The correct option is Deposition of heavy minerals by flowing water.
Explanation
Placer deposits are a specific type of mineral accumulation formed by surface weathering and the subsequent transport and concentration of heavy minerals by natural agents, primarily flowing water.
Analysis of Options:
- Deposition of heavy minerals by flowing water is Correct: Placer deposits occur when heavy, chemically resistant minerals (such as gold, silver, platinum, and tin) are eroded from their source rocks. As flowing water transports the sediment, these heavier minerals settle out of the current due to their high specific gravity, accumulating in riverbeds, sands, or valley floors. This process is known as gravity separation.
- Volcanic eruption and solidification is Incorrect: Volcanic eruption and solidification lead to the formation of igneous rocks. Minerals found in cracks, crevices, faults, or joints of these rocks are typically referred to as veins and lodes (e.g., copper, zinc, lead), not placer deposits.
- Metamorphism of limestone rocks is Incorrect: The metamorphism of limestone results in the formation of marble. This process involves the recrystallization of carbonate minerals under heat and pressure, unrelated to the sedimentary concentration of heavy metals.
- Evaporation of saline water in arid regions is Incorrect: The evaporation of saline water in arid regions leads to the formation of evaporite deposits, such as gypsum, potash, and common salt, rather than metallic placer deposits.
Key Takeaway: Placer deposits are alluvial accumulations of heavy, non-corrosive minerals like gold and platinum, concentrated by the mechanical action of flowing water in riverbeds and sands.