Queensland, Qld, Australia

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The Merlin high grade Mo-Re deposit is located 110 km south of Cloncurry, in the Mount Isa Block of northern Australia (#Location: 21° 39' 14"S, 140° 29' 39"E).

Merlin is a high grade development in the immediate footwall of the greater Mt Dore mineralised zone.

See also the separate Starra-Selwyn (Mt Dore, Merlin) and Cloncurry IOCG Province records for regional setting and context.

The Merlin deposit is hosted in Proterozoic metasedimentary rocks of the Mount Isa Block of northern Australia. These rocks comprise pelitic slates and phyllites interbedded with carbonaceous metapelites, underlain by intensely altered calc-silicate rocks, all of which are structurally overlain by the 1516±10 Ma Mt Dore Granite.

The deposit is hosted by the Kuridala Formation, which is several kilometres thick, and contains deep water turbiditic sediments and lower energy variably carbonaceous shales. These rocks were deposited no earlier than 1676±5 Ma, based on a tuff sample ~14 km east of the deposit (Page and Sun, 1998). The same formation also hosts several Cu and Cu-Au. Bedding parallel foliation in the more pelitic metasedimentary rocks was probably developed early during greenschist to amphibolite facies regional metamorphism, between 1650 and 1550 Ma (Rubenach et al., 2008). Widespread metasomatism and mineralisation and intrusion of the the Williams Batholith (which includes the Mt Dore Granite), as well as the local D3 and D4 shortening events dominated the late to post-orogenic stages from 1540 to 1500 Ma (e.g. Oliver et al., 2008; Rubenach et al., 2008). The latter deformation was responsible for the overthrusting of the granite onto of the metasedimentary package.

The 1516±10 Ma A-Type Mount Dore granite, (Pollard et al., 1997), of roughly 100 km2 (Carter et al., 1961), has a uniform composition, comprising equigranular K-feldspar, quartz, plagioclase (+biotite +titanite) ±zircon ±hornblende. The lower, thrust contact with the carbonaceous metapelites or phyllites is sharp and commonly brecciated, although all other contacts are intrusive with local contact metamorphism. Pelitic shales and phyllite facies, which are strongly foliated and moderately folded, are composed of muscovite +K feldspar +albite +quartz ±andalusite ±rutile. Interbedded carbonaceous metapelites have a similar mineralogy, although they also contain graphite, but no andalusite, and rare only albite. There are lateral and vertical stratigraphic variations in the relative proportion of quartz, muscovite and feldspar, which range from more silica-feldspar rich to more pelitic. The contact of the rocks described above and the underlying altered calc-silicate rocks is commonly brecciated and is the locus of the bulk of the Mo-Re mineralisation. The calc-silicates are banded and composed of amphibole, plagioclase, K feldspar, apatite, carbonates ±titanite ±magnetite, and have a general metasomatic zoning profile from sodic dominated near the base, to largely potassic near the top, although fracturing and brecciation complicate the alteration geometry. The lowermost section of the overall metasedimentary package comprises an intensely silicified rock, historically described as a quartzite.

The metasedimentary rocks host two main, partially overlapping, mineralisation zones: i). an upper, copper dominated zone, hosted primarily in the metapelites, and ii). a Mo-Re dominated zone that is structurally controlled and hosted by calc-silicates. Strong potassic alteration and silicification overprints a relatively early Na-(Ca) metasomatic event.

The Mo-Re mineralisation occurs as i). infill of matrix supported breccias with rounded clasts, but also ii). in small veins (commonly stylolitic) and iii). disseminated. The individual lenses of Mo-Re ore are 5 to 20 m thick, dip at ~45° E and have down-dip extents of 200 to 300 m.

The copper mineralisation can be divided into two zones: i). an upper zone, which is intensely weathered with cuprite, chrysocolla, chalcotrichite, pseudomalachite which gradationally becomes chalcocite dominated (Lazo and Pal, 2009); ii). a lower Cu zone, which is mostly associated to the pelitic shales and phyllites and carbonaceous metasedimentary rocks, concentrated in fractures with K-feldspar +quartz +sphalerite +tourmaline, and in breccias with a dolomite matrix containing fragments of pyrite +chalcopyrite ±sphalerite. The copper lodes have similar dips, down-dip extents to the Mo-Re lenses, and vary from 5 to 50 m in thickness.

Merlin and the nearby Mount Dore orebodies are multiphase hydrothermal deposits that have a complex history, involving multiple fluid sources. The Cu mineralisation has been shown by petrography and LA-ICPMS sulphide analyses to predate the Mo-Re, although it may have contributed sulphur to the Mo mineralisation (Babo et al., 2014). Sulphur isotope studies indicate magmatic signatures for the Cu mineralisation, suggesting it was derived from a magmatic-hydrothermal fluid that exploited dilatational, brittle fractures in the pelitic metasedimentary rocks, and was accompanied by potassic alteration. In contrast, the Mo-Re mineralisation shows both magmatic and sedimentary characteristics. Re-Os geochronology indicates the Mo-Re mineralisation is temporally coeval with the A-type granitoid intrusives of the district, although different fluid systems drove the Cu and Mo phases. The Mo mineralisation is interpreted to have been precipitated from a sedimentary-equilibrated fluid driven by the intrusion, and possibly precipitated by redox and/or pH changes due to wallrock interaction at the transition between the metapelitic sequence and the calc-silicates (Babo et al., 2014).

Indicated and inferred mineral resource of the Merlin deposit are quoted by Babo et al. (2014) as:
    13 Mt @ 0.8% Mo, 14 g/t Re, including
        a high grade pod of 15 000 t @ 13% Mo and 160 g/t Re (Brown et al., 2010)
JORC compliant mineral resources at September 2014 at a cut-off of 0.3% Mo (Chinova Resources, 2014) were:
    Measured resource - 0.8 Mt @ 2.3% Mo, 34 ppm Re;
    Indicated resource - 4.2 Mt @ 1.5% Mo, 26 ppm Re;
    Inferred resource - 1.4 Mt @ 1.1% Mo, 24 ppm Re;
    TOTAL resource - 6.4 Mt @ 1.5% Mo, 26 ppm Re.

This summary has been initially largely derived from Babo, J., Spandler, C., Rubenach, M., Oliver, N. and Brown, M., 2014 - The Merlin high grade Mo-Re deposit, Cloncurry District, Australia; EGRU Newsletter, School of Earth & Environmental Science, James Cook University, August 2014, pp. 10-12.

The most recent source geological information used to prepare this summary was dated: 2014.    
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
 References to this deposit in the PGC Literature Collection:
Babo, J., Spandler, C., Oliver, N.H.S., Brown, M., Rubenach, M.J. and Creaser, R.A.,  2017 - The High-Grade Mo-Re Merlin Deposit, Cloncurry District, Australia: Paragenesis and Geochronology of Hydrothermal Alteration and Ore Formation: in    Econ. Geol.   v.112, pp. 397-422.

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