minerals Article
The Arsenic Fault-Pathfinder: A Complementary Tool to Improve Structural Models in Mining Daniel Carrizo 1 , Carlos Barros 2 and German Velasquez 1, * 1 2
*
Advanced Mining Technology Center (AMTC), FCFM, University of Chile, 8320000 Santiago, Chile;
[email protected] Geomechanical Superintendence, Los Bronces Division, Anglo American Chile, 8320000 Santiago, Chile;
[email protected] Correspondence:
[email protected]; Tel.: +56-2-29771000
Received: 5 July 2018; Accepted: 17 August 2018; Published: 21 August 2018
Abstract: In a mining operation, the structural model is considered as a first-order data required for planning. During the start-up and in-depth expansion of an operation, whether the case is open-pit or underground, the structural model must be systematically updated because most common failure mechanisms of a rock mass are generally controlled by geological discontinuities. This update represents one of the main responsibilities for structural geologists and mine engineers. For that purpose, our study presents a geochemically-developed tool based on the tridimensional (3-D) distribution of arsenic concentrations, which have been quantified with a very high-density of blast-holes sampling points throughout an open pit operation. Our results show that the arsenic spatial distribution clearly denotes alignments that match with faults that were previously recognized by classical direct mapping techniques. Consequently, the 3-D arsenic distribution can be used to endorse the existence and even more the real persistence of structures as well as the cross-cutting relationships between faults. In conclusion, by linking the arsenic fault-pathfinder tool to direct on field fault mapping, it is possible to improve structural models at mine scale, focusing on geotechnical design and management, with a direct impact in the generation of safety mining activities. Keywords: mining structural models; mining planning optimization; geotechnical designs; geochemical tool; arsenic
1. Introduction In mining operations, the precise determinations of relevant fault geometries oriented to geotechnical design are a major concern for structural geologists and geotechnical engineers [1–5]. The principal uncertainties in structural and geomechanical models are mainly controlled by both limited data collection resolution [6] as well as the spatial distribution of data. Thus, in practice, mining engineers must complete large interpretations during complex structural modeling by using software that can build multiple data-consistent solutions. This scenario has strong consequences in the mining value chain [5,7] affecting not only the geotechnical design, but also the hydrological management, blasting strategies, among others. Additionally, the transition in the mining operation from open-pit to the underground, i.e., OP to UG transition [8,9], triggers a requirement to improve the quality and precision of the structural and geomechanical models for an in-depth mining for hard rock masses (i.e., volcanic rocks) in which brittle slope failures are difficult to predict [7]. In the subsurface, geological discontinuities are usually complex and their full characterization is a concern to be still investigated [5]. This is the main challenge for geological and mine engineers in current mining and requires considering an entire and comprehensive geological description of an ore deposit. In this way, geophysical techniques have been used to investigate structural attributes in the subsurface [5,10] and Minerals 2018, 8, 364; doi:10.3390/min8090364
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to propose the 3-D internal structure of a rock mass [10]. To our knowledge, this contribution represents the first approach to characterize fault attributes in the 3-D space using geochemical mine data. Arsenic (As) is an environmental polluting chemical element and its input to the atmosphere has been linked to emissions generated from copper smelters [11]. Arsenic-bearing minerals, mainly sulfides, have been recognized in most of the hydrothermal systems, such as porphyry copper deposits [11,12], requiring that arsenic concentrations must be controlled in routine mining practice. Consequently, in modern mining industries, special attention is paid to quantify arsenic in the mined material to be processed. It is also due to the concerns in metallurgical processing, including, among others: (i) an increase in the production costs; (ii) a decrease of the product purity, which affects the price and commercialization of the final product; and (iii) a possible generation of acid mine drainage, as well as high-contaminant passive and active wastes [13]. This scenario implies that a huge amount of samples from drill-holes and blast-holes must be systematically analyzed to quantify the arsenic concentration, and the sampling points must have a high-density tridimensional distribution, being it at least an order of magnitude greater than any other spacing used for collecting structural data. Thus, the distribution of arsenic concentration generates an extraordinary industry 3-D database which will not be affordable for a research group, representing an outstanding opportunity to use this information for purposes other than metallurgical and environmental issues, which can provide additional value to the investment made to quantify the arsenic concentration in the mined material. From this perspective, this paper describes how the tridimensional distribution of arsenic concentration can be used as a geochemical vector for elucidating structural attributes in a structural model, which frequently cannot be established using standard methods of structural characterization and mapping. The main goal of this study is to propose a complementary geochemically-developed tool to validate and to improve structural and geotechnical models, focused on efficiently reducing the intrinsic uncertainty of the models. The present research has been conducted taking the Los Bronces Mine as the study case, an open pit operation with a depth of about 800 m, which is considered as one of the deepest open pit mines in the world. The Los Bronces Mine is been developed at the “Los Bronces-Rio Blanco” porphyry Cu-Mo deposit, part of the most highly exploitable copper endowment in the Earth’s crust [14,15]. 2. Blast-Hole Arsenic Database Opportunity In a mining operation, the Arsenic (As) concentrations must be carefully controlled to determine the most suitable methods for mine extraction and processing as well as to establish the environmental safety conditions for tailings operation [11,13]. Beyond this operational mining condition, arsenic is an interesting element to be investigated in hydrothermal ore systems. For the case of meso- and epithermal deposits it is commonly proposed that As-rich sulfides play a role in scavenging gold [16–19]. Particularly, for porphyry systems, the geochemical behavior of arsenic has been recently investigated, showing that arsenic is geochemically decoupled from copper (Cu). In these hydrothermal systems [19,20], decoupling between copper and arsenic is due to changes in the composition of the hydrothermal fluid [20,21]. Copper and As selective geochemical partitioning [19–22] is considered the result of a fluid-phase separation process (or boiling), in which a single-phase ore-fluid is separated into a low-density vapor and denser brine phase (Figure 1). In this fluid separation, arsenic is preferentially concentrated into the low-density–low-salinity vapor phase, while copper is partitioned preferentially into the high-density saline brine fluid phase [23,24]. This fluid-phase separation takes place at the brittle-to-ductile transition (BDT) depths for porphyry systems (~1–3 km) [24,25], which is defined as the boundary-zone between the lithostatically pressured up-flow of hot magmatic fluids and the hydrostatically pressured convection of cooler meteoric fluids [25]. However, only the As-rich vapor phase is considered to be able to migrate upwards in the system through the permeable fracture network [21] developed in the upper crust (Figure 1). Interestingly, the As-rich vapor fluid phase migration can trigger fluid overpressure conditions during its ascent, resulting in fracturing in the shallowest parts of the hydrothermal system [21], and this process can be accompanied by a preferential
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conditions during its ascent, resulting in fracturing in the shallowest parts of the hydrothermal precipitation As-bearing minerals theaccompanied superficial fracture network (Figure 1). The copper-arsenic system [21],ofand this process caninbe by a preferential precipitation of As-bearing geochemical decoupling implies that Cu-bearing sulfides preferentially precipitate in the part minerals in the superficial fracture network (Figure 1). The copper-arsenic geochemicaldeeper decoupling ofimplies the porphyry system, while As-bearing sulfides would precipitate in shallower frequently that Cu-bearing sulfides preferentially precipitate in the deeper part of the parts, porphyry system, filling the permeable fracture network (Figure 1), accompanied by a minor proportion of Cu-bearing while As-bearing sulfides would precipitate in shallower parts, frequently filling the permeable sulfides [19–21]. fracture network (Figure 1), accompanied by a minor proportion of Cu-bearing sulfides [19–21].
Figure 1. The conceptual schema of an idealized porphyry Cu-Mo deposit showing the ore-fluid Figure 1. The conceptual schema of an idealized porphyry Cu-Mo deposit showing the ore-fluid phase phase separation and the As-Cu geochemical partitioning (simplified from Reference [23]). The separation and the As-Cu geochemical partitioning (simplified from Reference [23]). The brittle-to-ductile brittle-to-ductile transition (BDT)from zoneReference is simplified transition (BDT) zone is simplified [25].from Reference [25].
3. The Case of Study: Main Mineralogical Features of the Los Bronces-Río Blanco Porphyry 3. The Case of Study: Main Mineralogical Features of the Los Bronces-Río Blanco Porphyry Cu-MoDeposit Deposit Cu-Mo The“Los “Los Bronces-Río Blanco” porphyry Cu-Mo deposit is emplaced in the Miocene to early The Bronces-Río Blanco” porphyry Cu-Mo deposit is emplaced in the Miocene to early Pliocene Pliocene Andean magmatic arc and is developed in the frontal cordillera of the Central Andes Andean magmatic arc and is developed in the frontal cordillera of the Central Andes of Chile in Southof Chile in(Figure South America (Figure 2a) It is exploited in mining two neighbor mining operations America 2a) [12,14,15,26]. It [12,14,15,26]. is exploited in two neighbor operations (Figure 2b), (Figure 2b), namely the Los Bronces Mine, which is managed by the Anglo American Company and namely the Los Bronces Mine, which is managed by the Anglo American Company and the Andina the Andina Mine, managed by CODELCO (Corporación Nacional de Cobre, Santiago, Chile). Recent Mine, managed by CODELCO (Corporación Nacional de Cobre, Santiago, Chile). Recent discoveries by Anglo have that this porphyry deposit is world one ofclass the metal largest bydiscoveries Anglo American haveAmerican determined thatdetermined this porphyry deposit is one of the largest world class metal districts [12,14,15,26]. In the Los Bronces Mine, extraction is developed at ~4000 districts [12,14,15,26]. In the Los Bronces Mine, extraction is developed at ~4000 m above mean seam above mean sea (a.m.s.l.), by open-pit excavation an×open-pit ~3 km × 2 km (long × width), up to level (a.m.s.l.), by level excavation of an ~3of km 2 km (long × width), and up to ~800and m deep ~800 m3a). deep (Figure 3a). (Figure The main mineralization mineralization processes LosLos Bronces-Río Banco deposit (from(from 8.4 ± 0.5 The main processesatatthethe Bronces-Río Banco deposit 8.4to±4.3 0.5± 0.05 to Ma) are associated with the important development of hydrothermal breccias, which are 4.3 ± 0.05 Ma) are associated with the important development of hydrothermal breccias, which are superimposed by a late brittle deformation stage [15,27]. Mineralization is associated with the superimposed by a late brittle deformation stage [15,27]. Mineralization is associated with the depositionof of the andand the emplacement of theof Riothe Blanco-San Francisco deposition the volcanic volcanicFarellones FarellonesFormation Formation the emplacement Rio Blanco-San batholithbatholith [14,15,27]. The magmatic-hydrothermal system was developed in this region (between Francisco [14,15,27]. The magmatic-hydrothermal system was developed in this region16 and 4 Ma), as a result of a long-lived magmatic chamber activity, where the mass transference along (between 16 and 4 Ma), as a result of a long-lived magmatic chamber activity, where the mass the upper crust seems to have been controlled by (i) the Andean deformation regime that prepared transference along the upper crust seems to have been controlled by (i) the Andean deformation the upper structure; andcrustal (ii) the structure; fast exhumation in the region between ~6–3 Ma regime that crustal prepared the upper and (ii)rates the documented fast exhumation rates documented in the [15,27]. region between ~6–3 Ma [15,27].
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Figure 2. (a) The simplified tectonic map of South America showing the regional location of the Los 2. (a) The tectonic mapmap of South America showing the regional location of the Los Bronces-Rio Figure (a)Blanco Thesimplified simplified South America showing the of regional location of the Los Bronces-Rio District tectonic in relation toofthe metallogenic provinces the Central Andes. (b) Blanco District in relation to the metallogenic provinces of the Central Andes.of(b)the Leapfrog’s orthographic Bronces-Rio Blanco District in relation to the metallogenic provinces Central Andes. (b) Leapfrog’s orthographic projection of the Rio Blanco-Los Bronces district, showing the location of the projection of the Rio Blanco-Los Broncesof district, showing the location the Los Bronces Mine and the Andina Leapfrog’s the Rio Blanco-Los Bronces of district, Los Bronces orthographic Mine and the projection Andina Mine (modified from Reference [28]). showing the location of the Mine (modified from Reference [28]). Los Bronces Mine and the Andina Mine (modified from Reference [28]).
In the metallogenic framework, three ore-forming stages have been identified: (i) an early In metallogenic framework, three stages have been identified:hydrothermal (i) In the the metallogenic framework, three ore-forming ore-forming stages (ii) have been identified: (i) an an early early hydrothermal stage, including the porphyry Cu-Mo deposit; a superimposed hydrothermal stage, including the porphyry Cu-Mo deposit; (ii) a superimposed hydrothermal hydrothermal stage, including the porphyry Cu-Mo deposit; (ii) a superimposed hydrothermal breccia breccia complex, proposed as the main hydrothermal stage; and (iii) a polymetallic fault-vein (Figure 3b) breccia complex, proposed thetomain hydrothermal stage; and a polymetallic fault-vein (Figure 3b) complex, proposed as theasmain hydrothermal stage; and (iii)(iii) a polymetallic fault-vein 3b) emplacement phase associated the late hydrothermal stages [14,15]. Mineralogically, the(Figure early and emplacement phase associated to the late hydrothermal stages [14,15]. Mineralogically, the early and emplacement phase associated to the late hydrothermal stages [14,15]. Mineralogically, the early main mineralization stages are mostly constituted by copper (Cu) ± molybdenum (Mo)-bearing main mineralization stages are are mostly constituted bybycopper (Cu) (Mo)-bearing and main mineralization stages mostly constituted copper (Cu) ±±molybdenum molybdenum (Mo)-bearing sulfides, such as: (i) essentially chalcopyrite (CuFeS 2), accompanied by bornite (Cu5FeS4), sulfides, such as: (i) essentially chalcopyrite (CuFeS 2 ), accompanied by bornite (Cu5FeS4), sulfides, such as: (i) 2essentially chalcopyrite (CuFeS accompanied by bornite (Cu5 FeS molybdenite molybdenite (MoS ), and pyrite (FeS2) for the 2 ),early stage; and (ii) pyrite and4 ),chalcopyrite, molybdenite (MoS 2),(FeS and pyrite (FeS 2) stage; for theand early stage; and chalcopyrite, (ii) pyrite and chalcopyrite, (MoS (ii) pyrite and accompanied in 2 ), and pyrite 2 ) for the early accompanied in a minor proportion by chalcocite (Cu 2S) and covellite (CuS) for the main stage case. accompanied in a minor proportion by chalcocite (Cu 2S) and covellite (CuS) for the main stage case. a minor proportion by chalcocite (Cu S) and covellite (CuS) for the main stage case. While the 2 While the As-bearing sulfides (Figure 3c,d), such as enargite (Cu3AsS4), and tennantite While As-bearing sulfides (Figure 3c,d), such as enargite (Cu3AsS 4), and As tennantite As-bearing 3c,d), such as enargite (Cu tennantite [(Cu,Fe) 3 AsS4 ), and 12 4 S13 ] are [(Cu,Fe)the 12Assulfides 4S13] are (Figure only recognized in the late-stage polymetallic veins (Figure 3b), accompanied by [(Cu,Fe) 12As4S13] are only recognized in the late-stage polymetallic veins (Figure 3b), accompanied by only recognized in the late-stage polymetallic veins (Figure 3b), accompanied by pyrite, chalcopyrite, pyrite, chalcopyrite, bornite, sphalerite [(Zn,Fe)S], galena (PbS), and primary chalcocite and covellite pyrite, bornite, sphalerite [(Zn,Fe)S], galena (PbS), primary chalcocite and covellite bornite,chalcopyrite, sphalerite [(Zn,Fe)S], galena (PbS), and primary and covellite [29,30]. Interestingly, [29,30]. Interestingly, the main faults recognized in chalcocite the Losand Bronces Mine are filled by later [29,30]. Interestingly, the main faults recognized in the Los Bronces Mine are filled by the main faults recognized in the Los Bronces Mine are filled by later hydrothermal mineral paragenesis hydrothermal mineral paragenesis (Figure 3b), especially including arsenic-bearing sulfides,later i.e., hydrothermal mineral paragenesis (Figure 3b), sulfides, especially sulfides, i.e., (Figure 3b),and especially arsenic-bearing i.e.,including tennantitearsenic-bearing and enargite (Figure 3b–d). tennantite enargiteincluding (Figure 3b–d). tennantite and enargite (Figure 3b–d).
Figure 3. (a) (a)Photograph Photographof of Bronces pit.Photograph (b) Photograph of an outcrop a Figure 3. thethe Los Los Bronces openopen pit. (b) of an outcrop showing showing a principal Figure 3. (a) Photograph of the Los Bronces open pit. (b) Photograph of an outcrop showing a principal fault-vein filled byminerals. the As-bearing minerals. Scanning electron microscopy fault-vein filled by the As-bearing Scanning electron microscopy (SEM)-backscattered electron principal fault-vein electron filled (BSE) by images the As-bearing minerals. Scanning electron microscopy (SEM)-backscattered of an enargite (c) and a tennantite (d) crystal. (BSE) images of an enargite (c) and a tennantite (d) crystal. (SEM)-backscattered electron (BSE) images of an enargite (c) and a tennantite (d) crystal.
4. Without-Arsenic Structural Model 4. Without-Arsenic Structural Model Considering that structural models are first-order data required for geotechnical design and thatatstructural modelsMine are first-order required for geotechnical design and mineConsidering management, the Los Bronces this goal data has been developed by the Geotechnical mine management, at the Los Bronces Mine this goal has been developed by the Geotechnical Superintendence at the Los Bronces, Anglo American. It is based on a database with more than 200,000 Superintendence at the Los Bronces, Anglo American. It is based on a database with more than 200,000
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4. Without-Arsenic Structural Model Considering that structural models are first-order data required for geotechnical design and mine management, at the Los Bronces Mine this goal has been developed by the Geotechnical Minerals 2018, X, x FOR PEER REVIEW 5 of 12 Superintendence at the Los Bronces, Anglo American. It is based on a database with more than 200,000 single-points, including superficial and underground data (dip/dip-direction measurements) single-points, including superficial and underground data (dip/dip-direction measurements) (Figure 4). (Figure 4). The “Los Bronces structural-database” is the results of a data compilation following a The “Los Bronces structural-database” is the results of a data compilation following a systematic systematic characterization policy, which has been generated standard structural characterization characterization policy, which has been generated fromfrom standard structural characterization methods, mainly including (i) slope and drill-hole cores direct mapping; (ii) indirect slopes methods, mainly including (i) slope and drill-hole cores direct mapping; (ii) indirect slopes mapping mapping using photogrammetric technics (ShapeMetrix3D); and (iii) indirect structural mapping using photogrammetric technics (ShapeMetrix3D); and (iii) indirect structural mapping from from geophysical measurements of boreholes and acoustic tele-viewer). geophysical measurements of boreholes (optic(optic and acoustic tele-viewer).
Figure The 3-D schematic representation of the structural database acquired Los Bronces Figure 4. 4. The 3-D schematic representation of the structural database acquired for for the the Los Bronces mine. mine. Green dots show slope mapping data and magenta dots indicate drill-hole mapping Green dots show slope mapping data and magenta dots indicate drill-hole mapping information. information.
The on the the western western flank flankofofthe theCentral CentralAndes Andes TheRio RioBlanco-Los Blanco-LosBronces Bronces district district is is emplaced emplaced on structure structureisischaracterized characterized development an imbricated structure[28,31,32]. [28,31,32].This This structure byby thethe development of anof imbricated reversereverse fault fault system, defined as the West Andean Thrust (WAT; [32]). In this geodynamic context, the Cenozoic system, defined as the West Andean Thrust (WAT; [32]). In this geodynamic context, the Cenozoic volcanoclastic extensional volcanoclastic extensionalbasin basinwas wasinverted, inverted,accommodating accommodatingthe theorogenic orogenicshortening shorteningresponsible responsibleof theofAndean upliftuplift [32]. This was deformed as a fold-thrust belt, where syntectonic plutonic the Andean [32]. basin This basin was deformed as a fold-thrust belt, the where the syntectonic emplacement was controlled by the foldby axes San Francisco Figure 5a).Figure At the5a). district plutonic emplacement was controlled the(e.g., fold axes (e.g., San Batholith; Francisco Batholith; At scale, the deformation is characterized mainly by a profuse development of a NE-striking vein-fault the district scale, the deformation is characterized mainly by a profuse development of a NE-striking pattern (Figure 5b). This pattern a hybrid evidenced by strike-slip displacement and vein-fault pattern (Figure 5b). has This patternkinematics has a hybrid kinematics evidenced by strike-slip displacementfilling and [28,31,32]. hydrothermal [28,31,32]. pattern system cuts theincluding entire bedrock hydrothermal The filling vein-fault patternThe cuts vein-fault the entire bedrock a folded system including a folded volcanoclastic sequence of the the granitoid Farellonesrocks Formation, granitoid rocks volcanoclastic sequence of the Farellones Formation, of Santhe Francisco Batholith, of San Franciscoand Batholith, minor intrusions and the breccias mineralized hydrothermal minor intrusions the mineralized hydrothermal [26,28,31] (Figure breccias 5b). The[26,28,31] kinematic (Figure 5b). district The kinematic of explains the district 5c; [31]) explainsthe theregional relationship between model of the (Figuremodel 5c; [31]) the(Figure relationship between and mine scale the regional and mine scale structural patterns. The first-order deformation is represented by the structural patterns. The first-order deformation is represented by the blind west-vergent thrusts, blindthe west-vergent while observed the superficial observed in theoutcrops mine slopes and while superficial thrusts, deformation in thedeformation mine slopes and district is mainly district outcrops is mainly characterized by NE-ENE-strike dextral faults vein-faults, are characterized by NE-ENE-strike dextral faults and vein-faults, which areand interpreted as awhich secondary interpreted as a secondary deformation pattern developed in the shallower part of the orogenic deformation pattern developed in the shallower part of the orogenic fold-thrust belt structure [32]. fold-thrust belt structure [32]. Primary by mineralization was structures controlled (Figure by NS-strike deep Primary mineralization was controlled NS-strike deep 5c) and thestructures shallower (Figure 5c) and the shallower structures are related to post-mineral deformation stages, concomitant structures are related to post-mineral deformation stages, concomitant with the later hydrothermal with the later hydrothermal activity during the exhumation of the deposit. activity during the exhumation of the deposit. At the scale of the Los Bronces Mine (Figure 6), six single principal faults have been described in At the scale of the Los Bronces Mine (Figure 6), six single principal faults have been described in the currently-used structural model, which are associated to three main brittle deformation patterns the currently-used structural model, which are associated to three main brittle deformation patterns that control the open-pit slopes. The most important structural pattern labeled as the Central System that control the open-pit slopes. The most important structural pattern labeled as the Central System (Figure 6) and constituted by CS1, CS2, and CS3 faults, shows a NE-SW to E-W striking orientation (Figure 6) and constituted by CS1, CS2, and CS3 faults, shows a NE-SW to E-W striking orientation with a high angle of dip. In addition, an N-S oriented pattern is recognized in the eastern part of the with a high angle In addition, an N-S orientedbypattern is recognized in the eastern mine, labeled as of thedip. Eastern System and constituted ES1 and ES2 faults (Figure 6). The part thirdof structural pattern, labeled as the Diatreme System, is constituted by the DS fault, which exposes a ring-type geometry and is associated with a later diatreme intrusion shear-contact (Figure 6).
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the mine, labeled as the Eastern System and constituted by ES1 and ES2 faults (Figure 6). The third structural pattern, labeled as the Diatreme System, is constituted by the DS fault, which exposes a ring-type geometry and REVIEW is associated with a later diatreme intrusion shear-contact (Figure 6). 6 of 12 Minerals 2018, X, x FOR PEER
Figure 5. (a) A simplified east-west structural cross sections showing the tectonic context of the Rio Figure 5. (a) A simplified east-west structural cross sections showing the tectonic context of the Rio Blanco-Los Blanco-Los Bronces mining (after Reference [32]). Andean WAT: Western Andean Thrust, Paleozoic, Bronces mining district (afterdistrict Reference [32]). WAT: Western Thrust, Pz: Paleozoic, Mz:Pz: Mesozoic, Cz: Mz: Mesozoic, Cz: Cenozoic; (b) A simplified geological map of the Rio Blanco-Los Bronces district Cenozoic; (b) A simplified geological map of the Rio Blanco-Los Bronces district showing the main structural showingand thethe main structural andReference the mine-scale tensor (after Reference [28]); (c) patterns mine-scale strainpatterns tensor (after [28]); (c) strain Kinematic model showing the relationship Kinematic model showing the relationship between the different-scale structural patterns observed between the different-scale structural patterns observed in the mining district. The model explains the in the mining district. The model structures explains the kinematic of the different structures[32]). under kinematic relevance of the different under the platerelevance convergence vector (after Reference the plate convergence vector (after Reference [32]).
Even though this model is supported by a robust structural database (Figure 4), the nature Even though this model is supported by a robust structural database (Figure 4), the nature of of the fault patterns does not allow the recognition of 3-D correlations from the isolated structural the fault patterns does not allow the recognition of 3-D correlations from the isolated structural data. data. In consequence, the real persistence of structures, cross-cutting relationships, and clear fault In consequence, the real persistence of structures, cross-cutting relationships, and clear fault hierarchies are poorly constrained (Figure 6), especially in the subsurface. This lack of structural hierarchies are poorly constrained (Figure 6), especially in the subsurface. This lack of structural attribute characterization is commonly found in mining operations and usually, it is solved using attribute characterization is commonly found in mining operations and usually, it is solved using interpretations given by consultant structural geologists. However, uncertainties in the geotechnical interpretations given by consultant structural geologists. However, uncertainties in the geotechnical design can be not well-controlled by this approach. design can be not well-controlled by this approach.
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Figure Theschematic schematic representation the structural model currently used, the showing the Figure 6.6.The representation of theofstructural model currently used, showing recognized recognized principal faults. The main issues on the model are pointed out by question marks. CS: principal faults. The main issues on the model are pointed out by question marks. CS: Central System Central 1, 2 and 3; ES: Faults Eastern1 System Faults 1 and 2; DS: Diatreme Faults 1,System 2 and 3;Faults ES: Eastern System and 2; DS: Diatreme System Fault. System Fault.
5. Arsenic Database: A Complementary Tool 5. Arsenic Database: A Complementary Tool The arsenic database database is is constituted constituted by by 648,956 648,956 analysis analysis points points from from blast-hole blast-hole and and drill-hole drill-hole The arsenic samples. The3-D 3-Dspatial spatialdistribution distribution of data shows arrangements: (i) arsenic concentration samples. The of data shows two two arrangements: (i) arsenic concentration points points measured each 5 meter along the depth of geological drill-holes, which are heterogeneously measured each 5 meter along the depth of geological drill-holes, which are heterogeneously distributed distributed through the mine; and (ii) a single-point data collected from each blast-hole, which are through the mine; and (ii) a single-point data collected from each blast-hole, which are homogeneously homogeneously spaced as a block of ~7 × 7 × 15 m3 (Figure 7a). spaced as a block of ~7 × 7 × 15 m3 (Figure 7a). The Arsenic concentrations range from 2 ppm (the limit of detection of the analytical technique) The Arsenic concentrations range from 2 ppm (the limit of detection of the analytical technique) up to 13,000 ppm. The chemical analyses were performed by atomic absorption spectroscopy (AAS) up to 13,000 ppm. The chemical analyses were performed by atomic absorption spectroscopy (AAS) analytical technique, following standard protocols. Although this database was originally compiled analytical technique, following standard protocols. Although this database was originally compiled for metallurgical design and wastes control, it can be used as a complementary vector to track faults for metallurgical design and wastes control, it can be used as a complementary vector to track faults in in a 3-D space, as long as these faults are sealed by As-bearing mineral fillings. a 3-D space, as long as these faults are sealed by As-bearing mineral fillings. The Arsenic-database was processed using the Leapfrog Geo software (version 4.2.3, LeapFrog The Arsenic-database was processed using the Leapfrog Geo software (version 4.2.3, LeapFrog Enterprises Inc., Emeryville, CA, USA) [33], an implicit free-form drawing and modeling tool which Enterprises Inc., Emeryville, CA, USA) [33], an implicit free-form drawing and modeling tool which allows an easy management of big-data, offering a new opportunity to integrate multi-collected data allows an easy management of big-data, offering a new opportunity to integrate multi-collected data in a single and easy-handle 3-D representation. Implicit modeling is a technique that uses a radial in a single and easy-handle 3-D representation. Implicit modeling is a technique that uses a radial basis function which allows for the interpolation of borehole data, outcrop data, and field structural basis function which allows for the interpolation of borehole data, outcrop data, and field structural data [34]. data [34]. The Leapfrog’s output results (Figure 7a) reveal singular 3-D arsenic distributions from the The Leapfrog’s output results (Figure 7a) reveal singular 3-D arsenic distributions from the interpolated data. Interestingly, the highest As-concentrations, e.g., >200 ppm, suggest tabular interpolated data. Interestingly, the highest As-concentrations, e.g., >200 ppm, suggest tabular patterns, principally with a NE-SW orientation, which can be related to structurally controlled alignments (Figure 7a). The orthographic projection of the spatial distribution of arsenic concentrations (Figure 7a) from drill-holes (aligned points) and blast-holes (points with a high-density distribution) denotes
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patterns, principally with a NE-SW orientation, which can be related to structurally controlled alignments (Figure 7a). The orthographic projection of the spatial distribution of arsenic concentrations (Figure 7a) Minerals 2018, X, x FOR PEER REVIEW 8 of 12 from drill-holes (aligned points) and blast-holes (points with a high-density distribution) denotes alignmentsthat thatmatch matchthe themain mainfaults faults(Figure (Figure7a,b) 7a,b)described describedininthe thecurrently-used currently-usedstructural structuralmodel model alignments themine mine(Figure (Figure6). 6). Remarkably, Remarkably,these theseAs-rich As-richalignments alignmentscan canbe befollowed followedbeyond beyondextensions extensions ofofthe previously determined for the structures in the currently-used model (Figure 6), even in the subsurface, previously determined for the structures in the currently-used model (Figure 6), even in the validating their recognized and elucidating real persistence across the mineacross as well subsurface, validating their existence, recognized existence, andtheir elucidating their real persistence theas their cross-cutting relationships (Figure 7a,b). Additionally, the As-rich alignments also validate the mine as well as their cross-cutting relationships (Figure 7a,b). Additionally, the As-rich alignments real persistence of the faults down dip (Figure 7c). Even more interestingly, these As-rich alignments also validate the real persistence of the faults down dip (Figure 7c). Even more interestingly, these denotealignments three not previously recognized principal faults, labeled as As1, As2, and As3 faults, as and well As-rich denote three not previously recognized principal faults, labeled as As1, As2, as many other subordinate structures (Figure 7a,b), which can be incorporated in the improved As3 faults, as well as many other subordinate structures (Figure 7a,b), which can be incorporated in structural model. These model. arsenic-located structures correspond to correspond faults that were activated the the improved structural These arsenic-located structures to faults that in were brittle-ductile zone transition [25]. The zone fault [25]. activation process allows process the fluidallows circulation and activated in thetransition brittle-ductile The fault activation the fluid boiling [35], triggering arsenic-bearing minerals precipitation as fault filling. Faults that were not circulation and boiling [35], triggering arsenic-bearing minerals precipitation as fault filling. Faults activated at the late stages of late the hydrothermal event, for example, ES2, and DS1 faults, are that were not activated at the stages of the hydrothermal event,the for ES1, example, the ES1, ES2, and not shown in the orthographic projection of the arsenic distribution. DS1 faults, are not shown in the orthographic projection of the arsenic distribution.
Figure 7. 7. (a) Orthographic of the distribution of arsenic concentrations from drill-holes Figure Orthographicprojection projection of spatial the spatial distribution of arsenic concentrations from (aligned points) and blast-holes (points with a high-density distribution). (b) Sketch of(b) orthographic drill-holes (aligned points) and blast-holes (points with a high-density distribution). Sketch of projection showed in (a),showed outlininginthe previously main faults, three arsenic-denoted new orthographic projection (a), outliningrecognized the previously recognized main faults, three principal faults new (labeled as As1, As2,(labeled and As3asfaults) and and some arsenic-denoted subordinated faults. arsenic-denoted principal faults As1, As2, As3 faults) and some arsenic-denoted (c) A NNW-SSE cross-section showing the geometry of arsenic concentrations in depth. The insetinin subordinated faults. (c) A NNW-SSE cross-section showing the geometry of arsenic concentrations (c) outlines previously recognized and arsenic-denoted faults. CS: Central System Faults 2 and 3. depth. The inset in (c) outlines previously recognized main and arsenic-denoted main faults. CS:1,Central
System Faults 1, 2 and 3.
6. Arsenic-Improved Structural Model Using the Arsenic-database a spatial correlation has been performed between the arsenic distribution orthographic projection and the currently-used structural model (Figure 8a). The main improvements are (Figure 8b) (i) the existence of the principal faults, which were described in the
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6. Arsenic-Improved Structural Model Using the Arsenic-database a spatial correlation has been performed between the arsenic distribution orthographic projection and the currently-used structural model (Figure 8a). The main improvements are (Figure 8b) (i) the existence of the principal faults, which were described in9 of the Minerals 2018, X, x FOR PEER REVIEW 12 currently-used structural model (Central, Eastern and Diatreme systems), has been confirmed from an anindependent independent set set of ofdata; data;(ii) (ii)the thereal realpersistence persistenceof ofthe theprincipal principal fault fault has has been been validated, validated, not not only only along but also also down down the thedip. dip.ItItcan canbebeclearly clearly validated and highlighted case of along the the strike strike but validated and highlighted forfor thethe case of the the CS2 fault (Figure 6 vs. Figure 8a,b); (iii) the detailed cross-cutting relationships between the CS2 fault (Figure 6 vs. Figure 8a,b); (iii) the detailed cross-cutting relationships between the principal principal faults been determined and validated; and (iv) the arsenichas database has determined faults have beenhave determined and validated; and (iv) the arsenic database determined for the first for the first time the existence, real persistence and hierarchy of three new principal faults, as time the existence, real persistence and hierarchy of three new principal faults, named asnamed As1, As2, As1, As2, As3 (Figure 8b), corresponding to arsenic-filled fractures that had not been recognized in As3 (Figure 8b), corresponding to arsenic-filled fractures that had not been recognized in the former the former structural characterization, and therefore had been not been consideredinin the the currently-used structural characterization, and therefore had not considered currently-used geomechanical model. It is important to point out that there were direct structural data that indicated geomechanical model. It is important to point out that there were direct structural data that the existence of these newly-defined faults. Nevertheless, it was not enough to define their persistence indicated the existence of these newly-defined faults. Nevertheless, it was not enough to define their and hierarchy through the through open pit the mining arsenic-located structures correspond persistence and hierarchy openoperation. pit miningThese operation. These arsenic-located structures to faults activated during the late stages thestages hydrothermal event at theevent brittle-ductile transition correspond to faults activated during theof late of the hydrothermal at the brittle-ductile zone [25]. Other structures not indicated by the arsenic data are interpreted as (i) sealed faults transition zone [25]. Other structures not indicated by the arsenic data are interpreted as (i) which sealed were not reactivated during the late stages of the hydrothermal event, and/or (ii) faults reactivated or faults which were not reactivated during the late stages of the hydrothermal event, and/or (ii) faults propagated after the hydrothermal event. reactivated or propagated after the hydrothermal event.
Figure 8.8. The The Arsenic-improved Arsenic-improved structural structural model. model. (a) (a) Spatial Spatial correlation correlation between between the the As-database As-database Figure orthographic projection and the currently-used structural model. (b) Main progress in the structural orthographic projection and the currently-used structural model. (b) Main progress in the structural model, including including the the three three newly-revealed newly-revealed principal principal faults faults as as well well as as the the real real persistence persistence and and model, cross-cutting fault relationships. cross-cutting fault relationships.
7. Discussion Discussion and and Concluding ConcludingRemarks Remarks 7. The development development of of the the proposed proposed tool tool isis based based on on the the tectonic tectonic and andmetallogenic metallogenic conceptual conceptual The frameworkthat that explains the spatial correlation ore-fluid circulation mineral framework explains the spatial correlation betweenbetween ore-fluid circulation and mineral and precipitation precipitation in fractures developed as a network in the upper continental crust. In this study, it has been shown that the tridimensional distribution of arsenic concentrations can be used to confirm the existence and the real persistence of structures as well as the cross-cutting relationships between faults. This characterization of faults has a direct impact on the robustness of the total slope and inter-ramp geotechnical design as well as the slope stability analysis [3–5,7]. The implementation of the proposed tool also allows for the representation of other subordinated structural patterns, at a
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in fractures developed as a network in the upper continental crust. In this study, it has been shown that the tridimensional distribution of arsenic concentrations can be used to confirm the existence and the real persistence of structures as well as the cross-cutting relationships between faults. This characterization of faults has a direct impact on the robustness of the total slope and inter-ramp geotechnical design as well as the slope stability analysis [3–5,7]. The implementation of the proposed tool also allows for the representation of other subordinated structural patterns, at a scale from inter-ramp to slope; which can show evidence of fault characteristics such as kinematics and the magnitude of displacement. Our results show that linking the direct discontinuities characterization to the implementation of geochemically-developed tools can improve the quality of the structural models at a mine scale, allowing the development of more realistic representations of the rock-mass scenarios, which can minimize uncertainties in the geotechnical modelling. Finally, the proposed tool can be also comprehensively developed using other chemical elements, which can be systematically quantified in a mining operation. In a similar way, it can be implemented in other hydrothermal systems, including meso- and epi-thermal deposits, in which the fractures are sealed by hydrothermal ore paragenesis containing As-bearing minerals. Significantly, the use of a geochemical database to improve structural models represents an outstanding opportunity to enhance the investment made to quantify concentrations of chemical elements, other than the economically profitable metal, in mined rock-mass. In any case, it is necessary to make clear that the proposed methodology is not intended to replace the classical direct mapping of structures, which is a fundamental input to develop structural models in mining. Author Contributions: D.C. and C.B. conceived the main idea; D.C., C.B. and G.V. analyzed the data and proposed the tool; D.C., G.V. and C.B. wrote the paper; Conceptualization, D.C. and C.B.; Investigation, G.V.; Resources, C.B.; Writing-review & editing, D.C., C.B. and G.V. Funding: This research received no external funding. Acknowledgments: We wish to thank Anglo American T&S, particularly to M. Schellman (Principal Geomechanics) and P. Hodkiewicz (Head of Specialist and Integrated Geosciences), for allowing the preparation of this paper. We would also like to thank I. Vela (Geology Superintendent of the Los Bronces Underground) for his constructive discussions made on the manuscript. Thanks to LeapFrog Geo for supporting this research through a data analysis and modeling software endowment. Conflicts of Interest: The authors declare no conflict of interest.
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