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Mount Margaret Mine - E1, Monakoff

Queensland, Qld, Australia

Main commodities: Cu Au
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The Mount Margaret mining operation exploits the Monakoff and E1 group of IOCG-style copper-gold deposits in the Cloncurry District of NW Queensland, Australia.   Monakoff is located ~15 km ENE of Cloncurry, and is 20 km south of the Ernest Henry deposit. The E1 group of deposits are 22.5 km to the NNE of Monakoff and 8 km east of Ernest Henry.   The Mount Margaret ore is treated at the Ernest Henry mine (#Location: E1 20° 26' 36"S, 140° 47' 10"E; Monakoff 20° 37' 30"S, 140° 41' 20"E).

All of these deposits are hosted within the Eastern Succession of the Mount Isa Inlier, which consists of poly-deformed Palaeo- and Mesoproterozoic volcano-sedimentary Cover Sequences 2 and 3 (CS2 and 3), deposited between 1790 and 1690 Ma and from 1680 to 1610 Ma respectively.

For details of the regional setting and geology of the northeastern Cloncurry district, see the Ernest Henry record.

E1 Group

The four main E1 group deposits (E1 North, South, East and Central) are distributed over a NNW-SSE elongated area of approximately 1750 x 650 m. They are overlain by between 15 and 40 m of unconsolidated Mesozoic to Cenozoic cover, underlying an essentially flat pastoral landscape, and are hosted in a series of steeply dipping, folded and brecciated meta-sediments and meta-volcanic lenses that belong to the Soldiers Cap Group of CS3.

The host sequence in the deposit area comprises a lower unit of variously brecciated, silicified and 'red-rock' altered, meta-sedimentary and calc-silicate rocks, that include polymictic breccias. These pass up into a 100 to 200 m thick basalt, overlain by a trachyte unit with a similar thickness, followed in turn by black shale. It is likely that the 'red-rock' altered, meta-sedimentary and calc-silicate rocks belong to the Doherty/Corella Formation of CS2, and the basalt and trachyte units represent the Toole Creek Volcanics of the CS3 Soldiers Cap group.

In the immediate deposit area, this sequence has been tightly folded to form an 'S' shaped structure, representing two limbs of a NNW-SSE-trending syncline, with a tight, complexly faulted anticlinal axis to the NNW, and a faulted, but slightly less faulted synclinal axis to the SSE. The structure is truncated by two faults, the regional, north-south-trending Mount Margaret to the east, and another semi-parallel north-south to NNE-SSW-trending fault 1.5 km to the west.

The full sequence is represented on the western limb of the main syncline, with the black shale occupying the core of the structure. However, a fault parallel to, and just to the east of the fold axis, juxtaposes the black shale of the synclinal core, with brecciated metasedimentary rocks to the east, with both volcanic units being structurally removed. These rocks are then cut to the east by the Mount Margaret granite. Sill-like lenses of dolerite cut the brecciated metasedimentary rocks on the eastern limb of the syncline. Black shales are found to the east across the Mount Margaret Fault from the synclinal closure to the SSE.

Continuity of the host sequence to the west of the anticlinal closure in the NNW is unclear from the data available.

A folded, NNW-SSE-elongated, ~550 x 150 m structural inlier of undifferentiated mafic volcanic rocks, with intercalated black shales, is enclosed within the brecciated metasedimentary rocks in the northern part of the eastern limb, hosting the E1 East orebody. This enclave includes two steeply dipping (structurally repeated ?) 20 to 75 m thick banded-massive ironstone lenses that host the sulphide ore over strike length of up to 300 m, terminated at ~280 m below the Mesozoic unconformity by calc-silicate/polymict breccias.

The E1 North deposit is hosted by a series of steeply dipping lenses of magnetite-rich metasedimentary and metavolcanic rocks in the main faulted anticlinal closure to the NNW. The immediate deposit is bounded by two NNE-trending faults to the east and west, each dipping inwards and intersecting at depth, although mineralisation is repeated across these faults to the southeast. The main body plunges steeply over a down-plunge distance of >300 m and widths of 20 to 100 m. A high grade, flat lying blanket of supergene mineralisation straddles the base of oxidation, laterally outwards for up to 100 m from the steep hypogene ore zone, over thicknesses of 15 to 65 m, with intersections such as 64.6 m @ 2.87% Cu, 0.32 g/t Au, 153 ppm U3O8 from 31.4 to 96 m, including 16.6 m @ 9.04% Cu, 0.72 g/t Au & 280 ppm U3O8 from 31.4 to 48 m.

The E1 South deposit is contained in a parallel series of 6 nested/stacked, folded, parallel and concordant geologically and geochemically defined lenses of banded-massive ironstone. These lenses are folded as a NNW plunging synclinal structure, with the axial plunge shallowing to the NNW and with depth. Individual lenses are 10 to 30 m in thickness, and are largely restricted to the trachytic unit, from the contact with the underlying basalt at the base, to the margin with the overlying black shale. The most extensive of these lenses are truncated to the east by the axial fault (which removes the volcanic units of the eastern limb of the syncline, as described above), and extend around the fold nose to the western limb where most lens-out. These lenses are continuous over varying strike lengths of between 100 and 600 m, and to a maximum vertical depth of 320 m below the unconformity (Exco Resources, 2008).

The E1 Central deposit represents one (or more?) of the E1 South 'lenses' which is continuous over a strike length of more than 1 km along the NNW-trending western synclinal limb, to connect with the E1 North deposit. At the unconformity with the overlying Mesozoic to Cenozoic cover, this mineralised ironstone follows the contact between the basalt and trachyte units, and is underlain by a variably developed zone (up to a few metres thick) of carbonate alteration within the basalt.

The sulphide copper-gold mineralisation occurs predominantly within magnetite, pyrrhotite, and chalcopyrite-pyrite mineral assemblages. The principal components include magnetite (20 to 30%), micas (25 to 30%), quartz (12 to 15%), ankerite/calcite (5 to 15%), pyrite (3 to 12%), alkali feldspars (2 to 7%), fluorite (1.5 to 2.5%), apatite (0.5 to 2.5%), chalcopyrite (1.5 to 2.5%), bornite (0 to 0.1%), sphalerite (0 to 0.15%).

The source of information for the E1 group of deposits was drawn from reports by Exco Resources dated from 2007 to 2011, and sketch maps included therein.

Monakoff

Two major units predominate in the Monakoff area: the Soldiers Cap Group (1680 to 1660 Ma; Page and Sun, 1998) of the informally defined Maronan Supergroup, which is part of CS3, and the Corella Formation of CS2 (~1740 Ma; Page, 1988). Surface geology and seismic interpretation indicates the two to always be in tectonic contact. The two major contacts between these units are the Cloncurry Overthrust and the Pumpkin Gully Fault. The Soldiers Cap Group comprises a conformable carbonate-poor stratigraphic succession that passes upwards from the pelites and arenites of the Llewyllen Creek Formation, through the quartzo-feldspathic arenite, carbonaceous pelite, metagreywacke, metabasalt and iron formation of the Mount Norna Quartzite, to the metabasalt, minor carbonaceous metasedimentary rocks and iron formation of the Toole Creek Volcanics (Derrick et al., 1976). Sills and dykes of dolerite were widely intruded, predominantly prior to the main metamorphism, and in some cases before consolidation. The Corella Formation comprises well bedded scapolitic carbonate and quartzo-feldspathic layers, which is inferred to have been deposited within a shallow marine to evaporitic shelf environment (Reinhardt,1986). It has been subjected to strong intra-formationally brecciated and in places contains exotic clasts. Cu-Au mineralisation in the district is mostly hosted within the Soldiers Cap Group, with only minor occurrences adjacent to or within the Corella Formation breccias within 1 km of more significant accumulations.

Monakoff occurs within an unusual east-west trending belt of Soldiers Cap rocks, on the south-dipping limb of the Pumpkin Gully Syncline which is considered to be a regional east-west fold formed during D2 deformation. The deposit occurs along a tectonic contact between the Mount Norna Quartzite and the Toole Creek Volcanics (Ashley, 1983), and corresponds to a regionally extensive linear magnetic anomaly, with sporadic iron formations and ironstones as its outcrop expression. No granitic intrusives have been identified at Monakoff. The nearest granites, 2 km to the north, are outcropping hills of the Naraku Batholith, with the main metamorphic aureole to the batholith occurring a further 2 km NW.

The host Soldiers Cap rocks are separated from Corella breccia to the north and west by D1 thrust faults, while a splay of the northern thrust (the Monakoff Shear) hosts the Monakoff mineralisation (Davidson, et al., 2002). An east-west orientated fault separates the western and eastern zones of mineralisation. The Monakoff deposits comprise sulphide copper-gold mineralisation hosted by a well defined, east-west striking iron-rich alteration zone within steeply south dipping meta-sediments and amphibolites. The western zone of mineralisation forms a steeply east-dipping sheet 700 m long by 2 to 10 m thick, which is open at depth. The smaller eastern zone, which is 100 m northeast of the end of the western zone, forms a pipe-like breccia body that plunges very steeply to the west, with a 40 m strike length at the surface. Monakoff East is slightly deeper in the stratigraphy than the Monakoff West Zone (Davidson et al., 2002).

The upper 100 m of footwall lithologies comprise thinly (1 to 3 mm) intercalated magnetite-bearing muscovite pelites, psammopelites, and meta-dolerites of the uppermost Mount Norna Quartzite, overlain by massive porphyroblastic garnet-biotite schist (mostly of intrusive andesitic igneous origin, although the upper 3 to 5 m are vesicular), then a regionally persistent magnetite iron formation, and finally by strongly sheared metasediments immediately hosting the Monakoff West ore. The continuous, 1 to 2 m thick, prominently banded, quartz-magnetite±hematite iron formation can be traced through the whole prospect, occurring ~5 to 10 m above the top of the porphyroblastic garnet-biotite schist, hosted by pelitic metasediment. Banding varies from 1 to 2 mm 'wrinkly' lamination (possibly crenulations), to 0.5 cm interbands of quartz-barite, and spessartine-magnetite. The metadolerites are present as continuous sills, locally with angular contacts and variable epidote alteration, particularly adjacent to sediment contacts.

The hanging wall above the thin strip of sheared metasediment that hosts ore, commences with a 20 to 30 m thick interval of short strike-length metabasalts, minor iron formation, volcanic conglomerate, and breccia bearing clasts of limestone and meta-dolerites. A local pillow breccia outcrops close to the ore package near the centre of the deposit, while black carbonaceous limestone and stromatolitic cherts are also reported. More common rocks of the Toole Creek Volcanics occur above this complex, comprising massive medium- to coarse-grained meta-dolerite to gabbro in the west, and mixed meta-dolerite, meta-basalt, siliceous siltstone and minor iron formations in the east (Davidson et al., 2002).

The "ore package" is defined as the altered rocks adjacent to the ore, the ores themselves, and the overlying altered sediments and basaltic tuff, all of which young and dip to the south.   Near-ore alteration in the footwall commences with porphyroblastic spessartine-biotite-quartz-plagioclase-chloritoid-tourmaline-biotite development within the meta-andesite, with idioblastic garnet occuring as disseminations or irregular bands localised on peperite zones. Locally, pink spessartine is overgrown by coarse-grained almandine + quartz, indicating peak metamorphism exceeded the almandine isograd. The timing of almandine stability is uncertain (Davidson et al., 2002).

The major enriched components of the ore compared to the country rocks are: S (>20% S), Ba (>25% BaO), F (~2 to 10% F), Fe (>10% FeO) and Ca (~10% CaO), with significant enrichments of minor element including economic Co, Cu, Au and Ag, plus, Pb, Sb, As, W, U, La, Ce and Zn. P
2O5 abundances are low, with most ores containing only 0.2 to 0.4 wt.% P2O5. The highest values are systematically found in the iron formation, although no strong Fe-P correlation exists when all of the data is considered. La and Ce abundances are also very high, with values typically of >1000 ppm. Rather than apatite, the host phase for REE, U, and possibly P2O5, is monazite, which is common in ore intersections, and concentrates the LREE (Davidson et al., 2002).

Post-peak metamorphic alteration is abundant above the garnet-biotite schist, zoned around the ores, low-angle shears and tension gashes. The general alteration sequence distributed over widths of 0.5 to 10 m from mineralised zones is: i). an outer chlorite-spessartine; ii). biotite-magnetite±pyrite; to iii). siderite-magnetite±pyrite assemblages, as ore or sulphide-bearing veins/fractures are approached. When this alteration occurs within meta-siltstones it takes the form of pseudo-breccias where 'clasts' have distinct dark biotite-rich centres. Carbonate alteration also preferentially affects thin basaltic dykes or extrusives. Between the Monakoff West and East deposits, porphyroblastic garnet-biotite schist is replaced by unmineralised albite around a sinistral D3- to post-D3 fault, This assemblage is not seen elsewhere in the prospect, although regionally it is common in association with post-orogenic granites (Davidson et al., 2002).

The western Monakoff ore zone is enclosed by, and replaces, magnetite-bearing meta-siltstones, occurring at surface as a friable, but resistant, massive unit with variable black pyrolusite and malachite staining. At depth it occurs as a massive unit composed of barite, ponite (Fe-rhodochrosite), magnetite, chalcopyrite, pyrite, spessartine, fluorite ±K-feldspar, sphalerite, galena, arsenopyrite, mackinawite, molybdenite, brannerite/davidite, pentlandite and linnaeite (Ashley, 1983).

The eastern Monakoff ore zone appears to mainly comprise replacement of a medium- to coarse-grained amphibolite, within a tight D2 fold adjacent to an unexposed east-west fault, represented at surface by buff-coloured silica, studded with randomly oriented crystals of coarse magnetite, magnetite-hematite, and hematite, that are consistent with faithful pseudomorphing of meta-gabbro. The silica is most likely a surficial regolith replacement product of carbonate. At depth this rock-type is a package of comparatively thin massive meta-dolerites and intercalated peperitised sediments, with the contact between the two lithologies preferentially silicified, and an alteration zonation into the sediment of i). siderite-magnetite-pyrrhotite-chalcopyrite; ii).  magnetite-siderite-quartz, to iii). biotite-quartz-magnetite over 1 m or less, while the adjacent dolerite contains incipient siderite alteration. It is interpreted that in the core of the fold, severe alteration affected the interior of the folded metadolerite as well as the sediment margins. The fluids, which were tightly focussed by the fold structure, produced wholesale replacement of dolerite, and a narrow west-plunging pipe with grades of 1.4 to 3.0% Cu, consisting of siderite-barite-magnetite-chalcopyrite, which is likely to have considerable depth extent (Davidson et al., 2002 and sources quoted therein).

Resource and reserve figures for the Mount Margaret Operation deposits were:
E1 group of deposits (Exco Resources, 2010 - Xstrata, 2012 quotes the same tonnages and grades to lesser decimal poiints) :
    Measured resource - 9.17 Mt @ 0.87% Cu, 0.25 g/t Au
    Indicated resource - 24.7 Mt @ 0.71% Cu, 0.21 g/t Au
    Inferred resource - 14.2 Mt @ 0.64% Cu, 0.2 g/t Au
    Total resource - 48.1 Mt @ 0.72% Cu, 0.21 g/t Au
Monakoff + Monakoff East deposits (Exco Resources, 2010):
    Indicated resource - 2 Mt @ 1.39% Cu, 0.44 g/t Au
    Inferred resource - 2 Mt @ 1.3% Cu, 0.4 g/t Au
    Total resource - 4 Mt @ 1.32% Cu, 0.42 g/t Au
Monakoff + Monakoff East deposits (Xstrata, 2012):
    Indicated resource - 2 Mt @ 1.4% Cu, 0.4 g/t Au
    Inferred resource - 1 Mt @ 1.2% Cu, 0.4 g/t Au
    Total resource - 3 Mt @ 1.33% Cu, 0.4 g/t Au
Nearly 3500 t of U
3O8 is contained within these deposits. The average in-situ recoverable grade is ~112 ppm U3O8.

The most recent source geological information used to prepare this summary was dated: 2012.    
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:
Davidson G J, Davis B K and Garner A  2002 - Structural and geochemical constraints on the emplacement of the Monakoff oxide Cu-Au (-Co-U-REE-Ag-Zn-Pb) deposit, Mt Isa Inlier, Australia: in Porter T M (Ed), 2002 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, PGC Publishing, Adelaide,   v2 pp 49-75

   References in PGC Publishing Books: Want any of our books ? Pricelist
Davidson G, Davis B, Garner A, 2002 - Structural and Geochemical Constraints on the Emplacement of the Monakoff Oxide Cu-Au (-Co-U-Ree-Ag-Zn-Pb) Deposit, Mt Isa Inlier, Australia,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v2  pp 49-75
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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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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