WEATHERING OF REE-BEARING MINERAL PHASES IN THE SIERRA DE LAS MINAS: IMPLICATIONS FOR THE TOTAL CA FLUX

Recent work on weathering in streams draining La Sierra de Las Minas, Guatemala, shows that precipitation-corrected Ca/Na ratios in many of these streams draining felsic lithologies are greater than the Ca/Na ratio in feldspars, suggesting that weathering of other Ca-bearing phases in these rocks is important. Stream cobbles of the Paleozoic San Agustín gneiss were collected and analyzed for evidence of chemical weathering using an FEI Quanta FEG scanning electron microscope (SEM) equipped with a Bruker energy dispersive X-ray spectroscopy (EDX) detector to determine chemical composition. EDX chemical maps of weathered rock surfaces show that Ca is present primarily in feldspar (albite to oligoclase composition), epidote group minerals (epidote and allanite), apatite and monazite. Analysis by SEM shows evidence of dissolution, with euhedral etch pits observed on mineral surfaces and precipitation of secondary mineral phases (clays, iron oxyhydroxides, and phosphates). However, the Ca-bearing mineral phases appear to be more extensively etched than surrounding mineral phases. Epidote group minerals were frequently observed as inclusions within the sodic plagioclase phase, though epidote surfaces were indented below the albite surface due to preferential weathering of epidote compared to albite. Allanite, when observed, was extensively altered, exhibiting an almost sponge-like texture consistent with its metamict structure. Apatite and monazite showed evidence of dissolution with crystallographically controlled etch pits on their surfaces. Secondary rare earth element (REE) phosphate phases, florencite (REEAl₃(PO₄)₂(OH)₆) and rhabdophane (REEPO₄•H₂O), as well as cerianite (CeO₂), were observed in close proximity to weathered allanite, monazite and apatite, often infilling or replacing the dissolving phase. However, these secondary REE phases were also widely disseminated over the cobble surface, indicative of weathering of primary Ca-bearing REE and PO₄ phases and precipitation of secondary REE minerals. Results of the SEM analysis of the weathered rock surfaces suggest that even though trace Ca-bearing phases represent only a small fraction of the total Ca in the rock, their alteration textures indicate that they contribute disproportionately to the total weathering flux.