• Cooling and exhumation of the Late Paleozoic Tulasu epithermal gold system, Western Tianshan, NW China: implications for preservation of Pre-Mesozoic epithermal deposits

      Zhao, Xiao-Bo; Xue, Chun-Ji; Zhao, Wei-Ce; Seltmann, Reimar; Symons, David T.A.; Dolgopolova, Alla; Zhang, Yong (Geological Society of London, 2020-11-22)
      Epithermal gold deposits are rarely well preserved in pre-Mesozoic terranes because their low-temperature mineralization in shallow crust levels, and easily destroyed by subsequent erosion or depleted by tectonic events. However, several significant late Paleozoic epithermal gold deposits have been found in the Tulasu volcanic basin in NW China, forming one of the largest gold districts in the western Tianshan orogen. Here, we report new 40Ar/39Ar age from a monzonite porphyry enclave hosted in andesite and apatite fission track (AFT) data for 10 volcanic rocks from the Tulasu basin. These data, combined with the previous dataset, are used to perform inverse thermal modelling to quantify the district's cooling and exhumation history. Our modelling indicates a phase of burial reheating during late Paleozoic sedimentation following the mineralization, and subsequent a rapid exhumation in the Jurassic to Early Cretaceous (∼196–128 Ma), and a slow exhumation until to present. The Mesozoic exhumation is likely related to the far-field effects of the Cimmerian orogeny along the southern Eurasian margin. Therefore, we suggest that the quick burial by thick sediments and the slow protracted exhumation after mineralization were crucial for the preservation of the Paleozoic epithermal gold system at Tulasu.
    • Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK

      Catlos, EJ; Mark, DF; Suarez, S; Brookfield, ME; Giles Miller, C; Schmitt, AK; Gallagher, V; Kelly, A (Geological Society of London, 2020-09-21)
      The Ludlow Bone Bed (Welsh Basin) is a critical stratigraphic horizon and contains a rich assemblage of fish scales. Units above provide insights into the early evolution of animal and plant life. The bed has not yet been radioisotopically dated. Here, we report 207 secondary ion mass spectrometry (SIMS) ages from 102 zircon (ZrSiO4) grains from the Ludlow (n = 2) and stratigraphically higher Downton (n = 1) bone beds. SIMS ages are middle Ordovician (471.6 ± 20.7 Ma) to late Devonian (375.7 ± 14.6 Ma, 238U–206Pb, ±1σ analytical uncertainty). Cathodoluminescence images show that the youngest ages appear affected by alteration. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology was utilized to improve precision. Detrital zircon grains from Downton yield 424.91 ± 0.34/0.42/0.63 Ma and from Ludlow 424.85 ± 0.32/0.41/0.62 Ma (n = 5 each, 238U–206Pb, ±2σ analytical, tracer or systematic uncertainty). These ages provide a maximum deposition age. Results overlap the basal Přídolí age (423.0 ± 2.3 Ma) in its stratotype (Požáry Section, Reporyje, Prague, Czech Republic). The Ludlow Bone Bed marks the base of the local Downton Group, which has previously been correlated with the base of the Přídolí Series. The CA-ID-TIMS ages are older than those for other land arthropod-bearing sediments, such as the Cowie Harbour Fish Bed and Rhynie Chert.