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1.
Basem A. Zoheir 《Mineralium Deposita》2008,43(1):79-95
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. 相似文献
2.
Ore-forming fluids associated with granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong gold province, China 总被引:45,自引:0,他引:45
The Sanshandao gold deposit, with total resources of more than 60 t of gold, is located in the Jiaodong gold province, the most important gold province of China. The deposit is a typical highly fractured and altered, disseminated gold system, with high-grade, quartz-sulphide vein/veinlet stockworks that cut Mesozoic granodiorite. There are four stages of veins that developed in the following sequence: (1) quartz-K-feldspar-sericite; (2) quartz-pyrite±arsenopyrite; (3) quartz-base metal sulfide; and (4) quartz-carbonate. Fluid inclusions in quartz and calcite in vein/veinlet stockworks contain C-O-H fluids of three main types. The first type consists of dilute CO2–H2O fluids coeval with the early vein stage. Molar volumes of these CO2–H2O fluid inclusions, ranging from 50–60 cm3/mol, yield estimated minimum trapping pressures of 3 kbar. Homogenization temperatures, obtained mainly from CO2–H2O inclusions with lower CO2 concentration, range from 267–375 °C. The second inclusion type, with a CO2–H2O±CH4 composition, was trapped during the main mineralizing stages. These fluids may reflect the CO2–H2O fluids that were modified by fluid/rock reactions with altered wallrocks. Isochores for CO2-H2O±CH4 inclusions, with homogenization temperatures ranging from 204–325 °C and molar volumes from 55 to 70 cm3/mol, provide an estimated minimum trapping pressure of 1.2 kbar. The third inclusion type, aqueous inclusions, trapped in cross-cutting microfractures in quartz and randomly in calcite, are post-mineralization, and have homogenization temperatures between 143–228 °C and salinities from 0.71–7.86 wt% NaCl equiv. Stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin, most likely sourced from 120–126 Ma mafic to intermediate dikes. This is consistent with the carbonic nature of the fluid, and the cross-cutting nature of those deposits relative to the host Mesozoic granitoid.Editorial handling: R.J. Goldfarb 相似文献
3.
CO2-rich fluid inclusions containing opaque mineral crystals were found in the Fenghuangshan skarn-porphyry Cu–Fe–Au deposit
in Tongling, Anhui, China. These inclusions show variable CO2 contents and are accompanied by aqueous inclusions, both occurring as secondary inclusions in quartz and being locally associated
with chalcopyrite mineralization. Laser Raman microspectroscopic analyses confirm the predominance of CO2 in the vapor and the presence of H2S as high as 8 mol%, and identify the opaque mineral with yellow reflectance color in the inclusions as chalcopyrite. More
than half of the CO2-bearing inclusions contains chalcopyrite, whereas few of the associated aqueous inclusions do so. The chalcopyrite, occupying
less than 1% (volume) of the inclusions, is interpreted to be a daughter mineral, and calculated Cu concentrations in the
inclusions range from 0.1 to 3.4 wt%. Copper is inferred to have been transported in CO2-dominated fluids as HS− complexes. The occurrence of chalcopyrite daughter crystals in CO2-rich fluid inclusions indicates that CO2-rich vapor has the capacity of transporting large amounts of Cu, and possibly Au. This finding has significant implications
for metal transport and mineralization in hydrothermal systems enriched in CO2, such as orogenic-type and granitic intrusion-related gold deposits. 相似文献
4.
Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and
sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling
district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition
in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO2-H2O inclusions, which have a wide range in CO2 content and coexist with lesser primary CO2-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry
and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO2 in stage I inclusions and 10–20 mol% CO2 in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.%
NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th
total than, their coexistent CO2-H2O inclusions. The coexisting CO2-rich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a
homogeneous CO2-H2O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature
of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for
stage II fluids. The CO2-H2O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids.
The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility
in stage II veins.
Received: 15 May 1997 / Accepted: 10 June 1998 相似文献
5.
Relationship between CO2-dominated fluids,hydrothermal alterations and gold mineralization in the Red Lake greenstone belt,Canada 总被引:1,自引:0,他引:1
The Red Lake greenstone belt is one of the foremost Au mining camps in Canada and hosts the world-class Campbell-Red Lake Au deposit. Belt-scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with Au mineralization surrounded by distal calcite zones, both being accompanied by potassic alterations (sericite, muscovite, and biotite). At the Campbell-Red Lake and Cochenour deposits Au mineralization (in particular high-grade ore) is associated with silica and sulfides (especially arsenopyrite) that replace carbonate ± quartz veins and the host rocks. The prevalence of carbonic fluid inclusions and rare occurrence of aqueous-bearing inclusions in carbonate–quartz–Au veins in the Campbell-Red Lake deposit, and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages (FIA), have been interpreted to indicate that H2O-poor, CO2-dominated fluids were responsible for the carbonate veining and Au mineralization. Further studies of fluid inclusions in carbonate–quartz veins within and outside the deformation zone hosting the Campbell-Red Lake deposit (the Red Lake Mine trend) including the Cochenour Au deposit, the Redcon Au prospect, and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals. These studies indicate that CO2-dominated fluids were flowing through fractures during carbonate vein formation and Au mineralization both within and outside major structures. The carbonic fluid may have been initially undersaturated with water, or it may have resulted from phase separation of an H2O–CO2–NaCl fluid. In the latter case, phase separation modeling indicates that the initial fluid likely had XCO2 values larger than 0.8. Calculations based on hydrothermal mineral assemblages indicate XCO2 values in the host rocks from 0.025 to 0.85, reflecting a change from CO2-dominated fluids in the fractures (veins) to H2O-dominated fluids in the host rocks away from the fractures. The CO2-dominated fluids were likely advected from granulite facies in the deeper crust, whereas the H2O-dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies. Calculations based on CO2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in Au than the commonly assumed values of a few μg/L, which may explain why the Campbell-Red Lake deposit has a very high-grade of Au (average 21 g/t for the whole deposit and 81 g/t for the Goldcorp High-Grade zone). Fluid inclusion data suggest that the carbonate veining and Au mineralization likely took place at depths from 7 to 14 km. The development of crustiform–colloform structures in the carbonate ± quartz veins, which was previously interpreted to indicate relatively shallow environments, may alternatively have been related to extremely high fluid pressures and the CO2-dominated nature of the fluids, which could have enhanced the brittle properties of the rocks due to their high wetting angles. 相似文献
6.
Evandro Luiz Klein Reginaldo Alves dos Santos Kazuo Fuzikawa Rômulo Simões Angélica 《Mineralium Deposita》2001,36(2):149-164
Fluid inclusion and structural studies were carried out at the Guarim gold deposit in the Palaeoproterozoic Tapajós province
of the Amazonian craton. Guarim is a fault-hosted gold deposit cutting basement granitoids. It consists of a quartz vein,
which is massive in its inner portions, grading laterally either to a massive or to cavity-bearing quartz vein associated
with hydrothermal breccias. The wallrock alteration comprises chlorite, carbonate, white mica and sulphide minerals, with
free gold occurring within quartz grains and spatially associated with sulphide mineral grains. Petrographic, microthermometric
and Laser Raman investigations recognised CO2-rich, mixed H2O–CO2, and H2O fluid inclusions. The coexisting CO2 and H2O–CO2 inclusions were interpreted as primary immiscible fluids that formed the gold-bearing vein. The H2O inclusions were considered a product of later infiltration of fluids unrelated to the mineralising episode. The mineralising
fluid has CO2 ranging typically from 5–10 mol%, contains traces of N2, has salinities of ∼5 wt% NaCl equiv., and densities varying between 0.85 and 0.95 g/cm3. The P–T estimations bracket gold deposition between 270–320 °C and 0.86–2.9 kb; ƒO2–ƒS2–pH estimates suggest a reduced, near-neutral character for the fluid. Variations in the physico-chemical properties, as demonstrated
by the fluid inclusion study, resulted from a combination of fluid immiscibility and pressure fluctuation. This interpretation,
combined with textural and structural evidence, suggests the emplacement of the mineralised vein in an active fault and at
a rather shallow level (4–7 km). The geological and structural setting, deposit-scale textures and structures, wallrock alteration
and physico-chemical fluid properties are compatible with those of epizonal to mesozonal orogenic lode gold deposits.
Received: 3 March 2000 / Accepted: 21 October 2000 相似文献
7.
Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO2 ± CH4 ± H2O) and aqueous-carbonic (H2O–NaCl–CO2 ± CH4) inclusions along intragranular trails, whereas aqueous inclusions (H2O–NaCl ± CO2) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ34S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ34SΣs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO2 via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites. 相似文献
8.
Nicholas C. Williams 《Mineralium Deposita》2007,42(1-2):65-87
The Callie deposit is the largest (6.0 Moz Au) of several gold deposits in the Dead Bullock Soak goldfield of the Northern
Territory’s Tanami Region, 550 km northwest of Alice Springs. The Callie ore lies within corridors, up to 180 m wide, of sheeted
en echelon quartz veins where they intersect the 500-m-wide hinge of an ESE-plunging F1 anticlinorium. The host rocks are the Blake beds, of the Paleoproterozoic Dead Bullock Formation, which consist of a > 350-m-thick
sequence of lower greenschist facies graphitic turbidites and mudstones overlying in excess of 100 m of thickly bedded siltstones
and fine sandstones. The rocks are Fe-rich and dominated by assemblages of chlorite and biotite, both of which are of hydrothermal
and metamorphic origin. A fundamental characteristic of the hydrothermal alteration is the removal of graphite, a process
which is associated with bleaching and the development of bedding-parallel bands of coarse biotite augen. Gold is found only
in quartz veins and only where they cut decarbonized chloritic rock with abundant biotite augen and no sulfide minerals. Auriferous
quartz veins differ from barren quartz veins by the presence of ilmenite, apatite, xenotime, and gold and the absence of sulfide
minerals. The assemblage of gold–ilmenite–apatite–xenotime indicates a linked genesis and mobility of Ti, P, and Y in the
mineralizing fluids. Geochemical analysis of samples throughout the deposit shows that gold only occurs in sedimentary rocks
with high FeO/(FeO+Fe2O3) and low C/(C+CO2) ratios (> 0.8 and < 0.2, respectively). This association can be explained by reactions that convert C from reduced graphitic
host rocks into CO2 and reduce ferric iron in the host rocks to ferrous iron in biotite and chlorite. These reactions would increase the CO2 content of the fluid, facilitating the transport of Ti, P, and Y from the host rocks into the veins. Both CO2 and CH4 produced by reaction of H2O with graphite, effervesced under the lower confining pressures in the veins. This would have partitioned H2S into the vapor phase, destabilizing Au–bisulfide complexes; the loss of CO2 and H2S from the aqueous phase caused precipitation of gold, ilmenite, apatite, and xenotime. It is proposed that this process was
the main control on gold precipitation. Oxidization of iron in the very reduced wall rocks, resulting in reduction of the
fluid, provided a second mechanism of gold precipitation in previously decarbonized rocks, contributing to the high grades
in some samples. Although sulfide minerals, especially arsenopyrite, did form during the hydrothermal event, host rock sulfidation
reactions did not play a role in gold precipitation because gold is absent near rocks or veins containing sulfide minerals.
Sulfide minerals likely formed by different mechanisms from those associated with gold deposition. Both the fold architecture
and subsequent spatially coincident sinistral semibrittle shearing ensured that the ore fluids were strongly focused into
the hinges of the anticlines. Within the anticlines, a reactive cap of fine-grained, graphitic, reduced Fe-rich turbidites
above more permeable siltstones and fine sandstones impeded fluid flow ensuring efficient removal of graphite, and the associated
effervescence of CO2 from the fluid caused the precipitation of gold. Exploration for similar deposits should focus on the intersection of east–west
shear zones with folds and Fe-rich graphitic host rocks. 相似文献
9.
Nicolas Thébaud Pascal Philippot Patrice Rey Jean Cauzid 《Contributions to Mineralogy and Petrology》2006,152(4):485-503
Microthermometry and Raman spectroscopy techniques are routinely use to constrain ore-fluids δ18O and molar proportions of anhydrous gas species (CO2, CH4, N2). However, these methods remain imprecise concerning the ore-fluids composition and source. Synchrotron radiation X-ray fluorescence allows access to major and trace element concentrations (Cl, Br and K, Ca, Fe, Cu, Zn, As, Rb, Sr) of single fluid inclusion. In this paper, we present the results of the combination of these routine and newly developed techniques in order to document the fluids composition and source associated with a Mesoarchaean lode gold deposit (Warrawoona Syncline, Western Australia). Fluid inclusion analyses show that quartz veins preserved records of three fluid inclusion populations. Early fluids inclusions, related to quartz veins precipitation, are characterized by a moderate to high Br/Cl ratio relative to modern seawater, CO2 ± CH4 ± N2, low to moderate salinities and significant base metal (Fe, Cu, Zn) and metalloid (As) concentrations. Late fluid inclusions trapped in secondary aqueous fluid inclusions are divided into two populations with distinct compositions. The first population consists of moderately saline aqueous brines, with a Br/Cl ratio close to modern seawater and a low concentration of base metals and metalloids. The second population is a fluid of low to moderate salinity, with a low Br/Cl ratio relative to modern seawater and significant enrichment in Fe, Zn, Sr and Rb. These three fluid inclusion populations point to three contrasting sources: (1) a carbonic fluid of mixed metamorphic and magmatic origin associated with the gold-bearing quartz precipitation; (2) a secondary aqueous fluid with seawater affinity; and (3) a surface-derived secondary aqueous fluid modified through interaction with felsic lithologies, before being flushed into the syncline. Primary carbonic fluids present similar characteristics than those ascribed to Mesoarchaean lode gold deposits. This suggests similar mineralization processes for mid- and Mesoarchaean lode gold deposits despite contrasting fluid–rock interaction histories. However, in regard to the protracted history documented in the Warrawoona Syncline, we question the robustness of the epigenetic crustal continuum model, as ore-fluid characteristics equally support an epigenetic or a polyphased mineralization process. 相似文献
10.
The southern Tien Shan metallogenic province of Central Asia hosts a number of important gold resources including the Jilau
gold–quartz vein system in western Tajikistan. These deposits were formed at the late stages of continent–continent collision
in association with subduction-related magmatism, metamorphism and continental margin deformation attributed to the Central
Asian Hercynian Orogeny. Jilau is hosted by a Hercynian syntectonic granitoid intrusive that was emplaced into bituminous
dolomite country rocks. Economic mineralisation is associated with a dilational jog within a high-angle, oblique dextral-reverse
slip shear zone that was undergoing brittle–ductile deformation. The orebody takes the form of shear-zone subparallel quartz
veins and lenses that emanate from a steeply plunging ore shoot of veins and stringers within a silicified and sulphidised
granodiorite core. It is thought to have formed by a dynamic process in which fluid flow was governed by a fault-valve mechanism.
Numerous cycles of fluid pressure build-up, fault failure, jog dilation, fluid flow, phase separation of low salinity H2O–CO2–CH4(–N2) fluids, and sealing took place. Gold appears together with scheelite and bismuth minerals predominantly as inclusions in
arsenopyrite in quartz veins and altered wall-rock, and is mainly associated with quartz containing fluid inclusions enriched
in CH4. The correlation between high gold grades and high CH4 concentrations suggests that components of the mineralising fluids were derived from, or passed through, the reducing, carbonaceous
rocks in the contact aureole of the intrusive. The occurrence of Au and W in an adjacent Hercynian skarn deposit and in the
Jilau orebody, infers that the ore metals in both these systems were ultimately derived from a magmatic source.
Received: 15 April 1999 / Accepted: 30 December 1999 相似文献
11.
《Journal of African Earth Sciences》2008,50(2-4):234-254
The Paleoproterozoic terranes (Birimian) of West Africa are well known to host numerous economic gold mineralizations. The Angovia gold mineralization is located in a brecciated and mylonitic zone within the Birimian greenstones. The sulfide–gold mineralization is mainly represented by gold associated with pyrite and chalcopyrite. A fluid inclusion study undertaken on mineralized quartz veins revealed the presence of aqueous-carbonic (CO2–H2O) fluids, the association of carbonic (CO2) and early aqueous fluids, followed by later aqueous (H2O-salt) and finally nitrogen-rich fluids. Entrapment of the initial homogeneous aqueous-carbonic fluids prior to fluid immiscibility depicts the evolution of the P–T conditions during the exhumation of the terranes after the peak of green-schist metamorphism. The CO2 rich-fluid occurs especially in gold-bearing quartz, and are considered as the main evidence of the ore-forming process in the gold-bearing quartz veins. It is considered as a product of immiscibility of the CO2–H2O parent. The volatile fraction of carbonic and aqueous-carbonic fluid inclusions is dominated by CO2, containing minor amounts of N2, even smaller amounts of CH4 and sporadically, H2S. The aqueous-carbonic fluids have moderate salinity (3–10 wt.% eq. NaCl). Late aqueous and N2 – (CH4–CO2) fluids are considered as later, unrelated to the main ore stage, and were trapped during the cooling of the hydrothermal system from 300 to 200 °C.The immiscibility has been favored by a strong pressure drop, the main trapping P–T conditions being 320–370 °C and 105–135 MPa. The mineralizing process is likely related to the immiscibility event, which was probably favored by the release of the fluid pressure after fracturing along the main shear zones. The ore process is likely to have occurred along the main shear zones or related secondary structures affected by cycling of the fluid pressure and quartz sealing–fracturing processes. The superimposed process can also explain the relative complexity of the quartz textures and fluid inclusion microfractures, and the rather wide range in the density of both parent fluid and CO2-dominated fluid. 相似文献
12.
《Lithos》1986,19(1):1-10
Forming the southwestern segment of the Precambrian granulite facies terrain of the Indian shield, the Kerala region largely comprises charnockites, khondalites and migmatitic gneisses. Fluid inclusions in quartz from the charnockites show distinct distribution patterns consistent with three generations of inclusions. The early monophase type records entrapment of high-density CO2-rich fluid (0.95–1.0 g cm−3). A subsequent monophase type with lower-density CO2-rich fluid (0.65–0.75 g cm−3) coexists with CO2H2O inclusions having an average degree of filling of 0.2 (H2O = 20%; CO2 = 80%). Late aqueous biphase inclusions show coexistence with a second category of CO2H2O inclusions showing a degree of filling of 0.6 (H2O = 60%; CO2 = 40%). The CO2-isochores for early carbonic inclusions yield a pressure range of 4.6–6.1 kbar at granulite facies temperatures of 650–800°C, depicting the entrapment of fluids present during or close to the peak metamorphic stage. A definite sequence of fluid evolution is traceable for the subsequent stages. Thus, the coexisting CO2 and CO2H2O inclusions were entrapped at 510°C and 2.2 kbar, marking the waning of carbonic regime and the beginning of aqueous regime. At 330°C and 0.4 kbar, fluid unmixing occurred, leading to the simultaneous entrapment of mixed CO2H2O and H2O inclusions along rehealed microfractures. The data presented indicate that the metamorphic fluids evolved from early high-density carbonic through mixed carbonic-aqueous to late aqueous types. The dry granulite mineral assemblage of charnockites is a result of metamorphic equilibration under water-deficient and high-PCO2 conditions. 相似文献
13.
《Ore Geology Reviews》2009,35(4):580-596
The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO2-rich, H2O-rich and mixed H2O–CO2 fluid inclusions in the gold-bearing quartz veins. The H2O–CO2 inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ34S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO2. 相似文献
14.
High CO2 content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids? 总被引:4,自引:0,他引:4
Fluid inclusions were studied in samples from the Ashanti, Konongo-Southern Cross, Prestea, Abosso/Damang and Ayanfuri gold
deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralised veins of the Ashanti, Prestea, Konongo-Southern
Cross, and Abosso/Damang deposits contain almost exclusively volatile species. The primary setting of the gaseous (i.e. the
fluid components CO2, CH4 and N2) fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids. Microthermometric
and Raman spectroscopic analyses of the inclusions revealed a CO2 dominated fluid with variable contents of N2 and traces of CH4. Water content of most inclusions is below the detection limits of the respective methods used. Aqueous inclusions are rare
in all samples with the exception of those from the granite-hosted Ayanfuri mineralisation. Here inclusions associated with
the gold mineralisation contain a low salinity (<6 eq.wt.% NaCl) aqueous solution with variable quantities of CO2. Microthermometric investigations revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm3 at Ashanti, 0.85 to 0.98 g/cm3 at Prestea, up to 1.02 g/cm3 at Konongo-Southern Cross, and 0.8 to 1.0 g/cm3 at Abosso/Damang. The fluid inclusion data are used to outline the PT ranges of gold mineralisation of the respective gold deposits. The high density gaseous inclusions found in the auriferous
quartz at Ashanti and Prestea imply rather high pressure trapping conditions of up to 5.4 kbar. In contrast, mineralisation
at Ayanfuri and Abosso/Damang is inferred to have occurred at lower pressures of only up to 2.2 kbar. Mesothermal gold mineralisation
is generally regarded to have formed from fluids characterized by H2O > CO2 and low salinity ( ± 6 eq.wt.%NaCl). However, fluid inclusions in quartz from the gold mineralisations in the Ashanti belt
point to distinctly different fluid compositions. Specifically, the predominance of CO2 and CO2 >> H2O have to be emphasized. Fluid systems with this unique bulk composition were apparently active over more than 200␣km along
strike of the Ashanti belt. Fluids rich in CO2 may present a hitherto unrecognised new category of ore-forming fluids.
Received: 30 May 1996 / Accepted: 8 October 1996 相似文献
15.
Lukáš Ackerman Jiří Zachariáš Marta Pudilová 《International Journal of Earth Sciences》2007,96(4):623-638
Fluid inclusions, mineral thermometry and stable isotope data from two types of mineralogically and texturally contrasting
pegmatites, barren ones and lithium ones, from the Moldanubian Zone of the Bohemian Massif were studied in order to constrain
P–T conditions of their emplacement, subsolidus hydrothermal evolution and to estimate composition of the early exsolved fluid
and that of the parental melt. Despite the fact that the lithium pegmatites are abundant throughout the crystalline units
of the Bohemian Massif, data similar to this paper have not been published yet. The studied pegmatites are hosted by iron-rich
calcic skarn bodies. This specific setting allowed scavenging of calcium, fluorine and some other elements from the host rocks
into the pegmatitic melts and post-magmatic fluids. Such contamination process was important namely in the case of barren
pegmatites, as can be deduced from the variation in anorthite contents in plagioclase and from the presence of fluorite, hornblende
(with F content) or garnet in the contact zones of pegmatite dykes. Fluid inclusions were studied mostly in quartz, but also
in fluorite, titanite and apatite. Early aqueous–carbonic and late aqueous fluids were identified in both pegmatite types.
The P–T conditions of crystallization as well as the detailed composition of exsolved magmatic fluid, however, particularly differ.
The magmatic fluids associated with barren pegmatites correspond to H2O–CO2 low salinity fluids, composition of which evolved from 20 to 23 to <5 mol% CO2, and from 2 to 4–6 mol% NaCl eq. Sudden decrease in the CO2 content of the post-magmatic fluids (<5 mol% CO2) seems to coincide with the enrichment of the fluid in calcium (from the contamination process) and resulted in precipitation
of calcites (frequently found as trapped solid phases in fluid inclusions). The fluids associated with lithium pegmatites
are more complex (H2O–CO2/N2–H3BO3–NaCl). The CO2 content of early exsolved fluid is 26–20 mol% CO2 and remains the same in the next fluid generation. The main difference between the magmatic and the first post-magmatic fluids
is the presence of 7–9 wt% of H3BO3 (identified as daughter mineral sassolite) in the former. The second post-magmatic fluids are again CO2-poor (∼4 mol%) and more saline (∼4 mol% NaCl eq.). The composition of exsolved fluid was further used to constrain volatile
composition and content of the parental melts. Finally, P–T conditions of pegmatite crystallization are constrained: 600–640°C and 420–580 MPa for the barren pegmatites and 500–570°C
and 310–430 MPa for the lithium pegmatite. While the emplacement of the former occurred in thermal equilibrium with the Moldanubian
host rock environment, the emplacement of the later suggests substantial thermal disequilibrium. 相似文献
16.
P. A. Polito Y. Bone J. D. A. Clarke T. P. Mernagh 《Australian Journal of Earth Sciences》2013,60(6):833-855
The Junction gold deposit, in Western Australia, is an orogenic gold deposit hosted by a differentiated, iron‐rich, tholeiitic dolerite sill. Petrographic, microthermometric and laser Raman microprobe analyses of fluid inclusions from the Junction deposit indicate that three different vein systems formed at three distinct periods of geological time, and host four fluid‐inclusion populations with a wide range of compositions in the H2O–CO2–CH4–NaCl ± CaCl2 system. Pre‐shearing, pre‐gold, molybdenite‐bearing quartz veins host fluid inclusions that are characterised by relatively consistent phase ratios comprising H2O–CO2–CH4 ± halite. Microthermometry suggests that these veins precipitated when a highly saline, >340°C fluid mixed with a less saline ≥150°C fluid. The syn‐gold mineralisation event is hosted within the Junction shear zone and is associated with extensive quartz‐calcite ± albite ± chlorite ± pyrrhotite veining. Fluid‐inclusion analyses indicate that gold deposition occurred during the unmixing of a 400°C, moderately saline, H2O–CO2 ± CH4 fluid at pressures between 70 MPa and 440 MPa. Post‐gold quartz‐calcite‐biotite‐pyrrhotite veins occupy normal fault sets that slightly offset the Junction shear zone. Fluid inclusions in these veins are predominantly vapour rich, with CO2?CH4. Homogenisation temperatures indicate that the post‐gold quartz veins precipitated from a 310 ± 30°C fluid. Finally, late secondary fluid inclusions show that a <200°C, highly saline, H2O–CaCl2–NaCl–bearing fluid percolated along microfractures late in the deposit's history, but did not form any notable vein type. Raman spectroscopy supports the microthermometric data and reveals that CH4–bearing fluid inclusions occur in syn‐gold quartz grains found almost exclusively at the vein margin, whereas CO2–bearing fluid inclusions occur in quartz grains that are found toward the centre of the veins. The zonation of CO2:CH4 ratios, with respect to the location of fluid inclusions within the syn‐gold quartz veins, suggest that the CH4 did not travel as part of the auriferous fluid. Fluid unmixing and post‐entrapment alteration of the syn‐gold fluid inclusions are known to have occurred, but cannot adequately account for the relatively ordered zonation of CO2:CH4 ratios. Instead, the late introduction of a CH4–rich fluid into the Junction shear zone appears more likely. Alternatively, the process of CO2 reduction to CH4 is a viable and plausible explanation that fits the available data. The CH4–bearing fluid inclusions occur almost exclusively at the margin of the syn‐gold quartz veins within the zone of high‐grade gold mineralisation because this is where all the criteria needed to reduce CO2 to CH4 were satisfied in the Junction deposit. 相似文献
17.
The Samgwang mine is located in the Cheongyang gold district (Cheonan Metallogenic Province) of the Republic of Korea. It
consists of eight massive, gold-bearing quartz veins that filled NE- and NW-striking fractures along fault zones in Precambrian
granitic gneiss of the Gyeonggi massif. Their mineralogy and paragenesis allow two separate vein-forming episodes to be recognized,
temporally separated by a major faulting event. The ore minerals occur in quartz and calcite of stage I, associated with fracturing
and healing of veins. Hydrothermal wall-rock alteration minerals of stage I include Fe-rich chlorite (Fe/(Fe+Mg) ratios 0.74-0.81),
muscovite, illite, K-feldspar, and minor arsenopyrite, pyrite, and carbonates. Sulfide minerals deposited along with electrum
during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, argentite, pyrargyrite,
and argentian tetrahedrite. Only calcite was deposited during stage II. Fluid inclusions in quartz contain three main types
of C–O–H fluids: CO2-rich, CO2–H2O, and aqueous inclusions. Quartz veins related to early sulfides in stage I were deposited from H2O–NaCl–CO2 fluids (1,500–5,000 bar, average 3,200) with T
htotal values of 200°C to 383°C and salinities less than about 7 wt.% NaCl equiv. Late sulfide deposition was related to H2O–NaCl fluids (140–1,300 bar, average 700) with T
htotal values of 110°C to 385°C and salinities less than about 11 wt.% NaCl equiv. These fluids either evolved through immiscibility
of H2O–NaCl–CO2 fluids as a result of a decrease in fluid pressure, or through mixing with deeply circulated meteoric waters as a result
of uplift or unloading during mineralization, or both. Measured and calculated sulfur isotope compositions (δ34SH2S = 1.5 to 4.8‰) of hydrothermal fluids from the stage I quartz veins indicate that ore sulfur was derived mainly from a magmatic
source. The calculated and measured oxygen and hydrogen isotope compositions (δ18OH2O = −5.9‰ to 10.9‰, δD = −102‰ to −87‰) of the ore-forming fluids indicate that the fluids were derived from magmatic sources
and evolved by mixing with local meteoric water by limited water–rock exchange and by partly degassing in uplift zones during
mineralization. While most features of the Samgwang mine are consistent with classification as an orogenic gold deposit, isotopic
and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the Jurassic to Cretaceous
Daebo orogeny. 相似文献
18.
The Olympias Pb-Zn(Au, Ag) sulfide ore deposit, E. Chalkidiki, N. Greece, is hosted by marbles of the polymetamorphic Kerdilia
Formation of Paleozoic or older age. The geologic environment of the ore also comprises biotite-hornblende gneisses and amphibolites
intruded by Tertiary pegmatite-aplite dikes, lamprophyre dikes, the 30-Ma Stratoni granodiorite, and porphyritic stocks. Only
limited parts of the deposit display shear folding and brecciation; most of it is undeformed. Microthermometry of fluid inclusions
in gangue syn-ore quartz indicates three types of primary and pseudosecondary inclusions: (1) H2O-rich, 1–18 wt.% NaCl equivalent, <3.6 mol% CO2; (2) H2O-CO2 inclusions, <4wt.% NaCl equivalent, with variable CO2 contents, coexisting in both undeformed and deformed ore; (3) aqueous, highsalinity (28–32 wt,% NaCl equivalent) inclusions
found only in undeformed ore. Type 2 inclusions are differentiated into two sub-types: (2a) relatively constant CO2 content in the narrow range of 8–15 mol% and homogenization to the liquid phase; (2b) variable CO2 content between 18 and 50 mol% and homogenization to the vapor phase. Type 1 and 2b inclusions are consistent with trapping
of two fluids by unmixing of a high-temperature, saline, aqueous, CO2-bearing fluid of possible magmatic origin, probably trapped in type 2a inclusions. Fluid unmixing and concomitant ore mineralization
took place at temperatures of 350 ± 30 °C and fluctuating pressures of less than 500 bar, for both undeformed and deformed
ores. The wide salinity range of type 1 inclusions probably represents a complex effect of salinity increase, due to fluid
unmixing and volatile loss, and dilution, due to mixing with low-salinity meteoric waters. High solute enrichment of the residual
liquid, due to extreme volatile loss during unmixing, may account for high salinity type 3 inclusions. The Olympias fluid
inclusion salinity-temperature gradients bear similarities to analogous gradients related to Pb-Zn ores formed in “granite”-hosted,
low-T distalskarn, skarn-free carbonate-replacement and epithermal environments. 相似文献
19.
Prosper Andrianjakavah Stefano Salvi Didier Béziat Damien Guillaume Michel Rakotondrazafy Bernard Moine 《Mineralium Deposita》2007,42(4):385-398
The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed
2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites
and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite
is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated
by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually
<500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm
Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.%
eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes
variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same
area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO2 but shows that it may contain small amounts of CH4 (<1 mol%), H2S (<0.05 mol%) and traces of N2. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar.
The abundant CO2 and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H2S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility
was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences
with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing
fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation
as H2S partitioned preferentially into the carbonic phase. 相似文献
20.
Evidence for fluid phase separation in high-grade ore zones at the Porgera gold deposit, Papua New Guinea 总被引:1,自引:0,他引:1
Coexisting, liquid-rich and vapor-rich primary fluid inclusions in quartz provide direct evidence for fluid phase separation
in high-grade quartz–roscoelite–gold veins and breccias from the Porgera alkalic-type gold deposit. Vapor-rich fluid inclusions
are CO2-rich, and sometimes contain liquid CO2 at room temperature. The close spatial and paragenetic relationship between these “boiling assemblage” fluid inclusions and
gold suggests that gold was precipitated by phase separation, at least locally. Additionally, the occurrence of carbonate
and sulfate minerals in high-grade veins (reflecting pH increase and oxidation of the boiled fluid) and the appearance of
hydrothermal breccias, are consistent with the process of fluid phase separation. Liquid CO2-bearing fluid inclusions are rare in near-surface epithermal deposits, and indicate that the Porgera vein system was formed
at greater depths and pressures (our estimates suggest pressures between 250 and 340 bars). It is suggested that alkalic-type
gold deposits may be distinguished from other epithermal deposit types by the more gaseous nature of the ore-forming fluids,
in addition to their association with alkalic magmas.
Received: 24 February 2000 / Accepted: 6 April 2000 相似文献