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The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits
Jean Milot, Janne Blichert-Toft, Mariano Ayarzagüena Sanz, Nadège Fetter, Philippe Télouk, Francis Albarede
To cite this version:
Jean Milot, Janne Blichert-Toft, Mariano Ayarzagüena Sanz, Nadège Fetter, Philippe Télouk, et al..
The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits.
Chemical Geology, Elsevier, 2021, �10.1016/j.chemgeo.2021.120444�. �hal-03318657�
The significance of galena Pb model ages
1
and the formation of large Pb-Zn sedimentary deposits
2 3
Jean Milot1*, Janne Blichert-Toft1,Mariano Ayarzagüena Sanz2, 4
Nadège Fetter1, Philippe Télouk1, Francis Albarede1 5
6
1Ecole Normale Supérieure de Lyon, CNRS, Université de Lyon, 69007 Lyon, France 7
2Sociedad Española de Historia de la Arqueología, 28350 Ciempozuelos (Madrid), Spain 8
9
*Corresponding author: jean.milot@ens-lyon.fr 10
11
Abstract 12
13
In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, 14
we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the 15
former from ores and the latter separated from granites. All samples are from Spain and 16
the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With 17
few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, 18
a gap which is well outside the uncertainties of ~30 Ma assigned to the model. The histo- 19
gram of the new high-precision Pb isotope data shows prominent peaks of galena Pb 20
model ages at 94±38 Ma and 392±39 Ma. When the data are consolidated with literature 21
data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five 22
groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 23
395±40 (Middle Devonian), 90±34 Ma (Middle Cretaceous), and 613±42 Ma (Neoprotero- 24
zoic), with two minor peaks at 185+26 Ma (Jurassic) and 313±41 (Upper Carboniferous).
25
To a large extent, the model ages centered around these peaks correspond to distinct lo- 26
calities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan- 27
African tectonic events, which are the main tectonic episodes that shaped Iberian geology, 28
but rather match well-known global oceanic anoxic events. It is argued that surges of 29
metals weathered from continental surfaces scorched during anoxic events accumulated 30
and combined in anoxic water masses with unoxidized marine sulfide released by subma- 31
rine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope 32
compositions require that galenas from black shales are the source of the final ores. The 33
Revised manuscript with no changes marked Click here to view linked References
sulfides were later remobilized by large-scale convective circulation of basinal and hydro- 34
thermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas 35
and do not correlate with magmatic emplacement ages. It is suggested that they instead 36
reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb 37
isotopes can be used as a regional provenance tool, such an approach requires that the 38
data are considered in a fully 3-dimensional space.
39 40
1. Introduction 41
Stanton and Russell (1959) were the first to point out that the Pb isotope compositions of some 42
‘conformable’ strata-bound Pb-Zn 0-3.1 Ga old ore deposits could be accounted for by a single- 43
stage growth curve from the formation of the Earth to the known emplacement age of the ores.
44
Oversby (1974), however, reviewed the data existing at the time and concluded that a single- 45
stage closed-system evolution model was untenable. Using the concept of frozen lead, i.e. the 46
isotope composition of Pb locked into minerals with very low parent/daughter ratios, such as 47
galena in sulfide ores and K-feldspars, at the time of their formation, this issue received two 48
initially successful answers. One model uses a fit of conformable galena data with a two-stage 49
model in which the second stage starts at 3.8 Ga (Stacey and Kramers, 1975), while another 50
model uses a fit by a growth curve with a linear change in mantle U/Pb (Cumming and Richards, 51
1975), which is inconsistent with the evidence of constant U/Pb as demonstrated later by Alba- 52
rède and Juteau (1984) . The determination of Pb isotope compositions of K-feldspars from the 53
3.59 Ga old Amîtsoq gneisses in Greenland likewise led Gancarz and Wasserburg (1977) to 54
support a two-stage model. This model was strongly reinforced by evidence that, when the 238U- 55
206Pb and 235U-207Pb chronometers are considered one by one, the derived model 238U/204Pb () 56
values remain constant and are mutually consistent (Albarède and Juteau, 1984). The misfit 57
between the different models should not, however, be overemphasized. A model age is defined 58
as an apparent age calculated from measured isotopic abundances using simple assumptions, 59
typically closed-system behavior over one or two stages. The model ages calculated for a par- 60
ticular conformable galena from all the models discussed above and their deviation from the 61
respective emplacement ages agree in most cases to within ~30 Ma.
62 63
Conformable ores, however, only represent a small fraction of strata-bound Pb-Zn deposits.
64
Typically, major sedimentary exhalative deposits (SedEx) are syngenetic, i.e. formed at the 65
epigenetic, i.e. formed significantly after the deposition of their host rock (Wilkinson, 2014).
67
Leach et al. (2005, Appendix D) pointed out that the model ages of seven out of nine major 68
SedEx deposits are slightly to much older than their host rock ages. Metalliferous sediments 69
from the Atlantis II Deep in the Red Sea, which are considered an active equivalent of SedEx 70
deposits, derive their lead from underlying sediments, not from basalts (Dupré et al., 1988).
71
Leaching of Proterozoic lead from the basement is also the interpretation adopted for the MVT 72
deposits of the Phanerozoic Mississipi Valley ores (Doe and Delevaux, 1972; Goldhaber et al., 73
1995; Heyl et al., 1974; Paradis et al., 2007). It has been suggested that SedEx deposits of the 74
Phanerozoic reflect periods of global ocean anoxia (Goodfellow, 2004, 1987; Goodfellow et 75
al., 1993). The discordance between ‘frozen’ model ages and host rock ages means that ore 76
deposits did not form directly from a crustal source in a single stage, implying that a separate 77
stage of Pb-Zn preconcentration is a pre-requisite for Pb-Zn ore genesis, regardless of the ore 78
type and the environment involved.
79 80
Understanding large ore deposits therefore requires to dissociate the primary segregation of the 81
mined metals and the mechanisms by which these metals were transported to their current lo- 82
cations. Of course, a simple two-step representation of ore genesis does not do justice to the 83
idiosyncrasies of the mechanisms of ore deposition either. A two-step approach can, however, 84
be efficiently modeled and robustly tested by the U-Th-Pb system which is only determined by 85
two parameters, radiogenic ingrowth and U/Th/Pb fractionation events. The long-lived Pb- 86
based chronometers uses the rapidly changing proportions of the three isotopes 206Pb, 207Pb, and 87
208Pb upon radioactive decay of their parent elements U and Th.
88 89
The present work is mostly about what Pb model ages of galena and K-feldspar tell us about 90
the formation age and genetic environment of these minerals Although many model ages seem 91
compatible with geological evidence within the often generally large uncertainties on ore ages, 92
it is not always possible to assess a priori if a particular set of Pb isotope data is revealing the 93
genesis of the Pb carrier or the precise timing of its emplacement. These uncertainties have 94
direct implications on the field of archeology, which uses Pb isotopes for provenancing. For 95
decades, comparison of Pb isotope ratios measured on coins and other artefacts with Pb isotope 96
ratios measured on ores has been used to constrain the provenance of the metals used for pro- 97
ducing the objects in question (Stos-Gale and Gale, 2009). Model ages (Tm) and their associated 98
parameters ( and ), also derived from measured Pb isotope ratios, allow the blind comparison 99
of abstract Pb isotope compositions of ores and artefacts to be replaced by meaningful geolog- 100
ically identifiable environments (Albarède et al., 2012).
101 102
In the present study, we use a large set of high-precision Pb isotope data from the Iberian Pen- 103
insula and adjacent regions, gathered initially to establish the provenance of metals used for 104
silver artefacts and coins (Albarède et al., 2020), to propose an improved geological interpreta- 105
tion of model ages for galena ores and granitic K-feldspars. The motivation behind this work is 106
not to expand an already well-populated database of Pb isotopes on Iberian galenas and K- 107
feldspars, but to use a data set of uniform analytical quality to discuss how model ages can be 108
interpreted. This new framework places Pb isotope data onto a geological background and in 109
particular ties them to events of Pb-Zn ore formation.
110 111
2. What is a Pb model age?
112
A two-stage model of Pb isotope evolution can be summarized by the following equations:
113
𝑥0 = (206Pb
204Pb)
0
= 𝑥𝑇∗+ 𝜇(𝑒𝜆238U𝑇∗− 𝑒𝜆238U𝑇𝑚𝑜𝑑) 114
𝑦0 = (207Pb
204Pb)
0
= 𝑦𝑇∗+ 𝜇
137.79(𝑒𝜆235U𝑇∗− 𝑒𝜆235U𝑇𝑚𝑜𝑑) 115
𝑧0 = (208Pb
204Pb)
0
= 𝑧𝑇∗+ 𝜇𝜅(𝑒𝜆232Th𝑇∗− 𝑒𝜆232𝑇ℎ𝑇𝑚𝑜𝑑) 116
with 117
𝑥𝑇∗ = 𝑥0∗− 𝜇∗(𝑒𝜆238U𝑇∗− 1) 118
𝑦𝑇∗ = 𝑦0∗− 𝜇∗
137.79(𝑒𝜆235U𝑇∗− 1) 119
𝑧𝑇∗ = 𝑧0∗− 𝜇∗𝜅∗(𝑒𝜆238U𝑇∗− 1) 120
In these equations, subscript 0 indicates ratios measured today, T* is the age at which the second 121
stage began, and i is the decay constant of nuclide i. The asterisk represents parameters of the 122
reference models discussed below. The two parameters µ=(238U/204Pb)0 (or, short-hand, U/Pb) 123
and =(232Th/238U)0 (or, short-hand, Th/U) of the ore progenitor are time-invariant. This model 124
assumes that apparent radiogenic ingrowth is brought to a halt at Tmod, which is a standard 125
as sphalerite, pyrite, and chalcopyrite, also is low but not nearly as low as that of galenas. K- 127
feldspars contain significant amounts of initial lead but only little uranium, the latter of which 128
resides mostly in cracks in the form of oxides and phosphates, which, together with ingrown 129
radiogenic lead, can be efficiently removed by aggressive leaching (McNamara et al., 2017).
130
Each triplet (Tm, ) corresponds to a triplet of isotopic ratios. The Tmod, , representation 131
is nothing more than an alternative representation of the 3-dimensional Pb isotope space. Other 132
ratios with a different normalization isotope (e.g. 206Pb instead of the more standard 204Pb) can 133
be used, but the triplet (Tmod, ) remains unchanged.
134 135
The values 𝑥0∗, 𝑦0∗, and 𝑧0∗ of the so-called modern lead may be obtained in different ways.
136
Stacey and Kramers (1975) computed the intersection of isochrons from Archean terranes in 137
the conventional plot 207Pb/204Pb vs 206Pb/204Pb and then refined T* using the Pb isotope com- 138
position and geological ages of 13 ‘conformable’ galenas. One of the merits of this visionary 139
work was to validate the relevance of the two-stage evolution models. Albarède and Juteau, 140
(1984) took a different approach by considering the evolution of isotopic ratios independently 141
for the 13 galenas listed by Stacey and Kramers (1975), to which they further added the Pb 142
isotopic composition of an Early Archean galena from Isua, Greenland (Appel et al., 1978) and 143
plotted its value against eiTm1, where Tm is the accepted geological age of the sample, and 144
obtained excellent alignments. The intercepts provide the modern values 𝑥0∗, 𝑦0∗, and 𝑧0∗ of the 145
corresponding ratios. In addition, the slopes of the 206Pb/204Pb and 207Pb/204Pb alignments give 146
=(238U/204Pb)0 in the galena protolith, whereas the slope of the 208Pb/204Pb alignment gives the 147
associated =(232Th/238U)0. The and values obtained in this way are independent of the 148
(238U/235U)0 ratio of the Earth, a canonical value that was recently revised (Andersen et al., 149
2017; Goldmann et al., 2015). The value of =(232Th/238U)0 derived in this way is indistinguish- 150
able from the planetary value inferred by Blichert-Toft et al. (2010). The parameters derived 151
by Stacey and Kramers (1975) and Albarède and Juteau (1984) are listed in Table 1 and are 152
similar although they were derived in different ways. We assessed by Monte-Carlo error prop- 153
agation that model ages calculated using Stacey and Kramers (1975) and Albarède and Juteau 154
(1984) may differ by up to 30 Ma and the values by up to 0.3 units. Studies of deep-sea FeMn 155
nodules (Abouchami and Goldstein, 1995; Vlastélic et al., 2001; von Blanckenburg and Igel, 156
1999) and Quaternary loess (Sun and Zhu, 2010), two proxies of the ‘common lead’ of modern 157
upper crust, show that Albarède and Juteau’s (1984) model is a valid representation of the upper 158
for mantle-derived magmas plot to the right of the modern-lead secondary isochrons of Stacey 160
and Kramers (1975) and Albarède and Juteau (1984), which is tantamount to future model ages.
161
The conundrum of short Pb residence in the deep ocean (~80 y, Nozaki, 1986; Schaule and 162
Patterson, 1981) contrasting with the relative homogeneity of Pb isotopes in FeMn nodules (von 163
Blanckenburg and Igel, 1999) reflects that airborne particle deposition accounts for most of the 164
Pb input into the ocean (Schlosser et al., 2019). As for hydrothermal input from the ridge sys- 165
tem, the low Pb concentration of the oceanic asthenosphere and the rapid precipitation of Pb 166
next to oceanic spreading centers (Dasch, 1981) makes this source a minor contributor to the 167
sedimentary record.
168 169
More complex models, such as the popular Plumbotectonics model of Zartman and Doe (1981) 170
and Zartman and Haines (1988) and the detailed analysis by Kramers and Tolstikhin (1997), 171
provide a powerful illustration of possible Pb isotope evolution in the crust, but the multiple 172
stages and under-constrained parameters they entail make it difficult to assess the reliability of 173
the model ages inferred from these models.
174 175
3. The Iberian Peninsula: Geology and mining districts 176
Given the number and regional distribution of the samples in this study, only a short summary 177
of their Iberian background geology is presented here. Digitized geological maps of Spain, Por- 178
tugal, and France are freely available on the portals of the Geological and Mining Institute of 179
Spain (IGME), the National Laboratory of Energy and Geology of Portugal (LNEG), and the 180
French Geological Survey (BRGM).
181 182
The geology of the Iberian Peninsula was affected by Neoproterozoic to Neogene geodynamic 183
activity (Quesada and Oliveira, 2019) and involved three major orogenic cycles: the Cadomian 184
(Pan-African) events spanning the Ediacaran to the Cambrian, the Variscan (Hercynian) events, 185
and the Alpine events. The Pan-African (Cadomian) series of tectonic events occurred during 186
the late Neoproterozoic, about 650–550 Ma ago and led to the formation of the continent and 187
its collision with island arcs and other Proterozoic terranes. During the Variscan orogeny, 188
Gondwana collided with the northern Laurussia continent to form Pangaea. The resulting su- 189
percontinent gradually broke up during the Mesozoic Era and various fragments collided to 190
form Alpine orogens.
191
Figure 1 depicts the main features of Iberian geology while Fig. 2 positions the most commonly 193
used geological units. The bulk of Iberia was assembled and deformed during the Variscan 194
orogeny and constitutes the western expansion of the West European Variscides. The main 195
tectonic provinces underlain by pre-Variscan and Variscan basement are the South Portuguese, 196
Ossa Morena, Central Iberia, and West Asturian Leonese Zones, to which the internal zones of 197
the Pyrenees, the Betics, and the Catalan Coastal Range must be added. The pre-Variscan (Ne- 198
oproterozoic, Cadomian) basement includes fragments from the Pan-African, notably in west- 199
ern and northwestern Spain, the eastern Pyrenees, and the whole of the Ossa Morena Zone. The 200
Pyrenees is a prominent mountain range resulting from the Mid-Cretaceous and Eocene colli- 201
sion between Iberia and western Europe.
202 203
Variscan granites represent the most striking marker of the collision between Gondwana frag- 204
ments and Laurussia. Early petrological and chronological literature was reviewed by Castro et 205
al. (2002) and Paleozoic intrusive magmatism was recently re-assessed by Ribeiro et al. (2019).
206
The latter occupies a large surface area in northwestern and central Iberia, Catalonia, and the 207
Pyrenees. High 87Sr/86Sr assigns to all these rocks an unmistakably crustal origin (Castro et al., 208
2002). Most granitic rocks seem to have formed by reworking of Early- to Mid-Proterozoic 209
protoliths and were emplaced during the Variscan orogeny. Mesozoic and post-collisional Ne- 210
ogene volcanism from the Betic Cordilleras was reviewed by Gómez-Pugnaire et al. (2019).
211 212
In order to see through these overarching major tectonic events and grasp the relative im- 213
portance of other less imposing, but not necessarily less critical, geodynamic events, as well as 214
the significance of geological unit subdivision, we made use of zircon U-Pb geochronology of 215
detrital rocks of different stratigraphic ages, largely Paleozoic. In northern, northwestern, and 216
central Iberia and in the Ossa Morena Zone, the dominant age peaks are Ediacaran and Cam- 217
brian (Cadomian) (Ábalos et al., 2012; Fernández-Suárez et al., 2014; Martínez Catalán et al., 218
2008; Pastor-Galán et al., 2013; Silva et al., 2014; Talavera et al., 2012; Villaseca et al., 2016).
219
The same observation was made in the Betics (Esteban et al., 2017; Jabaloy-Sánchez et al., 220
2020; Santamaría-López and Sanz de Galdeano, 2018) and the Pyrenees (Filleaudeau et al., 221
2012). In contrast, the South Portuguese Zone south of the Rheic suture shows evidence of a 222
Silurian to Mid-Devonian event (380-420 Ma) superimposed on the dominant Cadomian event 223
(Pereira et al., 2017, 2014, 2012; Pérez-Cáceres et al., 2017). No zircon population with clearly 224
Alpine ages has been identified so far.
225
Overall, most of Iberia is a Variscan crustal segment with a strong heritage of the Pan-African 227
Gondwana supercontinent, which itself was built by the destruction of Proterozoic cratons. The 228
Variscan collision strongly deformed and metamorphosed this material with little addition of 229
juvenile magmas (Ribeiro et al., 2019). Only in the South Portuguese Zone is a Silurian-Devo- 230
nian geodynamic event interpreted as a short-lived intra-oceanic arc (Pereira et al., 2012) un- 231
questionably different from the rest of the Variscan domain. Alpine tectonics is restricted to the 232
boundaries between the microplates inherited from the breakup of Pangea.
233 234
A recent database (www.ehu.eus/ibercron/iberlid) collects Pb isotope compositions measured 235
by a variety of techniques over several decades on nearly 3000 geological and archeological 236
samples (de Madinabeitia et al., 2021). The work of these authors and their database can be 237
used to acquire some background on Iberian Pb-Zn mines and guide the reader through the 238
various geological environments of specific samples.
239 240
3.1. The mines of southeast Iberia 241
The southeastern region of the Iberian Peninsula can be divided into two major lithological 242
groups: (1) the Paleozoic and Mesozoic (Late Cretaceous to Paleogene) meta-sedimentary 243
rocks formed before the Betic Alpine orogeny under a subduction-related compressive regime 244
due to the convergence of European and African plates, and (2) the Miocene igneous rocks of 245
the Cabo de Gata-Cartagena volcanic belt associated with post-collisional extension (e.g. Arri- 246
bas and Tosdal, 1994; Esteban-Arispe et al., 2016; Turner et al., 1999). This Miocene volcanic 247
field results from the fusion of mantle materials metasomatized by subduction processes (e.g.
248
Benito et al., 1999). From southwest to northeast, interaction of rising melts with crustal rocks 249
led to the formation of calc-alkaline, high-K calc-alkaline and shoshonitic, ultrapotassic, and 250
rare alkaline basaltic rocks (Esteban-Arispe et al., 2016). Hydrothermal systems associated with 251
the calc-alkaline, high-K calc-alkaline, and shoshonitic rocks altered the volcanic rocks and 252
formed epithermal precious and base-metal deposits such as the San José, Rodalquilar, Carbon- 253
eras, Mazarrón, and Cartagena-La Unión deposits (e.g. Arribas et al., 1995; Arribas and Tosdal, 254
1994; Ruano et al., 2000).
255 256
3.1.1. The Cartagena-La Unión Range 257
The mining district of La Unión is located in the Sierra de Cartagena, which forms the eastern 258
end of the Betic Cordilleras (e.g. Oen et al., 1975; Sabaté et al., 2015). Depending on the loca- 259
tion, the mineralization consists mainly of Pb-Zn veins or stratabound deposits, associated with 260
hydrothermal alteration or carbonate replacement, hosted by volcanic, sub-volcanic, or meta- 261
morphic rocks (Sabaté et al., 2015). Galena is abundant in both vein and stratabound deposits 262
and has been intensively exploited since the Carthaginian expansion and subsequent Roman 263
times (e.g. Sanmarti et al., 2013; Soler et al., 2013). The Ag-bearing galena located in the Cabo 264
de Palos were, in ancient times, the richest in Hispania, containing between 5 and 6 kilograms 265
of silver per ton of lead.
266 267
3.1.2. Mazarrón 268
The Mazarrón mining district is located in the northern part of the Cabo de Gata-Cartagena 269
volcanic belt, at the eastern end of the Betic Cordilleras. This Zn-Pb-Cu-Ag-Fe sulfide deposit 270
is one of the best examples of epithermal mineralization associated with high-K calc-alcaline 271
and shoshonitic volcanics (Esteban-Arispe et al., 2016). Lead-bearing ores are represented by 272
lead sulfides and argentiferous galena, accompanied by sphalerite, pyrite, antimony and arsenic 273
minerals, and gypsum. The grade range between 3000 and 6000 grams per ton of ore, in some 274
places reaching 10,000 grams.
275 276
3.1.3. Sierra de Almagrera 277
The mining district of the Sierra de Almagrera is located in the central part of the Cabo de Gata- 278
Cartagena volcanic belt (e.g. Martinez Frias et al., 1992; Navarro et al., 2008; Suárez, 2016).
279
Ore deposits mainly consist of Ag-rich veins or stratabound ore bodies (Herrerias deposit) 280
hosted by Paleozoic schists (graphitic and quartz-rich phyllites) and quartzite of the Alpujárride 281
Complex (Internal Betic Zone). The major veins contain an assemblage of base metal sulfides 282
(Ag-bearing galena, sphalerite, pyrite, chalcopyrite, and marcasite) and Pb-Sb-Cu-Ag sulfosalts 283
(Arribas and Tosdal, 1994; Martínez-Frías et al., 1989). Locally, the strong supergene alteration 284
and secondary enrichment resulted in abundant native Ag and secondary minerals such as jar- 285
osite (Arribas and Tosdal, 1994).
286 287
3.1.4. The Gádor Range 288
The Sierra de Gádor is located in the western part of the Betic Cordilleras and belong to the 289
Gádor Range, with an extent of some twenty square kilometers. The mineralization consists of 291
lenticular and stratabound F-(Pb-Zn) deposits hosted by Middle Triassic carbonate rocks of the 292
lower Alpujárride Complex (Arribas and Tosdal, 1994; Fontboté et al., 1983). Mineral assem- 293
blages generally consist of fluorine, galena, sphalerite, pyrite, and bornite (Arribas and Tosdal, 294
1994). Although minerals occasionally appeared at the surface, the most important concentra- 295
tions of lead were located between 60 and 100 m depths.
296 297
3.2. The mines of the South Central Iberian Zone 298
Ore deposits from this area are associated with various lithological units, including slates and 299
shales of the Rumblar basin and Variscan granites. The igneous rocks are differentiated to var- 300
iable extents and include diorites, granodiorites, and porphyry. The veins in this east Lusitan- 301
Marianic region contain Cu-Pb, Pb, and Pb-Ag mineralizations, as well as some Sn-W and 302
epithermal deposits (Arboledas et al., 2014).
303 304
3.2.1. The Alcudia Valley 305
The Alcudia Valley ore district is located in the southwest of the Ciudad Real province and 306
belongs to a large metallogenic province known as the Sierra Morena (e.g. Palero-Fernández 307
and Martín-Izard, 2005; Santos Zalduegui et al., 2004). The most important geological feature 308
of this district is a succession of anticlines and synclines affecting sedimentary Neoproterozoic 309
and Ordovician-Early Carboniferous rocks. All the deposits of this district are related to Var- 310
iscan granite magmatism of Carboniferous age, in particular the intrusion of the Los Pedroches 311
Batholith in the southern part of the district (Palero et al., 2003; Palero-Fernández and Martín- 312
Izard, 2005). Ore deposits from the Alcudia Valley were classified according to their morphol- 313
ogy, host rock relations, structural setting, mineral assemblages, and geochemical parameters 314
(Palero et al., 1992, 1991). Most of the deposits of economic importance are hydrothermal Pb- 315
Zn-Ag veins with breccia textures located in tensional fractures within Neoproterozoic rocks 316
(Palero-Fernández and Martín-Izard, 2005). The ore mineral assemblage mainly consists of ga- 317
lena, sphalerite, chalcopyrite, pyrite, marcasite, and Ag-sulfosalts.
318 319
3.2.2. Linares-La Carolina 320
The mining district of Linares-La Carolina is located east of the Alcudia Valley, in the north- 321
western part of the Jaén province. This district is divided into two mineralized sectors, Linares 322
granite emplacement (e.g. Palero-Fernández and Martín-Izard, 2005; Santos Zalduegui et al., 324
2004). Deposits in Linares consist of hydrothermal veins of base-metal sulfides hosted mainly 325
by granites from the Linares pluton, and less commonly by Paleozoic rocks. The mineral as- 326
semblage contains Ag-poor galena, sphalerite, pyrite, and chalcopyrite in a quartz-barite-car- 327
bonate gangue. In La Carolina, deposits are base-metal sulfide veins hosted by Ordovician to 328
Silurian meta-sedimentary rocks, and more rarely by granites from the Santa Elena pluton. The 329
mineral assemblage contains Ag-bearing galena, sphalerite, and pyrite in a quartz gangue, 330
sometimes associated with ankerite and barite (Garcia De Madinabeitia, 2003).
331 332
3.3. The mines of southwest Iberia 333
The Iberian Pyrite Belt, located in the southwest of the Iberian Peninsula comprising part of 334
Portugal, Huelva, and the Seville province (Spain), contains diverse mining districts that have 335
been major sources of metals through centuries. As the Romans were unable to extract silver 336
from sulfides, silver was instead extracted from jarosites, which lie beneath the gossan, the 337
oxidised zone of sulfides. The ores are hosted by two geological zones: the Ossa Morena and 338
South Portuguese zones.
339 340
3.3.1. The Ossa Morena Zone 341
The Ossa Morena Zone (OMZ) is located south of the Central Iberian Zone and to the northeast 342
of the South Portuguese Zone. The geology of this tectonostratigraphic unit, described in detail 343
by Tornos and Chiaradia (2004), is particularly complex. It resulted from the accretion of ter- 344
ranes ranging from Late Riphean to Late Carboniferous in age. In short, the OMZ encompasses 345
(1) disseminated pre-Cadomian sequences of high-grade metamorphic and siliciclastic rocks, 346
(2) syn-orogenic Cadomian units of Neoproterozoic-Early Cambrian backarc to intra-arc se- 347
quences, (3) a volcano-sedimentary unit formed during Paleozoic intracontinental rifting, (4) 348
an Ordivician to Early Devonian passive margin sequence, and (5) syn-Variscan sedimentary 349
rocks (Tornos and Chiaradia, 2004). Ore deposits in the OMZ are related to three major oro- 350
genic and sedimentary events (Marcoux et al., 2002). First, during the Cadomian orogeny, small 351
volcanogenic massive sulfide (VMS) deposits of Upper Riphean to Lower Neoproterozoic age 352
were associated with calk-alcaline volcanism. Second, sedimentary-exhalative (SedEx) ore 353
lenses associated with alkaline volcanism are hosted by late Cadomian (Neoproterozoic-Cam- 354
brian) marbles. Third, Zn-Cu-Pb massive sulfide deposits of Upper Devonian to Permian age 355
are coeval with the giant deposits of the South Portuguese Zone described below and associated 356
with alkaline volcanism (Baeza-Rojano et al., 1981).
357 358
3.3.2. The South Portuguese Zone 359
The South Portuguese Zone (SPZ) counts many world-class giant and supergiant VMS depos- 360
its, mainly in the province of Huelva. This region includes, among others, the mines of Riotinto, 361
Tharsis, Cueva de la Mora, Sotiel Coronada, Castillo de Buitron, and La Zarza. Aznalcollar in 362
Sevilla, Aljustrel in the district of Beja, and Neves Corvo in southern Portugal are also consid- 363
erable in size. These deposits all belong to the Iberian Pyrite Belt that occupies the southwest 364
corner of the Iberian Peninsula (Almodóvar et al., 1997; Relvas et al., 2001; Sáez et al., 1996;
365
Tornos, 2006). The SPZ is interpreted to have been sutured to the Iberian Massif during the 366
Middle Carboniferous. The local stratigraphy includes three main units, from top to bottom: (1) 367
the Phyllite-Quartzite group (PQ), (2) the Volcanic-Siliceous (VS) Complex, and (3) the Culm 368
group (Sáez et al., 1996). The VMS deposits are hosted in the VS Complex which consists of 369
successive mafic and felsic volcanic sequences interbedded with mudstone and chemical sedi- 370
mentary rocks of Late Famennian to Early Late Visean age (Oliveira, 1990; Tornos, 2006). The 371
stratigraphy is locally modified by igneous intrusions and thrust faults (Tornos, 2006). Overall, 372
the major ore deposits hosted by the VS Complex consist of massive sulfide lenses that contain 373
up to 170 Mt of ore. Most of the deposits show a direct relationship with black shales that form 374
a significant part of the VS Complex (Tornos, 2006).
375 376
3.4. The mines of northern Iberia 377
The density of ore deposits in the northern part of the Iberian Peninsula is lower than in the 378
south. However, several mining districts are noteworthy in the Catalan Coastal Ranges (north- 379
eastern Iberia), the Pyrenees (northern edge of the Iberian Peninsula), the Basque-Cantabrian 380
Zone (north-central Iberia), and the West Asturian Leonese Zone (northwestern Iberia).
381 382
3.4.1. The Pyrenees 383
The Pyrennean chain, between France and Spain, formed during the Alpine orogeny which was 384
surperimposed on Paleozoic terranes and Precambrian basement previously deformed by the 385
Variscan orogeny (Munoz et al., 2016). The Alpine deformation of the Pyrenees is mostly lim- 386
ited to thrusting and faulting of the Paleozoic rock series. The Variscan phase of the Pyrenees 387
dates from the Late Carboniferous (Namuro-Wesphalian) and was accompanied by calc-alka- 388
line magmatism and high-temperature–low-pressure metamorphism (Denele et al., 2009;
389
Laumonier et al., 2010). The Paleozoic formations of the Axial Zone of the Pyrenees hosted 390
many SedEx Pb-Zn stratiform deposits. The more economically significant occur in Lower 391
Paleozoic meta-sedimentary rocks of Ordovician to Lower Devonian ages, such as the 392
Pierrefitte, Bentaillou, Aran Valley, and Benasque Pass deposits (e.g. Cardellach et al., 1996;
393
García-Sansegundo et al., 2014; Nicol et al., 1997), and are considered to have formed during 394
Early Paleozoic rifting. Additionally, Pb-Zn vein deposits with Ag-rich tetrahedrite, Les Ar- 395
gentières and Lacore deposits next to Aulus-Les-Bains (Munoz et al., 2016), are found in De- 396
vonian terranes, but formed by Mesozoic hydrothermal fluids circulating during the post-Var- 397
iscan extensional regime.
398 399
3.4.2. The Catalan Coastal Range 400
The Catalan Coastal Range is made of folded Variscan basement overlain by a Mesozoic to 401
Cenozoic sedimentary cover (e.g. Canals and Cardellach, 1997; Canet et al., 2003; Edel et al., 402
2015; Parviainen et al., 2008). The Variscan basement consists of Paleozoic sedimentary rocks 403
and Late-Variscan granites peneplaned during pre-Triassic erosion, then covered by Mesozoic 404
sedimentary sequences (Canals and Cardellach, 1997). Numerous mineralized veins including 405
barite, fluorite, and base-metal sulfides in variable proportions are hosted by the Paleozoic 406
basement and locally cut across Triassic red beds. A detailed classification of the different types 407
of deposits is given by Canals and Cardellach (1997). The overall relationships between the 408
type of enclosing rock and vein mineralogy are: deposits enclosed in Paleozoic meta-sedimen- 409
tary rocks have a complex mineralogy including Co, Ni, and Ag arsenides and sulfides, whereas 410
deposits hosted by granites are rich in fluorite and/or barite and depleted in base metal sulfides 411
412
3.4.3. The Basque-Cantabrian Zone 413
The Basque-Cantabrian basin is located west of the Pyrenees, in central northern Spain. This 414
region roughly consists of Mesozoic to Tertiary sediments overlying the Paleozoic Asturian 415
basement faulted and deformed during the Variscan orogeny (e.g. Velasco et al., 2003). Sedi- 416
ment-hosted Pb-Zn deposits are found in two geological settings (Velasco et al., 1996). The 417
first is Carboniferous clastic meta-sediments of the Cinco Villas massif (east of the Basque- 418
Cantabrian basin) which includes SedEx ore deposits affected by Variscan deformation and 419
metamorphism and associated remobilization (Pesquera and Velasco, 1993, 1989). The second 420
ore deposits (Herrero, 1989; Velasco et al., 1996). South of the Basque-Cantabrian zone, Pb- 422
Zn deposits also occur in the Paleozoic Sierra de la Demanda. Several deposits from this region, 423
such as Udias, Novale, La Florida, Mercadal, Comillas, Punta Calderon, and Reocin have been 424
historically exploited for Pb and Zn (e.g. Pašava et al., 2014; Velasco et al., 2003).
425 426
3.4.4. The West Asturian Leonese Zone 427
The West Asturian-Leonese Zone (WALZ) is located northwest of the Iberian Peninsula. The 428
Zn-Pb deposits are found in the calcareous Vegadeo Formation in the westernmost outcrops of 429
the WALZ. This formation constitutes a major carbonate intercalation in the mainly siliciclastic 430
Lower Cambrian sedimentary succession in this part of the Variscan Belt of Spain (Tornos et 431
al., 1996). In short, the Vegadeo Formation consists of a thick bed of carbonates with rare in- 432
tercalations of shales and sandstones of variable thickness. Two types of ore deposits are hosted 433
by this formation at different stratigraphic positions. Ores from the first type are disseminated 434
stratiform ores with thin layers of sphalerite and galena replacing earlier pyrite and are located 435
in the Lower Member of the Vegadeo formation. These deposits are thought to have a pre- 436
metamorphic origin with sulfur and fluids being derived from the host carbonates. The second 437
type, of greater economic importance, is located on top of the Vegadeo Formation always in 438
contact with the overlaying shales and sandstones. The mineral assemblage of this type is com- 439
posed of sphalerite and galena, with minor amounts of chalcopyrite, pyrite, Co-Ni-As sulfides, 440
bismuthinite, tetrahedrite, and Pb-Bi sulfosalts. Several indices suggest that these last Variscan 441
mineralizations result from the remobilization of the pre-metamorphic stratiform ores (Tornos 442
et al., 1996).
443 444
4. Analytical techniques 445
The high-precision Pb isotope compositions of the 64 galena and 52 K-feldspar analyzed in the 446
present study, all from Spain and the French Pyrenees, are listed in Tables S1 and S2, respec- 447
tively, of the Supplementary Material. In the following, we briefly summarize the analytical 448
techniques used at ENS Lyon for separating Pb from galena and K-feldspar and measuring the 449
Pb isotopic compositions my multiple-collector inductively-coupled plasma mass spectrometry 450
(MC-ICP-MS; Nu Plasma 500 HR).
451 452
4.1. Galena 453
Galena ores were crushed and powdered in a clean pre-conditioned agate mortar. About 0.1 g 454
of each sample was weighed into clean Savillex beakers for acid digestion. Samples were di- 455
gested in a wet chemistry laboratory at ENS Lyon (so as not to contaminate the low-blank clean 456
laboratory at ENS Lyon with the high Pb concentrations of galena) with 10 ml distilled con- 457
centrated HNO3 on a hotplate at 110°C for a several hours. Galena readily dissolves according 458
to the reaction:
459
PbS + 8 HNO3 = PbSO4 + 8 NO2 + 4 H2O 460
The NO2 gas was regularly evacuated from the closed beakers by slightly unscrewing their lids 461
as pressure built up until complete sample digestion was achieved. Once the reaction complete, 462
samples were evaporated to dryness on a hotplate at 90°C. The dry PbSO4 residue was then 463
dissolved in 20 ml distilled 3M HNO3 on a hotplate at 110°C and repeatedly placed in an ultra- 464
sonic bath until total dissolution. The elemental composition of the digested galena samples 465
was measured on an aliquot of known volume of the total dissolved sample using a Thermo 466
Scientific ICAP-Q quadrupole ICP-MS (inductively-coupled plasma mass spectrometer) at 467
ENS Lyon. The total procedural Pb blank of the wet laboratory part of the procedure was < 20 468
ng, which is negligible relative to the amount of Pb in the samples of the order of 10-100s of 469
mg. For each sample, an aliquot containing about 1 µg of Pb was taken for subsequent Pb 470
separation and purification under clean laboratory conditions for high-precision Pb isotopic 471
analysis.
472 473
Lead was eluted at ENS Lyon following a one-step column chromatography procedure that 474
uses microcolumns filled with 500 µL anion-exchange resin (AG1-X8, 100-200 mesh). The 475
sample aliquots containing 1 µg of Pb dissolved in distilled 3M HNO3 were first evaporated to 476
dryness on a hotplate at 110°C. The residues were then taken up in 1 mL distilled 6M HCl and 477
dried down to convert the samples to chloride form. The samples where thereafter dissolved in 478
1 mL double-distilled 1M HBr on a hotplate at 110°C and, once cooled down, loaded onto the 479
anion-exchange columns. Sample matrices were eluted with double-distilled 1M HBr and Pb 480
was collected with distilled 6M HCl. After evaporation to dryness of the pure Pb fractions, a 481
few drops of concentrated distilled HNO3 were added and the samples dried down again prior 482
to Pb isotopic analysis, ensuring that any traces of potentially interfering Br2 and organic ma- 483
terial were driven off. The total procedural Pb blank of the clean laboratory part of the procedure 484
was < 20 pg, which again is negligible compared to the starting amount of this step of 1 µg Pb.
485 486
The Pb isotopic compositions of the galena samples were measured on a Nu Plasma 500 HR 487
MC-ICP-MS at ENS Lyon following the procedures of Blichert-Toft et al. (2003) and White et 488
al. (2000). Instrumental mass bias was corrected using added Tl, and sample-standard bracket- 489
ing referred to the values of Eisele et al. (2003) for the NIST 981 Pb reference material. Re- 490
peated measurements of NIST 981, analyzed every second sample throughout the individual 491
analytical sessions, yielded an external reproducibility < 100 ppm (0.01%) for 204Pb-normalized 492
ratios and < 50 ppm (0.005%) for 207Pb/206Pb and 208Pb/206Pb. In-run analytical errors (2-sigma 493
mean, based on the measurement of 60 isotope ratios with 10 seconds integration time each for 494
each sample and standard run) being systematically smaller than the external reproducibility 495
for all samples, they are not reported with the data in Tables S1 (galenas) and S2 (K-feldspars) 496
so as to render the data tables more manageable and readable.
497 498
4.2. K-feldspar 499
The K-feldspars were separated from their respective granites via grinding, sieving, and hand- 500
picking under a binocular microscope. All subsequent leaching, dissolution, and Pb separation 501
procedures were done in the clean laboratory at ENS Lyon. The separated K-feldspars were 502
leached in hot distilled 6M HCl and rinsed in distilled water prior to dissolution in either a 503
3:1:0.5 mixture of double-distilled concentrated HF:HNO3:HClO4 or partially dissolved in cold 504
double-distilled 30% HF depending on how pure the K-feldspar separates were (i.e. to avoid 505
high-(U-Th)/Pb inclusions potentially present in trace amounts of quartz sometimes left behind 506
with the K-feldspar). After evaporation to dryness, the samples were taken up in distilled 6M 507
HCl to ensure complete dissolution, dried down again, then taken up in double-distilled 1M 508
HBr and run through the same Pb columns as described above for galena. The total procedural 509
Pb blank also was the same as for galena, < 20 pg. The Pb isotopic compositions of the K- 510
feldspars were measured as described above for galena.
511 512
5. Results 513
The new Pb isotope data for galena and K-feldspar are listed in Tables S1 and S2, respectively, 514
together with the relevant literature data. The new data and the complete data set built by con- 515
solidation of the new data with literature data are considered separately. The reason for this 516
distinction is that, since the early 2000s, MC-ICP-MS has significantly improved the accuracy 517
and precision of Pb isotope data. While data quality therefore may vary among studies in the 518
published literature, MC-ICP-MS data are consistently superior owing to the excellent control 519
of analytical mass bias allowed by this technique.
520 521
Although only few absolute datings of Pb-Zn mineralizations are available in the literature, 522
emplacement ages have been estimated for most of the ore deposits considered here. These ages 523
are reported in Table S1. For each mining region, several ore deposition events may have oc- 524
curred. As shown in Fig. 3, with rare exceptions, model ages are systematically older than em- 525
placement ages by up to 400 Ma, a time difference that largely exceeds the uncertainties on the 526
model ages.
527 528
We highlight this observation with a few examples, such as the Alcudia-Valley-los Pedroches 529
area, where five types of Pb-Zn deposits have been defined corresponding to distinct ore form- 530
ing events (Palero-Fernandez et al., 2003; Palero-Fernandez and Martin-Izard, 2005). The 531
model ages of the first four ore types which formed from the Silurian (450-420 Ma) to the 532
Upper Carboniferous (320-300 Ma) are Neoproterozoic (about 600 Ma). In contrast, the model 533
ages of galenas from the last ore-forming event of Permian to Triassic age (260-240 Ma) are 534
Devonian (380-400 Ma) (see Table S1). A similar pattern is observed in the Betic Cordilleras, 535
where three phases of ore formation have been identified (Arribas and Tosdal, 1994). A first 536
mineralisation type of Late Permian to Lower Triassic age (240-230 Ma) hosts galenas with 537
Devonian model ages (380-400 Ma), whereas the two Miocene types host galenas with Creta- 538
ceous to Tertiary model ages. Likewise, galenas from the Catalan Coastal Range have Devonian 539
model ages, whereas the ore-forming events occurred from Trias to the Lower Jurassic (225- 540
195 Ma) (Canals and Cardellach, 1997, 1993). Overall, with few exceptions, galenas from the 541
whole Iberian Peninsula have model ages significantly older than their host deposits (Fig. 3), 542
regardless of ore type deposit (i.e. SedEx, VHMS, MVT).
543 544
The maps of Fig. 4 (206Pb/204Pb (a), 207Pb/206Pb (b),208Pb/206Pb (c), Tmod (Pb model age) (d), 545
(apparent 238U/204Pb) (e) and (apparent 232Th/238U) (f)) reveal that these variables are 546
strongly controlled by geography. With the exception of the OMZ and the South Central Iberian 547
Zone, the data are consistent within the same region. 206Pb/204Pb and Tmod are negatively corre- 548
lated, whereas 208Pb/206Pb and are positively correlated. The map separates a province with 549
high values, the Pyrenees and the Catalan Coastal Range, from the rest of Iberia. Galenas with 550
the highest 206Pb/204Pb and the youngest Tmod are restricted to the Eastern Betic realm, whereas 551
galenas with the lowest 206Pb/204Pb, and hence the oldest, are found in León (northwestern 552
Spain), the South Central Iberian Zone, the OMZ (Extremadure), and the northern Pyrenees.
553
Other galenas from the OMZ, the South Central Iberian Zone, the Catalonia Batholith, and the 554
Pyrenees fall within an intermediate 206Pb/204Pb range. 207Pb/204Pb and Tmod mirror this distri- 555
bution and 208Pb/204Pb also shows the same strong contrast between the Betic realm, the OMZ, 556
and the Pyrenee-Catalonia domain.
557 558
As mentioned above, the present study focuses on the geological interpretation of Pb isotope 559
data using the model parameters Tmod, , and calculated from the equations given by Albarède 560
et al. (2012) and the parameters determined by Albarède and Juteau (1984). We used the re- 561
cently redetermined value of 137.79 for 238U/235U (Andersen et al., 2017; Goldmann et al., 562
2015). The provinciality of calculated model ages is particularly sharp (Fig. 4). The Tmod data 563
are also presented in 1-dimensional histogram form (Figs. 5 and 6), which will be discussed 564
below. In the 3-dimensional Tmod, , and space, the datawere treated as representing mixtures 565
of stochastic variables with normal distributions. The peaks are identified by a code (which can 566
be obtained from the senior author upon request) written in Matlab using the Statistical Toolbox 567
and the results are presented in Table 2.
568 569
The one-dimensional Tmod histogram of the new galena Pb isotope data identifies two particu- 570
larly significant peaks at 94±38 Ma and 392±39Ma (Fig. 5) with the significance level assessed 571
by a t-test, and a minor peak at ~614 Ma (Fig. 5). When the database obtained by combing the 572
new Pb isotope data with literature data (N=464) (Table S1), and and are added as additional 573
dimensions, the data set can be treated as a 3-dimensional population mixture: a five-component 574
mixture gives a best fit with well-defined model age peaks (Table 2). Two major model age 575
peaks are observed at 395±40 (N=237, Middle Devonian; Table 2) and 90±34 Ma (N=80, Mid- 576
Additionally, a broad Neoproterozoic to Early Paleozoic peak (N=74, 613±42 Ma; Table 2) and 578
two minor peaks at 185±26 Ma (Jurassic) and 313±41 (Upper Carboniferous) (Table 2) are also 579
identified. The average Th/U ratio of the galenas (3.99±0.13) matches the average value for 580
European and circum-Mediterranean galenas (Blichert-Toft et al., 2016).
581 582
For K-feldspars, emplacement ages and Pb model ages are not correlated (r = 0.14). Most 583
early Variscan granites dated at >300 Ma are located in Northwestern Iberia, the Eastern Pyre- 584
nees, and the Catalan Coastal Range and have model ages clustering between 402 and 449 Ma 585
(Upper Silurian-Devonian). The K-feldspar data from various types of granites together with 586
those from the literature reveal a picture very different from that of galena with well-defined 587
Tmod peaks at 329±24 Ma (Upper Carboniferous) and 417±36 Ma (Fig. 6). The Tmod histogram 588
also hints at a minor peak at 232±14 Ma (Fig. 6). The 3-dimensional (Tmod, , and ) analysis 589
(Table 2) reveals a four-component mixture with two dominant peaks at 336±30 Ma (N=46, 590
Carboniferous; Table 2) and 424±34 Ma (N=38, Devonian; Table 2) and two broad, less signif- 591
icant peaks at 214±53 Ma (N=12) and 185±180 Ma (N=7) (Table 2). The two major peaks de- 592
fined by K-feldspars are significantly different from the two major peaks defined by galenas 593
(Fig. 7). The average Th/U ratio of the K-feldspars (3.93±0.21) also matches the average value 594
for European and circum-Mediterranean galenas (Blichert-Toft et al., 2016), but a t-test 595
(p=310-12) shows that the difference in Th/U between galenas and K-feldspars is significant.
596 597
6. Discussion 598
As explained in Section 2, Pb isotopes do not have enough resolution and robustness to disen- 599
tangle the intricacies of the different mechanisms by which large amounts of metal are accu- 600
mulated in the upper crust but they do respond to the formation of large Pb segregations 601
(U/Th/Pb fractionation) and to the timing of the fractionation events in question. Lead isotopes 602
(Moorbath and Welke, 1968), often complemented by oxygen and strontium isotopes (Michard- 603
Vitrac et al., 1980), clearly help assess the relative contributions of mantle and crust to mag- 604
matic rocks and ore deposits. They also delineate model age provinces reflecting tectonique 605
cycles (Blichert-Toft et al., 2016). However, contrary to the case of VMS deposits (e.g.
606
McCallum et al., 1999; Piercey and Kamber, 2019), they appear of little help in refining the 607
SedEx and MVT genetic models of ore deposits. Due to the enormous fractionation factors of 608
Pb with respect to U and Th during ore formation, Pb isotopes provide testable time constraints 609
611
The Discussion will first address the issue of Pb model age significance in galenas and K- 612
feldspars and show that this new interpretation fits the Pb isotope data obtained at other locali- 613
ties.
614 615
6.1 A model of early Pb-Zn segregation and a new interpretation of Pb model ages 616
A first-order observation is that the Pb isotope compositions of the present galena and K-feld- 617
spar samples are clearly of crustal origin and distinct from mantle values, e.g. the values meas- 618
ured on basalts and compiled by Hofmann (1997). Mantle Pb sources may be suspected when 619
model ages are negative (Albarède et al., 2020). The origin of the original Pb stock therefore 620
firmly plots in the field of upper crustal rocks. Despite evidence that 207Pb/204Pb and values 621
are higher in the upper crust than in the mantle, the high 206Pb/204Pb values adopted for the 622
modern Upper Continental Crust by Zartman and Haines (1988) (19.33) and Kramers and 623
Tolstikhin (1997) (19.17) correspond to future model ages, as calculated in Albarède et al.
624
(2012), of 231 and 153 Ma, values that are unreasonably low with respect to observations on 625
galenas, whether conformable or not (Albarède and Juteau, 1984; Stacey and Kramers, 1975).
626
We therefore conclude that Pb model ages calculated from the two-stage modeling of conform- 627
able galenas provide a simple but consistent framework within which to discuss the age and 628
origin of the original Pb source of Pb-Zn ore deposits.
629 630
Lead-zinc strata-bound ores appear to form during certain geological periods. SedEx emplace- 631
ment ages form two clusters, one in the Mesoproterozoic and one in the Paleozoic (Leach et al., 632
2001). Likewise, there appear to be two major episodes of MVT formation, one in the Devo- 633
nian–Permian and the other in the Cretaceous–Tertiary. Even if reliable emplacement ages, and 634
even host rock ages, may not always be available for the Iberian Peninsula, the Pb model ages 635
obtained in the present work provide a different perspective on Pb-Zn pre-concentration. The 636
new galena data define two sharp peaks consistent with Middle to Late Devonian and Early to 637
Middle Cretaceous ages. These ages are regionally very consistent and seem to depend on the 638
size and type of the ore deposits. The former peak dominates the SPZ, the OMZ, the South 639
Central Iberian Zone, and the Catalan Coastal Range. Consolidating the new data and literature 640
data highlights a significant peak in the Neoproterozoic. Adding the and variables does not 641
improve age resolution in 2-dimensional histograms, but confirms the prominence of four 642
events occurring, in decreasing order of importance, in the Devonian, Mid-Cretaceous, Edia- 643
caran, and Carboniferous. Most samples defining the Ediacaran peak are located in the Pyre- 644
nees, northwestern Spain, and, sparsely, the OMZ and South Central Iberian Zone, which are 645
all localities where Neoproterozoic terranes are present, but predate the Cadomian collision.
646
The Late Devonian peak can exclusively be associated with U-Pb dates from the SPZ arc (Pe- 647
reira et al., 2017, 2012, 2012; Pérez-Cáceres et al., 2017). The case of the Early Cretaceous 648
peak is exemplary because, although this period coincides with the opening of the South Atlan- 649
tic, no major tectonic event or significant magmatic activity are known locally from this period 650
(Martín-Chivelet et al., 2002). Overall, the major peaks of galena model ages do not seem to be 651
associated with tectonic or magmatic events affecting the Iberian domain and emplacement 652
ages, where they can be assessed, are significantly older than the ages of the magmas and fluids 653
that carried the metal load. This case is clear for the Betic belt where Lower Cretaceous Pb 654
model ages are recorded in ores associated with Miocene volcanics.
655 656
It has long been accepted that the source of Pb ores is in the crust, and in particular in sedimen- 657
tary rocks (Heyl et al., 1974). For the Betics, Arribas and Tosdal (1994) suggested that the 658
protolith corresponds to the surrounding Paleozoic metasediments. Clearly, however, not all 659
sedimentary sequences foreshadow the formation of large Pb-Zn deposits. In addition, large- 660
scale circulation of groundwater is expected to average the Pb isotopic composition of the solute 661
to the ‘common lead’ of the time. Selective leaching of lead from some older formation(s) 662
existing at the time of formation of Pb-Zn deposits is therefore needed to ensure that their Pb 663
model ages are older than their emplacement ages.
664 665
Goodfellow and Jonasson (1986), Goodfellow (1987), and Turner (1992) advocated that sedi- 666
ment-hosted Pb-Zn ore deposits formed in a stratified ocean during anoxic or euxinic events.
667
Sáez et al. (2011) suggested a connection between the local anoxia of the local volcano-sedi- 668
mentary environment and the deposition of massive sulfide ores of Paleozoic age at Rammels- 669
berg (Germany), Tharsis (Southern Spain and Portugal), and Draa Sfar (Morocco). Here, Pb 670
model ages that are older than emplacement ages take the assumption of anoxic conditions one 671
step further by suggesting that if the well-defined peaks of model ages characterize unusual 672
sedimentary events, then the current ore deposits formed in a different environment. Remarka- 673
bly, the sequence of events identified by the five-component breakdown of this work (Table 2) 674
is strongly reminiscent of well-known episodes of anoxia in the Phanerozoic ocean. An appeal- 675
segregation, therefore is the effect of global oceanic anoxic events. Black shales are a typical 677
rock type deposited during anoxic events but the whole-rocks are not by themselves the likely 678
primary source of Pb and Zn ore deposits. In order for the sources of Pb-Zn ores to freeze in 679
the sharp peaks of model ages observed here, Pb isotope compositions require that the 680
=238U/204Pb) values of the parent rocks must be close to zero. Any source rock with >>0 681
inevitably leads to a broad spread in model ages due to variable radiogenic ingrowth. The lim- 682
ited number of available data on Pb isotope systematics of old black shales are indicative of 683
high values (from 9.6to 1000 and more) (Chen et al., 2009; Fetter et al., 2019; Jiang et al., 684
2006). Such elevated parent/daughter ratios render the black shale whole-rocks themselves un- 685
suitable sources of Pb because they would not ‘freeze’ the source Pb isotope compositions but 686
rather drive radiogenic ingrowth well beyond common lead and induce a broad scatter of iso- 687
topic ratios. Although mineralogical studies of shales are few, finely divided, but abundant ga- 688
lena has been described in black shales, often included in arsenopyrite (Abraitis et al., 2004;
689
Belkin and Luo, 2008), and would be more suitable Pb sources.
690 691
The case for sulfides deposited during these events, or in their immediate aftermath, is stronger.
692
As demonstrated for sedimentary Pb-Zn sulfidic ores deposited during the Silurian and Devo- 693
nian of North America (Goodfellow, 1987), the Neoproterozoic and Early Paleozoic at Imiter, 694
Morocco (Essarraj et al., 2016), and on the Yangtze Platform, China (Chen et al., 2009; Jiang 695
et al., 2006), oceanic anoxic events are commonly associated with the deposition of volcano- 696
sedimentary sulfide-rich ores. Oceanic anoxic events are known to coincide with major reduc- 697
tion and drawdown of marine sulfate, enhanced pyrite burial, and major shifts in the 34S of 698
marine barite (Paytan et al., 2004). Oceanic anoxic events further are associated with increase 699
in global weathering (Blättler et al., 2011; Percival et al., 2019), suggested to be triggered by 700
global wildfires (Brown et al., 2012; Kump, 1988), and development of euxinic conditions over 701
large expanses of the ocean bottom (Owens et al., 2013). A comparison between dates of major 702
anoxic events and the model age peaks observed in the present study shows the following cor- 703
respondences:
704 705
1. The oldest anoxic event relevant to the present study took place during the Cryogenian, 706
possibly at the end of the Marinoan glaciation (Sahoo et al., 2012): the largest silver 707
mine of Morocco, Imiter, was exploited since at least Medieval times and is associated 708
rather large spread of model ages with a peak at 613±42 Ma (Table 2), however, does 710
not warrant further elaboration.
711
2. The dominant model age peak at 395±40 Ma (Upper Devonian) (Table 2) is consistent 712
with the widespread period of anoxia known as the Kellwasser events (Frasnian-Fame- 713
nian extinction) (Buggisch, 1991) and dated at 372 Ma (Percival et al., 2018), possibly 714
triggered by eruption of the Viluy traps, Eastern Siberia (Courtillot et al., 2010; Percival 715
et al., 2019, 2018).
716
3. The Lower Toarcian global anoxic event (~186 Ma; Jenkyns, 1988) is less conspicuous 717
but still detectable as a model age peak in Iberian galenas (Table 2). Ages consistent 718
with this event are found elsewhere in other Pb-Zn ores, e.g. in Britain, France, and 719
Germany (Bode, 2008; Marcoux, 1987; Rohl, 1996).
720
4. In contrast, the well-documented oceanic anoxic events OAE1a (110 Ma), OAE1b 721
(~111 Ma), and OAE2 (93 Ma) (Jenkyns, 2010) overlap with the sharp Pb model age 722
peak in Iberian galenas (90±34 Ma; Table 2). The resolution and accuracy of Pb model 723
ages is insufficient to allow a more specific identification.
724 725
Although robust genetic models of SedEx and MVT ore deposits have become largely consen- 726
sual (Wilkinson, 2014), the original mechanisms leading to the original accumulation of the 727
Pb-Zn sulfide stock are not entirely clear. Galena is notoriously rare in oceanic hydrothermal 728
sulfides (Oudin, 1983) and Pb concentrations in mid-ocean ridge vent fluids are low (< 100 729
ppb, Chen et al., 1986; Michard et al., 1983), simply because Pb abundances in MORB and in 730
the upper mantle are very low (Albarède, 2005; Salters and Stracke, 2004). While the Pb source 731
of sediments is heavily controlled by erosion of continental surfaces (Percival et al., 2019), any 732
sulfide, whether hydrothermal or biogenic, is rapidly oxidized to sulfate in seawater under pre- 733
vailing periods of oxic conditions but not under anoxic conditions. As shown by 34S values in 734
sedimentary pyrite drawn from the euxinic levels of the Black Sea (Lyons, 1997), reduction of 735
marine sulfate during anoxic events does not account for the source of sulfur from massive 736
sulfide deposits (Seal, 2006). Sources of Pb and S therefore most likely are distinct: whenever 737
hydrothermal sulfide spouted from mid-ocean ridges and back-arc basins form VMS deposits, 738
their 34S falls within a relatively narrow range (1-7‰) close to the mantle value (Hannington, 739
2014; Seal, 2006). Anoxic water masses regularly injected by hydrothermal plumes rich in 740
mantle sulfur offer a potential mechanism by which Pb and Zn from continental runoff will 741
