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Mineralogy, lead isotope and metal geochemistry of gold-rich Tertiary VHMS deposits of the Western Cordillera of Ecuador
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
Macuchi, situated 70 km SSW of La Plata (Fig. 1), is hosted by the lower part of the Main Macuchi sequence (Fig. 2). The orebody is a lens of massive sulfides, 125 m long and up to 18 m thick, in a sequence of basaltic to basaltic andesite volcanic and volcaniclastic rocks with rarer felsic tuffs, porphyric andesites and dacites. The mineralisation occurs as replacement, stockwork and layered sulfides. The main alteration assemblage is quartz-sericite, prevailing over quartz-chlorite. The Macuchi mine was exploited from 1941 to 1950 with an average ore grade of 5% Cu, 7.6 g/t Au and 8.5 g/t Ag (Stoll 1962).
Correlation between the Warepan/Otapirian and the Norian/Rhaetian stage boundary: implications of a global negative δ13Corg perturbation
Published in New Zealand Journal of Geology and Geophysics, 2022
The Arawi Shellbeds and Ngutunui Formation are a succession of volcaniclastic sedimentary rocks dominated by thin sandstones, siltstones and shales, with minor but conspicuous conglomerates, tuffs and shellbeds (Figure 3). Limestones are not present, although the shellbeds approach coquina limestone composition in places within the Arawi Shellbeds (Grant-Mackie 1985), and there are no radiolarian cherts. Compositionally, the volcanic lithologies are broadly andesitic but range from basaltic andesite to dacite. The tuffs vary in vitric, crystal and lithic composition. In every respect, the sedimentary rocks in the Kiritehere section are typical of Murihiku Supergroup. They have been weakly metamorphosed to zeolite facies grade with conspicuous zeolite veining (laumontite, stilbite) and zeolite ‘cements’ (laumontite, heulandite, analcime). There are minor faults in places and also some ‘slumps’ (Grant-Mackie and Lowry 1964) but in general the stratigraphy is more or less ‘layer cake’, easy to recognise in the field, and the named formations and groups can be traced for many tens of kilometres. However, the sedimentary sequence as a whole has been folded, and a number of anticlines and synclines are recognised and named within a broader Kawhia Regional Syncline (e.g. Kear 1960; Edbrooke 2005). In the Kiritehere section, the sequence is dipping and younging to the east, and is part of a large homoclinal structure that may be interpreted as the west limb of a syncline (e.g. Kear 1960; Kear and Schofield 1964; Edbrooke 2005).