Dun Mountain, Nelson Region, South Island, New Zealand
|Latitude & Longitude (WGS84):||41° 20' 41'' South , 173° 21' 28'' East|
|Latitude & Longitude (decimal):||-41.34486,173.35787|
|Köppen climate type:||Cfb : Temperate oceanic climate|
Early Permian Ophiolite Belt. Copper mining mid 19th century, chrome mining closed 1902.
This locality has given its name to the ultrabasic rock dunite.
Dun Mountain is an important rock site, the type locality for the rocks dunite and rodingite.
Dunite is an olivine bearing peridotite, a metamorphosed ultrabasic intrusive igneous rock. The interior is olivine, grey-greenish, with sometimes black chromite masses, with a rust brown rind 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 named rodingite from the area, a grossular-diopside rock. Rodingite develops where the Little Twin spilite comes in contact with serpentinite, and there is much metamorphism, and 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 dunite, enstatite peridotite, pyroxenite, serpentinite, rodingite, 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 quartoze 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.
14 valid minerals.
Rock Types Recorded
Select Rock List TypeAlphabetical List Tree Diagram
Entries shown in red are rocks recorded for this region.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Jurassic - Triassic|
145 - 252.17 Ma
|Mesozoic sedimentary and volcanic rocks|
Age: Mesozoic (145 - 252.17 Ma)
Stratigraphic Name: Torlesse Supergroup
Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. 
|Early Jurassic - Triassic|
174.1 - 252.17 Ma
Age: Mesozoic (174.1 - 252.17 Ma)
Stratigraphic Name: Patuki Melange
Comments: Eastern Province (undifferentiated) Rocks
Reference: Edbrooke, S.W., Heron, D.W., Forsyth, P.J., Jongens, R. (compilers). Geology Map of New Zealand 1:1 000 000. GNS Science Geological Map 2. 
|Wuchiapingian - Artinskian|
254.14 - 290.1 Ma
|Undifferentiated Dun Mountain Ultramafics Group harzburgite, dunite and pyroxenite|
Age: Permian (254.14 - 290.1 Ma)
Stratigraphic Name: Dun Mountain Ultramafics Group
Comments: Basement (Eastern Province) igneous rocks. Age based on absolute age calculated from stratigraphic age range
Reference: Heron, D.W. . Geology Map of New Zealand 1:250 000. GNS Science Geological Map 1.