Fluid Inclusion and Oxygen Isotope Characteristics of Vein Quartz Associated with the Nabeba Iron Deposit, Republic of Congo: Implications for the Enrichment of Hypogene Ores

Abstract

The Nabeba iron ore deposit is located at the northern part of Congo Craton, Republic of Congo. The ore deposit consists of supergene and hypogene ores, both of which are hosted in the Precambrian Nabeba banded iron formation (BIF). This study focuses on the hypogene iron ore mineralization associated with quartz veins in the Nabeba deposit, for which two hypogene ore stages have been recognized based on geologic and petrographic observations: early-stage high‐grade hematite‐rich ore (HO‐1) and late-stage magnetite‐rich ore (HO‐2). Based on microthermometric measurements and laser Raman spectroscopy of the fluid inclusions, the H2O‐NaCl ± CO2 fluids interacting with the Nabeba BIF at the HO‐1 stage evolve from high‐to‐moderate temperatures (203−405 °C) and contrasting salinities (moderate-to-low: 1−15 wt. % NaCl equiv.; high: 30−35 wt. % NaCl equiv.) to H2O‐NaCl fluids of moderate‐to‐low temperatures (150−290 °C) and salinities (1−11 wt. % NaCl equiv.) for the HO‐2 ore stage. Assuming equilibrium oxygen isotopic exchange between quartz and water, the δ18Ofluid values range from 4.7−8.1‱ for the HO‐1 stage and −2.3‱ to −1.5‱ for the HO‐2 stage. This implies the ore‐forming fluid of initially-mixed metamorphic−magmatic origin, later replenished by seawater and/or meteoric water during the formation of the HO‐2 stage. These mixtures of different fluids, coupled with their interaction with the BIF lithology followed by phase separation, are responsible for the enrichment of hypogene iron ore in the Nabeba deposit.

Keywords

• hypogene ore
• banded iron formation
• nabeba deposit
• fluid inclusion
• oxygen isotope
• ivindo basement complex
• congo craton