Empirical guides to gold enrichment in Proterozoic quartz-pebble conglomerates in northern Zambia

Proterozoic quartz-pebble conglomerates and sandstones near the base of the Mporokoso Group in northern Zambia were deposited in a braided stream system and are known to be locally enriched in gold. A detailed sampling programme was carried out in small area of good exposure in order to identify empirical guides to gold enrichment and to test the hypothesis that the gold is of placer origin.

A total of 2350 rock samples were taken from nine vertical profiles across the main 50–100 m thick conglomeratic interval, spread over a horizontal distance of 4 km. These were analysed for gold by atomic absorption spectrophotometry. Detailed sedimentological logging was carried out and the gold analyses were correlated with sedimentary parameters. Gold enrichment preferentially occurs:

• 1.

  (1) in a 1–2 km longitudinal stretch of the 50–100 m thick conglomeratic interval;

• 2.

  (2) in massive conglomerates (facies Gm) and then in trough crossbedded conglomerates (facies Gt), and not in trough cross-bedded sandstones (facies St);

• 3.

  (3) in beds of facies Gm and Gt which have small pebbles and a low proportion of vein quartz pebbles;

• 4.

  (4) in sections with frequent vertical changes of facies.

These correlations are consistent with a placer origin for the gold and should help to identify prospective areas within the Mporokoso Group.