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Century |
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Queensland, Qld, Australia |
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Zn Pb Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Century zinc-lead-silver deposit is located 250 km to the NNW of Mt Isa and 375 km SW of McArthur River (#Location: 18° 43' 15"S, 138° 35' 54"E).
The Century deposit was discovered by CRA Exploration Pty Ltd in 1990. Pasminco bought it from CRAE in 1997, and in turn sold it to Zinifex in 2004. In 2008, Zinifex merged with Oxiana to become OZ Minerals, who sold the mine to MMG in 2009.
The mine commenced open-pit production in 1999, and the reserves were exhausted and mining ceased in August 2015, with the final processing of Century-sourced ore occurring in November 2015. Since closure, MMG had been rehabilitating the mine.
Between 1999 and 2015, production averaged 475 000 tpa zinc concentrate and 50 000 tpa lead concentrate. During these 16 years of operation, zinc and lead ore was mined and processed on site at Lawn Hill to produce zinc and lead concentrates. The concentrates were transferred in slurry form via a 304 km underground pipeline to Century's Port facility at Karumba for dewatering and drying, and shipping to smelters. The concentrates were shipped by barge from Karumba on the shallow Norman River to bulk carriers anchored in the Gulf of Carpentaria. The operation had been focussed on a high throughput processing plant that had only achieved relatively modest recoveries, which at closure in 2015 was only 74%, leaving ~2.287 Mt of contained zinc in 77 Mt of tailings containing 3.0% Zn, 12 g/t Ag.
In March 2017, Attila Resources Limited entered into an agreement with MMG Ltd and Century Bull Pty Ltd for the progressive acquisition of the Century Zinc Mine and associated infrastructure from MMG Ltd with the aim of retreating the Century Tailings, mining the nearby Silver King deposit and the East Fault Block deposit. The project is managed by a JV company New Century Resources. Independent metallurgical studies indicated the tailings could be reprocessed through the existing plant on site, with minor modifications, to achieve recovery of >60% of remaining zinc mineralisation into a saleable ~52% zinc concentrate.
New Century Resources restarted operations at Century in 2018, focused on the reprocessing of tailings and rehabilitation of existing disturbed areas, with an average annual production of 130 000 tpa zinc in concentrate (New Century Resources website, Nov., 2022).
Regional Setting
The Century deposit is one of number of major stratabound zinc-lead-silver deposits hosted within the late Palaeo- to Mesoproterozoic Mt Isa-McArthur succession that covers more than 50 000 km2 in northwest Queensland, the Mount Isa Inlier, and continues over an equally large area in adjacent Northern Territory (Blake et al., 1990). The others include Mount Isa, Hilton and George Fisher (Hilton North), Cannington, Dugald River, Lady Loretta, Century and McArthur River. Despite the abundance of major deposits, there is a paucity of documented zinc-lead occurrences in the region. Only ~80 occurrences are known throughout the entire Mount Isa Inlier, but >40 of these occur within a 20 km radius of Century.
The Mt Isa-McArthur succession is part of the Northern Australian Platform cover. It is a 5 to 15 km thick volcano-sedimentary succession deposited during the period 1800 to 1580 Ma in an intracontinental setting. Deposition took place in three super-basins which represent three nested cycles of deposition and exhumation, specifically the Leichhardt (1798 to 1738 Ma), Calvert (1728 to 1680 Ma) and Isa (1667 to 1575 Ma) super-basins, terminated by the 1590 to 1500 Ma Isan Orogeny, which was followed by the fourth and younger, ~1500 to ~1400 Ma Roper super-basin (Stewart, GeoScience Australia 2015; Betts et al., 2003). All of the major stratabound Zn-Pb-Ag deposits of the system, including Century, are hosted by 2 to 8 km thick successions of the Isa Superbasin, ranging in age from ~1660 Ma (Dugald River and Lady Loretta) to 1595 Ma at Century (Queensland Department of Mines and Energy, 2000).
Within the Mount Isa Inlier, these successions are divided into the Eastern and Western fold belt subprovinces by a 60 to 80 km wide north-south belt of Paleoproterozoic basement that forms the Kalkadoon-Leichhardt Subprovince. This belt largely comprises the 1860 to 1850 Ma felsic volcanic rocks of the ~1855 Ma Leichhardt Volcanics and the 1870 to 1850 Ma Kalkadoon and Ewen granites, which have been subjected to deformation and metamorphism, generally to amphibolite grade, during the Barramundi Orogeny, which was widespread in the North Australian Craton from ~1900 to ~1850 Ma (Etheridge et al., 1987; Betts et al., 2006). In the Mount Isa Inlier, this episode of orogenesis reflects east-west contraction (Blake and Stewart, 1992). The Leichhardt Volcanics overlie small windows of basement metamorphic rocks that may be Neoarchaean to Palaeoproterozoic in age, containing inherited 3.6 to 3.3 Ga zircons (Bierlein et al., 2008).
Basement is also exposed in the up to 25 x 165 km, ENE-WSW trending Murphy Inlier (or Domain) located ~110 km NW of Century. This tectonic ridge interrupts the Lawn Hill platform, and separates it from the McArthur Basin (to the north). The oldest rocks are the 1853±4 Ma Murphy Metamorphics, composed of quartz-mica schist, representing poorly sorted, immature, fine to coarse grained metasandstone, metasiltstone and shale. These are unconformably overlain by >4000 m of rhyolitic ignimbrites, and by alkali rhyolite lavas of the Cliffdale Volcanics, dated at 1851±3 and 1732±20 Ma respectively. All of these are intruded by the Nicholson Granite complex, a composite batholith of granite, adamellite and granodiorite dated at between 1845±3 and 1804±83 Ma (Ahmad and Wygralak, 1989; Geoscience Australia, 2016).
Two further granitic intrusion are recorded on the Lawn Hill platform, at 1820 and 1800 Ma, the Yeldham and Big Toby granites respectively, between the basement Barramundi phase magmatism and the commencement of deposition in the Leichhardt Superbasin.
The evolution of the Leichhardt superbasin, was characterised by widespread voluminous bimodal magmatism and fluvial sedimentation with transient shallow marine incursions (Eriksson et al., 1993; Simpson and Eriksson, 1993; O'Dea et al., 1997b; Rawlings, 1999; Jackson et al., 2000). This period of magmatism was followed by postrift clastic and carbonate sedimentation in the Mount Isa inlier between ~1755 and 1740 Ma (Derrick et al., 1980; Jackson et al., 1990; Page and Sun, 1998), then by a mid-basin inversion marking the transition from extension to local shortening of the Leichhardt superbasin (Bull and Rogers, 1996; Betts, 1999), resulting in the uplift of the Kalkadoon-Leichhardt Subprovince.
In the Western Fold Belt, the Leichhardt superbasin sequence is best developed within the north-south Leichhardt Rift (Derrick, 1982; O'Dea et al., 1997), which coincides with the Leichhardt River Domain of Withnall and Cranfield (2013), bordering the western margin of the Kalkadoon-Leichhardt Domain. In this domain, to the east of Mount Isa, the sequence includes the Mount Guide Quartzite, the ~8000 m thick, basaltic Eastern Creek Volcanics and the Myally Subgroup siliciclastics, overlain by the Quilalar Formation carbonates. Similar successions are recognised in the Eastern Fold Belt (the mafic Magna Lynn and Marraba Volcanics through to the carbonates of the Corella Formation) and the McArthur Basin (e.g., the Seigal Volcanics and associated sedimentary rocks) in the Northern Territory.
The final stages of the Leichhardt superbasin and inversion was accompanied by intrusion of the voluminous ~1740 to 1720 Ma Wonga Granite in the Mary Kathleen Domain that is juxtaposed along the eastern margin of the Kalkadoon-Leichhardt Subprovince, as well as the less extensive Gin Creek, Levian and Dipvale granites and the Mt Fort Constantine Volcanics further east in the Eastern Fold belt. This magmatic phase is known as the 'Big' event (Jackson et al., 2000).
Basin inversion in the Leichhardt superbasin was succeeded by the development of the Calvert superbasin, involving a renewal of extension, accompanied by fluvial and shallow marine sedimentation and bimodal magmatism (Derrick, 1982, 1996; Betts et al., 1998, 1999; Rawlings, 1999; Southgate et al., 2000; Jackson et al., 2000). In the Leichhardt River Domain, this sequence was deposited in half grabens that resulted from NW-SE directed extension (the Mount Isa rift event of Betts et al., 1998, 1999), which were superimposed on the pre-existing Leichhardt rift. The Calvert superbasin is temporally and areally more restricted than the other two superbasins, with thinner stratigraphic accumulations. It is found along the northern edge of the Lawn Hill platform (~1730 Ma Peters Creek Volcanics), and as the syn-rift unconformity separated Fiery Creek volcanic suite and overlying Surprise Creek Formation, followed by quartzite units in the Leichhardt River Domain and Lawn Hill platform. The 1711±3 Ma Weberra Granite coincides with the internal unconformity. In the Eastern Fold Belt, it is represented by the shallow marine sequence of the Staveley Formation, and following a brief inversion, the deposition of the lower part of the Kuridala and Soldiers Cap Groups.
A ~25 to 30 m.y. hiatus marked the end of deposition in the Calvert superbasin within the Mount Isa inlier. In the Western Fold belt, the linear, north-south trending, voluminous, 1670 Ma Sybella Granite, a foliated coarse porphyritic biotite granite, was intruded along the boundary between the Leichhardt River Domain and the Lawn Hill platform. The exposed and sub-outcropping batholith occupies an area of ~220 x 15 to 30 km and defines the Sybella Domain of Withnall and Cranfield (2013). The northern margin of this batholith passes into the NNE trending Mount Gordon Fault Zone which separates the Leichhardt River Domain from the Lawn Hill platform in this part of the Mt Isa Inlier. At the same time, the relatively small 1660 Ma Ernest Henry Diorite and a number of ~1680 Ma granitoids were intruded into the Eastern Fold Belt. This magmatic phase is known as the 'Gun' event (Jackson et al., 2000).
The depositional hiatus was followed by development of the Isa superbasin and the renewal of sedimentation from 1670 to 1590 Ma (Jackson et al., 2000). This superbasin is interpreted to have resulted from thermal subsidence of the lithosphere (the sag phase of Etheridge and Wall, 1994, and of Betts and Lister, 2001), which was most intense to the west and NW of the Leichhardt River Domain, where an extensive, thick blanket of carbonaceous shale, stromatolitic dolostone, and turbiditic sandstone and siltstone were deposited on the Lawn Hill platform (the ~10 km thick McNamara Group: Krassay et al., 2000; Southgate et al., 2000) and in the McArthur basin (the 5.5 km thick McArthur Group, which hosts the McArthur River deposit) in the Northern Territory (Rawlings, 1999). Within the narrower (40 to 90 km) Leichhardt River Domain, that is bounded to the east by the Kalkadoon-Leichhardt Domain, and the Lawn Hill platform to the west, the Isa superbasin sequence is thinner and and spans a narrower temporal range, represented by the up to 7.5 km thick, ~1670 to ~1647 Ma Mt Isa Group, which hosts the Mount Isa, Hilton and George Fisher (Hilton North) deposits. The Lady Loretta deposits are located within an equivalent sequence in the lower sections of the McNamara Group on the eastern margin of the Lawn Hill platform. Century is ~100 km to the NW, on the platform, hosted within the 1595±6 Ma Lawn Hill Formation towards the top of the McNamara Group, representing a depositional repetition of a comparable sequence to that of the Mt Isa Group and at Lady Loretta. The mineralisation at Century has been dated at 1575 Ma (Pb-Pb model age; Betts et al., 2003). The Cannington and Dugald River deposits are hosted by equivalent rocks in the Eastern Fold Belt (Betts et al., 2003).
Withnall and Cranfield (2013), Jell (2013) and Geological Survey of Queensland (2011) have divided the Lawn Hill platform into two parts, the Mount Oxide Domain to the SE, and the Century Domain the the NW, separated by the NE-SW trending Fiery Creek fault zone, ~65 km SE of Century. The Century Domain is distinguished on the presence of northeast structural trends interpreted to be associated with the Early Isan Orogeny.
Deposition in the Isa superbasin was terminated by the 1590 to 1500 Ma Isan Orogeny, which involved components of both north-south and east-west shortening strain and extensive plutonism. Peak metamorphism was from 1595 to1580 Ma, followed by emplacement of the Williams- Naraku batholiths and related intrusions between 1550 and 1500 Ma in the Eastern Fold Belt. In the Western Fold Belt, the distribution of strain was strongly influenced by the pre-existing fault architecture (O'Dea and Lister, 1995; Lister et al., 1999; Betts, 2001), with strain intensity generally decreased to the northwest. Betts et al. (2003) suggest the less intense basin inversion in the Lawn Hill platform contributed to the preservation of Century as the erosion level is still within the sag phase of the basin, in contrast to other parts of the Mount Isa inlier, which are characterised by intense basin inversion (e.g., the central Leichhardt River Domain and Eastern fold belt.
Deposition in the Roper super-basin commenced following the Isan Orogeny at ~1500 Ma. It is principally composed of the Roper and equivalent South Nicholson Groups (Jackson et al.,1999), and is largely confined to the northern Lawn Hill platform/South Nicholson and upper McArthur basins in the Northern Territory, overlying the Calvert and Isa superbasins <50 km NW of Century. This sequence was deposited between 1493±9 to 1280 Ma, and includes up to 5 to 9 km of mudstone-rich formations, punctuated by cross-stratified quartz arenite, with minor sedimentary breccia, fluvial sandstone, micritic and intraclastic limestone, and oolitic ironstone (Abbott et al., 2001).
The subdivisions of the late Palaeo- to Mesoproterozoic Mt Isa-McArthur succession into the Leichhardt, Calvert and Isa super-basin sequences (Jackson et al., 2000; Southgate et al., 2000; Neumann et al., 2006; Betts et al., 2003, 2006), supercedes, but is broadly correlatable with the Cover Sequence 1, 2 and 3 of Blake (1987), as follows. Cover Sequence 1 (1870 to 1850 Ma) corresponds to the basement of predominantly felsic volcanic sequences exposed in the Kalkadoon-Leichhardt Sub-province and Murphy Domain, whilst Cover Sequence 2 (1790 to 1690 Ma) correlates with the sequences within the combined Leichhardt (1790 to 1730 Ma) and Calvert (1720 to 1670 Ma) superbasins, and Cover Sequence 3 (1665 to 1590 Ma) closely parallels the sequence within the Isa Superbasin (1670 to 1590 Ma).
Geology and Structure
In the NW section of the Mt Isa Inlier in Queensland, the exposed units of the McNamara Group represent a 5000 to 10 700 m thick column of fine grained clastic sedimentary rocks with subordinate dolomitic clastic sediment, carbonate, volcanic rocks and chert (Hutton et al., 1981). The 1595±6 Ma host to the Century mineralisation, the Lawn Hill Formation, is the youngest preserved unit of the McNamara Group, and comprises between 1800 and 2200 m of shale, siltstone, tuff, tuffaceous siltstone and sandstone. It is subdivided into six members, Pmh1 to 6 (Sweet and Hutton, 1982). The Century mineralisation occurs over a 45 m interval, 80 to 100 m below the conformable and gradational boundary between the host unit Pmh4 and the overlying Pmh5 Widdallion Sandstone (Broadbent and Waltho, 1998).
Pmh4, which hosts the mineralisation, comprises ~850 m of siltstone, shale, carbonaceous shale and sandstone. From the base, this sequence overlies the sandstones of the Pmh3 Bulmung Sandstone, and commences with a fining upwards, succession of 300 to 400 m of carbonaceous, pyritic shale. These are followed by a sequence of interbedded siltstone, sideritic siltstone, shale and minor fine quartz-lithic sandstone that occur in the footwall of the mineralised sequence. A similar package is found in the hanging wall. The combined footwall, mineralised interval and hanging wall form an ~300 m thick, slightly coarsening upward package. Correlations within this package are difficult, due to the lack of lithologically or geophysically distinct markers, apart from a few reworked tuff horizons in the hanging wall siltstone-shale sequence, up to 30 m above the top of the mineralisation (Broadbent and Waltho, 1998).
The underlying Pmh3 sandstone and Pmh2 siltstone are ~120 m thick, and overlie ~220 m of Pmh1 black shale(Broadbent and Waltho, 1998).
The >150 m thick Pmh5 Widdallion Sandstone, the youngest member of the Lawn Hill Formation in the immediate area of the deposit, is overlain by intensely faulted and folded fragments of Cambrian carbonates which are from a considerable interval of the Georgina Basin succession (Broadbent and Waltho, 1998; Szulc, 1993).
The Century deposit is preserved within the core of a large scale fold structure, the Page Creek syncline, one of a number deforming the McNamara Group in the Lawn Hill area. This structure is a relatively gentle, open fold, with dips within most of the mineralised sequence of between 5 and 25°, but steepening to ~70° at the margins of the deposit (Broadbent and Waltho, 1998).
A major, long lived, NW-trending fault system, the Termite Range fault, passes immediately along the NE margin of the deposit, and coincides with a regional scale geophysical linear that can be traced from south of Mount Isa to McArthur River in the north. Various lines of analysis demonstrate the fault was active during Mesoproterozoic deposition (Andrews, 1998), throughout the later Isa Orogeny, and in a minor way at the end of the Cambrian. Numerous smaller scale faults parallel the major structure, with and a second, possibly conjugate set developed approximately perpendicular to the main structures. These fault sets display a combination of reverse and normal displacement, are generally steeply dipping, and have a significant affect on the distribution of the mineralised unit. More than 30 displace the orebody sufficiently to be regarded as significant to mine planning. The more significant NE-trending district and regional scale faults and their splays also host the quartz-siderite-lead-silver lodes mined historically in the Burketown mineral field (Broadbent and Waltho, 1998).
Deformation of the overlying Cambrian carbonate sequence has produced a chaotic considerably more complex fault and fold pattern, implying a possible thrust or extensional decollement surface at the base of the Cambrian sequence. A series of low angle, SW-dipping faults are mapped within the Cambrian sequence, responsible for juxtaposing blocks of Lawn Hill Formation from all parts of Pmh4 and Pmh5 as megaclasts between dismembered blocks of carbonate.
Three distinct blocks of mineralisation have been defined. The Southern and Northern blocks, which are separated by the shallowly (40 to 45°) north dipping Pandoras listrict fault, contain essentially all of the economically significant resource, whilst, the small Eastern block is ~100 m east of the NE limit of the Southern block. Throws across the Pandoras fault vary from nothing in the east, where the Northern and Southern blocks are juxtaposed, to >150 m to the west (Broadbent and Waltho, 1998).
The deposit covers an area of ~1200 m from east to west and 1400 m from north to south, and averages 40 to 50 m in thickness, with mineralisation closed in all directions by east trending normal faults, the unconformity between the Mesoproterozoic and overlying Cambrian sequences, or the present day land surface. The southern boundary of the Southern block is the Magazine Hill fault, a shallowly (~ 20°) north dipping, listric normal fault with a throw of at least 250 m that passes shallowly below the Southern block, to coalesce with the Pandoras fault below the Northern block (Broadbent and Waltho, 1998).
The northern margin of the Northern block is partly controlled by the steeply south dipping Nikki's fault, which has a normal throw of at least 70 m. The eastern boundary of the Southern block and all other margins of the Northern block are truncated by the Mesoproterozoic-Cambrian boundary. As such the deposit was almost entirely concealed, with a number of limited outcrops. On the western boundary of the Southern block, the upper portion of the mineralised sequence formed a prominent, but low ridge (Broadbent and Waltho, 1998).
Because the deposit is sphalerite rich and pyrite poor, no gossan was evident at surface, only a slightly pitted, banded, sideritic grey shale with a hint of limonitic staining.
Numerous, irregular, discordant, 0.1 to 34 m thick, intrusive carbonate breccia bodies are found throughout the deposit, preferentially developed along pre-existing fractures within both the Cambrian and Mesoproterozoic sequences, composed of Cambrian limestone and dolostone clasts in a carbonate matrix (Broadbent and Waltho, 1998).
Mineralisation
Unlike other major sediment hosted deposits of the Mt Isa-McArthur basin (e.g., McArthur River, Hilton and Mount Isa) that are associated with carbonate-rich sedimentary rocks, Century is hosted by a siliciclastic sequence (Broadbent and Waltho, 1998).
Mineralisation is sphalerite dominated, lacking the abundant accompanying diagenetic pyrite of those other shale-hosted deposits in the region, although Century does appear to have a weak distributed pyrite halo surrounding the main zinc-lead mineralisation. Instead of pyrite, the principal iron-bearing gangue phase is sideritic carbonate, with ~70% Fe. Sphalerite is particularly pure, containing >62% zinc, and is in part associated with significant quantities of pyrobitumen (Broadbent and Waltho, 1998).
Image above - Outcrop of the mineralised unit up-dip from the discovery diamond drill hole at Century in 1990. Note that as the deposit is hosted within siliciclastic rocks, and pyrite is essentially absent, with the only sulphides being sphalerite and lesser galena, there is no gossan as normally understood. The iron within the deposit is within an Mn-rich siderite, which is reflected in the manganese staining obvious in the image. The oxidised sphalerite and galena are represented by small, scattered, slightly limonitic voids. Where siderite is abundant, false gossan (i.e., one not due to weathering of sulphide) may form Image by Mike Porter, photographed 1990.
Economic mineralisation comprises fine grained sphalerite, galena and minor pyrite, occurring as delicate, bedding parallel lamellae in carbonaceous shale units, separated by distinctly less mineralised sideritic siltstone, or carbonaceous sideritic mudstone marker horizons. These sulphide rich sedimentary rocks make up the 40 to 50 m thick mineralised interval of Pmh4, which is otherwise sedimentologically similar to the siltstone and shale immediately above and below. The difference between the mineralised and unmineralised portions of this sequence is principally the presence of characteristic stylolitic bedding surfaces in siltstone beds, and thicker, more abundant black shale beds within the mineralised envelope. The stylolites appear to be the product of silicate dissolution from the matrix of the siltstone, and in many instances are filled with trace sphalerite and pyrobitumen. Feltrin et al., 2009) interpret these features as direct product of compaction-induced pressure solution (e.g. McBride, 1989; Dewers and Ortoleva, 1990), as they are broadly concordant with the stratigraphic layers, and preserve organic seams, which were most likely accumulated as insoluble material during basin dewatering.
Whilst the bulk of the sulphide mineralisation occurs as bedding parallel lamellae, the overall mineralised package transgresses stratigraphic layering, with the most intense mineralisation progressing gradually upward through the sequence from SE to NW. In addition, zinc grades generally decline across the deposit from NE to SW, away from the Termite Range fault. This grade change appears to be independent of any appreciable change in the thickness of the host shale. Zones of localised high lead and silver grades are found in the southeast part of the deposit, associated with discordant but stratabound galena veining (Broadbent and Waltho, 1998).
Two principal stratabound sphalerite types are recognised, 'porous' and 'non-porous', distinguished on the basis of their respective high and low pyrobitumen content. They appear to be cogenetic, with both being grey to white in colour and appear to be highly replacive. The porous variety has a preference for organic-rich microlaminae, whilst the non-porous is associated with siliceous microlaminae. Overall, porous sphalerite is slightly more common, but is much more common in lower grade sections of the deposit. Porous sphalerite also appears to decrease upwards in the mineralised sequence, although both types are still evident (Broadbent and Waltho, 1998).
Two additional varieties of sphalerite are found within the deposit, namely, i). coarser grained, yellow to honey-coloured sphalerite, which occurs as stratabound fracture fillings within the overall mineralised sequence; and ii). coarse grained, red-brown, higher iron sphalerite within brecciated carbonate veins developed along NEtrending faults. The latter is not commonly where these faults intersect stratabound mineralisation of the main Century deposit, but is common about the intersection of the Nikki's, Page Creek and Termite Range faults. The Page Creek fault is a large NE-trending structural zone bisecting the Northern block (Broadbent and Waltho, 1998).
Galena occurs in two main textural forms, i). fine, commonly zoned euhedra developed in bands within, and concordant to sphalerite lamellae, mostly in the upper portion of the mineralised sequence; and ii). coarse grained, irregular, stratabound veins, developed throughout the mineralised sequence. Thin bands of euhedral galena are distributed throughout the deposit whilst discordant, irregular galena veining is most common in the SE corner of the deposit where it appears to be associated with NE-trending faulting. Numerous similar faults throughout the Lawn Hill region control minor lode mineralisation. The galena veins are mostly stratabound within carbonaceous shale beds, appearing to overprint sphalerite lamellae, or are surrounded by a narrow band of shale in which no other sulphides are developed. Thin fracture coatings of galena, in both shale and siltstone, are frequently associated with this form of mineralisation (Broadbent and Waltho, 1998).
Silver occurs as a solid solution replacement in sphalerite, rather than as distinct silver minerals, although zone of high grade silver flank areas of discordant, but stratabound galena vein mineralisation (Waltho, Allnutt and Radojkovic, 1993).
The main iron-bearing gangue phase within the siltstone is siderite, which petrographically appears to postdate much of the compaction of the host siltstone, and is in part synchronous with stylolite development and sphalerite deposition. Microprobe analyses indicate very marked temporal changes in siderite composition. The earliest formed contain up to 20% Mn and 5% Zn, whilst later generations are more Fe- and Mg-rich with correspondingly less Mn and no Zn. This change appears to be coincident with the introduction of a mobile carbon phase (pyrobitumen interpreted to have then been liquid hydrocarbon) and the onset of major sulphide deposition (Broadbent and Waltho, 1998).
Image above - Low to medium grade Century ore. Laminae and cross-cutting veinlets of fine grained predominantly sphalerite (darker laminae) in carbonaceous shale. Note: the reflective faces are not sulphides, by reflection from the carbonaceous shale. Image by Mike Porter, sample collected 1999, photographed 2022.
Images above - Left - Weakly mineralised sideritic siltstone 'top unit' with minor sphalerite-galena veins. Right - Outcropping mineralisation from the discovery outcrop. Image by Mike Porter, samples collected 1999, photographed 2022.
Mine Restart Mineralisation and Deposits
A proposed restart of the mine by New Century Resources was based on the remaining mineralisation sources and resources as detailed in the Reserves and Resources section below:
Century Tailings - A single substantial tailings deposit exists at Century, generated from 16 years of large scale operations from the Century open pit. The historical focus was on throughput maximisation, as opposed to recovery maximisation. Substantial recoverable zinc mineralisation exists in the tailings at Century, Due to the historical focus on throughput maximisation as opposed to recovery maximisation resulted in relatively modest recoveries (74% in 2015) over the life of operations. Three independent metallurgical studies demonstrate the tailings may be reprocessed through the existing plant on site with minor modifications to achieve recovery of >60% of remaining zinc mineralisation into a saleable ~52% zinc concentrate (Attila Resources, 2017). The reprocessing of the Century tailings provides a mechanism for the economic rehabilitation of the mine site, with the tailings area representing a significant portion of the current rehabilitation requirements. After reprocessing, the tailings are planned to be deposited back into the original open pit and encapsulated via subaqueous deposition.
Silver King lead-zinc-silver deposit that is located is 1.5 km SW of the original Century open pit. Discovered in 1897, Silver King had been mined on a small scale until 1961, with 15 shafts and associated underground workings to a depth of approximately 60 m. Estimated historical production is 3149t of Pb and 3 t of Ag. Mineralisation comprises a series of moderately to steeply dipping quartz-galena-sphalerite-siderite veins associated with a NE trending dextral strike-slip fault. Further sphalerite and galena mineralisation occurs within shale hosted breccia also associated with the veins. With the implementation of a tailings reprocessing operations, potential exists to extract Silver King as a blending ore.
South Block deposit is located on the southernmost margin of the original Century ore body and directly adjacent to the existing processing plant. It is a lower grade continuation of the main deposit that was mined at Century. Metallurgical tests suggest recoveries of 82% zinc, 85% lead and 83% silver.
East Fault Block deposit is located 35 m below the surface of the run-of-mine stockpile area at the mine site and extends to a depth of 112 m and is an extension of the main deposit.
Watson's Lode deposit is located some 6 km SW of the Century Tailings dam. It comprises a vein system developed over a strike of close to 4 km and comprises a series of fault-filling epithermal, quartz-carbonate-sphalerite-galena veins containing high grade lead-zinc-silver within a broader mineralised envelope. Individual veins are up to 15 m thick and form an array that defines a zone that is up to 50 m wide. Within the larger vein system, multiple high-grade lead-zinc-silver intersections are concentrated over 1700 m long interval SW from the historic Watsons Shaft. This mineralised zone predominantly lies within the Century host rock sequence. Within this mineralised array, veins are usually steeply dipping, intersect other structures of different orientations and may be truncated by younger fault.
New Century Resources restarted operations at Century in 2018, focused on the reprocessing of tailings and rehabilitation of existing disturbed areas, with an average annual production of 130 000 tpa zinc in concentrate (New Century Resources website, Nov., 2022).
Reserves and Resources
The original pre-mining total in situ resource was (Broadbent and Waltho, 1998):
167.5 Mt @ 8.24% Zn, 1.23% Pb, 33 g/t Ag, including a higher grade resource of
105.1 Mt @ 12.10% Zn, 1.69% Pb, 46 g/t Ag at a 3.5% Zn cut-off.
Remaining resources at June 30, 2013 were as follows (MMG 2013 reserve and resource statement):
Century
Measured resource - 0.1 Mt @ 8.4% Zn, 1.3% Pb, 27 g/t Ag;
Indicated resource - 17 Mt @ 10.0% Zn, 1.5% Pb, 37 g/t Ag;
Inferred resource - nil
TOTAL resource - 17.1 Mt @ 10.0% Zn, 1.5% Pb, 37 g/t Ag;
East Block
Measured resource - nil
Indicated resource - 0.5 Mt @ 12.4% Zn, 1.0% Pb, 49 g/t Ag;
Inferred resource - nil
TOTAL resource - 0.5 Mt @ 12.4% Zn, 1.0% Pb, 49 g/t Ag.
Remaining resources at March 1, 2017 to support tailings retreatment and mining of remaining deposits (as described above) were as follows (Attila Resources ASX Release 1 March, 2017 and New Century Resources web site viewed Feb, 2018):
Century tailings
Indicated resource - 12.8 Mt @ 2.97% Zn;
Inferred resource - 58.2 Mt @ 2.68% Zn;
TOTAL resource - 71.0 Mt @ 2.73% Zn.
Silver King deposit
Inferred resource - 2.7 Mt @ 6.9% Zn, 12.5% Pb, 120 g/t Ag;
South Block deposit
Inferred resource - 6.1 Mt @ 5.3% Zn, 1.5% Pb, 43 g/t Ag;
East Fault Block deposit
Inferred resource - 0.52 Mt @ 11.6% Zn, 1.1% Pb, 48 g/t Ag.
Remaining Ore Reserves and Mineral Resources at 31 June 2022 (New Century Resources Annual Report, 2022) were:
Century tailings
Measured Mineral Resources - 41.3 Mt @ 3.1% Zn, 15 g/t Ag;
Proved Ore Reserves - 40.3 Mt @ 3.0% Zn, 15 g/t Ag.
Some 8.948889 Mt of tailings were mined in the 2021-22 financial year for 118 108 tonnes of zinc metal and 39.899 tonnes of silver in concentrate.
Silver King deposit
Measured + Indicated + Inferred resource - 3.7 Mt @ 4.5% Zn, 5.5% Pb, 44 g/t Ag;
South Block deposit
Indicated resource - 6.2 Mt @ 5.4% Zn, 1.5% Pb, 43 g/t Ag;
East Fault Block deposit
Indicated resource - 0.6 Mt @ 9.8% Zn, 1.1% Pb, 51 g/t Ag;
Watson's Lode deposit
Inferred resource - 1.7 Mt @ 7.7% Zn, 2.0% Pb, 10 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2017.
Record last updated: 2/2/2018
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.
Century
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Andrews S J 1998 - Stratigraphy and depositional setting of the Upper McNamara Group, Lawn Hill region, northwest Queensland: in Econ. Geol. v93 pp 1132-1152
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Betts P G, Giles D, Lister G S 2003 - Tectonic environment of shale-hosted massive sulfide Pb-Zn-Ag deposits of Proterozoic northeastern Australia: in Econ. Geol. v98 pp 557-576
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Betts, P.G., Giles, D., Mark, G., Lister, G.S., Goleby, B.R. and Ailleres, L., 2006 - Synthesis of the Proterozoic evolution of the Mt Isa Inlier: in Australian J. of Earth Sciences v.53, pp. 187-211.
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Broadbent G C, Myers R E, Wright J V 1998 - Geology and origin of shale-hosted Zn-Pb-Ag mineralisation at the Century deposit, Northwest Queensland, Australia: in Econ. Geol. v93 pp 1264-1294
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Broadbent G C, Waltho A E 1998 - Century zinc-lead-silver deposit: in Berkman D A, Mackenzie D H (Eds), Geology of Australian and Papua New Guinean Mineral Deposits The AusIMM, Melbourne pp 729-736
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Derrick G M 1996 - The geophysical approach to metallogeny of the Mt Isa Inlier - what sort of orebody do you want: in Proc The AusIMM Annual Conference, Perth, 24-28 March, 1996 The AusIMM, Melbourne pp 349-366
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Feltrin, J.G., McLellan, J.G. and Oliver, N.H.S., 2009 - Modelling the giant, Zn-Pb-Ag Century deposit, Queensland, Australia: in Computers and Geosciences v.35, pp. 108-133.
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Gibson, G.M., Hutton, L.J. and Holzschuh, J., 2017 - Basin inversion and supercontinent assembly as drivers of sediment-hosted Pb-Zn mineralization in the Mount Isa region, northern Australia: in Journal of the Geological Society v.174, pp. 773-786.
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Gibson, G.M., Meixner, A.J., Withnall, I.W., Korsch, R.J., Hutton, L.J., Jones, L.E.A., Holzschuh, J., Costelloe, R.D., Henson, P.A. and Saygin, E., 2016 - Basin architecture and evolution in the Mount Isa mineral province, northern Australia: Constraints from deep seismic reflection profiling and implications for ore genesis: in Ore Geology Reviews v.76, pp. 414-441.
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Glikson, M., Golding, S.D. and Southgate, P.N., 2006 - Thermal Evolution of the Ore-Hosting Isa Superbasin: Central and Northern Lawn Hill Platform: in Econ. Geol. v.101, pp. 1211-1229.
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Golding, S.D., Uysal, I.T., Glikson, M., Baublys, K.A. and Southgate, P.N., 2006 - Timing and Chemistry of Fluid-Flow Events in the Lawn Hill Platform, Northern Australia: in Econ. Geol. v101 pp 1231-1250
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Kelso, I., Briggs, T. and Basford, P., 2001 - The Century Deposit - Geological Update: in AIG Journal, January 2001, Paper 2001-03, 3p.
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Leach D L, Bradley D C, Huston D, Pisarevsky S A, Taylor R D and Gardoll S J, 2010 - Sediment-Hosted Lead-Zinc Deposits in Earth History : in Econ. Geol. v.105 pp. 593-625
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Lees, T.C., Murphy, F.C., Tomkins, A.G. and O Donohue, D., 2019 - Displacement of the Proterozoic Century Ore Deposit at the Edge of an Ordovician Meteorite Impact Crater, Queensland: in Econ. Geol. v.114, pp. 427-440.
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Polito, P.A., Kyser, T.K., Golding, S.D. and Southgate, P.N., 2006 - Zinc Deposits and Related Mineralization of the Burketown Mineral Field, Including the World-Class Century Deposit, Northern Australia: Fluid Inclusion and Stable Isotope Evidence for Basin Fluid Sources: in Econ. Geol. v.101, pp. 1251-1273.
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Polito, P.A., Kyser, T.K., Southgate, P.N. and Jackson, M.J., 2006 - Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relation to District-Wide Zn, Pb, and Cu Mineralization: in Econ. Geol. v.101, pp. 1159-1188.
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Williams P J 1998 - An introduction to the metallogeny of the McArthur River-Mt Isa-Cloncurry Minerals Province: in Econ. Geol. v93 pp 1120-1131
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Yang, J., Large, R.R., Bull, S. and Scott, D.L., 2006 - Basin-Scale Numerical Modeling to Test the Role of Buoyancy-Driven Fluid Flow and Heat Transfer in the Formation of Stratiform Zn-Pb-Ag Deposits in the Northern Mount Isa Basin: in Econ. Geol. v.101, pp. 1275-1292.
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Zhang Y, Sorjonen-Ward P, Ord A and Southgate P N, 2006 - Fluid Flow during Deformation Associated with Structural Closure of the Isa Superbasin at 1575 Ma in the Central and Northern Lawn Hill Platform, Northern Australia: in Econ. Geol. v101 pp 1293-1312
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