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Logtung |
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Yukon Territory, Canada |
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W Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Logtung scheelite-molybdenite occurrence in the Yukon Territory-British Columbia border, Canada, is hosted by a sequence of mid Devonian to lower Carboniferous (Mississippian) intercalated carbonates and argillites. These sediments were deposited on the Cassiar Platform which is on the eastern margin of the Omineca Crystalline Belt to the west of the Tintina Trench, and immediately to the south west of the Selwyn Basin. These sediments are believed to be equivalents of the Black Clastic Unit shales and cherts of the Selwyn Basin and represent a facies markedly similar to that encountered further east, at the transition from the platform carbonates of the Mackenzie Fold Belt to the basinal shales of the Selwyn Fold Belt.
Prospect Geology
The host sequence within the Logtung Prospect area comprises a shallow north-north westerly dipping sequence of fine clastic rocks and carbonates to calc-silicates of Devonian to Mississippian age. These sediments are intruded by Jurassic diorite, Cretaceous quartz-monzonite and a series of Cretaceous porphyry dykes.
The geological history of the prospect area may be summarised as follows:
TERTIARY
A few thin basalt dykes of Tertiary age cut the host sequence.
CRETACEOUS
Porphyry - A multiphase dyke swarm cuts the centre of the prospect area. These include quartz-porphyry, quartz-feldspar-porphyry, aplite, felsite-myrmekitic-porphyry, silicified porphyry and pegmatite. At the surface the undifferentiated porphyry is mapped as a U-shaped outcrop. The arms of this structure converge at depth into a pipe-shaped body.
Quartz-Monzonite - A small north-south elongated quartz-monzonite body outcrops to the north of the main prospect. This is a medium to fine grained (1 to 2 mm) rock with a granitic texture and occasional 2 to 3 mm quartz and pink feldspar phenocrysts. Both the quartz monzonite and porphyry dykes have been dated at 105 Ma.
JURASSIC
Diorite - Dioritic bodies are found to the east and west of the main prospect. The diorite is fine to medium grained with 1 mm crystals of mainly mafic minerals, with up to 30% pale feldspar and 1% disseminated pyrrhotite. The diorite has not been age dated. However, the porphyry dykes can be seen to cut the diorite.
MISSISSIPPIAN TO DEVONIAN
Black Clastic Unit Equivalents - The bulk of the sedimentary rocks in the main prospect area are fine clastics, with four main lenses of carbonates and calc-silicates.
Clastics - The clastics are mainly well bedded shales, dark cherts and argillites, with minor fine greywacke. In the main mineralised zone the sediments are altered to dark grey to green, banded, very fine grained, biotitic rock, with garnet and diopside bands up to 1 cm thick. These are cut by fine diopsidic veinlets.
Carbonates and Calc-silicates - Four carbonate-calc-silicate lenses have been mapped in the main prospect area. These range from 30 to 100 m in thickness and are separated by from 10 to 50 m of clastics. In the central section of the prospect, the carbonate-calc-silicate lenses account for 60% of the sequence to a depth of 300 m. They comprise un-altered limestone, and three main types of calc-silicate tactite namely; garnet-diopside, pyroxene and pale tactite.
The altered limestone is a grey banded limestone with 1 to 2 mm laminations apparent as pale and dark grey colour variations. Pyrite is present as sparse disseminations of 0.5 mm crystals.
The garnet-diopside tactite is a banded, pink, garnet rich, irregularly banded (up to 5 mm thick bands) tactite, with lesser dark green diopside and pale grey intercalated silica bands. These bands have diffuse margins. The rock has about 1% disseminated sulphides, mainly pyrrhotite and fine pyrite, with a 0.5 mm grain size. The quantity of each of the main components varies considerably from place to place.
The pyroxene tactite is banded, with alternating pale grey to white siliceous and dark green diopsidic 3 to 15 mm bands.
The pale tactite is a pale-grey to pale-green tinged cherty calc silicate, with approximately 1 mm banding between darker and paler grey-green bands. Veinlets of pyrrhotite and pyrite cut the bedding normally. At the surface, this rock is a pale-grey to off-white banded, highly siliceous cherty rock.
Mineralisation
Scheelite is found to varying degrees in all of the rock types in the prospect area, while molybdenite is largely restricted to the porphyry dykes and sedimentary sequence. The better grade scheelite is found within the calc-silicate-carbonate lenses and the clastic beds. Although sections of the quartz-monzonite and diorite do carry some disseminated scheelite adjacent to the main mineralised zone, they essentially limit the higher grade mineralisation.
Three main scheelite fluorescence types are present, namely yellow, blue and white. The yellow variety is predominantly the original disseminated variety while the white scheelite is found in quartz veins and on fracture surfaces. The blue variety is relatively rare and occurs in both forms.
Mineralisation within the main rock types can be summarised as follows:
Garnet-diopside Skarn - Scheelite occurs mainly as 0.2 to 0.5 mm, and lesser 0.5 to 1 mm grains disseminated evenly through the tactite, with no observable banding. Dense fine films of scheelite are found on fracture surfaces and in thin cross-cutting veinlets. These range up to a maximum of 1 mm in thickness and are oriented in three directions. They are spaced at an interval of one vein or fracture per 2 to 10 m in denser areas, and up to one per metre in others. As such they represent a minority of the overall contained scheelite. The better grade garnet-diopside tactite carries up to a maximum of 0.3% WO3.
Pyroxene Skarn - Scheelite occurs as 1 mm thick bands of 0.5 mm scheelite grains, developed parallel to the tactite bedding. These bands are cut by much lesser 1 mm thick scheelite coated fractures developed in two or three directions, but commonly at a lesser density than in the garnet-diopside tactite. In general grades range from 0.15 to 0.2% WO3.
Pale Skarn - Scheelite is present in a similar style, but overall lower grade, than the pyroxene tactite. In general the pale tactite carries around 0.09% WO3.
Banded Limestone - This lithology is usually barren, only having scheelite in the occasional tactite patches found within its confines.
Altered Argillite - The altered argillite carries minor disseminated scheelite with the majority being on thin (1 mm or less) fracture surfaces, totalling around 0.03% WO3.
Diorite - Only one drill hole has penetrated the diorite to date. This revealed white 1 to 2 mm scheelite grains concentrated on fractures and within 0.1 to 1 cm quartz veins. Minor disseminations are also present. The bulk grade within the diorite intersection was around 0.03% WO3.
Porphyry - The porphyry dykes in general are almost devoid of scheelite, but do carry significant molybdenite, probably the highest grade in the prospect. Occasional coarse 0.5 to 1.5 mm scheelite grains occur within the dykes.
Quartz Monzonite - This lithology carries minor disseminated scheelite and molybdenite, but is basically barren.
Surface soil geochemistry, partly in residual soils, but largely in glacial till, shows a broad arcuate anomalous zone, broadly conformable with the strike of the sedimentary sequence, but also covering the intervening sections of the diorite. The WO3 anomaly is more extensive than the molybdenum anomaly which falls within its outlines. Anomalous zinc levels are found at a lower stratigraphic level. The main geochemical anomalous tungsten zone defines an area of some 2000 x 600 m, while the drill indicated mineralised zone is around 500 x 400 m laterally, and up to 300 m vertically.
The tungsten grade in drill holes increases abruptly from <0.03% to >0.1% WO3 at the upper contact of the top-most carbonate-calc-silicate lens, while the MoS2 levels rise sharply to >0.03% some 50 m further below. The tungsten grades drop to <0.05% WO3 along strike to the south-west as the total carbonate-calc-silicate content of the sequence decreases. To the north-east, the intrusives sharply limit the mineralised zone. Minor crosscutting Pb-Zn massive sulphide veins have been worked to the south-east, in the zone of anomalous soil levels long before tungsten was recognised at the prospect.
The tactites and argillites carry minor disseminated pyrrhotite throughout, generally totalling between 0.1% and 1%. There appears to be a general correlation between the scheelite and pyrrhotite content at any one point.
Testing & Reserves
Drill testing to date has not been to any set pattern, but broadly conforms to a 200 to 250 m centres grid of vertical holes. Of the 12 holes drilled, only 8 have been in the main mineralised zone. These vary in depth from 90 to 300 m, averaging around 180 m. Grades range from around 0.03% WO3 in the altered argillites, to 0.09% in the pale tactite. Levels of 0.15 to 0.2% WO3 are encountered in the pyroxene and garnet-pyroxene tactites, with peaks of up to 0.3% WO3 in the latter. Bulked grades quoted for each of the drill holes range from 0.09 to 0.17% WO3 and 0.038 to 0.074% MoS2, with the majority being in the 0.11 to 0.15% WO3 and 0.036 to 0.04% MoS2 range.
Reserves are stated at:
200 Mt @ 0.12% WO3 and 0.051% MoS2
on the basis of this drilling. An eventual 100 to 125 m centres grid pattern is envisaged.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1984.
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.
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Selected References:
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Noble S R, Spooner E T C 1984 - The Logtung large tonnage, low-grade W (Scheelite)-Mo Porphyry deposit, south-central Yukon Territory: in Econ. Geol. v79 pp 848-868
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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