Columnar slags have been excavated from the Early Iron Age kurgan (a type of burial mound or barrow, heaped over a burial chamber) of Taksay 1 in western Kazakhstan. We have identified over 35 mineral phases in the slags, most of which are related to the disintegration products of sialic complexes. Among the mineral phases are several rare species, such as nickel-rich iron, native iron and nickel, schreibersite, Fe-rich armalcolite, troilite, and Cr-rich spinels, which are not typical for this region and are not known to be associated with metallurgical slags. Formation of the mineral associations requires elucidation of the genesis conditions of the slags. Some of the minerals are products of pyrometamorphism during the formation of slags in situ, accompanying the burning of the wooden burial chamber. The slags in the interior parts formed at high temperature, reaching 1250–1350 °C, which is confirmed by experimental data. Other slags contain relic nickel-rich iron, olivines, and Cr-rich spinels typical of ultrabasic and cosmogenic rocks. This slag formed by melting sand which filled in the roof of the burial chamber. The occurrence of nickel-rich iron, olivines, and Cr-rich spinels is proposed to be evidence of a meteorite substance in the sand. Nickel-rich iron with elevated Co (1–1.8 wt.%), Ge (50–140 ppm), and Ga (20–190 ppm) contents suggests a cosmogenic source.