URANIUM-RICH PHOSPHORITE BRECCIAS IN CAMBRIAN DOLOSTONE AS SOURCE OF ELEVATED URANIUM AND ALPHA RADIATION IN A BEDROCK AQUIFER, VERMONT, USA

Groundwater produced from the Clarendon Springs Formation, a Late Cambrian dolostone in northwestern Vermont, contains elevated uranium and alpha radiation to the extent that 23 % of bedrock wells (30 of 131 wells) in a 12 km² area produce concentrations of alpha radiation above the EPA’s Maximum Contaminant Level (MCL) of 15 pCi/L – mean gross alpha concentration in this area is 32 pCi/L, and > 90% of wells with elevated alpha radiation exceed the Vermont MCL for uranium of 20 ppb. Geologic mapping with a gamma ray spectrometer indicated that the origin of the radioactivity is a dolostone breccia containing black chips of phosphorite approximately 1 cm in length and 2 mm in thickness. The chips are frequently oriented parallel to bedding and appear to have been deposited as sedimentary rip-up clasts.

In order to better understand uranium speciation and origin in the rock, representative samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy with an energy dispersive spectrometer (SEM-EDS), inductively coupled plasma - atomic emission spectrometry (ICP-AES) and ICP-mass spectrometry. XRD indicates that the black chips are dominantly composed of flouroapatite, which contrasts sharply with the dolostone matrix of dolomite, quartz and calcite. ICP analysis indicates that black chips contain uranium ranging from 80 – 210 mg/kg compared to < 10 mg/kg in dolostone matrix. SEM-EDS confirms the presence of flouroapatite as the dominant mineral in phosphorite black chips, but even more importantly also reveals localized 5 – 10 micron crystals of U-bearing pyrite and a uranium-rich mineral that is likely autunite (suggested by EDS elemental ratios and XRD data). Oxidation of pyrite and dissolution of secondary autunite may be the main mechanisms by which U is released to groundwater.

The location of U-rich phosphorite breccias in carbonates deposited on a paleo-slope between the continental shelf and deep-water Franklin Basin implies either (1) a syn-depositional fixation of U or (2) deposition-related compositional controls (e.g. phosphate or organic matter in black chips [rip-up clasts]) that facilitated diagenetic fixation of U. Ongoing work consists of detailed XRD and SEM-EDS analysis designed to precisely understand U speciation and mobility.