• Confirmed near-resource expansion potential: Four holes tested the previously undrilled up-dip extension of the DMEA massive-sulfide chimney and intersected intense skarn alteration with copper, zinc, and lead sulfides.

• Delivered strong initial assay results: Drilling returned broad silver-enriched intervals, including 22.47 meters grading 86 g/t silver, with higher-grade subintervals and associated gold, copper, lead, and zinc values.

• Identified new massive sulfide mineralization at the Kentucky target approximately 250m westward of resource area: Three step-out holes demonstrated approximately 80 meters of down-plunge continuity, intersected about 15 meters of intense gossan, and left the mineralized zone open at depth.

• Advanced district-scale discovery targets: Drilling at the Bay State to the northwest, and on private claims to the southeast along trend intersected strongly altered, pyrite-rich schist and marble with localized skarn, improving the geological model and supporting additional exploration along the Laxey Marble trend.

Boise, Idaho--(Newsfile Corp. - September 17, 2026) - Thunder Mountain Gold, Inc. (OTCQB: THMG) (TSXV: THM) ("Thunder Mountain Gold" or the "Company") is pleased to report successful results from its 2026 exploration program at the South Mountain Project in southern Idaho, where drilling has successfully advanced multiple high-priority targets across the Company's expanding silver-zinc-gold-copper-lead district.

The Company's 8,500-foot diamond core program was designed to expand the established carbonate replacement deposit (CRD) massive sulfide resource while also testing compelling district-scale copper-gold porphyry targets identified through integrated geological and geophysical work. To date, ten drill holes have intersected mineralogic alteration with associated iron, copper, zinc, and lead sulfide mineralization reinforcing the potential to expand the known CRD system which is strongly associated with the Laxey Marble host rock formation (Figure 1).

Four of these holes tested the up-dip extension of the DMEA massive sulfide "chimney" towards the surface where there was no previous drilling. The holes intercepted intense hedenbergite skarn alteration containing copper-sulfide mineralization with associated zinc and lead sulfides. Assays have been received from three of these holes confirming strong enrichment in silver and gold within the massive sulfides (Table 1). The intercepts represent the first drill tests between the surface and the Laxey drift - a distance of 100 - 150 meters.

Table 1. Initial Assay Results from 2026 Drilling

Step-out drilling of three holes at the Kentucky Ridge target documents approximately 80 meters of down-plunge continuity from a surface gossan grading 7.6 meters at 125 g/t silver, 1.2% lead, and 1.7% zinc in a channel sample. Visual review of the drill intercepts notes approximately 15 meters of intense gossan and a correlated, but deeper intersection of intensely silicified, pyrrhotite-sphalerite (Zn sulfide) mineralization in a second drillhole. The mineralized zone remains open at depth. Company geologists believe this target may represent a high-level expression of deeper massive sulfide mineralization. Assays are pending.

Three additional holes targeted the important Laxey Marble including one hole testing a historic induced-polarization geophysical anomaly further northwest of the massive sulfide resource area at the Bay State target (Figure 1). Two additional holes, at the southeast end of the trend, tested the Laxey Marble approximately 400 meters to the southeast of the resource area. These holes intersected intensely sericitic, pyrite-rich brecciated schist and marble with localized skarn alteration. Assays are pending.

Table 2. Drillhole Coordinates

"This program accomplishes our goal which is to demonstrate that South Mountain has tremendous near-resource expansion potential while opening the door to larger district-scale discovery opportunities. The breadth of alteration and sulfide mineralization encountered in these first drill holes is a significant technical success for the Company. With most assays pending, and while drilling continues, we believe the 2026 program has greatly improved our understanding of the size and scale of the resource," commented Eric T. Jones, President and Chief Executive Officer of Thunder Mountain Gold.

Big Grass Fire Update

The Company also reports that the Big Grass Fire, which originated in Oregon and moved through Owyhee County and the South Mountain District in late July, resulted in no damage to Company structures or equipment. Apart from temporary evacuation delays, the fire has not had a material impact on the 2026 exploration drilling campaign.

Figure 1. Geologic map of the South Mountain polymetallic massive sulfide deposit resource area showing 2026 drillholes. 

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South Mountain Positioned as a U.S. Critical and Precious Metals Growth Asset

The South Mountain Mine is a polymetallic development project containing high-grade silver, zinc, gold, copper, and lead mineralization on private land approximately 70 miles southwest of Boise, Idaho. The project includes a long history of mining, more than 8,000 feet of historic underground workings, and approximately 4,000 feet of rehabilitated MSHA-compliant underground development.

Thunder Mountain Gold has advanced the project since 2007 and invested approximately US$25 million to date. The Company's expanded land position now totals approximately 13,061 acres, or roughly twenty square miles, following the purchase of 113 acres of private land contiguous with the main historic mine area, the staking of 423 mineral lode claims on adjoining BLM lands totaling approximately 8,200 acres, and a lease covering an additional 1,073 acres of private land in and around the project area.

This growing land package, combined with established infrastructure, historical production, a defined mineral resource, and multiple new exploration targets, positions South Mountain as a compelling U.S.-based critical and precious metals opportunity.

Sampling Methodology, Chain of Custody, Quality Control and Quality Assurance

Cutting and sampling of drill core was directed by Thunder Mountain Gold geologists. The drilling contractors transported the core samples to the Company's secure facility in nearby Jordan Valley, Oregon. An independent contractor delivered these and additional surface samples collected by geologists to MSA Labs for sample preparation in Elko, Nevada. The sample prep process included: dry, crush 1kg to 2mm, split 250 g and pulverize rock material to 85% at -75μm. Quality control was monitored by the insertion of numerous blind certified standard reference materials, duplicates, and blanks into each sample shipment. Drill samples were assayed by MSA Labs for gold by fire assay of a 30-gram charge with an AA finish (code FAS-111). Satisfactory results were achieved for all quality control samples related to the data reported herein. In addition, samples were submitted for multi-element analysis (48 elements) by 0.25g 4-acid digestion and ICP-MS ultra-trace finish (code IMS-230).

Technical Data in this Release
The technical information in this news release was reviewed and approved by Steven A. Osterberg, Ph.D., P.G., Q.P. consulting geologist, and is a "Qualified Person" as defined by the U.S. S-K 1300 regulations and National Instrument 43-101 standards. Dr. Osterberg is not independent of the Company as he is an officer.

Regarding Thunder Mountain Gold, Inc.
Thunder Mountain Gold Inc., founded in 1935, is a junior exploration company focused on advancing base and precious metals assets in the western United States. The Company's principal asset is the South Mountain Mine, a historic former Anaconda Copper Company development of zinc, silver, gold, lead, and copper located on private land in Owyhee County, Idaho. Thunder Mountain Gold also owns 100% of the Trout Creek Project, a gold exploration asset in Nevada adjacent to and surrounded by Nevada Gold Mines, a Barrick-Newmont joint venture. With a growing land position, established project infrastructure, and exposure to both critical and precious metals, Thunder Mountain Gold offers investors leverage to exploration success in premier U.S. mining jurisdictions. For more information, please visit the Company's website at www.Thundermountaingold.com, www.SEC.gov, and www.sedarplus.ca.

Cautionary Note to Investors
This news release is intended for distribution only in the United States. Neither the TSX Venture Exchange, the OTCQB, nor their respective regulation services providers accept responsibility for the adequacy or accuracy of this release.

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