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Juncal A.S.,Itasca Consultants AB | Ivars D.M.,Itasca Consultants AB | Brzovic A.,CODELCO Division El Teniente | Vallejos J.,University of Chile
Rock Engineering and Rock Mechanics: Structures in and on Rock Masses - Proceedings of EUROCK 2014, ISRM European Regional Symposium

Application of hydraulic fracturing for preconditioning of hard rock in underground caving mines has been demonstrated to be an effective means to induce and promote cave growth. Using Itasca's particle flow code PFC 2D, a real scale numerical test environment has been developed to assess the effect of preconditioning by hydraulic fracturing on primary fragmentation. The environment makes use of the synthetic rock mass (SRM) method to simulate the mechanical behavior of hard rock mass. This technique uses the bonded particle model for rock to represent intact material and the smooth-joint contact model (SJM) to represent the in situ discontinuities and the induced hydraulic fractures. A series of hypothetic scenarios of veined hard rock mass (stockwork) based on El Teniente mine has been simulated using this environment. The results emphasize the impact of preconditioning on primary fragmentation in veined hard rocks. © 2014 Taylor & Francis Group, London. Source

Vallejos J.A.,University of Chile | Suzuki K.,University of Chile | Brzovic A.,CODELCO Division El Teniente | Ivars D.M.,Itasca Consultants AB
International Journal of Rock Mechanics and Mining Sciences

Rock masses of the primary copper ore at the El Teniente mine fail mainly through the infill of preexisting veins during the caving processes, especially through those composed of less than 35% hard minerals (quartz and pyrite). In this study, the Synthetic Rock Mass (SRM) approach is used to reproduce the results of ten uniaxial compression tests on veined core-size samples of El Teniente Mafic Complex (CMET) lithology, from El Teniente mine, Codelco-Chile. At the scale of the tested samples it is observed that veins composed mostly of quartz dominate the failure process. The developed methodology considers generating a deterministic Discrete Fracture Network (DFN) based on the veins mapped at the surface of each core sample. Then, the micro-parameters of the Bonded Particle Model (BPM) are calibrated to represent the macro-parameters of the average block of intact rock within all samples. Next, the micro-parameters of the Smooth-Joint Contact Model (SJCM), which represent the mechanical properties of veins, are calibrated to reproduce the stress-strain curves and the failure modes of the veined core-size samples measured during the laboratory tests. Results show that the SRM approach is able to reproduce the behavior of the veined rock samples under uniaxial loading conditions. The strength and stiffness of veins, as well as the vein network, have an important impact on the deformability and global strength of the synthetic samples. Contrary to what was observed in the laboratory tests, synthetic samples failed mainly through weak veins. This result is expected in the modeling given that anhydrite veins are considered weaker than quartz veins. Further research is required to completely understand the impact of veins on the behavior of rock masses. © 2015 Elsevier Ltd. Source

Spencer E.T.,Imperial College London | Wilkinson J.J.,Imperial College London | Wilkinson J.J.,Natural History Museum in London | Creaser R.A.,University of Alberta | Seguel J.,CODELCO Division El Teniente
Economic Geology

The El Teniente Cu-Mo porphyry deposit, Chile, is one of the world's largest and most complex porphyry ore systems, containing an estimated premining resource of approximately 95 Mt Cu and 2.5 Mt Mo. Although Cu mineralization at the deposit is quite well studied, little work has focused specifically on the distribution and timing of Mo mineralization. Combined grade, vein, and breccia distribution analysis reveals that deposit-wide Mo grades of 0.01 to 0.06 wt % are strongly controlled by the abundance of main mineralization (type 6a) quartz ± molybdenite veins. These show a clear spatial relationship with several felsic-intermediate intrusions and appear to develop outward and upward into Cu-rich (type 6b-7b) quartz-chalcopyrite veins and (type 8) chalcopyrite-anhydrite ± bornite veins with sericitic alteration halos. High-precision Re-Os molybdenite dating reveals that these linked vein types did not develop in a single, deposit-wide evolution, but are diachronous, related to distinct episodes of hydrothermal activity associated with the emplacement of diorite finger porphyries and the composite Teniente Dacite Porphyry. These units acted as effective, short-lived (<100,000 years) conduits for pulses of Mo- and Cu-bearing hydrothermal fluids between 6.3 and 4.6 Ma. The rapid thermal contraction of each system during mineralization led to extensive overprinting of Mo-rich veins by their lower-temperature, Cu-rich equivalents. Separate pulses in magmatic-hydrothermal activity are separated by distinct gaps of up to 300,000 years, during which Mo-mineralizing activity appears to have gone into quiescence. Mo grades exceeding 0.06 wt % correspond to the presence of molybdenite-bearing, late mineralization-stage, tourmaline-cemented (type 9), and anhydrite-carbonate ± gypsum (type 10) veins and breccias. These are abundant at shallow mine levels and show a close spatial relationship with a series of concentric faults associated with the Braden Breccia Pipe. Mineralization in this paragenetic stage is relatively short-lived and occurs in all parts of the deposit between 4.80 and 4.58 Ma. The generally Cu poor nature of the late mineralization stage is attributed to the prior preferential extraction of Cu from the underlying magma chamber in earlier mineralizing events. This led to the late exsolution of oxidized, Mo-rich fluids that may have undergone further enrichment by remobilizing Mo from main mineralization-type veins associated with the Teniente Dacite Porphyry. The formation of the Braden Breccia Pipe is likely to have occurred in a single cataclysmic event at approximately 4.58 Ma, which cut the Mo-rich tourmaline breccias and created a distinct Mo-rich grade halo at shallow mine levels. With the exception of minor mineralization associated with small dacitic dikes at approximately 4.42 Ma, the Braden event marked the termination of Mo deposition. ©2015 Society of Economic Geologists, Inc. Source

Vry V.H.,Imperial College London | Vry V.H.,University of Tasmania | Wilkinson J.J.,Imperial College London | Wilkinson J.J.,University of Tasmania | And 2 more authors.
Economic Geology

The El Teniente copper-molybdenum deposit is hosted by the late Miocene Teniente Mafic Complex, a largely subvolcanic package of primarily basaltic-andesite porphyry sills and stocks that were emplaced within the mid-late Miocene Farellones Formation. In the late Miocene-Pliocene, a series of intermediate felsic plutons were intruded into the Teniente Mafic Complex. These are spatially associated with magmatichydrothermal breccias and multiple vein types that form individual mineralized complexes. Here we present detailed observations on mineralogy, textures, and intrusion, breccias, and vein crosscutting relationships to constrain the nature and relative timing of magmatic and hydrothermal events. Our revised classification defines 13 vein types, divided into three main stages: (1) premineralization biotite and/or K-feldspar ± quartz-anhydrite-albite-magnetite-actinolite-epidote veins that formed prior to emplacement of mineralized intrusions and breccias; (2) main mineralization stage veins that grade from gangue-dominated quartz-anhydrite veins ± potassic alteration halos into sulfide-dominated veins with phyllic alteration halos; and (3) late mineralization veins containing sulfosalts. Five breccia types have been observed in the deposit, typically forming individual, vertically zoned complexes, spatially associated with individual intrusions and overlapping in time: (1) igneous-cemented breccias, (2) K-feldspar?cemented breccias, (3) biotite-cemented breccias, (4) anhydrite-cemented breccias, and (5) tourmaline-cemented breccias. Breccia cements display a similar paragenetic evolution to main mineralization stage veins, indicating a close genetic link between them. A distinction can be made between early premineralization vein types (types 1-2) that represent veins formed, possibly deposit-wide, prior to emplacement of intrusion-breccia complexes, late pre- and main mineralization vein types (types 3-8), which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex, and late mineralization vein types (types 9-10) that represent a single event linked to the emplacement of the Braden Breccia Pipe. The geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow level, felsic-intermediate pipelike intrusions and the development of igneous and mineralized magmatichydrothermal breccias and vein halos. The magmatic-hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic-hydrothermal activity which followed rather similar evolution paths. Single, deposit-wide models of fluid evolution and mineralization are therefore inappropriate. We conclude that El Teniente represents a nested but otherwise rather typical porphyry Cu-Mo system, unusual only in that the Teniente Mafic Complex provided a particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic-hydrothermal breccia complexes and as an effective chemical trap for deposition of sulfides. The overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren, intermineral porphyries resulted in its unusual size and high hypogene grades. © 2010 Society of Economic Geologists, Inc. Source

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