The timing of deformation and associated gold mineralization in SE California, USA, is contentious, partly due to the challenges involved with dating ductile deformation. We therefore combine modern geo- and thermochronology with field and microscopic observations to show that the Cargo Muchacho Mountains preserve evidence of northward thrusting in a kilometer-scale ductile shear zone during the Late Cretaceous Laramide Orogeny, accompanied by hydrothermal fluid flow, gold mineralization, and pegmatite emplacement. Penetrative strain was largely accommodated within the Jurassic metavolcaniclastic Tumco Formation, whereas intrusive Jurassic granitoids behaved as competent bodies. Quartz microstructures suggest deformation at ∼500 °C, which is consistent with fabrics defined by amphibolite facies minerals. The timing of thrusting is constrained by dynamically recrystallized titanite with a U-Pb age of 68 ± 1 Ma and late syn-kinematic pegmatites that yield U-Pb zircon ages of 65.0 ± 4.2−63.2 ± 4.8 Ma. Syn-kinematic fluid flow was focused into a lateral thrust ramp where the shear zone foliation was deflected around a relatively rigid pluton, creating zones rich in magnetite-quartz veins and epidote, and precipitating gold associated with pyrite and chalcopyrite. Dating of these sulfides via Re-Os yields an age of 64.7 ± 0.8 Ma, which confirms a Laramide age for the gold mineralization. Together, apatite from the pegmatites and a nearby Jurassic granite yields a U-Pb age of 60.4 ± 3.5 Ma, reflecting cooling to below 530−450 °C. Comparison with published studies suggests that thick-skinned deformation in the Cargo Muchacho Mountains was driven by flat-slab subduction of the conjugate Hess Plateau, which occurred several million years after and to the south of flat-slab subduction of the conjugate Shatsky Rise. This suggests that the conjugate Hess Plateau may have been subducted up to several hundred kilometers farther north than previously thought. Metamorphic devolatilization of underplated Orocopia Schist likely generated the gold-bearing hydrothermal fluids, and anatexis of the schist formed the peraluminous pegmatites, which highlights the importance of schist underplating and devolatilization along much of the Californian and Mexican cordillera.
The crustal-to lithospheric-scale shear zones that accommodate relative plate motions play a major role in generating seismic hazard, influencing the evolution of orogens, and controlling the distribution of ore deposits, and are fundamental in enabling plate tectonics (Cox et al., 2006; Handy et al., 2007). Understanding the time period and conditions over which such structures are active is thus vital for quantifying the rheology of the lithosphere, modeling deformation, reconstructing the tectonic evolution of regions, and determining the economic prospectivity of areas (Huntington & Klepeis, 2018; Oriolo et al., 2018). However, estimating the pressure-temperature-time (P-T-t) conditions of deformation in large-scale shear zones remains challenging for several reasons: (a) deformation occurs over a range of different P-T conditions, which vary both spatially (with depth) and temporally (as material is advected, either by the fault zone itself
The polymetamorphosed Swartberg Cu-Pb-Zn-Ag deposit in the Namaqua Metamorphic Province of South Africa is a major metal producer in the region, yet its genesis remains poorly understood. The deposit comprises several stratiform to stratabound units, namely the Lower Orebody and Dark Quartzite, the overlying Barite Unit, and the Upper Orebody, all of which are folded by an F2 isoclinal syncline and refolded by an open F3 synform. A discordant Garnet Quartzite unit surrounds the Upper Orebody in the F2 hinge, where it overprints the Lower Orebody and Barite Unit. The Lower Orebody comprises sulfidic, pelitic lenses with fine-grained pyrite, sphalerite, galena, and lesser pyrrhotite, hosted by sulfide-poor but magnetite- and barite-bearing siliceous rock. The overlying Barite Unit is poorly mineralized and grades from massive magnetite-barite close to the F2 hinge to distal laminated baritic schist and quartzite. The Dark Quartzite is the stratigraphic equivalent of the Lower Orebody and Barite Unit but comprises siliceous quartzite and schist, with lenses of conglomerate and minor Fe-Mn-Zn phases. The Upper Orebody displays rapid zonations from massive magnetite-rich iron formation in the F2 hinge, rich in coarse galena, pyrrhotite, and chalcopyrite, to sulfide-poor, magnetite-bearing schist and quartzite. The Garnet Quartzite is dominated by quartz and almandine garnet and mineralized with pyrite and chalcopyrite. Geochemical discriminant plots show that the Lower Orebody has a significant detrital component, whereas the Upper Orebody and Barite Unit are strongly zoned, with the greatest chemogenic component close to the F2 hinge. This corresponds to a deposit-scale metal zonation from the Cu-rich F2 hinge to more Pb- and then Zn-dominated areas. Mineral assemblages and paleoredox proxies suggest generally oxic conditions, with a more reduced signature close to the hinge and in the sulfidic Lower Orebody lenses. The Lower Orebody is interpreted as a mixed chemogenic-pelitic unit, with sulfides deposited on or near the seafloor during stage 1 hydrothermal activity. The sulfidic lenses formed from fine mud and clay deposited in quiet seafloor depressions, in which warm, dense, reducing, Pb-Zn-Ba–rich stage 1 brines accumulated, while the siliceous portions formed from higher-energy clastic sediments on aerated seafloor highs. The Barite Unit forms a baritic cap to the Lower Orebody, while the Dark Quartzite is their shallower-water equivalent. Thereafter, clastic sediment with lesser hydrothermal input was deposited during stage 2a exhalations, forming the poorly mineralized portions of the Upper Orebody. During stage 2b hydrothermal activity, hot Cu-Fe–rich fluids invaded part of the Upper Orebody, creating the highly chemogenic protolith to the well-mineralized, magnetite-rich portion. Associated hydrothermal alteration in a discordant subseafloor feeder zone created the Garnet Quartzite protolith. The F2 hinge thus corresponds closely to the original vent zone. Swartberg therefore resembles a deformed and metamorphosed Selwyn-type sedimentary exhalative deposit, with both proximal- (Upper Orebody, Garnet Quartzite) and distal-style (Lower Orebody) mineralization. The close association of these styles suggests that differences in the mineralizing fluids and depositional environment, rather than proximity to a vent, determine the deposit style.
The Quxu batholith of the Gangdese magmatic belt, southern Tibet, comprises predominantly Early Eocene calc-alkaline granitoids that feature a variety of types of magmatic microgranular enclaves and dikes. Previous studies have demonstrated that magma mixing played a crucial role in the formation of the Quxu batholith. However, the specific processes responsible for this mixing/hybridization have not been identified. The magmatic microgranular enclaves and dikes preserve a record of this magma mixing, and are therefore an excellent source of information about the processes involved. In this study, mesoscopic and microscopic magmatic structures have been investigated, in combination with analyses of mineral textures and chemical compositions. Texturally, most of the enclaves are microporphyritic, with large crystals such as clinopyroxene, hornblende, and plagioclase in a groundmass of hornblende, plagioclase, and biotite. Two types of enclave swarms can be distinguished: polygenic and monogenic swarms. Composite dikes are observed, and represent an intermediate stage between undisturbed mafic dike and dike-like monogenic enclave swarms. Our results reveal three distinct stages of magma mixing in the Quxu batholith, occurring at depth, during ascent and emplacement, and after emplacement, respectively. At depth, thorough and/or partial mixing occurred between mantle-derived mafic and crust-derived felsic magmas to produce hybrid magma. The mafic magma was generated from the primitive mantle, whereas the felsic end-member was produced by partial melting of the preexisting juvenile crust. Many types of enclaves and host granitoids are thus cogenetic, because all are hybrid products produced by the mixing of the two contrasting magmas in different proportions. In the second stage, segregation and differentiation of the hybrid magma led to the formation of the host granitoids as well as various types of magmatic microgranular enclaves. At this stage, mingling and/or local mixing happened during ascent and emplacement. In the final stage, mafic or hybrid magma was injected into early fractures in the crystallizing and cooling pluton to form dikes. Some dikes remained undisturbed, whereas others experienced local mingling and mixing to form composite dikes and eventually disturbed dike-like monogenic enclave swarms. In summary, our study demonstrates the coupling between magmatic texture and composition in an open-system batholith and highlights the potential of magmatic structures for understanding the magma mixing process.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.