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The Khanneshin carbonatite is a deeply dissected igneous complex of Quaternary age that rises approximately 700 meters (m) above the Neogene sedimentary rocks of the Registan Desert, Helmand Province, Afghanistan. The complex consists almost exclusively of carbonate-rich intrusive and extrusive igneous rocks, crudely circular in outline, with three small hypabyssal plugs of leucite phonolite and leucitite outcropping in the southeast part of the complex. The igneous complex is broadly divisible into a central intrusive vent (or massif), approximately 4 kilometers (km) in diameter, consisting of coarse-grained sövite and brecciated and agglomeratic barite-ankerite alvikite; a thin marginal zone (<1 km wide) of outwardly dipping (5°–45°) and alkali metasomatized Neogene sedimentary strata; and a peripheral apron of volcanic and volcaniclastic strata extending another 3–5 km away from the central intrusive vent. Small satellitic intrusions of biotite-calcite carbonatite and rare leucite phonolite, no larger than 400 m in diameter, crop out on the southern and southeastern margin of the central intrusive vent. A zone of prospective light rare earth element (LREE) enrichment was delineated by Soviet geological teams in the mid-1970s. The area of LREE-enrichment is situated in extensively veined and dike-intruded barite-ankerite alvikite in the outer part of the central vent near its northeast contact with Neogene sedimentary rocks. In addition to having very high concentrations of LREE, the barite-ankerite alvikites are also highly enriched in barium and strontium. Three reconnaissance scoping missions to the Khanneshin carbonatite were led by scientists of the U.S. Geological Survey (USGS). Two of these were to LREE area of interest which is the primary subject of this report. Two types of LREE mineralization occur. Type-1 LREE mineralization consists of semiconcordant, symmetrically banded veins and discontinuous seams, as much as 0.5–0.7 m thick and several tens of meters long. These occur throughout a vertical thickness of at least 150 m. Type-1 banded veins and seams are yellow-weathering zones, symmetric about a dark central zone, that are enriched in khanneshite-(Ce), barite, strontianite, and secondary LREE minerals (synchysite-(Ce) and parisite-(Ce)). The dark central zone, consisting primarily of ankeritic dolomite, barite, apatite, and strontianite, also has trace khanneshite-(Ce). These type-1 veins and seams alternate with dark, meterthick layers of ankerite-barite alvikite (wall rock) over a vertical distance of approximately 150 m. In some veins LREE carbonate minerals form dense spherically shaped aggregrates (100 micrometers diameter), presumably crystallized from immiscible droplets, which constitute as much as 30 percent (by volume) of the vein. Type-1 veins and seams average 19.92 weight percent (wt. percent) Ba, 3.61 wt. percent Sr, and 2.78 wt. percent total LREE. The values of ∑ LREE (∑ LREE is the sum of La, Ce, Pr, and Nd) for eight average whole-rocks range from 6.23 to 1.83 wt. percent.

Rare earth element mineralogy, geochemistry, and preliminary resource assessment of the Khanneshin carbonatite complex, Helmand Province, Afghanistan