GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 288-3
Presentation Time: 2:15 PM


BASSOO Jr., Roy, Department of Geosciences, Baylor University, One Bear Place #97354, Waco, TX 76798 and BEFUS, Kenneth, Geology, Baylor University, One Bear Place #97354, Waco, TX 76798

Diamonds have been mined for the past century from alluvial gravels along the wild rivers deep within Guyana’s Amazon rainforest. The most productive gravels are spatially associated with the eastern escarpment of the Roraima Formation. This geographic proximity suggests that the diamonds are recycled grains eroding from the 2.3 to 2.0 Ga conglomerates of the Roraima Formation. If true, then Guyana’s diamonds must be even older, making them detrital evidence for some of Earth’s oldest kimberlites. No primary kimberlites have been discovered in Guyana, nor has there been any previous academic study of the diamonds. Exploration has been driven by artisanal miners, called pork knockers in the early 1900s, who prospected using detrital indicator minerals still referred to colloquially as “sweetman”, “tin”, and “carbon”, which are drusy quartz, rutile, and tourmaline, respectively. Typical kimberlite indicator minerals such as chrome diopside and pyrope are not found. Yearly production from the alluvial gravels averages 90,000 carats. Most diamonds are small, ranging from a few points to ~4 carats. Larger diamonds are sometimes found, with historically significant stones up to 57 carats. Guyana’s diamonds are predominantly colorless (G-J), cape yellow (K-M), and brown. Many diamonds have brown, green, and blue-green skins. Pink and green fancy body colors are rare. Diamonds are dodecahedral to octahedral, with lesser cubic and macle forms. Morphologies can be further subdivided into those with abraded surfaces and those not modified by abrasion. Abraded diamonds show various colors in CL whereas most non-abraded diamonds are CL blue. The blue CL response and non-abraded crystals suggest that a yet undiscovered kimberlite source may indeed exist. All diamonds are predominantly Type IaAB with moderate N aggregation and 109±235 ppm average total N; distinctly lower than diamonds from other locations across the Amazonian craton. N aggregation ratios and solid inclusion Raman thermobarometry indicate entrapment pressures of 5.9±0.5 GPa and residence temperatures of 1319±28 ºC. Forsterite is the most common inclusion, with lesser rutile, garnet, coesite and diopside. The inclusions and diamond δ13C ratios of -5.8±2.5‰ indicate upper mantle peridotite paragenesis with a subpopulation of eclogite paragenesis.