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Carr Boyd Rocks

Western Australia, WA, Australia

Main commodities: Ni Cu
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Carr Boyd Rocks was a gabbroid intrusive associated Ni-Cu deposit 80 km NNE of Kalgoorlie in Western Australia.

The deposit was mined from July 1973 to September, 1977.

Mineralisation is associated with a layered mafic-ultramafic complex that intrudes a Neoarchaean volcano-sedimentary sequence. That sequence has been divided into three cycles of mafic to felsic magmatism, represented by five formations, as follows from the base (after Purvis et al., 1972 and Schultz 1976):
• Cycle1 - Morelands Formation mafic volcanic rocks; and the Gindalbie Formation felsic volcanic and sedimentary rocks;
• Cycle 2 - Mulgabbie Formation mafic volcanic rocks; and the Gundockerta Formation felsic volcanic and sedimentary rocks;
• Cycle 3 - Kalpini Formation mafic volcanic rocks.
More recent interpretations equate these units with stratigraphy in the Kalgoorlie-Kambalda area as follows (see the Kalgoorlie Terrane section of the Yilgarn Craton overview record for stratigraphic detail). The Cycle 1 Morelands and Gindalbie formations are basement to the 2715 to 2690 Ma Kambalda Sequence which is correlated with the overlying Mulgabbie Formation. The succeeding Gundockerta and Kalpini formations are equivalents to the 2690 to 2660 Ma Kalgoorlie sequence, generally corresponding to the Black Flag and Merougil groups respectively (Tripp, 2013).

The host Carr-Boyd layered complex intrudes rocks of the Morelands Formation, and is, in turn intruded on its northern margin by an Archaean granite batholith. The immediate complex hosting the Carr Boyd deposit covers an area of ~75 km2, although a similar sized intrusion is located immediately to its east. The Carr-Boyd Complex is composed of a series lobate bodies. The western, southern and eastern lobes are composed of ultramafic rocks with broadly centripetal dips, while the central and northern sections are occupied by layered mafic rocks with some ultramafic units. The Carr-Boyd Rocks deposit occurs on the western margin of the complex.

In the deposit area, the layered complex is composed of five units, each of which is subdivided into separate layers as follows, from the base upwards:
Unit I - made up of bronzitite (150 m thick); dunite-harzburgite-bronzitite (50 m); bronzitite (150 m); dunite-harzburgite-bronzitite (50 m);
Unit II - a 200 to 800 m thick series of layers of norite, augite norite, olivine norite, harzburgite and dunite;
Unit III - Bronzitite (>200 m); Dunite (50 m); Dunite-harzburgite-bronzitite (200 m);
Unit IV - 250 m of dunite;
Unit V - a >1000 m succession comprising a basal suite containing minor dunite, bronzitite and augite norite, followed progressively by norite, olivine anorthosite and troctolite. The ore deposit is confined to a zone towards the base of Unit V.

Intrusive rocks of several lithologies cross cut the complex, including bronzite pegmatites and sulphide bearing noritic pegmatoids which accompany the ore breccia pipes, as well as dykes of micronorite, olivine-augite micronorite and andesite, and common irregular bodies of microgranodiorite. Most rocks have been partially to completely altered. Dunite and harzburgite are altered to serpentine-tremolite(-talc-chlorite) rocks; the bronzitites to tremolite(-chlorite) rock; and bronzite-pegmatites and bronzatite-sulphide pegmatoids to actinolite and actinolite-sulphide rocks respectively.

Three separate ore shoots were defined within a zone of pegmatoid intrusions that trend at 70° bisecting the intersection of two major faults. These shoots take the form of intrusive breccia pipes that plunge to the west and are cut by NNW striking faults. The most easterly of these, the No. 1 Shoot, averages 70 x 10 m in plan area and persists to a depth of 200 m. No. 2 Shoot, 120 m to the west, averages 50 x 10 m in plan area and continues down plunge for 120 m. No. 3 Shoot is generally down plunge from the latter and is developed between depths of 140 to >200 m below surface with plan dimensions of 70 x 12 m. No. 1 and 2 shoots both average ~1.47% Ni, 0.6% Cu, whilst No. 3 Shoot has 1.97% Ni, 0.49% Cu.

The ore pipes contain abundant, unmineralised clasts of the enclosing wall rocks, mainly troctolite-anorthosite, which amount for ~30% of the pipes and are up to several metres across. The matrix between these clasts is composed of bronzite-sulphide pegmatoid and comprise 25 to 30% massive to disseminated sulphides enclosing or interstitial to single silicate grains or aggregates. These grains or aggregates of silicate minerals are up to 20 cm across and constitute 40 to 45% of the rock. Bronzite is the principal silicate, accompanied in places by olivine and by interstitial clinopyroxene, hornblende, biotite and plagioclase, with chrome-titanomagnetite. Primary sulphides are monoclinic pyrrhotite, pentlandite, pyrite and chalcopyrite. Deep weathering has produced violarite and secondary pyrite, with minor marcasite and greigite.

Drilling in 1969 indicated a reserve of 1.3 Mt @ 1.65% Ni, 0.57% Cu (Schultz, 1975). Remaining reserves in 1981 were 0.77 Mt @ 1.49% Ni.

For more detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1981.    
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 & Additional Information
   Selected References:
Marston, R.J., Groves, D.I., Hudson, D.R. and Ross, J.R.,  1981 - Nickel sulfide deposits in Western Australia: a review: in    Econ. Geol.   v.76, pp. 1330-1363.
Purvis, A.C., Nesbitt, R.W. and Hallberg, J.A.,  1972 - The Geology of Part of the Carr Boyd Rocks Complex and Its Associated Nickel Mineralization, Western Australia: in    Econ. Geol.   v.67, pp. 1093-1113.
Schultz K  1975 - Carr-Boyd rocks nickel-copper deposits: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 125-128


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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