Origin and enrichment of borates in a Late Paleozoic alkaline lake-playa deposit, Junggar Basin, NW China

Reedmergnerite (NaBSi3O8) and searlesite (NaBSi2O6 center dot H2O) are rare borates minerals over the world. In the Late Paleozoic Fengcheng Formation of the NW Junggar Basin, NW China, the two borates, especially reedmergnerite, occur as locally abundant minerals of potential economic interest. Here, petrology, minerology, and boron isotopes of different reedmergnerite occurrences in the Fengcheng Formation were systematically studied to explore its enrichment mechanism in sedimentary rocks. Reedmergnerite in the Fengcheng Formation is a temperature-controlled (crystallizing at 55-180 degrees C, mainly at 90-110 degrees C) and diagenetic replacive mineral. In the lake center deposits of the Fengcheng Formation, reedmergnerite is confined within sodium carbonate beds or nodules and shows a clear replacement of sodium carbonate minerals. In the lake slope deposits, reedmergnerite is also very abundant and mainly replaces calcite or dolomite-ankerite within sub-horizontal fractures or shortite/eitelite crystal molds, occasionally seen replacing bedded searlesite or growing both replacively and displacively in organic-rich matrix. No significant reedmergnerite is found in the lake margin deposits. Boron isotopes of reedmergnerite-bearing samples (-0.04 parts per thousand to + 7.69 parts per thousand) are higher than those of lacustrine mudstone samples (-14.2 to + 2.79 parts per thousand) and overlap those of ignimbrite samples (+2.37 to + 3.87 parts per thousand). Boron for diagenetic reedmergnerite formation was primarily inherited from the original hydrothermal fluids that flowed into lakes and its concentration was influenced by lake water pH and salinity. Potential early boron reservoirs were searlesite, sodium carbonates, organic matter and tuff materials. Fluids responsible for reedmergnerite replacement were weakly acidic. CO2 generated by organic matter degradation-maturation and carbonate dissolution acted as a mineralizer to promote reedmergnerite formation at a lower temperature range than the previously synthetic temperatures (>270 degrees C). Enrichment of reedmergnerite in sedimentary rocks requires a primary volcanic-related evaporated lake with high organic productivity, and meanwhile a deep burial environment, at best reaching the depth of oil generation window.