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The Furtei high sulphidation epithermal gold deposit (Sardinia, Italy): mineral assemblage and its evolution
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
Most part of the volcano-sedimentary complex in the area has been affected by hydrothermal alteration and mineralization. At Furtei, alteration can be observed over an area of roughly 5 km2 and is controlled by tectonic structures and lithotypes. In particular the pyroclastic formations and breccias are intensely altered. Four types of alteration have been recognized: propylitization, argillic alteration, advanced argillic alteration, and silicification. Hydro-thermal alteration was overprinted by supergenic, secondary argillic alteration, mainly after oxidation of the sulphide phases. The propylitic alteration affected indiscriminately all rocks at various degrees and its products can be grouped into two main assemblages, i.e. a chlorite assemblage and a calcite assemblage. In the first type the main alteration minerals are chlorite and quartz, with variable amounts of calcite, pyrite, and sometimes epidote and sericite. The second type, comprises calcite, quartz and chlorite. Argillic alteration varies from intermediate to advanced argillic, depending on the local intensity of acid leaching. Intermediate argillic alteration commonly overlaps propylitic alteration. It is characterized by clay minerals, both montmorillonite and kaolinite, sometimes accompanied by quartz and pyrite. The advanced argillic alteration is commonly associated with silicification and pyritization. Kaolinite and dickite are the main minerals, usually accompanied by quartz and sometimes pyrite and pyrophyllite. Two main types of silicification can be recognized: silicification by residual enrichment and silicification by deposition, both as vein filling and as impregnation. Sulfides, including predominantly pyrite and clay minerals, mostly kaolinite and dickite, are normally associated with these alterations. Pyritization is widespread in all types of alterations already considered, ranging from slight, but almost always present, dissemination in the propylitic halo, to major concentrations occurring in the innermost areas of intense alteration, where it practically grades into the ore bodies. The gold-bearing mineralization is mainly associated with either massive or replacement silicification and with residual vuggy silica.
Geophysical signatures of New Zealand epithermal Au-Ag deposits, and methods for new exploration
Published in New Zealand Journal of Geology and Geophysics, 2019
Richard L. Kellett, Chris J. Bromley
The alteration zones associated with the mineralisation in the Hauraki Goldfield are characteristic of the adularia-sericite epithermal deposits with both lateral and vertical zonation (de Ronde 1986; Hedenquist et al. 2000; Simpson and Mauk 2011). The broad propylitic alteration zone around the hydrothermal system is characterised by the initial breakdown of the igneous minerals to clays (chlorite). As temperature increases closer to the fluid pathways, argillic alteration produces further breakdown of rock forming minerals to illite and sericite. Advanced argillic alteration around the silicic core of the deposit is dominated by a mixture of kaolinite, illite, and smectite. The stockwork and vein systems are brecciated and silicified. There can be a post-mineralisation profile of weathering superimposed on the epithermal signature. Identifying the geophysical characteristics of each of these zones is key to improving the success of geophysics in epithermal exploration (Williams 1997).
Synthesis of hydrothermal alteration, rock mechanics and geophysical mapping to constrain failure and debris avalanche hazards at Mt. Ruapehu (New Zealand)
Published in New Zealand Journal of Geology and Geophysics, 2021
Gabor Kereszturi, Lauren Schaefer, Stuart Mead, Craig Miller, Jonathan Procter, Ben Kennedy
In summary, rock properties and mechanics show that rocks at Mt. Ruapehu could fail at low stresses where there is sufficient or unfavourably oriented high porosity and/or pervasively altered rocks are present, or where macroscale fractures are sufficient to weaken the rock mass. Generally, rocks become stronger with depth (increasing confining pressure), however, rocks at depth transition from brittle to ductile failure, potentially resulting in different failure modes (Mordensky et al. 2019a), or an increase in deformation prior to failure (Schaefer et al. 2020). For example, the rocks beneath the Crater Lake’s active hydrothermal system dominated by advanced argillic alteration do not follow typical physical and mechanical property trends due to particularly high clay content (pervasively altered), low permeability, and low strength (Schaefer et al. 2020). This material deforms in a ductile manner driven by pore collapse, in contrast to intermediate argillic material, which deforms brittlely (Mordensky et al. 2019a; Schaefer et al. 2020). The influence of effective pressure (i.e. increasing depth) on permeability shows that decreases in permeability happen primarily in effective pressure conditions up to 5 MPa, and that above this threshold the effect is less prominent (Mordensky et al. 2019a). The extent of the decrease in permeability is controlled by how dominant microfractures are for a given the porosity of the geotechnical unit (Mordensky et al. 2019a). These findings highlight that caution should be made when assigning properties at depth, as surface sample properties and behaviour are likely not representative of subsurface samples.
Airborne hyperspectral characterisation of hydrothermal alteration in a regolith-dominated terrain, southern Gawler Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2021
A. S. Caruso, K. D. Clarke, C. J. Tiddy, M. M. Lewis
Overall, this study demonstrates that airborne hyperspectral imagery could be considered as an early analytical technique within a RDT to understand the occurrence and distribution of alteration across a landscape. The use of this imagery would allow rapid assessment of prospectivity prior to undertaking a more expensive exploration campaign. In the case of the southern Gawler Ranges, image analysis was able to identify mineral assemblages associated with advanced argillic and argillic alteration, consistent with previous claims that this area is prospective for porphyry mineralisation. The surface expressions mapped in this study show that some mineralogical signatures related to an epithermal–porphyry system are preserved in this area.