Fluid evolution and ore deposition in the Harz Mountains revisited: isotope and crush-leach analyses of fluid inclusions

David A Banks,Stefan De Graaf,Heike Kaden,Volker Lüders,Hubert B Vonhof,John J G Reijmer,Marta Sośnicka

doi:10.1007/s00126-019-00880-w

Abstract

Hydrothermal fluid flow along fault zones in the Harz Mountains led to widespread formation of economic vein-type Pb–Zn ore and Ba–F deposits during the Mesozoic. We reconstruct the fluid flow system responsible for the formation of these deposits using isotope ratios (δ2H and δ18O) and anion and cation contents of fluid inclusions in ore and gangue minerals. Building forward on extensive studies in the 1980s and 1990s, our new geochemical data reveal that seawater evaporation brines, which most likely originated from Zechstein evaporites, descended deeply into Paleozoic rocks to leach metals at depth. In Jurassic times, these metal-rich brines episodically recharged along fault zones and mixed with shallow crustal H2S-bearing brines. Primarily in the Upper Harz Mountains, this mixing system led to the formation of economic Pb–Zn–Cu mineralization, which locally shows banded textures with alternations of sulfide minerals and quartz or carbonate (mostly calcite). In the Middle and Lower Harz Mountains, Zechstein-derived brines interacted with K- and F-bearing basement rocks and/or magmatic rocks to deposit fluorite mineralization upon ascent in the Upper Cretaceous. The proposed model of mineralizing fluids originating as (evaporated) seawater has been shown to hold for numerous basin-hosted base-metal sulfide and fluoride deposits elsewhere in Europe.

Highlights

Hydrothermal vein-type deposits in the Harz Mountains (Harz Mts.) were mined from the Middle Ages until the end of the twentieth century and have been the subject of scientific research for over 100 years (Fig. 1)
Considerable variations in fluid inclusion isotope ratios and salinities suggest that various fluid reservoirs were involved in the deposition of a wide variety of vein-type mineralization in the Harz Mts
Zechstein brines are recognized to be of major importance throughout the main mineralization stages during the Mesozoic

Summary

Introduction

Hydrothermal vein-type deposits in the Harz Mountains (Harz Mts.) were mined from the Middle Ages until the end of the twentieth century and have been the subject of scientific research for over 100 years (Fig. 1). Whereas Stahl (1929) proposed a Mesozoic age for vein-type mineralization in the Upper Harz Mts., mid-twentieth century research generally assumed that the expulsion of magmatic fluids from Late Carboniferous granitic intrusions caused widespread deposition of sulfide, fluorite, and barite-rich veins (e.g., Dahlgrün 1950; Jacobsen and Schneider 1950; Wilke 1952). Möller and Lüders (1993) summarized the results of interdisciplinary research on the Pb–Zn, fluorite, and barite deposits in the Harz Mts. and concluded that mixing of metal-rich saline brines with reduced sulfur-bearing fluids gave rise to the deposition of quartz–sulfide and carbonate– sulfide ores along WNW–ESE striking fracture zones. The uplift lasted throughout the Tertiary and caused erosion of the Mesozoic cover and exposure of the Paleozoic rock sequences at the surface

Methods

Results

Discussion

Conclusion