Deep Hydrothermal Alteration in Porphyry Copper Systems: Insights from the Laramide Arc
AuthorRunyon, Simone Elizabeth
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PublisherThe University of Arizona.
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AbstractMultiple generations of normal faults dismembered, tilted, and exposed thicknesses of up to 15 km of the upper crust in portions of central and southern Arizona. This extension, variable in distribution and magnitude, was superimposed on the axis of the Laramide magmatic arc and dismembers many porphyry copper systems, allowing for detailed study of vertical and lateral zonation of alteration around these centers. This study examines tilted fault blocks containing portions of porphyry systems across Arizona, focusing on hydrothermal alteration deep and distal in these systems (3+ km paleodepth) to develop a more complete understanding of porphyry occurrences as larger geochemical systems. This study focuses on Na-Ca and coarse muscovite alteration in the roots of Laramide porphyry copper systems across Arizona (Ajo, Sierrita, Kelvin-Riverside, Mt. Grayback, Granite Mountain, Charleston, Globe-Miami, Sycamore Canyon, Copper Basin, Texas Canyon, and Copper Creek), provides a detailed study of Middle Jurassic coarse muscovite alteration at Luhr Hill in the Yerington district, Nevada, and documents the structural and hydrothermal evolution of the Ajo mining district in southwestern Arizona. Most areas in this study are interpreted to be highly extended, highly eroded, or both, allowing for study of deep hydrothermal alteration. Na-Ca alteration has been previously documented extensively along the Jurassic arc of the southwestern United States but less widely known in younger plutons, notably of Laramide age in Arizona. Coarse muscovite alteration previously has rarely been documented in porphyry copper systems, and this study shows that coarse muscovite alteration is likely present in systems where root zones are exposed at surface. Na-Ca alteration also is present in many in Laramide porphyry systems, though volumetrically minor, as no Laramide system contains more than a few volume perfect Na-Ca alteration in a given hydrothermal system. Na-Ca alteration in Laramide systems can include Ca, Na-Ca and Na alteration but is dominated by Na alteration (epidote-albite-chlorite ± actinolite). At Ajo, both Na-Ca and coarse muscovite alteration are present within the district due to superposition of temporally unrelated hydrothermal alteration, coupled with complex extensional deformation. This study shows that both Na-Ca and coarse muscovite alteration are more common in Laramide porphyry copper systems than previously recognized, that Na-Ca alteration is most commonly developed as shallower Na alteration (albite-epidote-chlorite ± actinolite), common deeper Na-Ca alteration (oligoclase-actinolite-epidote), and rare, deep Ca alteration (oligoclase-diopside-actinolite ± garnet ± epidote). Na-Ca alteration is commonly less voluminous in Laramide systems than documented in systems along the Jurassic arc. Coarse muscovite alteration, commonly termed greisen, occurs structurally below and commonly postdates potassic alteration and likely formed from late-stage, low-temperature, magmatic-hydrothermal fluids. Coarse muscovite alteration associated with more silicic magmatic compositions is developed at shallower depths and contains muscovite with higher trace element contents, coarse muscovite alteration with more variable mineral assemblages, and coarse muscovite veins that are better mineralized. Coarse muscovite alteration (greisen) occurs as the main mineralized veins at the tops of evolved metaluminous to peraluminous granites in W-Sn systems, as well mineralized veins in the cores and tops of Mo-Cu porphyry systems, and as poorly mineralized veins in the roots of porphyry copper systems. Detailed understanding of coarse muscovite alteration in a given district can, therefore, can be an indicator of depth or petrologic affiliation of a system. These results provide a better understanding of late-stage magmatic-hydrothermal alteration and hydrothermal alteration associated with the incursion of external fluids into the root zones of porphyry copper systems.
Degree ProgramGraduate College