IEC executive director and COO Mark McAndrew will this week take over the duties as acting CEO until a suitable replacement is made.
Group financial controller Kerry Angel will assist him.
Chairman Graeme Robertson expressed the company's sincere condolences noting that Brereton had steered Intra through a very difficult period of declining sales from import competition and had successfully positioned it to move towards energy diversification and electricity generation.
In addition to being awarded new lithium licenses in Tanzania, there have been discussions between the company and a number of parties who have a similar and complimentary interest to develop renewable energy projects in East Africa.
Robertson said: “IEC is expanding its business interests from coal to other energy sources, in particular battery development materials, lithium and graphite, and renewables, wind and solar.
“This is designed to offer shareholders alternatives in the field of energy while still focusing on the ongoing and expanding coal production and supply activities.”