资源环境与城乡规划管理外文翻译文献(文档含中英文对照即英文原文和中文翻译)Cyclical salt efflorescence weathering: an invisible threat to the recovery of underground mine environment for tourist exploitationJ. Matías Penàs Castejón • J. Francisco Maciá Sánchez •M a Pilar Jiménez Medina • M a Jesús Pen àlver MartínezReceived: 30 April 2013/Accepted: 25 January 2014/Published online: 11 February 2014○c Springer-Verlag Berlin Heidelberg 2014Abstract Enhancing the value of an underground mine environment for tourist exploitation involves altering the physico-chemical balance of stone materials whose original mechanical properties guaranteed the structural stability of the site’s galleries and chambers. Humidity and temperature changes caused by the public exhibition of this kind of assets are the main causes ofsuch disorders. After the intervention in the Agrupa-Vicenta mine in La Union (Spain) there were still runoff-water leaks into the mine. These water runoffs through the fault and schistosity planes of the enclosing rock mass give rise to salt precipitation over time. Adapting this mine and turning it into a museum have meant a decrease in relative indoors humidity and an increase in temperature. These variations have caused rocks, which were stable in the original conditions, to increase their rate of physico-chemical weathering due to the polycyclic supergene alteration of the metal sulfides they contain. The resulting release of sulfates into the solution and their subsequent precipitation as single and double salt efflorescence causes haloclasty, deteriorating the rock’s mechanical properties and diminishing the structural stability of the operation. This paper presents the results of characterizing the supergene mineral phases of salt efflorescence in the rock bed enclosing an underground sulfide mine value enhanced for tourist exploitation. Dangers for the structural stability of this type of architectural intervention, associated to the formation of efflorescences, are also identified; these efflorescences are caused by the weathering of rocks that make up its supporting structure.Keywords Mine environment . Haloclastic weathering . Salt efflorescence . Geoindicators . Stone decay . Agrupa-Vicenta mine . La Unio ´n (Spain)IntroductionThe withdrawal from mining activity in most last century mining areas and the subsequent economic and social crisis affecting these abandoned industrial areas are currently encouraging the recovery of mining heritage for tourist development (Hospers 2002).Projects for the public opening of underground mines face two great challenges: (1) to guarantee the stability and structural safety of the ground enclosure; and (2) to achieve the necessary conditions of habitability and comfort of the premises. The difficulties of tackling and executing such technical projects come mainly from operating in a dynamic environment, characterized by the mutability ofthe physico-chemical properties of its constituent materials.The new ventilation system, the new lighting conditions and the affluence of public cause variations in the original thermohygrometric balance conditions. These changes generate the appearance of salt efflorescences and polycyclical changes in their mineralogical phases.Saline efflorescence may cause the haloclastic rupture of the rocks that structurally support mine’s galleries and chambers. This calls for the determination of the origin and nature of such salts, so as to find suitable mechanisms that inhibit weathering processes from the very beginning. At present, different tubbing systems are being gradually but pervasively introduced to avoid collapse risks in these ‘‘museum mines’’.The rocks above and under the exploited mineral bed are called roof and bottom schists, respectively. The roof of the Agrupa-Vicenta mine is formed by chloritic micaschists and quartzites, intensely folded metamorphic rocks, and the bottom materials are graphitic schists and gray quartzites. Between both levels, a stratiform mineralization of sulfides used to be exploited, mostly pyrite inside a chlorite (and sometimes grenalite) gangue (Manteca and Garcı ´a 2005) (Fig.1).Despite the relative impermeability of the rocks that cover the mine, the intense rifting of the land has allowed for the infiltration and underground flow of rainwater over the course of time.Chemical weathering resulting from the supergene alteration of iron, lead and zinc sulfides in the mineral bed and in the schists of the bottom and roof of the mine has caused the release of anions and cations into the ground-water solution that continuously circulates through the network of pores, schistosity planes and the fracture net-work, after rain periods.When this water comes in contact with pyrite and marcasite, a series of chemical reactions take place, breaking down these minerals and forming iron sulfates and acid mine drainage.As a result, the fractures in the pyrite bed usually presented open holes stuffed with iron sulfate, ‘‘melanterite’’ (FeSO4 ·7H2O). Sulphuric acid was also reacting with other components of the mineral bed (e.g. chlorite), forming various hydrated sulfates-particularly ‘‘epsomite’’ (MgSO4·7H2O) which were precipitating on the bottom and roof of the mine in the form of needle-shaped white efflorescence.During the mining activity the working environment for miners was guaranteed through raise ventilation. Before opening the mine to the public, microclimatic conditions were measured along 1 year. Medium temperature and relative humidity were 23.5 ℃ and 95.3 %, respectively.After opening the mine to the public, it was necessary to install the mandatory air conditioning system. This auxiliary ventilation system takes air from the flow-through system (12,000 m 3 /h) and distributed it to the mines’galleries via two mounted ventilation fans. Ventilation system just operates during timetable public visits.As a consequence nowadays,the new medium temperature and relative humidity values are 18.5 ℃and 73.5 %, respectively.Therefore,the hydration and dehydration cycles of these salts intensified. The oversaturation of the solution also increased, due to higher evaporation produced by the descent in relative humidity and the increase in temperatureinside the main (Espinosa-Marzal and Scherer 2010).All of that implied an upsurge in the aggressiveness of the r ock’s physical weathering, as a result of both the increase in crystallization pressure exerted by salts and also of the increase in the number of cycles.Thus, the goals of our study were: (1) to characterize the salts involved in the physical weathering of the bottom and roof schists of this underground metal sulfide mine, as a result of the cyclical changes of temperature and humidity caused by its opening to the public; (2) to identify the risks for the structural stability of the mine, coming from salt formation; and (3) to propose corrective measurements to minimize those risks.Location and geologyThe Agrupa-Vicenta mine is situated in the Cuesta de Las Lajas of the Sierra Minera mountain range in La Unio ´n (Cartagena). This mountain ridge runs parallel to the Mediterranean sea from East to West, between the city of Cartagena and Cape Palos. It has an approximate length of 28 km and is 5 km broad. It once was one of the most important mining districts in Spain and the most repre- sentative of the Murcia Region, due to its iron, lead and zinc deposits (Fig. 2).After ceasing its mining activity in the year 1991, the city of La Unio ´n and the surrounding villages were immersed in a deep economic and social crisis.Local and regional governments have recently focused on preserving, restoring and giving mining a new meaning. Efforts have been focused, on keeping mining heritage as the economic engine of the area, although under a new approach that presents the mining heritage as a cultural and tourist attraction (Conesa et al. 2008). Thus, in 2006, the Town Hall of La Unio ´n decided to tackle the adaptation of the Agrupa-Vicenta underground mine, opening it to the public in 2009.The mineralization pattern of the Sierra Minera corre-sponds to that of the Nevado-Fila ´bride complex, possibly formed by the hydrothermal replacement of a level of Triassic marble located near the roof of this formation. This mineralization pattern is the most widely spread in the Sierra Minera, although it receives different names according to the prevalent mineral: pyrite bed, sphalerite bed or magnetite bed.The wall-to-roof vertical sequence of the geologic materials that form the Sierra conforms to three superposed geologic sets (Manteca and Ovejero 1992): Nevado-Fila ´- bride Complex (CNF), Alpuja ´rride Complex (CA) and Neogene (N) (Fig. 3).The CNF consists of intensely folded metamorphic rocks, with a lower basement formed by graphite schists from the Paleozoic, and an upper coverage formed by micaschists, quartzite and Permo-Trias marble.The CA is formed by lightly folded metamorphic materials above the underlying CNF. It shows at least two superposed beds over thrust, called lower CA (also known as San Gine ´s unit) and upper CA (a.k.a. Portma ´n unit). Each of them has a detritic basal tract, phyllites and quartzites of Permo-Trias age and an upper carbonated tract, limestones and dolostones, of Middle Trias age.The Neogene is made up of lightly folded Upper Mio- cene sedimentary rocks, conglomerates, sandstones and marls, and of magmatic rocks of Upper Miocene/Pliocene age, which traverse the whole series. Finally, modern alluvial sediments are found accumulated in depressions.Geologically, the Agrupa-Vicenta mine is on the eastern border of a ‘‘horst’’; i.e. a ro ck mass delimited by faults that has risen above the adjacent areas. The effects of erosion after its elevation eliminated the upper rocky levels, which explains why the Paleozoic substratum,—the most ancient materials of the Sierra-outcrop in this area.This area presents numerous dikes and chimneys of igneous rocks, andesites and rhyodacites, attesting to the magmatic activity that took place at the end of the Miocene. After the magmatic activity, thermal waters with dissolved sulfur and metals circulated across the rocks for thousands of years, giving rise to the formation of veins and beds.The kind of mineralization exploited in the Agrupa-Vi-centa mine corresponds to the ‘‘pyrite bed’’ type, since it consists mainly of pyrite (S 2 Fe), accompanied by s ome galena (SPb) and sphalerite (SZn), but in very low, industrially unprofitable proportions. Those sulfides are accompanied by a gangue of chlorite and quartz.The estimated average grades for this mineral are approximately 35 % S; with 0.3 % Pb and less than 0.5 % Zn (Oen et al. 1975).Apart from pyrite, this mine used to have a very small production of tin, after lodes of cassiterite (SnO 2 ) were found in 1913, while excavating the galleries in the wall schists (Arribas et al. 1984).Description of the mineThe Agrupa-Vicenta mine has an approximate extension of 4,800 m 2 , distributed in five altitude sublevels placed at 238, 235, 231, 228, and 223 m above sea level and average roof heights of 5 m.The mine’s configuration in descending sublevels is a c onsequence of the gradation of the mineral bed produced by geological rifting (Fernandez 2006).The mineral bed itself is a result of the replacement of a layer of Triassic marble, and its thickness in the mine ranges between 4 and 8 m.Because of the strong existing rifting, the mineral presents a tiered structure. The Agrupa-Vicenta was exploited by the traditional method of the Sierra Minera: ‘‘chambers and pillars’’ (Fig. 1). The height of the chambers does not exceed 5 m, except in the central sector, where the vaults of the chambers reach heights of up to 8 m. Chambers are supported by a total of 41 pillars of the same mineral bed (Manteca and Garcı ´a 2005).A stratiform sulfide mineralization used to be exploited in this mine. There was a predominance of pyrite inside a chlorite (and sometimes grenalite) gangue.From a geotechnical standpoint, although this pyrite bed presents optimal geomechanical conditions, the roof schists are hardly competent. That is why, when the mine was originally exploited, a ‘‘dome’’ of mineral of about 1 m thick above the roof was left in most chambers, so as to avoid contact with the talc-sericitic roof schists, since otherwise landslides would have occurred (Manteca and Garcı ´a 2005).In spite of that, the mine showed subsidence on several spots, before its refurbishment. The subsidence main spot was in the western sector of the mine, close to the entrance, where unstabilized cavitation of about 12 m height had appeared (Fig. 1).Besides, there were several detachments of rocky mineral plates from the roof in several areas of the mine. They had developed by conjunction of subvertical joints with the schistosity planes of the mineral bed.After computing the hollow volume, a maximum of 60,000 tons of mineral are estimated tohave been extracted from this mine throughout the different exploitation periods between years 1902 and 1970 (Fernandez 2006).Description of the interventionThe value enhancement of this former underground sulfide mine is part of a larger project: MINEU European cultural project within the CULTURE 2000 framework (2005). MINEU Project involved the creation of the tourist product ‘‘Mining Park of La Unio ´n’’.The election of the Agrupa-Vicenta mine for its value-enhancement was decided, first of all, given its safety and its acceptable conservation condition after 40 years of abandonment.Secondly, because of its closeness to the’’carretera del 33’’ (a well-known nearby road) and the existence of a convenient tunnel entry, which was a fundamental factor.And lastly, its geologic and mining interest: the ‘‘chambers and pillars’’ exploitation technique was particularly relevant within the traditional mining context of the Sierra Minera (Fig.4).The intervention would be conducted under the basic premise that only areas without structural problems—and in which safety conditions were sound—should be open to the public. To fulfill this goal, two technical documents had to be written, as an integral part of the Agrupa-Vicenta mine’s adaptation project: a Struc tural Consolidation Project and a Conditioning Project.Agrupa-Vicenta mine’s Structural Consolidation ProjectThe mine’s cavity has so far not collapsed due to the strength of the pillars and of the mineral crust that bears its roofs, which was left there to avoid contact with the weak roof schists.That is why the consolidation project resolved to keep the parts of mineral bed near the schists roof still in good condition, so that they served as containment and support. Prior to any further actuation, the roof needed to be systematically sampled, detaching all the plates presenting risk of collapse.For those areas identified in the prior geotechnical study, which for whatever reasons did not keep that mineral crust or in which it was weak or thin, three different types of tubbing systems were defined (Fernandez 2006) (Fig. 5).1. Wooden trusses. This is a tower-like structure of wooden logs to sustain the high chamber roofs that lack mineral crust.2. Wooden props or ‘‘mampostas’’. They are used in low areas with roofs where the mineral crust presents certain instability and a tendency to detach itself in the shape of plates.3. Bolted steel rebar reinforcement. This is a metallic mesh anchored to the rock by bolts fixed with resin. It is used in high roofs that have an unstable mineral crust with a tendency to the formation of plates.Figure 5 shows the tubbing systems defined in the Structural Consolidation project, which were initially applied in 2009, and those existing in January 2013 (Fig. 6).For a 3 years period since first opening to the public, it was necessary to gradually increase the number of tubbed areas, using wooden trusses and props. This was required due to the continuous fracturing of chloritic roof schists, which came from an increase in the aggressiveness of the formation process of saline efflorescence.Agrupa-Vicenta mine’s Conditioning ProjectThe goal of the mine’s Conditioning Project was to provide a space with the necessary elements and facilities to guarantee the safety of visitors, and the necessary degree of comfort for the correct exhibition and interpretation of the value-enhanced mining heritage. The following actions were tackled (Fernandez 2006):1.Accessibility improvement: So as to implement suitable comfort conditions of the tourist routes, the design needed to consider the mine’s configuration and its safety conditions. Bearing this purpose in mind and considering the existence of five sublevels, wooden staircases and protection barriers were designed.2. Ensuring suitable ventilation conditions: The scarce natural ventilation of the mine and the high degree of humidity before the intervention were solved drilling a 700 mm-diameter raise ventilation in the eastern sector of the mine, in front of the entry gallery that guarantees cross ventilation.3. Ensuring suitable lighting conditions: The electrical installation was carried out burying the cables and logs under the mine’s floor, which required the excavation of the corresponding grooves in the rock. Concerning lighting, tenuous indirect and warm-colored lights were installed to recreate the mine’s original atmosphere.4. Opening an emergency exit: An emergency exit was built making use of the general gallery for mineral transport and the chimney-hopper through which the mineral used to be thrown to a lowerextraction gallery with exit to the hillside of the mount.Two possible exit routes have been enabled. One with relatively easy access, consisting of a bypass through metallic stairs that retakes the entry tunnel from the general transport gallery. The second one consists of a zip-line system that descends through the chimney or drop pit and leads to the lower extraction gallery.Materials and methodsSalts efflorescences was sampled in eight points inside the mine according to the main occurrence pattern (Fig. 7).、After sampling, the specimen was placed immediately in a plastic bag and sealed for shipment to the laboratory.Mineralogical methodsSamples for XRD were gently ground by hand with agate mortar and pestle. Mineral phases were identified by powder XRD using a Bruker D8 Advance instrument in h–h mode (Bruker Corporation, Billerica, MA, USA), with CuKa radiation, 40 kV, 30 mA, and a 1-dimensional detector with a 18 window. Primary optics consisted of a 28 Soller slit, a 1 mm incidence slit due to high content in iron of soil samples, and an air scatter screen. Secondary optics included a 3 mm anti-scatter slit, a Ni filter and a 2.5°Soller slit.Sample was step scanned from 3 to 808 in 2h, with 0.058 stepping intervals, 1 s per step, and a rotation speed of 30 rpm. Powder sample was mounted in back loading plastic holder. Diffraction patterns were evaluated with DIFFRACplus software (in particular with EVA 12.0, a commercial package from Socabim, 2006) and powder diffraction files database PDF2 (ICDD 2000).Salts efflorescences were then morphologically and chemically characterized, using a combination of a low vacuum SEM (Hitachi S-3500N) for single particle analysis via an energy dispersive X-ray microanalysis system (EDX-Bruker AXS Microanalysis). Samples were studied on SEM with a thin film of platinum covered using a SC7610 instrument of Quorum Technologies. Microscope conditions for backscattered imaging were accelerating voltage of 15 kW and working distance of 10 mm.Elemental analysisAfterward, another portion of sample was dried at 60 ℃ for 24 h (to release most of the free water) and grounded in a disc mill for 1 min until a final particle size lesser than 40 lm. Although it is possible to separate the sulfate minerals by hand, picking up the crystals under the stereomi-croscope, it was preferred to study the sample as it occurred in the tailings dump, without any other manipulation.Samples for WDXRF were dried at 60 ?C for 24 h (to release most of the free water) and then ground in a disc mill for 1 min, to give a final particle size of less than 40 lm. Sample preparation involved the formation of pressed-powder pellets using 5 g of sample and 0.4 g of binder (Marguı ´ et al. 2009). The samples were analyzed using a commercial spectrometer (Bruker S4 Pioneer), equipped with a Rh anticathode X-ray tube (20–60 kV,5–150 mA, and 4 kW maximum), five analyzer crystals (LiF200, LiF220, Ge, PET, and XS-55), a sealed proportional counter for detection of light elements, and a scintillation counter for heavy elements. The energy resolution and efficiency for each analytical line were determined by both the collimator aperture and the analyzer crystal used. Analyses were performed in vacuum mode to avoid signal losses by air absorption, allowing the detection of low Z elements (Gonzalez-Fernandez and Queralt 2010).The recorded spectra were evaluated by the fundamental parameters method, using SPECTRAplus software linked to the equipment (specifically EVA 1.7, a commercial package from Bruker AXS and Socabim, (Bruker AXS GmbH 2006). A standard-less method was used owing to the lack of satisfactory certified reference materials with metal concentrations in the same range as the rocks analyzed in this study. The use of standard-less procedures in the fundamental parameters method has been described by Rousseau (Rousseau 2001).Thermal decomposition of the samples was performed in a TGA/DSC 1 HT thermogravimetric analyzer (Mettler-Toledo GmbH, Schwerzenbach, Switzerland) with a flowing nitrogen atmosphere (70 mL/min). Program temperature ranged from 30 to 1,100 ℃. All of the TG measurements were blank curve corrected, and alumina pans of 70 ll capacity, without lid, were used. The TG instrument was coupled to a BalzersThermostar mass spectrometer (Pfeiffer Vacuum, Asslar, Germany) for gas analysis. Only water vapor, carbon dioxide, and sulfur dioxide were analyzed. Dwell time for every ion was 1 s and cathode voltage in the ion source was 65 V. Quadrupole mass spectrometer model was QMS 200 M3, to evaluate the MS response, two different sequences of experiments were carried out. The first study dealt withthe influence of the heating rate on MS response. Heating rates of 5, 10, 20, 30, 40, and 50 ℃/min and sample mass of approximately 10 mg were used. The second study discussed the influence of sample mass on MS sensitivity. Sample masses close to 0.1, 0.5, 1, 3, and 10 mg and a heating rate of 30 ?C/min were used (Alcolea et al. 2010).Results and discussionAfter the rehabilitation process carried out in the Agrupa-Vicenta mine, the rate of chemical and physical weathering of roof and bed chloritic schists has increased, due to the polycyclical supergene alteration of the metal sulfides contained in them.It has been observed on the one hand, due to the number of detachments of rocky mineral plates from the roof; and the other hand, due to the increase of salt efflorescences that cover the galleries and chambers.Chemical weathering and mineralogyThe chemical weathering of pyrite (FeS 2 ) and pyrrhotite (Fe (1-x) S) provides primarysources of Fe and acidity that lead to the precipitation of secondary minerals or saline efflorescence through very diverse processes, including evaporation, oxidation, reduction, dilution, mixture and neutralization (Hammarstrom et al. 2003; Lottermoser 2007).As a result of these processes a great variety of hydrated simple salts are formed; worth mentioning are those that have divalent cations, in addition to trivalent mixed salts (Jambor et al. 2000).The formation of saline efflorescence inside the Agrupa- Vicenta mine is brought about by the presence of groundwater and excessive relative air humidity, which foster the alteration of the iron sulfides present in the roof and walls of the mine.Associated with the mineral bed, these mineralizations are also found disseminated in the shape of veins in roof chloritic micaschists. Therefore, according to the enclosing rock in which sulfide oxidation reactions take place, and the subsequent release into the solution of H 2SO4(acid mine drainage) and Fe 2+/3+ , two types of saline efflores-cence are formed. They can be grouped by their capacity to generate acidity and their physical characteristics can be defined by their hydration state.On the one hand, some come from the alteration of wall schists in the presence of the acidic solution originated from the supergene alteration of the pyrite and marcasite minerals that appear disseminated, and on the other hand others were strictly originated from the supergene alteration of the mineral bed.In the first case, the chlorite present in roof schists will weather in the presence of acid mine drainage,resultant from the oxidation of pyrite, and neutralize it. As a result, and considering the environmental conditions of the Agrupa-Vicenta mine (75 % ≤RH≤88 % and 20℃≤ T ≤23 ℃), SO4－and Mg 2+ are released into the solution. They precipitate formingsaline efflorescence consisting mainly of magnesium sulfate MgSO4 (Zehnder and Schoch 2009).Figure 8 shows the distribution of the magnesium sulfates which were abundantly observed at level 2, 3 and 4. These soluble salts precipitate as a bloom efflorescence covering all the chambers roof and pillars where schists are present. This massive encrustation of salts is characterized by their acicular or fibrous white crystals.The results of the XRD sample study and elemental analyze (Table 1) reveal the existence ofdifferent mineralogical phases of magnesium salt according to its hydration state: Kieserite (MgSO 4·(H2 O)), hexahydrite (MgSO4 ·6(H2 O)) and epsomite (MgSO4·7 ( H 2O)).In the second case,the oxide mineral bed mainly consists of a compact mass of pyrite (FeS2), marcasite (FeS2) and in minor quantity pyrrhotite (Fe (1-x)S). The oxidation of these mineral produces ferrous iron and sulfuric acid that decrease the pH values (PH 2,8) and makes it extremely acid (Nordstrom 1982). A simplified equation that explains this process is:2FeS2（s）+ 7O 2(g) +2H 2 O 2Fe2+(aq) +4SO 2-(aq)+4H+(aq) (1)4Fe2+(aq) + O2(g) + 4H+ (aq)Fe3+(aq)+ H2O (2)The Fe 3+ released in reaction (2) may hydrolyze to form ferric hydroxide:Fe3+(aq) + 3H 2 OFe(OH)3 + 3H + (aq) (3)or may oxidize additional pyrite by the reaction:FeS2（s）+ 14Fe3+(aq) + 8H2O 15Fe2+(aq) + 2SO 2-(aq) + 16H + (aq) (4) Other sulfides coexist with these ‘‘majority’’ sulfides, such as sphalerite (ZnS) and galena (PbS) which are equally altered, releasing the Pb 2+ and Zn 2+ cations into the solution.According to the results obtained from the mineralogical and elemental analysis of the samples (Table 1), the most frequent mineral groups are: rozenite (Fe2+(SO4)0.4(H2O)), melanterite (Fe 2+ SO 4 ·7(H 2 O)), halotrichite (Fe 2+Al 2 (SO 4 ) 4·22(H 2 O)), copiapite (Fe 2+ Fe 4 3+ (SO 4 ) 6 (OH) 2 ·20(H 2 O)) and jarosite (KFe 3 3+ (SO 4 ) 2 (OH) 6 ).Figure 7 shows the distribution of the iron and hydrous sulfates at levels 1 and 3. The iron and aluminium sulfates are at level 1. These ferric and ferrous salts precipitate as globules of different colors (green, white and yellow/ brown) where mineral bed is present (Fig. 9).The variety of mineral phases found demonstrates the hydration and dehydration processes that cyclically develop in the enclosure of the mine, as well as the chemical weathering of the sulfides contained in it (Valente and Leal Gomes 2009).Physical weathering and stone decayChemical and physical weathering processes take place simultaneously. The latter are due to the crystallization andrecrystallization processes of these secondary and even tertiary minerals。
