Geochronology and geochemistry of the Shilu Cu-Mo deposit in the Yunkai area, Guangdong Province, South China and its implication

摘要

Shilu is a large porphyry-skarn deposit in the Yunkai district in Guangdong Province, South China. The Shilu granitic intrusion in the mine area is a granodiorite which is genetically related to Cu mineralization. Plagioclase in the granodiorite has a zoned texture and is mainly andesine with minor amounts of labradorite, whereas the K-feldspars exhibit Carlsbad twins and some are also characterized by a zonal texture. K-feldspars from the granodiorite show high contents of Or (87-92 wt.%) with minor Ab (8-13 wt.%) and negligible An value of 0-0.3 wt.%. Biotite can be classified as magnesio-biotite, and is characterized by Mg-rich [Mg/(Mg + Fe) = 0.54-0.601 and Al-VI-low (average values = 0.11). Hornblende is chiefly magnesiohomblende and tschermakite. LA-ICP-MS zircon U-Pb age of the Shilu granodiorite is 107 +/- 0.7 Ma, which is consistent with molybdenites Re-Os age of 104.1 +/- 1.3 Ma. Geochemical data indicate that the Shilu granodiorite is silica-rich (SiO2 = 63.43-65.03 wt.%) and alkali-rich (K2O + Na2O = 5.45-6.05 wt.%), as well as calcium-rich (CaO = 4.76-5.1 wt.%). Trace element geochemistry results show enrichments in large ion lithophile elements (e.g., Rb, K, and Ba) and depletions in some high field strength elements (e.g., Nb, P, Ta, and Ti). The total rare earth element (REE) content of the granodioritic rocks is low (Sigma REE <200 ppm), and is characterized by light REE enrichment [(La/Yb)(N)> 9] and moderately negative Eu anomalies (Eu/Eu* = 0.83-0.90). These mineralogical, geochronological, and geochemical results suggest that the Shilu granodiorite has a mixed crust-mantle source with a geochemical affinity to 1-type granitoids. Hornblende thermobarometry yielded magmatic crystallization temperatures of 686-785 degrees C and aystallization pressures between 1.0 and 2.34 kbar, which is converted to depths in a range of 331 to 7.71 km. Biotite thermobarometry yielded similar temperatures and lower pressures of 680-780 degrees C and 0.8-2 kbar (depth 2.64-6.6 km), respectively. The parent magma had a high oxygen fugacity. The Shilu granodiorite has a relatively low epsilon(Nd)/t-t value and high (Sr-87/Sr-86)(i) value, and Nd isotopes yield two-stage depleted mantle Nd model ages of 969-1590 Ma. Our new data, combined with previous studies, imply that the granodiorite and the associated Shilu Cu-Mo deposit was formed in an extensional environment, closely related to remelting of residual subducted slab fragments in the Jurassic. (c) 2014 Elsevier B.V. All rights reserved.