Formation rates of calc-silicate minerals deposited inside drillhole casing, Ngatamariki geothermal field, New Zealand

Chips ejected during the initial openings of Ngatamariki well, NM2, in the Taupo volcanic zone, New Zealand, consist of euhedral crystals of wairakite, prehnite and Fe-rich epidote, plus rare quartz and pyrite. The shapes of some of the ejecta surfaces show that they formed inside the casing and liner slots of the well during the 472 days it was shut in. The calc-silicates were derived from a depth of 1580 to 1600 m where they grew from hot, dilute alkaline chloride water of near-neutral pH and low dissolved CO2 content.