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Sphalerite Geothermometry

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Sphalerite Geothermometry SPHALERITE GEOTHERMOMETRY BY C. BOP,a~SWA~,A RAo (G#olo#T D#p~tment, And~a Uni~#ratty, Wnltatr) Received November 12, 1964 (Communicated by Dr. C. S. Pichamuthu, ~.A.$c.) ABSTRACT Kullerud's method of formation-temperature determination has been applied to the polymetallie sulphide deposits of Sadon in Northern Caucasus, U.S.S.R. The sulplaide ore occurs at three places, separated by a few kilo- moters from each other. They ate Sadon, Zgid and Holst. Mineralogical assemblage is sphalerite, galena, pyrite and pyrrhotite. Ir is found thal sphalerites from the above three places were formed mainly within a tempe- rature range of 120-500 ~ C. The temperature data are represented in the forro of histograms. The occurrence of more than one maximum in each histogram may indicate ore deposition in more than one stage. From the histograms, it is concluded that in Sadon, ore deposition took place in three stages: in Zgid, in only one stage and in Holst, in two stages. These eonclusions are in agreement with those arrived at by others by minera- graphic studies. INTRODUCTION Trm temperature of formation of a rock body oran ore body is determined by various methods, the older methods being direr measurement from lavas and hot sprŸ and determination of crystallization temperatures of minerals. In recent times, new methods have been devised which cover a vast range of rock types and mineral deposits. With the help of inversion temperatures ofminerals like quartz (870 ~ C.), wollastonite (I 125 ~ C.) and argentite (179 ~ C.), it is possible to determine with great accuracy, the formation-temperature of any rock or ore deposit in which these minerals occur. Mineralogical composition of the ore body, exsolution temperature of two components, and temperature of recrystallization, ate applied for determining the forma- tion-temperature of any ore deposit. More recently, fluid inclusions have been used for determining the formation-temperatures of both rock bodies as well as ore bodies. Another recent method, particularly applicable to 152 Sphalerite Geothermometry 153 sulphide ore bodies, is the determination of formation-temperatures of sphale- rites frora their FeS content. The present paper deals with the application of this raethod to the polymetaUic sulphide deposits of Northern Caucasus region, U.S.S.R. 1VI~~OD Kullerud (1953) studied the phase equilibriura in the system FeS-ZnS at different teraperatures and pressures and found that the solubility of FeS in flZnS (of sphaleritic structure) depends on teraperature, the solubility increasing with rise of teraperature. Frora this it follows that ir the content of FeS in sphalerites is known, the formation-teraperature of sphalerites can be determined. However, this method requires that sphalerites, in the natural process of ore deposition, should be forraed in equilibriura with pyrrhotite. If the ore solutions do not contain enough quantity of sulphur which can combine with the raetallic elements like zinc, lead and iron, maxi- murn quantity of iron, at any particular teraperature, enters ZnS forraing mix- crystals, 13 (Fe, Zn) S of sphaleritic structure. The excess iron in the solution combines with sulphur forraing pyrrhotite with the forraula, Fel_xS. In case su~icient quantity of sulphur exists in the solutions, pyrite (FeS2) is formed, instead of pyrrhofite. During the forraation of pyrite in association with sphalerite, activity of iron is reduced by 2~ (Kullerud, 1953). In his work, Kullerud did not find noticeable difference in the coraposition of the raix- crystals forraed in equilibrium with FeS or with Fe~_z S or with Fel-zS +FeS2 if a little quantity of pyrrhotite is always left after the attainment of equilibrium. ORE BoDms The polymetallic sulphide deposits of Sadon are located in Northern Caucasus of Soviet Union, and coraprise three independent deposits separated by a few kiloraeters frora each other. The geology, structural aspects, para- genetic relationships and genesis of these ore deposits were studied in detail by Soviet geologists (Prokopenko, 1947; Adzhgirei, 1947; Bochkarev and Koblentz, 1958; Efreraov et al., 1958 ; Hetagyrov, 1958). These deposits, which are locally known as Sadon, Zgid and Holst deposits, are believed to have a common source and they are similar in raineralogical coraposition. The ore body consists of lead-zinc ore, the raain ore rainerals being sphalerite and galena. In all the three deposits, the associating rainerals with sphale- rite are pyrrhotite, pyrite and chalcopyrite. Co-existence of sphalerites with pyrrhotite and pyrite in these deposits allows determination of formation- teraperatures of sphalerites by the raethod of Kullerud. 154. C. BORRF.SWARARAO SAMPLE COLLECTION Samples of sphaleritie ore were collected from the above-mentioned dcposits, from diffcrent horizons and from all accessible placcs in order to make the samples as reprcscntativc as possible. The diffcrcnt varicties of sphalcritc~dark brown, black, honcy ycllow, and transparcnt ycllow colours ate represented in the collected ore material. Pure monomineralic fractions of sphalerites were obtained by final hand-picking under a binocular micro- scope of electromagnetically separated crude mineral fraction of -- 200 + 250 grade. Each final sphalerite fraction was thoroughly checked for its pu¡ ANALYTICAL PROCEDURE Owing to the fact that the final purc monomincralic fraction is small in quantity, cstimation of iron was carricd out by a spcctral mcthod. The ana- lyscs were carried out on a quartz-spectrograph, ISP-22 of mcdium dispcrsion (Ij.S.S.R. make). On account of the presence of iron in the samples between wide limits, from 0.01 ~ to about 10~ (values obtained from existing literaturc), spectral lines of iron of differcnt sensitivities werc selected for estimating iron percentage between these limits. Out of a number of lines of varying sr vities, two lines, one with low sensitivity, Fe-2389"971 A for estimating larger concentrations and the second, a more sensitive line, Fe-2598.369 Jk for determining smaller quantities of iron were selected. In this choice of lines, absence of interfering spectral lines due to other elements like Zn, G-c, Ga, Mn, Cd and In in the region of the chosen line and good reproducibility of results were ascertained. Standards containing different concentraUons of iron from 12~ to 0'001~ were prepared with a base of highly pure iron-free zinc sulphide (chemical), iron having been added in the form of FesOs. Lithium carbonate was used asa buffer material for smooth and stcady bum- ing of the arc. Ten milligrammes of standard or sample was mixed with an equal amount of Li~CO3 and loaded into the cavity (3 • 6 mm.) of the lower elcctrode of specpure carbon. Dircct current of 220 volts with a current strength of 15 aml0eres served asa sourcc. Anode-excitation was carried out for five minutes. On each spectral plate (Type II with a sensitivity of 16 Gost, U.S.S.R. make), spectra of diffcrent standards were photographed along with those of the samples, in duplicate. After dev.cloping the plates in a fresh developer for five minutes at 20 ~ C., and drying, spectral linc-density measurements were carried out on a microphotometer, MF-2 (U.S.S.R. make). Background intensity served as internal standard. Graduated curves were constructed by the method of "Three Standards" for lines, Fe-2389.971 A and Sphalerite Geothermometry 155 Fe-2598.369 A. By this method it is possible to determine iron content from 12~to 0.3~ by the low-sensitive line and from 1~ to 0"001~ by high-sensitive line. TEMPERATURE DETERMINATION The iron percentages were recalculated as FeS. The formation-tem- peratures were determined from the graph (Fig. 1), showing the relationship of FeS to temperature in mix-crystals q (Fe, Zn)S. The values thus obtaiHed represent tahose without pressure correction. Pressure influences, solubility of iron in the mix-crystals t3 (Fe, Zn)S. Kullerud (1953) has shown, by means of thermodynamical calculations, that for dissolving the same amount of FeS in q at different pressures, it is necessary to increase the temperature by 25 ~ C. for every 1,000 atmospheres. 900 800 7OO 6OO U/ soo ~4oo W I- 30Q 200 Ioo ! i ,,, ! o IO ~b 3o 4'o ~o FeS WT.% Fil3. 1. Relationship of FeS content in mix-crystals, q (Fe, Zn)S to tempr (Kullerud, 1953). In the area under study, Precambrian gran]tes ate overlain by sedimentary and volcanic rocks of Mesozoic period. The ore body was formed, according to Efremov et al. (1958), in the pre-Callovian period. It is mainly distributed in gran]tes and the overlying volcanic rocks. If it is assumed that the average thickness of these volcano-sedimentary rocks over the granites at the time of , 156 C. BORR.F.,SWA~ RAo ore formation did not exceed 2,000 meters (figure arrived at by summing up the thicknesses of individual beds), a correction for an average pressure of 500 atmospheres, equal to +13 ~ C. is necessary. The solubility of FeS in flZns is also influenced by the presence of other elements like manganese and cadmium. On account of a possible formation of solid solutions, MnS-ZnS (Schnaase, 1933; Kr6ger, 1939 a) and CdS-ZnS (Kr6ger, 1939 b), the presence of Mn and Cd in the solutions may affect the solubility of FeS in ZnS. In his work, Kullerud (1953) after studying the mix-crystals fl (Mn, Cd, Fe, Zn)S, found that very little quantities of MnS and CdS do not affect or influence to a very smaU degree, the solubility of FeS in flZns. According to hito, the iron content of sphalerite is not measurably influ- enced by the presence of cadmium or manganese as long as the latter elements do not exceed 2 mol.~ (Kullerud, 1956). The negligible influence of small quantities of MnS in the mix-crystals fl (Fe, Zn)S, on the solubility of FeS, is also clearly indicated by the data of Skinner (1959).
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