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 clearly 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.

Textural relations and thermodynamic considerations show that, for a time, wairakite, epidote, and prehnite grew together in apparent equilibrium from quartz-saturated water with log(a_Ca²⁺/$aH+2$⁠) : 8 and log(a_Fe³⁺/$aH+3$⁠): –8. The approximate minimum growth rates of individual crystals are estimated to be wairakite, 190 μm³ per day; prehnite, 0.2 to 0.5 pm per day in a direction parallel to the 110 faces, and epidote, 0.2 μm per day in a direction parallel to the 100 faces.