Public works quarry (Ohakune scoria pit), Ohakune, Ruapehu District, Manawatu-Whanganui Region, New Zealandi

Regional Level Types
Public works quarry (Ohakune scoria pit)	Quarry
Ohakune	- not defined -
Ruapehu District	District
Manawatu-Whanganui Region	Region
New Zealand	Country

Crusts of specular hematite crystals lining extinct fumaroles in scoria exposed in railroad ballast quarry in central North Island.

Okahune is a small town just beyond the southern border of the Tongariro National Park. It is largely used as a base for nearby ski fields, and walks (tramping) across the active volcanic mountainous zones to the north in summer.

This small quarry is 500 metres north-west of Okahune, on the northern side of the former railway line, now a bike path. Topographical maps name it the Rochfort Crater, however the extensive reference below does not name it this. The quarry shows an upper 16 metre sequence of a scoria cone, part of a previous volcanically active area consisting of a cluster of four vents, and a surrounding tuff ring. It was one of the parasitic vent areas from the presently active Mount Ruapehu (2797 metres), seen as a snow capped volcano just north of the town. The site represents the southern most point of the Taupo Volcanic Zone which extends north north-east across the North Island to White Island and beyond in the Bay of Plenty.

The Okahune volcanic complex was formed from the influence of a rapidly rising magma body to the surface, magmatic gas bubbles, vent geometry, and extensive water during volcanic activity. It is beyond the scope of Mindat to go into detail (see Houghton et. al, 1984), and this article will focus mainly on rock units, and potential specimens.

The vents formed across an east-west fault block in the underlying basal Pliocene-Miocene marine sediments. The surrounding land consists of a laharic ring plain surrounding Mount Ruapehu.

The scoria cone shows alternating lava flows, coarse welded and un-welded strombolian (small to medium volume, sometimes violent volcanic eruptions with ejecta) blocks, and bomb beds. Clasts in the scoria cone are dense, de-gassed, and oxidised. The un-gaseous nature was caused when the tuff ring blocked lava flow forming a lava pond, in the process de-gassing the lava as it rose to the surface more slowly. Earlier lava which rose quickly to the surface shows here as formerly gaseous, with now many small vesicles.

Red oxidised clasts of low silica andesite forms 95-99% of the deposit, (Houghten et.al) stating is unique for the area being plagioclase poor. There are two clast types: 1) well formed fusiform to ovoid bombs with moderately vesicular cores and 'breadcrust' rinds, and 2) dense angular non vesicular to micro vesicular blocks. Minor quartoze xenoliths, sedimentary inclusions, and well rounded boulders of plagioclase-phyric R andesite are found, the latter part of the surrounding laharic plain, rather than formed from the vents.

Houghton et.al studied the tuff ring from several small quarries which appear to be off Old Station Road, bordering the east side of the road, and surrounded by farmland, to the south around 500 metres from the specimen quarry.

The tuff ring consists almost entirely of pyroclastic beds, with a sequence of black bomb layered S beds, pale brown block and bomb layers, and dark brown ash beds. The S beds contain highly vesicular un-oxidised bombs. Paragonisation has formed the brown colour, with sagging noted at the contact with the underlying beds. The ring is 600 to 1000 metres wide, asymmetrical, surrounding the vent area.

A distal under 2 metre thick ejecta blanket is found up to 2 kilometres south and east of the vents and tuff ring.

The Mindat article noted below studied several specimens/rocks from the site, some matching Mindat photographs. These all consisted of brown to brick red scoria, variously described, including pumaceous areas, glassy melts, porcelainite-like, clay-like, burn't areas, clinkery, and garden variety scoria.

Attached to the scoria various species are noted including olivine as minute crystals, scattered, dark green massive, and a mat-like minute layer of olivine crystals. Hematite as dark grey to red zoned crystals often in vesicles, twinned crystals, black to metallic lustre, hematite oxide coatings on baked clay like scoria, thin foil like fragile hematite. White feldspar some altered to hyalite, a minute white matter suggested as possibly amorphous silica, minute plagioclase crystals, amorphous opal are also suggested.

Mindat Articles

Hematitic Scoria from an Ohakune Explosion Crater by Christine (Chris.) Johnson

Other Regions, Features and Areas containing this locality

References