In situ geochemistry and Fe–O isotopic composition of iron oxides from the Pha Lek Fe deposit, northwest Truong Son orogenic belt, Laos: implications for ore-forming processes

The Pha Lek Fe-polymetallic deposit in the southwest of the Truong Son orogenic belt records the evolution and metallogenesis of the southeastern Asia Tethys. Debates remain on the roles the Late Triassic volcanism played in the formation of the iron oxides of the deposit. Petrographic and SEM studies led to the recognition of two dominant iron oxides (magnetite and hematite) and three stages of mineralization (pre-ore, syn-ore, and post-ore stages). In situ LA–ICP–MS analyses for trace elements and Fe and O isotopic analyses were applied to understand the origin of the iron oxides. Generally, iron oxide of the syn-ore stage has higher total trace elements than post-ore stage, and magnetite has higher lithophile elements, whereas hematite has higher chalocophile elements. The trace-element abundance in magnetite is most comparable to porphyry-skarn-hydrothermal magnetite, and hematite is more comparable to supergene and secondary, non-magmatic hydrothermal chemistry. All δ56Fe values of the iron-oxide separates are above 0‰ and have an igneous signature, and syn-ore stage magnetite has relatively higher δ56Fe values than the post-ore stage hematite. All δ18O values of the iron-oxide separates are below 1‰ and plot within the non-igneous field, and show a similar decreasing trend from syn-ore stage magnetite to post-ore stage hematite. We propose that the Triassic volcanic activities played a limited role during the mineralization process; the deposit was formed because of the Late Carboniferous–Early Permian, porphyry-skarn-related, magmatic hydrothermal alteration; and the mixing of non-magmatic fluid and/or the supergene weathering caused the wide reprecipitation of Fe oxides. An evolutionary mineralization model related to the low-angle subduction for the deposit is also proposed.