The basement in the Ross area is the Greenland Group (Liard 1965), the oldest recognized sequence of rocks on the West Coast. These Ordovician turbidites get their name from Mount Greenland, which is located some 6 km south of the town of Ross. They form a band of monotonously thin bedded, slightly metamorphosed greywacke quartzite with subordinate metapelites stretching discontinuously for at least 300 km along the westerns side of the Alpine Fault.
This succession of greywacke and argillaceous rocks, termed the “Westland foreland province”, were deposited on the Tasman Geosyncline during the Cambrian-Ordovician period, and were subsequently intruded by granites during a prolonged Ordovician to Triassic period. (see Ross South Geology Map)
The Greenland Group rocks display blocky, close-spaced jointing, widespread shearing and ubiquitous quartz vein formation, often hosting some gold. The Greenland Group is assumed to be the source of the great majority of the alluvial gold at Ross. Greenland Group rocks have not been intersected by any of the exploratory drilling close to Ross. At least 400 to 600 m of cover is inferred, based on geophysics by others, over these rocks within and adjacent to the Ross basin. Early Tertiary, Late Tertiary and Quaternary groups of sediments rest upon these greywackes intruded by granite.
At least two major orogenic disturbances have affected the region since Oligocene times. These orogenic disturbances are a consequence of proximity to a plate boundary during the Devonian-Carboniferous (Tuhua Orogeny) and Cretaceous (Rangitoto Orogeny) together with the granitic emplacement. The first orogenic disturbance seems to have occurred at about the end of the Miocene, followed by further movements after the first episode of glaciation. There are indications to suggest that there have been two, if not three, distinct periods of ice advance. The first two of these periods were separated by a period of intense deformation, during which it was it was very likely that there was significant climate change amounting to an interglacial period, with the earliest glacial beds probably Upper Pliocene (Nukumaruan) in age.
These orogenic disturbances resulted in the rocks being metamorphosed as a result of high temperature and low pressure and highly deformed. Uplift and block faulting occurred during the Tertiary (Kaikura Orogeny) with deformation into broad synclines and anticlines. Sub-aerial erosion reduced the West Coast to a generally low elevation by the mid-Cretaceous so that the metasediments (Greenland group) and granites (Tuhua Group) are unconformably overlain by widespread, channelized terrestrial coal deposits (Cretaceous and Palaeocene), shallow marine deposits (Miocene to Pliocene-Blue Bottom Formation), tectonic gravels (Old Man Group) and glacial/fluvio-glacial gravels (Cockeye to Loopline Formations).
Marine transgression in the Eocene was accompanied by the accumulation of thick coal deposit sequences (the Brunner Coal Deposits). Quartz sands and coal were deposited on the hill slopes immediately south of the Ross area during the Eocene. Late Miocene to Pliocene yellow-brown, silty, fine glauconitic sands and silts occur to the south of the Birchfield pit and in Donnelly Creek as a 100-m thick sequence.
Marine incursion continued into the middle Oligocene so that the great majority of the West Coast was submerged up to approximately 1000 m below sea level, accumulating thick terrigenous and limestone sequences. Blue-grey mudstone, known as “papa” by the local miners, and limestone occur widely southeast of Ross, and a lime quarry operates south east of the Ross property.
Uplift during the Kaikura Orogeny resulted in erosion and denudation of these latter sediments from elevated areas and remnants are largely confined to low-lying areas and valleys. The rapid uplift of the Southern Alps commenced in the Pliocene at about 6.4 ma, and the associated fluvial erosion supplied massive volumes of coarse detritus, which flooded out from the rising mountains and filled what were already shallowing marginal marine basins. This led to the onset of sub-aerial deposition in the early Pliocene. This is the material that forms the Old Man Group, composed mainly of Torlesse greywacke and Alpine schist from east of the Alpine Fault, with a lesser component of granite, gneiss and Greenland Group clasts. Old Man Group sediments form the low hills southeast of Ross and appear to dip steeply west into the Ross Basin.
Locally, the combination of cooling global climate, rapidly rising topography and a westerly mid-latitude maritime atmospheric circulation caused the onset of glacial conditions (the Ross glaciation) at about 2.6 Ma. Moraines are preserved in the middle part of the Old Man Group some 200 m southwest of Ross. Post-450 Ka, a widespread glacial sequence is well preserved to the north of Ross, where piedmont glaciers flowed out well beyond the current shoreline. A moraine is preserved on the eastern flanks of Cemetery Hill near the eastern side of the Ross property. Its age is unknown, however it contains little locally derived debris and is not significantly weathered, suggesting an age less than approximately 100,000 years.
With time the early glaciers were constricted by erosional incision and by their own moraines and therefore became more “valley-form”. In the case of the Hokitika Glacier, north of Ross, from originally flowing seawards down the Totara River, this ice stream was slowly forced in a more northerly direction by accumulating lateral and terminal moraine so that its last major advance reach Woodstock, only 6 km south of Hokitika.
There are no significant glaciers anywhere near Ross today, and none of the relict terraces around the town, strongly modified by past mining, have been correlated to the main Hokitika sequence. The presence of significant quantities of reworked Old Man Group debris lends a brown cast to most of the gravels, precluding visual estimates of age based on weathering.
Old Man Group sediments in the Ross area pass up into a Pleistocene gravel sequence of very variable thickness. This has been named the Ross Flat and refers to all material filling the Ross basin above the Old Man Group. The only outcrops of this material are in the walls of the Birchfield pit.
At the mouth of Jones Creek, the Ross Flat formation gravels were perhaps 5 m thick at the commencement of mining, however 150 m to the northeast they exceed 90 m in the Scandinavian shaft and in excess of 110 m in the Ross United shaft 500 m to the north. These gravels are significantly younger than the Old Man Group and form thick, imbricated subangular to sub-rounded boulder beds of grey-brown, relatively unoxidized medium to coarse, predominantly Greenland Group derived gravel.
Alluvial gold can be found in almost all the constituents of the Late Tertiary and Quaternary groups. Well-defined and relatively spaced coarse beds, individually up to 20 m thick, within the Ross Flat formation are strongly enriched in alluvial gold and have been the target of most of the alluvial mining that has occurred and continues to occur at Ross. These are the so-called “bottoms”, which dip northward under the Ross fault. As these gravels accumulated early in the Pleistocene and tectonic uplift and faulting has been continuous during that period, the faults now bounding the eastern side of the Ross basin probably did not exist, and there is a strong possibility that the bottoms in the Ross basin extended east into the Ross east area at that time.
Interglacial and post-glacial erosion has dissected the moraines and outwash, and reconcentrated alluvial gold into beds of active fluvial systems. This alluvial gold continues to be recovered from these geological formations by various ongoing mining operations surrounding the Ross property. However, there are no records of the basal Tertiary conglomerates at Ross being explored for gold. Very old claims on the eastern slopes of Mount Greenland, on which foothills the town of Ross is established, near Cameron Creek appear to have worked locally derived greywacke detritus. On the floor of the Totara River valley and the hills to the southwest the recovered gold has been worked from glacial gravels and recent reconcentrations of glacial and R.8 gravels.
Recent floodplain deposits and perched remnants in older formations have also been worked in many locations. The claims are mainly operating in patches of glacial and R.8 gravels left between old claims, while tailings in Jones Creek have been successfully reworked. The alluvial gold in these gravels is generally rather fine, particularly closer to the coast where it can be extremely fine, however it has also been reported in R.6 conglomerates and R.5 sands, but not in R.7 silts. Some of the younger deposits further inland from the beaches contain very coarse, rough gold, suggesting they have not been subjected to long transportation by water.
Some of the Ross gold deposits have been enriched by successive concentrations, however at the same time it is evident that the workable deposits in Recent and Pleistocene beds have originated as a result of only a single cycle of erosion and deposition of gold derived from quartz veins in the vicinity. The Ross region is not only well known for alluvial gold, but also hard-rock gold deposits. Gold deposits in the Ross area include the Ross Flat deposit immediately east of the town and north of the Ross Property, and several ongoing alluvial gold workings around the district and along the coastline.
An historic shaft, the deepest in the area, has been located within the Ross South property. This shaft is thought to have accessed auriferous conglomerates between the historically rich Jones and Donnelly Creeks to the southeast of the town of Ross. A drilling program on the immediately adjoining property to the north in 2009 encountered high gold grades over intervals of up to 25 m thick, starting from surface.
Maori and pakeha gold-seekers probed the Totara River and Donnellys Creek late in 1864, the first year of the West Coast gold rushes, giving rise to the town of Ross. Jones's Creek (as it was known at first), a Donnellys Creek tributary, was rushed in August 1865 and mining continued on its bed and terraces for decades.
Little is known about Moye's shaft. There are historical references to a whim (and therefore presumably a shaft) on Moye's claim, whose location is unspecified, in 1867; ground sluicing at the foot of a high face in Moye's claim, Jones Creek, in 1868; a shaft worked by Moye, site unspecified, in 1871; and Michael Moye sinking a shaft for a party of which he was a member, holding ground on Jones and Guardian Flats in 1880.
Jones Creek was the scene of a sensation in 1909 when two prospectors found New Zealand's largest verified gold nugget, the 99-ounce 'Honourable Roddy' a short distance downstream from areas that will likely be the proposed drilling site.
