organic matter in high temp

8/4/2019 Organic Matter in High Temp

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Third ISAG, Sr Malo (Fra nce), 17-19/91 996

THE ROLE OF ORGANIC MATTER IN HIGH TEMPERATUREHYDROTHERMAL REGIMES

Gaston GIULIANI(~-), Alain CH EIL LET Z(~- ), Christian FRAN CE-LA NoRD (~)and Ftl ix RU ED A (~)

(1) ORSTO M, 213 rue La F ayette, 75480 Paris cedex 10, France(2) CRPG-CNRS , BP 20,54 520 Vandoeuvre-lb-N ancy, France,(3) ENSG-IN PL, BP 452 ,5400 1 Vandoeuvre-15s-Nancy,France,(4) MINERALC O S.A., calle 32, N013-07, Apartado aCreo 17878 , B og od , Colombia

KEY WORDS: organic matter, black shale, lhermochemical sulfale reduction, pyrite, Colombia

I N T R O D U C T I O NThe generation of petroleum in basins involves accumulation of organic matter (OM), its gradualburial, diagenesis breakdown and maturation via biodegradation or thermal cracking processes within a 75-150C "oil window" temperature range. In hydrotherm al regimes, as illustrated by active subm arine riftbasins (Simoneit, 1985), transformation of OM in sediments by high tem perature fluids, up to 360C, isan effective and more rapid process than in conventional basins. The stud y of such system s are of greatinterest to understand sulfate-hydrocarbon redox reactions specially for the thermochemical sulfatereduction (TSR), because the hydrothermal fluid discharge contains base-metal sulfides, sulfates,carbonates and petroleum. Moreover, the existence or a thermal gradient induced between ambient andsupercritical waters, permit to define the minimum temperature at which TSR reaction began.Hydrothermal experiments have shown that TSR take place a1 temperatures as low as 175OC whereasgeological evidences sug gest lower minimum tem peratures, 100-140C , as illustrated by the form ation ofthe Pine Point Mississippi Valley type deposit (Powell and Macqueen, 1984). This paper deals withcharacterization of OM associated with the Colombian emerald deposition and its key role in TSRthrough high temperature hydrothermal regimes.

T H E E X E MP L A R C A S E O F C O L O MB I A N E ME R A L D D E P O S I T SThe Colombian emerald deposits are found within two narrow zones, located along the two majorpolyphased thrusted limits of the Eastern Cordillera (EC), corresp ondin g to the original borders of an hugebasin in Cretaceous time. The eastern zone consists of the mining districts of Gachali, Chivor andMacanal and the western zone, of the distri cts of C oscuez, Muzo , La Palm a-Yacopi. Both are contained inEarly Cretaceous black shale (BS) series. Two distinct ages of formation or the emerald deposits havebeen obtained for the western and e astern emerald zones, respectively, 38-32 Ma for the Coscuez-M uzomines (Cheilletz et al., 1994). and 65 Ma for the Chivor mine (Cheilletz et al., 1995). The deposits resultfrom a two-stage cinematic process in which shorlening tectonics affects lhe two borders of the EC,leading to decollement planes, thrusting, and lhrust-fault related folds (Cheilletz and Giuliani, 1996).Stage 1 is characterized by decollement planes which focused the circulation of hydrothermal fluidsinducing albitisation and calcitisation of the BS. This metasomatism leads to leaching of major (Si, AI,K, Ti, Mg, P), trace (Ba, Be, Cr, V, C, B, U) and REE-elements from the enclosing BS; this stage isaccom panied by the deve lopm ent of a vein system filled by librous calcite and pyrite. Stage 2 is markedby the formation of breccias along thrust faults, and thrust related anticlines; it is characterized by


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Third ISAG, Sr Malo (France), 17-191911 996

extensional vein sets and hydraulic breccia development filled by muscovite, albite, rhomboedral calciteand dolomite, pyrite and finally by the precipitation in drusy cav ities of fluorite, apatite, parisite, REE-bearing dolomite, emerald and quartz.Microthermometric, Raman-probe and SEM analysis demonstrate the presence of H2O-NaCI-Ca C1 2-K Cl- C0 2-N 2 rich-brines trapped into emerald (Giuliani et al., 1992; Cheilletz et al., 1994;Ottaway et al., 1994), carbonate and pyrite (G iuliani et al., 1995). The trapping temperatures of fibrouscalcite from s tage 1 and rhomboed ric carbonates and emerald from stag e 2, are estimated respec tively, at150-200C and 300C (Cheilletz et al., 1994; Giuliani et al., 1995). Oxygen and carbon isotopecomposition of quartz and carbonates in all the deposits indicates a basinal formation water origin for themineralizing fluid (Giuliani et al., 1992), data confirmed by Ottaw ay et al. (199 4) for the M uzo mines.The 6 3 4 ~alues of H2S in solution in equilibrium with pyrite from emerald deposits (Giuliani et al.,1995) demonsk ate the evaporitic origin for the mineralizing brines. Cation analysis of fluid inclusions bycrush-leach technique (Banks et al., 1995) confirms that fluid in em erald, fluorite and quartz are derivedfrom the dissolution of primary halite and are predominantly Na-Cl-Fe-C a-K brines .

C H A R A C T E R I Z A T I O N O F S E D I M E N T A R Y A ND H Y D R O T H E R M A L O R G A N I CMA TTER RE LATE D TO EARLY CRE' I'ACEOUS BLACK SHALE S AND EMER ALDM I N E R A L I Z A T I O N

The EC of Colombia corresponds, in its central part, to a fold belt which thrusts, on the East,the Llanos basin and on the West, the Magdalena basin. The Cretaceous sediments were deposited in atwo arm basin, Tablazo-Magda lena at the west, and Cocuy at the east, separated by the Santander high.These basins were deform ed and inverted at Cen ozoic times. Fabre (1987) provides important estimationson burial and geothermal history of the EC basin during Cretaceous and Tertiary times. He show ed thatthe basin formed by extension on a strongly thinned lithosphere which provo ked a m ajor thermal eventcoupled with intrusion of mafic magm atism. Fabre and D elaloye (1983), dated these different basic rocksand showed that an Early Cretaceous thermal episode developed from 118 to 93 Ma, up to the Albian-Cenomanian boundary. The vitrinite reflectance data from different portions of the Cocuy paleobasin(Fabre, 1987) indicate increase of matury in sediments from Une (Albian; 0.5


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Third ISAG, St Malo (France), 17-191911 996 685

bitumen. The micro-transmission infrared microspectroscopy spectra show he lack of aliphatic andaroma tic bands and the disappearance of oxygen species. These results added t Rock-Eval data (HydrogenIndex< 1; 19 < Oxygen Index< 61 ) confirm the important ~he rm al racking suffered by these bitumen.613c values for bitumen range from -23.8 to -21.2 % and are similar to thoses obtained for the OM inthe BS. 6D alues are comprised between -73.2 and -122 960 and the H/C ratios are around 0.1. Theseisotopic data suggest a genetic relationship between OM and hydrothermal bitumen, and indicate thecomplete consum ption of organic hydrogen during degradation of OM .

T H E R O L E O F OR G A N IC M A T T E R I N T H E T H E R M O C H E M I C A L S U L F A T ER E D U C T I O N I N C O L O M B I A N E M E R A L D D E P O S I T SColombian emerald deposits have no magmalic connection and they can be considered as pyritedeposits hosted by carbonate veins, emerald being an accesso ry mineral. Sulfide-sulfur source is evaporiticbut the chemical process responsible for he reduction of sulfate in sulfide is still in debate (Ottaway etal., 1994; Cheilletz and Giuliani, 1996). The presence of both OM in the enclosing BS and in thehydrothe rmal carbonate-py rite veins opens to discussion of the possible role of OM in emerald deposit.Four main conditions are necessary to establish that OM has playcd a key role on TSR (modified from

Leventhal, 1990):1 - OM is now present in the or e body; i l is the case in colornbian deposits where hydrothermalbitumen are found,2- OM was present when the different stages of the hydrothermal system developed and at thetime of ore formation: in fact, hydrothermal bitumen precipitated during stages 1 and 2 and are found asdaugh ter minerals within primary fluid inclusions hosted by em erald,3- two types of OM a re present: in our case, original, i.e., OM in the BS , and hyd rotherma l, i.e.,bitume in the veins. The organic-matter-bearing BS enclosing the emerald mineralization has suffered athermal degrada tion due to burial and thermal history of the EC basin during Cretaceous tim e. The fluidsrelated to em erald formation at Cenozoic times rcmobilized this OM provoking chan ges, differences inchem ical comp osition and precipitation of bitum en,4- the chem ical change in the altered OM is compatible wilh TSR:4.1- sulfate reduction implies always ox ida ~io n f OM . Oxidation of OM contained inthe BS and subsequent carbonates prec ipitalion in cquilibrium wilh C 0 2 would result in the formation oflow 613C carbona tes. 613C of the carbonates will depend on thcir temperature of formation and the initialisotopic composition of OM. Calcite-graph ite 13C /'2C ractionation factors were calculated using thecalibration of Bottinga (1969). The difference of 4.5% in 6I3C found bctween OM from Macanal and Paja-Simiti (?) BS, implies that the 613C of carbonates from stages 1 and 2 of the eastern emerald zone arelighter than those of the western one. In fact, thcre are two isotopic populations of carbonates. The highe r6I3 C values correspond to Co scuez, Muzo, Yacopi deposits (-9.3 Rb + 2HCO3- + H2S whereC H 20 represents a carbohydrate and R(a,b) large organic molecules. HCO3- and H2S produced by thereaction reacted with ca 2 + and ~ e ~ +arried by the hydrothcrmal fluids to induce the precipitation of


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Third ISAG, St Malo (France), 17-191911 996

calcite and pyrite i.e., HC03- + c a 2 + -> Ca C0 3 (calcite) + H+ and 7H2S + ~ e ~ +S 0 4 - - -> 4FeS2(pyrite) + 4 H 2 0 + 4H+-

CONCLUS IONSThe geoc hem ical study of OM in the Colombian emerald deposits provided new inform ations andconfirmed previous data on the degree of maturity of the Early Cretaceous BS of the EC and the burialhistory of the ore-filled area. It constraints also the possible role of OM on the mode of formation ofemerald ore bodies. Bitumen appears to have been derived locally from the transformation of OM fromsediments by high temperature hydrothermal fluids, up to 300C. The question of genetic relationshipsbetween sulphides, OM and bitumen is solved. Sulfate-hydrocarbon redox reactions occurred at hightemperature as described in active submarine rift basins, and isotopic evidences are suggestive of theeffective role of OM in the formation of hydrogen sulfide necessary to produce sulfides (pyrite). Themechanism proposed involves the reaction between sulfate of evaporitic origin and OM, without theproduction of an intermediary elemental sulfur, to produce hydrogen sulfide. This redox reaction pro cesshas been already verified experimentally and invoked in dcssiminated or stratiform base metals orMississippi Valley type deposits.

REFERENCESBanks D.A., Yardley B.W .D., Giuliani G., Chcillctii A. and Rucda F. 1995. Chemistry and source of thehigh temperature brines in the Colombian emerald dcposits. Mineral deposits, Pasava, Kribek & Zak(eds), Balkema, Rotterdam, 557-560.Bottinga Y. 1969. Calculated fractionation factors for carbon and hydroge n isotope exch ange in thesystem calcite-carbon dioxide-graphite-methane-hydrogen-w atervapor. Geochim. Cosmochim. Acta, 33,49-64.Che illetz A., FCraud G., Giuliani G. and Rodriguez C.T. 1994. Time -pressu re-tem perature constraints onthe formation of Colombian emeralds: a laser-probe and fluid inclusion study. Econ. G a l . 89,2,36 2-380Cheilletz A., Giuliani G., Zimmermann J.L. and Ribeiro-Althoff A.M. 1995. Ages, geochemicalsignatures and origin of Brazilian and Colombian emerald dcposits: a magmatic versus sedimentarymodel. Mineral deposits, Pasava, Kribek & Zak (eds), Balkema, Rotterdam, 569-5 72.Cheilletz A. and Giuliani G. 1996. The genesis of Colombian emeralds: a restatement. MineraliumDeposita (in press).Fabre A. 1987 . Tecton ique e t gCnCration d'hydrocarbures: un modEle de I'Cvolution de la CordillereOrientale de Colomb ie et du bassin des Llanos p enda r~t e CrCtacC et le Tertiaire. Arch. Sc. G enkve, 40 ,2 ,145-190.Fabre A. and Delaloye M. 1983. Intrusiones basicas c red cica s en las sed imentitas de la parte cen tral de laCordillera Oriental. Geol. Noran dina, 6, 19-29.Giuliani G., Sheppard S.M.F., Cheilletz A. and Rodriguei, C.T. 1992. Fluid inclusions and 180/160,13C /12C sotope geochemistry contribution to the genesis of emerald deposits from the OrientalCordillera of C olombia. C. R . Acad. Sci. Paris, 314, sCrie 11 , 269-274.Giuliani G., Cheilletz A., Arboleda C., Carrillo V. , Rueda F. and Baker J. 1995. An evaporitic origin ofthe parent brines of Colomb ian emeralds: fluid inclusion and sulfur isotope evidence. Eu r. Journal ofMineralogy 7, 151-165.Leventhal, J.S. 1990. Organic matter and thermochemical sulfate reduction in the Viburnum trend,Southeast Missouri. Econ. Geol., 85,622-632.Machel H.G. 1987. Some aspects of diagenetic sulphate-hydrocarbon redox reactions. Geol. Soc. SpecialPub. "Diagenesis of Sedimentary sequences", Marshal1 J.D. (eds), 36, 15-28.Ottaway T.L., Wicks F.J., Bryndzia L.T., Kyser T.K. and Spo oner E.T.C. 199 4. Formation of the M uzohydrothermal emerald deposit in Colombia. Nature, 369,552-5 54.Powell T.G. and Macqueen R.W. 1984 . Precip itation of sulfide ores and organic matter: sulfate reactionsat Pine Point, Canada. Science. 224,63-66.Simoneit B.R.T. 1385. Hydrothermal petroleum: genesis, migration and deposition in Guaym as basin,Can. J. Earth Sci., 22, 1919-1929.

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