USING TITANITE COMPOSITIONS TO IDENTIFY INTRUSIVE PHASES AND MO-W MINERALIZATION PATTERNS IN THE OLIGOCENE LITTLE COTTONWOOD STOCK, UTAH

The Oligocene Little Cottonwood stock (LCS) of central Utah hosts two younger granitic intrusions, the White Pine (WP) and Red Pine (RP) porphyries. Low grade Mo-W mineralization and associated alteration form a crudely concentric pattern centered on the RP and several WP-hosted breccia pipes. Previous studies show that, rather than having differentiated from the LCS magma, both the WP and RP were generated from separate magma pulses ~2 m.y. later. However, while there are clear mineralogical and geochemical differences between the LCS and WP/RP, differentiating between the WP and RP intrusions is difficult due to similarities in whole rock composition and/or overprinting by alteration.

Magmatic titanite crystallized in each of the intrusive phases and, due to slow diffusion rates and high closure temperature, appears to have retained element patterns characteristic of the parental magma in primary or unaltered grains. Thus, electron microprobe and LA-ICP-MS analysis of titanite has been used to “fingerprint” their parent magma. Phaneritic RP titanite have lower Ti, Ca, Nb, Mo, and W than WP titanite, while porphyritic RP titanite have still lower concentrations (though higher Mn).

Applied to several samples with ambiguous affinities, these titanite fingerprints have expanded our spatial and temporal understanding of the magmatic system and accompanying mineralization by increasing the known extent of the WP and RP intrusions relative to the LCS, and in showcasing the reduced nature of the hydrothermal fluids via the breakdown of titanite in the WP and RP to minerals such as ilmenite and rutile. In contrast to some Mo-W systems, where titanite Mo values are high (120+ ppm) in the mineralizing granite, Mo values in titanite in the LCS-WP-RP system gradually decrease (averaging 45 to 29 to 18 ppm, respectively) towards the mineralizing RP intrusion. This gives an example of the potential misuse for titanite as an indicator mineral for Mo-W mineralization.