Geology of the Lower Parapara River

Abstract

The Waingaro Schist Zone is a band of tectonites formed along the boundary between North-west Nelson's Central and Eastern Sedimentary Belts during the Lower Devonian Tuhua Orogeny. The study area follows the Lower Parapara River and its tributaries as they cut a section through an isolated nappe of Waingaro Schist exposed to the south and west of the main body of the Central Sedimentary Belt. The metasedimentary nappe lithologies are dominated by psammitic to pelitic biotite schists with staurolite zone assemblages and layered amphibolites with lower amphibolite facies assemblages. Application of geothermobarometry to the mineral compositions in these assemblages indicate metamorphic conditions of 570 °C and 8.5 kbar of pressure. Their major element geochemistry suggests the biotite schists were deposited in a passive continental margin setting and that the amphibolitic schists have formed by decarbonisation of calcareous recycled volcanic arc sediments (para-amphibolites). Petrographic examination of the nappe lithologies reveled no evidence for the widespread dislocation metamorphism implied by their classification as part of the Waingaro Schist Zone. A highly deformed biotite micro-granite sill outcropping within the study area is the type locality of the Richmond Hill Porphyry. This granite is metaluminous and highly sodic with a dominantly I-type character. The trace element content of the granite is consistent with a continental margin, volcanic arc granite. U-Pb zircon dating of the granite, supported by its fabric and field relations, suggest it is Paleozoic in age and not Cretaceous as previously thought. A number of vertical screens of mineralised, biotite-garnet schist occur within the granite. This is one of numerous sites of gold, silver, lead, zinc, copper, and antimony mineralisation within the Parapara subdivision. Sulfide mineralogy is dominated by sphalerite, galena, tetrahedrite and boulangerite. The form and fabric of the ore bodies suggests that mineralisation was fault controlled and postdates both the emplacement and cooling of the Richmond Hill Porphyry and the thrusting event responsible for the emplacement of the nappe. Within the study area a planar body of mafic and ultramafic rocks outcropping along the base of the nappe have been mapped as part of the Cobb Intrusive Complex. The lithologies that make up the Cobb Intrusive Complex within the study area are dominated by antigorite serpentinite, talc-magnesite and quartz-magnesite. Subordinate metagabbro inclusions within the serpentinite have been altered to a Mg-rich chloritite by diffusion bimetasomatic reaction. The application of Gresens' equations to this alteration indicates it was accompanied by a 46% loss in total mass with the production of CaO, SiO2, Fe2O3 and a small amount of Al2O3 and the consumption of FeO, MgO, Cr2O3, NiO and H2O supplied by the alteration of serpentinite to talc.