# Distribution of trace elements in willemite from the Belgium non-sulphide deposits

Flavien Choulet, James Richard, Marie-Christine Boiron, Augustin Dekoninck, Johan Yans

► To cite this version:

Flavien Choulet, James Richard, Marie-Christine Boiron, Augustin Dekoninck, Johan Yans. Distribution of trace elements in willemite from the Belgium non-sulphide deposits. European Journal of Mineralogy, 2019, 31 (5-6), pp.983-997. 10.1127/ejm/2019/0031-2871. hal-02480880

HAL Id: hal-02480880

https://hal.univ-lorraine.fr/hal-02480880v1

Submitted on 26 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
and other minor elements during weathering of primary sulphides is poorly understood or even documented. While some minor metals are hosted in carbonates (e.g., cadmium; Schwartz, 2000), other mineral phases such as hydroxides and silicates are known to incorporate significant amounts of critical elements, and especially germanium (Bernstein, 1986; Lombaard et al., 1986; Saini-Eidukat et al., 2009; Choulet et al., 2017; Mondillo et al., 2018a, b). As direct replacement textures are frequently reported, the initial content of sulphide minerals is a determining factor for the final concentration of trace elements in the secondary minerals. Although the amount of Ge may locally reach several thousands ppm (e.g., Saini-Eidukat et al., 2009), this element can be heterogeneously distributed within host mineral phases (e.g., in willemite; Choulet et al., 2017). As mentioned above for sphalerite, this could indicate that the concentration of trace elements in secondary phases also depends on crystallographic parameters of the host mineral. External supply of minor elements by hydrothermal fluids and competition between secondary host minerals are also crucial features to determine the potential of the supergene zone. This has major implication for exploration and recovery of Ge during mineral and metallurgical processing (Drzazga et al., 2018).

Based on the example of the Belgian non-sulphide deposits, we attempt to evaluate the exploration potential of secondary non-sulphide minerals hosting strategic metals, taking into account their complex mineralogical evolution. Belgian non-sulphide deposits around the La Calamine locality are historical deposits, which have produced zinc from carbonate and silicate ores (Dejonghe et al., 1993), until to the development of flotation and the generalization of sulphide ore processing at the beginning of the 20th century. La Calamine is also the type locality for willemite (Cesàro, 1887) and, despite the total closure of mines and their flooding, numerous samples are currently available in museum collections. While previous studies focused on mineralogical inventorying or determining of conditions for Belgium non-sulphide ore formation (Coppola et al., 2008), the distribution of minor metals remains under-investigated, despite significant Ge and Ga concentrations in the primary sulphides (Goffin et al., 2015). This study presents detailed observation and analyses of sulphide and non-sulphide zinc ores and aims at diagnosing their potential for strategic metals such as Ge, as future exploration of these deposits is still a matter of debate in Belgium (Goossens, 2014).

# 2. Geological setting

## 2.1. Regional geology

Belgium has a long mining history mainly due to the occurrence of numerous Zn-Pb deposits within the Palaeozoic bedrock of the Dinant, Namur and Verviers synclinoria (Dejonghe, 1998). The geology of Belgium is closely associated to the evolution of the Variscan Belt and especially of the Rheno-Hercynian Zone (Franke, 2000). Successive tectonic (and minor metamorphic) events led to the formation of a composite Palaeozoic basement that was further

covered by Permian to Quaternary sediments (Fig. 1). The Palaeozoic basement includes Cambrian to Ordovician sedimentary rocks, further deformed and metamorphosed by Caledonian (sensu lato) events (Michot, 1980; Fielitz & Mansy, 1999). A first regional unconformity ("Ardennian") is the result of a tilting of the pre-Silurian basement, while a second regional unconformity ("Brabantian") represents the effect of tectonic inversion during the Early Devonian (Sintubin et al., 2009). Afterwards, a first cover made of Devonian to Carboniferous sedimentary rocks was deposited in a new platform associated with the development of the Rheic Ocean (Oncken et al., 1999). A new episode of inversion tectonics, related to the Variscan orogeny, started at the end of the Viséan and sedimentation was accompanied by the formation of regional inverse faults, such as the Midi Fault zone (Meilliez & Mansy, 1990). These thrusts accommodated the overthrusting of the southern Ardenne allochthon over the northern Brabant parautochthon. Consequently, regional-scale folded units, like the Dinant and Namur synclinoria developed and represent, with the Brabant parautochthon, the present-day structural zones of Belgium (Belanger et al., 2012). Further discontinuous Permian to recent sedimentary cover is recorded throughout the northern part of the country. Successive uplift episodes during the Mesozoic and the Cenozoic have progressively contributed to a partial erosion of this cover, and the subaerial exposure of the Paleozoic rocks (Vercautere & van den Haute, 1993; Quesnel, 2003; Thiry et al., 2006; Demoulin et al., 2018). To the northeast of Belgium (Fig. 1), Lower Cretaceous sandstone, siltstone and clays of the Aachen, Vaals and Gulpen formations cover the Early Palaeozoic rocks of the Verviers Synclinorium, the eastern prolongation of the "Dinant Synclinorium" (Graulich et al., 1984). Early Palaeozoic rocks include sandstones, shales, and carbonates; they are crosscut by two sets of faults: (1) NE-SW trending reverse faults related to the Variscan episodes and (2) NNW-SSE faults linked with the inception of the Rhine Graben (Fig. 1).

## 2.2. Metallogeny of Zn-Pb deposits in Belgium

The Verviers Synclinorium was the most productive mining area of metals in Belgium, representing around 75% of the total national past production of Pb-Zn ore concentrates (Dejonghe et al., 1993). Several types of primary sulphide ore deposits are distinguished: syn- to diagenetic veins and associated flats and palaeo-karst formations (Dejonghe, 1998). The distribution of ore deposits was largely controlled by the NNW-SSW faults (Fig. 1). The formation of sulphide orebodies resulted from hydrothermal events, which date back to the mid-Jurassic (Schneider et al., 1999). They were related to high salinity fluids derived from the late evolution of the sedimentary basin during the Late Palaeozoic; fluids were further involved in diagenetic processes but remained trapped at depth until to Jurassic tectonic episodes, which allowed their remobilization (Heijlen et al., 2001). The precipitation of sulphides resulted from mixing between metal-bearing fluids and fluids containing reduced sulfur (e.g., Sangster, 1990), leading to consider
<figure><img src="image_2.png" /><figcaption>Fig. 1. Geological map of northeastern Belgium, with location of old Zn-Pb mines. Note the distribution of deposits along NNW-SSE trending faults. Size of red circle is proportional to metal tonnage (data from Dejonghe, 1998). Modified after Dejonghe & Jans (1983).</figcaption></figure>

the Verviers ore deposits as Mississipi Valley type (MVT) deposits (Muchez et al., 1994, 2005). In addition, numerous non-sulphide ore deposits were formed by supergene weathering of the underlying primary sulphide deposits, with the exception of La Calamine (Dejonghe & Boni, 2005). The age of this oxidation event is probably Lower to Mid-Cretaceous (Coppola et al., 2008), consistent with various constraints on regional palaeo-weathering (e.g., Yans, 2003; Thiry et al., 2006; Demoulin et al., 2018) and the unconformity of Late Cretaceous rocks (Fig. 1). The La Calamine Mine, also referred to as Vieille Montagne or Moresnet, consists of a lenticular pocket with a sinkhole shape, hosted by Tournaisian dolomite and controlled by the position of a NE-SW thrust zone (Dejonghe, 1998). Sulphide minerals

were found only in traces (Dejonghe & Jans, 1983) and were considered as of supergene origin, although Coppola et al. (2008) provided mineralogical evidence for a partial replacement of sphalerite by willemite. The mineralization consists of a mixture of non-sulphide minerals, including early willemite and late smithsonite, hemimorphite and zinc clays, forming the typical “Calamine” ores (Coppola et al., 2008). Based on stable isotopes and fluid inclusions, Boni et al. (2005) and Coppola et al. (2008) proposed that it is presently not possible to decide between supergene weathering and low-temperature hydrothermal process for the formation of willemite. The formation of the late non-sulphide minerals (carbonates) is unequivocally related to supergene processes, though (Coppola et al., 2008).
## 3. Material and methods

Since Belgian deposits are inaccessible, characterization of mineralogy was made on samples housed at the Geological Survey of Belgium of the Royal Belgian Institute of Natural Sciences (RBINS). Most of the samples come from the historical mine of La Calamine (Table 1). One sample (IG32105-259) was also collected from the Fossey Mine, located to the south of La Calamine mine; this deposit was characterized by veins and flat lenses, at the limit between the Famennian sandstone and shale and the Tournaissian dolomite (Dejonghe & Jans, 1983). Another sample (RN3874) originates from the small Heggelsbrück deposit in the southwestern part of the district, where ore mineralization consists of non-sulphide veins hosted by the Famennian formations (Fig. 1; Dejonghe & Jans, 1983).

Samples were cut to prepare polished thin sections. Optical investigation using a Leica DMRX petrographic microscope was supplemented by electronic microscopy using a Scanning Electron Microscope (SEM) at the FEMTO-ST Institute (Université de Bourgogne Franche-Comté, France). Backscattered Electron (BSE) imaging operating at 15 kV in low vacuum conditions was combined with semi-quantitative analysis using Energy-Dispersive Spectrometers (EDS). Cold Cathodoluminescence (CL) observations using OPEA instrumentation hosted at Chrono-Environnement (Université de Bourgogne Franche-Comté) were made with an acceleration voltage of 18.5 ± 1.5 keV and a current of 350 ± 150 mA to investigate zoning in willemite and establish relationships with other minerals.

Major-element composition of sphalerite, carbonates and willemite was obtained by Electron Probe Micro-Analysis (EPMA) at the University of Lausanne, Switzerland, using a JEOL 8200 Superprobe equipped with five Wavelength-Dispersive Spectrometers (WDS). Carbon-coated polished thin sections were analyzed with a beam of 15 kV and a current of 10 nA. The resolution was ca. 1 µm. Oxides, silicates, and pure metals were used as standards. Jeol house-procedures were used for data reduction and the limits of detection were between 100 ppm and 300 ppm for detected elements. In addition, X-ray intensity hyperspectral mapping was realized to disclose the specific textures caused by the chemical features of minerals, especially in sulphide minerals.

The Laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) measurements of trace element contents within willemite and sphalerite were made on a quadripolar Agilent 7500c ICP-MS coupled with a 193-nm excimer (Geolas Q Plus System) laser hosted at GeoResources, Nancy, France. Laser ablations were performed with a constant 5 Hz pulse rate at 90 mJ laser energy. The laser spot size was 44 µm in diameter. The total acquisition time (background and signal) was limited to 100 s for one spot. Zinc content previously measured by EPMA was used as an internal standard for willemite and sphalerite. For willemite, NIST 610 (Ge, 426 ± 10 ppm; Ga, 438 ± 11 ppm; In, 441 ± 32 ppm) and NIST 612 (Ge, 35 ± 3 ppm; Ga, 36 ± 2 ppm; In, 43 ± 4 ppm) were used as external standards (Pearce *et al.*, 1997). For sphalerite, we used Mass-1 (Ge 58 ± 3 ppm, Ga 64 ± 11 ppm and In concentrations of about 50 ppm) as external standard (Wilson *et al.*, 2002). As the amount of ablated material is different for each analysis, the detection limits (d.l.) are different for each analysis and each element. The d.l. was calculated following the procedure detailed in Longerich *et al.* (1996). A 3σ criterion was used to estimate the d.l., minimum values of which are lower than 1 ppm for the trace elements analysed with an ablation spot diameter of 44 µm. The following isotopes were monitored to evaluate the Ge content: $^{70}$Ge, $^{72}$Ge, $^{73}$Ge, $^{74}$Ge, and $^{76}$Ge. $^{70}$Ge, $^{72}$Ge and $^{76}$Ge were not considered due to interference with $^{70}$Zn, $^{56}$Fe$^{16}$O and Ar isotopes, respectively. Following Belissont *et al.* (2014), we chose to retain $^{74}$Ge, the most abundant isotope (36.28%), for the quantification of Ge in willemite. Data reduction was carried out using Iolite 3.0 (Paton *et al.*, 2011).

## 4. Results

### 4.1. Optical and electron microscopy

Different morphologies of willemite can be observed in the non-sulphide samples. The most frequent one is marked by botryoidal sub-rounded aggregates that occur in druses. These 0.5 mm sized spherulites (Type W) consist of radially distributed elongated prisms (Fig. 2A) showing oscillatory zoning. In some cases, these oscillatory-zoned elongated prisms delimit a rim ($W_R$) around a hexagonal crystal remnant ($W_C$) in the core of the aggregate (Fig. 2B). This

<table><caption>Table 1. Sample location with brief description of macroscopic textures and mineralogy. Minerals are listed by their relative abundance.</caption><thead><tr><th>Sample number</th><th>Locality</th><th>Description</th><th>Mineralogy</th></tr></thead><tbody><tr><td>RN6483</td><td>La Calamine</td><td>Sulphide breccia with schalenblende</td><td>Sp, Gal, Py, Cal</td></tr><tr><td>RV2/837</td><td>La Calamine</td><td>Massive sulphide</td><td>Sp, Py, Gal</td></tr><tr><td>RN4019</td><td>La Calamine</td><td>Dissolution breccia with reddish willemite and clay filling</td><td>W, Sm, Cal, Sid, Clays, Qz, Zn-clays, Goe, Hem</td></tr><tr><td>RA5003</td><td>La Calamine</td><td>Massive microcrystalline willemite</td><td>W, Sm, Cal, Clays, Qz, Zn-clays, Zn-P</td></tr><tr><td>RA5232</td><td>La Calamine</td><td>Microglobular willemite aggregate with clay filling</td><td>W, Sph, Gal, Py, Clays, Qz Zn-clays, Dsc</td></tr><tr><td>RA6661</td><td>La Calamine</td><td>Microglobular willemite aggregate</td><td>W, Clays, Qz, Zn-clays, Goe, Hem, Zn-P</td></tr><tr><td>IG32105/259</td><td>Fossey</td><td>Carbonate-rich breccia</td><td>W, Qz, Sm, Sid</td></tr><tr><td>RN3874</td><td>Heggelsbrück</td><td>Porous microglobular willemite aggregate</td><td>W, Clays, Qz, Zn-clays, Goe, Hem, Zn-P</td></tr></tbody></table>

Cal, calcite; Dsc, descloizite; Gal, galena; Goet, goethite; Hem, hematite; Py, pyrite; Qz, quartz; Sid, siderite; Sm, smithsonite; W, willemite; Zn-P, undifferentiated zinc phosphates.
nucleus does not show regular internal zoning (Fig. 2C), but
has probably slightly different crystallographic parameters,
since interference colours are relatively distinct between
the core (first order) and the outer rim (second order)
(Fig. 2B). The core remnants can also be of larger size
and sub-rectangular shape, with tiny and fibrous willemite
in the rim that displays a comb-like texture (Fig. 2D).
Locally, the core exhibits very complex internal structures,
only revealed by CL imaging (Fig. 2E and F). Such cores
formed in two steps: (1) development of idiomorphic and
prismatic crystals with growth bands ($W_{C1}$), and (2) aggre-
gation and cementation by slightly zoned willemite ($W_{C2}$).
The formation of willemite may be accompanied by the for-
mation of tiny crystals of Pb-bearing minerals, such as
cerussite and galena (Gn) (Fig. 2G and H). Willemite also
forms massive areas ($W_2$), which consist of tiny sub-
euhedral crystals intergrown with euhedral prismatic quartz
crystals (Fig. 3A and B). Quartz (Qz) is characterized by a
two stage crystallization with a sub-rounded deep-blue lumi-
nescent core, surrounded by a rim exposing bright yellowish
colour in CL (Fig. 3A and C). Willemite ($W_1$), with distinc-
tive luminescence and contrast/brightness characteristics in
BSE imaging mode, is also observed in the massive facies
(Fig. 3A–C). Sometimes, this type is well developed and
we may observe internal zoning and growth banding
(Fig. 3D), very similar to that of willemite rims ($W_R$) in
other facies (Fig. 2).

The studied samples also include carbonates; they occur
in veins that crosscut silicates (Fig. 3A, B and D, E) or as
crystals coating willemite in druses. Veins include a mixture
of smithsonite (Sm) with deep red luminescence and succes-
sive bands of zincian siderite (Zn–Sd) and Fe-rich smith-
sonite (Fe–Sm), with darker luminescence (Fig. 3A and
D). Calcite (Cal) also occurs as late infilling within the sec-
ondary porosity formed by the dissolution of zinc and iron
carbonates (Fig. 3D). The druses are also filled with detrital
material including quartz and a mixture of K-rich clay min-
erals (Figs. 2B and 3F). Secondary minerals such as zinc
clays, probably fraipontite (Fig. 3F), zinc phosphates (hope-
ite, scholzite or parascholzite) and zinc vanadates
ITA
(descloizite) are also present as patches.

Sulphide samples are characterized by a “Schalenblende”
texture formed by collomorphic concretions of sphalerite
(Fig. 4A–D). The concretions are cemented by colourless
sphalerite (Sp) intergrown with calcite (Fig. 4A) and expose
rhythmic bands with various colours under the optical
microscope (Fig. 4B and C). Within *schalenblende*, galena
and pyrite (Py) crystals can also be observed (Fig. 4D
and E). Sphalerite also occurs as relics within willemite. A
rim of secondary galena coats the zinc sulphides, preserving
the protore from further weathering (Fig. 4F).

4.2. Chemical composition of willemite, carbonates
and sphalerite

A summary of willemite compositions is reported in Table 2.
For all data, the sum of oxides is between 98% and 102%
and the Si/Zn ratios are consistent with an ideal composition

of willemite, and significantly differ from hemimorphite.
Detailed analysis of individual spots is provided in Tables
S1 and S2 in the Supplementary Material linked to this arti-
cle and freely available at https://pubs.geoscienceworld.org/
eurjmin. All willemite types distinguished in the previous
section except W₂ have similar ZnO and SiO₂ contents, con-
sistent with ideal stoichiometry (73.04 wt% for ZnO and
26.96 wt% for SiO₂). For W₂, we can observe excess
ZnO, with a mean value reaching 74.12 wt%. Interpretation
of such zinc excess is unclear and it could be related to the
presence of crystal defects in the willemite structure or of
nano-inclusions of Zn-bearing minerals. No difference can
be seen for FeO and CaO. For the trace-element contents,
the following observations can be made: the P and As con-
tents are heterogeneous and may reach up to 4000 ppm.
Except outliers, the Mn content is generally low (less than
75 ppm). The Pb content in willemite is mostly between
400 ppm and 1400 ppm, but concentration can be unusually
high (W₁b) due to tiny galena inclusions (Fig. 2G and H). In
this case, higher Cu, Ag and Sb contents are also measured.
While the In content is below detection limits and the Ga
content does not exceed 4 ppm, the average Ge contents
range from 17 ppm to 68 ppm with a maximum value of
252 ppm. In zoned crystals, there is no clear difference in
composition between core and rim, except for higher P con-
tent in the core. W₂ remarkably differs from other willemite
types, as significantly lower Pb, As, Ge and Mn contents
(means of 23, 5, 17 and 12 ppm respectively) and higher
Cd contents (mean 47 ppm) are measured.

The chemical composition of carbonates was obtained by
EPMA and is reported in Table 3. Detailed analysis of indi-
vidual spots is provided in Table S3. In addition to pure cal-
cite, studied samples include zinc and iron carbonates.
Smithsonite contains major amounts of MgO (mean 2.0
wt%), but low amounts of CaO (mean 0.5 wt%) and FeO
(mean 0.9 wt%). Pure smithsonite exhibits a red lumines-
cence (Fig. 3A) and differs from dark luminescent smith-
sonite (Fig. 3D) in which the Fe content ranges from 9.0
wt% to 27.5 wt% (mean 15.5 wt%). The MgO and CaO
contents are also significantly higher (means 3.5 and 1.1
wt%). Siderite is also present in samples (Fig. 3D) and car-
ries high ZnO, MgO and CaO contents (means 6.9, 2.6 and
4.6 wt%, respectively).

The composition of sphalerite is provided in Table 4 and
detailed analyses can be found in Tables S4 and S5.
Sphalerite shows a variable Fe content, ranging from 0 wt
% to 5 wt%. Sphalerite commonly contains significant
minor elements including Cu, Ge, As, Cd, Sb and Pb. Dark
bands in *schalenblende* (Fig. 5A) are marked by high Fe
content, while light bands are iron-free (Fig. 5B). According
to element maps, there is a clear negative correlation
between Fe and Cd, at least in the inner parts of concretions
(Fig. 5C). The Ge content ranges between 19 ppm and
1325 ppm within sphalerite, and averages 269 ppm and
246 ppm in the two samples. The In content is below detec-
tion limit and the Ga content is less than 3 ppm. This vari-
ability is due to heterogeneous element distribution and
apparent correlation between the Fe and Ge contents
(Fig. 5D and E). In other sulphides (galena and pyrite),
<figure><img src="image_3.png" /><figcaption>Fig. 2. Photomicrographs of the willemite mineralisation. (A) Willemite spherulite (type W) showing radially distributed elongated prisms (sample RA5232, TL, nic+). (B) Willemite spherulite composed of a rim of elongated prisms ($W_R$) around a core ($W_C$) formed by hexagonal crystals (sample RA6661, TL, nic+). (C) Same view in CL mode, to illustrate oscillatory zoning in the $W_R$. (D) Prismatic remnant core ($W_C$) of willemite rimmed by sub-fibrous willemite crystals ($W_R$) (sample RN3874, TL, nic+). (E) Aggregate of anhedral crystals of willemite (type $W_C$) coated by fibrous willemite crystals ($W_R$) (sample RN3874, TL, nic+). (F) Same view in CL mode, to reveal the presence of idiomorphic crystal relics within cores. (G) Well-developed elongated crystals spotted by black inclusions of Pb-bearing minerals like galena (Gn), which delineate growth banding (sample RA5232, TL, nic+). (H) Same view in CL mode, to illustrate irregular zoning in the $W_R$ type. TL, transmitted light; CL, cathodoluminescence; nic+, crossed nicols. Red spots correspond to the location of LA-ICPMS analyses.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Photomicrographs of the willemite mineralisation. (A) Relics of early willemite ($W_1$) embedded in a mixture of late willemite ($W_2$) and zoned quartz ($Qz$); willemite mineralisation is crosscut by a vein of carbonates showing red luminescence (sample IG32-105, CL). (B) Same view in BSE mode; note the difference in grey tone between the two willemite types and the composite nature of vein carbonates (Sm, smithsonite; Fe–Sm, Fe-rich smithsonite; Zn–Sd, Zn-rich siderite). (C) Details of the polyphase willemite mineralisation, and the core-mantle texture of quartz mixed with late willemite (sample IG32-105, CL). (D) Late willemite aggregates with relics of early willemite, coated by a complex late carbonate (Cal, calcite) stage (sample RN4019-105, CL). (E) Same view in BSE mode. (F) BSE image of zoned smithsonite formed in druses further filled with detrital K-rich clays (dark grey), partly replaced by zinc clays (light grey) (sample RN4019-105). BSE, Back-scattered electron.</figcaption></figure>

the Ge content is below 3 ppm, while Ga and In are not detected.

# 5. Discussion

## 5.1. Polyphase willemite mineralization

Based on new observations of the non-sulphide ores, we propose the paragenetic succession presented in Fig. 6. As already noted by Coppola et al. (2008), three different mineralization stages can be distinguished. Whereas an early sulphide stage is locally preserved as massive sphalerite (Fig. 5A–E) or as tiny relics within silicates and carbonates

(Fig. 2F–H), the non-sulphide mineralization includes two successive ore stages: (1) the willemite stage and (2) the carbonate stage. Dejonghe & Boni (2005) and Coppola et al. (2008) interpreted the various-scale associations of willemite and sphalerite as the result of a direct replacement process. However, our observations support a combination of dissolution and cementation, as shown by the presence of secondary sulphides (galena) that armour partly dissolved sphalerite and predate willemite precipitation (Fig. 4F). The relative incompatibility of lead within the willemite structure could also explain such textures (Terracciano, 2008). The transition between willemite and carbonate stages is characterized by: (1) direct replacement process
<figure><img src="image_5.png" /><figcaption>Fig. 4. Photomicrographs (plane-polarised light) of the sulphide mineralisation. (A) General view of the banded *schalenblende* spherulite cemented by colourless sphalerite and calcite (sample RN6483, TL). (B) Detailed view of banded sphalerite (sample RN6483, TL). (C): General view of banded *schalenblende* spherulite (sample RV2-837, TL). (D) Galena crystal included in sphalerite (sample RN6483, RL). (E) Sulphide (galena and pyrite) inclusions in the core of *schalenblende* spherulite (sample RV2-837, RL). (F) Secondary galena armouring relics of sphalerite within the non-sulphide mineralisation (sample RA5232, BSE). RL, reflected light.</figcaption></figure>

(Dejonghe & Boni, 2005), (2) partial dissolution of willemite followed by carbonate precipitation (e.g., Fig. 3) and (3) carbonates filling veins in willemite (Coppola et al., 2008).

The coexistence of willemite and carbonate stages in a given mineralization is frequently reported in supergene Zn deposits (Terracciano, 2008; Choulet et al., 2017; Mondillo et al., 2018b). While the late mineral assemblage, including smithsonite, cerussite, hemimorphite, and hydrozincite, clearly indicates supergene weathering, the origin of the willemite stage in non-sulphide deposits has always been a matter of debate. On the basis of fluid-inclusion studies (e.g., Sweeney et al., 1991; Kamona & Friedrich, 2007; Terracciano, 2008), it is now widely accepted that willemite may also originate from hydrothermal-hypogene processes (Hitzman et al., 2003). In this case, the precipitation of willemite from mid to-high salinity hydrothermal fluids may

occur under: (1) oxidized conditions, (2) low sulfur activity, (3) moderate temperature (100–150 °C) and (4) pH > 6.5 (Brugger et al., 2003). Many Neoproterozoic to Cambrian willemite deposits from the southern hemisphere formed by such hypogene processes, e.g., Vazante in Brazil (Monteiro et al., 2007; Slezak et al., 2014), Zambezi prospects in Zambia (Terracciano, 2008; Boni et al., 2011) or Beltana in Australia (Groves et al., 2003). However, in several other deposits, such hypogene features are lacking, as exemplified by the willemite mineralization in Belgium (Coppola et al., 2008). Although two-phase fluid inclusions in willemite provided homogenization temperatures between 80 °C and 190 °C (Boni et al., 2005), monophase inclusions are predominant, suggesting that low temperature (50–80 °C) fluids have caused the precipitation of willemite. Coppola et al. (2008) considered that the high temperature obtained from two-phase inclusions is rather due to post-entrapment
<table><caption>Table 2. Major (EPMA, in wt%) and trace (LA-ICPMS, in ppm) element composition of willemite. < d.l.: maximum value lower than detection limit (see Materials and Methods section for d.l. calculation).</caption><thead><tr><th>Type</th><th>n</th><th>SiO<sub>2</sub></th><th>ZnO</th><th>FeO</th><th>CaO</th><th>Mg</th><th>Al</th><th>P</th><th>Ti</th><th>Mn</th><th>Cu</th><th>Ga</th><th>Ge</th><th>As</th><th>Ag</th><th>Cd</th><th>Sn</th><th>Sb</th><th>Pb</th></tr><tr><td></td><td></td><td colspan="10" style="text-align: center;">wt%</td><td colspan="10" style="text-align: center;">ppm</td></tr></thead><tbody><tr><td rowspan="3">W<sub>1</sub></td><td>66</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>27.7</td><td>72.4</td><td>0.1</td><td>0.0</td><td>71</td><td>289</td><td>1006</td><td>23</td><td>61</td><td>2.0</td><td>0.3</td><td>40</td><td>138</td><td>1.0</td><td>8.1</td><td>0.4</td><td>1.0</td><td>1367</td></tr><tr><td>Min</td><td>26.3</td><td>70.3</td><td>0.0</td><td>0.0</td><td>0.2</td><td>2.2</td><td>7.0</td><td>2.6</td><td>1.2</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>1.4</td><td>&lt;d.l.</td><td>4.1</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>12</td></tr><tr><td rowspan="3">W<sub>1b</sub></td><td>4</td><td>29.2</td><td>73.8</td><td>0.7</td><td>0.4</td><td>1910</td><td>2030</td><td>4110</td><td>251</td><td>1460</td><td>7.6</td><td>1.1</td><td>247</td><td>1195</td><td>26</td><td>19</td><td>2.0</td><td>13</td><td>7780</td></tr><tr><td>Mean</td><td></td><td></td><td></td><td></td><td>3.4</td><td>5.9</td><td>510</td><td>2.9</td><td>3.5</td><td>8.2</td><td>0.1</td><td>52</td><td>238</td><td>27</td><td>4.2</td><td>0.2</td><td>75</td><td>65175</td></tr><tr><td>Min</td><td></td><td></td><td></td><td></td><td>0.7</td><td>2.4</td><td>139</td><td>1.7</td><td>2.3</td><td>5.4</td><td>&lt;d.l.</td><td>8.8</td><td>177</td><td>22</td><td>3.6</td><td>&lt;d.l.</td><td>23</td><td>27800</td></tr><tr><td rowspan="3">W<sub>C</sub></td><td>44</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>27.6</td><td>72.5</td><td>0.2</td><td>0.0</td><td>364</td><td>3255</td><td>1909</td><td>87</td><td>76</td><td>2.5</td><td>1.0</td><td>37</td><td>354</td><td>0.6</td><td>14</td><td>0.8</td><td>0.8</td><td>413</td></tr><tr><td>Min</td><td>26.1</td><td>69.9</td><td>0.0</td><td>0.0</td><td>18</td><td>25</td><td>395</td><td>5.3</td><td>4.1</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>5.2</td><td>24</td><td>&lt;d.l.</td><td>3.0</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>102</td></tr><tr><td rowspan="3">W<sub>C1</sub></td><td>4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>28.7</td><td>73.5</td><td>2.0</td><td>0.1</td><td>1647</td><td>23800</td><td>3940</td><td>382</td><td>493</td><td>7.0</td><td>4.1</td><td>252</td><td>1466</td><td>2.3</td><td>30</td><td>2.7</td><td>2.7</td><td>1034</td></tr><tr><td>Min</td><td>27.7</td><td>72.7</td><td>0.0</td><td>0.0</td><td>6.9</td><td>31</td><td>76</td><td>4.5</td><td>2.8</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>7.0</td><td>19</td><td>&lt;d.l.</td><td>4.3</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>302</td></tr><tr><td rowspan="3">W<sub>C2</sub></td><td>4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>28.2</td><td>73.5</td><td>0.2</td><td>0.0</td><td>69</td><td>367</td><td>500</td><td>27</td><td>20</td><td>9.3</td><td>1.0</td><td>37</td><td>48</td><td>0.5</td><td>5.3</td><td>1.0</td><td>0.6</td><td>916</td></tr><tr><td>Min</td><td>27.7</td><td>73.3</td><td>0.0</td><td>0.0</td><td>24</td><td>214</td><td>1411</td><td>20</td><td>2.0</td><td>0.8</td><td>0.1</td><td>68</td><td>127</td><td>0.6</td><td>4.5</td><td>0.4</td><td>1.9</td><td>494</td></tr><tr><td rowspan="3">W<sub>R</sub></td><td>38</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>27.8</td><td>72.6</td><td>0.1</td><td>0.0</td><td>190</td><td>1369</td><td>666</td><td>45</td><td>50</td><td>1.6</td><td>0.7</td><td>22</td><td>461</td><td>0.6</td><td>12</td><td>0.7</td><td>1.2</td><td>413</td></tr><tr><td>Min</td><td>26.9</td><td>70.9</td><td>0.0</td><td>0.0</td><td>5.8</td><td>134</td><td>&lt;d.l.</td><td>7.2</td><td>4.2</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>2.8</td><td>87</td><td>&lt;d.l.</td><td>4.5</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>67</td></tr><tr><td rowspan="3">W<sub>2</sub></td><td>9</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>27.9</td><td>74.1</td><td>0.2</td><td>0.0</td><td>36</td><td>148</td><td>772</td><td>8.5</td><td>12</td><td>2.4</td><td>0.1</td><td>17</td><td>5.1</td><td>1.0</td><td>47</td><td>0.7</td><td>–</td><td>23</td></tr><tr><td>Min</td><td>24.9</td><td>72.8</td><td>0.0</td><td>0.0</td><td>1.2</td><td>0.8</td><td>99</td><td>3.5</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>1.0</td><td>d.l.</td><td>d.l.</td><td>26</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>2.1</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<table><caption>Table 3. Major (EPMA) element composition in wt% of carbonates. *SD* is for standard deviation.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Smithsonite (n = 9)</th><th colspan="4">Fe-rich smithsonite (n = 19)</th><th colspan="4">Zn-rich siderite (n = 6)</th></tr><tr><th>Mean</th><th>SD</th><th>Min</th><th>Max</th><th>Mean</th><th>SD</th><th>Min</th><th>Max</th><th>Mean</th><th>SD</th><th>Min</th><th>Max</th></tr></thead><tbody><tr><td>MgO</td><td>2.0</td><td>1.9</td><td>0.1</td><td>5.6</td><td>3.5</td><td>1.2</td><td>1.4</td><td>5.1</td><td>2.6</td><td>0.9</td><td>1.6</td><td>3.8</td></tr><tr><td>MnO</td><td>0.4</td><td>0.5</td><td>0.0</td><td>1.3</td><td>1.1</td><td>1.0</td><td>0.1</td><td>2.9</td><td>0.8</td><td>0.5</td><td>0.1</td><td>1.7</td></tr><tr><td>FeO</td><td>0.9</td><td>1.0</td><td>0.0</td><td>3.1</td><td>15.5</td><td>5.9</td><td>9.0</td><td>27.5</td><td>43.7</td><td>1.8</td><td>41.6</td><td>47.0</td></tr><tr><td>CdO</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.1</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td></tr><tr><td>CaO</td><td>0.5</td><td>0.4</td><td>0.0</td><td>1.1</td><td>1.1</td><td>0.6</td><td>0.1</td><td>1.8</td><td>4.6</td><td>0.6</td><td>3.8</td><td>5.4</td></tr><tr><td>ZnO</td><td>61.5</td><td>4.1</td><td>57.5</td><td>66.6</td><td>41.5</td><td>7.7</td><td>27.0</td><td>51.7</td><td>6.9</td><td>2.0</td><td>3.9</td><td>9.4</td></tr><tr><td>CO<sub>2</sub>*</td><td>36.9</td><td>1.1</td><td>35.7</td><td>39.4</td><td>37.5</td><td>1.0</td><td>35.7</td><td>39.2</td><td>38.0</td><td>0.3</td><td>37.7</td><td>38.5</td></tr><tr><td>Total</td><td>102.0</td><td>1.1</td><td>100.4</td><td>103.6</td><td>100.1</td><td>1.3</td><td>97.9</td><td>103.3</td><td>96.6</td><td>0.8</td><td>95.6</td><td>97.8</td></tr></tbody></table>

modifications than to primary hypogene process. Therefore, it is not possible to distinguish between low-temperature hydrothermal processes and a strict supergene origin for willemite mineralization.

This study also reveals the polyphase nature of willemite ores from the historical deposits of Eastern Belgium. Following previous studies, which reported idiomorphic crystals with hexagonal habits and botryoidal aggregates showing acicular radiated crystals (Boni et al., 2005; Coppola et al., 2008), two successive types, namely W<sub>R</sub> and W<sub>C</sub> are distinguished. We also noted that these two types have themselves recorded a complex evolution (e.g., W<sub>C1</sub> and W<sub>C2</sub>; Fig. 6). Although all these willemite types

have distinct morphological and internal zoning features, they are chemically similar (Table 2). Following a botryoidal stage, we also noted the precipitation of late willemite (W<sub>2</sub>), whose mineral chemistry significantly differs from those of other types. The latter occurs as a cement between early spherules and aggregates (W<sub>1</sub>), but predates the formation of zinc carbonates (Fig. 3C–E). Its precipitation is also texturally associated with the development of quartz rims around pre-existing quartz (Fig. 6).

The polyphase nature of willemite mineralization was also described in several other deposits throughout the world. In the Bou Arhous deposit in Morocco, several morphological types of willemite are reported, but only two separate

<footnote>*Calculated from stoichiometry.</footnote>
<table><caption>Table 4. Major (EPMA) and trace (LA-ICPMS) element composition of sphalerite. <d.l.: Maximum value lower than detection limit (see Materials and Methods section for d.l. calculation).</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2"></th><th colspan="3">wt%</th><th colspan="10">ppm</th></tr><tr><th>Zn</th><th>S</th><th>Fe</th><th>Co</th><th>Ni</th><th>Cu</th><th>Ga</th><th>Ge</th><th>As</th><th>Ag</th><th>Cd</th><th>Sn</th><th>Sb</th><th>Pb</th></tr></thead><tbody><tr><td rowspan="3">RV2-837</td><td>n = 17</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>66.8</td><td>32.1</td><td>0.7</td><td>0.5</td><td>9.1</td><td>1.1</td><td>0.1</td><td>269</td><td>667</td><td>3.3</td><td>2381</td><td>0.2</td><td>77</td><td>1805</td></tr><tr><td>Min</td><td>63.2</td><td>31.2</td><td>0.1</td><td>&lt;d.l.</td><td>5.9</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>20</td><td>55</td><td>&lt;d.l.</td><td>414</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>479</td></tr><tr><td rowspan="3">RN6483</td><td>n = 23</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>65.8</td><td>31.9</td><td>1.1</td><td>0.5</td><td>1.9</td><td>4.8</td><td>0.6</td><td>246</td><td>417</td><td>1.6</td><td>593</td><td>0.3</td><td>15</td><td>1874</td></tr><tr><td>Min</td><td>38.7</td><td>23.3</td><td>0.0</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>19</td><td>1.1</td><td>0.4</td><td>58</td><td>&lt;d.l.</td><td>0.8</td><td>36</td></tr><tr><td></td><td>Max</td><td>69.1</td><td>32.8</td><td>5.7</td><td>1.7</td><td>3.4</td><td>27</td><td>3.2</td><td>1325</td><td>4540</td><td>13</td><td>2109</td><td>1.0</td><td>70</td><td>10060</td></tr></tbody></table>

generations were clearly distinguished, based on the presence of an intermediate dissolution stage (Choulet *et al.*, 2017). In Namibian Berg-Aukas type Zn deposits (Misiewicz, 1988), up to three successive stages of willemite are distinguished, especially by the different cathodoluminescence colours (Terracciano, 2008). No difference of major-element chemistry between willemite generations is observed, and no trace-elements analysis is currently available (Terracciano, 2008). Willemite is considered to have precipitated from low-T hydrothermal fluids and predates a later strictly supergene carbonate stage. In the Kabwe deposit in Zambia, the two successive willemite generations are also reported. While Kamona & Friedrich (2007) have proposed an early hypogene stage followed by supergene stage, Terracciano (2008) proposed that willemite is of supergene or low-T hydrothermal origin. Recent oxygen isotope data obtained by Mondillo *et al.* (2018b) support that both generations of willemite and the late carbonates are not cogenetic in the Kabwe ores, ruling out a strictly supergene scenario for willemite precipitation.

Although we cannot distinguish between low-T hydrothermal and supergene origin for willemite in the Belgian deposits, all microscopic observations support that all willemite types, except $W_2$, are part of a single continuous stage. The differences in trace-element composition (Table 2) could reflect either a local source effect, impurities or a progressive change of composition for the mineralizing fluid. Such a change was observed in the willemites from Bou Arhous (Choulet *et al.*, 2017) and Tres Marias (Saini-Eidukat *et al.*, 2009), since oscillatory zoning is associated with changes of minor and trace elements composition.

## 5.2. Link between compositions of willemite and protore minerals

In situ measurements by LA-ICPMS revealed a significant amount of trace elements incorporated in willemite. Among them, high Pb content is correlated with other metal enrichment (especially Ag) but not with Zn content, suggesting that nano to micro-inclusions of Pb-rich minerals occur within willemite (Fig. 2G and H). A similar interpretation was proposed for the $W_1$ willemite from Kabwe (Mondillo *et al.*, 2018b). Conversely, high Pb contents (up to 2 wt%) were measured in willemite from Bou Arhous, Morocco

(Choulet *et al.*, 2017) and Tres Marias, Mexico (Saini-Eidukat *et al.*, 2009). In both cases, there is a strong negative correlation between Zn and Pb, suggesting that Pb substitutes for Zn in the willemite structure. This is confirmed by XANES spectrometry, which shows that Pb is present as eightfold-coordinated $Pb^{2+}$ in agreement with a lattice substitution model (Saini-Eidukat *et al.*, 2016).

Arsenic and phosphorous also occur in variable amounts within willemite (up to 3867 ppm and 3940 ppm, respectively). The concentration of As is higher than previously reported for willemite from Belgium (Coppola *et al.*, 2008). Such enrichments were reported for Tres Marias willemite with up to 0.76 wt% $P_2O_5$ and 0.8 wt% $As_2O_3$ contents (Saini-Eidukat *et al.*, 2009). In addition, these elements display heterogeneous distribution and can be used to discriminate between the willemite types with different morphological features. While the As contents generally average several hundred ppm in most willemite types (Table 2), very low As contents (less than 60 ppm) are measured in inherited core crystals ($W_{C1}$) and in late willemite crystals ($W_2$). Significant differences in As content are also reported between the two willemite generations from the Kabwe deposit (Mondillo *et al.*, 2018b). Further investigation of the P and As contents and their distribution could be used as geochemical fingerprints in order to better understand the nature of fluids involved in willemite mineralization.

The cadmium content differs between early (means ranging from 4 ppm to 14 ppm) and late generations of willemite (mean 47 ppm for $W_2$; Table 2). The latter value is consistent with the average values reported from Kabwe willemite (means 40–71 ppm; Mondillo *et al.*, 2018b). Indium concentrations are below detection limit, as measured for Kabwe willemite (Mondillo *et al.*, 2018b). The Ga content is lower than 4 ppm (Table 2), showing values significantly lower than those reported from the Kabwe deposit (up to 28 ppm; Mondillo *et al.*, 2018b). Despite the absence of obvious correlation between Ga and Al at Kabwe, both elements are positively correlated in samples from Belgium. This could reflect a contamination of several analyses by Al-rich phases (e.g., clay minerals), which are commonly reported either as inclusion or as secondary products after willemite (e.g., Choulet *et al.*, 2016). The Ge contents range between the detection limit and 250 ppm, with means of several tens of ppm depending on the willemite
<figure><img src="image_6.png" /><figcaption>Fig. 5. Elementary composition of sphalerite. (A–C) Chemical mapping (Cd and Fe) by EPMA of banded *schalenblende* spherulite with a pyrite inclusion in the core; note the first-order inverse correlation between Cd and Fe. (D and E) Elementary profiles, based on LA-ICPMS data, across banded sphalerite to illustrate the correlation between Fe and Ge domains.</figcaption></figure>

type. However, except for the low Ge contents in late willemite stage $W_2$, no significant differences in Ge content are observed between other willemite types (Table 2). Means of several tens of ppm of Ge were also measured in Kabwe willemite (Mondillo et al., 2018b), and in several willemite crystals from supergene deposits of New Mexico and Utah (Sheffer, 1966). As these values are significantly higher than the content measured in hypogene willemite from Franklin (<10 ppm; Sheffer, 1966), it is significantly lower than in several localities throughout the world, such as Tsumeb in Namibia (up to 1280 ppm; Lombaard et al., 1986) and Tres Marias in Mexico (4000 ppm; Saini-Eidukat et al., 2009). Such high Ge contents in willemite are explained by incorporation of $Ge^{4+}$ within the silica tetrahedra (Saini-Eidukat et al., 2016). Choulet et al. (2017) also reported Ge contents up to 1100 ppm in willemite from the Bou Arhous deposits. Interestingly, in the latter case, the content is highly heterogeneous and depends on the willemite type, defined by sector and oscillatory zoning (Choulet et al., 2017). In the case of Belgian deposits, we do not observe sector zoning like in Morocco. Together with oscillatory zoning, core and rim textures are frequently observed (Fig. 2), but the Ge content

does not differ between the various morphological and zoning features. Crystallographic controls on trace-element composition are therefore limited to particular textures, as also observed for a range of minerals (Reeder & Rakovan, 1999), e.g., sphalerite (Belissont et al., 2014) or pyrite (Chouinard et al., 2005).

In addition to crystal surface structure (e.g., Reeder & Rakovan, 1999) and kinetics effects (e.g., Putnis, 2009), the trace-element contents of willemite are directly influenced by the nature of mineralizing fluids and the initial contents of mineral phases predating willemite. The germanium concentrations are generally low (<0.1 ppb) in surface water (e.g., Höll et al., 2007). In thermal water, this concentration can reach up to 100 ppb (e.g., Kraynov, 1967; Simmons et al., 2018). Germanium is transported as hydroxide and chloro-complexes (Pokrovski & Schott, 1998). Although the Ge content of fluids increases with temperature (Bernstein, 1985), hypogene willemite from Franklin has low Ge content (10 ppm; Sheffer, 1966). At Kabwe, early willemite probably formed by hypogene processes is depleted in Ge compared to the late willemite derived from either hydrothermal or supergene processes (Mondillo et al.,
<figure><img src="image_7.png" /><figcaption>Fig. 6. Summary paragenetic sequence of the observed and inferred (from Coppola *et al.*, 2008) minerals in the non-sulphide mineralisation of eastern Belgium. Insert is a detailed view of the willemite types reported in the samples. See text for explanation of willemite typology. Thickness of the horizontal line corresponds to the relative abundance of each mineral species.</figcaption></figure>

2018b). This suggests that the geochemistry of the mineral-
izing fluid has only a very limited influence on the trace-
element content of willemite formed by supergene or
hydrothermal processes.

As reported in many studies, willemite occurs as a
replacement of primary sphalerite (Sweeney et al., 1991;
Coppola et al., 2008; Terracciano, 2008; Saini-Eidukat
et al., 2009). Sphalerite is known for hosting a large range
of trace elements (Cook et al., 2009), including germanium
or gallium. In general, a Ga-Ge-enriched group is distin-
guished from a Fe-Mn-In group and corresponds to low-
temperature deposits, such as MVT deposits (Belissont
et al., 2014; Frenzel et al., 2016). High concentrations of
Ge up to several thousands ppm have been reported in sam-
ples from Tres Marias (max. 1320 ppm; Saini-Eidukat et al.,
2009), Saint-Salvy (max. 2576 ppm; Belissont et al., 2014)
or Kipushi (5930 ppm; Belissont, 2016), but a correlated Ga
enrichment is not always reported (e.g., Cook et al., 2009;
Ye et al., 2011). In sphalerite from Belgium, the Ge content
ranges from 19 ppm to 1325 ppm with averages at 270 ppm
and 250 ppm, depending on samples (Table 4). The Ga con-
tent is lower than 3 ppm; these values are consistent with
previously published results from sphalerite collected in
eastern Belgium, with means for Ge and Ga at 302 ppm
and 2.2 ppm, respectively (Goffin et al., 2015). Other
sulphides (galena and pyrite) carry very low Ge and Ga con-
tents (Duchesne et al., 1983). As already pointed out by
Goffin et al. (2015), the Ge concentration in sphalerite
increases with iron content (Fig. 5). Such trend is also

observed in sphalerites from the Tres Marias MVT deposit (Saini-Eidukat et al., 2009), suggesting the following substitution 2Fe²⁺ + Ge⁴⁺ + □ ↔ 4Zn²⁺ (□ for vacancy; Cook et al., 2009, 2015). This trend is quite unusual because Ge (and Ga) enrichment generally occurs in Fe-poor bands or sectors in MVT deposits (Cook et al., 2009; Belissont et al., 2014; Frenzel et al., 2016). The Ge contents in willemite and sphalerite are of the same order (Tables 2 and 4). This is very similar to Ge-poor deposits such as Kabwe (3–91 ppm for willemite; 1–31 ppm for sphalerite; Mondillo et al., 2018b), or Ge-rich deposits at Tres Marias (up to 4000 ppm for willemite; up to 1320 ppm for sphalerite; Saini-Eidukat et al., 2009). This further indicates that the composition of primary sphalerite has a crucial role in the final Ge (and Ga) content of willemite. However, some exceptions are also reported, e.g., Ge-rich (up to 1100 ppm) willemite replacing Ge-poor sphalerite (<100 ppm) at Bou Arhous (Choulet et al., 2017). In this case, such a difference can be explained rather by selective incorporation of Ge in particular faces of the willemite crystals than by an external supply from Ge-rich hydrothermal fluids, which are extremely rare even in sulphide-rich mine waters (Goleva & Vorobeyeva, 1967).

An alternative control for trace-element enrichment in a
mineral phase is the competition with other minerals formed
earlier or synchronously. In all willemite-bearing deposits,
willemite is always the first mineral to be formed after the
protore stage. In some rare cases, it can be preceded by sec-
ondary sulphides (galena, greenockite, covellite), which can
armour sphalerite (Fig. 4F) and trigger minor elements such as Cd into greenockite (Choulet et al., 2014). Other mineral phases may co-precipitate in the oxidation zone and serve as potential hosts for trace elements and particularly Ge (Höll et al., 2007). In the oxidized ore of the Tsumeb deposit, willemite occurs together with numerous secondary minerals such as Ge oxides and Ge hydroxides (Lombaard et al., 1986; Melcher, 2003). At Tsumeb, iron hydroxides and various arsenates can also incorporate large amounts of Ge (up to 2.5 wt% in Melcher, 2003), limiting Ge content within willemite to 1280 ppm (Lombaard et al., 1986). Such competition between mineral phases is also illustrated in the supergene Cristal non-sulphide prospect in Peru, where Ge, initially present in sphalerite (mean at 140 ppm), is distributed between goethite (means between 100 ppm and 230 ppm) and hemimorphite (means between 40 ppm and 140 ppm) (Mondillo et al., 2018a). The incorporation of Ge in the minerals of the carbonate stage is limited to few ppm, though, as exemplified by low Ge contents in supergene smithsonite from Jbel Haounit, eastern Morocco (Verhaert et al., 2017). Such secondary distribution has important implication either for exploration of unconventional Ge targets or for optimized mineral processing to recover germanium.

## 6. Conclusion

This study presents new observations and chemical analyses of willemite mineralization from the historical "Calamine-type" deposits of eastern Belgium. Willemite occurs as a variety of types that continuously formed between the proto stage (sulphides) and the supergene stage (carbonates and hydrated phases). While the different types of willemite may be distinguished by their shape and zoning characteristics, changes of composition are slight, except for the late type, which probably represents a separate and later generation. Among trace elements, Ge contents up to 250 ppm are measured, consistent with other values reported from willemite mineralization throughout the world. Concentrations obtained in willemite are similar to those measured in sphalerite (ranging between 19 ppm and 1325 ppm, mean at 250 ppm).

Based on these results, we conclude that the non-sulphide zone of ore deposits has a real potential for exploration of minor metals and especially Ge. However, several features should be taken into account:

* The extent of the alteration/weathering zone itself depends on climate and tectonic factors, which control its development and its preservation through time;

* Ge enrichment is accompanied by a range of other minor elements (As, P, Cd, Pb), which may have a negative impact either on environmental issues or recovery process;

* willemite mineralization often occurs as a variety of forms, exposing variations of trace-element composition. The latter can change due to (1) the initial content of sulphide precursors, (2) the addition of metals by external fluids (hydrothermal or supergene), (3) a

competition between willemite and other phases [especially iron (hydr-)oxides] for hosting trace elements, and (4) fractionation within mineral phases with selective incorporation in the crystal structure.

**Acknowledgements:** Thanks are due to Marleen De Ceukelaire for assistance during sampling at the Royal Belgian Institute of Natural Sciences (Brussels). This work was partly associated to the project « Révision de la Carte géologique de Wallonie » of the Geological Survey of Wallonia (Service Public de Wallonie). The authors also warmly acknowledge the assistance of Didier Convert-Gaubier (Chrono-Environment, Besançon) for thin-section preparation and Martin Robyr (Université de Lausanne) for EPMA data acquisition. This study has benefited from financial support by INSU-CESSUR 2016 programme. Research activity of Marie-Christine Boiron is supported by the French National Research Agency through the "Investissements d'avenir" programme, reference ANR-10-LABX-21-LABEX. Access to SEM facilities of the MIMENTO centre was made in the frame of cooperative projects in the Renatech network. This paper has benefited from the careful handling and reviews of Patrick Cordier (Editor-in-Chief), Allan Pring (Associate Editor), Franck Melcher (reviewer) and an anonymous reviewer.

## References

Belanger, I., Delaby, S., Delcambre, B., Ghysel, P., Hennebert, M., Laloux, M., Marion, J.-L., Mottequin, B., Pingot, J.-L. (2012): Redéfinition des unités structurales du front varisque utilisées dans le cadre de la nouvelle Carte géologique de Wallonie (Belgique). *Geol. Belg.*, **15**, 169-175.

Belissont, R. (2016): Germanium and related elements in sulphide minerals: crystal chemistry, incorporation and isotope fractionation. PhD Dissertation, Université de Lorraine, Nancy, France, 210 p.

Belissont, R., Boiron, M.-C., Luais, B., Cathelineau, M. (2014): LA-ICP-MS analyses of minor and trace elements and bulk Ge isotopes in zoned Ge-rich sphalerites from the Noailhac-Saint-Salvy deposit (France): insights into incorporation mechanisms and ore deposition processes. *Geochim. Cosmochim. Acta*, **126**, 518-540.

Belissont, R., Munoz, M., Boiron, M.C., Luais, B., Mathon, O. (2016): Distribution and oxidation state of Ge, Cu and Fe in sphalerite by µ-XRF and K-edge µ-XANES: insights into Ge incorporation, partitioning and isotopic fractionation. *Geochim. Cosmochim. Acta*, **177**, 298-314.

Bernstein, L.R. (1985): Germanium geochemistry and mineralogy. *Geochim. Cosmochim. Acta*, **49**, 2409-2422.

— (1986): Geology and mineralogy of the APEX germanium-gallium mine, Washington County, Utah. *USGS Bull.*, **1577**, 1-9.

Boni, M. & Mondillo, N. (2015): The "Calamines" and the "Others": the great family of supergene nonsulfide zinc ores. *Ore Geol. Rev.*, **67**, 208-233.

Boni, M., Coppola, V., Dejonghe, L., Fedele, L. (2005): Willemite in the Belgian nonsulfide zinc deposits: a fluid inclusion study. *Period. Mineral.*, **74**, 87-100.

Boni, M., Gilg, H.A., Aversa, G., Balassone, G. (2003): The "Calamine" of SW Sardinia (Italy): geology, mineralogy and
stable isotope geochemistry of a supergene Zn-mineralization. *Econ. Geol.*, **98**, 731-748.

Boni, M., Terracciano, R., Balassone, G., Gleeson, S.A., Matthews, A. (2011): The carbonate-hosted willemite prospects of the Zambezi Metamorphic Belt (Zambia). *Mineral. Deposita*, **46**, 707-729.

Borg, G. (2009): The role of fault structures and deep oxidation in supergene base metal deposits. in "Supergene environments, processes and products", S.R. Titley, ed., *Econ. Geol. Spec. Publ.*, **14**, 121-132.

Brugger, J., McPhail, D.C., Wallace, M., Waters, J. (2003): Formation of willemite in hydrothermal environments. *Econ. Geol.*, **8**, 919-935.

Cesàro, G. (1887): Note sur quelques minéraux (dont la willémite fibroradiée de Moresnet). *Ann. Soc. Géol. Belg.*, **14**, 142-143.

Chouinard, A., Paquette, J., Williams-Jones, A.E. (2005): Crystallographic controls on trace-element incorporation in auriferous pyrite from the Pascua epithermal high-sulfidation deposit, Chile-Argentina. *Can. Mineral.*, **43**, 951-963.

Choulet, F., Charles, N., Barbanson, L., Branquet, Y., Sizaret, S., Ennaciri, A., Badra, L., Chen, Y. (2014): Non-sulfide zinc deposits of the Moroccan High Atlas: multi-scale characterization and origin. *Ore Geol. Rev.*, **56**, 115-140.

Choulet, F., Buatier, M., Barbanson, L., Guégan, R., Ennaciri, A. (2016): Zinc-rich clays in supergene non-sulfide zinc deposits. *Mineral. Deposita*, **51**, 467-490.

Choulet, F., Barbanson, L., Buatier, M., Richard, J., Vennemann, T., Ennaciri, A., Zouhair, M. (2017): Characterization and origin of low-T willemite (Zn₂SiO₄) mineralization: the case of the Bou Arhous deposit (High Atlas, Morocco). *Mineral. Deposita*, **52**, 1085-1102.

Cook, N.J., Ciobanu, C.L., Pring, A., Skinner, W., Danyushevsky, L., Shimizu, M., Saini- Eidukat, B., Melcher, F. (2009): Trace and minor elements in sphalerite: a LA-ICP-MS study. *Geochim. Cosmochim. Acta*, **73**, 4761-4791.

Cook, N.J., Etschmann, B., Ciobanu, C.L., Geraki, K., Howard, D. L., Williams, T., Rae, N., Pring, A., Chen, G., Johannessen, B., Brugger, J. (2015): Distribution and substitution mechanism of Ge in Ge-(Fe)-bearing sphalerite. *Minerals*, **5**, 117-132.

Coppola, V., Boni, M., Gilg, H.A., Balassone, G., Dejonghe, L. (2008): The “Calamine” nonsulfide Zn-Pb deposits of Belgium: petrographical, mineralogical and geochemical characterization. *Ore Geol. Rev.*, **33**, 187-210.

Dejonghe, L. (1998): Zinc-lead deposits of Belgium. *Ore Geol. Rev.*, **12**, 329-354.

Dejonghe, L. & Boni, M. (2005): The “Calamine-type” zinc-lead deposits in Belgium and West Germany: a product of Mesozoic palaeoweathering processes. *Geol. Belg.*, **8-3**, 3-14.

Dejonghe, L. & Jans, D. (1983): Les gisements plombo-zincifères de l'Est de la Belgique. *Chron. Rech. Min.*, **470**, 3-24.

Dejonghe, L., Ladeuze, F., Jans, D. (1993): Atlas des gisements plombo-zincifères du Synclinorium de Verviers (Est de la Belgique). *Mém. Cartes Géol. Min. Belg.*, **33**, 1-483.

Demoulin, A., Barbier, F., Dekoninck, A., Verhaert, M., Ruffet, G., Dupuis, C., Yans, J. (2018): Erosion surfaces in the Ardenne-Oesling and their associated kaolinic weathering mantle. in "Landscapes and landforms of Belgium and Luxembourg". A. Demoulin, ed. Springer International Publishing, Cham, 63-84.

Drzazga, M., Prajsnar, R., Chmielarz, A., Benke, G., Leszczyńska-Sejda, K., Ciszewski, M., Bilewska, K., Krawiec, G. (2018): Germanium and indium recovery from zinc metallurgy by-products—dross leaching in sulphuric and oxalic acids. *Metals*, **8**, 1041.

Duchesne, J.-C., Rouhart, A., Schoumaker, C., Dillen, H. (1983): Thallium, nickel, cobalt, and other trace elements in iron sulphides from Belgian lead-zinc vein deposits. *Mineral. Deposita*, **18**, 303-313.

Fielitz, W. & Mansy, J.-L. (1999): Pre- and synorogenic burial metamorphism in the Ardenne and neighbouring areas (Rheno-hercynian zone, central European Variscides). *Tectonophysics*, **309**, 227-256.

Franke, W. (2000): The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution. in "Orogenic processes: quantification and modelling in the Variscan Belt", W. Franke, V. Haak, O. Oncken, D. Tanner, eds., Geological Society, Special Publications. Geological Society of London, London, 35-61.

Frenzel, M., Hirsch, T., Gutzmer, J. (2016): Gallium, germanium, indium, and other trace and minor elements in sphalerite as a function of deposit type — a meta-analysis. *Ore Geol. Rev.*, **76**, 52-78.

Goffin, V., Evrard, M., Pirard, E. (2015): Critical metals in sphalerites from Belgian MVT deposits. in "Proceedings of 13th SGA Biennial Meeting" *Vol. 2*, A.S. André-Mayer, M. Cathelineau, P. Muchez, E. Pirard & S. Sindern, eds., 24-27 August, Nancy, France, 741-744.

Goleva, G.A. & Vorobeyeva, I.N. (1967): The migration of germanium in the ground waters of ore deposits. *Geochem. Int.*, **4**, 809-817.

Goossens, P.J. (2014): Zinc potential in Eastern Belgium. *Eur. Geol. Rev.*, **37**, 7-11.

Graulich, J.M., Dejonghe, L., Cnudde, C. (1984): La définition du Synclinorium de Verviers. *Bull. Soc. Belg. Géol.*, **93**, 79-82.

Groves, I., Carman, C.E., Dunlap, W.J. (2003): Geology of the Baltana willemite deposit, Flinders Ranges, South Australia. *Econ. Geol.*, **98**, 797-818.

Heijlen, W., Muchez, P., Banks, D.A. (2001): Origin and evolution of high-salinity, Zn-Pb mineralising fluids in the Variscides of Belgium. *Mineral. Deposita*, **36**, 165-176.

Hitzman, M.W., Reynolds, N.A., Sangster, D.F., Allen, C.R., Carman, C. (2003): Classification, genesis, and exploration guides for nonsulfide zinc deposits. *Econ. Geol.*, **98**, 685-714.

Höll, R., Kling, M., Schroll, E. (2007): Metallogenesis of germanium—a review. *Ore Geol. Rev.*, **30**, 145-180.

Kamona, A.F. & Friedrich, G.H. (2007): Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb-Zn deposit, Central Zambia. *Ore Geol. Rev.*, **30**, 217-243.

Kraynov, S.R. (1967): Geochemistry of germanium in the thermal carbonate waters (illustrated by examples from the Pamirs and Greater Caucasus). *Geochem. Int.*, **4**, 309-320.

Lombaard, A.F., Günzel A., Innes J., Krüger T.L. (1986): The Tsumeb lead-copper-zinc-silver deposit, South West Africa/Namibia. in "Mineral deposits of Southern Africa, Vol. 2", C.R. Anhaeusser & S. Maske, eds. Geological Society of South Africa, Johannesburg, 1761-1782.

Longerich, H.P., Jackson, S.E., Gunther, D. (1996): Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. *J. Anal. Atom. Spectrom.*, **11**, 899-904.

Meilliez, F. & Mansy, J.-L. (1990): Déformation pelliculaire différenciée dans une série lithologique hétérogène: le Dévono-Carbonifère de l'Ardenne. *Bull. Soc. Géol. France*, **8-6**, 177-188.

Melcher, R. (2003): The Otavi Mountain Land in Namibia: Tsumeb, germanium and snowball earth. *Mitt. Österreich. Mineral. Gesell.*, **148**, 413-435.

Michot, P. (1980): Le segment tectogénique calédonien belge. *Mém. Classe Sci. Acad. R. Belg.*, **1890**, 1-61.

Misiewicz, J.E. (1988): The geology and metallogeny of the Otavi Mountain Land, Damara Orogen, SWA/Namibia, with particular reference to the Berg Aukas Zn-Pb-V deposit – a model of ore genesis. MSc Thesis, Rhodes University, Grahamstown, ZA, 143 p.
Mondillo, N., Arfè, G., Herrington, R., Boni, M., Wilkinson, C.,
Mormone, A. (2018a): Germanium enrichment in supergene
settings: evidence from the Cristal nonsulfide Zn prospect,
Bongará district, northern Peru. *Mineral. Depos.*, **53**, 155–169.

Mondillo, N., Herrington, R., Boyce, A.J., Wilkinson, C., Santoro,
C., Rumsey, M. (2018b): Critical elements in non-sulfide Zn
deposits: a reanalysis of the Kabwe Zn-Pb ores (central
Zambia). *Mineral. Mag.*, **82**, S89–S114.

Monteiro, L.V.S., Bettencourt, J.S., Juliani, C., Oliveira, T.F.
(2007): Non-sulfide and sulfiderich zinc mineralizations in the
Vazante, Ambrosia and Fagundes deposits, Minas Gerais,
Brazil: mass balance and stable isotope characteristics of the
hydrothermal alterations. *Gondwana Res.*, **11**, 362–381.

Muchez, P., Slobodnik, M., Viaene, W., Keppens, E. (1994):
Mississippi Valley-type Pb-Zn mineralization in Eastern Bel-
gium: indications for gravity-driven flow. *Geology*, **22**, 1011–
1014.

Muchez, P., Heijlen, W., Banks, D., Blundell, C., Boni, M.,
Grandia, F. (2005): Extensional tectonics and the timing and
formation of basin-hosted deposits in Europe. *Ore Geol. Rev.*
**27**, 241–267.

Oncken, O., von Winterfeld, C., Dittmar, U. (1999): Accretion of a
rifted passive margin: the Late Paleozoic Rhenohercynian fold
and thrust belt (Middle European Variscides). *Tectonics*, **18**,
75–91.

Paton, C., Hellstrom, J., Paul, B., Woodhead, J., Hergt, J. (2011):
Iolite: freeware for the visualisation and processing of mass
spectrometric data. *J. Anal. Atom. Spectr.*, **26**, 2508. doi:
10.1039/c1ja10172b.

Pearce, N.J.G., Perking, W.T., Westgate, J.A., Gorton, M.P.,
Jackson, S.E., Neal, C.R., Chenery, S.P. (1997): A compilation
of new and published major and trace element data for
NIST SRM610 and NIST SRM612 glass materials. *Geostand.
Newslett.*, **21**, 115–144.

Pokrovski, G.S. & Schott, J. (1998): Thermodynamic properties of
aqueous Ge (IV) hydroxide complexes from 25 to 350 °C:
implications for the behaviour of germanium and the Ge/Si ratio
in hydrothermal fluids. *Geochim. Cosmochim. Acta*, **62**, 1631–1642.

Putnis, A. (2009): Mineral replacement reactions. *Rev. Mineral. Geochem.*, **70**, 87–124.

Quesnel, F. (2003): Paleoweathering and paleosurfaces from
northern and eastern France to Belgium and Luxembourg:
geometry, dating and geodynamic implications. *Géol. Fr.*, **1**,
95–104.

Reeder, R.J. & Rakovan, J. (1999): Surface structural controls on
trace element incorporation during crystal growth. in "Growth,
dissolution and pattern formation in geosystems", B. Jamtveit &
P. Meakin, eds. Springer, Dordrecht, 143–162.

Reich, M. & Vasconcelos, P.M. (2015): Geological and economic
significance of supergene metal deposits. *Elements*, **11**, 305–310.

Saini-Eidukat, B., Melcher, F., Lodziak, J. (2009): Zinc-germanium
ores of the Tres Marias Mine, Chihuahua, Mexico. *Mineral.
Deposita*, **44**, 363–370.

Saini-Eidukat, B., Melcher, F., Göttlicher, J., Steininger, R. (2016):
Chemical environment of unusually Ge- and Pb-rich willemite,
Tres Marias Mine, Mexico. *Minerals*, **6**, 20.

Sangster, D.F. (1990): Mississippi Valley-type and Sedex lead-zinc
deposits: a comparative examination. *Trans. Inst. Min. Metall.
(Sect. B, Appl. Earth Sci.)*, **99**, 21–42.

Schneider, J., Haack, U., Hein, U.F., Germann, A. (1999): Direct
Rb-Sr dating of sandstone-hosted sphalerites from stratabound

Pb-Zn deposits in the northern Eifel, NW Rhenish Massif,
Germany. in "Mineral deposits: processes to processing", C.J.
Stanley *et al.*, eds. Balkema, Rotterdam, 1287-1290.

Schwartz, M.O. (2000): Cadmium in zinc deposits: economic
geology of a polluting element. *Int. Geol. Rev.*, **42**, 445–469.

Sheffer, H.W. (1966): The occurrence of germanium in willemite.
*Geochim. Cosmochim. Acta*, **30**, 837–838.

Simmons, S.F., Kirby, S., Verplanck, P., Kelley, K. (2018):
Strategic and critical elements in produced geothermal fluids
from Nevada and Utah. in "Proceedings 43rd Workshop on
Geothermal Reservoir Engineering Stanford University", Stan-
ford, California, February 12–14. SGP-TR-213.

Sintubin, M., Debacker, T.N., Van Baelen, H. (2009): Early
Palaeozoic orogenic events north of the Rheic suture (Brabant,
Ardenne): a review. *C. R. Geosci.*, **341**, 156–173.

Slezak, P.R., Olivo, G.R., Oliveira, G.D., Dardenne, M.A. (2014):
Geology, mineralogy, and geochemistry of the Vazante North-
ern Extension zinc silicate deposit, Minas Gerais, Brazil. *Ore
Geol. Rev.*, **56**, 234–257.

Sweeney, M.A., Patrick, R.A.D., Vaughan, D.J. (1991): The nature
and genesis of willemite deposits of Zambia. in "Source,
transport, and ore deposition of metals", M. Pagel & J. Leroy,
eds. Balkema, Rotterdam, 139-142.

Terracciano, R. (2008): Willemite Mineralisation in Namibia and
Zambia. PhD Dissertation, Università degli Studi di Napoli
Federico II, Napoli, 178 p.

Thiry, M., Quesnel, F., Yans, J., Wyns, R., Vergari, A., Thévenaut,
H., Simon-Coinçon, R., Ricordel, C., Moreau, M.-G., Giot, D.,
Dupuis, C., Bruxelles, L., Barbarand, J., Baele, J.-M. (2006):
Continental France and Belgium during the Early Cretaceous:
paleoweatherings and paleolandforms. *Bull. Soc. Géol. Fr.*, **177**,
155-175.

Vercautere, C. & van den Haute, P. (1993): Post-Palaeozoic cooling
and uplift of the Brabant Massif as revealed by apatite fission
track analysis. *Geol. Mag.*, **130**, 639–646.

Verhaert, M., Bernard, A., Dekoninck, A., Lafforgue, L., Saddiqi,
O., Yans, J. (2017): Mineralogical and geochemical character-
ization of supergene Cu-Pb-Zn-V ores in the Oriental High
Atlas, Morocco. *Mineral. Deposita*, **52**, 1049–1062.

Wilson, S.A., Ridley, W.I., Koenig, A.E. (2002): Development of
sulfide calibration standards for the laser ablation inductively-
coupled plasma mass spectrometry technique. *J. Anal. Atom.
Spectrom.*, **17**, 406–409.

Yans, J. (2003): An overview of the saprolites of Belgium and their
potential kaolinite supplies to Mesozoic and Cainozoic sedi-
ments. *Géol. Fr.*, **1**, 33–37.

Ye, L., Cook, N.J., Ciobanu, C.L., Liu, Y.P., Zhang, Q., Liu, T.G.,
Gao, W., Yang, Y.L., Danyushevskiy, L. (2011): Trace and
minor elements in sphalerite from base metal deposits in South
China: a LA-ICPMS study. *Ore Geol. Rev.*, **39**, 188–217.

Yesares, L., Saez, R., Nieto, J.M., de Almodovar, G.R., Cooper, S.
(2014): Supergene enrichment of precious metals by natural
amalgamation in the Las Cruces weathering profile (Iberian
Pyrite Belt, SW Spain). *Ore Geol. Rev.*, **58**, 14–26.

Received 7 March 2019

Modified version received 29 May 2019

Accepted 3 June 2019