Geochemical Development of the Late Cenozoic Arc Volcanism in Northland and the Coromandel, and Implications for Geochemical Exploration in the Hauraki Goldfield

Abstract

Late Cenozoic subduction-related volcanism in northern New Zealand left a record in Northland, the Coromandel and, from 2 Ma, in the central North Island. Over 90% of the volcanic succession formed prior to the intensely studied central North Island volcanism. This study documents geochemical and isotopic characteristics of volcanic rocks of the Northland Arc and the Coromandel Volcanic Zone (CVZ). Rocks are basaltic, andesitic, dacitic to rhyolitic, with subordinate trachybasalts to trachydacites. All samples have an arc-type trace element signature, Northland Arc (23.5-15.5 Ma) western belt rocks are relatively mafic, and eastern belt rocks relatively felsic. The contrast possibly reflects variable crustal thickness across the Northland peninsula, with eastern belt magmas the products of more deep-seated magmatic systems. The CZV (18-1.95 Ma) erupted comparable andesites to Northland Arc eastern belt rocks, and also basalts, rhyolites and high-magnesium andesites. CVZ basalts resemble least differentiated andesites, but have relatively high Na2O and TiO2. CVZ rhyolites have distinct compositions, where the least differentiated rhyolites may represent partial melts derived from equivalents to CVZ basalts. Crystal and groundmass compositions suggest that andesites are mixtures of mafic and silicic components that to some extent resemble the compositions of CVZ basalts and rhyolites, respectively. With the recognition of high-magnesium andesites in the Kiwitahi chain, the CVZ includes all the major rock types that characterise the modern volcanic system. Hydrothermally altered CVZ andesites host c. 50 epithermal gold-silver deposits of the Hauraki goldfield. Trace elements used in exploration to locate such ore bodies are mostly confined to veins, so the range of geochemical exploration targeting can potentially be extended by a better understanding of major element mass changes that occur over larger distances. The unaltered rock dataset obtained in this study provides a baseline to quantify major element mass changes in altered rocks by using the immobile element ratio Zr/Ti to estimate protolith composition. A test case in the Waitekauri area alteration zone shows that maximum and average K and Rb gains consistently increase from periphery to core over a 3 km wide section, demonstrating that quantitative mass balance data can be a significant addition to geochemical exploration.