USING FLUID INCLUSION STUDIES TO INVESTIGATE THE SOURCE OF HYDROCARBON FLUIDS IN THE MIOCENE SNOQUALMIE GRANITE, NORTH CASCADES, WA, USA

Worldwide occurrences of biogenic hydrocarbons in fractured granitic and other crystalline basement rocks are well documented. The Green Ridge Breccia cuts the composite Miocene Snoqualmie Batholith in King County, WA, USA. The granite was emplaced at ~5km depth between ~17 and 20Ma and the crosscutting NW trending breccia contains large angular blocks of the host granite (<1 metre in longest dimension). The brecciated granite blocks are cemented by quartz-amethyst euhedra (<10 cm in longest dimension) bearing vugs. A notable feature is the presence of centimetric scale yellow oil inclusions. The Guye Sedimentary Member of the Naches Formation contains interbedded leaf-bearing carbonaceous argillites Eocene in age. The country rocks display extensive folding and faulting that are pre-, syn- and post- granite emplacement. Fluid inclusion studies using Transmitted Light Petrography, UV Microscopy, Microthermometry, Laser Raman Microspectroscopy and Gas Chromatography-Mass Spectrometry record the presence and fluid composition of three fluid inclusion types hosted by the euhedra: primary Type 1 (liquid rich two-phase (L+V) aqueous inclusions) and secondary Type 2 bituminous two-phase (S+L) inclusions and Type 3 yellow oil bearing two-phase (L_oil+L_aq) immiscible liquid inclusions. The pressure, temperature and fluid composition of the three fluid inclusion types indicate that the Green Ridge Breccia was the locus for convective hydrothermal fluid flow and that the quartz-amethyst vugs formed at T~390°C assuming a trapping pressure of ~1.65kb. Later, and at shallower depths (~3km) and lower temperatures (~100°C) hydrocarbon fluids migrated downwards from the roof source rock, the Guye Sedimentary Member and were trapped along microfractures in the euhedra. This was followed by unroofing of the batholith and exposure of the Green Ridge Breccia. This study highlights the potential for other oil accumulations within the Snoqualmie Batholith in areas where it forms the basement capped by the Guye Sedimentary Member.