The Dominga FE-CU deposit as an example of the interaction between faults, iron-oxide apatite, and ironoxid copper-gold mineralization, Northern Chile

Abstract

Understanding how fluids circulate in and around the presence of crustal faults is a fundamental topic when exploring ore deposits. This research presents a detailed description of the mineral texture of veins, hydrothermal breccias, and iron-rich layers from the Dominga deposit, aiming to determine, on the one hand, the role of fault zone architecture and rock permeability in ore occurrence, geometry, and the mechanisms of circulation of fluids through fault zones. On the other hand, the goal is to determine a possible spatial and temporal relationship between mineralization styles and to provide new insights into understanding how fluids evolve from one mineralization style to another. The Dominga Fe-Cu 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 and a subvolcanic unit. Here, the two structural systems that control Fe-Cu mineralization– the Early structural system (predating the Atacama fault system) and the El Tofo structural system (as a branch of the Atacama fault system), developed from a transtensional to a transpressional tectonic regime, respectively. Mineralization related to these structural systems can be divided into three stages: Early iron and Late iron stages associated with the Early structural system, and the Early copper stage associated with the El Tofo structural system. Magnetite belonging to the early iron stage represents the principal (and economic) Fe ore of the Dominga deposit. Detail mapping (1:20 scale) of six drill cores was used to collect representative samples and build structural sections highlighting the spatial distribution of the Fe-Cu occurrence and the mineral textures of veins and hydrothermal breccias within the fault zone architecture. The description of the mineralogy, texture, and internal structures of veins and hydrothermal breccias was conducted using an optical and digital microscope. Scanning Electron Microscope techniques in thin/polished sections were used to detail texture and analyze mineral inclusions. In addition, two oriented surface samples (diorite and volcaniclastic rock) were analyzed with X-ray computed micro-tomography and numerical fluid flow simulations using the Lattice-Boltzmann method to obtain (3D) permeability anisotropy associated with the early iron stage. A compositional study of magnetite from the early iron, late iron, early copper stage, and allanite from the late iron and the early copper stage was carried out using Electron microprobe analyses (EPMA). Mineralization from the early iron stage, hosted in diorite and volcaniclastic rocks, shows a paragenetic mineral assemblage that comprises magnetite+pyrite+biotite±titanite±actinolite. In diorite, mineralization occurs as the matrix of breccia (1A), magnetite veins (1B), and disseminated magnetite (1D), whereas in volcaniclastic rocks, mineralization occurs as magnetite-rich layers (1C), magnetite veins (1B), and disseminated magnetite (1D). Late iron stage in south Dominga is characterized by magnetite+apatite+actinolite±pyrite as the matrix of 2A breccia and magnetite±actinolite±biotite±quartz±apatite 2B syntaxial veins. In north Dominga, late iron stage is characterized by actinolite+apatite+magnetite±pyrite as the matrix of breccia 2C, actinolite+pyrite±magnetite±apatite±quartz±allanite 2D syntaxial veins, and actinolite+pyrite 2E antitaxial veins. Early copper stage occurs as syntaxial veins filled with quartz+K feldspar±anhydrite+tourmaline+allanite+pyrite+chlorite+chalcopyrite±magnetite veins (3A) and anhydrite+chalcopyrite+pyrite±magnetite±allanite±tourmaline±molybdenite hydrothermal breccia and vein (3B). Magnetite veins, breccias, and magnetite-rich layers associated with the early iron stage suggest that permeability plays a key role in the spatial distribution of the mineralization in the fault zone architecture, from proximal to distal in the fault core; and in fluid flow mechanisms, from outcrop down to a millimetric scale. Numerical fluid flow simulations of early iron stage 1B veins hosted in diorite and volcaniclastic rocks show a permeability anisotropy consistent with a transtensional tectonic regime. In diorite, 1B magnetite veinsact as a fluid-feeder for disseminated mineralization. Moreover, in volcaniclastic rocks, 1B magnetite veins are connected to 1C magnetite-rich layers. Numerical fluid flow simulations of 1C magnetite-rich layers show a higher permeability in horizontal directions (KHx, KHy) due to the natural permeability of volcaniclastic rocks parallel to bedding. However, the KVz value suggests a degree of structural control as millimetric 1B magnetite veins. This genetic relationship was observed from the outcrop down to the millimetric scale. These results indicate that the occurrence of early iron stage as veins and layers was controlled by the interaction of both primary permeability and structural permeability related to each lithology present at the Dominga Fe-Cu deposit. Variations in the modal abundance of act+ap+mag±py paragenetic mineral assemblage of late iron stage (2A to 2E) are interpreted as different parts of the same mineralizing episode. However, the mineral texture and spatial distribution of the mineralization in the south (2A breccias and 2B veins) and north Dominga (2C to 2E) suggests they were emplaced through different mechanisms due to local variations in the stress conditions. By contrast, the early copper stage has a more complex internal structure due to the recurrence of crack-seal episodes. The combination of various mineral textures and structures within 3A veins may record changes in fluid flow direction due to changes in the tectonic regime from transtensional to transpressional. Textural and compositional features of magnetite and allanite from the 2C breccias, 2D veins (late iron stage), and 3A veins (early copper stage) offer additional evidence of a transition between different structural systems (tectonic regimes) and different mineralization styles. These mineralization styles represent the same fluid that evolved from a high-temperature (~690° to ~480°C) magmatic-hydrothermal fluid to a mediumtemperature (500-360°C) hydrothermal system, indicating a genetic (spatial and temporal) relationship between the IOA and IOCG mineralization styles. On the other hand, the texture and composition of magnetite from the early iron stage are interpreted as an ironrich (and copper-poor) hydrothermal fluid, which shows no relation with the late iron and early copper mineralizing stages.