Formation of galena pseudomorphs after pyromorphite
Keim, Maximilian F.; Markl, Gregor
published: Sep 1, 2017
ArtNo. ESP154019403000, Price: 29.00 €
Galena pseudomorphs after pyromorphite, also called "Blaubleierz" (BBE, blue lead ore in German), is one of the most famous and exceptional European classics among museum mineral specimens. Despite the high interest it has in the museum and mineral collecting community, a detailed textural analysis and the quantitative understanding of the processes leading to its formation by modern mineralogical and hydrogeochemical techniques have not been undertaken yet. In this study, we focus on these remarkable pseudomorphs combining historical mineral samples with water analyses from the Kautenbach Mine near Bernkastel, Germany (the most famous locality of these specimens) and combine them with hydrogeochemical modelling to present a specific formation model for the Kautenbach locality. For textural comparison, mineral samples from the three other known BBE occurrences (Zschopau, Ore Mountains, Germany; Huelgoat, Brittany, France; Wheal Hope, Cornwall, UK) were investigated. Textures from the Kautenbach samples show that galena replacing pyromorphite occurs in symplectic/spongy intergrowth with fluorapa- tite. On some samples this texture is again overgrown by later pyromorphite. Calculated stability diagrams show that galena and fluorapatite can only coprecipitate in a small pH–fO2 range from a (Ca + F)-bearing fluid. The analyzed thermal waters from the Kautenbach mine fulfill these requirements and the formation of BBE is reproduced by a thermodynamic fluid path model using the water analyses. The spongy appearance of galena at the Kautenbach Mine is also observed at the BBE samples from Wheal Hope and Zschopau. This common textural feature speaks for a 1:1 conversion of pyromorphite to galena/(fluorapatite) caused by the different mineral densities. In contrast to Kautenbach, the spongy appearance at Zschopau and Wheal Hope is not caused by symplectic intergrowth with fluorapatite, but by the appearance of empty cavities. At Huelgoat, galena does not occur spongy, but compact. To form this texture without cavities, additional lead is needed because the amount of lead within prior pyromorphite is not sufficient to form massive galena. Since lead and sulfide cannot be transported by the same fluid due to the low solubility of galena, this feature could possibly be explained by a small-scale fluid mixing of an oxidized supergene lead-bearing fluid with a reduced sulfide-bearing fluid. In all cases, the formation of BBE impressively shows the drastic change of fluid regimes in these near-surface system. An oxidational milieu forming pyromorphite can change to a reduced sulfide-bearing one precipitating galena and vice versa. This study, motivates to look at sulfide minerals related to oxidation zones with a different view, especially in the vicinity of thermal fluids.