TY - THES

T1 - W-Sn mineralisation in calc-silicate rocks of the Basal Amphibolite unit at Messelingscharte (Felbertauern area, Austria)

AU - Ordosch, Alexander

N1 - embargoed until null

PY - 2017

Y1 - 2017

N2 - An unusual W-Sn mineralisation occurs at Messelingscharte in Early Palaeozoic amphibolites of the Basal Amphibolite unit in the Tauern Window. Four types of scheelite mineralisation are distinguished. (1) Scheelite-bearing calc-silicate pods, (2) Discordant, deformed scheelite-quartz veins, (3) Scheelite in concordant mylonitic quartz-amphibolite layers, (4) Scheelite on joint surfaces. Type 1 and 2 are assumed to be pre-Alpine (Variscan?) formations. The Basal Amphibolite consists of various amphibolites, hornblende-biotite schist, hornblende fels and hornblende-biotite gneiss; the latter preserves intrusive contacts with the amphibolites but is co-magmatic. The amphibolites exhibit geochemical signatures, similar to enriched back-arc basalts. The calc-silicate rocks occur as metre-sized irregular shaped pods. They are composed of major clinozoisite, quartz, and plagioclase with minor to accessory scheelite, titanite and chlorite. Bulk geochemistry reveals high concentrations of W (≤6.14 mass% WO3), Sn (≤1254ppm SnO2), Be (≤41ppm) and transition metals (Cu, Pb, Zn; ∑≤2500ppm) in these rocks. Three scheelite generations are distinguished based on micro-textures, zoning, Mo-content and luminescence colour. The first generation is coarse-grained and Mo-rich (0.82-1.7 mass% MoO3) and normally preserved in the cores of large scheelite porphyroblasts. It is interpreted as primary pre-Alpine (Variscan?) scheelite. Scheelite generations 2 and 3 from Messelingscharte are Mo-poor/free and are interpreted as metamorphic mobilisations and recrystallisation products formed during Variscan (?) and/or Young Alpine regional metamorphism. Hence, it is concluded that, similar to Felbertal scheelite deposit (Scheelite 1 and 2 there), pre-Alpine Mo-rich scheelite was overprinted by two stages of metamorphism. An unique feature of scheelite mineralisation at Messelingscharte is the association of W with Sn. Clinozoisite and titanite were identified as the main Sn-bearing phases (clinozoisite ≤3.00 mass% SnO2, =0.09 apfu; titanite ≤6.48 mass% SnO2). Sn-bearing clinozoisite (large anhedral grains with irregularly shaped Sn-rich lamella) was also affected by metamorphic recrystallisation; the fine-grained, euhedral metamorphic clinozoisite is Sn-free. Substitution of (Al,Fe)3+ by (Sn,Ti)4+ is explained by simultaneous incorporation of divalent cations like Fe2+. Titanite shows patchy irregular zoning defined by Sn-content and rarely hosts very small (

AB - An unusual W-Sn mineralisation occurs at Messelingscharte in Early Palaeozoic amphibolites of the Basal Amphibolite unit in the Tauern Window. Four types of scheelite mineralisation are distinguished. (1) Scheelite-bearing calc-silicate pods, (2) Discordant, deformed scheelite-quartz veins, (3) Scheelite in concordant mylonitic quartz-amphibolite layers, (4) Scheelite on joint surfaces. Type 1 and 2 are assumed to be pre-Alpine (Variscan?) formations. The Basal Amphibolite consists of various amphibolites, hornblende-biotite schist, hornblende fels and hornblende-biotite gneiss; the latter preserves intrusive contacts with the amphibolites but is co-magmatic. The amphibolites exhibit geochemical signatures, similar to enriched back-arc basalts. The calc-silicate rocks occur as metre-sized irregular shaped pods. They are composed of major clinozoisite, quartz, and plagioclase with minor to accessory scheelite, titanite and chlorite. Bulk geochemistry reveals high concentrations of W (≤6.14 mass% WO3), Sn (≤1254ppm SnO2), Be (≤41ppm) and transition metals (Cu, Pb, Zn; ∑≤2500ppm) in these rocks. Three scheelite generations are distinguished based on micro-textures, zoning, Mo-content and luminescence colour. The first generation is coarse-grained and Mo-rich (0.82-1.7 mass% MoO3) and normally preserved in the cores of large scheelite porphyroblasts. It is interpreted as primary pre-Alpine (Variscan?) scheelite. Scheelite generations 2 and 3 from Messelingscharte are Mo-poor/free and are interpreted as metamorphic mobilisations and recrystallisation products formed during Variscan (?) and/or Young Alpine regional metamorphism. Hence, it is concluded that, similar to Felbertal scheelite deposit (Scheelite 1 and 2 there), pre-Alpine Mo-rich scheelite was overprinted by two stages of metamorphism. An unique feature of scheelite mineralisation at Messelingscharte is the association of W with Sn. Clinozoisite and titanite were identified as the main Sn-bearing phases (clinozoisite ≤3.00 mass% SnO2, =0.09 apfu; titanite ≤6.48 mass% SnO2). Sn-bearing clinozoisite (large anhedral grains with irregularly shaped Sn-rich lamella) was also affected by metamorphic recrystallisation; the fine-grained, euhedral metamorphic clinozoisite is Sn-free. Substitution of (Al,Fe)3+ by (Sn,Ti)4+ is explained by simultaneous incorporation of divalent cations like Fe2+. Titanite shows patchy irregular zoning defined by Sn-content and rarely hosts very small (

KW - Wolfram

KW - Zinn

KW - Lagerstätte

KW - Hohe Tauern

KW - Tauern Fenster

KW - Skarn

KW - Kalksilikatgestein

KW - Amphibolit

KW - Basisamphibolit

KW - Geochemie

KW - Scheelit

KW - Klinozoisit

KW - Titanit

KW - tungsten

KW - tin

KW - mineral deposit

KW - Hohe Tauern

KW - Tauern Window

KW - skarn

KW - amphibolite

KW - Basal Amphibolite

KW - geochemistry

KW - scheelite

KW - clinozoisite

KW - titanite

M3 - Master's Thesis

ER -