International Geologiical Congress - Oslo 2008

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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts

 

Gold-bearing Ironstones within the Lower Ordovician black-shales of Valongo Anticline: a contribution to the understanding of their genesis

 

Helena Couto, Faculty of Sciences, University of Porto (Portugal)
 

 

In the Valongo Anticlinal (North-Western Portugal) overlaying the Lower Ordovician massive quartzites, black-shales occur in a sequence mainly composed of inter-bedded arenitic, siltitic and pelitic layers. These black layers composed of dark grey quartzitic beds and black shales with exhalative-sedimentary intercalations, contain oolitic ironstones and are significant gold bearing strata that have been exploited since roman times.
Concerning mineralization of the area, four discrete paragenetic associations were defined: Sb-Au, Au-As, Pb-Zn(Ag) and Sn-W. The Au-As type is controlled by the Lower Ordovician black-layers.
The oolitic ironstones are very predominantly composed of Fe-rich chlorite (chamosite), siderite, quartz, kaolinite, apatite and muscovite. Sometimes siderite dominates meanwhile the chlorite is dominant or exclusive in other places. Quartz is frequent and kaolinite rather frequent. Apatite is often present sometimes in algae skeleton, other times resulting from recristallization. Accessory minerals are tourmaline, zircon and monazite. Fossil alg (Botryococcus and Girvanella have been determined) and bryozoa are present. Some alg, are frequent and sometimes concentrated (cyanobacterial communities) in defined levels, containing alternating microbial and microbe free layers presenting the typical layered structure of stromatolites. Organic matter, represented by fusinitizated fragments sometimes with a graphitod texture, migrabitumens and hydrocarbures are also present, the later in relation with the algae.
Native gold silver-bearing is present most frequently in the sub-concordant quartz "veins" (exhalative- origin) interbedded in black shales. Two types of iron sulphides (pyrite and arsenopyrite) were identified: one syngenetic, the other epigenetic deposited by hydrothermal fluids related with the main mineralizing gold event that occurred in the area. The hydrothermal fluids improved a progressive sulphidation of oolitic texture resulting sometimes in a massive sulphide where oolits are not evident.
The oolitic ironstones and black layers have a greater thickness in the eastern limb of the Valongo anticline, with a very discrete presence in the periclinal termination, being absent in the western limb. In the normal limb of Valongo Anticline is noted that vers Oest the siderite dominates meanwhile the chlorite is dominant to East. These facts, among other evidences, show that in the Lower Ordovician the basin margin would be located to East.
Finally, the stratigraphic, palaeontological and geochemical data indicates that these chlorite and siderite bearing ironstones were formed in a nearshore shallow environment protected by barriers (stromatolites), with exhalative volcanic activity and being deeper to Oest. This depositional environment, part of a large platform located in the North Gondwana margin, acted as a structural and chemical trap for metals, and so was favourable to gold and iron precipitation.

 

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