Tectonically driven fluid flow and gold mineralisation in active collisional orogenic belts: comparison between New Zealand and western Himalaya D. Craw a, * , P.O. Koons a , T. Horton b , C.P. Chamberlain b a Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand b Department of Earth Sciences, Dartmouth College, Hanover, NH, USA Received 6 November 2000; accepted 12 February 2001 Abstract Hydrothermal activity and mesothermal-styled gold mineralisation occurs near the main topographic divide of most active or young collisional mountain belts. The Southern Alps of New Zealand is used in this study as a model for the mineralising processes. The collisional tectonics results in a two-sided wedge-shaped orogen into which rock is transported horizontally. Upper crustal rocks pass through the orogen and leave the orogen by erosion, whereas lower crustal rocks are deformed into the mountain roots. High relief drives meteoric water flow to near the brittle – ductile transition. Lower to upper greenschist facies metamorphic reactions, driven by deformation at the crustal decollement and in the root, release water-rich fluids that rise through the orogen. Intimate chemical interaction between fluid and rock results in dissolution and later precipitation of gold, arsenic and sulphur. Fluid flow and mineralisation in the topographic divide region is facilitated by a network of steeply dipping faults and associated rock damage zones where oblique strike-slip faults intersect the thrust faults that strike subparallel to the main mountain range. The Nanga Parbat massif of the western Himalaya is an example of an active collisional zone which hosts hydrothermal activity but no gold mineralisation. The lack of gold mineralisation is due to the following factors: CO 2 - dominated rising metamorphic fluid in dehydrated amphibolite-granulite facies metamorphic rocks does not dissolve gold and arsenic; hot (up to 400 jC) meteoric water confined to fractures in the gneiss limits dissolution of gold and arsenic; low density of hot water/dry steam, and low reduced sulphur content of fluid, restrict solubility of gold and arsenic; absence of fracture networks in the core of the massif and the small volumes of circulating fluid limit metal concentration; and lack of reactive rock compositions limits chemically mediated metal deposition. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gold; Tectonics; New Zealand; Himalayas; Fluid flow; Mesothermal 1. Introduction Crustal scale fluid flow is an integral part of the processes which occur during development of colli- sional mountain belts (Koons et al., 1998). The general patterns of this regional fluid flow have been identified in many active or young mountain belts (Nesbitt, 1992; Chamberlain et al., 1995; Koons et al., 1998), but the details of structural control on fluid flow at the more local scale are poorly known. This is because, despite the high relief and excellent exposure in active moun- tains, fluids rarely penetrate the surface as visible 0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0040-1951(01)00253-0 * Corresponding author. Tel.: +64-3-4797519; fax: +64-3- 4797527. E-mail address: dave.craw@stonebow.otago.ac.nz (D. Craw). www.elsevier.com/locate/tecto Tectonophysics 348 (2002) 135– 153