U-Pb age and ore mineralization of dike lamprophires of the Roсa Islands (Wilhelm Archipelago, West Antarctica)

Abstract

The dike of lamprophyres of the Roсa Islands chemically correspond to the basic rocks of the calc-alkaline series with high magnesian #mg 0.56. They have an increased content of Y (41.6 ppm) and Yb (11.5 ppm), which indicates the absence of garnet in the magmatic source. Rare earth elements are weakly differentiated — (La/Yb)N = 3.64). A deep negative European anomaly is distinguished — Eu/Eu*=0.36, which is probably due to the fractionation of plagioclase in the crustal magmatic source. Polymetallic mineralization for copper (445 g/t), zinc (207 g/t), lead (123 g/t) and tungsten (28.7 g/t) was found. Zircon from lamprophyres is represented by two types of crystals. The first type – transparent yellowish-pink individuals with a pyramidal-prismatic habit. In terms of quantity, it dominates; the second type is the formation of a flat outline. Dimensions are usually 0.3—0.7 mm along the L4 axis. Crystals of the first type were selected for geochronological research. It was found that the lamprophyre zircon contains very little lead, and a significant part of it is the lead isotope 204Pb. For this reason, age values for uranium-lead ratios of 238U/206Pb are more reliable. It was determined that the uranium-lead age of zircon from lamprophyres is within 50—60 Ma. Primary melt inclusions and less often mineral inclusions were found in zircon crystals. The former can sometimes occupy up to 30% of the crystal volume. Among the mineral inclusions, potassium feldspar, albite and potassium-sodium feldspar, apatite, and quartz were diagnosed. One primary inclusion of CO2 fluid was detected, the remaining inclusions are represented by primary crystallized melt inclusions. Rooting of the lamprophyre dyke is probably associated with the stress stresses experienced by the granodiorite plutons as a result of later tectonic movements.