Searching for the “next big thing”: Examining the potential for new feeder zone mineralisation in the western Navan area, County Meath, Ireland

Convery, Michael (2017) Searching for the “next big thing”: Examining the potential for new feeder zone mineralisation in the western Navan area, County Meath, Ireland. MRes thesis, University of Glasgow.

Abstract

This study focuses on the Navan Deposit, County Meath, Ireland – a carbonate-hosted Zn-Pb
sulphide deposit originally comprising 110Mt @ 8% Zn and 2% Pb, prior to depletion by mining.
Navan belongs to the “Irish-type” deposit group – Zn-Pb deposits hosted in the Irish Ore Field that
have characteristics of both MVT and SEDEX-style deposits, though cannot be easily be
categorised as either. The Irish-types formed from mixing of two fluids – a metalliferous
hydrothermal fluid, and a bacteriogenic-sulphide enriched brine. The hydrothermal fluid from
Irish-types in the southern Irish Midlands yield a respective hydrothermal fluid homogenisation
temperature and salinity range of 170-280oC and 12-18 wt.% NaCl. The Navan hydrothermal fluid
was notably lower temperature at 100-140oC and had a salinity of 5-10 wt.% NaCl. Mixing of this
moderate temperature fluid with a high salinity bacteriogenic-sulphur enriched brine (70-100oC,
20-25 wt.% NaCl) allowed high-grade Zn-Pb ore to precipitate along a shallow, extending sea basin
at approximately 345 Ma (Lower Carboniferous).

Navan ore has an average Zn/Pb ratio of 4, though low-grade, low Zn/Pb below 3 is observed in
the hangingwall of the Randalstown Fault 200 metres west of the mine site – an end-Variscan
reverse fault of at least several hundred metres throw and one kilometre dextral displacement.
Low Zn/Pb has been previously noted in Navan and other Irish-type deposits as an indication of a
nearby “feeder zone” – early-forming normal faults that allowed hydrothermal fluid upwelling.
Hydrothermal fluids then mixed with downwelling bacteriogenic sulphide-enriched brines on, or
close to the seafloor to produce high-grade Zn-Pb sulphide ore. Low Zn/Pb in western Navan,
therefore, is of particular interest and may point to a new, unidentified feeder. In addition, Cpy-
Qtz(-barite) veining has been identified in drillcore for the first time in the Navan area
approximately 850 metres west of the Randalstown Fault, coinciding with the low Zn/Pb area.
Notable chalcopyrite has been identified close to feeder-zone faults in other Irish-type deposits,
though is absent at Navan – until this discovery.

Mineralised samples were selected along four drillhole transects – The “A1 Transect” was
selected trending southeast along the hangingwall of the Randalstown Fault, while the “A2
Transect” trends southeast along the Randalstown Fault footwall. The “B and C Transects” trend
perpendicular and parallel to the Main Orebody, respectively. Sulphides from the Main Orebody
and western Navan share a similar paragenesis, suggesting that both may have derived from the
same hydrothermal fluid event. Sulphur isotope analysis constrains a δ34S range of -1.5 to 12.5‰
for Zn-Pb sulphides west of the Randalstown Fault. This range coincides with previous studies that
have constrained the δ34S range for the hydrothermal fluid (-4 to 16‰) that supplied Zn-Pb to the
Navan Main Orebody and is interpreted to represent hydrothermal fluid that migrated westward
during early normal faulting. Fluid inclusion analyses constrain a mineralisation temperature and
salinity range of 87-136.5oC, and 6.8-22.7 wt.% NaCl, respectively. This is broadly coincident with
the hydrothermal fluid temperature and salinity range. Higher fluid salinity is interpreted to
reflect mixing with limited downwelling seawater and/or in-situ pore water.

Cpy-Qtz(-barite) veining in western Navan is hosted in a 10.5 metre thick stratal dolomite and
formed from a separate mineralising fluid to the Navan Deposit. δ34S analysis of three
chalcopyrites yield a range of 0.8-3.3‰, results consistent with a hydrothermal fluid origin. Fluid
inclusion analysis of two-phase quartz-hosted inclusions yield a temperature range of 191-223oC,
while monophase inclusions constrained a salinity range of 10.2-22.8 wt.% NaCl. The assemblage
is both distinct from Navan and closer to higher temperature systems in the southern Irish
Midlands – an observation supported by a mineralisation temperature range that is closer to the
southern Irish-type deposit systems. The mineralising fluids were both higher temperature and
more saline than the Navan hydrothermal fluid, potentially pointing to the feeder zone of a new,
unidentified system.

• Item Type: Thesis (MRes)
• Qualification Level: Masters
• Keywords: Mining, exploration geology, Irish-type, Mississippi Valley-Type, sedimentary-exhalative.
• Subjects: Q Science > QE Geology
• Colleges/Schools: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Supervisor's Name: Boyce, Professor A.J., Newton, Dr. J., Ashton, Dr. J. and Blakeman, Dr. R.
• Date of Award: 2017
• Depositing User: Mr Michael Thomas Convery
• Unique ID: glathesis:2017-7900
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 23 Mar 2017 12:10
• Last Modified: 25 Apr 2017 13:46
• URI: https://theses.gla.ac.uk/id/eprint/7900