TSK 11 Göttingen 2006 Chatziliadou et al. Fracture sealing in lime- stones, a microstructural and mineralogical study Vortrag Maria Chatziliadou1 Christoph Hilgers2 Sven Sindern1 Introduction Fractures significantly enhance the flow rate in rocks, if fracture density is high (Taylor 1999, Cox et al. 2001). This leads to rapid flux along a hydraulic gra- dient from high to low pressure reser- voirs, and is represented in rocks as veins. Veins are precipitates from su- persaturated fluid, and are formed by a change in pressure, temperature or geo- chemistry. The solubility of vein form- ing minerals such as quartz, calcite or halite is generally low and thus large (and sometimes unreasonable) fluid vol- umes are required to account for the precipitated mass. Rapid ascent of so- lution may explain the high supersat- uration needed to seal fractures, ei- ther by fluid flow along deep reach- ing faults due to seismic ruptures, or mobile hydrofractures driven by pres- sure gradients in fluid filled fractured at deeper crustal sections (Bons 2001, Miller 2002). The vein microstructure is a unique tool to unravel the frac- ture sealing process. The most indica- tive microstructures are fractured min- erals, which were sealed by a fluid of dif- ferent composition. The repeated pres- ence of fluid and solid host rock inclu- sions in fibrous, stretched crystal type veins (minerals which extend across the vein and into the host rock) also indi- cate repeated fracture-sealing processes (Ramsay 1980), although their presence 1 Institut für Mineralogie und Lagerstät- tenlehre, RWTH-Aachen, Germany 2 Ge- ologie-Endogene Dynamik, RWTH-Aachen, Germany is not a sufficient criteria (Hilgers 2005). In this study, we outline the different fault sealing processes associated in a still seismic zone. The faults are lo- cated in Carboniferous limestones, and thus present an analogue for fault seal- ing processes in hydrocarbon reservoirs and an in-depth study of seismogenic faults. Geological Setting We studied lower Carboniferous lime- stones, which are truncated by sub- vertical, dm-wide calcite veins. These platform limestones are located on the NE-limb of the Stavelot-Venn Anti- cline, which is part of the Variscan fold and thrust belt. Our study area is the quarry Hastenrath, which is lo- cated app. 50m SW of the seismo- genic Sandgewand normal fault. Cal- cite veins locally contain lead- and zinc- sulfide ores and strike NW-SE parallel to the normal fault. We compare our re- sults of core data derived from the deep drilling project RWTH–1, a 2543m deep well drilled in 2004 in the city centre of Aachen. The core contains 20m of in- tensely deformed upper Devonian lime- stones at 1420m depth. These lime- stones also contain different sets of cal- cite veins, but are devoid of ore miner- alization. Microstructure and mineralogy Hastenrath The Hastenrath quarry exposes several approx. 20 cm wide subvertical veins in limestone with some first localised dolomitisation. Macroscopically, the veins show three to four different, sym- metrically arranged cements. #1 is close to the host rock, a pink hy- drothermal dolomite precipitated next 1 Chatziliadou et al. TSK 11 Göttingen 2006 to the host rock. The dolomite is a tectonic dilational breccia, which is ce- mented with calcite. The cathodolu- minescence shows fragments of euhe- dral calcite crystals outlined by differ- ent colours, and large areas where grains do not match with the variations in lu- minescence. Such calcite grains consist of subangular fragments of dark and or- ange luminescence. Calcite twin mor- phology indicates temperatures of app 250°C. Dolomite is overgrown by euhe- dral ankerite. Euhedral to subhedral galena crystals up to 1 cm in diame- ter indicate a later timing of growth. Euhedral quartz has a solid inclusion rich rounded centre and is present in all phases of zone #1. The intermediate zone # 2 shows up to 2 cm twinned calcite grains of twin type II which indicate a temperature of 150–300°C. The cathodoluminescence shows a pattern which does not corre- spond to calcite grain boundaries. The Figure 2: Microprobe image of a bro- ken fragment of dolomite with an ankerite seam. Galena is also present in the patch- work calcite matrix. central zone #3 is sealed with small twinned calcite crystals with a thin sphalerite vein. The sphalerite shows zones with high cadmium content. Both zone #1 and #3 contains chalcopyrite within and around galena and spha- lerite. Locally, organic material em- Figure 1: (a). A calcite vein with Pb-Zn mineralization, Hastenrath Quarry. The rim of the vein shows fragments of hy- drothermal dolomite and euhedral galena crystals. The central part of the vein is filled with organic material ( 36% C). (b). Horizontal section of the vein shows three fluid generations. 2 TSK 11 Göttingen 2006 Chatziliadou et al. placed in the central part of the vein. Vitrinite reflectance gives <150°C. RWTH Aachen The core material from the RWTH-1 well contains up to 0.3 cm thick veins in dolomite. The host rock indicates a late diagenetic dolomitisation with zoned Fe-rich seams around dolomite crystals. Two different quartz phases precipitates in the pores, as indicated by a blue luminescence (restricted to the host rock) and a later brown lumines- cence in veins and host rock. Pyrite crystals growth in the pores of the host rock as bacterial degradation in reduced milieus. The vein consists of dolomite of dark brown luminescence, transected and surrounded by bright orange cal- cite. Euhedral quartz crystals (brown luminescence) are present in dolomite, calcite and are enriched along the vein wall interface. Conclusions The sealed fractures show similar fluid events and -sequences in upper Devo- nian (RWTH–1) and lower Carbonif- erous (Hastenrath) limestones with an early dolomite vein, fractured and re- sealed by calcite and late euhedral quartz. The veins in Hastenrath show an even more complex sealing history because they contain beside brecciated dolomite and calcite, euhedral ankerite overgrowth, galena (approx. 150°C), eu- hedral quartz, calcite, sphalerite (ap- prox. 150°C) and calcite, later filled with organic material <150°C. Galena and sphalerite are associated with chalcopy- rite. The geochemistry of the dolomite is different in both localities, indicat- ing either a different fluid systems or strong influence of the host rock. Sim- ilar fluid types at both localities may suggest a homogeneous fluid system. However, marked differences exist be- tween hydrothermal mineral products of both localities. The complex fluid sys- tem in Hastenrath shows that the same fracture was sealed during different fluid events, possibly associated with normal faulting. Literatur Bons, PD (2001) The formation of large quartz veins by rapid ascent of fluids in mobile hy- drofractures. Tectonophysics 336(1–4), 1–17 Cox SF, Knackstedt MA & Braun, J (2001) Principles of structural control on permeabil- ity and fluid flow in hydrothermal systems. Reviews in Economic Geology 14, 1–24 Hilgers C & Urai, JL (2005) On the arrange- ment of solid inclusions in fibrous veins and the role of the crack-seal mechanism. Jour- nal of Structural Geology 27(3), 481–494 Miller SA (2002) Properties of large rup- tures and the dynamical influence of flu- ids on earthquakes and faulting. Jour- nal of Geophysical Research 107(B9), 2182, doi:10.1029/2000JB000032 Ramsay JG (1980) The crack-seal mechanism of rock deformation. Nature 284(5752), 135– 139 Sander B (1948) Einführung in die Gefügekunde der geologischen Körper. Erster Teil: Allgemeine Gefügekunde und Arbeiten im Bereich Handstück bis Profil. UND Zweiter Teil: Die Korngefüge. Springer, Wien Sibson RH (2004) Controls on maximum fluid overpressure defining conditions for meso- zonal mineralisation. Journal of Structural Geology 26, 1127–1136 Stoneley R (1983) Fibrous calcite veins, over- pressures and primary oil migration. AAPG Bulletin, Geological Notes, 1427–1428 Taylor WL (1999) Fluid flow in discrete joint sets: Field observations and numerical sim- ulations. Journal of Geophysical Research 104(B12), 28,983-29,006 3