The Canadian Mineralogist Vol. 55, pp. 1-14 (2017) DOI: 10.3749/canmin.1700005 SILICIFICATION OF HYDROTHERMAL GANGUE MINERALS IN Pb-Zn-Cu-FLUORITE- QUARTZ-BARYTE VEINS MATHIAS BURISCH § Institut f¨ ur Mineralogie, Technische Universit¨ at Bergakademie Freiberg, Brennhausgasse 14, 09599 Freiberg, Germany BENJAMIN F. WALTER AND GREGOR MARKL Institut f¨ ur Geowissenschaften, Universit¨ at T¨ ubingen, Wilhelmstrasse 56, 72074 T¨ ubingen, Germany ABSTRACT Late-stage pseudomorphous and perimorphous replacement of euhedral baryte and, to a lesser extent, fluorite and calcite by quartz is a common phenomenon in hydrothermal vein-type deposits. As a consequence of silicification, the primary mineral assemblage might be substantially altered, and therefore this process has a severe negative impact on the economic potential of mineral resources. Although these replacement textures are often reported and have a significant economic importance in mines producing baryte or fluorite of chemical grade, the process that causes this silicification is surprisingly poorly understood. In the present contribution, more than 40 Jurassic–Cretaceous and post-Cretaceous hydrothermal veins from the Schwarzwald mining district, including replacement textures of primary euhedral baryte, fluorite, and calcite, were investigated with respect to their macro- and microscopic textures. It appears that baryte is favorably replaced by pseudomorphs (bladed quartz), while fluorite and calcite are typically replaced by perimorphs. The textures indicate that the mode of replacement of the primary minerals happens continuously and after the initial vein formation. By combining these textural observations with calculated mineral solubilities, a detailed geochemical model has been developed. Existing fluid inclusion data indicate that substantial cooling of the hydrothermal solutions occurs after primary mineral formation. The calculated cooling path reveals opposing solubilities of quartz and the other gangue minerals (baryte, fluorite, and calcite) with decreasing temperature and explains the observed dissolution and precipitation textures. Furthermore, differences in temperature-solubility systematics between baryte on the one hand and fluorite and calcite on the other are responsible for the differences observed in the textures. This agrees with the occurrence of late-stage, low-temperature baryte crystals overgrowing primary baryte assemblages. Conversely, analogous late-stage calcite and fluorite assemblages are only rarely observed. In summary, silicification is a typical cooling effect in various hydrothermal vein-type deposits, but affects different gangue minerals in different ways depending on their temperature-dependent solubility. Keywords: silicification, mixing, post-ore cooling, pseudomorphous replacement, baryte, fluorite, calcite, bladed quartz. INTRODUCTION Pseudomorphous and perimorphous textures of quartz after the gangue minerals baryte, fluorite, calcite, and anhydrite seem to be typical of hydro- thermal vein-type deposits (Baier & Venzlaff 1961, Baumann et al. 2000, Leach et al. 2004) from various localities. Although primary minerals (especially baryte) are often replaced entirely by quartz, the original mineral texture is well preserved (‘‘zerhackter Quarz’’ is a classical German miner’s expression for this texture. Bladed pseudomorphous quartz is com- monly used in the English-language literature). Two different mineral replacement textures occur in the hydrothermal veins of the Schwarzwald, SW Germany: (1) the primary mineral is dissolved and replaced by another mineral phase throughout the entire dissolution process. This results in either the replacement of the entire primary mineral phase (Fig. 1C, F) or the replacement of only the outer rim, § Corresponding author e-mail address: mathias.burisch@mineral.tu-freiberg.de 1