The auriferous ore mineralisation and its zonal distribution around the Variscan Kłodzko–Złoty Stok granitoid pluton in the Sudetes (SW Poland) – an overview

Abstract

Due to a low level of erosion the Variscan (Carboniferous–Mississippian) Kłodzko–Złoty Stok (KZS) granitoid pluton and its metamorphic rock cover are unique examples of the ore mineralisation distribution pattern around a pluton. The KZS pluton formed from I-type magmas. It is composed mostly of high-K, metaluminous, biotite- and hornblende-rich granitoids of various compositions ranging from granodiorite to monzonite. The ore mineralisation in the KZS shows specific zonation that reflects interaction between post-magmatic mineralising fluids and country rocks of different composition. In the KZS pluton and its surroundings the mineralisation reveals high-temperature Ti-W oxides and As-Fe sulphide-arsenides plus Fe-oxides which are replaced outwards by Cu-Fe-Pb-Zn sulphides and subsequently, in the marginal parts of the pluton, by the Pb-Sb-Fe sulphide mineralisation. The specific elements and the isotopic composition of sulphur in ore minerals indicate that the metal-bearing postmagmatic-metamorphic hydrothermal fluids, despite their strong affinities to the KZS hybrid magmatic fluids, show also an influence of variable composition of country rocks. Variable alteration processes: hornfelsitisation, skarnisation, dolomitisation, serpentinitisation, pyroxenisation, biotitisation, K-feldspatisation, silicification, berezitisation, carbonatisation, prehinitisation, chloritisation, epidotisation, sericitisation, albitisation and sulphidisation developed along contacts of various rock types within the country rocks. In more fractured zones, transported elements were preferentially trapped by calcareous (Ca-rich) metamorphic rocks. Moreover, along the direct contact between granitoids and calcareous rocks a high temperature auriferous skarns with magnetite-pyrrhotite or/and löllingite-arsenopyrite mineralisation were formed. There are positive correlations between organic matter and Fe2O3 and MgO contents in skarns, suggesting remobilization of organic matter by hydrothermal fluids released during serpentinisation. The organic matter and carbonate samples have very variable δ13C and δ18O values. Most probably these are the result of isotopic re-equilibration between minerals under high temperatures. On the intimate contact of granitoids with less permeable amphibolitic hornfelses a high temperature titanite-scheelite mineralisation of veinlet-impregnation type occurred, while thermal and metasomatic alteration of the enveloping more permeable sandstones developed away from direct contact with granites, causing the formation of pyrite-rich beresites. As a rule, different types of sulphide ore mineralisation contain refractory gold captured by arsenopyrite, löllingite and/or pyrite. Moreover, the formation of contact-metasomatic ore mineralisation was accompanied by ore precipitation in veins and stockworks at a wide temperature range from 480 to 150°C during early and late fracturing stages. The younger stages also revealed variable concentration of gold. All those observations strongly indicate that the whole KZS pluton and its aureole was fertile in gold.