EXAMINING MINERALOGICAL INFLUENCES OF BEDROCK ON WATER CHEMISTRY THROUGH CHEMICAL WEATHERING EXPERIMENTS

The effects of the Waits River Formation (WRF) and the Hatfield Pluton (HP) on stream water chemistry is not fully understood. The WRF is a quartz-biotite schist containing beds of impure marble. The HP is tonalite dominated by plagioclase feldspar and quartz with hornblende, biotite, and minor clinopyroxene. Tributaries proximal to these formations have elevated acid neutralizing capacities (ANC) and SO₄ concentrations. Although calcite in marble from WRF can explain elevated ANC for some streams, the source of SO₄ is not clear. Additionally, low ANC values are expected for waters draining HP because of low chemical weathering rates of silicate minerals.

To examine these questions, we conducted weathering experiments of each formation with rain water (pH = 4.9) at room temperature for 2.5 weeks. Samples were passed through a 0.45μm filter and analyzed for pH, ANC, major cations and anions, total Fe and Al, and dissolved silica. Petrography and scanning electron microscopy (SEM-EDS) of thin sections from the freshest portion of samples informed rock compositions.

As expected, the marble of WRF yielded the highest Ca and ANC concentrations, and the largest increase in pH; Mg, K, and SO₄ increased in minor amounts. In contrast, the biotite-schist of WRF resulted in negative ANC with high concentrations of SO₄, total Fe, SiO₂, Ca, K, Mg, and detectable Al. These results suggest pyrite in schist, not visible in hand sample, is an important source of SO₄, and the generated acidity accelerates silicate weathering, particularly biotite. The presence of pyrite is supported by square-shaped pits containing sulfur, detected by SEM-EDS. On a watershed scale, acidity from pyrite in WRF is neutralized by calcite weathering.

HP showed less change in water chemistry and conflicting results. The light sample, which contained fewer mafic minerals, showed little change in water chemistry. In contrast, the dark sample contained elevated levels of Ca and ANC; however, low concentrations of Mg and SiO₂ suggests only minor weathering of mafic minerals. The results from the dark pluton are reflective of calcite weathering, yet no calcite veins were observed. Further work is needed to understand the full mineralogy of this formation and whether trace calcite and sulfur-bearing minerals may be present to explain stream water chemistry.