0393-000079
Magmatic-hydrothermal origin of the Triassic Laodou gold deposit, Western Qinling
Orogen, China: Implications for ore genesis of the Carlin-type gold deposits in the region
Corresponding author: Jian-wei Li, State Key Laboratory of Geological Processes and Mineral
Resources, Faculty of Earth Resources, China University of Geosciences, [email protected]
Co-authors:
Jin Xiao-ye, State Key Laboratory of Geological Processes and Mineral Resources, Faculty of
Earth Resources, China University of Geosciences, Wuhan 430074, China, [email protected]
Li Jian-wei, State Key Laboratory of Geological Processes and Mineral Resources, Faculty of
Earth Resources, China University of Geosciences, Wuhan 430074, China, [email protected]
Mesozoic gold deposits and intrusive rocks are widespread in the Western Qinling orogen (western
China) and have been extensively studied in the last two decades. The temporal and genetic
relationships between gold mineralization and magmatism, however, remain hotly debated. New
geochronological and geochemical data from the Laodou gold deposit in the Xiahe-Hezuo district
provide significant insights into this debated issue. This district is divided into the northern and
the southern belts by the NW-striking Xiahe-Hezuo Fault. The Laodou gold deposit occurs in the
northern belt and consists of auriferous lodes hosted in nearly NS-directed faults cutting through
the quartz diorite porphyry that intruded the Permian marine clastic and carbonate rocks.
Individual orebodies comprise auriferous quartz–tourmaline–stibnite veins and disseminated Aubearing pyrite or arsenopyrite within hydrothermally altered rocks on both side of the veins. Gold
occurs mostly as native gold and microscopic inclusion gold or solid solutions in pyrite and
arsenopyrite. Two ore-related sericite samples yield 40Ar/39Ar plateau ages of 249.1±1.6 Ma and
249.0±1.5 Ma (2σ), which are consistent within the analytical uncertainties with a zircon U-Pb age
of 247.6±1.3 Ma (2σ) for the quartz diorite porphyry.
Gold-related sericite grains have δDSMOW of -83.4 to -60.1 ‰ and δ18OSMOW of 8.5 to 12.3 ‰. The
values are respectively -69.2 to -54.4 ‰ and 11.3 to 13.6 ‰ for the tourmaline in the ores. The
calculated δDSMOW and δ18OSMOW for the ore-fluids ranges from -88.4 to -55.3 ‰ and from -1.45
to 7.3 ‰, respectively. The hydrogen and oxygen isotope data thus indicate a magmatic origin for
the ore fluids, with addition of minor amounts of meteoric components. This view is confirmed by
sulfur isotopic compositions of pyrite, arsenopyrite, and stibnite, which have δ34S values ranging
from -5.9 ‰ to 5.1 ‰ with a mean of -1.15 ‰. Boron isotopes of hydrothermal tourmaline
provided additional weight of evidence for the magmatic derivation of the ore fluids. Tourmaline
from auriferous lodes has δ11B of -11.19 to -0.89 ‰, indistinguishable from the values (-8.88 to 5.48 ‰) of magmatic tourmaline from the quartz diorite porphyry. Collectively, the stable isotope
and geochronological data confirm that the Laodou gold deposit was precipitated from magmatichydrothermal fluids exsolved from the coeval magmatism in the Xiahe-Hezuo district.
Numerous Carlin-type gold deposits have been recognized in the southern belt of the Xiahe-Hezuo
district, and most of them have a close relation to dioritic to granitic dikes emplaced into the early
Triassic marine calcareous siltstones and sandstones. Available data indicate that both the Carlintype gold deposits and associated dikes formed in the early-middle Triassic. The temporal
coincidence of gold mineralization and magmatism between the northern and southern belts
indicates that mineralizing fluids forming the Carlin-type gold deposits in the southern belt may
also have been derived from exsolution of the extensive early Triassic magmatism, which was
likely resulted from the northward subduction of the Paleo-Tethys oceanic slab.