Abstract:

“Porphyrite iron deposits” within the Ningwu and Luzong volcanic basins in the Middle-Lower Yangtze River Valley Metallogenic Belt are hosted in the early Cretaceous(~130 Ma) volcanic-intrusive rocks. They are characterized by mineral assemblages of magnetite-apatite-actinolite(diopside)and have geological characteristics similar to other iron oxide-apatite(IOA) or Kiruna-type deposits worldwide. Since the concept of porphyrite iron deposit was proposed in the 1970s, there are several hypotheses for ore genesis, that include immiscible iron-oxide melts of magmatic origin, magmatic-hydrothermal replacement, and a combination of magmatic and hydrothermal processes. This study presents an overview of recent advances in porphyrite iron deposits in China, including geochronologies of iron mineralization and the associated ore-related intrusions, sources of ore-forming materials, nature of early-stage ore-forming fluids, and possible genetic relationship between porphyrite iron deposits and skarn iron deposits as well as the newly discovered gold-copper mineralization at the periphery. Finally, we also make a comparison of those with global IOA deposits and comment on the potential problems of current genetic models. Recently, a large high-precision geochronological dataset has revealed that the IOA deposit in the Ningwu and Luzong basins intensively formed at ~130 Ma. It has been suggested that iron is dominantly derived from the subvolcanic intrusions, and the early stage ore fluids are characterized by high-temperature (550-780 ℃), ultra-high salinity (~90 wt% NaCl_eq). Such fluids are consistent with field evidence of the earliest, deposit-scale sodic alteration. On the other hand, S-Sr isotopes and in-situ analyses of the fluid inclusions indicate that external evaporite-derived fluids were involved in the iron mineralization processes, although it has not been known precisely, the role of evaporite during the mineralization. It should be also highlighted that the paragenetic sequence of hydrothermal alteration in the IOA deposit is similar to that of iron skarn deposits, implying a cryptic genetic link between the two types. We believe that the presence of different alteration phases possibly resulted from the interaction between similar ore-forming fluids and different country rocks at different conditions. During the past decade, IOA deposits have been attracted much more global attention and quite a few papers had been published in high ranked international journals. Except for traditional hypotheses involving Fe-oxide melts and magmatic-hydrothermal replacement, other models combined with magmatic and magmatic-hydrothermal processes have also been proposed. These new models include a magmatic magnetite-bubble suspension model and the model of ascent, degassing, and emplacement of hydrous immiscible Fe-Ca-P melts. Nevertheless, the current debate regarding the genesis of the IOA deposit still focuses on the transitional processes between magmatic and hydrothermal stage, and how iron and phosphorous separated from the magma or the intrusion, transported and concentrated. The proposed models, including iron oxide, melts (Fe-O or P-Ca-Fe-O), magmatic magnetite microlites (Fe₃O₄), Fe-rich hydrothermal fluids, or other enigmatic processes, remain open to be testified through further research.

Key words: Middle-Lower Yangtze River Valley Metallogenic Belt, porphyrite Fe deposit, Iron Oxide-Apatite deposit, iron skarn deposit, genetic model