The scheelite resources of the Glenorchy District, West Otago, New Zealand (print copy)

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Mutch, A.R. 1969 The scheelite resources of the Glenorchy District, West Otago, New Zealand. [Lower Hutt]: New Zealand Geological Survey. Report NZGS 40. 86 p

Abstract: The scheelite-bearing quartz lodes of the Glenorchy District are considered to be parametamorphic in origin, being produced by cavity filling of shear-zones or fissures in schist undergoing waning metamorphism during the last compressive phase of the Rangitata Orogeny in early Cretaceous time. (penecontemporaneous with the intrusion of the alpine-type ultramafics of the New Zealand Geosyncline into higher crustal levels). The strong deformation associated with these events allowed mineralisers to move into the shear-zones from the various schist formations. The tungsten is considered to have been derived from schist formations at deeper levels, the original sediments of which were rich in granitic detritus, like those of the adjacent Tasman Geosyncline. Variation in mineralisation within the lodes is considered to be a function of the degree of reactivity of the various schist lithologies. There are no measured reserves in the field, but indicated reserves in six estimate blocks, about one seventh the area mapped in detail, lie between 2,000-3,000 tons CaW)4 (146,000-210,000 units WO3 . The large ration between indicated and inferred reserves, not only shows the dearth of more precise information, but also the need to determine more closely these estimates. The small percentage difference (0.01% CaWO4 ) between the cut-off and estimated ore grades for all lodes worked by underground mining methods based on the post-1940 low price necessitates a most careful assessment of grade but precludes high confirmatory exploration costs. Nevertheless, the results from this initial feasibility survey show not only the quartz lenses but also the lodes as s whole to be at least 1.25% CaWO4 and are sufficiently encouraging to warrant further investigation for systematic mining. If, however, the future minimum price for scheelite can be based on the mid-1966 price, then the effect of high confirmatory exploration costs is not such a disadvantage and prospects are further enhanced. No scheelite reserves for dredging can be determined without drilling, but recent alluvial deposits forming fans in streams draining lode-bearing areas along the Rees River seem at present to offer the best prospects of mineable alluvial resources. (auth)

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