Only months after reporting a copper-gold intercept that rivaled the Olympic Dam discovery holes a quarter-century ago,
In November 2001, the Australian-listed junior, together with partner
The discovery hole was positioned on the northeastern edge of the gravity anomaly in a relatively weak portion of the feature where it narrows to about 150 metres. The lens-shaped gravity feature is just over 2 km long and ranges from 150 to 400 metres wide.
Drilled vertically to a depth of 708 metres, the discovery hole passed through 108 metres of younger barren sediments before intersecting a basement sequence of massive hematite-supported brecciated metasediments and volcanics.
The top 20 metres of the basement sequence ran 3.03 grams gold per tonne from 108 to 128 metres, followed further down-hole by: 107 metres of 1.94% copper plus 0.65 gram gold and 1.6 grams silver (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; and 152 metres of 1.1% copper, 0.61 gram gold, 2.6 grams silver and 267 parts per million (ppm) uranium from 429 to 521 metres.
The upper 90 metres or so of basement sequence consisted of brecciated, fractured and hematite-veined metasediments and tuffs, which converts, at 200 metres of depth, to a hematite-silica breccia with highly altered hematite-sericite-silica clasts. The copper mineralization begins at 200 metres with the appearance of the matrix-supported hematite breccia. The hematite breccia extends to a depth of 581 metres and is interrupted by an unmineralized, dolerite dyke and weakly brecciated metasediment from 307 to 429 metres.
The gravity anomaly was interpreted by Minotaur to reflect the outline of the hematite breccia. In January, Minotaur began to test the extent of the Prominent Hill discovery, by first stepping out to the southwest on 400-metre centres. The first two holes were drilled right into the heart of the gravity anomaly but encountered only narrow intercepts of weakly anomalous copper-gold mineralization.
The first hole was drilled vertically in an area where the gravity anomaly intensifies and broadens. However, poor drilling conditions forced Minotaur to shut down the hole much earlier than planned, at a depth of only 263 metres. The vertically drilled hole intersected highly brecciated siliceous volcanic rocks in the basement sequence with both “steely and earthy” hematite veining throughout. The hematite comprises a large percentage of the core, running as much as 69% Fe2O3. Down-hole geophysics confirms that the hematite iron-oxide is the source of the core gravity feature.
The second hole was collared a further 400 metres to the west on the northern flank of the anomaly and angled to the south. It entered into earthy-red hematite breccia at 97 metres before passing into steely-grey hematite-silica at 240 metres down-hole. Better intercepts included 16 metres of 0.16% copper at 159 metres of depth and 12 metres of 3 grams gold at 190 metres.
Minotaur believes the central portion of the Prominent Hill gravity anomaly represents a barren volcanic eruptive centre with sporadic copper and gold mineralization adjacent to the vent.
Working on the assumption that central steely hematite-quartz breccias form an unmineralized core to the mineralized system, Minotaur targeted the southern flank of the gravity anomaly by stepping back 330 metres south of hole 1. Angled at minus 60 north, hole 4 was drilled to a depth of 603 metres, intersecting andesitic volcanics and agglomerates below the younger sedimentary cover. Hematite alteration intensifies with depth, but the favourable matrix-supported hematite breccia was absent from the hole. One intercept returned 22 metres grading 0.25% copper, 0.03 gram gold and 1.1 grams silver at a depth of 530 metres.
Minotaur undercut the discovery hole with hole 3. Drilled from the southern edge of the anomaly, 170 metres south of the discovery hole, and aimed at minus 60 north, hole 3 passed through 124 metres of sedimentary cover before cutting a complex mix of highly brecciated tuffs, sediments, volcanics and matrix-supported hematite breccias. An upper 28 metres of the breccia ran 1 gram gold and 2.6 grams from 188 to 216 metres, followed down-hole by 235 metres of 1.05% copper, 0.57 gram gold, 2.9 grams silver and 130 ppm uranium between 400 and 635 metres. A further 54 metres averaged 0.5% copper and 0.46 gram gold from 635 to 689 metres.
Hole 3 has extended the main zone of copper-gold mineralization at least 120 metres north of the discovery hole. Using two rigs, Minotaur continues to test the northern boundary of the gravity anomaly with holes 5 and 6. The former was collared 200 metres east of hole 3; the latter, 400 metres northwest of the discovery hole.
As operator, Minotaur is carried with a 19% interest through to the first A$4 million in expenditures. Joint-ventre partner BHP is earning 51%. The current tenement holders,
The Mt. Woods joint venture covers 3,800 sq. km of exploration licences southeast of Coober Pedy. The Prominent Hill discovery is 125 km northwest of
The discovery lies on the southern margin of the Mt. Woods Inlier in the Gawler Craton. The Mt. Woods Inlier consists of Paleoproterozoic metasediments deposited in a shallow marine shelf environment overlying late Archean crystalline basement. Sediments include aluminous clastics, banded iron formation, quartzite-chert, carbonate, and calcsilicates. The complexly folded inlier is pervasively altered by iron-rich fluids sourced from the Mesoproterozoic Hiltaba Suite. Regional aeromagnetic and gravity data reflect the distribution of hematite-magnetite alteration.
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