Mesozoic Orogenic Gold Mineralization in the Jiaodong Peninsula, China: A Focused Event at 120 ± 2 Ma During Cooling of Pregold Granite Intrusions

Abstract

AbstractJiaodong gold deposits are mainly sited along faulted contacts between Upper Jurassic Linglong granite and Precambrian basement metamorphic rocks or Lower Cretaceous Guojialing granite. Long-standing controversies relate to timing of gold mineralization and granite-gold relationships. In this study, gold-related muscovite consistently provides concordant 40Ar/39Ar plateau ages of 120 ± 2 Ma (2σ) for the Jiaojia, Sizhuang, and Luoshan deposits. Analogous 40Ar/39Ar timing constraints from gold-related muscovite are provided by total gas and high-temperature ages from Fushan, concordant high-temperature ages from Rushan, and fusion-step ages from Xiadian deposits. These new 40Ar/39Ar ages, when combined with previous reliable 40Ar/39Ar and U-Pb age constraints for mineralization, including ages of pre- and postgold dikes, define a widespread gold mineralization event at 120 ± 2 Ma (2σ). Published zircon U-Pb ages for Guojialing and Aishan granite magmatism suggest an ~8-m.y. lag between peak intrusive activity and gold mineralization. This, together with lack of both high-temperature alteration assemblages and alteration and/or metal zonation, indicates that the structurally controlled Jiaodong deposits are orogenic rather than intrusion-related deposits. Despite this, granite intrusions are considered to have provided suitable fluid trap sites. New 40Ar/39Ar analyses of biotite from the Linglong and Guojialing granites show they had cooled to about ~300° ± 50°C by ca. 123 to 124 Ma, providing pressure-temperature conditions similar to those under which most orogenic gold deposits formed close to the ductile-brittle transition. This enabled the effective ingress of fluids at supralithostatic pressures at 120 ± 2 Ma, leading to intensive brecciation, alteration, and deposition of both vein-type and disseminated gold ores. New zircon (U-Th)/He dates together with apatite fission-track data indicate that preservation of the gold province is due to slow postmineralization uplift and exhumation.