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Ben Lomond

Queensland, Qld, Australia

Main commodities: U F Mo
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The Ben Lomond uranium deposit is associated with Late Palaeozoic felsic intrusives and volcanics in north-eastern Queensland, Australia.   Ben Lomond lies towards the north-western margin of the Townsville-Bowen volcanic field, in the overlap with the more northerly Newcastle Range-Featherbed volcanic field. The similar Maureen deposit is located towards the western margin of the latter field.   Ben Lomond is some 50 km WSW of the city of Townsville, while Maureen is a further 340 km to the NW (see the separate Maureen  record).

These deposits are the biggest of a large number of uranium-fluorine-molybdenum occurrences and radiometric anomalies that are associated with the extensive late Palaeozoic continental felsic volcanics and related intrusives of the Coastal Range Igneous Province that overlie and intrude the Georgetown-Coen Province of north-eastern Queensland and Cape Yorke Peninsular.

The Georgetown-Coen Province comprises the Georgetown, Einsaleigh and Gilberton Inliers, which are composed of variably deformed and metamorphosed Paleo- to Mesoproterozoic meta-sediments (from older calc-silicate gneisses, to andalusite-sillimanite-cordierite-mica-schists and quartzites, to younger, less metamorphosed mudstones, sandstones, shales and carbonaceous shales) and meta-volcanics (gabbros, dolerites and basalts low in the pile, to andesites and extensive rhyolites at the top), intruded by Mesoproterozoic granitoids.

The Proterozoic sequences are divided into the up to 15 000 m thick sequence of metamorphosed shallow water peltic to psammitic sediments and lesser mafic volcanics that comprise the Paleoproterozoic Etheridge Group, separated from the overlying early Mesoproterozoic Langlovale Group by the 1570 ±20 Ma Ewamin orogenic event. The Langlovale group, which is an at least 3000 m thick sequence of fluviatile to shallow marine psammites and pelites, and the coeval ~1550 Ma trondhjemite intrusives, were terminated by the 1470 ±20 Ma Jana Orogeny with associate S-type anatectic granitoids. These were unconformaby followed by the up to 2500 m thick, late Mesoproterozoic Croydon Volcanic Group, comprising early basaltic andesites followed by extensive rhyolites, and coeval S-type granitoids of the Forsayth Supersuite. These volcanics and intrusives were unconformably ovelain by the late Proterozoic to early Palaeozoic fluviatile sediments of the Inorunie Group.

These inliers are separated, overlain and intruded by Cambro-Ordovician, Siluro-Devonian, Carboniferous and Permian igneous rocks. The Cambro-Ordovician are laregly found to the south of the Georgetown Inlier, itself in the south, and include the Mt Windsor and Balcooma submarine to sub-aerial volcanics and volcaniclastics, which are largely felsics and comprise a sequence that is up to 7000 m thick.

The up to 100 to 150 km wide and 1000 km long, Siluro-Devonian Cape Yorke Plutonic Belt extends along the eastern margin of the Georgetown-Coen Province, along its faulted margin with the Palaeolzoic the thick Palaeozoic monotonous greywacke-shale sedimentary pile of the Hodgkinson Basin to the east. The most common rock type in this belt is biotite ± hornblende granodiorite occurring as composite, but not zoned batholiths, that are strongly discordant, foliated, sheared and intrude high grade metamorphcs with noobvious contact aureoles.

The Carboniferous and Permian igeous rocks of the Coastal Range Igneous Province extensively overprint the Georgetown-Coen Province and the sediments of the Hodgkinson Basin. The Carboniferous is largely represented by continental, welded rhyolitic ignimbrites, with minor more mafic components. The ignimbrite-dominated sequences are generally basinal and associated with concentric (ring) and/or linear fracture intrusive systems, many of which represent partial, single or composite cauldron collapse structures. The associated magma chambers are exposed as granitic batholiths. Extrusive rocks of the Newcastle Range-Featherbed volcanic field and intrusive equivalents are variably fractionated I-type in character. In many areas these have been divided into two suites, one predominantly dacitic to andesitic, the other, which is volumatrically dominant, of rhyolitic (or granitic) composition. Isotopic ratios suggest the intermediate suite was derived from old mantle, while the more acid suite were derived by anatectic reworking of Proterozoic basement rocks by invading mantle derived mafic magmas.

Early Permian igneous rocks are more widely distributed than the preceeding Carboniferous igneous complexes. In the Georgetown-Coen Province, the individual centres of igneous activity are characteristically thinner, less extensive and more heterogeneous than the underlying Carboniferous, with the intermediate to basic suites being relatively more voluminous. Thes volcanic sequences rest unconformably on Upper Devonian to Lower Carboniferous clastic sediments and Proterozoic basement, with much of the Carboniferous ignimbrite having been eroded. The Permian extrusives were deposited within broadly basinal structures, without associated concentric fracture system intrusives. The intrusives are dominantly sub-volcanic and of limited areal extent. Only to the east of the Georgetown-Coen Province did these basinal sediments overlap with more voluminous, felsic ignimbrite dominated volcanism with major cauldron subsidence, as exemplified by the Featherbed Volcanic Group. The Permian structural trend is predominantly NW, in contrast to the northerly structures of the Carboniferous. As in the Carboniferous, there are two suites represented, namely basaltic to andesitic and highly felsic rhyolitic rocks. The former suite is taken to represent an isotopically evolved mantle source, with the felsic suite being A-type derived from a depleted crustal source. There are also isolated putons of variably fractionated and zoned I-type granodiorite to granite with no preserved extrusive equivalents.

The Ben Lomond uranium-molybdenum-zinc deposit is located in the Hervey Range, North Queensland, Australia, approximately 50 km WSW of Townsville. It occurs within a fault-bounded block of Carboniferous calc-alkaline volcanics of the St James Volcanics,predominantly rhyolitic tuffs and lavas, which are part of the Glenrock Group. These overlie the Late Devonian to Early Carboniferous Keelbottom Group sediments and older basement sedimentary and volcanic rocks of the Paleoproterozoic Argentine Metamorphics. The St James Volcanics are unconformably overlain by andesitic and basaltic lavas and pyroclastics of the Upper Andesite Member. These are in turn unconformably followed by the Early Carboniferous Watershed North Rhyolite (previously known as the Cattle Creek Group), a crystal-rich to lithic-rich rhyolitic ignimbrite which is at least 400 m thick and is thought to have been deposited in a cauldron subsidence event.

In the immediate vicinity of the deposit, the youngest non-intrusive rocks are pyritic carbonaceous shales and sandstones of the Late Carboniferous Insolvency Gully Formation, which unconformably overlies both the St James Volcanics and Watershed North Rhyolite. The Insolvency Gully Formation is intruded by the Late Carboniferous to Early Permian Speed Creek Granite.

The St James Volcanic sequence has been subdivided into the following units, from the stratigraphic top to bottom:
i). Host unit, comprising from the top to base, the following sub-units: coarse grained quartz feldspar porphyry dyke; coarsely porphyritic, chloritic, flow banded rhyolite; maroon siltstone and tuffaceous sandstone with minor conglomerate and quartzite (the “Black Tuff Formation”); coarsely porphyritic dacite; rhyolite with siderite patches; porphyritic rhyolite; porphyritic lapilli tuff with rare fiamme; ppebbly porphyritic lapilli tuff with rare fiamme; coarse agglomerate; rhyolite; coarse porphyritic dacite; pebbly tuffaceous sandstone and siltstone; lapilli tuff; and porphyritic flow banded rhyolite,
ii). Flow-banded rhyolite,
iii). Welded tuffs and ash flows with local sedimentary intercalations,
iv). Basic-intermediate lava flows and pyroclastics with minor intercalated sediments,
v). Lahars of dacitic to rhyolitic composition.

The unconformably overlying lensoid Upper Andesite Member and the Watershed North Rhyolite which have been subdivided into the following units from the top:
i). Ignimbrite with classic fiamme texture,
ii). Lapilli tuffs, ignimbrites and minor vitric tuff,
iii). Lapilli tuffs and ignimbrites with subordinate crystal lithic tuffs,
iv). Ash flow tuffs with abundant shards and ejecta,
v). Welded tuffs and lapilli tuffs,
vi). Vitric tuffs and ignimbrites,
vii). Rhyolitic airfall tuffs, grading into coarse ignimbrites then lithic/vitric tuffs,
viii). Upper Andesite Member – Andesitic basalt flows.

The host volcanic units appear to have undergone at least two stages of pre-mineralisation alteration. The first was associated with cooling of the thick volcanic pile, while the second was the result of later convective hydrothermal activity associated with the shallow intrusion of the Pall Mall adamellite. The later hydrothermal activity produced tourmaline and dumortierite that preceded uranium-molybdenum-zinc mineralisation of the Ben Lomond deposit in the rhyolitic welded tuff at the top of the St. James volcanics.

The uranium-molybdenum mineralisation at Ben Lomond and its associated alteration halo, occurs within a strongly sheared east-west striking, steeply south dipping zone sub-parallel to the axial plane of a shallow plunging syncline, outcropping on the northern flank of Ben Lomond East Ridge. It has a shallow eastward plunges beneath the unconformably overlying Watershed North Rhyolite, and dips at 75°S, with a maximum width of 150 m, with an upper limit a few metres below the St James Volcanics/Watershed North Rhyolite unconformity. The mineralisation extends down dip for some 90 m overall, although the best grades are developed from 10 to 50 m below the unconformity. Within the overall mineralised envelope, a resource was initially, delineated over a 750 m strike length, although mineralisation and alteration continues below 100 to 400 m of Watershed North Rhyolite cover, for at least another kilometre.

The uranium-molybdenum mineralisation occurs as primary vein fillings of a complex system of steeply dipping fracture-infill veins, subordinate stockworks and brecciated zones, and as disseminations in the adjacent wallrock, The strongest grades are within subvertical vein filled structures, some with sharp contacts, although the majority have wispy diffuse contacts where the uranium minerals are disseminated through the wall rock immediately adjacent to the vein system. Veins are typically lenticular and discontinuous both along strike and down dip. The veins form a discontinuous swarm with individual veins rarely exceeding 200 m in length and 1 to 2 m in thickness. They are densely packed in the main zone of mineralisation such that higher grades persist over broad widths despite the intervening barren material between the veins. The vein zone is surrounded by an envelope of alteration, with a close relationship between uranium grade and the presence of haematite and quartz.

The hypogene mineralisation comprises a simple assemblage of pitchblende, coffinite, molybdenite and jordisite, with minor accompanying pyrite and arsenopyrite and trace marcasite, galena, sphalerite and chalcopyrite. The main gangue within the veins include fine grained quartz with subordinate sericite, chlorite and tourmaline, all of which also occur as disseminations in the wallrock.

The mineralised zone is characterised by strong silicic and hematitic alteration within the wallrock with associated peripheral chlorite and pervasive dolomite. In the west of the mineralised zone, where there is no capping of Watershed North Rhyolite, the depth of weathering ranges from a few to 30 m or more, generally from 15 to 20 m. The base of oxidation is gradational and typically sub-parallel to the topography, passing down into the underlying hypogene zone. Within the zone of oxidation, sporadic secondary accumulations of mineralisation occur, with the most common uranium minerals being umohoite and iriginite. In the eastern portion of the mineralised zone, where the mineralised zone passes below the Watershed North Rhyolite, oxidation is absent.
Deposit summary based on Vigar & Jones, (2005).

The estimated resource has been quoted by AAEC (McKay 1982) as 1.93 Mt @ 0.217% U for 4200 t of contained U.

Resources in July 2008 were quoted by Mega Uranium Ltd, 2008, at:
    Indicated resource - 1.33 Mt @ 0.27% U3O8 for 3597 t U3O8
    Inferred resource - 0.60 Mt @ 0.21% U
3O8 for 1275 t U3O8

The most recent source geological information used to prepare this summary was dated: 2005.    
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:
Vigar A and Jones D G  2005 - Ben Lomond uranium-molybdenum deposit, Queensland, Australia (excerpt): in    Publicly available report to Mega Uranium    27p


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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