GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 290-6
Presentation Time: 2:55 PM


MARTIN, Alec J., KEITH, Jeffrey D., CHRISTIANSEN, Eric H., KOWALLIS, Bart J., JENSEN, Collin G., HENZE, Porter, CHADBURN, Ryan, MARTIN, Samuel G. and WEBB, Haley D.M., Department of Geological Sciences, Brigham Young University, Provo, UT 84602

Hosted within the Oligocene Little Cottonwood stock (LCS) of central Utah are two younger granitic phases, the White Pine and Red Pine porphyries. Whole rock compositions, zircon U-Pb ages, and titanite compositions suggest that rather than having differentiated from the LCS magma, the White Pine was generated from a separate magma pulse, as was the Red Pine ~2 m.y. later.

Low grade Mo-W mineralization and associated alteration form a roughly concentric pattern centered on the Red Pine. Molybdenite is limited to the area of the Red Pine, more widespread scheelite mineralization cuts across portions of the White Pine, and phyllic-QSP alteration extends even farther into portions of the LCS. Magmatic titanite crystallized in each of the intrusive phases and, due to slow diffusion rates and high closure temperature, retained element patterns characteristic of the parental magma. Thus, titanite in diverse clasts in phreatomagmatic pebble dikes associated with the mineralization has distinctive geochemical fingerprints that allow us to identify clasts from both the LCS and the White Pine.

Magmatic titanite and hydrothermal overgrowths attest to relatively oxidized conditions during their formation. On the other hand, the breakdown of White Pine and Red Pine titanite to quartz, apatite, ilmenite, rutile, allanite, and monazite and the absence of magnetite or hematite (as seen in altered titanite in the LCS), along with the lack of wolframite and powellite, are evidence for lower fO2 at the time of alteration.

Titanite as an indicator mineral for metal fertility in intrusive bodies has also attracted attention in the last decade. However, complex intrusion patterns can obfuscate what metal concentrations in titanite indicate about economic potential. Moreover, care must be taken to distinguish between magmatic and hydrothermal zones of titanite which have significantly different compositions. LA-ICP-MS data on BSE- and EMPA-confirmed magmatic titanite shows that in the barren LCS, titanite averages ~40 ppm Mo and can reach over 60 ppm, whereas in the mineralizing Red Pine, titanite averages ~25 ppm Mo and rarely reaches above 40 ppm. This gives an incorrect indication of relative metal fertility in the LCS and Red Pine phases, which advises caution in the use of titanite as an indicator mineral.