Chronology of laramide magmatism and stockwork fracture filling at the Red Hills porphyry Mo-Cu deposit, Presidio County, Texas
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The Red Hills intrusive complex is the easternmost porphyry Mo-Cu system of Laramide age in southwestern North America. This study constrains the timing between igneous phases, and compares their geochemical characteristics with dikes from the Shafter silver deposit in order to support a possible link to a similar magmatic source. In addition, this study is the first to utilize SEM-CL techniques to document veining in a porphyry Mo system, comparing those to porphyry Cu systems. The high-K calc-alkaline igneous complex consists of early biotite porphyry (BP), biotite quartz monzonite porphyry (BQMP), quartz monzonite porphyry (QMP), quartz latite porphyry (QLP), and granodiorite porphyry (GP) with microgranular mafic enclaves. These magmas intruded Permian sandstones, siltstones, and dolostones, resulting in an extensive hornfels zone with local garnet skarn. U-Pb ages of zircon from the QLP and BQMP are 62.54 ± 0.49 Ma and 66.08 ± 0.31 Ma ages, respectively. When integrated with Gilmer's (2001) 60.2 ± 0.3 Ma molybdenite Re-Os age and ⁴⁰Ar/³⁹Ar sericite age of 61.58 ± 0.34 Ma, magmatism and hydrothermal activity may have spanned at least 6 My at Red Hills. Whole-rock and trace-element geochemistry suggest the Red Hills intrusions are sourced from an intermediate crustal melt and share a similar geochemical parent as dikes found in the Shafter carbonate-replacement-silver deposit. Potassic alteration is found in deep BP intercepts as mm-scale K-spar alteration halos and as shreddy biotite, which is pervasively overprinted by phyllic alteration. Intermediate argillic alteration caps the phyllic alteration zone and propylitic alteration extends east of the western mineralized part at Red Hills. Mo-Cu mineralization at Red Hills is confined to stockwork quartz veins and as disseminations within hydrothermally altered wallrock. The Red Hills intrusions have similar vein styles and, from youngest to oldest, the stockwork veining sequence involves: A veins (A1 and A2), B veins, D veins (D1 and D2), and hypogene alunite veins. Mo mineralization is confined to B veins and copper mineralization occurs later from phyllic overprinting in the form of copper sulfide inclusions in pyrite in wallrock and in D veins. SEM-CL of hydrothermal quartz reveals intricate quartz textures in relation to sulfide deposition within complex stockwork veins where fracture overprinting is common. A sequence of three stages of main-stage vein development and has been developed: 1) Fracturing in a semi-plastic magma formed sinuous veins filled with granular quartz that has internal wavy growth zones or homogenous CL texture. 2a) Fracturing under brittle conditions formed planar veins with euhedral, inward-oriented quartz that has oscillatory growth zones and bright luminescence. 2b) molybdenite deposition sites are associated with local dark-CL quartz in early quartz, while retaining primary growth textures. 3a) Dissolution of early quartz by retrograde solubility formed embayments along quartz grain boundaries. Following dissolution, euhedral, dark-CL quartz precipitated coevally with pyrite enriched in copper. When the hydrothermal activity ends, tectonic activity resulted in dark-CL microfractures and partial recrystallization may affect the stockwork veins.