Silver Breccias at San Marcial Span 1 Kilometer, Still Open at Depth

• The 2026 step-out program has extended mineralized breccia at San Marcial at least 300 meters (m) southeast and 75 m deeper than the boundary of the 2023 resource estimate.
• The known system now extends about 1 kilometer (km) along strike, and grade shows no sign of falling off at the deepest levels drilled.
• One new intercept returned 16.1 m at 162 grams per metric ton (g/t) silver, inside a wider 42.35 m zone grading 95 g/t silver.
• Grade concentrates where northeast-southwest structures cross the breccias, and silver falls away outside those crossings.
• A total of 6,292 m has been drilled in 21 holes of a 20,000-m program.

What Has Happened

The latest round of step-out holes at the San Marcial Area of the Plomosas Project in Mexico has pushed the known limits of a silver-bearing breccia system in two directions at once. Results reported on September 8, 2026, extend mineralized breccia at least 300 meters (m) southeast of the boundary drawn by the 2023 resource estimate, and 75 m below it. 

GR Silver Mining (TSXV: GRSL | OTCQX: GRSLF | FRANKFURT: GPE) now describes the mineralized breccias as extending over roughly 1 kilometer (km) of strike. Two of the 6 holes reported returned the higher grades: 16.1 m at 162 grams per metric ton (g/t) silver starting 375.05 m downhole, and 11.7 m at 147 g/t silver starting 275.55 m downhole. Both are included within much wider, lower-grade zones: 42.35 m at 95 g/t silver and 29.8 m at 87 g/t silver.

The Extent of the Breccia System & What Controls Grade Within It

Two separate variables govern what the drill program is finding at San Marcial: how far the breccia extends, and where inside it the silver concentrates. A breccia is a rock that has been shattered into angular fragments and recemented, in this case by hot mineral-bearing water. At San Marcial, the silver-rich version of that rock forms at the contact between an upper volcanic sequence of Oligocene age and a lower volcano-sedimentary sequence of Jurassic age, and the newest holes show it developing inside the Jurassic rocks themselves, not only along the contact. Two rock units host it, and both record repeated episodes of hot-fluid circulation. That repetition has concentrated the high-grade silver mineralization into wide, continuous breccia zones that continue along strike and downward.  

Where the grade goes inside those zones follows a rule that the geological team can now state. High-grade zones occur where northeast-southwest structural trends cross the northwest-southeast and east-west breccias, and silver grades drop outside those crossings. The newest holes also confirm an inclined plunge toward the southeast. A hole drilled roughly 80 m above another returned a broad, weak 36.0 m interval at 15 g/t silver, which places the system nearer the surface at that point, while width and grade both increase with depth.

President and Chief Executive Officer of GR Silver Mining, Eric Zaunscherb, counts the separate pulses of mineral-bearing fluid visible inside a single breccia: 

"It's quite exciting untangling all of that, even within the breccia itself. Some breccia, we see four, if not five, episodes, pulses of mineralization."

Each of those episodes deposited metal into rock that earlier episodes had already broken and filled, so the wide zones grade unevenly along their width.

What Changed Since the July Results 

The September holes tested the same ground the company reported on in July, and every measure it reports against moved outward. In July, the confirmed extension beyond the 2023 boundary was at least 150 m to the southeast, with no depth extension attached. That figure is now at least 300 m, with an additional 75 m of downward extension. The strike length of about 1 km is new; in July, the company gave no strike figure.

Grade at the wider intervals held broadly steady across the two rounds. July's thickest reported interval was 21.9 m at 168 g/t silver with 1.41% zinc, including 6.15 m at 447 g/t silver. September's equivalent was 16.1 m at 162 g/t silver, including 0.7 m at 1,344 g/t silver. What changed is the amount of low-grade rock around those intervals, which is far wider in the September holes. 

The program behind the results also moved. In July the company had drilled 15 holes, with results in hand for 8 of them; by September, the count stood at 6,292 m drilled in 21 holes, with assays outstanding on 7. Six more holes were drilled between the two rounds, and the pending count did not decline, so the queue has kept pace with the rig rather than shortening. Two of those outstanding holes belong to the Parallel Breccia target, a separate structure near the resource area. Drilling there in July returned 2.85 m at 340 g/t silver and 0.32 g/t gold from 63.00 m depth, and no result from that target has been reported since.

Signs of a Deeper System Below the Breccias 

One line in the September results concerns the rock the drill has not yet reached. Toward the southeastern end of the area, the holes are cutting more dikes, sheets of once-molten rock injected into the sequence, and those dikes themselves carry silver. Silver and lead-zinc mineralization cuts across the dikes, which places the mineralizing fluids after the intrusive complex rather than before it. The company reads that geometry as evidence of a closer relationship between the silver and an intrusive body underneath, and as a possible change with depth from an epithermal system, one formed by hot water circulating near the surface, into a porphyry system, the deeper and larger style of deposit that forms around a cooling intrusion. Geophysics has already mapped a chargeability anomaly beneath the deposit, a zone where the rock holds an electrical charge as sulfide minerals do, and the company interprets it as an intrusive-related porphyry setting.   

Nothing in either round of results tested that idea directly. What the newest holes do show is that the intensity of alteration, the minerals present, and the structures logged all continue at the bottom of the drilled interval, with no sign that grade or intensity is falling off.

Zaunscherb ties that vertical reach to boiling zones, the levels in a hot-water system where the fluid flashes to steam and drops its metal load:

"These boiling zones, if you're talking about one phase, are often 300 m in a typical system. But when you start having multiple systems, then they can stack, and you can get a much larger vertical representation." 

Stacked boiling zones would explain a deposit that keeps returning grade well below what a single episode would have exhausted, and geochemical and structural studies are underway alongside the drilling to guide the next holes toward it. 

Broader Context

None of this has reached a resource figure yet, because the only estimate on the property predates every hole discussed here.

In the 2023 estimate, the San Marcial Area holds 46 million ounces of silver in the indicated category, from 10 million metric tons grading 148 g/t, and a further 14 million ounces inferred from 3 million metric tons grading 145 g/t. Across the whole Plomosas Project, the figures are 55 million ounces of silver indicated and 22 million ounces inferred. Every intercept from the 2026 program lies outside the ground those numbers cover.   

The project covers 7,823 hectares on the southwestern edge of the Sierra Madre Occidental, at the border of Durango and Sinaloa. The district holds epithermal silver and gold mineralization in hydrothermal breccias and veins, and it has been mined before. IMMSA, a Grupo Mexico subsidiary, took more than 2.5 million metric tons of ore from the Plomosas underground mine between 1986 and 2000. Existing mine infrastructure, road access, and valid permits came with that history, and a five-year drill permit covering the San Marcial Area and the Plomosas Mine was granted in September 2025.  

What to Watch Next 

Three things will decide how much of this reaches the next resource estimate. The first is the remaining drilling: most of the 20,000-m program is still to come. The pace has been slowed by the extra distance to the project from the company's base in Durango, intermittent security issues in Sinaloa and the surrounding area, and the wet season. GR Silver Mining has launched initiatives to re-accelerate, targeting its impact in the fourth quarter.   

The second is the resource update, now placed in the first half of 2027, with a preliminary economic assessment (PEA) planned for the same window. The update is where the 300 m of lateral extension and the 75 m of depth extension either turn into ounces or do not. Third comes the bulk-sample test mining program at the past-producing Plomosas mine, with 21 underground areas accessible and pilot-plant engineering advancing in parallel.   

Zaunscherb places the direct test of the porphyry idea late in the program:  

"And ultimately, as we get further into this program, we're going to be putting deeper holes into that intrusive body to see if that porphyry model has merit." 

In the meantime, the drilling has settled which structures the high grades track, and the company is targeting the next holes on that basis.

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