Abstract
Gold mineralisation at Sams Creek occurs within an under-explored region with a rich history of alluvial gold prospecting dating back to at least the 1850s. Gold at Sams Creek is structurally hosted by sheeted veins and irregular fractures in an A-type peralkaline microgranite dike which extends 7km along strike and is up to 60 meters in thickness. The deposit was first discovered by CRA Exploration and was then further explored by OceanaGold, currently in a joint venture with MOD Resources. The majority of exploration has been conducted in the “Main Zone” including over 120 diamond drill holes.
At least three stages of hydrothermal activity have altered the dike including the precipitation of late arsenopyrite veins, and lesser amounts of pyrite, sphalerite, galena, and gold. Scanning Electron Microscopy (SEM) is used to show that gold occurs as an alloy with silver (~80-85% Au) as small growths (up to 40 μm) with pyrite and base metal sulphides. These phases are primarily found cementing and in-filling fractured and brecciated arsenopyrite (the most abundant “ore mineral”).
A reduced assemblage characterises the unaltered peralkaline microgranite dike which undergoes its first alteration (T1) under relatively oxidizing and ductile conditions during a late magmatic stage. Later, structurally controlled vein-related alterations (T2, T3, & T4), tied to regional deformation, occur under reduced and brittle conditions. Microstructural deformation during these stages is primarily characterised by a combination of extensional and shear related features.
This study isolates geochemical enrichments related to specific alteration and vein assemblages from analysis of a targeted, independent sample set collected for this research. Using exploration generated assays, from 1 m half-core sampling a kilometre of vertical relief, this study also defines spatial distributions and variations in geochemical signatures throughout the deposit scale. A Au-As-Sb assemblage is anomalous over the full scale of the deposit, and zoning patterns of Ag, Pb, and Zn are observed throughout the full range of elevations sampled. Most notably, Ag increases with elevation, while Pb and Zn increase with depth. Mo is also relatively elevated at depth.
Hydrothermal zircons first reported by Nazimova (2012) occur as clusters and stringers of <1-15μm anhedral crystals along grain boundaries of arsenopyrite and within the same micro fractures which gold grains occupy. These zircon are interpreted as forming from hydrothermally dissolved and reprecipitated zirconium present in the unaltered Sams Creek Dike. Zircons also occur in un-mineralised samples as larger ellipsoidal to faceted individual grains associated with primary mafic minerals and are interpreted as magmatic in origin. These zircons are up to ~25μm in size and many grains exhibit a spongy dissolution(?) texture.
U-Pb analysis of each zircon phase, by LA-ICP-MS, indicates a mid-Cretaceous age for both dike emplacement (109 ±9 Ma) and the main gold-bearing T4 hydrothermal event (114 ±6 Ma).
The presented Cretaceous emplacement age differs significantly from the published Carboniferous age (319 ±8 Ma) derived from Ar-Ar dating of amphibole by Tulloch and Dunlap (2006). A model is proposed for emplacement of the Sams Creek Dike under conditions of localised extension during the last stages of active convergence and subduction along the Pacific-Gondwana margin.
The Cretaceous mineralisation date presented here is in agreement with the age interpreted by Jongens (2013) from structural mapping and a resulting deformation-related model for mineralisation.
The debate about possible sources for gold-mineralising fluids is not resolved here. However, possible sources for the mineralising fluid are discussed along with their implications for regional exploration potential.