Final answer:
The statement is true; platinum deposits are typically associated with mafic and ultramafic rocks that originate from the mantle and become part of the crust through tectonic processes.
Step-by-step explanation:
True. Platinum deposits are indeed commonly associated with mafic and ultramafic rocks that rise from the mantle into the crust.
Platinum, a precious metal, is often found in nature alongside certain geologic formations. These formations include mafic and ultramafic rocks, which are rich in magnesium and iron. The earth's mantle, predominantly made up of ultramafic rocks such as peridotite, can sometimes produce mafic magmas that rise to form oceanic crust, composed of gabbro (intrusive) or basalt (extrusive). The association of platinum with these rock types is due to the process of magmatic differentiation and the particular affinity of platinum group elements (PGEs) for sulphide minerals, which are often concentrated in mafic and ultramafic layers.
During the cooling and solidification of these magmas within the earth's crust, platinum can become concentrated in sulphide minerals to form economically significant deposits. Ophiolites, which are sections of the oceanic crust and upper mantle thrust onto the continental crust through tectonic processes, often provide a source for platinum and other PGEs. As these rocks are derived from the mantle, they give us the closest representation of the types of rocks where one might expect to find platinum deposits.
The formation of such deposits is further influenced by specific geological settings. For example, large igneous provinces, which result from the melting of parts of the earth's mantle, can hold substantial concentrations of PGEs thanks to the large volumes of mafic to ultramafic magmas they produce. Also, the subduction of oceanic plates beneath continental plates, followed by collision and uplift during mountain building, can bring these deeper mafic and ultramafic rocks, along with their mineral riches, closer to the surface.