Constraints of Sr isotopic compositions of apatite and carbonates on the origin of Fe and Cu mineralizing fluids in the Lala Fe-Cu-(Mo, LREE) deposit, SW China

摘要

Numerous Fe-Cu deposits with mineralization styles similar to iron oxide-copper gold (IOCG) deposits form the Kangdian Fe-Cu metallogenic province, southwestern (SW) China. As one of the largest deposits in the region, the -1.0 Ga Lala Fe-Cu deposit is hosted in a Paleoproterozoic volcanic-sedimentary succession named the Hekou Group which is alternately intruded by -1.0 Ga doleritic plutons. This deposit has a paragenetic sequence evolving from Stage 1 of Na-alteration to Stage II of Fe mineralization, and finally to Stage III of Cu-(Mo, REE) mineralization, coeval with mafic-felsic intra-plate magmatism in the region. This study conducted in-situ Sr isotopic analyses on apatite and carbonate, aiming to resolve the long controversial issue regarding the origin of the Fe and Cu mineralizing fluids in the deposit. Apatite of Stage II has ~(87)Sr/~(86)Sr ratios varying from 0.71380 to 0.72733, much higher than those of synchronous igneous rocks in the region (0.7074 to 0.7091), but similar to the Paleoproterozoic host rocks (0.71368 to 0.71837 at ~ 1.0 Ga). This similarity indicates that radiogenic Sr of the Fe mineralizing fluid was dominantly sourced from the host rocks. Apatite and calcites of Stage III have ~(87)Sr/~(86)Sr ratios (0.75758-0.79293) much higher than apatite of Stage II and the host rocks but similar to the Ar-chean basement rocks (as high as 0.80 at -1.0 Ga) beneath the cover of the Yangtze Block, suggesting that the highly radiogenic Sr isotopic composition of the Cu mineralizing fluid was mainly inherited from the old basement rocks. In combination with previous C-O-S isotopic data indicating a magma-hydrothermal origin, it was suggested that the Fe mineralizing fluid was exsolved from a mafic magma that generated the -1.0 Ga doleritic plutons, and inherited radiogenic Sr from the host rocks during fluid-rock interaction. By contrast, the Cu mineralizing fluid might have been sourced from another pulse of magmatic, Cu-Mo-REE- and CO_2-rich fluid which have once interacted with Archean basement rocks prior to mineralization. The source of such a Cu-Mo-REE-rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma. We propose that intrusions of the bimodal magmas in Kangdian are responsible for regional hydrother-mal circulation which led to Fe-Cu-(Mo, REE) mineralization in the Kangdian province.