Ann: Gold Extraction Results from Ema and Tres Estados, page-21

Just ran a google search. Here is an abstract on gold leaching where the gold is very fine and tied in with the sulphide ore. . Quite a protracted process described here, but the interesting point is at the end where it refers to the time dependence of leaching ( 12 days in this instance) so the BBX time requirement seems reasonable I guess to achieve good recoveries

Abstract

A pyrite ore containing 3.2 g/ton of gold finely disseminated in the sulphide matrix was leached to recover this precious metal. The leaching was carried out in columns containing 200 kg of ore crushed to minus 10 mm. The ore was initially leached by solutions containing chemolithotrophic bacteria, ferric ions, dissolved oxygen and sulphuric acid to oxidize the pyrite and to expose the gold. The pH and the iron content of these solutions were maintained below 1.7 and 15 g/l, respectively, to prevent the precipitation of iron salts inside the columns and to avoid the partial inhibition of the bacteria. After bacterial pretreatment the ore was treated with alkaline solutions until the pH of the ore effluents was stabilized in the range of 9–10. The leaching of the gold was then started with solutions containing amino acids of microbial origin and thiosulphate as the gold-complexing agents. The final gold extraction depended on the extent of the prior pyrite oxidation. About 80% of the gold was leached from the pretreated ore within 12 days under optimum conditions, whilst only about 15 % was leached from the original, untreated ore.