Deciphering the Enigmatic Origin of Guyana's Diamonds

Abstract

Diamonds have long been mined from alluvial terrace deposits within the rainforest of Guyana, South America. No primary kimberlite deposits have been discovered in Guyana, nor have there been previous studies on the mineralogy and origin of the diamonds. Paleoproterozoic terranes in Guyana are prospective to diamond occurrences because the most productive deposits are associated spatially with the eastern escarpment of the Paleoproterozoic Roraima Supergroup. Geographic proximity suggests that the diamonds are detrital grains eroding from the <1.98 Ga conglomerates, metamorphosed to zeolite and greenschist facies. The provenance and paragenesis of the alluvial diamonds are described using a suite of placer diamonds from different locations across the Guiana Shield. Guyanese diamonds are typically small, and those in our collection range from 0.3 to 2.7 mm in diameter; octahedral and dodecahedral, with lesser cubic and minor macle forms. The diamonds are further subdivided into those with abraded and non-abraded surfaces. Abraded diamonds show various colors in cathodoluminescence, whereas most non-abraded diamonds appear blue. In all populations, diamonds are predominantly colorless, with lesser brown to yellow and very rare white. Diamonds are predominantly Type IaAB and preserve moderate nitrogen aggregation and total nitrogen concentrations ranging from trace to ~1971 ppm. The kinetics of nitrogen aggregation indicate mantle-derived residence temperatures of 1124 ± 100 °C, assuming residence times of 1.3 and 2.6 Ga for abraded and non-abraded diamonds, respectively. The diamonds are largely sourced from the peridotitic to eclogitic lithospheric upper mantle based on both δ13C values of –5.82 ± 2.45‰ (VPDB-LSVEC) and inclusion suites predominantly comprised of forsterite, enstatite, Cr-pyrope, chromite, rutile, clinopyroxene, coesite, and almandine garnet. Detrital, accessory minerals are non-kimberlitic. Detrital zircon geochronology indicates diamondiferous deposits are predominantly sourced from Paleoproterozoic rocks of 2079 ± 88 Ma.

Composition of the Sub‐Cratonic Mantle of the Guiana Shield Inferred From Diamond‐Hosted Inclusions

“…We infer that garnet inclusions were entrapped at greater depths than forsterite inclusions. Some variability in overall residual pressures reflects heterogeneous diamond populations mixed by alluvial processes that now coexist in the paleo‐placers (Bassoo et al., 2021). Many forsterite inclusions preserve residual pressures in the graphite stability field, lower than the entrapment pressures.…”

“…A collection of 665 diamonds, originally sourced from placer deposits along rivers in the Guyanese Amazon rainforest, were obtained from miners and a local diamond merchant (Kay's Diamond Enterprise Ltd.). The diamonds in the collection are small, ranging from 0.1 to 0.4 carats, and they primarily occur in octahedral and dodecahedral forms (Bassoo et al., 2021). Optical microscopy indicates 56 diamonds contained silicate inclusions.…”

“…Placer diamonds have been mined for the past century from Pleistocene‐to‐modern alluvial gravels in Guyana's remote Amazon rainforest (Bassoo et al., 2021). Although the diamonds are now part of the continent's sedimentary veneer, the diamonds' inclusions chronicle the conditions and evolution of the sub‐cratonic mantle of the Guiana Shield.…”

“…Olivine and perovskite are notably absent. A population of garnets displays some mantle affinity but diamond is the only certain cratonic mantle xenocryst (Bassoo et al., 2021). The diamonds are thought to erode from rocks of the Roraima Supergroup and are estimated to be >2 Ga (Bassoo et al., 2021).…”

“…A population of garnets displays some mantle affinity but diamond is the only certain cratonic mantle xenocryst (Bassoo et al., 2021). The diamonds are thought to erode from rocks of the Roraima Supergroup and are estimated to be >2 Ga (Bassoo et al., 2021). These diamonds and their inclusions could inform us of the composition and early formation of the sub‐continental lithospheric mantle of the Guiana Shield.…”