Fenitized Wall Rock Geochemistry of the First Carbonatite Dyke at Bayan Obo, Inner Mongolia, China

Abstract

The first carbonatite dyke at Bayan Obo is well exposed on the surface for a length and width of approximately 60 m and 1.1–1.5 m, respectively. Along its strike, the fenitized H1 (Qs) and H2 (Cs) quartzite is replaced by Na‐amphiboles, aegirines, and alkali‐feldspars, intermittently stretching as far away as 800 m in length. Based on petrographical characteristics, the dyke's fenitized wall rocks are divisible into different zones: (1) outer, (2) middle, and (3) inner. The outer zone is 5–17 m from the NW margin of the dyke. The middle zone is located at 3.5–5 m from the NW margin of the dyke. The inner contact zone is located between direct contact with the dyke and 3.5 m from the dyke. In the outer zone, upon visual examination, no evidence of outcrop fenitization was found and the major elemental rock composition is nearly identical to the unaltered H1 and H2 lithologies. In the thin sections, however, small amounts of Na‐amphibole and phlogopite are present. Despite relatively poor development throughout the 5 m of fenitization, the wall rocks have retained at least a small geochemical signature comparable to the original sedimentary protolith. The fenites occurring in the inner zone exhibit distinct variations, not only for the sharp contact at the outcrop scale, but also for variations in major, rare earth elements (REE), and trace elements and Sm‐Nd isotope composition. The wall rocks within 3.5 m have undergone strong fenitization, inheriting the geochemical signature derived from the carbonatite dyke. Fenitization in the middle zone was not as strong, at least compared to the inner zone, but was stronger than the outer zone. Compared to some trace elements and REEs, the major elements are relatively immobile during fenitization. The Sm‐Nd isotope data for the carbonatite dyke and the adjacent fenitized wall rocks, where the Sm and Nd originate solely from the dyke, plots as a six‐point isochron with an age of 1308±56 Ma. This age is identical to that of ore‐bearing dolomite carbonatite and the related ore‐forming events, indicating that there may be a petrogenetic link between the two. Based on Sr and Nd isotope compositional data, the first carbonatite dyke may be derived from an enriched mantle.

Niobium mineralization of sedimentary carbonates, Lewisian Complex, UK

“…The supra-subduction setting of the marble in a terrain intruded by alkaline rocks, and alteration of the marble to feldspar-rich rocks, suggests that the marble may be fenitized. Many examples of Nb mineralization, including mineralized limestones, are associated with fenitization (Elliott et al 2018), and fenitization is recorded at Bayan Obo (Wang et al 2018). Fenitization does occur in Northern Highlands of Scotland, cutting Moinian and Devonian rocks (Tanner and Tobisch 1972; Garson et al 1984) and attributed to latest Caledonian magmatism.…”

Post-Magmatic Fluids Dominate the Mineralization of Dolomite Carbonatitic Dykes Next to the Giant Bayan Obo REE Deposit, Northern China

“…The REE-bearing minerals are mainly hosted in the dolomitic rocks. The debate about whether the ore-hosting rock is sedimentary [1,[5][6][7][8][9][10][11][12] or igneous in origin [2][3][4][13][14][15][16][17][18][19][20][21][22][23][24][25] has lasted since the discovery of the deposit. Extensive application of advanced analytical techniques, such as in situ isotopic analysis, has demonstrated that the ore-hosting dolomitic rocks of the deposit are igneous in origin [26][27][28].…”

“…Based on the main constituent mineral, dykes in the Bayan Obo ore district can be classified into calcitic, dolomitic, and calcite-dolomitic carbonatites [21,29,30]. Apart from some studies (e.g., [21]) reporting the bulk rock geochemical characteristics of some dolomite carbonatite dykes (Dol-dykes), most previous research has concentrated on a calcite carbonatite dyke (Cal-dyke) named the Wu dyke in the northeast of East Orebody [12,18,30,31], as this dyke contains extremely high REE contents up to 20 wt.% [31] and caused prominent fenitization of its country rocks. However, rocks from Cal-dykes cannot be related directly to the ore-hosting dolomites as they may be in different evolution stages of carbonatitic magmas [21,28,29].…”