Zn-Chromspinels of Middle Timan and the Near-Polar Urals

摘要

The analysis of indicator-minerals from different-aged terrigenous rocks with a potential for native diamond prospects and intermediate collectors has revealed an interesting feature of the composition of accessory chromspinels: an anomalous Zn content that is tens or even hundreds of times higher than that in chromspinels from Alpine-type ultrabasic rocks. In addition to the typical Zn-poor varieties of chromspinels, xenomorphous grains and octahedral crystals (ZnO 1-27%) and gahnite ZnAl_2O_4 (Zn-spinel) were detected in some places. The Zn-bearing chromspinels have already been detected in the following four areas of Middle Timan: (1) Middle Devonian conglobreccia horizon in the Vol'sk-Vymsk Range (Ichet"yu diamond prospect); (2) Paleogene weathering crust in manganese prospects and Quaternary alluvial sediments along the entire eastern slope of the Chetlas Kamen Plateau; (3) Middle Jurassic sandstones of the Sysol Formation in the northern Ukhta area; and (4) lamprophyre dikes in the Chetlas Kamen Plateau. In addition, we found Zn-rich chromspinels in Upper Cambrian-Lower Ordovician sandstones (Al'kesvozh Formation) at upper reaches of the Balban'yu River. Two small (0.1-0.3 mm) octahedral diamond crystals were also detected in prospecting drifts in this area. The native source and genesis of Zn-chromspinels found in different areas and settings have long remained a puzzle. An elucidation of these aspects would have been helpful in solving the problem of the native sources of diamond.