Ore-forming Fluids And Ore Genesis Of Continental Subvolcanic Magnetite-apatite Deposits In Ningwu And Luzong Area, China

Continental subvolcanic iron deposit is an important type of deposits in the Middle and Lower Yangtze River metallogenic belt. The main orebodies of these deposits mainly occur in the contact zone between gabbro-diorite porphyry intrusion and overlying volcanic rocks. Former researches in this area concentrated on petrology, mineralogy, geodynamics of diagenesis and mineralization, geochemistry of mineral deposit, major elements, trace elements and isotope geochemistry, experimental geochemistry and chronology study and has established the famous "Ningwu porphyry iron deposit" model. These studies provide useful information for determining the spatial and temporal relationship between diagenesis and mineralization as well as sources of metallogenic materials and ore-fluids, estimating the physical and chemical conditions of mineralization, and understanding of ore genesis and the mechanism of alteration and mineralization. However, the sources of metallogenic materials and the connection between the ore deposits and the underlying Triassic evaporates did not reach a consensus, and one of the heated debate on the ore genesis focused on the whether the main massive orebodies of the iron deposits, especially Meishan and Gushan iron deposits, formed by iron-oxide melt or fluid. Based on systematic field survey, Meishan iron deposit in Ningwu basin, Nihe and Luohe iron deposits in Luzong baisn were selected to conduct petrology, major elements, trace elements and rare earth elements geochemistry, C, H, O, S and Pb isotope geochemistry and fluid inclusion studies to discuss these questions. The decrepitation temperature of fluid inclusions trapped in magnetite were analyzed in these deposits, while fluid inclusion petrology study and microthermometry study were carried out on fluid inclusions trapped in transparent minerals associated with magnetite, and compositions of single primary fluid inclusions in the early stage pyroxene and garnet were analyzed by Raman spectroscopy, electron microprobe and LA-1CP-MS. The LA-ICP-MS analyses of single fluid inclusions were carried out at Bayerisches Geoinstitut, Germany, and the compositions of natural fluid inclusions were quantified based on LA-ICP-MS analysis of the synthetic fluid inclusions trapped in quartz in the modeling environment of Meishan and Nihe iron deposits. The following understandings were acquired from this work:1. The Ca content obviously increase, whereas Mn, K and Ti contents slightly decrease in pyroxene from the subvolcanic intrusion to the mineralized alteration zones, and the species of pyroxene change from augite to diopside and sahlite, while garnet are all andradite in both subvolcanic intrusion and the mineralized alteration zones. Since pyroxene and garnet formed nearly at the same time in alteration zones in all the three deposits, the species of them suggest that the ore-fluids, from which they crystalized, have relatively low acidity and high oxygen fugacity. The pyroxene in mineralized alteration zones at Meishan deposit are mostly diopside, and are enriched in Fe and depleted in Mg relative to Nihe and Luohe deposits, the pyroxenes of which are mostly sahlite. Combined with the larger proportion of andradite in the constitute of garnet in Meishan deposit than in Luohe deposit, the oxygen fugacity and alkalinity of the ore-fluids in Meishan deposit may be a little bit higher than those of the ore-fluids in Nihe and Luohe deposits. The apatite in the alteration zones in the three deposits are mainly fluorapatite, which formed in volatile-rich fluids during pneumatolytic or hydrothermal process. Their trace elements and rare earth elements characters are similar with those of apatite in intermediate-basic rocks among different types of rocks.2. Magnetite in these iron deposits can be divided into four generations. Magnetite of the first generation are the magnetite in subvolcanic rocks as accessory minerals; Magnetite of the second generation are fine-grained, subhedral-anhedral and disseminated, and are mainly distributed in the subvolcanic rocks; Magnetite of the third generation are coarse-grained, sometimes pegmatitic, enhedral-subhedral and veined-net veined which crystalized from volatile-rich and high temperature pneumatolytic hydrothermal fluids, and are mainly distributed in the contact zone between subvolcanic rocks and overlying volcanic rocks; The fourth generation magnetite are the magnetite of fine-grained massive ores of the main orebody in Meishan deposit, while are absent in Nihe and Luohe deposits. The forming temperatures of magnetite gradually decrease from the first generation (>600℃～800℃) to the fourth generation (350℃～500℃), and the pegmatitic magnetite of the third generation may formed in a higher temperature environment relative to veined magnetite of the same generation, the forming temperature of which are about 400℃～550℃. All the magnetite (generation II, III and IV) in mineralized alteration zones are formed by hydrothermal fluids based on electron microprobe and LA-ICP-MS analysis of magnetite. Some of the disseminated, veined-net veined and pegmatitic magnetite show transition characters from magmatic to hydrothermal magnetite because of their relatively high contents of TiO2, V2O5 and low Ni/Cr ratios, which are similar with compositions of altered magnetite in gabbro-diorite, and the Ti concentrations of these magnetite have an increasing tendency from shallow to deep, indicating that some elements, such as Ti, in magnetite in subvolcanic rocks can be activated and migrated during hydrothermal activities. Therefore, magnetite from alteration zones are considered to be mainly precipitated from high temperature hydrothermal fluids rather than magma, but retaining some composition characters of the magmatic magnetite due to replacement of the first generation magnetite by ore-forming fluids. On the contrary, the TiO2 contents are quite low in fine-grained massive magnetite, demonstrating that they formed in relatively lower temperature hydrothermal fluids. From the above, in these iron deposits, iron could partly derived from the extraction of the crystalized gabbro-diorite by ore-fluids.3. Fluid inclusions in pyroxene and garnet, which formed at the same time with or slightly earlier than magnetite, are mostly multi-phase fluid inclusions containing various daughter minerals, e.g. anhydrite, hematite, magnetite and halite, while none melt inclusions were identified. Observation and analyses of these inclusions indicate that the initial ore-fluids of the iron deposits are dominated by Na, K, Ca, Fe, Cl and S, and are high temperature (740℃～810℃) and hypersaline (>80 wt%NaCl) fluids with high oxygen fugacity and strong capability to carry metals. The ore-fluids of Meishan deposit have higher contents of Na and Cl and lower contents of S and Sr than Nihe and Luohe deposits are consistent with the larger scale of sodic alteration in Meishan deposit and more abundant apatite and anhydrite in Nihe and Luohe deposits.4. There are several metallogenic stages of the mineralizing process. From early to late, the ore-forming fluids trend to high-intermediate or intermediate-low temperature (120℃～300℃) and intermediate-low salinity (0.2～11.5 wt%NaCl), whereas the types of fluid inclusions changes from multi-phase inclusions containing various daughter minerals, multi-phase inclusions containing 1 or 2 daughter minerals, vapor-rich or liquid-rich two phase fluid inclusions to pure liquid inclusions. During these processes, the ore-fluids may partially undergo boiling twice. The first boiling of the fluids may be generated by degassing of volatiles at the time of early stage disseminated magnetite forming, while the second boiling of the fluids occurred approximately at the time of later stage veined and massive magnetite precipitation, which may result from a locally sudden decrease in pressure caused by sudden increase of the activity space of ore-forming fluids. Nevertheless, the boiling of the fluids may not intense and extensive enough to be the main mechanism responsible for large amounts of magnetite precipitation in this area.5. The ore-fluids at the early stage are mainly composed of magmatic fluids and tend to be mostly meteoric waters by progressive mixing with meteoric waters during late stage hydrothermal activities after mineralization, revealed by H-0 isotopic composition of the ore-fluids. The C-0 isotopic composition of siderite in Meishan deposit indicates that part of the CO2 in fluids may come from deep marine carbonates.6. Compositions of the ore-fluids in all the researched iron deposits are significantly different with typical magmatic fluids, analyzed in single fluid inclusions in pyroxene and garnet by LA-ICP-MS. The very high Cl/Br, Na/Br, Na/B, S/B ratios and relatively high Ca/Na ratio of the fluids suggest the contribution of halite- and anhydrite-bearing evaporites. Additionally, the C-O isotopic compositions of siderite, the S isotopic compositions of sulfides and sulfates and the Pb isotopic compositions of ore minerals and subvolcanic rocks indicate that the metallogenic materials have mixed sources of mantle and upper crust. Based on these evidences, the regional deep marine carbonates and Triassic evaporites may be assimilated and mixed into magma during magma ascent. The addition of large amounts of Na, Cl, Ca and S from evaporites could promote the exsolution of high salinity magmatic fluids from magma and prominently enhance the capability of these fluids to extract and transport iron, so that help the enrichment and precipitation of magnetite and provide a number of metallogenic materials (mainly S). Therefore, the addition of evaporites may be a key step in forming of iron deposits in the research area since it can, in some extent, promote the iron mineralization process, so that the evaporite-bearing basins are more promising for discovery of these continental subvolcanic iron deposits than those basins without evaporites.7. The continental volcanic iron deposit in Ningwu and Luzong areas are related to continental rifting. The diagenesis and mineralization both occurred at early Cretaceous. These deposits are characterized by typical magnetite-apatite-pyroxene/actinolite mineral assemblage, large scale of Na-Ca alteration and high temperature, high salinity, high oxygen fugacity ore-fluids, which have been affected by deep evaporites. All the above characteristics of these iron deposits are most similar with IOCG-clan deposits among the general types of deposits. But the enrichment of sulfides and sulfates, the high concentrations of Ti in iron oxides and the close temporal and spatial relationship with contemporaneous gabbro-diorite are distinct characters of these deposits, which are obviously different with the typical IOCG deposits characterized by depletion of sulfur, low contents of Ti in iron oxides and generally no direct connection with specific intrusions.