Abstract

The Mineral Mountains intrusive complex records the interaction since Oligocene time between magmatic, structural, and hydrothermal processes. These processes continue to the present, as exemplified by the Roosevelt Hot Springs geothermal system, which is structurally controlled and driven by a pluton emplaced during the Pleistocene. The intrusive rocks of the complex show changes in composition from an early calc-alkaline suite (Oligocene) to a slightly alkaline main intrusive sequence (Miocene) that makes up most of the range. At about 9 Ma, the intrusive activity was associated with low-angle faulting. The oldest volcanic rocks are dated at ∼7 Ma. Volcanic activity continued in the region until ∼0.5 Ma and may be active still. Examples of the youngest volcanism include a sequence of rhyolite flows, domes, and pyroclastic deposits that were erupted along the crest of the range between 0.8 and 0.5 Ma. Minor basaltic flow and pyroclastic eruptions also were localized in the northern part of the range.

A major low-angle detachment fault, containing as much as 200 m of cataclasite, is preserved in the southern Mineral Mountains. This fault separates lithologies of the Mineral Mountains intrusive complex from an overlying Paleozoic sedimentary sequence. We propose that this zone developed at the brittle-ductile transition and has been exposed because of the extreme uplift of the range. The range also preserves a series of listric faults which may sole into a major low-angle fault at depth. High-angle normal faults strike both north and east. Both sets have important controls on the flow of fluid in the Roosevelt Hot Springs geothermal field. The east-west structures are presently seismically active.

Hydrothermal activity has been associated with all intrusive events in the Mineral Mountains. Base-metal mineralization accompanied the older events, whereas precious-metal mineralization accompanied the younger.

We propose an evolution of the intrusive complex, involving the interaction of magmetic, structural, and hydrothermal components. From oldest to youngest, each phase of activity reflects emplacement of magma into progressively higher levels of the crust. Uplift reflected by this progression has been quite rapid at times, as shown by fission track dating. As a consequence, the Mineral Mountains constitute a profound structural high in the region. This extreme uplift probably was caused by diapiric rise of this area under the influence of repeated magmatic intrusion.

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