AMMONIUM MINERALS IN THE ALTERATION HALOS OF EPITHERMAL GOLD-SILVER DEPOSITS IN THE HAURAKI GOLDFIELD, NEW ZEALAND

Ammonium has been detected in some epithermal Au-Ag deposits, including in Nevada, Japan, Argentina, Mexico, and New Zealand, using short-wave infrared (SWIR) spectrometry. When ammonium is present, it is concentrated around veins. This study examined the distribution and occurrence of ammonium in three epithermal low-sulfidation vein-type deposits in the Hauraki goldfield of New Zealand: Martha (>6.7Moz Au, >42.1Moz Ag), Favona (>0.6Moz Au, >2.36Moz Ag), and the recently discovered Wharekirauponga (WKP; 0.42Moz Au, 0.8Moz Ag) deposit. The Martha and Favona auriferous quartz-adularia veins are hosted by late Miocene to Pliocene andesite, whereas auriferous veins at WKP are hosted by late Miocene to Pliocene rhyolite. SWIR spectrometry identified ammonium at Favona and WKP but not at Martha. The wallrock is altered to form quartz, illite, mixed layer illite/smectite, adularia, chlorite, and pyrite. Kaolinite occurs at WKP. Ammonium contents are low in veins (<94 ppm) but are commonly high in the altered wallrocks. The wallrock ammonium values at Favona (<10,117 ppm) are much higher than at both Martha (<192 ppm) and WKP (<982 ppm). Leaching experiments using a 2N KCl solution shows that over 90% of ammonium is in mineral structures. Samples with high ammonium (>1,000 ppm) show significant absorption at 1980 nm and 2100 nm in the SWIR spectra, suggesting hydrous alteration minerals as the major host of ammonium. This is supported by a positive correlation of ammonium concentrations with LOI (0.6-16.3 wt%) and with K₂O (1.3-8.0 wt%), Buddingtonite (ammonium feldspar) was identified in a few samples at WKP. At Favona, SWIR identified a halo of ammonium minerals extending ~100m from veins which corresponds to a zone of high ammonium (990-10117 ppm). The footwall illite altered zone contains 990-4301 ppm ammonium. In the hanging wall, within 100 m of the mineralization in the mixed layer illite/smectite alteration zone, ammonium values range from 1827-10117 ppm. Outside of 100 m from the mineralization in the hanging wall, ammonium values range from 107-301 ppm with the smectite-bearing samples being the lowest (107 ppm). δ¹⁵N values for all wallrock samples (n=54) range from +0.5 to +7.9 ‰, suggesting the derivation of nitrogen from the Jurassic greywacke basement or sediments intercalated with volcanic rocks.