矿床地球化学国家重点实验室知识库

Many similar to 260 Ma mafic-ultramafic layered intrusions, including Hongge and Panzhihua, in the Emeishan large igneous province, southwestern China host world-class Fe-Ti-V oxide ore deposits. These two most important ore-bearing intrusions show differences in lithology and mineral chemistry. The most important orebodies in the Hongge intrusion occur as concordant layers in the middle part of the intrusion, closely associated with clinopyroxenites. Titanomagnetite and Mg-rich ilmenite are the major ore minerals of the Hongge deposit. Coexisting clinopyroxene contains >1.7 wt % TiO2. These data indicate high Ti parental magma for the Hongge ore-bearing clinopyroxenites. In the Panzhihua intrusion, the most important orebodies also occur as concordant layers but in its lower part instead of middle part. In contrast with the Hongge deposit, the most important host rocks of the Panzhihua deposit are gabbros, not clinopyroxenites. In addition, ilmenite is rare and titanomagnetite is predominant in the Panzhihua deposit. Coexisting clinopyroxene in the Panzhihua deposit contains <1.6 wt % TiO2. The contrasting lithologic and mineral compositions indicate that the parental magma for the Hongge deposit has higher TiO2 than that for the Panzhihua deposit. The compositions of clinopyroxene from the Panzhihua and Hongge deposits indicate that their parental magmas are also different in MgO/FeO and trace element ratios as well. Calculations using average experimental Mg-Fe exchange coefficient and trace element partition coefficients between clinopyroxene and magma show that the Hongge parental magma has higher MgO/FeO ratios and more fractionated mantle-normalized trace element patterns than the Panzhihua parental magma. The estimated compositions of the parental magmas for the Hongge and Panzhihua ore-bearing lithologies resemble the average compositions of the Longzhoushan-type high Ti basalts and Ertan-type intermediate Ti basalts in the Emeishan large igneous province, respectively. The new trace element data from this study, together with available Sr-Nd isotope data for the ore-bearing intrusions from literature, support a new petrogenetic model involving selective assimilation of newly subducted, stagnant oceanic gabbroic slab above the deep-seated Emeishan mantle plume. This process and subsequent contamination with the upper crust played an important role in the variation of parental magma compositions between the Hongge and Panzhihua magmatic oxide ore deposits. Abundant Fe-Ti oxide ore deposits associated with less evolved basaltic magma in the Emeishan large igneous province than elsewhere in the world are attributed to selective assimilation of newly subducted, stagnant oceanic lithospheric slab by the ascending mantle plume-derived picritic magma that was originally undersaturated with Fe-Ti oxides.