Mesozoic gold deposits in the Shandong Peninsula, P. R. China

摘要

The Jiaodong Peninsula of Shandong Province is located in the east portion of the North China Craton and is the largest district of gold mining and production in China. The Peninsula possesses over 25% of China's total exploited gold reserves and currently provides more than one fourth of China's total gold production per annum. The basement of the Jiaodong Peninsula consists of Archean - Proterozoic metamorphosed rocks, mainly amphibolite, granulite, gneiss, schist, marble, quartzite and sandstone. The rocks were subjected to multiple stages of granulite - to greenschist - facies metamorphism during the timespan 2945～1674 Ma. The basement was reactivated by the Mesozoic Yanshanian orogeny (Jurassic to Cretaceous), which was an important tectono - magmatic - mineralization event in the area. The Early Yanshanian (130 to 160 Ma) granitoids were formed by anatexis of the basement rocks. More than 80% of the lode gold deposits in the Peninsula are hosted within the granitoids and/or along the contact zones between the granitic and basement rocks, and together constitute the largest granitoid - related gold province in the world. Two types of gold deposits are recognised: (1) lode gold - quartz - sulphide veins (i. e., the Linglong type); and (2) shear - zone hosted disseminated sulphides (i. e., the Jiaojia type). Both types of gold deposits are similar in their geology and geochemistry. Ore mineral assemblages are dominated by pyrite, followed by galena, sphalerite, chalcopyrite, pyrrho-tite and arsenopyrite. Gold - silver - bearing minerals are mainly composed of native gold and electrum. Four stages of mineralization include (1) pyrite-quartz, (2) gold - bearing pyrite - quartz, (3) gold - bearing multiple metallic sulphides and quartz and (4) quartz - carbonate. The gold is concentrated in stages 2-3. Alteration minerals consist of sericite, mus-covite, sulphides (mainly pyrite, pyrrhotite and arsenopyrite), carbonates, K - feldspar, chlorite, and kaolinite. Radiogenic ages of mineralization, dated by Rb - Sr and K - Ar methods on hydrothermal and ore minerals and inclusion fluids, are mostly between 100 ± 4 to 135 ± 5 Ma for eight major gold deposits. These ages are 5 to 20 million years less than those of corresponding host granitic intrusions in the main ore belt. Mineralizing fluids are H_2O - CO_2 - NaCl solutions (salinity: < 12 wt.% NaCl equiv.) at minimum T- P of 250—350℃ and 0.3～1 kbar. Isotopic compositions of ore minerals or inclusion fluids are: δ~(13)C (0 to -5 per thousand), δ~(34)S (+4 to + 12 per thousand), δD and δ~(18)O (-20 to-110 per thousand; -10 to +8 per thousand). On the basis of the geological, geochemical and isotopic data, it is suggested that the gold mineralization in the Jiaodong Peninsula post - dated the granitic emplacement and was formed mainly by evolved meteoric waters in the waning stages of the Mesozoic Yanshanian orogeny at c. 100 to 130 Ma.