The expansion at the Bauxite Hills operation that'll increase export capacity from about 3.5 million tonnes per annum to 6Mtpa is expected to cost $51.4 million.
The vast bulk of the capital will go towards a floating terminal that can load larger, ungeared ships offshore from Skardon River at Bauxite Hills.
The term of the loan being offered by NAIF is up to nine years.
The potential loan's coupon was not disclosed.
A final Metro board decision will follow completion of the detailed engineering and design work.
Earlier this year, Metro reported a 2018 gross profit for the Bauxite Hills operation of $9.6 million on the back of about 2Mt of exports.
Metro's cash on hand and trade receivables at the September was $43.5 million, compared to $29.5 million at the end of the June.
Shares in Metro were up 12% to 13.5c in morning trade, capitalising the company at $187 million.