Abstract

The Karangahake deposit is the third largest gold producer in the Hauraki goldfield, Coromandel, New Zealand. Production was mainly from the Maria and Welcome/Crown veins that were mined over a strike length of >1,300 m and vertical interval of 700 m. The veins are hosted in andesite that is strongly altered over an area of approximately 4.2 × 2.7 km. By using petrography, qualitative and quantitative X-ray diffraction (XRD), and automated mineralogy from a scanning electron microscope (SEM), hydrothermal alteration has been characterized in detail for surface rocks exposed along the Ohinemuri and Waitawheta rivers and along the underground Keillors crosscut, which links the Maria and Welcome/Crown veins. The chemistry of hydrothermally altered rocks was analyzed using portable X-ray fluorescence (pXRF). The automated mineralogy reveals alteration mineral occurrences, abundances, and some textural relationships that are not as apparent from conventional studies.

Hydrothermal alteration of the andesite is typically strong to intense, with 100% conversion of primary to secondary minerals, but decreases to moderate to weak, comprising <25% secondary minerals toward the west and east, where the margins of alteration are exposed. The andesite is altered by combinations of quartz, chlorite, adularia, albite, illite, pyrite, calcite, mixed-layered illite-smectite, smectite, and rare epidote. Quartz and chlorite are common and present in both strongly and weakly altered rocks. Adularia is widespread and extends laterally up to 500 m from the Maria and Welcome/Crown veins. Illite is broadly coextensive with and locally occurs beyond the adularia halo. The zone of illite alteration is bordered to the east by patchy mixed-layered illite-smectite and outlying smectite alteration. Along the Keillors crosscut, the rocks between the Maria and Welcome/Crown veins are altered to adularia, albite, illite, chlorite, and rare calcite and epidote.

Automated mineralogy and quantitative XRD mineral estimates indicate many rocks have been altered by >30% adularia (max. 74%) and that the amount of adularia can exceed that of quartz. Adularia and albite occur throughout the Keillors crosscut, with adularia most abundant within 20 to 50 m of the Maria, Mystery, and Welcome/Crown veins, whereas albite is more abundant between these veins.

Hydrothermal alteration has resulted in the variable gains in K and Rb forming anomalous halos that surround veins, which are attributed to adularia and illite. By contrast, Ca, Sr, and to a lesser extent Fe are variably lost around veins in broadly overlapping zones of depletion.

The key hydrothermal alteration minerals are illite, illite-smectite, smectite, adularia, and albite. The systemscale zonation of illite, illite-smectite, and smectite outlines the broad thermal structure, with illite stability indicating >230°C, illite-smectite indicating 130° to 230°C, and smectite indicating <130°C. Adularia indicates zones of inferred high permeability and the central upflow of boiling hydrothermal fluids, whereas albite indicates lower inferred permeability and peripheral-style alteration. Adularia represents a broad-scale vector toward veins, and its occurrence and abundance can be determined using field portable XRD or chemically by proxy from the K/Al value.

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