Barrick Gold Corp (ABX.TO), the world's biggest gold producer, forecast an output drop over the next four years on Wednesday, with mining costs flat to higher.

Barrick, which reported quarterly earnings that matched market expectations, said its reserves of unmined gold dropped by 25 perc ent last year to 64.5 million ounces, as it sold off mine stakes and changed its plan for a South American asset.

Falling production and reserves are a concern for the industry, as finding new deposits becomes ever harder and the non-renewable resource is mined out.

Tight exploration and development budgets and a focus on cash flow in recent years, as companies have worked to repair damaged balance sheets, have also slowed the building of new mines.

Toronto-based Barrick said it expects to produce between 4.5 million and 5 million ounces of gold this year, at a cost of between US$765 and US$815 an ounce on an all-in sustaining basis, the industry benchmark.

That is down from 5.32 million ounces of gold in 2017, when costs were a comparable US$750 per ounce. Barrick is one of the industry's lowest cost producers.

Further out, Barrick, which has mines in the Americas, Australia and Africa, expects to produce between 4.2 million and 4.6 million ounces of gold a year between 2019 and 2022 at costs of US$750 to US$875 per ounce.

To help stem the slowdown in production, Barrick expects to increase its capital spend on project development by more than 60 per cent in 2018, to as much as US$550 million.

In the fourth quarter, Barrick reported adjusted net earnings of US$253 million, or 22 cents U.S. a share, largely unchanged from US$255 million, or 22 cents U.S. a share, in the same period a year ago.

That was in line with the 21 cents U.S. a share that analysts had expected, according to Thomson Reuters I/B/E/S.

On a net basis, Barrick reported a loss of US$314 million, or 27 cents U.S. a share, as it wrote down the value of unmined gold at its stalled Pascua-Lama project on the Chile-Argentina border, after its plans for developing the project changed.

Last month, Chilean environmental regulators ordered Barrick to close existing infrastructure on the Chilean side of the project, where Barrick had planned to develop an open pit mine. That has been replaced with a plan to develop an underground mine.