SEG 45 Final_qx4 - Society of Economic Geologists

22 SEG NEWSLETTER No 53 • APRIL 2003
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Economic Geology Commentary (Continued)
Economic Geology Commentary
observations made primarily on the
subeconomic, and, most importantly,
relatively undolomitized Cooleen zone
(near Silvermines), have recognized five
main breccia types. Their type I polymictic
and roughly concordant breccia is
“...interpreted to have formed through
episodic, fault-controlled synsedimentary
slumping to create debris flows during
formation of the Waulsortian mudbanks...”
(Lee and Wilkinson, 2002, p.
659). Their type III breccia is described
as comparable to the black matrix breccia
at Lisheen and is regarded by these
authors as being originally a type I breccia
that has been overprinted and
altered by regional dolomitization and
hydrothermal processes. The conclusions
of Lee and Wilkinson (2002) are
challenged by Reed and Wallace (2003)
but are stoutly defended by Wilkinson
and Lee (2003). Obviously there is no
consensus at this time.
Whether the Lisheen BMB is of
hydrothermal (Hitzman et al., 2003) or
synsedimentary origin (Lee and
Wilkinson, 2002) does not alter the fact
that these bodies are closely associated
with sulfides and, therefore, will continue
to offer exploration targets many
times the size of the associated orebodies.
However, if future detailed studies of
the Lisheen black matrix breccia confirm
the synsedimentary origin of these
breccias, the significance of the regional
dolomitization and the resulting
increase in permeability to facilitate the
flow of hydrothermal fluids will be
reduced. Hydrothermal fluids, instead,
will be seen as favoring the highly permeable
debris flows and forming sulfide
bodies by open-space filling and
replacement of these bodies.
Hitzman et al. (2002) conclude that
formation of the Lisheen orebody
required the following: (1) the presence
of suitable host rocks; (2) an effective
plumbing system; and (3) metalliferous
NOTE:
hydrothermal fluids. The first of these,
the regional dolomitization, occurred
no earlier than the Chadian and
resulted in a significant increase in
porosity and permeability. Regional
dolomitization is considered to have
controlled the size and geometry of the
Lisheen deposit. The second, synsedimentary
normal faulting during late
Courceyan to Chadian provided the
necessary channels for ingress of
hydrothermal fluids. Although the
nature of the hydrothermal fluids is not
fully clear, they appear to have been
somewhat acidic, and caused initial formation
of the Lisheen breccias followed
by sulfide precipitation. Mineralization
is estimated to have occurred “...at least
10 m.y. after deposition of the
Waulsortian complex host rocks”
(Hitzman et al., 2002, p. 1653).
One final note deserves attention:
Hitzman et al. (2003) allude to the difficulty
in correlating radiometric dates
and stratigraphic ages in the
Carboniferous time scale, and three different
time scales proposed by George et
al. (1976), Haq and van Eysinga (1998),
and Okulitch (1999), vary by up to 10
m.y. in the absolute positioning of the
various stratigraphic boundaries. These
uncertainties have led to some confusion
in the literature over the timing of
mineralization in the Irish Zn-Pb
deposits (e.g., Lee and Wilkinson, 2002;
Boyce et al., 2003). It is suggested that
improved absolute age definition of this
part of the Carboniferous time scale
would help resolve some of the outstanding
controversies regarding these
important base metal deposits.
REFERENCES
Boyce, A.J., Little, C.T.S., and Russell, M.J.,
2003, A new fossil vent biota in the
Ballynoe barite deposit, Silvermines,
Ireland: Evidence for intracratonic sea-floor
hydrothermal activity ~352 Ma: Economic
Geology, v. 98, in press.
George, T.N., Johnson, G.A.L., Mitchell, M.,
Prentice, J.E., Ramsbottom, W.H.C.,
Sevastopulo, G.D., and Wilson, R.B., 1976,
A correlation of Dinantian rocks in the
British Isles: Geological Society of London
Special Reports 7, 87 p.
Haq, B.U. and van Eysinga, F.W.B., 1998,
Geological time scale—fifth edition:
Amsterdam, Elsevier Science.
Hitzman, M.W., Redmond, P.B., and Beaty,
D.W., 2002, The carbonate-hosted Lisheen
Zn-Pb-Ag deposit, County Tipperary,
Ireland: Economic Geology, v. 97, p.
1627–1655.
Lee, M.J. and Wilkinson, J.J., 2002,
Cementation, hydrothermal alteration,
and Zn-Pb mineralization of carbonate
breccias in the Irish Midlands: Textural evidence
from the Cooleen zone, near
Silvermines, County Tipperary: Economic
Geology, v.97, p. 653–662.
Mullane, M.M. and Kinnaird, J.A., 1998,
Synsedimentary mineralization at the
Ballynoe barite deposit, near Silvermines,
Co. Tipperary, Ireland: Transactions of the
Institute of Mining and Metallurgy, v.107,
p. B48–61.
Okulitch, A.V., 1999, Geological time scale,
1999: Geological Survey of Canada Open
File 3040, National Earth Sciences Series,
Geological Atlas.
Reed, C. and Wallace, M., 2003,
Cementation, hydrothermal alteration,
and Zn-Pb mineralization of carbonate
breccias in the Irish Midlands: textural evidence
from the Cooleen zone, near
Silvermines, County Tipperary—a discussion:
Economic Geology, v. 98, 191–193.
Wilkinson, J.J. and Lee, M.J., 2003,
Cementation, hydrothermal alteration,
and Zn-Pb mineralization of carbonate
breccias in the Irish Midlands: textural evidence
from the Cooleen zone, near
Silvermines, County Tipperary—a reply;
Economic Geology, v. 98, 194–198. 1
Commentary that appears in this column is
solicited by the technical editor.
Dr. Sangster retired from the Geological
Survey of Canada in 1997; he is a past
President of SEG.
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