Mineralogical, Geochemical, And Geochornologcal Constrains On The Genesis Of Iron Deposits In The Daye District, Eastern China

The Chengchao and Daye iron skarn deposit is located in the northern portion of the Daye Fe-Cu mineralizing district, western Middle-Lower Yangtze River Metallogenic Belt along the northeastern margin of the Yangtze Craton This metallogenic belt contains more than 200 deposits and has been one of the most important Fe-Cu-Au producers in China in the last three decades. The Daye district is well known for its resource endowment, with numerous Fe, Fe-Cu-(Au), Cu-Au, and Cu-Mo-(W) skarn or porphyry deposits. Genesis of the Chengchao and Daye iron skarn deposits, as well as many other contemporaneous equivalents in the Daye district, has long been a matter of debate, although most deposits have been extensively described and discussed. In view of the presence of vein-or dike-like magnetite ore bodies that have sharp contact with host rocks and very thin or no alteration halos, some authors proposed that the high grade, magnetite-dominated veins or dikes were directly crystallized from iron oxide melts. In other studies, however, these vein-type magnetite ores were considered as replacement deposits along fractures penetrating the Triassic carbonates, or being precipitated from iron-rich fluids facilitated by fluid immiscibility due to an abrupt decrease in pressure of the fluids when they fluxed into the fractures. In this study, mineralogy and geochemical, isotope of magnetite and skarn minerals of the Chengchao and Daye iron deposits were systematically studied to constrain on the genesis of the iron deposits in Daye mining district.A variety of re-equilibration processes, including oxy-exsolution, re-crystallization, and dissolution and re-precipitation, have been, for the first time, systematically demonstrated and widely recognized in iron skarn deposits. The dissolution and re-precipitation of magnetite is present in most of the samples investigated, whereas oxy-exsolution occurs only in the high Ti magnetite. Some of the high Ti magnetite grains have also been re-equilibrated by subsequent dissolution and re-precipitation. In addition, magnetite grains from some samples have triple junction grain boundaries (foam-like texture), indicating that they have recrystallized due to the subsequent hydrothermal activities. The textural evidence indicates that, in many cases, magnetite from iron skarn system represents the final product of mineralization, with the early phases being partly obscured or completely erased during the re-equilibration processes. These re-equilibration processes have significantly modified the trace element compositions of magnetite, particularly for Si, Mg, Ca, Al, Mn, and Ti. The high-Ti magnetite from the iron skarn deposits mostly plot in the fields of IOCG and porphyry deposits in the Ti+V vs. Ca+Al+Mn diagram, whereas some low-Ti magnetite and their secondary products fall into the BIF or IOCG fields. Mixing of external saline fluids with magma tic-hydro thermal solutions, increasing of temperature, and decreases in fO2 and pressure are the potential triggers for dissolution and re-precipitation reactions, and further for the recrystallization, whereas decreasing temperature and an increase in fO2 might cause the oxy-exsolution process. Fluid-assisted dissolution could have enhanced the porosity and permeability of magnetite, further contributing to the DRP processes. The re-equilibration processes presented here are likely ubiquitous in magnetite formed in skarn systems and many other hydrothermal environments, resulting in much more complex textures than previously recognized. The applicability and reliability of the widely used trace element discrimination plots (e.g., Ti+V versus (Ca)+Al+Mn diagram) for magnetite should be re-evaluated. Detailed textural characterization must be undertaken before in situ compositional analysis of magnetite grains, so that the trace element compositions can be reasonably interpreted and properly used as an indicator for the genesis of magnetite and associated ore deposits.Base on the detailed field observations and systematic sampling, we found allanite as the major ore mineral that account for the rare earth mineralization in both Chengchao and Daye iron deposits. Rare earth mineralization mainly occurs in diopside skarn. Some magnetite ore also contains small amount of allanite. Rare earth mineralization was related to both the Quartz diorite and granite in Chengchao iron deposit. The allanite rich diopside skarn in Chengchao iron deposit contain up to 22477 ppm total REE content which has reached the industrial grade. Rare earth mineralization in Daye iron deposit is much wilder and the grade of REE mineralization is much higher than that in Chengchao iron ore deposits. REE mineralization is mainly related pyroxene diorite in Daye deposit. Through the detailed observations and systematic chemical analysis of drill cores, we have identified 7.9 m and 9.1 m thick REE ores with a averaged ∑LREE2O3 grade of 2.99 wt.% in dill holes CK-4 and CK-3, respectively. The precipitation of REE-rich mineral may due to the interaction between hydrothermal fluids and carbonate strata, which cause abrupt up lift of PH of the ore-forming fluid leading sudden decomposition of REE complexes.LA-ICPMS U-Pb dating of titanite in endoskarn yield an age of 131.4±0.2 Ma (n=42; MSWD=1.7) which is marginally consistent with a titanite U-Pb age of 131.4±0.2 Ma and the zircon U-Pb age of 130±2 Ma for the quartz diorite intrusion. Zircon U-Pb dating of pyroxene diorite and quartz diorite yield an age of 140±1 Ma and 139±2 Ma consisting with the age (141 ±1Ma) that obtained from titanite form magnetite ore, indicating the Daye iron skarn deposit were formed at around 140Ma. U-Pb dating of titanite that coexisted with allanite in Chenchao deposit provide an age of 131±0.4 Ma, whereas U-Pb dating of allanite in Daye deposit has an age of 141±1 Ma, suggesting two independent REE mineralization events which was also consist with two iron-copper mineralization episodes in the region.Some ore bodies of the Chengchao and Daye iron deposits show features resembling Kiruna-type ores crystallized from immiscible iron oxide melts, such as lack of alteration halos on both sides of the magnetite-dominated veins, development of vesicular structure in the ores. Textural and compositional data indicate that those ore bodies form the Chengchao and Daye iron deposits are of hydrothermal origin, rather than being crystallized from immiscible iron oxide melts as previously suggested. Magnetite from those ore bodies has high Co/Ni radio (usually more than 1) and low V2O3 (generally less than 0.1) which is consistent with the geochemical characteristics of hydrothermal magnetite from typical skarn ore bodies, but significantly differenct from geochemisty of magmatic from ore related intrusions. The re-equilibration of magnetite from those iron ores must have resulted in an increase in the amount of grain boundaries or effective porosity that is in turn favorable to fluid infiltration, further promoting dissolution and re-precipitation of the magnetite. The overprinting of high temperature saline fluids derived from the cooling magmas is indentifiled as an important mechanism for triggering the dissolution and re-precipitation of magnetite in addition to fluxes of external fluids, most likely basinal brines or meteoric waters dissolving evaporitic rocks, can potentially cause this process, as confirmed by the recent geochronology data.The mineralization characteristics, including the ore forming age, stable isotope of Chengchao iron deposit resemble those in Jinshandian iron skarn deposit and Wangbaoshan iron deposit. In addition, similar characteristics also found in the Longqiao and Baixiangshan iron deposits form Ningwu and Luzong basin in Middle-Lower Yangtze River Metallogenic Belt. This reflects that Chengchao iron deposit is the typical iron porphyry deposit. In this study, we have recognized apatite-rich diposide skarns and iron ores (locally up to 20 vol.% apatite) in the Daye deposit, with mineral assemblages and geochemistry resembling the Kiruna-type ores. Apatite forms mass of aggregates coexisting with magnetite or prograde skarn minerals such as diposide and garnet, or occurs as veinlets crosscutting the magnetite. Many magnetite grains are characterized by orientated ulvospinel exsolutions. The diopside and garnet have chemical compositions typical of iron skarn deposits. However, apatite contains high F, LREE, and magnetite with ulvospinel inclusion contains high Ti and V, characters similar to the Kiruna-type deposits. We suggest that the IAO ores from the Daye deposit have a similar origin to the Kiruna deposits and possibly represent the missing link between Kiruna-type and iron skarn mineralization. It also indicates that magnetite-apatite formed in the high temperature progade stage and the classical ore-forming stages of skarn iron deposit need to be revised. Our findings also indicate that the Kiruna-type iron deposit may have formed from high temperature magmatic-hydrothermal fluids.