Paragenesis, Stable Isotopes, and Molybdenite Re-Os Isotope Age of the Lala Iron-Copper Deposit, Southwest China

Abstract

The Lala Fe-Cu deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, SW China, is hosted in the Paleoproterozoic Hekou Group. This volcanic-sedimentary sequence has undergone upper greenschist-lower amphibolite facies metamorphism at ~850 Ma. The deposit contains more than 200 million metric tons (Mt) of Fe-Cu ores with an average grade of 13 wt % Fe and 0.92 wt % Cu. Orebodies are roughly strata bound but occur along foliation, lithologic contacts, and shear zones. The paragenetic sequence of the Lala deposit includes metamorphosed (I, II, and III) and unmetamorphosed (IV and V) stages. Stage I is characterized by extensively pervasive Na alteration, forming abundant albite but minor scapolite. Stage II occurs as massive and banded replacements, consists of Ti-V–poor magnetite and apatite with minor disseminated sulfide minerals, and was associated with pervasive K-Fe alterations forming K-feldspar, chlorite, and actinolite. Stage III occurs predominantly as bands and disseminations confined along foliations with minor veinlets cross-cutting hosting rocks. This stage is characterized by abundant Fe-Cu-Mo sulfides intergrown with angular magnetite and minor titanite and allanite and is associated with alteration of carbonate, quartz, fluorite, and mica. Stage IV has an assemblage dominated by chalcopyrite with variable pyrite and bornite, occurring as veins of sulfides + quartz + calcite + biotite ± muscovite ± fluorite. Stage V occurs chiefly as veins of hematite ± calcite ± quartz and also results in extensive hematization. Stages II and III have fluids with high δ 18 O (7.2–11.7‰) and δ 34 S values (0–4‰), indicative of a magmatic-hydrothermal origin. However, the stage III fluids have δ 34 C values (~0.5‰) close to the least altered marble (0–2‰) of the hosting rocks, indicating a possible contribution of strata-sourced carbon. The stage IV fluids have δ 34 C values similar to and δ 18 O (4.1–6.7‰) lower than the stage III fluids, and a wider range of δ 34 S values (−9.0 to +10.5‰), consistent with significant contributions of strata-sourced sulfur. Fluids of the late barren vein stage (V) have relatively low δ 34 C (−4.0 to −2.4‰) and δ 18 O values (−2.9 to +0.6‰), indicative of significant contributions of meteoric and oxidized fluids. Molybdenite from stage III has an Re-Os isotope age of 1086 ± 8 Ma, providing tight constraints on the timing of the main stage of mineralization. This age indicates that the Lala deposit is coeval with the ~1100 Ma intraplate magmatism in the western Yangtze Block. We propose that the majority of the Fe-Cu deposits in the Kangdian IOCG metallogenic province may have formed in an intraplate extensional setting, although it is possible that some of the deposits may have involved in multiple events including modification by the Neoproterozoic metamorphism forming the mineral assemblages of stage IV.