Summary:
Three types of mineral deposits are recognized at Atacocha, described as either: skarn (Exo and Endo Skarn); replacement (lithological and structurally controlled); or hydrothermal veins (and collapse breccias).
Mineralization
At Atacocha, mineralization is characterized as a skarn, intermediate sulphidation epithermal vein/breccia-style, or porphyry mineralization.
Skarn/metasomatic-related mineralization spatially associated with the Santa Bárbara stock or San Gerardo stock is paragenetically earlier, followed by the hydrothermal mineralization. Skarn-related mineralization is commonly associated with the garnet and silica-skarn-chlorite assemblages, comprising pyrite, chalcopyrite, sphalerite, galena, and lesser pyrrhotite, pyrite, bournonite, covelite, orpiment, and realgar occurring within the Pucará Group sediments around the Santa Bárbara stock.
Skarn-related mineralization is characterized by pyrite, chalcopyrite, sphalerite, galena, with lesser bismuthinite and a variety of sulphosalts (Bi-bearing) and pyrrhotite, bornite, and covellite at lower elevation. Molybdenite may occur proximal to the skarn-related mineralization. Elevated Bi and Au are reported to be associated with skarn-related mineralization. Veins and veinlets with pyrite, chalcopyrite, sphalerite, galena, with quartz and carbonate occur within marble units, and are spatially associated with skarn bodies.
The intermediate sulphidation epithermal mineralization occurs in the upper part of the system (i.e., at higher elevations) as veins and breccias, characterized by galena, Ag-bearing sulphosalts, sphalerite, and free Au. Gangue mineralogy comprises quartz (silica), pyrite, specularite/hematite, adularia, rhodochrosite, rhodonite, and alabandite. The silica-sericite halloysite alteration assemblage is generally associated with this epithermal system. These veins and breccias are mostly between the San Gerardo stock and Fault 1, appearing to be at least partly controlled by the northwest-trending Fault 13. Breccias have been grouped into either Ag-Pb-Zn hydrothermal breccias or siliceous breccias based on their mineralogical assemblages and textural characteristics.
Siliceous breccias are further sub-divided into three groups: silica, granular (“terrosa”), and massive. Collectively the siliceous breccias may form large, relatively continuous bodies based on various interpretations; some appear to be spatially associated with the San Gerardo and Santa Bárbara intrusives and structurally controlled by the Milpo-Atacocha, Longreras, 13, 1, and Laquia faults. Veinlets of pyrite, galena, sphalerite, and possibly other fine undistinguished sulphides may occur within the variable silica-sericite-clay matrix.
The silica breccia comprises clasts of milky white to grey opaline silica, sub-rounded to sub angular, from millimetre to centimetre size; as well as less common sandstone and limestone clasts. The silica-breccia clasts are cemented by a white granular silica, with occasional cross cutting thin white silica veinlets.
The massive (siliceous) breccia forms zones of pervasive alteration comprising predominantly fine grained and massive white silica.
The granular-(siliceous) breccia comprises loose white to grey silica grains, within a poorly cemented clay (undifferentiated) matrix.
The Ag-Pb-Zn breccias are sub-divided into calcareous, polymictic-monomictic, and karst/collapse. Breccia clasts include limestone, marble, silica (massive), and skarn; the composition of the clasts indicates that brecciation occurred later than skarn development. Massive silica alteration may cross-cut skarns.
The calcareous breccia comprises sub-angular to sub-rounded clasts of limestone and marble; cemented by a grey to dark grey calcareous matrix, with occasional bituminous material and rare pyrite. Pyrite, sphalerite, galena, and other sulphides/sulphosalts including orpiment, realgar, tetrahedrite, alabandite, stannite, as well as quartz, calcite, rhodochrosite, and rhodonite occur within the matrix. Minor mineralization may occur within marble where the calcareous breccias cross-cut these units. Geochemical/mineralogical zonation is apparent whereby galena (Pb) and Mn-bearing minerals are more abundant distally relative to sphalerite (Zn). Bi- and Sn-bearing minerals are more elevated proximally.
The polymictic to monomictic breccias are overall grey, and comprise sub-angular clasts of black limestone, mudstone, white silica with silica-pyrite veinlets, and marble with silica wollastonite and calcite. Monomictic breccias comprise sub-angular clasts of limestone or predominantly intrusive rocks. Both polymictic and monomictic breccias clasts vary in size and are cemented with a black amorphous material with disseminated pyrite. Pyrite, chalcopyrite, sphalerite, galena, and possibly other sulphides occur within the matrix forming veins/veinlets, pockets, or disseminations.
Karst/collapse breccias contain clasts of limestone, marble, silica, skarn, and intrusive rocks, are sub-angular to sub-rounded, within a matrix of sub-horizontal laminated limestone and silica-sericite-clay material.
The Cu-Au±Mo porphyry mineralization is found in the Santa Bárbara stock. It consists of porphyry quartz monzodiorite with potentially economic grades (i.e., 0.3% Cu, 0.3 g/t Au) of Cu Au ± Mo. The San Gerardo stock is made of the same quartz monzodiorite, but with weaker alteration and much lower grades of all metals of interest. In the Milpo stock, the porphyry potential was poorly investigated, however, A- and B-type stockwork veinlets with low Cu grades were recorded in the past and the stock warrants future investigation.
Summary:
Atacocha operates two mines: the Atacocha underground mine and the San Gerardo open pit mine. The underground mine is currently suspended, but mining continues in the San Gerardo open pit mine. The underground mine is planned to restart production in 2027 at which point the open pit mine will be depleted.
Once the pit is depleted in 2027, operations at the underground Atacocha mine will be re-started. The underground mine will be integrated with the El Porvenir underground mine, with both mines feeding the El Porvenir processing plant. The Atacocha processing plant and mine site will be shut down and all operations will be transferred to the El Porvenir mine site.
The Atacocha underground mine is mined by the overhand Cut and Fill (C&F) and Sub-Level Stoping (SLS) mining methods. C&F stopes are 20.0m high consisting of 4.0m high cuts and a minimum mining width of 4.0m. C&F stopes are located 55 m from the main ramps and accessed by stope access ramps with grades varying from -15% to +15%. Production is achieved by horizontal drill and blast, and backfilled using unconsolidated waste fill or hydraulic backfill. The SLS mining method has demonstrated increased productivities and reduced unit costs as compared to the C&F mining method. SLS stopes are located a minimum of 40m from infrastructure, are 20.0m high, 30.0m long and have a minimum mining width of 4.0m. Production is achieved by vertical blastholes and backfilled using unconsolidated waste fill. Mined-out stopes are backfilled using unconsolidated backfill and hydraulic fill using tailings.
San Gerardo is an open pit operation located at the top of mineralized zone and is mined by 6.0m high benches. Since the temporally suspension of Atacocha underground mine, the open pit production is responsible by the metallurgical plant feed. Operations are carried out by contractor with a mining fleet that allows selectivity in the loading process.
Atacocha Open Pit (San Gerardo)
San Gerardo is a conventional open pit drill and blast operation using excavators and trucks operating on bench heights of six metres. The open pit is operated by a contractor, that provides operators, equipment, and ancillary facilities required for the mining operation.
Mill feed material produced by San Gerardo is hauled to the orepass at the east of the pit that reports to the 3600 Level of Atacocha UG, where it is trammed by locomotive and rail car to the Atacocha plant. Waste is hauled to the San Gerardo waste dump, which is adjacent to the Atacocha TSF dam. Average single direction haul distances for ore and waste are approximately 1.0 km and 4.7 km, respectively.
Open Pit Design
The San Gerardo pit is excavated with variable pit floor elevations and an ultimate pit bottom at 4,114 MASL. Ramps are 12 m wide at a 10% gradient that allow bi-directional haul truck travel. The mine is designed with a six-metre bench height, utilizing 127 mm vertical drill holes on 4.8 m x 5.8 m drill pattern with 0.5 m of sub-drilling.
Open Pit LOM Production Schedule
The open pit Mineral Reserve estimate supports an open pit LOM production plan of approximately four years before the underground mine starts production in 2027. Total material mined does not exceed 10 Mtpa (approximately 27 ktpd).
The LOM plan assumes that Atacocha UG will be operational starting in 2027 and will ramp up from 0.5 Mtpa in 2027 to 1.0 Mtpa in 2029.
Summary:
Crushing
The mined ore is stored in four coarse underground hoppers located at the 3600 Level. Hopper No. 1 has a capacity of 1,600 t. Hopper No. 2 has a capacity of 1,900 t. Hopper No. 3 has a capacity of 800 t, and Hopper No. 5 has a capacity of 700 t.
The ore accumulated in the coarse hoppers is fed to the crushing section by conveyor. The new No. 1 and No. 2 coarse hoppers each work with a 42 in. x 12 ft Comesa apron feeder; the No. 3 and No. 5 coarse hopper each work with 60 in. x 16 ft Comesa apron feeders.
The ore is conveyed to a 1,100 mm x 850 mm, Nordberg C110 B primary jaw crusher. The crusher product is conveyed to the primary Simplicity double deck screen. The screen undersize is conveyed to the fine ore storage bins. The upper deck screen oversize feeds the secondary Sandvik CH660 cone crusher. The lower deck screen oversize discharges onto belt No. 4, which is joined with the product of the secondary CH660 crusher and both products are conveyed to the secondary 8 ft x 20 ft double deck banana type screen via belts 5, 6A, and 6B.
The secondary banana screen – 3/8 in. bottom deck screen undersize material reports to the fine ore storage bin and the top and bottom deck screen oversize material feeds the tertiary Nordberg HP-500 cone crusher. The tertiary crusher product is conveyed to the tertiary Simplicity screen via conveyors 8, 5, and 6. The tertiary Simplicity screen undersize reports to the fine ore bins and the screen oversize from both decks feeds the tertiary crusher, closing the tertiary crushing circuit.
Product conveyor No. 9 distributes the ore to each of the six 450 t fine ore bins using a tripper conveyor.
Grinding
The grinding circuit comprises six ball mills, two Hardinge 8 ft x 5 ft conical mills and four Comesa 8 ft x 10 ft mills as primary mills, and one Hardinge 8 ft x 5 ft conical ball mill used as a secondary mill.
Five of the primary mills operate in closed circuit. The discharge of each mill is fed to an SK-80 flash flotation cell that recovers coarse lead concentrate which is pumped directly to lead concentrate storage. The tailings from these cells are pumped to a 15 in. hydrocyclone classifier. The cyclone underflow feeds the primary ball mill and the hydrocyclone overflow feeds a 10 ft dia. by 12 ft high lead-copper flotation conditioner in the flotation circuit.
The sixth primary mill operates in series with a secondary mill. The primary mill discharge feeds an SK-80 flash flotation cell. The SK-80 tailings are pumped to a 15 in. hydrocyclone. The cyclone underflow feeds the single secondary ball mill, which is closed by a 20 in. hydrocyclone.
Each primary mill is fed by variable speed conveyor belts and tonnage is automatically controlled by Ramsey scales. The following flotation reagents are added to each primary mill: zinc sulphate (depressant), sodium cyanide (depressant), and in each flash flotation cell: xanthate (Z11 or Z14) and MIBC are added.
Summary:
The Atacocha concentrator utilizes a conventional crushing, grinding, and sequential flotation scheme to produce lead and zinc concentrates with an average daily processing rate of approximately 4,400 tonnes. A flash flotation step is included in the grinding circuit that recovers lead at a grade sufficiently high to report directly to the final lead concentrate. The majority of gold and silver report are into the lead concentrate. Despite the plant having capacity to produce copper concentrate, the circuit of the plant has been bypassed and the Atacocha metallurgical plant produces only zinc and lead concentrates.
Flotation
The flotation plant consists of three circuits that are controlled by a Courier 6SL inline analyzer that was installed in September 2004 (currently only the multiplexer operates). The bulk rougher circuit produces a bulk copper-lead concentrate that is then separated in the lead circuit to produce the copper and lead concentrates. The zinc circuit processes the tailings from the copper and lead circuits, and including a regrinding stage, produces the zinc concentrate and the final tails.
Bulk Rougher Circuit
The bulk rougher flotation circuit treats the whole ore, obtaining lead concentrates, made up of the concentrates of the grinding circuit SK-80 flash flotation cells plus the OK-8 and OK-20 cleaner concentrates and the OK-30 I and 1st OK-8 scavenger concentrates. To this are added the concentrates of the RCS-30 and 2nd + 3rd OK-8 scavenger cells (may or may not include cleaners); the cleaner tailings are recycled to the bulk rougher feed. The feed to the cleaners is made up of rougher concentrates. The concentrates from the OK-30 I cells are fed to the OK-3 cells (lead cleaning), the concentrate from these cells also forms the lead concentrate and the tailings returns to the rougher feed.
Bulk-Scavenger Flotation Circuit
The tailings from the bulk rougher OK-30 II cells feed the bulk scavenger flotation circuit consisting of an OK-20 cell followed by a bank of six OK-8 cells and one OK-16 cells. The OK 20 scavenger concentrate flows to a bank of five cleaner scavenger OK-8 cells. The bulk scavenger tailings feed the zinc circuit.
Pb/Cu Separation Circuit
Since July 2019, no Pb/Cu separation has been carried out due to a copper head grade of less than 0.10%, ranging from 0.06% to 0.08%. The Pb/Cu separation can be resumed when the copper head grade improves.
Zinc Circuit
The zinc rougher circuit processes the tailings from the bulk lead-copper scavenger circuit with conditioning in two 16 ft dia. x16 ft agitated tanks operated in series. The zinc rougher circuit consists of one OK-100 cell and one OK-50 cell operated in series. The scavenger circuit consists of a bank of three OK-16 cells, a bank of four OK-16 cells and one OK-50 cells, and the cleaning circuit consists of one stage using an OK-10 cell and a bank of five OK-3 cells. The zinc scavenger tailings are final tailings. The first rougher concentrate and the cleaner concentrate are the final zinc concentrates.
The scavenger concentrates return to the second scavenger feed (OK-16) and tailings from the first scavenger are recycled to the second rougher feed.
Thickeners and Filters
Lead an zinc flotation concentrate are pumped to two parallel concentrate thickeners for each metal for dewatering. The thickener underflow slurry from the lead and zinc thickener underflows are pumped to the lead and zinc concentrate storage tanks respectively for density control. The slurry is then pumped to a CC-45 ceramic disc filter, which reduces the moisture in each of the concentrates to approximately 8.2% for transportation. Filter aid at 20 g/t is added to improve the filtration process. There are two drum filters that can be used for either lead or copper concentrate dewatering. The filtered concentrates are discharged to the concentrate storage areas for shipping. The concentrates are loaded into trucks by front end loader when being despatched to customers.
Tailings Pumping System
Atacocha's tailings transport system is a high pressure pumping system using Geho positive displacement pumps designed to pump thickened tailings at a density 55%-65% solids to the Atacocha tailings dam. A project is being developed for a treatment rate of 5,000 dry tpd, i.e. 4,500 tpd of tailings.
The concentrator's general tailings are fed to the 125 ft thickener at a density of 1,250 g/L to reduce the total volume of material that must be transported to the Atacocha TSF. The tailings pulp is temporarily stored in the thickener-clarifier. Flocculant is dosed at a rate of 30 L/min at a concentration of 0.025%. The underflow of the thickener is recirculated with HR-200 pumps until it reaches 55% to 65% solids. When the pulp reaches the appropriate density, it is fed into the linear sieve, to store clean pulp of adequate granulometry in the storage tanks, and when the density decreases, the pulp is recirculated to the thickener until it reaches the adequate density again. The overflow with < 50 ppm solids is stored in a tank and then recirculated to the concentrator, and excess water is sent to the settling ponds.
The pumped tailings are disposed of in the basin of the Atacocha TSF.
Recoveries & Grades:
| Commodity | Parameter | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2017 |
|
Zinc
|
Recovery Rate, %
| 75.9 | 79.1 | 76.3 | 75.3 | 77.5 | 77.9 | 79 |
|
Zinc
|
Head Grade, %
| 0.77 | 0.89 | 0.88 | 1.2 | 1.43 | 1.43 | 1.43 |
|
Zinc
|
Concentrate Grade, %
| 50.7 | 50.8 | 50.4 | 50.6 | 49 | 50 | 51.4 |
|
Silver
|
Recovery Rate, %
| 82.7 | 77.7 | 76 | 76.4 | | | |
|
Silver
|
Head Grade, g/t
| 37.6 | 32.7 | 31.4 | 43.2 | 47.3 | 44.2 | 44.5 |
|
Silver
|
Concentrate Grade, g/t
| 2,006 | 1,658 | 1,802 | 1,844 | | | |
|
Lead
|
Recovery Rate, %
| 85.7 | 85.5 | 83.3 | 83 | 84 | 85.3 | 86.7 |
|
Lead
|
Head Grade, %
| 0.93 | 0.97 | 0.82 | 1.15 | 1.3 | 1.18 | 1.22 |
|
Lead
|
Concentrate Grade, %
| 52.9 | 53.9 | 51.7 | 53.5 | 53.7 | 55.3 | 55.8 |
|
Gold
|
Recovery Rate, %
| 56.2 | 67.9 | 69.6 | 54 | | | |
|
Gold
|
Head Grade, g/t
| 0.31 | 0.31 | 0.31 | 0.31 | 0.37 | 0.47 | 0.59 |
|
Gold
|
Concentrate Grade, g/t
| 11.2 | 20.2 | 22.1 | 10.3 | | | |
|
Copper
|
Head Grade, %
| | | | 0.05 | 0.08 | 0.1 | 0.09 |
|
Copper
|
Recovery Rate, %
| | | | | 5.99 | 9.61 | 8.46 |
|
Copper
|
Concentrate Grade, %
| | | | | 18.5 | 20.7 | 18.8 |