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Preliminary analyses by microthermometry of fluid inclusions in detrital quartz of the Upper Cambrian Lamotte Sandstone revealed the occurrence of 2 distinct groups of aqueous fluid inclusions. Specific salinity signatures and homogenization temperatures may be used to distinguish specific granite types of the source rock terrain. The inclusions chosen for analysis occur in isolated clusters or are randomly distributed within a grain, commonly in association with mineral inclusions of zircon, sphene, rutile, and/or tourmaline. Secondary inclusions are present in the Lamotte but are not included in this study.
The first group of inclusions is characterized by low salinities (< 1.0-8.0 wt. % eq. NaCl), the second by high salinities (12.1-29.6 wt. % eq. NaCl). Final melting temperatures as low as -30.6°C indicate the presence of divalent ions in these inclusions. Both groups yield homogenization temperatures of between 150°C and 220°C. The low-salinity inclusions occur predominantly in subrounded to well-rounded sand less than 1.0 mm in size that is derived from a distal source. The brine inclusions occur exclusively in subangular to angular gravel 2.0-3.0 mm in size, implying a more proximal source area.
A comparison of these inclusions with inclusions found in the granites of the apparent source terrain indicates that a medium-silica amphibole-orthoclase granite (Slabtown type) or a low-silica amphibole-plagioclase granite (Sivermines type) or both are the primary source rocks for this quartz. These granite types have limited areal distribution in the present-day St. Francois mountains and the identification of these granite types as the source rock for the locally derived quartz has broad implications for reconstructing Cambrian depositional environments and paleostructure of the ancient St. Francois mountains.
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