An Overview of the European Kupferschiefer Deposits *
Borg Gregor, Adam PiestrzyŃski, Gerhard H. Bachmann, Wilhelm Püttmann, Sabine Walther, Marco Fiedler, 2012. "An Overview of the European Kupferschiefer Deposits ", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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The Kupferschiefer of northern central Europe is not only one of the largest sediment-hosted accumulations of copper ores worldwide (largest 1% of deposits with >60 Mt contained Cu) but has also one of the longest continuously documented mining histories, starting from at least 1,199 A.D. in the Mansfeld district of Germany. Kupferschiefer ores are currently mined in Poland from several large underground mines with active near-mine exploration and possible downdip extensions at a planning stage. Kupferschiefer mines in the Mansfeld and Sangerhausen districts of Germany had been largely exhausted by 1990 but a new exploration campaign is currently targeting a major deep Kupferschiefer resource near Germany's eastern border with Poland.
The Cu-rich part of the Kupferschiefer mineralization is dominated by chalcocite, chalcopyrite, and bornite and is hosted by several rock types including footwall sandstone and conglomerate, black shale, carbonate rocks in the immediate hanging wall, and anhydrite even higher in the hanging wall. Orebodies can range in thickness from 0.3 m, contained largely within the black shale of the Kupferschiefer sensu stricto, up to more than 50 m, where sublevel stoping, backfilling, and pillar mining reflect the pervasive mineralization. The ore zone can occur at various stratigraphic levels from (1) as low as some 35 m below the Kupferschiefer sensu strict, to (2) within and immediately adjacent to the black shale unit, to (3) several tens of meters above the base of the Zechstein limestone. Economic mineralization also occurs locally where no black shale has been deposited at all, for example, above Weissliegend sand dunes at the basin margin of the Kupferschiefer Sea that were never covered by the black euxinic mud. Ore textures include disseminated ores, disseminated replacement of dia-genetic and framboidal pyrite, crosscutting and bedding parallel veinlets, impregnation and replacement ore of carbonate and anhydrite cements, replacement of fossil shells, and even replacement of detrital feldspar and feldspar in lithic clasts.
All copper deposits share a marked metal and ore mineral zonation pattern adjacent to a major secondary redox front, the so-called Rote Fäule. This three-dimensionally, roughly hemispherically zoned mineralization system is transgressive and locally even steeply crosscutting to stratigraphy. It grades from an Fe3+ zone (hematite), through a locally developed precious metal (Au, Pt, Pd) zone, an always redox-proximal Cu zone (chalcocite, bornite, chalcopyrite), a locally overlapping Pb and Zn zone, into a distal Fe2+ zone of preore, commonly framboidal or early diagenetic pyrite. The oxidized part of the zoned orebodies commonly originates from permeable zones such as fault structures or sand dunes, which might have acted as valves through the relatively impermeable Kupferschiefer.
In general, the Kupferschiefer mining districts occur exclusively within an arcuate belt that is situated above basement rocks of magmatic arc origin, the Mid-European Crystalline High, typically at the intersections with major NW-SE– and NNE-SSW–trending fault structures. Local and regional studies have shown that regional metal distribution, orebody geometry, and metal grades are largely structurally controlled, although divergent opinions were originally expressed as to the timing of metal introduction via these conduits. An absolute age of ca. 255 Ma is generally accepted as the sedimentation age for the “Kupferschiefer” black shale. However, recent paleomagnetic age dating of mineralization at Sangerhausen has revealed late epigenetic mineralization ages of 149 and/or 53 Ma. The results argue for a new metallogenic model, which involves two major epigenetic pulses of metal introduction to the Kupferschiefer ores as impregnations, replacements, and subsequent veins and breccias.
A holistic understanding of the Central European Basin, which hosts the Kupferschiefer ores in its lower part of the stratigraphy, from the basin's origin in the Late Carboniferous to the Tertiary, and particularly the various related extensional and compressive tectonic events helps to put the individual stages of Kupferschiefer mineralization into a European plate tectonic perspective. The time span from Late Jurassic to Mid-Cretaceous was a period of major crustal rearrangement with the break-up of Pangea and the potential for the remobilization of major pulses of metalliferous brines. Both the main quantity of the Kupferschiefer ores and the giant Mississippi Valley-type (MVT) Pb-Zn ores of Upper Silesia appear to have formed at this stage. The younger, Tertiary, mineralizing event is also noted in both base metal provinces and was probably, again, related to crustal movement that involved metalliferous fluid flow. Additionally, this period was accompanied by magmatic pulses in the wider area of the Kupferschiefer metalliferous belt. Locally, late vein-type Co-Ni-rich mineralization, upgrading preexisting impregnation and replacement ores, gives evidence for this latest hydrothermal event, for example, in the German mining districts of Spessart/Rhön and Richelsdorf.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.