Agnew - Waroonga-Kim-Emu, Redeemer, Crusader-Deliverer-Cox, Genesis-New Holland, Songvang, Lawlers
Western Australia, WA, Australia
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The Agnew cluster of gold deposits are located 320 km NNW of Kalgoorlie, 128 km north-west of Leonora, 23 km west of Leinster, in the Kalgoorlie Terrane of the Archean Yilgarn Craton (#Location: 27° 59' 8"S, 120° 29' 49"E).
Gold was first discovered at Agnew in 1895, with sporadic production to 1948. Western Mining Corporation began exploring the district in 1948, but did not delineate sufficient resources to commence full scale mining until 1986. A number of separate deposits were subsequently discovered or purchased to maintain mill throughput, including Redeemer (1985), Cox-Crusader (1987) and Genesis (1990), New Holland (1991) and Fairyland (1997) before underground mining commenced at New Holland in 1998. In 2001 the Agnew operation was acquired by Gold Fields Australia and continuing discoveries and extensions have maintained operational resource levels to the present. These included the Kim South lode at Waroonga in 2002, and Songvang OP, with production commencing in 2002 and 2004 respectively. Further discoveries were made at Fairyland UG (2009) and FBH (Fitzroy, Bengal and Hastings) and Cinderella at Waroonga in 2012. The neighbouring Lawlers mine was purchased fron Barrick and amalgamated into the Agnew operation in 2013.
These deposits fall within the Kalgoorlie Terrane (or Norseman-Wiluna Greenstone Belt) of the Eastern Goldfields in the Archaean Yilgarn Craton of Western Australia. They are hosted by the 2.72 to 2.69 Ga granite-greenstone succession of the Kalgoorlie Terrane, which is situated immediately to the east of the Proto-Yilgarn craton, represented by the late Mesoarchean Youanmi Terrane, separated from the Kalgoorlie Terrane by the major Waroonga Shear (or Ida Fault), 2 to 4 km to the west of Agnew. The Kalgoorlie Terrane is the westernmost division of the Neoarchean Eastern Goldfields Superterrane. This succession is composed of abundant tholeiitic and komatiitic volcanic rocks, chert, sulphidic and albitic sedimentary rocks, and a chain of discrete felsic volcanic centres, all of which have been metamorphosed to upper greenschist to lower amphibolite metamorphic facies.
The Agnew deposits are hosted by the Agnew-Wiluna Greenstone Belt, the northernmost seqment of the Kalgoorlie Terrane. It has a coherent basal stratigraphy of metamorphosed basalt, gabbro, dolerite and ultramafic flows of the Lawlers Greenstone Formation, which are faulted or in unconformable contact with the overlying ultramafic- and mafic-clast conglomerates, arenites and turbidites of the Scotty Creek Formation. The bulk of the deposits are concentrated on the steep western limb of the large, open, 30°N plunging Lawlers Anticline. Gabbro of the Lawlers Greenstone Formation has been dated at 2795±38 Ma, while a tonalite intrusion in the core of the anticline has been dated as 2652±20 Ma. Late leucogranite, which succeeds the main greenschist facies D1 deformation has an age of at 2576±14 Ma. The western limb of the Lawlers Anticline is cut obliquely by the prominent NNE trending, D2 Emu Shear corridor, which separates the meta-sedimentary and the meta-volcanic sequences to the west and east respectively. This shear corridor also hosts many of the main Agnew deposits distributed over a 15 km interval, including the Genesis-New Holland; Waroonga-Emu-Kim-Rajah-Cinderella-Main-Fitzroy-Bengall-Hastings-New Woman; Redeemer; Crusader-Claudius-Pilgrim-Deliverer; and Songvang deposit complexes. However, economic mineralisation also occurs on the eastern limb (e.g. Vivien), in axial planar positions to the north (e.g., Cams) and in the adjacent Mount White Syncline (e.g. Maria).
Gold mineralisation dominantly occurs in quartz breccia lodes, quartz tension-veins and disseminated with arsenopyrite-pyrite-biotite alteration assemblages. These mineralisation styles are principally controlled by competency contrasting lithological contacts. High-grade ore shoots plunge steeply to the north in dilational zones formed at the intersection of steeply dipping north-south axial planar structures and these contacts. The stronger the contrast in rock competency, the greater the potential for mineralisation. Mineralisation is not restricted to any particular section of the sequence, but is distributed throughout the stratigraphic column. To the south, at Songvang, mineralisation is concentrated at the contact between granitic rocks and the basal basalts. The Crusader complex deposits are localised at the contact between the basal basalts and the overlying ultramafic suite, and locally within the latter. The Redeemer and Waroonga lodes are mainly found at contacts on the margins of ultramafic- and mafic-clast conglomerates at the base of the Scotty Creek Formation, whilst, further north at New Holland-Genesis competency contrast layers in the overlying sandstones localise lodes.
Examples of the setting at some of the key deposit groups are as follows:
The Redeemer Main Lode which is hosted in a sequence dominated by lower Scotty Creek Formation meta-sedimentary rocks. The succession commences with up to 30 m of basal ultramafic-clast conglomerate, composed of strongly deformed ultramafic, mafic and rare felsic clasts set in a matrix of well foliated chlorite-talc-actinolite. This is overlain by 50 to 60 m of predominantly mafic paraconglomerate containing pebble to boulder size ultramafic, mafic and granitic clasts in a clastic mafic matrix. These units are capped by an upward-fining sandstone-greywacke sequence. The Redeemer Main Lode is hosted by a structurally complex open flexure within overturned mafic-clast conglomerate. It has a 150 m strike length, an average width of 50 m, and persists down a 60 to 70°N plunge for >1 km. Mineralisation occurs as lenses within the mafic-clast conglomerate, concentrated proximal to both the ultramafic-clast conglomerate and sandstone contacts, and joined by cross lodes along anastomosing shear zones within the mafic conglomerate. A steeply north plunging foliation that parallels the Lawlers Anticline axis, is overprinted by a subordinate NW-trending foliation. The lineation formed by the intersection of these two foliations parallels the plunge of the Redeemer Main Lode. Gold mineralisation is accompanied by variably intense biotite-amphibole alteration and minor secondary sulphides, silica and/or carbonate minerals. Secondary magnetite is found within the core of the lodes. Gold often occurs as <100 µm grains interstitial to, or within coarse actinolite and biotite, as well as in association with bismuth tellurides, arsenopyrite, chalcopyrite, molybdenite and scheelite. The zoning of mineralisation comprises an inner biotite-amphibole core, an enveloping partially mineralised, amphibole-biotite-chlorite zone and peripheral, weakly mineralised amphibole-chlorite.
The Redeemer Zone 2 and 3 Lodes are hosted by two felsic conglomerates of the Scottys Creek formation, bounded between arenite and greywacke units and are within 100 m to the west of the Main Lode. The Zone 2 conglomerate averages 5 m in thickness and comprises granule to cobble clasts of tonalite within a quartzo-feldspathic matrix, with lesser interstitial biotite-chlorite-actinolite alteration. The conglomerate is discontinuous and grades laterally into medium to coarse-grained sandstone. The Zone 3 conglomerate, which averages 12 m in thickness, has a similar matrix to the Zone 2 unit and clasts of clasts of both granite and tonalite. Both lodes are related to flexures that are analogous to that at Redeemer Main Lode and have the same plunge to the north. They are associated with semi-brittle deformation formed by dilational shearing. The gold mineralisation is spatially with arsenopyrite-bearing quartz veins and are is associated with an increase in hydrothermal alteration.
The Waroonga Complex, comprises two strings of steeply NNW plunging lodes, 7 km north of Redeemer. This complex of deposits appears to be developed within Scotty Creek mafic- and ultramafic-clast conglomerates, similar to Redeemer. The largest, the Kim Lode, extends from immediately below the northern floor of the Waroonga open pit to >1400 m below surface, with a strike length of 100 to 250 m. A second parallel string of discontinuous and branching lodes including Rajah, Main North and South, Hastings, Bengal and Fitzroy occurs below the southern end of the same open pit, and have been traced to depths of >1 km below surface.
The Crusader Complex lodes, which are developed 2 to 3 km south of the Redeemer deposits, are hosted by mafic and ultramafic rocks of the Lawlers Greenstone Formation. The host sequence comprises a basal tholeiitic pillow basalt with interflow sulphidic carbonaceous shale and fine dolerite sills. It is overlain by 40 to 60 m of ductile ultramafic rocks, chiefly serpentinised and/or carbonated dunite and orthocumulate peridotite. These are followed by 30 to 110 m of ultramafic komatiite flows on the opposite side of the 10 to 14 m thick Pilgrim Shear. The latter structure includes a wedge of intense talc-magnesite-carbonate-biotite alteration. The komatiite flows are succeeded by an ~80 m thick, suite of high-magnesium basalt with interflow fine marine sedimentary beds. The sequence is capped by 110 m of sheared mafic-clast conglomerate, the southern continuation of the Redeemer Main Lode host rock. Gold is predominantly localised within the basal tholeiitic basalt, at or close to its upper contact with the cumulate ultramafic unit, and is commonly associated with interflow sedimentary rocks and dolerite. It occurs as fine to very fine disseminations, with associated alteration comprising epidote-magnetite-chlorite±actinolite-biotite, consistent with a formation temperature of >400°C. Compressional tectonics produced prominent flexures, concentrated at structural intersections with rheological contrast contacts. Multiple, lensoid lodes are developed within the complex with thicknesses of a few to 30 m and individual down plunge extents that vary from ~70 to ~300 m.
Cox-Crusader is 3 km south of Redeemer, on the western limb of the Lawlers Anticline and has a 20 to 30°N trending plunge associated with a flexure in the ultramafic rock-basalt contact. The lodes are 2 to 30 m thick and extend 50 m down dip, with mineralisation in three parallel lodes, with high grade pods in flexures, linked by lower grade limbs. The orebody contains very fine grained disseminated native gold with associated epidote-magnetite-chlorite±actinolite biotite alteration.
The Cox Deliverer and Pilgrim lodes are 300 m north of the main Cox pit and are hosted by the basal tholeiitic basalt. The Deliverer orebody comprises three high grade shoots near the ultramafic-basalt contact, plunging 45°NW, related to a prominent flexure in the westerly dipping contact. Lodes are lensoid to elliptical and 5 to 12 m thick and 70 m down dip, while the zone persists more than 300 m down plunge. It is associated with chlorite-epidote-magnetite alteration. The Pilgrim orebody is in the ultramafic rock hangingwall of the Deliverer lodes, within the heavily chloritised Pilgrim Shear. The ore zone varies from 5 to 7 m in true thickness and dips 60°W.
The Cox Claudius Creek lode is 700 m north of the main Cox open pit, and is hosted by the basal tholeiitic basalt, paralleling a flexure in the basalt-ultramafic contact which plunges 30°NNW. It comprises five parallel lodes, 3 to 8 m thick. Mineralisation comprises very finely disseminated native gold, with amphibole-chlorite-biotite alteration.
The Emu deposit is around 7 km north of Redeemer and hosted by the Emu Shear zone. The mine sequence starts with i). an ultramafic flow, ii). 20 to 40 m ultramafic conglomerate, iii). flow banded felsic porphyry and volcanics, iv). ultramafic flow sequence, v). ultramafic conglomerate, vi). a polymictic ultramafic conglomerate, and vii). arenite and wacke.
An approximately 100 m wide zone of shearing hosts the mineralisation which is present as two lode zones, with a cross lode between the two. The Hangingwall Lode dips at 60° W at the contact between the arenite and wacke unit and the underlying polymictic ultramafic conglomerate and is 5 to 30 m wide over a strike length of 1 km. It comprises 0.1 to 1 m thick quartz veins and/or vein stockworks with pyrite, pyrrhotite and actinolite-chlorite ± talc selvages.
The Footwall Lode is 600 m long and 5 to 20 m wide at the schistose contact between the polymict ultramafic and the underlying ultramafic conglomerates, where a thin felsic unit is also developed. This zone is flanked by amphibole-chlorite-talc ±quartz and arsenopyrite alteration.
The Hunters Lode is hosted by the lower ultramafic conglomerate which is actinolite altered. This lode is 0 to 35 m east of the Footwall Lode and grades into the latter to the north. It has a strike length of around 350 m and is 10 to 20 m wide.
The 120° trending Cross Lode connects the Footwall and Hangingwall lodes and is 5 to 15 m wide and is generally high grade.
A 100 000 tonne supergene zone was mined at Emu with a mean grade of 28.5 g/t Au.
Total production from Emu to 1989 was - 2.257 Mt @ 6.75 g/t Au for 15.26 t Au.
The current reserve and resource figures for the Agnew deposits at 31 December, 2006, quoted by Gold Fields in 2007 were:
measured + indicated + inferred resource - 15.2 Mt @ 5.9 g/t Au, for 90 t of Au, including,
proved + probable reserve - 3.1 Mt @ 6.7 g/t Au, for 19.7 t of Au.
Total Mineral Resources at December 2016 were 12.5 Mt @ 5.31 g/t Au, inclusive of Ore Reserves of 3.0 Mt @ 5.39 g/t Au. Total historic production has been in excess of 163 t of Au, which, with remaining reserves comprises an endowment of ~230 t.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 2/8/2017
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Aoukar N, Whelan P 1990 - Emu Gold deposit, Agnew: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 323-329|
Broome J, Journeaux T, Simpson C, Dodunski N, Hosken J, de Vitry C and Pilapil L, 1998 - Agnew gold deposits: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 161-166|
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