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Meggen |
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Germany |
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Zn Pb
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The Meggen stratabound, sediment hosted zinc-lead deposit is located in the Sauerland region of North Rhine-Westphalia in Germany, around 100 km east of Köln and 60 km WSW of Korbach. The deposit was exploited from 1852 until 1985 when the mine was closed and subsequently rehabilitated.
It lies within the "Rhenisch Schiefergebirge", a NE-SW striking Hercynian zone of folded Palaeozoic sedimentary and minor volcanic rocks.
The oldest rocks within the Sauerland are Ordovician slates and siltstones which were folded during the Caledonian orogeny in the Silurian after which the palaeogeography was dominated by the Old Red continent to the north and a subsiding depression, the Variscan Trough to the south. During the Lower Devonian this trough was the site of rapid subsidence and the deposition of a thick (>1000 m) sequence of high energy, oxygenated, shallow marine, coarse clastics, the Rhenish facies. Quartz keratophyric volcanism is represented by up to 300 m of tuffs in the late Lower Devonian (Emsian), originating from a centre to the south-east of Meggen.
These rift phase sediments were overlain by a complex Middle Devonian sequence, represented to the north on the shelf by coarse clastics, and to the south in the trough by variable thicknesses of pelitic sediments. The two facies were separated by and east-west trending Middle Devonian limestone reef.
In the Variscan trough there is a sudden change in the sedimentation at the base of the Eifelian to the deep marine pelitic rocks of the Wissenbach Shales (known locally as the Lennehelle Beds) which are around 500 m thick. At the same time on the shelf, around 1500 m of clastics derived from the Old Red continent were deposited on the shelf to the north-west, extending into the trough as a delta front forming the anomalous Ramsbeck Beds. Along the Altenbüren Lineament which separates the shelf and trough facies, there are mafic sills and rhyolite domes of Eifelian age.
Beneath the Meggen deposit the Wissenbach Shales are anomalously thin (around 200 m), possibly reflecting a palaeo-rise, the site of the earliest of the Middle Devonian reefs mentioned above. During the Givetian, deposition in the trough became more silty, represented locally by the Meggen Beds which immediately underlie the Meggen deposit. These beds are around 20 m thick at Meggen where they contain fragments of the Meggen Reef. On the shelf to the north the equivalents are 1000 to 1500 m thick. This was followed by renewed reef development along the shelf-trough margin later in the Givetian and Upper Devonian, while other reefs were established on the shelf itself. Black shales were deposited in inter reef and back reef settings. During this same interval the trough was starved of sediment and only thin pelites were deposited.
The Meggen sulphide-barite deposit was emplaced along the boundary between the Middle and Upper Devonian, grading laterally into, and partially overlain by 4 to 6 m of pelagic limestone known as the Lagerkalk, taken to represent a local starved basin, protected by an adjacent reef complex. At the same time, spilitic volcanism is recorded in a number of localities within the region associated with reef developments. Following the end of the Middle Devonian, the rate of subsidence in the trough changes dramatically with up to 600 m of silty shales being deposited, while only 20 to 200 m are laid down on the shelf. The Upper Devonian hanging wall sediments of the Meggen mineralisation, the Adorf Shales, comprise an up to 120 m thick, dark grey sequence of well banded shales with tuffite layers, and two layers of nodular limestone. To the southwest these same beds are more oxidised, red, green and calcareous. The Adorf shales include a carbonaceous unit (either a fetid limestone or black shale), the Kellwasser Limestone and a sequence of grey, green and red, sometimes calcareous shales and siltstones, overlain by Carboniferous black, often pyritic, Alum shales (the Alaunschiefer) and bedded cherts of the Kieselschiefer and then the siltstone of the Posidonia Schiefer.
There are no Variscan intrusives within the Rhenisch Schiefergebirge, unlike in the Harz mountains near Rammelsberg. The Variscan Orogeny reached its climax in the Upper Carboniferous, resulting in the folding of the Palaeozoic sequence, typically forming NW verging folds with steep or overturned NW anticlinal limbs and shallow SE limbs, a well developed uniform cleavage and SE dipping thrust planes.
The Meggen ore deposit occured as a conformable 3.5 to 4 m thick sheet like body that extended over a total strike length of around 6 km. It is structurally divided into two orebodies, the Old and the New. The Old orebody occupied the two limbs of the NE-SW trending Meggen syncline, separated by a sharp anticlinal axis from the New orebody immediately to the NW on the southern, overturned and faulted limb of the adjacent syncline. The pre-folding sulphide orebody was estimated to have dimensions of around 4500 m east-west and 1200 to 2200 m north-south, bounded to the north and south by a 500 m wide apron of barite.
The stratigrahic footwall is formed by the siltstones and sandstones of the Meggen Beds, and the hangingwall by fine grained micritic limestones of the Lagerkalk which contain thin alkaline tuffite bands. The orebody is in late Middle Devonian (Upper Givetian) position, overlain by Upper Devonian sediments.
The ore thickness, sulphide and Fe content are highest in the western parts of the ore deposit, while the highest concentrations of Zn and Pb are 500 to 1000 m east of the peak Fe interval. There is a vertical zonation from pyritic sediments of the upper Meggen Beds, to banded nodular pyrite and a metre of strongly nodular pyrite, then to the laminated pyrite with laminated sphalerite and galena, which is in turn overlain by a thin black shale (a few cm thick) and then the Lagerkalk. The lateral zonation from the sulphide to barite zone is quite narrow, generally of only 10 to 20 m wide.
The thicker, sulphide rich core of the deposit is composed of fine grained nodular and finely banded pyrite and marcasite with intergrown sphalerite and galena with a gangue of silty quartz, clay and carbonate interbeds. Barite mineralisation is peripheral to the main sulphide mass, occurring as fine grained, massive and dark grey barite.
The sulphide mineralisation is enriched in Hg (1100 to 8500 ppb), Tl (around 230 ppm), although the Ag content is low and was not recovered. Relatively enriched Mn values, up to >1500 ppm, form a lateral halo around the deposit in the equivalent micrite for up to 5 km along strike from the ore.
The original, pre-mining ore deposit is estimated to have contained 50 Mt @ 8% Zn (Max of >12% in the centre of the deposit), 1% Pb, 0.02% Cu, plus 10 Mt @ 95% BaSO4.
The most recent source geological information used to prepare this decription was dated: 1994.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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