The Visitor mine “Drei
Kronen und Ehrt“
Drei
Kronen und Ehrt is now a show mine and was in active use until 1990. During the
time of active minig, the pit was called Grube Einheit. Stratigraphically, the
place belongs to the geological unit Elbingerode Complex.
The
Volcanic-hosted massive sulphide mineralization at the Einheit deposit in the
Harz Massif as part of the German Variscan is one of only two occurences of this
mineralization type in Germany. It is associated with Middle Devonian acid
volcanics in sequences. The mineralization consists almost entirely of pyrite,
and was exploited as a sulphur-source for manufacturing acid.
History
Mining
of the iron ores of the Elbingerode Complex is documented back into the 10th
century, new archaeological findings even give hints on iron smelting in the 4th
century. The earliest document mentioning the sources of the “Great Graben”
originates from 1582. At this time the primary pyrite ores have probably already
been discovered after centuries of mining of the overlaying gossan. In the 18th
and 19th century the mining was transferred completely to underground
work at up to 328 single collieries.
The
most successful mining period in the Elbingerode Complex began after World War
II. The three mines Büchenberg, Braunesumpf and Einheit delivered iron ores
during this period till 1970. After this, only “Grube Einheit” continued
mining of the massive sulfides for further 20 years. The peak level of
production was reached in the early 70s with 381.144 t/year raw material and
56.559 t/year sulfur content.
In 1990 the production ceased because it was no longer economic. During the time
of activity, about 13 Mt pyrite-rich material were raised from this mine; the
sulfides from the whole Elbingerode Complex contained 50 Mt Fe.
Geology
and ore deposites of the Elbingerode Complex
This
stratigraphic unit hosts some of the most important ore
deposits in the Harz Mts. Over centuries the mining of ironores has been of
economical importance for the region, while today the recovery
of Devonian limestones dominates.
The
Elbingerode Complex consists of volcanics and reef carbonates, which are framed
by shales and greywackes. The foot wall are the Wissenbacher Shales. In Middle
Devonian, the magmatic activity of the early Variscan orogeny was initiated and
submarine volcanic outflows developed along NE-SW striking crack systems. The
bulky volcanic series of acid and basic rocks was deposited during three main
phases. The ore horizon is located in the upper part of these series above
trachitic lavas. These differ in
color from light green-grey to violet-grey, which
indicates a changing redox potential. Between the phases of volcanic activity
and after its extinction, large reef carbonate complexes developed to a maximum
thickness of about 600 m. The growth of the reefs ended in the Upper Devonian.
During
further geological history, the volcanics-reef-complex in the sedimentary trough
was covered by shales and greywackes. The Variscan orogeny (Upper Devonian to
Carboniferous) deformed the thick sedimentary layers together with the
reefstones and volcanics into a disharmonic fold pattern.
A
further multiphase tectonic deformation occurred during the Mesozoic, resulting
mainly in NW-SE and N-S striking faults and fracture zones. Following elevation
and erosion of the Harz Mountains removed the mesozoic sediments and exposed the
reef limestones and volcanics to the surface.
Mineralizations
In
the Upper Givet a late magmatic hydrothermal circulation systems developed at
the seafloor, and caused the deposition of synsedimentary iron ores, bound to
the middle and upper volcanic group. A sulphidic and an oxidic ore type can be
distinguished, existing with direct transitions. The massive sulfides mainly
consist of fine grained pyrite precipitations up to 45 m in thickness. The
stratiform orebody is overlain by the reef carbonates. At the contact to the
hanging wall, at the top of the sulphidic orebody the
hematitic transition zone exists. Also in lateral vicinity to the massive
sulfides, hematitic iron ores can be found. The contact to the footwall is
gradual, the sulfides exceed into the volcanic rocks as impregnations, crack
fillings, veins and debris.
Beside
the pyrite, which makes up about 80% of the sulfide ore, other sulfides, like
sphalerite, exist in very small amounts, trace elements are rare as well. Trace
element concentrations suggest a uniform iron source for both ore types. The
composition of the oxidic orebody differs in its chemistry and mineralogy.
Directly on top of the volcanics are often red siliceous hematite ores. In the
upper parts mainly magnetite-chlorite-ores and magnetite-siderite-pyrite-ores
can be found. The important preconditions for the hydrothermal activity and the
enrichment of the iron, mobilized from the volcanic basement rocks are the
thermal influence of the magmatic source and the tectonic position, which is
characterized by cross cutting and deep fault systems.
Dipl.-Geol.
Friedhart Knolle, Goslar, 2006
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