The Relationship Between The Late Mesozoic Magmatism And Iron Poly Metallic Deposits In Southwestern Fujian Depression Belt,China

This study gives insights into the metallogeny of strata-bound Makeng type deposits and the genesis of associated magmatic rocks, based on the geology, geochemistry, isotope systematics, and geochronology of ores and associated (magmatic) rocks.The granites have high SiO2 contents and are peraluminous. Chondrite-normalized REE patterns of the granites show relative enrichment of light rare earth elements (LREEs) and weakly negative Eu anomalies. Most of the granites in the Makeng deposit district (Juzhou and Dayang) and some of the granites in the Luoyang deposit district are A-type, whilst the granitoids in the Pantian, Dapai, and some of the granites of the Luoyang ore district granites are hybrid I-type granite, having crustal and mantle components contaminated by interaction with the upper crust.The overall U-Pb Zircon dating of intrusions (mainly granites, but include a diabase also) associated with the deposits divulged multiple stages of magmatism in this area; 185-170; 162-145; 145-135; 135-120 Ma.90 zircon grains from Cretaceous (stage 120-135 Ma) granites yielded 176Hf/177Hf ratios varying from 0.2821 to 0.2827 and ?Hf(t) varying from -21.63 to-0.86, with an average of -7.62 and a calculated two-stage Hf model ages varying from 1.24 to 2.55 Ga (average age:1.67 Ga). The ?Hf(t) data suggest that these granitoid are derived from an early Paleoproterozoic crust. Zircon grains of an older (Jurassic stage 162-145) granite yielded ?Hf(t) values from -24.43 to -9.81, with an average value of -16.81 meanwhile the calculated two-stage Hf model ages varies from 1.561 to 3.57 Ga, with an average age of 2.25 Ga. Mafic inclusions, the aluminium saturation index, and the raise in the ?Hf(t) of the younger granitoids gives a strong suggestion for a crust-mantle magma mixing during the formation of the Cretaceous granites.Pyrite Rb-Sr dating of the Pb-Zn ores (Dapai deposit) yielded a mineralization age of 178.6±3.6 Ma, a high 87Sr/86Sr ratios (0.708 to 0.716) and 87Sr/86Sr initial ratio (Isr) of 0.710. The ?34SCDT values of galena, pyrite and sphalerite (from -2.6‰ to 1.5‰) and their 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios (18.486?18.537‰,15.665?15.712‰ and 38.823-38.979%o, respectively) suggest a mixture of upper mantle and upper crust magmas during the Mid-Jurassic. This metallogenic event is related to the oblique subduction of the paleo-Pacific plate (with Izanagi plate) under South China Block (SCB) in the Jurassic.The ?56Fe and ?57Fe of magnetite (average value -0.052‰ and -0.085‰, respectively) coupled with ?18O of magnetite (average value 3.53%o) and garmet (average value 4.8%o) show that Fe mineralization is cogenetic to granitic magmatism. A-type granites and diabase dikes that are related to the Fe mineralization are suggested to have been formed in an extensional setting, responding to the slab rollback and concomitant retreating arc system of the paleo-Pacific plate within the SCB. This suggests that main Fe mineralization period is but not restricted to 145-135 MaThe period 130-135 Ma (based on Re-Os Mo dating in Makeng, Dapai, and Luoyang deposit) represents an Mo mineralization. The mantle-crust magma mixing was probably responsible for the widespread hybrid granites and Molybdenite mineralization of the Makeng type deposits in the Cretaceous (130-135 Ma). It is related to the Cretaceous subduction of the paleo-Pacific plate (with Izanagi plate) under SCB.Therefore, a three-phase evolution of the Makeng type deposits during the Yanshanian is proposed, whereby different metal associations were enriched in a large-scale skarn system in response to the tectonic evolution and vigorous magmatism during the Yanshanian period.