Geochemical compositions of apatites from the Xuejiping and Disuga porphyries in Zhongdian arc: Implications for porphyry Cu mineralization

Triassic calc-alkaline granitic rocks are widely distributed in the Zhongdian arc, SW China. Some of these rocks host porphyry Cu deposits of different sizes, but most of them are barren. In order to understand the key factors of mineralization, and to provide fertility discriminators for future mineral deposit exploration in the region, this study presents new whole-rock and apatite geochemical data of the ore-bearing porphyries from the large Xuejiping deposit and Disuga prospecting area. These data have been compared with and integrated with previously published data of the ore-bearing porphyry from the giant Pulang deposit and the barren Triassic Xiuwacu biotite granite. The four intrusions are I-type granitoids with adakitic affinities, characterized by high whole-rock Sr/Y and La/Yb ratios and high Sr/Y and EuN/EuN* ratios in apatites. Coherent whole-rock geochemical variations and changes in Sr contents and REE ratios in apatites indicate that the four intrusions underwent similar fractional crystallization. All the rocks show enrichment of LILEs and LREEs and depletion of HFSEs with relatively wide ranges of whole-rock Sr-Nd isotopes ((⁸⁷Sr/⁸⁶Sr)t = 0.7043–0.7070; ε_Nd(t) = -6.8–0.2). These geochemical features together with apatite Sr-Nd isotopes suggest that their parental magma may originate from partial melting of the slab-derived fluid metasomatized mantle wedge and mixing with melt of lower crust. Compared to the barren Triassic Xiuwacu biotite granite, three ore-bearing porphyries have higher Eu_N/Eu_N*, lower Ce_N/Ce_N* ratios of apatites, indicating higher oxygen fugacity. Besides, the ore-bearing porphyries have higher magmatic S contents than that of the barren one. The differences in magmatic oxygen fugacity and S content are key factors controlling the economic potential of magmas in this arc. Further, the Disuga porphyry from prospecting area shares similar characteristics with the other two ore-bearing porphyries in magma source, fractionation process, oxidation state, and S content. Therefore, the Disuga porphyry may have the potential to form a large porphyry Cu deposit. No large deposit has been found in the Disuga area is probably due to the intensive denudation in this area.