6.1. Ore-Forming Stages
The concentration of fine-grained, framboidal and euhedral pyrite in black shales during the early stage is a rather common sedimentary/diagenetic process [
20,
21] during which the formation of pyrite requires bacterial reduction of Fe
3+ coupled with oxidation of organic matter and marine sulfate reduction [
22]. The availability of abundant Fe
2+ and SH
2 in pore waters at oversaturated concentrations [
23] would favor nucleation rather than crystal growth, promoting the formation of pyrite framboids. Nevertheless, this low-oxygen fugacity conditions contrast with the presence of hematite-rich rocks which imply low SH
2 and high reactive Fe concentrations. The absence of clear observations on the structural and textural relationships between fine-grained pyrite- and magnetite-rich rocks hinders an appropriate discussion on the genetic relationships and the relative chronology between both types of rocks in terms of physicochemical changes in the sedimentary environment and/or variations in the chemistry of low temperature hydrothermal solutions supplied to the basin with concentrations of H
2 and SH
2 close to the stability fields of pyrite and hematite [
24]. Preliminary petrographic and geochemical data of the siltstones and shales hosting the Castellanos deposit point to chemical changes in the low-temperature hydrothermal system, similar to the proposal by [
24,
25] for some VMS deposits where similar zones rich in pseudomorphic magnetite formed after hematite occur.
The formation of the Zn-Pb ore during the main stage took place via the superimposition of successive mineralizing events associated with the input of metal-rich brines at increasing temperature and/or decreasing metal saturation degree. Estimations made by [
12] on the basis of the reflectance of vitrinite and the crystallinity of clay minerals in the host black shales constrain temperature conditions at <200 °C. The presence of ferroan dolomite alteration at such low temperatures suggests that the fluids responsible for the formation of the main mass of sphalerite and galena ore were oxidized, nearly neutral brines [
4]. The described textural sequence also suggests an early input of oxidized, saline hydrothermal fluids capable of transporting Zn and Pb up to oversaturation which, in turn, gave way to colloform precipitation of sphalerite and skeletal galena when the ore fluids encountered reduced (generated thermochemically and/or by means of bacteria) sulfur on or below the seafloor in the host euxinic basin [
26,
27]. Throughout the mineralizing process, slight increase in temperature and putative metals undersaturation of fluids would promote crystal growth leading to the development of coarse-grained euhedral sulfides. Metals undersaturation of the fluids could also be reached at the end of each one of the individual crystallization events. The formation of colloform sphalerite (+galena) enveloped by colloform pyrite promoted metal depletion in the residual fluids thus giving rise to the development of subhedral pyrite overgrowing the outer colloform pyrite bands of sphalerite-pyrite aggregates.
The mineralogy and texture described in the basal pyrite-rich zone support the existence of replacement processes of the early Zn-Pb sulfide ores associated with the precipitation and growth of pyrite. Pyrite also shows pseudomorphic textures after replacement of laminar magnetite, as [
25] propose for VMS deposits of the Iberian Pyrite Belt. Since dissolution of sphalerite and galena is favored by heating [
24], the formation of the lower pyrite-rich zone can be the consequence of the thermal evolution proposed for the mineralizing process during the main stage of ore formation. During the thermal maximum reached at the end of the main stage (probably slightly above 200 °C), the abundance of H
2S generated by the dissolution of sphalerite and galena [
24] promoted extensive crystallization of pyrite in the ore and replacement of laminar pseudomorphic magnetite in the host rock. These mineral assemblages and textures have also been observed in the pyrite veins of the underlaying stockwork.
The onset of the fracturing and brecciation stage is marked by the crystallization of a new generation of colloform banded pyrite shortly followed by early barite (
Figure 6) which is usually intergrown with calcite (
Figure 5E) and locally associated with anhydrite. Later on, remobilization of galena, precipitation of at least three younger generations of barite filling fractures, and a silicification event took place. Finally, crystallization of chalcopyrite occurred at the brecciated zones (even within pyrite veins of the stockwork). The limited mobility of Cu at low temperature (<350 °C) [
28,
29] suggests that the hydrothermal system created during this stage allowed late supply of deep, high-temperature brines to the ore-forming environment. Although these brines played a minor role in the formation of the Castellanos ore, they might have been predominant in the formation of the Cu vein-type ores of the Matahambre deposit (
Figure 1 and
Figure 2) [
13,
14].
6.2. Timing of Au Precipitation
It has been stated that the redox state of sulfur in shale-hosted, clastic-dominated zinc-lead deposits (especially in those interpreted as sedimentary exhalative) can control the proportion of Au and base metals in ore brines at 100–200 °C [
1,
2,
5]. According to these authors, the higher the H
2S content in the fluid, the lower the solubility of Zn and Pb. Furthermore, there is a general consensus on transport of base metals (Fe, Cu, Zn and Pb) in these brines at low temperature having taken place predominantly as chloride complexes whereas Au is mobilized as bisulfide complexes [
30]. Thus, the formation of the main-stage ore at Castellanos requires the input of highly saline, oxidized hydrothermal fluids with low H
2S contents. These fluids could have concentrated zinc and lead by leaching the oxidized fluvial-deltaic clastic sediments of the San Cayetano Formation as has been proposed for other shale-hosted, clastic-dominated deposits elsewhere [
1,
5]. Conversely, transport of Au and Ba requires more reduced conditions, moderate to high H
2S contents and low SO
42+ concentration [
1,
5,
31]. The source of such fluids must be linked with organic carbon-rich lithologies such as the black shales occurring intercalated in the San Cayetano Formation, where H
2S would have been produced via thermochemical or bacteria sulfate reduction coupled with decomposition of organic matter [
22,
32].
The correlation between Au content maxima and modal amount of early barite suggests that the beginning of the fracturing and brecciation stage implied major change in the redox state of hydrothermal fluids to more reducing conditions, due to the input of slightly deeper brines buffered by black shales. Moreover, the heterogeneous distribution of Au in single samples and textural types of pyrite, as well as at the scale of mineral aggregates and crystals, further suggests variations in the redox state of sulfur in successive fluid batches. Thus, we hypothesize that early supply of small amounts of Au to the ore-forming environment would have coincided with the crystallization of colloform pyrite (Py IIa) at the beginning of the main stage, and that Au input would have become progressively more abundant as the proportion of batches of H2S-rich, reduced fluids increased up to the formation of the banded colloform pyrite (Py III) coeval with the onset of fracturing and prior to the crystallization of early barite. Each colloform band with its own growing layer of euhedral pyrite (Py IIb) would represent single batches of fluids buffered with different country rock levels and, hence, with different budget of dissolved metals. This evolution of fluids chemistry seems to be better recorded in the ore assemblages formed towards the top of the orebody.
As fracturing became a widespread phenomenon, the incoming fluids were progressively more oxidized, promoting the crystallization of calcite and the remobilization of galena. Fracturing also allowed late flow of warmer fluids (>350 °C) from deeper zones of the San Cayetano Formation, which were suitable for transporting Cu and precipitate chalcopyrite in restricted brecciated areas of the Castellanos orebody and in some pyrite veins of the underlying stockwork. Nevertheless, the presence of several generations of barite veins even within the stockwork reveals the existence of discrete inputs of reduced, SO
4-free fluids during the late fracturing stage too. These fluids could also supply additional Au transported in the form of chlorite complexes [
33].
The comparison of the Castellanos ore with that from similar shale-hosted, clastic-dominated Zn-Pb deposits shows that the former has average whole-rock Au abundances around 1 ppm, similar to the Pb-Zn-Ag-Ba ± Au ores of medium-sized, sedimentary-exhalative deposits [
1] whose average grades vary from 0.5 ppm in the Faro deposit (Anvil district, Selwyn Basin) [
34] to 2 ppm in the Triumph deposit (Milligen Formation, Central Idaho) [
35]. These values are also comparable with those reported for many VMS deposits, since most of them (~78%) have Au grades <2 ppm [
36]. Higher Au contents in VMS deposits are generally linked with the calcoalkaline to transitional nature of the associated volcanism as well as the volume of felsic volcanics [
36,
37], thus highlighting the predominant magmatic origin of Au. The absence or very restricted occurrence of igneous rocks in the stratigraphic successions including shale-hosted, clastic-dominated deposits limits the supply of the ore-forming metals (Zn and Pb but also Au and Ag) to their effective leaching from the terrigenous lithologies that dominate such successions [
30]. Black shale intercalations in the stratigraphic sequence can contribute to buffer ore-forming fluids, producing reduced, H
2S-rich brines and supplying Au if they content enough amounts of Au-bearing sedimentary pyrite, as is the case of the Selwyn basin [
38]. Actually, some of the few shale-hosted, clastic-dominated deposits containing significant amounts of Au are located in the Anvil district, within the Selwyn basin [
1].
6.3. Au Distribution in Zn-Pb Ores
The results presented above point to a preferential concentration of gold in pyrite where Au atoms should become fixed on the Sb-, Ag- and As-rich pyrite growth zones (
Figure 8 and
Figure 9). The incorporation of such trace elements in pyrite promoted the development of structural distortions and the formation of Fe-deficient surfaces thus favoring the fixation of Au in the pyrite lattice or on its surface via chemisorption processes [
39,
40]. The substitution of S by As and Sb from the early H
2S-bearing fluids resulted in the creation of local reduced surfaces on pyrite [
41] thus contributing to destabilize Au-bisulfide complexes and Au deposition. Nevertheless, Au contents in pyrite of samples SR-185/0.6–0.7, SR-208/5.5–5.8 and CE-61 do not account for their whole-rock amounts. Assuming that all Au is hosted by pyrite, that the average Au content of pyrite in these samples is 0.59, 1.66 and 4.97 ppm respectively (
Appendix A,
Table A2) and that pyrite represents ~77%, ~33% and ~13% respectively of the ore modal composition, the expected whole-rock Au amounts should be around 0.45, 0.54 and 0.65 ppm respectively. Conversely, their respective whole-rock Au contents would be 1.18, 1.85 and 1.55 ppm. Since trace element analyses of sphalerite and galena did not reveal significant Au concentrations, this discrepancy would be explained either by the presence of Au minerals or native Au in the ore assemblage or assuming higher heterogeneity in the distribution of Au in pyrite, having zones with much higher concentrations than those analyzed in this study. Although the first explanation cannot be totally discarded, it is not supported by the results of our systematic EDS analyses performed by ESEM and FE-ESEM on particles with high mean-atomic weight. These particles were detected and analyzed during scanning of the total surface of the 54 polished sections used for petrographic studies and none of them showed any spectrum of Au minerals. In contrast, the heterogeneous distribution of Au in different textural types of pyrite in the analyzed samples and even within single growth bands in colloform aggregates and euhedral crystals (
Figure 8 and
Figure 9, and
Appendix B,
Figure A1 and
Figure A2) supports the possibility of finding pyrite crystals or aggregates with higher Au contents, which would contribute to balance the trace amounts analyzed in pyrite with whole-rock contents. The superimposition of different events of sulfide crystallization involving dissolution-precipitation processes would promote local Au remobilization and subsequent concentration in preferential sites.
To our current knowledge, there are no data available in the literature on trace Au contents in pyrite, sphalerite and galena from shale-hosted, clastic-dominated deposits. Similarly, only few papers report LA-ICP-MS results on pyrite from VMS deposits [
42,
43,
44], in spite of the important Au endowment of many deposits [
36,
37] and the need-to-know Au speciation for effective recovery. Average Au content in pyrite from different deposits of the Bathurs Mining Camp varies from 0.07 ppm in Key Anacon East Zone to 1.86 ppm in Louvicourt and always concentrates in As-rich pyrite [
43]. Furthermore, in the Brunswick No 12 deposit from the same camp, [
42] reports increasing average Au contents in pyrite (from 0.39 ppm to 2.6 ppm) towards the hanging wall of the deposit associated with concentration of As, Sb and Ag in the sulfide ores. This evolution is consistent with similar Au enrichments recognized in other VMS deposits, including Kuroko in Japan [
21], Rosebery, Que River and Hellyer in Australia [
33] and Neves Corvo, Rio Tinto, Sotiel and Migollas in the Iberian Pyrite Belt [
45,
46]. Nevertheless, whereas Au contents in arsenian pyrite mostly account for the whole-rock contents of the different sulfide ores at Brunswick No 12 [
42], Au mainly occur in the form of native gold and Au-Ag-Hg alloys in the Iberian Pyrite Belt [
46,
47]. Preliminary LA-ICP-MS results show that pyrite from several deposits of the Iberian Pyrite Belt may contain significant amounts of Au and that different textured-pyrite host variable Au contents [
44]. This author shows that average Au content increase from framboidal pyrite (1.26 ppm) to colloform pyrite (1.36 ppm) and significantly decrease in recrystallized pyrite (0.56 ppm), while Au contents in all these textural types of pyrite positively correlate with As, Sb, Ag and Ni.
In absence of published studies on Au distribution in shale-hosted, clastic-dominated Zn-Pb deposits, a comparison of Castellanos with VMS deposits reveals a somewhat similar bottom to top enrichment of Au, As, Ag and Sb in pyrite. However, the described enrichment at Castellanos is more pronounced than in VMS deposits since the average Au contents of recrystallized pyrite (Py IIb) vary from 0.82 ppm to 9.02 ppm, reaching maxima (17.58 ppm average.) in the colloform banded pyrite (Py III) from the top of the orebody. The high Au contents of pyrite from Castellanos in comparison with those from VMS deposits like Brunswick No 12 where it is the principal host for Au also suggest that most Au at Castellanos could be hidden in pyrite.