J. Parnell et al., Regional fluid flow and gold mineralization in the Dalradian of the Sperrin Mountains, northern Ireland, ECON GEOL B, 95(7), 2000, pp. 1389-1416
Citations number
74
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
Gold vein mineralization occurs in the metamorphosed and deformed Dalradian
(Neoproterozoic) rocks of the Sperrin Mountains, Northern Ireland. Two str
uctures exerted a control on the location of the mineralization; the north-
south Omagh lineament and the west-northwest-east-southeast Curraghinalt la
teral ramp in the footwall of the northeast-southwest Omagh thrust. These a
re Caledonian structures resulting from the thrusting of Dalradian rocks ov
er 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 precipita
tion of each quartz phase and associated sulfide and precious metal mineral
ization. The initial phase (QI) appears to have been associated with the ma
in Caledonian metamorphic event (ca. 470 Ma) and is nonauriferous. The seco
nd phase (Q2) forms an extensive cement to brecciated early quartz and is b
elieved to have involved a fluid (similar to 15 wt % CO2 10 wt % NaCl + KCl
equiv) with a significant magmatic component of 470 to 400 Ma, which under
went phase separation and dilution with a cooler formation water. This proc
ess resulted in precipitation of the main phase of gold mineralization char
acterized by an assemblage of electrum, pyrite, arsenopyrite, chalcopyrite,
tennantite-tetrahedrite, and Various tellurides. Similar fluids are observ
ed on a regional scale, concentrated within the hanging wall of the Omagh t
hrust, indicating an extensive fluid-flow event. The relative abundance of
gold at the Curraghinalt and Cavanacaw prospects is thought to be due to hi
gher fluid fluxes in favorable zones of dilation and closer proximity to th
e fluid source.
The deposit was subsequently reactivated with the precipitation of later qu
artz (Q3-Q4) from a formation water believed to be resident in the Dalradia
n metasediments, which mixed with a low-temperature, high-salinity basinal
brine, probably during Carboniferous basin inversion. Brine flow resulted i
n the remobilization of earlier electrum, reducing its fineness, and also i
ntroduced base metal sulfides, carbonates, and barite. Again, brine flow is
localized by the Omagh thrust, indicating the long-lived role of this stru
cture in controlling regional fluid migration.