Sumbawa Timur Mining’s Onto Project Could Rival Freeport Indonesia Grasberg
Fri 14 Aug 2026, 16:02 PM
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Sumbawa Timur Mining’s Onto project in West Nusa Tenggara is still under exploration, with its copper-gold resource estimated at more than 2 billion tons.
Indonesia still has significant untapped gold potential across several regions, with one exploration-stage project in West Nusa Tenggara potentially holding a larger gold resource than PT Freeport Indonesia’s Grasberg operation in Central Papua, according to the Indonesian Mining Institute (IMI).
Indonesian Mining Institute Chairman Irwandy Arif said the potential highlights the scale of Indonesia’s remaining mineral resources and the need for continued exploration to identify new mining deposits.
One of the projects highlighted by Irwandy is the Hu’u Project, operated by PT Sumbawa Timur Mining (STM) in Hu’u District, Dompu Regency, West Nusa Tenggara.
“Its potential is still very large. In West Nusa Tenggara, we have Sumbawa Timur Mining. Its reserves could potentially exceed those of Freeport. We have also identified the same mineralization corridor in that area, although it was known before,” Irwandy said at the Mindialogue 2026 event in Jakarta, as quoted Thursday (August 13).
STM is currently exploring the Onto copper-gold deposit, which forms part of its Hu’u Project. The project remains in the development stage and has not yet entered commercial production. Vale Base Metals describes Hu’u as a tier-one copper-gold development district, with the Onto deposit discovered in 2013 and currently progressing through a pre-feasibility study.
STM’s own data estimates the Onto deposit contains more than 2 billion tons of mineral resources. The company says the project is one of the world’s major copper-gold discoveries and remains open laterally and at depth.
STM has held a seventh-generation Contract of Work for the Hu’u area since 1998. Exploration activities began in 2010, while the Onto deposit was discovered in 2013. The company subsequently entered the pre-feasibility study stage in 2019.
STM is majority-owned by Vale through Eastern Star Resources, which holds 80%, while state-owned mining company PT Aneka Tambang Tbk (ANTM) owns the remaining 20%.
The company has continued to advance the project toward feasibility. Local government officials said in April 2026 that STM had entered the feasibility-study stage as part of preparations for eventual production.
The scale of the Onto resource has attracted attention because the deposit contains both copper and gold. A 2021 resource estimate cited by Petromindo put the total mineral resource at around 2.1 billion tons, comprising 1.1 billion tons of indicated resources and 1 billion tons of inferred resources.
The project is therefore not yet comparable to an operating mine in terms of proven production or commercially recoverable reserves. Its potential will depend on the completion of further exploration, feasibility studies, permitting and eventual mine development.
Freeport Indonesia, meanwhile, operates the Grasberg mining complex in Central Papua, one of the world’s largest copper and gold mining operations.
The comparison made by IMI therefore refers specifically to the potential scale of STM’s gold resources relative to Freeport Indonesia’s operations in Papua, rather than to Freeport’s global parent company.
The Hu’u Project is currently being advanced as a future copper-gold mining operation, with STM continuing technical studies and exploration before any final investment and construction decisions are made.
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