Geochemistry of felsic metavolcanic rocks from

Geochemistry of felsic metavolcanic rocks from the Finlayson Lake volcanogenic massive sulphide
district, Yukon-Tanana terrane, southeastern Yukon.
Stephen J. Piercey*
(Mineral Deposit Research Unit,
Earth and Ocean Sciences, University of British Columbia,
Vancouver, B.C., V6T 1Z4, [email protected])
Suzanne Paradis
(Mineral Resources Division, Geological Survey of Canada, Sidney, B.C.)
The Finlayson Lake volcanogenic massive sulfide (VMS) district of the Yukon, is one of Canada's most
recent VMS discovery regions with nearly 30 Mt of massive sulfide ore discovered since the mid 1990's.
Felsic metavolcanic-hosted deposits (Wolverine and Kudz Ze Kayah) comprise nearly 20 Mt of ore
associated with a metamorphosed and deformed Devono-Mississippian volcano-sedimentary sequence of
the Yukon-Tanana terrane (YTT). These discoveries have led to a resurgence in research and exploration
interest within the YTT and numerous new geological, tectonic and metallogenic ideas have been
proposed for this region (e.g., Murphy and Piercey, 1999, 2000; Piercey et al., 1999, Piercey and Murphy,
2000). This poster presents the results of a regional lithogeochemical study of least altered felsic volcanic
rocks that host the Wolverine and Kudz Ze Kayah VMS deposits.
The felsic metavolcanic rocks (rhyolites, dacites and related tuffaceous rocks) have variable SiO2 contents
(67.90-79.90 wt%) with weakly to moderately peraluminous A/NK (1.08-2.34) and A/CNK (1.02-2.15)
ratios. Their trace element signatures have features similar to calc-alkaline arc rocks including LREE
enrichment (Ce/Ybn = 3.47-17.87) with variably negative Eu (Eu/Eu*=0.12-0.73) and Ti anomalies
(Ti/Ti* = 0.03-0.11), coupled with elevated Th/Nb values (0.66-1.75) and negative Nb anomalies
(Nb/Nb* = 0.11-0.33; Fig 1). In contrast, they contain elevated HFSE (Zr = 130-572 ppm; Hf =3.813.0ppm; Nb = 11.0-40.0 ppm; Y =19.0-68.0 ppm; Ga = 12.0-24.0 ppm) and REE (except Eu REE = 114471 ppm) contents, elevated Nb/Y (0.49-1.13; Fig. 2a) and Ga/Al (104Ga/Al = 1.9-3.5; Fig. 2b) ratios, and
Nb-Ce-Y systematics (Fig. 2c) typical of within plate environments (non-arc) and akin to fractionated Itype (volcanic-arc) to A-type (non-arc) granitoids. These geochemical characteristics are similar to those
of felsic metavolcanic rocks within the Tetagouche Group of the Bathurst VMS district, New Brunswick,
Canada (Figs. 1 and 2), and are typical of rocks within extensional regimes in rifted arcs and ensialic
back-arc basins. This style of magmatism is associated with felsic metavolcanic rocks with elevated
emplacement temperatures (T >800oC; Fig. 3) and extensional tectonic activity, which would provide
elevated regional heat flow and ground preparation required for the generation and maintenance of
hydrothermal systems. Our regional geochemical database indicates that geochemistry alone cannot
locate a specific horizon favorable for VMS mineralization, however it could be used to identify districtscale regions with prospectivity for VMS mineralization when combined with other conventional
exploration techniques such as geological mapping and geophysical surveys.
Felsic volcanic geochemistry, Finlayson Lake VMS district
1000
Kudz Ze Kayah Succession
Rock / Primitive Mantle
Rock / Primitive Mantle
1000
100
10
1
Bathurst District
Lentz (1996, 1999)
Whalen et al. (1998)
(a)
Wolverine Succession
100
.1
10
1
Bathurst District
Lentz (1996, 1999)
Whalen et al. (1998)
(b)
.1
Th
Nb
La
Ce
Pr
Nd
Sm Hf
Ti
Tb
Y
Yb
Zr
Eu Gd Dy Er
Lu
Th
Nb
La
Ce
Pr
Nd
Sm Hf
Ti
Tb
Y
Yb
Zr
Eu Gd Dy Er
Lu
Figure 1. Primitive mantle normalized trace element plots for the Finlayson Lake felsic
metavolcanic rocks, including: Kudz Ze Kayah succession (a) and Wolverine succession (unit 5)
samples (b). Shown for comparison are felsic metavolcanic rocks from the Tetagouche Group,
Bathurst mining camp, New Brunswick from Lentz (1996), Lentz (1999) and Whalen et al. (1998).
1000
1000
(b)
(a)
A-type
ge
&
te rid
pla ean
n
i
c
o
th
wi lous
ma
o
an
100
S-type
syncollsional
10
I-type
volcanic arc
Zr (ppm)
Nb (ppm)
A-type
within plate
100
I&S-types
OR-type
ocean ridge
M-type
10
1
1
10
100
1
1000
4
10 *Ga/Al
Y (ppm)
Nb
10
(c)
A1
mantle A-type
A2
crustal A-type
Y
Ce
Figure 2. Trace element plots for felsic metavolcanic rocks from the Finlayson Lake district. (a) NbY discrimination plot, modified from Pearce et al. (1984), exhibiting the fractionated I -type
(volcanic arc) to A-type (within plate) characterstics of the Finlayson Lake felsic metavolcanics; (b)
Ga/Al-Zr discrimination of A-type versus other granite types (Whalen et al., 1987); (c) Nb-Ce-Y
discrmination plot of Eby (1992). Symbols and shading of Bathurst fields as in Figure 1.
o
T ( C)-Zircon Saturation
Felsic volcanic geochemistry, Finlayson Lake VMS district
950
900
850
KZK Succession
800
Wolverine Succession
750
SRPS
700
0
10
20
30
40
50
Nb (ppm)
Figure 3. Zircon saturation temperature (Watson and Harrison, 1983) as a function of Nb content
illustrating the HFSE-enriched, A-type rhyolitic rocks from Finlayson Lake have elevated zircon
saturation temperatures when compared to typical VMS-barren calc-alkaline arc-related granitoids of
the younger (~345-355 Ma) Simpson Range Plutonic Suite (SRPS; data from Grant, 1997). This
feature suggests that the HFSE-enriched magmas from the Finlayson Lake region have elevated
emplacement temperatures that likely induced and maintained hydrothermal fluid flow required for
formation of the Kudz Ze Kayah (KZK) and Wolverine VMS deposits.
References
Eby, G.N., 1992. Chemical subdivision of the A-type granitoids: Petrogenetic and tectonic implications:
Geology, 20: 641-644.
Grant, S.L., 1997. Geochemical, radiogenic tracer isotopic, and U-Pb geochronological studies of YukonTanana Terrane rocks from the Money Klippe, southeastern Yukon, Canada: Unpublished M.Sc. thesis,
University of Alberta, 177 p.
Lentz, D.R., 1996. Trace element systematics of felsic volcanic rocks associated with massive-sulfide
deposits in the Bathurst Mining Camp: Petrogenetic, tectonic and chemostratigraphic implications for
VMS exploration; In Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulfide
Exploration. Edited by D.A Wyman. Geological Association of Canada, Short Course Notes Volume 12,
pp.359-402.
Lentz, D.R., 1998. Petrogenetic evolution of felsic volcanic sequences associated with Phanerozoic
volcanic-hosted massive sulfide systems: the role of extensional geodynamics: Ore Geology Reviews, 12:
289-327.
Lentz, D.R., 1999. Petrology, geochemistry and oxygen isotopie interpretation of felsic volcanic and
related rocks hosting the Brunswick 6 and 12 massive sulfide deposits (Brunswick Belt), Bathurst Mining
Camp, New Brunswick, Canada: Economic Geology, 94: 57-86
Murphy, D.C., and Piercey, S.J., 1999. Finlayson project: Geological evolution of Yukon-Tanana Terrane
and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon; in
Yukon Exploration and Geology; Exploration and Geological Services Division, Department of Indian
and Northern Affairs, p. 47-62.
Felsic volcanic geochemistry, Finlayson Lake VMS district
References (continued)
Murphy, D.C., and Piercey, S.J., 1999. Finlayson project: Geological evolution of Yukon-Tanana Terrane
and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon; in
Yukon Exploration and Geology; Exploration and Geological Services Division, Department of Indian
and Northern Affairs, p. 47-62.
Murphy, D.C., and Piercey, S.J., 2000. Syn-mineralization faults and their re-activation, Finlayson Lake
massive sulphide belt, Yukon-Tanana terrane, southeastern Yukon. In Yukon Exploration and Geology
1999; Exploration and Geological Services Division, Department of Indian and Northern Affairs, pp. 5366.
Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984. Trace element discrimination diagrams for the
tectonic interpretation of granitic rocks: Journal of Petrology, 25: 956-983.
Piercey, S.J., and Murphy, D.C., 2000. Stratigraphy and regional implications of unstrained DevonoMississippian volcanic rocks in the Money Creek thrust sheet, Yukon-Tanana Terrane, southeastern
Yukon. In Yukon Exploration and Geology 1999, Exploration and Geological Services Division,
Department of Indian and Northern Affairs, pp. 67-78.
Piercey, S.J., Hunt, J.A., and Murphy, D.C., 1999. Lithogeochemistry of meta-volcanic rocks from
Yukon-Tanana Terrane, Finlayson Lake region, Yukon: Preliminary results. In Yukon Exploration and
Geology 1998, Exploration and Geological Services Division, Department of Indian and Northern
Affairs, pp. 125-138.
Watson, E.B., and Harrison, T.M., 1983. Zircon saturation revisited: temperature and composition effects
in a variety of crustal magma types: Earth and Planetary Science Letters, 64: 295-304.
Whalen, J.B., Currie, K.L., and Chappell, B.W., 1987. A-type granites: geochemical characteristics,
discrimination and petrogenesis: Contributions to Mineralogy and Petrology, 95: 420-436.
Whalen, J.B., Rogers, N., van Staal, C.R., Longstaffe, F.J., Jenner, G.A., and Winchester, J.A., 1998.
Geochemical and isotopic (Nd, O) data from Ordovician felsic plutonic and volcanic rocks of the
Miramichi Highlands: petrogenetic and metallogenic implications for the Bathurst Mining Camp:
Canadian Journal of Earth Sciences, 35: 237-252.
Biographical Note
Steve Piercey obtained a B.Sc.(Hons) (1996) and M.Sc. (1998) both from Memorial University of
Newfoundland. Presently he is a Ph.D. candidate at the Mineral Deposit Research Unit at the University
of British Columbia. His research interests are in the application of field, geochemical, and isotopic
techniques to better understand the interelationships between magmatism, tectonics and hydrothermal
system generation with particular emphasis on volcanogenic massive sulphide deposits.

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