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1.
Evidence for a magmatic origin for Carlin-type gold deposits: isotopic composition of sulfur in the Betze-Post-Screamer Deposit, Nevada, USA 总被引:6,自引:0,他引:6
We report here new sulfur isotope analyses from the Betze-Post-Screamer deposit, the largest Carlin-type gold deposit in the
world. Carlin-type deposits contain high concentrations of arsenic, antimony, mercury, tellurium and other elements of environmental
interest, and are surrounded by large volumes of crust in which these elements are also enriched. Uncertainty about the source
of sulfur and metals in and around Carlin-type deposits has hampered formulation of models for their origin, which are needed
for improved mineral exploration and environmental assessment. Previous studies have concluded that most Carlin-type deposits
formed from sulfide sulfur that is largely of sedimentary origin. Most of these studies are based on analyses of mineral separates
consisting of pre-ore diagenetic pyrite with thin overgrowths of ore-related arsenian pyrite rather than pure, ore-related
pyrite. Our SIMS spot analyses of ore-related pyrite overgrowths in the Screamer zone of the Betze-Post-Screamer deposit yield
δ34S values of about −1 to 4‰ with one value of about 7‰. Conventional analyses of realgar and orpiment separates from throughout
the deposit yield δ34S values of about 5–7‰ with one value of 10‰ in the Screamer zone. These results, along with results from an earlier SIMS
study in the Post zone of the deposit and phase equilibrium constraints, indicate that early arsenian pyrite were formed from
fluids of magmatic origin with variable contamination from sulfur in Paleozoic sedimentary rocks. Later arsenic sulfides were
formed from solutions to which sulfur of sedimentary origin had been added. The presence of Paleozoic sedimentary sulfur in
Carlin-type deposits does not require direct involvement of hydrothermal solutions of sedimentary origin. Instead, it could
have been added by magmatic assimilation of Paleozoic sedimentary rocks or by hydrothermal leaching of sulfur from wall rocks
to the deposit. Thus, the dominant process delivering sulfur, arsenic, gold and mineralizing fluids to Carlin-type systems
and their surrounding country rocks was probably separation of fluids from a magmatic source.
Editorial handling: G. Beaudoin 相似文献
2.
Miriela María Ulloa Santana Márcia Abrahão Moura Gema R. Olivo Nilson Francisquini Botelho T. Kurtis Kyser Bernhard Bühn 《Mineralium Deposita》2011,46(1):91-104
The Camagüey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently,
the La Unión prospect was discovered in the southern part of the district, containing gold and minor copper mineralization
interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite
intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guáimaro Formation. The porphyry is affected by
propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz,
and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously
replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring
as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion
and oxygen isotope data are consistent with a H2O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary
fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling
may have played an important role in metal precipitation. Pyrite δ34S values for the La Unión prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The
fluids in equilibrium with the mineralized rocks have estimated δ18O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Unión prospect,
its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature
of the alteration minerals and fluids indicate that the La Unión gold mineralization is similar to the porphyry gold type,
even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or
phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper,
which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis,
represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold
porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba. 相似文献
3.
Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera.
The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural
evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole
by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole.
While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of
hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich
actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular
Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures.
The δD values of hydrous silicates vary from −63 to −105‰. Most δ18O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ18O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central
Andean Volcanic Zone (δ18O = 7.0–7.4‰). Only one sample in this study (δ18O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary
minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El
Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric
fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile.
Received: 8 July 1998 / Accepted: 15 April 1999 相似文献
4.
At Sams Creek, a gold-bearing, peralkaline granite porphyry dyke, which has a 7 km strike length and is up to 60 m in thickness, intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the Late Ordovician Wangapeka formation. The lamprophyre dykes occur as thin (< 3 m) slivers along the contacts of the granite dyke. δ18Omagma values (+5 to +8‰, VSMOW) of the A-type granite suggest derivation from a primitive source, with an insignificant mature crustal contribution. Hydrothermal gold–sulphide mineralisation is confined to the granite and adjacent lamprophyre; metapelite country rocks have only weak hydrothermal alteration. Three stages of hydrothermal alteration have been identified in the granite: Stage I alteration (high fO2) consisting of magnetite–siderite±biotite; Stage II consisting of thin quartz–pyrite veinlets; and Stage III (low fO2) consisting of sulphides, quartz and siderite veins, and pervasive silicification. The lamprophyre is altered to an ankerite–chlorite–sericite assemblage. Stage III sulphide veins are composed of arsenopyrite + pyrite ± galena ± sphalerite ± gold ± chalcopyrite ± pyrrhotite ± rutile ± graphite. Three phases of deformation have affected the area, and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding, the youngest of which is Early Cretaceous. Locally preserved early-formed fluid inclusions are either carbonic, showing two- or three-phases at room temperature (liquid CO2-CH4 + liquid H2O ± CO2 vapour) or two-phase liquid-rich aqueous inclusions, some of which contain clathrates. Salinities of the aqueous inclusions are in the range of 1.4 to 7.6 wt% NaCl equiv. Final homogenisation temperatures (Th) of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355°C, which are not too different from vein formation temperatures of 340–380°C estimated from quartz–albite stable isotope thermometry. δ18O values of Stage II and III vein quartz range from +12 and +17‰ and have a bimodal distribution (+14.5 and +16‰) with Stage II vein quartz accounting for the lower values. Siderite in Stage III veins have δ18O (+12 to +16‰) and δ13C values (−5‰, relative to VPDB), unlike those from Wangapeka Formation metasediments (δ13Cbulk carbon values of −24 to −19‰) and underlying Arthur Marble marine carbonates (δ18O = +25‰ and δ13C = 0‰). Calculated δ18Owater (+8 to +11‰, at 340°C) and
(−5‰) values from vein quartz and siderite are consistent with a magmatic hydrothermal source, but a metamorphic hydrothermal origin cannot be excluded. δ34S values of sulphides range from +5 to +10‰ (relative to CDT) and also have a bimodal distribution (modes at +6 and +9‰, correlated with Stage II and Stage III mineralisation, respectively). The δ34S values of pyrite from the Arthur Marble marine carbonates (range from +3 to +13‰) and Wangapeka Formation (range from −4 to +9.5‰) indicate that they are potential sources of sulphur for sulphides in the Sams Creek veins. Another possible source of the sulphur is the lithospheric mantle which has positive values up to +14‰. Ages of the granite, lamprophyre, alteration/mineralisation, and deformation in the region are not well constrained, which makes it difficult to identify sources of mineralisation with respect to timing. Our mineralogical and stable isotope data does not exclude a metamorphic source, but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source. Assuming that the hydrothermal fluids were sourced from crystallisation of the Sams Creek granite or an underlying magma chamber, then the Sams Creek gold deposit appears to be a hybrid between those described as reduced granite Au–Bi deposits and alkaline intrusive-hosted Au–Mo–Cu deposits. 相似文献
5.
Mineralogy and geochemistry of El Dorado epithermal gold deposit, El Sauce district, central-northern Chile 总被引:1,自引:0,他引:1
J. Carrillo-Rosúa S. Morales-Ruano D. Morata A. J. Boyce M. Belmar A. E. Fallick P. Fenoll Hach-Alí 《Mineralogy and Petrology》2008,92(3-4):341-360
Summary The El Dorado Au-Cu deposit is located in an extensive intra-caldera zone of hydrothermal alteration affecting Upper Cretaceous
andesites of the Los Elquinos Formation at La Serena (≈ 29°47′S Lat., 70°43′W Long., Chile). Quartz-sulfide veins of economic potential are hosted by N25W and N20E
fault structures associated with quartz-illite alteration (+supergene kaolinite). The main ore minerals in the deposit are
pyrite, chalcopyrite ± fahlore (As/(As + Sb): 0.06−0.98), with electrum, sphalerite, galena, bournonite-seligmanite (As/(As
+ Sb): 0.21−0.31), marcasite, pyrrhotite being accessory phases. Electrum, with an Ag content between 32 and 37 at.%, occurs
interstitial to pyrite aggregates or along pyrite fractures. Pyrite commonly exhibits chemical zonation with some zones up
to 1.96 at.% As. Electron probe microanalyses of pyrite indicate that As-rich zones do not exhibit detectable Au values. Fluid
inclusion microthermometry shows homogenization temperatures between 130 and 352 °C and salinities between 1.6 and 6.9 wt.%
NaCl eq. Isotope data for quartz, ankerite and phyllosilicates and estimated temperatures show that δ18O and δD for the hydrothermal fluids were between 3 and 10‰ and between −95 and −75‰, respectively. These results suggest
the mineralizing fluids were a mixture of meteoric and magmatic waters. An epithermal intermediate-sulfidation model is proposed
for the formation of the El Dorado deposit.
Author’s present address: J. Carrillo-Rosúa, Dpto. de Didáctica de las Ciencias Experimentales, Universidad de Granada, Campus de Cartuja, 18071, Granada, Spain 相似文献
6.
Numerous stable isotope studies of whole rocks and mineral separates in epithermal systems indi-cate that even though meteoric waters are dominant components in epithermal systems ,fluids of other origins,such as sedimentary or meta-sedimentary fluids,magmatic waters and even evolved meteoric waters ,may also play a role in the formation of epithermal ore deposits.Usually the more depleted the wall rocks,the larger the size of ore deposits ,and the least depletion degrees in whole rocks for economic mineralization are by about 3.5‰.The depletion in δ^18O in wall rocks,however,may be complicated by the superimposition of low temperature-hydration over high-temperature altera-tion or vice versa,the existence of primary low-^18O and high-^18O magmas ,and alteration by vol-canic gases.The depletion in δ^18O in wall rocks is controlled by the composition and nature of flu-ids,the temperature of fluids,the elevation of rocks at the time of alteration ,lithology,boiling effects of fluids ,and alteration style,as well as by water/rock ratios.In addition ,the fluids re-sponsible for epithermal deposits have experienced positive δ^18O shifts .It seems that when the above complications and controlling factors are well defined,oxygen isotope studies would be a promising and powerful exploration tool. 相似文献
7.
Robert R. Seal II Robert A. Ayuso Nora K. Foley Sandra H. B. Clark 《Mineralium Deposita》2001,36(2):137-148
The Barite Hill gold deposit, at the southwestern end of the Carolina slate belt in the southeastern United States, is one
of four gold deposits in the region that have a combined yield of 110 metric tons of gold over the past 10 years. At Barite
Hill, production has dominantly come from oxidized ores. Sulfur isotope data from hypogene portions of the Barite Hill gold
deposit vary systematically with pyrite–barite associations and provide insights into both the pre-metamorphic Late Proterozoic
hydrothermal and the Paleozoic regional metamorphic histories of the deposit. The δ34S values of massive barite cluster tightly between 25.0 and 28.0‰, which closely match the published values for Late Proterozoic
seawater and thus support a seafloor hydrothermal origin. The δ34S values of massive sulfide range from 1.0 to 5.3‰ and fall within the range of values observed for modern and ancient seafloor
hydrothermal sulfide deposits. In contrast, δ34S values for finer-grained, intergrown pyrite (5.1–6.8‰) and barite (21.0–23.9‰) are higher and lower than their massive counterparts,
respectively. Calculated sulfur isotope temperatures for the latter barite–pyrite pairs (Δ=15.9–17.1‰) range from 332–355 °C
and probably reflect post-depositional equilibration at greenschist-facies regional metamorphic conditions. Thus, pyrite and
barite occurring separately from one another provide pre-metamorphic information about the hydrothermal origin of the deposit,
whereas pyrite and barite occurring together equilibrated to record the metamorphic conditions. Preliminary fluid inclusion
data from sphalerite are consistent with a modified seawater source for the mineralizing fluids, but data from quartz and
barite may reflect later metamorphic and (or) more recent meteoric water input. Lead isotope values from pyrites range for
206Pb/204Pb from 18.005–18.294, for 207Pb/204Pb from 15.567–15.645, and for 208Pb/204Pb from 37.555–38.015. The data indicate derivation of the ore leads from the country rocks, which themselves show evidence
for contributions from relatively unradiogenic, mantle-like lead, and more evolved or crustal lead. Geological relationships,
and stable and radiogenic isotopic data, suggest that the Barite Hill gold deposit formed on the Late Proterozoic seafloor
through exhalative hydrothermal processes similar to those that were responsible for the massive sulfide deposits of the Kuroko
district, Japan. On the basis of similarities with other gold-rich massive sulfide deposits and modern seafloor hydrothermal
systems, the gold at Barite Hill was probably introduced as an integral part of the formation of the massive sulfide deposit.
Received: 17 August 1998 / Accepted: 12 October 2000 相似文献
8.
9.
E. Marcoux 《Mineralium Deposita》1997,33(1-2):45-58
Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (206Pb/204Pb), 15.622 (207Pb/204Pb) and 38.191 (208Pb/204Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (206Pb/204Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt. 相似文献
10.
The strata-bound Cu−Pb−Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist phase of the middle-upper
Proterozoic Langshan Group in central Inner Mongolia. δ34S values for sulfides range from −3.1‰ to +37.3‰, and an apparent difference is noticed between vein sulfides and those in
bedded rocks. For example, δ34S values for bedded pyrite range from +10.6‰ to +20.0‰, while those for vein pyrite vary from −3.1‰ to +14.1‰. δ34S of bedded pyrrhotite is in the range +7.9‰–+23.5‰ in comparison with +6.5‰–+17.1‰ for vein pyrrhotite. The wide scatter
of δ34S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H2S as a result of bacterial reduction of sulfates in the sea water. Sulfur isotopic composition also differs from deposit to
deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides
in response to regional metamorphism and magmatic intrusion led to the formation of vein sulfides.
δ18O and δ13C of ore-bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of lower
values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks.
Pb isotopic composition is relatively stable and characterized by lower radio-genetic lead. The age of basement rocks was
calculated to be about 23.9 Ma and ore-forming age 7.8 Ma.207Pb/204Pb−206Pb/204Pb and208Pb/204Pb−206Pb/204Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle.
It is believed that the deposits in this region are related to submarine volcanic exhalation superimposed by later regional
metamorphism and magmatic intrusion. 相似文献
11.
Peter Neumayr John Walshe Steffen Hagemann Klaus Petersen Anthony Roache Peter Frikken Leo Horn Scott Halley 《Mineralium Deposita》2008,43(3):363-371
Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia
indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced
alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits)
and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor
in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized
deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused
on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface.
Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite
hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and
temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly
synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different
mineral assemblages reflect differences in redox conditions. The δ
34S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized
magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial
and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are
highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite
to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred
where fluids of contrasting redox state mixed. 相似文献
12.
The Iberian Pyrite Belt, located in the SW Iberian Peninsula, contains many Paleozoic giant and supergiant massive sulphide
deposits, including the largest individual massive sulphide bodies on Earth. Total ore reserves exceed 1500 Mt, distributed
in eight supergiant deposits (>100 Mt) and a number of other smaller deposits, commonly with associated stockwork mineralizations
and footwall alteration haloes. Massive sulphide bodies largely consist of pyrite, with subordinated sphalerite, galena and
chalcopyrite and many other minor phases, although substantial differences occur between individual deposits, both in mineral
abundance and spatial distribution. These deposits are considered to be volcanogenic, roughly similar to volcanic-hosted massive
sulphides (VHMS). However, our major conclusion is that the Iberian type of massive sulphides must be considered as a VHMS
sub-type transitional to SHMS.
This work is an assessment of the geological, geochemical and metallogenic data available up to date, including a number of
new results. The following points are stressed; (a) ore deposits are located in three main geological sectors, with the southern
one containing most of the giant and supergiant orebodies, whereas the northern one has mainly small to intermediate-sized
deposits; (b) ore deposits differ one from another both in textures and mineral composition; (c) Co and Bi minerals are typical,
especially in stockwork zones; (d) colloidal and other primary depositional textures are common in many localities; (e) a
close relation has been found between ore deposits and some characteristic sedimentary horizons, such as black shales. In
contrast, relationships between massive sulphides and cherts or jaspers remains unclear; (f) footwall hydrothermal alterations
show a rough zoning, the inner alteration haloes being characterized in places by a high Co/Ni ratio, as well as by mobility
of Zr, Y and REE; (g) 18O and D values indicate that fluids consist of modified seawater, whereas 34S data strongly suggest the participation of bacterial-reduced sulphur, at least during some stages of the massive sulphide
genesis, and (h) lead isotopes suggest a single (or homogeneized) metal source, from both the volcanic piles and the underlying
Devonian rocks (PQ Group).
It is concluded that, although all these features can be compatible with classical VHMS interpretations, it is necessary to
sketch a different model to account for the IPB characteristics. A new proposal is presented, based on an alternative association
between massive sulphide deposits and volcanism. We consider that most of the IPB massive orebodies, in particular the giant
and supergiant ones, were formed during pauses in volcanic activity, when hydrothermal activity was triggered by the ascent
and emplacement of late basic magmas. In these conditions, deposits formed which had magmatic activity as the heat source;
however, the depositional environment was not strictly volcanogenic, and many evolutionary stages could have occurred in conditions
similar to those in sediment-hosted massive sulphides (SHMS). In addition, the greater thickness of the rock pile affected
by hydrothermal circulation would account for the enormous size of many of the deposits.
Received: 8 September 1998 / Accepted: 4 January 1999 相似文献
13.
Ana M. Dreher Roberto P. Xavier Bruce E. Taylor Sérgio L. Martini 《Mineralium Deposita》2008,43(2):161-184
The Igarapé Bahia Cu–Au deposit in the Carajás Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks
of the Igarapé Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiúnas Supergroup,
in which other important Cu–Au and iron ore deposits of the Carajás region are also hosted. The orebody at Igarapé Bahia is
a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and
chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly
concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of
coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix
and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals,
and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite,
amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarapé Bahia
is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped
in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl2 equiv), together with carbonic (CO2 ± CH4) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from
the fragmental layer have δ13C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ34S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic
or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic
ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ18O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction
with 18O-rich, probably sedimentary rocks. The late veins of the Igarapé Bahia deposit area were formed from saline aqueous fluids
(2 to 60 wt% NaCl + CaCl2 equiv) with δ18Ofluid compositions around 0 per mil that indicate contribution from meteoric fluids. With respect to geological features, Igarapé
Bahia bears similarity with syngenetic, volcanic-hosted massive sulfide (VHMS)-type deposits, as indicated by the volcano-sedimentary
geological context, stratabound character, and association with submarine volcanic flows, hyaloclastite, and exhalative beds
such as BIF and chert. On the other hand, the highly saline ore fluids and the mineral assemblage, dominated by magnetite
and chalcopyrite, with associated gold, U- and LREE-minerals and scarce pyrite, indicate that Igarapé Bahia belongs to the
Fe oxide Cu–Au (IOCG) group of deposits. The available geochronologic data used to attest syngenetic or epigenetic origins
for the mineralization are either imprecise or may not represent the main mineralization episode but a later, superimposed
event. The C, S, and O isotopic results obtained in this study do not clearly discriminate between fluid sources. However,
recent B isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit (Xavier, Wiedenbeck, Dreher,
Rhede, Monteiro, Araújo, Chemical and boron isotopic composition of tourmaline from Archean and Paleoproterozoic Cu–Au deposits in the Carajás Mineral
Province, 1° Simpósio Brasileiro de Metalogenia, Gramado, Brazil, extended abstracts, CD-ROM, 2005) provide strong evidence of the
involvement of a marine evaporitic source in the hydrothermal system of Igarapé Bahia. Evaporite-derived fluids may explain
the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit. Evaporite-derived fluids also
exclude a significant participation of magmatic or mantle-derived fluids, reinforcing the role of nonmagmatic brines in the
genesis of Igarapé Bahia. Considering this aspect and the geological features, the possibility that the deposit was generated
by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated. 相似文献
14.
The Arroyo Rojo deposit, located in Tierra del Fuego, is the most important polymetallic, volcanic-hosted massive sulphide in the rhyolitic belt of the Fuegian Andes. The best intercepts in drill holes indicate a true thickness of 18.6 m and concentrations of 2.2% Cu, 3.9% Pb, 14.5% Zn, 140 g/t Ag, 1.1 g/t Au). This deposit, located near the town of Ushuaia, is hosted in a Middle Jurassic volcanic and volcanoclastic sequence. Massive and semimassive bodies display stacked lenticular morphologies with disseminated mineralization in both the footwall and hanging wall. The associated hydrothermal alteration system is partially conformable with the layering of the volcanic rocks. The ores and host rocks display a penetrative tectonic foliation and were metamorphosed to greenschist facies.Previous studies have not resulted in a consensus regarding the nature and the source of ore-forming fluids and the style of deposition of the sulphides at Arroyo Rojo. In this study, both stable and radiogenic isotopes were used develop a better understanding of these aspects of the deposit.Hydrogen and oxygen isotopes indicate that an evolved seawater mixed with significant contributions from other fluid reservoirs such as magmatic and/or metamorphic waters was the most likely source of the ore-forming fluids. These fluids underwent significant interaction with the underlying volcanic and sedimentary rocks, which promoted partial (Sr isotopes) or full (Pb isotopes) homogenization of radiogenic isotopes. δ34SCDT values suggest that the sulphur was derived from several sources: biogenic reduction of seawater sulphate (BSR) in a restricted to closed basin was mixed with a heavier component derived from inorganic reduction of seawater sulphate (TRS) and possibly from sulphur leached from igneous footwall rocks and/or direct contribution from magmatic fluids.Lateral infiltration of hydrothermal fluids resulted in the formation of a halo of semimassive to disseminated ore due to the replacement of porous, reactive glassy and breccia tuffs.As a result of the hydrothermal circulation, two styles of mineralization are observed in the Arroyo Rojo deposit: a stringer zone and a halo of semimassive to disseminated ore corresponding to sub-seafloor replacement, and syn-sedimentary mineralization consisting of massive sulphides.This model is consistent with the geodynamic context of the study area: a narrow, deep-marine volcano-tectonic rift parallel to the Andean side of South America and related to the initial break-up of Gondwana (ca. 145 Ma). 相似文献
15.
The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include
Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenite-hosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens
and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped
into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed
by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively
(events II–IV). The δ18O-δD and δ34S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event I fluids are of mixed
origin. They are metamorphic fluids (H2O-CO2-CH4-NaCl; δ18O=4.7 to 9.3‰; δD ab.−34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H2O-NaCl, 4.5–6.4 wt% NaCl eq.; δ18O=7.3–7.8‰; δD=−77 to −74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H2O-NaCl, <8 wt% NaCl eq.; δ18O=−0.4 to 3.4‰; δD=−75 to −58‰). They were probably formed by local hydrothermal cells that were spatially and temporally
related to the youngest Variscan granites, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor
influence of magmatic fluids confirms that the intrusion of these granites was essentially water-undersaturated, as most of
the hydrothermal fluids were external to the igneous rocks. The fluids involved in the younger hydrothermal systems (events
II–III) are very similar. The waters involved in the formation of episyenites, chlorite-rich greisens, retrograde skarns and
phyllic and chlorite-rich alterations in the shear zones show no major chemical or isotopic differences. Interaction of the
hydrothermal fluids with the host rocks was the main mechanism of ore formation. The composition (H2O-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and the δ18O (−4.6 to 6.3‰) and δD (−51 to −40‰) values are consistent with a meteoric origin, with a δ18O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective
cells and were then channelled along major crustal discontinuities. In these shear zones the more easily altered minerals
such as feldspars, actinolite and chlorite had their δ18O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower
portions of the hydrothermal systems, basement-cover fluorite-barite-base metal veins formed by mixing of these deep fluids
with downwards percolating brines. These brines are also interpreted as of meteoric origin (δ18O< ≈ −4‰; δD=−65 to −36‰) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hosted in the post-Variscan
sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper
Paleozoic meteoric waters in central Iberia had very negative δD values (≤−52‰) whereas those of Early Mesozoic age ranged
between −65 and −36‰.
Received: 9 June 1999 / Accepted: 19 January 2000 相似文献
16.
Whole rock oxygen isotope data are presented for the Panorama district, in the Archean Pilbara Craton of Western Australia,
where near-perfect exposure reveals a cross section through a complete volcanogenic massive sulfide (VMS) hydrothermal alteration
system. The δ18O values decrease with depth in the volcanic pile, across semi-conformable alteration zones, to values below 6‰ immediately
above a large (180 km2) subvolcanic intrusion. Altered rocks in the upper parts of the subvolcanic intrusion have lower δ18O values (6–8‰) than least altered granite (8‰), apart from sericite–quartz altered zones, which are slightly higher (8–10‰).
Corridors of low δ18O values crosscut this regional zonation, and are coincident with transgressive feldspar-destructive alteration zones, which
underlie VMS mineralization. The whole rock oxygen isotope distribution patterns are interpreted to represent alteration temperature,
where high δ18O values correspond to low temperature alteration and low δ18O values correspond to high temperature alteration. Alteration temperatures, which were calculated using modal alteration
mineral abundances and an assumed fluid δ18O, are consistent with this interpretation. Increasing temperatures with depth in the volcanic pile and high temperatures
in transgressive corridors leading up to VMS deposits, are consistent with a convective hydrothermal model, in which heat
from the subvolcanic intrusion drove seawater through the volcanic pile. Granite-hosted sericite–quartz alteration zones are
18O-enriched, and are tentatively interpreted to have formed from a mixed magmatic-evolved seawater fluid.
Received: 12 April 1999 / Accepted: 6 April 2000 相似文献
17.
Thomas Wagner Martin Okrusch Stefan Weyer Joachim Lorenz Yann Lahaye Heiner Taubald Ralf T. Schmitt 《Mineralium Deposita》2010,45(3):217-239
The Spessart district (SW Germany), located at the southwestern margin of the Permian Kupferschiefer basin in Central Europe,
hosts abundant stratabound and structurally controlled base metal mineralization. The mineralization styles identified are
(1) stratabound Cu-Pb-Zn-(Ag) ores in Zechstein sedimentary rocks, (2) structurally controlled Cu-As-(Ag) ores in Zechstein
sedimentary rocks, (3) crosscutting Co-Ni-(Bi)-As and Cu-Fe-As veins, (4) stratabound metasomatic Fe-Mn carbonate ores in
Zechstein dolomite, (5) barren barite veins, and (6) Fe-Mn-As veins in Permian rhyolites. Building on previous work that involved
mineralogical, textural, and chemical characterization of the major mineralization types, we have performed a comprehensive
sulfur isotope study that applied both conventional and novel laser-ablation multi-collector inductively coupled plasma mass
spectrometry techniques. The δ34S values of sulfide minerals from the different ore types are consistently negative and highly variable, in the range between
−44.5‰ and −3.9‰, whereas the δ34S values of barite are all positive in the range between 4.7‰ and 18.9‰. Remarkably, stratabound and structurally controlled
mineralization in Zechstein sedimentary rocks has the least negative δ34S values, whereas vein-type deposits have consistently more negative δ34S values. The observed pattern of sulfide δ34S values can be best interpreted in terms of fluid mixing at the basement-cover interface. Hydrothermal fluids originating
from the crystalline basement migrated upward along subvertical fault zones and were periodically injected into groundwaters
that were flowing in the post-Variscan sedimentary cover. These groundwaters had interacted with the Zechstein sedimentary
rocks, resulting in fluids characterized by elevated concentrations of reduced sulfur (with negative δ34S values) and alkaline pH. Repeated mixing between both chemically contrasting fluids caused rapid and efficient precipitation
of sulfide ore minerals in hydrothermal veins with highly variable but distinctly negative δ34S values. 相似文献
18.
Hypogene Zn carbonate ores in the Angouran deposit,NW Iran 总被引:1,自引:0,他引:1
Maria Boni H. Albert Gilg Giuseppina Balassone Jens Schneider Cameron R. Allen Farid Moore 《Mineralium Deposita》2007,42(8):799-820
The world-class Angouran nonsulfide Zn–Pb deposit is one of the major Zn producers in Iran, with resources estimated at about
18 Mt at 28% Zn, mainly in the form of the Zn carbonate smithsonite. This study aims to characterize these carbonate ores
by means of their mineralogy and geochemistry, which has also been extended to the host rocks of mineralization and other
local carbonate rock types, including the prominent travertines in the Angouran district, as well as to the local spring waters.
Petrographical, chemical, and stable isotope (O, H, C, Sr) data indicate that the genesis of the Zn carbonate ores at Angouran
is fairly distinct from that of other “classical” nonsulfide Zn deposits that formed entirely by supergene processes. Mineralization
occurred during two successive stages, with the zinc being derived from a preexisting sulfide ore body. A first, main stage
of Zn carbonates (stage I carbonate ore) is associated with both preexisting and subordinate newly formed sulfides, whereas
a second stage is characterized by supergene carbonates (Zn and minor Pb) coexisting with oxides and hydroxides (stage II
carbonate ore). The coprecipitation of smithsonite with galena, pyrite and arsenopyrite, as well as the absence of Fe- and
Mn-oxides/hydroxides and of any discernible oxidation or dissolution of the sphalerite-rich primary sulfide ore, shows that
the fluids responsible for the main, stage I carbonate ores were relatively reduced and close to neutral to slightly basic
pH with high fCO2. Smithsonite δ18OVSMOW values from stage I carbonate ore range from 18.3 to 23.6‰, while those of stage II carbonate ore show a much smaller range
between 24.3 and 24.9‰. The δ13C values are fairly constant in smithsonite of stage I carbonate ore (3.2–6.0‰) but show a considerable spread towards lower
δ13CVPDB values (4.6 to −11.2‰) in stage II carbonate ore. This suggests a hypogene formation of stage I carbonate ore at Angouran
from low-temperature hydrothermal fluids, probably mobilized during the waning stages of Tertiary–Quaternary volcanic activity
in an environment characterized by abundant travertine systems throughout the whole region. Conversely, stage II carbonate
ore is unambiguously related to supergene weathering, as evidenced by the absence of sulfides, the presence of Fe-Mn-oxides
and arsenates, and by high δ18O values found in smithsonite II. The variable, but still relatively heavy carbon isotope values of supergene smithsonite
II, suggests only a very minor contribution by organic soil carbon, as is generally the case in supergene nonsulfide deposits. 相似文献
19.
Lauren C. Duarte Leo A. Hartmann Luiz H. Ronchi Zsolt Berner Thomas Theye Hans J. Massonne 《Mineralium Deposita》2011,46(3):239-255
Stable isotopes (C, O, S) and mineralogical studies of the world-class amethyst-geode deposits of the Los Catalanes gemological
district, Uruguay, constrain processes operative during mineral deposition. The mineralized basaltic andesites from the Cretaceous
Paraná volcanic province are intensely altered to zeolites (clinoptilolite) and clay minerals. Variations in the δ18O values of silica minerals in geodes (chalcedony, quartz, and amethyst) are much larger and the values generally somewhat
lower (21.2–31.5‰) in the Uruguayan deposits than in the Ametista do Sul area of southern Brazil. The range of δ34S values (−15.0 to −0.3‰) of altered basaltic rocks requires (in addition to sulfur of magmatic origin) the involvement of
34S-depleted sedimentary sulfur from bacterial sulfate reduction. The results delimit the mineralizing processes to a post-eruption
environment characterized by low temperature and strong interaction of the lava flows with meteoric water. 相似文献
20.
Jan-Marten Huizenga Jens Gutzmer David Banks Lynnette Greyling 《Mineralium Deposita》2006,40(6-7):686-706
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. 相似文献