TURAIF, Saudi Arabia  - Saudi Arabia's King Salman on Thursday inaugurated the Waad Al-Shamaal mining project, which is expected to boost the kingdom's gross domestic product by 24 billion riyals ($6.4 billion) and its non-oil GDP by around 3 percent.

The kingdom will invest 85 billion riyals in the project, a 440-square-km (150-square-mile) city for mining industries in the northern region, the Ministry of Energy, Industry and Mineral Resources said in a statement.

The completion of the next phase will lift the kingdom's phosphate fertiliser production to 9 million tonnes annually, Energy Minister Khalid al-Falih said.

"This will make the kingdom the second largest producer of phosphate fertilisers in the world," Falih added.

Mining is key to the kingdom’s efforts to diversify its economy away from hydrocarbons, as the government aims to more than triple the sector’s contribution to the nation’s economic output by 2030.

Saudi authorities estimate the region holds 500 million tonnes of phosphate ore, around 7 percent of global proven reserves, mainly in the Al Jalamid and Umm Wu’al areas between Arar and Turaif.

The energy ministry estimates the kingdom’s unused mineral resources to be valued at 5 trillion riyals ($1.33 trillion).

Aramco CEO Amin Nasser said on Thursday that the state oil giant had invested more than 10 billion riyals in developing the first system in the kingdom for producing and supplying unconventional gas.

Such investment is aimed at providing sufficient energy at competitive prices to develop the Waad Al-Shamaal mining project, a statement released by Aramco said.

Saudi Arabia’s efforts to build an economy that does not rely on oil and state subsidies involves a shift towards mining vast untapped reserves of bauxite, the main source of aluminium, as well as phosphate, gold, copper and uranium.

(Reporting by Marwa Rashad, additional reporting by Nayera Abdallah; Writing by Tuqa Khalid; Editing by Emelia Sithole-Matarise, Jason Neely and Adrian Croft) ((Tuqa.Khalid@thomsonreuters.com; +971521047568;))