ANALYSIS OF TITANITE IN AN OLIGOCENE GRANITIC INTRUSIVE COMPLEX IN CENTRAL UT: IMPLICATIONS FOR MAGMATIC AND HYDROTHERMAL SYSTEM EVOLUTION AND MO-W MINERALIZATION

The Oligocene Little Cottonwood stock (LCS) of central Utah hosts two smaller, younger granitic phases: the White Pine intrusion (WP) and Red Pine (RP) porphyry. Low grade Mo-W mineralization and associated alteration form a crudely concentric pattern centered on the RP and several WP-hosted breccia pipes. Rather than differentiating from the ~30 Ma LCS magma, both the WP and RP were generated from separate magma pulses ~3 and ~4 Ma later, respectively. However, discriminating between the WP and RP intrusions is difficult due to similarities in whole rock composition and/or overprinting by alteration. Magmatic titanite crystallized in all three of the intrusive phases and, due to slow diffusion rates and a high closure temperature, retained element patterns characteristic of the parental magma pulse. Thus, electron microprobe and LA-ICP-MS analysis of titanite has been used to “fingerprint” respective parental magmas.

Despite their classifying differences, the geochemical similarities between WP and RP magmatic titanite are nonetheless further evidence of their close petrogenetic relationship first revealed by extensive whole rock data and zircon ages (Jensen, 2020). The presence of resorbed ilmenite nucleation core (RINC) textures in titanite from all three intrusions suggests the existence of a common feeder chamber at depth that witnessed the injection(s) of mafic ilmenite-bearing magma, directly attested to by mafic enclaves found within the LCS. Trends of increasing F and Al and decreasing REE+Y, Mo, Sn, Zr, Sc, Ba, Th, and Hf/Ta within the titanite from the three intrusions likely reflect the general evolution of the feeder chamber. Despite the obscuring proximities in time (1.5-0.5 Ma within error) and space between the WP and RP intrusions, cross-cutting relationships suggest derivation from separate pulses from the lower feeder chamber, and that the WP appears to have been at least mostly crystalized by the time the RP pulse arrived. The majority of the RP intrusion likely crystalized below the WP, causing the overlying WP intrusion to be the primary host for alteration, mineralization, and hydrothermal features such as breccia pipes and pebble dikes, all caused by RP fluids. The intrusive system thus provides insights into magmatic processes that are closely related in space, composition, and time.