2.1. Regional framework

The study area is situated at the junction of three physiographic provinces (Figure 1): (a) the Sierra Madre Oriental (SMO), (b) the Sierra Madre Occidental (SMOc), and (c) the Mesa Central (or Central Plateau, CP) (Figure 1). The SMO is a fold-and-thrust belt that resulted from the deformation of Mesozoic sedimentary rocks by the Laramide orogenesis in late Eocene times (^{Eguiluz-De-Antuñano et al., 2000}). On the other hand, the SMOc is a silicic volcanic arc resulting from the subduction of the former Farallon plate beneath North America, and it consists of a thick sequence of volcanic rocks, mostly andesites, rhyolites, and ignimbrites, with ages ranging from Upper Cretaceous to Eocene (e.g.,^{Ferrari et al.,2005}).

The CP is an elevated plateau, with an average altitude of about 2000 m, bounded to the north and east by the SMO, to the west by the SMOc, and to the south by the E-W El Bajío Fault (^{Nieto-Samaniego et al., 2005}). Both the SMOc and the CP are richly endowed with ore deposits, particularly of precious and base metals (^{Damon et al., 1981}; ^{Clark et al., 1982}; ^{Camprubí, 2009}). According to Nieto-Samaniego et al. (2005), mineral deposits from the CP and in its vicinities are associated with major fault systems, which also control the emplacement of volcanic and hypabyssal rocks.

2.2. Geology of the study area

The rocks of the study area correspond mainly to a predominantly carbonate transgressive Cretaceous sequence related to the opening of the Gulf of Mexico. This sequence was deposited unconformably on the meta-volcanosedimentary Nazas Formation, Upper Triassic to Lower Jurassic in age (^{SGM, 1997}) (Figure 1). This unit, which does not crop out in the study area, consists of intermediate to silicic tuffs and lavas associated with red beds and affected by low-grade metamorphism, and it is crosscut by dikes of basaltic to andesitic composition (^{Barboza-Gudiño et al., 2004}). The rocks of the Nazas Formation represent a continental volcanic arc, developed from the Upper Triassic to the Lower Jurassic at the western margin of North America (^{Barboza-Gudiño et al., 1998}).

The Cretaceous transgressive sequence is represented in the study area by three formations: from the base to the top (^{Kelly, 1936}; ^{SGM 1997}), (a) the Aurora Formation (Albian), with a thicness of 300-400 m, composed of thin-layered limestones rich in chert nodules; (b) the Cuesta del Cura Formation (Albian-Cenomanian), ~300 m in thickness, consisting of interbedded limestone and lutite; and (c) the Indidura Formation (Turonian), ~80 m in thickness, a flysch rhythmic alternation of mudstone and organic-matter-rich lutite beds. This sequence is unconformably overlain by the Ahuichila Formation (Eocene), with an average thickness of ~450 m, and which consists of continental conglomerates with carbonate cement and clasts coming from the underlying Cretaceous sequence (^{Rogers et al., 1961}).

Magmatic activity started in the Oligocene and continued through the Miocene, producing welded rhyolitic tuffs followed by andesitic and basaltic lavas (SGM, 1997). The rhyolitic tuffs form a ~160 m thick succession that fills the Cuencamé graben and are covered by up to 300 m of Quaternary alluvial sediments (Figure 1). Eocene magmatism also produced felsic to intermediate (granitic to dioritic) hypabyssal stocks and NW-SE-trending dikes that crosscut the entire Mesozoic sequence and the Eocene Ahuichila conglomerates (Figures 1 and 2). ^{Spurr and Garrey (1908)} first noticed the relationship between ore mineralization and hypabyssal rocks in the Velardeña mining district.

These authors described the intrusive bodies and labeled them as “alaskites” and trachytes. A K-Ar age of 33.4 ±1.7 Ma (ca. Eocene-Oligocene boundary) was determined for magmatic biotite of the hypabyssal rocks associated with the ore mineralization in the Velardeña mining district by ^{Felder (1979)}. As a whole, this district has been interpreted as a large volcano-plutonic complex that resulted from the overlapping of the SMOc arc magmatism and the Basin and Range extension (^{Levich, 1973}; ^{Gilmer et al., 1986}; ^{Gilmer et al., 1988}). Due to the thick pyroclastic deposits accumulated in the Cuencamé graben (among other facts), ^{Aguirre-Díaz et al. (2008)} suggested the occurrence of a “graben-type caldera” volcanic center.

2.3. Skarns in the Velardeña mining district

The Santa María mine has been studied by several authors as a notable example of Zn-Pb skarn deposits (^{Spurr and Garrey, 1908}; ^{Ambriz, 1979}; ^{Levich, 1973}; ^{Gilmer et al., 1986}, ¹⁹⁸⁸). Reyna de Cobre is also a skarn with mainly Pb-Zn mineralization, despite its name (“Copper Queen” in Spanish), which comes from the striking blue-green secondary copper minerals that occur on the surface. In both areas, the mineralization is associated with intrusive contact of dikes and stocks with the calcareous strata and, occurs (a) as irregularly shaped replacement masses with a paragenesis dominated by calc-silicates and sulfides, and (b) as high-grade calcite-fluorite-sphalerite (-adularia, -quartz) veins placed in the skarn sequence.

The ore deposit-related igneous rocks of the Velardeña mining district have been petrographically classified as monzogranites with porphyritic to granular textures (^{Jiménez-Franco, 2012}). They contain subhedral phenocrysts, up to 4 mm in length, consisting of quartz and K-feldspar with subordinate plagioclase and biotite. K-feldspar usually appears partly altered to sericite. A few meters away from the contact with the carbonate rock, an endoskarn zone is developed on the intrusive bodies, showing pervasive silicification with wollastonite, epidote, chlorite and calcite, and disseminated pyrite, chalcopyrite and sphalerite.

The exoskarn is irregular in shape and thickness, up to 50 m; it consists of a calc-silicate-rich assemblage, variable in grain size, with replacive, inequigranular texture, and contains the volumetrically higher sulfide mineralization. The predominant calc-silicates are calcic garnet and clinopyroxene (plus subordinate wollastonite), garnet being the most abundant, and are accompanied by a later assemblage of calcic amphibole, chlorite, calcite and quartz with associated sulfides. The sulfides are principally sphalerite (marmatite variety), galena and chalcopyrite, with abundant pyrite and pyrrhotite, and occur (a) as massive, centimeter-sized, coarse-grained aggregates, (b) disseminated; and (c) within veinlets in the exoskarn.

Most of the veins in the Velardeña mining district are not exposed at the surface, they are hosted in the carbonate sequence, generally strike NW-SE and steeply dip to the NE; they have a maximum thickness of 20 m and extend for several km. Some of them suffered brecciation in the proximity of intrusive rocks. They are characterized by an assemblage of calcite and fluorite with subordinate adularia and quartz. The ore minerals contain sphalerite (Fe-poor, pale variety), arsenopyrite and pyrite, with minor galena and chalcopyrite. According to ^{Pinet and Tremblay (2009)}, these veins are epithermal in origin and are fault controlled. The mineralized structures are cut by NW-SE-striking normal faults related to the Basin and Range extension (Oligocene). The biggest one is a calcite vein-fault located on the boundary between the Antares dike and the Valle de Cuencamé (Figure 2A).

4.1. Paragenetic sequence

Ore-bearing mineral assemblages of Velardeña are rich in calc-silicates and show replacement textures that suggest an epigenetic, sequential mineral deposition (figures 3, 4, and 5) caused by a complex, multistage hydrothermal event. On the basis of the textural microscopic analysis, three paragenetic stages linked to hydrothermal activity can be distinguished (Figure 6): (a) prograde skarn, (b) retrograde skarn, (c) vein filling, and (d) supergene stage.

Figure 3 Petrographic images of mineral assemblage representative of the Velardeña ore deposits. (A) Euhedral andradite crystals with concentric zoning, clinopyroxene inclusions, and interstitial calcite (sample C-23-N, transmitted light). (B) Euhedral andradite crystals showing anomalous anisotropy and partial replacement to a calcite-chlorite cryptocrystalline assemblage (sample C-05-C, transmitted light, plane polarized light). (C) Retrograde skarn assemblage with actinolite, adularia, and calcite (sample V-26-B, transmitted light, plane polarized light). (D) Sulfide assemblage of pyrite, arsenopyrite, pyrrhotite, chalcopyrite, and minor Fe-rich sphalerite (Sp-1) (sample V-20-D, reflected light). (E) Sulfide paragenesis of galena, pyrite, sphalerite, and arsenopyrite (sample C-04-F, reflected light). (F) Lillianite prismatic inclusions within chalcopyrite (sample C-05-C, reflected light). (G) Crystals of Fe-poor sphalerite (Sp-2) with chalcopyrite disease (sample VE-60-V, reflected light). (H) Scheelite and clinozoisite from the retrograde skarn assemblages (sample VW-158-AB, transmitted light, plane polarized light). (I) Zoned, subhedral crystals of Fe-poor sphalerite (Sp-2) with calcite, from late vein mineralization (sample V-129-G, transmitted light). Mineral abbreviations: Amp: amphibole (actinolite); Apy: arsenopyrite; Cal: calcite; Chl: chlorite; Clz: clinozoisite; Cpx: clinopyroxene (diopside); Cpy: chalcopyrite; Gn: galena; Grt: garnet (andradite); KF: K-feldspar (adularia); Llt: lillianite; Po: pyrrhotite; Py: pyrite; Sch: scheelite; Sph: sphalerite. 

Figure 4 SEM-BSE (secondary electron microscopy in back scattering electrons image) of mineral assemblages of the Velardeña ore deposits. (A) Euhedral andradite crystals with concentric zoning, clinopyroxene inclusions, and interstitial calcite (sample C-23-N). (B) Relic andradite replaced by a quartz-calcite assemblage (sample V-122-F). (C) Actinolite acicular crystals within a sphalerite groundmass (sample V-26-B). (D) Pyrite-arsenopyrite paragenesis followed by a late (supergene), replacive siderite-marcasite assemblage (sample V-88-J). (E) Sulfide paragenesis of galena, pyrite, arsenopyrite, and marcasite, (sample C-04-F). (F) Assemblage of pyrite, arsenopyrite, and galena crystals; galena is located in fractures and grains boundaries (sample C-04-F). Mineral abbreviations: Amp: amphibole (actinolite); Apy: arsenopyrite; Cal: calcite; Cpx: clinopyroxene (diopside); Gn: galena; Grt: garnet (andradite); Mc: marcasite; Py: pyrite; Qz: quartz; Sd: siderite; Sph: sphalerite. 

Figure 5 (A) Core sample showing epithermal vein of calcite (white) and fluorite (purple gray) cutting the skarn assemblage from the Santa Maria deposit (sample VW-158 AB). (B) Epithermal vein inside the Santa Maria mine, where several generations of growth are exhibited. (C) Photomicrograph of the sphalerite crystals in calcite vein (sample V-129-G, transmitted light, plane polarized light). Cal: calcite; Sph: sphalerite. 

Figure 6 Comparative paragenetic tables of the Velardeña skarn and vein deposits. Left: Santa Maria deposit. Right: Reyna del Cobre deposit. Symbols: quartz: circles; calcite: squares; amphibole: triangles; garnet: hexagon; sphalerite: diamond. All colored symbols are related to temperatures in Figures 10 and 12. Black bars thickness corresponds to relative abundance of minerals. 

During the prograde stage calcic clinopyroxene and garnet formed in the exoskarn; clinopyroxene usually occurs as inclusions in poikilitic garnet crystals, both in Reyna de Cobre and in Santa María (figures 3a, 3b, and 4a), which implies that garnet formed relatively later than clinopyroxene (Figure 4a). Garnet crystals are euhedral with concentric zoning and anomalous anisotropy, and rarely exceed 3 mm in diameter, whereas clinopyroxenes occur as short prismatic crystals, up to 300 μm in length. Wollastonite, which is particularly abundant in the endoskarn, occurs as veinlet infills and pseudomorphs after plagioclase, and also occurs with quartz and calcite.

During the retrograde stage hydrous silicates (calcic amphibole, epidote and chlorite), sulfides (pyrite, arsenopyrite, sphalerite, galena, chalcopyrite, and pyrrhotite) and sulfosalts (lillianite, Pb₃Bi₂S₆; and boulangerite, Pb₅Sb₄S₁₁), as well as scheelite, hematite, quartz and calcite were deposited (figures 3c to 3h). Such assemblages occur interstitially with respect to clinopyroxene and garnet and may pseudomorphically replace the latter (Figure 4b).

Calcic amphibole occurs mostly as acicular crystals, up to 1 mm in length, which are generally included in sphalerite and galena grains (Figure 4c), as single crystals, or forming radial aggregates. Epidote forms prisms up to 700 μm long (Figure 3h) and commonly replaces calcic garnets in the exoskarn and plagioclase phenocrysts in the endoskarn. Chlorite is found as radial aggregates, up to ~50 μm in diameter, replacing calcic garnets and filling veinlets (Figure 3b). Sphalerite and galena are the most abundant sulfides. Sphalerite occurs through all the mineralized zones. In veins, sphalerite develops anhedral crystals, up to a few centimeters across, with curvilinear equilibrium grain boundaries. In the massive or semimasive mineralization it forms mosaic aggregates and intergrowths with galena, pyrrhotite, pyrite and chalcopyrite, which can be interpreted as a cogenetic association (^{Gilmer et al., 1988}). Galena is generally associated with sphalerite and commonly has boulangerite intergrowths. Galena occurs in all the mineralized zones. At the microscopic level, galena veinlets crosscut pyrite, sphalerite, chalcopyrite and pyrrhotite.

Pyrite and pyrrhotite are widespread in the massive ores, occurring in a large variety of textures and grain sizes. The grain size of pyrite is up to 0.5 mm, while pyrrhotite is up to 200 µm. In the exoskarn, pyrite occurs as coarse-grained aggregates of euhedral crystals with interstitial calcite, chlorite and quartz in cogenetic association with sphalerite and minor galena (Figures 3d and 3e). In the endoskarn pyrite occurs as pseudomorphs after plagioclase phenocrysts (Figure 4d).

The final hydrothermal stage produced the vein and breccia mineralization that crosscut the previous skarn paragenesis (Figure 5b). This late mineralization is also complex, being the product of a succession of events, which resulted in multiple vein generations and in a variety of structures as brecciation, open-filling texture and crustiform banding. Veins are mostly filled by calcite and quartz, with fluorite and adularia also abundant (Figure 5a). Quartz occurs both as microcrystalline anhedral aggregates and as chalcedony, whereas calcite commonly forms mosaic aggregates. Adularia develops rhomb-shaped crystals, up to 1 mm across, and occurs disseminated within the calcite aggregates, associated with quartz. Veins hosted in porphyritic rocks show alteration halos with sericite, chlorite and chabazite (Figure 3c).

Some of the veins are rich in fluorite, which occurs as monomineralic bands alternating with quartz. A pale, Fe-poor variety of sphalerite is also characteristic of vein mineralization and forms two different open-filling textures (Figure 5c): (a) subhedral crystals, up to 1 cm across with concentric zoning (Figure 3i), and (b) botryoidal aggregates. Other ore minerals that are present in veins and breccias are arsenopyrite, pyrite and marcasite, in addition to subordinate galena and chalcopyrite.

4.2. Mineral chemistry

4.2.1. Calcic amphibole

The Fe content in the amphiboles ranges from 0.844 to 0.745 atoms per formula unit (apfu), while Mg varies between 4.048 and 4.072 apfu (Table 2). According to the classification of ^{Leake et al. (2004)}, all the analyzed crystals correspond to actinolite (Figure 7a).

Additionally, according to the latest nomenclature of the amphibole group of ^{Hawthorne et al. (2012)}, the crystals plot between the cummingtonite and actinolite fields, because they are depleted in manganese (0.04 to 0.055 apfu; Table 2 and Figure 7b).

Figure 7 Composition diagrams of silicates (electron-microprobe data) from the Velardeña skarns. (a) Amphibole plot of ^{Leake et al. (2004)}. (b) Magnesium-iron-manganese amphiboles classification of calcium amphiboles of ^{Hawthorne et al. (2012)}. (c) Mg vs. Fe + Mn correlation in calcic clinopyroxenes. (d) Calcic clinopyroxenes plotted on johannsenite (Jh)-diopside (Di)-hedenbergite (Hd) triangular diagram. (e) Calcic garnets plotted on a pyralspite (Pyr)-grossular (Gr)-andradite (Ad) triangular diagram; dashed area indicates the composition range for Zn skarn deposits worldwide compiled by ^{Meinert et al. (2005)}. (f) K-feldspar (adularia) plotted on orthoclase-albite-anorthite triangular diagram. Black diamonds correspond to Reyna de Cobre samples, while empty diamonds correspond to Santa María samples; apfu: atoms per formula unit. 

Table 2 Chemical composition and structural formulas of calcic amphiboles from the Velardeña skarn deposits (sample V-26-B, Table 1). 

		#1	#2	#3
SiO2	wt.%	57.48	58.72	57.96
Al2O3		0.71	0.80	0.95
TiO2	.	0.03	0.02	0.00
CaO	.	13.90	13.02	13.03
Na2O	.	0.15	0.17	0.17
K2O	.	0.14	0.12	0.17
MnO	.	0.47	0.45	0.38
FeO	.	6.81	6.54	7.37
MgO	.	19.70	20.05	19.82
NiO	.	0.00	0.00	0.01
Cr2O3	.	0.00	0.00	0.00
H2O*	.	2.17	2.20	2.19
Total	.	101.54	102.08	102.04
Si	apfu	7.92	8.00	7.94
Al	.	0.12	0.13	0.15
Ti	.	0.00	0.00	0.00
Ca	.	2.05	1.90	1.91
Na	.	0.04	0.04	0.04
K	.	0.02	0.02	0.03
Mn2+	.	0.05	0.05	0.04
Fe2+	.	0.79	0.75	0.84
Mg	.	4.05	4.07	4.05
Ni	.	0.00	0.00	0.00
Cr	.	0.00	0.00	0.00

Apfu: atoms per formula unit. H₂O based on structural formula 22 O, 2 OH

4.2.4. Sulfides

The ore assemblage of Velardeña essentially consists of sphalerite, galena, chalcopyrite, pyrite, arsenopyrite, pyrrhotite, and Bi- and Sb-sulfosalts (figures 3, 4d, and 4e).

Pyrrhotite EMP analyses yield a chemical composition close to Fe₇S₉ (ca. 44.4 atom % of Fe; 55.6 atom % of S). Such composition corresponds to the monoclinic polytype of pyrrhotite (cf. ^{Lusk et al., 1993}).

Arsenopyrite forms up to 0.5 mm long euhedral crystals. Chemically, the crystals approximate the ideal formula of pure arsenopyrite, FeA_sS, with 32.6 to 33.4 atom % of As.

The galena composition matches that of pure galena, PbS, although the analyses revealed a silver contents ranging between 0.40 and 0.21 wt.% (figures 3e, 4e, and 4f).

Two modes of occurrence of sphalerite, corresponding to different stages of the mineralization, can be recognized (Figure 8): (a) Sp-1, associated with calc-silicates, corresponding to the marmatite (dark) variety; and (b) Sp-2, in calcite-fluorite veins and breccias, corresponding to the pale variety. In both cases, sphalerite is intergrown with galena, and Sp-2 has “chalcopyrite disease” texture. Chemically, there are significant differences between Sp-1 and Sp-2, particularly in the iron contents (Figure 8b; Table 4). Iron shows a wide range of variation in the crystals (from 2.69 to 12.61 wt.% Fe), so that the highest concentrations (16-22 mol % FeS) correspond to sphalerite that is associated with garnet skarns (Sp-1), whereas sphalerite from veins (Sp-2) is iron-poor (4-6 mol % FeS), which is in agreement with the two varieties that were optically identified. In contrast, other elements such as cadmium (<0.223 atom % of Cd) and manganese (0.136-2.219 atom % of Mn) are in low concentrations and do not show significant variations between the two sphalerite types.

Figure 8 Composition diagrams of all sphalerite crystals (n = 67, electron microprobe data) from the Velardeña skarns and veins. (a) Zn vs. Fe + Cd + Mn plot, where 2 sphalerite types are plotted. (b) Histogram of iron contents, in X% FeS. 

Table 4 Chemical composition and structural formulas of selected sphalerite from the Velardeña skarn and vein deposits (#1 to #2 from sample V-39-A; #3 to #6 from V-18-E; #7 to #9 from V-129-G; 10 to 12 from VE-02-AD, Table 1). 

		Sp-1	Sp-1	Sp-1	Sp-1	Sp-1	Sp-1	Sp-2	Sp-2	Sp-2	Sp-1	Sp-1	Sp-1
		#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	#11	#12
S	wt. %	33.89	33.58	33.79	33.78	33.62	33.61	33.05	33.6	33.62	33.92	33.47	33.78
Mn		0.75	0.72	0.24	0.21	0.21	0.18	2.46	0.49	0.56	0.44	0.39	0.38
Cu		0	0.02	0.02	0.07	0.07	0.02	0	0.06	0.05	0.01	0	0.02
As		0.019	0.002	0.05	0	0.03	0.02	0	0	0	0	0	0.01
Bi		0.02	0	0	0	0.02	0.03	0	0	0.07	0	0.14	0
Fe		12.08	12.06	11.43	11.65	11.68	11.59	2.77	3.16	3.08	11.4	11.3	11.41
Cd		0.08	0.04	0.06	0.09	0.03	0	0.52	0.23	0.33	0.14	0.06	0.01
Zn		53.081	53.242	54.2	53.9	53.9	54.4	61.16	62.6	62.47	54.239	54.31	54.2
Sn		0	0	0	0	0	0.02	0.06	0.04	0.09	0	0	0
Ag		0	0	0	0	0	0	0	0	0	0	0	0
Sb		0.01	0	0	0	0	0	0	0	0	0.02	0.03	0.01
Pb		0	0	0	0.05	0.09	0.06	0.017	0	0	0	0	0
Total		99.93	99.66	99.81	99.75	99.65	99.92	100.03	100.16	100.26	100.17	99.69	99.82
S	atom %	50.34	50.09	50.34	50.36	50.2	50.11	49.9	50.53	50.57	50.36	50.06	50.31
Mn		0.65	0.63	0.21	0.19	0.18	0.15	2.17	0.43	0.49	0.38	0.34	0.33
Cu		0	0.02	0.01	0.05	0.05	0.01	0	0.04	0.04	0	0	0.01
As		0.01	0	0.03	0	0.02	0.01	0	0	0	0	0	0.01
Bi		0	0	0	0	0.01	0.01	0	0	0.02	0	0.03	0
Fe		10.3	10.32	9.78	9.97	10	9.92	2.4	2.73	2.66	9.71	9.71	9.76
Cd		0.03	0.01	0.03	0.04	0.01	0	0.22	0.1	0.14	0.06	0.02	0
Zn		38.65	38.93	39.6	39.39	39.5	39.75	45.28	46.16	46.06	39.47	39.82	39.57
Sn		0	0	0	0	0	0.01	0.02	0.02	0.04	0	0	0
Ag		0	0	0	0	0	0	0	0	0	0	0	0
Sb		0	0	0	0	0	0	0	0	0	0.01	0.01	0
Pb		0	0	0	0.01	0.02	0.01	0	0	0	0	0	0

The sphalerite geobarometer of ^{Hutchinson and Scott (1981)} has been applied to three analyses (#10, 11, and 12 of Table 4) that were selected because they correspond to Sp-1 crystals and where found in paragenesis with pyrite and pyrrhotite (figures 3d and 3e). The pressure thus obtained is 0.8-0.9 kbar, corresponding to a depth of ~3-4 km.

4.3. Fluid inclusions

The FI have regular morphologies and are distributed following growth zones of crystals, or isolated, ranging in size from 5 to 25 μm. In the skarn assemblages, the FI are aqueous with two phases (liquid + vapor) (Figure 9a). In late vein calcite, some FI are poly-phased (liquid + vapor + solid), the solid being always halite (figures 9b, 9c, and 9d). CO₂ was detected in some FI, particularly in the veins, through the formation of clathrates, which implies that this gas is above 3.7 wt.% (cf. ^{Hedenquist and Henley, 1985}).

Figure 9 Photomicrographs of fluid inclusions from the Velardeña skarns and veins. (A) liquid + vapor fluid inclusions in quartz (sample C-07-M). (B) FIA (fluid inclusion association) in sample RC-N9 # 5; (C) Fluid inclusion in calcite with liquid (L) + vapor (V) + solid (S) (halite) (sample V-81-S). (D) Fluid inclusion in calcite with liquid + vapor + solid (halite) (sample RC-N9 # 5); Qz: quartz. 

The microthermometric data are summarized in Table 5 and plot on a T_H versus salinity diagram shown in Figure 10.

Figure 10 Diagram showing salinity (wt.% NaCl eq.) vs. homogenization temperatures (TH) of the individual fluid inclusions analyzed in the Velardeña skarn and vein deposits (Table 2). (A) FIAs from The Reyna de Cobre deposit. (B) FIAs from Santa Maria Deposit. The assignment of the analyzed minerals to skarn formation stages is shown in Figures 6 and 12. 

4.3.3. Sulfur isotopes

Sulfur isotopic composition of sulfides is presented in Figure 11 and Table 6. All δ³⁴S values are distributed in a short range, between -1.1 and +2.6‰, except one value of -3.9‰ that corresponds to sphalerite of the late generation (Sp-2), from a vein of the Santa María mineralized area (sample VASM-22 of Table 6).

Table 6 Sulfur isotope data, δ³⁴S (‰), for sulfide minerals from the Velardeña skarn deposits. 

				δ34S(CTD)
Sample	Location	Stage	Classification	Cpy	Gn	Sph	Py
C-O2-D	Reyna de Cobre	prograde	Prograde skarn with Zn-Cu-Pb		0.9		2.2
C-O4-F		vein	Prograde skarn with Zn-Cu-Pb			1.3
C-O4-F		prograde	Prograde skarn with Zn-Cu-Pb				2.6
C-O4-F		vein	Prograde skarn with Zn-Cu-Pb		-0.0
V-18-E	Santa María	retrograde	Calcite vein			0.7
V-20-D		prograde	Massive mineralization	0.8
V-20-D		prograde	Massive mineralization		-0.9
V-26-B		prograde	Prograde skarn with Zn-Cu-Pb			-1.1
V-39-A		prograde	Massive mineralization		0.9
V-39-A		prograde	Massive mineralization	1.0
V-39-A		prograde	Massive mineralization		-0.5
VE-60-V		vein	Calcite vein			-0.9
V-75-R		prograde	Prograde skarn with Zn-Cu-Pb			0.6
V-75-R		prograde	Prograde skarn with Zn-Cu-Pb	-0.7
V-78-P		prograde	Endoskarn				-0.5
V-88-J		retrograde	Massive mineralization				1.0
V-122-AC		prograde	Endoskarn	0.9
V-129-G		vein	Calcite vein			1.8
VE-200T		vein	Calcite vein				0.7
UASM-22		retrograde	Massive mineralization			-3.9

Key: Cpy, chalcopyrite; Gn, galena; Py, pyrite; Sph, sphalerite.

Figure 11 Histograms showing δ³⁴SCDT (in ‰) values of sulfides from the Velardeña skarns and vein deposits (Table 6). (Top) Values from Reyna de Cobre samples. (Bottom) Values from Santa María samples. Py: pyrite, Sp: sphalerite; Cpy: chalcopyrite; Gn: galena. 

In one sample from Reyna de Cobre (C-04-F of Table 6), pyrite and galena occur apparently in paragenesis (figures 3e, 4e, and 4f) and have been isotopically analyzed, with δ³⁴S of +2.6 and 0.0‰, respectively. The temperature of formation of this mineral pair was calculated with the isotopic geothemometer of ^{Ohmoto and Rye (1979)}, with a result of 346 ±25 °C.

5.1. Type of deposit

The two studied skarn occurrences of Velardeña (Santa María and Reyna de Cobre) are very similar in terms of structure and mineral composition. In both areas, skarns occur along the intrusive contact between stocks (and dikes) and limestones and are largely composed by andradite garnet accompanied by Zn-Pb (Cu) sulfides. These characteristics allow us to categorize them as zinc skarns (^{Einaudi et al., 1981}; ^{Meinert et al., 2005}). Considering (a) the nature of the precursor magmatism (dikes and stocks of porphyritic texture; granitic to granodioritic composition), and (b) that skarns develop as aureoles around the intrusive contact, the studied mineralization can also be characterized as proximal, dike-related skarn deposits (Einaudi et al., 1981).

The calculated pressure for the Velardeña skarns using the sphalerite geothermometer of ^{Hutchinson and Scott (1981)}, 0.8-0.9 kbar, confirms that they formed at hypabyssal depths in the crust (i.e., less than 5 km). Moreover, shallow skarns (<1 kbar) are typically Zn- and Pb-rich, this being the case of Velardeña, whereas deeper skarns (1-3 kbar) are generally Cu-rich (^{Shimazaki, 1975}).

On the other hand, the chemical composition of garnets plotted in a ternary diagram (“pyralspite”-andradite-grossular) fall, for both studied occurrences, in the field of Zn skarn deposits summarized by ^{Meinert et al. (2005)} (Figure 7e).

5.2. Processes and conditions of formation

Skarn deposits form by metasomatism, through a succession of replacive mineralizing events triggered by a magmatic-hydrothermal system, generally including a prograde (high temperature) stage and a subsequent retrograde (lower temperature) stage (e.g.,^{Einaudi et al., 1981}). The prograde stage is dominated by progressive reactions that produce high temperature minerals, typically calcic garnet and clinopyroxene (e.g.,^{Logan, 2000}; ^{Canet et al., 2009}, ^2011a). In the Velardeña skarns, clinopyroxene has a relict character and occurs as inclusions within poikilitic garnet, which indicates that garnet formed after clinopyroxene (Figures 3a and 4a). Two possible reactions leading garnet formation, regardless of the chemical variability of the calc-silicates involved, are (Figure 12):

Figure 12 Temperature vs. O2 fugacity diagram (Ca-Fe-Si-C-O-H system at 500 kbar; after ^{Bowman 1998}) showing suggested general paths for prograde (A, B, and C) and retrograde (D) stages of formation of the Velardeña skarns. Dashed lines: univariant curves at f (CO₂) of 1.5 MPa; solid lines: univariant curves at f (CO₂) of 6 MPa. Mineral abbreviations: Ad: andradite; Cal: calcite; Cpx: clinopyroxene (diopside); Grt: garnet (andradite); Hem: hematite; Hd: hedenbergite; Mag: magnetite; Qz: quartz; Sph: sphalerite; Wo: wollastonite. 

2 hedenbergite + calcite + 0.5 O₂ → andradite + quartz + CO₂ (1)

9 hedenbergite + 2 O₂ → 3 andradite + magnetite + 9 quartz (2)

Reaction (1) implies an increase in O₂ fugacity but is little dependent on temperature and is affected by CO₂ fugacity (^{Bowman, 1998}). Besides, it takes place within a relatively narrow range of high temperatures, of ~450-525 °C, which may shift to ~390-490 °C in low f (CO₂) conditions (Figure 12). On the other hand, reaction (2) has a greater dependence on temperature and the minimum temperature at which it can take place drops down to 335 °C (Bowman, 1998) (Figure 10). According to our FI data, the replacement of clinopyroxene by garnet implies a significant decrease of temperature, taking place at ~375 °C, and the T_H range obtained in garnet (337-376 °C) is well below the temperature range for reaction (1), even in low f (CO₂) conditions (Figure 12a). Therefore, reaction (2) better accounts for the evolution of the prograde skarn. The only drawback of this reaction is the absence of magnetite in the studied paragenesis, although this can be explained by (a) enhanced f (O₂) conditions, falling up to the stability field of hematite (present in the studied mineral assemblages); and/or (b) the low Fe²⁺/Mg ratio in the clinopyroxene crystals, so they are diopside instead of hedenbergite (Figure 7d).

Wollastonite is another mineral of the prograde stage of the Velardeña skarns, occurring both in the endoskarn and in the exoskarn. Its temperature of crystallization strongly depends on CO₂ partial pressure, being of at least 540 °C with high f (CO₂) or about 475 °C low at low f (CO₂) conditions (^{Bowman, 1998}). Therefore, wollastonite may have been the first-appearing mineral phase of the skarn, having formed during or shortly after the intrusion of granitic magmas into the carbonate rocks, through a reaction that involves calcite and quartz and that is independent from f (O₂) (Bowman, 1998; Figure 12a):

calcite + quartz → wollastonite + CO₂ (3)

Our microthermometry analysis revealed that in the skarns of Velardeña calcite formed in a wide range of temperature conditions (corresponding, therefore, to different FIAs), with T_H reaching values as high as ~470 °C (Table 5), just below the univariant line of reaction (3) at low f (CO₂) conditions (Figure 12b). Besides calcite, quartz formed at high temperature (T_H: 390-400 °C), therefore during the prograde stage (Table 5; Figure 10).

Retrograde assemblages in Zn skarn deposits essentially consist of hydrous silicates (commonly epidote, calcic amphiboles, and chlorite) accompanied by quartz and calcite, and by base metal sulfides (e.g.,^{Einaudi et al. 1981}; ^{Canet et al., 2009}, ^2011a). The petrography and SEM study of the Velardeña skarns confirmed the above mineralogy for the retrograde stage, finding also hematite and scheelite, and revealing the pseudomorphic alteration of garnet.

This mineral replacement could be explained by the following reaction (Figure 12):

andradite + 3 CO₂ → 3 quartz + 3 calcite + hematite (4)

This reaction is independent of the f (O₂) and takes place at 390 °C and high f (CO₂) or at 335 °C and low f (CO₂) (^{Bowman, 1998}; Figure 12c); taking into account that calcic garnets in Velardeña formed at temperatures (T_H) down to 337 °C, the later conditions account for the replacement of garnets and, therefore, the temperature boundary between prograde and retrograde stages should be at ~335 °C (figures 10 and 12). It is noteworthy that most T_H values obtained from FI in calcite and quartz are lower than 335 °C, indicating that these minerals were formed in large amounts when the temperature dropped below this limit (Figure 10). Regarding the sulfides, iron-rich sphalerite (Sp-1) was deposited around this temperature limit, according to the T_H measurements (average: 345 °C; Table 5), as well as galena and pyrite, with isotopic (sulfur) equilibrium temperatures of 346 °C.

On the other hand, the lower temperature limit of the retrograde stage could be around 220 °C (Figure 12d), according to the lowest T_H values obtained from calcite other than that from veins (Figure 10).

Accompanying the temperature decline involved in the evolution of the paragenesis of the Velardeña skarns, from over 475 °C to ~220 °C (which comprises the prograde and retrograde stages), there is a general increase in f (O₂) (Figure 12). These changes can be explained by mixing with cooler, oxidizing and dilute water (cf. ^{Hedenquist and Henley, 1985}; ^{Wilkinson, 2001}; ^{Canet et al., 2011b}).

Veins rich in calcite, fluorite, quartz and adularia are found both in Santa María and Reyna de Cobre mineralized areas. These mineralized structures are similar to the typical Mexican low-sulfidation epithermal deposits (cf. ^{Camprubí and Albinson, 2007}) and show conditions of formation that are contrasting with those of skarns, remarkably (a) temperatures (T_H) below 200 °C; (b) high CO₂ pressure, revealed by the formation of clathrates during the freezing of FI; and (c) strong variations in salinity (in FI), from 7 to 23% NaCl eq. (figures 9b, 9c, and 9d). This variation in salinity (FIA “d” in Santa Maria and “c” in Reyna de Cobre), along with the occurrence of adularia indicates that boiling affected the hydrothermal system (Figure 10), probably controlling metal deposition (cf. ^{Canet et al., 2011b}; ^{Cruz-Pérez et al., 2016}). With respect to the isotopic composition of sulfur, differences between skarn and vein mineralizations were found too. The general range of variation of δ³⁴S in sulfides from the skarns, between -1.1 and +2.6‰ (Table 6), unequivocally indicates a magmatic origin for sulfur (^{Ohmoto and Rye, 1979}). ^{Gilmer et al. (1988)} reported δ³⁴S values from -3.5‰ to 2.5‰ from Santa María, roughly overlapping the results in this study.

In Reyna de Cobre, δ³⁴S values follow the order pyrite > sphalerite > galena (Figure 11), which agrees with the different ³⁴S fractionation factors between these minerals and H₂S (^{Ohmoto and Rye, 1979}). In Santa María, where the number of analyses is much higher, no regularity is observed in the distribution of δ³⁴S values per mineral (Figure 11), likely reflecting the effects of differences among stages of mineralization.

On the other hand, the only vein sulfide analysis corresponds to a sphalerite grain (from Santa María) that gave us the lowest δ³⁴S value, of -3.9‰. This ³⁴S depletion relative to the skarn sulfides may account for higher f (O₂) conditions in the vein mineralizing fluid (which sulfate-sulfide fractionation is significant) even starting from an equal total sulfur isotopic composition, which would correspond, in both cases, to a magmatic source (cf. ^{Ohmoto and Rye, 1979}).

Significant differences in ore characteristics, particularly in sphalerite (Fe content, isotopic values and T_H) between the skarn (prograde and retrograde stages) and the epithermal veins suggest an overprinting process. Normal faulting should have allowed veins formed at high crustal levels to overprint the deeper and earlier skarn deposits, resulting in a telescoped mineral system (cf. ^{Sillitoe, 1994}). Moreover, ^{Camprubí (2013)} mentioned several other telescoping processes in the Mexican metallogenetic history, not only for skarns (e.g., Cosalá, Sinaloa), but also in other epithermal deposits (Ixtacamaxtitlán, Puebla). According to this, telescoped deposits in the Mexican context are typically polymetallic and/or Ag-rich, and some are overprinting skarn and/or porphyry-type ores, whether they are stacked or telescoped.