The Anatomy of an Alkalic Porphyry Cu-Au System: Geology and Alteration at Northparkes Mines, New South Wales, Australia

Abstract

The Late Ordovician--Early Silurian (~455--435 Ma) Northparkes system is a group of silica--saturated, alkalic porphyry deposits and prospects which developed within the Macquarie Island Arc. The system is host to a spectacular and diverse range of rocks and alteration--mineralization textures that facilitate a detailed understanding of its evolution, in particular into the nature and controls of porphyry--related propylitic alteration.The first intrusive phase at Northparkes is a pre--to early--mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite (BQM) to alkali feldspar granite (AFG).Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite (QMZ) that marks the onset of a fertile fractionation series. Towards its upper levels, the QMZ is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of syn--mineralization quartz monzonite porphyries (QMP) were emplaced. The QMP intrusions have a distinct pipe--like morphology and are ubiquitously K-feldspar altered with a crystal--crowded porphyritic texture. The textures of the QMPs and common occurrence of porphyry--cemented contact breccias indicate they were forcibly emplaced and of relatively

Magmatic Fluids Implicated in the Formation of Propylitic Alteration: Oxygen, Hydrogen, and Strontium Isotope Constraints from the Northparkes Porphyry Cu-Au District, New South Wales, Australia

“…The Middle to Late Ordovician age Goonumbla Volcanics, dominated by trachyandesites, are intercalated with rare marine carbonates and contain peperitic textures, indicating a dominantly submarine environment (Simpson et al, 2005;Pacey et al, 2019). However, within the Late Ordovician to earliest Silurian Wombin Volcanics, trachytic welded ignimbrites and volcanic agglomerates are present (Kolkert, 1998;Simpson et al, 2005;Pacey et al, 2019), indicating volcanism was at least partly subaerial by that time. Therefore, based solely on geological evidence, there is the possibility that both meteoric water and seawater could have been present during alteration at Northparkes.…”

“…Therefore, based solely on geological evidence, there is the possibility that both meteoric water and seawater could have been present during alteration at Northparkes. Porphyry--related intrusive rocks are constrained as latest Ordovician to earliest Silurian (Lickfold et al, 2003;Lickfold et al, 2007), hosted within the Wombin Volcanics, with which they appear broadly comagmatic (Lickfold et al, 2003;Pacey et al, 2019). A cluster of five Cu--Au orebodies (E22, E26, E27, E48 and GRP314), as well as numerous prospects, occur in the district.…”

“…Such alteration-mineralization is developed around the margins and apices of complex, multiphase quartz monzonite porphyry intrusions. These intrusions are narrow (~50 to 100 m in diameter) but vertically elongate (>1.4 km) and were emplaced within and above a large (~12 km diameter) stock that underlies much of the mining area (Pacey et al, 2019). The latter varies in composition between an equigranular biotite quartz monzonite and plagioclase--phyric alkali feldspar granite.…”

“…The latter varies in composition between an equigranular biotite quartz monzonite and plagioclase--phyric alkali feldspar granite. Magmatic--hydrothermal phenomena, such as UST textures, are common in the quartz monzonite intrusions (Heithersay and Walshe, 1995;Lickfold et al, 2003;Pacey et al, 2019).…”

“…Our study is based on a suite of very well--constrained samples from the Northparkes porphyry Cu--Au district of Central West NSW, Australia, principally around the Endeavour (E)48 deposit. Details of the geology and alteration in this system are available in Pacey et al (2019) and a study of the zonation of propylitic mineral chemistry in the district is provided by Pacey et al (this volume).…”