Development of the Texas Canyon Granite Magmatic-Hydrothermal System, Little Dragoon Mountains, Arizona

The Texas Canyon granite is an Early Cenozoic (57 Ma) two-mica granite present within the Laramide magmatic arc in Arizona in an area modified by superimposed Cretaceous to early Tertiary compression and mid- to late Cenozoic extension. It is simultaneously associated with two distinct metallogenic regimes present in the western Cordillera of North America: lithophile element (W) mineralization and greisen (quartz + muscovite ± fluorite ± apatite) alteration associated with two mica granites, and porphyry-Cu style mineralization with potassic alteration typically associated with biotite-hornblende bearing metaluminous granites. The dual presence of both copper and tungsten ore deposits makes it important to recognize the controls and genetic factors which made the Texas Canyon Granite favorable for both episodes of mineralization. This study characterizes the unusual magmatic-hydrothermal system associated with the Texas Canyon granite through detailed mapping, structural analysis, petrography and, mineral and whole rock chemistry. Palinspastic reconstruction and removal of Cenozoic normal faulting, tilting is further presented to reveal the original geometry of the system at the time of its formation. The main megacrystic phase of the Texas Canyon Granite is mineralogically and chemically homogenous over its entire exposed area of ~ 7 x 11 km. Main phase Texas Canyon Granite is transitional in both major element, biotite chemistry, and fO2 as implied by biotite Fe/Fe+Mg+Mn between Laramide granites associated with porphyry Cu-Mo mineralization and highly evolved two mica granites associated with lithophile element mineralization. Its mineralogy shows ubiquitous sub-solidus equilibration with development of two generations of Fe-Ti oxides representing distinct temperature regimes (~700°C and ~500°C), secondary white mica, and feldspars indicating low temperatures of formation (250-350°C). Main phase Texas Canyon Granite was intruded by several generations of late magmatic dikes with consistent cross-cutting relationships. The most voluminous are aplitic dikes which show a continuous gradation between early Bt + Ms to later Ms ± Gt. Late magmatic dikes show N-S, E-W and NE-SW orientations aligned to a regional stress field imposed by the waning stages of Laramide compression with local modification by intrusion-centered stress. REE patterns of Bt + Ms dikes and Ms ± Gt dikes evidence progressive differentiation of late-stage melts by fractional crystallization. Bt + Ms aplites become increasingly subordinate to Ms ± Gt aplites going from NE to SW in the pluton, evidencing diachronous opening of tensional fractures sourcing increasing evolved late stage melts. Swarms of late magmatic dikes are concentrated in a 1.5 km wide zone which trends NE-SW and is oriented along strike with a large porphyry Cu deposit on the pluton’s eastern margin. Late greisen (Qtz + Ms + Py ± Fl) veins are present throughout the pluton at low background abundance (1-2/m), with scattered 10-200 m wide, NE-SW and E-W trending vein swarm zones localizing pervasive greisen alteration. NE-SW oriented swarm zones in particular localize hubnerite ± scheelite mineralization. Greisen veins are dominantly oriented NE-SW with lesser NNE-SSW and WNW-ESE orientations, related to the regional Laramide compressional stress field with transient flips in σ1 after release of tensional stress and magmatic fluid pressure with fracture formation. Chemistry of greisen white micas is distinct compared to sub-solidus white micas in nominally unaltered Texas Canyon Granite, having higher F concentration and greater component of celadonite. Distinct white mica chemistry is recognized between greisen vein swarms associated with abundant W-mineralization and barren vein swarms, with those in mineralized systems having a lower component of celadonite for a given F concentration. Mineralized vein swarms also have white micas with conspicuously low Fe/Fe+Mg+Mn ratios relative to barren vein swarms. The pluton has been affected by differential Cenozoic tilting to the NE related to a series of gently SW-dipping normal faults. Present orientations of inferred strike-slip lineations on greisen veins and dips of NNE-SSW, WNW-ESE striking greisen veins indicate that the central and western parts of the pluton are only gently tilted (10-15°) while the eastern margin has been moderately tilted (45-55°). Reconstruction removing tilting reveals the presence of a granite cupola which restores to a position below the porphyry-style Cu mineralization, and distal Pb-Ag-Au mineralization present to the east of the Texas Canyon Granite. Two distinct fracturing and fluid release events are postulated to explain the formation of the two contrasting ore deposit types. Early, shallow porphyry-Cu style alteration-mineralization is located along strike of tensional fracture systems that formed above (now east of) the cooling magma chamber that concentrated swarms of late magmatic dikes which then accumulated as a porphyry phase at the apex of the pluton. Post-dating diking and porphyry-style mineralization, fracturing, greisen alteration and tungsten mineralization affected the entire exposed area of the Texas Canyon Granite pluton. This is inferred to have resulted from late-stage magmatic fluid release perhaps enhanced by a change in stress regime. Fluids were likely sourced from the widespread, small volume residual melt which may have concentrated tungsten, fluorine, and other acid species. Contrasting white mica chemistry between barren and hubnerite-bearing quartz vein swarms (celadonite-rich, high Fe/Fe+Mg+Mn vs muscovite-rich, low Fe/Fe+Mg+Mn) suggests that mineralizing fluids were warmer and more sulfidizing than those responsible for barren greisen. Early fracturing and fluid release events created the mineralized NE-SW oriented vein swarms, with later E-W trending vein swarms localizing lower temperature fluids relatively poor in tungsten. Evaluation of other tungsten-mineralized granitoids worldwide suggests that opening of tensional fracture zones late in a plutons cooling history under the influence of tectonic stress is a common feature in the formation of economic deposits