Abstract
The Nuocang Pb-Zn deposit is a newly discovered polymetallic skarn deposit in the southern Lhasa subterrane, western Gangdese, Tibet. The skarn occurs at the contact between the limestone of Angjie Formation and the Linzizong volcanic rocks of Dianzhong Formation (LDF), and the subvolcanic granite porphyry intruding those formations; the contact metasomatic skarn is well zoned mineralogically and texturally, as well as geochemically. The skarn minerals predominantly consist of an anhydrous to hydrous calc-silicate sequence pyroxene–garnet–epidote. The endoskarn mainly consists of an assemblage of pyroxene, garnet, ilvaite, epidote, and quartz, whereas the exoskarn is characterized proximal to distally, by decreasing garnet, and increasing pyroxene, ilvaite, epidote, chlorite, muscovite, quartz, calcite, galena, and sphalerite. Geochemical analyses suggest that the limestone provided the Ca for all the skarn minerals and the magmatic volatiles were the main source for Si (except the skarnified hornfels/sandstone, and muscovite-epidote-garnet-pyroxene skarn possibly from the host sandstones), with Fe and Mn and other mineralizing components. During the hydrothermal alteration, the garnet-pyroxene skarn and pyroxene-rich skarn gained Si, Fe, Mn, Pb, Zn, and Sn, but lost Ca, Mg, K, P, Rb, Sr, and Ba. However, the skarnified hornfels/sandstone, and muscovite-epidote-garnet-pyroxene skarn gained Fe, Ca, Mn, Sr, Zr, Hf, Th, and Cu, but lost Si, Mg, K, Na, P, Rb, Ba, and Li. The REEs in the skarn were sourced from magmatic fluids during the prograde stage. Skarn mineral assemblages and geochemistry indicate the skarn in the Nuocang deposit were formed in a disequilibrated geochemical system by infiltrative metasomatism of magmatic fluids. During the prograde stage, garnet I (And97.6Gro1.6) firstly formed, and then a part of them incrementally turned into garnet II (And64.4Gro33.8) and III (And70.22Gro29.1). The subsequent substitution of Fe for Al in the garnet II and III indicates the oxygen fugacity of the fluid became more reduced, then resulted in formation of significant pyroxene. However, the anisotropic garnet IV (And38.5Gro59.8) usually replaced the pyroxene. In the retrograde stage, the temperature decreased and oxygen fugacity increased, but hydrolysis increased with epidote, ilvaite, chlorite I, and muscovite forming with magnetite. The continuing decreasing temperature and mixing with meteoric water lead to Cu, Pb, and Zn saturation as sulfides. After the sulfides deposition, the continued mixing with large amounts of cold meteoric water would decrease its temperature, and increase its pH value (neutralizing), promoting the deposition of significant amounts of calcite and chlorite II. The geological, mineralogical, and geochemical characteristics of Nuocang skarn, suggest that the Nuocang deposit is of a Pb-Zn polymetallic type. Compared to the other typical skarn-epithermal deposits in the Linzizong volcanic area, it indicates that the Nuocang deposit may have the exploration potential for both skarn and epithermal styles of mineralization.
Highlights
Skarn deposits, typically formed by the interaction of carbonate-bearing host rocks and magmatic hydrothermal fluids, occupy an important position among the sources of Fe, Cu, Pb, Zn, and Au.In comparison to the proximal Fe-Cu skarn, the Pb-Zn skarn are formed at relatively long distances from the causative magmatic rocks, forming in a lower temperature environment [1,2,3,4]
It indicates that the Nuocang deposit may have the exploration potential for both skarn and epithermal styles of mineralization
The ilvaite I in the Nuocang deposit plotted close to W-Mo deposits, whereas the ilvaite II plotted close to the Pb-Zn deposits, indicating that the Nuocang deposit is of a Pb-Zn polymetallic type (Figure 9)
Summary
Typically formed by the interaction of carbonate-bearing host rocks and magmatic hydrothermal fluids, occupy an important position among the sources of Fe, Cu, Pb, Zn, and Au.In comparison to the proximal Fe-Cu skarn, the Pb-Zn skarn are formed at relatively long distances from the causative magmatic rocks, forming in a lower temperature environment [1,2,3,4]. The infiltrative skarn deposits show affinity with magmatic hydrothermal activity, and their compositions positively correlate with that of the generating magmatic rocks [1,5]. They are characterized by the overall zonation that is based on the mineral assemblages in space, from the causative plutons through the proximal skarn zone towards the distal skarn zone, the wall-rocks. They are usually divided into the prograde and retrograde stages, reflecting the fluid evolution and their physical and chemical conditions. The whole-rock geochemistry of skarn and its mineralogical aspects yield a relatively unique fingerprint, which could provide a record of the element migration and enrichment in different zones, and could reflect the environmental variation characteristics in the formation process [2]
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