Structural key features of bismuth and Sb-As sulfosalts from hydrothermal deposits—micro-Raman spectrometry
Introduction
The sulfosalt minerals from hydrothermal deposits show a great importance for understanding the ore forming conditions and to establish the mineralization distribution. Certain sulfosalts can provide also information on temperature of deposition. The bismuth minerals may be used as gold indicators, since the incorporation of gold in bismuth melts is a well-known mechanism of concentration [1], [2], [3], [4].
Bismuthinite belongs to the bismuthinite-aikinite isomorphic series that has a general formula of CuxPbyBi8-1/2(x+y)S12 [5]. The lillianite homologues comprise Pb-Bi-Ag sulfosalts with layers of “galena-like” structure, the differences between homologues series being the size of these layers, i.e. the number of octahedra that forms the slabs. The lillianite-gustavite series (N = 4) forms a solid solution with the general formula AgxPb3-2xBi2+xS6 [6]. Heyrovskyite is the lillianite homologous member with the order number N = 7 and Pb12Bi4S18 formula. Cosalite can be described by the general formula CuxAgi+sPb8-2s-0.5(x+i)Bi8+sS20, where s is formula units of Ag and Bi that replace Pb, x and i for Cu and Ag respectively that substitutes the Me2 site [7].
The tetrahedrite-tennantite (“fahlores”) series (Cu10(Fe,Zn)2Sb4S13—Cu10(Fe,Zn)2As4S13) is one of the most common sulfosalt group that is found in a wide variety of deposits. The Ag-rich members are the most frequent economic ore minerals of silver [8]. Also common in hydrothermal deposits are the members of the bournonite-seligmannite solid solution—PbCu(Sb,As)S3. Semseyite (Pb9Sb8S21) was discovered and described for the first time at Baia Sprie deposit, Romania, and it belongs to plagionite group together with fülöppite, plagionite and heteromorphite, forming a homologous series.
Most of these minerals were not studied so far by means of Raman spectrometry, with the exception of bismuthinite, tetrahedrite-tennantite series and bournonite [8], [9]. Yet, Raman spectra of tetrahedrite-tennantite phases with high Ag content (∼25 wt%) were not reported in literature.
The Baia Sprie ore deposit is located in the NW part of Romania within the metallogenetic district of Baia Mare, Neogene volcanic chain of the Eastern Carpathians. This epithermal deposit has one of the greatest vein systems, with two main veins: Principal and Southern Veins. The Principal vein is more than 5 km in length with a thickness of 0.5–22 m and vertically size of more than 800 m [10], [11]. Almost 90 mineral species were described within the Baia Sprie deposit, and it represents the type locality for 6 minerals (andorite, semseyite, felsobanyaite, dietrichite, klebelsbergite and szmikite) [11]. The mineralization of Baia Sprie deposit was triggered by an intense volcanic activity and a series of major tectonic events. The mineralization is spatially associated with an amphibole-pyroxene andesitic block and with subvolcanic intrusions and it is developed along the faults at magmatic-sedimentary contacts or near the intrusions. Towards the surface several branches are observed within andesitic body and also within lavas, breccias and andesitic agglomerates [10], [11], [12]. Two metallogenetic stages were considered for the formation of Baia Sprie deposit. The first one is a cupriferous stage with an assemblage of pyrite, chalcopyrite, magnetite, hematite, scheelite, wolframite, chlorite, quartz, ankerite and barite. The second stage has a polymetallic character (Pb-Zn and Au-Ag): pyrite, sphalerite, galena, stibnite, tetrahedrite, quartz, calcite and barite. The Principal Vein displays a well-defined vertical zoning: copper-rich mineralization in the deeper part of the deposit, lead-zinc mineralization within the intermediate levels, and gold-silver in the upper part [10], [11].
This study aims to present for the first time the Raman spectra and interpretation of several sulfosalt minerals: lillianite-gustavite series, heyrovskyite, cosalite, Ag-rich tetrahedrite, semseyite, that are essential for a better understanding of hydrothermal deposits. The main goals are: (1) to assign the observed Raman bands to specific vibrational modes; (2) to establish the structural changes due to chemical variations through the solid solution series; and (3) to outline the influence of different cationic sites on the Raman spectra appearance. Moreover, semseyite samples from the type locality were analyzed and the Raman spectra were discussed.
Section snippets
Materials and methods
The samples used in this study belong to the Baia Sprie ore deposit, from different levels and parts of the mineralization. The samples were prepared as polished sections in order to identify minerals of interest by using reflected-light microscopy. The polished sections were prepared at Technical University Cluj-Napoca, North University Center of Baia Mare. For the reflected-light microscopy a Meiji ML9430 polarizing microscope was used.
Several minerals of interest were identified and subject
Chemical composition
The chemical composition and calculated formula units of the investigated samples are shown in Table 1, Table 2. The average chemical formula of bismuthinite is Pb0.21Cu0.22Fe0.05(Bi7.59Sb0.03)7.62(S11.83Se0.05)11.88. The Bi concentration varies between 76.21 and 77.74 wt%. Other elements with significant contents are Pb (2.14 wt%) and Cu (0.68 wt%). For the Bi sulfosalts, the concentration of bismuth is lower and ranging in the 30.06–44.73 wt% interval. The lillianite-gustavite phases are
Conclusions
The micro-Raman study on hydrothermal deposits of sulfosalts provides useful information regarding their structure behavior related with the chemical composition, outlined by the Raman spectra. Such an approach is essential since these minerals are very important for the interpretation of ore forming conditions. Only few Raman studies were carried out on these sulfosalts and proved to be a suitable method, especially when other structural measurements are not available, due to the small grain
Acknowledgements
This work was supported by the “Alexandru Ioan Cuza” University of Iasi, within “Grants for UAIC young researchers” competition (project code GI-2015-09). Thanks are also extended to the staff of electron microanalysis laboratory (from the State Geological Institute of Dionýz Štúr, Bratislava, Slovakia): Dr. Konečný Patrik, Dr. Ivan Holický and Dr. Viera Kollárová for the EPMA analyses. We extend our gratitude to the editor Mr. Professor Gabor Keresztury and to anonymous reviewers for their
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