This study, via combined analysis of geophysical and geochemical data, reveals a lithospheric architecture characterized by crust-mantle decoupling and vertical heat-flow conduits that control orogenic gold mineralization in the Ailaoshan gold belt on the southeastern margin of Tibet. The mantle seismic tomography indicates that the crust-mantle decoupled deformation, defined from previous seismic anisotropy analysis, was formed by upwelling and lateral flow of the asthenosphere, driven by deep subduction of the Indian continent. Our magnetotelluric and seismic images show both a vertical conductor across the Moho and high Vp/Vs anomalies both in the uppermost mantle and lowest crust, suggesting that crust-mantle decoupling promotes ponding of mantle-derived basic melts at the base of the crust via a heat-flow conduit. Noble gas isotope and halogen ratios of gold-related ore minerals indicate a mantle source of ore fluid. A rapid decrease in Cl/F ratios of lamprophyres under conditions of 1.2 GPa and 1050°C suggests that the ore fluid was derived from degassing of the basic melts. Similar lithospheric architecture is recognized in other orogenic gold provinces, implying analogous formational controls.
Ore systems are located in zones of enhanced and focused heat and fluids flux within the lithosphere. In order to target deep ore deposits under cover, a better understanding of lithospheric architecture is essential, especially in relation to magmatism and fluids. Here we attempt an integrated approach using zircon LuHf isotopic mapping (455 samples with 5049 zircon analyses, including 1021 new data), combined with whole-rock geochemistry and isotopes of mantle-derived mafic rocks, high-resolution seismic tomography from 325 seismic stations and new thermochemical modelling, to establish the lithosphere architecture in southeastern Tibet. The integrated data suggest lithospheric refertilisation accompanied by heat flux from the asthenosphere, and also reveal the evolutionary pathway of the volatile components. The approach adopted in our study can be used in exploration for porphyry CuAu, orogenic-Au and rare earth element deposits in Southeastern Tibet, and illustrate the usefulness of lithosphere-architecture mapping as a useful tool for mineral exploration.
Abstract Quartz cathodoluminescence (CL) images are commonly combined with trace element concentrations to decipher complex histories of hydrothermal systems. However, the correlations between aluminum content and CL zoning of low-temperature hydrothermal quartz and their genesis remain controversial. In this contribution, a multiparametric study was carried out on CL-aluminum zoning of low-temperature hydrothermal quartz (<350 °C) from the Shihu and Rushan quartz-vein type Au deposits in the North China Craton. The results show that aluminum concentration correlates negatively with CL intensity in quartz from the Shihu Au deposit. CL-dark quartz zoning has significant Al concentrations as well as detectable Al-H bonds. However, in the Rushan Au deposit, the correlation is positive, and aluminum is enriched in the CL-bright quartz zoning. The Al content is positively correlated with K content with r2 = 0.769. Combined with the electron backscatter diffraction (EBSD), X-ray single crystal diffraction (XRD), and transmission electron microscope (TEM) data, we infer that the genesis of CL zoning in the low-temperature hydrothermal quartz is closely related to Al3+-H+ and Al3+-K+ concentrations. The Al3+-K+ may act as the CL-activator, while the Al3+-H+ may act as the CL-dampener. Where Al3+-Si4+ substitution is charge balanced by hydrogen, the intensity of CL response decreases; where Al3+-Si4+ substitution is charge balanced by potassium, the intensity of CL response increases. The correlations between CL intensity and aluminum concentration in the low-temperature hydrothermal quartz reflect pH fluctuations of hydrothermal system.
In order to get deep information on the Tibetan collisional orogen, and to constrain the orogeny processes during Indus—Asian continental collision, the isotopic geochemistry of helium emitted from geothermal springs in the southern Tibetan was systemically studied. Our researches and previously-published data show that the isotope composition of helium have a wide range (R/Ra = 0.017~5.38; R is 3He/4He value of sample, Ra is 3He/4He value of air), which defines two kinds of principal helium variation domains, i.e., mantle helium domain (R/Ra = 0.11~5.38) and crustal helium domain (R/Ra = 0.017~0.072). The former mainly distributes in the Tengchong Rehai geothermal field (R/Ra = 0.40~5.38) near the eastern Himalayan syntax, the Shiquanhe geothermal field (R/Ra = 0.27~0.30) near the western Himalayan syntax, and the Lhasa hydrothermal activity zone (R/Ra = 0.11~0.98) to the east of longitude 89°E. The latter mainly present in the Ngamring hydrothermal activity zone (R/Ra = 0.017~0.072) to the west of 89°E. Near the eastern and western syntaxes, where the hydrothermalism is controlled by large-scale strike-slip faults, there are 50% mantle-derive helium's contribution in geothermal helium. While in the hinterland of the plateau, where NS-trending rift control the development of hydrothermal zone, the value of R/Ra has obvious variation in east—west direction rather than in south—north direction. Both two helium domains are bounded by longitude 89°E in EW-direction, but all bestride the Yarlung Zangbu suture in SN-direction. Synthetic analysis on the helium isotopic data and available geophysical data shows that modern hydrothermal activity was mainly driven by magmatic chambers or partially molten layers occurring as patches in the upper crust, but the contribution of mantle heat and mass (helium) to spring gases has been recognized to the east of 89°E. As a consequence, we proposed that the Indus continent is underthresting obliquely northwards as a whole with an inconsistent underthrusting front, which shows different slab styles in two sides of 89°E. To the west of 89°E, the continental slab downgoing towards NNE in a gentle slope has probably passed the Yarlung Zangbu suture and reached to the Bangong—Nujing suture; while to the east of 89°E, underthrusting slab front, likely to tear down along the Yadong—Gulou rift valley, is downgoing in a steep slope, thus has not spanned the Yarlung Zangbu suture.
Adakitic rocks occur in a variety of tectonic settings and are key to understanding the tectonic evolution and geodynamics of orogenic belts. We investigated latest Oligocene (23.5–22.5 Ma) quartz monzonites and granites from the western segment of the Urumieh–Dokhtar magmatic belt in Iran, which are likely to have formed in response to the early stages of Arabia–Eurasia collision. The studied rocks have the geochemical characteristics of typical adakites, such as high SiO 2 (60.18–68.82 wt%) and Sr (499–793 ppm) contents, low Y (8.90–17.1 ppm) and Yb (0.88–1.58 ppm) contents, and high Sr/Y (26.1‒67.8) and (La/Yb) N (21.9‒32.9) ratios. They have variable K 2 O (3.88–5.09 wt%), MgO (0.44–2.74 wt%; Mg# = 33.7–52.5), Cr (4.27–40.59 ppm), Ni (4.28–35.68 ppm) and Th (9.56–59.59 ppm) contents, and relatively depleted Sr–Nd isotopic compositions [( 87 Sr/ 86 Sr) i = 0.70450–0.70516; ε Nd ( t ) = 2.1–2.7]. These characteristics indicate that the quartz monzonites were derived from the partial melting of delaminated lower crust that interacted with mantle peridotite with high MgO, Cr and Ni contents and depleted Sr–Nd isotopic compositions and suggest that the granites were formed by the fractional crystallization of quartz monzonitic magma. The geochemical features of the studied adakitic rocks could therefore have been affected by magmatic processes (e.g. fractional crystallization), which might be misleading in interpretations of their petrogenesis and related tectonic settings. The geochemical features of the studied rocks indicate that the crust of the western segment of the Urumieh–Dokhtar magmatic belt was thickened to c. 50 ± 4.43 km during the latest Oligocene ( c. 23.5 Ma) as a result of Arabia–Eurasia collision. Supplementary material: Whole rock geochemical and Sr–Nd isotopic compositions, and zircon U–Pb and Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.6188328
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