The site is called the Middlepits project and the interest is being bought from Metswedi Mining.
Once the deal is closed, Tango will be responsible for all further exploration and development expenditure.
The site consists of one prospecting licence (PL) and was explored between 1974 and 1976 by diamond major De Beers, between 1978 and 1980 by Falconbridge and between 1993 and 1997 by Southern Africa Minerals Corporation.
Work by De Beers resulted in the identification of a 100 sq km area of river terrace gravels containing diamonds and heavy mineral concentrations, mainly garnets and ilmenites.
The diamond content of the gravels has never been assessed systematically, however diamonds of up to 0.55 carats were recovered, the firm said.
Tango's technical team will undertake to work with Metswedi to develop a comprehensive exploration and development program in the first quarter of 2018.
Metswedi is in the process of renewing the prospecting license and closing of this deal is dependent on completing that
The licence covers 876 sq km and upon renewal will cover around 435 sq km and will be valid for two years, with a further option to renew.