Black Bear Minerals (ASX:BKB) is expanding its review of historic drill core following encouraging results from the Shafter silver project in Texas.

Shafter is a formerly producing underground silver mine located in Presidio County, near the Mexico border, with historical operations dating back to the early 20th century. The project last operated commercially under previous owners before being placed on care and maintenance, making Black Bear's restart ambitions a potential revival of a past-producing asset rather than a greenfield discovery.

The ongoing re-sampling and check analysis program forms part of a historic core review aimed at validating the 17.5 million ounce (Moz) foreign mineral resource estimate (MRE) at Shafter. Because the existing resource was compiled under a non-JORC reporting framework, conversion to JORC (2012) compliance is a regulatory prerequisite for an ASX-listed entity reporting resources to the market, making this work central to the company's ability to publicly disclose and rely on the estimate.

Results from two additional historic 2012-series underground diamond holes continue to demonstrate an uplift in contained silver for drillhole intercepts when compared to the original database.

CEO Dennis Lindgren stated that the results reinforce the company's confidence in the quality of the Shafter mineralised system. Notable returns include 8.5 metres at 525 grams per tonne (g/t) silver, with a peak grade of 1,360 g/t silver. Another historic intercept showed a 147% increase in contained silver.

"In addition, we continue to see previously un-assayed polymetallic potential, with significant gold mineralisation as high as 0.7 g/t gold," Lindgren said.

"We will continue to extend the historic core review across the broader resource footprint as we progress towards JORC conversion, MRE upgrade and rapid mine restart study."

The review process has involved site geologists assessing historical core and cutting the remaining half-core into quarter cores for sampling at the same interval lengths recorded in the historical database. Where necessary, whole core was cut to half core to extend sampling into the wall rock.

Core was re-measured to confirm that sample intervals were correct and could be accurately replicated. In cases where ambiguity prevented confirmation of a sample boundary, no sample was taken.

Results to date provide data covering seven drill holes and 174 comparable assay pairs, enabling a preliminary comparison between the historic dataset and re-assay results from the same core intervals.

Recent grab and rock chip sampling by the company has demonstrated that mineralisation at the project is polymetallic, including gold, zinc, lead, and vanadium. The presence of multiple payable metals could bear on future project economics, as by-product credits from base metals and gold can offset standalone silver production costs — a consideration relevant to the company's mine restart study.

Black Bear Minerals plans to broaden check-analysis across additional historic drill programs where core is available and in suitable condition, and to extend re-analysis coverage across the full MRE footprint.

BKB opened steady at 56 cents, with a market capitalisation of $86.88 million prior to markets opening.