ARTICLE A. S. Templeton á D. Craw á P. O. Koons á C. P. Chamberlain Near-surface expression of a young mesothermal gold mineralizing system, Sealy Range, Southern Alps, New Zealand Received: 21 May 1997 / Accepted: 30 June 1998 Abstract Carbonate-limonite veins formed in steeply dipping fractures in the upper few hundred metres of basement greywacke in the actively rising Southern Alps of New Zealand. The veins are found commonly in ex- tensional fractures near to, but not in, major faults as- sociated with mountain uplift, and/or sinistral faults which bound mountain ranges. Some of the veins con- tain sulphides and minor gold deposited as part of in- crementally formed fracture ®llings. Oxygen isotope ratios of calcite range widely between +6 and +24&, and calcite d 13 C PDB  )5.5 to )11.5&. The veins formed from isotopically exchanged crustal ¯uid with a probable meteoric water component. The shallow vein network is the near-surface expression of a tectonically induced hydrothermal system which has deposited gold- bearing veins with a mesothermal style over several vertical kilometres. This vein network has formed in a dilatational zone of the oblique collisional orogen where near-vertical fractures tap deep-sourced ¯uids. Similar processes acting at the southern end of the Southern Alps in the Miocene resulted in locally rich mesothermal quartz-gold veins. Introduction Mesothermal gold deposits are a major source for the world's historical gold production, and are extensively mined in many parts of the world. They typically consist of a set of auriferous quartz veins cutting greenschist facies rocks in exhumed metamorphic belts. The veins generally formed at middle crustal depths (Colvine 1989; Bottrell et al. 1990), although some shallower level (upper few kilometres) deposits occur also (Hagemann et al. 1994), and some have formed under essentially metamorphic conditions (Barnicoat et al. 1991). The best-known and most productive deposits are hosted by Archean greenstone belts (Colvine 1989), but most low-grade metamorphic belts have some mesothermal veins and some such Phaner- ozoic belts were rich producers. Because mesothermal gold deposits formed at several kilometres depth and are only exposed after several kilometres of erosion, no evidence is preserved of geological processes occurring in the surface region while the veins formed. As a consequence, much is known about geometry and geochemistry of meso- thermal systems (Sibson et al. 1988; Colvine 1989), but little or nothing is known about the overlying topog- raphy, the structural setting, or the surface expression of the mineralizing systems. This study attempts to de®ne the nature of the upper portions of mesother- mal deposits by examining evidence for hydrothermal activity in the Southern Alps of New Zealand. This region comprises a collisional mountain belt which has been active throughout the late Cenozoic and is still forming. Mesothermal gold deposits formed during the late Cenozoic deformation, and variable uplift has exposed auriferous veins at dierent crustal levels. This work focuses on the near-surface expression of the Southern Alps hydrothermal system by presenting new ®eld and laboratory data on previously undescribed veins from the Sealy Range (Fig. 1), and examining that data in the context of mesother- mal vein systems formed at deeper levels. We then extend our comparisons to historically exploited Miocene mesothermal veins farther south in the Otago Schist. Mineralium Deposita (1999) 34: 163±172 Ó Springer-Verlag 1999 Editorial handling: J. Hedenquist A.S. Templeton 1 á C.P. Chamberlain Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA D. Craw (&) á P.O. Koons Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand e-mail: dave.craw@stonebow.otago.ac.nz Present address: 1 Lawrence Berkeley National Laboratory, M.S. 70A-3363, 1 Cyclotron Rd., Berkeley CA 94720, USA