Geology, Fluid Inclusion and Isotopic Study of the Neoproterozoic Suoi Thau Copper Deposit, Northwest Vietnam

The Sin Quyen‐Lung Po district is an important Cu metallogenic province in Vietnam, but there are few temporal and genetic constraints on deposits from this belt. Suoi Thau is one of the representative Cu deposits associated with granitic intrusion. The deposit consists of ore bodies in altered granite or along the contact zone between granite and Proterozoic meta‐sedimentary rocks. The Cu‐bearing intrusion is sub‐alkaline I‐type granite. It has a zircon U‐Pb age of ～776 Ma, and has subduction‐related geochemical signatures. Geochemical analysis reveals that the intrusion may be formed by melting of mafic lower crust in a subduction regime. Three stages of alteration and mineralization are identified in the Suoi Thau deposit, i.e., potassic alteration; silicification and Cu mineralization; and phyllic alteration. Two‐phase aqueous fluid inclusions in quartz from silicification stage show wide ranges of homogenization temperatures (140–383°C) and salinities (4.18wt%–19.13wt%). The high temperature and high salinity natures of some inclusions are consistent with a magmatic derivation of the fluids, which is also supported by the H‐O‐S isotopes. Fluids in quartz have δD values of ?41.9‰ to ?68.8‰. The fluids in isotopic equilibrium with quartz have δ¹⁸O values ranging from 7.9‰ to 9.2‰. These values are just plotted in the compositional field of magmatic‐hydrothermal fluids in the δD_water versus δ¹⁸O_water diagram. Sulfide minerals have relatively uniform δ³⁴S values from 1.84‰ to 3.57‰, which is supportive of a magmatic derivation of sulfur. The fluid inclusions with relatively low temperatures and salinities most probably represent variably cooled magmatic‐hydrothermal fluids. The magmatic derivation of fluids and the close spatial relationship between Cu ore bodies and intrusion suggest that the Cu mineralization most likely had a genetic association with granite. The Suoi Thau deposit, together with other deposits in the region, may define a Neoproterozoic subduction‐related ore‐forming belt.