Valhalla, Skal

Queensland, Qld, Australia

Main commodities: U V
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The Valhalla and Skal uranium deposits are located 40 km northwest of Mount Isa in north-west Queensland, Australia, 24 kilometres north of the Hilton and George Fisher lead-zinc-silver ore bodies, which straddle the Mt Isa Fault Zone, and 40 km south of the Gunpowder copper mine.

The deposits are reflected by ironstone gossans, large radiometric, magnetic and geochemical anomalies and several small historic uranium workings including the Valhalla prospect. The presence of significant zones of uranium and copper mineralisation, the proximity to the regional Mt Isa Fault zone, the proximity to granite intrusives and the mineralogy of the known deposits have been taken to imply that the mineralisation has an IOCG association.

The Valhalla, Skal and related uranium and vanadium mineralisation is hosted within a NNW striking sequence of intercalated meta-basalts, laminated meta-shales to siltstones and minor associated tuffaceous rocks which dips at around 70° SW.   This sequence has been correlated with the Eastern Creek Volcanics of the regional Cover Sequence 2 of the the Mt Isa Inlier, part of the multiple scale rift/sag episodes that are thought to have occurred between 1740 and 1680 Ma within the latest Paleoproterozoic.

The basalts are tholeiitic to intermediate in composition reflecting the zonation within a series of lava flows. The individual flows are of the order of 10 to 30 m thick and display characteristic flow top textures grading down into coarser grained intermediate lavas in the middle, to finer, more mafic extrusives at the base. The mineral assemblage in these volcanics is characteristic of lower amphibolite facies metamorphism and includes variable amounts of hornblende, feldspar, chlorite, epidote, magnetite, carbonate and quartz, accompanied by pyrite and chalcopyrite.

The metasediments comprise laminated carbonaceous shales and siltstones, commonly composed of chlorite, sericite, graphite, carbonate, feldspar ±pyrite ±magnetite, with frequent up to 5 cm thick laminae and beds of dolomitic sand. Planar and cross bedded sedimentary structures are both common.

The host sequence has been subjected to regional scale hydrothermal magnetite carbonate alteration. Coarse grained euhedral pyrite and magnetite porphyroblasts are associated with the basalt contracts.

The Valhalla uranium-vanadium mineralisation outcrops over a ~350° trending strike length of ~600 m and down dip/plunge extent of up to 450 m within a sub-vertical body which plunges ~50° S. The mineralised zone is discordant to the stratigraphy and averages around 60 m in thickness, as defined at a 100 ppm U3O8 cutoff.
    The uranium mineralisation is structurally controlled and is associated with distinct iron, carbonate and sodic metasomatism related to a zone of intense mylonitic/cataclastic shearing and hydraulic brecciation, where the more ductile textures are overprinted by a later brittle event. The uranium is predominantly contained within brannerite and metamict zircon with minor coffinite, uraninite and uranophane.
    Alteration associated with U mineralisation can be divided into an early, main and late stage (Polito et al., 2009). The early stage is dominated by laminated and intensely altered rock comprising albite, reibeckite, calcite, (titano)magnetite ± brannerite. The main stage of mineralisation is dominated by brecciated and intensely altered rocks that comprise laminated and intensely altered rock cemented by brannerite, apatite, (uranoan)-zircon, uraninite, anatase, albite, reibeckite, calcite and hematite. The late stage of mineralisation comprises uraninite, red hematite, dolomite, calcite, chlorite, quartz and Pb-, Fe-, Cu-sulphides. Brannerite has U-Pb and Pb-Pb ages that indicate formation between 1555 and 1510 Ma, with significant Pb loss evident at ~1200 Ma, coincident with the assemblage of Rodinia. The oldest ages of the brannerite overlap with
40Ar/39Ar ages of 1533±9 Ma and 1551±7 Ma from early and main-stage reibeckite and are interpreted to represent the timing of formation of the deposit. These ages coincide with the timing of peak metamorphism in the Mount Isa area during the Isan Orogeny (Polito et al., 2009).
    The orientation and extent of individual higher grade (>450 ppm U
3O8) zones within the broad mineralised envelope (100 ppm U3O8) is a complex, occurring as anastomosing lenses, both along strike and down dip, although consistent down plunge. There is a persistent high grade zone on the Footwall (eastern most) contact. Thickening of higher grade zones may be due to: i). local thickening of geology due to folding; ii). sigmoidal or en echelon stacking of the high grade lenses; iii). structural and/or lithological control of sites of preferential mineralisation.

The Skal deposit, which is approximately 10 km SE of Valhalla, is hosted by silicified and brecciated calcareous and jasperoidal sediments within the foliated argillaceous sediments and basalt sequence. The mineralised zone averages around 30 m in thickness. Both the discordant lodes and sub-parallel stratigraphy strike approximately NNE and dip at around 50°W. The lodes plunge to the south. The North and South lodes outcrop over a distance of some 300 m, separated by cross faulting. The primary uranium mineralisation occurs as 0.001 to 10 mm diameter grains of brannerite, with minor carnotite associated with hematite near surface. Minor chalcopyrite and pyrite have been recorded, although locally up to 2% Cu has been encountered. Gangue minerals are fine grained quartz, calcite and iron oxides.

Published resources are as follows (Summit Resources, 2006):

    Indicated + Inferred cut-off 0.23 kg/t U
3O8 - 33.3 Mt @ 0.77 kg/t U3O8 = 25900 t U3O8
    Indicated + Inferred cut-off 0.64 kg/t U
3O8 - 16.3 Mt @ 1.13 kg/t U3O8 = 18600 t U3O8.

The most recent source geological information used to prepare this summary was dated: 2007.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Polito P A, Kyser T K and Stanley C  2004 - The Proterozoic, albitite-hosted, Valhalla uranium deposit, Queensland, Australia: a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole V39: in    Mineralium Deposita   v44 pp 11-40
Polito P A, Kyser T K and Stanley C,  2009 - The Proterozoic, albitite-hosted, Valhalla uranium deposit, Queensland, Australia: a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole V39: in    Mineralium Deposita   v.44 pp. 11-40

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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