Sul­fur. Geo­log­i­cal fea­tures and min­ing prospects

Sul­fur deposits are an impor­tant nat­ur­al resource with broad appli­ca­tions across var­i­ous indus­tries, rang­ing from chem­i­cal and oil to agri­cul­ture and phar­ma­ceu­ti­cals. Ukraine, pos­sess­ing some of the largest reserves of native sul­fur in the world, plays a cru­cial role in the glob­al min­ing indus­try. A sig­nif­i­cant por­tion of the deposits is con­cen­trat­ed in the Pre-Carpathi­an sul­fur-bear­ing basin, as well as in the Dnieper-Donets depres­sion and the Kerch Penin­su­la. This arti­cle exam­ines the geo­log­i­cal fea­tures of Ukrain­ian sul­fur deposits, their min­er­al char­ac­ter­is­tics, and the cur­rent state of extrac­tion. Spe­cial atten­tion is paid to the gen­e­sis of the deposits and the poten­tial prospects for fur­ther study and exploita­tion.

General Information

The aver­age sul­fur con­tent in the Earth­’s crust is approx­i­mate­ly 3×10⁻⁴%. High­er con­cen­tra­tions (up to 2.4×10⁻³%) are char­ac­ter­is­tic of clay and car­bon­ate rocks. In nature, sul­fur occurs both in bound forms—as sul­fates and sulfides—and in its free state. It is also found in oil, coal, nat­ur­al gas, and cer­tain min­er­al waters.

Sul­fur is includ­ed in the list of nation­al­ly sig­nif­i­cant min­er­als approved by the Cab­i­net of Min­is­ters of Ukraine on Decem­ber 12, 1994, under Res­o­lu­tion No. 827, as refrac­to­ry raw mate­ri­als, flux mate­ri­als, and raw mate­ri­als for the pro­duc­tion of build­ing stone.

Native sul­fur can be both crys­talline and amor­phous. The most com­mon form in nature is rhom­bic sul­fur (α‑sulfur), sta­ble at tem­per­a­tures below 95.5°C. Mon­o­clin­ic sul­fur (β‑sulfur) is sta­ble between 95.5°C and 119°C, while amor­phous (µ‑sulfur) is metastable and tran­si­tions into rhom­bic form under nor­mal con­di­tions. The melt­ing tem­per­a­tures of these sul­fur forms are 110–112.6°C, 114.5–119°C, and 114.5°C, respec­tive­ly. Sul­fur’s ther­mal and elec­tri­cal con­duc­tiv­i­ty is very low, and its hard­ness ranges from 1 to 2. Sul­fur is prac­ti­cal­ly insol­u­ble in water and acids but dis­solves well in car­bon disul­fide, oil, gaso­line, and oth­er organ­ic liq­uids.

Upon melt­ing (at tem­per­a­tures between 114°C and 119.8°C), sul­fur turns into a mobile yel­low liq­uid. At tem­per­a­tures above 160°C, it dark­ens and becomes vis­cous, turn­ing into a thick, dark-brown mass. At 300°C, sul­fur becomes liq­uid again. It boils at 444.6°C, and its igni­tion tem­per­a­ture in air ranges from 214°C to 280°C. At 360°C and above, sul­fur active­ly reacts with oxy­gen, form­ing SO₂. When com­bined with hydro­gen at 400°C, it forms H₂S, which decom­pos­es into water and sul­fur at 1690°C.

The main com­pounds of sul­fur include sul­fur diox­ide (SO₂), hydro­gen sul­fide (H₂S), and sul­fu­ric acid (H₂SO₄). Sources of these com­pounds can be native sul­fur, oil, nat­ur­al gas, sul­fide and sul­fate ores (gyp­sum and anhy­drite), bitu­mi­nous sands, and fos­sil coal. Native sul­fur is the most impor­tant resource for the nation­al econ­o­my.

Areas of Use and Raw Material Requirements

In the chem­i­cal indus­try, sul­fu­ric acid is used to pro­duce phos­phor­ic, hydrochlo­ric, and oth­er acids. In agri­cul­ture, it is essen­tial for the pro­duc­tion of phos­phate fer­til­iz­ers (approx­i­mate­ly 400 kg of sul­fu­ric acid is required to pro­duce 1 ton of super­phos­phate). In the oil refin­ing indus­try, sul­fu­ric acid is used to puri­fy kerosene and petro­le­um oils, and in met­al­lur­gy, it is used for met­al pick­ling. Large quan­ti­ties of sul­fu­ric acid are used in ura­ni­um ore extrac­tion, soap pro­duc­tion, deter­gents, paints, and pig­ments.

Arti­fi­cial fiber pro­duc­tion (vis­cose) is also a major con­sumer of sul­fur, par­tic­u­lar­ly for the pro­duc­tion of CS₂. In the paper indus­try, sul­fur in the form of SO₂ is used to treat wood pulp (bisul­fite method). A sig­nif­i­cant amount of sul­fur is also used in the vul­can­iza­tion process of rub­ber, where the quan­ti­ty of sul­fur deter­mines the prop­er­ties of the rub­ber, from soft to hard (ebonite). Sul­fur is also used in the chem­i­cal-phar­ma­ceu­ti­cal indus­try to pro­duce sul­fon­amides and oint­ments, as well as in the man­u­fac­ture of ultra­ma­rine. In the tex­tile, food, starch, and molasses indus­tries, sul­fur and its com­pounds are used for bleach­ing, pre­serv­ing fruits, and in refrig­er­a­tion pro­duc­tion. Addi­tion­al­ly, sul­fur is used in match pro­duc­tion, pyrotech­nics, the glass indus­try, and leather pro­cess­ing.

New areas of sul­fur use include the pro­duc­tion of sul­fur asphalts, con­crete, ceram­ics, foam sul­fur, and sul­fur coat­ings. Sul­fur min­ing and uti­liza­tion are impor­tant indi­ca­tors of a coun­try’s indus­tri­al devel­op­ment.

Sul­fur can be extract­ed from any sul­fur-bear­ing rocks that con­tain at least 5–8% native sul­fur. Based on sul­fur con­tent, the ores are clas­si­fied into rich (over 25%), medi­um (10–25%), and poor (5–10%) cat­e­gories. Depend­ing on their min­er­al com­po­si­tion and litho­log­i­cal prop­er­ties, sev­er­al types of ores are dis­tin­guished, with the most com­mon being lime­stone (cal­cite), cal­cite-dolomite, marl, and clayey ores. Less com­mon­ly found are siliceous-sandy and so-called opalite ores and sul­furous quartzites. Harm­ful impu­ri­ties in native sul­fur include gyp­sum, bitu­men, arsenic, and sele­ni­um. Sul­fur intend­ed for sul­fu­ric acid pro­duc­tion should con­tain no more than 5% impu­ri­ties, with organ­ic mat­ter mak­ing up no more than 1%. Par­tic­u­lar­ly strin­gent require­ments apply to sul­fur used for the pro­duc­tion of vis­cose (harm­ful impu­ri­ties include bitu­men and arsenic), cel­lu­lose (Se 0.05%), rub­ber (arsenic and bitu­men), phar­ma­ceu­ti­cal prod­ucts, and gun­pow­der (unac­cept­able impu­ri­ties include sand).

Deposits of native sul­fur are devel­oped either by open-pit min­ing or by under­ground smelt­ing using the Frasch method. Under­ground min­ing is com­pli­cat­ed by sulfur’s flam­ma­bil­i­ty, the release of tox­ic sul­fur gas­es, and the explo­sion haz­ard of fine sul­fur dust. The Frasch method involves inject­ing super­heat­ed water (165°C) into the ore body through bore­holes to melt the sul­fur in situ. The liq­uid sul­fur is then pushed to the sur­face using com­pressed air. The main require­ments for deposits suit­able for this method include the water per­me­abil­i­ty of sul­fur-bear­ing rocks, imper­me­abil­i­ty of the over­ly­ing and under­ly­ing lay­ers, sul­fur con­tent over 10%, and a recov­ery rate of at least 40%.

Rich ores (over 25% sul­fur) are sent direct­ly for pro­cess­ing, while poor ores are enriched using flota­tion. Sul­fur is melt­ed in auto­claves using super­heat­ed steam, result­ing in the pro­duc­tion of “lump sul­fur.” The cost of tech­ni­cal sul­fur ranges from $60 to $240 per ton, with an aver­age price of around $120 per ton.

Genetic and Geological-Industrial Types of Deposits

Sul­fur deposits form under var­i­ous geo­log­i­cal con­di­tions, asso­ci­at­ed with diverse process­es such as mag­mat­ic and hydrother­mal for sul­fide deposits, and sed­i­men­ta­ry for sul­fate deposits.

For native sul­fur, two main groups of deposits are dis­tin­guished: Vol­canogenic, locat­ed in zones of cur­rent or recent vol­canic activ­i­ty (exha­la­tion, hydrother­mal, vol­canogenic-sed­i­men­ta­ry); Exo­genic, main­ly asso­ci­at­ed with lagoon­al or lagoon­al-marine rocks (syn­genet­ic and epi­ge­net­ic).

Vol­canogenic deposits are sig­nif­i­cant, par­tic­u­lar­ly exha­la­tion deposits, formed by fuma­role and sol­fa­tara activ­i­ty. Sul­fur is trans­port­ed as gas­es and hydro­gen sul­fide from vol­canic cen­ters and deposit­ed through con­den­sa­tion on the cold walls of cracks in tuffs and lavas. The ores from these deposits are usu­al­ly of high qual­i­ty, with the deposits form­ing small but clus­tered pock­ets.

Hydrother­mal deposits, relat­ed to exha­la­tion, form through the action of hot sul­fu­ric waters, lead­ing to the intense alter­ation of rocks (kaolin­iza­tion, opal­iza­tion, alu­ni­ti­za­tion). Sul­fur is formed by the chem­i­cal inter­ac­tion of H₂S and SO₂, while rocks are altered to sec­ondary quartzites con­tain­ing quartz, opal, sul­fur, alu­nite, and kaolin. These deposits exhib­it ver­ti­cal zon­ing, where facies of quartzites vary from the upper to low­er sec­tions. For exam­ple, monocryp­tic quartzites tran­si­tion into sul­fur-alu­nite quartzites, then into kaolin and sericite quartzites. This zon­ing is due to the chang­ing acid­i­ty of the solu­tions.

Vol­canogenic-sed­i­men­ta­ry deposits form at the bot­tom of crater lakes, where hot springs and sul­fur gas­es emerge through frac­tures. The ores of these deposits con­sist of sul­fur, opal, and black iron pyrite. Exo­genic deposits have the great­est prac­ti­cal sig­nif­i­cance, as they account for about 90% of the world’s known reserves of native sul­fur. They are divid­ed into syn­genet­ic (sed­i­men­ta­ry-bio­chem­i­cal) and epi­ge­net­ic (infil­tra­tion-bio­chem­i­cal) deposits.

Syn­genet­ic deposits form in lakes and lagoons, where sul­fate salts pre­cip­i­tate. Anaer­o­bic bac­te­ria, devel­op­ing in the organ­ic-rich bot­tom lay­ers of the basins, cause sul­fate reduc­tion and the for­ma­tion of hydro­gen sul­fide, which ris­es to the sur­face and oxi­dizes to native sul­fur. These deposits have a lay­ered struc­ture and are asso­ci­at­ed with anti­clines and dome-like uplifts. Typ­i­cal exam­ples include native sul­fur deposits in the Carpathi­an region.

Epi­ge­net­ic deposits form in frac­tured and porous rocks sat­u­rat­ed with sul­fate waters con­tain­ing organ­ic mat­ter. Hydro­gen sul­fide, formed dur­ing sul­fate reduc­tion, oxi­dizes to sul­fur in mix­ing zones of deep and sur­face waters. These deposits are often asso­ci­at­ed with oil and gas regions and con­tain hydro­car­bon com­pounds such as ozokerite and asphalt bitu­mens. An exam­ple is the Truskavets deposit in Ukraine.

Sulfur Deposits in Ukraine

Ukraine holds one of the lead­ing posi­tions glob­al­ly in terms of proven native sul­fur reserves. Accord­ing to the state reg­is­ter, there are ten deposits with total reserves of 196 mil­lion tons, of which con­firmed reserves (cat­e­gories A+B+C1) amount to 128.3 mil­lion tons, and cat­e­go­ry C2 amounts to 2.3 mil­lion tons. Until recent­ly, min­ing activ­i­ties were con­duct­ed at three deposits in the Pre-Carpathi­an sul­fur-bear­ing basin (Podor­ozh­nyan­sk, Yazivske, and Nemyrivske), while sev­en oth­er deposits (Zaghaip­il­sk, Grymne, Shevchenkivske, Lyuben­sk, Teis­arivske, Tlumatsk, Zhukivske) remain reserved. Min­ing at the Podor­ozh­nyan­sk deposit ceased in 1997, activ­i­ties at the Nemyrivske deposit were sus­pend­ed, and extrac­tion con­tin­ues only at the Yazivske deposit. Sul­fur extrac­tion vol­umes declined from 1.8 mil­lion tons in 1991 to 79 thou­sand tons in 2003. Sul­fur is also found in oil fields in Sumy and Lviv regions, where total reserves amount to 430,000 tons and 47,000 tons, respec­tive­ly.

The Pre-Carpathi­an sul­fur-bear­ing basin stretch­es as a nar­row strip through Lviv and Ivano-Frankivsk regions and into Poland, along the south­west­ern edge of the East Euro­pean Plat­form and its junc­tion with the Pre-Carpathi­an depres­sion. The deposits in this basin belong to the infil­tra­tion-meta­so­mat­ic type and are locat­ed in clay-car­bon­ate-sul­fate for­ma­tions of the Tor­ton­ian and Sar­ma­t­ian stages, with thick­ness­es rang­ing from 10 to 400 meters. Neo­gene deposits are over­lain by a thin lay­er of Qua­ter­nary sed­i­ments. Indus­tri­al sul­fur deposits are con­fined to Ratyne lime­stones, in which sul­fur occurs as fine­ly dis­persed cryp­tocrys­talline seg­re­ga­tions.

The cumu­la­tive thick­ness of pro­duc­tive lay­ers varies from 2 to 30 meters, with sul­fur con­tent reach­ing up to 91.4%. The ores are clas­si­fied into two types: lime­stone and clay-lime­stone. The largest deposits in the basin are Roz­dol and Yazivske. The pro­duc­tive lay­ers con­sist of sul­fate-car­bon­ate deposits of lagoon­al ori­gin, to which the main sul­fur con­cen­tra­tions are linked. The high­est sul­fur con­cen­tra­tion is observed in the low­er part of these deposits, decreas­ing toward the upper lay­ers with an increas­ing amount of ter­rige­nous mate­r­i­al in the lime­stones.

Truskavets deposit is formed by car­bon­ate-sandy-clay rocks of the Low­er Vorotyshne For­ma­tion of the Miocene, which form an anti­cli­nal struc­ture. Sul­fur-poly­metal­lic min­er­al­iza­tion is dis­tinct­ly strat­i­fied and con­cen­trat­ed in a pack­age of brec­ciat­ed coarse-grained phyl­litic sand­stones and gray clays. The unique min­er­al asso­ci­a­tions and for­ma­tion con­di­tions of these deposits result in dif­fer­ent the­o­ries regard­ing their gen­e­sis (mag­mat­ic, hydrother­mal, sed­i­men­ta­ry, exo­genic). Most researchers clas­si­fy the deposit as an infil­tra­tion type formed by brine ore gen­e­sis process­es.

Native sul­fur is also found in oth­er loca­tions, such as the Yefremivs­ka, Olek­si­ivs­ka, and Petro­vskaya salt dome struc­tures, where sul­fur forms inclu­sions or veins in rock salt and clay-car­bon­ate rocks. At the Korul­sky dome, sul­fur occurs between lay­ers of Pale­o­gene lig­nite. For exam­ple, the Novod­mytrivske deposit, locat­ed in the south­east­ern part of the Dnieper-Donets Depres­sion (DDD), con­tains lens-like native sul­fur deposits in sul­fate-car­bon­ate rocks.