Origin of the inconsistent apparent Re-Os ages of the Jinchuan Ni-Cu sulfide ore deposit, China: Post-segregation diffusion of Os

摘要

Apparent Re-Os ages of some magmatic sulfide ore deposits are older than the zircon and baddeleyite U-Pb ages which are interpreted as the formation age of the host intrusions. The Jinchuan Ni-Cu-PGE deposit of China, the world's third largest, is such a case. We report apparent Re-Os isochron ages of 1117 +/- 67 Ma, 1074 +/- 120 Ma and 867 +/- 75 Ma with initial Os-187/Os-188 ratios of 0.120 +/- 0.012, 0.162 +/- 0.017 and 0.235 +/- 0.027 for disseminated ores, sulfides from the disseminated ores and massive ores from Jinchuan, respectively. Using these data and Re-Os ages from the literature, we find that the oldest apparent Re-Os age and lowest initial Os isotope ratio are from disseminated ores which contain small amounts of sulfide minerals, the highest initial Os isotope ratios and youngest apparent Re-Os ages, consistent with the zircon and baddeleyite U-Pb ages, are from massive ores containing 90-100 modal% sulfide, and net-textured ores with about 25 modal% sulfides yield apparent Re-Os ages and initial Os ratios intermediate between those of the disseminated and massive ores. Because Os diffusion between sulfides is inhibited by the intervening silicates even at high temperatures, re-equilibration did not occur in the disseminated ore and the samples retained the Os ratios of the contaminated magma, leading to geologically meaningless ages that are older than the formation age of the rocks. While Os-bearing sulfide minerals and magnetite show low closure temperatures of Os diffusion and the sulfide minerals in the massive ore are closely connected with each other, facilitating fast diffusion of Os, re-equilibration of Os was achieved during cooling of the ore from about 850 degrees C after the segregation to about 400 degrees C. Thus, an age corresponding to the formation time and an elevated initial Os ratio were yielded by the massive ore. Os isotopes in the net-textured ore behave in the way intermediate between the disseminated and massive ores. Pb isotope data support the Os results. Disseminated ores have heterogeneous Pb isotope ratios whereas Pb in the massive ores is more uniform, consistent with Pb isotopic equilibration in the massive ores, but not in the disseminated ores.