Hydrothermal Alteration: Separating Fact from Hyperbole

Abstract

The value of information that can be gleaned from hydrothermal mineral studies in geothermal exploration and development projects is well known. Identification and interpretation of hydrothermal mineral assemblages is an important aspect of surface and subsurface geothermal exploration, as it may provide insight into the present and/or past nature of the geothermal reservoir. Observational and analytical tools available to the exploration geothermal geologist are wide and increasingly sophisticated. For several decades, the significance of mineral occurrences (e.g., indicators of temperature conditions at the time of mineral formation), textural relations (e.g., insights into system longevity and evolution, or influx of cool marginal fluid ingress) and chemistry (e.g., indicators of low pH conditions, potential for natural self-sealing or development-induced scaling etc.) has been recognised. However, at a time of geothermal renaissance in many parts of the World, it is a concern that the petrological skills are not used to their maximum potential, or are even being ignored. Low petrological skills of entrance-level geologists, and poor geothermal exploration and drilling practice knowledge of decision-makers new to the geothermal industry, mean potentially vital exploration and development information is being overlooked or superficially considered. This is of concern because petrological information could help resolve the conceptual hydrological-chemical (e.g., past vs. present reservoir characteristics) and geophysical-geological (evolution) framework of a system. This paper provides an opportunity to highlight proven petrological practices and insights, including conventional techniques and recently developed tools, and knowledge that can be gleaned from hydrothermal minerals studies that, to some degree, are at risk of been lost or devalued. Whilst highlighting the value of information that can be obtained, we also discuss the pitfalls, mineralogical misrepresentation and ambiguities within the exploration and development-stages of the conceptual model.