Exploration Potential, page-71

Maybe VTX will follow…$2 billion+

Historical Shallow Mining and Supergene Enrichment

Gold was first discovered at Ora Banda in 1893, and early mining focused on near-surface, high-grade ore shoots. Both the early 1900s underground workings and later 1980s open-pit operations were largely confined to shallow depths (generally within the top ~50–100 m). In fact, most Australian open pits (including Ora Banda’s Gimlet South pit and satellite deposits) were less than 100 m deep. The regolith profile at Ora Banda is deeply weathered and lateritic, reaching ~40 m depth in unmineralized basalts and extending 80–120 m along the mineralized lodes. Within this weathered zone, gold was often remobilized and depleted near surface but reconcentrated just below the oxide horizon. This process created supergene-enriched zones at the base of oxidation that boosted gold grades in the shallow subsurface. Supergene gold in the oxide zone is typically associated with goethite-limonite (iron oxides) in the laterite profile . Historical open pits in the 1980s were designed to exploit this oxide and supergene ore; for example, approximately 3 million tonnes of oxide ore were processed from Ora Banda between 1985 and 1990, mainly from the shallow Gimlet South open cut . Supergene enrichment was an important contributor to early production – several deposits in the Ora Banda district (e.g. the Racetrack and Golden Funnel prospects) are cited as examples where gold grades were enhanced in the oxidized cap above primary mineralization . By the time easily worked oxide and supergene ore was exhausted, deeper primary zones remained largely untapped by historic miners.

Modern Exploration Targeting Primary Gold at Depth

In recent years, exploration companies (notably Ora Banda Mining Ltd, ASX:OBM) have turned their focus to the primary (fresh rock) gold mineralization beneath the old workings. Modern drilling programs are extending known orebodies to much greater depths than earlier mining. For instance, the main lodes at Ora Banda have been shown to persist for at least 500 m down-dip, with steeply plunging high-grade shoots continuing well into fresh rock . Ora Banda Mining’s 2020 Definitive Feasibility Study initially focused on open pits, but the company has since pivoted to evaluating underground potential.

At the Riverina deposit (north of Ora Banda), recent drilling confirmed high-grade continuity over 300–500 m below the open pit mine plan, far beyond the oxide zone . This led to a maiden underground resource of ~303,000 ounces at ~4 g/t Au at Riverina, demonstrating significant primary mineralization at depth . The resource remains open in all directions, and Phase 2 drilling is probing even deeper extensions .

Other deposits in the Ora Banda area show similar potential: for example, the Golden Eagle deposit (near Davyhurst) was historically mined to ~100 m depth, but recent diamond drilling by OBM intersected strong gold zones well below those levels (discussed further below) and confirmed that mineralization remains open down-plunge and at depth . In summary, ongoing exploration is successfully identifying substantial primary gold beneath the old oxide pits, indicating that the historic shallow mining only scratched the surface of a larger gold system.