When birds meet bauxite: Supergene Al-phosphate formation via guano infiltration in the Connétable Islands (French Guiana)

• New geochemical, mineralogical and geochronological data of the Connetable Islands. • Detailed model of formation of bauxite-hosted Al-phosphate and sulfate minerals. • Impact of birds’ guano in the formation rare Al-phosphate minerals. • Oligocene lateritization to Mio-Pliocene bauxitization. The Grand Connétable (GC) and Petit Connétable (PC) Islands in French Guiana are iconic sites of geological heritage and hotspots of marine biodiversity. They are notable for their peculiar Al-phosphate deposits, which were previously exploited and are related to deep weathering profiles at sea level. Although the mineralogy of some Al-phosphate minerals has been described, there is no geological model for the formation of these weathering profiles and ore deposits. Therefore, we acquired geochemical, mineralogical, and geochronological data on the GC and PC islands to address this issue. Our findings revealed that the GC island, which rises up to ∼ 70 m above sea level (a.s.l.), consists of a weathering profile exceeding 35 m in thickness, dominated by gibbsitic regolith (>30 m), and capped by an Fe-Al-P-duricrust. This bauxite deposit is thicker than those found on land in French Guiana, raising questions about the role of basement rock type, morphological factors, and ocean proximity in the development of bauxite deposits. The classic bauxitic profile at GC has recently been infiltrated by phosphoric acid from marine birds’ guano derived. This has reacted with gibbsite to form rare Al-phosphate minerals such as variscite, meta -variscite and planerite, as well as Fe-phosphate minerals in a lower extent, down to the base of the profile. Ca-K-Al-sulphate minerals have been recognised to a lesser extent and can be traced back to the impact of sea spray in providing additional elements to the regolith. The PC Island, which is ∼2 m high a.s.l. is made of a somital part of gibbsitic regolith, with only the Fe-Al duricrust is emerging. Hematite and goethite (U-Th)/He geochronology indicate that successive laterization (since the Oligocene) and bauxitization (since end of the Miocene) processes occurred in the area between ∼30 and ∼3 Ma, before the formation of the islands. The geochronological results are similar to those obtained from dated weathering profiles in French Guiana and Suriname, indicating a regional shift toward a more humid climate at the end of the Miocene. This information enables us to better define and discuss the Al-phosphate ore deposit over time, as well as its regional geological and paleoclimatic implications.