GEOCHEMICAL VARIATIONS IN THE SAMS CREEK GOLD PROSPECT, NW NELSON, NZ; U-PB DATING OF A GOLD BEARING HYDROTHERMAL EVENT

Gold mineralization at Sams Creek, 20 km east of the paleo Gondwana margin, lies amongst an under-explored region with a rich history of alluvial gold prospecting dating back to the 1850s. Gold at Sams Creek is hosted by an A-type peralkaline microgranite dike that extends 7km along strike and is up to 60m thick. Over 120 diamond drill holes sample a kilometer of vertical relief from which we recognize large scale mineralogical and geochemical variations. Analyses of 1m half-core reveal a Au-As-Sb assemblage to be anomalous over the full scale of the deposit. Specific zoning patterns of Ag, Pb, and Zn are observed throughout the range of elevations sampled. Most notably Ag increases with elevation, while Pb and Zn increase with depth. Mo is also relatively elevated at depth.

Multiple stages of hydrothermal activity have altered the dike including the precipitation of late arsenopyrite veins, base metal sulphides, and gold. Gold-silver alloy (~80-85% Au) occurs as small growths (up to 40 µm) cementing and in-filling brecciated and fractured arsenopyrite (the most abundant ore mineral). All samples from the Sams Creek Dike are highly enriched in LREEs, moderately enriched in HREEs, have a distinct negative Europium anomaly, and show no significant variation between stages of alteration. This suggests the source of mineralizing fluids shares a similar trace element chemistry to that of the dike.

Hydrothermal zircons occur as clusters and strings of <1-15µm crystals along grain boundaries of arsenopyrite and within the same micro fractures which gold grains occupy. We interpret these as hydrothermally remobilized magmatic zircons. Magmatic zircons also occur in un-mineralized samples as relatively large ellipsoidal individual grains. They exhibit a spongy dissolution texture and grain size of up to ~25µm. U-Pb analysis of the hydrothermal zircons by LA-ICP-MS assign a Cretaceous age for hydrothermal mineralization.