CRD: King of grade and continuity
- CRDs are known for its high grades of silver, zinc and lead.
- Mining engineers love CRD‘s continuous ore-mining widths of a few metres (up to 10 m and sometimes even more; thickening at depth; ideally suited for underground mining).
- It‘s rare to find unmined CRD outcrops today, simply because if someone had seen just one in the long past of humanity, it would most likely be mined out artisanally or professionally by now. That‘s because they strike the eye with its almost pure, not-uncommonly-90%-pure-metal content.
- Strip Ratio: Without much barren gangue material or deleterious elements, a lot of metal is concentrated in a small tonnage, perfectly suited for low-cost mining and processing methods to boost economics. If you can find +10 million tonnes of high-grade CRDs at or near surface, you can consider yourself fortunate.
- Some CRDs host +100 million tonnes.
- Most CRDs are blind deposits; only rarely with surface exposure. To drill such deposits, deeply and blindly, takes more time and money than to drill into outcrops to quickly chase them with the drill all the way to the source a bit deeper (hopefully not too deep for mining reasons): A magmatic intrusion, typically in form of a porphyry, either barren or enriched with copper, gold and/or molybdenum.
- All CRDs are driven by an intrusion (e.g. porphyry). If you can find both deposit types on your property at mineable depths with economic grades, you are off to the races for the world‘s most attractive deposits.
CRDs (Carbonate Replacement Deposits) differ from other polymetallic deposit-types such as VMS (Volcanogenic Massive Sulfides) and MVT (Mississippi-Valley-Type):
CRDs offer much larger tonnage potential than VMS and MVT deposits while at the same time typically hosting higher grades of silver, zinc, lead, and copper, sometimes with appreciable credits of gold and other metals.
Tonnage
CRD:
10-150 million t
VMS: (811 deposits, 1999)
0.3-33 million t with 13 deposits >100 million t
MVT: (30 largest deposits, 2009)
10-128 million t (average: 38 million t)
Grade
CRD:
Zinc: 3-25%
Lead: 3-25%
Copper: 0.2-5%
Silver: 150-1,500 g/t
Au, Cd, Ge, In, W, Mo, PGE credits
VMS: (811 deposits, 1999)
Zinc: 0.3-6%
Lead: 0.1-2.9%
Copper: 0.9-4.8%
Silver: 5-238 g/t
Gold: 0.8-3 g/t
MVT: (30 largest deposits, 2009)
Zinc: 1-14%
Lead: 0.3-13%
Copper: 0-0.4%
Silver: 0-161 g/t
CRDs offer much larger tonnage potential than VMS and MVT deposits while at the same time typically hosting higher grades of silver, zinc, lead, and copper, sometimes with appreciable credits of gold and other metals.
CRDs are responsible for roughly 40% of Mexico’s 10 billion ounce historic silver production and are characterized by massive to semi-massive silver-lead-zinc-sulphide replacements of limestone. CRDs occur along major regional structures that hosts several of the largest CRDs in Mexico: “The Carbonate Replacement Deposit (CRD) model evolved from Dr. Peter Megaw’s PhD studies at Santa Eulalia: repeatedly validated worldwide…The Santa Eulalia District ranks as one of Mexico’s chief silver and base metal producers, and its largest CRD. Historic district production (1703-2020) amounts to 51 Mt of ore at average grades of 310 g/t Ag, 8.2% Pb, 7.1% Zn, yielding a total of 500 Moz Ag, 4 Mt Pb and 3.6 Mt Zn.“ (Source)
“CRDs are the second largest contributor to the historic silver production of Mexico. CRDs are the backbone of Mexico’s world-class underground lead-zinc mining industry. The country contains many Ag-Pb-Zn (Cu, Au) CRDs, which occur along the intersection of the Mexican Thrust Belt and Sierra Madre Occidental Magmatic Belt. The biggest CRD deposits appear to lie along inferred deep crustal structures.“ (Source)
While VMS deposits oftentimes host metallurgically complex ores, CRDs and MVTs are rather uncomplicated metallurgically. The total average operating costs (for mining, milling and processing) are generally lower for CRDs and MVTs than for VMS and Sedex-type deposits or even vein-type deposits. (Source)
Moreover, CRDs typically form as a result of a near-by porphyry intrusion, thus offering potential to add large tonnage from mining such deposit aside from CRDs. These days, most of such projects worldwide are facing the challenge of many companies controlling small portions of a CRD-porphyry system, oftentimes under option agreements and/or with underlying royalty liabilities (NSRs; Net Smelter Royalties; somewhat unattractive for majors). This makes it difficult for majors to acquire the full extent of such large CRD-porphyry systems to better understand the regional geology and structure of the complex with district-scale exploration programs. With Blue, Core Assets owns 100% of a very large, district-scale property (1083 km2), royalty-free. Once the geology and structure of the CRD showings at surface are better delineated with drilling, this potential CRD mineralization (chimneys, mantos, and/or skarns) is targeted to lead to the source – possibly a large and well-mineralized copper porphyry enriched with gold or molybdenum.
CRDs are polymetallic replacement deposits (also known as high-temperature carbonate-hosted silver-lead-zinc deposits) formed by the replacement of sedimentary (usually carbonate rock) by metal-bearing solutions in the vicinity of igneous intrusions (e.g. copper or molybdenum porphyries).
When the CRD orebody of metallic minerals forms a blanket-like body along the bedding plane of the rock, it is commonly called a manto ore deposit. Other ore geometries are chimneys and veins.
The mineralogy changes with distance from the intrusion: Closest to the intrusion is the copper-gold zone; next is the lead-silver zone, then the zinc-manganese zone. In many instances, CRDs can be considered as the distal part of a continuum with skarn deposits, and in some cases terminology may be misused.
Although similar in orebody geometry, host-rock lithology, and the presence of lead and zinc, MVTs (Mississippi Valley Type; also known as carbonate-hosted lead-zinc deposits are considered a different type of ore deposits. MVTs oftentimes lack silver and gold mineralization, are lower temperature, and are not associated with nearby igneous intrusions.
Hermosa
One of most recent major CRD transactions was the Hermosa CRD Project (formerly Taylor-Hermosa / Hardshell Project).
On June 17, 2018, for an All-Cash-Offer of $1.8 billion CAD, ASX-listed South32 Ltd. took over TSX-listed Arizona Minerals Inc. (offer price of $6.20 CAD per share represented a 50% premium to previous day closing).
“Our all cash offer for Arizona Mining will allow us to optimise the design and development of one of the most exciting base metal projects in the industry. We have been a major shareholder in Arizona Mining since May 2017 and an active participant in the Hermosa Project with representation on the operations committee and a nominee on the board of directors. Our deep understanding of this high grade resource and surrounding tenement package, and extensive experience at Cannington, makes us the natural owner of this project and ensures we are well positioned to bring it to development, delivering significant value to our shareholders.”
According to South32 (2022):
“Our work at Hermosa is part of our strategy to reshape our portfolio and increase exposure to the base metals critical for a low carbon future. Zinc is used in renewable energy infrastructure and to protect against corrosion, while silver is used in solar panels and electric vehicles.“
“Hermosa comprises the zinc-lead-silver Taylor sulphide deposit (Taylor Deposit), the zinc-manganese-silver Clark oxide deposit (Clark Deposit) and an extensive, highly prospective land package with the potential for further polymetallic and copper mineralisation.“
“We have completed a Pre-feasibility Study (PFS) for the Taylor Deposit, our first development option at Hermosa. It is a large, carbonate replacement massive sulphide deposit with a Mineral Resource estimate of 138Mt, averaging 3.82% zinc, 4.25% lead and 81 g/t silver. The deposit remains open at depth and laterally, offering further exploration potential. We are designing the Taylor Deposit to be our first ‘next generation mine’, using automation and technology to minimise our impact on the environment and to target a carbon neutral operation, in line with our goal of net zero operational carbon emissions by 2050.“
“A scoping study for the spatially linked Clark Deposit has confirmed the potential for a separate, integrated underground mining operation producing battery-grade manganese, as well as zinc and silver. The Clark Deposit has a reported resource of 55 million tonnes, averaging 2.31% zinc, 9.08% manganese and 78g/t silver.“
Peñasquito
Newmont-Goldcorp (NYSE: NEM) owns 100% of the Peñasquito Gold-Silver-Zinc-Lead Deposit, Mexico‘s largest gold mine and second largest silver mine as well as one of the country‘s largest producers of zinc and lead: “Two diatreme pipes, Peñasco and Brecha Azul, are the principal hosts for gold-silver-zinc-lead mineralization at Peñasquito. The pipes flare upward and are filled with breccia clasts in a milled matrix of similar lithological composition. The diatremes are surrounded by coalesced halos of lower grade, disseminated sphalerite, galena, and sulphosalts containing gold and silver. Garnet skarn hosted polymetallic mineralization has been identified at depth between the Peñasco and Brecha Azul diatremes. The skarn has horizontal dimensions of approximately 1,000 metres by 1,200 metres and is open at depth.“ (Source)
Newmont-Goldcorp‘s Peñasquito Gold-Silver-Zinc-Lead Mine produced 272,000 ounces of gold in 2018, accounting for about 17% of the combined companies‘ net asset value, according to Scotiabank. Peñasquito is the world‘s 5th largest silver mine (2nd largest in Mexico) and is Mexico‘s largest open-pit operation, comprising of 2 pits (Peñasco and Chile Colorado), expected to produce 1.6 million GEOs (Gold-Equivalent-Ounces) annually between 2020-2024 (2020E-GEO-breakdown: 37% gold, 28% zinc, 25% silver, 10% lead).
Proven & Probable Reserves (2020):
“A major expansion was undertaken at the mine with pre-stripping starting in August 2007. The expansion was completed and officially inaugurated in March 2010. Full production was achieved by the end of 2011. The expansion included the construction of two separate processing facilities, an oxide ore facility and a sulphide ore facility. The oxide plant commenced production in February 2008 and first gold was poured in May 2008... The project consists of a leach facility, and is capable of processing 25,000t/day of oxide ore and 65,000t/day of sulphide ore.“
Platosa
Excellon Resources Inc. (TSX: EXN) aquired the Platosa Silver-Zinc-Lead Deposit (Durango, Mexico) in 1996; the property now encompasses 110 km2 of prospective land. Platosa is Mexico’s highest-grade silver producer with resources of 1,000 g/t AgEq; Over 800,000 t mined since underground production commenced in 2005; Most of the mining occurred from flat-lying massive sulphide bodies (mantos); Ore produced from the mine is crushed on site and transported 200 km south for processing at the Miguel Auza Mill; Post-pandemic suspension; Indicated resources: 317,000 t averaging 485 g/t Ag, 5.3% Pb and 5.5% Zn (contained: 5 Moz Ag, 37 MLbs Pb, 39 MLbs Zn).
“The Platosa Mine exploits a series of typical, though very high-rade, massive sulphide, distal CRD silver, lead, zinc manto deposits located strategically in the middle of the prolific Mexican CRD Belt. Diamond drilling results in 2013 and 2014 continued to confirm that the Platosa property holds considerable potential for the discovery of additional high-grade manto mineralization and for the discovery of large tonnage, though lower grade, proximal skarn mineralization. CRDs are epigenetic, intrusion-related, high-temperature, sulphide-dominant, lead-zinc-silver-copper-gold-rich deposits that commonly occur in clusters associated with major regional geologic features. The Mexican CRD Belt is perhaps the world‘s best developed CRD cluster with typical deposit sizes within the belt ranging from 10-15 million tonnes.“
Silvertip
As the crow flies only ~180 km east from Atlin, BC, you need to travel ~370 km by car to reach the Silvertip Mine. Soon after the initial discovery in 1955, a 26 km long access road (gravel) was built to reach one of the world‘s highest grade CRD projects. The long history of exploration shows how difficult it was in the past to find this buried (blind) deposit at vertical depths ranging from 30 m to 475 m, whereas “the resources identified to date likely represent the distal portions of the CRD system and that the higher-grade feeder ‘chimneys’ and the proximal copper-gold skarn portions of the system have not been found,“ according to a 2019-Technical-Report.
In 2017, Chicago-based Coeur Mining Inc. (NYSE: CDE) paid $250 million USD to acquire privately owned JDS Silver Holdings Ltd., which owned and developed Silvertip. For this price, Coeur acquired a newly built mine with 2.35 million t @ 352 g/t silver, 9.4% zinc and 6.7% lead (Indicated).
“While this newly constructed operation came with a rich resource base and an operation that Coeur has ramped up to commercial production, a technical report and mine plan to elevate the high-grade silver resources to reserves had yet to be completed for Silvertip. With the information gleaned from a rigorous drill program carried out over the 14 months since its acquisition, Coeur has converted some 58 percent of the resources that came with Silvertip into enough reserves to support the mine for about 4.5 years [with a 1,000 t/day mill on-site]… Over that initial span, the Silvertip Mine is expected to average around 2.9 million oz of silver, 45 million lb of zinc and 37 million lb lead, or roughly 8.2 million oz silver-equivalent, per year. This would put Silvertip on par with the second highest producing operation in Coeur‘s portfolio, the Rochester silver-gold mine in Nevada.“ (Source, 2019)
According to Coeur‘s current ”Silvertip” project website: “The Silvertip silver-zinc-lead mine is an underground operation… Silvertip is one of the highest-grade silver-zinc-lead operations in the world and sits within a highly prospective land package… Coeur temporarily suspended mining and processing activities at Silvertip in early 2020… [but] has since conducted additional technical work and exploration activities to evaluate and support a potential expansion and restart of the operation.“
Reserves (Proven & Probable, 2021):
1.8 million t @ 252 g/t silver, 8.21% zinc and 5.36% lead, containing:
- 14.6 million ounces of silver [in-situ-value today: $280 million USD]
- 296 million pounds of zinc [in-situ-value today: $379 million USD]
- 193 million pounds of lead [in-situ-value today: $164 million USD]
Resources (Measured & Indicated, 2021):
2.8 million t @ 291 g/t silver, 10.46% zinc and 5.55% lead, containing:
- 26.3 million ounces of silver [in-situ-value today: $505 million USD]
- 589 million pounds of zinc [in-situ-value today: $754 million USD]
- 313 million pounds of lead [in-situ-value today: $266 million USD]
Resources (Inferred, 2021):
2.4 million t @ 236 g/t silver, 8.98% zinc and 4.27% lead, containing:
- 17.8 million ounces of silver [in-situ-value today: $342 million USD]
- 422 million pounds of zinc [in-situ-value today: $540 million USD]
- 201 million pounds of lead [in-situ-value today: $171 million USD]
With a total tonnage (reserves plus resources) of around 7 million t, the contained metals have an in-situ-value of $3.4 billion USD at market prices as of November 2022.
Waterpump Creek
As a more recent example, the Waterpump Creek CRD in Alaska gained much attention in 2022, when Western Alaska Minerals Corp. announced 102.7 m @ 159 g/t silver, 5.4% zinc and 5.3% lead after 147 m of core length, including 7.2 m @ 349 g/t silver, 7.3% zinc and 9.7% lead after 159 m of core length (gross metal value: $603 USD/t).
As above image shows, hole WP-84-029 was drilled vertically in 1984, yielding a mineralized thickness of 1.4 m, however widths increase considerably at greater depths to the south.
In 2021, Western Alaska drill-tested Waterpump Creek for the first time since 1984: Hole WPC21-09 intersected 10.5 m @ 522 g/t silver, 22.5% zinc and 14.5% lead after 109 m of core length (gross metal value: $1,229 USD/t; silver equivalent grade: 1,991 g/t).
As the figures above and below indicate, CRD mineralization typically widens at depth.
Stardust
NorthWest Copper Corp. (TSX: NWST) owns 100% of the Stardust Copper-Gold-Silver-Zinc Deposit (northern BC, Canada) with no royalties or other underlying encumbrances: “BC’s newest, high-grade copper gold deposit and one of the highest-grade copper-gold deposits in BC… Stardust is a high-grade copper-gold deposit, hosted within a large, mineralized system with significant opportunity for growth. The property features a 2.2-kilometre corridor of mineralization including four mineralization styles typical of a Carbonite Replacement Deposit (CRD) system: porphyry, skarn, manto and epithermal vein. Stardust is one of the few CRD systems in the world with all CRD components fully intact. Having the ability to view the overall structure of the system, provides us the opportunity to better predict how to most efficiently and successfully explore it.“
Indicated Resource (2021):
2 Mt @ 1.31% Cu, 1.44 g/t Au, 27 g/t Ag (2.59% CuEq)
Inferred Resource (2021):
5.8 Mt @ 0.86% Cu, 1.17 g/t Au, 20 g/t Ag (1.88% CuEq)
“Carbonate Replacement Deposits (CRD) are bodies of metallic minerals formed by the reaction of hot magmatic fluids with carbonate rocks, such as limestone or dolomite. The hot, metal-bearing fluids are driven upward from the heat source, an igneous intrusion (Glover Mo-Cu Porphyry) and dissolves the reactive carbonate rocks when they encounter them. That reaction changes the chemistry of the fluid, causing it to precipitate metallic sulphide minerals that often contain copper, zinc, lead and significant precious metals such as gold and silver. The metallic sulphide minerals effectively “replace” the carbonate host creating massive sulfide mineralization. Nearer to the source of the fluids, mineralization is often in the form of copper - gold skarn (The 421 and overlying Canyon Creek Zones), transitioning to zinc - lead - silver mantos (4b Manto Zone; No.3 Manto Zone; No.2 Manto Zone) and chimneys. In the periphery of the system, the cooler fluids drop out precious metals silver and gold veins (No.1 Vein Zone Ag-Au ±Zn±Cu) form in the edges of the system.“
Sun Metals‘ exploration model:
“Carbonate Replacement Deposits (CRD) are bodies of metallic minerals formed by the reaction of hot magmatic fluids with carbonate rocks, such as limestone or dolomite. The hot, metal-bearing fluids are driven upward from the heat source, an igneous intrusion (Glover Mo-Cu Porphyry), and dissolves the reactive carbonate rocks when they encounter them. That reaction changes the chemistry of the fluid, causing it to precipitate metallic sulphide minerals that often contain lead, zinc, copper and precious metals such as gold and silver. The metallic sulphide minerals effectively “replace” the carbonate host as the reactive fluids dissolve a pathway through it.
Nearer to the source of the fluids, mineralization is often in the form of copper – gold skarn (Canyon Creek Skarn Deposit), transitioning to zinc – lead – silver mantos (4b Manto Zone; No.3 Manto Zone; No.2 Manto Zone) and chimneys. In the periphery of the system, the cooler fluids drop out precious metals silver and gold veins (No.1 Vein Zone Ag-Au ±Zn±Cu) form in the edges of the system. The fluid path is always continuous, and therefore the metallic bodies are too.
Stardust is a large mineralized system with true district potential. It features a 2.2-kilometre corridor of mineralization including four mineralization styles typical of a CRD system: porphyry, skarn, manto and epithermal vein. Stardust is one of the few CRD systems in the world with all CRD components fully intact.“
In March 2021, Serengeti Resources Inc. announced the completion of a merger with Sun Metals Corp. (operator of Stardust) and changed its name to NorthWest Copper Corp. Two years earlier, Sun Metals‘ CEO Steve Robertson said in an interview with The Northern Miner:
“We hit a home run with our first at-bat. Hole 421 was clearly head and shoulders above any intercept that has ever been seen on the property before, and quite frankly, we’re viewing this as a disruptive discovery.” The company reported assays for hole 421 in November 2018. The hole cut 100 metres grading 2.51% copper, 3.03 grams gold and 52.5 grams silver from 517 metres downhole. The company says that hole indicates a possible extension of its Canyon Creek Skarn zone. “I don’t think anyone was expecting to find the highest-grade gold mine in the world at Eskay Creek when they found it in northern British Columbia. Nobody was expecting that Chuck Fipke was going to find a diamond mine when he was scoring for diamonds in the Northwest Territories, and here we are with 100 metres of 5% copper equivalent in a carbonate-replacement system in northern B.C., where nobody would have expected that you’d actually find one,” Robertson says. “It changes our thoughts about what the potential is. We’re thinking of a much bigger scale system now, and I really believe that this is going to change the way that people think about carbonate-replacement systems in British Columbia, as well.” The company says the intercept may indicate a “major mineralized pathway” that leads to the heart of a high-grade system.“
Cinco de Mayo
Many CRDs globally are buried (blind) deposits and occur at significant depths, which makes a discovery a lengthy and costly venture. For example, it took 15 years of prospecting and research to finally make the Cinco de Mayo CRD Project in Mexico a high-priority drill target for MAG Silver Corp.
In early 2007, MAG Silver announced assays from the first 9 holes at Cinco de Mayo, with the best one yielding 1.88 m @ 190.9 g/t silver, 7.9% zinc and 4.3% lead after 355 m of core length. At metals prices as of November 2022, this hole has a gross metal value of $421 USD/t.
A year later in 2008, MAG Silver announced the discovery of the José Manto in hole #20: 6.8 m @ 254 g/t silver, 7% zinc and 6.4% lead after 471 m of core length (gross metal value: $435 USD/t).
It was not until 2012, when MAG Silver announced the discovery of the Pegaso Zone: 61.6 m @ 89 g/t silver, 0.78 g/t gold, 0.13% copper, 7.3% zinc and 2.1% lead after 928 m of core length. Before this deep discovery was made, an inferred resource was published: At 2022’s metal prices with an in-situ-value of $4.1 billion USD, contained in just 12.5 million t of rock.
Cinco de Mayo: A World-Class CRD at Depth without Surface Access
“Small scale mining took place in the Property area in at least twelve locations sometime prior to the 1990s. In the mid-1990s, an affiliate of Industrias Peñoles S.A. de C.V. (Peñoles) drilled six reverse circulation holes for a total of 1,368 m to test several silicified zones. In 1992, the area was visited by Peter Megaw on behalf of Teck Corporation (Teck) as part of a reconnaissance program in Chihuahua State carried out from 1991 to 1994. Megaw determined that the area exhibited characteristics favourable for large CRDs. Teck’s field work included reconnaissance mapping and detailed sampling of the jasperoid veins along Cinco de Mayo Ridge. Teck transferred the Property to Cascabel in early 2000 with no retained interest. Cascabel continued to stake claims until 2003. In 2004, MAG optioned the ground from Cascabel.“ (Source)
The discovery of the buried José Manto on the Cinco de Mayo Property in Mexico was not an overnight success. It was only after 15 years of research and systematic exploration when it emerged as a high-priority target for MAG Silver Corp. in 2007.
- In February 2007, assays from the first 9 holes were announced: 1.88 m @ 190.9 g/t silver, 7.9% zinc and 4.3% lead (starting after 355 m of core length)
- In February 2008, assays from discovery hole #20 at José Manto were announced: 6.8 m @ 254 g/t silver, 7% zinc and 6.4% lead (after 471 m of core length)
- In July 2008, more assays were announced (drilled near discovery hole): 3.46 m @ 612 g/t silver, 13.2% zinc and 11.59% lead (after 520 m of core length, which is at a vertical depth of ~400 m)
- In September 2008, more assays with significant gold credits were announced: 1.5 m @ 370 g/t silver, 6.89 g/t gold, 9.46% zinc and 19.15% lead (after 750 m of core length)
- In December 2008, more assays were announced: 5.57 m @ 487 g/t silver, 16.46% zinc and 8.97% lead (after 460 m of core length)
- In January 2009, more assays were announced: 2.72 m @ 462 g/t silver, 13.62% zinc and 10.06% lead (after 485 m of core length)
Despite the extensive alluvial cover, the 2007/2008-drilling program intersected significant mineralization within a laterally traceable, low-angle structural host over an area of more than 8 km2.
- In September 2009, first assays from drilling at Pozo Seco (4 km southwest of the José Manto discovery) were announced: 75 m @ 0.307% molybdenum (beginning near surface), including 34.5 m @ 0.637% molybdenum and 0.467 g/t gold (after 47 m of core length)
“The Pozo Seco moly-gold results may be a strong indicator that a causitive intrusive [porphyry] center is located nearby.“ (MAG Silver in September 2009)
- In November 2009, more assays from the Pozo Seco moly-gold discovery were announced: 57.34 m @ 0.35% molybdenum and 0.73 g/t gold (after 19 m of core length)
- In July 2012, first assays from newly discovered, deep Pegaso Zone were announced: 61.6 m @ 89 g/t silver, 0.78 g/t gold, 0.13% copper, 7.3% zinc and 2.1% lead (after 928 m of core length)
“[This hole at the Pegaso Zone] has cut by far the strongest mineralization yet seen at Cinco de Mayo. The size and geological characteristics we see are the kind of major mineralization centre/source we have long expected at Cinco de Mayo, and it is open in all directions. This is a real victory for our systematic exploration methodology and we are very pleased to deliver another discovery of this magnitude to our shareholders, this time 100% owned.“ (MAG Silver in July 2012)
MAG Silver started aggressive drilling programs at Cinco de Mayo in 2007, which continued until September 2012. In November 2012, MAG Silver announced that “landholders from the local community of Ejido Benito Juarez (the “Ejido“) voted […] to expel MAG from its Cinco de Mayo property and establish a 100 year mining moratorium over the area.“
According to MAG Silver‘s current “Cinco de Mayo“ project website: “As MAG Silver has been unable to negotiate a renewed surface access agreement with the local Ejido controlling the surface access to key portions of the property, a full impairment was recognized in the year ended December 31, 2016. MAG Silver continues to believe that the Cinco de Mayo Property has significant geological potential and will continue to maintain these mineral concessions in good standing.“
Excerpts from MAG Silver‘s Annual Information Form (2016):
- MAG has made a major carbonate replacement deposit (CRD) discovery at its 100% owned Cinco de Mayo Property in northern Chihuahua, Mexico.
- The Upper Manto Pb-Zn-Ag (Au) deposit, formerly known as the Jose Manto-Bridge Zone deposit, consists of two parallel and overlapping manto deposits referred to as the Jose Manto and the Bridge Zone. The Property also hosts the Pozo Seco Mo-Au deposit […] the two deposits host distinctly different mineralization with different commodities, are separated by four kilometres and small mountain range, will potentially be mined by different methods, underground for Upper Manto and open pit for Pozo Seco, and have no significant synergies between them.
- A total of six zones were modelled, with the largest, Manto M10, extending from the northwest part of the Jose Manto deposit to partway into the Bridge Zone and measuring 2,500 m long by 350 m down dip by 1.5 m to 9 m thick. The zones extend from surface to a depth of 950 m below surface
Upper Manto resource (Inferred, 2012):
12.45 million t @ 132 g/t silver, 0.24 g/t gold, 6.47% zinc and 2.86% lead, containing:
- 52.7 million ounces of silver [in-situ-value today: $1 billion USD]
- 96,000 ounces of gold [in-situ-value today: $158 million USD]
- 1.77 billion pounds of zinc [in-situ-value today: $2.3 billion USD]
- 785 million pounds of lead [in-situ-value today as of November 2022: $667 million USD]
The 61.6 m of massive sulphide intercept, known as the Pegaso Zone located deeper in hole CM-12-431, was not included in this resource estimate. That intercept indicates excellent potential for a much larger resource at depth, however, additional drilling is required to establish the geometry of the Pegaso Zone.
Excerpts from “Cinco de Mayo: A new silver, lead, and zinc discovery in northern Mexico“ (2009):
- CRD deposits account for roughly 4 billion ounces or 40% of the 10 billion total silver ounces produced in Mexico. They are second only to Mexico’s epithermal veins in historic silver production.
- Major CRDs average 10-13 million tonnes with the largest exceeding 50 million tonnes of high-grade ore. Grades typically range from 60-600 g/t silver, 2-12% lead, 2-18% zinc, with trace copper and gold. CRD ore bodies are generally straightforward metallurgically, amenable to low-cost underground mining methods, and their environmental footprint is minimal.
- The vast majority of CRDs found to date outcropped and were followed from surface to depth. Such opportunities are now mostly exhausted. [As such,] CRDs are difficult exploration targets… Focusing the search on areas where known CRDs have adequate size and grade potential and then applying modern and appropriate exploration techniques is essential.
High-Temperature carbonate replacement ores in the Mexican Altiplano are characterized by metals-rich Tertiary intrusive bodies intruding structurally prepared carbonate host rocks, either limestone or dolomite... Mantos, chimneys and pods of massive sulphides are the three principal morphological types and are generally remote from intrusive centers. CRD´s are large systems ranging from 25-100M tonnes of high-grade ores and are significant contributors of silver lead and zinc. To date, ten CRD in the Altiplano have published resources (all inclusive) for over 1.7M Oz Au, 300M Oz Ag, 3.1B Lb Cu, 147M Lb Mo, 2.4B Lb Pb and 9.6B Lb Zn. First Majestic’s La Encantada has the largest silver resource with over 92.5M oz Ag at a grade of 213 g/t Ag, while La Platosa from Excellon Resources boasts the higher grade, with 12.3 M oz Ag at a grade of 786 g/t Ag. Other significant properties are Tayahua owned by Frisco with resources of over 970K Oz Au, 57M Oz Ag, 2.7B Lb Cu, and 1.4B Lb Zn at grades of 0.26g/t Au, 15.1 g/t Ag, 1% Cu, and 0.5% Zn, and MAG Silver´s Cinco de Mayo with resources of over 96K oz Au, 52.7M Oz Ag, 785M Lb Pb and 1.7B Lb Zn, at grades of 0.24 g/t Au, 132 g/t Ag, 2.86% Pb and 6.47% Zn. Pozo Seco (part of Cinco de Mayo property), also owned by MAG Silver, is the only property containing moly resources for over 147.2 M Lb at a grade of 0.13% Mo. The table [above] shows tonnages, grades, as well as gold, silver, copper, moly, lead, and zinc resources (all inclusive) for each of these deposits.“
“The Santa Eulalia Mining District is the largest of a number of important Ag-Pb-Zn-Cu-Au Carbonate Replacement Deposits that occur along the intersection of the Laramide-aged Mexican Thrust Belt and the Tertiary volcanic plateau of the Sierra Madre Occidental. Santa Eulalia and comparable districts form a spectrum ranging from stock contact skarns, through dike and sill contact skarns, and massive sulfides, to massive sulfide chimneys and mantos. The entire spectrum may be manifested on a district scale in highly elongated systems, or a significant portion of the spectrum may be displayed by single orebodies in highly telescoped systems. Santa Eulalia is a highly elongated system in which the distal (mantos to dike and sill contact skarns) parts of the spectrum have been encountered and exploited. The proximal, stock-related portions of the spectrum have never been found...“
Dr. Peter Megaw is the co-founder of Minera Cascabel and MAG Silver. His Ph.D. work was an exploration-focused geological study of the Santa Eulalia Ag-Pb-Zn District in Mexico and CRDs in general. He has published extensively on CRDs in both geological and mineralogical journals and books. Dr. Megaw and his team are credited with the significant discoveries at Platosa, Durango (Excellon Resources), Juanicipio-Fresnillo, Zacatecas (JV between Fresnillo and MAG Silver), Pozo-Seco/Cinco de May (MAG Silver).
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More InformationThe CRD Exploration Model evolved from Dr. Peter Megaw‘s PhD studies at the Santa Eulalia CRD Mining District – the model has been repeatedly validated worldwide as it has been the basis for a number of CRD discoveries in North America in the past decade, including Arizona Mining’s Taylor Deposit, MAG Silver’s Cinco de Mayo amongst others. Using this model, Dr. Megaw has an exceptional track record of CRD discoveries. With 35 years of involvement in Mexican geology, he has become well-known in the Mexican mining industry and is a frequent speaker at international academic and technical symposia. His Ph.D. work at the University of Arizona was an exploration-focused geological/ geochemical study of the Santa Eulalia Ag-Pb-Zn District, Chihuahua and Carbonate Replacement Deposits (CRDs) in general. He has published extensively on CRDs in both geological and mineralogical journals and books. Peter’s primary exploration focus are CRDs and Epithermal Vein Deposits and Porphyry Copper Deposits. He and his team are credited with the significant discoveries at Platosa, Durango; Juanicipio-Fresnillo, Zacatecas; and Cinco de Mayo, Chihuahua. Peter was awarded the Society of Mining Engineers 2012 Robert M. Dreyer Award for excellence in Applied Economic Geology. Peter has been a passionate mineral collector since first setting foot in Santa Eulalia in 1977. He moved to Tucson in 1979 and quickly joined the Tucson Gem and Mineral Society. His mineral collecting has come to focus almost exclusively on Minerals of Mexico. He won the Desautels Trophy in 2006 for the Best Case of Minerals at the Tucson Show for a case of Mexican Minerals. He has also three times been awarded the Romero Trophy for the best Mexican specimen on exhibit in the Tucson Show. His mineralogical writing has also been rewarded: he was the first recipient of the FM Student Paper Award in 1985 with his paper on the mineralogy of the East Camp of Santa Eulalia; in 2003 he was co-recipient with Tom Moore of FM’s Best Paper in Mineralogical Record award for their article on the Ojuela Mine, Mapimi, Durango Mexico; and he received the 2010 FM Best Paper in extraLapis award for his article on “The Geology and Genesis of (most) Collectible Smithsonite.” He has written for Mineral News since its inception and is a consulting editor and occasional author for Rocks and Minerals. He is photo-moderator for submissions on Mexico to Mindat and co-moderator of the FMF Mineral Forum, often with tongue planted firmly in cheek. A combination of the above led him to be awarded the Carnegie Mineralogical Award for 2009.
Megaw-Publications: www.researchgate.net/profile/Peter-Megaw
Stephan Bogner
Contact
Rockstone News & Research
Stephan Bogner (Dipl. Kfm., FH)
Müligässli 1, 8598 Bottighofen
Switzerland
Phone: +41-71-5896911
Email: info@rockstone-news.com
Disclaimer: This article is for informational purposes only and does not constitute investment advice or a recommendation to buy or sell commodities. The author holds physical gold and silver, stored in Central Switzerland through Elementum International AG. The author does not hold any direct interests or financial instruments related to other commodities or companies mentioned in this article. All views and forecasts reflect the state of knowledge at the time of publication and are subject to change. There is no guarantee that future developments will unfold as described. Investing in commodities involves risks. Consultation with a licensed financial advisor is strongly recommended.
