The Rammelsberg deposit

This SGA Mineral Deposit Archive (MDA) comprises a series of 34 slides and presentation notes that explain the regional geology, structural setting, ore petrology, geochemistry and Devonian paleogeography of the Rammelsberg deposit.

The author of this description of the Rammelsberg deposit is Andreas Mueller.

Download the Rammelsberg deposit MDA (pdf,  6.7 Mb)

 

Summary: 

The Rammelsberg Cu-Zn-Pb sulfide-barite deposit in the Harz mountain range, northern Germany, was mined almost continuously for more than a 1000 years (968-1988 AD). The mine, located south of the medieval town centre of Goslar, is now a UNESCO world heritage site. The Rammelsberg is a type locality for shale-hosted, sedimentary-exhalative (SEDEX) Zn-Pb-Ag deposits but is unusual because of the high grade (27 Mt at 19% Zn, 9% Pb, 160 g/t Ag) and high copper-gold content (1% Cu, 0.5-1 g/t Au) of the sulfide ore.

The Harz is part of the unmetamorphosed slate belt of the Variscan orogen, formed in the Carboniferous during the collision of the paleo-continents Laurussia and Gondwana. The Rammelsberg deposit occurs in a NE-striking, overturned isoclinal syncline of Middle Devonian calcareous black shale, which is enclosed in sand-banded black shale and structurally overlain by Lower Devonian shelf sandstone. In the structural hanging wall but stratigraphic footwall of the sulfide ore, the black shale is altered to a hard quartz-chlorite-ankerite rock termed Kniest. The tightly folded Kniest wedge spans the entire width of the deposit. Pyrite, arsenopyrite and sphalerite disseminations in the Kniest, and sulfide mantos and spotted zones in the Lower Devonian sandstones define a broad zone of epigenetic footwall mineralization. The high-grade massive sulfide, located in the overturned fold limb beneath the Kniest is strongly deformed, recrystallized to a tectonic banding, and separated into two major lenses by reverse movement of the Kniest mass. 

The massive sulfide grades laterally into a fringe of shale-banded ore (2 Mt at 6.5% Zn, 3.5% Pb) and is compositionally zoned, stratigraphically higher sulfide-gangue lenses spreading beyond the lower ones. The lowermost lens consists of low-grade pyrite + Fe-dolomite + quartz, overlain by pyrite + Mn-dolomite with layers of chalcopyrite and sphalerite, and blanketed by gold-rich chalcopyrite-sphalerite-galena ore containing 5-10% Fe-dolomite and barite. The uppermost and most extensive layer consists of silver-rich sphalerite-galena ore with intercalated barite beds. Another two beds of sulfide-poor barite occur stratigraphically above the massive sulfide, separated by about 30 m of black shale. Laterally, the sulfide ore grades into the dolomite-rich ore horizon, marked by beds of felsic tuff and traced in drill holes 3 km to the northwest. The ore horizon contains more metal (13 Mt Zn + Pb) than the deposit itself (7-8 Mt Zn + Pb) defining a huge sedimentary-exhalative dispersion halo. The Kniest feeder system, ore textures, and sulfur isotope ratios suggest vent-proximal deposition of sulfide muds in a brine pool by a reduced, H2S-bearing fluid discharging at about 300°C. Radiogenic lead and osmium isotope data indicate deep fluid circulation and metal leaching from the thick pile (>1000 m) of Lower Devonian shelf sandstones and from paragneisses in the continental crust below.

Paleogeographic reconstructions of the Middle Devonian show that the Rammelsberg deposit formed at the faulted margin of an euxinic basin, part of the basin-and-ridge topography of a marine back-arc rift located at the southern margin of the Laurussian continent. Spilitized alkali basalt and trachyte/rhyolite, associated with hematite ore and pyrite mineralization on volcanic ridges, indicate high heat-flow and extensive seawater circulation. The plate-tectonic setting is remarkably similar to that of the present northwest Pacific, where the Okinawa Trough and the Sea of Japan represent sediment-filled marine rift basins opened in continental crust behind active arc-trench systems. The Japanese Kuroko volcanogenic massive sulfide deposits display ore grades and sulfide-gangue zones almost identical to those of the Rammelsberg, providing a genetic link between VMS and SEDEX, the two main classes of syn-volcanic and syn-sedimentary sulfide deposits.