Uranium mineralization in the Hawks Crag Breccia of the Lower Buller Gorge Region, South Island, New Zealand

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Abstract
Bedded uraniferous deposits have been discovered in the middle (?) Cretaceous Hawks Crag Breccia formation of the lower Buller Gorge region of the South Island of New Zealand, and in three areas to the south (Bullock Creek, Waitahu River, and Fox River mouth). The characteristic feature in all the areas is the restriction of the mineralization to the arkosic facies of the Hawks Crag Breccia. On the north side of the Buller Gorge, at least ten uraniferous horizons are known, ranging in width from a few inches to several feet. Within each horizon the mineralization is patchy. The main primary uranium mineral is coffinite which has been introduced along with pyrite, carbonate (largely ferroan dolomite), and red hematite staining of the clastic feldspar. Secondary uranium minerals are generally inconspicuous. The deposits are considered to result from low-temperature mineralization by epigenetic solutions, but it is still uncertain whether the uranium originated from dispersed sources in the arkosic sediments or was introduced in hypogene solutions from lamprophyres and porphyries. Minor uranium concentrations are associated with a poryphyry dyke in Hawks Crag Breccia, and with quartz veinlets and lodes in granite-gneiss and Greenland sediments. A comparison is made between mineralization in the Buller Gorge and on the Colorado Plateau.

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