Sec ond ary ar senic min er als from the Z³oty Stok As-Au aban doned mine (SW Po land)
Rafa³ SIUDA1, * and Anna MACIOCH1
1 Uni ver sity of War saw, In sti tute of Geo chem is try, Min er al ogy and Pe trol ogy, ¯wirki i Wigury 93, 02-089 Warszawa, Po land
Siuda R., Macioch, A., 2018. Sec ond ary ar senic min er als from the Z³oty Stok As-Au aban doned mine (SW Po land). Geo log i - cal Quar terly, 62 (4): 925–940, doi: 10.7306/gq.1442
As so ci ate ed i tor: Stanis³aw Wo³kowicz
Sec ond ary ar senic min er als (SAM) formed re cently in aban doned adits of the for mer Au-As mine at Z³oty Stok (SW Po land) con sti tute two as sem blages. The first con sists of two types of scorodite, pitticite, kaòkite, hörnesite, picropharmacolite and mi nor amounts of jarosite and gyp sum. For ma tion of the Fe ar se nates took place un der acidic con di tions (pH ~3–4) as a re - sult of löllingite, ar seno py rite and py rite ox i da tion. Hörnesite and picropharmacolite crys tal lized as prod ucts of in ter ac tions be tween acidic ar senic-rich pore so lu tions with Mg-Ca car bon ates from rocks that sur round the ore min er ali sa tion. The in ter - ac tion of car bon ates with acid pore so lu tions caused a rapid in crease in pH that reached neu tral or weakly al ka line val ues.
The chem i cal com po si tions of hörnesite and picropharmacolite cor re spond well to their ideal com po si tions:
(Mg3.17Ca0.07)S3.24(AsO4)1.90× 8H2O and Ca4.31Mg0.92(HAsO4)1.91[(AsO4)1.99(SO4)0.01]S2.00× 11H2O, re spec tively. The sec ond as - sem blage of SAM com prises ex clu sively the Mg-en riched erythrite [(Co1.66Mg1.03Ni0.28Ca0.05Zn0.02)S3.03(AsO4)1.99× 8H2O)] – anna ber gite [(Ni1.48Mg0.94Co0.66Ca0.12Fe0.01Zn0.01)S3.20(AsO4)1.92 × 8H2O] se ries. These min er als crys tal lized from slightly acidic (pH ~5–6) to neu tral me dia. Dis so lu tion of SAM and other sec ond ary phases (e.g., schwertmannite) causes the re - lease of ar se nate and sul phate ions into mine wa ters. These ions can be re duced un der an aer o bic con di tions by dif fer ent strains of bac te ria. The prod uct of this pro cess is orpiment.
Key words: erythrite–hörnesite–anna ber gite se ries, picropharmacolite, sec ond ary ar senic min er als.
INTRODUCTION
Ar senic is an ex am ple of toxic el e ment re leased dur ing de - com po si tion of its pri mary min er als. Sec ond ary ar senic min er - als (SAM) are among the main As sinks in the supergene en vi - ron ment. Their crys tal li sa tion strongly de pends on the physico chemical prop er ties of the weath er ing zones of As-rich de pos its (e.g., Drahota and Filippi, 2009). Changes in these con di tions lead to the dis so lu tion of SAM. This pro cess re - leases ar senic into un der ground and sur face wa ters, thus lead ing to their con tam i na tion (Marsza³ek and W¹sik, 2000).
Par tic u lar spe cies of SAM are well-stud ied be cause of ar senic tox ic ity, which stron gly de pends on the ox i da tion state of this met al loid. Thus, un der stand ing SAM sta bil ity plays an im por - tant role from the en vi ron men tal point of view. Sec ond ary phases from the Z³oty Stok de posit have been poorly rec og - nized in pre vi ous re search. Only sev eral weath er ing min er als are known from this lo cal ity (Kubisz, 1964; Budzyñska, 1971).
This pa per is the first to pres ents the re sults of de tailed study
of re cently form ing SAM found within aban doned adits of the Au-As mine at Z³oty Stok. At ten tion is paid to the chem i cal di - ver sity of SAM and their re la tion ships to the known crystallo - chemical solid so lu tions. We also re con struct the crys tal li sa - tion con di tions of the SAM un der study.
GEOLOGY
The Z³oty Stok Au-As de posit is lo cated in the cen tral part of the Lower Silesian Block (¯elaŸniewicz et al., 2011) at the con - tact be tween the Z³oty Stok-Skrzynka Shear Zone and the K³odzko-Z³oty Stok granitoid pluton. The Z³oty Stok-Skrzynka Shear Zone de vel oped dur ing Variscan de for ma tion of dif fer ent meta mor phic rocks (Cymerman, 1996; Mazur et al., 2006). It is com posed of blastomyllonites, gneiss es, mica schists and lepti - nites in ter ca lated with mar bles, am phi bo lites and serpentinites;
the lat ter co ex ist ing with mi nor nephrites (Kowalski, 1969;
Smulikowski, 1979; Gil et al., 2015; Fig. 1). The K³odzko-Z³oty Stok granitoid pluton con sists of rocks rang ing from granodiorite to monzonite (Mikulski et al., 2013; Mikulski and Wil liams, 2014). Dif fer ent types of hornfelses, skarns and skarnoids have been formed in the granitoid con tact zone due to the ther mal and metasomatic in flu ence of the pluton (Wierzcho³owski, 1976). Metasomatic events re lated to the Variscan magmatism
* Corresponding author, e-mail: siuda@uw.edu.pl
Received: February 15, 2018; accepted: October 30, 2018; first published online: December 18, 2018
were also re spon si ble for the main ore min er al iza tion in the Z³oty Stok de posit (Mikulski and Speczik, 2008, 2016).
Three types of ore min er al isa tion oc cur in the Z³oty Stok Au-As de posit. The first type con tains löllingite-ar seno py rite min - er al isa tion as so ci ated with di op side skarns and ser penti nites sur round ing dolomitic mar bles. The sec ond type is a pyrrhotite - -mag ne tite ore with mi nor löllingite and ar seno py rite re sid ing within di op side-tremolite rocks. The third va ri ety is a dis sem i - nated löllingite-ar seno py rite ore that oc curs in side black ser pen - tin ite. Sub-mi cro scopic and mi cro scopic gold oc curs in all the ore types (Mikulski and Speczik, 2016). Other ore min er als, such as py rite, ga lena, sphalerite, chal co py rite, schee lite, glaucodot, cobaltite, cubanite, marcasite, bis muthi nite, bis muth, ar senic, and Ni-Fe-Co sulphoarsenides oc cur as small ag gre gates of no eco nomic sig nif i cance (Mikulski, 1996; Muszer, 2011).
The Z³oty Stok de posit has a long min ing his tory. The first in for ma tion about gold mines dates back to 1341 in this area (Dziekoñski, 1972). Pro duc tion of ar senic and syn thetic ar senic pig ments started at the be gin ning of the 17th cen tury. Due to the poor qual ity of the ar senic ores, min ing and ar senic-gold smelter ac tiv ity ceased in 1961 (Muszer, 2011).
METHODS AND SAMPLES
Min eral sam ples were col lected from the Gertruda and Wniebowziêcie adits of the for mer Au-As Z³oty Stok mine (Fig.
1). The most di verse supergene min er al isa tion was found in the Gertruda adit (~842 m from the en trance). A small, up to 30 cm in size, ore lens sur rounded by bar ren rocks rich in cal cite and do lo mite, was found there. The pri mary min er als of this lens (löllingite, ar seno py rite and py rite) have been al most com - pletely de com posed by weath er ing, lead ing to the for ma tion of sev eral char ac ter is tic zones with dif fer ent min eral com po si tions
(Fig. 2). The first, cen tral zone is dom i nated by pow dery scorodite that con tains relicts of the ore min er als. In the mar - ginal parts of this scorodite type oc curs its sec ond form rep re - sented by bot ry oi dal or spher i cal green ag gre gates. It is fol - lowed by pitticite, next to which the kaòkite ac cu mu la tion zone is found. Segregations of Mg and Ca ar se nates oc cur in the bar - ren rocks sur round ing the ore lens. In this zone, large amounts of biofilms with col o nies of mi cro or gan isms are seen. The other min eral sam ples were taken from the Wniebowziêcie adit, which is sit u ated to the south of the Gertuda adit. Co-Mg-Ni ar - se nates form small ag gre gates on the sur face of the weath er ing Co-rich löllingite. In Wniebowziêcie adit, schwertmannite was also found; how ever, it does not oc cur with SAM. The sam ples vary from 0.5 to 10 cm in di am e ter. To avoid hu mid ity changes, all min eral sam ples were placed in tightly-sealed plas tic con - tain ers. The sam ples were trans ported to the lab o ra tory and stored at a tem per a ture of about +10°C.
Quan ti ta tive chem i cal data were ac quired us ing a Cameca SX 100 elec tron microprobe of the Inter-In sti tute An a lyt i cal Com plex from Min er als and Syn thetic Sub stances, Uni ver sity of War saw. The fol low ing stan dards, an a lyt i cal lines and crys - tals were used: V2O5 – V (Ka, LIF), rhodochrosite – Mn (Ka, LIF), Fe2O3 – Fe (Ka, LIF), chal co py rite – Cu (Ka, LIF), ZnS – Zn (Ka, LIF), al bite – Na (Ka, TAP), di op side – Mg (Ka, TAP), orthoclase – Al (Ka, TAP), di op side – Si (Ka, TAP), GaAs – As (La, TAP), ap a tite – P (Ka, PET), orthoclase – K (Ka, PET), di - op side – Ca (Ka, PET), NiO – Ni (Ka, LIF), and me tal lic Co – Co (Ka, LIF). Anal y ses were con ducted us ing the ac cel er at ing volt - age of 10 kV and beam cur rent of 5 nA. The beam di am e ter was 5–10 mm. ZAF cor rec tions were ap plied. El e vated an a lyt i cal to - tals for min er als with hydroxyl groups or crystallisation wa ter are gen er ally caused by evap o ra tion of wa ter un der high vac uum con di tions or by its evap o ra tion due to heat ing of the ana lysed area by the elec tron beam. Lower to tals are due to the po rous na ture of some min er als. The pres ence of all min er als was con - Fig. 1. Geo log i cal map of the Z³oty Stok area, mod i fied af ter Cwojdziñski (1974);
lo ca tion of adits is based on old min ing maps
firmed by pow der X-Ray dif frac tion (PXRD). Iden ti fi ca tion of those min er als was per formed us ing a pow der X-ray diffracto - meter X`Pert PRO MPD by the DSH method. The pa ram e ters of the X-ray beam were as fol lows: CoKa wave length, fi nally mono chro matized by par a bolic X-rays mir rors, emit ted from the X-Ray tube with 40 mA and 40 kV cur rent ap plied. X-ray pat - terns were re corded in 2.5 to 75.99 °2q range, with a step size of 0.02 °2q. The re sults were pro cessed us ing the X-ray anal y - sis soft ware X`Pert Plus HighScore (ver. 2.2e) and ICDD PDF-2 da ta base. Mor phol ogy of the sam ples was stud ied at the Fac ulty of Ge ol ogy, Uni ver sity of War saw, us ing a FE-SIGMA VP (Carl Zeiss Mi cros copy GmbH) scan ning elec tron mi cro - scope with an en ergy-dispersive (EDS) de tec tor (Quantax XFlash 310, Bruker Nano GmbH). IR ab sorp tion spec tra were re corded with a Nicolet Magna 550 specrometer from 4000 to 400 cm–1, us ing KBr pel lets (Fac ulty of Chem is try, War saw Uni - ver sity). Raman spec tra were re corded on a Jobin spec trom e - ter -Yvone-Horiba cou pled with a con fo cal mi cro scope Olym - pus BX40 (AGH Uni ver sity of Sci ence and Tech nol ogy). The spec tra were col lected from min eral sam ples at room tem per a - ture, us ing la ser with a wave length of 514.5 nm in the range of 4000-50 cm–1 at a res o lu tion of 3.5 cm–1 and a mea sure ment time from 30 to 360 s.
SECONDARY MINERALS FROM THE GERTRUDA ADIT
SCORODITE FeAsO4 × 2H2O
In the Gertruda adit, this min eral forms two types of ag gre - gate. The first is pow dery and grey-green and oc curs in the cen -
tral part of the strongly ox i dized ore lens, al most com pletely re - plac ing the pri mary löllingite-ar seno py rite-py rite ag gre gates.
The di am e ter of these type-I scorodite ag gre gates reaches tens of centi metres. Even by ap ply ing very large mag ni fi ca tions, no crys tals of this min eral can be ob served. The chem i cal com po - si tion of the type-I scorodite does not strongly dif fer from the the o ret i cal com po si tion (Ta ble 1). The iron con tent var ies from 0.84 to 0.99 apfu. This el e ment is re placed by a small amount of alu mi num (0.01–0.08 apfu) and mag ne sium (up to 0.02 apfu).
In the tet ra he dral site the ar se nate ion (0.66–0.99 apfu) is re - placed by sul phate an ion (0.02–0.28 apfu) ac com pa nied by a phos phate ion (0.01 apfu). The cal cu lated em pir i cal for mula of the type-I scorodite (based on four ox y gen at oms per for mula unit) is (Fe0.92Al0.02)S0.94[(AsO4)0.75(SO4)0.22(PO4)0.01]S0.98× 2H2O.
The sec ond type of scorodite (scorodite II) forms green spher i - cal ag gre gates up to 100 mm in di am e ter (Fig. 3A). They ap pear out side the earthly scorodite and co ex ist with pitticite. The iden - ti fi ca tion of the scorodite II was based only on the PXRD method (Fig. 4A).
KAÒKITE FeAsO4 × 3.5H2O
Kaòkite is an other iron ar se nate. It forms semi-crys tal line green coat ings on the bar ren rock sur face. These encrustations are built of very fine, thin tab u lar crys tals up to 20 mm in size (Fig. 5A). The chem i cal com po si tion of kaòkite is shown in Ta - ble 2. It shows a rel a tively small vari a tion in iron con tent (from 0.88 to 0.98 apfu). This el e ment is ac com pa nied by alu mi num (from 0.04 to 0.14 apfu) and mag ne sium (from 0.01 to 0.03 apfu). The ar se nate con tent var ies from 0.86 to 0.97 apfu. This an ion is sub sti tuted to a small ex tent by sul phate (from 0.01 to 0.04 apfu) and phos phate (0.01 apfu) ions. Based on mi - Fig. 2. Sec ond ary Fe and Mg-Ca ar se nates on the wall of the Gertruda adit
cro-area anal y ses, the em pir i cal for mula of kaòkite (cal cu lated based on four ox y gen at oms) is (Fe0.93Al0.08Mg0.02Ca0.01)S 1.05[(AsO4)0.93(SO4)0.03(PO4)0.01]S0.97× 3.50H2O. The pres ence of kaòkite was con firmed by the PXRD method (Fig. 4B).
PITTICITE
Pitticite ap pears be tween scorodite II and kaòkite. It oc curs as honey-yel low or brown ish crust-like ag gre gates with a dis - tinc tive conchoidal frac ture and vit re ous to greasy luster (Fig.
3B). It is also very brit tle and strongly cracked. The chem i cal com po si tion is shown in Ta ble 3. The iron con tent var ies from 33.15 to 36.92 wt.% Fe2O3. Small amounts of alu mi num (from 0.20 to 0.31 wt.% Al2O3) and mag ne sium (up to 0.09 wt.%
MgO) were also de tected. Ar senic, rang ing in con tent from 36.72 to 39.95 wt.% As2O5, is ac com pa nied by sul phur (10.45 to 11.36 wt.% SO3), phos pho rus (from 0.09 to 0.26 wt.% P2O5) and sil i con (up to 0.06 wt.% SiO2).
HÖRNESITE Mg3(AsO4)2 × 8H2O
Hörnesite crystallises on bar ren rock sur faces (Mg-en - riched mar ble) on the out side of the kaòkite ac cu mu la tions zone. It forms very fine, white, spher i cal ag gre gates up to 4 mm in size (Fig. 3C). Snow ball-like ag gre gates of hörnesite are built of automorphic lath-like crys tals. The size of the crys tals is up to
20 mm. At large mag ni fi ca tions, a mo saic struc ture of the crys tal ter mi na tions is vis i ble (Fig. 5B). Hörnesite ag gre gates are usu - ally sur rounded by mu cous prod ucts of mi cro bi o log i cal me tab o - lism pro cesses (Fig. 5C). The chem i cal com po si tion of hörne - site from the Gertruda adit is also close to the the o ret i cal value (Ta ble 4). There is only a small amount of cal cium. The chem i - cal anal y sis of hörnesite yields the for mula (Mg3.17Ca0.07)S 3.24(AsO4)1.90× 8H2O (cal cu lated based on eight ox y gen at oms).
The Raman spec tra of hörnesite from the Gertruda adit are shown in Fig ure 6A. The re sults of the Raman anal y ses are re - ported in Ta ble 5. Bands at 3055, 3044 and 3480 cm–1 are as - so ci ated with O-H stretch ing vi bra tions in the wa ter. The po si - tion of these bands is very sim i lar to those re ported by Frost et al. (2003). Sev eral broad bands at 2817–1141 cm–1 are ob - served. Very strong bands at 876 and 807 cm–1 are as signed to the v3 and v1 anti-sym met ric and sym met ric starch ing modes at ar se nate groups, re spec tively. The band at 559 cm–1 re sults from the Mg-O stretch ing vi bra tions. The Raman spec trum of the low-wavenumber re gion in cludes sev eral sharp bands.
Their pres ence is linked to v4, v2 modes and AsO bend ing and lat tice modes (Makreski et al., 2015). The pres ence of hörnesite was con firmed by the PXRD method (Fig. 7A).
PICROPHARMACOLITE Ca4Mg(HAsO4)2(AsO4)2 × 11H2O
This min eral co ex ists with hörnesite. It ap pears as acicular crys tals (Fig. 5D) form ing spher i cal ag gre gates on bar ren rock
T a b l e 1 Chem i cal com po si tion [wt.%] of type-I scorodite from the Gertruda adit
Anal y sis
num ber 1 2 3 4 5 6 7 8 9 10
Fe2O3 29.73 30.56 36.01 36.93 37.12 35.02 36.26 34.04 34.63 35.13
Al2O3 1.78 1.80 0.16 0.17 0.19 0.20 0.19 0.20 0.18 0.22
MnO 0.00 0.00 0.03 0.00 0.00 0.14 0.00 0.00 0.00 0.00
MgO 0.39 0.37 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
As2O5 49.68 50.80 38.40 37.68 35.49 37.21 38.01 39.00 39.04 38.51
SO3 1.34 0.89 9.75 10.24 10.44 10.65 9.93 10.51 10.25 10.22
P2O5 0.38 0.35 0.15 0.16 0.15 0.14 0.18 0.16 0.13 0.17
SiO2 0.00 0.04 0.12 0.09 0.08 0.06 0.11 0.07 0.08 0.08
H2O* 15.90 16.11 17.04 17.21 16.88 16.94 17.08 17.08 17.08 17.08
To tal 99.20 100.92 101.66 102.47 100.36 100.35 101.76 101.06 101.40 101.40 apfu
Fe3+ 0.84 0.86 0.95 0.97 0.99 0.93 0.96 0.90 0.91 0.93
Al3+ 0.08 0.08 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
Mn2+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Mg2+ 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
S A side 0.94 0.96 0.96 0.98 1.00 0.95 0.97 0.91 0.92 0.94
AsO43- 0.98 0.99 0.71 0.69 0.66 0.69 0.70 0.72 0.72 0.71
SO42- 0.04 0.02 0.26 0.27 0.28 0.28 0.26 0.28 0.27 0.27
PO43- 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01
SiO44 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
S T side 1.03 1.03 0.97 0.96 0.94 0.98 0.97 1.00 0.99 0.98
H2O 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
*H2O was cal cu lated from the ideal con tent in the for mula H2O = 2; Ca, Ni, Co, V be low de tec tion limit
sur faces (Fig. 3D). The in ves ti gated picropharmacolite is char - ac ter ized by high chem i cal pu rity (Ta ble 6). Only small amounts of sul phur (up to 0.03 apfu) were found, re plac ing the ar se nate an ion. The quan ti ta tive chem i cal anal y sis of picropharmacolite gives the fol low ing em pir i cal for mula based on 16 ox y gen at - oms: Ca4.31Mg0.92(HAsO4)1.91[(AsO4)1.99(SO4)0.01]S2.00× 11H2O.
The pres ence of picropharmacolite was con firmed by the PXRD method (Fig. 4C).
JAROSITE KFe3(SO4)2(OH)6
Yel low, fine-grained, pow dery ag gre gates (up to 0.2 mm in di am e ter) of jarosite were iden ti fied at the bound aries of the kaòkite and Mg-Ca ar se nate zones. Jarosite ac cu mu la tions are com posed of automorphic crys tals up to 10 mm in size. Re sults of the chem i cal anal y sis are given in Ta ble 7. Site A is dom i - nated by po tas sium, vary ing from 0.79 to 0.90 apfu. It is ac com - pa nied by small amounts of so dium (from 0.02 to 0.03 apfu) and the hydronium ion (from 0.08 to 0.19 apfu). In the tet ra he dral site, a mi nor sub sti tu tion of the sul phate ion (1.85 to 2.04 apfu) by the phos phate ion is ob served (from 0.02 to 0.04 apfu). The cal cu lated em pir i cal for mula of jarosite (based on 14 ox y gen at - oms per for mula unit; the amount of OH- and H3O+ ions was cal -
cu lated based on the charge bal ance) is: (K0.83H3O+0.15Na0.02)S 1.00Fe3.01[(SO4)1.97(PO4)0.03]S2.00(OH)6.00. This min eral was also iden ti fied by the PXRD method (Fig. 4D).
GYPSUM CaSO4 × 2H2O
This min eral oc curs as small plate-like crys tals, up to 20 mm in length, form ing spher i cal ag gre gates. They were iden ti fied at the con tact of the kaòkite and Mg-Ca ar se nate zones. The pres - ence of gyp sum was con firmed only by PXRD.
ORPIMENT
In the Gertruda adit, un iden ti fied ar senic sul phide is quite com mon. It oc curs as im preg na tions on wooden min ing sup - ports sub merged in wa ter. It com prises bright yel low earthy ac - cu mu la tions. The ag gre gates are formed of thin crys tals up to 3 mm in di am e ter (Fig. 5E), which co ex ist with bac te rial col o nies.
The chem i cal com po si tion is shown in Ta ble 8. The ana lysed ma te rial shows high vari abil ity of ar senic and sul phur con tent (57.71–64.55 wt.% As, 21.23–31.67 wt.% S). The pres ence of orpiment was con firmed by the PXRD method (Fig. 4E).
Fig. 3. For ma tion of sec ond ary ar senic min er als from the Gertruda adit
A – green ag gre gates of scorodite II, B – ac cu mu la tions of pitticite, C – white ag gre gates of hörnesite, D – sprays of picropharmacolite
SUPERGENE MINERALS FROM THE WNIEBOWZIÊCIE ADIT
ERYTHRITE Co3(AsO4)2 × 8H2O – HÖRNESITE Mg3(AsO4)2 × 8H2O – ANNABERGITE Ni3(AsO4)2 × 8H2O SOLID SOLUTION
In the Wniebowziêcie adit, two types of spher i cal ag gre - gates of Co-Ni-Mg ar se nates were found. The first type is pur - ple and con sists of plate-like crys tals up to 30 mm in size. Pur ple ag gre gates con tain higher amount of co balt and ac cu mu late on the weath er ing sur face of Co-rich löllingite (av er age con tent of co balt is 4.5 wt.%). The sec ond type of Co-Ni-Mg ar se nate is light pink in col our and formed on gersdorffite ag gre gates as so - ci ated with löllingite. The chem i cal com po si tion of crys tals that build the pur ple ag gre gates is shown in Ta ble 9. Their com po si - tion is vari able. The oc ta he dral site is dom i nated by co balt and mag ne sium, with the con tents vary ing from 1.39 to 2.01 apfu and from 0.61 to 1.28 apfu, re spec tively. These el e ments are ac com pa nied by small amounts of nickel (from 0.23 to 0.34 apfu), cal cium (from 0.03 to 0.07 apfu), and zinc (up to 0.04 apfu). The an a lysed crys tals be long to the erythrite-hörnesite se ries with a slight en rich ment in nickel (Fig. 8). The em pir i cal for mula of this phase was cal cu lated based on eight ox y gen at - oms to be (Co1.66Mg1.03Ni0.28Ca0.05Zn0.02) 3.03(AsO4)1.99 × 8H2O. In the case of the sec ond type of ag gre gates, the dom i nant role in the cat ion site is played by nickel, whose con tent var ies from 1.38 to 1.62 apfu (Ta ble 10). In the struc ture, it is par tially re - placed by mag ne sium (from 0.87 to 1.01 apfu) and co balt (from 0.60 to 0.70 apfu). They are ac com pa nied by small amounts of
cal cium (from 0.09 to 0.15 apfu), zinc (up to 0.02 apfu) and iron (up to 0.01 apfu). The em pir i cal for mula of the an a lysed phase is (cal cu lated based on eight ox y gen at oms per for mula unit) (Ni1.48Mg0.94Co0.66Ca0.12Fe0.01Zn0.01) 3.20(AsO4)1.92 × 8H2O. Thus, the phase rep re sents an Mg- and Co-en riched va ri ety of anna - ber gite (Fig. 8).
The Raman spec trum of Mg-en riched erythrite from the Wniebowziêcie adit (Fig. 6B) dif fers slightly from that of erythrite from Bou Azzer (Fig. 6C). The an a lysed sam ple shows the char ac ter is tic pres ence of three strong bands with max ima at 3456, 3228 and 3068 cm–1. Com pared to the Raman spec trum of pure hörnesite from the Gertruda adit, bands in this range are more vis i ble. The bands ob served at 856 and 796 cm–1 ap pear from the anti-sym met ric and sym met ric As-O4 vi bra tions. The po si tions of these bands are as so ci ated with the cat ion type at po si tion A (Frost et al., 2003). In pure hörnesite, they ap pear at higher wave num bers (Table 5) than in pure erythrite. The po si - tion of the same bands for Mg-en riched erythrite from the Wniebowziêcie adit is in ter me di ate and sim i lar to the po si tions noted by Dumañska-S³owik et al. (2018). This shift is due to sub sti tu tion be tween Mg2+ - Co2+ and Ni2+ ions in the cat ion site.
GOETHITE FEOOH AND FERRIHYDRITE Fe10O14(OH)2
These min er als co ex ist with each other and form small sta - lac tites and drapes of up to sev eral centi metres in length. In the nat u ral hu mid ity state, they are soft and be come brit tle af ter dry - ing. The pres ence of these min er als has been con firmed only by PXRD.
Fig. 4. PXRD pat terns of sec ond ary min er als from the Gertruda adit A – scorodite II, B – kaòkite, C – picropharmacolite, D – jarosite, E – orpiment
SCHWERTMANNITE Fe16(OH, SO4)12-13O16 ·10-12H2O
Schwertmannite oc curs only in nar row zones (up to sev eral centi metres in size) as so ci ated with strongly ox i dized py rite min er al isa tion. It forms small, up to 0.5 cm, sta lac tites. At large mag ni fi ca tions, typ i cal thin, nee dle-like crys tals of this min eral are vis i ble. The pres ence of this min eral has been con firmed by PXRD.
DISCUSSION
The min eral com po si tions of SAM parageneses are strictly de pend ent on the con di tions of their crys tal li sa tion (Drahota
and Filippi, 2009). The main fac tors that af fect the crys tal li sa - tion are the chem i cal com po si tion of pri mary ore min er al isa - tion and sur round ing bar ren rocks, the hu mid ity, pH and Eh, and the bi o log i cal ac tiv ity of mi cro or gan isms. Vari abil ity of the SAM crys tal li sa tion con di tions can be ob served in the ox i dized ore lens ex posed in the Gertruda adit. Ar sen ides and sulphi - des of iron (löllingite, ar seno py rite and py rite) are un sta ble un - der supergene con di tions. In the pres ence of ox y gen and wa - ter, they un dergo de com po si tion (Walker et al., 2006). This pro cess is rap idly ac cel er ated by Fe3+ ions – a strong ox i diz ing agent (Mo ses et al., 1987; Holmes and Crundwell, 2000).
These ions are among the prod ucts of mi cro bial ac tiv ity of some bac te ria strains (e.g., Schippers et al., 1996; Jones et al., 2003, Devasia and Natarajan, 2010; Vera et al., 2013; Tu et al., 2017). The pres ence of such bac te ria was also con - Fig. 5. SEM im ages of sec ond ary ar senic min er als from the Gertruda adit
A – spher i cal ag gre gates of kaòkite on pitticite; B – mo saic ter mi na tion of a hörnesite crys tal; C – hörnesite crys - tals sur rounded by a mi cro bial mat; D – nee dle-like crys tals of picropharmacolite; E – tab u lar ag gre gates of an un - iden ti fied ar senic sul phide from an old wooden min ing con struc tion, the ar row in di cates bac te rial col o nies; F – mi cro bial col o nies min er al ised by pitticite
T a b l e 2 Chem i cal com po si tion [wt.%] of kaòkite from the Gertruda adit
Anal y sis
num ber 1 2 3 4 5 6 7 8 9 10
Fe2O3 30.16 31.32 29.06 29.47 29.16 29.51 30.36 30.00 31.99 30.90
Al2O3 1.96 2.87 1.94 1.63 2.66 1.58 1.20 1.04 1.28 0.90
CaO 0.41 0.15 0.18 0.15 0.12 0.17 0.12 0.10 0.24 0.18
MgO 0.35 0.54 0.35 0.38 0.47 0.29 0.24 0.16 0.17 0.19
ZnO 0.08 0.22 0.34 0.17 0.06 0.11 0.00 0.00 0.00 0.05
K2O 0.00 0.00 0.02 0.00 0.00 0.09 0.01 0.03 0.00 0.06
As2O5 42.42 40.53 45.56 43.59 42.53 43.26 46.24 43.82 44.17 43.03
SO3 1.10 1.32 0.82 1.28 1.36 0.79 0.87 0.41 0.34 0.55
P2O5 0.23 0.30 0.32 0.32 0.26 0.22 0.22 0.23 0.40 0.28
SiO2 0.15 0.08 0.02 0.06 0.01 0.22 0.08 0.07 0.11 0.06
H2O* 25.51 25.79 26.06 25.62 25.63 25.21 26.22 24.89 25.83 24.98 To tal 102.36 103.11 104.67 102.67 102.25 101.46 105.57 100.76 104.53 101.18
apfu
Fe3+ 0.93 0.96 0.88 0.91 0.90 0.92 0.91 0.95 0.98 0.98
Al3+ 0.09 0.14 0.09 0.08 0.13 0.08 0.06 0.05 0.06 0.04
Ca2+ 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.01
Mg2+ 0.02 0.03 0.02 0.02 0.03 0.02 0.01 0.01 0.01 0.01
Zn2+ 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00
K+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
S A side 1.07 1.14 1.01 1.02 1.06 1.03 0.99 1.02 1.06 1.04
AsO43- 0.91 0.86 0.96 0.93 0.91 0.94 0.97 0.97 0.94 0.95
SO42- 0.03 0.04 0.02 0.04 0.04 0.02 0.03 0.01 0.01 0.02
PO43- 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
SiO44 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00
S T side 0.96 0.92 1.00 0.99 0.96 0.98 1.00 0.99 0.97 0.98
H2O 3.50 3.50 3.50 3.50 3.50 3.50 3.50 3.50 3.50 3.50
* H2O was cal cu lated from the ideal con tent in the for mula H2O = 3.5; Mn, Ni, Co, V be low de tec tion limit
T a b l e 3 Chem i cal com po si tion [wt.%] of pitticite from the Gertruda adit
Anal y sis
num ber 1 2 3 4 5 6 7 8 9 10
Fe2O3 34.14 33.6 33.94 33.15 35.03 34.44 36.92 34.94 36.43 36.28
Al2O3 0.2 0.22 0.3 0.28 0.31 0.31 0.22 0.24 0.28 0.25
MgO 0.04 0.05 0.00 0.04 0.00 0.08 0.09 0.00 0.00 0.00
As2O5 37.23 37.67 36.72 38.34 39.95 39.88 38.42 38.38 38.64 38.11
SO3 10.67 10.45 10.57 10.6 10.84 10.85 11.36 10.62 10.96 11.13
P2O5 0.12 0.09 0.20 0.17 0.23 0.14 0.15 0.18 0.20 0.26
SiO2 0.02 0.03 0.03 0.04 0.06 0 0.06 0.01 0.04 0.05
To tal* 82.42 82.11 81.76 82.62 86.42 85.70 87.22 84.37 86.55 86.08
* Ex clud ing wa ter con tent; Mn, Ca, Ni, Co, V be low de tec tion limit
firmed in the mi cro bial mats found in the Gertruda adit (e.g., Drawniak et al., 2010). De com po si tion of pri mary ar senic min - er als leads to the re lease of sol u ble forms of ar senic (such as ar se nate and arsenite spe cies). Ar se nate ions dom i nate in en -
vi ron ments with high Eh po ten tials. Arsenite ions can be rap - idly ox i dized by Fe3+ ions or dur ing mi cro bi o log i cal pro cesses (Nich o las et al., 2003; Dre wniak et al., 2012) to the AsO43- spe - cies (Mandl and Vyskovsky, 1994; Wiertz et al., 2006; Qin et al., 2013). In the course of iron sul phide ox i da tion, large amounts of sul phate ions are re leased into the en vi ron ment, thus trig ger ing a sig nif i cant re duc tion in pH.
Un der low pH con di tions and very high ac tiv ity of Fe3+ and AsO43- ions, type-I scorodite is formed. This min eral re places ar - senic ore min er als in situ. Only small relicts of pri mary min er als are pres ent within the scorodite ac cu mu la tions. Scorodite is sta ble in the rel a tively nar row pH range: from 3 to 4 (Krause and Ettel, 1989; Bluteau and Demopoulos, 2007; Paktunc and Bruggeman, 2010). As the pH de creases, scorodite un der goes con gru ent dis so lu tion which leads to the for ma tion of Fe3 +– and AsO43--rich pore so lu tions. These so lu tions mi grate out side the type-I scorodite ac cu mu la tions. Scorodite II crys tal lises from these so lu tions. Spher i cal ag gre gates of this min eral are typ i cal of scorodite which crys tal lises slowly from pore so lu tions rich in the As(V) and Fe(III) spe cies (Haffert and Crow, 2008). Sit u a - tions of de creas ing pH, lead ing to dis so lu tion of scorodite I, are likely in sig nif i cant and short-term. The ab sence of zýkaite and bukovskýite con firms this ob ser va tion, as these min er als are char ac ter is tic of low pH en vi ron ments (2.5–3.5) (Gas`kova et al., 2008; Majzlan et al., 2012b, 2015).
The scorodite II-dom i nated zone con tacts with pitticite ac cu - mu la tions. Pitticite is an amor phous hy drated sulpho arse nate of iron. It is a typ i cal prod uct of weath er ing of pri mary ar senic min er - als which takes place in acidic en vi ron ments (e.g., Kocour ková et al., 2011b; Parviainen et al., 2012). The pres ence of highly min - er al ised bac te rial col o nies within the pitticite ac cu mu la tions (Fig.
Anal y sis
num ber 1 2 3 4 5 6 7 8 9
MgO 27.06 24.94 25.71 26.70 26.66 26.38 25.58 24.35 24.31
CoO 0.07 0.08 0.03 0.00 0.06 0.01 0.00 0.00 0.00
ZnO 0.00 0.22 0.00 0.00 0.00 0.00 0.00 0.00 0.00
CaO 1.04 0.98 0.93 0.79 0.84 0.69 0.26 1.21 0.52
FeO 0.00 0.06 0.03 0.03 0.03 0.00 0.00 0.00 0.00
As2O5 43.33 44.55 43.89 42.44 43.82 43.80 44.36 44.74 45.89
H2O* 29.50 29.00 29.01 28.84 29.38 29.18 28.91 28.81 29.01
To tal 100.99 99.83 99.60 98.80 100.78 100.05 99.11 99.11 99.72 apfu
Mg2+ 3.28 3.08 3.17 3.31 3.25 3.23 3.17 3.02 3.00
Co2+ 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Ni2+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Ca2+ 0.09 0.09 0.08 0.07 0.07 0.06 0.02 0.11 0.05
Fe2+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
S A side 3.38 3.17 3.26 3.38 3.32 3.29 3.19 3.13 3.04
AsO43- 1.84 1.93 1.90 1.85 1.87 1.88 1.92 1.95 1.98
H2O 8 8 8 8 8 8 8 8 8
* H2O was cal cu lated from the ideal con tent in the for mula H2O = 8; Al, Mn, Ni, Co, S, V, P, Si be low de tec tion limit
Fig. 6. Raman spec tra of hörnesite from the Gertruda adit (A), erythrite-hörnesite from the Wniebowziêcie adit (B)
and erythrite from Bou Azzer (Marocco) (C)
5F) in di cates the in volve ment of mi cro or gan isms in the pre cip i ta - tion of this phase. In re la tion to crys tal line iron ar se nates, the amor phous hy drated sulphoarsenate of Fe is un sta ble. Mat u ra - tion of pitticite leads to the for ma tion of crys tal line Fe ar se nates (Paktunc et al., 2008). On the other hand, pitticite can dis solve due to a slight de crease in ar se nate or sul phate ions con cen tra - tion (Chukhlantsev, 1956; Rob ins, 1987; Lang muir et al., 2006).
For this rea son, it may be a pre cur sor to the co ex ist ing kaòkite.
Kaòkite crystallises pri mar ily in post -min ing or un der ground min - ing ex ca va tions (Majzlan et al., 2014). No di rect crystallisation of kaòkite has been ob served on the weath er ing sur faces of ar -
senic ore min er als. This ob ser va tion may in di cate that crystallisation of kaòkite is re lated to mi gra tion of ar se nate-rich po rous so lu tions. Fur ther more, kaòkite is found only in very highly hu mid en vi ron ments (Kato et al., 1984; Haffert et al., 2010;
Majzlan et al., 2014). Kaòkite from the Gertruda adit is a youn ger phase in re la tion to scorodite and pitticite (Fig. 5A). This con firms the ob ser va tions of other au thors who re lated crystallisation of this min eral to dis so lu tion of pre vi ously formed sec ond ary iron ar - se nates (Haffert et al., 2010; Kocourková et al., 2011a). Kaòkite co ex ists with a small amount of jarosite, which is an in di ca tor of low pH. Ex per i men tal data show that kaòkite is metastable with T a b l e 5 Po si tion and as sign ment of the bands [cm–1] in the Raman spec tra of Mg and Co ar se nates from Z³oty Stok,
and com par i son with lit er a ture data Hörnesite
from the Gertruda adit
Hörnesite
(Frost et al., 2003) Hörnesite
(Makreski et al., 2015) Erythrite from the Wniebowziêcie adit
Erythrite from Bou Azzer (this work)
Erythrite
(Frost et al., 2003) Band as sign ment
68 68 70
118 117 113 113 Lat tice modes
138 Lat tice modes
149 149 Lat tice modes
160 159 158 162 Lat tice modes
181 180 180 189 188 Lat tice modes
205 206 205 206 Lat tice modes
211 209 Lat tice modes
222 218 223 Lat tice modes
234
241 243 242 240 245 Lat tice modes
250 249 Lat tice modes
262 262 263 262 263 Lat tice modes
273 273 271 Lat tice modes
304 303 301 303 302 301 v2(AsO4)
365 363 365 v2(AsO4)
377 378 v2(AsO4)
389 383 v2(AsO4)
391
404 403 403 v4(AsO4)
430 429 430 v4(AsO4)
437 442 439 v4(AsO4)
446
459 457 v4(AsO4)
467 467 468 467 v4(AsO4)
508
657 660 651 652
727
796 793 792 v1(AsO4)
807 807 808 v1(AsO4)
856 852 852 v3(AsO4)
875 875 875 v3(AsO4)
907 906 902
2817-1141 2280-1140 poor
band in ten sity
3055 3030 3068 3065 3052 OH stretch ing
3166
3228 3202 3200 OH stretch ing
3337
3404 OH stretch ing
3456 OH stretch ing
3480 3479 OH stretch ing
re spect to liq uid wa ter or wa ter vapour (Majzlan et al., 2012a).
De com po si tion of this min eral may be the source of the AsO4 3 -
ions for other sec ond ary ar se nates to form. The for ma tion of Hörnesite and picropharma colite is as so ci ated with the re ac tion of acidic ar senic-rich pore so lu tions with car bon ates from bar ren rocks. These rocks con tain vari able amounts of cal cite and do lo -
mite, which are the source of Mg2+ and Ca2+ ions. The in ter ac tion of car bon ates with acid pore so lu tions causes a rapid in crease in pH. Based on ob ser va tions of crystallisation con di tions of Mg-Ca ar se nates in south ern France (Juillot et al., 1999) it can be as - sumed that the cor re spond ing pro cess in the Ger trude adit takes place un der neu tral or weakly al ka line pH con di tions. Sul phate Fig. 7. PXRD pat terns of Mg, Co, Ni ar se nates from Z³oty Stok
A – hörnesite (from the Gertruda adit), B – erythrite (from the Wniebowziêcie adit), C – anna ber gite (from the Wniebowziêcie adit)
Ta b l e 6 Chem i cal com po si tion [wt.%] of picropharmacolite from the Gertruda adit
Anal y sis
num ber 1 2 3 4 5 6 7 8 9 10
CaO 24.28 28.03 25.10 25.16 25.13 25.29 25.97 25.76 27.20 27.71
MgO 3.89 3.51 4.12 4.09 3.68 3.87 4.16 4.16 4.11 4.29
As2O5 47.37 49.56 47.89 46.62 45.00 46.52 50.10 50.51 48.83 48.90
SO3 0.12 0.00 0.14 0.11 0.20 0.28 0.00 0.20 0.00 0.13
H2O* 22.56 23.80 23.26 22.68 22.19 22.69 23.70 23.97 23.62 23.85
To tal 98.22 104.89 100.52 98.65 96.21 98.65 103.93 104.59 103.75 104.87 apfu
Ca2+ 4.15 4.54 4.16 4.28 4.37 4.30 4.22 4.14 4.44 4.48
Mg2+ 0.93 0.79 0.95 0.97 0.89 0.91 0.94 0.93 0.93 0.97
S A side 5.07 5.33 5.11 5.24 5.26 5.21 5.17 5.07 5.37 5.44
AsO43- 1.99 2.00 1.98 1.99 1.98 1.97 2.00 1.98 2.00 1.98
SO42- 0.01 0.00 0.02 0.01 0.02 0.03 0.00 0.02 0.00 0.02
(HAsO4)2- 1.97 1.92 1.89 1.88 1.84 1.89 1.98 1.99 1.89 1.87
S T side 3.97 3.92 3.89 3.88 3.84 3.89 3.98 3.99 3.89 3.87
H2O 11.00 11.00 11.00 11.00 11.00 11.00 11.00 11.00 11.00 11.00
* H2O was cal cu lated from the ideal con tent in the for mula H2O = 11 and charge bal ance; Fe, Al, Mn, Ni, Co be low de tec tion limit
ions that ac com pany ar se nate ions are bound in gyp sum crys tals that co-ex ist with hörnesite and picropharmacolite. A sim i lar pro - cess of the for ma tion of Mg-Ca ar se nates has been de scribed from aban doned mine gal ler ies in Jachymov, Czech Re pub lic (Ondruš et al., 1997a, b). Crystallisation of hörnesite (and the ac - com pa ny ing picropharmacolite) un der neu tral or alkalescent pH con di tions is also con firmed by the ex is tence of hörnesite in soils with high ar senic con tent (Voigt et al., 1996; Fos ter et al., 1997).
It is also worth not ing that hörnesite ag gre gates are of ten as so ci - ated with col o nies of mi cro or gan isms (Fig. 5C). Their role in the for ma tion of this min eral has not been fully es tab lished. There is a pos si bil ity that As and Mg2+ con cen tra tions within such col o - nies some how in crease to the value at which crystallisation of hörnesite is pos si ble.
Supergene Co-Mg-Ni ar se nates of the erythrite-hörnesite - -anna ber gite solid so lu tion oc cur be yond the above-de scribed ore lenses at the Wniebowziêcie adit. Their crys tal li sa tion is re - lated to the de com po si tion of ar sen ides con tain ing co balt and nickel. Weath er ing of these min er als does not cause a sig nif i - cant re duc tion in pH. This is be cause the de com po si tion of
Co-Ni ar sen ides does not re lease sulphuric acid but only poorly dis so ci ated ar senic acid. The ar senic acid re acts with the re - leased Co2+ and Ni2+ ions, which in turn leads to the ap pear ance of sec ond ary min er als di rectly at the sur face of the ores. Mg2+
ions are sup plied to the sys tem from do lo mite-con tain ing bar ren rocks. The for ma tion of the erythrite-hörnesite-anna ber gite solid so lu tion is very sim i lar to the mech a nism de scribed by Markl et al. (2014) to oc cur in supergene zones of var i ous Ger - man polymetallic mines. Ac cord ing to data ob tained by Langmuir et al. (1999) and Mahoney et al. (2007) these ar se - nates crystalised from slightly acidic me dia (pH ~5–6). These data were con firmed by Yuan et al. (2005), who proved that the sol u bil ity of anna ber gite de creases with in creas ing pH (min i mal at pH = 9). The same re la tion ship was con firmed for erythrite (Zhu et al., 2013). In sev eral parts of the Wniebowziêcie adit, schwertmannite was found. Its oc cur rence is lim ited to small zones con tain ing dis sem i nated py rite min er al isa tion (with out ar seno py rite and löllingite) which ap pears in side blasto myllo - nites. More over, some SAM were found with schwertmannite.
This min eral is an in di ca tor of acidic en vi ron men tal con di tions T a b l e 7 Chem i cal com po si tion [wt.%] of jarosite from the Gertruda adit
Anal y sis
num ber 1 2 3 4 5 6
K2O 6.63 7.20 6.95 7.12 7.83 7.61
Na2O 0.16 0.16 0.13 0.09 0.12 0.15
Fe2O3 43.54 44.01 43.12 46.54 45.20 44.40
SO3 24.92 31.14 30.57 30.04 28.85 30.09
P2O5 0.37 0.53 0.40 0.33 0.35 0.34
H2O* 9.30 10.66 10.45 10.67 10.14 10.35
To tal 84.92 93.70 91.62 94.79 92.49 92.94
apfu
K+ 0.84 0.80 0.79 0.79 0.90 0.86
Na+ 0.03 0.03 0.02 0.02 0.02 0.03
H3O+ 0.13 0.18 0.19 0.19 0.08 0.11
S A side 1.00 1.00 1.00 1.00 1.00 1.00
Fe3+ 3.24 2.88 2.88 3.05 3.06 2.96
SO42- 1.85 2.03 2.04 1.96 1.95 2.00
PO43- 0.03 0.04 0.03 0.02 0.03 0.03
S T side 1.88 2.07 2.07 1.98 1.97 2.02
OH- 6.93 5.45 5.48 6.14 6.20 5.80
*H2O con tent was cal cu lated based on charge bal ance; Al, Mn, As be low de tec tion limit
Ta b l e 8 Chem i cal com po si tion [wt.%] of orpiment from the Gertruda adit
Anal y sis
num ber 1 2 3 4 5 6 7 8 9
S 22.08 21.23 31.67 23.07 25.85 27.78 25.63 23.44 20.72
As 59.87 57.71 64.55 56.41 62.44 60.44 63.54 59.53 49.73
To tal 81.95 78.94 96.22 79.48 88.29 88.22 89.17 82.97 70.45
Anal y sis
num ber 1 2 3 4 5 6 7 8 9 10
CoO 25.93 25.30 24.35 21.71 21.33 21.14 20.30 19.10 19.03 19.18
MgO 4.26 4.77 5.02 8.47 7.15 7.22 9.09 8.97 9.11 9.43
NiO 3.91 3.47 3.54 3.53 3.59 4.47 3.50 3.75 3.06 3.43
ZnO 0.31 0.40 0.17 0.00 0.18 0.38 0.16 0.14 0.00 0.63
CaO 0.47 0.28 0.30 0.44 0.59 0.40 0.38 0.44 0.59 0.68
FeO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
As2O5 39.51 39.14 39.80 39.61 39.99 39.79 40.68 40.27 40.32 42.02 H2O* 26.40 26.19 26.75 26.52 26.78 26.43 26.92 26.61 26.62 26.48 To tal 100.78 99.55 99.93 100.27 99.61 99.82 101.02 99.28 98.73 101.85
apfu
Co2+ 2.01 1.98 1.90 1.64 1.63 1.61 1.51 1.44 1.44 1.39
Mg2+ 0.61 0.69 0.73 1.19 1.02 1.02 1.26 1.26 1.28 1.27
Ni2+ 0.30 0.27 0.28 0.27 0.28 0.34 0.26 0.28 0.23 0.25
Zn2+ 0.02 0.03 0.01 0.00 0.01 0.03 0.01 0.01 0.00 0.04
Ca2+ 0.05 0.03 0.03 0.04 0.06 0.04 0.04 0.04 0.06 0.07
S A side 3.00 3.01 2.95 3.13 3.00 3.05 3.07 3.04 3.02 3.02
AsO43- 2.00 2.00 2.02 1.95 2.00 1.98 1.97 1.98 1.99 1.99
H2O 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00
* H2O was cal cu lated from the ideal con tent in the for mula H2O = 8; Mn, S, P, V, Si be low de tec tion limit
Fig. 8. Ter nary plot of the chem i cal com po si tion of Mg-Co-Ni ar se nates from the Gertruda and Wniebowziêcie adits (in at oms per for mula units)
(pH = 3–4) and high ac tiv ity of sul phate ions (Parafiniuk and Siuda, 2006). Schwertmannite is a metastable min eral, sus cep - ti ble to en vi ron men tal changes (Yu et al., 2002). In hu mid pe ri - ods of the year, an in creased flow of me te oric wa ters of neu tral pH re sults in trans for ma tion of schwertmannite into ferrihydrite and goethite. This causes the schwertmannite-bound sul phate ions to be re leased into mine wa ters.
Ar se nate and sul phate ions of mine wa ters can be re duced un der an aer o bic con di tions. Bac te ria play a key role in this pro cess (Nich o las et al., 2003). Ar senic sulphides of var i ous As:S ra tios stand for the prod ucts of such bac te ri ally-driven re - duc tion (e.g., Newman et al., 1997; Ledbetter et al., 2007;
Battaglia -Bru net et al., 2012; Ro dri guez-Freire et al., 2014, 2016). Dif fer ent strains of bac te ria, which re duce As(V) to As(III) were iden ti fied in the mi cro bial mats from the Gertruda adit (Drewniak et al., 2010, 2012; Tomczyk-¯ak et al., 2013, C³apa³a et al., 2016). In this pro cess, the ar senic and sul phate ions are elec tron ac cep tors, whereas or ganic mat ter from the old wooden min ing con struc tion acts as an elec tron do nor.
Then, ar senic sul phide is formed and pre cip i tates only in the in ter stices of wood im preg nated with the bac te rial col o nies, which clearly in di cates that this pro cess is re lated to or ganic mat ter and mi cro bial ac tiv ity.
CONCLUSIONS
Sec ond ary ar senic min er als (SAM) from the As-Au mine at Z³oty Stok are a unique as sem blage of supergene min er als in Po land. These min er als re cord changes in en vi ron men tal con - di tions in the aban doned mine gal ler ies. Un der ox i diz ing con di - tions, the dom i nant solid phases are ar se nates. Iron ar se nates oc cur only in ar eas of low pH and high ac tiv ity of Fe3+ ions. Such
zones de velop when the pri mary ar senic min er als and py rite are weath ered. The first min eral to crys tal lise is scorodite I. This min eral is partly dis solved and recrystallized as scorodite II. Ar - se nate and fer ric ions re leased in this pro cess are bound up in pitticite and kaòkite. The bar ren rocks, which sur round the weath er ing ore min er als, con tain cal cite and do lo mite that are a source of Ca2+ and Mg2+ spe cies.
Hörnesite and picropharmacolite are the prod ucts of the re - ac tion be tween Ca2+, Mg2+ and AsO43-
ions. These min er als crys tal lise un der neu tral or slightly al ka line pH con di tions, with in creas ing pH val ues re sult ing from car bon ates dis so lu tion. Ar - se nates of the erythrite-anna ber gite-hörnesite solid so lu tion are pro duced by de com po si tion of pri mary Co-Ni ar sen ides. This pro cess does not cause a sig nif i cant pH re duc tion. For this rea - son, these min er als crys tal lise un der slightly acidic pH con di - tions. Un der anoxic con di tions ar se nate and sulphate ions pres - ent in the mine wa ters un dergo bac te rial re duc tion. The ar senic sulphides form in this way. Mi cro or gan isms take part in the for - ma tion of sec ond ary ar senic min er als. They ac cel er ate both re - ac tions: the ox i da tion of re duced ar senic forms and the re duc - tion of ar se nate ions. The aban doned adits of the Z³oty Stok As-Au mine are the first doc u mented place in Po land where the SAM re cord the full cir cu la tion of ar senic in a supergene en vi - ron ment.
Ac knowl edge ments. The study was fi nanced by an IGMiP pro ject. Au thors would like to thank A. Borkowski, T. Segit, M. Ka³aska and M. Syczewski for their help in the field work. Spe cial thanks go to E. Szumska for en abling our re - search in the Z³oty Stok aban doned gold mine. Au thors also would like to thank R. Mac don ald for cor rect ing the Eng lish ver sion of this ar ti cle.
Ta b l e 1 0 Chem i cal com po si tion [wt.%] of anna ber gite from the Wniebowziêcie adit
Anal y sis
num ber 1 2 3 4 5 6 7 8 9
NiO 18.51 18.80 19.96 19.92 19.58 19.30 21.06 19.25 18.19
CoO 8.97 9.01 7.96 8.55 8.47 9.20 7.76 8.70 9.23
MgO 7.07 7.27 6.12 7.11 6.11 6.52 6.11 6.67 6.95
ZnO 0.22 0.04 0.00 0.00 0.17 0.00 0.00 0.00 0.30
CaO 1.38 1.53 1.02 1.15 0.85 1.29 0.91 1.21 1.38
FeO 0.07 0.00 0.16 0.01 0.07 0.14 0.08 0.01 0.04
As2O5 39.23 38.65 38.56 38.50 39.16 40.05 38.42 37.83 38.64
H2O* 25.70 25.67 24.99 25.53 25.18 25.93 25.05 24.93 25.38
To tal 101.14 100.96 98.76 100.75 99.59 102.43 99.39 98.60 100.11 apfu
Ni2+ 1.39 1.41 1.54 1.51 1.50 1.44 1.62 1.49 1.38
Co2+ 0.67 0.68 0.61 0.64 0.65 0.68 0.60 0.67 0.70
Mg2+ 0.98 1.01 0.88 1.00 0.87 0.90 0.87 0.96 0.98
Zn2+ 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.02
Ca2+ 0.14 0.15 0.10 0.12 0.09 0.13 0.09 0.12 0.14
Fe2+ 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00
S A side 3.20 3.26 3.15 3.26 3.12 3.16 3.19 3.24 3.23
AsO43- 1.91 1.89 1.94 1.89 1.95 1.94 1.92 1.90 1.91
H2O 8 8 8 8 8 8 8 8 8
* H2O was cal cu lated from the ideal con tent in the for mula H2O = 8; Mn, S, P, V, Si be low de tec tion limit
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