Proceedings of the 6th New Zealand Geothermal Workshop 1985. Karangahake fossil geothermal field : an epithermal gold-silver mineralised system. New Zealand Geothermal Workshop [7-9 Nov. 1984, New Zealand].

Anstiss, Richard G.

[1984]

CIENA
171

0-86869-056-2

A subsurface investigation of the Maria Lode and associated hydrothermal alteration of andesites facilitated reconstruction of the local evolutionary sequence in the once active Karangahake geothermal field. The evolutionary sequence involved two main phases of activity : phase A (dominant) - initial development of the Maria fissure by a progressive fault related hydrofracturing mechanism forming a breccia complex comprised of hydrothermally altered and hydrofractured andesite in a quartz matrix. Contemporaneous with this development was almost all the hydrothermal alteration of the host andesites forming three lateral zones around the Lode. The mineralogy in these zones was directly related to their distance from the Lode because with decreasing distance the increasing fluid/andesite ratio permitted greater attainment of equilibrium of the fluids with the thermodynamically stable secondary mineralogy (decreased buffering by andesite). The progressive mixing at ¸250°C of initial alkali-chloride fluids with the basal zone of the near surface acid-sulphate fluids during phase A caused a change in fluid composition resulting in deposition of gold and silver (as lectrum) within the quartz matrix of the breccia complex. Phase B (waning) - silicification and self-sealing of the near surface during phase A culminated in a hydrothermal eruption and onset of local waning..

Presenta diagrs., gráfs. y maps.

NUEVA ZELANDIA; ENERGIA GEOTERMICA; CAMPOS GEOTERMICOS; MINERALES; EVOLUCION; ANALISIS HISTORICO; GEOQUIMICA; ALTERACIONES HIDROTERMALES;