Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

Figures & data

Fig. 1  Location map of the South Island showing the topography of the Southern Alps (DEM from geographx.co.nz), the current plate tectonic setting and the locations of gold-bearing vein systems along the mountains. Specific mineralised localities mentioned in this study are indicated and described in Table 1 and the text (MDFZ: Main Divide Fault Zone).

Fig. 2  Geological map showing the principal lithotectonic terranes in the South Island, focusing on the rocks which underlie the Southern Alps (Fig. 1). A belt of schists containing abundant metabasites and scattered ultramafic pods is shown extending from Aspiring Terrane to the Pounamu belt (after Cooper 1976), although this continuity is not well established.

Table 1  Geological setting of vein material from which sulphide samples were taken for Pb isotope analyses in the Otago and Alpine schists

	Copper Creek	Macetown	Smyth Glacier	Callery River	Whataroa River	Fiddes vein
Metals	Au, Sb, As	Au, Sb, As	Au, Sb, As	Au, W, As	Au, As	Au, As, Cu
Style	Vein, breccia, ankeritic alteration	Vein, breccia, ankeritic alteration	Vein, breccia, ankeritic alteration	Veinlets, disseminated, greenschist alteration	Vein, breccia, ankeritic alteration	Stringer veins, minor breccia, little alteration
Attitude	NW strike, 60° NE dip	NW strike, steep dip	E to NE strike, vertical	NE strike, variable dip	NE strike, steep dip	N strike, 55° W dip
Host rock	Micaceous schist, greenschist facies	Micaceous schist, greenschist facies	Pumpellyite-actinolite facies of schist	Biotite zone micaceous schist	Pumpellyite-actinolite facies of schist	Prehnite-pumpellyite facies metagreywacke
Host terrane	Aspiring	Aspiring	Rakaia	Rakaia	Rakaia	Rakaia
Local structural setting	Associated with Miocene folding of Oligocene marine sediments and schist foliation, and Moonlight Fault	Crosscut Miocene folds of schist foliation, associated with Moonlight Fault	Swarm of small structures containing ankeritic carbonate, cuts across steeply dipping schist foliation	Cut across Alpine Schist folds of schist foliation	Cut schist foliation associated with faults that juxtapose rocks of differing metamorphic grade	Cut metagreywacke slices in active Main Divide Fault Zone
Regional structural setting	Folds and extensional zones related to inception of Alpine Fault in early Miocene	Folds and extensional zones related to inception of Alpine Fault in early Miocene	Associated with Pliocene-Recent faults that affect topographic development of mountains	Host rock exhumed from metamorphic depths and folded in Quaternary	In hanging wall of Main Divide Fault Zone	In vein swarm that cuts across metamorphic zones uplifted along Alpine Fault, near Hope Fault intersection
Lamprophyre relationship	Veins fill same structures as Miocene lamprophyres to N	Veins fill same structures as Miocene lamprophyres to N	None apparent; 20 km from main locus of Miocene intrusion	None	None	None
Crosscutting structures	Late stage fractures	Late stage fractures	Late stage fractures	Quaternary adularia-quartz veins	Late stage fractures	Active Main Divide Fault Zone
References	Cooper et al. (1987); Craw et al. (2006)	Cooper et al. (1987); Craw (2006)	Craw et al. (2003); Craw (2006)	Craw et al. (1987); Teagle et al. (1998)	Craw et al. (1994); Craw & Campbell (2004)	Becker et al. (2000)

Table 2  Lead isotopic compositions of sulphide minerals from gold-bearing vein occurrences in the Otago and Alpine schists (errors given at a 2σ level)

Sample	Mineral	^206Pb/^204Pb	Error (%)	^207Pb/^204Pb	Error (%)	^208Pb/^204Pb	Error (%)	^207Pb/^206Pb	Error (%)	^208Pb/^206Pb	Error (%)
Copper Creek	sb	18.713	0.11	15.633	0.15	38.560	0.19	0.8354	0.051	2.0606	0.092
Copper Creek	sb	18.773	0.09	15.657	0.14	38.738	0.18	0.8340	0.046	2.0635	0.091
Copper Creek	py	18.727	0.06	15.630	0.07	38.613	0.10	0.8347	0.039	2.0620	0.050
Macetown	py	18.740	0.1	15.646	0.09	38.674	0.14	0.8349	0.06	2.0637	0.089
Fiddes	asp	18.724	0.07	15.652	0.09	38.611	0.10	0.8360	0.019	2.0622	0.038
Smyth Glacier	sb	18.842	0.27	15.719	0.27	38.950	0.28	0.8343	0.052	2.0672	0.050
Callery-1	py	18.689	0.16	15.642	0.16	38.526	0.17	0.8370	0.022	2.0615	0.041
WB-1	py	18.739	0.07	15.641	0.08	38.602	0.10	0.8347	0.019	2.0601	0.038
WB-2	py	18.816	0.07	15.705	0.08	38.801	0.10	0.8346	0.020	2.0621	0.038
WB-3	py	18.785	0.04	15.671	0.06	38.714	0.08	0.8342	0.023	2.0609	0.039

Fig. 3  (A, B) Lead isotopic compositions of sulphides from gold-bearing veins and vein breccias in the northwestern Otago and Southern Alps, together with fields for sulphide Pb compositions from gold-bearing veins and shear zones in the main part of the Otago Schist belt (from Mortensen et al. 2010). The present-day fields for Pb isotopic compositions of Pahau Terrane greywackes on North Island, New Zealand, are shown for reference (data from Graham et al. 1992; McCulloch et al. 1994). Age-corrected fields for Pb isotopic compositions of Pahau Terrane greywackes at 25 Ma are also shown; these were calculated using Pb evolution parameters as discussed in the text.

Fig. 4  Lead isotopic compositions of sulphides from gold occurrences in northwestern Otago and Southern Alps. Fields for Pahau Terrane greywacke Pbs as in previous figure.

Fig. 5  Lead isotopic compositions of sulphides from gold occurrences in northwestern Otago and Southern Alps (symbols as in Fig. 2, 3) together with Pb isotopic compositions of lamprophyres in the Alpine (= Westland) Dikes (from Barreiro & Cooper 1987; Hoernle et al. 2006). The Stacey & Kramer (1975) growth curve is shown for reference, along with fields for 25 Ma compositions of Pahau Terrane greywacke Pbs as in previous figures. Black arrow points to 0 Ma on an average mantle growth curve (from Zartman & Doe 1981).

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