Infos Guyane blog brings you an unusual metamorphic diamond-bearing rock which is part of the 2.11 Ga metavolcanics of the Lower Paramaca series in the Dachine area of the southern greenstone belt of French Guiana.
A mineral assemblage corresponding to greenschist facies metamorphism makes up the bulk of the host rock. Mantle-derived rninerals are a rninor component.
This study examines the characteristics of the host rock and defines an assemblage of the possible mantle-derived minerals using samples from outcrop, drill core and drill core concentrate from eight locations on the body. Special attention is given to the compositions of minerals that are used to forecast the diamond potential of tiraditional diamond deposits in order to determine if conventional diamond exploration techniques are valid at Dachine.
Microprobe analyses and petrographic description of the host rock classifies it as an albite-actinolite-carbonate-chlorite-talcs schist that has been locally affected by hydrothermal alteration and extensively weathered.
At least two deformational events are evident. Pseudomorphed relict olivine crystals and chromite with a metamorphic overprint are the only obvious original igneous phases.
Relict igneous texture suggests a pyroclastic precursor, varying from a crystal ash tuff to lapilli tuff. Variations in mineralogy and relict igneous texture suggest deposition occurred as a series of multiple events.
Major and rare-earth element composition of the host rock implies an ultramafic igneous precursor with no enrichment in the light rare-earth elements, comparable to komatiite.
[…] This is the first documented occurrence of diamonds and other xenocrysts in a rock that resembles a pyroclastic metakomatiite. The unusual nature of this deposit may be a consequence of unusual conditions in the mantle under the Guiana shield.