Criteria for recognition of localization and timing of multiple events of hydrothermal alteration in sandstones illustrated by petrographic,fluid inclusion,and isotopic analysis of the Tera Group,Northern Spain

Stratigraphic relations, detailed petrography, microthermometry of fluid inclusions, and fine-scale isotopic analysis of diagenetic phases indicate a complex thermal history in Tithonian fluvial sandstones and lacustrine limestones of the Tera Group (North Spain). Two different thermal events have been recognized and characterized, which are likely associated with hydrothermal events that affected the Cameros Basin during the mid-Cretaceous and the Eocene. Multiple stages of quartz cementation were identified using scanning electron microscope cathodoluminescence on sandstones and fracture fills. Primary fluid inclusions reveal homogenization temperatures (Th) from 195 to 350°C in the quartz cements of extensional fracture fillings. The high variability of Th data in each particular fluid inclusion assemblage is related to natural reequilibration of the fluid inclusions, probably due to Cretaceous hydrothermal metamorphism. Some secondary fluid inclusion assemblages show very consistent data (Th = 281–305°C) and are considered not to have reequilibrated. They are likely related to an Eocene hydrothermal event or to a retrograde stage of the Cretaceous hydrothermalism. This approach shows how multiple thermal events can be discriminated. A very steep thermal gradient of 97–214°C/km can be deduced from δ¹⁸O values of ferroan calcites (δ¹⁸O −14.2/−11.8‰ V-PDB) that postdate quartz cements in fracture fillings. Furthermore, illite crystallinity data (anchizone–epizone boundary) are out of equilibrium with high fluid inclusion Th. These observations are consistent with heat-flux related to short-lived events of hydrothermal alteration focused by permeability contrasts, rather than to regional heat-flux associated with dynamo-thermal metamorphism. These results illustrate how thermal data from fracture systems can yield thermal histories markedly different from host-rock values, a finding indicative of hydrothermal fluid flow.     

Stable isotope and fluid inclusion evidence for the origin of the Brandberg West area Sn–W vein deposits, NW Namibia

The Brandberg West region of NW Namibia is dominated by poly-deformed turbidites and carbonate rocks of the Neoproterozoic Damara Supergoup, which have been regionally metamorphosed to greenschist facies and thermally metamorphosed up to mid-amphibolite facies by Neoproterozoic granite plutons. The meta-sedimentary rocks host Damaran-age hydrothermal quartz vein-hosted Sn–W mineralization at Brandberg West and numerous nearby smaller deposits. Fluid inclusion microthermometric studies of the vein quartz suggests that the ore-forming fluids at the Brandberg West mine were CO₂-bearing aqueous fluids represented by the NaCl–CaCl₂–H₂O–CO₂ system with moderate salinity (mean=8.6 wt% NaCl_equivalent).Temperatures determined using oxygen isotope thermometry are 415–521°C (quartz–muscovite), 392–447°C (quartz–cassiterite), and 444–490°C (quartz–hematite). At Brandberg West, the oxygen isotope ratios of quartz veins and siliciclastic host rocks in the mineralized area are lower than those in the rocks and veins of the surrounding areas suggesting that pervasive fluid–rock interaction occurred during mineralization. The O- and H-isotope data of quartz–muscovite veins and fluid inclusions indicate that the ore fluids were dominantly of magmatic origin, implying that mineralization occurred above a shallow granite pluton. Simple mass balance calculations suggest water/rock ratios of 1.88 (closed system) and 1.01 (open system). The CO₂ component of the fluid inclusions had similar δ ¹³C to the carbonate rocks intercalated with the turbidites. It is most likely that mineralization at Brandberg West was caused by a combination of an impermeable marble barrier and interaction of the fluids with the marble. The minor deposits in the area have quartz veins with higher δ ¹⁸O values, which is consistent with these deposits being similar geological environments exposed at higher erosion levels.     

“Extreme boiling” model for variable salinity of the Hokko low-sulfidation epithermal Au prospect, southwestern Hokkaido, Japan

The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2 wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150° to 200 °C. The results obtained from stable isotope analysis of  δ¹⁸O in quartz vein material showed a gradual decrease in  δ¹⁸O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ¹⁸O_H2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions. We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid, before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable salinities at the same time, and in close proximity to each other. This is also reflected in the  δ¹⁸O_H2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within the Hokko vein system also occurred at this time as a result of boiling and gas loss. Received: 30 May 1997 / Accepted: 6 January 1998     

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H₂O-CO₂-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V₁) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ¹⁸O across the batholith (9.5 ± 0.4‰n = 12) suggests V₁ precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V₁ indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H₂O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ¹⁸O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V₂) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H₂O-NaCl-CaCl₂-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ¹⁸O (−3 to + 1.2‰), δ¹³C_CO2 (−19 to 0‰) and δ³⁴S_sulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V₃). Correlations of salinity, temperature, δD and δ¹⁸O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ¹⁸O (1.2–2.5‰), high-δ¹³C_CO2 (> −4‰), low-δ³⁴S_sulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ¹⁸O (−5.4 to −3‰), low-δ¹³C (<−10‰), high-δ³⁴S_sulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V₃ veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997     

The effect of fluid salinity on infiltration-driven contact metamorphism of carbonate rocks

Calculated phase equilibria involving minerals and H₂O–CO₂–NaCl fluid lead to predictions of how infiltration of rock by H₂O–NaCl fluids with X _NaCl in the range 0–0.3 (0–58 wt% NaCl) drives the reactions calcite + quartz = wollastonite + CO₂ and dolomite = periclase + calcite + CO₂. Calculations focus on metamorphism in four aureoles that together are representative of the normal P–T conditions and processes of infiltration-driven contact metamorphic reactions. The effect of salinity on the spatial extent of oxygen isotope alteration was also computed. The time-integrated input fluid flux (q°) that displaces the mineral reaction front an increment of distance along the flow path always increases with increasing X _NaCl. For input fluids with salinity up to approximately five times that of seawater (X _NaCl ≤ 0.05), values of q° required to explain the spatial extent of decarbonation reaction are no more than 1.1–1.5 times that computed assuming the input fluid was pure H₂O. For more saline fluids, values of q° may be up to 1.4–7.9 times that for pure H₂O. Except for reaction in the presence of halite and vapor (V), infiltration of H₂O–NaCl fluids expands the region of oxygen isotope alteration relative to the size of the region of mineral reaction. The expansion is significant only for saline fluids with X _NaCl ≥ ~0.1. Immiscible fluid phase separation and differential loss of the liquid (L) or V phase from the mineral reaction site increases the amount of reactive fluid required to advance the mineral reaction front compared to conditions under which equilibration of minerals and fluid is attained with no loss of L or V. Decarbonation reactions driven by infiltration of fluids with even modest seawater-like salinity can explain the occurrence of salt-saturated fluid and solid halide inclusions in contact metamorphosed carbonate rocks.     

Infrared microthermometric and stable isotopic study of fluid inclusions in wolframite at the Xihuashan tungsten deposit,Jiangxi province,China

The Xihuashan tungsten deposit, Jiangxi province, China, is a world-class vein-type ore deposit hosted in Cambrian strata and Mesozoic granitic intrusions. There are two major sets of subparallel ore-bearing quartz veins. The ore mineral assemblage includes wolframite and molybdenite, with minor amounts of arsenopyrite, chalcopyrite, and pyrite. There are only two-phase aqueous-rich inclusions in wolframite but at least three major types of inclusions in quartz: two- or three-phase CO₂-rich inclusions, two-phase pure CO₂ inclusions and two-phase aqueous inclusions, indicating boiling. Fluid inclusions in wolframite have relatively higher homogenization temperatures and salinities (239–380°C, 3.8–13.7 wt.% NaCl equiv) compared with those in quartz (177–329°C, 0.9–8.1 wt.% NaCl equiv). These distinct differences suggest that those conventional microthermometric data from quartz are not adequate to explain the ore formation process. Enthalpy–salinity plot shows a linear relationship, implying mixing of different sources of fluids. Although boiling occurred during vein-type mineralization, it seems negligible for wolframite deposition. Mixing is the dominant mechanism of wolframite precipitation in Xihuashan. δ³⁴S values of the sulfides range from −1.6 to +0.1‰, indicative of a magmatic source of sulfur. δ¹⁸O values of wolframite are relatively homogeneous, ranging from +4.8‰ to +6.3‰. Oxygen isotope modeling of boiling and mixing processes also indicates that mixing of two different fluids was an important mechanism in the precipitation of wolframite.     

Hydrocarbon- and ore-bearing basinal fluids: a possible link between gold mineralization and hydrocarbon accumulation in the Youjiang basin,South China

The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO₂-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C₂H₆, vapor CH₄, CH₄–H₂O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and ⁴⁰Ar/³⁹Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system.     

Ore-forming process of the Huijiabao gold district,southwestern Guizhou Province,China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO₂ inclusions, CO₂–H₂O inclusions, hydrocarbon inclusions, and hydrocarbon–H₂O inclusions. The ore-forming fluids are characterized by a H₂O + CO₂ + CH₄ ± N₂ system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H₂O + NaCl → H₂O + NaCl + CO₂ + CH₄ ± N₂ → H₂O + NaCl ± CH₄ ± CO₂ with a slight decrease in homogenization temperature and salinity. The δ¹⁸O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δD_H2O and calculated δ¹⁸O_H2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H₂O–NaCl–CO₂ fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.     

Ore fluid geochemistry of the Jinlongshan Carlintype gold ore belt in Shaanxi Province, China

THEJINLONGSHANGOLDOREBELTINZHEN’ANCOUN TY,SOUTHERNSHAANXIPROVINCE,ISLOCATEDINTHEWEST ERNQINLINGGOLDPROVINCE(NO.16INFIG.1;CHEN YANJINGETAL.,2004).ITWASDISCOVEREDINTHEDEVO NIANSTRATAINTHELATE1980S).ITSGEOLOGICALSETTING ANDMETALLOGENICEVOLUTIONARESIMILARTOT…     

Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit,Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T _h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ³⁴S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ¹⁸O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.     

Helium and argon isotopic compositions of the Longquanzhan gold deposit in the Yishu fault zone and their geological implications

The Longquanzhan gold deposit hosted in granitic cataclasites with mylontization of the foot wall of the main Yishui-Tangtou fault. 3He/4He ratios in fluid inclusions range from 0. 14 to 0. 24 R/Ra,close to those of the crust-source helium. 40Ar/36Ar ratios were measured to be 289-1811, slightly higher than those of atmospheric argon. The results of analysis of helium and argon isotopes suggested that ore-forming fluids were derived chiefly from the crust. The δ18O values of fluid inclusions from vein quartz range from -1.78‰ to 4.07‰, and the δD values of the fluid inclusions vary between -74‰ and -77‰. The hydrogen and oxygen isotope data indicated that the ore-forming fluid for the Longquanzhan gold deposit had mixed with meteoric water in the process of mineralization. This is consistent with the conclusion from the helium and argon isotope data.     

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

Quartz–carbonate–chlorite veins were studied in borehole samples of the RWTH-1 well in Aachen. Veins formed in Devonian rocks in the footwall of the Aachen thrust during Variscan deformation and associated fluid flow. Primary fluid inclusions indicate subsolvus unmixing of a homogenous H₂O–CO₂–CH₄–(N₂)–Na–(K)–Cl fluid into a H₂O–Na–(K)–Cl solution and a vapour-rich CO₂–(H₂O, CH₄, N₂) fluid. The aqueous end-member composition resembles that of metamorphic fluids of the Variscan front zone with salinities ranging from 4 to 7% NaCl equiv. and maximum homogenisation temperatures of close to 400°C. Pressure estimates indicate a burial depth between 4,500 and 8,000 m at geothermal gradients between 50 and 75°C/26 MPa, but pressure decrease to sublithostatic conditions is also indicated, probably as a consequence of fracture opening during episodic seismic activity. A second fluid system, mainly preserved in pseudo-secondary and secondary fluid inclusions, is characterised by fluid temperatures between 200 and 250°C and salinities of <5% NaCl equiv. Bulk stable isotope analyses of fluids released from vein quartz, calcite, and dolomite by decrepitation yielded δD_H2O values from −89 to −113 ‰, δ¹³C_CH4 from −26.9 to −28.9‰ (VPDB) and δ¹³C_CO2 from −12.8 to −23.3‰ (VPDB). The low δD and δ¹³C range of the fluids is considered to be due to interaction with cracked hydrocarbons. The second fluid influx caused partial isotope exchange and disequilibrium. It is envisaged that an initial short lived flux of hot metamorphic fluids expelled from the epizonal metamorphic domains of the Stavelot–Venn massif. The metamorphic fluid was focused along major thrust faults of the Variscan front zone such as the Aachen thrust. A second fluid influx was introduced from formation waters in the footwall of the Aachen thrust as a consequence of progressive deformation. Mixing of the cooler and lower salinity formation water with the hot metamorphic fluid during episodic fluid trapping resulted in an evolving range of physicochemical fluid inclusion characteristics.     

Genesis of the Assif El Mal Zn–Pb (Cu,Ag) vein deposit. An extension-related Mesozoic vein system in the High Atlas of Morocco. Structural,mineralogical, and geochemical evidence

The Assif El Mal Zn–Pb (Cu–Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, amphibole, chloritoid, and garnet. The Assif El Mal Zn–Pb (Cu–Ag) mineralization forms subvertical veins with ribbon, fault breccia, cockade, comb, and crack and seal textures. Two-phase liquid–vapor fluid inclusions that were trapped during several stages occur in quartz and sphalerite. Primary inclusion fluids exhibit T _h mean values ranging from 104°C to 198°C. Final ice-melting temperatures range from −8.1°C to −12.8°C, corresponding to salinities of ∼15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater. Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions and homogenization temperatures ranging from 75°C to 150°C. The δ¹⁸O and δD fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived organic waters. Low δ¹³C_VPDB values ranging from −7.5‰ to −7.7‰ indicate a homogeneous carbon source, possibly organic matter disseminated in black shale hosting the Zn–Pb (Cu–Ag) veins. The calculated δ³⁴S_H2S values for reduced sulfur (22.5‰ to 24.3‰) are most likely from reduction of SO₄ ²⁻ in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical sulfate reduction in which organic matter was oxidized to produce CO₂ which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent with fluid–rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif El Mal formed during the Jurassic opening of the central Atlantic Ocean.     

Fluid inclusion study of the Wunugetu Cu–Mo deposit,Inner Mongolia,China

Hydrothermal alteration and mineralization at the Wunugetu porphyry Cu–Mo deposit, China, include four stages, i.e., the early stage characterized by quartz, K-feldspar and minor mineralization, followed by a molybdenum mineralization stage associated with potassic alteration, copper mineralization associated with sericitization, and the last Pb–Zn mineralization stage associated with carbonation. Hydrothermal quartz contains three types of fluid inclusions, namely aqueous (W-type), daughter mineral-bearing (S-type) and CO₂-rich (C-type) inclusion, with the latter two types absent in the late stage. Fluid inclusions in the early stage display homogenization temperatures above 510°C, with salinities up to 75.8 wt.% NaCl equivalent. The presence of S-type inclusions containing anhydrite and hematite daughter minerals and C-type inclusions indicates an oxidizing, CO₂-bearing environment. Fluid inclusions in the Mo- and Cu-mineralization stages yield homogenization temperatures of 342–508°C and 241–336°C, and salinities of 8.6–49.4 and 6.3–35.7 wt.% NaCl equivalent, respectively. The presence of chalcopyrite instead of hematite and anhydrite daughter minerals in S-type inclusions indicates a decreasing of oxygen fugacity. In the late stage, fluid inclusions yield homogenization temperatures of 115–234°C and salinities lower than 12.4 wt.% NaCl equivalent. It is concluded that the early stage fluids were CO₂ bearing, magmatic in origin, and characterized by high temperature, high salinity, and high oxygen fugacity. Phase separation occurred during the Mo- and Cu-mineralization stages, resulting in CO₂ release, oxygen fugacity decrease and rapid precipitation of sulfides. The late-stage fluids were meteoric in origin and characterized by low temperature, low salinity, and CO₂ poor.     

The Paleoproterozoic carbonate-hosted Pering Zn–Pb deposit, South Africa. II: fluid inclusion, fluid chemistry and stable isotope constraints

The Pering deposit is the prime example of Zn–Pb mineralisation hosted by stromatolitic dolostones of the Neoarchean to Paleoproterozoic Transvaal Supergroup. The hydrothermal deposit centers on subvertical breccia pipes that crosscut stromatolitic dolostones of the Reivilo Formation, the lowermost portion of the Campbellrand Subgroup. Four distinct stages of hydrothermal mineralisation are recognised. Early pyritic rock matrix brecciation is followed by collomorphous sphalerite mineralisation with replacive character, which, in turn, is succeeded by coarse grained open-space-infill of sphalerite, galena, sparry dolomite, and quartz. Together, the latter two stages account for ore-grade Zn–Pb mineralisation. The fourth and final paragenetic stage is characterised by open-space-infill by coarse sparry calcite. The present study documents the results of a detailed geochemical study of the Pering deposit, including fluid inclusion microthermometry, fluid chemistry and stable isotope geochemistry of sulphides (δ³⁴S) and carbonate gangue (δ¹³C and δ¹⁸O). Microthermometric fluid inclusion studies carried out on a series of coarsely grained crystalline quartz and sphalerite samples of the latter, open-space-infill stage of the main mineralisation event reveal the presence of three major fluid types: (1) a halite–saturated aqueous fluid H₂O–NaCl–CaCl₂ (>33 wt% NaCl equivalent) brine, (2) low-salinity meteoric fluid (<7 wt% NaCl) and (3) a carbonic CH₄–CO₂–HS⁻ fluid that may be derived from organic material present within the host dolostone. Mixing of these fluids have given rise to variable mixtures (H₂O–CaCl₂–NaCl ±(CH₄–CO₂–HS⁻), 2 to 25 wt% NaCl+CaCl₂). Heterogeneous trapping of the aqueous and carbonic fluids occurred under conditions of immiscibility. Fluid temperature and pressure conditions during mineralisation are determined to be 200–210°C and 1.1–1.4 kbar, corresponding to a depth of mineralisation of 4.1–5.2 km. Chemical analyses of the brine inclusions show them to be dominated by Na and Cl with lesser amounts of Ca, K and SO₄. Fluid ratios of Cl/Br indicate that they originated as halite saturated seawater brines that mixed with lower salinity fluids. Analyses of individual brine inclusions document high concentrations of Zn and Pb (∼1,500 and ∼200 ppm respectively) and identify the brine as responsible for the introduction of base metals. Stable isotope data were acquired for host rock and hydrothermal carbonates (dolomite, calcite) and sulphides (pyrite, sphalerite, galena and chalcopyrite). The ore-forming sulphides show a trend to ³⁴S enrichment from pyrite nodules in the pyritic rock matrix breccia (δ³⁴S = −9.9 to +3.7‰) to paragenetically late chalcopyrite of the main mineralisation event (δ³⁴S = +30.0‰). The observed trend is attributed to Rayleigh fractionation during the complete reduction of sulphate in a restricted reservoir by thermochemical sulphate reduction, and incremental precipitation of the generated sulphide. The initial sulphate reservoir is expected to have had an isotopic signature around 0‰, and may well represent magmatic sulphur, oxidised and leached by the metal-bearing brine. The δ¹⁸O values of successive generations of dolomite, from host dolostone to paragenetically late saddle dolomite follow a consistent trend that yields convincing evidence for extensive water rock interaction at variable fluid–rock ratios. Values of δ¹³C remain virtually unchanged and similar to the host dolostone, thus suggesting insignificant influx of CO₂ during the early and main stages of mineralisation. On the other hand, δ¹³C and δ¹⁸O of post-ore calcite define two distinct clusters that may be attributed to changes in the relative abundance in CH₄ and CO₂ during waning stages of hydrothermal fluid flow.     

Implications from inclusions in topaz for greisenisation and mineralisation in the Hensbarrow topaz granite, Cornwall, England

Textural and geochemical studies of inclusions in topaz from greisens in the Hensbarrow topaz granite stock (St. Austell, Cornwall) are used to constrain the composition of fluids responsible for late stage greisening and mineralisation. The topaz contains an abundant and varied suite of inclusions including aqueous liquid + vapour (L + V), quartz, zinnwaldite, albite, K-feldspar, muscovite, ilmenorutile, apatite, columbite, zircon, varlamoffite [(Sn, Fe)(O, OH)₂] and qitianlingite [(Fe⁺²,Mn⁺²)₂(Nb,Ta)₂W⁺⁶O₁₀]. Primary L + V inclusions in topaz show relatively high T _h (mainly 300 to >500 °C) and a narrow range of salinities (23–30 wt % NaCl equivalent) compared with those in greisen quartz (150–450 °C, 0–50 wt % NaCl equivalent). Textures indicate that topaz formed earlier than quartz and the fluid inclusion data are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation, mixing with meteoric waters and a decrease in pressure causing intermittent boiling. The presence of early-formed albite and K-feldspar as inclusions in the topaz is likely to indicate that the greisen-forming fluid became progressively more acid during greisenisation. The most distinctive inclusions in the topaz are wisp- and bleb-shaped quartz, < 50 μm in size, which show textural characteristics indicating former high degrees of plasticity. They often have multiple shrinkage bubbles at their margins rich in Sn, Fe, Mn, S and Cl and, more rarely, contain euhedral albite, K-feldspar, stannite or pyrrhotite crystals up to 40 μm in size. The quartz inclusions show similar morphologies to inclusions in topaz from quartz-topaz rocks elsewhere which have been interpreted as trapped “silicate melt”. Their compositions are, however, very different to those expected for late stage topaz-normative granitic melts. From their textural and chemical characteristics they are interpreted as representing crystallised silica colloid, probably trapped as a hydro gel during greisenisation. There is also evidence for the colloidal origin of inclusions of varlamoffite in the topaz. These occurrences offer the first reported evidence in natural systems for the formation of colloids in high temperature hydrothermal fluids. Their high ore carrying potential is suggested by the presence of varlamoffite and the occurrence of stannite, pyrrhotite and SnCl within the quartz inclusions. Received: 9 April 1996 / Accepted: 12 November 1996     

Fluid origin and structural enhancement during mineralization of the Jinshan orogenic gold deposit, South China

The Jinshan orogenic gold deposit is a world-class deposit hosted by a ductile shear zone caused by a transpressional terrane collision during Neoproterozoic time. Ore bodies at the deposit include laminated quartz veins and disseminated pyrite-bearing mylonite. Most quartz veins in the shear zone, with and without gold mineralization, were boudinaged during progressive shear deformation with three generations of boudinage structures produced at different stages of progressive deformation. Observations of ore-controlling structures at various scales indicate syn-deformational mineralization. Fluid inclusions from pyrite intergrown with auriferous quartz have ³He/⁴He ratios of 0.15–0.24 Ra and ⁴⁰Ar/³⁶Ar ratios 575–3,060. δ¹⁸O_fluid values calculated from quartz are 5.5–8.4‰, and δD values of fluid inclusions contained in quartz range between −61‰ and −75‰. The δ¹³C values of ankerite range from −5.0‰ to −4.2‰, and ankerite δ¹⁸O values from 4.4‰ to 8.0‰. The noble gas and stable isotope data suggest a predominant crustal source of ore fluids with less than 5% mantle component. Data also show that in situ fluids were generated locally by pervasive pressure solution, and that widespread dissolution seams acted as pathways of fluid flow, migration, and precipitation. The in situ fluids and fluids derived from deeper levels of the crust were focused by deformation and deformation structures at various scales through solution-dissolution creep, crack-seal slip, and cyclic fault-valve mechanisms during progressively localized deformation and gold mineralization.     

Tectono-sedimentary evolution of active extensional basins controlling the deposition of the Middle Miocene Kareem Formation,southwestern Gulf of Suez,Egypt

The Naozhi Au–Cu deposit is located on the continental margin of Northeast China, forming part of the West Pacific porphyry–epithermal gold–copper metallogenic belt. In this paper, we systematically analyzed the compositions, homogenization temperatures, and salinity of fluid inclusions as well as their noble gas isotopic and Pb isotopic compositions from the deposit. These new data show that (1) five types of fluid inclusions were identified as pure gas inclusions (V-type), pure liquid inclusions (L-type), gas–liquid two-phase inclusions (W-type, as the main fluid inclusions (FIs)), CO₂-bearing inclusions (C-type), and daughter-mineral-bearing polyphase inclusions (S-type); (2) W-type FIs in quartz crystals of early, main, and late stage are homogenized at temperatures of 324.7–406.7, 230–338.8, and 154.6–308 °C, with salinities of 2.40–7.01 wt% NaCl_eq, 1.73–9.47 wt% NaCl_eq, and 6.29 wt% NaCl_eq, respectively. S-type FIs in quartz crystals of early stage are homogenized at temperatures of 328.6–400 °C, with salinities of 39.96–46.00 wt% NaCl_eq; (3) Raman analysis results reveal that the vapor compositions of early ore-forming fluids consisted of CO₂ and H₂O, with H₂O gradually increasing and CO₂ being absent at the late mineralization stage; (4) fluid inclusions in pyrite and chalcopyrite have ³He/⁴He ratios of 0.03–0.104 Ra, ²⁰Ne/²²Ne ratios of 9.817–9.960, and ⁴⁰Ar/³⁶Ar ratios of 324–349. These results indicate that the percentage of radiogenic ⁴⁰Ar* in fluid inclusions varies from 8.8 to 15.5 %, containing 84.5–91.2 % atmospheric ⁴⁰Ar; (5) the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁶Pb/²⁰⁴Pb ratios of sulfides are 18.1822–18.3979, 15.5215–15.5998, and 38.1313–38.3786, respectively. These data combined with stable isotope data and the chronology of diagenesis and metallogenesis enable us suppose that the ore-forming fluids originated from the melting of the lower crust, caused by the subduction of an oceanic slab, whereas the mineralized fluids were exsolved from the late crystallization stage and subsequently contaminated by crustal materials/fluids during ascent, including meteoric water, and the mineral precipitation occurred at a shallow crustal level.

     

Formation of the Vysoká–Zlatno Cu–Au skarn–porphyry deposit, Slovakia

The central zone of the Miocene Štiavnica stratovolcano hosts several occurrences of Cu–Au skarn–porphyry mineralisation, related to granodiorite/quartz–diorite porphyry dyke clusters and stocks. Vysoká–Zlatno is the largest deposit (13.4 Mt at 0.52% Cu), with mineralised Mg–Ca exo- and endoskarns, developed at the prevolcanic basement level. The alteration pattern includes an internal K- and Na–Ca silicate zone, surrounded by phyllic and argillic zones, laterally grading into a propylitic zone. Fluid inclusions in quartz veinlets in the internal zone contain mostly saline brines with 31–70 wt.% NaCl eq. and temperatures of liquid–vapour homogenization (Th) of 186–575°C, indicating fluid heterogenisation. Garnet contains inclusions of variable salinity with 1–31 wt.% NaCl eq. and Th of 320–360°C. Quartz–chalcopyrite veinlets host mostly low-salinity fluid inclusions with 0–3 wt.% NaCl eq. and Th of 323–364°C. Data from sphalerite from the margin of the system indicate mixing with dilute and cooler fluids. The isotopic composition of fluids in equilibrium with K-alteration and most skarn minerals (both prograde and retrograde) indicates predominantly a magmatic origin (δ¹⁸O_fluid 2.5–12.3‰) with a minor meteoric component. Corresponding low δD_fluid values are probably related to isotopic fractionation during exsolution of the fluid from crystallising magma in an open system. The data suggest the general pattern of a distant source of magmatic fluids that ascended above a zone of hydraulic fracturing below the temperature of ductile–brittle transition. The magma chamber at ∼5–6 km depth exsolved single-phase fluids, whose properties were controlled by changing PT conditions along their fluid paths. During early stages, ascending fluids display liquid–vapour immiscibility, followed by physical separation of both phases. Low-salinity liquid associated with ore veinlets probably represents a single-phase magmatic fluid/magmatic vapour which contracted into liquid upon its ascent.     

Metallogeny of the Shilu Fe–Co–Cu deposit,Hainan Island,South China: Constraints from fluid inclusions and stable isotopes

The Shilu deposit is a world-class Fe–Co–Cu orebody located in the Changjiang area of the western part of Hainan Island, South China. The distribution of Fe, Co, and Cu orebodies is controlled by strata of the No. 6 Formation in the Shilu Group and the Beiyi synclinorium. Based on a petrological study of the host rocks and their alteration assemblages, and textural and structural features of the ores, four mineralization stages have been identified: (1) the sedimentary ore-forming period; (2) the metamorphic ore-forming period; (3) the hydrothermal mineralization comprising the skarn and quartz–sulfide stage; and (4) the supergene period. The fluid inclusions in sedimentary quartz and/or chert indicate low temperatures (ca. 160 °C) and low salinities from 0.7 to 3.1 wt.% NaCl_eq, which corresponds to densities of 0.77 to 0.93 g/cm³. CO₂-bearing or carbonic inclusions have been interpreted to result from regional metamorphism. Homogenization temperatures of fluid inclusions for the skarn stage have a wide range from 148 °C to 497 °C and the salinities of the fluid inclusions range from 1.2 to 22.3 wt.% NaCl_eq, which corresponds to densities from 0.56 to 0.94 g/cm³. Fluid inclusions of the quartz–sulfide stage yield homogenization temperatures of 151–356 °C and salinities from 0.9 to 8.1 wt.% NaCl_eq, which equates to fluid densities from 0.63 to 0.96 g/cm³.Sulfur isotopic compositions indicate that sulfur of the sedimentary anhydrite and Co-bearing pyrite, and the quartz–sulfide stage, was derived from seawater sulfate and thermochemical sulfate reduction of dissolved anhydrite at temperatures of 200 °C and 300 °C, respectively. H and O isotopic compositions of the skarn and quartz–sulfide stage demonstrate that the ore-forming fluids were largely derived from magmatic water, with minor inputs from metamorphic or meteoric water. The Shilu iron ore deposit has an exhalative sedimentary origin, but has been overprinted by regional deformation and metamorphism. The Shilu Co–Cu deposit has a hydrothermal origin and is temporally and genetically associated with Indosinian granitoid rocks.