Australian-listed
Minotaur jumped from A18 to more than A$2 before settling in at A$1.86 on news that it had pulled a 107-metre intercept averaging 1.94% copper and 0.65 gram gold per tonne from its first hole on the Prominent Hill prospect, a discrete gravity anomaly measuring 1,500 by 500 metres. The discovery hole passed through 107.8 metres of younger sediments before intersecting a sequence of mineralized hematitic breccias. The upper 20.2 metres of the breccia ran 3.03 grams gold per tonne, followed farther down-hole by 107 metres of 1.94% copper and 0.65 gram gold (including 35 metres at the base of the intercept grading 3.86% copper, 0.82 gram gold and 4.5 grams silver) at a depth of 200-307 metres. The vertically drilled hole was shut down in mineralization at 450 metres of depth, pending assay results and mineralogical assessment. The final 21 metres of the hole averaged 0.9% copper and 0.46 gram gold. The hole will soon be deepened.
Minotaur reports that original rock textures in the breccia are almost totally destroyed. The siliceous, sericitically altered rock fragments vary in size from rock-flour to clasts several centimetres in diameter. Hematite mineralization dominates both the matrix and breccia clasts.
Copper chalcocite mineralization occurs as disseminations, stringers and veinlets throughout the hematite matrix. The chalcocite is concentrated at a depth of about 300 metres.
The gravity anomaly partially coincides with a well-defined magnetic anomaly of similar size. Two holes tested the magnetic feature in the early 1990s and intersected an altered ultramafic skarn.
Prominent Hill is the fourth of six iron-oxide, copper-gold targets on the Mt. Woods joint venture to be tested since drilling began in August. Minotaur has entered into an agreement with BHP Billiton (through Billiton Exploration Australia) regarding 600,000 sq. km in South Australia and western New South Wales. Minotaur is responsible for generating prospects with potential for large tonnage copper-gold and/or zinc-lead mineralization, whereas BHP Billiton has first rights to acquire joint-venture interests in these projects.
Minotaur and BHP Billiton are currently involved in four joint ventures, including Mt. Woods, which covers 3,800 sq. km of exploration licences along a major northwest-striking structure between Coober Pedy and Roxby Downs. The Prominent Hill discovery is 125 km northwest of
Olympic Dam is the world’s sixth-largest copper deposit and largest uranium deposit. Proven and probable underground reserves at the end of 2000 stood at 707 million tonnes grading 1.7% copper and 0.05% U3O8, plus 0.5 gram gold. Total resources are estimated at 2.5 billion tonnes grading 1.3% copper, 0.04% U3O8 and 0.5 gram gold. Last year, the mine produced 200,423 tonnes of refined copper and 4,539 tonnes of uranium oxide, as well as nearly 70,000 oz. gold and 625,100 oz. silver. Cash operating costs were less than US30 per lb. of refined copper, whereas fully allocated costs were under US55 per lb. The operations achieved an operating profit of A$165.7 million in 2000.
For the first six months of 2001, Olympic Dam posted an operating profit of A$85 million on the back of 103,889 tonnes of refined copper, 2,292 tonnes of uranium oxide, 59,000 oz. gold and 460,000 oz. silver.
Minotaur is earning a 19% interest in Mt. Woods as the initial operator, and BHP Billiton is earning 51% by spending A$4 million on exploration.
History
Over the past 20 years, the Mt. Woods area has been explored by Normandy, CRA Exploration and Burmine. Their efforts led to the identification of large skarns and widespread iron-oxide breccias anomalous in base metal mineralization. Early drill holes encountered intercepts of 287 metres grading 0.23% copper and 186 metres of 0.13% copper in magnetite breccia.
Minotaur focused its efforts on the more hematite-dominant alteration systems characteristic of the Olympic Dam sub-domain. Following a review of Normandy’s geophysical and drilling databases using BHP Billiton’s proprietary three-dimensional inversion software modelling, Minotaur outlined six targets scattered over an area measuring 30 by 15 km.
Each of the underexplored targets shows size potential plus evidence of mineralization and large-scale hydrothermal alteration with associated signatures of iron, copper, gold and rare earth elements. The structural setting of each target is suitable for a high-level, oxidized intrusion environment containing the required reactive host rocks.
Two holes drilled on the Armstrong North and Blaze targets showed strong hematite alteration, as well as intense hematite and carbonate veining, but no significant values. Drilling at the Neptune prospect, 10 km southeast of Prominent Hill, intersected a 540-metre sequence of hematite and magnetite altered brecciated volcanics anomalous in copper and gold. Mineralized sections ran as high as 0.18% copper and 68 parts per billion gold over lengths of up to 87 metres.
Characteristics
Olympic Dam-type deposits are still poorly understood, though are known to have the following characteristics:
– Mineralogy — The mineralization is generally dominated by iron oxides (either magnetite or hematite).
– Metal suite — Iron is the dominant metal, whereas copper occurs as the next most abundant. Individual deposits may contain significant gold, silver, uranium and rare earth elements. Significant trace metals may include cobalt and molybdenum.
– Alteration — The host rocks are generally intensely altered. The alteration assemblage depends on host lithology and depth of formation, but there is a general trend from sodic (albite-actinolite) alteration at deep levels to potassic (potassium-feldspar-biotite) alteration at intermediate-to-shallow levels to sericitic (sericite-limonite) alteration and silicification at very shallow levels.
– Tectonic setting — The deposits lie in areas that were cratonic or continental margin environments, and most occur along major structural zones.
Although many Olympic Dam-style deposits occur in silicic-to-intermediate igneous rocks of anorogenic type, the host rocks can be sedimentary. Both morphology and the extent of mineralization and alteration appear to be largely controlled by permeability along faults, shear zones and intrusion contacts, or by permeable horizons in sequences of volcanic flows.
Minotaur has 18.1 million shares outstanding, or 22.8 million fully diluted. BHP Billiton holds a 6.9% stake in the junior.
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