Uranium and microcracks in a 1,000-meter core, Redstone, New Hampshire

Abstract

The spatial distribution and mineralogical association of uranium in 30 samples of a 1,000 meter core from the Redstone (NH) Quarry were examined with fission track techniques, backscattered electron imaging, and energy dispersive X-ray spectrometry with the following results for the Conway and Mount Osceola granites: (1) Uranium occurs in microcracks sealed with siderite and/or with Ca+ RE fluorocarbonate minerals; the uranium contents of siderite and the RE fluorocarbonates are 1 to 30 ppm and 500–1,500 ppm, respectively. (2) Siderite and to a lesser extent RE fluorocarbonate minerals occur as replacement minerals of amphibole and biotite; the RE fluorocarbonates are the main alteration minerals of allanite. (3) The degree of alteration and the extent of uranium redistribution are greater in samples with major uraniferous microcracks. (4) The contents of Ca and Mn are approximately constant for siderite in sealed cracks throughout the Conway and Mount Osceola granites for the entire section penetrated by the corehole, approximately 700 meters. — From these observations, we conclude that an extensive set of fractures throughout a significant volume of the Mount Osceola and Conway granties were the conduits through which hydrothermal fluids containing CO2, uranium, and REs circulated. Microcracks facilitated fluid-rock interactions resulting in alteration and uranium redistribution. Certain primary uraniferous accessory minerals, i.e., allanite and ilmenorutile may have been the source for at least part of the uranium and REs that now occur in secondary minerals in these granites.