Mapping magma prospectivity for Cordilleran volcanogenic ...spiercey/Piercey_Research...Piercey et...
Transcript of Mapping magma prospectivity for Cordilleran volcanogenic ...spiercey/Piercey_Research...Piercey et...
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Mapping magma prospectivity for Cordilleran volcanogenic massive sulphide (VMS) deposits using Nd-Hf isotopes:
Preliminary results
S.J. Piercey*, L.P. Beranek and J.M. Hanchar Department of Earth Sciences, Memorial University of Newfoundland
Piercey, S.J., Beranek, L.P. and Hanchar, J.M., 2017. Mapping magma prospectivity for Cordilleran volcanogenic massive sulphide (VMS) deposits using Nd-Hf isotopes: Preliminary results. In: Yukon Exploration and Geology 2016, K.E. MacFarlane and L.H. Weston (eds.), Yukon Geological Survey, p. 197-205.
ABSTRACT
Preliminary whole rock Nd-Hf isotopic data for porphyritic rhyolitic intrusive rocks from the Wolverine volcanogenic massive sulphide (VMS) deposit are presented herein. Pre-VMS (~352 Ma) quartz-feldspar porphyritic intrusive rocks (QFP) have Nb/Ta ratios (~12) and lower eNdt and eHft values, compared to syn-VMS (~347 Ma) feldspar porphyritic intrusive rocks (FP), which have higher Nb/Ta ratios (~17) and lower eNdt and eHft. Both suites have Proterozoic to Archean depleted mantle model ages indicative of crustal inheritance; however, the FP suite has a more juvenile signature. The progression from the crustal-dominated QFP suite, to a more basalt-influenced FP suite reflects the progressive opening of the Wolverine back-arc rift where early QFP magma was dominated by continental crustal melting, whereas the FP magma reflects greater back-arc basin extension, upwelling of basaltic magma beneath the rift, and enhanced continental crust-juvenile basalt mixing. Basalt upwelling beneath the Wolverine basin likely created the elevated geothermal gradient required for Wolverine VMS deposit formation.
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resistant minerals (e.g., zircon, monazite, apatite) record similar geochemical and isotopic signatures. Presented herein are preliminary results from bulk rock Nd and Hf isotope geochemistry of pre and syn-VMS high-level (~synvolcanic) footwall porphyritic intrusive rock (i.e., porphyries) from the Wolverine deposit, which provide initial data to test the “hot” rift hypothesis.
GEOLOGICAL SETTINGThe Finlayson Lake district is located northeast of the Tintina fault in southeastern Yukon (Fig. 1). The district is underlain by pericratonic rocks of the Yukon-Tanana terrane and oceanic rocks of the Slide Mountain terrane, which are juxtaposed against rocks of the North American continental margin along the Inconnu thrust of post-Late Triassic age (Fig. 2; Murphy et al., 2006). The Finlayson Lake district is subdivided into several informal fault and unconformity-bounded groups and formations; the Wolverine deposit is hosted by the Wolverine Lake group within the Big Campbell thrust sheet (Fig. 2; Murphy et al., 2006). The Wolverine Lake group consists of Mississippian felsic metavolcanic and high level meta-intrusive rocks, metasedimentary units, and locally mafic metavolcanic and metaplutonic rocks (Figs. 2 and 3; Murphy et al., 2006).
In the Wolverine deposit area, the Wolverine Lake group consists of a footwall dominated by felsic tuffaceous rocks, meta-intrusive rocks, and interlayered black shale (Figs. 3 and 4; Bradshaw et al., 2008; Piercey et al., 2008, 2016). The hanging wall consists of interlayered felsic siltstone, black shale, iron formation, carbonate exhalative rocks, felsic siltstone breccia, and mafic intrusive and lesser volcanic rocks (Figs. 3 and 4; Bradshaw et al., 2008; Piercey et al., 2008, 2016).
In the immediate footwall to the deposit there are very distinctive felsic porphyritic intrusive rocks. These coherent intrusive rocks occur at the Wolverine deposit proper, as well as along strike at other VMS prospects, including the Fisher, Puck, and Sable zones (Figs. 3 and 4). Previous work has documented the field relationships and ages of the porphyries and documented an older, ~352 Ma pre-VMS suite of quartz feldspar porphyries, and a younger ~347 Ma, syn-VMS suite of feldspar porphyritic rocks (Figs. 4 and 5; Piercey et al., 2008).
INTRODUCTIONVolcanogenic massive sulphide (VMS) deposits are important sources of base (Cu, Zn, Pb) and precious metals (Ag, Au) to the Canadian economy (e.g., Galley et al., 2007). Volcanogenic massive sulphide deposits in Yukon have been the focus of past exploration and production (e.g., Wolverine), and are the current focus of exploration in some locales (e.g., BMC Minerals Ltd. at Kudz Ze Kayah, Finlayson Lake district). Both globally and in Yukon examples, VMS deposits are associated with specific geodynamic environments and specific magma clans (e.g., Piercey, 2011). In particular, VMS deposits are associated with extensional geodynamic environments (e.g., ridges, arc rifts, back arc basins) and with magma that was emplaced at high temperatures (T >900oC; Piercey, 2011). More generally, extensional geodynamic environments provide the structural conduits and permeability required for hydrothermal fluid recharge, and eventual discharge onto the seafloor (e.g., Franklin et al., 2005; Galley et al., 2007; Piercey, 2011), whereas high temperature magmatism provides the heat engine required to drive hydrothermal circulation critical to VMS formation (Lesher et al., 1986; Barrie, 1995; Lentz, 1998; Piercey et al., 2001, 2008; Hart et al., 2004; Piercey, 2010, 2011).
In the Wolverine deposit, recent research illustrates that even in felsic-dominated successions mantle heat and upwelling of juvenile basaltic magma beneath a rift may be an important driver of VMS hydrothermal circulation (Piercey et al., 2008). Furthermore, the signature of mantle upwelling and juvenile crust-mantle mixing is recorded in the trace element signature of felsic rocks, and could be used as a proxy for heat flow of a given VMS environment. In particular, Piercey et al. (2008) illustrated that higher temperature felsic rocks have elevated high field strength element (HFSE) and rare earth element (REE) contents, but also have element ratios (e.g., Nb/Ta, Ti/Sc) indicative of evolved crust – juvenile basalt mixing. These authors also argued that this record of juvenile basalt involvement may be a critical feature for delineating prospective “hot” rifts in felsic dominated successions.
To test the significance of juvenile magmatism and “hot” rifts, a project was initiated at the Wolverine deposit utilizing bulk rock Sm-Nd and Lu-Hf tracer isotopes, and in situ mineral-scale analyses of Lu-Hf and U-Pb in zircon, to identify the “mantle” trace element signatures recorded in Wolverine porphyritic rhyolite, and to determine if
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were undertaken. Throughout the course of the study repeat analysis of the JNDi-1 reference material yielded an average 143Nd/144Nd value of 0.512100 ± 6 (1 s.d, n = 27); the published value of Tanaka et al. (2000) is 0.512115. Lu-Hf isotopes were prepared using standard method of spiking, sample dissolution, column chemistry and subsequent analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) following the methods of Vervoort et al. (2004). During the course of this study the JMC-475 Hf standard yielded an average value of 176Hf/177Hf = 0.282171 ± 17 (1 s.d, n = 12); the published values are 176Hf/177Hf = 0.282160 (Vervoort and Blichert-Toft, 1999). Initial 143Nd/144Nd ratios and eNdt were calculated at 350 Ma, the mid-point between the ages of the two porphyry suites presented in this paper. The chondritic uniform reservoir (CHUR) values used for
LITHOGEOCHEMISTRY AND RADIOGENIC ISOTOPE GEOCHEMISTRYSamples of the quartz-feldspar and feldspar porphyries were analyzed previously for lithogeochemistry as part of the study by Piercey et al. (2008). The samples were taken from drill core from the footwall in various zones of the Wolverine deposit (Fig. 4), and the full dataset will be released in future publications. A subsample of this dataset were subsequently analyzed for Sm-Nd and Lu-Hf isotopes at Memorial University of Newfoundland following the methods outlined in Phillips (2015). For Sm-Nd, standard methods of spiking, sample dissolution, column chemistry and subsequent analysis by isotope dilution thermal ionization mass spectrometry (ID-TIMS)
128°W
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PALEOZOIC PERICRATONICASSEMBLAGES
CONTINENT MARGINASSEMBLAGES
Devonian - Permian Stikine assemblage
Devonian - Jurassicoceanic assemblages
PermianKlondike SchistPennsylvanian - Permian Klinkit Devonian - Mississippianarc assemblages
Neoproterozoic - Devonian basinal facies
Neoproterozoic - Missrift and basinal facies
Proterozoic - Devonian platformal faciesNeoproterozoic - Paleozoicother cont margin assembl
Devonian - Mississippian mineral districts
Yukon-Tanana terrane
Figure 1. Terrane map of northern Cordillera showing location of Finlayson Lake district. Modified from Colpron et al. (2006). Abbreviations: Wh = Whitehorse; Fb = Fairbanks; WL = Watson Lake; ST + CC = Stikine and Cache Creek; E = eclogite (Permian); e = eclogite (Mississippian); and b = blueschist (Permian).
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VHMS & VSHMSDEPOSITS
Kudz Ze Kayah
Fyre Lake
Wolverine
Ice
GP4F5
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INCONNU INCONNU
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JCFJCF
JCFJCF
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PellyPelly RiverRiver
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FireLakeFireLake
Campbell HighwayCampbell Highway
YUKON
Finlayson Lake(105G)
Frances Lake(105H)
Ross River~ 40 km
Watson Lake~ 110 km
TINTINA
FAULT
Mississippianeclogite (KMC)
North Klippen
Money Klippe
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ultramafic and mafic intrusions
polymictic conglomerate, sandstone,siltstone, mafic and felsic volcanic rocks,limestone
basalt and varicolored chert
dark phyllite and chert, varicoloredchert, chert-pebble conglomerate,sandstone, limestone, felsic and maficmetavolcanic rocks
YUKON-TANANA TERRANE
YUKON-TANANA TERRANE
POST - YTT / SMTAMALGAMATION
SLIDE MOUNTAIN TERRANE
SLIDE MOUNTAIN TERRANELEGEND
INTRUSIVE ROCKS
INTRUSIVE ROCKS
SIMPSON LAKE GROUP
Campbell Range formation
Early Permian
granite, quartz monzoniteaugen granite
Late Devonian to Early Mississippian
Lower Permian
granite, quartz monzonite,granodiorite
SIMPSON RANGE PLUTONIC SUITE
GRASS LAKES PLUTONIC SUITE
ultramafic and mafic intrusions,Big Campbell and Cleaver Lake thrust sheets
FORTIN CREEK GROUP Carboniferous to Permian?
undifferentiated intrusions
undifferentiated volcanic rocks
Mesozoic and Cenozoic
grey shale, siltstone and limestone
Triassic
Permian - Triassic
dark shale, siltstone and limestone
Triassic
NORTH AMERICANCONTINENTAL MARGIN
undifferentiated formations of SelwynBasin, McEvoy Platform, Earn Groupand Mt. Christie Formation
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limestone and quartzite
LAYERED ROCKS
Gatehouse formation Lower to Middle Permian
dark phyllite and sandstone, chert,chert-pebble conglomerate, diamictite
LAYERED ROCKS
Money Creek formation Lower Permian
massive bioclastic limestoneFinlayson Creek limestone
Mid-Pennsylvanian to Lower Permian
massive bioclastic limestoneWhitefish limestone
Upper Mississippian
intermediate, felsic and mafic volcanicrocks, sandstone, chert, limestone
Tuchitua River formation Lower Mississippian
undifferentiated mafic and felsic volcanic rocks and dark clastic rocksof the Fire Lake, Kudz Ze Kayah andWind Lake formations
GRASS LAKES GROUP
calc-alkaline basalt, rhyolite, chertand volcanic-derived sandstone
Cleaver Lake formation
North River formation
felsic to intermediate metavolcanicrocks and carbonaceous phyllite
Waters Creek formation
Upper Devonian to Lower Mississippian
Pre-Upper Devonian
quartzose metaclastic rocks, marbleand non-carbonaceous pelitic schist
undifferentiated mafic and felsic volcanic rocks and dark clastic rocks
WOLVERINE LAKE GROUP
green and pink chert, limestone, sandstone, conglomerate, mafic metavolcanic rocks
undifferentiated White Lake and King Arctic formations Upper Mississippian to mid-Pennsylvanian
Figure 2. Geological map of Finlayson Lake district showing locations of volcanogenic massive sulphide deposits (from Murphy et al., 2006).
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eNdt calculations are 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967 (Hamilton et al., 1983). Depleted mantle model ages (TDM) are calculated using the values of 143Nd/144Nd = 0.513163 and 147Sm/144Nd = 0.2137 (Goldstein et al., 1984). Initial 176Hf/177Hf ratios and eHft were calculated at 350 Ma similar to the Nd isotopic data. The chondritic uniform reservoir (CHUR) values used for eHft calculations are 176Hf/177Hf = 0.282772 and 176Lu/177Hf = 0.0332 (Blichert-Toft and Albarede, 1997). Depleted mantle model ages (TDM) are calculated using the values of 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0334 (Vervoort and Blichert-Toft, 1999).
RESULTS
Sm-Nd and Lu-Hf results are shown in Figures 6 and 7. In Figure 6 Wolverine quartz-feldspar and feldspar porphyritic rhyolite samples are plotted against the whole-rock Nb/Ta ratio. The Nb/Ta data are from Piercey et al. (2008) and are shown as they are reflective of the source region (i.e., upper crust vs. juvenile basalt) for the felsic rocks. Nb and Ta are geochemical twins and rarely fractionated from one another during partial melting and fractional crystallization, with upper crustal rocks and rocks derived therefrom having Nb/Ta ~12, whereas Nb/Ta ~17 for rocks derived from more juvenile,
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LegendGeology
Campbell Range FormationSulphidezone
Slide Mountain terrane
Yukon-Tanana terrane
Wolverine Lake Group
Kudz Ze Kayah Formation
Permian ultramafics
Money Creek Formation
hanging wall basalt
footwall carbonaceous sedimentary rocksfootwall felsic volcaniclastics and iron fmnshanging wall aphyric rhyolite & volcaniclastics
footwall basal clastic rocksGrass Lake Group
normalfault
Symbols
Figure 5
Figure 3. Geology of local Wolverine deposit area, illustrating locations of different zones. Geology from Murphy et al. (2006).
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massive basalt
basaltic volcaniclasticinterbedded graphitic argillite & greywackerhyolitic siltstone brecciamagnetite iron formation
carbonate exhalitemassive sulphide
finer grained rhyolitic siltsone/tuff
coarser grained rhyolitic siltsone/tuffcoarse grained felsic volcaniclastic (± qtz, fsp crystals)
graphitic argillitetrace of diamond drill hole
Figure 4. Geological cross sections Wolverine deposit illustrating deposit stratigraphy and the presence of feldspar porphyritic rhyolites in the deposit footwall. Sections modified from Bradshaw et al. (2008).
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mantle sources (e.g., basalt; Green, 1995; Barth et al., 2000; Kamber and Collerson, 2000; McLennan, 2001). The pre-VMS QFP (352 Ma) suite have Nb/Ta ~12 and eNdt ranging from -7.7 to -11.5, eHft ranging from -12.4 to -19.0, and with TDM(Nd) ages of 1.66-2.58 Ga and TDM(Hf) ages of 1.59-2.22 Ga (Figs. 6 and 7). The younger, syn-VMS FP (347 Ma) suite of porphyries have higher Nb/Ta ratios and eNdt ranging from -7.9 to -8.1, eHft ranging from -13.4 to -18.0, and with TDM(Nd) ages of 1.59-1.67 Ga and TDM(Hf) ages of 1.62-1.97 Ga (Figs. 6 and 7).
DISCUSSION AND SUMMARYPorphyritic rocks from the Wolverine deposit have distinct variations in Nd-Hf isotopic signatures that are indicative of varying contributions of upper crust vs. juvenile (basaltic) material to their genesis. Both suites of high level intrusive rocks have been influenced by upper crustal materials
as indicated by their negative eNdt and eHft values and Proterozoic to Archean depleted mantle model ages (Figs. 6 and 7); it is implied that these rocks melted continental crustal sources, or assimilated continental crustal material during emplacement (Lentz, 1998; Piercey et al., 2008). These data are also consistent with previously reported data for the felsic rocks throughout the Yukon-Tanana terrane, which show strong evidence of crustal inheritance from Laurentian-derived, peri-continental material (e.g., Piercey et al., 2006). There are notable differences between the suites, however. The older QFP suite has lower Nb/Ta ratios, lower eNdt and overlapping, but generally lower eHft, compared to the younger FP suite (Figs. 6 and 7). Piercey et al. (2008) suggested that the lower Nb/Ta in the QFP suite reflected a greater upper crustal contribution relative to the FP suite, which had a greater juvenile contribution, likely derived from underplated basaltic magma at the base of the Wolverine back-arc rift. The isotopic results herein support this hypothesis and the FP suite is shifted towards higher eNdt and eHft, consistent with a greater juvenile contribution to its genesis relative to the QFP suite.
The progression from the more crustally-derived QFP suite at 352 Ma to the FP suite with a greater juvenile contribution at 347 Ma is interpreted to represent the progressive opening of the Wolverine back-arc basin, upwelling of juvenile basaltic magma beneath the rift, and greater crust-basalt mixing as the basin widened (Piercey et al., 2008). The latter authors also argued that the upwelling of mantle-derived magma beneath the rift was also critical in increasing heat flow to the basin, which in turn enhanced hydrothermal circulation and ultimately led to the formation of the Wolverine deposit.
%quartz, size, %feldspar, size
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QFP
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or argillite
QFP withmm-scale wisps
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carbonaceous
2-3%, <2 mm, 5%, 2-3 mm
2-3%, <1 mm, 5-7%, 2-3 mm
10-12%, 4-6 mm, <3%, 2 mm-1.3 cm
10-12%, 4-6 mm, 4-5%, 2 mm-1.3 cm
10-12%, 4-6 mm, 3-5%, 2 mm-1 cm
10-12%, 3-5 mm, 5-7%, 1-1.5 cm
10-12%, 4-6 mm, 3-5%, 2 mm-1.1 cm
10-12%, 4-6 mm, 3-5%, 2 mm-1.1 cm
10-12%, 4-6 mm, ~3%, 2-9 mm
10%, ~5 mm, ~3%, 2 mm-1 cm
10%, ~5 mm, <1% feldspars destroyed
10-12%, 3-6 mm, 5%, up to 1.5 cm
10-12%, ~4 mm, 3%, 3 mm
10-12%, ~3 mm, 3%, 2 mm
10%, <2 mm, 3-5%, <3 mm
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6-8%, 4-8 mm, up to 1.1 cm
WV97-81 (Wolverine/Lynx zone)Type FP Section
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Figure 5. Graphic logs of type sections of (a) quartz-feldspar porphyries and (b) feldspar-porphyries from the Wolverine deposit. Figure modified from Piercey et al. (2008).
a b
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The presence of distinctive, more juvenile eNdt-eHft
signatures in the FP suite, which is intimately associated with mineralization and syn-VMS formation, suggests that rhyolitic rocks with similar juvenile signatures within the Yukon-Tanana terrane and other pericratonic terranes may also be prospective for VMS mineralization. This hypothesis requires further testing and will be the focus of ongoing research by the authors. Moreover, future work will focus on comparing both bulk rock Nd-Hf and in situ Hf-Nd-U-Pb signatures of heavy mineral phases (e.g., zircon, monazite, apatite) within the pericratonic terranes to see if there are important isotopic differences between barren and VMS-bearing assemblages, testing geochemical and isotopic relationships as a function of VMS deposit grade and tonnage, and seeing if such methods allow mapping of VMS potential and fertility of felsic-dominated assemblages.
ACKNOWLEDGEMENTSFieldwork on this research was undertaken in 2000 and 2005 and logistical support was provided by Yukon Zinc Corporation. Discussions with Geoff Bradshaw, Gilles Dessureau, Jason Dunning, Harold Gibson and Jan Peter were very helpful during the fieldwork phase of this research. The research was previously funded by an NSERC Discovery Grant and the NSERC-Altius Industrial Research Chair at Memorial University, and ongoing research is being funded by the Yukon Geological Survey and the Geological Survey of Canada through the Targeted Geoscience Initiative 5 program. Will Bradford, Anne Westhues, and Sherri Strong are acknowledged for assistance with Nd-Hf isotope data acquisition. Ongoing discussions with Mo Colpron and Don Murphy on the Yukon-Tanana terrane and Finlayson Lake geology are
gratefully acknowledged. We thank Alana Hinchey for her review of this manuscript. A special thanks to Karen MacFarlane for her patience and for accommodating the late arrival of this paper!
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Figure 6. (a) Nb/Ta versus eNdt and (b) Nb/Ta versus eHf
t
for porphyritic rhyolitic rocks from the Wolverine VMS deposit. Nb/Ta values ~12 reflect crustal sources, whereas Nb/Ta ~17 reflect more basaltic/juvenile sources. While all samples have negative eNd
t and eHf
t, the younger, FP suite
tends towards more juvenile signatures relative to the QFP suite.
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Figure 7. Depleted mantle model ages for Hf and Nd for the Wolverine porphyritic rhyolites. Notably all samples have Proterozoic to Archean model ages indicative of crustal inheritance from old upper crustal sources.
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