A fair amount of magnetite occurs both in unaltered and specific altered facies of granodiorite porphyry which comprises a major host rock of the porphyry copper deposit at Michiquillay, Peru. Magnetite occurs as sporadically disseminated discrete grains in unaltered rocks, and both as disseminated discrete grains and veinlet fillings in altered rocks.
Microprobe analysis of minor elements (Ti, V, Al) of magnetite has been carried out for 17 specimens of unaltered and altered rocks. The alteration types are petrographically determined in the hand specimens and are classified into 4 categories; unaltered, potassic, incipiently retrograded potassic, and retrograded potassic-I. The latter two are interpreted to have been formed by the superimposition of the later stage alteration on the earlier potassic assemblage.
The results obtained are as follows: (1) Remarkable differences are observed between magnetite in unaltered rocks and that in altered rocks in the contents of minor elements and in the occurrence of titanium. (2) fairly systematic changes in minor elements are observed among the groups of magnetite from different alteration types.
The writer suggests, though further studies and confirmation are necessary, that these mineralogical characteristics might be applicable for exploration purposes as a tool to locate and distinguish altered and unaltered facies, and to establish alteration zones and/or sub-zones, even in case that hypogene alteration minerals have been obliterated by the superimposition of supergene-or later stage-alteration.
