Potassic Magmatism And Ore-forming Processes Of Beiya Gold Deposit In Western Yunnan

The Beiya gold deposit located in the western margin of the Yangtze block,Yunnan, SW China, and was situated in the middle segment of a belt of theJinshajiang-Ailaoshan potassic porphyry intrusion. It occupied an explored goldreserve about127.28t (the type of333and above) by the end of2010. Orebodiesmainly are massive magnetite-hematite-limonite ores, commonly called Fe-Auorebodies. The Fe-Au orebodies were mostly located along the boundary of theCenozoic potassic porphyry and within the faults in the Triassic limestone. In theopen-pit of the Wandongshan in Beiya gold deposit, quartz-K-feldspar porphyrylacking alteration and mineralization inside was intruded by the monzograniticporphyry with pyritization. In addition, some lamprophyre intruded into it.The two kinds of porphyries show potassic adakitic features in geochemistry, asexpressed by high K2O/Na2O (2.195~26.963), Sr/Y (53.2to143.2) and (La/Yb)N(4.934~39.426) ratios. They have identical zircon LA-ICPMS U-Pb ages from34Mato37Ma and similar εHf(t) ranging from-6.82to+2.65of the crystalline zircons. Thelamprophyres have extremely low Nb/U ratios of1.6~4.8, as is supportive that themafic intrusions were derived from a metasomatized subcontinental lithosphericmantle (SCLM). The geochemical features of the porphyries and the lamprophyre arecompatible with the explanation that the Beiya porphyries were formed via lowercrust melting and mixing with a K-rich mafic magma, which was derived frommetasomatized SCLM.Lots of inherited zircons entrained in the porphyries have euhedral shape,representing the parent rocks for the zircons were formed in the vicinity of the Beiyagold deposit. The inherited zircons have U-Pb ages clustered at peaks of800Ma,500Ma, and280Ma. The inherited zircons with ages around800Ma have εHf(t) rangingfrom largely negative to elevated positive values, which support the theory that thisgroup of zircons was formed via a slab subduction around800Ma. The detritalzircons with ages around500Ma and280Ma represent two other episodes of themantle interaction with thickened lower crust. These features can be explained by that the northwestern Yangtze experienced metasomatization of SCLM at~800Mafollowed by other episodic mantle-crust interactions. The barrenness of lamprophyresuggests that the metals might have accumulated in the lower crust, and then theywere carried into the upper crust via the felsic magma.The analysis via the Fe-Au orebody and porphyries suggested a three-episodeore-forming process. In the first episode, a small amount of skarn was developedlocally in the contact, and a few magnetite and sulfide. In the second episode, theoxidized elemental fluid migrated upwards, which perhaps relate to the alteredmonzogranite porphyry, and mostly pyrite were formed, as the upper crusts wasextensioning. In the third episode, the Fe-Au orebody suffered from intenseweathering and forming the laterite-type orebody.