Oxidation Condition and Metal Fertility of Granitic Magmas: Zircon Trace-Element Data from Porphyry Cu Deposits in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) hosts a number of porphyry Cu deposits, all associated with calc-alkaline granitic rocks and ranging in size from giant to small. Major- and trace-element compositions of whole rocks and zircons grains were measured from 13 ore-bearing intrusions in nine porphyry Cu deposits (with 0.6 to 12 Mt Cu), including Bozshakol, Nurkazghan, Kounrad, Borly, Aktogai, and Koksai in Kazakhstan, Baogutu, and Tuwu-Yandong in China, and Erdenet in Mongolia. All zircon grains show high Ce 4+ /Ce 3+ ratios, ranging from 29 to 592. Higher Ce 4+ /Ce 3+ ratios are recorded at a given crystallization temperature from deposits with larger Cu tonnages. Large (>4 Mt Cu) and intermediate (1.5–4 Mt Cu) size porphyry Cu deposits are associated with granitic intrusions that have zircons with Ce 4+ /Ce 3+ ratios greater than 120. There is also a clear relationship between calculated log ( f O 2 ) values and the size of deposits, with NNO + 2 values separating large and intermediate porphyry deposits from small deposits. The data of zircon Ce 4+ /Ce 3+ ratios and associated oxygen fugacity values in magma from ore-bearing intrusions indicate that more oxidized magmas are associated with the formation of larger porphyry Cu deposits. Such a conclusion may potentially be used in regional exploration for porphyry Cu deposits in the CAOB.