Alteration Processes as Exploration Parameter for Lead-Zinc Ores in the Graz Paleozoic

The Graz Paleozoic is part of the Drauzug-Gurktal nappe system in the Austroalpine unit of the eastern Alps (Austria). It consists of Silurian to Carboniferous metasedimentary successions representing a differentiated basin evolution. Stratiform SEDEX-type Pb-Zn-Ag-Ba occurrences are situated in the Schönberg formation of the Schöckl nappe and are hosted by polyphase deformed greenschist facies metapelitic, metacarbonate and metavolcanoclastic lithologies. Alteration zones and elemental zonation surrounding sulphide mineralisation were investigated. For this purpose, drill core sections with sulphide ore horizons of a 1970-80s exploration program at Haufenreith and Guggenbach as well as from former mining districts were analysed. The investigated sulphide ore horizons are composed of mainly galena, sphalerite, pyrite, pyrrhotite and in parts of baryte and magnetite. Minor ore phases are chalcopyrite, ullmannite, pentlandite, Ni-Co-Fe sulpharsenides, argentiferous phases such as hessite and argentopentlandite, and other rare ore minerals. These major and minor ore phases are also present disseminated or in veinlets in small quantities throughout the alteration zone. The alteration zone around sulphide mineralisation forms a diffuse halo within metamorphosed background sedimentation. It occurs both in the footwall and hanging wall of ore horizons indicating vent proximal ore deposition and is traceable for tens of metres even in weakly mineralised ore horizons of drill cores. Albitization occurs commonly beneath ore horizons. Carbonate alteration is present mainly but not exclusively in the hanging wall of sulphide mineralisation and consists of dominating dolomite-ankerite, and minor siderite. The chemical compositions of ferrous carbonates, chlorite and white mica in the alteration zone show a general enrichment in Fe as well as Mg. Carbonates are additionally enriched in Mn. A zone of initial Mg enrichment at the outer rim of the alteration zone is followed by an inner zone with Mg depletion as well as Fe enrichment peaking at sulphide mineralisation, where carbonates become Mn enriched. Moreover, Ba bearing white mica is present in the alteration zone reaching highest Ba concentrations at the location of mineralisation. Barium rich feldspar, celsian and hyalophane, is present in samples of the mineralisation at Arzwaldgraben and Peggau-Taschen as well as in drill cores from Haufenreith, where they are accompanied by K-feldspar, which sometimes occurs also as minor constituent of ore horizons and in sulphide veinlets as gangue phase. High Field Strength Element (HFSE) and Rare Earth Element (REE) rich phases frequently accompany ore and alteration minerals. Observed HFSE minerals are rutile, ilmenite, titanite, (urano-)thorite, zircon and others. Discrete niobium rich minerals are columbite-(Fe) and euxenite-(Y), which are exclusively associated with ore horizons at the Kaltenberg-Burgstall district. REE minerals encompass abundant monazite-(Ce), minor fluorocarbonates and xenotime-(Y) as well as rare allanite-(Ce). Besides, fluorapatite occurs often with HFSE and REE minerals. Observed alteration mineralogy and the frequency of alteration related phases correlate well with whole rock geochemistry of investigated drill cores. The detailed mineralogical and geochemical investigation of drill core material illustrates type and extent of wall rock alteration associated with the metamorphosed stratiform sulphide ore and can be applied as a tool in regional exploration programs.