Fracturing, fluid processes and mineralisation in the Cretaceous continental magmatic arc of Northern Chile (25°15'-27°15'S)
The structural geology of the Coastal CordiIIera of northern Chile (25°15'-27°15'S) is dominated by three fracture systems: (1) the margin-parallel, trench-linked Atacama Fault Zone (AFZ), (2) a northwest¬trending network of faults, and (3) northeast-trending discrete fracture zones. The margin-oblique fracture sets appear to relate to magnetic fabrics imaged deep in the lithosphere and suggests that these fault zones utilise long-lived zones of weakness. The interaction of these fractures was an important structural control on the focusing of melts and ore fluids' in the leading edge of the Andean margin. Magnetite-dominated deposits comprise one major style of genetically-related mineral deposits within the study area. Primary' fluids from a magnetite-apatite deposit (Carmen) are moderate to highly saline (c. 17 to 37wt% NaCl eq.), Na-Ca-Fe±Mg-Cl-H20 brines. Magnetite from the main-stage of mineralisation was precipitated from a fluid with a high alSo content of circa lO%lIvs SMOW, at temperatures of -300 to 500°C and a mean depth of -2.5km, indicating a magmatic provenance of these magnetite deposits. Comb and dendritic textures indicate precipitation from a low viscosity ore fluid, consistent with fluid densities of 0.9 to 1.lg/cm3 and high contents of Cl, F and H20. These data point to a submagmatic origin of the ores. The second major type of fracture-controlled mineralisation in the Coastal CordiIIera are fault-hosted veins and breccia-pipes of specular haematite-cha1copyrite ore. They are found in close spatial relationship with magnetite-dominated styles of mineralisation and are thought to have formed by cooling and oxidation of an originally magnetite-bearing ore fluid, by mixing with a carbonate-bearing, extraneously sourced fluid. This explains the intergrowth of ore minerals with calcite and/or siderite, and the geological setting of these deposits, under a carbonate-precipitating, shallow-marine marginal basin, which is tentatively suggested to source the extraneous fluid. Further investigation is needed to confirm this. A first-order structural control on the distribution of magnetite-dominated deposits is imposed by the Atacama Fault Zone, due to its role in the emplacement of dioritic and granodioritic magmas, which are the likely sources of the magnetite-mineralisation. Magnetite-dominated deposits were emplaced along broadly north-south-trending brittle and mylonitic segments of the sinistral AFZ. The structural controls on the mineralisation imposed by brittle faults are not fully understood. In several of the larger deposits, east¬northeast-fractures are important, although their precise role remains obscure. Magnetite-dominated mineralisation along the mylonitic shear zones of the AFZ and Chivato Fault Zone, appear associated with cyclic deformation. This is linked to pulses of hydrothermal fluids expelled from an intrusion, or episodic fluid flow accompanying high coseismic strain rates, experienced by the mylonites due to their high crustal level. The structural control on the haematite-cha1copyrite breccias is imposed by northwest trending faults. Breccia-pipes and veins are located in dilationaI sites along these faults, such as jogs and/or fault bends. Their textures may also interpreted to relate to the earthquake cycle. Along the AFZ belt of brittle and mylonitic shear zones, haematite-cha1copyrite-bearing, northwest-trending fractures commonly cross-cut the north-south oriented, magnetite bodies. This represents a change in the predominant structures which focused hydrothermal fluids throughout the evolution of a mineralising system. A model is proposed in which, 'locking-up' of the mylonite occurs as the temperature passes through the quartz plastic-brittle transition temperature (-300°C). This causes deformation to switch from localised displacements along the mylonitic shear zones of the AFZ, to more distributed deformation along 'basement-controlled' northwest faults. These faults are more favourable to accommodate displacements due to their orientation and that they are probably nucleated upon inherited zones of crustal weakness. The temperature of the brittle-ductile transition is approximately intermediate between the formation temperatures of the two classes of Fe-oxide deposit, hence switching of the fracture kinematics is broadly contemporaneous, with the change in mineralisation style.