The 9 Mile Deposit of the Barama-Mazaruni Greenstone Belt of the Guiana Shield: geochemistry, geochronology and regional significance

Bassoo, Roy; Murphy, J. Brendan

doi:10.1590/2317-4889201820180072

Cited by 2 publications

(1 citation statement)

References 19 publications

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“…Primary and placer diamond deposits are shown with octahedron and rounded octahedrons, respectively (AM-Amatuk, BV-Boa Vista, CO-Los Coquitos, DC-Dachine, EK-Ekereku, GU-Guaniamo, JW-Jawalla, KM-Kamarang, KU-Kurupung, KW-Konawaruk, MK-Maikwak, MM-Monkey Mt., NA-Nassau, RO-Rosebel, TE-Tepequém). Figure is modified from Santos et al (2003), Nadeau and Heesterman (2010), Daoust et al (2011), Kroonenberg et al (2016), Bassoo and Murphy (2018), and Bassoo et al (2021).…”

Section: Regional Geologymentioning

confidence: 99%

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

Bassoo

Befus

2021

Geochem Geophys Geosyst

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The composition of diamond‐hosted inclusions provides insight into the character of the sub‐cratonic lithosphere of the Guiana Shield. Guyana's Paleoproterozoic diamonds preserve an inclusion suite comprised of forsterite (Fo ∼89.3–91.8), enstatite, chromite, and Cr‐pyrope. Raman thermobarometry of entrapped olivine and pyrope inclusions indicate entrapment pressures of ∼5.3–7.0 GPa. Unpolarized Fourier transform infrared spectroscopic measurements of forsterite and enstatite inclusions produce low absorbances from OH. Using established calibrations, those absorbances indicate forsterite and enstatite contain median values of 26 ± 14 and 14 ± 18 ppm H2O, respectively, and suggest a high effective viscosity of 1023.7 ± 2.1 Pa∙s for the lithospheric mantle. When combined with inclusion thermobarometry, diamond residence temperatures suggest paleo‐geotherms ranging from 35 to 40 mW m−2. Low‐to‐moderate Fe2O3 content (1.9 ± 0.8 wt.%) and low oxygen fugacity (log ƒO2 (ΔFMQ) −1.6 ± 1.1) determined from chromite inclusions indicate crystallization in reducing conditions. Forsterite and chromite inclusions retain evidence for metasomatic alteration, including Mn‐enrichment in forsterite and chromite rich in Zn. These characteristics indicate that the sub‐cratonic lithosphere of the Guiana Shield experienced episodes of partial melting and fluid‐driven metasomatism of dry, strongly viscous, and moderately depleted garnet‐spinel harzburgite. The Guiana Shield has been relatively stable since the Paleoproterozoic, meaning diamond inclusions may also provide the best means for understanding current conditions in the region's lithospheric mantle.

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Section: Regional Geologymentioning

confidence: 99%

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

Bassoo

Befus

2021

Geochem Geophys Geosyst

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Deciphering the Enigmatic Origin of Guyana's Diamonds

Bassoo

Befus

Liang

et al. 2021

American Mineralogist

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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.

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The 9 Mile Deposit of the Barama-Mazaruni Greenstone Belt of the Guiana Shield: geochemistry, geochronology and regional significance

Cited by 2 publications

References 19 publications

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

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

Deciphering the Enigmatic Origin of Guyana's Diamonds

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