ABSTRACT
The Birthday Reef was the most productive gold producer during historic mining of the Reefton goldfield on the west coast of the South Island of New Zealand. Deep exploration drill holes (up to 1.6 km long) intersected the mineralised quartz vein zone and adjacent hydrothermal alteration halo beneath the historic mine workings. The Paleozoic metasedimentary host rocks contain between 0.5 and 8 ppb Au and between 4 and 30 ppm As, and metamorphic pyrite typically contains 0.1 to 1 ppm Au in solid solution. The Au and As halo, above these background values, extends <20 m from the Birthday Reef, and other petrographic indicators of alteration are also confined to this narrow envelope. Porphyroblasts of pyrite and arsenopyrite, with minor Au enrichment, grew across the metamorphic cleavage in the alteration halo before emplacement of the Birthday Reef and associated shearing, and reflect an earlier, late metamorphic precursor to the main gold mineralisation phase.
Most orogenic gold deposits are dominated by metre-scale quartz veins that constitute a small target for explorationists. There is typically some alteration of the immediate wall rock to the veins, which can increase the width of the exploration target. However, the widths of these alteration zones vary as a result of the structural, lithological and geochemical processes involved in mineralisation. Hence, each individual deposit has its own characteristics, which hinders accurate prediction of the scale of exploration targets at the surface and in drill holes.
The Reefton goldfield of New Zealand (Figure 1) is a dismembered portion of the Lachlan Orogen of southeastern Australia, a well-endowed gold-bearing terrane with abundant orogenic vein deposits. Several attempts have been made to define the scale of wall rock alteration in Lachlan Orogen gold deposits, but because each deposit is unique, the characteristic footprint features defined in one deposit are rarely extendable to other deposits. Similarly, within the Reefton goldfield itself, there is a wide range of styles of mineralisation and resultant alteration footprints, which range from only centimetres to several tens of metres from veins
Historically one of the richest gold-producers in the Reefton goldfield, apparently has an extremely narrow zone of adjacent alteration (metre scale), and this alteration zone is the focus of this study. The Birthday Reef is essentially a single quartz vein, typically c. 0.6 m wide, with a steep dip that was the target for deep historic underground mining. Modern exploration activity has required deep drilling beneath the historic workings, and drill holes up to 1600 m long have been emplaced as part of the deepest gold-related drilling programme in New Zealand.
In this study, we used material from some of these deep drill holes to define the width of the alteration zone adjacent to the Birthday Reef and determine the hydrothermal features that constitute the footprint of the Birthday Reef mineralisation. We use low detection limit gold analyses to determine the extent of the gold enrichment halo in relation to background gold concentrations. We then integrate the gold halo with other geochemical and petrographic features of the alteration zone.
General geology
The Reefton goldfield lies on the western side of the South Island of New Zealand, and is hosted by Paleozoic metasediments of the Greenland Group in the Western Province.
These host rocks in the Reefton goldfield consist of turbiditic greywackes and argillites that have been metamorphosed to lower greenschist facies, preserving bedding and some sedimentological features.
A pervasive metamorphic cleavage cuts bedding at a range of angles associated with tight upright folding on the kilometre scale, with a north to northeast trend. Granitoid intrusives were emplaced into the Greenland Group in the late Paleozoic and Cretaceous. The Greenland Group in the Reefton goldfield was exhumed in the late Paleozoic, and is locally unconformably overlain by Devonian and early Cenozoic sediments. Gold mineralisation in the Reefton goldfield was controlled by faults that locally cut across, but are partially related to, the upright to steeply inclined fold hinges.
The Birthday Reef strikes north–northeast, sub-parallel to the fold axial surfaces of the hosting folds and the bedding that defines those folds. Known strike length is >1 km, and historic mining extended to >800 m below the surface. The historic mining extracted >700,000 ounces of gold (>22 tonnes), and typical gold grades were in excess of 15 g/tonne, before the mine closed in 1951. Recent exploration drilling has intersected the Birthday Reef a further 700 m below the last worked level of the mine, leading to an inferred resource estimate of c. 700,000 ounces of gold for this delineated portion of the reef.
https://www.tandfonline.com/doi/full/10.1080/00288306.2016.1274332
