GENESIS OF THE KASKANAK BATHOLITH, AND THE ROLE OF TITANITE AND OTHER ACCESSORY MINERALS ON TRACE ELEMENT DISTRIBUTIONS

The Kaskanak batholith (89-91 Ma) is a composite granodiorite complex associated with the giant Pebble porphyry Cu-Au-Mo deposit in SW Alaska. Careful petrologic studies and drill core logging have allowed for identification of several generations of porphyry dikes associated with ores, and chronologic studies suggest that 2-3 pulses of magmatism and hydrothermal activity associated with the batholith occurred over a span of 2 m.y. Geochemical zonations of accessory mineral phases record late-stage magmatic processes and provide supporting evidence for a complex magmatic history.

Mass balance calculations of Nb, Ta, and REEs in accessory mineral phases suggest that titanite is the dominant sync for these trace elements. Late-stage magmatic differentiation of these trace elements in the melt is therefore governed by titanite crystallization and fractionation. Generally, core to rim MREE profiles of titanite differentiate from convex upward profiles with negative Eu anomalies to strongly depleted concave patterns with neutral or positive Eu anomalies. These zonations generally correspond with decreases in core to rim calculated Zr-in-titanite temperatures from 760-700˚C and depletion in Nb and Ta. The preferential removal of REE³⁺ and exclusion of Eu²⁺ by titanite is reflected in whole rock compositions of differentiated porphyry dikes associated with mineralization and may have important implications for the interpretation of the Eu anomaly (e.g., utility as a redox-sensitive tracer).

However, accessory mineral rims also record complex zoning patterns. Late-stage geochemical fluctuations in some titanite rims record replenishment of Nb and Ta, enrichment in Th and U, and modification of the Nb/Ta and Th/U ratios. These reversely-zoned trace elements fluctuations are broadly correlative with increased Zr-in-titanite temperatures and may suggest magmatic recharge or periodic injections by small volumes of andesitic melts. Small volumes of andesite porphyry have been identified on the flanks of the Kaskanak batholith and may have provided the necessary sulfur and trace metals to form a world class porphyry deposit.