Metamorphic Remobilization Research Articles

Nature and origin of primary ore-forming fluids in the highly metamorphosed Archean Hongtoushan VMS deposit, North China: Insights from in situ S isotopes and trace elements of sulfides

Many ancient volcanogenic massive sulfides (VMS) deposits underwent various degrees of deformation and metamorphism, which modified the primary orebody geometries and sulfide textures, resulting in the great challenge of revealing the source and physicochemical properties of primary ore-forming fluids and original mineralization processes. Here, we observed the textural evolution of sulfides in the highly metamorphosed Archean VMS Hongtoushan Cu-Zn deposit, China, to constrain the histories and P-T conditions of metamorphic remobilization and to evaluate the influence of metamorphism on geochemical characteristics of sulfide. Further, the in situ sulfur isotopic and trace elemental analyses were utilized to reveal the source and physicochemical conditions of initial fluids. During metamorphism, pyrite developed elongated texture, bulging recrystallization, coarse porphyroblasts, and foam texture. These textures all occurred at a temperature of 550 – 700 °C, at which sulfide remobilization and deformation occurred in an anhydrous environment. The geochemical exchange among sulfides did not happen during metamorphism. The sulfur isotope and trace elements of sulfides thus can be used to track the source and physicochemical properties of primary ore-forming fluids. In situ sulfur isotopic analyses of sulfides yielded values of δ34S approaching zero and Δ33S situating mass-dependent isotope fractionation (MDF) area, indicating that the sulfur of the original submarine hydrothermal system was mainly derived from the magmatic fluids. The content of Se in pyrite revealed the primary fluids were acidic and reductive and were used to constrain the temperature of ore-forming fluids, which was higher than 250 °C. The ratio of Co/Ni of pyrite was higher than 10, indicating primary fluids were of magmatic-hydrothermal origin. The high content of Se and As in pyrite and Cd and Hg in sphalerite documented that these elements were partially derived from the magmatic volatiles or magma degassing process. Overall, our study demonstrates that coupling textures and in situ geochemical features of sulfides could be a helpful tool for tracking the primary ore-forming processes of the metamorphosed massive sulfide deposits.

Sulfide texture and geochemistry of the Neoarchean Hongtoushan Cu-Zn deposit (NE China): Implication for mixed-state metamorphic remobilization

The Hongtoushan Cu-Zn deposit in the Neoarchean Qingyuan greenstone belt of the North China Craton is the oldest volcanogenic massive sulfide (VMS) deposit in China. The ore samples are classified by massive sulfide ores, disseminated sulfide ores, and sulfide-bearing quartz veins. Four types of massive sulfide ores are further identified. Type A massive ore, characterized by the granular quartz and euhedral dark mineral aggregates in the massive sulfides matrix, is interpreted as the result of partial melting of primary sulfides, and type B might record the process of partial melting and mechanical migration. Type C massive ore, formed by the recrystallization of type B massive ore, is characterized by coarse pyrite and pyrrhotite porphyroblasts. Type D massive ore undergoes deformation and upgrades by elements released during recrystallization. The relic anhydrite in type A massive ore is the preserve of the primary mineral from VMS mineralization. Most sulfide in massive ores has δ34S values closing to ∼ 0 ‰, showing magmatic sulfur characteristics. The intergrowth of pyrrhotite and magnetite with higher δ34S (5.5–8.3 ‰) is suggested to be the dissolution product of primary anhydrite. The sulfides from the tourmaline-bearing quartz veins have negative δ34S (-6.0 – −2.6 ‰) values, indicating that the sulfur was released from the desulfurization of massive pyrite. The ore remobilization is dominated by partial melting of sulfide and mechanical migration during the peak metamorphism. The fluid metasomatism is activated at the retrograde metamorphism. This mixed-state metamorphic remobilization plays a key role in the formation of metamorphosed massive sulfide ore and ore upgrading.

Multistage Genesis of the Haerdaban Pb-Zn Deposit, West Tianshan: Constraints From Fluid Inclusions and H-O-S-Pb Isotopes

The Haerdaban Pb-Zn deposit is located on the western edge of the Chinese Western Tianshan Orogen. This deposit consists of stratiform and veined mineralization hosted in Proterozoic carbonaceous and dolomitic limestone. Three metallogenic stages were recognized: an early sedimentary exhalative stage (stage 1), an intermediate metamorphic remobilization stage (stage 2), and a late magmatic-hydrothermal stage (stage 3). Fluid inclusions (FIs) present in stage 1 are liquid-rich aqueous, with homogenization temperatures of 206–246 C and salinities of 5.9–11.6 wt% NaCl eq. FIs present in stage 2 are also liquid-rich aqueous, with homogenization temperatures of 326–349 C and salinities of 3.4–6.6 wt% NaCl eq. FIs present in stage 3 include halite-bearing, vapor-rich aqueous, and liquid-rich aqueous FIs. Homogenization temperatures for these FIs span a range of 249–316 C. Halite-bearing, vapor-rich aqueous, and liquid-rich aqueous FIs yield salinities of 33.8–38.9, 2.6–3.5, and 4.2–8.1 wt% NaCl eq., respectively. Oxygen and hydrogen isotopic data (δ18OH2O = 2.6–13.6‰, δDH2O = −94.7 to −40.7‰) indicate that the ore-forming fluids of stages 1–3 were derived from modified seawater, metamorphic water, and magmatic-meteoric mixed water, respectively. Sulfur isotopic data (δ34S = 2.1–16.3‰) reveal that ore constituents were derived from mixing of marine sulfate and magmatic materials. Lead isotopic data (206Pb/204Pb = 17.002–17.552, 207Pb/204Pb = 15.502–15.523, 208Pb/204Pb = 37.025–37.503) reveal that ore constituents were derived from a mixed crust-mantle source. We propose that the Haerdaban deposit was a Proterozoic sedimentary exhalative deposit overprinted by later metamorphic remobilization and magmatic-hydrothermal mineralization.

The Occurrence of Breithauptite and Nisbite-Like Sb-Ni Phases At Sindesar-Khurd, Rajasthan, India: Implications For Melt-Assisted Sulfide Remobilization

Abstract Breithauptite (NiSb) generally occurs in magmatic Ni-Cu-PGE ores and in hydrothermal veins associated with Ni and Co arsenide minerals. Its presence in the Rajpura-Dariba–Sindesar-Khurd–Bethumni metallogenic belt, Rajasthan, India, may be ascribed to metamorphic remobilization as observed elsewhere, e.g ., Sulitjelma, Norway. From the coexistence of breithauptite with probable Sb-Ni melt phases at Sindesar-Khurd, a temperature of approximately 930–970 °C is indicated from the phase relations in the Ni-Sb binary phase diagram. However, this temperature is substantially above the peak metamorphic temperature of 580–600 °C estimated from equilibrium silicate phase assemblages in the rocks around the Rajpura-Dariba ore body, which is situated six kilometers to the south of Sindesar-Khurd. We have conducted experiments at 600 °C in the Ni–Sb system in the presence of Ag to understand if Ag is responsible for such a drastic decrease of the temperature of melting. The presence of other phases including freibergite [(Ag,Cu,Fe,Zn) 12 Sb 4 S 13 ], Ag-rich tetrahedrite [(Cu,Ag,Fe,Zn) 12 Sb 4 S 13 ], ∼2FeS.Ag 2 S, and ∼CuPbSbS 3 .Ag 2 S along with breithauptite at Sindesar-Khurd indicates the selective incorporation of Ag. In a second assemblage nisbite (NiSb 2 ) instead of breithauptite was observed along with 3PbS.Ag 2 S, 2Ag 2 S.FeS, (Fe,Ag)S, a Sb-Ni-Ag-Fe-S phase, and a Ag-Sb-Ni-Cu-Fe-S phase. Nisbite is stable only below 628 °C, suggesting that these assemblages formed below this temperature, and the presence of several additional phases of variable compositions further indicates disequilibrium and probable rapid crystallization from a melt. Our results suggest that Ag significantly depresses melting temperatures in the Ni–Sb system, making possible the existence of an Ag-Sb-Ni melt at 600 °C. In the presence of S and other elements such as Fe, Pb, Cu, etc . the melting temperature would be further depressed. Thus the removal of Ag from the melt through formation of Ag-bearing phases, e.g ., 2FeS.Ag 2 S, 3PbS.Ag 2 S, 2Ag 2 S.FeS, or CuPbSbS 3 .Ag 2 S, may result in a residual metastable melt at the ambient temperature. Such a melt may quench to a fine-grained assemblage containing breithauptite and nisbite-like Sb-Ni solid phases, such as encountered at Sindesar-Khurd. The second assemblage, which is nisbite-bearing, could result from a relatively Ag-rich Ni-poor melt fraction that becomes extremely Sb-rich due to the removal of Ag.

Fluid inclusion and stable isotopic constraints on fluid sources and evolution of the Luojiahe Cu deposit in the southern margin of the North China Craton

The Luojiahe Cu deposit in the Zhongtiaoshan region is located in the southern margin of the North China Craton. The orebodies are hosted in the mafic volcanic-sedimentary sequences of the metamorphosed (greenschist-facies) Neoarchean Songjiashan Group. The Luojiahe Cu mineralization can be divided into the primary volcanogenic massive sulfide (VMS) mineralization stage (Stage I, banded or stockwork ores) and the subsequent metamorphic remobilization stage (Stage II, coarse-vein ores).Three types of quartz selected for fluid inclusion (FI) studies were collected from the Stage I banded (Q1) and stockwork (Q2) ores and Stage II coarse-vein (Q3) ores. Four types of FIs were identified: (1) liquid-rich FIs (L-type), (2) pure vapor and vapor-rich FIs (V-type), (3) daughter mineral-bearing FIs (S-type), and (4) CH4-H2O FIs (C-type). Systematical microthermometric and H-O isotopic studies show that the Stage I ore-forming fluids consist predominantly of high salinity evolved seawater (125–220°C; 23.9–27.9wt.% NaCl equiv.) and some magmatic-hydrothermal fluids (249–339°C; 34.5–42.2wt.% NaCl equiv.). The two fluid end-members are represented by the L-type FIs in Q1 and the S- and V-type FIs in Q2. The temperature- and salinity variation trends of the L-type FIs in Q1 indicate a mixing process between the hot evolved seawater and cold seawater at Stage I. Furthermore, the V- and S-type FI coexistence in Q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at Stage I. In contrast, the Stage II ore-forming fluids consist of CH4-rich metamorphic fluids (192–350°C; 10.6–43.2wt.% NaCl equiv.). Carbon isotopic analysis of the Stage II calcite (−4.58 to −10.83‰) and graphite (−32.01 to −39.16‰) in the ore-hosting chlorite schist indicates that the metamorphic ore-forming fluids had exchanged carbon isotope with graphite. The generation of CH4 may have resulted from the interaction between H2O (released by metamorphic devolatilization) and graphite. The continuous consumption of H2O in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the CH4-NaCl-H2O system. In addition, the VMS metallogenic environment is generally favorable for microbial communities. It is considered that the graphite at Luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems, as also supported by carbon isotopic data.We propose that at Stage I, the main mineralization may have been resulted from 1) fluid mixing of hot evolved seawater and cold seawater in the near-surface environment; and 2) fluid unmixing caused by the percolation of magmatic fluids into syn-volcanic faults, forming the stockwork ores. At Stage II, the interaction between H2O and graphite may have resulted in the reduction of ore-forming fluids and Cu precipitation, and fluid unmixing in the CH4-NaCl-H2O system may have further promoted the Cu mineralization.

The multistage genesis of the giant Dongshengmiao Zn-Pb-Cu deposit in western Inner Mongolia, China: Syngenetic stratabound mineralization and metamorphic remobilization

The genesis of the giant Dongshengmiao in the northern margin of the North China Block has been debated since its discovery in the 1950s, because it shows geological and geochemical characteristics with both syngenetic and epigenetic signatures. It has geological settings and sulfur and lead isotopic compositions that are similar with typical SEDEX (sedimentary exhalative) deposit, while the Zn-Pb-Cu mineralization was controlled by shear deformation and metamorphism, showing similarities with orogenic-type deposits. In this contribution, both the syngenetic and epigenetic features of the Dongshengmiao are envisaged, and accounted for in the context of a genetic model with two metallogenic periods. Massive pyrite at the Dongshengmiao was mostly recrystallized during metamorphism, but fine-grained texture was locally preserved, indicating its syngenetic origin. On the contrary, all the Zn-Pb-Cu ores observed in this study show characteristics of epigenetic hydrothermal mineralization that controlled by metamorphism and accompanying shear deformation. The sulfur and lead isotopic compositions of sphalerite and galena indicate that they were in situ remobilized from a syngenetic stratabound source, and the oxygen and hydrogen isotopic ratios of ore-fluid indicate that the large-scale remobilization was assisted by metamorphic fluid. The thermodynamic modeling indicates that the ore-fluid during remobilization has a great potential of transporting Cu. This may account for the abnormally enriched Cu in the remobilized SEDEX deposit. The metamorphic fluid might strip Cu from the fluid source during devolatilization, and overprint it on the Zn-Pb orebodies during remobilization. A secondary flow-through modeling reveals that Zn- and Cu-sulfides would be preferentially redistributed in Fe-rich carbonates during remobilization, as a result of fluid-rock interaction. Conclusively, a multistage genetic model is proposed. During the development of the Proterozoic rift, stratabound Zn-Pb mineralization took place in a SEDEX ore-forming system. The syngenetic sulfides subsequently underwent a large-scale fluid-assisted remobilization during the early Cretaceous metamorphism and thrusting, forming the shear zone-controlled epigenetic orebodies. During the remobilization process, Cu was scavenged from the source of metamorphic fluid, and deposited accompanying remobilized Zn-Pb sulfides. Shear structures and Fe-rich carbonates are ideal sites for redistribution and re-deposition of remobilized sulfide.

Breithauptite: A rare antimonide in the Dariba-Rajpura-Bethumni belt, Rajsamand district, Rajasthan

Breithauptite (NiSb), a rare antimonide, is generally found associated with hydrothermal vein ores of Ni and Co. In India, it has so far been recorded only from the gold-quartz veins of the Kolar Gold Field. Occurrence of this antimonide in a sediment-hosted milieu however, has seldom been noted. We report here the identification and characterization of this rare phase in both the stratiform and the vein-type ores of the sediment-hosted Zn-Pb sulphide deposits and prospects within the Dariba-Rajpura-Bethumni metallogenic belt in Northwestern India. Metamorphic remobilization from the stratiform ores of the belt has been postulated as a possible mechanism for the conspicuous segregation of breithauptite as a discrete phase within these ores.

Metamorphism, graphite crystallinity, and sulfide anatexis of the Rampura–Agucha massive sulfide deposit, northwestern India

Located adjacent to the Banded Gneissic Complex, Rampura–Agucha is the only sulfide ore deposit discovered to date within the Precambrian basement gneisses of Rajasthan. The massive Zn–(Pb) sulfide orebody occurs within graphite–biotite–sillimanite schist along with garnet–biotite–sillimanite gneiss, calc–silicate gneisses, amphibolites, and garnet-bearing leucosomes. Plagioclase–hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720–780°C, whereas temperatures obtained from Fe–Mg exchange between garnet and biotite (580–610°C) in the pelites correspond to postpeak resetting. Thermodynamic considerations of pertinent silicate equilibria, coupled with sphalerite geobarometry, furnished part of a clockwise P–T–t path with peak P–T of ∼6.2 kbar and 780°C, attained during granulite grade metamorphism of the major Zn-rich stratiform sedimentary exhalative deposits orebody and its host rocks. Arsenopyrite composition in the metamorphosed ore yielded a temperature [and log f(S 2)] range of 352°C (−8.2) to 490°C (−4.64), thus indicating its retrograde nature. Contrary to earlier research on the retrogressed nature of graphite, Raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals (490 ± 43°C with a maximum at 593°C). The main orebody is mineralogically simple (sphalerite, pyrite, pyrrhotite, arsenopyrite, galena), deformed and metamorphosed while the Pb–Ag-rich sulfosalt-bearing veins and pods that are irregularly distributed within the hanging wall calc–silicate gneisses show no evidence of deformation and metamorphism. The sulfosalt minerals identified include freibergite, boulangerite, pyrargyrite, stephanite, diaphorite, Mn–jamesonite, Cu-free meneghinite, and semseyite; the last three are reported from Agucha for the first time. Stability relations of Cu-free meneghinite and semseyite in the Pb–Ag-rich ores constrain temperatures at >550°C and <300°C, respectively. Features such as (1) low galena–sphalerite interfacial angles, (2) presence of multiphase sulfide–sulfosalt inclusions, (3) microcracks filled with galena (±pyrargyrite) without any hydrothermal alteration, and (4) high contents of Zn, Ag (and Sb) in galena, indicate partial melting in the PbS–Fe0.96S–ZnS–(1% Ag2S ± CuFeS2) system, which was critical for metamorphic remobilization of the Rampura–Agucha deposit.

Mass‐independent fractionation of sulfur isotopes in sulfides from the pre‐3770 Ma Isua Supracrustal Belt, West Greenland

ABSTRACTRedox chemistry of the coupled atmosphere–hydrosphere system has coevolved with the biosphere, from global anoxia in the Archean to an oxygenated Proterozoic surface environment. However, to trace these changes to the very beginning of the rock record presents special challenges. All known Eoarchean (c. 3850–3600 Ma) volcanosedimentary successions (i.e. supracrustal rocks) are restricted to high‐grade gneissic terranes that seldom preserve original sedimentary structures and lack primary organic biomarkers. Although complicated by metamorphic overprinting, sulfur isotopes from Archean supracrustal rocks have the potential to preserve signatures of both atmospheric chemistry and metabolic fractionation from the original sediments. We present a synthesis of multiple sulfur isotope measurements (32S, 33S and 34S) performed on sulfides from amphibolite facies banded iron‐formations (BIFs) and ferruginous garnet‐biotite (metapelitic) schists from the pre‐3770 Ma Isua Supracrustal Belt (ISB) in West Greenland. Because these data come from some of the oldest rocks of interpretable marine sedimentary origin, they provide the opportunity to (i) explore for possible biosignatures of sulfur metabolisms in early life; (ii) assess changes in atmospheric redox chemistry from ∼3.8 Ga; and (iii) lay the groundwork to elucidate sulfur biogeochemical cycles on the early Earth. We find that sulfur isotope results from Isua do not unambiguously indicate microbially induced sulfur isotopic fractionation at that time. A significantly expanded data set of Δ33S analyses for Isua dictates that the atmosphere was devoid of free oxygen at time of deposition and also shows that the effects of post‐depositional metamorphic remobilization and/or dilution can be traced in mass‐independently fractionated sulfur isotopes.

Metamorphic ore remobilization in the Hällefors district, Bergslagen, Sweden: constraints from mineralogical and small-scale sulphur isotope studies

The marble- and metavolcanic-hosted Pb-Zn- (Ag-Sb-As) deposits of the Hallefors district, located in the Palaeoproterozoic Bergslagen ore province, south central Sweden, comprise both stratabound sulphides and discordant, Ag-rich sulphide-sulphosalt veins. The complex sulphide-sulphosalt assemblages of the Alfrida- Jan Olof mines at Hallefors were investigated by a combination of ore microscopy, electron-microprobe analysis, and in situ laser sulphur isotope analysis. The massive ore is characterized by positive and homoge- neous d 34 S (+1.4& to +2.7& V-CDT), whereas vein- hosted sulphides and sulphosalts exhibit similar, but generally less positive to slightly negative d 34 S( 0.6& to +2.0&). Comparison of the observed ore mineral assemblages with calculated phase equilibria in the sys- tem Fe-As-S-O-H and isotopic fractionation as a function of temperature, oxygen fugacity and pH indi- cates that the vein-type mineralization was formed from relatively reduced and rather alkaline hydrothermal fluids. At these reduced conditions, fractionation of d 34 S via changes of fO2 is insignificant, and thus the isotopic signatures of the vein minerals directly reflect the com- position of the sulphur source. We therefore conclude that the vein-type ore essentially inherited the sulphur isotope signature from the pre-existing massive sulp- hides via metamorphic remobilization at approximately 300-400C and 2-3 kbar. Scales of remobilization observable are on the order of about 5 mm to 30 cm. Overall, the sulphide-sulphosalt assemblages from the Alfrida-Jan Olof mines exhibit d 34 S values which are comparable to a majority of metasupracrustal-hosted deposits in the Bergslagen province, thereby suggesting a common origin from ca. 1.90-1.88 Ga volcanic-hydro- thermal processes.

“Siderite mineralization of the Gemericum superunit (Western Carpathians, Slovakia): review and a revised genetic model” [Ore Geology Reviews 24, 267–298]—a discussion

Radvanec et al. (2004) provided a revised version of their metamorphic-hydrothermal model for siderite and associated mineral deposits of the Western Carpathians. Compared to the previous version (Grecula, 1995; Grecula et al., 1996), the upgraded model emphasizes the dominant role of retrograde Variscan metamorphic processes and admits remobilization of the Variscan siderite and other ore components during the Alpine orogenic cycle. Similar to the previous versions, the Variscan ore-forming processes genetically related to metamorphism are believed to dominate not only in the siderite vein and metasomatic deposits, but also in the superimposed vein sulphides (p. 292). The revised model is unsustainable in light of recent knowledge. The major pitfalls are summarized and commented as follows.

Laser microprobe sulphur isotope analysis of arsenopyrite: experimental calibration and application to the Boliden Au–Cu–As massive sulphide deposit

Metamorphic remobilization of arsenopyrite-rich ores is a globally important process which can lead to significant concentrations of gold. In order to understand this and related processes, relations of sulphur isotopes can give a number of important clues. To resolve such relations in detail, we have successfully calibrated and applied a laser combustion system for in situ analysis of sulphur isotopic compositions of arsenopyrite. Experimental calibration of the laser fractionation factor (+0.4‰) was obtained by using compositionally and isotopically homogeneous natural samples from ore deposits at Boliden (Sweden) and Freiberg (Germany); subsequent to detailed microscopic study, the S isotope ratios of these samples were measured by conventional and laser combustion techniques. The present application to different types of arsenopyrite in the Palaeoproterozoic metamorphosed VHMS ores of the Boliden Au–Cu–As deposit, Skellefte district, northern Sweden, shows that the sulphur isotope composition of arsenopyrite is essentially unmodified during medium-grade metamorphic recrystallization and remobilization. Here, massive arsenopyrite ore is crosscut by later veins that carry a complex quartz–sulphosalt–sulphide assemblage. The latter ore type is markedly Au-rich compared to the host ore, and thus of significant economic interest. We find that both ore types exhibit very similar sulphur isotope compositions, ca. +2‰ to +3‰ (V-CDT), which is similar to most massive sulphide deposits in the Skellefte district. Thus, the crosscutting Au-rich vein ore has inherited the sulphur isotope composition from sulphur liberated by metamorphic reactions affecting the massive ore, and most likely also inherited the Au through this mechanism. The latter finding clearly has important implications both for the general discussion on, and the prospecting for, similar high-grade Au ores in this world-class mining region.

142Nd evidence for early Earth differentiation

We measured 142Nd/ 144Nd in metabasalts and metagabbros from the western and southern parts of the Isua Supracrustal Belt, West Greenland. The samples were selected based on field and thin section criteria so as to minimize metamorphic effects on the isotope systematics. We developed a new multi-stage chemical separation scheme and a new isotope measurement technique using multiple-collector inductively coupled plasma mass spectrometry and, upon replication of measurements, achieved a precision better than 20 ppm. Out of eight samples, three show a positive 142Nd anomaly of 30 ppm consistent with a previous measurement by Harper and Jacobsen [Nature 360 (1992) 728–732]. These samples also define a 147Sm– 143Nd isochron age that agrees with the commonly accepted U–Pb zircon age of 3.78 Ga for Isua, while the non-anomalous samples show substantial dispersion. The latter lack of coherence is interpreted as indicating either isotopic heterogeneities in the Isua upper mantle or redistribution of Nd by fluids during late Archean metamorphic events. Scatter among the non-anomalous samples is also observed for the Lu–Hf isotope systematics and confirms the severity of metamorphic remobilization. If the 147Sm– 143Nd and 146Sm– 142Nd systems are explained by a two-stage model, it is required that the upper mantle went through a major differentiation event within a few tens of millions of years after planetary accretion. If early Archean convection decoupled the two Nd isotope systems, this necessitates that the episode of differentiation took place within no more than 150 Myr of planetary accretion. One interpretation of the high-Sm/Nd reservoir is that it represents the remnants of a stagnant lithospheric lid overlying a magma ocean. Because the lid is shattered by impacts, it is unable to sink. Alternative explanations, such as an ultra-deep magma ocean at the core–mantle boundary or massive subduction of early crust, call for the early burial of enriched material and the ubiquitous presence of a 142Nd anomaly in the early Archean mantle.

Partial melting of the Broken Hill Galena-Sphalerite ore: Experimental studies in the system PbS-FeS-ZnS-(Ag2S)

For many years the genesis of the Broken Hill Pb-Zn-Ag deposit has been, and continues to be, hotly debated, mostly because many of the original characteristics of the deposit have been obscured by high-grade metamorphism and deformation. Metamorphism not only overprinted the deposit but fundamentally modified the sulfides through metamorphic remobilization. Based on experimental studies it had also been speculated that partial melting of the sulfides may have occurred. The lack of experimental data at elevated pressures in the PbS-FeS-ZnS system has limited this suggestion to mere speculation. We present the results of an experimental study of the melting behavior of sulfides, which show that Broken Hill ores must have been partially molten under peak metamorphic conditions.

On the interpretation of the oceanic variations in87Sr/86Sr as recorded in marine limestones

The present published inventory of fluvial Sr and87Sr/86Sr data, combined with new information from the big rivers of Eastern Siberia (a combined total of ∼ 1,000 measurements), is used to investigate the probable origin of the large rise in the marine isotopic ratio, recorded in limestones, over the last ∼ 20 million years. With the exception of the data from the Ganga-Brahmaputra all measurements fall on what is proposed to be called the “Wickman trend”, essentially a mixing line between the limestone sink for Sr, with the integrated marine ratio, and the flux from the weathering of average continental crust. However, time-variations along this trend, i.e. changes in relative weathering intensity, cannot explain the observations from limestones. They can only be caused by very high and radiogenic fluxes of Sr as are occurring from the present Himalayan orogeny, lying far above the Wickman trend and caused by metamorphic remobilization of radiogenic Sr during underthrusting and subsequent unroofing associated with the collision of India with Eurasia. In general the variations in the ratio are therefore caused by specific tectonic events, not by general climatic variations in the intensity of aluminosilicate weathering.

Gold distribution in the Mobrun volcanic-associated massive sulfide deposit, Noranda, Quebec; a preliminary evaluation of the role of metamorphic remobilization

The Mobrun Zn-Cu-Au-Ag orebodies show metal zonation typical of volcanic-associated massive sulfide deposits, although some metamorphic remobilization of metals has occurred. The nature and distribution of primary facies are the result of synvolcanic deposition and reworking of sulfides by hydrothermal fluids. Secondary facies resulting from metamorphism and deformation locally overprint primary facies and include transgressive veins which contain remobilized Au as electrum. The distribution of secondary facies was controlled by the mechanical properties of the mineralization as well as of the surrounding rocks; the mechanical response of different facies to deformation influenced the development and distribution of zones containing easily recoverable gold. Preservation of primary textures is best in the Main and Satellite lenses. The 1100 lens also shows significant preservation of primary facies; however, development of secondary facies is pronounced. Textural, structural, and chemical evidence indicates that the 1100 lens was subjected to more extreme metamorphic recrystallization and deformation than the other lenses. Metamorphic remobilization was important in releasing refractory Au from sulfides and locally concentrating it in a recoverable form (electrum).

Geology of the Bousquet 2 Mine; an example of a deformed, gold-bearing, polymetallic sulfide deposit

The Bousquet 2 mine is a deformed pyritic polymetallic Archean gold deposit located within a zone of intense ductile-brittle deformation composed of highly sheared and altered metamorphic tectonites of contrasting rheological properties. The ore occurs in a metamorphosed zone of aluminous alteration and is composed of foliation-parallel massive and semimassive pyritic lenses overprinted by late en echelon extensional veinlets containing Au-Ag-Cu mineralization.Structural and textural analyses show that the gold-bearing sulfide bodies and enclosing host rocks contain discrete tectono-metamorphic features resulting from two principal episodes of metamorphism, M 1 and M 2 . The earlier prograde event, M 1 , and coeval regional deformation, D 1 , were accompanied by overall ductile deformation of sulfides and by recrystallization and remobilization of the mineralization parallel to foliation. M 1 caused development of a pervasive east-west-trending metamorphic layering of the pyritic mineralization, transposition and elongation of the pyritic bodies parallel to mineral stretching L 1 lineations, meter-scale folding of sulfides, sulfide brecciation, coarsening and annealing recrystallization of pyrite, and ductile remobilization of incompetent sulfides and gold into interstices and triple-point junctions of juxtaposed pyrite grains. Extensive differential remobilization of ductile sulfides (mainly sphalerite, pyrrhotite, galena, chalcopyrite, chalcocite, and bornite) and gold took place during M 1 and gave rise to planar zones enriched in Au-Ag-Cu mineralization along the interfaces of pyritic lenses and their adjacent silicate rocks.M 2 metamorphism is chronologically related to a late brittle incremental stage of the progressive deformation and is characterized by high fluid influx, pervasive retrogression of peak metamorphic minerals, and remobilization of sulfides and gold into transgressive structures. The retrograde mineralization displays a wide range of small-scale structures resulting from fluid-assisted remobilization of ductile sulfides, including piercement and injection structures, as well as foliation-oblique extensional veinlets. Metamorphic remobilization of ore constituents took place mainly at ore-wall rock interfaces. Fe-, Pb-Zn-, and Au-Ag-Cu- bearing extensional veinlets generally occur in foliation-parallel protores of similar mineralogical composition and are interpreted as having resulted from in situ metamorphic remobilization.Gold and copper show a corelationship throughout the deposit; the highest gold and copper values occur within extensional veins overprinting previously Au-Cu-enriched planar zones. Mineralogical zoning in the sulfide masses is structurally controlled and can be related directly to abundant Au-Cu-bearing veinlets that represent mineralized segments of a systematic joint system.Structural and textural evidence suggests that gold mineralization with its present textural relations formed during M 1 and M 2 , but that a large proportion of the structurally controlled vein mineralization resulted from in situ remobilization rather than from additions from an external source.

A multi-stage, multi-process genetic hypothesis for greenstone-hosted gold lodes

Abundant new data about the geology and geochemistry of greenstone-hosted gold lodes have emerged during the worldwide, intensive search for these ores for the last decade. Unfortunately, neither the resulting descriptions of their field relationships nor the data from fluid inclusion, light stable isotopic and radiometric age studies are definitive of a single, unique genetic process. Instead, some of these relationships are best explained by an epigenetic process, others by a synsedimentary one, and many are equivocal and explainable by either. A generally acceptable genetic hypothesis for the many, various forms of these lodes must account not only for all field and laboratory data but, also, must explain the sources of the various components of the ores, and the mechanisms of their mobilization, transport and deposition. A multi-stage, multi-process hypothesis is necessary to provide such a comprehensive explanation. The initial genetic stage involved deuteric reactions between seawater and thick, subaqueous tholeiitic basalts low in the greenstone belt successions. These reactions did not involve deep, sub-seafloor convective circulation of seawater, as in the generation of volcanogenic massive sulfide deposits higher in the successions, but occurred at shallow depths under the strongly reduced conditions of Archean time. They leached gold, silica, carbonate and other components of the ores from the lavas and moved upward, carbonatizing and variably altering the sea floor strata that they penetrated. They were discharged passively over broad areas to the sea floor, where their leached components were deposited as auriferous exhalites, variably sulfidic, ankeritic, magnetitic and siliceous. In places these were pure chemical precipitates, but elsewhere were mixed with variable amounts of clastic or pyroclastic detritus.This accounts for the widespread low, but anomalous gold content of Algoma type iron-formations and for the absence of highly altered rocks marking focused discharge vents beneath the few exhalites that contained enough gold to be ore. Reworking of the auriferous exhalites and enclosing altered lavas along the flanks of growing Archean volcanic complexes shed rounded clasts of auriferous sulfide and ankeritized, fuchsitic rocks into flanking basins, forming pre-metamorphic conglomerates that are essentially coeval with, but locally disconformable or unconformable on their source greenstones. These weakly auriferous strata were important pre-enriched source-rocks during multiple, superimposed stages of metamorphic remobilization, re-deposition and further concentration of gold. Successively, these stages include; burial diagenesis involving compaction, dehydration and lithification; syn-kinematic deformation during subsidence; intrusion of small, sub-volcanic porphyritic plutons that were late but co-magmatic in greenstone belt evolution; orogenic plutonism that accompanied deep-seated granulitization of the belts, and; post-orogenic intrusions. All these formed late, discordant, epigenetic veins, vein systems or stockworks that cut all earlier rocks and one another. Most fluid inclusion and light stable isotopic studies are of minerals from these late, epigenetic veins, not from the earlier, fine-grained syn-sedimentary strata; and their results cannot be generally applied to the latter. The studies show that the late veins were formed by CO 2-rich fluids at 200°–400°C and depths of 3–4 km. The same fluids formed not only mineralized veins in the pre-enriched source-rocks but, also, barren veins widespread throughout most Archean terranes where the source-rocks were absent. The source of these fluids remains problematical. They may have originated from dehydration of rocks undergoing deep-seated granulitic metamorphism, from reactions between fluid and earlier carbonate-rich rocks of the greenstone belts at lower metamorphic rank, by magmatic crystallization-differentiation, or from all of these.

Geological, mineralogical and geochemical aspects of Archean Banded Iron-Formation-hosted gold deposits: Some examples from Southern Africa

This paper reports on different styles of gold mineralization observed in Archean gold deposits hosted by Algoma-type Banded Iron-Formations (BIF) in southern Africa. Genetic aspects of various occurrences are discussed in the context of mineralogical as well as geochemical data of BIFs from the greenstone terranes of the Zimbabwe and Kaapvaal cratons. The study revealed that, in spite of their different provenance and age (3.5 to 2.6 Ga), the BIFs are geochemically similar, whereas observed mineralogical differences reflect various degrees of metamorphic overprint. Generally, the BIFs belong to mixed oxide-carbonate-(±sulfide)-facies. REE distribution patterns of the investigated Archean BIF samples exhibit positive Eu-anomalies, which suggest a strongly reducing nature of the solutions which also provided the distinctive element contents now present in the chemical sediments. Irrespective of their formation, gold enrichment in BIF only occurs if the S- and/or As-contents of the BIFs exceed specific threshold values, i.e. gold mineralization is always associated with increased contents of the iron-sulfides pyrite, arsenopyrite and pyrrhotite. The studies indicate that BIF-hosted gold occurrences are not products of a single universal metallogenic process, but may be explained by several different genetic processes such as primary syn-sedimentary formation, diagenetic changes, metamorphic remobilization, and epigenetic hydrothermal emplacement.

Synvolcanic and syntectonic gold mineralization at the Bousquet Mine, Abitibi greenstone belt, Quebec

The Bousquet pyrite gold deposit, consisting of disseminated- and vein-type mineralization in sheared and altered greenschist facies Archean volcanic rocks, is hosted by two vein sets: foliation-parallel veins in ore zone 3 and foliation-oblique veins in zones 1, 2, 4, and 5. Foliation-parallel veins are composed of pyrite and lesser amounts of chalcopyrite, sphalerite, pyrrhotite, arsenopyrite, galena, bornite, rutile, tellurides, and native gold. The foliation-oblique veins of zones 1 and 2 are composed of pyrite, chalcopyrite, and sphalerite, with subordinate amounts of pyrrhotite, arsenopyrite, galena, stannite, gudmundite, rutile, ilmenite, and gold. Quartz and muscovite are the principal gangue minerals in both vein types in ore zones 1, 2, and 3. In zone 4, foliation-oblique veins are composed of pyrite, magnetite, pyrrhotite, and minor amounts of chalcopyrite and gold. The gangue minerals are calcite, ankerite, white mica, chlorite, and tourmaline. The veins of zone 5 contain pyrite, chalcopyrite, sphalerite, bornite, magnetite, rutile, and gold in a gangue of quartz, carbonate, white mica, and chlorite. Disseminated auriferous pyrite occurs in zones 3 and 5.Mineralogical and textural features indicate that gold emplacement and associated alteration occurred during two events, one predating peak regional metamorphism and another postdating peak metamorphism. Early, premetamorphic mineralization and related synvolcanic (?) hy-drothermal alteration produced initial auriferous sulfides and an advanced argillic alteration. Subsequent deformation, recrystallization, and metamorphic remobilization of early sulfides resulted in the currently observed, structurally controlled vein-type gold deposits. The vol-canogenic event is suggested by highly aluminous rocks closely associated spatially with the mineralized zones, by textures preserved through the overprint of metamorphic textures, and by the occurrence of stannite in ore zones 1 and 2.The late, postmetamorphic gold mineralization is considered to be contemporaneous with retrograde alteration of metamorphic minerals formed during late deformation. This alteration produced a pervasive kaolinite-pyrophyllite-diaspore gangue mineral assemblage involving the replacement of syntectonic andalusite porphyroblasts in felsic rocks. In mafic host rocks, retrograde alteration is characterized by carbonate, chlorite, muscovite, pyrite, and gold. The formation of a white mica-diaspore-carbonate-chlorite assemblage and the emplacement of associated gold are interpreted as epigenetic metamorphic events because these late retrograde minerals overprint the greenschist facies mineral assemblage.