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Geometry and geology
Published in Xavier Emery, Serge Antoine Séguret, Geostatistics for the Mining Industry, 2020
Xavier Emery, Serge Antoine Séguret
The deposit is also famous among geologists for its diatreme, a weakly mineralized intrusion with the shape of an inverted vertical cone of 1 km height and a maximum diameter of 1 km at its highest point (Skewes et al., 2002; Maksaev et al., 2004). Known as the ‘Braden pipe’ or ‘Braden breccia’ by Chilean geologists and miners, this unit crosses the deposit and constitutes the internal limit of the mining operations (Figure 1.1). It is important to determine the surface of its contour. This problem refers to the geometry and precedes the prediction of copper grade, which is often conditioned by the defined geometry, as will be seen later.
The role of hydrovolcanism in the formation of the Cenozoic monogenetic volcanic fields of Zealandia
Published in New Zealand Journal of Geology and Geophysics, 2020
A basaltic diatreme, Ngatatura diatreme (Heming 1980), exposed on the Waikato coastline, (Figure 4A) has been K-Ar dated at 1.83–1.54 Ma (Briggs et al. 1989; Briggs et al. 1994, 1997). However, new K-Ar dating reporting ages of 3.34 ± 0.06–1 Ma are well outside of the commonly accepted Ngatatura ages and similar to those known from Okete Formation and Alexandra Volcanics (Van Niekerk 2016). The diatreme itself is dominated by pyroclastic breccias (Figure 4B) and cannot be linked to any surface expression of a volcano nearby (van Niekerk et al. 2015b). A recent study has revealed the rock defined as Ngatatura diatreme is part of a complex volcano with a history of multiple events potentially spanning a few hundreds of thousands of years and contributing to a record of dramatic changes in the coastal region where near-sea level, coastal conditions have dominated and controlled small volume volcanism over time (van Niekerk et al. 2014, 2015a). As a result, the Ngatatura diatreme and enclosing deposits reveal a history of shallow, subaqueous volcanism that produced hyaloclastite (Figure 4C) and pillow lavas subsequent to the diatreme formation (Van Niekerk 2016).
The Dunedin Volcanic Group and a revised model for Zealandia's alkaline intraplate volcanism
Published in New Zealand Journal of Geology and Geophysics, 2020
James M. Scott, Alessio Pontesilli, Marco Brenna, James D. L. White, Emanuele Giacalone, J. Michael Palin, Petrus J. le Roux
Rocks on the western and northwestern side of the field near Middlemarch (Figure 2) testify to phreatomagmatic eruptions. For example, the Crater is the spectacular high-level vent diatreme facies of a ∼1 km wide maar excavated through Otago Schist (Figure 4E) (Coombs et al. 1986; Németh and White 2003). Other diatremes include: Foulden Maar, which had a ∼1.5 km wide crater lake that filled to > 100 m with a laminated diatomite that preserves a remarkable Miocene fossil and climate record (Lindqvist and Lee 2009; Fox et al. 2015; Kaulfuss 2017); Gladsmuir, in which poorly exposed basanite and pyroclastic rocks may represent the top of a diatreme; and a cluster of 4 buried maars at Hindon reaching up to 1000 m in diameter, aligned in a NE trend and containing up to 8 m of diatomite (Kaulfuss et al. 2018). Other eruptive centres in this area, such as Slip Hill, Bald Hill, Abbortsford, Mt Stoker and Fortification Peak, are basanitic lavas.
Basanite cobbles in Pleistocene sediments in Central Otago and their implications for intraplate volcanism and Clutha River paleo-drainage
Published in New Zealand Journal of Geology and Geophysics, 2023
James M. Scott, Alan F. Cooper, Dave Craw, Petrus J. le Roux, Hayden B. Dalton, Marshall C. Palmer
Our geochemical data favour derivation from the northwest, as previously inferred for river drainage by Youngson et al. (1998) and Craw et al. (2012, 2013). However, a probable precise source location remains unclear. To date, Alpine Dike Swarm basanite has only been found in the West Wanaka diatreme (Perry 2022), which is situated in an area surrounded by dikes and diatremes of lamprophyre. Since the Galloway location lacks lamprophyre, at least at the scale we sampled, it is unlikely this diatreme was the source. Furthermore, most of the basanites in the diatreme are heavily contaminated by Otago Schist material (Perry 2022).