Talk-serpentinforekomster i Sparbu, Nord-Trøndelag.
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The report presents a technical description of certain talc-serpentine deposits occuring within the Gula Schist Group (Cambrian) of the Trondheim region. No form of contact metamorphism of the schist can be observed along the serpentinite contacts. Some of the serpentinites are strongly brecciated, and cemented by secondary magnesite. Border zones consisting of talc, magnesite and chlorite occur in this type. Non-brecciated serpentinite bodies show no, or very narrow, secondary border zones. The talc-magnesite zones are considered to have formed from antigorite by reaction with CO2-rich hydrothermal fluids. Antigorite was attacked by these solutions along the breccia fissures, with the resultant formation of magnesite, which fills the cracks and replaces the massive serpentine. This alteration caused a considerable increase in volume, so that newly-formed talc and magnesite were squeezed out of the serpentinite body to form the border zones. In addition to CO2, the hydrothermal solutions carried K, together with traces of B, F and Rb. Aggregates of coarsely crystalline smaragdite, and some Cr-free secondary magnetite are found in the talc zone. The serpentine body contains Cr-bearing magnetite as well as some Ni, partly as sulphides but mostly contained in Mg-silicates. This Ni reappears in the newly-formed talc-magnesite, and in pentlandite.