The Cornelia pluton, Ajo, Arizona

Extreme tilting and faulting of the Cornelia pluton has exposed rocks that once lay far below the mineralized apex, as has been pointed out by Gilluly (1946). Petrographic study of specimens from 65 localities in the deeper zone, spaced 0.25 mi apart in a grid, has identified a sequence of temporally discrete intrusive episodes. Lithologic varieties are, from oldest to youngest, quartz diorite, granodiorite, equigranular quartz monzonite, porphyritic quartz monzonite, and porphyritic microquartz monzonite. The shape of the last-named centrally situated body is similar to that of the New Cornelia ore body, and the unit is lithologically correlative with those that predominate in the mine area, suggesting that it represents the conduit.