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Internal Structure and Petrography of the Mineralized Faults in the Radomiro Tomic Deposit

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Internal Structure and Petrography of the Mineralized Faults in the Radomiro Tomic Deposit O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 Internal Structure and Petrography of the Mineralized Faults in the Radomiro Tomic Deposit Erik Jensen*, Gabriel González, Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile *Contact email: [email protected] Abstract. The interaction between faults and hydrothermal largest ore-bearing fault systems in the world (DFS). fluids is studied observing the internal characteristics of the Aimed to unravel the temporal and genetic relationship faults in RT deposit. Distribution and petrography of fault- between faulting and fluid-induced alteration, in order to related rocks, grouped in units (studied with optical understand their effects on the evolution of the crustal microscope and XRD) unravel their timing and genesis. rheology. Subsequently, nature of pore fluids and evolution of mechanical properties of faults are deduced. Both rock deformation and circulation of fluids in the deposit Case of study were conducted through the successive reutilization of the same structures (N30-60E/~90). Initially quartz veins and The case of study RT, is located within a fore-arc trench- afterwards fault planes. During the nucleation of faults, parallel fault system, called Domeyko Fault System (DFS). phyllic fluids permeated the deposit, adding pyrite The location of several porphyry deposits along it suggest (+chalcopyrite) to the fractures and transforming feldspar to high permeation of hydrothermal fluids trough the crust, at illite in the haloes. Subsequently, these faults were sheared 3-5 km depth (subsequently exhumed). The deposit is by cataclasis, forming black gouges and pyrite breccias. hosted by granitic porphyric rocks from the Chuquicamata Later on, gouge was cemented by quartz becoming Intrusive Complex, 35-34 Ma (U/Pb zircon dating) cataclasite. Finally, hydrothermal illite and remaining (Cuadra & Rojas 2001, Diaz et al. 2009). The feldspars were partially converted to kaolinite. hydrothermal alteration in the deposit is separated in three Mechanically, the deformation started as semi-plastic stages. (1) Early K-Silicate Alteration or Background hydro-fracturing, but as the temperature decreased, Potassic Alteration (32.7 Ma). (2) Quartz/Sericite deformation evolved to a brittle regime of fracturing and Alteration (31.8 Ma). (3) Argillic Alteration (youngest). shearing. Sericitc alteration, by phyllic fluids, weakened the Both metallic mineralization and hydrothermal alteration rocks and facilitated the slip on faults. Finally, silicification products are spatially related to sub-vertical faults striking hardened the faults locking the slip. Later formation of kaolinite may have weakened the rocks, however no N30-60E. These faults conform the main structural deformation is recorded after it. system in the deposit, and are of second order with respect to the two major regional-scale faults: Messabi Fault and Keywords: Radomiro, Tomic, Fault-Fluid, Interaction West Fissure (N00-10E/sub-vertical). Objective Methodology Knowledge about fluid-rock interaction within faults of the After checking, the available in-mine routes and observe Earth’s crust is key to understanding a wide range of the accessible outcrops, two sites where selected for geological processes, such as earthquake generation, analysis. Both selected sites are mine walls perpendicular crustal strength and distribution of economic minerals. to the fault strike and bellow the oxidized zone. In order to characterize the internal structure of the faults, outcrop- Crustal scale faults are widely considered to control fluids scale maps were conducted and samples taken. The flow and enhance their chemical interaction with rocks observed rocks were grouped according their macroscopic (Caine et al. 1996, López & Smith 1996, Wibberley & petrographic properties, defining "Fault-related Units". Shimamoto 2003, Evans & Chester 1995, Goddard & Samples of each unit where then analyzed under Evans 1995, Janssen et al. 2004, Schulz & Evans 1998). petrographic microscope and XRD. Contrastingly, most of the published research on petrography of fault-rocks has been conducted in faults Petrographic Description with weak hydrothermal activity, away from any important hydrothermal deposit. (Braathen et al. 2009, Chester & The components of the studied faults are separated in 5 Chester 1998, Evans & Chester 1995, Faulkner et al. 2003, Fault-Related Units and the surrounding rocks are grouped Janssen et al. 2004, Wibberley & Shimamoto 2003). into a Granitic Protolith Unit. Even though the protolith is not fault-related, it is analysed in the same terms as the Here, we present mineralogical (XRD) and petrographic fault-related units, in order to compare and understand (micro- and macroscopic) analysis of fault-related rocks background process. Distribution of these units is shown in within a crustal-scale hydrothermal system. In one of the Figure 2. 182 AT 1 GeoloGía ReGional y Geodinámica andina stage, temperatures around 450-550°C are expected Granitic Protolith: Igneous porphyric and euhedral (Sillitoe 2010), allowing crystal plasticity of quartz and texture. Composed of quartz (25-40%), plagioclase (20- subsequent ductile creeping at relatively slow strain rates. 30%), orthoclase (25-35%), clays (5-20%) and biotite (5- This plastic deformation is the responsible for the 10%) // Overgrowth orthoclase exhibit inclusions of quartz subgrains in quartz and plagioclase and irregular wavy and plagioclase. shapes of quartz veins. The repetitive brittle extension (banded quartz veins) is consequence of flashing pulses of Clay-Altered Granit: Igneous porphyric and euhedral ascending fluids. These pulses suddenly increase the fluid texture. Composed of quartz (30-40%), k-feldspar (25- pressure and the strain-rate, favouring the brittle fracture 35%), clays (25-30%) and pyrite (~1%) // Plagioclase is over the plastic creep at the same temperature (Fournier replaced by illite and kaolinite, forming pseudomorphs. 1999). Figure 1A. Orthoclase is only partially replaced to clay. The euhedral crystals of quartz reflect precipitation under White Proto-cataclasites: Igneous porphyric to aphanitic enough fluid pressure to prevent the walls from yielding to texture, fragmentally deformed. Composed of quartz (50- lithostatic pressure and close (between the pulses). Those 80%), clay (15-50%) and pyrite (~1%) // Quartz intercrystalline spaces are mainly aligned parallel to the phenocrystal are euhedral and unaltered, all the other walls, providing the highest permeability direction. Some minerals are replaced by patchy quartz, kaolinite and of these spaces are partially filled with euhedral crystals of sericite. molibdenite, brought by posterior fluids pulses. Figure 1B. Quartz Vein: Composed of euhedral and anhedral quartz Striation on molibdenite crystals, and the abundance of (95%) , pyrite (3%) and molibdenite (2%)// Quartz is Quartz Vein fragments in the Black Cataclasites/Gouge are banded parallel to the walls interlayered with metallic consequence of brittle faulting after the vein was fully sulphides. Irregular crystal boundaries and sub-grains formed. Most of the shear bands are located along the (plastic deformation) are heterogeneously distributed. quartz veins. This is consistent with a mechanical Metallic sulphides (molibdenite, pyrite, chalcopyrite) are anisotropy induced by a relatively strong vein crossing filling the spaces between euhedral quartz crystals and through a weaker rock (Propagating fractures across the some are striated. veins require more energy than along them). This indicate that the structures started as opening fractures (Mode I) Black Cataclasite/Gouge: Fragmental foliated texture. and subsequently were sheared to become faults. This Composed of quartz (50%), clays (20-40%), pyrite (5- requires a relative rotation of the faults with respect to the 15%) and chalcopyrite (~1%) // Illite is foliated and stress field. (Figure 1c). deformed by shear. Quartz is found as fragments of quartz-vein and as cement (between fragments and clays). At the time when the fractures started to form along veins Pyrite is distributed in bands of comminuted fragments. and walls, hydrothermal fluids were channelled along The foliation and sigmoidal asymmetries indicates right- them, considerably increasing the mineral alteration and lateral kinematics. precipitation. Pyrite is spatially related to the shear bands and fractures, suggesting that sulphide rich hydrothermal Pyrite Breccia: Coarse fragmental texture, roughly fluids penetrated the faults during brittle shearing. A minor foliated. Composed of quartz (30-45%), clays (20-40%) amount of pyrite is also disseminated within and pyrite (10-30%) // Fragments are 0.1-5mm size: intercrystalline spaces of the porphyric host-rock, Quartz vein, squared blocks of clay (undeformed suggesting that previous pyrite mineralization could have plagioclase pseudomorph) , pyrite (with jigsaw puzzle also occurred before the main one. However, as the major texture) and fragments of foliated gouge. amount of sulphides is related to fractures and zones of cataclasis, brittle shearing is considered the main process Fault evolution and interaction with fluids driving the precipitation of hydrothermal sulphides. Illite (and smectitie) from Quartz-Sericitic alteration is also According to the distribution and boundary relationships of spatially related to the faulted veins. In the haloes: (altering the described
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