MULTI-STAGE CONSTRUCTION OF THE LITTLE COTTONWOOD STOCK, UTAH: INTRUSION, VENTING, MINERALIZATION, AND MASS MOVEMENT

The Little Cottonwood Stock (LCS) is an Oligocene, calc-alkaline granitic intrusion in Utah’s Wasatch Mountains. The stock contains smaller, Mo-W mineralized intrusions at the apex: the Red Pine Porphyry (RP) and the White Pine intrusion (WP) (Sharp, 1958). It hosts various compositions of crosscutting aplite and rhyolite dikes, pebble dikes, and mafic enclaves. Our study is to determine the likely origin and age of the various lithologies, the architecture and development of the pluton, and the connection to the proposed East Traverse Mountain mega-landslide.

The RP is the most differentiated intrusive unit, based on ME and TE geochemistry. It is almost universally associated with strong QSP (quartz-sericite-pyrite) alteration, and is manifest in three forms with similar trace element geochemistry, but different textures: phaneritic, aphanitic-porphyritic, and Mo mineralized. All three contain large quartz phenocrysts, few mafic phases, and substantial pyrite. Zircons yield U-Pb ages of ~25.8-27.2 Ma.

The WP is similar to the LCS in texture, but lacks hornblende and titanite visible in hand sample. It is usually subjected to QSP alteration. Defining it with chemical and mineralogical parameters is difficult, and QSP overprint obscures many igneous features. However, Fe/Al ratios in titanite confirm this was a separate magma from the LCS. U-Pb ages of likely WP samples range from 26-26.9 Ma.

U/Pb ages and cross-cutting relationships of pebble dikes and mineralization help constrain the construction of this pluton. The LCS intruded 34-30 Ma, followed by scattered pulses of WP and then RP, until the system mostly solidified ~26 Ma. Before complete solidification, pebble dikes sampling multiple lithologies were formed in association with the emplacement of the WP-RP stocks, probably with volcanic activity as well, based on quench textures in the RP. QSP alteration, concurrent with continued pebble dike emplacement, mineralized the entire apex of the intrusion. Eventually the hydrothermal system cooled to cause propylitic and then argillic alteration in a wider halo than the QSP. This alteration may have weakened the overlying sedimentary and volcanic rocks, contributing to the proposed East Traverse Mountain mega-landslide.