Garnet U-Pb and O isotopic determinations reveal a shear-zone induced hydrothermal system

Abstract

The absolute crystallization ages of minerals from hydrothermal fluids measured in situ can unravel the timing of key events leading to the formation of, for instance, ore deposits and hydrothermally derived geological terrains. In this study, a skarn iron deposit from northwest (NW) China is shown to have U-Pb garnet and U-Pb zircon ages of 254.2 ± 1.7 Ma and 255.5 ± 1.0 Ma, respectively, which are both significantly younger than magmatism and metamorphism of the region. This skarn age instead correlates with the occurrence of strike-slip and thrust faulting in the region. The water/rock mass ratio of 0.065~0.115 suggests the δ 18 O garnet composition is ~1‰ at temperatures ranging from 250–450 °C. The low oxygen isotopic composition indicates the role of meteoric water in the garnet formation. These measurements can be interpreted as the shear along faults circulating meteoric water ~10 km below the hanging wall of meta-volcanic sedimentary rock. Meteoric water in this hydrothermal system would leach cations from the meta-volcano-sedimentary rocks necessary for mineralization. Silica-rich hydrothermal fluid reacts with calcic-rich materials in the meta-volcano-sedimentary rocks, depositing the garnet and magnetite. Our work suggests that the shear zone is rich in ores, rendering this deposit for NW China a prospective source for future mineral resource exploration.

Role of Hydrothermal Fluids in the Formation of the Kamioka Skarn-Type Pb–Zn Deposits, Japan

“…Although the δ 18 O values of ore minerals in magmatic hydrothermal skarn deposits may be similar to those of magmatic water (+5.5‰ to +9.0‰; [21]) at high temperatures, the δ 18 O value of garnet in Mengku skarn-type iron deposits in China [22] is as low as +1‰. This is inconsistent with a magmatic origin, suggesting instead a shear-zone induced hydrothermal system with meteoric water [22].…”

“…Although the δ 18 O values of ore minerals in magmatic hydrothermal skarn deposits may be similar to those of magmatic water (+5.5‰ to +9.0‰; [21]) at high temperatures, the δ 18 O value of garnet in Mengku skarn-type iron deposits in China [22] is as low as +1‰. This is inconsistent with a magmatic origin, suggesting instead a shear-zone induced hydrothermal system with meteoric water [22]. Even if there was early-stage magmatic fluid for the Takatori hypothermal tungsten-quartz vein deposit in Japan, the δ 18 O value of the fluid decreased with decreasing temperature because the fluid was mixed with meteoric water [23].…”

“…Thus the δ 13 C and δ 18 O values of crystalline limestone in the Kamioka deposits can be used as an indicator of the influence of hydrothermal fluids on the ore mineralization. Meteoric water was efficiently transported downward along faults at shear zones in the Mengku skarn-type iron deposit in China [22]. There are several fracture systems in the Kamioka district.…”

Fluid circulation in the South Tibetan Detachment System: Evidence from fluid inclusions and oxygen isotope data of quartz veins in the Ramba Dome, North Himalayan Gneiss Domes

“…Fluids from different sources have distinct isotopic signatures (Yardley & Bottrell, 1992). A relatively low hydrogen (δ 2 H) and oxygen (δ 18 O) isotopic composition of fluid can be indicative of a meteoric component (e.g., Zang et al, 2019), and thus used as a palaeoaltimeter (e.g., Campani, Mulch, Kempf, Schlunegger, & Mancktelow, 2012;Dusséaux, Gébelin, Ruffet, & Mulch, 2021;Gébelin et al, 2013;Gébelin, Mulch, Teyssier, Page Chamberlain, & Heizler, 2012;Mulch & Chamberlain, 2007;Rowley & Garzione, 2007). Low δ 2 H and δ 18 O ratios recorded in extensional detachment systems indicate that meteoric fluids can circulate into different depths and even into ductile detachment zones (e.g., Gottardi et al, 2015;Morrison & Anderson, 1998).…”

Mineral Chemistry and U-Pb Garnet Geochronology of Strongly Reduced Tungsten Skarns at the Pampa de Olaen Mining district, Córdoba, Argentina

“…Isotopic measurements were carried out in an Agilent 7500a (Agilent Technologies, Japan) mass spectrometer coupled with an excimer 193 nm laser ablation system at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Detailed analytical procedures and experimental parameters are described by Yang et al (2018), which were successfully applied to a skarn ore deposits in China (Zang et al, 2019). Standards were two matrix-matched external references andradite (Willsboro and Magana Mali) and one internal standard NIST 612 glass.…”