# Syn-volcanic mineralized skarn vs. syn-metamorphic barren skarn: a clarification for the Central Rhodope

Lison Moynier, Nathan Cogné, Yannick Branquet, Pierre Gautier, Boris

Ivanov, Ianko Gerdjikov, Philippe Boulvais

► To cite this version:

Lison Moynier, Nathan Cogné, Yannick Branquet, Pierre Gautier, Boris Ivanov, et al. Syn-volcanic mineralized skarn vs. syn-metamorphic barren skarn: a clarification for the Central Rhodope. Ore Geology Reviews, 2025, 184, pp.106755. 10.1016/j.oregeorev.2025.106755. insu-05150333v2

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# Syn-volcanic mineralized skarn vs. syn-metamorphic barren skarn: a clarification for the Central Rhodope

Lison Moynier<sup>a, *</sup>, Nathan Cogné<sup>a</sup>, Yannick Branquet<sup>a</sup>, Pierre Gautier<sup>a</sup>, Boris Ivanov<sup>b</sup>, Ianko Gerdjikov<sup>b</sup>, Philippe Boulvais<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Pb-Zn deposits

Barren skarns

Mn-skarns

Carbonate replacement ore bodies

U-Pb dating

Central Rhodope

## ABSTRACT

The concept of mineral system is used to understand the formation of ore deposits at different scales and gives keys for targeting and exploration strategy. Applying this strategy to the Pb-Zn system of the Central Rhodope Region (CRR) helps clarify several ambiguities remaining in current models. In CRR, the economic Pb-Zn deposits occur in the form of Carbonate Replacement Ore Bodies (CROB) associated with ore-bearing faults in spatial relationship with Mn-rich skarns. We focused on the Laki district (Bulgaria), where we performed petrographical, geochronological and structural analyses on both the mineralized and unmineralized mineral assemblages observed in mines and at surface. We described primary barren skarns (dated at ca. 42–41 Ma, U-Pb titanite and apatite), sulphide-poor Mn-skarns (32.4 ± 1.8 Ma, U-Pb epidote), and sulphide-rich CROB (30.8 ± 2.6 Ma, U-Pb apatite). We show that barren skarns formed in an extensional context associated with the Middle Rhodope Detachment activity. Disconnected from this extensional history, the Pb-Zn mineralization formed synchronously with the volcanic activity recorded within the Borovitza caldera. The Mn-skarns mark the peak of volcanic activity whereas the CROB formed in its aftermath. The ore faults are conjugated strike-slip faults associated with a N-S horizontal shortening, which is consistent with previous works realized in the CRR Madan district Altogether, this comprehensive study of the Pb-Zn mineral system of the CRR allows identifying two periods of fluid activity, the first one leading to metamorphic-induced barren skarns (42–41 Ma), and the second one leading to volcanic-induced bi-pulsed Pb-Zn mineralization (32–29 Ma).

## 1. Introduction

Over the past years the concept of mineral systems has been developed to understand and to predict ore deposit formation at different scales, rather than focusing solely on the deposit (e.g. McCuaig et al., 2010; McCuaig and Hronsky, 2014). The aim of this approach is to provide pieces of information on three main topics: (i) fertility, which includes the metal and fluid sources as well as depositional mechanisms; (ii) favorable geodynamics, which control the formation of fluid drains and the fluid driving forces (e.g. heat, pressure gradient), (iii) the lithosphere architecture, which constrains the fluid sources, the pathways and the screens.

Such approach in the metal rich Rhodope massif has never been applied. This Massif has been identified as a metallogenic province of first importance in Europe (Marchev et al., 2005; Menant et al., 2018; Gourcerol et al., 2022), especially the metamorphic terrains of the

Central Region of the Rhodope (CRR). The CRR hosts several Pb-Zn deposits that have been mined in Roman times and again from the 1920s to the present day (Radoslavov, 1920). On the Bulgarian side of the CRR, the Pb-Zn deposits are distributed in three main districts named Madan, Laki, and Davidkovo, in order of economic interest (Fig. 1), and most of the previous studies have focused on the first two. There, the deposits occur in the form of sub-vertical Pb-Zn bearing ore faults (called ore fault later in the text) from which massive Carbonate Replacement Ore Bodies (CROB) formed by replacement of the marble levels within the metamorphic basement (Vassileva et al., 2009). CROB are spatially associated with skarns. These skarns are characterized by their high content in calc-silicate minerals, including or not Mn-rich species such as johannsenite or rhodonite (e.g. Vassileva and Bonev, 2001; Vangelova et al., 2007; Vassileva et al., 2009; Buret, 2012; Vassileva et al., 2020; Hantsche et al., 2021; Milenkov et al., 2022). Previous studies provide precise ages of the Pb-Zn economic mineralization ($^{40}$Ar-$^{39}$Ar, Kaiser-

<footnote><sup>a</sup> Géosciences Rennes, UMR 6118, CNRS Université de Rennes 35042 Rennes, France</footnote>

<footnote><sup>b</sup> Dpt. of Geology, Sofia University "St. Kliment Ohridski", 1504 Sofia, Bulgaria</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: lison.moynier@univ-rennes.fr (L. Moynier).</footnote>
Rohrmeier et al., 2004, Buret et al., 2013), petrographic description of skarns, CROB and ore faults (e.g. Kolkovski et al., 1996; Vangelova et al., 2007; Buret, 2012; Hantsche et al., 2021; Vassileva et al., 2020), data constraining the nature of the fluids involved and the potential sources of metals (e.g. McCoy, 1995; Marchev et al., 2005, Marchev and Moritz 2006; Buret, 2012) and information on the geodynamic and magmatic activity at the time of ore deposition (e.g. Gerdjikov et al., 2010; Kaiser-Rohrmeier et al., 2013; Marchev et al., 2013; Kounov et al., 2020). However, in the light of these studies, several critical issues remain to be clarified to understand the Pb-Zn system at both the local and regional scales. (i) On a local scale, the paragenetic sequence of the skarns is not well established with respect to the nature of calc-silicates. A first problem concerns the inclusion or not of Mn-rich mineral species. This brings ambiguity on the existence of a single generation of skarns variably enriched in Mn (Vangelova et al., 2007; Buret, 2012) or of two generations that must be distinguished between barren skarns and Mn-skarns. A second issue concerns hydrous minerals that are either described as replacing the prograde calc-silicates in a skarn retrograde stage (Vassileva and Bonev, 2001: Buret, 2012; Hantsche et al., 2021), or as synchronous with the prograde minerals (Vangelova et al., 2007). This is of first importance since some studies suggest that the economic sulphides come in close relationship with the retrograde stage of the Mn-skarns (Vassileva and Bonev, 2001; Vassileva et al., 2009; Milenkov et al., 2019; Hantsche et al., 2021). Consequently, the genetic link between the skarns (enriched or not in Mn) and the mineral assemblages bearing Pb-Zn sulphides must be better constrained. (ii) At a larger scale, based on structural studies of Madan ore faults (Atanassov et al., 1972; Manev, 1975a; Dokov and Popov, 1963), the syn-ore tectonic context

was first thought to correspond to N-S shortening (Manev, 1975b). Later, after the identification of several detachments in the Rhodope massif including the Middle Rhodope Detachment on the northern side of the CRR, some authors rather suggested that the ore faults correspond to the last increments of the regional extension (Kaiser-Rohrmeier et al., 2013). Consistently, most studies indicate that the vertical ore faults post-date the normal faults (e.g. Kaiser-Rohrmeier et al., 2004; Marchev and Moritz, 2006) while others argue for the opposite (Buret, 2012). (iii) The analysis of the late Cenozoic tectonic contexts in the CRR leads to the reinvestigation of the magmatic system providing fluids as well as heat necessary for their circulation. Currently, two sources are debated, the syn-extensional partial melting of metamorphic terranes and the volcanic activity occurring within the Borovitza caldera (Singer and Marchev, 2000; Ovtcharova et al., 2003; Kaiser-Rohrmeier et al., 2004, 2013; Marchev et al., 2005; Marchev and Moritz, 2006).

In this study, we aim to address these issues to clarify the mineralization events in the Central Rhodope Region. Focusing on the Pb-Zn Laki district (Fig. 1), we provide new petrographical data on both CROB and mineralized and unmineralized skarns to refine the paragenetic sequences. We performed a structural analysis not only of the ore faults but also of the skarns and CROB. We performed U-Pb dating on each mineral assemblage to provide time constraints on the whole system, including the pre-economic stages. With all the new and pre-existing data we propose a comprehensive mineral system approach for the formation of the skarn-related Pb-Zn deposits of the CRR.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** A) Simplified tectonic map of the Central Rhodope Region (modified after Kounov et al., 2020) with main Pb-Zn ore veins represented in red after Ivanov et al. (2000). B) Detailed map of the Laki district, modified after Ivanov et al. (1986). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
## 2. Geological and metallogenic backgrounds

### 2.1. The Central Rhodope Region (CRR)

The CRR essentially consists of high-grade metamorphic rocks sha-
ped by a series of syn-metamorphic thrusts and extensional detachments
related to the Alpine orogen (e.g., Ivanov et al., 2000; Kounov et al.,
2020). In the Laki area, the main Alpine structure is a regional north-
dipping shear zone, hereafter called the Middle Rhodope Detachment
(MRD). The MRD is interpreted as either a post-orogenic (Ivanov et al.,
2000; Kaiser-Rohrmeier et al., 2013; Kounov et al., 2020) or a syn-
orogenic (Gerdjikov et al., 2010) detachment. Its hangingwall corre-
sponds to the Asenitza unit that is mainly made of marble. The footwall
comprises the Arda unit that hosts the main part of the Pb-Zn deposits
and consists of migmatitic gneiss with subordinate marble layers and
metabasic lenses. In this unit, top-to-north shearing along the low-
dipping detachment produced a sequence of ductile-then-brittle fault
rocks crosscut by steeper north-dipping normal faults. The emplacement
of pegmatite related to near-peak metamorphic conditions is dated at
39.9 ± 2.3 Ma at Govedarnika (U-Pb on titanite; Georgieva et al., 2022)
(Fig. 1). Shearing under decreasing temperatures occurred at ca. 34–36
Ma (Kaiser-Rohrmeier et al., 2004, 2013; Gerdjikov et al., 2010).

The metamorphic units of the CRR are overlain by volcano-
sedimentary rocks that seal the MRD (Fig. 1). The Borovitza caldera,
which is a major E-W elongated volcanic structure of 1150 km², is
located only a few km northeast of Laki (e.g. Dhont et al., 2008; Yanev,
2017; Marchev et al., 2023). Most of the faults controlling the caldera
are E-W striking. Structural analysis of the southeastern border of the
caldera evidenced the role of the NNW striking Sarnitza dextral fault
(Dhont et al., 2008). Volcanic rocks from the caldera have been dated
between 33.5 and 31 Ma with a peak of activity recorded at ca. 32 Ma (e.
g. Marchev et al., 2013), and rhyolitic dykes coming from the caldera
have age of ca. 32.5 Ma (Kaiser-Rohrmeier et al., 2013).

### 2.2. CRR Pb-Zn ore fields

The Madan ore field is located on the Southern part of the Bulgarian
CRR (Fig. 1A). There, the Pb-Zn mineralization corresponds to CROB
developed laterally to vertical ore faults and spatially associated with
Mn-skarns acting as precursors for ore deposition (e.g. Vassileva and
Bonev, 2001; Vassileva et al., 2009; Hantsche et al., 2021). The ore veins
have been described as a conjugated strike-slip system made of dextral
NW-striking faults, and minor sinistral NE-striking faults (Atanassov
et al., 1972; Manev, 1975a; Dokov and Popov, 1963). The mineraliza-
tion has been dated between ca. 31 and 30 Ma ($^{40}$Ar-$^{39}$Ar on muscovite;
Kaiser-Rohrmeier et al., 2004). Pb isotope analyses on galena suggest a
magmatic origin for the mineralizing fluids (e.g. Marchev and Moritz,
2006).

The Laki ore field is located on the Northern edge of the CRR (Fig. 1).
It includes two active mines named Dzhurkovo and Govedarnika, as well
as several others that have ceased operations, such as Chetroka and
Pilevo (Fig. 1B). As in Madan, the economic mineralization corresponds
to CROB and associated ore faults. No detailed structural analysis of the
Laki ore faults exists. They are sub-vertical and have been described
either as ramified veins or as a vein array corresponding to NNE-striking
fault zones (Ivanov et al., 2000). Ore occurrences are mainly hosted
within the Arda unit, although historical maps indicate that some veins
extended into the Asenitza unit (Ivanov et al., 1986, 2000) (Fig. 1).
Other ore veins are hosted within volcanics of the southwestern border
of the Borovitza caldera where they appear to follow the ring faults (i.e.
the Chetroka ore field; Fig. 1B) (Yanev, 2017).

Previous petrographical studies led to the establishment of a para-
genetic sequence of the vertical ore faults (Vangelova et al., 2007; Buret,
2012). Three main phases have been identified: (i) a quartz-pyrite stage,
(ii) a polymetallic stage which is itself subdivided into three sub-stages
based on the dominant sulphides (galena, galena-pyrite-sphalerite,

chalcopyrite-galena-pyrite), and, (iii) a final quartz-carbonate stage. The main gangue mineral accompanying the sulphides is quartz. The latter comes with chlorite, siderite, epidote and hematite that had their maximum development during the final quartz-carbonate stage. The Pb-Zn CROB formed within marble layers can extend up to tens of meters from the ore faults and, given their size, have greater economic interest than ore faults. The CROB mineral assemblage is comparable to that of the ore faults as it comprised galena, sphalerite, pyrite, and chalcopyrite in a quartz-carbonate gangue with additional chlorite, hematite, magnetite, and siderite (Vangelova et al., 2007; Buret, 2012). The fluid circulation induces chloritic and calcic alterations accompanied by the formation of adularia and hematite within the silicate rocks at the contact with the replaced marble (Buret, 2012). This alteration of the silicate rocks is also observed around the ore faults. Finally, the marble layers of the Arda unit locally contain a skarn mineral assemblage including various species of calc-silicates, mainly garnet and pyroxene, among which Mn-rich minerals have been identified (e.g. johannsenite, rhodonite, pyroxmangite; Vangelova et al., 2007). These assemblages are considered as pre-ore (Vangelova et al., 2007; Buret, 2012).

The muscovite grown on galena grains from the Dzhurkovo mine yielded $^{40}$Ar-$^{39}$Ar ages of ca. 29–30 Ma (Kaiser-Rohrmeier et al., 2004). Buret et al. (2013) obtained $^{40}$Ar-$^{39}$Ar ages on hydrothermal K-feldspar coming from different Pb-Zn assemblages that range from 32 to 30 Ma On the other hand, older U-Pb ages on garnet from Dzhurkovo skarns of 39.5 ± 4.6 Ma and 38.56 ± 3.6 Ma suggest a link with the syn- extensional partial melting of the silicate rocks (Vassileva et al., 2020).

The Pb isotope composition of both Dzhurkovo galena and Chetroka pyrite (Marchev and Moritz, 2006), as well as the fluid inclusions analysis on quartz from Dzhurkovo ore faults (Buret, 2012) point to a magmatic origin for the mineralizing fluids. The role of metamorphic fluids in leaching lead from the country rocks is proposed on the basis of the Sr isotope compositions (Marchev and Moritz, 2006).

## 3. Method

We conducted petrographical and structural analysis of barren and
mineralized structures in natural outcrops, old trenches and in under-
ground mines. Based on our observations, we carried out a representa-
tive sampling of the different facies, including host rocks, CROB, and
mineralized or unmineralized skarn assemblages. A total number of ca.
40 samples were collected. As we aimed to provide precise paragenetic
sequences with geochronological constraints, we performed U-Pb dating
on at least one mineral phase of each paragenesis. When possible, we
selected samples for dating analysis at both distance (away from the
mine) and close to (in mine) hydrothermal deposits to constrain the
impact of fluid circulation on obtained dates. Mineralogical determi-
nation, petrographic analysis and “in context” U-Pb dating (LA-ICP-MS)
were carried out on 70 µm-thick polished thin sections. We have not
sampled volcanic of the Borovitza caldera that we consider well dated
and described.

### 3.1. Mineralogy and petrography

Optical microscopic observations were supplemented by SEM anal-
ysis (CMEBA platform, Rennes, France). Additional EPMA was per-
formed on the dated minerals and on phases of the mineralized
parageneses at the IFREMER platform (Cameca SX-100 electron probe
micro-analyzers, Brest, France).

Silicates measurements were performed with a 3 µm diameter beam, a 15 kV acceleration voltage and a beam current of 20 nA. Elements Na, Mg, Si, Al, Ca, K, Ti, Fe, Mn, P, Cr and Ni were analyzed. Apatite measurements were realized with a 5 µm diameter beam, a 15 kV acceleration voltage and a beam current of 20 nA. Elements P, Cl, F, Ca, and Na, Si, Mn, Y, Nd, and S were analyzed. The OH content was estimated by charge balance of F and Cl assuming the halogen site to be full (Piccoli and Candela, 2002). For the sulphides, we analyzed S, Fe, Cu, Ti, Cr, Co,
Ni, As, Si, Zn, Mo, Sb, Pb and Mn elements with a 3 µm diameter beam, a 15 kV acceleration voltage, a beam current of 40nA and a counting time of 20 s on peak. For carbonates, measurements were realized with a 5 µm diameter beam, a 15 kV acceleration voltage and a beam current of 10nA. Elements Sr, Mg, Ba, Si, Ca, Fe and Mn were analyzed. For dated minerals, punctual EPMA was completed by elemental mapping. Detailed analytical conditions are presented in [supplementary material A](#).

## 3.2. U-Pb geochronology

Before U-Pb analyses on apatite, titanite and epidote, the minerals were localized under Zeiss Axio Imager M1 optical microscope. Additional cathodoluminescence was performed for apatite. All U-Pb analyses have been carried out by LA-ICP-MS at the GeOHeLiS analytical platform of Rennes University (France). The number of analyses per sample varies from 15 to 60 spots, depending on the number of mineral grains available on the thin section. The detailed analytical conditions are given in [supplementary material B](#). The reduction of the data was performed on Iolite4 (Paton et al., 2011) using the "VizualAge_U-comPbine" data reduction scheme (DRS; Chew et al., 2014). Dates calculation and Tera-Wasserburg diagram were produced with IsoplotR software (Vermeesch, 2018). All dates are quoted at the 2σ level throughout the text, tables, and figures.

For apatite dating, laser ablation spots were 50 µm in diameter, with 5 Hz repetition rate and 5.2 J/cm² energy during 45 s plus 15 s of blank measurement. Madagascar apatite (Thomson et al., 2012) was used as primary standard while Durango (McDowell et al., 2005) and McClure (Schoene and Bowring, 2006) were used as quality control materials. In IsoplotR, McClure regression was anchored to a common $^{207}$Pb/$^{206}$Pb (Pb$_c$) value of 0.874 (Krestianinov et al., 2021) while Durango was anchored to a value of 0.83763 (Chew et al., 2014 from Stacey and Kramers, 1975 Pb evolution model). For analytical sessions in which trace elements were analyzed in addition to U-Pb, we kept the same ablation parameters but added the NIST612 for trace element primary external standard (Jochum et al., 2011) and used $^{43}$Ca as internal standard. McClure apatite was used as quality control material (Cogné et al., 2024). The data reduction was performed on Iolite4 using the "Trace Elements" DRS. Reduced data were exported with detection limit of Longerich et al. (1996).

For the titanite grains, the ablation spots were 40 µm in diameter. The repetition rate was 5 Hz and the energy 5 J/cm² with a measurement duration of 45 s plus 15 s of blank. OLT1 (Kennedy et al., 2010) was used as primary standard while McClure and FCT titanite (Schoene and Bowring, 2006; Phillips et al., 2022) were used as quality control materials, regressions being anchored to 0.8746 and 0.8444 respectively.

For epidote, LA-ICP-MS ablation spots were of 50 µm diameter, with 5 Hz repetition rate and 5.2 J/cm² energy during 45 s plus 15 s of blank measurement. Tara (Gregory et al., 2007; Peverelli et al., 2021) was used as primary standard while Bona was used as quality control material using its U-Pb age (Von Blackenburg, 1992; Gregory et al., 2007), in accordance with the protocol outlined by Peverelli et al. (2021). Pb$_c$ composition of 0.8339 (Von Blackenburg, 1992) was used to anchor the regression of Bona.

We also attempted to date calcite of with U-Pb method from the different skarns and on both prograde and retrograde paragenesis (see below). However, the low U/Pb$_c$ ratios and/or low count rates yielded unresolvable dates.

## 4. Petrography of skarns and CROB

In this study we focus on the three main mineral assemblages encountered in the Laki mining district: (i) barren skarns; (ii) poorly mineralized Mn-skarns; (iii) CROB associated with ore faults. Based on surface and mine observations, Fig. 2 is a synthetic representation of

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Synthetic cross section showing the spatial relationships between the different skarns and CROB encountered in the Laki ore field. Compiled from surface exposures and from the Dzhurkovo and Govedanika mine galleries. (?) mark that the contacts between CROB and Mn-skarns have not been observed.</figcaption></figure>

those three mineral assemblages with respect to a major vertical ore fault. skarns and CROB are concentrated in the marble layers and at their contacts with surrounding gneiss. Crosscutting and inclusion relationships indicate that barren skarns formed before Mn-skarns and CROB. Clear contacts between CROB and Mn-skarns have not been observed in galleries. Nonetheless, Mn-skarns are always observed in the vicinity of the CROB.

### 4.1. Barren skarns

Barren skarns have been observed both at the surface (Fig. 3A) and in the Dzhurkovo and Govedanika mines (Fig. 3B). They correspond to centimeter-thick unmineralized skarns at the contact between marble and silicate rocks such as gneiss, amphibolite, or pegmatite that are mostly of quartz-feldspath composition with additional biotite and amphibole (Fig. 3). The barren skarns are characterized by a prograde mineral assemblage (–P suffix, Fig. 3) mainly made of anhydrous calc-silicates, and a retrograde mineral assemblage (–R suffix, Fig. 3) containing hydrous minerals and oxides locally observed in fractures (–F suffix, Fig. 3).

The prograde phase of the barren skarns is characterized by crystallization of xenomorphic diopside (Di-P) accompanied by titanite (Ttn-P), apatite (Ap-P), biotite (Bt-P), quartz (Qz-P), few garnet (Gt-P) and the recrystallization of the marble with newly-formed fine-grained calcite (Cal-P) (Fig. 3C). Elongated rhombs of titanite-P align with the foliation (Fig. 3C). Titanite-P grains are locally truncated in a direction parallel to N-S horizontal stretching. Apatite-P appears as isolated rounded grains in the marble. However, a fraction of these apatite grains crystallized with minerals of the prograde skarn which allows to assign apatite-P to this stage (Fig. 3D). The xenomorphic garnet-P, with an andradite-grossular composition, are associated with diopside-P and observed along the contact between silicate rocks and marble. Feldspar is absent from skarns developed at the expense of marble (exo-skarns) but present in endo-skarns (on the silicate rock side).

The skarn retrograde stage is well developed in the endo-skarn. It is marked by the formation of epidote (Ep-R) with automorphic or acicular shapes forming starry aggregates. Epidote-R can be also observed replacing diopside-P. This epidote-R takes dark color and crystallised together with cubic magnetite (Mag-R) parallel to the foliation (Fig. 3E). Magnetite-R is oxidized into hematite (Hem-R). Apatite grains with various shapes, among which some are slightly elongated parallel with the foliation, are associated with the mineral of this retrograde stage of the barren skarn but most of these apatite grains are found within gneiss. Altogether, they appear different than apatite-P and we call them apatite-R (Fig. 2F). The retrograde assemblage is also characterized by the newly crystallization of syn-foliation automorphic amphibole (Amp-R; Fig. 3D) and the alteration of the prograde minerals by calcite (Cal-R).
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** The barren skarns with –P suffix standing for prograde minerals, –R suffix for retrograde minerals and –F suffix for fracture minerals. Mineral abbreviations are explained in the text. A) Barren skarns that formed at the contact between silicate protoliths (gneiss, amphibolite, granitic leucosome and pegmatite) and marble. Within marble, all stronger lithologies, including barren skarns, are boudinaged. B) Gneiss boudins within marble. The dark borders of gneiss boudins are made of barren skarn assemblage. C) Skarn micro-boudin with prograde mineral assemblage formed after gneissic protolith observed within marble. Zoom on the relationships between diopside (Di-P) and titanite (Ttn-P). D) Growth of rounded apatite (Ap-P) with biotite (Bt-P). Skarn retrograde paragenesis in marble protolith marked by amphibole (Amp-R). E) Skarn retrograde paragenesis in gneissic protolith marked epidote (Ep-R) partly replacing Di-P and cubic magnetite (Mag-R) aligned along the foliation. F) Skarn retrograde paragenesis on the gneissic protolith marked by Ep-R, Mag-R and a new generation of apatite (Ap-R). G) Epidote (Ep-F) and pyrite (Py-F) fracture and Py-F along metamorphic foliation. H) Paragenetic sequence of the barren skarns at the contact marble/silicate rocks.</figcaption></figure>
and chlorite (Chl-R).

Finally, barren skarns are affected by fractures corresponding to normal «en echelon» E-W-striking extensional veinlets. The latter formed preferentially on the silicate rock side and are filled by calcite (Cal-F) or epidote-pyrite assemblage (Ep-F and Py-F) (Fig. 3G). Such fractures have also been observed filled by chlorite and crosscutting the titanite-P. Near the fractures, pyrite and epidote locally crystallize along the foliation. The synthetic paragenetic sequence of the barren skarns is presented in Fig. 3H.

## 4.2. Poorly mineralized Mn-skarns

In the Govedarnika mine, pinkish-to-greenish sulphide-poor Mn-skarns replacing marble layers are abundant close to ore faults and CROB. At the outcrop scale, Mn-skarns develop within the marble layers along gneiss, barren skarn boudins, and fractures affecting the marble (Fig. 4A). Mn-skarns are characterized by successive mineral assemblages forming halos that crosscut the metamorphic foliation (Fig. 4B). Locally, boudins of gneissic rocks and barren skarns are partially replaced by Mn-skarn minerals.

At the microscopic scale, the Mn-skarns halos are defined by successive mineral assemblages that progressively replace each other. In the text, the minerals belonging to the Mn-skarn parageneses will be denoted with -K suffix (Fig. 4). The first assemblage comprises quartz (Qz-K) and disseminated acicular epidote forming starry aggregates (Ep-K) (Fig. 4C). When Mn-skarns replace silicate rocks along micro-cracks, quartz-epidote-K is the main mineral assemblage observed. This silicate-dominated assemblage is affected by chloritization (Chl-K) and additional hematite (Hem-K) is observed as product of epidote-K alteration. The second mineral assemblage is made of Mn-rich platy calcite (Cal-K) and Mn-rich silicates such as rhodonite (Rdn-K) and johannsenite (Jhn-K). The platy calcite-K growth preferentially by replacing the quartz-K as shown by the remaining quartz-K fringe around the epidote-K (Fig. 4C). The rhodonite-K grains are fine radiating needles forming disorganized nests in the core of quartz-K (Fig. 4D₁). These rhodonite-K nests grow and expand until they merge together, then completely replacing the quartz-K (Fig. 4D₂). Finally, an amorphous phase made of undetermined reddish oxides invades the rhodonite-K-rich zones (Fig. 4D₃). The johannsenite-K grains are automorphic with acicular and lath habits that grow centrifugally to the preceding fronts (Fig. 4E). It is noteworthy that even if former marble are no longer observed as residue inside the Mn-skarn, some apatite grains with morphologies similar to that of apatite-P of the barren skarn, are preserved within the Mn-skarn matrix.

Disseminated sulphides without economic interest are observed distributed parallel to the Mn-skarn fronts, often along the forefront. These sulphides are pyrite (Py-K), sphalerite with chalcopyrite disease (Sp-K), galena (Ga-K) and chalcopyrite (Cpy-K). Acicular hematite (acicular Hem-K) is associated with the sulphides (Fig. 4F). Petrographic relations suggest that sphalerite-K grains grow first, while the other sulphides develop around them. Galena-K and sphalerite-K grew locally along with cleavages of the platy calcite-K (Fig. 4G). Rhodonite-K needles have been observed growing inside sphalerite-K (Fig. 4H) suggesting on-going crystallization of Mn-rich calc-silicates after sulphide deposition. Interestingly, as shown in the paragenetic sequence (Fig. 4I), these Mn-skarns do not present a typical retrograde stage characterized by late hydrous minerals such as amphibole (Fig. 4I).

## 4.3. Pb-Zn ore faults and carbonate replacement ore bodies (CROB)

Along strike and vertically, the ore faults are not homogenous in thickness, textures and mineral fillings. The hydrothermal mineral assemblage of the ore faults is similar to that of the CROB. When intersecting marble, the ore faults can be meter-thick and controls the formation of CROB laterally (Fig. 5A) that are characterized by a fine zebra-like texture formed of alternating ribbons of quartz and sulphides crosscutting the metamorphic foliation (Fig. 5B). Within gneiss, ore

faults are thinner, composed of multiple segments with slices of gneiss gouges integrated in between thin quartz and sulphides ribbons.

At the outcrop scale, the CROB develops preferentially on the top of marble layers (Fig. 5C). The minerals belonging to the CROB paragenesis are quoted with the -Z suffix. Massive silicification (pre-ore Qz-Z) preceded the marble replacement, and progressed along the contacts of the calcite grains and micro-cracks (Fig. 5D). This silicification was followed by the sulphide-rich phase starting with pyrite (Py-Z). The others sulphides, galena (Ga-Z), sphalerite (Sp-Z with Cpy disease), chalcopyrite (Cpy-Z), and additional oxides (mostly hematite; Hem-Z), grew simultaneously around and inside fractures affecting pyrite-Z crystals (Fig. 5E). The principal gangue mineral accompanying the sulphides is quartz (syn-ore Qz-Z), but additional calcite (Cal-Z), chlorite (Chl-Z), muscovite (Ms-Z) and epidote (Ep-Z) are present.

The CROB developed in the marble but the circulation of hot hydrothermal fluids also affected the surrounding gneissic rocks with a lower replacement rate. The gneiss and the barren skarns, formed at the transition between marble and gneiss, are affected by chloritic (Chl-Z) and calcic (Cal-Z) alterations (Fig. 5F), the former yielding to a thin dark fringe around contacts, boudins rims and ore faults (Fig. 5B). Advanced alteration of gneiss and barren skarns boudins leads to argillization of boudin cores with crystallization of scarce sulphides. Chlorite-Z, along with syn-ore quartz also fills cracks cutting through the silicate rocks. This alteration is accompanied by hematite (Hem-Z) and adularia (Adu-Z). Inside chloritic fringes and halos, rounded apatite grains (Ap-Z) are widespread. The largest apatite-Z grains display fractures (Fig. 5G). The synthetic paragenetic sequence is presented in Fig. 5H.

# 5. Structural analysis

In the Laki ore field, the mineralization crosscuts the low-dipping foliation and replaces the marble layers of the Arda unit. Prior to mineralization, the metamorphic rocks of the Arda unit underwent progressive exhumation beneath the MRD. We thus observe a retrograde evolution starting with partial melting, expressed by the presence of migmatites and pegmatite, to mylonitization and intense boudinage associated with top-to-north shearing. It is noteworthy that this boudinage also affects the barren skarns (Fig. 3 A, B, C) and that some of the skarn minerals align with the metamorphic foliation (e.g. titanite-P; Fig. 3C). The retrograde evolution continues with brittle extensional deformation expressed by N-dipping normal faults (Fig. 6A, B) and ar-rays of «en echelon» extensional veinlets filled with chlorite-F, epidote-F and pyrite-F (Fig. 3G).

On the contrary, the Mn-skarns and CROB show no signs of deformation during or after their formation. The E-W striking normal faults associated with MRD last brittle increments, exhibit cataclastic to gougy core zones and do not include any Pb-Zn bearing clasts. Mine observations show that the normal faults abut against ore faults (Fig. 6A) or are displaced by them. Moreover, CROB are nor crosscut (Fig. 6B) nor modified by the normal faults. Therefore, the mineralization postdates the activity of the MRD, and none of the extensional structures are percolated by mineralizing fluids.

Our observations both at the surface and in the Dzhurkovo and Govedarnika mines allow us to characterize the orientation and kinematics of the ore faults. Most ore fault planes are sub-vertical and present sub-horizontal slickensides of quartz or striation on shear planes affecting galena (Fig. 6C). Their kinematics have been determined by using criteria such as slickenfibers (Fig. 6C), «en echelon» arrangement of extensional veins (Fig. 6 C, D, E) or pull-aparts filled with mineralization (Fig. 6F). Fault measurements show that at the scale of the Laki district, the NNE faults are predominant (Fig. 1). Most of the NNE-striking faults are sinistral while most of the NNW-striking faults are dextral (Fig. 6G). This fault network documents bulk NNE-SSW horizontal shortening during mineralization.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Poorly mineralized Mn-skarn with –K suffix standing for minerals of this paragenesis. A) Development of the Mn-skarn in marble along fractures and foliation-parallel boudins of gneiss transformed in barren skarns (cf. Fig. 3B). B) Hand specimen of Mn-skarn front developing within marble. C) Starry aggregates of epidote (Ep-K) altered by chlorite (Chl-K) and hematite (Hem-K). The platy calcite (Cal-K) progressively replaces the quartz (Qz-K) and Ep-K. D) Mn-skarn front showing progressive Qz-K replacement by rhodonite (Rdn-K) needles. This begins with the growth of Rdn-K nests and ends by the formation of numerous spheroidal nests within an amorphous matrix of oxides (undetermined). E) Successive fronts of acicular johannsenite (Jhn-K). F) Disseminated sulphides assemblage of Mn-skarns. G) Sulphide growth guided by platy Cal-K cleavages. H) Rdn-K needles inside sphalerite (Sp-K). I) Paragenetic sequence of the poorly mineralized Mn-skarns.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Carbonate replacement ore bodies (CROB) with “Z” suffix standing for the minerals of this paragenesis. A) Development of CROB in marble cross cutted by an ore fault. B) Vertical ore fault passing through CROB with irregular convoluted sulphide front and alteration of gneiss/barren skarn boudins. C) Development of a CROB at the top of marble with gneiss/barren skarn layers acting as fluid screen. D) Progression of a silification front (micro pre-ore Qz-Z) front in marble by micro-fracturing and Cal dissolution. E) Relationships between ore minerals in a gangue of quartz. F) Chloritic and calcic front in silicate rocks (gneiss or barren skarn) in contact with CROB. Apatite grains (Ap-Z) are abundant in the chlorite fringe. G) Zoom on Ap-Z in the chloritic fringe with overgrow of pyrite on one apatite grain. H) Paragenetic sequence of CROB deposition.</figcaption></figure>

## 6. U-Pb geochronology and mineral composition

U-Pb dating by LA-ICP-MS has been performed on datable minerals constrained in the paragenetic sequence of skarns and CROB. Minerals

dated are: (i) titanite-P (Fig. 3C), apatite-P (Fig. 3D) and apatite-R (Fig. 3F) from the prograde and the retrograde stages of the barren skarns; (ii) epidote-K from the Mn-skarns (Fig. 4C); (iii) apatite-Z found in the chloritic fringes affecting gneiss around the CROB (Fig. 5 F, G).
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Characteristics and structural analysis of ore faults of the Laki ore field. A) A N-dipping normal fault (orange half-arrow) crosscut by a subvertical NNE-striking ore fault. B) E-W-striking N-dipping normal faults crosscut by a Pb-Zn CROB formed at the interface between marble and gneiss layers. Stereogram (Wulf, lower hemisphere) of the normal fault planes with arrows for striations. C to G: characteristics, orientation and kinematics of ore faults with sinistral (in red) and dextral (in blue) kinematics. In D to F, pairs of half-arrows indicate the sense of shear, but because the views are toward the roof of a mine gallery, the actual kinematics of these strike-slip fault zones is the opposite; for instance, the apparent right-lateral displacement in D corresponds to a sinistral fault. C) Near horizontal quartz slickenfibers on an ore fault indicative of right-lateral displacement. D) Main ore vein within ore fault showing subordinate extensional veins indicative of sinistral sense of shear (roof picture). E) Thin ore fault system in gneiss composed of brittle shear veins (with clasts) associated with «en échelon» arrays of extensional veins indicating a dextral sense of shear (roof picture). F) Sinistral pull-apart along a subordinate mineralized shear vein (roof picture). G) Lower hemisphere, equal-area projection summarizing ore faults orientation and kinematics from the Laki ore field. Shortening and stretching domains were calculated using the Faultkin software (Allmendinger, 2020). Compass measurements were corrected from 4° of magnetic declination. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<caption>**Table 1**</caption>

Summary of the U-Pb ages on apatite-P, apatite-R, apatite-Z, titanite-P and epidote-K.

<table><thead><tr><th>ID</th><th>Age (Ma)</th></tr></thead><tbody><tr><td colspan="2"><strong>U-Pb Apatite</strong></td></tr><tr><td colspan="2"><strong>Apatite P (Barren skarn)</strong></td></tr><tr><td>GVD23_1</td><td>41.2 ± 0.9 Ma | MSWD = 3.3 | n = 26</td></tr><tr><td>GC3c</td><td>40.9 ± 0.8 Ma | MSWD = 0.81 | n = 15/16</td></tr><tr><td>GC4a</td><td>40.9 ± 1.5 Ma | MSWD = 1 | n = 20</td></tr><tr><td>GVD24_3</td><td>41.6 ± 1.1 Ma | MSWD = 4.3 | n = 31</td></tr><tr><td colspan="2"><strong>Apatite R (Barren skarn)</strong></td></tr><tr><td>V1C</td><td>34.8 ± 1.4 Ma | MSWD = 0.96 | n = 27/28</td></tr><tr><td>V3B</td><td>34.0 ± 4.8 Ma | MSWD = 0.90 | n = 45/46</td></tr><tr><td>M2D_gneiss</td><td>34.3 ± 2.6 Ma | MSWD = 0.91 | n = 37</td></tr><tr><td>M3B</td><td>34.1 ± 1.0 Ma | MSWD = 1.4 | n = 50</td></tr><tr><td colspan="2"><strong>Apatite Z (CROB)</strong></td></tr><tr><td>M2D_Chl</td><td>30.8 ± 2.6 Ma | MSWD = 1.1 | n = 15</td></tr><tr><td colspan="2"><strong>U-Pb Titanite</strong></td></tr><tr><td colspan="2"><strong>Titanite P (Barren skarn)</strong></td></tr><tr><td>M6</td><td>41.7 ± 1.4 Ma | MSWD = 1.2 | n = 30</td></tr><tr><td>V3B</td><td>41.7 ± 0.9 Ma | MSWD = 1.1 | n = 26/29</td></tr><tr><td colspan="2"><strong>U-Pb Epidote</strong></td></tr><tr><td colspan="2"><strong>Epidote K (Mn-skarn)</strong></td></tr><tr><td>GVD23_1</td><td>32.4 ± 1.8 Ma | MSWD = 0.76 | n = 30</td></tr></tbody></table>

Vxx samples: 41° 46'39"N; 24° 49'23"E (outside of the mines)

GVDxx and GCxx samples: Govedarnika mine

Mxx samples: Dzhurkovo mine

Results are summarized in Table 1. Raw data of samples and standards are presented in supplementary materials D.

## 6.1. U-Pb on titanite

Barren skarn titanite-P analyzed for U-Pb dating are parallel to the foliation (Fig. 3C). Sample V3B has been taken away from the Pb-Zn mineralization out of mines. In contrast, sample M6 comes from the Dzhurkovo mine, close to a Pb-Zn CROB. Both samples yield the same lower intercept date of 41.7 ± 0.9 Ma for V3B and 41.7 ± 1.4 Ma for M6 (Fig. 7A). The SEM-BSE images show that titanite-P are homogeneous (Fig. 7 B, C).

## 6.2. U-Pb and chemical composition of apatite

All apatite types have a fluoro-apatite composition and are chemically homogeneous without distinct zoning (Fig. 8). Apatite-P from prograde stage of barren skarn reworked by Mn-skarns show no enrichment in Mn and no chemical zonations (Fig. 8B). EPMA analyses show no difference between the apatite-R and the apatite-Z for major element composition (Fig. 8A). However, in cathodoluminescence, they have different colors denoting different trace element contents (Fig. 8C, D). It is noteworthy that both apatite-R and apatite-Z were observed in the same thin section (M2D; Fig. 5F).

For the first group (apatite-P; Fig. 3D), apatite grains were analyzed from four samples of the Govedarnika mine. Apatite-P of three of the dated samples (GVD24_3, GVD23_1 and GC3c) are found in marble replaced by Mn-skarn. Apatite-P from the last sample (GC4a) are found

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** A) Titanite U-Pb data represented in Tera-Wasserburg diagram. The color of the ellipses refers to the name of the sample. White ellipses are not considered for age calculation. Mineral composition of titanite (EPMA) is available in supplementary material C. Detailed U-Pb data are available in supplementay material D. Individual Terra-Wasserburg plots are presented in supplementary material E. B and C) SEM-BSE image of Ttn-P from samples V3B and M6 respectively.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** A) Laki ore field apatite composition (EPMA; raw data are presented in supplementary material C) in ternary diagram F/OH/C. B) EPMA-BSE image and Mn-mapping of apatite-P. C) Cathodoluminescence and EPMA-BSE image of Ap-R. D) Cathodoluminescence and EPMA-BSE image of apatite-Z.</figcaption></figure>

in a non-replaced marble. However, all the apatite-P gave comparable lower intercept dates (Fig. 9A): 41.6 ± 1.1 Ma (GVD24_3), 40.9 ± 0.8 Ma (GC3c), 41.2 ± 0.9 Ma (GVD23_1) and 40.9 ± 1.5 Ma (GC4a). Two of these samples, GVD24_3 and GVD23_1, show a relatively high MSWD likely due to a perturbation of the U-Pb system.

The apatite of the second group (apatite-R: Fig. 3F) have been analyzed from four different samples: V1C (34.8 ± 1.4 Ma) and V3B (36.4 ± 2.1 Ma) come from a surface outcrop far from the Pb-Zn mineralization, and, M3B (34.1 ± 1.0 Ma) and M2D_gneiss (34.3 ± 2.6 Ma) come from the Dzhurkovo mine. All the samples show homogeneous U-Pb dates at the 2σ uncertainty level (Fig. 9B).

The third apatite group (Ap-Z; Fig. 5F, G) was analyzed on sample M2D_Chl at ca. 1 cm away from the M2D_gneiss apatite-R (i.e. on the same thin section). The apatite-Z are found in the chloritic alteration fringe of a gneiss boudin in contact with CROB. The M2D_Chl date of 30.8 ± 2.6 Ma seems slightly younger than the apatite-R, although similar at 2σ uncertainty level (Fig. 9C). However, on the Tera-Wasserburg diagram (Fig. 9C), the ellipses of M2D_Chl plot closer to the lower intercept than the ones of M2D_gneiss (Fig. 9B), denoting a higher U content for M2D_Chl apatite grains. Moreover, the upper intercept of the regression with the y-axis gives a common Pb composition for M2D_Chl of 207Pb/206Pb = 0.769 ± 0.026, significantly lower than the apatite-R values (ca. 0.835).

The U-Pb dataset of the three apatite groups was completed by trace element analyses performed on the apatite grains coming from samples

GVD24_3 and GC3 for the apatite-P group, samples M3B, M2D_gneiss, and V3B for the apatite-R group, and sample M2D_Chl for the apatite-Z group (corresponding ellipses are represented in color in Fig. 9). Apatite-P, as well as being distinguished by their older U-Pb dates, show clearly different Sr-REE-Y spectra compared to the two other groups (Fig. 9D). The ca. 41 Ma apatite-P is ca. 100x depleted in the REE and 10x in Sr than apatite-R and apatite-Z. In apatite-P, Sr is enriched compared to LREE while it is depleted in other apatite groups. Also, apatite-P displays a negative Ce anomaly (average Ce/Ce* = 0.6 with the formula Ce/Ce* = CeN / 2PrN – NdN) that is not observed in other apatite groups. Sr-REE-Y spectra of apatite-R and apatite-Z are similar. However, apatite-Z displays a higher negative HREE slope (GdN/LuN = 9.44), than apatite-R (GdN/LuN = 3.56).

### 6.3. U-Pb on epidote

Only one sample yielded a resolvable lower intercept age for epidote U-Pb dating. Acicular epidote-K from the Govedarnika mine, that crystallized with the first mineral front of Mn-skarn (Fig. 4C), provide a date of 32.4 ± 1.8 Ma (sample GVD23_1; Fig. 10A). The SEM-BSE images illustrate that the dated epidote grains are homogeneous (Fig. 10B). Despite their crystallization within a Mn-rich paragenesis, epidote-K do not incorporate large amount of Mn (see Mn-mapping image in supplementary material C).
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Apatite U-Pb data represented in Tera-Wasserburg diagram. All the individual diagrams are available in supplementary material F. Colored ellipses indicate that Sr-REE-Y data are available for the same analysis, reported in D. Grey ellipses indicate that only U-Pb data are available for the analysis. For samples with colored and grey ellipses, the lower intercept calculation takes them all into account. Only white ellipses are not considered in age calculations. A) Ap-P from the prograde barren skarns. B) Ap-R from the retrograde stage of barren skarns. The color of the concordia curve for a given sample refers to the color of the discordia and ellipses of this sample. C) Ap-Z from the CROB chloritic fringes. D) Sr-REE-Y spectra obtained after normalization to chondrite composition (McDonough and Sun, 1995). Sr-REE-Y raw data are presented in supplementary material G.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** A) Epidote U-Pb data represented in Tera-Wasserburg diagram. B) SEM-BSE image illustrating the textural position of dated epidote-K and the absence of chemical zonation. The composition in major elements (EPMA) of the dated epidote and an EPMA Mn-map are given in supplementary material C.</figcaption></figure>

## 6.4. EPMA of sulphide-bearing paragenesis

During the replacement of marble by Mn-skarns and CROB, mineral compositions are influenced by their parent fluid compositions. Fig. 11 shows that gangue and sulphide minerals of Mn-skarns are systemically enriched in Mn with respect to the CROB and the marble protolith minerals. Calcite-K are x100 richer in Mn than calcite from marble and x10 than calcite-Z. Sphalerite-K are x10 richer in Mn than sphalerite-Z. The diagram shows that for calcite the Fe content does not vary accordingly to their parageneses. Mn-skarn sphalerite-K are slightly enriched in Fe compared to CROB ones.

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Mn vs. Fe composition (EPMA) of calcite (dot) and sphalerite (square) depending on the paragenesis (CROB in green, Mn-skarn in pink, and protolithic marble in blue). Logarithmic axes with weight % oxide for calcite and weight % element for sphalerite. Raw data are presented in supplementary material C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

# 7. Discussion

## 7.1. Significance of U-Pb dates

### 7.1.1. Titanite

The U-Pb dating on titanite-P grains coming from a sample located away from the Pb-Zn mineralization yields a date of 41.7 ± 0.9 Ma (V3B; Fig. 7). Titanite-P grains coming from a mine sample, located closer to the mineralization yield a comparable date of 41.7 ± 1.4 Ma (M6; Fig. 7). This demonstrates that neither regional heat source nor regional fluid flow post-dating ca. 42 Ma did disturb the titanite-P U-Pb system. The titanite-P crystallized during the prograde phase of the barren skarn because some grains are observed inside diopside-P and biotite-P indicating co-crystallization of the minerals (Fig. 3C). We thus argue that the age of ca. 42 Ma obtained on titanite-P corresponds to their crystallization during the prograde phase of the barren skarns. This is consistent with the two ages of 39.5 ± 4.6 Ma and 38.56 ± 3.6 Ma obtained by U-Pb on garnet grains coming from skarns of the Dzhurkovo mine (Fig. 12; Vassileva et al., 2020).

### 7.1.2. Apatite

Three groups of fluoro-apatite (apatite-P, apatite-R and apatite-Z; Fig. 8) distinguishable by their morphologies and host rock have been identified. U-Pb dating and Sr-REE-Y chemistry highlight their different origins (Fig. 9).

The dating of apatite-P was performed on four different samples that all yielded comparable ages ranging from 41.6 ± 1.1 Ma to 40.9 ± 1.5 Ma (Fig. 9A, 12). However, two samples show an important dispersion (sample GVD23_1 and GVD24_3, Fig. 9A). Such dispersion is likely indicative of perturbations of the U-Pb system probably by later fluid flows, for instance during the formation of the Mn-skarn. However, BSE-EPMA image of apatite-P do not show any chemical zonation or alteration and Mn EPMA mapping showed that apatite-P are Mn-poor (Fig. 8B). This suggests that the apatite-P did not recrystallize even if the surrounding marble have been completely replaced. We assume that, despite of individual isotopic perturbations, the discordia dates have a geological significance and thus, at the district scale the ca. 41 Ma age is significant. Most of apatite-P grains are found in marble close to the margins of barren skarn lenses or boudins, but some co-crystallized with
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Compilation of available geochronological data for the Laki and Madan area (with 2σ errors). Data from this study are represented in bold with larger symbols. (Se = Sericite).</figcaption></figure>

prograde biotite-P (Fig. 3C, D). Thus, as apatite-P and titanite-P dating yields similar ages, we argue that the U-Pb dates refer to the crystallization age of apatite-P during the prograde phase of the barren skarn (Fig. 12). The apatite-P grains are surprisingly depleted in REE, especially compared to the apatite-R and apatite-Z (Fig. 9D). This difference seems to reflect the lithology of their host rock (Ismail et al., 2014). Apatite-P crystallizes in marble that are not suitable for incorporating REE (O'Sullivan et al., 2020), and that have likely induced Ce negative anomaly, the latter being uncommon for apatite (Ochiai et al., 1993). Moreover, it has been shown that prograde skarn minerals have low REE concentrations compared to magmatic or metamorphic equivalent minerals (Ochiai et al., 1993). It has also been demonstrated that garnet, titanite, feldspath, and epidote act as REE pumps (e.g. Gromet and

Silver, 1983; Henrichs et al., 2018). Thus, both the depletion in REE and the Ce anomaly are in accordance with an apatite-P crystallization in marble synchronously with prograde barren skarn assemblage at ca. 42–41 Ma.

The dating of apatite-R was also realized on four samples. The obtained dates range from 34.0 ± 4.8 Ma to 34.8 ± 1.4 Ma. These dates may correspond to either the neo-crystallization of the apatite-R at ca. 34.5 Ma, cooling ages or the resetting of gneissic protolithic apatites under the effect of later hydrothermal fluid circulations. Cathodoluminescence images of apatite-R show no zonation or corrosion gulf typical of recrystallized or dissolved apatite (Fig. 8C). In addition, the effect of Pb-Zn bearing hydrothermal fluids on the U-Pb system was tested by the sampling strategy as both distant (V3B and V1C) and close
to the Pb-Zn mineralization (M2D gneiss and M3B) apatite-R have been dated. There are no chemical nor geochronological arguments indicating that this fluid circulation impacted the apatite-R (Figs. 8 and 9B, D). Petrographical observations show that apatite-R grains are located in gneiss and more or less elongated in the foliation planes. Some of them crystallized in association with epidote-R during the retrogression of barren skarns (Fig. 3F). Apatite-R are x100 richer in REE and 10x in Sr than apatite-P, and their spectra do not display the Ce negative anomaly (Fig. 9D). The differences in Sr-REE-Y spectra could come from the nature of the host rock of apatite. Apatite-R did not crystallize in marble but in gneissic protoliths (Fig. 2F) that have higher REE content than marble. As retrograde skarn stage is classically associated with a fluid influx that triggers hydration and alteration of the prograde skarn minerals, REEs might have been leached and incorporated within growing apatite-R. Therefore, apatite-R was likely newly crystallized during the retrograde stage of barren skarns at ca. 34.5 Ma.

The last type of apatite (apatite-Z) is observed within the chloritic fringe that formed along gneiss/marble contacts and around gneiss boudins in association with the CROB formation (Fig. 5F). U-Pb dating yielded a date of 30.8 ± 2.6 Ma, which is similar to the one of apatite-R at 2σ uncertainty level. Apatite-R and apatite-Z are both encountered within gneissic protoliths. However, apatite-Z have a lower common Pb composition and more uranium than apatite-R (Fig. 9C). With respect to apatite-R, apatite-Z presents a more pronounced HREE negative slope despite of a similar Sr-LREE spectra (Fig. 9D). These differences are visible on a centimetric scale in sample M2D (Fig. 5F). Moreover, petrographic observations show that apatite-Z can be overgrowth by sulphides and that the largest grains present fractures (Fig. 5G). This suggests that the parent-fluids of CROB induced the recrystallization of the apatite-R into apatite-Z within the chloritic fringes at ca. 31 Ma. These fluids might have leached preferentially LREEs rather than LREEs. Therefore, the U-Pb date on apatite-Z corresponds to the age of the CROB formation. This is in accordance with the ca. 31–29 Ma ages obtained by $^{40}$Ar-$^{39}$Ar on muscovite from the CROB (Kaiser-Rohrmeier et al., 2004).

### 7.1.3. Epidote

In the Mn-skarns, based on petrographical observations of a dozen samples, we identified four successive mineral assemblages (Fig. 4 C, D, E). The mineral assemblages represent successive fronts replacing each other following a sequence that is always the same (Fig. 4I). However, one assemblage of the sequence may miss. In such a metasomatic system, the various advancing fronts are formed from the same fluid, that evolves locally at different velocities as a result of dissolution-crystallization processes (e.g. Zharikov et al., 2006; Putnis and Autshreim, 2010). These local evolutions might likely induce the lack of a given mineral assemblage. Therefore, even if local variations may exist, the paragenesis proposed in Fig. 4I is consistent at the scale of the Govedarnika mine. We dated the epidote-K grains that are part of the first mineral assemblage of the Mn-skarns (Fig. 4C, 10B). The growth of rhodonite-K needles into sphalerite-K (Fig. 4H), as well as the growth of sulphides within platy calcite-K cleavages (Fig. 4G) indicate that all the minerals constituting the Mn-skarn crystallized sub-synchronously. Thus, the ca. 32 Ma date obtained by U-Pb dating of epidote-K (Fig. 10A) is the age of the marble replacement by the Mn-skarns, and therefore the age of the sulphides deposition together with those Mn-rich minerals.

## 7.2. Distinction of the three mineral assemblages

### 7.2.1. Differentiating barren skarns from Mn-skarns

Based on our observations in the Laki district we distinguish a Mn-free barren skarn (Fig. 3) from a later distal Mn-rich skarn (Fig. 4). The barren skarns are boudinaged with some minerals crystallized parallel to the foliation (Fig. 3A, B, C), whereas the Mn-skarns show no signs of deformation and cross-cut the metamorphic foliation (Fig. 4A).

This reflects different strain regimes during crystallization of both skarns. Barren skarns are coarse-grained (Fig. 4C) whereas the Mn-rich skarns minerals are micrometric in size with fine acicular habitus (Fig. 4D, E). Moreover, mineralogical and chemical differences suggest that an external fluid input is required for the formation of the calc-silicates and sulphides of the Mn-skarns. Thus, the Mn-skarns correspond to distal advective skarns formed near fluid drains (i.e. sub-vertical Pb-Zn ore faults) (e.g. Meinert, 1987; Megaw et al., 1988; Shu et al., 2024) whereas barren skarns formed in a diffusion-dominated closed system (Meinert et al., 2005; Zharikov et al., 2006). None of these mineralogical differences can be explained by fluid evolution due to, for instance, the distance from the fluid source or local change in the protolith composition. Our U-Pb dating also supports the distinction between the two skarns with about 8 Ma gap between the both ones (Fig. 12). In the Dzhurkovo mine, Vassileva et al. (2020) identified two types of garnet-bearing skarns, the first being barren diopside and the second being sulphide-associated andradites. U-Pb dating of each garnet-type grains yielded the same age of respectively 39.5 ± 4.60 and 38.56 ± 3.60 Ma (Fig. 12). Here we suggest that both skarns studied by these authors correspond to barren skarn. The barren diopside skarn is located in a marble not replaced by a CROB and the sulphide-associated andradite skarn is located at the contact between CROB and gneiss. Thus, both garnet-bearing skarns are coeval, barren and pre-ore.

In brief, on the contrary of some authors, who describe skarn assemblage mixing Mn-free calc-silicates with Mn-rich species (e.g. Vangelova et al., 2007; Buret, 2012), we argue for the distinction of two skarn generations: (i) barren skarns without Mn-rich calc-silicates having composition, structural characteristics and age comparable to the skarns described by Vassileva et al. (2020); (ii) Mn-skarns characterized by the presence of Mn-calc-silicates. This has implications for the relation of the skarns with the Pb-Zn sulphides including the CROB ones. The barren skarns are ubiquitous in the metamorphic pile and therefore can be observed in the mineralized zones and sometimes altered by the Mn-skarns or the CROB (e.g. when found in the chloritic fringes). Nonetheless, the calc-silicates of the barren skarns must be disconnected from the paragenetic sequence of both the Mn-skarns and the CROB.

### 7.2.2. Differentiating sulphides bearing paragenesis: Mn-skarns and CROB

Two sulphide-bearing mineral assemblages including sphalerite, galena, pyrite and chalcopyrite with additional hematite (Figs. 4, 5) have been identified in the Laki mines. The Mn-skarns are always observed in the vicinity of the CROB and of ore faults, but the reverse is not true. We dated the Mn-skarns at ca. 32 Ma (U-Pb epidote-K; Fig. 10), whereas the CROB are dated at ca. 31–29 Ma (this study U-Pb apatite-Z, Fig. 9C; $^{40}$Ar-$^{39}$Ar muscovite: Kaiser-Rohrmeier et al., 2004). Atop of these ages, there are several arguments to distinguish these two sulphide-bearing paragenesis. Their gangue minerals are different as Mn-skarn sulphides come together with rhodonite-K and johannsenite-K compared to CROB sulphides that come with gangue minerals depleted in anhydrous calc-silicates (Fig. 4I and 5H). Gangue minerals such as calcite, which are present in both sulphide-bearing assemblages, display differences in major element composition especially in Mn content which is higher in the Mn-skarn minerals than in the CROB ones (Fig. 11). Similarly, sphalerite-K from Mn-skarns are richer in Mn than sphalerite-Z from CROB (Fig. 11). Finally, Mn-skarns are sulphides-poor compared to CROB and associated ore faults that represent the main economic targets. We thus argue that the sulphides observed in Mn-skarns and CROB belong to two different paragenesis coming from two distinct fluid circulations (Fig. 13).

A distal sulphide-bearing Mn-rich skarn has been described in the Madan district (Kolkovski et al., 1996; Vassileva and Bonev, 2001; Milenkov et al., 2019; Hantsche et al., 2021). It presents a mineralogy comparable to the Mn-skarns we described in this study for the Laki district. However, in the case of the Madan ore field, prograde and retrograde phases have been identified and sulphides are interpreted to crystallize during the retrograde stage of the Mn-skarns, reaching
<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Schematic scenario illustrating the evolution of the Laki Pb-Zn district at outcrop (left) and district (right) scale. See text for explanation.</figcaption></figure>

economic concentrations (Hantsche et al., 2021). This corresponds to typical models of Pb-Zn ore skarns that are often associated with distal Mn-skarns (e.g. Einaudi et al., 1981; Abrecht, 1985; Palinkas et al., 2013; Chang et al., 2019; Shu et al., 2021). By contrast, in the case of the Laki Mn-skarns, we have shown that sulphides crystallized together with the prograde anhydrous Mn-rich calc-silicates, in non-economic concentrations, and we did not identify any typical retrograde alteration (e. g. late amphibole, biotite, or epidote; Fig. 4I). Moreover, in Laki, the economic mineralization comes with the CROB. Despite of the presence of scarce hydrous minerals (chlorite-Z and epidote-Z) coming together with the sulphides, CROB cannot be considered as the retrograde stage of the prograde Mn-skarn. We never observed the CROB minerals altering the Mn-skarns minerals and we never observe any relict of Mn-

skarns within CROB. Thus, in the Laki district, none of the two sulphide-bearing mineral assemblages present or correspond to a retrograde skarn stage.

### 7.3. Metamorphic-induced barren skarn: recordings of the Middle Rhodope detachment activity

#### 7.3.1. Prograde barren skarns as a result of high-grade metamorphism

Close to the Middle Rhodope Detachment, emplacement of pegmatite related to near-peak metamorphic conditions is dated between ca. 40–38 Ma (Gerdjikov et al., 2010; Georgieva et al., 2022) (Fig. 12). Prograde phase of skarns formed when the heat supplied by a magmatic intrusion inside carbonate rocks induces metasomatic processes (i.e.
elemental exchanges between the carbonate rock and the magma itself or a nearby silicate rock) (e.g. Meinert, 1992; Meinert et al., 2005). This is what we observe for the barren skarns that formed both in contact with pegmatite and as reaction-skarn in contact with gneiss (Fig. 3A, B). The age of the prograde stage of the barren skarns is constrained between ca. 42 and 39 by our titanite-P and apatite-P ages (Fig. 7A, 9A), as well as by the garnet U-Pb ages of Vassileva et al. (2020) and is consistent with the age of partial melting recorded in the Laki district (Fig. 12).

Petrographic observations show that titanite-P are lenticular-shaped and their longer axis aligns with the regional metamorphic foliation (Fig. 3C). At both outcrop and microscopic scales, gneiss, leucosome and barren skarns are boudinaged within marble (Fig. 3A, B, C). Boudins indicate a N-S sub-horizontal stretching consistent with the top-to-the-north vergency of the MRD. Such ductile boudinage of newly-formed skarns is common within marble in extensional tectonics and detachment settings (e.g. Serifos Fe-skarns, Ducoux et al., 2017). Therefore, we argue that the prograde barren skarns are the result of the partial melting of the upper part of the Arda unit during extensional deformation accommodated and localized by the MRD.

### 7.3.2. Retrograde barren skarn as a result of retrograde metamorphism

After the partial melting and the temperature peak affecting the Arda unit, on-going activity of the MRD is responsible for exhumation of the unit under colder conditions. Exhumation of metamorphic rocks under detachment is accompanied by retrograde metamorphic sequences notably marked by the formation of green minerals such as chlorite or epidote (Rehrig and Reynolds, 1980). As shown above, part of the apatite-R are found in the gneissic side of the barren skarns, crystallizing with its retrograde minerals (e.g. epidote-R; Fig. 3F) and geochronological data show that apatite-R are not disturbed by later fluid circulations after their crystallization. Such skarn retrograde minerals formed as the result of fluid input leading to the hydration of the prograde minerals (e.g. Meinert, 1992; Meinert et al., 2005). In Laki, these fluids likely come from the retrograde metamorphism, inducing the crystallization of apatite-R and hydrous minerals including chlorite and epidote. Those minerals crystallized both at the expense of prograde skarn minerals (i.e. retrograde skarn stage) but also in silicate rocks resulting for instance in chloritic gneiss (i.e. retrograde metamorphism). Finally, the coldest increments of the exhumation of the Arda unit beneath the MRD are marked by fractures filled by calcite-F, epidote-F and pyrite-F (Fig. 3G). Thus, the retrograde skarn stage and the retrograde metamorphism are coeval and recorded by the crystallization of apatite-R at ca. 34 Ma. This is in accordance with the ca. 36–34 Ma age of the retrograde metamorphism ($^{40}$Ar-$^{39}$Ar dating of muscovite and sericite; Kaiser-Rohrmeier et al., 2004; Gerdjikov et al., 2010).

## 7.4. The Pb-Zn mineral system of the Laki district

### 7.4.1. Mn-skarns and CROB induced by volcanism

The geochronological, petrographical and mineralogical data show that the CROB and the Mn-skarns come from two distinct fluids circulation. However, both ages are compatible with the volcanic activity that is recorded in the Borovitza caldera between ca. 33.5 and 31 Ma with a peak at ca. 32 Ma (e.g. Marchev et al., 2013; Fig. 12). Mn-skarns are dated with epidote-K at 32.4 ± 1.8 Ma in this study. While at 2σ uncertainty level this age is compatible with our apatite-Z age on CROB (30.8 ± 2.6 Ma), it is significantly older than the more precise $^{40}$Ar-$^{39}$Ar ages obtained by Kaiser-Rohrmeier et al. (2004) on CROB muscovite (the oldest being 29.36 ± 0.34 Ma). We thus argue that the Mn-skarns probably sign the peak of the Borovitza volcanism, whereas the CROB formed in its aftermath (Fig. 12).

Available fluid inclusion on syn-ore quartz from ore faults (Buret, 2012) and Pb isotopic analyses perform on galena-K (Dzhurkovo) and pyrite-K (Chetroka) from the economic Pb-Zn paragenesis indicate that the mineralizing fluids are of magmatic origin (Marchev and Moritz,

2006). Moreover, the Chetroka Pb-Zn ore faults are disposed parallel to the Borovitza caldera ring faults attesting that mineralizing fluids circulated within the caldera during the volcanic activity (Yanev, 2017, Fig. 1). We therefore argue for a genetic link between the Oligocene Borovitza magmatism and the Pb-Zn deposits, rather than with the partial melting occurring within the Arda Unit during its exhumation beneath the MRD.

There are no isotopic analyses performed on Mn-skarn minerals. Nevertheless, we suggest that the Pb-Zn mineralization is polyphased, induced by at least two successive hydrothermal fluid pulses governed by the same volcanic episode. As suggested by the geochronological data (Fig. 12), the magmatic-hydrothermal fluid coming from the Borovitza caldera is first enriched in Mn leading to the formation of Mn-skarns, and then enriched in Pb, Zn and Fe leading to the formation of CROB at lower temperatures. The enrichment of magmatic fluids in Mn, Pb and Zn, whereas it is depleted in Mg and Al is acquired during the circulation of the fluid within metamorphic rocks and explain why Mn-skarns and Pb-Zn deposits are often observed at some distance of magmatic intrusions (Einaudi et al., 1981). Such multi-stage Pb-Zn deposits has already been described for the Mozumi skarns (Japan), where five successive sulphide phases, linked to the same magmatic event, have been identified (Mariko et al., 1996).

### 7.4.2. Structural control of the Pb-Zn deposits

Structural data show these faults are sub-vertical strike-slip faults that formed conjugate systems (Fig. 6G). The largest and dominant faults, with a more or less cataclastic character, are mainly NNE-striking with a sinistral kinematics. The dextral NNW-striking conjugate faults are present but less abundant. Structural data available in the literature indicate that the opposite is observed in Madan with the NNW-striking dextral faults prevailing over the NNE-striking sinistral faults (Atanassov et al., 1972; Manev, 1975a; Dokov and Popov 1963). Therefore, both Laki and Madan ore fields present a similar conjugate system of strike-slip ore faults indicating a N-S-trending horizontal shortening at the CRR scale during the mineralizing process. (Fig. 13). These results contradict the models suggesting that the Pb-Zn ore faults formed in a post-orogenic extensional setting (Kaiser-Rohrmeier et al., 2013).

Noteworthy, the N-S horizontal shortening we evidenced here, is compatible with the Borovitza caldera formation and structural controls. Indeed, as described by Dhont et al. (2008) this caldera appears to emplace in a strike-slip tectonic regime. Those authors described a horse-tail termination of a major NNW-striking dextral strike-slip fault (Sarnitza fault) along the southeastern edge of the caldera (see their Fig. 8B). The Mn-skarn fluids, which initiated the hydrothermal period, percolated through drains that may be lithological contacts or fine fractures. These fractures can be pre-existent or synchronous with the marble replacement (e.g. Putnis, 2009). As all mine outcrops of Mn-skarns are located close to CROB and ore faults, these fractures might correspond to damage zones of incipient ore faults strike-slip system (Fig. 13). Thus, the CROB and the Mn-skarns likely formed under the same N-S shortening tectonic regime.

### 7.4.3. The role of the inherited extensional structures in the hydrothermal circulation

Ore faults crosscut and/or displace the E-W-striking normal faults (Fig. 6A, B). The former formed under N-S horizontal shortening which is incompatible with the top-to-the-North extensional kinematics of the MRD and associated E-W-striking normal faults. This indicates that the MRD and extensional tectonics stopped before the sulphide deposition. Furthermore, E-W normal faults, oriented normal to the N-S trending horizontal shortening, are maintained closed and impermeable for fluid circulation (see for example Rhys et al., 2020 for such mechanism). This is in accord with the absence of mineralization inside these E-W normal faults even when they are in direct contact with CROB or ore faults (Fig. 6A, B).
However, the syn-extensional flat metamorphic foliation, ductile boudinage and late normal faults highly influenced the permeability architecture of the Arda unit and therefore the later hydrothermal fluid circulation. For instance, concerning the Mn-skarn, the boudinage represents a redistribution of lithological contacts used by fluids to percolate. Concerning the CROB, Fig. 5C shows that the fluids circulate at the top of the marble levels that are delimited by barren skarns and gneiss. The boudinage opens up pathways for fluids to flow to higher marble levels. In the same way, large scale boudinage of gneiss layers within marble of the Arda metamorphic pile favors vertical connections for fluids (Fig. 13). Moreover, as the brittle E-W-striking normal faults offset the marble level, in a N-S direction CROB stops abruptly against gneiss panels. Thus, even if the MRD and related extensional structures are no more active at the time of the mineralizing hydrothermalism, brittle and ductile deformation linked to extension have modified the permeability architecture of the metamorphic pile and thus the fluid flow pattern.

## 8. Conclusion

In this study, we propose a mineral system for the Pb-Zn deposits of the Laki district. We distinguished three mineral assemblages: (i) a primary and widespread barren skarn with a prograde phase dated at ca. 41 Ma (U-Pb titanite-P and apatite-P) and a retrograde phase dated at ca. 34 Ma (U-Pb apatite-R); (ii) a sulphide-poor Mn-skarn dated at 32.4 ± 1.8 Ma (U-Pb epidote-K); (iii) a sulphide-rich CROB associated with Pb-Zn ore faults dated between ca. 31 and 29 Ma (U-Pb apatite-Z and previously published ⁴⁰Ar-³⁹Ar ages on muscovite).

The structural analysis of the barren skarn shows that their syn-deformation formation is compatible with the Middle Rhodope detachment activity. Geochronological data suggest that the prograde stage of the barren skarn occured during metamorphic peak-temperature. Subsequent retrograde metamorphism enhanced the replacement of the prograde mineral assemblage by retrograde skarn minerals. Thus, the barren skarns, widespread in the metamorphic pile and then present in mineralized zone, is metamorphic and anatectic in origin and is disconnected from the mineralization history.

We have shown that Pb-Zn hydrothermalism is at least bi-pulsed and geochronological data suggest that they are synchronous with the volcanic activity recorded in the Borovitza caldera. The Mn-skarn mark the peak of the volcanic activity whereas the CROB formed in ots aftermath. Therefore, the Pb-Zn hydrothermalism is volcanic-induced.

Structural analysis of the subvertical ore faults shows that they formed in a strike-slip conjugate system associated with N-S horizontal shortening. This strain orientation is incompatible with the E-W striking extensional structures, explaining they remain impermeable to fluids. Therefor the extension only influences Pb-Zn deposition by reworking the permeability network prior to fluid circulation.

Finally, in this study we implemented the mineral system of the skarn-related Pb-Zn deposits of the Central Rhodope Region by distinguishing regional barren from hydrothermal fertile skarns, clarifying the structural controls and plumbing systems of the Pb-Zn mineralization, in a new and precise temporal framework.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This project was funded by the RILA program (PHC France-Bulgaria, project 51273PB), the OSUR (AO 2023, Axe Ressources Naturelles), and the University of Rennes (AAP Défis scientifiques 2024). We warmly thank Laki Invest-Dzhurkovo EOOD for giving us access to the mines. We are grateful to Xavier Le Coz for high quality thin sections. Jessica

Langlade and Loïc Joanny are thanked for EPMA and SEM analyses. Dominique Bavay for its helps with some technical issues.

## Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.oregeorev.2025.106755.

## Data availability

Data will be made available on request.

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