Manxman, Joes Dam
South Australia, SA, Australia
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The Manxman A1 magnetite body, which contains anomalous copper, gold and uranium is located ~100 km SSE of Coober Pedy, 20 km WNW of the Prominent Hill IOCG Cu-Au deposit, and 650 km NNW of Adelaide in northern South Australia (#Location: 29° 40' 12"S, 135° 23' 30"E).
It is located in the Joe's Dam Sub-domain, 5 km from the southern edge of the Mount Woods Domain (MWD) on the northeastern margin of the preserved Gawler craton, and also lies within the Olympic IOCG Province that encloses all of the significant IOCG and related deposits of the craton.
For details of the geology and Sub-domains of the southern half of the Mount Woods Domain, see the description in the Prominent Hill record, and for background on the Olympic IOCG Province see the Gawler Craton and Olympic IOCGU Province record.
The Joe's Dam Sub-domain is characterised by extremely high amplitude magnetic lineaments on its margins, more subtle concentric magnetic zoning in its centre, and a very pronounced and complex gravity signature, largely reflecting widespread magnetite rich intercalations within the basement metasedimentary quartzo-feldspathic gneisses, and mafic intrusive rocks belonging to the White Hill Igneous Complex and related intrusive events, as described below.
The Manxman A1 deposit is reflected by a ~2 x 1 km, tear-shaped magnetic anomaly (tapering to the west) on the southeastern extremity of a ~7 km long, WNW-ESE trending, ~1 km wide magnetic ridge, as defined by the 3000nT anomaly. The A1 anomaly has a peak of over 6000 nT, and is separated from the remainder of the ridge by a deep saddle. The peak of the A1 magnetic high coincides with a ~4 milligals above background gravity anomaly (Finch, 1986; Sugden 1991). It is one of a series of near coincident bulls-eye magnetic and gravity anomalies along the ridge, and surrounding the A1 anomaly, which have been numbered A1 to A6. A2, A3 and A4 are 2 km south, 1.25 km NNE and 2.4 km NW of A1 respectively. A4 is on the eastern end of the main magnetic ridge, while A6 is a small negative response, immediately to the NE of A4. Two other anomalies, Joe's Dam and Joe's Dam West are 5 km ESE and 3 km east of A1 respectively. All have been drill tested, including the peak in the western half of the main magnetic ridge. Another cluster of magnetic and gravity anomalies 2 to 5 km to the NE of Manxman A1 is referred to as Manxman B.
Some 7 diamond core holes were drilled into the Manxman A1 magnetic feature, through 80 to 100 m of cover, and completed to depths of 226 to 595 m, over an east-west interval of ~1 km, testing a width of up to ~600 m perpendicular to the axis of the anomaly.
The entire area of these mineralised systems is masked (with the exception of a few localised, deeply weathered outcrops) by 80 to 100 m of cover, mainly comprising the glaciogene, lacustrine and marine sedimentary sequences of the Permian Boorthana Formation, Jurassic Algebuckina Sandstone, and the Cretaceous Cadna-Owie Formation and Bulldog Shale. In the Manxman A1 area, only the buff to white silts and underlying black carbonaceous claystones of the Bulldog Shale are present. This younger cover is underlain by a 10 to 20 m thick palaeoregolith developed within the underlying Proterozoic basement rocks. Consequently all of the basement geology is based on drill intersections testing the mineralised systems and geophysical anomalies, and hence is skewed towards altered rocks.
The basement encompass a range of medium to high grade metamorphic rocks of sedimentary and igneous provenance, including leucocratic gneiss, porphyroblastic garnet gneiss, "banded iron formation", cordierite rich "granofels" and gneiss, and metaconglomerate (Freeman and Tomkinson, 2010), which have been grouped into a suite of pink leucocratic gneisses and a sequence of metasedimentary granulites (psammitic and pelitic quartzo-feldspathic gneisses). The metasedimentary granulites include both non-magnetic metasediments with defined intercalated magnetic psammite ("iron formations") and a terrane of gneisses that are pervasively enriched in magnetite. The metasedimentary granulite suite includes meta-conglomerates which overlie the pink leucocratic gneisses. U-Pb zircon analyses on cordierite-garnet-bearing pelite and magnetite psammite surface samples to the north of the Manxman area suggest maximum depositional ages of 1751±6 Ma and 1752±6 Ma respectively (Jagodzinski et al., 2007).
Within the MWD, the gneisses that are pervasively enriched (regionally altered ?) in magnetite, as compared to non-magnetic sedimentary gneisses with defined iron formation lenses, are restricted to a WNW-ESE elongated 30 x 15 km terrane that is confined by the major Skylark Fault to the north, and the Southern Overthrust to the south (Betts et al., 2003). The latter structure marks the southern margin of the MWD, over which it has been thrust onto the latest Archaean to earliest Palaeoproterozoic metamorphic rocks of the Christie Domain. The Manxman mineralised system is located towards the southeastern extremity of this terrane of magnetic gneisses. It is uncertain whether this magnetic terrane is a reflection of the composition of the original protoliths, or is due to metasomatic alteration, or a combination of the two.
Peak metamorphism of the metasediments reached upper amphibolite to lower granulite facies (Ambrose and Flint 1981; Betts et al., 2003). U-Pb SHRIMP dates on a migmatitic segregation related to this event suggests a melting age of 1736±14 Ma (Finlay 1993; Daly et al., 1998).
Both the leucocratic gneisses and metasedimentary granulites were intruded by the syn-metamorphic Engenina Adamellite (~1691±25 Ma; Finlay 1993; Daly et al., 1998), which have only been weakly deformed and metamorphosed, with a weak to moderate, biotite-defined foliation, similar in orientation to the metamorphic fabric in surrounding metasedimentary granulites (Chalmers, 2007). The bulk of the mapped Engenina Adamellite is confined to the terrane defined by the pervasive magnetic gneisses described above (Betts et al., 2003).
All of these rocks have been intruded by the un-metamorphosed and un-deformed Hiltaba Suite equivalent Balta Granite which crystallised at 1584±18 Ma (Fanning 1997), and the similarly un-metamorphosed extensive mafic White Hill Igneous Complex, predominantly of gabbroic composition, with fractionated cumulate phases including gabbro, hypersthene gabbro, oxide-rich norite, oxide-apatite-rich gabbro and orthopyroxene-magnetite-ilmenite-rich cumulates (Cibej and Freeman, 2009). On the basis of the lack of deformation, relationship to the Balta Granite and Mount Woods metamorphics, the 1587±4 Ma dating (U-Pb; Jagodzinski, 2005) of similar mafic intrusives nearby, and an age of 1582±5 Ma for veining cutting gabbro-norite of the complex (Belperio et al., 2007), Cibej and Freeman (2009) conclude that the White Hill Igneous Complex is of Hiltaba age.
All of these rocks have been intruded by the magnetic Neoproterozoic Gairdner Dyke swarm which exploit the pre-existing WNW-ESE tectonic fabric of the region.
The rock-types encountered within the overall Manxman-Joe's Dam mineralised systems have mostly been strongly altered, often resulting in the obliteration of any textures that might identify the protolith. The following lithologies have been encountered in Manxman and Joe's Dam drilling:
Felsic gneisses - composed of pink to green to green-grey fine-grained, silicified and chloritised, K feldspar-quartz gneiss with or without magnetite-rich foliations/bands, or as medium-grained, banded biotite-K feldspar±magnetite gneiss to quartzite. These rocks have textures that vary from gneisses to granulites. These gneisses are sometimes interbedded with felsite. They vary from granoblastic to weakly to well foliated textures. Quartz and feldspar often occur as either euhedral porphyroblasts or as anhedral grains, with the mafic minerals in the interstices.
Acid volcanics/felsite - banded and foliated, very siliceous, brick-red to grey-green quartz-feldspar felsite, grading to a banded/laminated and foliated, highly siliceous pink (potassic altered) rock that is also described as a quartzite. In places the highly potassic altered and siliceous rock is also impregnated with fine magnetite to become a magnetite quartzite.
Adamellite - occurring as an orange to pink to grey, fine- to medium- to coarse-grained, weakly foliated, granoblastic, quartz-K feldspar-plagioclase (variably altered to K feldspar) porphyry to felsic-gneiss with 2 to 15 mm K feldspar and ~4 mm quartz and plagioclase phenocrysts and <5% mafic minerals (chlorite, hematite, magnetite). It can also display an orbicular texture. Within the mineralised system, this rock type varies in composition and crystal size and is largely subjected to magnetite, chlorite and potassic alteration.
Gabbro - only encountered at Joe's Dam, where it comprised a coarse- to medium-grained, unaltered gabbro with accessory magnetite, biotite, apatite and ilmenite. In places magnetite segregations within the gabbro were as high as 85% of the rock.
Felsic breccia - composed of angular to sub-angular to sub-rounded to rounded clasts that may include one or more of felsic gneiss, adamellite, felsite/quartzite and lesser mafic rocks, that range from grit sized to 4 to 5 cm in diameter, set in a matrix that may vary from fine- to medium-grained quartz and felspar to a pink silica, with common chloritisation. The matrix may contain dispersed magnetite, grading to massive fine grained or crystalline magnetite with or without included clasts. The clasts may be altered and include segregations of magnetite. The magnetite-rich matrix can carry variable amounts of sulphide (pyrite and chalcopyrite), and is often cut by chlorite, magnetite and/or calcite veining. In places the finer breccias have the appearance of meta-grit to meta-tuff. These breccias range from weakly fractured and magnetite veined felsite/quartzite or gneiss to well developed breccias.
Magnetite, magnetite-chlorite and magnetite-sulphide breccia - comprises clast- to matrix-supported breccia in which the matrix is dominantly composed of magnetite with chlorite (and sometimes amphibole), and varying amounts of accompanying sulphide. The sulphides comprise 0 up to 5 to 20%, to local massive sulphide over tens of cms, and are mainly pyrite, lesser chalcopyrite, and some pyrrhotite, which are finely disseminated, or as coarse blebs or fine stringers. The clasts are as described for the felsic breccia above that may be up to 20 cm across, and may also include quartz-magnetite. They may be strongly chlorite or potassic altered and be cut by magnetite veins. In some places, the clasts may be zoned, with outer margins of amphibole-chlorite surrounding a potassic rim which in turn encloses a core of less altered felsite/gneiss or 'quartzite'. The percentage of clasts varies, with intervals of one to several tens of metres of massive magnetite with no or few (or ghosts of) clasts. These breccias grade into massive magnetite and magnetite-sulphide rocks where the clasts are observed to be replaced by and assimilated into the magnetite-sulphide matrix. The most strongly altered of these magnetite breccias have fluidised, contorted textures. These breccias have been intersected over intervals of more than 200 m.
Hematite-chlorite breccia to hematite-magnetite-chlorite breccia - which is mostly found in the upper 10 to 20 m below the unconformity at Manxman A1. It contains angular to rounded clasts of chloritised, silicified and potassic altered felsite in a matrix of chlorite and chloritic clays, hematite and manganese oxides, with increasing magnetite at depth. This breccia is cut by carbonate veining and jarosite boxworks.
Hematite-magnetite-chlorite-quartz breccia - that occurs well below the base of palaeoweathering. In this breccia, medium crystalline hematite may comprise around 80% of the rock, and appears to replace and pseudomorph magnetite. Adjacent gneisses and felsites may also be similarly hematite altered with associated thin chlorite and quartz stringers.
Dolomitic breccia - only encountered in the lower levels of Manxman A2, below the mineralised zone, where it is matrix supported, composed of small (generally 1 to 2, but may be 5 to 10 cm) angular clasts of occasionally altered (potassic, with minor chlorite), banded gneiss in a dolomitic carbonate matrix.
The mineralised system at each of the anomalies may be summarised as follows:
Manxman A1 - Modelling of the magnetic and gravity data over the widest and most intense point of the anomalies towards the eastern end of the tear shaped anomaly suggested the presence of two, parallel, steeply dipping, generally east-west trending, prisms of rock with a high density and magnetic susceptibility, each ~200 m wide and separated by a gap of ~100 m. The detailed magnetic data also suggested the magnetic anomaly was hook-shaped, with the main east-west-trending southern limb hinging to the east to form a short NW-SE trending northern limb (Mackee 1986, reported in Finch 1986), considered to represent the nose of an F2 fold (Hampton, 1997).
North-south oriented drilling over the peaks of the anomalies revealed a mineralised system with a width of ~600 m (with a possible central gap occupied by poorly mineralised felsic gneisses), composed of interspersed thick intersections of magnetite-sulphide breccias and magnetite-sulphide altered breccias. This mineralised zone was traced over a strike length of ~1 km, with a width of ~150 m in the western-most drill section.
Within the Manxman A1 deposit area, the mineralised system is dominantly represented by magnetite-sulphide breccia and magnetite-sulphide altered breccias, with only a small proportion of hematite-magnetite breccias. All of these breccias normally carry between 0.15 and 0.23% Cu throughout, with accompanying 0.04 to 0.13 g/t Au, e.g., DD88EN43 (declination -60°N, azimuth north), through the steeply south-dipping mineralised zone, with 402 m @ ~35% Fe, including 287 m @ 0.23% Cu, 0.06 g/t Au, 32% Fe; or DD88EN61 with a similar declination and azimuth which intersected two separate zones of 89.5 m @ 0.17% Cu, 0.13 g/t Au, >40% Fe, and 84.8 m @ 0.17% Cu, 0.08 g/t Au, >20% Fe. These long low grade zones encapsulate thin, very sparse local zones of a few metres @ 0.8 to 1.66% Cu, 1 to 10 m @ 0.5 to 1 g/t Au and similar widths of weak ~200 ppm U. The hematite-rich breccias appear to carry the same tenor of mineralisation as those in which magnetite dominates.
Mineralisation accompanies magnetite and comprises 5 to 20%, to local massive sulphide over tens of cms. It is mainly pyrite, with lesser chalcopyrite, and some pyrrhotite, which are finely disseminated, or as coarse blebs or fine stringers. Uranium is present as sparse uraninite. Bulking of analytical results suggest that the elevated gold levels (as indicated above) decrease from north to south across the mineralised system. The volume of 30 to 50% magnetite within the system is appreciable, and the deposit has been promoted as a low grade magnetite iron ore deposit.
On the basis of the 7 drill holes into Manxman A1, it would appear to be a body that is ~1 km long from east to west, developed over a width of up to 600 m to the east, tapering to ~150 m in the last drill hole in the west. Mineralisation has been traced to depths of more than 350 m below the unconformity, and appears to take the form of two parallel, steeply south dipping prisms separated by a narrow zone of less mineralised felsic gneisses.
Hampton (1997) records the alteration assemblages at Manxman A1 as essentially including an early albite, ferro-actinolite, scapolite and diopside, followed by K feldspar, biotite, with associated chlorite, apatite, epidote, allanite, quartz, carbonate and fluorite.
Manxman A2, A3, A4, A6 - While similar magnetite-sulphide breccias, magnetite altered breccias and host rocks are found at all of the other Manxman anomalies listed above, only Manxman A1 has high background mineralised (but sub-economic) zones. In all of the others, the assay values for the magnetite and sulphide-bearing breccias are generally <50 ppm Cu (with some intervals of 50 to 200 ppm) and <0.05 ppm Au. The best results outside of Manxman A1 was from Manxman A2 in drill hole DD86EN37, which included a 0.5 m interval of 1.26% Cu for from 184.5 m, and a 5.03 m width of 2.05 g/t Au from 330 m, and 8.4 g/t Au over 0.52 m from 335.41 m.
The sequence encountered at Manxman A2 comprised psammitic (quartz-feldspar-magnetite±biotite) and pelitic gneiss (biotite and/or K feldspar-magnetite gneiss) dominating from the unconformity at 132 to 178.3 m. Below that depth, the sequence was characterised by alternating bands of pelitic (magnetite-biotite-amphibole) gneiss and intervals of massive hematite/magnetite to 286 m, then an interval of psammitic gneiss, followed by alternating intervals of well banded psammitic gneiss and massive intermixed massive magnetite and hematite to 358 m. The remainder of the hole, to the total depth of 372 m was occupied by a dolomitic breccia, as described above.
Manxman B - a drill hole testing the main coincident magnetic and gravity anomalies passed through the unconformity into basement at 62.4 m, where it encountered sugary psammitic gneisses which were often hematitic and chloritic, including a 5.3 m interval of clast- to matrix-supported hematite-quartz-feldspar breccia. The breccia was clay altered, and the clasts, as well as the matrix, have been pervasively hematised and totally replaced, with little or no evidence remaining of any form of open space fill. Textures indicate a polymictic protolith, possibly a shear breccia (Freeman and Tomkinson, 2010). This sequence persisted from the unconformity to 103 m. The next 23 m were occupied by a massive hematite zone, with a second similar intersection from 189.3 to 202.2 m, separated by more sugary textured gneissic hematitic quartzite. From 202.2 m, the hole passed through felsic gneisses before encountering a third hematite zone from 245.6 and 250.4 m, then into felsic gneisses, pegmatites and adamellite. The second massive hematite band shows evidence that the hematite replaced earlier magnetite (Finch 1987), supported by the observation that then hematite sections are magnetic (Freeman and Tomkinson, 2010). The massive hematite sections were locally recrystallised into coarse grained aggregates and veins of coarse grained to steely crystalline hematite up to 30 cm thick, and has no evidence of a deformational fabric, suggesting crystallisation after peak metamorphism. Outside of the massive accumulations, hematite is present as bands that follow gneissosity (Finch, 1987; Freeman and Tomkinson, 2010).
Throughout the drill hole, copper values were mostly <20 ppm, with a few elevated values of up to 50 ppm. All gold values were below the limit of detection of 0.05 ppm, and uranium was below 10 ppm throughout. The hematite zones carried 33 to 45% Fe (Finch, 1987).
Joe's Dam and Joe's Dam West - A single drill hole into Joe's Dam West testing the coincident gravity and magnetic responses passed through 100 m of Cretaceous cover, and passed into a a succession of quartz-feldspar-magnetite-biotite microgneiss, believed to represent a metamorphosed pelitic protolith. From the unconformity to a depth of 270 m, magnetite is common, forming up to 80% of the rock, accompanied by sulphides (mainly pyrite and chalcopyrite) which may locally reach 20%. Chalcopyrite is often intricately intergrown with silicate minerals. Anomalous metal contents of 34 m @ 0.39% Cu, from 186 to 220 m, and 10 m @ 0.28 g/t Au from 212 to 222 m were encountered in an upper zone, and 6 m @ 0.68% Cu from 272 to 278 m, and 4 m @ 0.29 g/t Au from 272 to 276 m defined the lower zone. Below 270 m, the magnetite content reduced substantially to only be a few grains distributed through the rock fabric. Unlike Manxman A1, copper values outside of these two zones above the lower zone was generally low, mostly <100 ppm, with a few values up to 300 ppm. Below the lower zone it quickly dropped to <10 ppm Cu.
Drilling at Joe's Dam encountered massive magnetite (locally containing up to 20% sulphide) interspersed with both magnetite altered felsite of the Mount Woods Metamorphics, and gabbro, presumed to belong to the White Hill Igneous Complex. The bulk of the intersections in the five drill holes was of magnetite-rich gabbro. The best assays in the magnetite were from 375 to 1000 ppm Cu over intervals of 2 to 12 m, separated by low background values. Hematite dominated the top 4 m in each hole that passed into 'mineralisation' immediately below the unconformity (Mackee, 1984).
Petrologic studies in the vicinity of Joe's Dam (reported in Freeman and Tomkinson, 2010) suggest the alteration paragenesis comprised: i). Early Na metasomatism, in the form of (grey-white) albitised metasediments occurring as selvages extending from brittle fractures, but preserving foliation, accompanied by hematite dusting of the albite veining; ii). Reopening of the same fractures, and open space fill of diopside, actinolite-tremolite and minor magnetite cutting the early albite selvages; iii). Additional opening of existing, and development of new, fractures which are infilled with massive to coarsely crystalline magnetite veining, accompanied by magnetite alteration selvages developed out from the new fractures; and iii). Late stage re-brecciation of the magnetite fill and deposition of biotite/phlogopite and/or K feldspar with pyrite±chalcopyrite, and anomalous Au.
The rocks of the Manxman area represent a deeper level within the MWD at the time of mineralisation than in the vicinity of Prominent Hill. At the latter, the mineralisation was emplaced within Gawler Range Volcanics (GRV), which had only recently been extruded at the surface, with the mineralisation being accompanied by dominant hematite-sericite alteration and only lower greenschist grade metamorphism. In contrast, the hosts at Manxman have been metamorphosed to upper amphibolite to granulite facies with no GRV equivalents preserved above the mineralised interval. The mineralisation is also accompanied by a magnetite-rich assemblage and by sodic, potassic and chloritic alteration suggesting a deeper regime. Although the Manxman mineralised systems has been locally overprinted by hematite alteration, this does not seem to have resulted in the addition of further Cu or Au.
The most recent source geological information used to prepare this summary was dated: 2010.
This description is a summary from published sources, the chief of which are listed below.
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