PLIOCENE UPLIFT AND PALEOCLIMATIC EVOLUTION IN THE NORTHEASTERN TIBETAN PLATEAU: EVIDENCE FROM 40AR/39AR AND STABLE ISOTOPE DATA OF SUPERGENE JAROSITE

Supergene jarosite is widely distributed in weathering profiles that were developed on primary sulfides ores of the early Paleozoic Baiyin deposit currently situated at over 1900 meters elevation on the NE Tibetan plateau. This volcanogenic massive sulfide deposit has typical pyrite, chalcopyrite, minor amounts of galena and sphalerite, and the traces of barite. Jarosite occurs at depths of up to 10 meters beneath paleo-weathering surfaces.

We are studying jarosite from this and other deposits on the NE Tibetan Plateau to gain insights into the timing and evolution of climate during the uplift of the NE Tibetan Plateau and the subsequent development of monsoon weather patterns. Here we present preliminary results from laser incremental-heating ⁴⁰Ar/³⁹Ar geochronology and stable isotope analyses of jarosite.

Twenty-two jarosite grains from fourteen samples yield well-defined plateau or plateau-like ages ranging from 4.6 ± 0.1 Ma to 3.07 ± 0.12 Ma (2σ). The ⁴⁰Ar/³⁹Ar ages indicate that the sulfide ores in the Baiyin deposit were weathered at least in the Early Pliocene, which is consistent with independent age estimates for uplift of the northern Tibetan Plateau.

These jarosite samples have δ³⁴S values between 5.0 and 6.5‰ that are slightly greater than the δ³⁴S values for the hypogene pyrite (3.4 to 4.0‰). This slight difference in δ³⁴S values between jarosite and pyrite most likely reflects the contribution of aqueous sulfate derived from the dissolution of a small amount of hydrothermal barite during weathering. The δ¹⁸O_Total values of jarosite range from -2.2 to -0.7‰. The narrow range for δ¹⁸O_Total values probably reflects oxidation of sulfide by ground water and the formation of parent aqueous sulfate under uniform conditions.

The δD values for the jarosite range from -163 to -148.6‰, with the lowest δD values having the youngest ⁴⁰Ar/³⁹Ar age. This may reflect formation of younger jarosites at increasingly higher elevations. The lowest δD_H2O value of the parent fluid for the youngest jarosite (-103‰) is clearly lower than the present-day ground water (-78 ± 10 ‰) in an area that currently has less than 8 cm of annual precipitation, suggesting that since 4.6 Ma the monsoon-arid environment pattern in NW China has been established and the annual mean temperature of precipitation of the NE Tibetan Plateau has fluctuated.