Abstract
The Zhangbaoshan (ZBS) super-large Rubidium deposit, located in the Eastern Tianshan, is a typical granite-type Rb deposit. The ZBS deposit is mainly hosted in the highly evolved Baishitouquan (BST) pluton enriched in F and Rb, which exhibits five lithological zones from the bottom to the top: leucogranite (zone-a), amazonite-bearing granite (zone-b), amazonite granite (zone-c), topaz-bearing amazonite granite (zone-d) and topaz albite granite (zone-e), as well as minor small lodes of amazonite pegmatite. Two types of zircon were identified from the BST pluton. Type-I zircons mainly occur in the zone–a, are characterized by obvious oscillatory zoning, high Zr contents (47.4–67.3 wt% ZrO2) and Zr/Hf ratios (21.72–58.23), low trace element concentrations, and heavy rare earth elements (HREE)–enriched patterns with prominent positive Ce anomalies (Ce/Ce* = 1.21–385) and strong negative Eu anomalies (Eu/Eu* = 0.008–0.551), indicative of early magmatic zircon. Type–II zircons mainly occur in the upper zones (zone-c to zone-e), exhibit porous and dark Cathodoluminescence images, inhomogeneous internal structure, plenty of mineral inclusions, low Zr (38.7–51.0 wt% ZrO2) and Zr/Hf ratios (3.35–11.00), high Hf (34,094–85,754 ppm), Th (718–4,980 ppm), U (3,540–32,901 ppm), Ta (86.7–398 ppm), Y (1,630–28,890 ppm) and rare earth elements (REEs) (3,910–30,165 ppm), as well as slightly HREE–enriched patterns and significant M–type tetrad patterns with t3 values (quantification factor of tetrad effect) of 1.51–1.69. It is suggested that the type–II zircons are crystallized from a deuteric F–rich fluid coexisted with the highly evolved residual magma during the transition from the magmatic to the F–rich hydrothermal stage of the BST pluton. The F–rich fluid exsolution during the magmatic–hydrothermal transition is one of the most important factors controlling the modification of highly evolved granite and related Rb enrichment and mineralization. The type–I zircon samples from zone–a yield concordant ages of 250 ± 2.5 Ma and 250.5 ± 1.7 Ma, respectively, indicating that the BST pluton was emplaced in the Early Triassic. The type–II zircons from zone–c to zone–e yield lower intercept U–Pb ages between 238 and 257 Ma, which may represent the age of F–rich fluid–melt interaction during the transition from the magmatic to the hydrothermal stage. The mineralization of the ZBS super–large Rb deposit should have occurred shortly after emplacement of the BST pluton in the Early Triassic. Combined with available data, it is suggested that the Triassic is an important period for granitic magmatism and rare metal metallogeny in the Eastern Tianshan.
Highlights
As a common accessory mineral in a variety of rocks, zircon has long been recognized as an excellent geochronometer due to its high U, Th and low common Pb contents, and high closure temperature for the U–Th–Pb system (Geisler et al, 2007; Harley et al, 2007)
Type–II zircons from zone–c to zone–e show lead loss on the U–Pb concordia diagrams, yielding lower intercept U–Pb ages between 238 and 257 Ma (Figures 7E–H), which could indicate the age of the F–rich fluid–melt interaction above solidus during the transition from magmatic to hydrothermal conditions
Type–II zircons are hydrothermal zircons crystallized from deuteric F–rich fluid that coexisted with residual magma
Summary
As a common accessory mineral in a variety of rocks, zircon has long been recognized as an excellent geochronometer due to its high U, Th and low common Pb contents, and high closure temperature for the U–Th–Pb system (Geisler et al, 2007; Harley et al, 2007). Zircon can incorporate considerable amounts of temperature–or process–sensitive trace elements, including Hf, Y, Ti and REEs during crystallization, which make it a useful tool to fingerprint the mineral–melt–fluid processes during crust formation and evolution, hydrothermal alteration and mineralization (Belousova et al, 2006; Geisler et al, 2007; Hanchar and Van Westrenen, 2007; Harley et al, 2007). Zircon is ubiquitous in granitoids and has generally been considered as magmatic in origin (e.g., Hoskin, 2005; Jiang et al, 2020) Zircons of both magmatic and hydrothermal origin commonly occur in highly evolved granite and pegmatite (e.g., Yang et al, 2014; Li H. et al, 2018; Li X.-C. et al, 2018; Huang et al, 2020). Zircons in F–rich highly evolved magmatic systems are generally complex, and caution should be taken to interpret their elemental and isotopic data
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