The nature and origin of gold mineralization at Sams Creek, north-west Nelson

Gold mineralization is hosted by a peralkaline granite dyke which outcrops over about eight kilometers in the area of Sams Creek, a tributary of the Takaka River, north-west Nelson. The granite is riebeckite and acmite bearing, with antiperthitic alkali feldspars; it has A-type chemistry, implying an origin by partial melting at a temperature > 800°C. Associated with the granite is a suite of lamprophyre dykes; now highly altered, they probably range from original camptonites to quartz - bearing calc-alkaline types. A quartz- bearing lamprophyre has intruded along the footwall contact of the granite over much of the observed outcrop length; it may have been a source rock for gold. Granite intrusion is inferred to have occurred before or during the 0 2 deformation in the surrounding shales and quartzites, suggesting a minimum age of 400Ma. Deformation involving E-W compression has caused the granite to break into essentially rigid pieces, of the order of 50m in size, surrounded by ductilely deforming sediments. Strain has been concentrated into the footwall lamprophyre, which has also served as a conduit for hydrothermal fluids. Locally, small brittle - ductile shear zones occur within the granite; some of them post - date the mineralization. Tectonic imbrication of the granite and the development of fracture permeability have probably controlled the locus of gold mineralization. Two styles of hydrothermal alteration are observed: in Main Zone, hydrolysis of riebeckite produced the assemblage magnetite+ siderite; an influx of Fe, As, S and associated Au then produced the assemblage quartz + albite + K-feldspar + pyrite + arsenopyrite. Gold is present as the native element, poikilitically enclosed by arsenopyrite. In the western part of the area, iron has been lost during the early stages of alteration to produce the assemblage quartz - albite - K-feldspar; with further reaction, feldspars alter to sericite, and pyrite and arsenopyrite are deposited. Gold values are lower than in Main Zone. The mineralizing fluids are inferred to have been metamorphic in origin. Geothermometry based on the assemblage arsenopyrite - pyrite - sphalerite suggests a temperature of 370 ±50 °C for Main Zone. Fluid inclusions in the western part of the area show evidence of immiscibility between H2O and CO2; their homogenization temperature directly yields an estimate of 260 ± 25°C for the hydrothermal fluid. Water-bearing inclusions from Main Zone, taken in conjunction with the arsenopyrite – sphalerite geothermometer, suggest a pressure of between one and two kilobars for the mineralization. In Main Zone, progressive hydrolysis of granite was accompanied by falling aO2 , which resulted in the precipitation of arsenopyrite; this in turn caused a localized fall in total. dissolved sulphur which caused gold to co-precipitate. The fluid pH was ~ 9. Further west, in the sericitically altered zone, the fluid pH was buffered to between 5 and 5.4 by the alteration of K-feldspar to sericite. The lower pH has been critical in limiting gold mineralization in the western part of the area