The Geology and Mineralogy of the Beiya Skarn Gold Deposit in Yunnan, Southwest China

Abstract

The Beiya skarn gold deposit is located in the eastern Tethyan orogenic belt in western Yunnan province, China. It is one of the largest gold deposits in China, with significant amounts of silver and base metals. To the end of 2014, the estimated resources are 125 million tonnes (Mt) of ore, grading 2.42 g/t Au, 0.48 wt % Cu, 25.5 wt % Fe, 38.85 g/t Ag, 1.24 wt % Pb, and 0.53 wt % Zn. Skarn alteration and mineralization are related to shoshonitic quartz monzonite porphyries that were emplaced in Triassic carbonates (Beiya Formation). Re-Os dating on molybdenite from a skarn orebody indicates an ore-forming age of 36.82 ± 0.48 Ma, which is consistent with previous dating results of the quartz monzonite porphyries. At least two paragenetic stages of skarn minerals and associated sulfides were recognized, with the early stage typified by garnet ± pyroxene, magnetite, and calcite, and the late stage characterized by epidote, amphibole, chlorite, quartz, and calcite, containing up to 70% sulfides (pyrite, chalcopyrite, and minor pyrrhotite). The early skarn is dominated by anhydrous minerals, which were replaced by hydrous minerals formed during the late stage. The garnet in the Beiya deposit is andradite rich (Ad36-97Gr3-61), and pyroxene is relatively diopside rich (Di8-91Hd7-89). This mineral assemblage indicates an oxidized skarn system, similar to other Au-Cu, Fe-bearing skarn deposits around the world. Fluid inclusions from pyroxene indicate precipitation from high-temperature and high-to moderate-salinity fluids (420°-530°C, 11.1-43.3 wt % NaCl equiv), which probably results from boiling of a moderately saline magmatic fluid. Cooler (180°-365°C) and moderate-to low-salinity fluids (1.6-16.5 wt % NaCl equiv) were trapped in garnet and quartz and are interpreted to be responsible for gold deposition. Chlorite chemistry indicates ore-forming temperatures between 300° and 340°C, in agreement with fluid inclusion data. It appears that gold was transported as chloride complexes under oxidized conditions and was deposited at temperatures of about 300°C, when transport of chloride complexes as gold carriers was less efficient.

Eocene porphyry copper deposits in the eastern Tibetan Plateau, China: Uplift, denudation, and implications for mineral exploration

“…The emplacement age is 35 Ma, and the cooling age is 33 Ma (Guo et al, ; W. Y. He, Yu, Dong, & Huang, ; Peng, Bi, Hu, Wu, & Sang, ). The following differential uplift event resulted in an exhumation of up to 4–5 km to the surface, followed by a three‐stage tectonic uplift in the eastern Tibetan Plateau, with an overall uplift of up to about 4.4 km.…”

LA-ICP-MS U-Th-Pb Dating and Trace Element Geochemistry of Allanite: Implications on the Different Skarn Metallogenesis between the Giant Beiya Au and Machangqing Cu-Mo-(Au) Deposits in Yunnan, SW China

“…In the retrograde alteration and quartz-sulfide stages, the gold is also hosted by magnetite and sulfides (pyrite, chalcopyrite and bismuthinite), respectively. Compositions of the Beiya skarn minerals suggest a typical oxidized skarn system [28]. Fluid inclusion studies indicate medium to high temperatures (186-372 • C) and high salinities (7.5-19.8 wt % NaCl equiv.)…”

“…Lamprophyre dikes are also documented in the area. Most of the porphyries were emplaced at around 36.9 to 33.3 Ma [24][25][26]28,[56][57][58][59][60], except for the quartz albite porphyry (65 Ma) and the biotite orthoclase porphyry (3.8 Ma) [45]. The quartz syenite porphyry in the Beiya deposit has been LA-ICP-MS zircon U-Pb dated to be 36.07 ± 0.43 Ma [24], and the retrograde-stage hydrothermal titanite yielded a weighted average 206 Pb/ 238 U age of 33.1 ± 1.0 Ma [61].…”

“…138-531 m thick). The Beiya Formation is deposited in a NS-trending basin upon which a broad NS-trending syncline is developed [28]. [41,42].…”

“…The Emeishan flood basalt is exposed in the southeastern part of the mining area. The regionally-widespread Cenozoic alkaline porphyries include those of (quartz) syenite, biotite-K-feldspar and quartz-albite [26,28,[43][44][45]], yet they are rarely exposed in the Beiya mining district. Major exposed rocks in the Beiya deposit include the Lower Triassic Qingtianbao Formation (175-350 m thick), the Middle Triassic Beiya Formation and Quaternary sedimentary rocks ( Figure 2).…”