Ore Paragenesis Research Articles

A possible genetic model of the Shuangwang hydrothermal breccia gold deposit, Shaanxi Province, central China: Evidence from fluid inclusion and stable isotope

The Shuangwang gold deposit (with a gold resource of approximately 70t Au), hosted in a NW-trending breccia belt, is located in the Fenxian-Taibai fore-arc basin in the West Qinling Orogen of central China. Four stages of ore paragenesis are identified, demonstrating mineral assemblages of ankerite–quartz–albite, quartz–albite–pyrite–ankerite, pyrite–calcite–quartz, and fluorite–dickite–gypsum, respectively. Fluid inclusions hosted in stages I, II, and III hydrothermal minerals yield homogeneous temperatures of 300–463°C, 220–340°C and 100–279°C, with salinities lower than 22.7wt% NaCl equiv. Trapping pressures estimated from CO2–H2O fluid inclusions show a gradual decrease from 100–170MPa (KT8 ore body) to 17–55MPa (KT2 ore body), corresponding to mineralization depths from 3.8–6.4km (KT8) to 0.6–2.1km (KT2). Hydrogen and oxygen isotopic data suggest that the ore-forming fluids evolved from metamorphic water to magmatic water, and lately meteoric water. Sulfur and carbon isotope compositions show that these fluids might have originated from interaction with the host rocks with minor additional magmatic source. Based on geochemical investigations, with combination of regional and ore deposit geology, a possible genetic model with a three-step ore-forming process is proposed. The Devonian Xinghongpu sedimentary rocks are characterized by a relatively high gold content, which might provide the initial gold source. Linear folds and faults formed during Triassic orogenic processes provided the subsequent pathways for ore-forming fluids and suitable space for gold mineralization. Postorogenic magmatic activity induced voluminous hydrothermal fluids that mixed with the basinal fluids and may have started the ore formation process. Over pressure led to hydrofracturing and the subsequent pressure drop promoted fluid boiling, which in turn resulted in abundant gold deposition. Induced by postorogenic magmatic hydrothermal activity, the Shuangwang gold deposit is considered a special type of orogenic gold deposit formed in a compression–extension transition.

Ore microscopic characterization of mineralized rocks at the Bi’r Tawilah gold prospect, Saudi Arabia

Ore microscopic investigation of core samples from the Bi’r Tawilah gold prospect enables to distinguish three main groups of ore paragenesis from oldest to youngest, namely magmatic, hydrothermal and supergene. The ore minerals are encountered in sheared and mineralized ophiolitic serpentinites and a variety of hydrothermally altered granitic rocks that include quartz–diorite, granodiorite, monzogranite and quartz–feldspar porphyry. Magmatic ore minerals are chromite, magnetite, ilmenite and pyrrhotite. Hydrothermal ore minerals are the most common (>90%) and they follow the paragenetic sequence: rutile, sphalerite, chalcopyrite, pyrite (with visible gold and electrum inclusions) and arsenopyrite. Supergene ore minerals are ferric Ca-arsenate and ferric oxyhydroxide (±goethite) which represent the weathering products of pyrite and arsenopyrite, respectively. The non-arsenian nature of pyrite and late formation of arsenopyrite all together suggest mobility of As in the hydrothermal fluids until late stages. There are no remarks of ore metamorphism as all events took place post-orogenically despite zonal arrangement of inclusions in pyrite that can result from paucity of crystallization. Ore textures indicate change from early relatively reducing hydrothermal fluids at low fS2 that crystallize pyrrhotite which is converted into pyrite when the fluid becomes relatively more oxidizing at high fS2 and pH <5. On the other hand, the supergene enrichment zone at the uppermost 40m is dominated by exclusively oxidizing fluids (pH >7) at low temperature up to ∼75°C. Supergene ferric Ca-arsenate is a cellular-structured hydrous phase with distinct As5+↔Fe3++Ca2+ ionic substitution in three distinct domains.

The Geology and Mineralogy of the Loulo Mining District, Mali, West Africa: Evidence for Two Distinct Styles of Orogenic Gold Mineralization

Loulo is a world-class orogenic gold mining district in the Birimian terrane of western Mali. Orebodies are located along second or higher order shears associated with the Senegal-Mali shear zone, with gold mineralization largely linked to a transtensional event. The mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics. One group is typified by the Gara deposit, whereas the other by the Yalea deposit. Both deposit styles are hosted by similar rock types (calcareous graywackes and calciticdolomitic marbles). Gara-style orebodies occur as sulfide disseminations or ankerite-rich shear vein stockworks, hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the Senegal-Mali shear zone. These deposits are characterized by intense multi-stage albitization and tourmalinization (pre-, syn- and postmineralization). Gold lodes are Fe-rich (dominated by nickeloan pyrite), contain Cu-Ni ± Co minor and trace sulfides (e.g., chalcopyrite, gersdorffite, pentlandite, cobaltite, millerite), and show consistently high levels of P-REE-W–bearing phases (apatite, monazite, xenotime, and scheelite). Base metal concentrations show a marked increase in marble host rocks, with the formation of nickeloan pyrite-cobaltite-clausthalite ores. In contrast, Yalea-style deposits are associated with quartz ± ankerite vein lodes and disseminated sulfide stringer zones. Mineralization occurs along highly altered (tourmaline-absent), brittle-ductile, shears up to 8 km away from the Senegal-Mali shear zone. Wall-rock alteration is characterized by addition of K 2 O, CaO, CO 2 , H 2 O, and SiO 2 , with mineral assemblages consisting of chlorite-sericite-carbonate-quartz ± albite. Ore paragenesis is enriched in As, mainly as multistage growth of arsenopyrite and arsenian pyrite. Base metal sulfides, scheelite, and (REE)-phosphates are extremely rare. The diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region. The As-rich Yalea-style deposits have characteristics typical of Birimian gold mineralization in Ghana, and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism. On the other hand, the polymetallic, Fe-B–rich, Gara-style orebodies show atypical features for Birimian gold mineralization. Instead, field relations and the mineralogy and geochemistry of the Gara-style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks, with possible links between gold mineralization and iron oxide skarn development in the region. The data collected at Loulo highlight the diverse nature of orogenic gold deposits, especially in West Africa. This style of ore deposit can form from a variety of fluid sources, both metamorphic and magmatic.

Natural stabilization of mine waste-dumps — Evolution of the vegetation cover in distinctive geochemical and mineralogical environments

The present work is focused on remediation promoted by natural vegetation at distinctive mine waste-dumps. Three mines, located in different climatic, hydrological, and paragenetic environments, were selected as case studies (Cerdeirinha, Lapa Grande and Penedono, N Portugal). They were analyzed with the following objectives: i) mapping of cover vegetation to monitor the success of environmental rehabilitation; ii) to estimate the value of local flora for regional phytostabilization; and iii) to study the influence of geochemical and mineralogical parameters for the establishment of spontaneous vegetation. Remote detection tools showed the effective mapping of the evolution of the vegetation for a thirty year period (between 1980 and 2011). Results also suggest that the ore deposits have a strong influence on the evolution of the mine wastes and, consequently, on the establishment of conditions for sustainable revegetation. A complex ore paragenesis, the presence of varied clay minerals and geochemical evolution of the wastes in order to form stable supergenic minerals, seem to enhance the potential for successful revegetation. Combining information regarding environmental mineralogy with data about substrate and vegetation allows the monitoring of the evolution of distinctive mining environments and the recommendation of the appropriate species for phytostabilization purposes.

Geochemistry, mineralogy and genesis of the Ayazmant Fe–Cu skarn deposit in Ayvalik, (Balikesir), Turkey

The Ayazmant Fe–Cu skarn deposit is located approximately 20 km SE of Ayvalık or 140 km N of Izmir in western Turkey. The skarn occurs at the contact between metapelites and the metabasites of the Early Triassic Kınık Formation and the porphyritic hypabyssal intrusive rocks of the Late Oligocene Kozak Intrusive Complex. The major, trace, and rare earth-element geochemical analysis of the igneous rocks indicate that they are I-type, subalkaline, calc-alkaline, metaluminous, I-type products of a high-level magma chamber, generated in a continental arc setting. The 40Ar– 39Ar isochron age obtained from biotite of hornfels is 20.3 ± 0.1 Ma, probably reflecting the age of metamorphic–bimetasomatic alteration which commenced shortly after intrusion into impure carbonates. Three stages of skarn formation and ore development are recognized: (1) Early skarn stage (Stage I) consisting mainly of garnet with grossular-rich (Gr 75–79) cores and andradite-rich (Gr 36–38) rims, diopside (Di 94–97), scapolite and magnetite; (2) sulfide-rich skarn (Stage II), dominated by chalcopyrite with magnetite, andraditic garnet (Ad 84– 89), diopside (Di 65– 75) and actinolite; and (3) retrograde alteration (Stage III) dominated by actinolite, epidote, orthoclase, phlogopite and chlorite in which sulfides are the main ore phases. 40Ar– 39Ar age data indicate that potassic alteration, synchronous or postdating magnetite–pyroxene–amphibole skarn, occurred at 20.0 ± 0.1 Ma. The high pyroxene/garnet ratio, plus the presence of scapolite in calc-silicate and associated ore paragenesis characterized by magnetite (± hematite), chalcopyrite and bornite, suggests that the bulk of the Ayazmant skarns were formed under oxidized conditions. Oxygen isotope compositions of pyroxene, magnetite and garnet of prograde skarn alteration indicate a magmatic fluid with δ 18O values between 5.4 and 9.5‰. On the basis of oxygen isotope data from mineral pairs, the early stage of prograde skarn formation is characterized by pyroxene (Di 94–97)-magnetite assemblage formed at an upper temperature limit of 576 °C. The lower temperature limit for magnetite precipitation is estimated below 300 °C, on the basis of magnetite–calcite pairs either as fracture-fillings or massive ore in recrystallized limestone-marble. The sulfide assemblage is dominated by chalcopyrite with subordinate molybdenite, pyrite, cubanite, bornite, pyrrhotite, galena, sphalerite and idaite. Gold–copper mineralization formed adjacent to andradite-dominated skarn which occurs in close proximity to the intrusion contacts. Native gold and electrum are most abundant in sulfides, as fine-grained inclusions; grain size with varying from 5 to 20 µm. Sulfur isotope compositions obtained from pyrrhotite, pyrite, chalcopyrite, sphalerite and galena form a narrow range between − 4.8 and 1.6‰, suggesting the sulfur was probably mantle-derived or leached from magmatic rocks. Geochemical data from Ayazmant shows that Cu is strongly associated with Au, Bi, Te, Se, Cd, Zn, Pb, Ni and Co. The Ayazmant mineralizing system possesses all the ingredients of a skarn system either cogenetic with, or formed prior to a porphyry Cu(Au–Mo) system. The results of this study indicate that the Aegean Region of Turkey has considerable exploration potential for both porphyry-related skarns and porphyry Cu and Au mineralization.

Structural controls, temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine, south Eastern Desert, Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (LCO2 + Laq ± VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2 ± VCO2) and aqueous H2O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H2O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f S2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f O2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.

Geology and petrography of the Masatdagi diasporic bauxites, Alanya, Antalya, Turkey

There are more than fifteen bauxite outcrops in the study area located in the southern part of the Central Taurides. The bauxites are developed in the Upper Permian Cebireis Formation (carbonate rocks interbedded with schist) and Lower Triassic Asmaca Formation (limestone, dolomite, quartzite and schist). The bauxite outcrops are observed as parts of two or three independent bauxite blocks that were broken apart by faulting and an erosional surface. Based on outcrop appearances, four ore types can be defined; black, earthy, clayey, and Fe- and Mn-bearing bauxites. Oolitic textures are common in all ore types, except the earthy ones. Bauxites are represented by a mixture of diaspore and clay in varying proportions. Diaspore, boehmite, gibbsite, corundum, hematite, goethite, amorphous iron hydroxide, magnetite, chamosite, pyrite, marcasite, psilomelane, rutile, kaolinite, illite, muscovite, hydro-muscovite, chlorite, chloritoid, rectorite, ephesite, albite, microcline, quartz, K–Mg–Al silicate, hydrated K–Al silicate, and dolomite are present in the ore paragenesis. Bauxites, which are observed at or near the contact between the Cebireis and Asmaca formations, are thought to be fillings of cavities within and/or on Late Triassic—early Cretaceous carbonate rocks.

Genesis of sediment-hosted stratiform copper–cobalt deposits, central African Copperbelt

The Neoproterozoic central African Copperbelt is one of the greatest sediment-hosted stratiform Cu–Co provinces in the world, totalling 140 Mt copper and 6 Mt cobalt and including several world-class deposits (⩾10 Mt copper). The origin of Cu–Co mineralisation in this province remains speculative, with the debate centred around syngenetic–diagenetic and hydrothermal-diagenetic hypotheses. The regional distribution of metals indicates that most of the cobalt-rich copper deposits are hosted in dolomites and dolomitic shales forming allochthonous units exposed in Congo and known as Congolese facies of the Katangan sedimentary succession (average Co:Cu = 1:13). The highest Co:Cu ratio (up to 3:1) occurs in ore deposits located along the southern structural block of the Lufilian Arc. The predominantly siliciclastic Zambian facies, exposed in Zambia and in SE Congo, forms para-autochthonous sedimentary units hosting ore deposits characterized by lower a Co:Cu ratio (average 1:57). Transitional lithofacies in Zambia (e.g. Baluba, Mindola) and in Congo (e.g. Lubembe) indicate a gradual transition in the Katangan basin during the deposition of laterally correlative clastic and carbonate sedimentary rocks exposed in Zambia and in Congo, and are marked by Co:Cu ratios in the range 1:15. The main Cu–Co orebodies occur at the base of the Mines/Musoshi Subgroup, which is characterized by evaporitic intertidal–supratidal sedimentary rocks. All additional lenticular orebodies known in the upper part of the Mines/Musoshi Subgroup are hosted in similar sedimentary rocks, suggesting highly favourable conditions for the ore genesis in particular sedimentary environments. Pre-lithification sedimentary structures affecting disseminated sulphides indicate that metals were deposited before compaction and consolidation of the host sediment. The ore parageneses indicate several generations of sulphides marking syngenetic, early diagenetic and late diagenetic processes. Sulphur isotopic data on sulphides suggest the derivation of sulphur essentially from the bacterial reduction of seawater sulphates. The mineralizing brines were generated from sea water in sabkhas or hypersaline lagoons during the deposition of the host rocks. Changes of Eh–pH and salinity probably were critical for concentrating copper–cobalt and nickel mineralisation. Compressional tectonic and related metamorphic processes and supergene enrichment have played variable roles in the remobilisation and upgrading of the primary mineralisation. There is no evidence to support models assuming that metals originated from: (1) Katangan igneous rocks and related hydrothermal processes or; (2) leaching of red beds underlying the orebodies. The metal sources are pre-Katangan continental rocks, especially the Palaeoproterozoic low-grade porphyry copper deposits known in the Bangweulu block and subsidiary Cu–Co–Ni deposits/occurrences in the Archaean rocks of the Zimbabwe craton. These two sources contain low grade ore deposits portraying the peculiar metal association (Cu, Co, Ni, U, Cr, Au, Ag, PGE) recorded in the Katangan sediment-hosted ore deposits. Metals were transported into the basin dissolved in water. The stratiform deposits of Congo and Zambia display features indicating that syngenetic and early diagenetic processes controlled the formation of the Neoproterozoic Copperbelt of central Africa.

Mineralization of La Salvadora deposit, Province of Mendoza, Argentina

Fil: Rubinstein, Nora Alicia. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires; Argentina

ZONALIDADE VERTICAL NO CORPO MINERALIZADO DA MINA DE OURO SCHRAMM (SC) - DETALHAMENTO DO MODELO GENÉTICO

The petrography of ore samples collected from level 140 to 180 of the Schramm gold mine displayed a hydrothermal phase not previously discerned, occurred before the ore genesis, which crystallized magnetite-hydrotermalites. A vertical zonation, was also displayed, characterized by three different ore paragenesis. Bellow level 155, the carbonated ore is hosted by the magnetite-hydrotermalites and it is composed by lamellar hematite, magnetite, sphalerite and gold, with subordinated pyrite, calcopyrite, arsenopyrite, gersdorffite and siegenite. This paragenesis typifies an environment where the primary ore fluid was oxidized and desulphurized, what caused gold deposition. Around the 155 level, the carbonated ore has few sulfides and a paragenesis in which siegenite, gersdorffite and sphalerite are very common minerals, with gold and subordinated pyrite and calcopyrite. At this level ore is hosted by ultrabasic granulites, and gold was deposited as a consequence of the low pH-intermediate fO 2 environment. Above the 155 level, the main ore minerals are Bi-rich galena, sphalerite and gold, with subordinated gersdorfite, pirrotite, pyrite and cosalite. Gold is mainly in carbonate but also in quartz veins hosted by granulites. These minerals characterize a reduced fluid and a high pH environment. The ore body was formed through at least four phases: (1) Genesis of magnetite-hydrothermal veins, at temperatures around 230°C. (2) Genesis of the oxidized ore inside magnetite-hydrothermal veins and the siegenite-millerite-gersdorffite ore hosted by ultrabasic granulites, at temperatures between 260° and 360°C. (3) Period that the main vein stay opened, filled by fluids that precipitated euhedric, prismatic, siderite, quartz and chlorite with little gold, at temperatures between 310° and 320°C. (4) With the vein walls coated by minerals crystallized previously, the primary, reduced, ore fluid could not react with host rocks. Gold deposited due the temperature reduction caused by mixing of ore fluid and meteoric water. This new fluid precipitated milky-quartz until the vein closure and the end of the hydrothermal system.

Zonalidade vertical no corpo mineralizado da mina de ouro Schramm (SC) – detalhamento do modelo genético

The petrography of ore samples collected from level 140 to 180 of the Schramm gold mine displayed a hydrothermal phase not previously discerned, occurred before the ore genesis, which crystallized magnetite-hydrotermalites. A vertical zonation, was also displayed, characterized by three different ore paragenesis. Bellow level 155, the carbonated ore is hosted by the magnetite-hydrotermalites and it is composed by lamellar hematite, magnetite, sphalerite and gold, with subordinated pyrite, calcopyrite, arsenopyrite, gersdorffite and siegenite. This paragenesis typifies an environment where the primary ore fluid was oxidized and desulphurized, what caused gold deposition. Around the 155 level, the carbonated ore has few sulfides and a paragenesis in which siegenite, gersdorffite and sphalerite are very common minerals, with gold and subordinated pyrite and calcopyrite. At this level ore is hosted by ultrabasic granulites, and gold was deposited as a consequence of the low pH-intermediate fO 2 environment. Above the 155 level, the main ore minerals are Bi-rich galena, sphalerite and gold, with subordinated gersdorfite, pirrotite, pyrite and cosalite. Gold is mainly in carbonate but also in quartz veins hosted by granulites. These minerals characterize a reduced fluid and a high pH environment. The ore body was formed through at least four phases: (1) Genesis of magnetite-hydrothermal veins, at temperatures around 230oC. (2) Genesis of the oxidized ore inside magnetite-hydrothermal veins and the siegenite-millerite-gersdorffite ore hosted by ultrabasic granulites, at temperatures between 260o and 310oC. (3) Period that the main vein stay opened, filled by fluids that precipitated euhedric, prismatic, siderite, quartz and chlorite with little gold, at temperatures between 310o and 320oC. (4) With the vein walls coated by minerals crystallized previously, the primary, reduced, ore fluid could not react with host rocks. Gold deposited due the temperature reduction caused by mixing of ore fluid and meteoric water. This new fluid precipitated milky-quartz until the vein closure and the end of the hydrothermal system.

Geochemistry and mineralogy of Platinum-group elements in the Ransko gabbro–peridotite massif, Bohemian Massif (Czech Republic)

The Ransko gabbro–peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex of Lower Cambrian age. The complex hosts low grade Ni–Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites and, to a much lesser extent, in both pyroxene and olivine gabbros and plagioclase-rich peridotites. The ore zone is characterized by strong serpentinization and uralitization. The total Ni + Cu locally reaches up to 4 wt%. Anomalous concentrations of platinum-group elements (PGE's) (maximum 532 ppb Pd, 182 ppb Pt, 53 ppb Rh, 15 ppb Ru, 41 ppb Ir) were detected in samples of Cu–Ni and Ni–Cu ores (maximum 2.63 wt% Ni and 2.31 wt% Cu) from the Jezirka orebody. The main ore paragenesis includes pyrrhotite, pentlandite, chalcopyrite, cubanite, pyrite, magnetite, mackinawite, valleriite, ilmenite and sphalerite. During this work, michenerite, froodite, sperrylite, gold, native bismuth, altaite, tsumoite, hessite, an unnamed Bi–Ni telluride, cobaltite–gersdorffite and galena were newly identified. The host rocks originated through partial melting of a slightly depleted mantle source with noble metals scavenged from this primitive magma prior to the development of these rocks.

Geochemical characteristics across the oxic/anoxic interface ( Rote Fäule front) within the Kupferschiefer of the Lubin-Sieroszowice mining district (SW Poland)

Samples of profiles from Lower Zechstein in the Lubin-Sieroszowice mining district (SW Poland) were collected within and at different distance to the oxidized facies (Rote Fäule), developed within the Zechstein strata around the Zary Pericline. The samples include Kupferschiefer, underlying Weissliegendes sandstone and overlying Zechstein Limestone from Rote Fäule zones, adjacent Cu-mineralized strata, and drill cores from the Pb/Zn-mineralized zone more distant to the Rote Fäule front. The Kupferschiefer samples show differences in ore paragenesis and composition of organic matter, which are accompanied by variations in base metal and trace element contents, and isotope ratios of organic carbon and carbonates. The results from Rock-Eval analyses provide evidence for an oxidative alteration of organic matter in Kupferschiefer samples within and adjacent to the Rote Fäule zones related to base metal mineralization. The extent of this alteration is reflected by differences in the temperature of maximum pyrolysis yield ( T max), hydrogen and oxygen indices (HI, OI). In contrast to Kupferschiefer from Pb/Zn-mineralized areas, organic matter is characterized by lower hydrogen equivalents, and by higher T max values. The changes in the isotopic composition of organic carbon and carbonates reflect the alteration of organic matter. Kerogen within, or close to, Rote Fäule zones is enriched in 13C caused by the preferential release of isotopically light organic compounds through progressive degradation of organic matter. The opposite tendency towards lower δ 13C values of calcite provides evidence for organic matter remineralization. Isotopic exchange reactions of carbonate with the isotopically distinct oxidizing brines at temperatures between 100 and 120 °C caused decreasing δ 18O values of calcite. The regional zonation in the Kupferschiefer with respect to sulfide paragenesis and base metal composition in the mines is explained by metal precipitation from oxidizing brines through decreasing Eh conditions during the infiltration of the brines into the reduced Kupferschiefer. Relationships between selected trace elements, organic carbon and sulfur contents, and HI values are consistent with that model of ore formation.

Fluid evolution and chemical controls in the Fazenda Maria Preta (FMP) gold deposit, Rio Itapicuru Greenstone Belt, Bahia, Brazil

The Fazenda Maria Preta (FMP) gold deposit is confined to two regional-scale, sinistral-oblique brittle–ductile shear zones which are located in a greenschist facies metamorphosed volcano-sedimentary sequence in the northern sector of the lower Proterozic Rio Itapicuru greenstone belt, NE Brazil. Alteration is pervasive throughout the shear zones and characterized by carbonatization and sericitization of intermediate volcanic, volcaniclastic, and sub-volcanic rocks. The orebodies are mainly shear veins hosted by, or directly associated with, highly deformed carbonaceous volcaniclastic rocks, breccia and stockwork systems or disseminations in felsic and mafic sub-volcanic bodies. Native gold appears as free grains finely disseminated in the quartz veins or closely related to arsenopyrite, pyrite, pyrrhotite, sphalerite, and Fe-chlorite in the carbonaceous wallrocks. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy revealed that the vein quartz is largely dominated by primary, pseudosecondary and rarely secondary, populations of CO 2–(±CH 4±N 2) inclusions (type I), whereas primary groups of low salinity (<6 wt.% eq. NaCl) H 2O–CO 2–(±CH 4±N 2) inclusions (type II) comprise the dominant inclusion type in only a few veins. The ThCO 2 (L–V→L) data indicated a variation in the CO 2 density of 1.05 to 0.60 g/cm 3 for the type I inclusions and, conversely, a narrower range of 0.76–0.73 g/cm 3 for the type II inclusions. The broader variation of the CO 2 density for the CO 2-rich fluid could have been the result of trapping of the vein fluid under a variable pressure regime, or by its re-equilibration during continuous deformation within the shear zone domains and reduction of the overburden pressure during uplift. The CO 2-rich fluid inclusions provide no evidence that they have been the result of H 2O loss either during infiltration (e.g., hydration reactions) or after trapping of an original H 2O–CO 2 fluid. Accordingly, the CO 2±(CH 4+N 2) and the H 2O–CO 2–(±CH 4±N 2) fluids are considered as representatives of two distinct fluid regimes, which were active during the gold mineralizing events within the shear zones. Both types of ore fluids are interpreted as part of a deep metamorphic–magmatic hydrothermal system in which (i) a CO 2-rich fluid, probably originated in the mantle, was transported to, and released in, higher structural levels mainly by tonalitic–granodioritic and alkaline magmas, whereas (ii) a H 2O–CO 2 fluid, generated by devolatilization reactions during the regional metamorphism, was either directly channeled into favorable structural sites (i.e., metamorphic fluid), or absorbed and later exsolved by the crystallization of felsic magmas (i.e., magmatic fluid sensu lato). In this context, gold deposition occurred between 320°C and 420°C and 2.1 to 4.4 kb and calculated values of fO 2 between 10 −26.5 and 10 −32.4 bar reveal the relatively reducing nature of the mineralizing fluids. The ore paragenesis constrains fS 2 to the range of 10 −10.1–10 −6.5 bar for these fluids. Under such conditions, the gold was transported mainly as bisulfide complexes by the H 2O–CO 2 fluid, but in the case of the CO 2-rich fluid, the role of thiocomplexes or some other types of ligands is obscured by the lack of high temperature experimental data. Deposition of the metal occurred in response to redox changes, which accompanied fluid–carbon interaction, particularly a decrease in fO 2 and fS 2 of the fluid. On a microscopic scale, phisisorption and chemisorption processes at the fluid–iron sulfide interface may have further enhanced the deposition of the gold, particularly on the surfaces of precipitating pyrite and arsenopyrite.

Geochemistry of the Ore Fluids in the Bonfim and Itajubatiba Gold Skarn Deposits, Northeastern Brazil: A PIXE Method Study

Skam deposits are characterised by their high variety of metals and contents. The variety of metals become an interesting economic characteristic of these deposits, that could be an important source of many metals. In this scenario, each particular skam can show different geochemical associations. Different ore types and their complex geochemical association reflect the parental fluid, the composition of the host rocks and the temperature and pressure of ore formation (Meinert, 1992). The characterisation of the ore fluids can give fundamental information to elaborate a metaUogenic model and to provide important criteria for ore exploitation, in each particular deposit. Detailed characterisation of the ore fluids composition can be obtained by mineral chemistry and fluid inclusions studies of the ore paragenesis. The advance of new techniques allows, for example, to measure traces elements in minerals, in a non destructive way. That is the case of the PIXE Method (Particle Induced X-Ray Emission), where the highly intense photon beam from the synchrotron allows deep atoms to be excited by the photons remaining after the attenuation by the matrix (Volfinger et al., 1997). This contribution reports PIXE analyses carried out on the samples from the ore paragenesis of two gold skarn deposits, in Northeastern Brazil: Itajubatiba (Au) and Bonfim (W-Au). The principal objective is to determine the composition of the ore fluids and their geochemical association. Its purpose is to estab-lish a geochemical and metallogenic model for these deposits and elaborate some be used in a more rational exploitation of them.

Multistage gold deposition in the Archaean Maria Lázara gold deposit (Goiás, Brazil)

The Maria Lazara gold deposit in the Archaean Guarinos greenstone belt, central Brazil, consists of quartz-carbonate veins formed in a ductile shear zone. Study of the sulphides and gold-bearing quartz-carbonate veins and veinlets of the potassic hydrothermal alteration zone reveals two stages of ore paragenesis. The first stage corresponds to S-rich arsenopyrite deposition and the second one to the precipitation of As-rich arsenopyrite and Bi-Te-Au-S phases. Modes of occurrence, textures and Ag contents of native gold define two types of gold. The first type is native gold with Ag contents > 5 wt % postdating the S-rich arsenopyrites. The second type has Ag contents < 3 wt % and is contemporaneous with Bi-Te-S phases.

The W-Mo deposit of Perda Majori (SE Sardinia, Italy): a fluid inclusion study of ore and gangue minerals

The W-Mo deposit of Perda Majori (SE Sardinia) consists of a NW-SE trending quartz vein that crops out for about 500 m in the Salto di Quirra region. Hercynian low-grade metamorphic rocks, which host the orebody, largely dominate the geology of the area and are themselves intruded and contact metamorphosed by late Hercynian leucogranites and leucogranite porphyries. Field evidence supports a genetic link between the orebody and the emplacement of the leucogranites. The ore paragenesis consists chiefly of wolframite, molybdenite, scheelite, pyrite and calcopyrite, while the gangue is formed of quartz, K-feldspar, albite, topaz, micas, fluorite and calcite. The highest Th (320-420 o C) were found in coarse-grained quartz (Qz I) which also records signs of boiling. Inclusions found in wolframite, giving Th 310-330 o C and the highest recorded salinities (6-7 wt.% NaCl eq.), suggest that ore deposition closely followed the boiling

Sulphur-isotope and elemental geochemistry studies of the Hemlo gold mineralization, Ontario: sources of sulphur and implications for the mineralization process: Discussion

Undoubtedly, geochemical studies can provide key information on the genesis of an ore deposit but it is important to constrain these studies by reliable ore paragenesis. This is particularly important for the Hemlo gold deposit, which is characterized by a long history of deformation and metamorphism. In their paper the authors have used sulfur-isotope studies to determine the temperature of isotopic equilibrium and to speculate on the possible source of sulfur of barite and sulfides. In our opinion the methodology of their study is very questionable because they seem to give no consideration to the ore paragenesis and the relative timing of deformation, metamorphism, and gold mineralization. In the Golden Giant mine Michibayashi (1991) has recorded the presence of two distinct types of barite: highly folded and sheared-banded barite and massive barite filling fractures within the feldspathic-altered ore bodies. The massive barite appears to have formed after the potassic alteration as a result of syntectonic remobilization of the banded barite. The massive barite is characterized by microstructures (polygonal to dynamically recrystallized) indicating that barite was deformed plastically. Based on the mesoscopicand microscopic-scale structures the pyrite grains can be divided into three types: sub to euhedral pyrite, polygonal pyrite, and elongate pyrite with their long axes parallel to the schistosity. Onetching, the individual pyrite grains commonly show zoned structure with a core overgrown by a later generation of possibly metamorphic pyrite. In the Golden Giant mine, microstructural studies reveal a difference in the timing of formation of molybdenite and stibnite. molybdenite has formed synchronously with the development of S, and S, cleavages and was associated with the formation of potash-feldspar and muscovite. Stibnite occurs along rnicrofractures that cut across both S, and S, cleavages and is coeval with the development of S, (Michibayashi 1991). Gold is associated with both molybdenite and stibnite mineralization and appears to be unrelated to barite. The authors have used the equilibrium fractionation between barite and pyrite to determine the temperature. According to them, the temperatures determined by sulfur-isotope fractionation (varying between 492 and 557°C) are close to those determined by the oxygen-isotope fractionation between quartz and muscovite, although the oxygen-isotope geothermometer

Origin of emerald deposits of Brazil

Precambrian emerald deposits of Brazil are found in a typical geologic setting with Archean basement and supracrustal, ultramafic, granitoid and rocks. Volcano-sedimentary series occur as imbricated structures or as bodies affected by complex folding and deformation. Emerald mineralization belongs to the classic biotite-schist deposit, which formed by the reaction of pegmatitic veins within ultrabasic rocks. At the same time, pegmatite-free emerald deposits linked to ductile shear zones are also known. Emerald formation is attributed to infiltrational metasomatic processes provoking a K-metasomatism of the ultrabasic rocks and also a desilication of the pegmatites. A new classification based on the geological setting, structural features, and ore paragenesis is proposed.

Characterization and evolution of ore-forming fluids at Fazenda Brasileiro Gold Mine, Rio Itapicuru Greenstone Belt, BA

Fluid inclusion studies by microthermometry and Raman spectroscopy carried out on the main hosts of the gold mineralizations at Fazenda Brasileiro Mine, southern part of Rio Itapicuru greenstone belt define the mineralizing fluid s as relatively dense (0.85-0.95 g/cm3 ) and hot (&gt; 400°C) solutions composed essentially of CO2 (89.7-85.3 mole%) and minor amounts of CH4 and N2, which gradually evolved to more a90ueous fluids (40 to 62.5 moles% H20) of low salinity(&lt; 10 eq. wt% NaCl) at lower temperatures (250-300°C). The isochores of the carbonic and H20-C02 fluids and the temperature of total homogeneization combined with geothermometric data on the are paragenesis, point to at least two periods of gold deposition: 1. 380-419° C and 2.2-3.2 kb in the quartzofeldspathic and arsenopyrite. Pyrite quartz veins and 2. 270-300°C and 1.2-1.4 kb in the massive quartz vein. Based on experimental work on the solubility of gold complexes, chemical characteristics of mineralizing fluids, ore paragenesis and Au/Ag ratio, it was possible to speculate that the transport of gold occurred mainly by reducing (high ΣH2S/ ΣS04), slightly neutral to alkaline solutions, as thio-complexes such as Au(HS)2, HAu (HS)2 or AU2(HS)2S-2. In the earliest stage of mineralization conditions of gold deposition were probably attained due to fluid/ ock reactions which led to a decrease in the activity of S-2 by precipitating sulphides (arsenopyrite, pyrite, pyrrhotite, etc). The predominance of the H20 regime over the C02 regime in the latest stage might have been the cause of CO2 dilution, decrease in pH and oxidation and, as a consequence, the precipitation of gold. The fluid inclusion data also suggest a metamorphic origin for the mineralizing fluids, most likely through the devolatilization of the basal sequences of the volcano-sedimentary pile. The devolatilization process would be able produce low salinity, H20-C02 fluids which would later migrate through favourable structural sites and deposit their metal content.