In a nutshell to my mind there are a few variations on the Pilbara cong plays:
1. Gold in cong at the base of the Mt Roe Basalt (MRB). This is the host for the watermelon seed nuggets at Purdeys Reward (ARV/NOVO) and high grade at 67g/t from a bulk sample, Comet Weĺl (NOVO), Loudens Patch, Scotty Well (DEG). This cong unconformably overlies much older rocks (250-350Ma older) that I believe may be the source of the mineralisation.
2. Younger Fortescue Gp congs such as the Hardey Formation that overly the MRB. This is the host for NOVOs Beatons Ck deposit which is lower grade at 2-3g/t over wide exposures.
3. Cong found in the older rocks underlying the MRB. There have been alluvial watermelon seed nuggets found overlying older rocks at a distance from MRB eg at Egina (NOVO) and Womerina (PIO). There is a school of thought on HC that the older rocks have shed these nuggets relatively recently and are the primary target.
The above do not address the origin of the gold and the relationship to the Wits deposits in SA.
In the end all of these may have some merit but I favour number 2 and have invested in stocks with tenements containing, or adjacent to, basal MRB as this correlates with the current higher grade cong finds.
