Western Australia, WA, Australia
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The Blendevale deposit, re-named Pillara, is located some 35 km and 250 km respectively to the south-east of Fitzroy Crossing and Derby in north-western Western Australia. It is one of a number of carbonate hosted Zn/Pb occurrences within middle to late Devonian carbonates on the Lennard Shelf of the Canning Basin. Unlike the nearby smaller stratabound but higher grade Cadjebut orebody, Pillara is distinctly transgressive.
During a regional exploration program the seventh hole of an 800 m centres reconnaissance drilling program covering a 50 sq. km area in the Pillara-Blendevale district intersected several mineralised zones between 210 and 440 m, including 8m @ 8.9% Zn, 3.5% Pb. This was the 136th hole in the Pillara area. Follow-up drilling delineated the deposit to a minimum 10 Mt which was then subjected to a preliminary feasibility study and found to be sub-economic. As a consequence exploration continued resulting in the discovery of Cadjebut. The Blendevale deposit was subjected to further drilling, a full reserve delineation and a feasibility study in 1985, but still failed to meet economic requirements. This work outlined a geological resource of:
20 Mt @ 8.3% Zn, 2.5% Pb, 17g/t Ag,
based on a cutoff of 3% Zn+Pb, from a 50 m spaced testing grid. Between 1976 and 1985 some 410 diamond drill holes have been cored in the Pillara-Blendevale area, 150 of which were to delineate the Blendevale deposit.
Total past production from June 1997 to October 2003 was 10.3 Mt @ 6.9% Zn, 2.3% Pb. Current (2006) reserves and resources are:
Probable reserve - 3.0 Mt @ 7.3% Zn, 1.8% Pb
Measured resource - 1.4 Mt @ 8.9% Zn, 2.4% Pb
Indicated resource - 1.1 Mt @ 7.9% Zn, 1.8% Pb
Inferred resource - 0.2 Mt @ 9.6% Zn, 1.9% Pb
The following is derived from Murphy, et al., (1986), Murphy, et al., (1990) and Murphy (1990).
See the 'Regional Geology' segment of section from the record on the nearby Cadjebut orebody.
The Blendevale deposit is situated at the northern end of the Limestone Billy Hills, the exposed portion of a limestone platform/atoll reef complex located north of the Pillara Range. The Pillara Range and Limestone Billy Hills are middle to upper Devonian limestones developed over Precambrian granitic basement highs that protruded above the surrounding lower Palaeozoic (mainly Ordovician) sediments of the Lennard Shelf.
The platform/atoll complex outcropping in the Blendevale area comprises a series of lithologies of the Devonian Pillara Limestone and the fore-reef/marginal-slope equivalents belonging to the Sadler Limestone. The basin and inter-reef facies comprise the Gogo and Virgin Hills Formations, which are dominantly shales and siltstones with thin interbedded limestones. Geologists have sub-divided the Pillara Limestone into a number of mappable units which make up the limestone complex hosting the Blendevale deposit. The comprise the following, from the oldest,
Lower Platform/Bank Facies - made up of,
* Lower Flaggy Limestone (or Unit 1c), up to 90m thick - which rests unconformably on the irregular Precambrian granitic basement and comprises well bedded flaggy calcareous siltstone alternating with thin bedded stromatoporoid limestone which contains corals, gastropods and pelecypods.
* Lower Fenestral Limestone (or Unit 1b), 160 to 180m thick - overlies the Lower Flaggy Limestone at Blendevale and comprises a massive to thick bedded, pale grey 'birds-eye' fenestral micritic limestone.
* Lower Cyclic Limestone (or Unit 1a), up to 150m thick - which is an irregular cyclic unitcontaining upward shoaling cycles commonly 2 to 10m thick. The cycles are characterised by a basal dark carbonaceous shale/siltstone containing branching and colonial corals, overlain by micritic stromatoporoid limestone and capped with micritic fenestral limestone.
* Lower Stromatoporoid Limestone (or Unit 2), up to 140m thick - overlies the Lower Cyclic Unit and is a bedded stromatoporoid limestone, commonly containing algal oncolites and displaying residual stylolitic textures.
Upper Reef/Atoll Facies - subdivided into,
* Lower Reef Cap (or Unit 3), 5 to 15m thick - a massive white limestone containing brachiopods, laminar stromatoporoids and some Renalcis. It marks the first true reefal facies and forms the base of the main atoll complex which is fringed by a steep reef wall. It is also an important marker unit.
* Middle Stromatoporoid Member (or Unit 4), 10 to 130m thick - a medium grey, bedded, reef fringed, stromatoporoid limestone.
* Upper Fenestral Limestone (or Unit 5), 100 to 120m at Blendevale, but up to 240m thick to the east - a massive to thick bedded, clean, pale coloured micritic limestone with well developed fenestrae and interbeds of stromatoporoids.
* Upper Stromatoporoid Limestone (or Unit 6), up to 200m thick - a pale grey massive to crudely bedded stromatoporoid limestone containing a few beds of sedimentary breccia. Near the reef unit it inter-tongues with the Upper Reef Margin Limestone.
* Upper Reef Margin (or Unit 7) - which forma the steep reef margin, varying from 10 to 80m wide, and is a massive white limestone with Renalcis, laminar stromatoporoids and brachiopods. Towards the top this reefal facies regresses over the back-reef facies and forms an irregular cap to the atoll.
* Sedimentary Breccia (or Unit 8) - a thin irregular sedimentary breccia covering the atoll complex, largely composed of stromatoporoid limestone fragments, possibly derived from local active fault scarps.
* Gogo Formation (or Unit 9), up to 75m preserved at Blendevale - dark, thin bedded pelagic calcareous mudstone which overlies the preceding units and represents a drowning of the reef complex.
* Virgin Hills Formation (or Unit 10) - green and brown thin bedded mudstone with minor pelagic limestone bands.
The only post Devonian sediments in the Blendevale area are small outliers of Permian Grant Group quartz arenites filling localised karst depressions in the weathered limestones. It is probable that the Devonian-Permian unconformity surface was approximately parallel and not far above the current land surface.
The Limestone Billy Hills complex has been tilted and now dips to the north at 10š to 20š, such that to the north the complete sequence is preserved, with progressively older units are exposed to the south, until the garnitic basement is seen in outcrop. Only very mild folding is evident with the majority of dips being depositional or compactional, modified only by the northerly tilting.
The sequence is however cut by a series of north-south to NE-SW trending normal faults, which at Blendvale have resulted in the main structural feature a NNE-SSW trending graben. The west dipping Eastern Fault forms the eastern margin of the graben with a west-side-down displacement of 50 to 60m. The Western Fault which forms the western edge of the graben has a sub-parallel strike to that of the Eastern Fault, but is east dipping with an east-side-down displacement of 100 to 120m.
To the north, the Western Fault splays and a subsidiary fault, known as the F10, branches off and lies approximately 100m in the footwall of the Western Fault. Their combined vertical displacement in the north approximates the total movement on the Western Fault in central Blendevale.
To the south both the Eastern and Western Faults are truncated by the NE-SW trending Osprey-Spinifex fault system. In addition a further series of lesser faults have also been recognised within the Blendevale prospect area, including the F8 and F9 as shown on plan no. WAa ddd.
The main tectonic events influencing the Blendevale area appear to have accelerated towards the end of the platform limestone development during the early upper Devonian and probably continued into the lower Carboniferous. There is evidence that some of the faults in the Blendevale area were active during the atoll stage, as evidenced by local disconformities and unit thickness variations.
The mineralisation within the Blendevale deposit is principally hosted by tectonically controlled breccia zones formed by the bounding fault zones of the Blendevale Graben within platform/atoll facies carbonates. The fault controlled orebodies typically have a core of mineralised breccia with limestone clasts rimmed by sphalerite, galena, marcasite and sparry calcite. The brecciation is tectonically controlled, but is interpreted as being solution modified either prior to, or as an integral part of, the mineralising phase. Vein networks and fracture fillings with similar mineral assemblages are associated with and envelope the fault controlled mineralisation.
Disseminated sphalerite and galena within the host carbonates are known, but of limited economic significance. However the extensive distribution of surface geochemical anomalies over an area of more than 30 km2 with values of greater than 400ppm Zn+Pb in dense drainage sampling suggests wide spread dissemination of primary sulphides within the Pillara Limestone, possibly to levels of several hundred to several thousand ppm. Similarly the equally widespread IP anomalies indicate regionally dispersed pyrite/marcasite surrounding the ore. These disseminated sulphides appear to occupy volumes of rock which are several orders of magnitude greater than that of the higher grade mineralisation.
While the principal control on mineralisation appears to be structural, there is evidence that particular lithologies are more favourable than others. In particular, within the Western and Eastern Fault zones Unit 5 is host to extensive mineralised breccia and vein network developments, which also extend along the bed well into the hangingwalls of the fault zones (as shown on plan no. WAa ddd). These more 'stratabound' extensions are characterised by predominantly sub-vertical veins and fracture fillings, and minor disseminations, of sphalerite and galena.
The majority of the mineralisation is contained within the Blendevale Graben formed by the Eastern, Western and F19 Faults occurring as three mineralised zones which may be traced for 2.4 km within the platform and atoll carbonates of the Pillara Limestone. These mineralised zones are as follows,
* Western Fault Body - in which mineralisation extends for 2200 m along the fault and dips to the east at between 55 and 60°. Mineralisation is found over the full dip extent of fault zone within the carbonate sequence, although it only approaches economic grades over an interval of 150 to 250 m, mainly within units 4 and 5. The main higher grade mineralised zone plunges gently to the north at around 15°, following the trace of unit 5 in the hangingwall of the fault. The thickness of mineralisation varies substantially up to widths of 25 to 30 m, but averages 7 m at a cutoff of 3% Zn+Pb. The thicker mineralisation is concentrated in two intervals in the north and central sections with respective strike lengths of 300 m and 500 m. To the south the mineralisation gradually diminishes and fades out before reaching the Osprey Fault, while to the north grades and thickness decrease fairly rapidly after the thick high grade interval. Higher grades tend to be associated with the lower margins of the mineralised zone. The Western Fault Mineralisation accounts for a reserve of,
10.7 Mt @ 7.5% Zn, 2.2% Pb, 16 g/t Ag, 141 ppm Cd.
* Eastern Fault Body - parallels the strike of the Western Fault mineralisation and dips at 60° to the west with a strike length of 1000m. The mineralisation varies in width between 2.5 m and 15 m, averaging 5m, and also follows the plunge of unit 5 in the hangingwall of the fault. Poorly developed mineralisation may be followed to the north and south of the interval indicated above. The Eastern Fault Mineralisation accounts for a reserve of,
3.3 Mt @ 7.8% Zn, 2.4% Pb, 17 g/t Ag, 124 ppm Cd.
* F10 Fault Body - which is a splay of the Western Fault in the northern section of the Blendevale prospect. Mineralisation extends over a strike length of 700m where the fault cuts units 1 and 2. The main mineralisation has a vertical extent of around 100m in the plane of the fault and plunges to the north at around 15° paralleling the base of unit 2 within the hangingwall of the fault. In several drill intersection the mineralisation is massive with colloform sphalerite, galena, marcasite, melnikovite pyrite and sparry calcite, with no limestone clasts. This is believed to represent open space filling within the carbonates, with grades in excess of 20% and some as high as 35 to 40% Zn+Pb. At depth the mineralisation rapidly decreases to in width to a narrow fault zone less than 1m thick. Slickensides are common in the F10 Fault zone, and it may represent later stage movement than observed in the Eastern and Western Faults. Above the high grade zone mineralisation is more similar to that found in the Eastern and Western Fault zones with sulphide cemented limestone breccias and vein networks extending vertically over 150 to 200m. The F10 Fault Mineralisation accounts for a reserve of,
3.3 Mt @ 14.1% Zn, 4.4% Pb, 28 g/t Ag, 205 ppm Cd.
Mineralisation is also present on at least three other fault surfaces, the F1, F8 and F9 faults, all of which have low to moderate Zn/Pb grades associated with breccia in the upper atoll units and are similar in style to the Western and Eastern bodies. The mineralisation in these structures accounts for a reserve of,
2.7 Mt @ 5.1% Zn, 1.5% Pb, 8 g/t Ag, 90 ppm Cd.
For the Western, Eastern Fault and F10 mineralisalised zones the Zn:Pb ratio ranges between 3.2 and 3.4:1, although no distinct variation patterns are discernable. The Fe content of the Western and Eastern zones is relatively low, at approximately 2% Fe, but tends to increase with depth. In contrast the F10 Fault has >5% Fe and contains melnikovite pyrite.
The ore mineralogy consists of medium and coarse grained colloform sphalerite and intergrown galena with gangue marcasite and late stage sparry calcite. The sphalerite varies from pale brown low Fe varieties (<0.02% Fe) to dark brown (2.2 to 5.5% Fe). In some cases the sphalerite has been found to contain purplish patches which are believed to represent a hydrocarbon phase. The galena is coarser grained than the sphalerite and is normally found as subhedral to euhedral crystalline aggregates, although a finely banded colloform variety is lso present associated with sphalerite. Sulphides generally occur as rims to clean limestone clasts, or fill open space cavities as colloform layers. High angle veins or vein networks occur as sulphide filled fractures on the margins of the fault breccias, particularly in the hangingwall, where it does not attain high grades.
Sparry calcite occurs both as late stage open space filling and as an early phase prior to the sulphide mineralisation. Within the reefal units laminar calcite filled vugs and fractures are common. Dolomite is not recognised at Blendevale. Marcasite is found in association with base metals and as disseminations and vein networks in the adjacent lithologies with a preference for the lower units and the footwalls of the mineralised zones. It occurs in two forms, a medium to coarse grained subhedral to euhedral crystal aggregates and a fine grained colloform variety.
Cd is present associated with sphalerite and Ag occurs with both galena and colloform sphalerite. As, Sb and Bi concentrations are negligible, while minor barite and rare fluorite are associated with the Western Fault zone. Trace chalcopyrite has been intersected close to the basement below the main mineralisation.
The paragenesis is typically marcasite and/or colloform sphalerite, sphalerite with subordinate fien grained galena intergrowths, coarse subhedral galena and sparry calcite. Galena commonly occurs as coarse subhedra on the outer surface of the colloform sphalerite. More complex paragenetic sequences are obvious in the veins and breccias. Two and possibly three calcite phases are indicated and possibly more than one for sphalerite. Brecciation of the mineralisation is apparent in many intersections with clasts of colloform sphalerite, and disseminated sulphides forming the matrix to the breccia.
Coarse calcites from mineralised veins contain fluid inclusions with homogenisation temperatures of 45° to 90°C and salinities of 5 to 20 wt% NaCl, plus some contained hydrocarbons. Fluid inclusions from sphalerites indicate homogenisation temperatures of 55° to 110° (mean 80°) C and salinities of 17 to 27 wt% NaCl. Sulphur isotopes suggest morethan one S source, while the Fe sulphides appear to have a different S isotope signature, and possibly different source, to those of the galena and sphalerite. However the isotope studies suggest reduction of sulphates as the source.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1995.
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.
Christensen J N, Halliday A N, Vearncombe J R, Kesler S E 1995 - Testing models of large-scale crustal fluid flow using direct dating of Sulfides: Rb-Sr evidence for early dewatering and formation of Mississippi Valley-type deposits, Canning Basin, Australia: in Econ. Geol. v90 pp 877-884|
Murphy G C 1990 - Lennard Shelf Lead-Zinc deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1103-1109|
Symons D T A and Arne D C 2005 - Paleomagnetic constraints on Zn–Pb ore genesis of the Pillara Mine, Lennard Shelf, Western Australia: in Mineralium Deposita v39 pp 944-959|
Vearncombe J R, Dentith M, Dorling S, Reed A, Cooper R, Hart J, Muhling P, Windrim D, Woad G 1995 - Regional- and prospect-scale fault controls on Mississippi Valley-type Zn-Pb mineralization at Blendevale, Canning Basin, Western Australia: in Econ. Geol. v90 pp 181-186|
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