Ford, Colin Victor
The integration of petrologic and isotopic data from the Boulder Conglomerate to determine the age of the Navan orebody, Ireland.
PhD thesis, University of Glasgow.
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The Boulder Conglomerate is the Lower Carboniferous sedimentary allochthon deposited in the second order half graben created by major listric normal faults in the Navan area. The base of the Boulder Conglomerate comprises erosive-based, highly immature rock avalanches which are interbedded with hemipelagic limestone. Sulphide mineralisation in the autochthon, the Pale Beds Ore, is truncated by the fault-generated unconformity and is reworked as clasts and matrix into the basal Boulder Conglomerate. Matrix-deficient breccia beds deposited on the unconformity possess mineralisation as cement to interclast porosity. This cement is contiguous with replacement and fracture-cementing mineralisation in limestone strata beneath the unconformity. Mineralisation associated within the interbeds to the Boulder Conglomerate occurs as early diagenetic concretions and as stratiform replacement. The 34S of these sulphides ranges from -21‰ to +1‰.
The rock avalanches are overlain by clast-bearing carbonate mud strata. The carbonate mud is replaced by pyrite and paragenetically later sphalerite and galena. The isotopic range of 34S for pyrite (-32‰ to -35‰) is distinct from that of the zinc and lead sulphides (-18‰ to +6‰).
The uppermost Boulder Conglomerate is characterised by slumped and brecciated mineralisation which is dominated by pyrite. The 34S values of the in situ and reworked mineralisation range from -40‰ to -23‰.
The Thinly Bedded Unit overlies the Boulder Conglomerate and comprises interlayered shales and calcarenite and laterally impersistent, erosive-based, graded bioclastic and oolitic microbeccias. Mineralisation occurs as selective replacement to the microbreccia and as stratiform sphalerite and pyrite mud. The latter occur in-situ as laminae and as channelised debris flows. The reworked sulphide mud supports plant fragments which exhibit exceptional preservation by pyrite, sphalerite and galena. The 34S of the laminated and reworked sulphides ranges from -37‰ to -41‰. (DXN008,313)
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