Origin of the Bada porphyry Cu–Au deposit, eastern Tibet: Geology and isotope geochemistry (C–O–S–Pb) constraints

Abstract

Bada is a newly discovered Cu–Au deposit located in the south part of the Yulong porphyry Cu (–Mo–Au) belt (YPCB), eastern Tibet. The Cu–Au mineralization, mainly hosted by the alkali-rich quartz monzonite porphyry (QMP), with minor by the fine-grained monzonitic porphyry (MP) and surrounding strata of the Nanxin Formation. The hydrothermal alteration assemblages recognized at Bada mainly include potassic, phyllic, propylitic, and argillic. The Cu–Au mineralization occurs within the potassic and phyllic alteration zones. Three main metallogenic stages have been identified: stage I ore-barren quartz ± dolomite ± K-feldspar ± pyrite veins, stage IIa dolomite ± quartz + pyrite veins, stage IIb dolomite ± quartz + polymetallic sulfide veins, and stage III dolomite ± calcite veins. The dolomite δ13CPDB values from stage I to III range from –2.2 – –1.8 ‰, –6.8 – –3.4 ‰, to –4.2 – 1.9 ‰. Their δ18OSMOW values vary from 7.7 – 9.8 ‰, 6.5 – 11.9 ‰, to 7.8 – 11.4 ‰, respectively. The corresponding calculated δ18Ofluid values ranging from 6.0 to 6.3 ‰ for stage I, and –1.1 to 4.3 ‰ for stage II, and –1.7 to 1.9 ‰ for stage III. The C–O isotopic compositions imply that the ore-forming fluids were dominated by magmatic water with the input of meteoric water. The narrow δ34S value range of sulfides (1.43 to 6.61 ‰, mean 4.64 ‰) reflects a predominantly magmatic origin for the sulfur. The Pb isotopic compositions of most sulfide samples coincide with the alkali-rich porphyries in the region, indicating that the alkalic porphyries provided the main source of Pb. Combined with the zircon U–Pb age of the ore-bearing porphyry (ca. 35.0 Ma), the Bada deposit is defined as a porphyry Cu–Au deposit in a late-collisional setting. The study of the genesis of Bada provides a better understanding of the Cu–Au mineralization potential of the southern YPCB.

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