Magmatic Sources of Copper in Porphyry Copper Deposits

During crystallization of a granitic magma, copper is shown to prefer octahedral sites, be they in the melt or in crystalline phases. Burns and Fyœe (1964) showed that the AleOa/(KeOq- Na._,O q-CaO) ratio of the magma is proportional to the number of octahedral sites available in the melt. A relatively large alumina/alkali ratio will mean more octahedral sites in the melt, a higher retention of copper in the melt, and, thus, a greater likelihood of porphyry-type mineralization, as the copper will still be in the melt at the hydrothermal stage. A comparison of this alumina/alkali ratio for mineralized and barren intrusives in the southwestern U.S. porphyry belt and in the Caribbean indicates that this relationship holds true. Generally speaking, for Laramide intrusives, mineralized stocks have a normative corundum content 2.5 weight percent greater than that of barren intrusives. This result suggests that in composite intrusives the more aluminous ones are better targets for porphyry mineralization and that early formed ferromagnesian minerals from mineralized intrusives should be copper deficient compared to barren systems (Kesler et al., 1975a)