Dun Mountain, Nelson Region, New Zealandi

Regional Level Types
Dun Mountain	Mountain
Nelson Region	Region
New Zealand	Country

Early Permian Ophiolite Belt. Copper mining was conducted mid 19th century, and later chrome mining closed 1902.

Dun Mountain is an important rock site, the type locality for the rocks dunite and rodingite.

Dunite is a peridotite, a very olivine-rich intrusive ultrabasic igneous rock. The fresh rock is grey-greenish, with some black chromite grains, with a rust brown rind forming from weathering. The rock type was discovered by Dr. Ferdinand von Hochstetter in 1859, when asked by the Nelson Provincial Government to investigate the mineral potential of the province.

Bell, Clark and Marshall in 1911 also named rodingite from the area. This is a metamorphosed igneous rock composed mostly of grossular, diopside, chlorite and vesuvianite. Rodingite develops where the Little Twin spilite (a basaltic rock) comes in contact with serpentinite, and there is intense metasomatism. Fine grained rodingite is common to the north and north-west of Dun Mountain. Rodingite is very hard, light grey to white, specific gravity 3.35, showing sometimes white flecks or blotchy grey-white. It is often crossed by grossular veins, and some green vesuvianite. Microscopically, titanite, magnetite, diopside, grossular, vesuvianite, prehnite, and rarely ilmenite, have been found in the rocks.

Dun Mountain is 10 kilometres south-east of Nelson. It can be reached by a popular mountain bike trail loop, leaving Brook Street on the outskirts of Nelson. The trail takes 4-6 hours, covering 38 kilometres. The rock belt in question, can easily be identified due to a marked change in vegetation along the track, where forest and scrub changes to tussock grass and small bushes.

It is part of an ophiolite belt containing serpentinite, dunite and harzburgite, stretching from D'Urville Island in the north, trending south-west for 90 kilometres.

Two rock groups form an anticline, the axis along the Dun Mountain-Little Twin Ridge. The Te Anau Group includes argillite, and spilite of the Little Twin members, and Pelorus greywacke. The lower Maitai Group, includes the Rangitoto Formation of volcanic breccia, marble and greywacke, and the Greville Formation consisting of argillite. Dykes and veins of metamorphic and metasomatised serpentinite cut across much of the above.

The sequence of rocks from the surface is generally the following. Late Cretaceous or early Tertiary Captain Amphibolite of coarse and fine grained amphibolite and talc; then the Dun Member carrying dunite, enstatite peridotite, pyroxenite, serpentinite and rodingite. There has been some mining of chrome and copper ores from the Dun Member. The Greville slaty banded quartoze argillite; Rangitoto flaggy impure marble with bands of greywacke, arkosic calcareous greywacke, and hematitised volcanic breccia. Fine grained massive spilite, grossular-diopside, tuffaceous spilite, greywacke, and quartzose argillite of the Little Twin Members. Finally the Pelorus Greywacke.

The area shows a complex variety of rocks that space here does not allow us to go into detail. While not a site for mineral specimens, the following may give some indication of species found in the rocks. Purplish-red hematite rock outcrop on the Little Twin Ridge, Wooded Peak and elsewhere. Rocks can contain hematite veins and clots, hematite-epidote, and hematite-chlorite veins. Generally across the area, chlorite, prehnite, and calcite veins are common, also relict pyroxene and titanite, and impregnations of grossular. Serpentine dykes are common, coloured dark grey, with a green hue, cross-cut by albite, green hornblende, chlorite and epidote veins. The material sometimes contains vugs of prehnite, and patches of iron ore, and calcite.

Also found is albite-epidote amphibolite, talc-chlorite, spilite with phenocrysts of albite, serpentinised olivine set in aggregates of pyroxene and feldspar needles, bastite serpentinite pseudomorphs in a mesh serpentinite. The Rangitoto volcanic breccia as dark grey to black pebbles to 5 cms in a fine grained hematite matrix. The Rangitoto Marble outcrops north and north-west of Dun Mountain, but is often obscured by a thin layer of carbonate. Fresh calcite marble is light grey to light green, or finely banded of these colours. Many calcite veins cross it, sometimes containing minute limonite cubes after pyrite.

The Maitia River has its headwaters on Dun Mountain. Chrysotile is noted, with calcite, and larger veins of garnet-serpentine-chlorite-rodingite. Also in this area are dykes of vesuvianite-chlorite-rodingite; and titanite rock, cross-cut with chlorite and carbonate veins; tuffaceous sandstone with quartz, feldspar, chlorite and epidote; marble (some grey-green banded) with acicular calcite crystals, and minor quartz, epidote, feldspar and chlorite; as well as many of the rocks mentioned above for Dun Mountain.

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References