The stratigraphic succession of the IPB has traditionally been subdivided into three major lithostratigraphic units [31]. In ascending order these are: the Phyllite–Quartzite Group (PQ), the Volcano–Sedimentary Complex (VSC), and the Culm Group.

The PQ Group consists of a thick sequence of shales and quartz arenites of late Middle to Late Devonian (i.e., late Givetian–late Famennian) age, deposited on a quiescent shallow siliciclastic platform sporadically affected by storm events [15,17]. The increase in the sand/lutite ratio towards the top of the PQ Group reflects basin shallowing as consequence of the Late Devonian regression. Also at the top, the coeval occurrence of discontinuous, unconnected carbonate lenses and high-energy conglomerates indicates an abrupt change in the depositional regime, related to the onset of the Variscan orogenic activity [17]. The seismic activity associated to these initial orogenic movements is recorded by such sedimentary and tectonic features as sediment liquefactions, slidings, synsedimentary faults, collapses, tsunamite records, etc. [19]. The initially extensional tectonism [33] produced collapse and compartmentation of the previously stable shelf into a series of horst and grabens [17]. The resultant complex palaeogeography strongly influenced the sedimentation of the succeeding CVS series.

The basal part of the CVS, conformably succeeding the PQ Group, is characterized by a sequence of black shales and subordinate volcanic and volcaniclastic rocks. This sequence, of Latest Famennian age, represents the main host horizon of the massive sulphide deposits. The remainder of the VSC is composed of diverse bimodal volcanics and subvolcanics intercalated within shales and volcaniclastic rocks. Chert and carbonate lenses also occur intermittently throughout the series. The lithological heterogeneity of the CVS and its frequent and abrupt lateral thickness and facies variations are the response to an intricate relationship between sedimentation, volcanism and tectonic activity in a compartmentalized basin. Toward the top of the VSC, a mixed volcanodetritic and shaly sequence, termed the Basal Shaly Formation [16], is transitional to the overlying Culm Group. This Group is made of a thick sequence of shales, lithic greywackes and rare conglomerates of Late Viséan to Bashkirian age. It represents the final post-volcanic, synorogenic turbiditic infill of the IPB basin.

During the Asturian phase of the Variscan orogeny, the entire IPB succession was folded, thrust and metamorphized to very low-grade [1,20,32].

2.1 Las Herrerías

2.2 La Torerera

Forty-six and 23 samples were collected respectively from the Las Herrerías and La Torerera mines. The Las Herrerías samples are from the shaly sequence at the base of the massive sulphides, as exposed in the northern wall of Guadiana open pit (Fig. 3C). In La Torerera, the shales analyzed are those of the hosting sequence as exposed south of the main shaft (Fig. 3C). All the samples were processed by means of conventional palynological preparation techniques [35]. These entailed treatments with hydrochloric and hydrofluoric acids to digest the carbonate and silicate constituents respectively, followed by immersion (for varying, but usually very brief, time intervals) in fuming Schulze solution in order to oxidize the organic-rich residue. Seven samples from Las Herrerías and two from La Torerera proved to be palynologically productive (Fig. 3). The palynological assemblages are generally sparse and their preservational quality varies from poor to fairly good. Trilete miospores (mainly simple, laevigate forms) of terrestrial derivation are predominant, while the subordinate marine component consists principally of prasinophyte phycomata representative of the genus Maranhites Brito. Selected species of miospores and organic-walled microphytoplankton recovered from the two collection sites are illustrated in Fig. 4.

The miospore assemblages from Las Herrerías (Fig. 5) are characterized by Punctatisporites planus Hacquebard, Retusotriletes incohatus Sullivan, Auroraspora macra Sullivan and Densosporites sp. cf. D. spitsbergensis Playford. Other species present include Pustulatisporites dolbii Higgs, Clayton and Keegan, Endosporites micromanifestus Hacquebard, Endosporites tuberosus González, Playford and Moreno, Grandispora echinata Hacquebard emend. Utting, Knoxisporites sp. cf. K. literatus (Waltz) Playford, Retispora lepidophyta (Kedo) Playford, Rugospora flexuosa (Kedo) Streel, Verrucosisporites nitidus Playford and Vallatisporites verrucosus Hacquebard. The microphytoplankton content is dominated by the prasinophyte phycomata species Maranhites brasiliensis Brito and M. mosesii (Sommer) Brito, together with undifferentiated forms assigned to Tassmanites Newton, and rare specimens of the acritarch Dupliciradiatum crassum González, Moreno and Playford (the latter identified in two samples only). A marine depositional environment is therefore inferred for the analyzed shaly sequence at Las Herrerías.

In Western Europe, the Retispora lepidophyta–Verrucosisporites nitidus (LN) miospore Biozone is defined by the first appearance of Verrucosisporites nitidus Playford. Vallatisporites verrucosus Hacquebard and Tumulispora malevkensis (Kedo) Turnau are likewise introduced at the base of the LN Biozone [10,34]. The biozone’s upper limit is marked by the disappearance of Retispora lepidophyta (Kedo) Playford and other palynostratigraphic indices, notably R. flexuosa (Kedo) Streel, Diducites versabilis (Kedo) emend. Van Veen, and Crassispora catenata Higgs [10].

Accordingly, the co-occurrence of V. nitidus Playford and R. flexuosa (Kedo) Streel in the lowest productive sample of Las Herrerías (HER44), and that of Retispora lepidophyta (Kedo) Playford and Vallatisporites verrucosus Hacquebard in the uppermost sample (HER17), enable the whole sequence sampled from the Guadiana open pit to be assigned to the LN Biozone.

The palynofloras from the La Torerera samples (Figs. 4 and 5) are less diverse than those obtained from Las Herrerías. They similarly suggest a marine depositional environment, including both terrestrial and marine palynomorphs. The former are mostly dominated by the trilete miospores Punctatisporites planus Hacquebard, Retusotriletes incohatus Sullivan and R. flexuosa (Kedo) Streel, while the scarce marine component is represented by the genera Tasmanites Newton and Maranhites Brito.

Densosporites sp. cf. D. spitsbergensis Playford, which occurs in the lower sample (TOR02), is smaller than the specimens originally described by [27] (pp. 627–628, pl. 89, Figs. 1–5) and lacks their differential distribution of spinae on the cingulum. However, forms akin to Densosporites sp. cf. D. spitsbergensis Playford have been reported from several other areas of the IPB [5,7,8,24,25] in assemblages judged to be compatible with an LN biozonal assignment. Moreover, the debut of Densosporites spitsbergensis Playford is known to occur elsewhere in Western Europe at or close to the base of the LN Biozone. Accordingly, the association of Densosporites sp. cf. D. spitsbergensis Playford and R. flexuosa (Kedo) Streel in the lowermost La Torerera sample suggests its attribution to the LN Biozone, despite the absence of the two index species Retispora lepidophyta (Kedo) Playford and V. nitidus Playford. Given the occurrence of R. flexousa (Kedo) Streel in the succeeding sample, it is reasonable to assign the upper assemblage (TOR09) also to the LN Biozone, because the disappearance of this species, as stated above, coincides with the LN Biozone’s upper limit.

In chronostratigraphic terms, the LN spore Biozone is the uppermost Devonian biozone defined by [34] and [10] in Western Europe. Because its upper limit coincides approximately with the Devonian/Carboniferous boundary, a Latest Devonian (Late Famennian or Strunian) age can be assigned to the Las Herrerías and La Torerera shaly sequences. Fig. 6 shows the chronostratigraphic position of the LN Biozone together with the stratigraphic ranges of selected species recorded from the two studied areas.

The Las Herrerías and La Torerera palynofloras are therefore coeval with those described from the shaly sequence hosting the giant deposits of Neves-Corvo [23], Aznalcóllar [25], Tharsis [7], and Sotiel-Coronada [8]. Other age-equivalent assemblages have been reported elsewhere from non-sulphide bearing sequences of the IPB; for instance at the PQ-CVS boundary in the Calañas area [18].

Las Herrerías and La Torerera are two small to medium sized massive sulphide deposits located in the central part of the IPB that show a very complicated structural framework and include palynologically datable black shales associated with the ore rocks. The Late Famennian age assigned to the shaly sequences hosting both ore-bodies enables correlation with all the major massive sulphide deposits dated so far in the region. This provides further support for the hypothesis of a single mineralizing event having been responsible for the genesis of most of the IPB’s massive sulphides [7]. The Las Herrerías and La Torerera chronostratigraphic determination is at the same level of precision as that attained for the larger sulphide ores (i.e., by attribution to the same short-term LN Biozone); hence this reaffirms the conception of a regional mineralizing event of extraordinarily short duration. In other words, during a relatively brief interval close to the Devonian–Carboniferous boundary, the IPB basin as a whole clearly provided the optimal magmatic, tectonic, sedimentologic and palaeoenvironmental conditions for the widespread generation and preservation of a major accumulation of massive sulphide deposits.

In such a region as the IPB, where the palaeontological record (particularly of megafossils) is very meagre, high-resolution palynostratigraphic studies like that presented here are of particular importance, not only for dating and correlation, but also for ore-exploration purposes, by enabling potential hosting sequences to be identified.