Constraints on the formation of the Baogutu reduced porphyry copper deposit (West Junggar, NW China): Assessing the role of mafic magmas in mineralization

Abstract

The Baogutu porphyry copper deposit is a typically reduced porphyry copper deposit (RPCD), characterized by reduced, calc-alkaline, felsic shallow intrusions. A newly discovered gabbro, which contains 1182–1260 ppm Cu, provides an ideal window to reveal the genesis and formation of the Baogutu RPCD. A combined study of the petrological, geochronological, geochemical and isotopic characteristics of the Cu-bearing gabbro and granitoids in the Baogutu RPCD has been completed. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of zircons from a gabbro, diorite and porphyritic diorite are 318.2 ± 0.8 Ma (MSWD = 0.01, n = 16), 315.8 ± 1.0 Ma (MSWD = 1.50, n = 16), and 314.9 ± 0.6 Ma (MSWD = 0.12, n = 20), respectively. The geochemical and isotopic characteristics of the gabbro, diorite, and porphyritic diorite at Baogutu RPCD are all metaluminous, calc-alkaline series rocks with depletion in Nb, Ta, and Ti and enrichment in light rare earth elements (LREE) and large ion lithophile elements consistent with an arc-related setting. The relatively high Nd and Hf isotopes (εNd(t) = +6.9 to +7.5, εHf(t) = +8.8 to +14.4) and wide variations of major and trace elements suggest that the gabbro and diorite in the Baogutu RPCD were all derived from a mixture of melts of metasomatized mantle wedge, the hydrous slab, and old crustally derived materials. The high Mg andesites (HMA)/sanukite features (high Mg#, Cr and Ni content) of the gabbro suggest it was derived mainly from a metasomatized mantle-hydrous slab-derived melt source with only slightly contamination by the old crustal materials, whereas adakitic features of the diorite (high Al2O3 and Sr contents and Sr/Y ratios) suggest a greater role the melting of residual subducted oceanic crust with juvenile mantle materials. The porphyritic diorite, which inherited the geochemical characteristics of diorite, was probably derived from a similar parental magma but the low εNd(t) of +1.4, old T2DM (Nd) age of 961 Ma and enrichment in Zr and Hf suggest a greater degree of crustal contamination. The high content of Cu and high oxidation state of the gabbro is consisted with it having formed early from an unusually Cu-rich parental magma. This magma would have provided abundant metal and sulfides which would have been mobilized and reconcentrated by later ore bearing fluids linked to the intermediate to acid porphyry intrusions in the Baogutu RPCD.