Paper No. 13
Presentation Time: 4:40 PM
PALEOPLACER GOLD POTENTIAL IN THE EARLY PROTEROZOIC RORAIMA GROUP
The Roraima Group, comprising quartz-pebble conglomerates, sandstones and siltstones, is preserved over an area of 73 000 square kilometres on granite-greenstone terrain of the Guiana Shield. The topography is deeply dissected along faults and joints and all the drainages carry diamond and gold concentrates associated with quartz-pebble gravels. Various criteria favor a paleoplacer source: the provenance comprises extensive granite-greenstone terrain which contains lode-gold deposits; the depository is seven times the size of the Witwatersrand basin; the sedimentary rocks represent braided fluvial sediments deposited in alluvial plain to sub-aerial braided delta settings; oxygen isotope ratios of the quartz-pebble assemblage indicate that 85 percent of the population exceeds a d18
O value of 11.5 permil (v-smow). Only 45 percent of the Witwatersrand quartz pebbles exceed this 11.5 permil value, which is considered to be a gauge of gold provenance; crystalline gold from vein-quartz in the basement and detrital gold particles extracted from Roraima saprolite have been dated at 2000 Ma by rhenium depletion ages; the age of sedimentation has been established by means of a single zircon U - Pb age of 1901 ±1 Ma obtained from a tuff bed conformably overlying an auriferous conglomerate. These ages confirm that the Roraima gold is older than the host sediment.
Recent exploration utilising concepts derived from paleoplacer experience on Witwatersrand deposits has identified unconformities, paleocurrent distribution and relative sediment transport activity in order to determine optimal concentration sites, and then these sequences have been channel sampled and fire assayed for gold using ICPMS technology to obtain parts per billion accuracy. A background gold content of 10ppb is prevalent (Witwatersrand sandstones contain 6 ppb gold on average) and conglomeratic beds commonly contain over 100 ppb gold. Shallow diamond drilling, designed to recover fresh intersections of unexposed parts of the stratigraphy, has sampled unconformities that contain up to 23 g/t gold. These results indicate extensive paleoplacer gold potential.