals from a silicate magma. The process is intrinsically linked to the evolution of magmas in the Earth’s crust and mantle, and understanding their formation is crucial for effective exploration and resource management. These deposits are typically associated with mafic and ultramafic intrusions, but can also occur in association with felsic systems.

Formation of Magmatic Sulphide Deposits

The formation of magmatic sulphide deposits is a multi-stage process involving several key steps:

1. Magma Source and Sulphur Content

The source magma plays a critical role. Mafic and ultramafic magmas derived from the mantle are generally richer in sulphur than felsic magmas derived from the crust. Sulphur enters the magma through assimilation of crustal sulphur-bearing rocks (like sediments) or directly from the mantle. The initial sulphur content is crucial for subsequent sulphide saturation.

2. Sulphide Saturation

Sulphide saturation occurs when the magma can no longer dissolve additional sulphide minerals. This can happen through several mechanisms:

  • Cooling: As magma cools, its solubility for sulphur decreases, leading to saturation.
  • Pressure Decrease: A decrease in pressure (e.g., during ascent) also reduces sulphur solubility.
  • Magma Mixing: Mixing of sulphur-rich and sulphur-poor magmas can induce saturation.
  • Fractional Crystallization: Removal of early-formed silicate minerals can increase the proportion of remaining sulphur, eventually leading to saturation.

3. Sulphide Segregation and Accumulation

Once sulphide saturation is achieved, several processes lead to the segregation and accumulation of sulphide liquids:

  • Immiscibility: Sulphide liquids are immiscible with silicate melts, meaning they separate into distinct phases, similar to oil and water.
  • Gravity Settling: Sulphide liquids are denser than silicate melts, causing them to settle towards the bottom of the magma chamber. This is a primary concentration mechanism.
  • Convection Currents: Convection within the magma chamber can also aid in the transport and accumulation of sulphide liquids.
  • Flow and Channeling: Sulphide liquids can flow along pathways created by magma movement, concentrating in specific areas.
  • Replenishment: Repeated injections of fresh magma can replenish the sulphide liquid supply and promote further concentration.

4. Post-Magmatic Processes

After initial accumulation, post-magmatic processes can further modify the deposit:

  • Hydrothermal Alteration: Interaction with hydrothermal fluids can alter the sulphide minerals and introduce new elements.
  • Remobilization: Sulphide minerals can be remobilized and re-concentrated by later hydrothermal activity.

Characteristic Mineral Assemblage of Magmatic Sulphide Deposits

Magmatic sulphide deposits exhibit a characteristic mineral assemblage, varying based on the magma composition and evolution. The typical assemblage includes:

1. Primary Sulphide Minerals

  • Pyrrhotite (Fe1-xS): The most abundant sulphide mineral, often constituting a significant portion of the deposit.
  • Pentlandite ((Fe,Ni)9S8): The primary nickel-bearing sulphide mineral.
  • Chalcopyrite (CuFeS2): The main copper-bearing sulphide mineral.
  • Pyrite (FeS2): Common, but generally less economically important than pyrrhotite and pentlandite.

2. Platinum Group Minerals (PGMs)

These are highly valuable and often occur in association with pentlandite. Common PGMs include:

  • Platinum (Pt)
  • Palladium (Pd)
  • Osmium (Os)
  • Iridium (Ir)
  • Ruthenium (Ru)
  • Rhodium (Rh)

3. Accessory Minerals

Other minerals commonly found in these deposits include:

  • Olivine ((Mg,Fe)2SiO4): Often present as a silicate component.
  • Pyroxene ((Mg,Fe,Ca)2Si2O6): Another common silicate mineral.
  • Amphibole: Present in some deposits, particularly those associated with more evolved magmas.
  • Apatite (Ca5(PO4)3(OH,Cl,F)): A phosphate mineral.

The relative abundance of these minerals varies depending on the specific deposit. For example, the Norilsk-Talnakh deposits in Russia are renowned for their exceptionally high PGE content, while the Sudbury Basin in Canada is known for its large nickel-copper sulphide deposits.

Conclusion

Magmatic sulphide deposits represent a crucial source of strategically important metals. Their formation is a complex process governed by magma properties, saturation mechanisms, and segregation processes. Understanding the characteristic mineral assemblage is vital for exploration and resource evaluation. Continued research into the genesis of these deposits will be essential for ensuring a sustainable supply of these critical materials in the future.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Sulphide Saturation
The point at which a magma can no longer dissolve additional sulphide minerals, leading to the exsolution of a separate sulphide liquid phase.
Immiscibility
The inability of two or more liquids to mix to form a homogeneous solution. In the context of magmatic sulphide deposits, it refers to the separation of sulphide liquids from silicate melts.

Key Statistics

Global nickel reserves are estimated to be around 94 million metric tons (as of 2023), with a significant portion contained within magmatic sulphide deposits.

Source: US Geological Survey, Mineral Commodity Summaries 2024

South Africa holds approximately 70% of the world’s known reserves of platinum group metals (PGMs), largely concentrated in the Bushveld Igneous Complex, a major magmatic sulphide deposit.

Source: Johnson Matthey, Platinum Group Metals Review 2023

Examples

Sudbury Basin, Canada

The Sudbury Basin is a world-class example of a magmatic sulphide deposit, formed by the impact of a large asteroid approximately 1.85 billion years ago. This impact generated a large magma chamber, leading to the formation of massive nickel-copper-PGE sulphide deposits.

Frequently Asked Questions

▶What is the difference between orthomagmatic and stratiform sulphide deposits?

Orthomagmatic deposits are directly related to the crystallization of a primary magma, typically associated with mafic/ultramafic intrusions. Stratiform deposits, while also magmatic in origin, are often associated with layered intrusions and exhibit a more distinct layered structure.

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