Postgraduate Report
OROGENIC LODE GOLD DEPOSITS SHORT COURSE & FIELD TRIP
TO NORTHERN IRELAND GOLD DEPOSITS
Megan Hough
Monash University, Vic
2007 SMEDG - AIG Postgraduate Bursary Winner
I would like to thank the Australian Institute of Geoscientists and the Sydney Mineral
Exploration Discussion Group for the 2007 SMEDG-AIG Postgraduate Bursary Award.
The bursary contributed to my attendance to the Orogenic Lode Gold Deposits short
course and the Northern Ireland Gold field trip, associated with the Ninth Biennial SGA
Meeting held at Trinity College in Dublin during August 2007. My attendance to the
SGA Meeting in Dublin was also funded by a Monash Postgraduate Travel Grant and
SGA Student Grant.
At the “Mineral Exploration and Research: Digging Deeper” meeting, I had the
opportunity to present my PhD research on the gold deposits of the Walhalla – Woods
Point region in Victoria, in the “Gold Metallogenesis: Americas & Australiasia”
session.
My research project is supported by ARC Linkage funding, in collaboration with
Goldstar Resources NL and Geoscience Victoria. The aim of the research is to apply
predictive 3D modeling to gold exploration for anomalously large deposits associated
with a Devonian dyke swarm within the Walhalla-Woods Point Goldfield in eastern
Victoria. This unique goldfield within Victoria allows the comparison of (i) western and
eastern Victorian gold mineralization; (ii) sediment-hosted, dyke-associated and dykehosted gold mineralization; and (iii) structural and geochemical factors for gold
mineralization.
The ten highest yielding deposits within the goldfield are either hosted within, or
spatially associated with, intrusions of the Woods Point Dyke Swarm, due to the high
chemical reactivity of the dolerite dykes, and the high rheological contrast between the
dolerite dykes and surrounding greenschist facies meta-sedimentary units. Based on
current research, consistently favourable sites for gold mineralisation in the WalhallaWood’s Point Goldfield are dyke-hosted quartz breccia zones. Historical production
concentrated on visible gold within the shear-zone hosted laminated reef systems,
although recent assay results have highlighted the increased levels of visible gold, and
also the presence of disseminated gold, especially along dyke margins.
Attending the Orogenic Gold Workshop with Professor David Groves of the University
of Western Australia was extremely beneficial. The short course reviewed the nature of
orogenic gold provinces around the globe and deposits at different crustal levels, and
then applied this understanding to gold exploration. I was particularly interested in
Professor Groves’ work on the host-rock and structural controls on orogenic gold
deposits, and the methods of quantifying the chemical reactivity of host rocks and
rheological contrasts between the lithologies within the rock sequence.
The two day field trip to the Northern Ireland Gold Deposits was organized by Garth
Earls of the Geological Survey of Northern Ireland. The group first visited Cashel Rock
of the Ordovician Tyrone Igneous Complex, north of Omagh in Northern Ireland. The
next day the group visited the Galantas Gold Open Cut Mine near Cavanacaw, 5 km
WSW of Omagh, followed by the Tournigan Gold Project near Curraghinalt, 15 km NE
of Omagh. The Cavanacaw and Curraghinalt orogenic gold deposits are hosted within
an inlier of Neoproterozoic Dalradian metasediments. The gold mineralization is
associated with the later stages of the Caledonian Orogeny (~470 – 400 Ma), and is
structurally controlled by the N-S Omagh lineament and the NE-SW trending Omagh
Thrust (an extension of the Caledonian Highland Boundary Fault in Scotland). In the
past, gold exploration has been hampered by the cover of peat bog over most of
Northern Ireland, and also the very limited use of explosives for mine site development.
Cashel Rock of the Ordovician Tyrone Igneous Complex is a brecciated rhyolite
surrounded by tuff, and hosts volcanogenic massive sulfide mineralization. It is part of
an ophiolite complex which obducted due to the closing of the Iapetus Ocean during the
Caledonian Orogeny. The Neoproterozoic inlier of Dalradian meta-sediments which
host the gold mineralization, was thrust over the Ordovician Tyrone Igneous Complex.
The Curraghinalt gold deposit is a high-grade mesothermal quartz-sulphide gold vein
system, with visible gold and negligible arsenopyrite. The gold mineralization formed
close to the surface, and is highly oxidized, is copper-rich and the wallrock alteration
extends no more than 0.5 metres from the veins. The current resource estimate is
262,000 ounces of gold in 527,700 tonnes, grading at 15.45 g/t.
The lower temperature Cavanacaw gold deposit is characterized by gold-bearing
sulphides and clay gouge associated with steeply east-dipping quartz veins. The largest
and most continuous mineralized structure is the Kearney Structure. Most gold is
associated with pyrite, arsenopyrite and galena, with little to no visible free gold. The
current proven/probable ore reserve is 367,310 tonnes at 7.52 g/t for 88,827 ounces of
gold, and the current indicated resource is 1,183,830 tonnes at 7.02 g/t for 267,150
ounces of gold.
The opportunity to attend the Ninth Biennial SGA Meeting, and associated short course
and field trip, allowed me to meet and learn from world-reknowned gold researchers,
and to visit differing styles of orogenic gold deposits. I have learnt more on the
structural and host rock controls on gold mineralization, which I can now apply directly
to my research. On the fieldtrips, I was also able to view drillcore from both deposits
and to access two mines - many photos were taken of the geology and mineralization for
comparison with my research!
I am also very grateful for the opportunity to present my research at a SMEDG Meeting
in Sydney at the end of October.