Scotgold Resources plans to build an underground operation with an initial production capacity of 12,000 ounces of gold annually for up to 17 years.
Scotland Mining News
With this deal, Rio has now agreed to more than $1.3bn in divestments this year.
Scotgold believes gold and silver from its Cononish project near Loch Lomond in Trossachs National Park would attract a "significant premium".
Scotland has stopped generating electricity from coal for the first time in more than 100 years.
The finding is more than double the age of the previous oldest known occurrence of reidite, believed to have formed about 450 million years ago.
More than 100 objects, including solid gold jewellery, arm bands and silver ingots are part of the trove, one of the largest discoveries of Viking treasure in Scotland.
A triumph of the "Yes Scotland" campaign would have to be followed by negotiations with London over a raft of assets.
The Finnish firm rejected a sweetened multibillion-dollar bid on the grounds that it was "significantly" undervalued and appeared "opportunistic."
The combined equipment manufacturer would be listed on both the London and Helsinki Stock Exchanges.
The mine is expected to produce 10 million tonnes of coal over 10 years.
Preferred bidder also bought mines from second largest open pit coal miner in Scotland in December.
Move follows liquidation of Britain's number one coal supplier last week.
In what is claimed to be a world first, a Scotland-based independent malt whisky producer will start using bacteria to feed on the "leftovers" from the booze making process in order to produce fuel for cars.
Scotgold Resources announced Tuesday it expects to receive a mining lease from the Crown for the Cononish gold and silver project, the only one of its kind in the UK, in May this year. Production could start early in 2013.