The Broken Hill zinc - lead - silver orebodies are located 40 km east of the South Australian border in far western New South Wales, approximately 950 km WNW of Sydney and 450 km north-east of Adelaide.
The Broken Hill deposits lie within the Willyama block which extends from far western New South Wales into eastern South Australia and is composed of Paleo- to Mesoproterozoic metamorphic rocks and minor intrusives. The Willyama block is overlain by Neoproterozoic sedimentary cover which separates it from the similar rocks of the Euriowie Block to the east, while the equivalent Olary block to the west in South Australia is separated from the Willyama Block by younger cover.
The Willyama block is largely occupied by the metamorphics of the ~7 km thick, ~1720 to ~1640 Ma Willyama Supergroup which comprises amphibolite to granulite facies schists and gneisses ranging from andalusite-muscovite, through sillimanite-muscovite, sillimanite-potassium feldspar-garnet to hornblende-granulites, with lesser amphibolite and basic granulite. It has been interpreted to represent a metamorphosed protolith of clayey and sandy sediments with intercalated basic and felsic volcanics and minor sub-volcanic intrusives. Basement rocks to the Willyama Supergroup are not exposed, but inheritance indicates the presence of Archaean and older Palaeoproterozoic crystalline crust.
The lower Willyama Supergroup consists of an oxidised and albitised succession including granitic gneisses, amphibolite sills, and psammitic, pelitic, and calc-silicate rocks. The upper succession is dominated by metapelites with graphitic layers deposited in a reducing environment.
Two periods of folding accompanied the prograde metamorphism in the Broken Hill and Euriowie Blocks, namely D1 which produced very large recumbent folds/nappes with a transport direction from the SE, and D2 which produced large folds with steeply dipping axial planes. Both D1 and D2 resulted in sillimanite grade axial plane schistosity. The peak metamorphism associated with these deformations appears to be at 1660±10 Ma. A third phase of deformation, D3, produced folds on all scales throughout the Broken Hill Block, generally with near vertical axial planes and narrow to broad zones of retrograde shearing, commonly parallel to the fold axes and usually coinciding with F3 anticlinal axial planes. Major D3 monoclines fold the sequence.
The D3 shear zones range from a few metres in width to several hundred metres, while in some cases they constitute complex retrograde belts several kms wide. All of these wide complex belts are characterised by numerous, close spaced, anastomosing, intense, retrograde schist zones; by pervasive retrogression of intervening high grade rocks and by an abundance of granitoids, pegmatite, aplite, migmatite and granite. These shear zones are generally aligned in a NE-SW or NW-SE direction, while some are E-W. They appear to have been generated early in the D3 phase, before 1570 Ma, but were reactivated during the Delamerian event at 500±20 Ma.
All of the economic mineralisation at Broken Hill is hosted by the Broken Hill Group (Suite 4), of the Willyama Supergroup, which is composed of the following, from the base, the:
Clevedale Migmatite (Suite 1) composed of >425 m of leucocratic quartzo-feldspathic migmatites and composite gneisses (quartz-albite-K feldspar-biotite±cordierite rocks), with meta-sedimentary migmatites, leucocratic sodic plagioclase-quartz rocks and concordant to transgressive basic gneiss. The Suite 1 Redan Gneiss has been dated at 1710±4 Ma (Page et al., 2005);
Thorndale Composite Gneiss (Suite 2), comprising 1000 to 2000 m of poorly to well bedded psammitic to psammo-pelitic meta-sedimentary composite gneisses, composed of quartz-feldspar-biotite-sillimanite±cordierite±garnet, interlayered with subordinate sodic, plagioclase-quartz rock and concordant to transgressive basic gneiss;
Thackaringa Group (Suite 3), 1000 to 3000 m, averaging 1500 m thick, dominantly composed of quartzo-feldspathic gneisses with lesser pelitic and mafic gneisses. Rocks in the basal and upper parts of this unit have been dated at ~1704 and 1683±3 Ma respectively (Page et al., 2005);
Broken Hill Group (Suite 4) composed predominantly of pelitic to psammitic gneisses with minor quartzo-feldspathic and mafic gneisses. This unit has been dated at between 1693±4 (Ettlewood Calc-silicates) and 1685±3 Ma (Hores Gneiss, which hosts the Broken Hill orebody; (Page et al., 2005);
Sundown Group (Suite 5), 5 to 1350 m thick, mainly pelitic to psammitic gneisses, all of which occur in tightly folded repetition. This group represents a major change in the Broken Hill Domain, from the underlying oxidised and albitised succession, to deposition of largely pelitic and psammitic units in a reducing environment. Maximum depositional ages of 1688±6 and 1672±7 Ma have been determined from different parts of the Inlier (Page et al., 2005);
Paragon Group (Suites 6 to 8), characterised by graphitic meta-sediments and is best developed in the lower grade northern sections of the Broken Hill Block. Maximum depositional ages of 1655±4 and 1657±4 Ma have been determined from different parts of the Inlier (Page et al., 2005).
The Broken Hill Group (Suite 4) is 1000 to 1500 m thick, and from its base, marks a significant stratigraphic change from the dominantly feldspathic meta-sediments of suites 1, 2 and 3, to the more pelitic meta-sediments, intercalated with basic gneisses, which characterise the upper half of the Willyama Supergroup, in which no further significant feldspathic gneisses/rocks occur. The Broken Hill Group also contains scattered zoned calc-silicates nodules. It has been sub-divided, from the base, into:
Allendale Meta-sediments, 0 to 1500 m, generally 500 m thick - thinly bedded pelitic, to psammo-pelitic/psammitic, to locally psammitic meta-sediments/gneisses with minor basic gneiss, quartz-gahnite rock, tourmaline-quartz rock and well the bedded calc-silicates of the 0.5 to 10 m thick Ettlewood Calc-silicate Member near the base of the unit, generally occuring as a well bedded quartz-Ca plagioclase±clinopyroxene±amphibole±epidote calc-silicate rock, containing high background Pb, Zn & W, and is interpreted as an original impure dolomitic limestone,
Purnamoota Subgroup, 600 m thick, which makes up the remainder of the Broken Hill Group, and is characterised by meta-sediments intercalated with basic gneiss (Fe rich, commonly with abundant garnet and pyroxene), felsic gneiss/rock and quartz-gahnite and quartz-garnet rocks 'lode horizons'. In the northern section of the Block the subgroup is well developed and can be readily sub-divided, while in the south these subdivisions are not as obvious. To the NW it is dominated by basic gneisses which apparently have well developed relict igneous textures. Across the thick developments of the Thackaringa Group granite gneisses around the Stephens Creek the subgroup is thin with poorly developed felsic and basic gneisses. It may be further sub-divided into,
Parnell Formation, 150 to 500 m thick - comprising mainly pelitic, to psammo-pelitic /psammitic metasediments, intercalated with massive to thinly layered basic gneiss which is Fe rich with abundant garnet or orthopyroxene. The basic gneisses are associated with lenticular units of quartz-plagioclase-biotite-garnet±K feldspar gneiss. Small occurrences of Broken Hill type Pb- Zn-Ag mineralisation and associated quartz-gahnite, garnet-quartz and banded iron formation 'lode horizon' are widespread, while tourmaline-quartz rocks and garnet-epidote-amphibole calc-silicates commonly host stratabound W mineralisation.
Freyers Meta-sediments, 50 to 190 m thick -composed of well bedded pelitic, to psammo-pelitic /psammitic metasediments, which are thinly and planar bedded, with rare graded bedding and sporadic calc-silicate nodules, and contain rare basic gneiss, quartz-gahnite and quartz-tourmaline rocks. In the mine area they are pelitic to psammitic, containing banded iron formation lenses and directly underlie the orebodies.
Hores Gneiss, 60 to 400 m thick - is similar to the felsic gneisses of the Parnell Formation, and is generally a quartz-plagioclase-biotite-garnet±K feldspar gneiss. It contains variable proportions of intercalated meta-sediments and is typically associated with minor amphibolite, tourmaline-quartz rock (usually anomalous in W) and rare quartz-gahnite rock. At Broken Hill it is unusually rich in meta-sediments, and consists mainly of pelitic meta-sediments/gneisses intercalated with lenticular, elongate bodies of quartz-feldspar-biotite-garnet gneiss, thin (few cm's to a few metres thick) banded iron formations and the main Broken Hill orebodies.
Silver King Formation, <1 to 50 m thick - the Hores Gneiss passes laterally and upwards into the Silver King Formation with an increasing proportion of basic gneiss and meta-sediments. It occurs mainly in the NW of the Broken Hill Block where massive, extensive and conformable basic gneisses predominate, and rarely in the central section of the Block. It comprises mainly pelitic to psammo-pelitic/psammitic meta-sediments and abundant amphibolite, with subordinate lenticular bodies of quartz-feldspar-biotite±garnet gneiss, in places with tourmaline-quartz rock and minor quartz-gahnite rock. The Silver King mineralisation is hosted by this unit. The top of the formation is marked by the abrupt termination of basic gneiss by the meta-sediments of the Sundown Group.
In the vicinity of the Broken Hill mine the Broken Hill Group has been broken into units 4.1 to 4.8, where 4.1 corresponds to the lower sections of the Allendale Meta-sediments; 4.2 the Ettlewood Calc-silicate Member which is largely absent in the mine area; 4.3 averages 80 m in thickness and is a coarsely garnetiferous gneisses; 4.4 a suite of mafic, hornblende rich basal amphibolite which grade upwards into felsic and garnetiferous variants and averages 25 m in thickness; 4.5 is composed of banded pelitic to psammitic meta-sediments, with massive blue-quartz rich psammites, interbanded with thick poorly to well banded pelites, become more common upwards,with minor zones of disseminated sphalerite, pyrrhotite, galena and chalcopyrite within the psammitic horizons towards the top; 4.6 the Freyers Meta-sediments, described above, contains local lenses of banded iron formation that are few cms to 2 m thick; Unit 4.7, the 60 to 400 m thick (averaging 150 m) Hores Gneiss, which contains all of the economic Broken Hill orebodies, is generally a psammitic gneiss, characterised by the presence of lode rocks, comprising garnet-quartzite, garnet sandstone, blue quartz-gahnite lode, lode pegmatite, calc-silicate lenses and sulphide orebodies; 4.8 corresponds to the Silver King Formation as described above.
Ore occurs as both massive and disseminated sulphides comprising almost exclusively course sphalerite (martite) and galena with minor pyrrhotite, chalcopyrite and loellingite. Apart from a local pyrrhotite mass on one margin, there are very few other sulphides, nor is magnetite an important constituent of the ore. The gangue mineralogy includes quartz, calcite, garnet, calcium and manganese pyroxenes and pyroxenoids. There is a gross, but not well defined banding to the ore as defined by gangue and sulphides, although the principal foliation is evident in low grade mineralisation. The sulphides are generally medium to coarse grained, seldom <1 mm in crystal size. Gangue seldom forms distinct bands with the ore, generally being present as irregular zones and blocks within the higher grade ore. Large calcite or quartz veins with variable amounts of sulphide are found cutting the high grade ore.
The nine, folded, linear, elongate Broken Hill ore lodes are distributed over a strike length of 7.3 km, a 'stratigraphic thickness' of less than 150 m and down dip extent of 300 to 1000 m. The lodes include, from the structural footwall, upwards, the No.s 3, 2 and 1, the A, B and C lodes and their splits. The No. 3, 2 and 1 Lodes (the 'Lead Lodes') are characterised by massive, coarsely-crystalline, galena and sphalerite, with average grades of 8 to 16% Pb, 12 to 22% Zn and 50 to 300 g/t Ag in a gangue dominated by similarly coarsely-crystalline quartz, calcite and rhodonite. The structurally higher, or 'Zinc Lodes' contain a higher proportion of sphalerite are less massive, not as coarse, and have a predominantly quarz and garnet gangue, with grades of 3 to 4% Pb, 6 to 12% Zn and 30 to 35 g/t Ag. The 'Lead Lodes' are best developed and predominate in the northern sections of the line of lode, while the 'Zinc Lodes are more significant in the Southern Leases.
Each of the lodes overlaps the lode above and below along the direction of elongation. They are steeply dipping, but exhibit a shallowly doubly plunging 'coat hanger-like' structure extending below the surface to the north and south of the exposed apex. The orebodies are restricted to two adjacent tight 'drag folds' over the full length of the line of lode. Within these structures, the ore occupies the keel of an eastern synform, the crest of a western antiform, their common limb and a shear zone merging into the eastern limb of the synform. The elongation and plunge of the lodes and the alignment of their vertical en echelon development parallels the F2 and F3 fold axes, both locally and regionally, while the elongation of the lodes corresponds closely to the orientation of the L1-2 lineation and L1 where it is distinguishable from L2.
Within the Broken Hill orebody, markedly transgressive high grade ore extend upwards and downwards from the orebody. These developments are known locally as 'droppers', and carry up to 42% combined metal, although typically assaying around 15% Zn, 10% Pb, 80 g/t Ag. Droppers range from simple cusp features to major piercement structures up to 50 m or more in length. The droppers are often irregularly developed along strike, varying greatly in thickness between mine sections, generally with strike lengths of up to 120 m. Droppers may comprise up to 10 m thick of high grade ore on one section, and be a barren shear on the next 20 m spaced section.
The orebodies origninally outcropped as a >1 km long by 30 to 40 m wide manganiferous gossan on a long narrow ridge formed by the resistant siliceous lode rocks which stood approximately 50 m above the surrounding gneisses. The gossan was characterised by coronadite, limonite, pyrolusite, psilomelane, quartz and garnet with minor cerussite, silver halides, pyromorphite and a little galena, that assayed from 60 to 900 g/t Ag and 10 to 15% Pb, although leaching of Zn was almost complete. Generally oxidation was complete to 100m and in places below 200m, while the outcropping zone of Fe and Mn oxides extended to an average depth of 75m.
Away from the main orebody, 'lode rocks' with small massive sulphide accumulations and anomalous sulphide mineralisation has been traced over a cumulative strike length of 300 km. Within this mineralised trace, there are a number of other smaller zones of higher grade mineralisation, including the Potosi and Pinnacles and Silver Peak prospects and deposits. Potosi and Silver Peak are approximately 2 km north-east of the North Mine.
The Potosi deposit, which is also hosted by the Hores Gneiss, has been exploited as an open pit and occurs on the opposite limb of the 'Hanging Wall Synform' to the Broken Hill deposit which is to the south-west. The 'Hanging Wall Synform'is cut by two major sub-parallel structures, the Globe-Vauxhall and Western shear zones, and occurs between these two structures and are limited by the Potosi shear zone to the south-west. Mineralisation is dominantly coarse black, iron-rich sphalerite (marmatite) with lesser galena, chalcopyrite and pyrrhotite in a gangue of gahnite, garnet and blue-quartz. It occurs in four main ore types, i). stringers to sub-massive marmatite with 3 to 20% Zn; ii). 4 to 5 m thick zones of massive marmatite with a 'pebbly' durchbewegung texture, generally containing 30% Pb+Zn; iii). varying amounts of disseminated galena-chalcopyrite with fine veinlets and stringers of marmatite with lesser galena and chalcopyrite in a blue quartz, garnet and gahnite gangue, typically with 7% Pb+Zn; and iv). stringer marmatite in a blue quartz psammite, averaging 10% Pb+Zn. To the SW the mineralisation occupies a synformal structure, while to the NE it becomes a west dipping flexure as it approaches the Western Shear. In 1996, prior to mining the proved + probable reserve quoted by Pasminco was 1.0 Mt @ 2.3% Pb, 9.1% Zn, 28 g/t Ag in a 400 x 200 by 90 m deep pit outline. The indicated + inferred resource outside of the pit amounted to 1.8 Mt @ 3.7% Pb, 13.7% Zn, 45 g/t Ag, 0.28% Cu.
The Pinnacles Mine, some 17 km to the south-west of Broken Hill comprises 0.15 Mt @ 2.5% Zn, 11% Pb, 500 g/t Ag which has been mined from one of six generally 2 to 4 m (locally up to 15 m) thick concordant lodes distributed over a 150 m stratigraphic thickness. The lodes are siliceous (quartzites) with garnet, gahnite, hedenbergite, galena and sphalerite with minor pyrrhotite, chalcopyrite, arsenopyrite and pyrite within or adjacent to a quartz-magnetite rock. The mineralisation occurs in a strongly folded interval between two retrograde schist zones. The host has prominent quartz-magnetite and fine sodic plagioclase-quartz rock components. This lode zone and the other similar occurrences within the Willyama Inlier, is within the Suite 3 Thackaringa Group, which lies below the Broken Hill Group that hosts the main Broken Hill lodes.
Origin - Large and Crawford (2010) note that the Broken Hill Pb-Zn-Ag deposits are hosted by high metamorphic grade arkosic sediments within a regional sequence that varies from quartzo-feldspathic to pelitic dominant. Tholeiitic metagabbroic sills form a significant component of the regional host package. In the immediate deposit area, the meta-arkosic sediments are accompanied by amphibolite, felsic gneiss, minor units of BIF and quartz-garnet-gahnite lithologies. They note that the deposit has a pronounced alteration halo that extends over at least 3 to 4 km along strike beyond the line of lode, and may be up to 500 m thick, defined by garnet spotting and increasing enrichment in K-Rb-Fe-Mn-Pb, and depletion of Na-Ca-Mg-Sr, towards mineralisation. Early interpretations favoured syn-kinematic or post-kinematic, often replacement-related origin for the mineralisation, in which ore fluids were closely linked with the granulite to amphibolite facies metamorphism and deformation. However, subsequent studies indicated that both the ores and their surrounding pronounced stratabound alteration halo existed prior to peak metamorphism, and that mineralisation was present some 80 m.y. before the major deformation event. Since then, models evolved from an essentially syn-sedimentary exhalative process (SEDEX), to a dominantly inhalative processes just below the seafloor, forming the stacked ore lenses over a 5 to 6 m.y. period, synchronous with or shortly after deposition of the host sediments (cf., Laisvall in Sweden). Neither of these models is consistent with the restricted range of d34S isotopic values from the Broken Hill lode sulphides, which cluster around 0, and magmatic S/Se values. These values are homogeneous, and totally mantle-like, with minimal evidence for seawater-derived S, suggesting an igneous source for the S in the deposit.
A detailed study of the trace element and isotopic geochemistry of the extensive mafic sills in the Broken Hill Block (Crawford & Maas, 2009) has resulted in the development of an alternative model. Crawford & Maas (2009) argue that the Broken Hill Pb-Zn ore metals and S were ultimately derived from very strongly fractionated rift ferrotholeiite magmas emplaced along a major detachment around 1685 Ma that was associated with the extension and commencement of breakup of the Nuna-Columbia super-continent. They propose that highly evolved Fe-rich melts were forced out of intercumulus sites in gabbros up the detachment by synkinematic fractionation, to crystallize as very Fe-rich oxide gabbros. They also state that fractional crystallisation very effectively separates Cu from Pb and Zn, since Cu partitions strongly into a magmatic vapour phase from the early stages of fractionation, whereas lithophile Pb and Zn remain in the magma as fractionation proceeds. They suggest late magmatic fluid evolution from these Fe-rich magmas which transported large amounts of Pb and Zn via a saline-rich hydrothermal fluid that ultimately deposited those metals as massive and/or disseminated sulphide lenses beneath the seafloor, higher in the section along the trace of the extensional shear system. The abundant Mn, a characteristic of the Broken Hill ores and alteration halo, is ascribed to alteration of ilmenite (typically 0.5 to 2% MnO), a major phase in oxide gabbros. The major Thorndale gravity anomaly, centred east of, and bordered on its western margin by the Broken Hill Line of Lode, probably reflects the presence of these ferrogabbros on a major E-dipping crustal structure, the outcrop of which may define the line of lode. This model does not require the involvement of seawater-derived fluids, nor of metal extraction from the metasedimentary pile. New Pb isotope data for the metagabbroic sills (amphibolites) in the Broken Hill Block (Crawford and Maas, 2009) strongly supports this model. (The preceding paragraphs are paraphrased from Large and Crawford, 2010).
In addition, however, the structural relationships within the lode suggest structural remobilisation of ore, while Frost et al., 2011, observe that the massive sulphides exhibit strong evidence of melting, and suggest that during peak metamorphism, at temperatures of >750°C and 5 kbars pressure, the main Broken Hill orebody was partially molten, with much of the Pb, Cu, Ag, Sb and As in the melt, and the Fe and Zn residing in the restite. They also suggest that the main sulphide melt froze after the last penetrative deformation at temperatures of <720°, with a geometry and distribution determined by the structural dynamics during peak metamorphism.
History - The main Broken Hill deposit was recognised by Charles Rasp in 1883 as a gossan outcrop, following the discovery in 1882 of high grade Ag and Pb-Ag at Thackaringa and Silverton 20 to 30 km to the west. Charles Sturt had first collected samples from the Broken Hill ridge in 1844, but abandoned them when in danger of perishing in the desert later in the expedition. Mining has been continuous from 1883 to the present. Early mining was concentrated on the exposed gossans of the central hump of the line of lode by the company that became the Broken Hill Proprietry Company Limited (BHP). The northern down-plunge limb of the deposit was worked by a series of mines, which in 1885 were consolidated into the North Mining Company, later to become North Broken Hill Ltd (NBH). In 1885, Broken Hill South Limited (BHS) acquired leases adjacent to the BHP mines, and by 1940 had purchased the BHP leases to control the central part of the line of lode, the Central Leases, between the NBH and the Southern Leases. BHP had mined 13.43 Mt @ 12.5% Pb, 470 g/t Ag, 5.0% Zn before their departure from the field in 1940. The other early mines acquired by BHS in the Central Leases are estimated to have mined around 13.21 Mt of high grade ore.
Zinc Corporation Limited commenced mining on the Southern Leases over the southern down-plunge extensions of the line of lode in 1911. The associated New Broken Hill Consolidated (NBHC) mine started operations in 1937 on the continuation of the southern limb of the line of lode immediately to the south of the Zinc Corporation leases. A third shaft complex, the Southern Cross mine, was established as part of the ZC-NBHC operation as the southern continuation of the NBHC mine in the 1970's.
The four mines, NBH, BHS, ZC and NBHC, worked the line of lode on a large scale until BHS ceased operation in 1972. The Central Leases of BHS were acquired by Minerals, Mining and Metallurgy Limited (MMM) who commenced an open pit to extract lower grade ore, remnants and mineralised dumps until 1990. Until it ceased operations, BHS had extracted 19.77 Mt @ 12.8% Pb, 170 g/t Ag, 7.7% Zn. By 1990, the total production on the Central Leases, including that of BHP, BHS, MMM and other early mines in the Central Leases had totalled 51.9 Mt of ore.
In 1988, the operations of NBH, and the ZC-NBHC mines of ZC Mines Pty Ltd were amalgamated, with other operations of the same companies to form Pasminco Mining Ltd. Until its closure in 1993, the North Mine, operated initially by NBH and later by Pasminco, had mined approximately 34 Mt @ 14% Pb, 230 g/t Ag, 11.5% Zn to a depth of 1700 m. By 1987, just prior to the formation of Pasminco the Southern Mines of ZC-NBHC had produced more than 73.5 Mt @ 11.1% Zn, 10.2% Pb, 78 g/t Ag.
In late 2001, Pasminco was liquidated and the company's Broken Hill operations were purchased by Perilya in early 2002 who have continued the operation until the present (2008).
The 3.8 km long Central Leases held by CBH Resources Ltd contain the Kintore open pit mine developed on the main upper sections of the line of lode, and the down dip 'Western Mineralisation' zone that was too low grade to have been exploited when held by BHS.
The original pre-mining ore deposit (as quoted by Morland and Webster, 1998) consisted of 185 Mt of mineable ore, including 150 Mt @ >20% combined Pb+Zn. It has been estimated that approximately 60 Mt of ore was eroded from the original ore deposit. In addition there are around 100 Mt of >3% Pb+Zn mineralisation.
In late 1997 proven + probable reserves totalled 33.5 Mt @ 8.2% Zn, 5.1% Pb, 51 g/t Ag.
In mid 2007 the reserves and resources quoted by Perilya (2007) for the Northern and Southern Leases sections of their operation were:
Southern Operation - measured + indicated + inferred resource: 11.47 Mt @ 9.9% Zn, 7.6% Pb, 73 g/t Ag,
proved + probable reserve: 11.198 Mt @ 6.7% Zn, 4.9% Pb, 49 g/t Ag,
North Mine - measured + indicated + inferred resource: 0.165 Mt @ 6.6% Zn, 6.5% Pb, 108 g/t Ag,
proved + probable reserve: 0.046 Mt @ 4.5% Zn, 4.0% Pb, 87 g/t Ag,
North Mine Deeps - measured + indicated + inferred resource: 3.7 Mt @ 11.3% Zn, 13.5% Pb, 219 g/t Ag,
proved + probable reserve: 0.046 Mt @ 4.5% Zn, 4.0% Pb, 87 g/t Ag,
Potosi North - indicated + inferred resource: 0.304 Mt @ 12.7% Zn, 7.2% Pb, 64 g/t Ag,
Potosi Extended - inferred resource: 1.729 Mt @ 12.6% Zn, 2.9% Pb, 45 g/t Ag,
Flying Doctor - inferred resource: 0.52 Mt @ 5.4% Zn, 7.1% Pb, 72 g/t Ag,
Silver Peak - inferred resource: 0.335 Mt @ 5.9% Zn, 11.1% Pb, 91 g/t Ag,
Total Resource - 18.223 Mt @ 10.2% Zn, 8.4% Pb, 101 g/t Ag,
Total Reserve - 11.224 Mt @ 6.7% Zn, 4.9% Pb, 50 g/t Ag.
In mid 2007 the remaining resources of the Central Leases to a depth of 600 m were quoted by CBH Resources (2007) as:
10.1 Mt @ 4.9% Zn, 3.5% Pb, 43 g/t Ag.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2012)