Question

Which of the following types of deposits is/are formed by fractional crystallization of magma?

• A. Komatiite hosted Ni-Cu
• B. eridotite hosted Cr
• C. Leucogranite hosted U
• D. Anorthosite hosted Ti-Fe

Hint:

Remember: Magmatic fractional crystallization commonly produces Cr (chromite), Fe–Ti oxides, and P-rich deposits. In contrast, Ni–Cu sulfides form by immiscible sulfide segregation, and U mineralization in granites is usually hydrothermal.

Answer 1

The Correct Option is B, D

Step 1: Understanding fractional crystallization.
- Fractional crystallization is the process where different minerals crystallize from cooling magma at different temperatures.
- As crystallization progresses, early-formed minerals separate from the remaining melt, leading to concentration of specific elements and formation of magmatic ore deposits. Step 2: Evaluate each option.
- (A) Komatiite hosted Ni–Cu:
Komatiite-hosted Ni–Cu deposits are typically formed by sulfide segregation from high-Mg ultramafic magmas, not mainly by fractional crystallization. $\Rightarrow$ Incorrect.
- (B) Peridotite hosted Cr:
Chromite (Cr) deposits in layered peridotite complexes (e.g., Bushveld Complex) are classic examples of fractional crystallization deposits, where chromite crystallizes early from magma. $\Rightarrow$ Correct.
- (C) Leucogranite hosted U:
Uranium mineralization in leucogranites is largely related to partial melting and hydrothermal processes, not dominantly fractional crystallization. $\Rightarrow$ Incorrect.
- (D) Anorthosite hosted Ti–Fe:
Ilmenite and magnetite ores associated with anorthosite bodies are typical results of magmatic fractional crystallization, concentrating Ti–Fe oxides. $\Rightarrow$ Correct.
Step 3: Final Answer.
The deposits formed by fractional crystallization of magma are: \[ \boxed{\text{(B) Peridotite hosted Cr and (D) Anorthosite hosted Ti–Fe}} \]