Lithium developer Ioneer has reported a fourfold increase in ore reserves at its Rhyolite Ridge lithium/boron project in Nevada, US.

The ore reserve has surged by 308% to 246.6-million tonnes, containing 1.92-million tonnes of lithium carbonate equivalent (LCE) and 7.68-million tonnes of boric acid equivalent (BAE), the company said in a statement on Monday.

Nearly half of the mineral resource has now been converted to reserve.

“Today’s updated reserve and mine plan reinforces the importance of Rhyolite Ridge’s remarkable mineralogy,” said MD Bernard Rowe. “It allows Ioneer to match prevailing market conditions and blend or prioritise ore to produce a valuable boric acid co-product, whose market is uncorrelated with the project’s primary lithium product. No other lithium project offers this level of flexibility and economic advantage.”

The company plans to prioritise high-boron ore in the first 25 years of the 95-year mine life, enabling average yearly production of about 19 200 t of LCE and 116 400 t of boric acid. Boric acid is expected to contribute an average one quarter of total revenue during this period, supporting margins in weaker lithium price environments.

“In periods of low cycle lithium pricing, like today, we plan to prioritise the high-boron ore production to optimise the relative proportion of total revenue derived from boric acid,” Rowe said.

The updated mine plan and economics reaffirm the project as a large-scale, long-life, low-cost operation. Ioneer estimates an all-in sustaining cost of $5 745/t LCE, placing Rhyolite Ridge in the bottom quartile of the global cost curve. The direct cost, net of expected boric acid credits, is forecast at $3 858/t LCE in the first 25 years.

Lithium hydroxide prices are forecast at $23 040/t over the first 25 years of production, while boric acid is projected at $1 296/t.

Capital expenditure to develop the project is pegged at $1.67-billion, including a 10% contingency. The company has completed about 70% of the project's engineering and adopted a more conservative stance on plant availability and maintenance assumptions, leading to lower but more reliable economic projections.

Updated economic metrics include an after-tax unlevered net present value of $1.37-billion and an internal rate of return of 14.5%.