| Description |
The similar timescales and field relationships in magmatic systems suggest common processes, whether the system is mineralized or has associated volcanism. Though often overlooked because of alteration and their relatively small size, the magmatic-hydrothermal environment may prove key to a holistic model of magmatic systems. Seven age distributions from the Morgan Creek pluton were determined by LA MC-ICP-MS U-Pb zircon analysis and the ages of six molybdenite samples from the adjacent Pine Creek tungsten skarn were determined by Re-Os molybdenite analysis to examine the timing, duration, and evolution of a mineralized magmatic system. The molybdenite ages range from 91.0 to 107.2 Ma. Ages for three of the six samples are identical within error (±0.5 Ma) at ~94 Ma. Ages of 91.0 and ~94 Ma may represent distinct mineralizing events, whereas the oldest two ages from disseminated molybdenite in the skarn may indicate disturbance of the Re-Os system. The U-Pb data show dominant populations at ~96-98 and ~94 Ma. The main facies of the Morgan Creek pluton, comprising over 95% of the pluton, can be divided into three groups based on mafic mineral abundance and whole rock major and trace element geochemistry. Major element geochemistry is very similar to that of plutons in nearby intrusive suites. However, trace element geochemistry (Ba, Rb, Sr, Zr, and REE) suggest a distinct genesis from typical intrusive suite magmas. Trace element models suggest the magmas that formed the Morgan Creek pluton were derived from partial melting of upper crustal rocks. The Morgan Creek pluton was emplaced either as a single batch of magma, or through multiple rapid pulses that led to a single magma body. Fractional crystallization of a relatively homogenous volume of magma led to the chemical variations seen within the pluton. The age and compositional data show that the Morgan Creek pluton is not a part of the Lake Edison Granodiorite, as was previously interpreted. Instead, the Morgan Creek pluton represents an early stage of the John Muir Intrusive Suite, possibly correlative with the Basin Mountain pluton which is commonly interpreted to be part of the Lake Edison Granodiorite. |
