MINERALOGICAL EVIDENCE FOR HYDROTHERMAL ORIGIN OF BRANDON RESIDUAL FORMATION, VERMONT?

The Brandon Residual Formation (Brandon, VT) is part of a broken chain of kaolin deposits trending along the Green Mountain Front from Bennington to Monkton, Vermont. The unlithified sediments of the Brandon Residual Formation unconformably overlie the contact between the Cambrian Cheshire Quartzite and Dunham Dolomite, and are associated with extensive lignite deposits. Paleobotanical evidence derived from fossil flora found within the lignite deposits, indicate Early Miocene age limits and a depositional environment similar to that of the present day Gulf Coast, USA. The associated kaolin deposits are therefore likely linked with an intense weathering environment.

X-ray diffraction analyses of both oriented and randomly oriented clay samples associated with the lignite were performed to assess the weathering origin of the kaolin deposits. Preliminary XRD patterns show sharp basal peaks indicating that the Brandon kaolins are well ordered, well crystalline, and show few defects. Preliminary patterns also indicate a predominance of well crystalline kaolinite coexisting with minor amounts potassium feldspar, illitic mica, and plagioclase. Several samples also show R1-ordered illite/smectite.

The presence of well ordered/crystalline kaolinite coexisting with potassium feldspar, illitic mica, and plagioclase, as well as the existence of R1-ordered illite/smectite in several samples indicate a possible hydrothermal origin for the Brandon Residual Formation. If this is true, it implies Mesozoic or Paleozoic origin of the Brandon Residual Formation, and might indicate that Miocene coal forming swamps formed atop a previously existing, clay-rich substrate. Scanning electron microscope work will provide further analysis of the crystal morphology and chemical composition of the kaolin deposits.