Aluminum precipitates control the hydrochemistry and mineralogy of a broad variety of environments on Earth (e.g., acid mine drainage, AMD, coastal wetlands, boreal and alpine streams, tropical acid sulfate soils, laterites and bauxites, …). However, the geochemical and mineralogical processes controlling Al (and other associated metals and metalloids) transport and removal in those environments are not fully understood. The geochemical system of Paradise Portal (Colorado, USA) comprises sulfate-rich mildly acidic waters, the hydrochemistry of which is directly controlled by the massive precipitation of hydrobasaluminite Al4(SO4)(OH)10·12-36H2O. Three connected but discernible aluminum precipitation stages were identified and described: 1) nanoparticle formation and size decrease along the creek, 2) hydrobasaluminite neoformation on the riverbed, and 3) precipitate accretion and accumulation on the riverbed leading to Al and Fe banded formations. The co-occurrence of Al and Si in the system was observed, recording significant amounts of Si accompanying the three different components of the system (i.e., nanoparticles and fresh and aged Al-precipitates). Also, abrupt and minor changes in the sedimentary record were described and proposed to be the response of the system to seasonal and interannual changes in AMD chemistry. Concerning the mobility of other metals and metalloids, P, Th, V, W, Ti and B showed a tendency to be preferentially incorporated into hydrobasaluminite, while others like Be, As, Se or Ba tend to remain dissolved in the water. © 2019 Elsevier B.V.

This study was partially financed by the program Fondecyt Iniciación N°11150002 and U-Inicia from the University of Chile. M.A.C. was financially supported by the Spanish Ministry of Education through the Post-doctoral International Mobility Subprogramme I+D+i 2008–2011. M.A.C. gratefully acknowledges the support from the Advanced Mining Technology Center of the University of Chile. The authors thank M. Cabañas and R. Bartrolí (IDAEA-CSIC), Chris Winkler and Stephen McCartney (NanoEarth-NCFL), Cristobal Cantero (Servicios Centrales I+D-UHU) for their analytical assistance. RBW and PLV were supported by the Mineral Resources Program of the U.S. Geological Survey. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement of the U.S. Government. We would also like to thank Dr. Karen Johannesson (Editor) and two anonymous reviewers for their suggestions and comments that significantly improved the quality of the original manuscript.

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