The evolution of the Dominga Fe-Cu deposit (northern Chile): Insights from mineral textures and micro-CT analysis

Abstract

The Fe-Cu Dominga deposit (2082 Mt at 23% Fe, 0.07% Cu), located in the Coastal Cordillera of northern Chile, is hosted by volcanic rocks of the Punta del Cobre Formation (131.5 ± 1.5 Ma zircon U-Pb) and into subvolcanic units (Dioritic Complex, 131.6 ± 1.0 Ma zircon U-Pb). The Fe-Cu mineralization is controlled by three structural systems which developed from a transtensional to a transpressional tectonic regime and can be divided into three groups: Early iron, Late iron and Early copper ores. Early iron ores are comprised of magnetite + pyrite + biotite breccia (1A ore), veins (1B ores), layers (1C ores) and disseminated ores (1D ores). Late iron ores are characterized by two groups of magnetite-apatite-actinolite hydrothermal breccias (2A, 2C ores) and syntaxial/antitaxial veins (2B, 2D, 2E ores). Early copper ores occur as syntaxial K-feldspar and quartz + epidote + chalcopyrite veins (3A ores), and as anhydrite + chalcopyrite-rich matrix breccia and veins (3B ores). This work presents a detailed mineral texture study of veins, hydrothermal breccias and disseminated iron-rich layers of the Dominga deposit which aims to determine fluid flow mechanisms associated with both iron and copper ores. The description of vein and breccia textural and internal structures was conducted in thin/polished sections perpendicular and parallel to the wall using optical and Scanning Electron Microscope techniques. In addition, two oriented surface samples were analyzed by computerized X-ray microtomography and numerical fluid flow simulations through the Lattice-Boltzmann method to obtain (3D) permeability anisotropy associated with early iron ores. Microtextures associated with Dominga iron and copper ores suggest that the main mass transfer fluid flow mechanism corresponds to advection (channelized and pervasive fluid flow), regardless of the tectonic regime. However, early copper ores have more complex mineral textures and internal structure due to the recurrence of crack-seal episodes. We propose that the various mineral textures and structures indicate changes in fluid flow direction over time, controlled by the permeability anisotropy of each tectonic regime. Results from numerical fluid flow simulations of early iron ore (1B) veins show a higher value of structural permeability in the vertical direction (kVz), which is consistent with a transtensional tectonic regime and the formation of vertical veins. Moreover, permeability related to 1C layered ore is higher in horizontal directions (kHx, kHy) rather than vertical (kVz) because of the natural permeability anisotropy of volcaniclastic rocks parallel to bedding. However, the kVz value suggests that the development of such layered ores also exhibits a degree of structural control at several length scales consistent with a transtensional regime. These results indicate that the occurrence of early iron ores as veins and layers may be controlled by both primary permeability anisotropy related to each lithology present at the Dominga Fe-Cu deposit and to the tectonic regime at the time. Finally, the Dominga Fe-Cu deposit attests to long lived hydrothermal activity with a transition from early fluids capable of precipitating iron ores under a transtensional system to later-stage fluids which precipitated copper ores under a transpressional regime that generated multiple crack and seal episodes.