West Peko Au-Cu prospect, Tennant Creek, Barkly Region, Northern Territory, Australia

Latitude & Longitude (WGS84):	19° 40' 21'' South , 134° 15' 49'' East
Latitude & Longitude (decimal):	-19.6725644632, 134.26372524

The West Peko prospect is 1.8 kilometres west of the established and abandoned Peko Mine, and 6 kilometres east of the Tennant Creek town site. It was discovered by BMR, and drilled by Geopeko up to 1987. The deposits depth of up to 500 metres, and relatively low overall grade of 10 g/t Au, has discouraged development.

Country rock is greywacke, siltstone, shale, and hematite shale. The lode is a quartz-hematite-magnetite ironstone lode typical of Tennant Creek deposits, containing Au-Cu-Bi values. Quartz-chlorite porphyry underlies the ironstone. There are two ironstone bodies, named Lode 1 and Lode 2, up to 40 metres thick, and 150 metres long each, striking east-west, dipping steeply north in the upper parts, and steeply south in the lower parts.
The ironstone bodies are found in a reverse shear-fault jog on the south limb of a regional syncline.

Hematite is found in the ironstone with magnetite, finely banded magnetite-quartz rock, and jasperoidal bands. Magnetite pseudomorphs come in a variety of types including bladed, acicular, spindle, lamellar and diamond shape in the ironstone. Magnetite is also found as laths up to 5 mms long, radiating aggregates, or diamond or trapezoidal crystals. Some crystals are skeletal filled with silicates. Magnetite is also found as dustings, or filling veinlets.

Pyrrhotite is the most abundant sulphide in the ironstone, as extension veins up to 1 cm wide in magnetite-quartz-minnesotaite zones, or irregular shaped aggregates adjacent to enveloping magnetite. Pyrrhotite is also found in chalcopyrite rich zone with sphalerite, gold, un-named Pb-Bi sulphosalts, chlorite, greenalite, and lastly as pyrrhotite bands with greenalite, siderite, rhodochrosite and calcite.

Chalcopyrite is found interstitial to magnetite as massive aggregates, or extension vein arrays, containing also pyrrhotite, sphalerite, un-named Pb-Bi sulphosalts, gold, native bismuth, greenalite, stilpnomelane and talc.

Pyrite is minor and sporadic in ironstone, mostly at its margins, and in some chalcopyrite-pyrrhotite-gold zones. Marcasite is uncommon, replacing pyrrhotite. Pyrite may form spongy cellular fine grained masses with rare marcasite within. Arsenopyrite as 1-5 mm wide bands in magnetite-quartz-minnesotaite zones. Red, orange and yellow sphalerite minor but widespread associated with chalcopyrite.

Bismuthinite, galena, cosalite, native bismuth are the most abundant Bi minerals at the deposit in the Cu-Au rich zones.

Gold is said to not be hand visible, and occurs as irregular shaped grains associated with sulphides, mostly chalcopyrite. The upper part of the Au-Cu-Bi lode contains magnetite-stilpnomelane, central part magnetite-chlorite- biotite-siderite, and lowest part magnetite-chlorite-stilpnomelane. The deposit is also zoned outwards from a central Au-Cu-Bi with magnetite-chlorite, or magnetite-stilpnomelane-biotite-hematite-siderite-calcite; then magnetite-stilpnomelane or magnetite-talc-stilpnomelane; then magnetite-quartz-minnesotaite banded; then magnetite-stilpnomelane-chlorite at the ironstone contact; then chloritised wall rock grading out to quartz-chlorite-sericite and less altered country rock.

Chlorite is found in the central ore zone of the No. 2 Lode intergrown with magnetite, and more Fe rich chlorite at the ironstone fringes. Small tabular equant brown, olive and green biotite crystals. Stilpnomelane is a major phase in the magnetite areas, as fine grained rosettes and aggregates interstitial to the magnetite. Minnesotaite is a major phase with magnetite, quartz, and minor talc, often associated with low grades of Au-Cu-Bi. It also forms fibres, wispy vein like bodies, radial aggregates, and needles.

Talc-magnetite is found adjacent to the main ore zone as rosettes replacing quartz, veinlets, tension gashes. Greenalite is the most widespread of the silicates as blades and acicular crystals at the margins of chalcopyrite veins. Tremolite/actinolite is abundant in the deeper parts of the deposit as blades up to 1 mm long.

Trace calcite and Mn-calcite rimmed by siderite in the core of the ore zone, intergrown with various other species. There is a 5 metre wide massive ankerite and Fe-dolomite zone in the lower parts of the No. 1 Lode.

Mineral List

31 valid minerals.

Rock Types Recorded

Entries shown in red are rocks recorded for this region.

References