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The gold of Granada: Geology and history
Abstract
Gold of alluvial origin is found in Granada (SE Spain), occurring as fine, millimetre- to centimetre-sized plates. It has been panned in the Genil and Darro rivers since Roman times, and was also mined in the conglomerates of the 'Alhambra Formation'. The 'Alhambra Formation' is an alluvial-fan deposit, up to 200 m thick, of lower Pliocene age (~5 My). The area of the mine corresponds to a poorly-sorted, well-rounded conglomerate, with clasts of metamorphic rocks. The mine is Roman and was exploited using the 'Ruina Montium' method: a huge scar on top of the hill, from which the man(Roman)-induced landslide started, can still be recognized. This mine, with an average gold content of 0'5 gr/m3, was worked again between the years 1875 and 1877 by a French company: they dismantled the gold-bearing conglomerate using a powerful water jet, and separated the gold by amalgamation with mercury. The origin of the gold is closely related to that of the 'Alhambra Conglomerate', which comes in turn from the erosion of an older, Miocene conglomerate. The source rocks for the latter are those of the metamorphic Sierra Nevada core. Gold concentration took place during successive processes of erosion, transportation and deposition (sedimentary cycles). The Miocene conglomerate formed first; then the 'Alhambra Conglomerate', and, finally, the placer-deposits linked to the Genil and Darro rivers. Gold has been panned exclusively in these rivers at those places where they cut across the 'Alhambra Conglomerate'.
... The palaeoenvironmental model for Subunit E1 ('Alhambra system') is that of coalescent alluvial fans, placed at the base of the palaeo-Sierra Nevada and palaeo-Sierra Arana reliefs at the eastern side of the basin (Martín 2000), passing laterally to the North to a fluvial system and this, in turn, to a lake (Fig. 3). ...
... In Crevillente the Apodemus meini Zone (Middle Turolian) has in its upper part Fig. 7. Unit E. ᮀA. Sediment distribution (lithofacies) and position of the studied sections (data from Elorza et al. 1978;Dabrio & Fernández 1986;Fernandez & Soria 1987;Lupiani & Soria 1985a, b;Martín 2000; and this study). ᮀB. ...
The Late Miocene and Pliocene continental sediments in the Granada Basin (southern Spain) have yielded large amounts of fossil small mammals in 37 localities from 11 sections. The aim of this paper is to integrate faunistic, stratigraphic, and sedimentary criteria to unravel the geological history of the continental infilling of the basin.
The palaeontological study has led to a detailed biozonation on the basis of rodents, which helps to correlate in detail the different sedimentary units found in the basin, and to follow the changes of the different sedimentary systems and their palaeogeographical evolution through time.
Combination of the proposed biostratigraphy and the reinterpretation of the magnetostratigraphic analyses of the Barranco del Purcal section allows us to assign an absolute age slightly older than 5.23 Ma to the Turolian–Ruscinian boundary (MN13-MN14).
Soil organic matter has important implications in dissolution reactions of clay minerals and their transformation. Under acidic conditions increased dissolution rates are generally reported. However, in high pH environments, organic matter (OM) might have a passivation effect on the mineral surface, thereby hindering dissolution and transformation substantially. The reduced reactivity of clay minerals is an important consideration when clays are used as backfill materials in nuclear waste disposals or when lime is used for soil stabilization. Here the outcome of a comparative study is presented using soil, containing common clay minerals such as smectite, interstratified illite–smectite, illite and kaolinite, that was treated with highly concentrated alkaline solutions in the presence and absence of naturally occurring organic matter. The results clearly show that clay mineral dissolution and the formation of new zeolitic phases at room T are delayed in the presence of OM. The soils' OM content is, thus, an important parameter to be considered in systems where clay minerals are exposed to highly alkaline environments.
Granada and its metropolitan area lie on the eastern edge of a basin where the foothills rise into the adjacent Sierra Nevada (3,482 m). On the west the valley is bordered by several faulted, Quaternary-age alluvial fans and by dissected terraces of the Genil River. Landscape evolution from about Tortonian to Pleisto-cene time is reflected by relict Quaternary glaciers preserved on a deeply eroded lower Paleozoic terrain and by terrace remnants and channels resulting from rapid Late Pleistocene and Holocene fluvial incision. This particular geological, geomorphic, and active tectonic setting, combined with a Mediterranean climate under continental influence, and the rapid urban development in the last 60 years have increased exposure to several natural hazards. Urban expansion has increased flash-flood and mass-wasting vulnerabil-ity, and seismic risk is similarly increasing with the 1884 Andalusian Earthquake (M 6.3), reflecting the last of several large, ,500-year recurrence temblors that have affected the area since at least Roman times. Modern Granada and its metropolitan area, with about a half million inhabitants, are currently increasingly challenged by the highest levels of exposure to natural hazards, by strain on traditional society resulting from adapting to the changes produced by the three decades since the advent of democracy (1978) and by the consequences of the deep economic crisis.
The Rb/Sr whole-rock age of granitoid gneiss interlayered with marbles at the top of the Nevado-Filabride Complex cover is 247 ± 11 Ma (Permian-Triassic boundary). Tourmaline-rich metasediments were metasomatized by B-rich fluids from the igneous body. Hence the igneous rocks were emplaced in the sedimentary rocks that now host the gneiss body, precluding a later tectonic emplacement. Therefore, the entire Nevado-Filabride Complex is Palaeozoic or older and may have been the stratigraphic basement of the immediately overlying Alpujarride units, which include younger rocks. The consistent evolution of the Alpine high-pressure metamorphism in the Nevado-Filabride and Alpujarride complexes might therefore be due to their original stratigraphic relationship.
The litho-stratigraphic development of the Mulhacen and Alpujarride Complex, in the eastern Sierra de los Filabres, is described in detail. The Mulhacen Complex comprises three thrust-sheet units, formed during a process of crustal-scale underthrusting. The thrust-sheets are composed of a dark coloured series of inferred Paleozoic age, covered by light coloured metasediments of inferred Triassic age. The Alpujarride Complex comprises two tectonic units, constituted by light coloured meta-sediments of Triassic age. It appears that well-defined litho-stratigraphic units can be traced for tens of kilometres along strike, despite the complicated tectono-metamorphic history.
The most extensively developed unit from the Mulhacen Complex is the Nevado-Lubrin unit. It comprises a clastic series of inferred Triassic age covered by a sequence dominated by carbonates. The clastic series probably represents shelf deposits in an open marine environment. The overlying sequence marks an abrupt transition to a lagoonal environment, with reef and littoral facies. This abrupt facies shift might have been caused by a regional phase of crustal extension.
Annexes; 1 full colour geological map and 3 full colour profiles at 1/20 000.
Este trabajo da una primera interpretación sedimentaria y una reconstrucción paleogeográfica de los materiales conglomeráticos neógenos de parte de la Depresión de Granada. El modelo sedimentario propuesto consiste en abanicos aluviales que penetran en medio marino. En una fase posterior, se establece un régimen fluvial que desemboca en zonas lacustres, eventualmente evaporÍticas. [ABSTRACT] This paper offers a first sedimentary interpretation aod a paleogeographic reconstruction for the neogenic conglomerates in a part of Granada Basin. The proposed sedimentary ruodel consists on alluvial fans going into marine environment. Later, a braided fluviatile pattern connected downstream with lacllstrine areas with episodic evaporite deposits establishes in this place.
Middle Miocene coral-oyster patch reefs crop out at Murchas. They are irregularly shaped masses of coral-oyster boundstone, up to 18 m wide and 3-4 m high, that developed on the outer part of a homoclinal ramp, seaward of some sand shoals, in a mixed carbonate-terrigenous environment. Heliastrea is the predominant coral. Porites, Tarbellastraea and the phaceloid coral Mussismilia are also important components. These corals show no clear pattern in their distribution and appear embedded in a silty (bioclastic) matrix. Oysters in the reef community belong to the species Hyotissa aquarrosa.
In the Betic Cordillera, the westernmost of the Alpine Chains, the Alpujárride Complex is the only one in which the Middle-Upper Triassic is represented predominantly by carbonate marine facies. The correlation of various stratigraphic columns, together with the position of the facies belts at different levels, reveal that these carbonate deposits could only have originated in a basin which opened westwards to the sea. In contrast to previous hypotheses, the Triassic Alpujárride basin is thought to have opened towards an Atlantic in the west and not towards the Tethys in the east.Although some connection clearly existed between the Alpujárride and Tethys basins during the Middle Triassic, when the extension of the Alpujárride carbonate deposits was at its greatest, the Upper Triassic saw a separation of the Alpujárride basin from the Tethys, which is reflected in the very specific peculiarities in the endemic flora and differences in the types of bioconstructions which evolved in this and in coetaneous basins.Thus the Alpujárride carbonate deposits originated in an early Atlantic and were tectonically emplaced in the western Mediterranean by transform movements and later by overthrust.