Mather is a director of The Mather Foundation, a philanthropic auxiliary foundation trust fund that sold SolGold shares this week at 21.25p each for £180,625 (US$237,279).
"The sale was finalised to address obligated philanthropic and charitable donations to prostate cancer research, mental health and homelessness organisations and emergency flight care services in Australia," the company said.
Due to the end of financial year, it said the Mather Foundation was required to monetise assets each year and make cash donations to third-party charitable organisations.
"The sale is in no way an indication of a diminished outlook in respect of the value of SolGold or its growth outlook in the future," Mather said.
The company said the sale represented less than 1% of the total shareholdings associated with Mather in SolGold.
Mather holds around 6.6% of SolGold. He also holds around 8% of DGR Global, SolGold's second-largest shareholder with 12.2% of the company.
SolGold last month had 12 rigs drilling at its 85%-owned Cascabel copper-gold project in Ecuador where it is aiming to produce a resource update.
The company also has exploration underway at its fully-owned 73 other concessions in Ecuador.
SolGold is capitalised at around £370.2 million in London.