Correct Response: C. Hydrothermal alteration of minerals in the country rock surrounding a pluton occurs as hot mineral-rich fluids move from a cooling pluton into the surrounding country rock. The process alters the chemistry and makeup of the surrounding rock and can deposit precious metals within fractures as the fluids cool. By adding, removing, and redistributing chemical elements in the surrounding country rock, hydrothermal fluids alter the minerals in the rock and can concentrate precious metals, such as gold and silver. Isostatic readjustments (A) that cause vertical changes in the crust do not directly affect local geochemical processes involved in the formation of economically valuable minerals. Magmatic differentiation changes the magma's composition as minerals crystallize during cooling (B), but does not directly form concentrations of precious metals. Chemical weathering (D) does not occur extensively within county rock and therefore does not play a role in concentrating precious metals found in country rock.
