| Abstract |
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 main phases of activity : Phase A (dominant) - initial development of the Maria fissure by a progressive fault related hydrofracturing mechanism forming a breccia complex p rise d of hydrothermally altered and hydrofractured andesite in a quartz matrix. Contemporaneous with this development was almost a l l the hydrothermal alteration of the host andesites three lateral zones around the Lode. The mineralogy in these zones w a s directly related to their distance from the Lode because with decreasing distance the increasing ratio permitted greater attainment of equilibrium of the fluids with the thermodynamically stable secondary mineralogy (decreased buffering by and e site ). The progressive mixi n g at of initial a l k a l i- chloride fluids with the basal zone of the near surface acid- sulphate fluid s duri n g Phase A caused a change in fluid composition r e s u l ti n g in deposition of gold and silver (as within the quartz matrix of the breccia complex. Phase B (waning) - silicification s e l f- sealing of the near surface during Phase A culminated in a hydrothermal eruption and onset of local waning. Phase B there w a s minimal interaction, fluids remained at but of slightly different mixed composition than that at the end of Phase A. The hydrothermal eruption e j e c t e d Phase A breccia complex material forming a lensoid conduit in the Maria Lode up which t here were fast flowing fluids. These fluid s deposited hematitic potassium aluminosilicates on the walls of the conduit until the flow rate dropped due to r e - s i l i c i f i cation and re- selfsealing in the near surface. This w a s followed by a second hydrothermal eruption which caused an implosion in the conduit breccia t in g and milli n g the hematitic potassium aluminosilicates. A fine immature lensoid shaped breccia w a s formed containing a small irregular conduit up which there were flowing fluid s ( last locally ) depositing p y r i t i c potassium aluminosilicates until the fluid flow in the ceased altogether due to total silicification with temperatures beginning to fall. Throughout The Karangahake geothermal field later became ext in c t, eroded down 150-200 near tne summit of Karangahake and mined a s a gold- silver deposit. While much data from New Zealand active geothermal fields can applied to the fossil m a l field tne greater access i b i l i t y of the latter perm i t s further investigation of some features . |