Uranium-lead dating of hydrothermal zircon and monazite from the Sin Quyen Fe-Cu-REE-Au-(U) deposit, northwestern Vietnam

The Sin Quyen deposit in northwestern Vietnam contains economic concentrations of Cu, Au and LREE, and sub-economic concentration of U. In this deposit, massive and banded replacement ores are hosted in Neoproterozoic metapelite. The paragenetic sequence includes sodic alteration (stage I), calcic-potassic alteration and associated Fe-REE-(U) mineralization (stage II), Cu-Au mineralization (stage III), and sulfide-(quartz-carbonate) veins (stage IV). The Sin Quyen deposit experienced an extensive post-ore metamorphic overprint, which makes it difficult to precisely determine the mineralization age. In this study, zircon and monazite U-Pb geochronometers and the Rb-Sr isochron method are used to constrain the timing of mineralization. Zircon grains in the ore are closely intergrown or texturally associated with hydrothermal minerals of stage II (e.g., garnet, allanite, and hedenbergite). They may contain primary fluid inclusions and display irregular zoning in cathodoluminescence (CL) images. Zircon grains are rich in U (688 to 2902 ppm) and poor in Th (0.2 to 2.9 ppm). Their delta O-18(V-SMOW) values range from 11.9 to 14.0aEuro degrees, higher than those of typical magmatic zircon. These textural and compositional features imply that zircon precipitated from O-18- and U-rich hydrothermal fluids, coeval with the minerals of stage II. Monazite occurs in close association with stage II magnetite and allanite and has low contents of Th (< 2700 ppm), indicative of a hydrothermal origin. Hydrothermal zircon and monazite have indistinguishable U-Pb ages of 841 +/- 12 and 836 +/- 18 Ma, respectively, representing the timing of Fe-REE mineralization. There is no direct isotopic constraint on the timing of the Cu-Au mineralization, but geological observations suggest that the Cu-Au and Fe-REE ores most likely formed within a single evolved hydrothermal process. In the plot of Rb-87/Sr-86 vs. Sr-87/Sr-86, the composition of bulk-ore and biotite separates from ore lie along a reference line for 30 Ma, which is consistent with the timing of metamorphism in the region. The mineralization age of the Sin Quyen deposit falls within the overall age range (740 to 860 Ma) of the regional Neoproterozoic igneous rocks. This temporal linkage, in combination with the magmatic-like sulfur isotopes of sulfide minerals (delta S-34(V-CDT) = -0.8 to 3.1), indicates that the mineralization may have a close genetic association with the Neoproterozoic igneous activity

Neoproterozoic granitoids from the Phan Si Pan belt, Northwest Vietnam: Implication for the tectonic linkage between Northwest Vietnam and the Yangtze Block

Several Neoproterozoic granitic intrusions, including the Phin Ngan intrusion and some smaller ones from the Sin Quyen Cu mine, have been identified in the Phan Si Pan belt, Northwest Vietnam. Whole rock geochemical and Sm-Nd isotopic data and zircon U-Pb age and Hf-O isotopes have been determined for these intrusions in order to constrain their ages, petrogenesis and tectonic implications. The Phin Ngan intrusion, mainly composed of syenogranite, was emplaced at 824 +/- 4 Ma. Rocks from this intrusion have high SiO2 (72.1-73.5 wt.%), and K2O (5.14-5.52 wt.%), with A/CNK values ranging from 1.02 to 1.06. They have negative whole-rock epsilon(Nd)(t) (-8.4 to -5.2) and zircon epsilon(Hf)(t) values (-8.1 to -5.5), and high zircon delta O-18 values (9.7-10.9 parts per thousand). These geochemical features suggest that the Phin Ngan intrusion was derived from ancient, K-rich crustal sources. Granitic intrusions from the Sin Quyen mine occur as stocks or dykes intruding ores and ore-hosting rocks. They are composed of monzogranite and granodiorite, emplaced between 736 +/- 8 and 758 +/- 6 Ma. They have high SiO2 (68.2-76.7 wt.%) and (K2O + Na2O) (6.93-8.54 wt.%), and low MgO (0.19-0.87), with A/CNK values ranging from 0.91 to 1.14. Their whole-rock epsilon(Nd)(t) values range from -6.6 to 0, mostly clustered between -6.6 and -4.3. Their zircon epsilon(Hf)(t) values vary from -6.7 to +11.1, mostly lower than -2.0. Such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials, coupled with subordinate involvement of mantle-/juvenile crust-derived components. The Phin Ngan intrusion and intrusions from the Sin Quyen mine are sub-alkaline in nature, and have arc-like trace-element compositions. Compiled with other broadly synchronous igneous rocks with arc-like affinities in the region, the Neoproterozoic magmatism in the Phan Si Pan belt was generated in a subduction-related setting. The Neoproterozoic intrusions in the Phan Si Pan belt are geochronologically and geochemically correlated with the Neoproterozoic subduction-related igneous rocks along the western Yangtze Block. Thus, the Phan Si Pan belt represents the southern most part of the giant arc system of the western Yangtze Block. (C) 2017 Elsevier B.V. All rights reserved