Volume 71, Nº 5 (2016)

As a resulting of comprehensive studies of Paleoproterozoic igneous complexes a regular remagnetization trend is revealed. It is shown that the remagnetization front propagated from the northwest to southeast. The obtained remagnetization trend probably reflects the direction of the collision in the Paleoproterozoic. Correlation between remagnetization processes and rock compositions was found. The degree of conservation of secondary earlier and later magnetization components is likely associated to both the compositions of protoliths and different transformation conditions of rocks, including their fluid saturation.

Based on the results of more than 600 electron microprobe analyses of 25 minerals the distribution pattern of the Cr⁶⁺ impurity in vanadates, phosphates, and arsenates collected in oxidation zones of six ore deposits of the Urals was studied. Among them are Pb minerals of the brackebuschite, apatite, adelite, and tsumcorite groups and alunite supergroup, as well as carminite, cornwallite, and bayidonite. Vanadates and arsenates with brackebuschite-type structures show a high affinity to Cr⁶⁺. The maximum content of the Cr⁶⁺ impurity is characteristic of minerals with specified Fe³⁺ trivalent cations (ferribushmakinite, arsenbrackebuschite, and gartrellite) or Al³⁺ (plumbogummite and bushmakinite). The prevailing scheme of isomorphous substitution, according to which chromium enters into the compositions of these minerals, is heterovalent: Cr⁶⁺ + M²⁺ → Т⁵⁺ + M³⁺ (where Т = V, As, P; M³⁺ = Fe, Al; M²⁺ = Сu, Zn), whereas the role of isovalent substitutions Cr⁶⁺ → S⁶⁺ and Cr⁶⁺ → Mo⁶⁺ in oxosalts that formed in mineral occurrences of the Urals is insignificant.

A magnetotelluric (MT) data processing algorithm that demonstrates high robustness to intense electromagnetic noise that occurs in measured MT data has been developed. The key features of the algorithm are a specific approach to estimating different transfer functions and the ability to utilize all four channels acquired at remote reference stations. The code utilizes various techniques to reduce the estimation errors, including the robust Huber estimator, jack-knife approach, improved remote reference technique, and compensating for overestimation of power spectra. The proposed algorithm has high efficiency in processing data with a low signal-to-noise ratio.

The results of electrical exploration carried out using the method of magnetotelluric sounding at a deposit in Western Kazakhstan are presented. Based on processing and interpretation of the electrometric data, a digital geoelectric model of the exploration area is developed and a geoelectric section of the test profile is constructed. It is shown that MT soundings, along with a seismic survey and well-log data, can be effectively used to define a geological structure and to identify prospective oil-and-gas bearing segments in Mesozoic terrigeneous-sedimentary complexes.