Ore-forming fluid for the formation of banded iron formation-hosted high-grade magnetite ores of the North China Craton: Constraints from pyrite trace elements and sulfur isotopes

Abstract

Banded iron formation-hosted high-grade iron ore deposits are among the most important global sources of iron. Although less common than hematite ores, high-grade magnetite ores represent another significant type, occurring mainly in northern North China Craton. These ores are generally considered to have formed through the interaction of high-temperature hydrothermal fluids with ∼2.55 Ga BIF protore, yet the nature and origin of ore-forming fluids remain uncertain. Two types of high-grade magnetite deposits have been classified based on their size and alteration features: the Gongchangling type and Qidashan type. In situ trace-element and sulfur isotopic analyses were conducted on pyrite from BIFs, high-grade magnetite ores, and altered rocks surrounding high-grade orebodies. In high-grade magnetite deposits, sulfur isotopes may be influenced by multiple factors, making them less effective in constraining the source of ore-forming fluids, whereas trace element analysis of pyrite could provide more precise insights into fluid nature and source. The ore-forming fluids in both deposits exhibit characteristics of hydrothermal systems associated with deep-seated magmatic activity. In the Gongchangling deposit, the ore-forming fluids were primarily derived from magmatic activity at ∼1.85 Ga, with additional contributions from fluids leaching Paleoproterozoic Co-Ni-rich geological bodies. In contrast, the Qidashan deposit is dominated by As-Se-rich fluids originating from magmatic-hydrothermal activity around ∼2.5 Ga. We propose that deep-seated magmatic activity is fundamental for forming high-grade iron deposits, whereas the development of large-scale deposits, such as the Gongchangling deposit, requires multi-phase hydrothermal activity involving Co-Ni-rich ore-forming fluids.