Oxidized fluid superimposition triggered high-grade Cu-Au mineralization: Apatite and titanite constraints from the Pangkham Cu-Au deposit, northwestern Laos

Constraining the temporal and redox evolution of magmatic-hydrothermal fluids is essential for understanding the genesis of sulfide-metal deposits. However, linking high-precision geochronology to specific fluid transformations remains challenging due to the scarcity of alteration minerals preserving both reliable ages and redox-sensitive signatures. Here, we identified two generations of hydrothermal apatite (Ap1 and Ap2), together with hydrothermal titanite coeval with Ap1, associated with ore minerals in the Pangkham deposit, a major Cu-Au resource in the Loei fold belt of Laos, and conducted in situ dating and geochemical analyses. Ap1 occurs with albite, epidote, chlorite, and minor sulfides, and is characterized by relatively high rare earth element contents and negative Eu anomalies, whereas coeval titanite displays positive Eu anomalies. These contrasting Eu signatures likely reflect different crystal-chemical controls on Eu incorporation, together with feldspar-buffered fluid-rock interaction during the early alteration stage. In contrast, Ap2 coexists with magnetite, hematite, and abundant sulfides, and exhibits less negative to positive Eu anomalies, indicating crystallization from a relatively more oxidized hydrothermal fluid during the later mineralizing stage. In situ U-Pb dating reveals that Ap1 and titanite formed at 304.7 ± 6 Ma and 306.9 ± 3.5 Ma, respectively, whereas Ap2 yields a younger age of 296.4 ± 5.1 Ma. The early hydrothermal activity (ca. 307−305 Ma) was broadly synchronous with arc magmatism related to Paleo-Tethys subduction, whereas the main mineralization (ca. 296 Ma) is interpreted to have formed during the continued subduction with possible slab rollback, which promoted the influx of highly oxidized fluids. This study highlights that the superimposition of late-stage oxidized fluids onto a fertile magmatic-hydrothermal system was a critical mechanism for high-grade Cu-Au formation, thereby refining the metallogenic framework of the Loei fold belt.