Geochemistry and oxygen isotope composition of magnetite from the Zhangmatun deposit, North China Craton: Implications for the magmatic-hydrothermal evolution of Cornwall-type iron mineralization

- The abnormal Si, Ca, Mg, Na and K concentrations are attributed to the inclusions in magnetite.
- The carbonate country rocks have incorporated into the ore-forming fluids.
- The iron in the ores was mainly derived from leaching of the solidified ore-related gabbro.
- Fluid/rock interaction exerted significant control on the precipitation of magnetite.

The Zhangmatun deposit is a high-grade skarn-type iron deposit genetically linked with a gabbroic intrusion, and it has been recognized as a typical example of “Cornwall-type” deposits in China. Electron microprobe (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data on magnetite show decreasing contents of Ti, V, Cr, Mn, Co and Ga, from the gabbro, through the skarn, and to the iron ores. Additionally, magnetite grains from the gabbro, skarn and iron ores have similar primary mantle normalized signature and chondrite-normalized REE pattern, suggesting that the Fe in them might have be derived from a common source. The δ18OV-SMOW values of magnetite from iron ores mainly range from 4.3‰ to 6.4‰, markedly higher than that of magmatic magnetite (3.0‰). The significantly elevated oxygen isotopes of both magnetite and the corresponding water in the hydrothermal system are considered to reflect oxygen isotopic exchange between the ore-forming fluids and 18O-rich carbonate rocks at relatively high temperature. Thus, the magnetite in the iron ores is inferred to be predominantly precipitated from the modified ore-forming fluids, rather than directly from the unreacted primary magmatic fluids. In the time-resolved analytical signals of LA-ICP-MS, the abnormal signals of incompatible elements such as Si, Ca and Mg are attributed to the sub-microscopic silicate mineral relicts. The inference is also supported by the BSE images and chemical maps of representative magnetite. The abnormal signals of Na and K are possibly related to the micro- to nano-scale salt-bearing fluid inclusions in magnetite. Based on the geochemical characteristics of different magnetite grains and varying FeOT in the fresh and altered gabbroic rocks, we propose that the iron in the ores was mainly derived from leaching of the solidified ore-related gabbro during the contact metasomatism by hydrothermal fluids. The interaction between hydrothermal fluids and carbonate country rocks and the evolution of the ore-forming fluids, including the changes in geochemical composition, decreasing temperature and pressure, and increasing oxygen fugacity, provided the conditions for precipitation of magnetite from ore-forming fluids. Our study has potential implications in understanding the origin of 'Cornwall-type' deposits elsewhere world.

Time-resolved analytical signals of LA-ICP-MS analyses. (a) Signals of pure magnetite. (b) Signals of representative magnetite with Ca-Mg silicate mineral relict. (c) Signals of representative magnetite with Na, K-rich fluids inclusions.116