Topaz granites in northern Kazakhstan

The paper presents the first data obtained on topaz granites from dikes in columbite-bearing granites of the Kokchetav block, northern Kazakhstan. The columbite-bearing granites (398 ± 3 Ma, Rb-Sr method) compose large massifs and ubiquitously contain columbite as an accessory mineral in amounts from a few to a few dozen grams per ton. The topaz granites make up dikes up to 70–100 m long and 0.4–1.5 m thick, which have sharp contacts with the host granites. The composition of the topaz granites is as follows: albite ∼50%, potassic feldspar ∼24–26%, quartz ∼20%, topaz 3–5%, biotite and muscovite ∼1%. Topaz occurs in inner contact zones in the form of large (3–4 cm) crystals, which grew inward from dike contacts and is ubiquitous in the fine-grained groundmass as small (no larger than 0.05 mm) crystals. The ore minerals in the topaz granites (zircon, thorite, bastnaesite, Y-cerite, monazite, Y-fluorite, W-ixiolite, tantalite, columbite, and uraninite) developed as very small grains, which can be identified only under a microprobe and electron microscope. W-ixiolite was found only in topaz crystals. The topaz granites are rich in albite, contain Na > K and F concentrations from 0.2 to 0.97 wt %. Compared to the host columbite-bearing granites, the topaz granites are 10–15 times richer in Ta and Li, 3-8 times richer in Rb, Cs, Nb, Sn, F, and Be but 1.2–2 times poorer in U, Th, and REE. The columbite-bearing granites have Ta/Nb = 1: 20, while this ratio in the topaz granites is 1: 3, a fact highlighting the enrichment of the granites in Ta and their geochemical autonomy with respect to columbite-bearing granites.