Integrated geological and geophysical exploration methods for cryptocrystalline magnesite
Research output: Thesis › Doctoral Thesis
Cryptocrystalline magnesite (CM) occurs predominantly in ultramafic rocks tectonically controlled as veins and stockworks (Kraubath type; KT) or in sediments associated with ultramafites in the vicinity (Bela Stena type; BST). The representative magnesite deposit Dutluca/Kobal contains veins at the lowest part of the pit, stockwork in big amounts at shallow levels, zebra ore in highly weathered serpentinite in the vicinity to the paleosurface and magnesite layers in ultramafic conglomerates and Miocene sediments. CM is related to serpentinite and occurs in faults with well developed fault planes and joints which are related to extensional domains (antithetic Riedel planes, joints) of major fault zones (e.g. Eskişehir Fault zone). The fault zones are commonly steep structures with a dextral strike slip and a normal component. Extensional structures are therefore essential for magnesite formation. The serpentinite shows an alteration from magnesium rich, dark serpentinite with relicts of olivine and ortho-pyroxene to altered serpentinite with a relative enrichment of iron against magnesium. Weathered serpentinite contains smectite minerals which are a replacement product of serpentine minerals. Chrome spinels indicate that the ophiolite has formed in a suprasubduction zone. Altered serpentinite contains carbonate crystals which have replaced olivine and subordinarily bastite which has replaced ortho-pyroxene. The alteration of serpentinite is an important indicator for magnesite formation. Stable isotope studies of 14 different CM deposits in Turkey and the type locality in Kraubath/Austria show that two different systems of magnesite formation occur. Positive δ13CV-PDB values indicate a shallow deposit which has formed in alkaline environments such as evaporation or biogenic fermentation. Since these processes are related to the surface, the horizontal extension of the deposit will be important for magnesite exploration. CM deposits with negative δ13CV-PDB ratios indicate that the carbon source was derived either from decarboxylation of organic carbon or volcanic degassing, indicating that these deposits have been supported by uprising fluids. Negative δ13CV-PDB ratios are getting even heavier the closer the fluid is getting to the surface. This will be on one side due to the CO3-2 degassing where lighter isotopes vapourise first, leaving heavier carbon isotopes in the fluid. On the other side uprising fluids will be influenced by the groundwater. Thus magnesite layers have always slightly higher δ13CV-PDB ratios than the stockwork below. Geomagnetic survey is using the phenomenon that serpentinite is highly susceptible but not homogeneous. When the serpentinite is fractured due to tectonic structures, an abrupt change in the susceptibility causes an anomaly in Earths magnetic field. The fact that magnesite is diamagnetic does only have an influence if the magnesite body is directly on the surface and only covered by soil. Geophysical parameters like natural remanent magnetisation or anisotrope magnetic susceptibility have an influence on Earth's magnetic field as well. The combined data enables the modelling of the geological structures in the underground. Geomagnetic survey is a quick and effective method to discover structures which are covered by soils and to detect magnesite itself if the body is bigger than the distance between the measurements. Paleomagnetism enables the determiation of tectonic blocks which have been rotated to each other and enables the determination of zones which might have been fractured with no indication on the surface. Nevertheless geomagnetic survey is only able to detect structures on the surface. The signal is reduced by sedimentary cover. The thickness of the sedimentary cover can be investigated by geoelectric survey.
|Translated title of the contribution||Integrierte geologisch-geophysikalische Prospektionsverfahren auf kryptokristallinen Magnesit|
|Publication status||Published - 2012|