Hydrothermal Alteration and Veins at the Epithermal Au-Ag Deposits and Prospects of the Waitekauri Area, Hauraki Goldfield, New Zealand

摘要

The Waitekauri area of the Hauraki goldfield, New Zealand, contains several adularia-sericite epithermal Au-Ag deposits and prospects. From west to east, the area contains the Sovereign, Jubilee, Scimitar, Scotia, Teutonic, and Jasper Creek deposits and prospects, which are hosted by andesitic and dacitic flows, breccias, and localized pyroclastic and air fall deposits. Drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3-km-long composite cross section through the area. Most host rocks are intensely altered, with 100 percent of the igneous minerals replaced by hydrothermal minerals, although the alteration intensity becomes more variable and weaker toward the east. Alteration minerals include quartz, adularia, albite, chlorite, pyrite, illite, interstratified illite-smectite, smectite, calcite, hematite, and minor epidote. Many of these minerals have zoned distributions; adularia is widespread at Sovereign, but is restricted to shallow levels at both Scotia and Jasper Creek. Albite occurs in a discrete zone below adularia at Scotia, and minor epidote is restricted to Sovereign and Jubilee. Illite occurs throughout Sovereign and Jubilee and at the western margin of Scotia and Scimitar, where it grades eastward into interstratified illite-smectite and smectite at Teutonic and Jasper Creek. Veins are typically less than 10 cm wide, but have diverse mineralogy with zoned distributions. Quartz veins dominate at Sovereign and Jubilee, whereas calcite veins are more abundant at Scotia, Scimitar, and Jasper Creek. Laumontite occurs at Scotia and locally at Scimitar, whereas veins of clinoptilolite and mordenite ± calcite occur at Jasper Creek and stilbite veins occur at Teutonic. Fluid inclusions in quartz and calcite homogenized between 132o and 310oC and trapped a dilute solution with an apparent salinity of less than 2.6 wt percent NaCl equiv. Homogenization temperatures are highest at Sovereign (avg. 241oC), Jubilee (avg 239oC), and Scimitar (avg 236oC), lower at Scotia (avg 204oC) and lowest at Teutonic (avg 168 oC) and Jasper Creek (avg 162oC). Estimated positions of the paleowater table above Sovereign, Jubilee, Scimitar, Scotia, Jasper Creekg and Teutonic relative to present elevations was at least 690, 750, 575, 450, 225, and 150 m above sea level, respectively; the deposits and prospects, therefore, span a 600-m vertical interval. Individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west. Alteration intensity, alteration and vein mineral distributions, and fluid inclusion microthermometry are interpreted to indicate that Sovereign and Jubilee formed at relatively high temperatures, whereas Teutonic and Jasper Creek formed at relatively cooler temperatures. Several hydrologic reconstructions are possible, including (1) a single hydrothermal system with an inclined water table and significant lateral outflow to the east, or 2) a single low-relief hydrothermal system with a flat-lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10o to the east. Regardless of the preferred reconstruction, the Sovereign and Jubilee deposits appear to have formed in the main zone of fluid upflow, whereas the Teutonic and Jasper Creek prospects appear to have formed toward the margin. Moreover, the greatest erosion has occurred at the Jubilee and Sovereign deposits (~300–400 m), and these may represent the roots of a more extensive vein network that has largely been eroded.