TITANITE TRACE ELEMENT TRENDS IN A LOW-F PORPHYRY MO-W DEPOSIT IN THE OLIGOCENE LITTLE COTTONWOOD STOCK, UTAH

Mo and W mineralization occurs in the White Pine porphyry and Red Pine porphyry units hosted in the larger and older Little Cottonwood stock in the Wasatch Range of Utah. Krahulec (1981) estimated ore-grades for this small, low-F porphyry Mo deposit as 0.16% Mo and 0.02% W. Mineralization occurs in a roughly concentric pattern with molybdenite in the center and scheelite more disseminated throughout.

Although the surrounding Little Cottonwood stock is relatively rich in titanite, as proximity to the mineralization center in the Red Pine porphyry increases, titanite becomes increasingly scarce; much of it is altered, and most remnants are partially replaced and rimmed by oxides such as magnetite and rutile. Trace element characteristics in titanite have been known to reveal details on mineralization patterns in granites.

Twenty-three titanite grains in thin sections from two scheelite-bearing samples of the intermediately mineralized White Pine porphyry were analyzed by SEM and LA-ICP-MS. Trace element data revealed three groups of titanite in the system, all of which are distinct from the titanite of the Little Cottonwood stock based on Al/Fe ratios. Along with younger U-Pb zircon ages, this suggests that a separate magma pulse formed the ore-hosting porphyries; cross-cutting relations show that mineralization occurred post-magmatically in the porphyries.

The two largest groups of titanite are (1) clean, relatively unaltered magmatic or primary titanite, and (2) oxidized and variably altered titanite. The oxidation of the titanite led to a depletion of REE, although concentrations of ore metals Mo, W, and Sn remained relatively unchanged. A third, smaller group consists of rim overgrowths and oxidized outliers that likely did not fully equilibrate with the hydrothermal fluid. They are more variable in composition, but have consistently lower concentrations of Mo, W, Sn and REE than either of the other two groups. The textures and compositions suggest that hydrothermal oxidation related to QSP mineralization caused partial loss of REE in titanite’s Ca-site, but Mo, Sn, and W in the Ti-site remained relatively immobile until titanite was completely destroyed and replaced by Fe-Ti oxides. This appears to have released additional ore metals into the mineralizing hydrothermal system.