Broken Hill and Broken Hill type (BHT) Mineralisation

SCI tenements are located within the Broken Hill and Euriowie Blocks of the Willyama Supergroup. SCI consider the most important style of mineralisation in to be BHT, due to its potentially high intrinisic in-ground value. The host rocks comprise a suite of metamorphosed quartz and feldspar-rich sedimentary sequences and intercalated felsic and basaltic volcanic rocks. The metamorphosed sandy sediments are referred to as psammites, and the finer silty sediments, pelites.

The rocks were laid down in extensional rift basins between 1710 and 1640 million years ago and intruded by thick sills of granite and sills and dykes of dolerite. Hydrothermal fluids enriched in metals and sulphur, either derived from the basins or from a magmatic source, ascended within large rift-bounding faults (Figure 1). Sulphide minerals, galena (PbS) and sphalerite (ZnS) precipitated from these fluids either onto the sea-floor as chemical sediments (exhalites) or within permeable layers of semi-consolidated sandy sediment below the sea-floor (inhalites).

Geochronological studies suggests the bulk of these minerals were formed at the same time as a specific unit known as the Hores Gneiss which is thought to be a metamorphosed felsic volcanic rock about 1685 million old.

Since the deposition of the sulphide-bearing sedimentary package, the rocks have undergone at least three periods of tectonic deformation which have included deep crustal burial, heating, folding and faulting. This has resulted in the rocks being highly metamorphosed, intensely folded and dismembered by faults. Some of the rocks present today are variably described as metamorphic rocks such as gneisses, granite-gneisses or amphibolites. Under high pressures and temperatures sulphide minerals are ductile and move to areas of less strain. These areas can include new open fractures/faults which have an epigenetic, vein-like character, or the hinge zones of folds where they concentrate and form thicker accumulations of sulphide-rich ore minerals compared to the attenuated limbs of folds. This accumulation and thickening process is considered to be an important mechanism in the formation of economic widths of sulphide ores at Broken Hill. This is an important feature in exploration, as very narrow sulphide zones can become very thick ore zones over short distances.

Importance of “Lode Rocks” in Mineral Exploration

The Broken Hill ore lenses are hosted within an envelope of companion lithologies called "lode rocks". These are thought to be metamorphosed chemical sediments and include rocks known as garnet quartzite, garnet sandstone, blue quartz-garnet-gahnite rock, spotted psammopelite, quartz-magnetite, banded iron formation and lode pegmatite. The garnet is often manganese rich and weathers at surface to dark black manganese oxide minerals. Feldspars within pegmatites are often greenish in colour due to their high lead content. These lode rocks, when mapped at surface or encountered in drilling, provide one of the best indicators to potential proximity of sulphide-bearing ore. In this regard their accurate location on maps and their morphology is an important element in successful exploration. Not all lode rocks are mineralised but in places can host high grade mineralisation in small isolated or discontinuous areas of outcrop. These can signal the beginning of an ore shoot.

Regionally, there are at least three main corridors of mapped lode rocks which coincide with and host known occurrences of BHT mineralisation. One to the south encompasses the Little Broken Hill-Copper King trend, the central corridor covers the Broken Hill Line of Lode itself, and the third, a much larger and folded corridor, extends north-eastward through the Purnamoota prospect to the Allendale prospect where it swings southward to encompass Maybell, Stephens Trig, Nine Mile and Corruga in the south. This latter corridor follows a complex fold in the Broken Hill Group rocks and lies almost entirely within SCI tenements. It is about 65km long; some 3 times the length of the Broken Hill Line of Lode corridor and SCI estimates it hosts well over 100 kilometres of complexly folded and faulted lode rock. It provides an outstanding target zone for high grade metal sulphide deposits (Figure 3).

The Broken Hill District has the potential to host deposits other than the Broken Hill type including:

Iron Oxide Copper-Gold Deposits (IOCG's)

This style of mineralisation is typified by the giant Olympic Dam Deposit in the Gawler Craton to the west of the Curnamona Province. Others of this type include Prominent Hill and Carapateena in South Australia, Ernest Henry and Selwyn in the Mount Isa District and the deposits of the Tennant Creek district in Central Australia. A number of smaller prospects have been recognised to the west of Broken Hill within the Curnamona province. These include Portia which is currently being mined and Kalkaroo. They are characterised by abundant iron oxides, magnetite and hematite, and contain accumulations of copper, gold, uranium, silver, molybdenum and often rare earth elements. They are associated with magmatic/volcanic activity. The Copper Blow project within SCI tenure is a typical IOCG deposit with strong biotite-magnetite alteration and high grade copper mineralisation.

Mt Isa Sediment-hosted Stratiform Lead-Zinc-Silver

Geochronology of the Paragon Group in the Broken Hill district, a sequence of carbonaceous pelitic rocks suggests that it is at least in part the same age as the Urquhart Shale in the Mt Isa district. The Urquhart Shale is the host to the large Mt Isa Pb-Zn-Ag deposit. The prospectivity of the Paragon Group is derived from this chronological correlation and the presence of weakly mineralised rocks at this stratigraphic position in the Paragon Group. As yet no significant mineralisation has been recognised. SCI tenements which host the Paragon Group include Willyama and Aragon.

Mississippi Valley Type Lead-Zinc (MVT's)

Published data on a drilling program by another explorer at the Dome Five prospect some 35km northwest of Broken Hill, has shown that MVT styles of mineralisation occur in dolomitic rocks within the Neoproterozoic rocks of the Adelaidean Group, which unconformably overlie the Willyama Supergroup. This is the first time this style of mineralisation had been described in the district and opens up large tracts of land where Adelaidean rocks outcrop for future exploration. 

High Grade Veins

Late structural events in the history of the Willyama Supergroup have led to the development of both silver, and copper-gold rich vein systems.

The silver-rich veins known as Thackaringa-type veins are of particular interest to SCI as they occur throughout the district and have historically returned extremely high grade ores from oxidised zones.

The mineralogy of the Thackaringa veins, where they are weathered and close to the surface, makes them amenable to low cost beneficiation techniques. Coalescing vein structures and stockwork veins will be targeted during exploration. The Euriowie Block hosts a number of quartz-rich vein-like structures which are known to host both gold and copper mineralisation. The Golden King mine is an example.

Sediment Hosted Copper Deposits

A number of sediment-hosted copper occurrences, locally associated with magnetite-rich rocks, are located in the Euriowie tenement. These occurrences impart a strong copper metallogeny on the Euriowie Block. The model for their formation is thought to be similar to that of Broken Hill, but with copper as the dominant commodity.

BROKEN HILL TYPE (BHT) MINERALISATIONIRON OXIDE COPPER GOLD (IOCG) MINERALISATION
Host Rocks
Variably metamorphosed chemical sediments incl. garnetites, siliceous gahnite rocks, reduced iron formation facies and / or altered calcsilicates. Occur within meta-sedimentary and / or meta-volcanic to volcaniclastic sequences.
Host Rocks
Hosted in variable rock types including felsic to intermediate/basic metavolcanics, metasediments, banded iron formations and intrusives.
Igneous Association
Altered basic (basaltic) to intermediate (rhyodacitic) extrusives and intrusives or bimodal volcanics.
Igneous Association
Association with igneous activity and usually spatially and temporally related to significant magmatic events.
Structural Setting
Structurally modified, remobilised when massive due to later folding and shearing. Occur in rift situations.
Structural Setting
Usually localised along high and low-angle faults which are often splays off major crustal scale faults.
Deposit Morphology
Stratiform to stratabound. Progressively remobilised with increased metamorphic grades and later shear events. Folded and brecciated via cataclastic remobilisation. Complex ore shapes.
Deposit Morphology
Deposits occur in a variety of morphologies ranging from stratabound sheets, to irregular stockwork breccia zones. Virtually all deposits formed by the replacement of the host rock.
Mineralogy
Galena, sphalerite, chalcopyrite +/- pyrrhotite, pyrite. Silver mainly in tetrahedrite included in galena.  Characteristic gangue minerals are Mn, Fe, Si, Ca, F, Ba, Mg and P enriched. Indicator minerals in host rocks include gahnite, Mn garnet and blue quartz.
Mineralogy
Characterised by the presence of iron oxide minerals, usually magnetite + hematite and there is a comparative lack of iron sulphides. Almost all the deposits contain rare earth elements and may contain significant carbonate, Ba, P or F.
Alteration
Increased Mn, Fe, Si, Ti, P, Ca, Bi, B, Al, Mg, K, Rb and S, and Na depletion. Expressed in host rocks as increased garnet, quartz, biotite, chlorite, K-feldspar, sillimanite, sericite, barite, tourmaline, or as other silicates, Ti-oxides, or disseminated sulphides.
Alteration
Host rocks are typically intensely altered. Generally associated with sodic, potassic or hydrolytic (sericite-carbonate) alteration; pre-mineralisation sodic and sodic-calcic alteration systems tend to be spatially extensive.
Metals
Mainly Pb, Zn, Ag and Cu. By-products may include Au, Sb, Cd, As, Co and Bi.
Metals
Deposits are generally mined for Cu with Au (or other metals as a credit). Au grades are generally less that 1g/t Au. Chalcopyrite is the dominant sulphide.  Minor metals can include U, Ag, Mo, Co, As and Zn.
Geophysical Signature
Magnetic surveys useful in locating associated magnetite bearing chemical sediments and some ore types. Useful in tracking broader host stratigraphy and associated iron formations.  Usually insufficient pyrrhotite to produce meaningful anomalies.
  • Electrical methods have limited application from surface. IP may be useful in outlining disseminated pyritic haloes and galena rich stringer types. EM limited to galena rich ores. Pyrrhotite enrichments generally non-diagnostic. Zinc rich sulphides poorly to non-responsive to EM and IP.
  • Down hole MMR, EM and IP possibly useful in tracking sulphide rich zones intersected by drill holes.
Geophysical Signature
The geophysical signatures are variable but often include:
  • Anomalously high magnetic responses but the deposits are not always coincident with the anomalies.
  • Gravity highs are often coincident with the mineralisation.
  • Some deposits have a high radiometric response although radiometrics are not frequently used as an exploration tool.
  • Conductivity anomalies as the sulphides as well as magnetite and hematite are all conductive. IP and resistivity methods have been successfully used in exploration.

 

MISSISSIPPI VALLEY TYPE (MVT) MINERALISATIONMOUNT ISA-STYLE MINERALISATION
Host Rocks
Hosted by marine carbonate rocks.
Host Rocks
Hosted in pyritic and carbonaceous dolomitic siltstone.
Igneous Association
No obvious igneous activity associated with the deposits. Mineralising fluids were basinal brines.
Igneous Association
No obvious igneous activity associated with the deposits. Mineralising fluids were basinal brines.
Structural Setting
Typically associated with growth faults, especially basin margin faults.
Structural Setting
Typically associated with major growth faults.
Deposit Morphology
Replacement and karst-fill deposits, massive to irregular.
Deposit Morphology
Stratiform to slightly cross-cutting mineralisation, with stacked tabular bodies.
Mineralogy
Sphalerite with or without galena, fluorite and barite.
Mineralogy
Mostly pyrite, galena and sphalerite.
Alteration
Deposits are commonly associated with a "dolomite front" alteration in which dolomite (Ca Mg carbonate) partly replaces original Ca carbonate
Alteration
Haloes of Zn, Pb, Cu, Ag, Tl, Hg, and Mn enrichment extending for km laterally, and anomalous Tl above and below.
Metals
Predominantly Zn, Pb
Metals
Predominantly Pb, Zn with some Ag.
Geophysical Signature
Geophysical methods are generally ineffective in direct detection although gravity may be able to distinguish higher density metal accumulations.
  • Aeromagnetics and gravity methods may help delineate basin morphology and growth faults.
Geophysical Signature
The deposits are non-magnetic but are highly conductive. Graphitic host rock would tend to mask both EM and IP responses.
  • Down-hole electromagnetics ("DHEM") is an effective tool.

 

THACKARINGA TYPE VEINSCOPPER-GOLD VEINS
Host Rocks
Willyama, wide range of ages, preferentially hosted in schist zones, shallow-moderate east to southeast dips
Host Rocks
Willyama, wide range of ages.
Igneous Association
None.
Igneous Association
None
Structural Setting
In retrograde shears. Implication is these are younger than BHTs
Structural Setting
Mylonitic or retrograde shear zones
Deposit Morphology
Discontinuous, generally short strike length, narrow viens
Deposit Morphology
Discontinuous but with persistent strike to 800, narrow veins
Mineralogy
Non-laminated, crustiform siderite-(barite)-quartz gangue with silver-rich galena, sphalerite,pyrite, arsenopyrite, chalcopyrite, tetrahedrite pyrargyrite. High grade silver minerals especially chlorargyrite in oxidised weathered zone.
Mineralogy
Quartz gangue. Veins, vein stockwork, stringers, locally chalcedonic and laminated. Pyrite, chalcopyrite, pyrrhotite, gold.
Alteration
None
Alteration
Narrow silicification halos. Chlorite. Possibly tourmaline.
Metals
Ag, Pb, Zn, Cu, As
Metals
Cu, Au and Ag
Geophysical Signature
None known to date.
Geophysical Signature
IP reported to be effective at Golden King.