QEMSCAN AND MICRO-XRF ANALYSES REVEAL THE LOCATION OF AU IN BANDED EPITHERMAL VEINS

Colloform-crustiform banded low to intermediate sulfidation epithermal veins are important sources of gold and silver globally. These veins predominantly contain the gangue minerals quartz and calcite and form from near-neutral pH low-temperature hydrothermal solutions. Detailed micro-XRF (x-beam) and QEMSCAN (e-beam) mineralogical and elemental mapping, combined with reflected light microscopy, and SEM, EPMA, and LA-ICP-MS analyses, are used to document the presence and textural relationship of Au- or Ag- bearing minerals to gangue minerals in classic epithermal veins from the Hauraki Goldfield of New Zealand.

The micro-XRF can analyze large rock samples and detect the presence of micron sized Au- or Ag-bearing minerals on or just below the surface. For each sample, a rapid (<60 minutes) scan with coarse pixel spacing (100-200 µm) was used to identify areas of interest that were subsequently analyzed at a fine pixel spacing (5-20 µm). Results show that the Au and Ag minerals are not evenly distributed throughout the veins, occurring primarily within specific bands. These bands typically have quartz that is much finer-grained than the quartz in adjacent bands, which has been interpreted to reflect flashing of hydrothermal solutions. In veins that formed at deeper levels, electrum commonly coexists with base metal sulfide minerals such as sphalerite and galena.

The most common and abundant gold-bearing mineral is electrum, whereas other gold-bearing minerals such as uytenbogaardtite, fischesserite, and petzite are rare. The LA-ICP-MS analyses show that Au also occurs in solid solution in arsenian pyrite. Silver-bearing minerals include sulfides, with less common selenides, sulfosalts, and tellurides.

The restriction of gold to specific bands may reflect: (1) input of precious metals from a magmatic fluid that is more metal-rich than the meteoric water that normally dominates the hydrothermal system, or (2) extreme physical processes (e.g. flashing) that led to efficient metal deposition. Available isotopic and fluid inclusion data from New Zealand and elsewhere suggest that both causes are likely, and that a metal-rich hydrothermal fluid is a necessary precursor to form rich concentrations of gold within specific vein bands.