Hydrothermal Alteration
The interaction between the circulating thermal fluids and rocks results in water-rock reactions that produce assemblages of secondary (hydrothermal) minerals that are often used as a guide to present or past subsurface temperatures and fluid type. Permeability and porosity of the host rock strongly influence the water-to-rock volumetric ratio and the degree to which the original minerals are altered. The suit of secondary minerals is less controlled by rock type than by temperature and composition of the fluids (Browne, 1978).
The typical hydrothermal alteration found in most geothermal systems is the prophyllitic type, which consists of several mineral assemblages. In order of increasing depth and temperature these are as follows: a) clay-zeolite, b) calcite- mixed clays-chlorites and c) chlorite-epidote.
Classification of the hydrothermal mineral types associated with deposition environment in the geothermal system is as follow:
1- Acidic type: Near surface boiling, the release of H2S into the gas phase and the influx of oxygenated meteoric water combine to yield abundant H+ and an alteration assemblage consisting of aluminosilicates such as alunite, kaolin etc. is the acidic type
2- Alkaline type: A neutral Ph, Na-Cl type circulating fluid in the reservoir can produce alkaline type mineral assemblage by water-rock interaction either ion exchange and/or ion dissolution and precipitation in fracture and pores.
One of the important parameters to form hydrothermal minerals is the meteoric water infiltrated through the fissures to deeper and deeper, and interacted with rocks in the earth crust. Temperature, pressure and chemical changes in water and rock strongly affect the type of hydrothermal minerals as simply illustrated in Fig. 1
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Alteration minerals can be used as a guide to evaluate the reservoir temperature as well as the evolution history of geothermal systems. The important point is that to notice the mineral in samples is altered mineral or original rock forming mineral. In order to realize this one should has experience on study of alteration such as to collect the samples both from in-situ and well cuttings or cores and how to analyse it.
Some hydrothermal minerals and their forming temperature ranges are below in Fig. 2.
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Tags: geothermal system, hydrothermal alteration, hydrothermal mineral types, secondary minerals