ABSTRACT

Crust-form aragonite and needle-shape aragonite crystallized in cavities in magnesites as well as vein magnesite from ultramafic rocks related to the Sleza ophiolite (NE part of Bohemian Massif) have been examined for carbon and oxygen isotope ratios and for some trace elements. δ¹⁸O_SMOW values of magnesite, corrected for acid-decomposition fractionation, comprise a narrow range of 23.7 to 27.9‰, which suggests near-surface temperature, high water-to-rock ratio, and meteoric origin of the ore-forming solution. This inference is further supported by δ¹³C values (–18.7 to –6.4‰) which indicate a biogenic origin of carbon. Moreover, chemical analyses of the magnesites showing lack of elements characteristic for intrusionrelated hydrothermal deposits support the meteoric-biogenic origin of the magnesite deposits of the Gogolow-Jordanow massif. The two types of aragonites are uniform in δ¹³C (–15.0 to –16.3‰) and δ¹⁸O_SMOW (23.9 to 25.4‰). The Δ¹³C_mg-ar = –3.1 and –3.5‰ of two samples of magnesite-aragonite aggregates show the absence of carbon isotope equilibrium in this system. However, the Δ¹⁸O_mg-ar = 4.0 and 1.8‰ for the respective samples suggest that the aragonites formed in the magnesites have possibly crystallized in oxygen isotope equilibrium with the host magnesite.