Plomosas Test Mining Targets 1 to 2 Years Off San Marcial's Path to Cash Flow

• Plomosas, the former mine about 5 kilometers from San Marcial, holds its major permits, and Mexico's environmental regulator ruled in May 2026 that no new Environmental Impact Authorization is required.
• Bulk sample test mining can draw on 21 accessible underground areas at a mine that produced more than 2.5 million tonnes of ore between 1986 and 2000.
• Management prefers toll milling or minegate sales over a pilot plant that would cost roughly $10 million, citing lower capital, a shorter timeline, and lower execution risk.
• The program is sized to the 60 to 100 tons per day that local power supports, against a historical permitted rate of 600 tons per day.
• Management states that cutting 1 to 2 years from San Marcial's timeline would lift its net present value, with a resource update and preliminary economic assessment targeted for the first half of 2027.

Plomosas Holds the Permits for Test Mining

GR Silver Mining Ltd. (TSXV: GRSL | OTCQX: GRSLF | FRA: GPE) owns 100% of the Plomosas Project in Mexico, which includes the San Marcial silver discovery and the former Plomosas underground mine, about 5 kilometers north of San Marcial. The mine holds the permits and underground access for a bulk sample test mining (BSTM) program that the company is now studying.

The company describes Plomosas as a past producer with all major permits in place. In May 2026, Mexico's environmental regulator, SEMARNAT, confirmed that no new Environmental Impact Authorization (EIA) is required, so the existing approvals cover the work GR Silver has in mind.

IMMSA, a Grupo Mexico subsidiary, mined more than 2.5 million tonnes of ore at Plomosas between 1986 and 2000 using room-and-pillar methods, producing more than 67,000 tonnes of lead concentrate and 31,000 tonnes of zinc concentrate. The company reports the stopes are largely in excellent condition, with 21 underground areas accessible for BSTM and aimed at higher-grade silver zones. Engineering and metallurgical studies are advancing.

Toll Milling Targets Lower Capital & Execution Risk

Several mills near Plomosas have been assessed for processing material from the program, and the company is evaluating pilot plant options alongside toll milling. Selling material through a mill or at the mine gate changes the program's capital requirements and timeline.

President & Chief Executive Officer of GR Silver Mining, Eric Zaunscherb, described the economics of that route:

"The idea is that, rather than spending $10 million to buy a small pilot plant, we have a much lower capex and a much shorter timeline to the point where that material gets sold onward. And then, very importantly to us, is lower execution risk if we sell that material at minegate."

The mine was permitted to 600 tons per day, and available power points to 60 to 100 tons per day. Management says Plomosas has precious metals content alongside lead and zinc, and that production there points to lead and zinc concentrates.

Social License Connects Plomosas to San Marcial

Management describes social license as the main value of the Plomosas program. The project involves 4 levels, from local cooperatives to the federal government.

The company has separate agreements with 2 local agricultural cooperatives, and the next levels are the municipality of Rosario, the state of Sinaloa, and the federal government. Management describes the municipality and the state as keen to receive mining revenue.

The company once employed more than 150 people locally, and management is considering a return to local hiring, along with clean water and training. Management says San Marcial would have an even larger community effect, and it hopes good corporate citizenship can take 1 to 2 years off San Marcial's path to cash flow.

San Marcial's Timeline Drives the Value Case

San Marcial holds 46 million ounces of indicated and 14 million ounces of inferred silver in the 2023 resource. Management puts the standalone project 5 to 7 years from any potential cash flow if the plan proceeds as intended.

Zaunscherb tied a shorter timeline at San Marcial to net present value (NPV):

"The much bigger prize is San Marcial. That is the future of our company. And so if we can cut the timeline down by a year or two at San Marcial, that has a massive impact on the NPV."

An access tunnel at San Marcial would serve as a drill station and bulk sample source for metallurgical studies. If management applies for the permit at year-end, it would be looking at underground drilling in late 2028. At Plomosas, BSTM and pilot plant engineering are 2 of the company's 5 planned catalysts, and the company is targeting a resource update and a preliminary economic assessment (PEA) for the first half of 2027.

The Investment Thesis for GR Silver Mining

• Plomosas holds its major permits, and Mexico's environmental regulator confirmed that no new Environmental Impact Authorization is required, allowing GR Silver to test bulk-sample mining under existing approvals.
• Toll milling or minegate sales would let the company sell material onward without spending roughly $10 million on a pilot plant, and management says this route lowers capital, shortens the timeline, and reduces execution risk.
• The program is sized to the 60 to 100 tons per day that local power supports, and it targets a return to local employment that once exceeded 150 people.
• Management is using Plomosas to demonstrate good corporate citizenship ahead of San Marcial, where it describes the community effect as even larger.
• Management says cutting 1 to 2 years from San Marcial's timeline would have a large positive effect on net present value, versus a standalone path of 5 to 7 years to potential cash flow.
• The company holds 55 million ounces of indicated and 22 million ounces of inferred silver in its 2023 resource, and it is targeting a resource update and preliminary economic assessment for the first half of 2027.

Plomosas combines permits, existing underground infrastructure, and a low-capital route to a first material sale. Each step supports the shorter San Marcial timeline management is building.

TL;DR

GR Silver's Plomosas mine holds its major permits, with no new EIA required, and BSTM can access 21 underground areas. Management prefers toll milling or minegate sales to a roughly $10 million pilot plant and sizes the program to 60 to 100 tons per day. It states that cutting 1 to 2 years from San Marcial's 5 to 7-year path to potential cash flow would lift NPV, with a resource update and PEA targeted for the first half of 2027.

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