A miscibility gap in trioctahedral Mn-Mg-Fe chlorites: Evidence from the Lienne Valley manganese deposit, Ardennes, Belgium

Abstract

Low-temperature veinlets crosscutting low-grade manganiferous ironstones of Ordovician age contain four texturally distinct types of chlorites with nearly constant Al/Si-ratios that form two separate populations regarding their Mn/Mg/Fe-ratios: One with low iron contents (<1.5 w.% FeO) and molar Mg/Mn-ratios just below unity (magnesian pennantites), the other with higher iron (7–11 w.% FeO) and Mg/Mn≳4 (manganoan clinochlores). The two populations, which can be distinguished readily by their characteristic optical elongation and dispersion colors, are intimately intergrown and have formed partly during consecutive stages of a chlorite crystallization sequence, partly by simultaneous growth and possibly even as exsolution products of a pre-existing homogeneous chlorite phase of intermediate composition. These findings indicate a miscibility gap in the chlorite solid solutions beginning along the binary Mg-Mn series and extending into the ternary system. There may be a solvus relationship with the miscibility gap closing at higher temperatures (400° C?). One very intensely colored chlorite type of the pennantite population may contain Fe3+ or Mn3+ or both. Additional minerals in the veinlets are spessartine, kutnahorite, quartz, and an allanite-piemontite phase. Crystallization began near the centers of the present veins with Mn-rich minerals and continued towards their edges and into the extremely thin ends of the developing fractures with the deposition of the more Fe-rich chlorites that are in equilibrium with the ironstone.