Multiple episodes of gold mineralization in the East Kunlun Orogen, western Central Orogenic Belt, China: Constraints from Re-Os sulfide geochronology

Abstract

The Gouli goldfield (>110 t Au), located in the East Kunlun Orogen, western Central Orogenic Belt of China, is one of the most important goldfields in this area. In the last decade, a number of orogenic gold deposits (e.g., Guoluolongwa and Annage) have been shown to be hosted by rock units of different lithology and ages. Rhenium-osmium (Re-Os) geochronology of sulfides from gold-bearing veins was performed to define the chronologic relationships between gold mineralization present in the metamorphic rocks (Proterozoic and Silurian) of the East Kunlun Orogen. Sulfides (pyrite and chalcopyrite) from pyrite-quartz vein and polymetallic sulfides-quartz vein in the Guoluolongwa gold deposit yield Re-Os isochron dates of 374 ± 15 Ma (MSWD = 4.6; initial 187Os/188Os ratio (Osi) = 0.06 ± 0.22) and 354 ± 7 Ma (MSWD = 0.18; Osi = 0.13 ± 0.01), respectively. Similar ages are also revealed by the pyrite mineral separates from the Annage gold deposit (383 ± 8 Ma and 349 ± 6 Ma). These ages are interpreted to record the timings of the formation of the two vein types in these deposits, which are nominally separated by ~20 Ma. The new Re-Os ages presented here identify the first two Late Paleozoic (Devonian and Early Carboniferous) gold-mineralizing events in the East Kunlun Orogen and thus indicate at least two mineralization epochs in this area given ages (Late Triassic) of other gold systems and field observations. Considering the geological background and temporal distribution of gold deposits in adjacent areas (western Qinling and Qaidam-Qilian), we suggest that gold deposits in the western Central Orogenic Belt were formed in collisional/post-collisional settings being controlled by common tectonic-magmatic activities related to the evolution of both the Prototethys Ocean (Proterozoic – Paleozoic) and Paleotethys Ocean (Paleozoic – Early Cenozoic). Further, the initial Os (Osi) obtained from the Re-Os isochron suggest that for the two vein types in the Guoluolongwa gold deposit the Os and by inference the ore metal (Au) were derived from a mantle-like source (Osi values = ~0.12–0.13), which should be related to the contemporaneous mantle-like magmatism. In contrast, the pyrite-quartz vein in the Annage gold deposit possesses a significantly radiogenic Osi value (3.65 ± 0.51). Given the similar timing of mineralization between the Guoluolongwa and Annage deposits, it is considered that the ore metal likely has a similar origin, i.e., a mantle-like source, however at Annage the hydrothermal fluid interacted with the Proterozoic metamorphic host rocks and leached radiogenic Os that masks any evidence of a mantle-like source.