However, it balanced the updated offer against what it describes as Lonmin's ongoing financial limitations.
It has now offered one Sibanye-Stillwater share for every one Lonmin share, which is up 3.4% from the initial December 2017 offer of 0.967 Sibanye-Stillwater shares for every Lonmin share.
After completion, Lonmin shareholders will own about 10.9% of the enlarged Sibanye-Stillwater group.
The revised offer values Lonmin shares at 77.4p (US99.89c) and its existing issued ordinary share capital at about £226 million based on Sibanye-Stillwater's closing price of R14.33 (US99c) on the Johannesburg Stock Exchange on April 23 and an exchange rate of £1:R18.511.
This compared to the previous offer valuing Lonmin shares at £1 and the company at about £285 million based on Sibanye-Stillwater's 30-day trading volume weighted average of R18.67 on the JSE for the period ended December 13 at an exchange rate of £1:R18.056.
The offer is subject to approvals from both companies' shareholders at May 28 general meetings, as well as clearance by relevant South African competition authorities and courts, but is expected to become effective on June 7, 2019.
The UK Competition and Markets Authority unconditionally cleared the proposed acquisition in June.
Sibanye-Stillwater's (SJ:SGL) shares were trading at R14.31/share Thursday, 33% higher than six months ago, while Lonmin's (LSE:LMI) shares were at 74.9p, up 54.4% from half a year ago.