Gold mineralization without quartz veins in a ductile-brittle shear zone, Macraes Mine, Otago Schist, New Zealand

Greenschist facies schist which hosts the Macraes Mine in East Otago, New Z ealand has been pervasively altered by post-metamorphic (lower greenschist facies) fluids over a 120 m thick section perpendicular to foliation. Metam orphic titanite has been replaced by rutile, and epidote has been replaced by a variety of metamorphic minerals including siderite, chlorite, muscovit e and calcite. The early stages of this alteration occurred during developm ent of a ductile cleavage associated with kilometre scale recumbent folding . The cleavage was widely overprinted by a subparallel set of spaced (mm sc ale) microshears which are locally enriched in rutile and hydrothermal grap hite. Strain was then concentrated into narrow (m scale) zones where more i ntensely deformed portions of the rock are crossed and highly disrupted by closely spaced (100 mu m scale) microshears. The highly strained rocks show a combination of mylonitic and cataclastic microstructures, including crys tal-plastic grain size reduction and recrystallization of micas to form a n ew foliation. Muscovite has grown at the expense of albite in the mylonitic cataclasites. Hydrothermal alteration was accompanied by addition of pyrit e, arsenopyrite and gold without development of quartz veins. Gold precipit ated with sulphides during reduction of the fluid by hydrothermal graphite. The whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold, scheelite, rutile, pyrite and arsenopyrite . This deposit displays a continuum of postmetamorphic processes and hydrot hermal fluid Bow which occurred during uplift of the schist belt.