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The Älgtrask intrusive-hosted Au (-Cu) deposit, Sweden
Boliden AB.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
Boliden AB.
2008 (English)Conference paper, Presentation (Other academic)
Abstract [en]

The Algtrask gold deposit is located in the northern part of the Skellefte mining district, northern Sweden. It constitutes a major gold and copper exploration target for the mining company Boliden Mineral AB with an inferred mineral resource of 1.6 Mt grading 3 g/t Au. The deposit is situated in the southern margin of the Jorn Granitoid Complex (JGC), a composite, I-type, calc-alkaline batholith, which intruded into an island arc or continental margin arc volcanic succession in the early Proterozoic. The area has been affected by regional metamorphism of lower greenschist facies.Mineralization occurs as veins and disseminations in meter wide, steeply dipping, sub-parallel ENE-NE striking deformation zones within a coarse grained quartz-porphyritic granodiorite. The deposit has proximal silicic/phyllic and distal propylitic alteration. The altered zones commonly show ductile deformation, and where they are sulphide bearing, form IP-anomalies. Gabbro, equigranular tonalite, quartz-feldspar porphyritic dacitic dykes, basaltic dykes, aplitic dykes, quartz-calcite veins and episyenite have been recognized close to mineralization. The quartz-feldspar porphyritic dykes are in part altered and mineralized in a similar manner to the granodiorite.Two main mineralized zones have been distinguished at Algtrask; both contain pyrite, chalcopyrite, sphalerite and arsenopyrite and minor molybdenite, bornite, pyrrhotite, magnetite, gold and tellurides (e.g. petzite, hessite, altaite, coloradoite). Traces of native silver and bismuth have also been observed. The gold occurs as electrum (ca 80% Au) and calaverite closely associated with other tellurides. Gold and tellurides commonly occur as inclusions or in fractures in pyrite, within quartz-calcite veins or zones of phyllic-silicic alteration. Thicker bands of arsenopyrite are common in the southern zone, but do not occur in the northern zone, which suggests that the chemistry and sulphur fugacity of the ore forming fluids were different in the two mineralised zones.The Tallberg porphyry copper deposit is situated only 3 km west of Algtrask. Tallberg is characterized by quartz stockworks with disseminated pyrite, chalcopyrite, pyrrhotite and magnetite, hosted in propylitic and phyllic altered equigranular tonalite of the JGC. The similar settings, alteration and close spatial relationship, suggest a genetic link between the two deposits. The Algtrask deposit might represent a higher level epithermal system genetically linked to the Tallberg porphyry system. Ductile deformation took place during subsequent tectonic events, probably causing remobilization of sulphides and precious metals. Alternatively, a syntectonic model can be proposed in which the Algtrask deformation zones have overprinted an earlier porphyry copper system. In this model, syntectonic hydrothermal fluids may have "leached" gold from the earlier porphyry system during deformation and deposited gold in the deformation zones.

Place, publisher, year, edition, pages
Research subject
Ore Geology
URN: urn:nbn:se:ltu:diva-38585Local ID: d06d09c0-6f95-11dd-8151-000ea68e967bOAI: diva2:1012085
International Geological Congress : 06/08/2008 - 14/08/2008
Godkänd; 2008; 20080821 (pawe)Available from: 2016-10-03 Created: 2016-10-03Bibliographically approved

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