Geochemistry and geochronology of the intrusive rocks of the central Wasatch Mountains igneous belt, Utah, USA: implications for porphyry mineralization

The central Wasatch Mountains of Utah host a number of mineralized intrusions including the White Pine Fork and Park Premier porphyry deposits. The western stocks of the central Wasatch igneous belt (WIB) include the Clayton Peak, Alta and Little Cottonwood stocks, and the eastern stocks are the Flagstaff, Mayflower, Ontario, Glencoe, Valeo, Pine Creek and Park Premier stocks. New geochronological results comprising zircon LA-ICP-MS ²³⁸U-²⁰⁶Pb Concordia intercept ages and molybdenite TIMS ¹⁸⁷Re-¹⁸⁷Os ages show that the igneous rocks of the central Wasatch Mountains were emplaced in at least two separate events between ~36 to 29 Ma. The oldest phase is associated with the easternmost intrusions, including the Valeo (35.38 ± 0.66 Ma), Flagstaff (35.00 ± 0.64 Ma) and Pine Creek (34.85 ± 0.65 Ma) stocks. The easternmost Park Premier stock has a slightly younger age of 33.69 ± 0.79 Ma. Ignimbritic flows that form part of the Keetley volcanic rocks and igneous clasts found within the Keetley Volcanics are coeval with this phase. A volcanic clast, an ignimbrite, and a flow were sampled separately to yield zircon ages of 35.38 ± 0.66 Ma, 34.9 ± 0.44 Ma, and 34.6 ± 0.39 Ma, respectively. The Clayton Peak stock, which is located in the centre of the suite of stocks, is similar in age to the eastern stocks (34.64 ± 0.73 Ma). The Alta stock, which is located west of the Clayton Peak stock in the central-west part of the district, has a younger crystallization age (32.82 ± 0.65 Ma). The westernmost intrusion, the Little Cottonwood stock, has the youngest crystallization dates (three different samples of the stock yield ages of 29.91 ± 0.56, 29.63 ± 0.52, and 29.45 ± 0.54 Ma). The White Pine intrusion is a late crystallizing phase of the Little Cottonwood Stock. This intrusion hosts Mo mineralization in a breccia with granitic clasts that yield a U-Pb age of 26.61 ± 0.47 Ma. The Mo mineralization in the breccia yields a Re-Os age of 30.21 ± 0.14 Ma with a repeated age of 29.84 ± 0.15 Ma. Duplicate ages (25.30 ± 11 and 28.3 ± 0.13 Ma) were obtained from a second sample of the Mo mineralization.

The western stocks (Little Cottonwood, Alta, and Clayton Peak) range in composition from diorite through to granite (53–74 wt% SiO₂). The eastern stocks (Flagstaff, Mayflower, Ontario, Park Premier, Pine Creek and Valeo) and Keetley volcanics have a narrower compositional range (59-65 wt% SiO₂). All rocks sampled are calc-alkaline in composition. The Clayton Peak stock has the most mafic composition; whereas, the youngest of the western stocks, the Little Cottonwood stock, is the most felsic. This variation in composition is consistent with increasing melt evolution with decreasing age. The igneous stocks have εNd(t) compositions that range from εNd(t) = 12.8 (Alta stock) to εNd(t) = 18.5 (White Pine intrusion). The easternmost samples display the most isotopically-enriched compositions, with fSm/Nd(t) values of -0.58 to 0.56, compared to the rest of the igneous stocks that range from -0.53 to -0.49.

The geochemical data suggest a more complex evolution for the porphyry-related intrusions of the WIB than previously recognised. The western stocks evolved to more felsic compositions over time with increased crustal contamination not observed in the eastern stocks. The increased contamination in the western stocks over time suggests that the magmas were ponding at depth and assimilating older crust, consistent with their greater depth of emplacement. Increasing (La/Yb)cn ratios in the youngest and most felsic rocks, which were emplaced during regional crustal thinning, suggest a deeper melt source and identify fertile rocks for porphyry mineralization.