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1551912 B855.Pdf UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Mineralogical generations and orientation of fractures, based on drill cores from the West-Link Project, Gothenburg Vladimir Medan ISSN 1400-3821 B855 Master of Science (120 credits) thesis Göteborg 2015 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Mineralogical generations and orientation of fractures, based on drill cores from the West-Link Project, Gothenburg. Vladimir Medan, Göteborg University, Department of Earth sciences; Geoscience, Box 460, SE-405 30 Göteborg Abstract Core-drilled boreholes have been drilled in Proterozoic bedrock in the central parts of Gothenburg, SW Sweden, during the preliminary investigation for tunneling a railway in the West-Link Project. The Swedish transport administration is responsible for the planning of the project and to guarantee the safety, therefore significant information about the bedrock is obtained through preliminary investi- gations before excavating the tunnel. The aim of this study was to investigate the fracture mineralogy and the fracture orientations and if possible use as a predictive tool for future tunnel projects. Five generations of fracture minerals were identified that were precipitated during at least three major epi- sodes of fluid migration in fractured Proterozoic bedrock, western Sweden. Data were used from the preliminary investigations of the West-Link Project and samples were taken from drill cores. The min- erals identified were grouped into generations based on cross-cutting relationships for their relative age. Three major events of fluid migration have been identified in the West-Link. The first event is thought to have occurred during the Sweconorwegian Orogeny and formed generation (1) cataclasite fracture filling NW and NE striking fractures. During the later stages of the Sweconorwegian orogeny hydrothermal alteration of primary rock minerals such as biotite and plagioclase occurred and formed generation (2) mineralogy with E-W striking fractures and reusing previous fractures. During genera- tion (3) and (4) the mineral assemblage indicates lower temperatures being precipitated somewhere between the Caledonian Orogeny and the Paleozoic producing low temperature minerals such as quartz, adularia and calcite in reused fracture groups. Subsequent to the Caledonian Orogeny shallow marine sediments were deposited in the foreland basin producing an environment suitable for genera- tion (5) mineralogy comprising clay minerals in reused fracture groups. Analysis of the fracture orien- tations show that later generations typically reactivate the fractures of the previous generations, indi- cating that there is no easy and direct correlation between mineralogy and fracture orientation on which to prognose rock mass stability. Keywords: West-Link, fracture minerals, hydrothermal alteration, fracture orientation ISSN 1400-3821 B855 2015 Mineralogiska generationer och riktningar av sprickor, baserat på borr- kärnor från Västlänken, Göteborg. Vladimir Medan, Göteborgs Universitet, Institutionen för geovetenskaper; Geologi, Box 460, SE-405 30 Göteborg Sammanfattning Kärnborrhål har borrats i Proterozoisk berggrund i den centrala delen av Göteborg, sydvästra Sverige, i samband med preliminära undersökningar till den planerade järnvägsförbindelsen Västlänken. Tra- fikverket ansvarar för planeringen av projektet samt säkerheten, därför är det viktigt att skaffa inform- ation om bergets egenskaper under de preliminära undersökningarna innan man börjar byggnationen av tunneln. Målet med denna studie är att undersöka sprickornas mineralogi samt sprickorienteringar och om möjligt använda informationen som en prognosmodell för framtida tunnelbyggen. Fem olika sprickgenerationer har identifierats som utfälldes under minst tre större episoder av hydrotermal cirku- lation i spricksystemen i den Proterozoiska bergarten. Data som användes i rapporten är från de preli- minära undersökningarna från projekt Västlänken samt att prover hämtades från borrkärnorna. Minera- lerna som identifierades grupperades i generationer baserat på korsande relationer mellan mineral för att ta reda på deras relativa ålder. Tre olika händelser av utfällning har identifierats i Västlänken. Första händelsen tros ha skett under Svekonorvegiska Orogenesen vilket fällde ut generation (1) ka- taklasit sprickfyllnad med NV och NÖ strykande orientering. Under den senare fasen av Svekonorve- giska Orogenesen skedde det en hydrotermal omvandling av de primära mineralen i sidoberget som biotit och plagioklas vilket producerade generation (2) mineral som har en Ö-V strykning samt återan- vänder äldre sprickgrupper. Under generation (3) och (4) indikerar mineralen längre temperaturer som utfälldes mellan Kaledoniska Orogenesen och Paleozoikum som producerade lågtemperatur mineral som kvarts, adularia och kalcit i reaktiverade sprickgrupper. Efter den Kaledoniska orogenesen avsat- tes grundhavssediment i förlandsbassängen som bildade en miljö som gjorde det möjligt att bilda ge- neration (5) mineral som består av lermineral i reaktiverade sprickgrupper. Analyser av sprickor visar att senare generationer använder sig av spricksystem bildade i tidigare generationer, detta antyder att det inte finns något enkelt och direkt samband mellan mineralogi och sprickorientering som man kan använda för att göra en prognos för bergets hållfasthet. Nyckelord: Västlänken, sprickmineral, hydrotermal omvandling, sprickorientering ISSN 1400-3821 B855 2015 Contents 1. Introduction ......................................................................................................................................... 1 1.1 General background ...................................................................................................................... 1 1.2 Aims and objectives ...................................................................................................................... 5 2. Background ......................................................................................................................................... 5 2.1 Geological setting .......................................................................................................................... 5 2.1.1 Proterozoic to Mesozoic geological evolution of the SW Sweden ......................................... 5 2.2 Earlier studies ................................................................................................................................ 9 3. Methods ............................................................................................................................................. 10 3.1 Sampling and preparation for microscopy .................................................................................. 10 3.2 Petrography and mineralogy........................................................................................................ 10 3.2.1 Macroscopically ................................................................................................................... 10 3.2.2 Microscopy and scanning ..................................................................................................... 10 3.2.3 SEM-EDS ............................................................................................................................. 10 4. Results ............................................................................................................................................... 11 4.1 Relative sequence of fracture mineralisations ............................................................................. 11 4.2 Fracture filling types ................................................................................................................... 11 4.3 Identified fracture fillings ............................................................................................................ 12 4.4 Fracture generations .................................................................................................................... 28 4.4.1 Generation 1 ......................................................................................................................... 28 4.4.2 Generation 2 ......................................................................................................................... 30 4.4.3 Generation 3 ......................................................................................................................... 34 4.4.4 Generation 4 ......................................................................................................................... 36 4.4.5 Generation 5 ......................................................................................................................... 41 4.5 Local derivation of hydrothermal mineralization ........................................................................ 41 4.6 Observed alteration processes in the West-Link ......................................................................... 43 5. Discussion ......................................................................................................................................... 44 5.1 Generation 1 ................................................................................................................................ 44 5.2 Generation 2 ...............................................................................................................................
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