Abstract

Paleomagnetic and 40Ar/39Ar age-spectrum data from most stocks of the Red Mountain intrusive system, in the northwest Colorado mineral belt, provide an improved understanding of the structural and cooling history of the suite of intrusions host to a world-class molybdenum deposit. Paleomagnetic data from five stocks at the surface and eight younger stocks exposed in the subsurface Henderson Mine support field observations (for example, dike and vein orientations, stock geometries, and distribution of zones of mineralization) that imply moderate tilting (15°-25° down to the east-southeast) since latest Oligocene time after cooling and mineralization. Surface stocks contain magnetizations carried by both magnetite and hematite. The Red Mountain stock is the youngest surface intrusion and contains mostly normal polarity magnetizations (for example, D = 321°, I = 59°, α95 = 19°, k = 9, N = 6 samples, site RM9), whereas older East Knob and Rubble Rock breccia intrusions contain a nearly antipodal, well-characterized magnetization (East Knob stock: declination = 161°, inclination = -47°, α95 = 13°, k = 23, average of five site means). Polarity changed from reverse to normal during emplacement and cooling of the Red Mountain intrusions exposed at the surface. 40Ar/39Ar age-spectrum data on biotite and orthoclase from the Red Mountain stock and stocks of the Henderson Mine indicate the reversal to be older than 30 Ma. All Henderson Mine stocks have normal polarity magnetizations (Primos stock: D = 333°, I = 51°, α95 = 5°, k = 44, average of six site means) which, on the basis of 40Ar/39Ar age spectra from orthoclase and biotite, were blocked between 28.7 and 27.6 Ma. Magnetite and maghemite are the major carriers of magnetization in these rocks.

On the basis of an 40Ar/39Ar thermochronologic study of the Red Mountain intrusive system, thermal activity started at or just before 29.9 ± 0.3 Ma and ended at 26.95 ± 0.08 Ma. The age-spectrum data are interpreted to indicate that the porphyry of Red Mountain, one of the oldest stocks, was emplaced before 29.9 ± 0.3 Ma (possibly before 30.38 ± 0.09 Ma). Nearby lamprophyre dikes were emplaced at 29.8 ± 0.1 Ma; rhyolite dikes intruded at 29.4 ± 0.2 Ma. The Urad and Seriate stocks intruded after 29.8 Ma but before emplacement of the Vasquez stock at 28.71 ± 0.08 Ma. The system core cooled below 280 ± 40 °C (the argon closure temperature of biotite) at 27.59 ± 0.03 Ma. The last period of thermal activity involved pulses of magnetite-sericite alteration around the Seriate stock between 27.51 ± 0.03 and 26.95 ± 0.08 Ma; this activity did not thermally overprint unaltered parts of the intrusive system.

Tilting of the Red Mountain area is implied by a comparison between a grand mean (on the basis of 10 stock means, D = 333°, I= 49°, α95 = 5°, k = 78) and a mid-Tertiary reference field. The Red Mountain intrusive system and host Precambrian rocks probably were deformed along a nearly north-south horizontal axis in response to northwest-side down, strike-slip faulting with displacement largely along the Woods Creek fault zone. Late Tertiary deformation of Precambrian-cored parts of the Front Range, host to numerous mineral deposits, was more complicated than simple, near-vertical uplift of the crust.

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