Genesis of the Weibao banded skarn Pb-Zn deposit, Qimantagh, Xinjiang: Insights from skarn mineralogy and muscovite 40Ar-39Ar dating

Abstract

The Weibao Pb-Zn deposit is located in the Qimantagh area, Xinjiang (NW China). At Weibao, stratiform Pb-Zn ore bodies are hosted by the skarn-altered Mesoproterozoic Langyashan Formation. Both of the skarn rocks and Pb-Zn ores are banded. The skarn bands are normally 0.5 to 5cm wide and comprise alternating garnet and pyroxene zones. Systematic mineralogical studies indicate that the Weibao banded skarn was formed by infiltrative metasomatism of magmatic fluids in a disequilibriated geochemical system, rather than by the bimetasomatism between marble and Si-Al units or the metamorphism of calc-silicate-rich submarine exhalative beds. This infiltrative metasomatic process can be further divided into four stages, namely the prograde skarn, retrograde skarn, sulfide and carbonate stage. Three garnet types (Garnet I, II and III) and two pyroxene types (Pyroxene I and II) have been identified at Weibao. Garnet I has commonly Fe-rich core and Al-rich rim (Ad34Gr65 to Ad96Gr3), while Garnet II has a wider composition range (Ad23Gr73 to Ad97Gr2; average: Ad59Gr41). Garnet III is compositionally more homogeneous and Mn-rich (Ad17Sp3Gr80 to Ad55Sp6Gr39; average: Ad47Sp4Gr49). Pyroxene I is diopside-rich (Hd20Jo1Di79 to Hd36Jo2Di62; average Hd29Jo1Di70), while Pyroxene II has lower diopside component (Hd34Jo3Di63 to Hd55Jo13Di32; average: Hd48Jo5Di47). Garnet I and Pyroxene I may have formed by a small volume of high-temperature and volatile-rich fluids exsolved from the magmas at its initial crystallization stage. Further fractionation generated abundant magmatic fluids that reacted with the impure carbonates through infiltrative metasomatism, forming Garnet II and Pyroxene II in the banded skarn. Garnet III occurs as garnet+sphalerite+galena+calcite veinlets crosscutting the early-stage pyroxene and garnet zones, and may have been a product of late hydrothermal superimposition.Sulfide stage is the main ore-forming stage at Weibao. Sulfide minerals in this stage precipitated mainly through replacing anhydrous (e.g., pyroxene) and hydrous (e.g., amphibole) minerals, or as interstitials among early-formed minerals. A minor amount of the sulfides also occurs as sulfide veins. The sulfide precipitation may have been controlled chiefly by physicochemical changes of ore-forming fluids and the characteristics of early-formed skarn minerals. Specifically, hydrothermal replacement of skarn minerals has provided the space for sulfide to precipitate and thus controlled their distribution, and further shaped the mineralization into a banded structure. 40Ar-39Ar dating of the syn-mineralization muscovite yielded a Late Triassic age (ca. 226Ma), which was coeval to the massive post-collisional intermediate-felsic granitoids emplacement in the Qimantagh area. Magmatic fluids evolved from these intermediate-felsic granitoids have induced the regional-scale magmatic hydrothermal mineralization, including that at Weibao.