Sedimentary ore deposits

Abstract

Summary Until the last two decades the magmato-hydrothermal theories for the origin of ores were considered to be predominant. The change in viewpoint over 20 years or so towards a general acceptance of various syngenetic and epigenetic concepts in ore genesis is outlined in the opening section of this review. In the following section the concentration of metals particularly Al, Ni and to some extent Au in the weathering crust is described. The ways in which metals are concentrated by physical sedimentation are outlined, with particular attention to gold and tin placer deposits and the importance of concepts such as geomorphic thresholds, complex response, and episodic erosion is emphasized. For these deposits it is shown that following the release of elements from bedrock sources they are cycled through the sedimentary system. Essentially, elements move from proximal to distal fluvial sites and from stratigraphically lower to higher positions. Resultant orebodies trend from high grade/low tonnage to lower grade/higher tonnage in a cycle. Chemical sediments, and particularly ironstones are then discussed. Oolitic ironstones occur in two major phases in the Phanerozoic and although their formation in nearshore sheltered sites is becoming established the precise mode of formation, whether primary or diagenetic, is still uncertain. The mainly Precambrian ‘banded iron formations’ pose the problems of the source and transport mechanism for such vast volumes of iron-bearing sediment. Various hypotheses are discussed and a palaeoenvironmental approach adopted. The possibility is emphasized that the unroofing of mafic Archaean basement in the early Proterozoic may well have been a critical factor in their formation and time distribution. Ore deposits of lead, zinc, copper and uranium are found in continental or marine sediments. The metals are derived from bedrock, pass through the weathering crust and are then incorporated into the sedimentary prism as ‘fluid’ or detrital oxide complexes. Migration takes place in interstratal solutions and they are finally precipitated as ores in the diagenetic environment. A broad 2-fold environmental division can be recognized, with some ores forming in fluviatile, lacustrine or paralic environments (Cu, U and to a lesser extent Pb, Zn) and secondly those in nearshore to offshore basin or platform sites (Pb, Zn). The movement of metallic fluids out of basinal shales and into basin margin facies, often up growth faults, is described and a model is outlined to account for lead-zinc deposits associated with carbonates and black shales. The formation of copper and uranium deposits in continental facies associations and above sub-redbed unconformities, the origin of uranium-rich conglomerates in the Proterozoic, the formation of sediment-hosted copper deposits and the copper-gold-uranium association in sedimentary rocks are all discussed in terms of a ‘fluid unroofing’ model. In the final section it is emphasized that we need to refine ‘fluid unroofing’ models, and have a better understanding of the way metal is cycled through the sedimentary — diagnetic environment. The importance of sophisticated facies analysis, the relevance of ‘maturity’ with regard to metals as well as oils, and the significance of biological mediation in ore formation are all emphasized. Finally the critical importance of the weathering horizon in establishing the potential metal fertility of any sedimentary basin is noted.