2025 Thayer Lindsley Award

For the discovery of the Onto copper-gold deposit in Sumbawa, Indonesia.

The Onto Discovery Team was selected for the 2025 Thayer Lindsley Award for their outstanding discovery of the Onto copper-gold deposit in the Hu'u project, Sumbawa, Indonesia. This discovery is considered one of the most significant copper-gold discoveries globally in the past decade, notable for its sheer size, grade, and the challenging exploration conditions. The Onto deposit is a Tier-1, world-class copper-gold deposit containing approximately 2.1 billion tonnes of mineralization with an average grade of 0.86% copper and 0.48 grams per tonne of gold. This indicated and inferred resource contains 17.6 million tonnes of copper and 32 million ounces of gold. 

Members of the Onto Discovery Team at the time worked for Vale Exploration Pty Ltd, PT Vale Eksplorasi Indonesia or PT Sumbawa Timur Mining. PT Sumbawa Timur Mining is an Indonesian joint venture company owned by Eastern Star Resources Pty Ltd (80%), a 100% owned subsidiary of Vale International, and PT Aneka Tambang Tbk (20%).

In 2006, Vale Exploration Pty Ltd decided on a porphyry exploration strategy to identify areas of advanced argillic alteration in lithocap environments that typically occur above porphyry systems, and test for concealed porphyry mineralization. One of the first areas identified and visited in 2008 was the Hu’u Project Contract of Work (CoW). An option agreement was signed, and exploration commenced in 2010. The first drill hole intersected a small porphyry deposit in a small diorite stock with chalcopyrite-pyrite ± bornite in biotite-magnetite alteration and a quartz vein stockwork that was drilled in 20 holes. Subsequent exploration advanced to the southwest looking for similar targets.  In June 2013, VHD034 was drilled at Onto testing a weak geochemistry anomaly and magnetic high and intersected 287 meters of copper and gold mineralization dominated by pyrite and covellite; a totally new style of mineralization for this region.

The discovery was made in dense, mountainous terrain without any surface mineralization, with deep drilling (800m holes), following mapping, stream sediment and soil sampling and airborne geophysical surveys (Lidar, magnetics and radiometrics. The deposit occurs 500 meters below the surface, hidden under unmineralized volcanic rocks, making it a remarkable "blind" discovery.

The economic significance of the Onto deposit, combined with the technical challenges and innovative exploration methods employed, earned the team recognition for their contribution to the global mining industry.