There is a little more evidence of the events of the early part of the Paleozoic era in New Zealand. In west Nelson and Fiordland thick sequences of Cambrian, Ordovician, Devonian, and probably Silurian rocks are preserved. In both areas the rocks include great thicknesses of sedimentary and volcanic rocks that appear to have accumulated in a major geosyncline. Along with these is a wide variety of slates, schists, gneisses, quartzites, and marbles formed by metamorphism of similar sedimentary rocks, and these have been invaded by granites and other intrusive igneous rocks.
In West Nelson deposition in the early Paleozoic geosyncline began with the accumulation of the Cambrian Haupiri Group. The earliest of these rocks was a large thickness of basic volcanic rocks — lavas, agglomerates, and tuffs; later these were covered by grey mudstones with patches of limestone that have yielded New Zealand's oldest fossils — middle Cambrian trilobites, shells, and sponges, discovered in the Cobb Valley. These were succeeded by the great thickness of Ordovician strata of the Aorere and Mount Arthur Groups which include slaty argillites containing fossil graptolites that closely resemble those of Victoria, Australia. The Mount Arthur Group includes the extensive Mount Arthur marble, quarried for many purposes at Kairuru. In the Devonian, sedimentation in the geosyncline slowed down, and quartzites and richly fossiliferous limestones were laid down. Today, fossiliferous Devonian strata are preserved only in the Baton River area near Wangapeka and at Reefton.
In Fiordland the Cambrian sediments, some 9,000 ft thick, have been converted to metamorphic rocks; they appear originally to have been a succession of basic volcanic rocks and sediments that included sandstones, greywacke, and limestone. The Ordovician sediments, more than 20,000 ft thick, have also suffered metamorphism; they appear originally to have been a varied sequence of sandstones, greywackes, mudstones, and tuffs. The Preservation Formation includes abundant graptolites of Lower Ordovician age, in beds of slaty argillite.
Both in Nelson and in Fiordland the geological structure is extremely complex. The intense folding and thrusting of the Lower Paleozoic sediments, their conversion in part to metamorphic rocks, and the invasion of them by large and small masses of intrusive rocks, took place at various times during the Paleozoic. In West Nelson there were several major episodes of deformation. It was perhaps in the Devonian period that Cambrian sediments were transported as great overturned folds (nappes) so that they came to rest on top of younger Ordovician rocks. Further major folding in other directions took place, possibly in the Carboniferous, and again in the Carboniferous and Permian. Ultramafic and basic intrusive rocks, mainly serpentines, peridotites, and dolerites, were intruded, probably in the Cambrian, as a sill; they are exposed now between the lower Cobb and Takaka Valleys. Talc-magnesite and asbestos are minerals won from this ultramafic mass.
Part of the conversion of the Lower Paleozoic sediments of West Nelson to a complex assemblage of metamorphic rocks took place as a result of these foldings. Later, emplacement of the great mass of the Separation Point granite raised the temperatures and increased the degree of metamorphism. The Karamea, Paparoa, and many other granite masses in Nelson and Westland may have been intruded also in late Paleozoic times.
Many minerals of value were formed during Paleozoic metamorphism and intrusion in Nelson and Westland. The gold veins of the Aorere and Wangapeka districts are in Lower Paleozoic rocks: gold and sulphides of lead, zinc, copper, and molybdenum have been worked in Paleozoic rocks at Johnson's United Mine, where the mineralisation apparently occurred along the sole of a Paleozoic thrust plane. The Richmond Hill silver deposits are found in a similar environment. Molybdenite and other sulphide deposits of Mount Radiant, near Karamea, occur in masses of Paleozoic metamorphic rocks within granite. Metamorphism of Mount Arthur marble by Separation Point granite has produced a wollastonite body near Motueka, small deposits of barite and fluorite near Wangapeka, and magnetite iron ore near Canaan, Nelson.
In Fiordland the main phase of metamorphism and plutonic intrusion is thought to have been late Paleozoic; the distribution of the major groups of these rocks is summarised later in the section on Southland. Metamorphism of Fiordland limestone produced marble, some of which is very handsome and suitable for building stone.