Soil gas geochemistry in the southern Hauraki Goldfield, New Zealand

Abstract

The high mobility of soil gases has made soil gas geochemistry an attractive exploration tool in search for structures related to mineral deposits covered by thick postmineral cover. Soil gas sampling was undertaken within the southern portion of the Hauraki Goldfield, including Waihi East, WKP, the Hauraki Rift and Golden Cross. There were three key goals for this study: (1) to detect subsurface mineralised veins or intersecting faults that may enhance gas migrations into shallow soil; (2) to identify the major influences on soil gas compositions; and (3) to recognise how soil gas can be used with other exploration methods. There were 379 samples collected from an area of 2 x 2 km at Waihi East. Fifteen samples were obtained from a single line at WKP that was compared with 39 samples collected in 2011. An additional 35 samples were taken along short lines in the Hauraki Rift. Soil gas Hg anomalies are offset, but roughly follow the trends for mineralised veins and faults extrapolated from Martha and Favona. Anomalies are also within 25 m and 50 m, respectively to mapped faults in the Hauraki Rift and WKP. This study demonstrates that soil gas Hg can be used to confirm or identify faults with a high degree of accuracy. Most sites with anomalously enriched CO2/O2 ratios do not contain anomalous Hg concentrations. The variation of CO2 and O2 is primarily an indication of microbial degradation of organic matter and of aeration, and therefore, co-spatial CO2/O2 and Hg anomalies are coincidental. Soil gas was sampled five times on a monthly basis along short lines at Waihi East and Golden Cross. Within a warm temperate to subtropical climate, rainfall appears to be the most important variable for controlling concentrations of soil gas Hg, creating up to two orders of magnitude variations. In contrast, the CO2 and O2 contents of soil gas are relatively unaffected by rainfall. The NH3, H2S, and CO soil gases, if present, are at concentrations below detection limits, except for in wet soils. At Waihi East, soil gas Hg anomalies identify a narrower region within a larger zone of anomalous soil Au, Ag, As and Sb that presumably reflect mineralised veins. Anomalies of soil gas Hg also overly high amplitude resistivity that may indicate silicified rocks. However, the area is covered by thick postmineral cover, where the underlying rock type needs validation. Results in this thesis can be used to enhance interpretation of soil gas data, and to develop better guidelines for future soil gas analyses alone, or in conjunction with other exploration techniques.