"Guides" to potentially economic porphyry copper deposits are essential in their preliminary appraisal. Secondary biotite in dioritic host rocks and significant amounts of copper in stream sediments are the most significant in the early assessment of these deposits.

It is generally being recognized that zones with strong secondary biotite is generally coincident with ore—grade porphyry copper mineralization. Secondary biotite is one of the diagnostic minerals of the innermost zone, the zone of potassic alteration. Case A, B and C show the close association between areas of intense secondary biotite alteration and strong copper mineralization.

Although stream sediment sampling. particularly for cold extractable copper, is fairly established as an efficient and economical tool for prospecting for porphyry copper deposits, there is still need for more geochemical data particularly those pertaining to hot extractable copper and total copper in stream sediments in and around mineralized areas. Case B and C indicate that HxCu and total copper in stream sediments over mineralized zones is generally greater than 1000 ppm. Copper in sediments outside of mineralized areas is generally less than 1000 ppm.

The presence of these features over a wide area suggests deposits with economic potential.