Dominican Larimar Mining—Current State and Future Prospects

: This paper presents basic mineralogical and petrological data of the rocks hosting larimar deposit from the Sierra de Bahoruco, in the SW part of the Dominican Republic. Larimar is a unique ornamental stone with a wealth of decorative qualities. The Las Filipinas Larimar Mine in Barahona Province is the only place of this type in the world. Its production and distribution require radical action; hence, the paper also provides current information on the extraction of this raw material and prospects for the future. The paper indicates the possibility of using a new adit support system based on a mixed support (arch support and shotcrete). Numerical analysis shows the need to create a shotcrete coating around the entire circumference, about 15 cm thick. The variable geotechnical situation around the excavation, caused by the planned operation, prompts the use of slightly different solutions in the ﬁeld of support. The article presents an outline of the technology using Polish experience. It is a support-bolt casing system with the latest achievements in this ﬁeld.


Introduction
Larimar (lorimar) is a unique gemstone of the world, also called Dolphin Stone, Atlantis Stone and Stefilian's Stone. It was accidentally encountered on 22 November 1916 by Father Miguel Fuertes Domingo Lorén near the city of Barahona (Barahona Province) in the SW part of the Dominican Republic [1]. After nearly 60 years, it was rediscovered by Miguel Méndez in the Sierra de Bahoruco region and named after his daughter Larisalari-and the Spanish word for sea-mar [1][2][3][4][5][6]. Currently, the importance of larimar for the Dominican Republic is evidenced by the fact that the nature reserve El Monumento Natural Miguel Domingo Fuertes was named after its first discoverer. Additionally, in March 2018 the Dominican National Congress was established on 22 November as the "National Day of Larimar" (Día Nacional del Larimar).
The Las Filipinas Larimar Mine, the world's only mine of its type, is located in the aforementioned Sierra de Bahoruco. The larimar is found within the effusive and piroclastic rocks of the Dumisseau Formation (DFm) of the Caribbean Volcanic Province [7][8][9][10]. The uniqueness of this stone is the result of structural heterogeneity, mineral composition, color, physical properties and high ornamental qualities. Also unique is the larimarized fossil charcoal found in the larimar deposit [11,12].
Larimar is an example of complex mineralization in which the main component is blue pectolite with other accompanying phases, e.g., calcite, natrolite, prehnite, clay and SiO 2 group minerals and phosphates (apatite). It is referred to by some researchers as a Geologically, Hispaniola Island represents one of the numerous uplifts of the oceanic lithosphere in this region above the water level of the Atlantic Ocean and the Caribbean Sea, which are covered by marine carbonate sediments. The crystalline substrate of the island is mainly basalt cover and numerous rocks of the deeper zones (also from the upper mantle of the Earth), which can be seen in the form of outcrops of metamorphic rocks, including amphibolites. This region is seismically and volcanically active due to the continuous movement of tectonic plates [37][38][39][40][41][42].
The Sierra de Bahoruco consists of large anticlinal structures coexisting with smaller anticlines, forming anticlines extending for 5 and 10 km. The Las Filipinas larimar deposit (approximately 32 hectares) is located on one of the hills of the mountain range, at about 750 m above sea level, in Los Chupaderos town, about 7 km from the Barahona and about 10 km from La Ciénaga ( Figure 1). This area is a part of the larger extensive Hôte-Selle-Bahoruco tectonostratigraphic unit (NW-SE), having a strong magnetic anomaly associated with overlying basaltic rocks (DFm). They are associated with the deep-sea volcanism, radiometrically dated at 88-92 Ma (Cenomanian-Turonian) [12,27,43,44]. The DFm unit is one of the emerged fragments of the Caribbean Large Igneous Province (CLIP), uplifted and accreted on the northern margin of the Caribbean Plate [7].
In the study area, the Dumisseau Formation rocks developed in the EW direction of the regional fault zone [5,19,25,27]. These are mainly basaltic rocks occurring in the form of several land eruption cycles accompanied by pyroclastic rocks [21,[45][46][47][48][49][50]. Altogether they form a complex more than 1.5 km thick, intersected by dolerite dikes [51]. According to Ar 40 /Ar 39 analyses, the age of the volcanics is ca. 74 Ma (Miocene or Late Cretaceous), while that of the dolerite dykes was estimated at 52 Ma [10]. The volcanics are overlain by thick marine pelagic carbonate sequences of the Neiba Formation (NFm), aging from Paleocene to Middle Miocene, formed by cyclic marine transgressions [53]. In the contact zone between the volcanic and pyroclastic rocks, irregularly spread tree fragments (trunks and branches) as well as fine plant detritus with various degrees of fossilization were found [11,12,24]. As a result of post-magmatic hydrothermal activity, the tree fragments underwent complex mineralization, mainly with pectolite, calcite and prehnite, creating the globally unique larimar deposit.

Current Mineralogical and Petrological Data of the Rocks Hosting Larimar Deposit
In the lower part of the Demisseau Formation (DFm) complex, exposed in the area concerned, basalts with vesicular structure, volcanic breccias, granulometrically differentiated volcanic flow material and basaltic lava pillows were found. The upper series (with a thickness of 15-30 m) is formed by polygenic sediments of pyroclastic origin, lying unconformably on extrusive rocks. Both effusive and pyroclastic rocks were strongly transformed [21,31]. Between them there is a level of the paleosoil, composed mainly of clay minerals, and a layer of sands, indicative of the gradual immersion of the area [10,12,31,54].
Macroscopically, basaltic rocks appear massive [20]. They are black in color, sometimes dark green. Under the optical microscope they show a typical porphyritic texture, sometimes poorly ordered ( Figure 2). The mineral composition is dominated by The volcanics are overlain by thick marine pelagic carbonate sequences of the Neiba Formation (NFm), aging from Paleocene to Middle Miocene, formed by cyclic marine transgressions [53]. In the contact zone between the volcanic and pyroclastic rocks, irregularly spread tree fragments (trunks and branches) as well as fine plant detritus with various degrees of fossilization were found [11,12,24]. As a result of post-magmatic hydrothermal activity, the tree fragments underwent complex mineralization, mainly with pectolite, calcite and prehnite, creating the globally unique larimar deposit.

Current Mineralogical and Petrological Data of the Rocks Hosting Larimar Deposit
In the lower part of the Demisseau Formation (DFm) complex, exposed in the area concerned, basalts with vesicular structure, volcanic breccias, granulometrically differentiated volcanic flow material and basaltic lava pillows were found. The upper series (with a thickness of 15-30 m) is formed by polygenic sediments of pyroclastic origin, lying unconformably on extrusive rocks. Both effusive and pyroclastic rocks were strongly transformed [21,31]. Between them there is a level of the paleosoil, composed mainly of clay minerals, and a layer of sands, indicative of the gradual immersion of the area [10,12,31,54].
Macroscopically, basaltic rocks appear massive [20]. They are black in color, sometimes dark green. Under the optical microscope they show a typical porphyritic texture, some-times poorly ordered ( Figure 2). The mineral composition is dominated by phenocrysts within an almost completely recrystallized (devitrified) groundmass. Locally, traces of volcanic glass are visible, often replaced by a mixture of chlorite and calcite. The majority of phenocrysts include prismatic pyroxenes (diallage) and olivines, both strongly serpentinized, and also fairly numerous basic plagioclases (~n65) with poikilitic (xenoikitic) structures and intensive signs of alteration (garnetization). In the marginal zones of those individuals, chlorites and small admixtures of saponite and celadonite also appear (SEM-EDS). Some of the plagioclase plates have been replaced by a mixture of zeolites (natrolite) and clay minerals (e.g., illite). Quite often carbonate assemblages (calcite) occur, sometimes forming veins, as well as prehnite and pectolite (SEM). Fine rims and veins of hematite occur in most pseudomorphs after pyroxenes. Among the opaque minerals, ilmenite dominates, although the presence of other Fe (e.g., magnetite, hematite) and Ti (e.g., rutile, titanite) oxides cannot be excluded. In conclusion, the volcanics described above are characterized by structural-textural peculiarities and mineral composition that indicate the formation of these rocks under specific conditions. In basalts, the quantity and type of secondary phase minerals are highly diversified, and these minerals include: natrolite, pectolite, prehnite, calcite, the serpentine group minerals, clay minerals (chlorite, chlorite/smectite), celadonite, magnetite, ilmenite, pumpellyite and garnets with an admixture of Ti. The majority of them were developed in hydrothermal processes (200-400 °C). On the other hand, pyroclastic material after deposition and cementation changed into tuff, genetically related to spilite-like assemblage formed in deep rift zones. This is indicated by the elevated sodium contents of these rocks and also the high quantities of glass (palagonite). The spilite-derived pyroclastics (spilite tuffs) were later affected by many physicochemical processes.
Based on the above considerations it appears that the original environment for the formation of pectolite mineralization was associated with the marine reservoir in which volcanic phenomena occurred [20]. Strongly heated basalt magma cooled down, out- The described basaltic rocks are intersected by dark green dolerite veins of varying thickness [20]. Under the optical microscope, the texture of the rocks is sub-ophitic, typical of the diabase type. In SEM images, fine-crystalline pyroxene grains are enclosed in large plagioclase plates. Some of the plagioclase pseudomorphs are fully filled with grains of calcium-ferruginous garnets. The grains of pyroxene are considerably smaller (less than 1 mm), strongly serpentinized and partly chloritized. Previously existing empty spaces between the plagioclase plates and pyroxene grains have been filled with clusters of secondary minerals: celadonite, chlorite, prehnite and calcite. Calcite together with prehnite and pectolite are also components of small veins intersecting the dolerites and forming thin incrustations on the rock surface. The rocks contain also grains of ilmenite that considerably dominate over magnetite and very fine grains of copper sulfides. In comparison to basalts, the quantity of the opaque grains seems to be lower, but their sizes are clearly larger.
Pyroclastic rocks are dark green or brownish [20]. They are composed of rather strongly altered components, i.e., dominant litho-and crystalloclasts and minor vitroclasts.

•
Lithoclasts are represented by fragments of volcanics (basalts). In the groundmass, only pyroxenes are preserved a state that allows their primary nature (mainly orthopyroxenes, bronsite-hypersthene) to be determined. Some pyroxene grains show a lamellar structure resulting from exsolution of diopsides. In many places, they are accompanied by accumulations of pectolite and garnets. Plagioclase plates are almost completely altered. Many are filled with isotropic, usually irregular garnet grains. Sometimes they are intergrown with pyroxene prisms, which clearly indicate the order of crystallization of the different mineral phases. They are accompanied by numerous iron and titanium oxide clusters (e.g., magnetite, ilmenite, rutile), well visible on SEM images. Titanite grains are also quite numerous. Secondary products include calcite, serpentine group minerals, pectolite, pumpellyite, prehnite and garnets.
In conclusion, the volcanics described above are characterized by structural-textural peculiarities and mineral composition that indicate the formation of these rocks under specific conditions. In basalts, the quantity and type of secondary phase minerals are highly diversified, and these minerals include: natrolite, pectolite, prehnite, calcite, the serpentine group minerals, clay minerals (chlorite, chlorite/smectite), celadonite, magnetite, ilmenite, pumpellyite and garnets with an admixture of Ti. The majority of them were developed in hydrothermal processes (200-400 • C). On the other hand, pyroclastic material after deposition and cementation changed into tuff, genetically related to spilite-like assemblage formed in deep rift zones. This is indicated by the elevated sodium contents of these rocks and also the high quantities of glass (palagonite). The spilite-derived pyroclastics (spilite tuffs) were later affected by many physicochemical processes.
Based on the above considerations it appears that the original environment for the formation of pectolite mineralization was associated with the marine reservoir in which volcanic phenomena occurred [20]. Strongly heated basalt magma cooled down, outgassed and contracted, causing the formation of numerous internal fractures. This environment ensured the inflow of Na + and Ca 2+ ions, making it possible to form the silica aggregates but first of all specific hydrated silicates in which such ions as K + , Fe 2+ , Mg 2+ and Al 3+ were built in the structures, and Cu, Fe and Ti were dispersed as extra-structural admixtures in forming future larimar specimens.

Current State of Larimar Mining
The Las Filipinas Larimar Mine is located high in the mountains, surrounded by a subtropical forest. The Dominican government transferred the right to extract the stone to the local population [54]. Starting from the 1970s, the exploitation of larimar in the Sierra de Bahoruco area continued until the mine was closed in August 2021. From the beginning, larimar was exploited with very primitive mining techniques, using small dimension cross-section shafts hollowed from the surface, at the bottom of which extraction galleries or drifts were bored in the mountainside, and from these, smaller shafts were drilled. (Figure 3).

Perspectives for Future
In view of the closure of Las Filipinas mine for safety reasons, it is necessary to develop a new operations strategy to return to the process of larimar mining. This strategy should take into account the objective possibilities derived from the quality and stability of the rock mass, as well as the knowledge accumulated after the use of the previous exploitation tunnel. Mining was mostly carried out with shovels and pickaxes, without the use of explosives. Currently, mining activities can still be called craftsmanship, despite the introduction of electric hammers and three-wheeled motorcycles for the transport of both output and gangue. The excavated material and waste rock (waste) were transported vertically through shafts by hand or by electric winches and along the pavements in buckets and wheelbarrows. Both the exploration and production works were carried out in a less systematic manner, without clear planning. The ventilation and drainage systems were very primitive. In the area of the Las Filipinas mine, which is of interest to us, including the main drift providing access to the deposit, mining was carried out for over 30 years [28]. Due to the low standards of work safety, which resulted in a large number of accidents, the Dominican authorities decided to intervene and build a new tunnel, 1000 m long, which was to facilitate access to the larimar deposit and improve safety conditions by improving drainage, ventilation and creating an escape route in in the event of an emergency [28].
The construction of the adit started in June 2007 with the support of European programs supporting mining in the Caribbean. The project was completed after 426.7 m with a slope of 2% of an adit built to the west of the main entrance at an elevation of 749.65 m above sea level. At the end (764.92 m asl), a ventilation gallery rises to the surface for 25 m. The western end of the drift was connected to the surface by a 55 m long shaft, which was both the exit route and the natural ventilation shaft [55]. The bottom of the tunnel is inclined towards the main entrance. During construction, steel arches with a steel mesh were used, as well as anchoring the ceiling and sprayed concrete. Construction was completed in 2010, but the excavation was closed for several years. In 2012, during a storm, there was a flow of debris and soil, which flooded the western end of the tunnel, blocking access to the shaft inside the tunnel for about 20 m. The rubble was removed in 2013.
The exploitation of the larimar with the use of the adit started in October 2015. The exploitation was carried out using the old techniques, i.e., by boring sidewalks from the adit and then further shafts and pavements in the deposit area. From the very beginning of the operation, the first symptoms of adit tightening appeared, which intensified in 2016. There were deformations and buckling of the casing arches, sprayed concrete detachment and adit cross-section clamping.

Perspectives for Future
In view of the closure of Las Filipinas mine for safety reasons, it is necessary to develop a new operations strategy to return to the process of larimar mining. This strategy should take into account the objective possibilities derived from the quality and stability of the rock mass, as well as the knowledge accumulated after the use of the previous exploitation tunnel.
Undoubtedly, it is necessary to excavate a new main adit, maintaining the shoring system that was used in the old main adit (arch supports and shotcrete). As a main support system, five elements of yieldable steel arch support were chosen. The proposed system allows for a few centimeters of displacement before the set locks. Steel sets will be placed in 1m intervals. Depending on the rock mass quality, the aforementioned standing support will be reinforced with a layer of shotcrete, and its thickness will change with rock quality and depth. Initial numerical simulations indicate that in deeper sections of the main adit, a steel set invert will have to be installed to ensure stability of the design. One of the proposed designs is presented in Figure 4. placed in 1m intervals. Depending on the rock mass quality, the aforementioned st support will be reinforced with a layer of shotcrete, and its thickness will chang rock quality and depth. Initial numerical simulations indicate that in deeper sec the main adit, a steel set invert will have to be installed to ensure stability of the One of the proposed designs is presented in Figure 4. Two different shotcrete thicknesses were tested with a simplified FLAC3D The first of them included 10 cm while the second was reinforced with 15 cm of sh The goal of the simulation was to find the maximum depth which satisfied two acce criteria: (1) Maximum main adit strain of 2% (ratio between opening width and observ placement) [56]; (2) Maximum bending moment observed in steel set <70 kNm.
Each model was 40 m wide, 40 m high and 16 m long. Additional stresses corre ing with analyzed depth were applied to the upper boundary of the model, and a statical stress state was assumed. The Mohr-Coulomb failure criterion was chosen cohesion and friction angle of the rock mass were recreated based on an RMR valu to 19. Young's modulus was derived from Equation (1)   Two different shotcrete thicknesses were tested with a simplified FLAC3D model. The first of them included 10 cm while the second was reinforced with 15 cm of shotcrete. The goal of the simulation was to find the maximum depth which satisfied two acceptance criteria: (1) Maximum main adit strain of 2% (ratio between opening width and observed displacement) [56]; (2) Maximum bending moment observed in steel set <70 kNm.
Each model was 40 m wide, 40 m high and 16 m long. Additional stresses corresponding with analyzed depth were applied to the upper boundary of the model, and a hydrostatical stress state was assumed. The Mohr-Coulomb failure criterion was chosen, while cohesion and friction angle of the rock mass were recreated based on an RMR value equal to 19. Young's modulus was derived from Equation (1) [57][58][59]: The rock mass properties are summarized in Table 1. Steel sets were represented with beam elements and their properties matched those of V32 (32 kg of steel per linear meter, Wx-121.4 cm 3 ) section type produced with S480W steel (Re ≥ 480 MPa, Rm ≥ 650 MPa, A5 ≥ 17%). Sliding between different elements was not allowed. Simulation was divided into 16 steps, each recreating the advance of the main adit equal to 1 m. This included steel set installation and shotcrete spraying. The modeling procedure could be used to vary the Young's modulus of the shotcrete, thus recreating its curing process. Given the 5 m main adit width, the 2% main adit strain acceptance criteria translate into the maximum allowable loss of width of the main adit equal to 10 cm. The calculations commenced with the first design including the steel set at a 1 m interval reinforced with a 10 cm layer of shotcrete reaching a maximum allowable depth of 30 m for the design. Between 30 and 50 m below, the surface the structure was reinforced with another 5 cm of shotcrete. Figure 5 presents negligible horizontal displacement for the aforementioned design at 50 m below the surface. This depth is also a turning point for floor stability, as vertical displacements reached almost 4 cm. Simulation was divided into 16 steps, each recreating the advance of the main adit equal to 1 m. This included steel set installation and shotcrete spraying. The modeling procedure could be used to vary the Young's modulus of the shotcrete, thus recreating its curing process. Given the 5 m main adit width, the 2% main adit strain acceptance criteria translate into the maximum allowable loss of width of the main adit equal to 10 cm. The calculations commenced with the first design including the steel set at a 1 m interval reinforced with a 10 cm layer of shotcrete reaching a maximum allowable depth of 30 m for the design. Between 30 and 50 m below, the surface the structure was reinforced with another 5 cm of shotcrete. Figure 5 presents negligible horizontal displacement for the aforementioned design at 50 m below the surface. This depth is also a turning point for floor stability, as vertical displacements reached almost 4 cm.   Simulation was divided into 16 steps, each recreating the advance of the main adit equal to 1 m. This included steel set installation and shotcrete spraying. The modeling procedure could be used to vary the Young's modulus of the shotcrete, thus recreating its curing process. Given the 5 m main adit width, the 2% main adit strain acceptance criteria translate into the maximum allowable loss of width of the main adit equal to 10 cm. The calculations commenced with the first design including the steel set at a 1 m interval reinforced with a 10 cm layer of shotcrete reaching a maximum allowable depth of 30 m for the design. Between 30 and 50 m below, the surface the structure was reinforced with another 5 cm of shotcrete. Figure 5 presents negligible horizontal displacement for the aforementioned design at 50 m below the surface. This depth is also a turning point for floor stability, as vertical displacements reached almost 4 cm.  Based on the preliminary numerical modeling results, the analyzed shoring system should be characterized by the parameters shown in Table 2. The aforementioned support scheme was calculated assuming hydrostatic stress state and no stress concentration induced by mining activity and was based on scarce data. A slightly different approach to the housing of the new main adit is presented below. It is a proposal based on the experience of Polish mines in difficult geological and mining conditions. The method is used especially in a situation where the excavation is subject to the influence of active or closed exploitation. The method is based on the use of a steel arch support and reinforcement of the rock mass with rebar anchors and cable bolts [60][61][62]. In this method, it is assumed that the rebar anchors cooperate only with the rock mass and the cable bolts with the rock mass and the support casing. During the drifting of the excavation, after the opening (with a heading machine or explosive materials), the exposed space around the entire perimeter is secured with a steel mesh. Then, rebar anchors are installed in a quantity determined by the design and local geological conditions. Anchors with a diameter of 22 mm and a length of 2.5 to 3.0 m are usually used. The load capacity of these anchors ranges from 250 to 300 kN, and the diameter of the holes in which they are installed is 27 mm. The anchors are glued with adhesive charges (polyester resin cartridge) along the entire length of the opening. Then, the steel arch set is installed on the face, spaced according to the design. Proper contact with the rock mass is an extremely important element that guarantees the correct operation of the support casing. This is achieved by performing mechanical lining in the bullflex system [63,64]. A system of fabric containers is placed behind the support casing door frames, which are filled with a mineralcement binder. The binder fills the entire void between the rock mass and the casing. After hardening, it guarantees an even method of load distribution on the support casing arches and accelerates their cooperation with the rock mass. The next stage of strengthening the support and the rock mass is carried out several or several dozen meters behind the drift face. Cable bolts are installed here in the roof or in the roof and walls. In the main adit conditions, high load-bearing capacity anchors with pre-tensioning and injection gluing along the entire length should be used. It seems optimal to use one of the three types of anchors-Titan Bolt (by DSI Underground, Bennetts Green, Australia), Sumo Bolt (by Jennmar, Canonsburg, PA, USA) and IR7 (by Interram, Ogrodzieniec, Poland) with a length of 6 to 10 m, Figure 7.
adit) at 104 m below the surface.
Based on the preliminary numerical modeling results, the analyzed shoring system should be characterized by the parameters shown in Table 2. The aforementioned support scheme was calculated assuming hydrostatic stress state and no stress concentration induced by mining activity and was based on scarce data. A slightly different approach to the housing of the new main adit is presented below. It is a proposal based on the experience of Polish mines in difficult geological and mining conditions. The method is used especially in a situation where the excavation is subject to the influence of active or closed exploitation. The method is based on the use of a steel arch support and reinforcement of the rock mass with rebar anchors and cable bolts [60][61][62]. In this method, it is assumed that the rebar anchors cooperate only with the rock mass and the cable bolts with the rock mass and the support casing. During the drifting of the excavation, after the opening (with a heading machine or explosive materials), the exposed space around the entire perimeter is secured with a steel mesh. Then, rebar anchors are installed in a quantity determined by the design and local geological conditions. Anchors with a diameter of 22 mm and a length of 2.5 to 3.0 m are usually used. The load capacity of these anchors ranges from 250 to 300 kN, and the diameter of the holes in which they are installed is 27 mm. The anchors are glued with adhesive charges (polyester resin cartridge) along the entire length of the opening. Then, the steel arch set is installed on the face, spaced according to the design. Proper contact with the rock mass is an extremely important element that guarantees the correct operation of the support casing. This is achieved by performing mechanical lining in the bullflex system [63,64]. A system of fabric containers is placed behind the support casing door frames, which are filled with a mineral-cement binder. The binder fills the entire void between the rock mass and the casing. After hardening, it guarantees an even method of load distribution on the support casing arches and accelerates their cooperation with the rock mass. The next stage of strengthening the support and the rock mass is carried out several or several dozen meters behind the drift face. Cable bolts are installed here in the roof or in the roof and walls. In the main adit conditions, high load-bearing capacity anchors with pre-tensioning and injection gluing along the entire length should be used. It seems optimal to use one of the three types of anchors-Titan Bolt (by DSI Underground, Bennetts Green, Australia), Sumo Bolt (by Jennmar, Canonsburg, PA, USA) and IR7 (by Interram, Ogrodzieniec, Poland) with a length of 6 to 10 m, Figure 7. All the above anchors are characterized by a similar structure, parameters and method of installation. The parameters of these bolts are summarized in Table 3. The bolt lock is a typical barrel and wedge clamp. The anchors are installed in two stages. In the first stage, after drilling a hole with a diameter of~42 mm, the anchors are installed at the bottom of the hole on loads of resin with a total length of up to 1 m. Immediately after gluing, the washer and the barrel and wedges are attached, and with the use of a hydraulic cable tensioner, the initial tension is applied to the anchor (usually from 100 to 300 kN) [65]. The second stage of installation is usually immediately after the first or with a slight delay. At this stage, the mineral binder or a liquid two-component synthetic resin is fed under pressure. An adhesive connection of the rock mass with the prestressed anchor is thus obtained along the entire length, and additionally, the gaps that were not closed in stage I are filled, and thus the rocks are glued together. The section of the anchor protruding into the excavation is used to strengthen the support casing arches. This is done by building V-shaped stringers with holes and building another barrel and wedge clamp ( Figure 8).
All the above anchors are characterized by a similar structure, parameters and method of installation. The parameters of these bolts are summarized in Table 3. The bolt lock is a typical barrel and wedge clamp. The anchors are installed in two stages. In the first stage, after drilling a hole with a diameter of ~42 mm, the anchors are installed at the bottom of the hole on loads of resin with a total length of up to 1 m. Immediately after gluing, the washer and the barrel and wedges are attached, and with the use of a hydraulic cable tensioner, the initial tension is applied to the anchor (usually from 100 to 300 kN) [65]. The second stage of installation is usually immediately after the first or with a slight delay. At this stage, the mineral binder or a liquid two-component synthetic resin is fed under pressure. An adhesive connection of the rock mass with the prestressed anchor is thus obtained along the entire length, and additionally, the gaps that were not closed in stage I are filled, and thus the rocks are glued together. The section of the anchor protruding into the excavation is used to strengthen the support casing arches. This is done by building V-shaped stringers with holes and building another barrel and wedge clamp ( Figure 8). As a result, the rope anchor performs two important functions at the same time-it strengthens the rock mass and the arch support frame. An exemplary scheme of securing the new main adit according to the above technology is shown in two projections in Figure  9. As a result, the rope anchor performs two important functions at the same time-it strengthens the rock mass and the arch support frame. An exemplary scheme of securing the new main adit according to the above technology is shown in two projections in Figure 9. Minerals 2022, 12, x FOR PEER REVIEW 12 of 16 Figure 9. Proposal for a support system in the new main adit.

Summary
This paper presents the casing protection concept for the larimar mining process. In the Las Filipinas larimar (blue pectolite) deposit area, mafic igneous rocks (basalts), typical of eruptive formations, accompanied by pyroclastic rocks are observed [1,21]. The structural and textural features of the volcanics and the specific stress marks, internal cracks and deformation present in them indicate that the formation of these rocks occurred rapidly, under special marine conditions. These structural and textural features, as well as the characteristic mineral composition and the great variety of secondary products in them, should be taken into account when designing new technological solutions for mining this raw material.
In the work, the authors present several solution options. They propose a return to the shoring system used in the old main adit (arch support and shotcrete) for the shoring of the new main construction. However, an analysis of the available geological and geotechnical data is required for this project to allow for: (a) The viability of implementing the design to be determined; (b) The design to be optimized in the future.
General conclusions about the situation at the Las Filipinas mine could be grouped into several topics. The main ones are: poor knowledge of the deposit geology, defects in the construction of the drift support and uncontrolled exploitation in shafts and galleries affecting the rock mass surrounding the drift.

Summary
This paper presents the casing protection concept for the larimar mining process. In the Las Filipinas larimar (blue pectolite) deposit area, mafic igneous rocks (basalts), typical of eruptive formations, accompanied by pyroclastic rocks are observed [1,21]. The structural and textural features of the volcanics and the specific stress marks, internal cracks and deformation present in them indicate that the formation of these rocks occurred rapidly, under special marine conditions. These structural and textural features, as well as the characteristic mineral composition and the great variety of secondary products in them, should be taken into account when designing new technological solutions for mining this raw material.
In the work, the authors present several solution options. They propose a return to the shoring system used in the old main adit (arch support and shotcrete) for the shoring of the new main construction. However, an analysis of the available geological and geotechnical data is required for this project to allow for: (a) The viability of implementing the design to be determined; (b) The design to be optimized in the future.
General conclusions about the situation at the Las Filipinas mine could be grouped into several topics. The main ones are: poor knowledge of the deposit geology, defects in the construction of the drift support and uncontrolled exploitation in shafts and galleries affecting the rock mass surrounding the drift. The presented support system should be a sufficient protection for the new tunnel. In Polish mines of hard coal, it is successfully used in difficult geological conditions at a depth of more than 1000 m. Naturally, when necessary, the load-bearing value of the above-described casing system can be increased by a reduction in frame spacing, installation of additional cable bolts, closing the support casing by building a steel floor bar or imposing a layer of shotcrete around the perimeter of the tunnel. To summarize, it should be emphasized that the design of the support, taking into account the selection of the appropriate system, including the selection of the type and spacing of the support arches, anchoring screens and length of the bolts, requires proper recognition of the geological properties of the rock mass. Such research should be carried out in two ways: laboratory and in situ. Control tests should also be carried out at the stage of excavation. It will also be necessary to prepare an appropriate monitoring system for the support and the rock mass for early decision-making on strengthening the support for the excavation.
The new main adit will be excavated in a rock mass that requires preliminary geophysical surveys. The BGC report (2018) stated that the RMR76 value of the rock mass in the area of the old main adit ranged from 19 to 34. The main adit will reach a maximum depth of 104 m.

1.
The paper contains general assumptions of one of the variants of securing the main adit in the Las Filipinas Larimar Mine. To some extent the presented solution draws on the previous experience in the field of maintenance of the main works of this mine.

2.
The selection of the potentially most effective method of supporting the mining pits requires a series of geophysical and geotechnical surveys. The tests to be conducted will enable the preparation of design assumptions for the new deposit access structure and the planning of operating works. The option of increasing the role/share of vertical shafts in accessing the deposit is also being considered.

3.
A test site covering the entire field will be designed together with the Dominican side.