Rock magnetic properties and their anisotropy from the Outokumpu assemblage in the Outokumpu deep drilling, Finland

该项目的目的是在考虑古地磁场矢量(倾角)以及磁化率和剩磁的方向相关性的情况下提供岩石和古地磁数据，以解释磁测井数据(见Hördt和Leuven的提案)。我们重点研究了乌托昆普深井1314米至1515米之间的乌托昆普组合。该组合具有几个地球物理异常，包括极高的磁化率和剩余磁化作用，是Outokumpu型铜-钴-锌-镍(-银-金)矿床的赋矿岩石。蛇纹岩中的剩余磁化作用由磁铁矿和磁黄铁矿携带。蛇纹岩不同程度的硅碳酸盐蚀变导致磁铁矿分解，亚铁磁性转变为反铁磁性磁黄铁矿，并伴随着镍硫化物的形成，这在蛇纹岩单元中表现为强烈的磁化率变化。虽然我们没有发现岩石磁性与化学成分之间的相关性，但我们观察到与硅碳酸盐蚀变有关的矿物学和磁性之间的明显关系。这一发现也解释了由Hördt和他的同事测量到的被调查的Outokumpu组合内的强磁场变化。

The aim of this project is to provide rock and paleomagnetic data under consideration of paleomagnetic field vectors (inclination) and of the directional dependence of magnetic susceptibility and magnetic remanence for the interpretation of the magnetic logging data (see proposal by Hördt and Leuven). We focus in our study on the Outokumpu assemblage between 1314 and 1515 m of the Outokumpu deep drilling. This assemblage is characterized by several geophysical anomalies including both very high magnetic susceptibilities and remanent magnetizations and is the host rock of the Outokumpu-type Cu-Co-Zn-Ni(-Ag-Au) deposits. The remanent magnetization in the serpentinites is carried by magnetite and pyrrhotite. Different degrees of silica-carbonate alteration of serpentinite caused a decomposition of magnetite and a transformation of ferrimagnetic into antiferromagnetic pyrrhotite along with Ni-sulfide formation, which is seen in strong susceptibility variation in the serpentinite unit. Although we found no correlation between rock magnetic properties and chemical composition, we observed a clear relationship between mineralogy and magnetic properties in relation to the silica-carbonate alteration. This finding also explains the strong magnetic field variation within the investigated Outokumpu assemblage measured by Hördt and co-workers.