Sunstone Metals (ASX:STM) said a scoping study for its Bramaderos gold copper project in Ecuador has outlined an initial 23-year mine life producing 135,000 gold equivalent ounces per year for the first eight years, according to a Tuesday Australian bourse filing.

The company said the study returned a post-tax net present value of $900 million at a gold price of $3,500 per ounce, with an all-in sustaining cost (AISC) averaging $1,499 per ounce after credits.

Sunstone Metals said the operation is a conventional single open pit feeding a 10-million-tonne-per-year plant at a life-of-mine strip ratio of 1.4, with capital payback estimated at 34 months at $3,500 per ounce gold and 19 months at spot price.

The company said potential for significant growth in inventory could underpin a possible stage 2 expansion.