In what environment would you expect to find placer deposits of gold, platinum, and titanium?

• A. placer deposits
• B. ore veins in bedrock
• C. disseminated deposits
• D. bedded cherts

Answer: (A)

Placer deposits form when a transporting medium, typically a river or stream, slows down and drops out the heavier minerals from suspension. Gold, platinum, and titanium minerals are very dense, so they tend to accumulate in the sediments at a river’s bend, in downstream gravels, or along coastal or desert shorelines where currents lose energy. Over time these heavy grains become concentrated in the surrounding sand and gravel, creating a placer deposit.

Other modes—ore veins in bedrock, disseminated deposits, or bedded cherts—involve different processes: hydrothermal fluids filling fractures in solid rock for veins, widespread low-grade sulfide mineralization for disseminated deposits, and layered silica-rich rocks from chemical or biological sedimentation for bedded cherts. These do not describe the gravity-driven concentration of dense minerals that characterizes placers.