MINERALOGIA – SPECIAL PAPERS, 38, 2011 www.mineralogia.pl MINERALOGICAL SOCIETY OF POLAND POLSKIE TOWARZYSTWO MINERALOGICZNE
Mineral chemistry of the As-bearing ore minerals from Z oty Stok nephrites – preliminary results Grzegorz GIL Institute of Geological Sciences, Faculty of Earth Sciences and Environmental Management, Wroc aw University, pl. Maksa Borna 9, 50-204 Wroc aw, Poland, e-mail:
[email protected]
The As-bearing minerals from the contact aureole of K odzko-Z oty Stok granitoid Intrusion Unit (Sudetes Mts) are known since medieval ages. The main As-ore bearing minerals in this deposit are arsenopyrite and löllingite (J!drzejewska, Sa aci"ski 1997). Nephrite occurs there occasionally, forming thin veins within pyroxenites. The microprobe study of the As-ore minerals from the Z oty Stok nephrites was the main goal of this research. After the field sampling, the conventional petrographic studies were made and some samples were marked out for microprobe analysis . The 18 electron microprobe analyses of the As-ore bearing minerals were carried out using the Cameca SX 100 microprobe analyzer in the Microscopy and Microprobe Laboratory at the Warsaw University,. The obtained results showed that the As-ore bearing minerals from Z oty Stok nephrites contain of 0,697-3,359 wt% S; 67,573-70,525 wt% As; 27,960-29,134 wt% Fe; 0,178-0,317 wt% Se; respectively – 1,481-6,832 at% S; 59,680-64,064 at% As; 33,897-34,269 at% Fe; 0,149-0,270 at% Se. Other elements occur in trace quantities. For the comparison, the chemical composition of the major As-ore minerals from selected Au-bearing arsenic deposits and experimentally synthetized minerals are presented below: Arsenopyrite from Fäboliden deposit, northern Sweden, analyses after Bark (2008): 17,31-18,40 wt% S; 47,19-48,71 wt% As; 33,80-34,63 wt% Fe. Arsenopyrite from eastern Kazakhstan Au deposits (Kovalev et al. 2011): 18,3422,19 wt% S; 42,45-47,85 wt% As; 32,59-34,67 wt% Fe; respectively – 32,73-36,71 at% S; 30,34-34,43 at% As; 32,51-32,94 at% Fe. Arsenopyrite experimentally synthetized (Fleet, Mumin 1997): 14,70-39,90 wt% S; 24,20-53,50 wt% As; 30,40-35,20 wt% Fe; respectively – 26,60-3,20 at% S; 27,7051,50 at% As; 31,60-32,70 at% Fe; results depending on synthesis temperature. Löllingite from Fäboliden deposit, northern Sweden (Bark 2008): 2,98-3,18 wt% S; 67,59-68,06 wt% As; 27,17-27,38 wt% Fe; 0,74-1,06 wt% Ni, Se absent in all analyses. Löllingite experimentally synthetized (Fleet, Mumin 1997): 2,00 wt% S; 70,40 wt% As; 27,40 wt% Fe; respectively – 4,20 at% S; 62,90 at% As; 32,80 at% Fe.
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The composition of the analyzed As-ore minerals from the Z oty Stok nephrites is typical for löllingite and similar to e.g. cases from northern Sweden. Some samples have chemical composition similar to synthetic löllingite. Crystals with the highest amount of S (and adequately the lowest As values) probably represent the solid solutions between major löllingite and minor marcasite, because of their structural similarity (O’Day 2006). Some samples with the highest Fe and lower As content, can be an effect of the As substitution by Fe (Radcliffe, Berry 1968). The occurrence of löllingite, arsenopyrite, pyrrhotite and native gold mineral paragenesis indicates the crystallization at the temperature of 525 to 650oC and the pressure between 3 to 4 kbar (0,3-0,4 GPa) (Bark, Weihed 2007). In the experiments on FeAs-NaCl-H2O system, löllingite occurs at the temperature range from 500 to 600oC. Below granulite facie conditions, the most common As-bearing ore mineral is arsenopyrite, usually occurring with pyrrhotite or pyrite (Barnicoat et al. 1991, vide Bark, Weihed 2007). During the progressive metamorphism, the desulfidation of arsenopyrite takes place, resulting in formation of löllingite and pyrrhotite (Bark, Weihed 2007). The process can be illustrated by the chemical reaction: 4 FeAsS 4 × arsenopyrite
# #
2 FeAs2 2 × löllingite
+ +
2 FeS 2 × pyrrhotite
+ +
S2 sulphur
The occurrence of löllingite and the absence of arsenopyrite and pyrite are the arguments for the ore mineralization in pyroxenite and nephrite at a temperature of 500 to 650oC , which is probably higher than in other rocks from Z oty Stok deposit. This can also be a temperature of the whole rock formation – higher than in the major parts of the deposit, however, more study is needed to prove or deny this argument. The absence of other sulfide minerals is unexplained; perhaps it is related to the specific conditions during the mineralization in studied rocks or it may be a result of too small number of studied samples. The abundance of Se in löllingite is difficult to explain and gives the opportunity for further study. References Bark G. & Weihed P. (2007). Orogenic gold in the new Lycksele-Storuman ore province, northern Sweden; the Palaeoproterozoic Fäboliden deposit. In Bark G. On the origin of the Fäboliden orogenic gold deposit, northern Sweden. Doctoral Thesis 2002:72, (35-55), Luleå University of Technology. Bark G. (2008). Appendix 3, Electron microprobe data. In Bark G. On the origin of the Fäboliden orogenic gold deposit, northern Sweden. Doctoral Thesis 2008:72, (125-130) Luleå University of Technology. Fleet M. E. & Mumin A. H. (1997). Gold-bearing arsenian pyrite and marcasite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis. American Mineralogist 82, 182-193. J!drzejewska A. & Sa aci"ski R. (1997). Przejawy okruszcowania strefy kontaktowej intruzji k odzkoz otostockiej ze stref$ %cinania Z oty Stok-Trzebieszowice. In Konf. Nauk. Jarno tówek 19-21 czerwiec 1997, Metale szlachetne w NE cz!%ci Masywu Czeskiego i w obszarach przyleg ych – geneza, wyst!powanie, perspektywy: (116-121). Instytut Nauk Geologicznych Uniwersytetu Wroc awskiego. Kovalev K. R., Kalinin Y. A., Naumov E. A., Kolesnikowa M. K. & Korolyuk V. N. (2011). Gold-bearing arsenopyrite in eastern Kazakhstan gold-sulfide deposits. Russian Geology and Geophysics, 52, 178-192. O’Day P. A. (2006). Chemistry and Mineralogy of Arsenic. Elements, 2, 77-83. Radcliffe D. & Berry L. G. (1968). The safflorite-loellingite solid solution series. American Mineralogist, 53, 1856-1881.
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