Abstract
The Yao’an gold deposit is located in the middle of the Jinshajiang-Ailaoshan alkali-rich metallogenic belt, and this belt hosts many porphyry-type Cu-Au-Mo deposits formed at 46–33 Ma. Yao’an porphyry gold-mineralization is intimately associated with biotite syenite porphyry, whereas the contemporaneous quartz syenite porphyry is barren. In this study, we compared the major and trace elements of apatite and zircon and isotopic compositions of zircon from the biotite syenite porphyry and quartz syenite porphyry, to explore their geochemical differences that may affect their mineralization potential. The results show that both porphyries were derived from the partial melting of the thickened lower crust, which has been modified by slab-derived fluids, but has different mineral crystallization sequences, magma fluid activities, and magma oxidation states, respectively. REE contents in apatite and zircon can be used to reveal the crystallization sequence of minerals. A rapid decrease of (La/Yb)N ratio in apatite from both porphyries may be caused by the crystallization of allanite. Large variation of Cl contents and negative correlation between F/Cl and (La/Yb)N in apatite from fertile porphyry indicate that it has experienced the exsolution of Cl-bearing hydrothermal fluid. Higher Y/Ho and lower Zr/Hf in zircon from fertile porphyry indicate a stronger fluid activity than barren porphyry. The high S, V, As contents, δEu, low δCe in apatite, as well as high Ce4+/Ce3+ and log(fO2) estimated from zircon geochemistry from fertile porphyry, indicate high a oxidation state of fertile porphyry, similar to other fertile porphyries in this metallogenic belt. High fluid activity and fluid exsolution are conducive to the migration and enrichment of metal elements, which are very important for mineralization. High oxygen fugacity inhibits the precipitation of metal in the form of sulfide, thereby enhancing the mineralization potential of rock. Therefore, the exsolution of Cl-bearing hydrothermal fluid and high oxygen fugacity are the key factors promoting mineralization in Yao’an area.
Highlights
Apatite and zircon are ubiquitous accessory minerals that can accommodate a variety of elements (REE, Sr, U, Th, Mn, S) in igneous rocks [1,2,3,4]
We have obtained a total of 40 major elements and 40 trace elements in apatites. 50 trace elements and 28 Hf isotope were analyses for zircon
The results show that all of the apatite grains are enriched in F relative to Cl, which we classified them as fluorapatite and have magmatic feature (Figure 4a)
Summary
Apatite and zircon are ubiquitous accessory minerals that can accommodate a variety of elements (REE, Sr, U, Th, Mn, S) in igneous rocks [1,2,3,4]. Such types of features make them powerful petrogenetic and ore-forming indicators in deciphering magma evolution history and genesis [5,6,7,8,9,10,11]. The change of REE and halogen contents in apatite can be used to trace magma composition variation and the crystallization of other minerals [14,15,16]. Apatite and zircon are widely used in U-Pb dating [20,21,22,23]
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