Because tungsten minerals are heavy, exploration can be done using stream sediment samples. Prospectors looking for gold have found scheelite or wolframite in their placer concentrates, and in some cases this has led them to make important tungsten finds.

Geology is a science of general tendencies with frequent exceptions, not one of hard and fast rules that are always true and never vary. For almost every well-accepted rule of gold deposits, I can point you to a number of important exceptions.