Inhaltszusammenfassung:
Fluids (H2O ± CO2) that flow through the earth's crust are major transporters of dissolved elements, such as Mg, Na, Ca, Fe, Ba, as well as more precious elements, such as Pb, Zn, Cu, Co, Ag and Au. Fluids in the crust can also carry significant amounts of heat. Hydrothermal fluids are, by definition, fluids that are hotter than the surrounding rocks and are associated with many ore deposits, for example the huge Olympic Dam U-bearing iron oxide-copper-gold (IOCG) deposit in South Australia and the abundant small deposits in the German Schwarzwald. When such fluids are associated with the formation of breccias, they are referred to as hydrothermal breccia. Hydrothermal fluids are often involved in the process of dolomitization. When crustal fluids (hydrothermal) cause fractures, they may cause significant dilation to create breccia.
Breccias can form in various ways, such as through sedimentation, igneous activity, meteorite impact, tectonic diminution, collapse, or fluid. These can be grouped into three broad categories: (i) igneous, (ii) sedimentary, (iii) tectonic. To classify them prefixes are commonly used, such as volcanic, igneous, hydrothermal, chert, fault, impact and seismic breccias. Hydrothermal breccias may thus share similarities with tectonic, dissolution, as well as collapse breccias. A distinguishing feature of a hydrothermal breccia is that the fracturing processes is dominated by a high fluid pressure and not primarily by elevated differential stresses as is the case for fault and impact breccias.
Apart from their significance for fundamental research, breccias are very important host rocks for ore deposits, and can also be zones of high permeability and thus serve as potential aquifers and reservoirs. One major setback on their study, the sub-classes, is that it’s often difficult to determine based solely on their origin, especially in terms of the genetic and/or textural classification under a major type. Improved definition of the diagnostic signature of the individual breccia types requires improved knowledge on their brecciation mechanisms, as well as their genetic associations and geological setting. This has resulted in hydrothermal breccia receiving a considerable attention in structural geology.
This research discusses the flow regime, fracture initiation and growth in rocks and place emphasis on fluid-assisted fracturing which extensively result in total disintegration (brecciation) of rock and potential fluid mixing and suggest ways in estimating the amount of fluid (pressure/energy) needed to cause or initiate a hydrofracture and fluid-assisted fracture dynamics and estimate sources of fluid (hydrothermal) that is released at depth by for example
dehydration reactions, exhumation or changes in the tectonic stress field. The study also analysed the clast size distribution of some hydrothermal breccias samples and discuss the conversion between different expressions of the size distributions. The new clast-size analyses of hydrothermal breccias from Pozolagua in NW Spain and the SW-German Black Forest is then present. Finally, this research presents model to explain why the power-law exponent is smaller in hydrothermal, fluid-pressure induced, breccias.
Method used is a numerical modelling of fluid flow in a rigid matrix to simulate hydrothermal fluid transport and fracture growth and dynamics, which constrains the minimum amount of pressure/energy the ascending fluids must provide. For the breccia clasts, a statistical analysis of various breccia samples to gauge size distribution using the clast area (2-D) dimension. Fractures begin to form (rock failure) at some stage when there is continued increasing fluid overpressure. Unstable states that could lead to brecciation at much shallower levels are only reached at high fluid escape through large hydrofractures. Fractures and fluid (hydrothermal) transport dynamics may explain the abundance of deep sourced fluid inclusions and hydrofractures in much shallow levels in the subsurface. results further show that fracture growth varies significantly with proportionately constant fluid overpressure and that deep sourced fluid mixing is possible with well-connected fractures. Finally, as the formation of hydrofractures can dictate local effective permeability, intensive fracturing can lead to brecciation and mineral precipitation at much shallower levels as hydrofractures aid in upwelling deep sourced fluid to the near surface. It appears that clast sizes in all cases tend to follow a power-law (mean n-value of ~1.68) distribution, but power-law exponents can vary significantly, even within a single setting. Like the clast size distribution, fractures from the simulation also follows a power law, except those large ones that reached the surface. It appears that a rapidly ascending hydrofractures will tap fluids from all levels of the infiltrated rock column, and as well mix these fluids during ascent.
Hydrothermal Fluids