Abstract:
40Ar/39Ar dates of the Martha, Empire, Welcome, and Moonlight veins in the Waihi vein system, Coromandel Peninsula, New Zealand, yield insight into the evolution of this epithermal Au-Ag deposit. The preferred ages of each vein are: 6.19 ± 0.05 Ma for Moonlight vein, 6.08 ± 0.07 Ma for Martha vein, 5.85 ± 0.02 Ma for Welcome vein, and 5.83 ± 0.05 Ma for Empire vein. These ages suggest the Moonlight vein formed first, followed by the Martha vein, and subsequently, vein splays of Martha (Empire and Welcome vein). The timing for formation of veins at Union Hill is unknown, but these may have formed during the same upflow event that initiated the formation of the Martha vein splays. Dates from two bands in the Martha vein are 5.93 ± 0.06 Ma and 6.08 ± 0.05 Ma, indicating at least two separate hydrothermal pulses occurred during its formation. The lifespan of formation for the Waihi vein system is suggested to be 590,000 years, extending the lifespan indicated in previous studies. Dates obtained in this study provide quantitative evidence for an episodic formation for the Waihi vein system. Quartz textures identified in hand sample, thin section and cathodoluminescence (CL) imaging, together with trace element incorporation in quartz, indicates an episodic vein formation at a timescale unresolvable by current radiometric dating. Quartz textures show lateral and vertical variability throughout the Waihi vein system, indicating changes in the depth of boiling and the level of vein emplacement in the system. Cathodoluminescence imaging reveals several quartz textures that are not observed in thin section or hand specimen, many of which have not been seen in previous studies and are therefore described here for the first time. Trace elements incorporated in quartz have a strong control over the CL intensity. Aluminium and calcium act as both activators and quenchers, iron acts as either an activator or quencher, and titanium acts solely as a quencher in all samples. The combined results of this study indicate that the Waihi vein system formed from episodic mineralisation related to at least two upflow zones within one hydrothermal system, or as a result of two discrete hydrothermal systems.
