Helium and argon isotope geochemistry of alkaline intrusion-associated gold and copper deposits along the Red River–Jinshajiang fault belt, SW China

Abstract

The Red River–Jinshajiang strike-slip fault zone on the eastern margin of the Tibetan plateau was originally produced by the India–Eurasia collision ∼60–70 Myr ago. Numerous post-collisional, mantle-derived alkaline igneous rocks, with ages of ∼40–30 Ma, have been intruded along this fault zone. In recent years, several copper and gold deposits associated with the alkaline intrusions of this region were discovered, such as the Yao'an and Beiya gold deposits and the Yulong and Machangqing copper deposits studied in this paper. The mineralised intrusions are felsic, with SiO 2 ranging from 61.4 to 67.7 wt.%, K 2O+Na 2O from 8.1 to 11.5 wt.% and K 2O/Na 2O>1. The deposits are located at both the exo- and endo-contact zones of the intrusions. The mineral deposits are of hydrothermal origin, with the ore-forming temperatures mainly in the range 150–450 °C. This paper presents He and Ar isotope analyses of these four deposits. The concentrations of 4He trapped in fluid inclusions of pyrites from the ores are (0.7–54.1)×10 −6 cm 3 STP g −1, and those of 40Ar are (0.6–7.3)×10 −6 cm 3 STP g −1, 3He/ 4He ratios are 0.3–2.5 Ra (Ra represents the 3He/ 4He ratio of air, 1.39×10 −6), 40Ar/ 36Ar ratios are 316–1736, and 3He/ 36Ar ratios are 0.2–11.2×10 −3. Generally, the 3He/ 4He, 40Ar/ 36Ar and 3He/ 36Ar ratios for the gold deposits are higher than those for the copper deposits. We suggest that the ore-forming fluids of both gold and copper deposits were differentiated from the mantle-derived alkaline magmas, but were diluted by modified air-saturated water (MASW) that experienced intensive interaction with crustal rocks. However, the magmatic fluids responsible for the gold deposits were less extensively diluted by MASW, resulting in higher 3He/ 4He, 40Ar/ 36Ar and 3He/ 36Ar ratios than the copper deposits.