Wulkanity hercyńskie w rejonie Wojcieszowa

Abstract

Przedstawiono charakterystykę geologiczną i zróżnicowanie petrograficzne skat wulkanicznych na wzgórzu Żeleźniak koło Wojcieszowa. Wulkanity te reprezentowane przez ryolity cechują silne zmiany hydrotermalne.

Ryolity przecięte są żyłami andezytu i wykazują mineralizację siarczkową. W obrębie skał

wulkanicznych stwierdzono granit drobnoziarnisty. Wiek wulkanitów określa się przypuszczalnie jako górnokarboński.

HERCYNIAN VOLCANIC ROCKS IN THE WOJCIESZÓW AREA

The studied area is situated in south-eastern part on the Góry Kaczawskie Mts, built of epimetamorphic Cambro-Devonian rocks (Fig. 1). The metamorphic series is accompanied by younger igneous rocks, both bathyal (granite) and volcanic ones. The latter form a row of intrusive bodies (trunks, dykes and apophyses), generally discordant in relation to surrounding strata. Igneous activity found in this area was related to Hercynian diastrophic cycle.

Volcanic rocks are mainly concentrated in area of the Żeleźniak and Bukowinka Hills. Moreover, they form a row of apophyses radially arranged around the latter hill, in distance ranging from a few to about a dozen kilometers (Fig. 2). Analyses of borehole columns and geological data and results of petrographic studies make it possible to assume that the largest occurrence of these rocks, building the Żeleźniak Hill, has a from of laccolith formed at relatively shallow depths (Fig. 3).

The body is highly varying in petrographic development. It is mainly built of aphanic rocks and the share of cleary grained ones with porphyry structure and random texture appears subordinate. The rocks display numerous phenocrysts of quartz, orthoclase, oligoclase and biotite (25 mm in size). Feldspars display effect of advanced transformation. In accordance with the currently used petrographic classifications, the rocks should be assigned to rhyolites (Fig. 4). They are cut by veins of intermediate rocks, represented by andesites. Volcanic rocks of the Bukowinka Hill display more uniform structure, clearly grained character and higher share of biotite. Petrographic development and chemical composition of volcanic rocks from the vicinities of Wojcieszów appear close to those of similar rocks from the areas of Bolków and Świerzawa (Fig. 1) which, however, display less advanced hydrothermal transformations.

The similarities suggest that igneous rocks from these areas have been either formed under similar conditions or from the same source.

A hydrothermal mineralization, represented by ore mineral assemblages mainly connected with veins infilling joint fissures and tectonic fractures, appears related to the Żeleźniak volcanic rocks. The occurrences of ore minerals, exploited in the past, are nowadays without economic value.

The studied volcanic rocks include some bathyal ones – granites. In the Żeleźniak Hill granite has been found in the borehole XI, at depths 55.4 119.0 m (Figs. 2, 3). The granite is light-gray in colour and with random texture. In northern part of the Bukowinka Hill, there was found light-gray various- grained porphyry granite. The volcanic activity found in this region is assumed to be of the Late Carboniferous age. Position of the granite among these volcanic rocks remains unclear as its geometry is still unknown. The granite may represent either erratics of crystalline basement or veins indicating presence of granitoid intrusion beneath the metamorphic series