WHAT'S WEATHERING IN LA SIERRA DE LAS MINAS?

Chemical weathering of silicate rock in the terrestrial environment and the subsequent precipitation of carbonate minerals in marine sediments serve as the primary geological long-term sink of CO₂ from Earth’s atmosphere. Previous studies of chemical weathering in small mountainous rivers have shown their disproportional role in chemical weathering and delivery of dissolved and particulate materials on a global scale. Our group’s studies of chemical weathering in small mountainous rivers focus on the stream water chemistry but require knowledge of the mineralogy, petrology and lithology of the rocks which are the source of the dissolved products of chemical weathering. La Sierra de las Minas, the mountain range that is the focus of this study, is located in east central Guatemala and rises approximately 3000 m above sea level. Petrologic analysis of thin sections made from stream bed cobbles show the rocks are a felsic gneiss, composed primarily of quartz, alkali feldspar (albite to oligoclase composition), biotite, hornblende, with trace amounts of epidote, allanite, monazite and apatite. Analysis of these samples by light and scanning electron microscopy shows that the calcium bearing phases are generally more highly weathered than other mineral phases and contribute disproportionately to the total weathering flux. With bulk chemistry data gathered using x-ray fluorescence analysis the chemical index of alteration was shown to increase with decreasing elevation. Further investigation showed trace amounts of carbonate in lower elevation rocks, in parallel with results found in previous studies. While accessory carbonate has been found in thin section, which would suggest the precipitation of carbonate from solution, initial PHREEQC results show a low saturation index for calcite, -2.52 to -4.64, and an even lower saturation index for dolomite from -5.25 to -9.83. Geochemical modeling with PHREEQC combined with a knowledge of the petrology of collected stream cobbles will determine the lithology from which the observed weathering products originated.