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| Titel |
Evaluation of anisotropy in physical/mechanical properties of metabasalts from Gadag (Southern India) – implications for vein emplacement and gold mineralization |
| VerfasserIn |
C. Satheesan Vishnu, Manish A. Mamtani |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130925
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| Publikation (Nr.) |
 EGU/EGU2016-11255.pdf |
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| Zusammenfassung |
| Foliated rocks have anisotropic physical and mechanical properties. In the case of foliated
metamorphic and bedded sedimentary rocks, it is easy to decipher this anisotropy. However,
this is not readily possible in the case of massive rocks. Vishnu et al. (2010) used Anisotropy
of Magnetic Susceptibility (AMS) analysis to identify magnetic fabric in massive quartzites
and established that the rocks have a lower strength parallel to the magnetic foliation,
than perpendicular to it. In the present study, the authors have extrapolated the
same concept to the metabasalts of Gadag region (West Dharwar Craton, Southern
India), which is replete with quartz veins that are gold bearing. These metabasalts are
massive and are devoid of visible foliation. Mondal and Mamtani (2013, 2014) did
AMS analysis of the metabasalts and showed that the magnetic foliation is NW-SE
striking, and the quartz veins also have similar strike. It was inferred by the above
authors that the magnetic foliation developed during D1/D2 deformation (regional
NE-SW compression) and the veins emplaced during D3 (NW-SE compression). This
implies (a) dilation of pre-existing anisotropic elements during D3 and (b) rock
strength anisotropy must have been important in controlling this dilation. To test this
oriented blocks (each approximately 50 × 40 × 25 cm in size) of metabasalts were
collected. A portion of the sample was used for AMS analysis. Subsequently, the
magnetic foliation identified from AMS analysis was marked on the remaining sample
block, and NX size (Diameter = 54.7 cm) cylindrical cores were drilled parallel and
perpendicular to the magnetic foliation for various rock physical/mechanical tests - uniaxial
compressive strength (UCS), point load test (PLT), P-wave velocity and Brazilian
tensile strength. Results so far indicate that average point load index parallel and
perpendicular to the foliation is 8.47 MPa, and 9.93 MPa, respectively, while UCS is
172.77 kN and 212.95 kN, respectively. This anisotropy of strength is a proof of the
difference in physical property of the metabasalts with respect to magnetic foliation.
This is also manifested in the result of P-wave velocity measurements, which is
lower perpendicular (∼5000 m/s) to foliation than parallel (∼5700 m/s) to it. These
results imply that the foliation developed during D1/D2 regional deformation dilated
during D3 and resulted in emplacement of quartz veins, some of which are gold
bearing.
References:
Mondal, T.K., Mamtani, M.A. (2013). 3-D Mohr circle construction using vein
orientation data from Gadag (southern India) - implications to recognize fluid pressure
fluctuation. Journal of Structural Geology 56, 45-56.
Mondal, T.K., Mamtani, M.A. (2014). Fabric analysis in massive rocks of the Gadag
region (southern India) – implications to decipher time relationship between regional
deformation and gold mineralization. Tectonophysics 629, 238-249.
Vishnu, C.S., Mamtani, M.A., Basu, A. (2010). AMS, ultrasonic P-wave velocity and
rock strength analysis in quartzites devoid of mesoscopic foliations – implications for rock
mechanics studies. Tectonophysics 494, 191-200. |
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