ARTICLE R.L. Brathwaite á M.P. Simpson á K. Faure D.N.B. Skinner Telescoped porphyry Cu-Mo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins in the Thames District, New Zealand Received: 28 August 2000 / Accepted: 2 April 2001 / Published online: 7 June 2001 Ó Springer-Verlag 2001 Abstract Porphyry Cu-Mo-Au mineralisation with as- sociated potassic and phyllic alteration, an advanced argillic alteration cap and epithermal quartz-sulphide- gold-anhydrite veins, are telescoped within a vertical interval of 400±800 m on the northeastern margin of the Thames district, New Zealand. The geological setting is Jurassic greywacke basement overlain by Late Miocene andesitic-dacitic rocks that are extensively altered to propylitic and argillic assemblages. The porphyry Cu- Mo-Au mineralisation is hosted in a dacite porphyry stock and surrounding intrusion breccia. Relicts of a core zone of potassic K-feldspar-magnetitebiotite al- teration are overprinted by phyllic quartz-sericite-pyrite or intermediate argillic chlorite-sericite alteration assemblages. Some copper occurs in quartz-magnetite- chlorite-pyrite-chalcopyrite veinlets in the core zone, but the bulk of the copper and the molybdenum are asso- ciated with the phyllic alteration as disseminated chal- copyrite and as molybdenite-sericite-carbonate veinlets. The advanced argillic cap has a quartz-alunite-dickite core, which is enveloped by an extensive pyrophyllite- diaspore-dickite-kaolinite assemblage that overlaps with the upper part of the phyllic alteration zone. Later quartz-sphalerite-galena-pyrite-chalcopyrite-gold-anhy- dritecarbonate veins occur within and around the margins of the porphyry intrusion, and are associated with widespread illite-carbonate argillic) alteration. Multiphase ¯uid inclusions in quartz stockwork veins associated with the potassic alteration trapped a highly saline 50±84 wt% NaCl equiv.) magmatic ¯uid at high temperatures 450 to >600 °C). These hypersaline brines were probably trapped at a pressure of about 300 bar, corresponding to a depth of 1.2 km under lithostatic conditions. This shallow depth is consistent with textures of the host dacite porphyry and recon- struction of the volcanic stratigraphy. Liquid-rich ¯uid inclusions in the quartz stockwork veins and quartz phenocrysts trapped a lower salinity 3±20 wt% NaCl equiv.), moderate temperature 300±400 °C) ¯uid that may have caused the phyllic alteration. Fluid inclusions in the quartz-sphalerite-galena-pyrite-chalcopyrite-gold- anhydritecarbonate veins trapped dilute 1±3 wt% NaCl equiv.) ¯uids at 250 to 320 °C, at a minimum depth of 1.0 km under hydrostatic conditions. Oxygen isotopic compositions of the ¯uids that deposited the quartz stockwork veins fall within the 6 to 10& range of magmatic waters, whereas the quartz-sulphide-gold-an- hydrite veins have lower d 18 O water values ±0.6 to 0.5&), re¯ecting a local meteoric water ±6&) in¯uence. A d 18 O versus dD plot shows a trend from magmatic water in the quartz stockwork veins to a near meteoric water composition in kaolinite from the advanced argillic alteration. Data points for pyrophyllite and the quartz- sulphide-gold-anhydrite veins lie about midway between the magmatic and meteoric water end-member compo- sitions. The spatial association between porphyry Cu- Mo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins suggests that they are all genetically part of the same hydrothermal system. This is consistent with K-Ar dates of 11.6±10.7 Ma for the intrusive porphyry, for alunite in the advanced arg- illic alteration, and for sericite selvages from quartz-gold veins in the Thames district. Introduction A porphyry Cu-Mo-Au prospect was discovered at Ohio Creek, 3 km northwest of the Thames gold bonanza zone Fig. 1), by Amoco Minerals, and was drill-tested Mineralium Deposita 2001) 36: 623±640 DOI 10.1007/s001260100182 Editorial handling: P. Lattanzi R.L. Brathwaite &) á M.P. Simpson á K. Faure á D.N.B. Skinner Institute of Geological & Nuclear Sciences Ltd., P.O. Box 31-312, Lower Hutt, New Zealand E-mail: B.Brathwaite@gns.cri.nz Present address: M.P. Simpson, Department of Geology, The University of Auckland, Private Bag 92019, Auckland, New Zealand