Tectonic controls on gold deposits and their environmental impact, New Zealand

New Zealand is an active orogenic belt which varies along its length from c ontinental collision to continent-ocean subduction tectonics. Mesothermal g old deposits, with rare mercury, have formed in collisional settings from l ate Mesozoic to Pleistocene, with youngest deposits along the axis of the a ctively rising Southern Alps mountain chain. Epithermal gold and mercury de posits have formed, and are still forming associated with calcalkaline volc anism above the subduction zone, and with basaltic volcanism associated wit h extensional faulting. The environmental impact of mineral deposits is rel ated to tectonic setting, mineralogy, and climate, all of which are governe d by the geometry of the orogen. Mesothermal mineralisation added carbonate s to calcite-bearing host rocks in what is now cool semiarid or extreme Alp ine settings, and oxidation of sulphides does not result in significant aci dification of the environment. Arsenic is the principal metal of environmen tal significance, and is readily mobilised from these deposits at neutral t o alkaline pH. Mercury may be leached from cinnabar and/or gold on geologic al time scales. In contrast, epithermal mineral deposits have hydrothermal clay alteration, locally further clay-altered by deep temperate to subtropi cal weathering, and calcite is generally subordinate to sulphides. Acidific ation accompanies oxidation of these deposits, and copper, cadmium, lead an d zinc are readily mobilised into the environment. Mercury can be mobilised by these acid solutions as well, where mercury occurs in minerals other th an cinnabar. (C) 2001 Elsevier Science B.V. All rights reserved.