Large-scale gold mineralization in eastern China induced by an Early Cretaceous clockwise change in Pacific plate motions

Abstract

Orogenic gold (Au) deposits are the most important type, accounting for more than half of the world's proven Au reserves. They are mainly controlled by three key factors: (1) abundant andesitic rocks (SiO 2 of 55-60 wt.%) at depth, which have systematically higher Au contents than other rock types; (2) a pervasive transition from greenschist facies to amphibolite facies metamorphism within a short period, which releases S 2− -rich fluids that may scavenge Au from host rocks; and (3) deformation and fracturing under a compressive/transpressive tectonic regime. Orogenic belts at convergent margins are the best places for such mineralization because convergent margins are rich in andesites; the transition from greenschist to amphibolite facies recrystallization commonly occurs as a result of collision, compression, and thickening at convergent margins, forming large amounts of Au-rich fluids within a short period of time; and strong deformation and fracturing during orogenic processes provide channels for fluid transportation. Moreover, the overlying plate is injected and enriched by auriferous fluids released during amphibolite facies metamorphism of the subducting plate. The Pacific plate changed course by ∼80• (from SW to NW) at approximately 125-122 Ma, reflecting an altered thermal structure and the elevation of the South Pacific plate attending the appearance of the plume head that formed the Ontong Java large igneous province. Consequently, the tectonic regime changed from extension to compressive/transpressive in eastern China, causing deformation, thickening, and metamorphism of the overriding plate, especially along weak crustal belts (e.g. overlying plates of palaeosutures), which resulted in world-class mineralization of orogenic Au deposits. During this process, pyrite changed to pyrrhotite during the transition from greenschist to amphibolite facies, releasing sulphur. Sulphur mobilized and scavenged Au and other chalcophile elements into metamorphic ore-forming fluids. A series of NE-trending compressive faults were formed at ∼120 Ma as a result of continuous compression of the subducting Pacific plate, releasing these ore-forming fluids. Auriferous carbonate-rich quartz veins and/or metasomatized Au-bearing wall rocks were formed due to the decompression of the ascending ore-forming fluids. Orogenic belts along the margins of the North China craton and the Jiangnan block were the most favourable regions for mineralization. Compared with the former, the latter has much smaller proven Au reserves. However, more exploration is needed along the margins of the Jiangnan block. Promising targets include accessory faults and kink points of large, NE-trending Cretaceous faults that transect greenschist facies metamorphic rocks of the Niuwu and Jingtan Groups, etc.

Initiation and evolution of the South China Sea: an overview

“…In the case of the SCS, backarc extension may have started after the steering of the whole Pacific Plate from northward drifting to northwestward at *50 Ma (Sharp and Clague 2006;Sun et al 2007Sun et al , 2013, because the major change in the drifting direction of the Pacific Plate was coupled with a dramatic decrease in northward compression of the tectonic regime of the SCS region.…”

“…Alternatively, it was proposed that backarc basins around the Eurasian continent, opened at 32-17 Ma, were induced by either rapid eastward migration of the western Pacific trench system or by oblique subduction of the Pacific Plate beneath Asia (Yin 2010). The Pacific Plate was subducting roughly northward between 100 and 50 Ma (Sun et al , 2013, which was likely responsible for the northsouth extension of the Japan Sea/East Korean Sea and the Philippine Sea. Nevertheless, the subduction regime associated with the SCS was quite different from that of the Japan Sea/East Korean Sea.…”

Decratonic gold deposits

“…(4) The Early Cretaceous gold deposits in the NCC emerged in a short period, with the Eastern and Western Belts having mineralization ages of 130 and 120 Ma, respectively Sun et al, 2007Sun et al, , 2013. Orogenic gold deposits usually have a prolonged mineralization history, lasting up to 50-70 Ma or even longer as revealed from the Cordillera orogenic belt and the Abitibi greenstone belt (Goldfarb et al, 2001;Robert et al, 2005).…”

“…(4) Mafic magma can provide heat sources for metallogenesis and circulation of crustal fluids as well. Some Au in metamorphosed mafic volcanic rocks of the NCC was leached out by the existing fluid circulation (Sun et al, 2013). The mineralization process is depicted in Figure 8.…”

U–Pb dating of hydrothermal zircon from the Dongping gold deposit in North China: Constraints on the mineralization processes

“…Moreover, some other workers argued that the low salinity, CO 2 -rich ore fluids reflect a metamorphic hydrothermal origin (Hart et al, 2002;Mao et al, 2003), which is typical of most orogenic gold deposits (e.g., Goldfarb et al, 2001). Orogenic gold deposits are usually associated with greenschist phase metamorphic rocks (e.g., Goldfarb et al, 2001), which is likely due to the pyrite to pyrrhotite transition during prograde metamorphism from greenschist to amphibolite phases (e.g., Sun et al, 2013). The Dongping gold deposit, however, is located in the central part of the Shuiquangou syenite, which is not the usual geologic setting for orogenic gold deposits.…”