Quartz-amethyst hosted hydrocarbon-bearing fluid inclusions from the green ridge breccia in the snoqualmie granite, North Cascades,WA, USA

Martin Feely*, Alessandra Costanzo, Franzisca Lindner, Joe George, John Parnell, Stephen Bowden, Mas’Ud Baba, Peter Owens

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)
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The Green Ridge Breccia cuts the composite Miocene Snoqualmie Batholith in King County, WA, USA. The granite was emplaced at ~5 km depth between ~17 and 20 Ma and the crosscutting NW trending breccia contains large angular blocks of the host granite (<1 m 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 amber coloured oil inclusions within the quartz-amethyst crystals. Fluid inclusion studies using Transmitted Light Petrography, UV Microscopy, Microthermometry, Laser Raman Microspectroscopy and Gas Chromatography-Mass Spectrometry record the presence and the 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 amber coloured oil bearing two-phase immiscible liquid inclusions. The Green Ridge Breccia was the locus for convective hydrothermal fluid flow that formed the quartz-amethyst vugs formed at T~390 _C assuming a trapping pressure of ~1.65 kb. Later, hydrocarbon fluids migrated downwards from the roof source rock (e.g., the Guye Sedimentary Member) and were trapped 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 migrations into the Snoqualmie Batholith in areas where it forms the basement capped by the Guye Sedimentary Member.

Original languageEnglish
Article number174
Pages (from-to)1-16
Number of pages16
Issue number9
Publication statusPublished - 19 Sept 2017

Bibliographical note

We wish to acknowledge the constructive criticisms made by the three anonymous reviewers.Their suggestions and comments greatly improved the first version of the manuscript.


  • Hydrocarbon-bearing fluid inclusions
  • North cascade
  • Quartz-amethyst euhedra
  • Snoqualmie granite


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