Nineteenth-century observations of the Dun Mountain Ophiolite Belt, Nelson, New Zealand and trans-Tasman correlations

Abstract

AbstractThe Nelson Mineral Belt, part of the Dun Mountain Ophiolite Belt of Permian age, in the north of the South Island of New Zealand, was the subject of intense interest soon after the European settlement of New Zealand owing to the discovery of copper and chromite mineralization. The first geologist to survey the Mineral Belt in east Nelson was Thomas Ridge Hacket (c. 1830–1884), who arrived from Britain in 1857, although he was primarily interested in the mineralization. The first scientific description of the belt followed the visit in 1859 of the geologist Ferdinand von Hochstetter (1829–1884) of the Austrian Geological Survey who regarded the belt as a sill. He recognized that Dun Mountain was not serpentinite, like the rest of the belt, but an olivine-rich rock that he named dunite. von Hochstetter tentatively considered that thick mafic rocks of the Brook Street Volcanics, that were then considered to be intrusives rather than sedimentary, may have been a correlative of the Mineral Belt. Between the belt and the Brook Street rocks he mapped the Wooded Peak Limestone and Maitai Slates. von Hochstetter tentatively assigned a Mesozoic age to all of the above rocks and for nearly 100 years Mesozoic sedimentary rocks in many parts of New Zealand were regarded as a correlative of the Maitai Slates.Both Hacket and von Hochstetter confirmed an abundance of chromite in the Mineral Belt, a finding that resulted in the building of New Zealand's first railway. Mining was short-lived but hopes were raised that gold might be found following Hacket's move to Queensland in the late 1860s. He recognized that the Gympie gold-fields had many similarities with Dun Mountain and reasoned that east Nelson might also be auriferous. This was taken up by men in Nelson such as Sir David Monro (1813–1877) and William Wells (1810–1893) who were keen geological observers, and by Monro's son in law Dr, later Sir James, Hector (1834–1907) who was director of the New Zealand Geological Survey. In 1870, Hector arranged for Edward Heydelbach Davis (1845–1871) to undertake a detailed examination of the Permo-Triassic rocks of east Nelson. Davis, like Hacket, regarded the Mineral Belt as metamorphosed Maitai rocks. As well as demonstrating that the east Nelson rocks did not contain economic concentrations of gold, copper or chromite, Davis showed that they were distinct from what had been thought to be similar rocks in the Coromandel gold-fields in North Island of New Zealand. This was the first, if not fully appreciated, step to limit the Maitai rocks. Hacket's correlation of the Gympie and Dun Mountain rocks was forgotten until the advent of plate tectonics. The various major rock units in Nelson are now recognized as terranes that accumulated as a result of convergent plate tectonics along the Mesozoic margin of Gondwanaland.