Petrogenesis of the A.O. Porphyry Copper Complex in Jackson and Grand Counties, Northwestern Colorado
Karimpour, M. H.; Atkinson, W. W. Jr.
published: Jan 1, 1982
ArtNo. ESP136000201008, Price: 19.00 €
The A.O. Complex porphyry copper deposit is located in northwestern Colorado. It consists of four main intrusive units (monzonite), which were emplaced about 23.6 m.y. ago. The alteration consists of a potassic core, which is surrounded at depth by a propylitic zone and above by a sericitic zone. Copper and gold occur in the potassic zone and to a lesser extent with the sericitic zone. Anhydrite occurs mainly within the potassic zone and has a magmatic origin. Major and minor elements indicate that the magma underwent slight differentiation. Relative to chondritic abundances, the light rare-earth elements are enriched. The initial Sr ratio (0.704) and rare-earth elements pattern suggest that the primary magma for the A.O. Complex originated by partial melting of eclogite and was contaminated in the crust. Copper and other metals were incorporated into the magma from mantle sulfides and melting of hornblende. During fractional crystallization, the heavy metals were partitionated preferentially into crystals as traces, so that they never reached a sufficient concentration to form ore. The reasons may be an initial low metal content of the A.O. magma and the chemistry of the magma.