| Abstract |
In recent years, the large number of geothermal systems explored has allowed a close comparison to be made between active and extinct systems so that many hydrothermal ore deposits now have an active counterpart. However, there are still several notable differences between active and extinct geothermal systems; for example,many of the very much larger in area, and show few effectsdue to waning. Despite this, a knowledge of the processes that occur in active geothermal systems can be a very useful guide in evaluating a particular hydrothermal ore deposit or prospect. Particularly important are mineralogical and textural guides to temperature, fluid type and salinity), thermal change and the flow regimes that operated. Interpretations from petrographic and structural observations can then be used to deduce the paleohydrology of a hydrothermal mineral deposit and to conditions under which mineralisation However,the upper levels of many ore deposits have been removed by erosion, and the of this important section of a system is often unknown. For this reason, epithermal geologists need to recognise and understand processes that can occur in the near-surface levels (upper m) of active geothermal systems. Shallow processes in a geothermal system are controlled by the prevailing terrain which, in turn, affects the reservoir hydrology and its thermal regime. A vertical distribution of different fluid types results, and this is reflected in the alteration mineralogy. For example, where hot chloride water reaches the usually at lowest elevations, different types of silica form geyserite, ripples, dunes, At some locations, such as Champagne Pool, Waiotapu,New Zealand, silica sinter is accompanied by of "ore grade" concentrationsof gold and silver because earlier-formedamorphous antimony and precipitates scavenge theseprecious metals from fluid in them. Where the alkali chloride water becomes appreciably diluted with fluids or has travelled long distances, it is incapable of precipitating silica but, in some cases,calcium precipitates instead, due to loss of dissolved Above the alkalichloride water, waters of slightly acid commonly form calcite, siderite and interlayeredillitemontmorillonite. Where ascending oxidises the hydrothermal alteration processes are essentially of a destructive nature, producing silica residue, kaolin, hematite, alunite, and other sulfates. Most constituents needed to farm thesephasesderive the rocksnearby, with only and, ond o n s , ammonia and mercury, depositingfrom ascendingvapour. Hydrothermal eruptions arecommon events in active geothermal fields,and the presence of breccias and rocks with hydraulically textures shows that they were also during the lives of many hydrothermal ore deposits. Recent studies of hydrothermal eruptions in active geothermal fields in New Zealand show that these eruptions start at the surfaceand downwards. It is importantto recognise this since eruption breccias and brecciated rocksarecommon hosts for major ore mineralisation. |