DOCUMENTING THE NATURE AND TIMING OF FLUID-ROCK INTERACTION IN NEOPROTEROZOIC SILICICLASTIC ROCKS OF NORTHERN UTAH

The timing and extent of fluid-rock interaction in siliciclastic rocks is critical data for tectonic and basin analysis studies, and understanding crustal migration of geofluids. Evidence for fluid flow is often cryptic in these rock types, preserved as diagenetic cements and alteration products. Geochronological study of authigenic minerals forming syn-fluid-rock interaction provide quantitative information about the spatial and temporal extent of fluid-flow. We report field, petrographic, and geochronologic evidence for post-depositional geofluid circulation and attendant mineral growth in quartzite and volcaniclastic deposits in northern Utah. These units experienced a protracted history of repeated burial and unroofing, and near-field and far-field tectonism resulting in fluid-rock interaction.

The Mutual and Browns Hole formations (Fm) are successive units of the late Neoproterozoic-early Cambrian Brigham Group, a sequence of marine, fluvial, and lacustrine strata. The Mutual Fm is a regionally extensive, hematite cemented orthoquartzite with interlayered mudstones, locally overlain by the Browns Hole Fm. Near Huntsville, UT, the Browns Hole Fm comprises hematite-cemented volcaniclastic sandstones and siltstones coarsening up into volcaniclastic conglomerates and basalt flows. This is overlain by quartz arenite of the Terracotta Member of the Browns Hole Fm. Basalt in this interval exhibits secondary hematite mineralization in a northwest trending m-scale swath. Teardrop shaped vesicles, mm- to cm-scale, are filled with µm-scale specular hematite, quartz, and likely zeolites. Initial microscale observations of the quartzite units from the Huntsville section reveal <5-10 μm diameter authigenic monazite associated with altered mm-scale potassium feldspar. These monazites exhibit irregular shapes, and electron microprobe analysis (EMPA) indicate some contain compositional domains, perhaps reflecting multiple precipitation events. Pilot EMPA U-Th-total Pb data of these monazites suggest at least two episodes of growth at 500 ± 300 Ma and 300 ± 20 Ma. Additional petrology, monazite U-Th-Pb geochronology, and hematite (U-Th)/He dating of coarse-grained specularite plates is underway to document neomineralization related to fluid infiltration and fluid-rock interaction.