Geology And Geochemistry Of Makeng Fe-Mo Deposit Fujian

The Makeng Fe-Mo deposit in the polymetallic belt of southwestern FujianProvince is the largest iron deposit in South China. It is a large magnetite deposithosted in the interlayer fractures of Huanglong Formation(C2h)carbonates and LindiFormation(C1l)clastic rocks at the exo-contact zone of the Juzhou-Dayang granite.Skarn has genetic relationship with ore fromation. Zonation can be found almostanywhere, and occurs on scales from kilometers to micrometers. The formation ofMakeng iron deposit can be calssified into skarn stage, retrograde stage, and quartzsulfide and carbonate stage, among which the retrograde stage was the mainmineralization stage.Zircon LA-ICP-MS U-Pb dating indicates that the Dayang granite, the Juzhougranite and diabase in the Makeng deposit were formed at (127±1Ma and 128±1Ma),(125M±1a, 126±1Ma, 129±1Ma, 130±1Ma and 130±1Ma) and (303±2Ma and64±1Ma), respectively; The SHRIMP ziron U-Pb age of Dayang granite is 133±1Ma.Lithospheric thinning related to paleo-Pacific plate (subduction) is a likelyresponsible geodynamic background for Juzhou-Dayang granites'formation. Crackor rift valley is a likely responsible geodynamic background for Late Carboniferousand early Paleocene diabases.According to its geochemical compostion such as high silicon and alkali, lowcalcium and magnesium, and high differentiation index, the Juzhu-Dayang granitecan be identified as weak peraluminous-metaluminous granite; The rocks have highand remarkably varying REE, and their distribution patterns show LREE enrichmentwith gentle right oblique deviation, and a"V"model characterized by significantnegative Eu anomaly; The trace elements compositions are strongly enriched in Rb,U, Th and La and considerably depleted in Ba, Sr, P and Ti; Petrogeochemical andisotopic characteristics indicate that the Juzhu-Dayang granite is crust-derived, andhas experienced high differentiated evolution. The magma sources of theDayang-Juzhou granites were mainly derived from Proterozoic crustal materials, butalso involved some proportion of EMII components.Skarn mineral assemblages consist of pyroxene, garnet and bustamite, andretrograde minerals comprise chlorite, epidote, amphibole, quartz. Electronmicroprobe analyses show that the pyroxenes are mainly diopside and hedenbergite,with minor johannsenite, whereas associated pyroxenoids are bustamite andrhodonite. The garnet is dominated by andradite, with minor grossular. Theamphibole in the Makeng iron deposit belongs to calcic amphiboles.Garnet, pyroxene and magnetite have similar chondrite-normalized REEdistribution patterns with light REE(LREE) enrichment, moderate Eu positiveanomaly and absent Ce anomaly, indicating that they were formed under theoxidation environment, and there was genetic relation between ore formation andskarns. The REE geochemistry of marble and alterated rocks varied regularly, andthat alteration is featured by metasomatism and magmatic hydrothermal solution,and its ore-forming materials might come from surrounding strata and diabase partly.Fluid inclusion studies show that the inclusion types of Makeng deposit are complex, mainly with vapor- and liquid-rich, daughter mineral-bearing and CO₂-richfluid inclusions, among which the liquid-rich type is predominant. Gas compositionof fluid inclusions mostly includes CO₂, H₂O, N₂and O₂, with minor CH₄, C₂H₄,C₂H₆and a small amount of C₂H₂. Cations of liquid composition are dominant withNa⁺, K⁺, Mg²⁺, followed by Ca²⁺and a small amount of Li+. Anions are mainlyconsist of SO₄^2-, F^-, Cl^-, with minor Br-and NO3-. Homogenization temperatures ofthree group fluid inclusions from early to late stage are 460～600℃, 260～540℃,160～400℃, respectively. Salinities are (6～24 wt.%, 32～44 wt.%), (4～16%,36～44%), (0～4% ) , respectively.H, O, C and S isotopic geochemistry shows that the ore-forming fluids of theskarn stage were mainly derived from magmatic water, whereas the fluids of thequartz-sulfide stage mixed with the meteric water with variable proportions. Themetallogenic material was mainly derived from mantle or a deep-seated place, butother factors, such as country rocks, contributed as well. The characteristics ofskarnminera and REE indicate that they mainly formed under oxidizd conditions.Re-Os isotopic dating of the molybdenites symbiosis with magnetites obtainedmodel ages of 133.0±0.8Ma.The main mineralization stage was of the EarlyCretaceous, which was consistent with the emplacing age of the host rockJuzhou-Dayang granitic intrusion. These features indicated that they were results ofthe same magma-structure-fluid activity. The mineralogenetic epoch of MakengFe-Mo deposit may be related to the lithospheric extension event of the back arc ofthe southeastern margin of Eurasian continent caused by the Pacific plate subduction.Phase separation of magmatic water, fluid mixing with meteoric water, and buckboiling may be the main causes of mineralization.