The Marmato gold deposit consists of a structurally controlled epithermal vein system with a mineral assemblage dominated by pyrite, arsenopyrite, black iron (Fe) rich sphalerite, pyrrhotite, chalcopyrite and electrum in the Upper Zone (UZ), and a mesothermal veinlet system with a mineral assemblage dominated by pyrrhotite, chalcopyrite, bismuth minerals and visible gold in the MDZ.

The MDZ mineralization consists of a network of thin, less than 5 centimeters, sulfide veinlets, mainly pyrrhotite+chalcopyrite, hosted in weak argillic and deeper potassic alteration which is related to a previous event and rimmed by a thin sodium-calcitic alteration halo, which is related to the mineralization. Recent geological reports on MDZ (Sillitoe, 2019) concluded:

• Gold grade distribution in the Zona Baja (MDZ) mineralized orebody is unrelated to the presence of distinct porphyry phases and is entirely dependent on the intensity of structurally localized veinlets;
• Potassic alteration, represented chiefly by biotite, is progressively better preserved at depth in the Zona Baja, raising the possibility that early potassic alteration could also be gold bearing, but further work is required to confirm this theory;
• Gold distribution appears to be exclusively a product of veinlet intensity and orientation related to structural controls during orogenesis. The veinlets responsible for much of the Zona Baja gold are those containing quartz, pyrrhotite and traces of chalcopyrite and having prominent albite alteration halos; and
• The presence of visible gold is also noted in the core and, as expected, relates to increased assay values when present.

The MDZ type corresponds to several sets of N60-70W/90 veinlets that run parallel to each other as sheeted veins, these are controlled by the main trend stress tensor WNW-ESE, as explained in the chapters of regional and local structural geology. For this reason, these veinlets are considered to represent stress fractures at a very early stage in the porphyritic intrusion, which may represent a greater extension of this mineralization to that currently explored, modeled and estimated.

The MDZ gold mineralization consists of a network of thin, less than 5 cm thick sulfide veinlets, mainly consisting of pyrrhotite+chalcopyrite, and typically rimmed by a thin sodium-calcitic alteration halo, all hosted in weak argillic and deeper potassic alteration related to a pre-gold mineralization event (SRK, 2019).

The Mesothermal veinlets within the MDZ mineralization follow a standard pattern, presenting a predominantly NW orientation between 40 to 62°, with steep dips (between 70 to 84°). Minor variations in the trend from NNW to E-W have also been noted but are less common and within borehole MT-IU009, another family of mesothermal veins with an N10W tendency and a dip of 47° is identified.

The upper portion of the MDZ has been exposed in Level 21 of the existing Caldas mining operations, while deeper sections have been observed in drillcore, both of which have been confirmed as different styles of mineralization. The lowest levels of the mine have currently intersected a combination of the porphyry domain, where the gold is associated with pyrite veinlets, and the MDZ where gold is associated with pyrrhotite. There is a transition zone existing between the two domains, which is observed to some extent in the current mine workings with overprinting of the epithermal system on the MDZ. The vertical extent of the transition is not clearly defined from the current drilling. Currently, underground mining at the Caldas- operated mine remains focused on the vein structures located in the central portion (Zona Baja) of the Marmato deposit.

MDZ zone is indicated to be continuous along strike for approximately 500 m and has a confirmed down dip extent that reaches up to 800 m, with a thickness that varies between 35 and 150 m.

For this PFS, there are three different mining methods, separated into three distinct zones as follows:
- The first zone is the mineralized vein material between 950 m elevation to 1,300 m elevation, referred to as the Veins. This is the current mine and will be mined using the current conventional cut and fill stope method.
- The second zone is the wider porphyry material between 950 m elevation and 1,050 m elevation, referred to as the Transition Zone. A modified longhole stoping method will be used in this area. The stope size is 15 m wide by 15 m high with varying length of up to 26 m. These stopes are mined in a primary-secondary sequence with paste backfill for the primary stopes and unconsolidated waste rockfill for the secondary stopes. Where waste rock is unavailable, hydraulic sand fill will be used to fill the secondary stopes.
- The third zone is the porphyry material below 950 m elevation, referred to as MDZ. There is a 10 m sill pillar left in situ between the MDZ and the UZ (Veins plus Transition area). The MDZ material can be mined using a longhole stoping method with stope sizes that are 10 m wide by 30 m high, with varying lengths of up to 30 m. The MDZ area is currently not developed.

The first two zones (Veins and Transition) are considered the UZ, and the material is processed in the existing processing facility. The third zone is considered the MDZ and the material is envisioned to be sent to a new processing facility. Separate mine plans are presented for the UZ area and MDZ area.

MDZ Mining
The MDZ area is currently in the exploration phase and has not been developed. Mineralization is located approximately 600 to 1,200 m below the surface (480 masl to 1,100 masl). Based on geomechanical information and mineralization geometry, an underground longhole stoping method (LHS) is suitable for the deposit. Cut-and-fill vein mining will continue above the MDZ area, but it is not a method that will be used in the MDZ area.

The MDZ deposit will be mined in blocks where mining within a block occurs from bottom to top with the use of paste backfill. Sill pillars are left in situ between blocks. The backfill will have sufficient strength to allow for mining adjacent to filled stopes without the need for dip pillars. The stopes will be 10 m wide and stope length will vary based on mineralization grade. A spacing of 30 m between levels has been used. In the top mining block, a higher grade core is extracted first, mined from bottom to top. Subsequently, additional stopes are mined from the bottom of the block up, mining adjacent to (but not underneath) backfilled stopes.

The mine will be accessed by a decline drift with mineralization transported from stopes via truck to an underground crusher and then to surface by conveyor. Internal intake and exhaust raises will be developed using raisebore machines and air will flow into dedicated intake and exhaust ventilation drifts to surface. A new 4,000 t/d process facility using gravity concentration and cyanidation of the gravity tailings will be constructed to process material from the MDZ. In addition, a new DSTF will be constructed to receive approximately 55% of the total LoM tailings from the plant. The other 45% of tailings will go back underground into the mine as cemented paste backfill.

Stoping
Stopes will be mined using the longhole open stoping method. Individual stope blocks are designed to be 10 m wide, up to 30 m long, and will have a transverse orientation. Levels are spaced 30 m apart and each stope block will have a top and bottom access (4.5 m by 4.5 m flat back drifts).

Stopes will be drilled downward from the top access using 114 mm diameter holes (stope slots and stope production rings will be drilled with an ITH drill). A bottom up, primary/secondary extraction sequence will be followed. Primary stopes will be backfilled with high strength paste backfill and secondary stopes will be backfilled with RoM waste from the underground operation and low strength paste backfill as needed when waste rock is not available.

Stope extraction will occur in two steps. During the first step, a slot will be mined at the far end the stope using a drop raise and 28 fan-drilled slash holes. The slot is required to create sufficient void space for the remainder of the stope to be blasted. During the second step, production rings will be blasted three rows at a time (13 blastholes per ring) until the stope is completely extracted. The number of three-row blasts in a given stope will depend on the length of the stope. All blasting will be performed with bulk emulsion.

Ore will be remotely mucked from the bottom stope access using a 7.3 m3 (17 t) LHD. The LHD will transport the ore to a re-muck bay to maximize the efficiency of the stope mucking operations. A second LHD and a fleet of 45 t haul trucks will be used to transport ore from the re-muck bays to the grizzly feeding the underground material handling system. Multiple re-muck bays will be used on each level to avoid interference between the stope loader and the haul trucks.

UG Material Handling System
The underground material handling system is designed to size the rock, provide surge and storage capacity, and be an efficient, automated system for moving the rock to surface via conveyor. During operations, ore will be brought to the dump point and fed via a rock breaker protected grizzly into an infeed ore pass that loads into the crusher station. The ore pass from the grizzly is sized to hold approximately 2,000 t (approximately ½ half day mill feed). A feeder will load a vibratory grizzly which will separate fine and coarse sections and feed the coarse (oversize) material into a jaw crusher. Undersize will be fed to the main transfer conveyor and then onto the main conveyor to surface.

Development
Lateral development includes main conveyor ramp, interlevel truck ramps, ventilation drifts, level accesses, stope accesses, and short connecting drifts for ventilation. Interlevel ramps and levels accesses will be located in the footwall and have been designed to avoid crossing fault zones to the maximum extent possible. Stope accesses are oriented perpendicular to the strike of the orebody.

Haulage
The mine plan assumes that 7.3 m3 (17 t) LHDs will load 45 t haul trucks from re-muck bays that will be strategically located throughout the development workings. Ore and waste haulage distances and cycle times were calculated using the haulage profile module in Vulcan and are based on estimated underground truck speeds. All waste material mined through the end of 2028 is sent to surface for surface construction purposes. In 2029, and through the end of the mine life, the availability of mined-out secondary stopes was evaluated to determine haulage distances for waste material.

Backfilling
The mine production sequence includes the use of cemented paste backfill to fill the voids left by the stopes to maintain the mine structural integrity. The mine utilizes a high strength backfill paste that has a 7% cement content in the primary stopes. A lower strength paste with 4% cement is used to backfill the secondary stopes. A backfill operations crew installs barricades in the lower access drift to the stopes, extends the pipe delivery system in the upper access drift into the stopes, and monitors the backfill as the stope fills. Once the stope is filled the backfill is allowed to cure (seven days) to design strength of over 1 MPa prior to blasting on the adjoining stope.

Marmato Process Plant (Current Operations)
Crushing Circuit
RoM ore is hauled by rail from the mine and dumped into a hopper where a slusher is used to move the material to a 5 m by 7 m feed hopper that feeds a vibrating grizzly to remove the -3/8 inch material, prior to feeding the primary jaw crusher. The discharge from the jaw crusher is conveyed to a doubledeck vibrating screen fitted with a 7/8 inch upper deck and a 3/8 inch lower deck. The screen oversize from the top deck is conveyed to a Nordberg 1352 Omnicone which is operated in closed circuit with the vibrating double-deck screen. Ore retained on the second deck is conveyed to a Nordberg HP300 cone crusher, which is also operated in closed circuit with the vibrating screen. The -3/8 inch screen undersize discharges to the fines bin. The -3/8 inch undersize from the vibrating grizzly is further classified in a spiral classifier. The classifier oversize is fed directly into the primary ball mill and the classifier undersize is thickened and then pumped to the primary hydrocyclones. The crushing circuit has an operating capacity of 1,600 t/d.

Grinding
Crushed ore (-3/8 inch) is fed from the fines bin and then transported on a conveyor fitted with a belt, scale to the 9.5 ft by 14 ft primary ball mill (600 hp).

Expansion Plans
Install a refurbished 15.5 ft by 22 ft ball mill (3000 hp) to replace the current primary ball mill (600 hp) and secondary ball mill (300 hp)

MDZ Process Plant
Primary/Secondary Crushing and Stockpile
RoM ore is transported to the primary crusher located underground and dumped into a RoM bin. The RoM bin has a 600 mm by 600 mm static grizzly installed to keep large oversize from plugging the primary crusher feed cavity, and rock breaker to treat any oversize. A vibrating grizzly feeder with 100 mm bar spacing ahead of the of the primary jaw crusher is used to screen out the finer material and feed the jaw crusher the grizzly oversize material. The jaw crusher will produce a product with an 80% passing size of 128 mm. The crushing circuit is designed for an annual operating time of 5869 h/a or 67% availability.

The primary crusher product along with the grizzly feeder undersize is conveyed along a short sacrificial belt and transferred to a longer conveyor to the surface and discharge into a surge bin for the secondary crusher. A pan feeder will feed the secondary crusher screen, with the oversize fed to a secondary cone crusher, which produces a product with an 80% passing size of 36 mm. The screen undersize and the secondary cone crusher discharge (fine ore product) are combined and conveyed to the crushed ore stockpile. The stockpile allows for 24 hours of continuous milling operation at the nominal feed rate. Crushed ore is 100% passing 65 mm and 80% passing 32 mm. The crushing and grinding circuits are configured in a 2CSAB circuit, which is two stage crushing followed by SAG (semi-autogenous grinding mill) and ball mill.

Crushed ore will be withdrawn from the stockpile by two variable speed belt feeders. The belt feeders are sized such that during maintenance, one of the feeders can provide the full mill-feed capacity of 181 tonnes per hour (t/h).

Grinding
Crushed ore from the stockpile is transferred to the SAG mill via the mill feed conveyor. The conveyor is equipped with a belt scale to provide feed rate data for feed control to the SAG mill.

A SAG mill media bin located adjacent to the mill will add media to the mill on a pre-set scheduled basis. A similar media bin is located adjacent to the ball mill to add media to the mill at a determined charging schedule.

Primary grinding is provided by a 6 m diameter by 3.6 m EGL SAG mill, with an installed pinion power of 2 MW. The SAG mill trommel undersize will report to the cyclone feed hopper, with the oversize recirculated back to the SAG mill feed conveyor by means of a high-angle pebble conveyor. There is a contingency for a future pebble crusher installation at the head end of the high-angle pebble conveyor. Secondary grinding is provided by a 4.9 m diameter by 7.3 m ball mill, with an installed pinion power of 3.1 MW and operated in closed circuit with hydrocyclones. The classification circuit will operate at a nominal circulating load of 300% which is a typical for ore of similar characteristics and target grind size of 80% passing 105 µm. To avoid damage to the cyclone feed pumps and cyclone clusters, ball mill discharge is screened through a trommel screen to scalp off oversized particles and broken grinding media.

CGM operates a 1,200 t/d process plant to recover gold and silver values from material produced from current Marmato mining operations in the UZ and plans to expand this facility to a 1,500 t/d capacity in the next couple of years. In addition, CGM is evaluating the development of the MDZ, which is below the current mining operations and the construction of a new 4,000 t/d plant to process material solely from the MDZ.

Marmato Process Plant (Current Operations)
Gravity Concentration Circuit
The primary ball mill discharges to a Knelson gravity concentrator to recover coarse gravity recoverable gold. The tailings from the gravity concentrator is pumped to the cyclones. The cyclone underflow discharges to the secondary ball mill (300 hp), which is operated in closed circuit with the cyclones and the overflow advances to the flotation circuit. The gravity concentrate is combined with the flotation concentrates prior to advancing to the regrind and cyanidation circuits ........