The sedimentology and petrography of Lower Carboniferous limestones and dolomites: host-rocks to the Navan zinc-lead deposit

[摘要] Navan Zn-Pb ore deposit is located along the SW margin of the Longford Down Lower Palaeozoic Inlier approximately 40 km NW of Dublin. The orebody is hosted by Lower Carboniferous (Courceyan) limestones which dip gently to the SW. The folded Lower Palaeozoic basement is overlain unconformably by Red Beds. These are interpreted as reflecting deposition in a fluviatile system. Deposition was not continuous, periodic abandonment of the flood plain led to the formation of caliche. The overlying Laminated Beds reflect alternations of barrier sandstones, lagoonal mudstones and siltstones, and tidal flat sediments. At least 2 episodes of emergence occured, the first is represented by a pedogenic green clay, the second produced nodular anhydrite (reflecting a sabkha environment). The Muddy Limestone which follows is believed to represent a clastic influenced lagoon. The limestones above, the Pale Beds, host 97% of the ore body and are sub-divided into a lower Micrite Unit and an upper Grainstone Unit. The Micrite Unit consists of approximately 20 metre scale lagoon tidal-flat shallowing-upwards cycles, with oolitic grainstones reflecting periodic incursions of more open marine conditions. Many cycles culminate in emergent surfaces. Three of these surfaces are important because they host high grade mineralization. The lowest is an in situ breccia overlain by a pedogenic green clay, the second prominent surface, 10 m above, is also a breccia surface. The youngest surface is a karren type palaeokarst. The upper surface of the Micrite Unit is bounded by a palaeotopography, which includes a NW striking channel. The Grainstone Unit blankets the Micrite Unit. It reflects a sequence of open marine oolitic and bioclastic sand shoals. Several strati graphic marker horizons are present within the Grainstone Unit; the Lower Dark, Nodular and Upper Dark Markers. These are believed to represent lagoonal sediments, suggesting alternate regression and transgression. The Grainstone Unit contains several solutionally modified surfaces similar to the palaeokarst surfaces in the Micrite Unit below, suggesting that deposition was again punctuated by subaerial erosion. The sequence overlying the Pale Beds is Late Courceyan to Arundian in age and consists of Shaley Pale and Argillaceous Bioclastic Limestones and Waulsortian Limestones. Together they reflect continued deepening. These are truncated by an erosion surface which cuts down through the entire sequence at Navan. This is overlain by the Boulder Conglomerate, believed to reflect a submarine debris flow system, perhaps associated with synsedimentary faulting. The overlying Upper Dark Limestones represent basinal turbidites. Cementation of lime sediment began early and was in part contemporaneous with sedimentation. Features of shallow marine and meteoric cementation are present but both are over-printed by mechanical compaction which is itself over-printed by blocky (burial) calcite cement. The final result was a tightly cemented limestone with low porosity and permeability Dolomitzation in the Pale Beds is confined to a limited area, having a linear NE-SW trend and an overall plume shaped geometry. Relic fractured grains within dolomite and replacement of both calcite cements and stylolites indicates that dolomitization post dates sediment compaction and calcite cementation. Three stages of dolomitization are recognised, separated by dissolution events. The geometry and distribution of dolomitziation, dolomite rock textures, fluid inclusion homogenization temperatures of 59.7 °C to 159.3 °C and oxygen isotope values of δ18O-6.1 to -10.4‰ PDB indicate that dolomitizing solutions were heated. The most permeable part of the sequence at Navan is the Pale Beds, high porosity and permeability are not pervasive but confined to subaerial erosion surfaces separated by tightly cemented limestones. In addition grainstones have controlled the distribution of dolomite, which, relative to grainstones, has a high porosity and permeability. The Pale Beds are enclosed in an envelope of low porosity and permeability consisting of the Laminated Beds and the sequence overlying the Pale Beds. The location of ores in the western mine area is strongly dependent on ground conditions and ores are located along karren type palaeokarst surfaces. This view contrasts with that of Anderson (1990) who believed that the cavities were the result of soft sediment pulling away from the base of dolomite 'crusts' during burial and compaction. Mineralization occured between stages of dolomitization. However, alteration of dolomite by sulphide, replacement of dolomite by sulphide and the occurrence of sulphide filling fractures and brecciation within dolomite indicate that mineralization continued after dolomitization was complete. The linear NE-SW trend of the dolomitization is contiguous with the outline of the orebody at Navan supporting the view that dolomitization and mineralization are genetically related. At Navan the Red Beds pinch out and the Pale Beds contain evidence of subaerial erosion, both suggest a basin margin position, supporting the geophysical data of Brown & Williams (1985). Apparent lack of alteration of Lower Palaeozoic rocks, leaching of feldspars and mafics in the Red Beds accord with derivation of ore fluids from the basin which underlies the Irish Midland region, following the stratal aquifer model of Lydon (1986) for ore genesis.

[发布机构] University:University of Glasgow

[关键词] QE Geology