A middle Precambrian uranium- and apatite-bearing horizon associated with the Vihanti zinc ore deposit, western Finland

Abstract

A uranium- and apatite-bearing horizon was found in close association with the pyrite, zinc, and copper ores of the middle Precambrian Lampinsaari ore complex, Vihanti. The characteristic rock types in the horizon are phosphorite-banded dolomite and fine-grained apatite-bearing quartz-plagioclase gneiss, a metamorphic derivative of phosphatic tuff. Parts of the phosphatic metatuff contain pyrite bands and, on the basis of position and geochemical characteristics, the pyrite-banded metatuff is regarded as a termination of the pyrite ore proper and hence, at least partly, the U-P rocks belong to the same horizon.Uranium shows a strong correlation with apatite in both distribution and content. In the phosphorite bands uranium seems to be mainly bound in uraninite and in the phosphatic metatuff mainly in apatite. The mean uranium content in the horizon is 0.03 percent and the mean P 2 O 5 content of grade 3 to 5 percent. The thorium content was found to be less than 10 ppm.The isotopic composition of sulfide sulfur was determined in 43 analyses and found to be strongly dependent on the lithology of the horizon. The changes in pH of the source fluids are regarded as the principal factor behind this interdependence. The hydrothermal temperature range is assumed for the formation of at least the phosphorite-banded dolomite. A volcanic exhalation mechanism is favored by the delta 34 S pattern of the sulfides as well as by the paleolithology of the uranium- and apatite-bearing horizon.