The Compositions and Routes of the Fluids Generating the Navan Giant Base-Metal Orebody

Blakeman, Robert James (2002) The Compositions and Routes of the Fluids Generating the Navan Giant Base-Metal Orebody. PhD thesis, University of Glasgow.

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Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesothermal (though non-magmatic) deposition of ore began when a supernatant seawater brine still had access to the host sediments (during the early to mid-Lower Carboniferous), has been challenged by recent suggestions favouring a later (mid-Lower to Upper Carboniferous) mineralisation derived from cooler fluids traversing either the Carboniferous basin, or the underlying basement from the south. These models, characterized here as Irish Type and Mississippi Valley Type (MVT) respectively, are interrogated as to their particular predictions with regard to metal distributions and sulfur isotope patterns associated with various fault geometries. An examination of the temporal relationship between mineralisation and known structural events has revealed that the onset of mineralisation occurred prior to, or coincident with the initial developments of the major ENE trending semi-listric extensional faults of early Lower Carboniferous age, that now control the general disposition of the orebody. The basal 5 lens of the Navan Zn+Pb deposit contains -70% of the known tonnage of the -90 Mt orebody and thus is the focus of the metal distribution examination. Lead distribution patterns especially suggest that migration of metal-bearing fluids was principally directed up early to mid Mississippian, near vertical NNE, NE and ENE minor normal faults. These faults predate or are coeval with the major extensional, partly listric, ENE faults which now control the general disposition of the deposit. Only where these major ENE faults cross putative deep-seated, NE (Caledonoid) and NW structures are they associated with lead enrichments. A systematic delta34S survey in the 5 lens across five minor NNE through to ENE- trending faults associated with distinct lead enrichments, and one ENE trending, partly listric, major extensional fault adjacent to that trend, revealed positive values (+1 to +18‰) for galena, sphalerite and marcasite sampled within 3 m of all the faults on the profile. Sulfides with positive delta34S values associated with the deep- seated, metal-bearing fluid generating the Navan deposit have been highlighted by previous workers (Anderson et al., 1998). The evidence reported here strongly suggests that the metal-bearing fluids rose through all the fractures. Conversely negative delta34S values (-1 to -26‰) were returned in galena and sphalerite sampled 3 m or more from these faults. These negative values indicate that locally derived bacteriogenic sulfide, reduced from seawater sulfate, dominated away from these faults. Pyrite delta34S values suggest a background level of -29+/-3.0‰ across the profile. However, pyrite delta34S values as low as -34+/-2.7‰ were recorded in one sample collected from within 1 m of a fault. Thus fluids containing highly fractionated, bacteriogenic sulfide also gravitated into these faults on at least one occasion. There is also evidence suggesting that the metal-bearing solutions periodically displaced the locally derived bacteriogenic sulfide-bearing fluid in and near the faults. Mineral sulfide petrography is used to contextualize the sampling and to give a qualitative indication of the degree of chemical disequilibrium of the system. Mineral textures demonstrating comminution and dissolution are revealed by this study which, when coupled with evidence of isotopic overprinting, force the conclusion that mineralising fluids first invaded the host lithologies during early Lower Carboniferous times, coincident with active faulting. There is no evidence of reactivation of the minor fault sets encountered in the study area during post Chadian tectonism, though the major, partly listric, ENE extensional faults were reactivated at that time. A genetic model, based on the hypothesis of Russell, 1974, and Russell, 1986, for the Navan deposit is presented. Highly saline early Lower Carboniferous seawater was allowed access to the basement via regionally major NE trending (Caledonoid) fault systems. These faults remained the focus of down-welling seawater throughout the duration of the mineralising system. Thus the formation waters at depth were re-charged and metals leached from the basement. Ore deposition was effected by bacteriogenic sulfide (reduced from early Lower Carboniferous seawater sulfate) reacting with the rising, metal-bearing mesothermal fluids.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Mike Russell
Keywords: Geology
Date of Award: 2002
Depositing User: Enlighten Team
Unique ID: glathesis:2002-75967
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 17:11
Last Modified: 19 Nov 2019 17:11

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