Correct Option
The correct option is Silver.
Explanation
Minerals occur in various geological forms, including veins, lodes, beds, layers, and residual masses. Placer deposits are a specific mode of occurrence where minerals are found as alluvial deposits in the sands of valley floors and at the base of hills. These concentrations are formed by the gravity separation of heavy minerals during sedimentary processes.
Detailed Analysis
For a mineral to occur as a placer deposit, it must possess two key characteristics: high specific gravity and resistance to corrosion by water and weathering.
- Silver (Option a): Along with Gold, Tin, and Platinum, Silver is a classic example of a mineral found in placer deposits. These minerals are chemically inert (do not corrode easily in water) and are heavy enough to settle in riverbeds while lighter sediments are washed away.
- Manganese (Option b): Manganese deposits are typically sedimentary in origin or formed through the decomposition of surface rocks (residual concentration), not as alluvial placers.
- Zinc (Option c): Zinc is generally found in veins and lodes within igneous and metamorphic rocks. It is often associated with lead and does not typically survive the transport processes required to form placer deposits.
- Limestone (Option d): Limestone is a sedimentary rock formed in beds or layers through the accumulation of organic debris or chemical precipitation. It is not a placer mineral.
Key Takeaway: Placer deposits consist of heavy, non-corrosive minerals such as Gold, Silver, Tin, and Platinum, which accumulate in alluvial sands of valley floors.