Regional Fluid Flow and Gold Mineralization in the Dalradian of the Sperrin Mountains, Northern Ireland
2000
Gold vein mineralization occurs in the metamorphosed and
deformed Dalradian (Neoproterozoic) rocks of the Sperrin Mountains, Northern
Ireland. Two structures exerted a control on the location of the
mineralization; the north-south Omagh lineament and the
west-northwest-east-southeast Curraghinalt lateral ramp in the footwall of
the northeast-southwest Omagh thrust. These are Caledonian structures
resulting from the thrusting of Dalradian rocks over a possibly still active
Ordovician arc. Cathodoluminescence microscopy distinguishes four phases
of vein quartz in the Curraghinalt gold prospect. Fluid inclusion studies
and stable isotope geochemistry have defined the probable fluids responsible
for the precipitation of each quartz phase and associated sulfide and
precious metal mineralization. The initial phase (Q1) appears to have been
associated with the main Caledonian metamorphic event (ca. 470 Ma) and is
nonauriferous. The second phase (Q2) forms an extensive cement to brecciated
early quartz and is believed to have involved a fluid (~15 wt % CO 2 ,
10 wt % NaCl + KCl equiv) with a significant magmatic component of 470 to
400 Ma, which underwent phase separation and dilution with a cooler
formation water. This process resulted in precipitation of the main phase of
gold mineralization characterized by an assemblage of electrum, pyrite,
arsenopyrite, chalcopyrite, tennantite-tetrahedrite, and various tellurides.
Similar fluids are observed on a regional scale, concentrated within the
hanging wall of the Omagh thrust, indicating an extensive fluid-flow event.
The relative abundance of gold at the Curraghinalt and Cavanacaw prospects
is thought to be due to higher fluid fluxes in favorable zones of dilation
and closer proximity to the fluid source. The deposit was subsequently reactivated with the
precipitation of later quartz (Q3-Q4) from a formation water believed to be
resident in the Dalradian metasediments, which mixed with a low-temperature,
high-salinity basinal brine, probably during Carboniferous basin inversion.
Brine flow resulted in the remobilization of earlier electrum, reducing its
fineness, and also introduced base metal sulfides, carbonates, and barite.
Again, brine flow is localized by the Omagh thrust, indicating the
long-lived role of this structure in controlling regional fluid migration.
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