Uranium-lead and lead-lead investigations of minerals from the Broken Hill lodes and mine sequence rocks

CSIRO Div. Mineral., North Ryde, N.S.W., Australia

A regression analysis of combined rock and lode apatite data for the ²⁰⁷ Pb/ ²⁰⁶ Pb vs. ²⁰⁴ Pb/ ²⁰⁶ Pb plot gives an age of 1,565 + or - 20 m.y. (2sigma ) which is similar, within the experimental error limits, to the minimum U-Pb ages for monazite and Pb-Pb age for sphene from the mine sequence rocks of about 1,595 m.y. The possible differences in apparent age of the monazite-sphene and apatite may reflect the different blocking temperatures of the U-Pb systems in the two minerals. Cooling ages indicate that the Broken Hill Block remained at temperatures in excess of 600 degrees C for over 60 to 70 m.y. and would have allowed extensive concentration and/or recrystallization of the sulfide orebodies. On a concordia plot, the U-Pb data for zircons from two samples of upper granite gneiss and two of Potosi gneiss lie on an array with an apparent age of 1,663 (super +9) (sub -8) m.y. (2sigma ) and with a mean square of weighted deviates of 24.6. This age is interpreted as the time of granulite facies metamorphism. The least magnetic zircon fractions of samples close to mineralization contain from 2.2 to 16 ppm of common Pb. The abundant common lead in the zircons indicates that the sulfides were already in existence by 1,660 m.y., even if only in a disseminated form. An alternative explanation of the isotopic data is that there have been two major metamorphic events at Broken Hill during the middle Proterozoic: the granulite facies at approximately 1,660 m.y. and another at approximately 1,595 m.y. Thirty-five analyses of 21 sphalerites and pyrrhotites show uniform lead isotope ratios throughout the orebody because of the presence of galena inclusions.--Modified journal abstract.

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