Gold-hosting high Ba-Sr granitoids in the Xincheng gold deposit, Jiaodong Peninsula, East China: Petrogenesis and tectonic setting

摘要

The Xincheng deposit is the only large gold deposit with a proven reserve of ＞2001 gold hosted by the Early Cretaceous granitoids in northwest Jiaodong Peninsula, East China. The granitoids hosting this ore deposit comprise an inner medium- to fine-grained quartz monzonite and an outer medium- to coarse-grained monzogranite with distinctive K-feldspar megacrysts. LA-ICP-MS zircon dating yields U-Pb ages of 128 ± 1 to 132 ± 1 Ma and 127 ± 2 to 129 ± 1 Ma, for the quartz monzonite and the monzogranite, respectively. The Early Cretaceous ages obtained in our study are comparable with the 126-130 Ma age range reported for the Guojialing granitic suite. The monzogranites, typical high Ba-Sr granites, possess high SiO_2 (70.89-73.35%), K_2O (3.85-4.32%), total alkalis (K_2O + Na_2O = 8.08-8.68%), Sr (634-888 ppm), Ba (1395-2111 ppm) and LREE (59.43-145.88), with low HREE and HFSE contents and insignificant Eu anomalies. The rocks display markedly high Sr/Y (114-297) and (La/Yb)_N (20-79) ratios. They have low MgO (0.23-0.62%), Cr (0.4-8.33 ppm) and Ni (0.47-2.92 ppm) contents. The typical high Ba-Sr signatures of the outer acidic monzogranites are also shared by the inner intermediate-acidic quartz monzonites, with a relatively higher abundance of these elements. The plagioclases in the quartz monzonites and monzogranites are oligoclase-andesine with An contents of 11.7-44.5%, and oligoclase with An contents of 12.9-29.3%, respectively, which both show the reverse zoning texture. The quartz monzonites have zircon ε_(Hf)(t) values of-21.3 to -13.9 (average -18.7), which are less negative and show larger variations than those of the monzogranites {ε_(Hf)(t) = -24.7 to -18.1, average -19.5). Detailed elemental, mineralogical and isotopic data suggest that the high Ba-Sr quartz monzonites and monzogranites were most likely generated by partial melting of the basement rocks of the Jiaobei terrane accompanied by crustal assimilation, with minor addition of the intermediate magma derived from the partial melting of juvenile mafic lower crust formed by the earlier underplating of mantle magma, and the quartz monzonites may represent the path of intermediate magma inputting into felsic magma. In combination with previous investigations, we suggest subduction of the paleo-Pacific slab beneath the North China Craton (NCC) and associated asthenosphere upwelling were most likely the mechanism associated with the generation of the high Ba-Sr granites.