Rb-Sr Alter und Sr-Pb Isotopencharakteristik von Gangmineralisationen in paläozoischen Gesteinen am Nordrand des linksrheinischen Schiefergebirges (Raum Stolberg-Aachen-Kelmis) und Vergleich mit den rezenten Thermalwässern von Aachen-Burtscheid

Vein mineralizations of different age and formation were studied in the northern Eifel area and its foreland. The focus was put on microstructure, geochemistry and isotopic composition to characterize discrete fluid flow events and to date their generations. Samples from the geothermal well RWTH – 1 representing Variscan vein mineralizations, from the Pb – Zn vein mineralizations of the mining district Stolberg – Aachen – Kelmis with special emphasis on veins from the limestone quarry Hastenrath of Postvariscan age and from the recent thermal springs of Aachen and Burtscheid were investigated. RWTH – 1 sank down in the town of Aachen in 2004 is situated within the Aachen fold and thrust belt and reached a final depth of 2544 m. From top to the base the bore hole exposed Upper Carboniferous, Upper Devonian and Lower Devonian carbonate and siliciclastic sediments. Three core cuts display intensive hydrothermal veining with variable orientations. These veins are mm to cm wide and represent extension structures with characteristics of deformation and recrystallization in minerals of the rim parts. Calcite, ankerite, dolomite, chlorite, quartz and occasionally pyrite are the typical vein minerals with chlorite found along the vein margins and the carbonates generally in the centre. Crack-seal processes indicate brittle deformation and multiple reactivations of the opening. The chemical compositions of the chlorite from veins of the first and second cored section of the well indicate formation temperatures of 290°C to 370°C. Homogenization temperatures of fluid inclusions found in the vein minerals are < 390°C (Lögering, 2008). These temperatures reflect typical Variscan metamorphic temperatures along the northern part of the Rhenohercynian fold belt (Behr et al., 1993, Muchez et al., 2000). The precipitation of the carbonate minerals within the veins is attributed to cooling of a fluid phase and its change in pH by reaction with the carbonate-bearing wall rocks during uprise in the fault systems of the rocks (compare also Lögering, 2008). The pronounced factionation of rare earth elements suggests that the components of the precipitated carbonate minerals in the veins cannot be derived from the immediate wall rocks aside the veins but must have come from more distant rock volumes. Independant from the stratigraphic level these carbonates are characterized by a significant Eu anomaly which is interpreted to show a carbonate formation at increased temperatures. Initial 87Sr/86Sr ratios of calcite and chlorite of the veins cannot become identical at any times. Thus a Rb-Sr isochrone age of the mineral assemblage of the veins cannot be obtained. Model calculations for the chlorite – carbonate mineral pairs using a paleo-mixing of different Sr isotope compositions suggest a model age of 218±15 Ma (Beiss, 2008) which is similar to 40Ar/39Ar ages of the vein chlorites of 182±18 Ma (Sindern et al., 2008). Of course these "ages" do not correspond to the Variscan formation of the veins. The initial 87Sr/86Sr ratios of carbonate minerals and chlorite, both from the veins, and the fractions of the corresponding host rocks soluble in HCl show a considerable variation for Carboniferous times. Calcite varies from 0,71107±1 to 0,72119±1, Chlorite from 0,71513±4 to 0,72628±3 and the soluble part of the host rocks from 0,71049±1 to 0,72138±1. This variation in Sr composition may be an effect of intensive interaction of the fluid phase with the corresponding wall rocks of the veins but can also be interpreted as distinct fluid pulses with individual Sr compositions. In any case, since chlorite is always higher in 87Sr/86Sr than the associated carbonates the two phases cannot be precipitated simultaneously. Chlorite may have formed by interaction of a fluid phase with the country rocks or, chlorite experienced a late alteration. Likewise, the carbonate components are not exclusively derived from the wall rocks. Pb isotope compositions of pyrite (206Pb/204Pb = 18,229 – 18,254; 207Pb/204Pb = 15,589 – 15,592) characterize the Variscan mineralizations. The Lower Carboniferous limestone of the Hastenrath quarry shows 87Sr/86Sr ratios of 0,70812±1 and 0,70817±1 which is typical for a marine formation. For the source of C delta13C of this rock suggests formation waters. Diagenetic processes within this limestone are documented by small-scale branching structures composed of dolomite. The structures are cross cut by non-oriented calcite veins with Sr isotope compositions distinctly higher radiogenic than the surrounding country rocks. Sr of these veins has an unknown source. The structures of the Paleozoic limestones are block faulted perpendicular to their strike by NW –SE trending faults which exhibit a Pb – Zn vein mineralization typical for the Stolberg – Aachen – Kelmis mining district. Three different mineralized zones can be distinguished within the veins (zones 1, 2, 3) which indicate at least three crack – seal processes and thus at least three periods of fluid flow. The brecciated zone 1 marks the first fracture sealing process. It contains fragments of galena crystals, chalcopyrite, bornite, quartz, dolomite, ankerite and blocks of the wall rocks fixed in a calcitic matrix. The second fracture sealing phase (zone 2) is composed exclusively of large crystals of calcite. Zone 3 is made of calcite which is overgrown by collomorphic sphalerite. Within this ZnS inclusions of galena, chalcopyrite and bornite are found. In parts of the sphalerite the Cd content is very high (up to 6,5 wt.-%). Within one of the investigated veins the minerals are symmetrically deposited on the limestone wall rocks. The central part of this veins contains a small filling of lignite coal. It marks the fourth fracture sealing phase. The transport of this lignite coal into the centre of the vein may be explained by tectonic shearing along the fault. The homogenization temperatures of fluid inclusions in calcite from sealing zones 1, 2 and 3 are distinctly different. The total span covers 80,7°C to 179,7°C. The salinity varies from 10,24 to 23,08 wt.-% NaCl equivalent and thus are within the range of Postvariscan NaCl – CaCl2 – H2O fluids of other occurrences of the northern margin of the Rhenohercynian fold belt. The REE distribution patterns of calcite of zones 1, 2 and 3 are similar to those of the wall rocks. All carbonate minerals exhibit a small negative Eu anomaly indicating a reducing character of the fluid phase. delta18O of calcite increases from zone 1 to zone 3, delta13C decreases. The delta are characteristic for hydrothermally crystallized carbonate minerals. delta13C of calcite from zone 3 (-7,53 - -8,70%) may be interpreted as a mixture of C from the wall rocks and C from a hydrothermal source higher in temperature. Late stage calcite formations in limestone caves of the Hastenrath quarry show similar C and O isotopic compositions than calcite from zone 3 of the veins. Sphalerite from zone 3 occurrences were dated by Rb – Sr. The isochron reflects an age of 134,3±1,3 Ma which defines a period of formation at the turn from the Jurassic to the Cretaceous. The large range of 87Sr/86Sr ratios for calcite even from single zone 1, 2 or 3 indicates the complex conditions of formation. The range of Pb isotope ratios of calcite of the veins, galena and sphalerite is small and very similar to other occurrences of Postvariscan mineralizations of the northern Eifel area. The Pb ratios suggest a crustal source of this lead and a possible remobilization by repeated hydrothermal pulses from mixed and homogenized sources. Pb–Zn vein mineralizations of Diepenlienchen, Albertsgrube, Altenberg and Bleiberg in Belgium and Thermae 2002, Netherlands, represent equivalents to the mineralization of the limestone quarry Hastenrath. Sphalerite in its collomorphic appearance, galena, pyrite, marcasite, the carbonates calcite, dolomite, ankerite and siderite, and quartz are prominent minerals. In contrast to Hastenrath the sphalerite here is rich in Fe (< 5,81 wt.-%). This Fe enrichment is interpreted to show elevated temperatures during crystallization. Carbonate minerals of Bleiberg are characterized by a positive Eu anomaly while die other occurrences show negative Eu’s. The Sr isotope compositions of carbonates vary considerably, and similar to Hastenrath also for Bleiberg several fracture sealing generations of calcite are recognized (Muchez et al., 1994). The Pb isotope compositions of galena, sphalerite and pyrite plot into the field of Postvariscan mineralizations defined by Krahn (1988) as do these minerals of Hastenrath. The Rb–Sr systematics date the crystallization of sphalerite to 129,9±9,7 Ma for Diepenlienchen, to 137,1±1,7 Ma for Altenberg, and to 134,5±4,1 Ma for Thermae 2002. Thus, within the limits of error, all these vein-type Pb – Zn mineralizations of the mining district Stolberg – Aachen – Kelmis took place at the turn from the Jurassic to the Cretaceous as defined by Odin (1994). This age marks a time of block faulting in Central Europe which is related to phases of opening of the North Atlantic Ocean. It is characterized by intensive hydrothermal activities and a wide-spread formation of ore deposits. The thermal spring waters along the Aachen and the Burtscheid thrust system document recent fluid flow. Their chemical composition is used here for a comparison with the Variscan and Postvariscan fluid flows. The mineralizations of the springs reflect several geochemical processes. Subrecent sinter formations are a direct segregation product of the springs. Catchment areas, transport routes and geochemical enrichment and depletion processes can be modelled by new Sr and Pb isotope compositions of the thermal water and, thus, are used to refine the genetic models of Pommerening (1993) and Herch (1997). The 87Sr/86Sr of the thermal water ranges from 0,71607±1 to 0,71618±1 and reflects mixtures of several ground water types which took part in different water-rock interactions. In any case, the Sr compositions are distinctly more radiogenic than sea water during the whole Phanerozoic. Sr sources of the Aachen and Burtscheid waters will be Paleozoic siliciclastic and carbonate rocks, possibly also evaporitic sediments. The spring precipitations exhibit 87Sr/86Sr ratios of 0,71600±1 to 0,71615±1 which falls into the range of Sr compositions of the waters. Pb isotope compositions of the spring waters and their precipitations scatter over a wide range. This scatter may be caused by complex mixing of different ground waters characzerized by different Pb sources. This is suggested since Pb compositions of the Variscan and Postvariscan ore deposits described above scatter in a very similar way. An anthropogenic contamination of the springs by lead, however, cannot be excluded. The Variscan, Postvariscan and recent fluid flow documented for the NW Rhenohercynian belt is correlated to deformation processes, block faulting and fold and thrust processes. The results of geochemical and isotope investigations presented here refer to multiple fluid mobilizations and complex genetical processes which are related to spatial – temporal variations of tectonics and fluid supply.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Chatziliadou, Maria

Contributors dc:contributor

• Kramm, Ulrich

• info:eu-repo/classification/ddc/550
• Mineralchemie
• Geochemie
• Datierung
• Isotopendatierung
• Vererzung
• Zinkblende
• Carbonate
• Geowissenschaften
• Gangmineralisation
• Rb-Sr Datierung
• Pb-Zn Vererzung
• Strontiumsignaturen
• rezente Thermalwässer
• Isotopencharakteristik
• Fluidfluss
• veins
• fracture sealing
• stable isotopes
• sphalerite dating
• thermal waters
• Rb-Sr age
• vein mineralisation
• ore
• fluid flow

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113503%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51191