Preliminary radiogenic isotope study on the origin of the Khanneshin carbonatite complex, Helmand Province, Afghanistan

Abstract

The REE-mineralized Khanneshin carbonatite complex in southern Afghanistan is an economically light rare earth element (LREE)-enriched carbonatite complex of Quaternary age. Geological studies have suggested that the LREE-enriched rocks at Khanneshin are comparable in grade to many economic REE deposits, including parts of the world-class Bayan Obo (China), and Mountain Pass, California (United States) deposits. The extent of mineralization at Khanneshin is unknown; however, mineralized areas delineated as part of a reconnaissance study are estimated to contain ~1.29Mt REE ore. We have determined for the first time the radiogenic isotope compositions (Pb–Sr–Nd) of bulk rocks and mineral separates (barite, strontianite, carbonate, etc.) of this carbonatite complex and propose the likely source of radiogenic isotope reservoirs associated with the REE mineralization. This study provides an opportunity to establish fundamental isotopic features of this highly mineralized system to aid in regional exploration efforts and to identify other comparable mineralized systems that shared similar source reservoirs, origins, and evolution in southern Afghanistan.The carbonatite complex consists of coarse-grained sövite and brecciated and agglomeratic barite–ankerite alvikite. Small satellitic intrusions of biotite–calcite carbonatite and rare leucite phonolite also crop out. The massif is encircled by alkali metasomatized Neogene sedimentary strata. A peripheral apron of volcanic and volcaniclastic strata extends farther away from the complex. Present-day Pb isotopic compositions of bulk rocks (dikes, welded tuffs, sövite, and mineralized rocks) and mineral separates exhibit a narrow range in 206Pb/204Pb (18.814–18.877), 207Pb/204Pb (15.616–15.674), and 208Pb/204Pb (38.892–39.094). The results show that the source region of the carbonatite had a moderate compositional variation (most samples overlapping within the level of analytical precision). Present-day neodymium isotopic compositions from the Khanneshin carbonatite show a range in 143Nd/144Nd (0.512374–0.512462). Present-day strontium isotopic compositions also have a moderate range in 87Sr/86Sr (0.708034–0.709577); the mineral separates and ore are somewhat less radiogenic in 87Sr/86Sr (0.707954–0.708686) than the whole rocks.A fundamental feature of the Khanneshin carbonatite is the overall high degree of isotopic homogeneity evident for the Nd–Sr–Pb isotopic systems and we regard this consistency as a diagnostic feature related to its evolution. The carbonatite constitutes a unique isotopic target for identification of other mineralized systems that might have shared the same source. The bulk rocks and minerals in the deposit are generally analytically isotopically indistinguishable, regardless of the mineralogical complexities. All rocks and minerals shared radiogenic isotopic reservoirs, a common evolution, and have remained in isotopic equilibrium. Isotope variations eliminate the source of MORB rocks, or depleted mantle (DMM), as a significant reservoir to the Khanneshin carbonatite. Compared to model mantle reservoirs, the Pb and Nd isotopes of the Khanneshin carbonatite roughly suggest source combinations of enriched mantle, type EMI and HIMU. Sr isotopic data highlight the contribution of another source (EMII?) to account for the relatively high values of 87Sr/86Sr. The radiogenic isotope compositions of the Khanneshin carbonatite differ from world-class REE-mineralized systems such as Bayan Obo (China) and Mountain Pass (California).