Aug. 7, 2019
Research Highlight

The Origin of Barite Deposits from the Archean

A study on the Barberton Greenstone Belt in South Africa

Astrobiologists supported in part by the Exobiology Program have pieced together the story of how barite deposits in the Barberton Greenstone Belt (BGB) of South Africa may have formed. The BGB dates back to the Archean, and can provide information about the environmental conditions that were present during this time in Earth’s history when life was first gaining a foothold on our planet.

Barite blades growing within a layer of barite sand
Barite blades growing within a layer of barite sand. The lowest barite blades are truncated their tops (red arrow) indicating that they were already present when the overlying layer was deposited. Above this horizon, the lower sand layers (yellow arrows) show no evidence that the blades were present when the sand was deposited. Toward the top, the barite sand shows thickening and disruption around the blades (white arrows) suggesting that the blades were present during sand deposition. The blades evidently grew upward within the sand concurrently with or very shortly following sand deposition. Several layers of small barite crystals occur as crusts within the barite sand (black arrows) and the barite layer is capped by gray chert.Image credit: Lowe et al. (2019).

The researchers identified five different morphological types of barite at the site, all of which were associated with active faults and fractures in the underlying rock. This indicates that the barite originated from fluids containing barium that reached the surface from below. The research team believes that small, cool springs allowed shallow subsurface waters to vent to the surface. These waters then interacted with sulfate-bearing water, which triggered barite precipitation. This would mean that the barite deposits were not formed in open marine waters, and therefore hold little information about the composition of seawater in the Archean.

The study, “Crustal fracturing, unconformities, and barite deposition, 3.26–3.23 Ga, Barberton Greenstone Belt, South Africa,” was published in the journal Precambrian Research. The work was supported by NASA Astrobiology through the Exobiology Program.