Submarine metalliferous carbonate mounds in the Cambrian of the Baltoscandian Basin induced by vent networks and water column stratification

Abstract

Two massive precipitation events of polymetallic ore deposits, encrusted by a mixture of authigenic carbonates, are documented from the Cambrian of the semi-enclosed Baltoscandian Basin. δ³⁴S (‒9.33 to ‒2.08‰) and δ³³S (‒4.75 to ‒1.06‰) values from the basal sulphide breccias, sourced from contemporaneous Pb–Zn–Fe-bearing vein stockworks, reflect sulphide derived from both microbial and abiotic sulphate reduction. Submarine metalliferous deposits were triggered by non-buoyant hydrothermal plumes: plumes of buoyant fluid were trapped by water column stratification because their buoyancy with respect to the environment reversed, fluids became heavier than their surroundings and gravitational forces brought them to a halt, spreading out laterally from originating vents and resulting in the lateral dispersion of effluents and sulphide particle settling. Subsequently, polymetallic exhalites were sealed by carbonate crusts displaying three generations of ikaite-to-aragonite palisade crystals, now recrystallized to calcite and subsidiary vaterite. T_h of fluid inclusions in early calcite crystals, ranging from 65 to 78 ºC, provide minimum entrapment temperatures for carbonate precipitation and early recrystallization. δ¹³C_carb (‒1.1 to + 1.6‰) and δ¹⁸O_carb (‒7.6 to ‒6.5‰) values are higher than those preserved in contemporaneous glendonite concretions (‒8.5 to ‒4.7‰ and ‒12.4 to ‒9.1‰, respectively) embedded in kerogenous shales, the latter related to thermal degradation of organic matter. Hydrothermal discharges graded from highly reduced, acidic, metalliferous, and hot (~ 150 ºC) to slightly alkaline, calcium-rich and warm (< 100 ºC), controlling the precipitation of authigenic carbonates.