143-1 Alkaline Lake Systems and Critical Metal Enrichment: Characterizing Na-Carbonate Mineral Assemblages in the Permian Fengcheng Formation, Junggar Basin, NW China

Abstract:

Alkaline lake systems are unique depositional environments that host diverse Na-carbonate mineral assemblages and are increasingly recognized as important sources of critical metals, including lithium, rubidium, and boron. The Lower Permian Fengcheng Formation in the Junggar Basin, NW China, is a well-documented alkaline lacustrine deposit containing abundant sodium carbonates. However, the potential for critical metal enrichment in this formation has not been systematically evaluated, and the relationship between Na-carbonate mineral assemblages and metal endowment remains poorly constrained.

This study presents new data from Well Xiayun 1 (XY1) in the Mahu Sag, where a 12.24 m continuous core was recovered from the Feng-2 Member of the Fengcheng Formation (5376.94–5389.18 m). Integrated petrographic, mineralogical, and geochemical analyses, including XRD, SEM-EDS, EPMA, and ICP-MS, were conducted to characterize Na-carbonate mineral assemblages and quantify critical metal concentrations.

XRD analysis identifies a diverse Na-carbonate assemblage including shortite (Na₂Ca₂(CO₃)₃, ~11%), northupite (Na₃Mg(CO₃)₂Cl, ~1.4%), and trona (Na₃H(CO₃)₂·2H₂O, ~7%), interbedded with dark-colored tuffite composed of K-feldspar, plagioclase, quartz, and pyrite. The core exhibits cyclic alternations of light-colored alkaline beds and dark tuffite, recording episodic volcanic input and evaporative concentration under arid climatic conditions. Well log signatures (low GR, high AC, very high resistivity) are diagnostic of Na-carbonate evaporites.

ICP-MS analyses reveal significant critical metal enrichments: Li up to 380 ppm, Rb up to 450 ppm, and B up to 1,200 ppm—the first such documentation in the Fengcheng Formation. SEM-EDS and EPMA identify shortite as the primary host for Li, authigenic K-feldspar and illite as hosts for Rb, and searlesite/reedmergnerite as hosts for B. These findings indicate that critical metal enrichment occurred alongside Na-carbonate precipitation, controlled by a combination of hydrothermal input, evaporative concentration, and diagenetic incorporation into specific mineral phases.

A genetic model is proposed linking alkaline lake evolution, episodic volcanism, and hydrothermal activity to coupled Na-carbonate mineralization and critical metal enrichment. This study establishes the Fengcheng Formation as a prospective alkaline lake-hosted critical metal system and provides exploration criteria applicable to other ancient evaporite basins globally.

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