Characterization of Ammonium Minerals in the Alteration Halos of the Favona, Martha, and Wharekirauponga (WKP) Low Sulfidation Epithermal Gold-silver Deposits in New Zealand

Abstract

Ammonium has been detected in and around several epithermal Au-Ag deposits, including those in Nevada, Japan, Argentina, Mexico, and New Zealand, using short-wave infrared (SWIR) reflectance spectroscopy. 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. The wallrock of all three deposits is altered to form quartz, illite, smectite, adularia, chlorite, and pyrite +/- kaolinite. Ammonium contents are enriched (>137 ppm) in wallrock samples from all three deposits and low (<94 ppm) in vein samples. Ammonium contents are higher at Favona (<10,675 ppm) than at Martha (<192 ppm) and WKP (<2,783 ppm). Leaching experiments using a 2N KCl solution show that most ammonium is in mineral structures (>90% at Favona, >80% at Martha, >70% at WKP). There is a positive correlation of ammonium contents with LOI (0.6 – 16.3 wt%) and with K2O (1.3 – 8.0 wt%) in all samples which suggest a hydrous potassium mineral as the major host of the ammonium. This is supported by the SWIR data obtained by previous workers of these samples where they show an absorption at ~1410 nm due to OH. At Favona, samples with high ammonium (>990 ppm) are reported to have significant absorption at ~2000 nm and ~2100 nm in the SWIR spectra likely due to ammonium. High ammonium contents (990 – 10,675 ppm) are found in rocks less than ~100m from the Favona vein which occur within an ammonium-bearing zone identified by previous workers based on SWIR. Samples outside of this zone contain low ammonium (107 – 301 ppm) with the smectite altered samples being the lowest. Ammonium contents within the hangingwall (1,827 – 10,675 ppm) of the Favona vein tend to be higher than in the footwall (990 – 4,301 ppm) and are highest within the most intensely illite altered rocks. At WKP, the intensely adularia +/- minor illite altered samples within 100m of the main East-Graben (EG) vein contain low ammonium (<200 ppm). The intensely illite altered samples away from the EG vein (>100m) have higher ammonium contents (200 – 800 ppm). This relationship of high ammonium contents to high illite abundance confirms illite as the major host of ammonium in these deposits. δ15N values for all samples (n=54) including near and far from auriferous veins range from +0.5 to +7.9 ‰, suggesting the derivation of most of the ammonium from the Jurassic greywacke basement or sediments intercalated within the volcanic rocks.