Quartz-chlorite Veins Research Articles

Диарсениды и сульфоарсениды Fe, Co, Ni, пространственно связанные с Лево-Сеймканским гранитным плутоном (Магаданская область, Россия)

The Ni-Co mineralization in the Levo-Seimkan ore cluster has been studied. In xenoliths of gabbro-diorites from of the Levo-Seimkan granite massif, the disseminated sulfoarsenide and sulfoantimonide mineralization with predominant antimonial nickeline and with the gersdorfitekrutovite series minerals has been revealed, with a constant Sb admixture in gersdorfite (from 1.1 to 2.4 wt. %) and nickeline (Sb 3.2-12.2 wt. %) in association with pyrrhotite. In a new ore occurrence (Puglivy Creek, Arman River basin, 2 km from the mouth), in a stockwork of quartz-chlorite veins localized directly in the granites of the Levo-Seymkan massif, the Co-As-Bi-Te-Au mineralization has been established. Ore mineral nests and inclusions are represented by glaucodote (Ni under 3.28 wt. %), associated with native Bi, bismuthine, tellurovismuthite, hedleyite, and native Au. Ni-Co mineralization of disseminated ores from gabbro-diorite xenoliths from the Levo-Seymkan massif and from the stockwork localized in the massif granites (roof outshot in the Puglivy Creek), as well as from veins and veinlets in contact hornfelses (Puglivoye and Obkhod deposits). It has been found that mineral phases are not as characteristic for mineralization in granites and Ni hornfelses, and that the Co/Ni increases with distance from the Levo-Seimkan massif due to the lower migration ability of Ni relative to that of Co in the course of successive geochemical differentiation.

Depositing >1.5 Mt of Tin Within <1 m.y. of Initial Granitic Intrusion in the San Rafael Tin (-Copper) Deposit, Southeastern Peru

Abstract The San Rafael Sn (-Cu) deposit, located in the Eastern Cordillera of southeast Peru, is one of the world’s largest cassiterite-bearing vein systems (>1 Mt Sn produced since 1969). The deposit consists of a quartz-cassiterite-chlorite-sulfide lode system spatially associated with an upper Oligocene (ca. 24 Ma) S-type granitic pluton. Based on a revised paragenetic sequence for the deposit, we interpret the temporal setting of both magmatic (biotite, K-feldspar) and hydrothermal (muscovite, adularia, cassiterite) minerals analyzed by 40Ar/39Ar step-heating and U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) geochronology. The least-disturbed biotite sample from the megacrystic monzogranite yielded a 40Ar/39Ar plateau age of 24.10 ± 0.26 Ma (2σ), which constrains the time of cooling of the upper part of the pluton to below 300°C. Greisen developed on top of the granitic cupola and its immediate metamorphic aureole dated at 24.24 ± 0.24 Ma (2σ; 40Ar/39Ar muscovite average plateau age) is interpreted to be contemporaneous with the emplacement of pre-ore quartz-tourmaline veins and breccias. In situ U-Pb dating of cassiterite, including both botryoidal cassiterite (“wood tin”) and coarse-grained cassiterite in quartz-chlorite veins and breccias, constrains the timing of the main Sn ore stage to between 24.10 ± 0.37 and 23.47 ± 0.53 Ma (2σ). Botryoidal and coarse-grained cassiterite are characterized by similar trace element compositions with fluctuating metal concentrations across growth banding, suggesting significant changes of physicochemical conditions of the hydrothermal system during cassiterite precipitation, likely caused by rapid and repeated mixing between magmatic fluids and meteoric groundwaters. Polymetallic sulfide-rich veins and quartz-carbonate veins are constrained to have formed between 22.72 ± 0.11 and 22.29 ± 0.24 Ma (2σ), based on adularia 40Ar/39Ar plateau ages. The latter overlap partially reset 40Ar/39Ar age spectra for K-feldspar megacrysts in the host granite and thus reflect pervasive alteration by hydrothermal fluids. Collectively, the results show the magmatic-hydrothermal system spanned at least 2 m.y. with the main Sn ore stage representing <1 m.y. in the lifetime of the deposit. The latest polymetallic stages postdate the main Sn ore stage by ca. 1 m.y. and reflect the waning of the hydrothermal system, accompanied by additional incursion of meteoric groundwaters. This study provides further evidence that the present-day exposed level of the San Rafael granite was a passive host for the Sn mineralization and only provided the structural focusing for the mineralizing fluids derived from a deeper part of the magmatic system.

Emplacement of sharp-walled sulfide veins during the formation and reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario

Broken Hammer is a hybrid Cu–Ni–Platinum Group Element (PGE) footwall deposit located in Archean basement rocks below the impact-induced Sudbury Igneous Complex (SIC), Canada. The deposit consists of massive chalcopyrite veins surrounded by thin epidote, actinolite, and quartz selvedges and low-sulfide, high-PGE mineralization consisting of disseminated chalcopyrite (<5%) and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite, and quartz. The veins are grouped into five steeply-dipping sets, striking northeast-, southwest-, southeast-, south-, and east–west, which were emplaced along impact-related fractures that were reactivated multiple times during stabilization of the crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip shattered the quartz–chlorite veins into fragments that were incorporated in massive sulfide veins that crystallized from fractionated sulfide melts or from high temperature (400–500 °C) hydrothermal fluids, which migrated outward into the basement rocks from a cooling and crystallizing SIC melt sheet. Hydrothermal fluids syn-genetic with the epidote–actinolite–quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGMs from the sulfide veins and redistributed them to form low-sulfide, high-PGE zones in the footwall rocks. During post-impact tectonic events, slip at temperatures below the brittle–ductile transition for chalcopyrite (<200 °C to 250 °C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu–Ni–PGE footwall deposits formed by the reactivation of impact-related fractures that provided conduits for the migration of melts and hydrothermal fluids.

Trace element geochemistry of magnetite and pyrite and sulfur isotope geochemistry of pyrite and barite from the Thanewasna Cu-(Au) deposit, western Bastar Craton, central India: Implication for ore genesis

The Thanewasna copper belt in Bastar Craton of central India is well-known for Cu ± Au mineralization. In this study, we use major and trace element chemistry of magnetite, hematite, and pyrite, as well as the sulfur isotope composition of chalcopyrite, pyrite and barite from the deposit to characterize the mineralization and to constrain its petrogenesis. The mineralization is hosted in quartz-chlorite veins associated with 2.5 Ga and 1.6 Ga granitoids. The mineralization is focused along the NW-SE trending Thanewasna brittle-ductile shear zone and comprises disseminated and vein-hosted Cu-sulfides, mainly chalcopyrite and pyrite intergrown with minor magnetite, hematite, and sulfide-free barite. The associated hydrothermal alteration is characterized by pervasive potassic-sodic alteration and chloritization. Calcite and sulfide-bearing barite veins postdate the Fe-oxide and Cu-sulfide ores. At least two generations of magnetite, hematite, and pyrite were identified based on their textural and trace element characteristics.Texturally early magnetite (Mag-I) displays oxy-exsolution of ilmenite and is of magmatic origin, representing the pre-mineralization igneous assemblage of the host rocks. Texturally later magnetite (Mag-II) is of hydrothermal origin and replaces Mag-I. It has high Co and Mg concentrations and shows dissolution and re-precipitation texture as well as evidence for high-temperature annealing. Martitisation of Mag-I and II associated with chloritization and Cu-Au mineralization produced hematite.The δ34SVCDT of syn-mineralization pyrite and chalcopyrite (−0.77‰ to −4.28‰) is suggestive of magmatic sources for the sulfur. Post-mineralization barite veins have δ34SVCDT between +10.31‰ and +17.55‰ which indicate that the sulfur was derived from seawater sulfate without going through an intermediate stage of reduction. Variation in the sulfur isotopic compositions of sulfide and sulfate was the result of dilution and cooling of the metalliferous fluid after interaction with meteoric fluids, which triggered the deposition of Cu ± Au along structural weak planes.The widespread presence of iron-rich breccias, association with crustal-scale shear zone, pervasive K-Na alteration, chloritic and advanced argillic alteration, the involvement of magmatic and non-magmatic fluid and Cu-Fe-Au-Ag-Ba-REE metal association are all suggestive of IOCG-style mineralization at Thanewasna. This supported by the Ti, Al + Mn, Ti + V concentrations and Ti/V, Ni/Cr, Ni/(Cr + Mn) ratios of Mag-II and hematite. The Co/Ni ratios of pyrite further supports a hydrothermal origin of the deposit, unrelated to skarns or copper porphyry, having similarity with known IOCG deposits. The identification of IOCG-type deposits in the Thanewasna region gives a boost to deeper sub-surface exploration in this belt as well as in similar geological setting elsewhere.

A shear-hosted Au-Cu-Bi metallogenic event at ~1660 Ma in the Tennant Creek goldfield (northern Australia) defined by in-situ monazite U-Pb-Th dating

High-grade Au, Cu and Bi ores in the Tennant Creek goldfield have been mined from hydrothermal magnetite- and/or hematite-rich ironstone bodies. Less well known is a style of Au-Cu-Bi mineralisation hosted by quartz vein systems within shear zones outside ironstones. We report ion probe (SHRIMP) U-Pb-Th ages of 1659 ± 13 Ma and 1659 ± 15 Ma for hydrothermal monazite associated with this mineralisation style at the Orlando East Au-Cu-Bi deposit and Navigator 6 Au prospect, respectively. The dated monazite at Orlando East is intergrown with Cu and Bi sulfides and Au in quartz-chlorite veins within a shear zone. At Navigator 6 the monazite occurs both in late-stage chalcopyrite-bearing quartz-hematite veins and within earlier-formed sheared chloritic breccia that also contains anomalous Au.Although the full significance of the ~1660 Ma monazite ages has yet to be determined, the results indicate the introduction of Au, Cu and Bi in quartz vein systems at ~1660 Ma, or remobilisation of earlier, ~1850 Ma, main-stage Au-Cu-Bi from mineralised ironstones and into quartz veins 10 s–100 s of metres from the initial mineralisation. In either case, hydrothermal fluids capable of transporting significant concentrations of ore metals were present in the Tennant Creek goldfield at ~1660 Ma, synchronous with deformation. A key implication is that rock sequences up to ~200 million years younger than the ironstone-hosted Au-Cu-Bi deposits may now be considered prospective for a second stage of Au ± Cu ± Bi mineralisation. The results also validate non-ironstone shear-hosted deposits in the Tennant Creek region as an alternative exploration target to the well known high-grade but low-tonnage Tennant Creek ironstone-hosted deposits.

Characteristics of hydrothermal chlorite from the Niujuan Ag-Au-Pb-Zn deposit in the north margin of NCC and implications for exploration tools for ore deposits

The Niujuan Ag-Au-Pb-Zn deposit is located in the northern Hebei Province of North China, which is hosted by Permian coarse-grained granite. The orebodies are controlled by a NE striking fault. Chloritization is the most extensive alteration with increasing alteration intensity from the distal to proximal end of the orebody. Chlorite and biotite from twenty-one altered samples from different depths of four drill holes were analyzed using electron microprobe (EMPA) and laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Empirical geothermometry shows that chlorite formed at 206–340 °C. Through comparing with compositions of unaltered biotite, two biotite converted to one chlorite (2Bt → 1Chl) is the main chloritization mechanism in weakly and moderately altered granites during the hydrothermal alteration in the Niujuan deposit. During the alteration, hydrothermal fluid mainly brought in Fe, Sr, Th, Pb and CO2 and took out Mg, Ti, Si, F, K, Li, Zn, Sc, Ga, Co from the biotite, which was accompanied by the formation of quartz, rutile, calcite and fluorite during the chloritization. Chlorite in quartz-chlorite veins was precipitated directly from the hydrothermal fluids. Veined chlorite in intensely altered granites formed by the dissolution-transport-precipitation mechanism. Under the high fluid/rock conditions, veined chlorite has nearly the same compositions as chlorite in quartz-chlorite veins. According to the compositions of chlorite from intensely altered granites and quartz-chlorite veins, we infer that the hydrothermal fluid had low HFSEs and transition elements, such as Ti, Ga, Sc and V, and low fO2. From weak to intense chloritization granites, FeO, Sr, Th and Pb contents of chlorite increase, while MgO, TiO2, Li, Zn, Sc, Ga, V, Co and Nb contents decrease. The composition variations of chlorite are controlled by the composition of biotite, the compositions of fluids and the intensity of fluid-rock interaction during alteration. This indicates that the chemical variations of chlorite could be as a tool for mineral exploration in the moderate-low temperature Ag-Au-Pb-Zn deposit. Comparison with chlorite of different genesis shows that compositions of chlorite mainly differ in Mg, Fe and AlIV, which are mainly controlled by the composition of host rocks and the formation temperature.

The Koudia El Hamra Ag–Pb–Zn deposit, Jebilet, Morocco: Mineralogy and ore fluid characterization

The Koudia El Hamra silver - base metal (Pb-Zn) deposit is located in the Upper Visean - Namurian Sarhlef schist, Jebilet Hercynian Massif (Morocco). Ore bodies consist of N-S to NE-SW quartz-chlorite veins hosted by Hercynian black shale. Mineralogical and paleo-fluid investigations (microthermometry, Raman spectroscopy) and stable isotope data (O, C, S) have shown that high salinity brines (30 to 35 wt% NaCl+CaCl2 equiv.), likely from sedimentary origin deposited base metal (Zn) - carbonate assemblage at minimal temperature around 220°C. Then brines deposited silver ore - galena assemblage at lower salinity (9 to 15 wt% NaCl equiv.) and similar average minimal temperature (around 220°C). The main driving mechanism for silver ore deposition is the dilution of ore-bearing brines by low-salinity fluids probably combined with reduction of brines in the host black shale. Ore brines likely have come from the Permian-Triassic Atlasic basins related to the extension coeval with the Central Atlantic opening. Those brines probably have percolated into the basement through Hercynian faults (N-S to NE-SW) reworked during the Permian-Triassic to Lower Jurassic extensional events. The source of silver seems to be in black shale and/or in abundant mafic to intermediate magmatic rocks (i.e. microdiorite dykes) consistently observed close to the ore bodies. Silver ores are superimposed on the same lineament to preexisting Fe-As-(Sn) assemblages related to Hercynian events in the central Jebilet and mainly due to the circulation of magmatic and/or metamorphic fluids. However, silver ores have no genetic relationship with magmatic-metamorphic events as suggested by former models. The metallogenic model proposed for the Koudia El Hamra deposit is similar to those proposed for several other silver - base metal (Pb-Zn) deposits from Morocco like the Roc Blanc silver deposit (Jebilet massif) and major silver deposits from the Anti-Atlas belt (Imiter, Zgounder and Bou Azzer) and from Western Europe.

Iron Oxide-Copper-Gold Mineralization at Thanewasna, Western Bastar Craton

Iron oxide-copper-gold (IOCG) at Thanewasna, Maharashtra, India is a new genetic type of ore deposit, being reported from the western margin of Bastar craton, based on integrated field, drilling, mineral chemistry and Raman microprobe studies. It is the fourth such IOCG type being reported from India. Hydrothermal mineralization is structurally confined to en echelon dilatational quartz-chlorite veins along NW-SE trending brittle-ductile shear zone hosted in calc-alkaline granitoid. The mineralization is characterized by chalcopyrite, magnetite and barite which occur as dissemination, stringers and veins associated with hydrothermal K-alteration and chlorite alteration. Chemical analysis shows significant amounts of Cu, Fe, Ba and anomalous Au content. Ore petrography and scanning electron microscope and electron probe micro analyser studies show assemblages of Cu-Fe-Au-Ag-Ni-Ba-REE minerals typical of IOCG type deposits at Thanewasna. Ore textures, mineralogy and alteration characteristics are typical of IOCG-type deposits, further supported by mineral chemistry of magnetite (V versus Ti/V) using EPMA, and thus define a IOCG metallogenic province in Thanewasna area with significant implications for future exploration.

The evolution of a Precambrian arc-related granulite facies gold deposit: Evidence from the Glenburgh deposit, Western Australia

Gold deposits are rare in upper-amphibolite to granulite facies environments. Known examples commonly attract debate about whether they formed under these conditions or instead represent metamorphosed, metasomatic, or superimposed (retrograde) mineralization. The Glenburgh gold deposit is located in the Paleoproterozoic upper-amphibolite to granulite facies Glenburgh Terrane in the southern Gascoyne Province of Western Australia. Gold at the Glenburgh deposit is free and disseminated within quartz–biotite–garnet gneiss, amphibolite, and (post-gold) quartz–chlorite veins. No clear association with a specific host lithology has been identified and mineralization does not have a visually distinct proximal alteration assemblage.The rocks hosting the deposit represent a distinct sedimentary package that was deposited, mineralized, buried, and metamorphosed, all during arc magmatism. Features within the internal structure of gold grains, such as high-purity gold veinlets, incoherent twinning, and low silver content, suggest the gold has been through post-depositional processes such as metamorphism and deformation. Abundant sulfide minerals are interpreted to have formed by sulfidation of the host rock contemporaneously with gold mineralization, and the presence of rounded sulfide inclusions within garnet porphyroblasts illustrates the presence of a sulfide phase prior to peak metamorphism. Geochronology of zircon and monazite constrains the timing of mineralization to be younger than c.2035Ma—the maximum depositional age of the metasedimentary host rocks—but older than c.1991Ma—the peak of M1 metamorphism during the Glenburgh Orogeny; these events were synchronous with arc magmatism. Rocks at the Glenburgh deposit were likely deposited in a fore-arc or accretionary wedge, a favourable setting for porphyry Cu–Mo–Au, epithermal Au, polymetallic (Sn, W) skarn, and orogenic Au mineralization. Phase equilibria modelling of a pelitic migmatite constrains peak P–T conditions to be 865–885°C, 6.8–7.6kbar, consistent with elevated thermal gradients within the arc, followed by conductive cooling of arc magmas. Partial melting during peak M1 metamorphism possible caused gold remobilization. The lack of an alteration assemblage further suggests that the alteration assemblage and mineralization were recrystallized during deformation and metamorphism. However, increases in Ca and K abundance and magnetic susceptibility decreases toward mineralization, suggesting that they may constitute ore vectors.

Chloritisation along the Thanewasna shear zone, Western Bastar Craton, Central India: Its genetic linkage to Cu–Au mineralisation

The Cu±Au mineralisation of Thanewasna, Bastar Craton, Central India, is hosted in quartz–chlorite veins traversing granitoid rocks. Mineralisation and associated alteration are controlled by a NW–SE trending brittle–ductile shear zone. Large scale alkali magmatism in the form of K-rich granite emplacement is found in the vicinity of the shear zone. The emplacement of this granite (1587±14Ma) along the northern shoulder of the Godavari rift produced a high geothermal gradient, resulting in intense chloritisation and mineralisation. Field and petrographic studies show that the chlorite formed in two possible modes, viz. a) replacement of ferromagnesian minerals of the host rock, and/or b) precipitated directly from hydrothermal solutions. Mineral chemistry (EPMA) coupled with SEM and XRD studies reveals that chamosite with minor clinochlore varieties of chlorite is commonly associated with Cu and Au mineralisation. Chlorite shows variation in Fe/(Fe+Mg) ratios from 0.54 to 0.60, 0.55 to 0.64 and 0.49 to 0.55 and AlIV from 1.88 to 2.40, 1.91 to 2.50 and 1.80 to 1.94apfu respectively at the surface and in the sub-surface mineralisation and host rock.Empirical and thermodynamic geothermometers have been used to determine the temperature of chlorite formation based on chemical composition. Six empirical methods, based on AlIV, AlVI and Fe/Fe+Mg ratio showed a temperature range of 187°C to 381°C. Temperatures of chlorite formation have also been determined by the thermodynamic solid solution method. The thermodynamic equations of three intra-crystalline equilibria at 2kbar pressure show that temperature ranges from 215° to 454°C for low Si contents (<2.8apfu) and 110°–133°C for higher Si contents (Si>2.8apfu). The thermodynamic method of Vidal et al. (2001) and (2005) was not so compatible for an estimation of temperature of hydrothermal Fe-rich chlorites associated with Cu–Au mineralisation at the Thanewasna copper deposit (TCD).The temperature of chlorite formation increases from the host rock (mean—201°C) to proximal mineralisation zone (mean—234°C) to distal K–Na alteration zone (mean—251°C). Integrating field and laboratory results suggested that TCD chlorite is part of a structurally controlled high sulphidation epithermal system. This characterization would help in the regional exploration for base and precious metals and the results are applicable for the entire Western Bastar Craton (WBC) of Central India and similar deposit elsewhere.

Crack-seal microstructure evolution in bi-mineralic quartz–chlorite veins in shales and siltstones from the RWTH-1 well, Aachen, Germany

In core samples from the deep geothermal well RWTH-1 we studied Variscan quartz–chlorite veins formed by crack-seal processes in siliciclastics at the brittle to ductile transition. These sheared veins are common in sections of the well, which are interpreted as Variscan thrust zones based on image logs and seismic data. Microstructures interpreted to reflect different stages in the evolution of such crack-seal veins suggest the veins started in microcracks sealed by quartz and chlorite, to veinlets crossing multiple grains, and bundles of veinlets evolving by progressive localization into low-angle extensional shear veins and high-angle dilational jog veins. In the sheared veins, chlorite and quartz ribbons show evidence for crack-seal and simultaneous ductile shearing during vein evolution, forming peculiar fin-shaped microstructures in quartz ribbons. In high-angle dilational jogs fibrous crystals of quartz and chlorite point to multiple crack-seal events with simultaneous growth of two different mineral phases. This is interpreted to be the basic microstructural process in the veins. We extend earlier models of polycrystal growth in fractures and present a series of 2D simulations of the kinematics of crystal growth in these bi-mineralic veins for both localized and non-localized cracking. Results are compared with the observed microstructures. We show that when the relative growth rates of the two mineral phases are different, serrated grain boundaries evolve. The similarities between observation and model suggest that the assumption of our model is valid, although many second order processes require a more detailed study. We propose that the principles observed here can be applied to other bi-mineralic crack-seal veins.

Gold Mineralization at the Anomaly A Deposit, Clarence Stream Area, Southwestern New Brunswick: Distal Deposits of a Syn-Deformational Intrusion-Related Gold System?

Gold-bearing mineralization at Anomaly A occurs in polydeformed Ordovician turbidites of the Kendall Mountain Formation of the St. Croix belt in southwestern New Brunswick, and is hosted by WSW-trending, shallow-dipping, brittle–ductile high-strain zones associated with the latest stage of the second of four regional deformation events. Gold-bearing syn-deformational stockwork, massive, and multiply brecciated quartz-sulfide veined zones form irregular pods and sheets up to several meters thick, which contain arsenopyrite, pyrrhotite, pyrite, stibnite, and a variety of rare minerals that are gold-bearing. Mineralized zones are enveloped by locally auriferous, weakly to intensely altered wall rock. Vein paragenesis includes: (1) early (pre- to syn-D 2 ) quartz-chlorite veins that are largely the product of migration of quartz by pressure solution; (2) crosscutting, multiphase, vuggy quartz-sulfide veins that contain most of the gold and are interpreted to be structurally controlled; (3) late, crosscutting, nondeformed, non-gold-bearing veins containing laumontite, chlorite, muscovite, quartz, fluorite, and base metal sulfides. Carbonate-poor alteration associated with zones of gold mineralization is lithology-specific, and appears to reflect overprinting of multiple events. Major elements removed during alteration include Si, Na, Mn, and Mg. The minor and trace elements removed include Co, Li, Sc, and possibly Sr, Ni, and Zn. Minor and trace elements added include Au, Ag, As, Sb, S, and possibly F, Cu, Pb, and Zr. On the basis of proximity, time constraints, and a similar ore mineral assemblage, gold mineralization at Anomaly A is interpreted to be a distal equivalent of the intrusion-related gold mineralization at the nearby Clarence Stream Main zone deposit.

Uranium Deposits of the Mount Isa Region and Their Relationship to Deformation, Metamorphism, and Copper Deposition

The Mount Isa region contains over 20 uranium deposits, few of which contain economic concentrations of uranium. They are thought to be significant in the exploration for new copper deposits, based on the model that the uranium deposits are located in the pathways of oxidized hydrothermal fluids that formed the postmetamorphic Mount Isa copper orebodies. There are two uranium deposit types, both hosted in volcanosedimentary rocks. We suggest that they represent one mineralization event but different metamorphic histories. Examples in the eastern low-grade metamorphic zone are dominated by a brannerite-anatase-apatite assemblage and are low in uranium. The examples in the western high-grade metamorphic zone are characterized by uraninite and titanite with higher uranium concentrations. The chemical association of U, Ti, and Ca is consistent for all deposits; however, phosphate is only important at low metamorphic grade. The most likely mechanism of formation in the lower metamorphic grade Anderson’s Lode example is the complete replacement of chlorite in the sedimentary host-rock matrix by the mineralizing assemblage. In this model, chlorite acted as a reductant, allowing the deposition of uranium from an oxidized fluid. This mechanism of uranium deposition is similar to that proposed for unconformity-type uranium deposits. We conclude that these uranium deposits are metamorphosed equivalents of Proterozoic unconformity-type uranium deposits found elsewhere in northern Australia. At Anderson’s Lode, brannerite and apatite were concentrated in veinlets by pressure dissolution after the deposition of the original assemblage. These pressure dissolution seams are cut by premetamorphic quartz-chlorite veins. Examples at higher metamorphic grade have textures that suggest uranium deposition was pre- to synmetamorphic at the latest. This precludes any genetic or temporal connection between the uranium deposits and the postmetamorphic Mount Isa copper deposit.

San Rafael, Peru: geology and structure of the worlds richest tin lode

The San Rafael mine exploits an unusually high grade, lode-type Sn–Cu deposit in the Eastern Cordillera of the Peruvian Central Andes. The lode is centered on a shallow-level, Late Oligocene granitoid stock, which was emplaced into early Paleozoic metasedimentary rocks. It has a known vertical extent exceeding 1,200 m and displays marked vertical primary metal zoning, with copper overlying tin. The tin mineralization occurs mainly as cassiterite–quartz–chlorite veins and as cassiterite in breccias. The bulk of it is hosted by a K-feldspar megacrystic, biotite- and cordierite-bearing leucomonzogranite, which is the most distinctive phase of the pluton. Copper mineralization occurs predominantly in the veins that straddle the metasedimentary rock–intrusion contact or are hosted entirely by slates. Both tin and copper mineralization are associated with strong chloritic alteration, which is superimposed on an earlier episode of sericitization and tourmaline–quartz veining. Based on the distribution of alteration and ore mineralogy, cassiterite deposition and subsequent chalcopyrite precipitation are believed to have been the result of a single, prolonged hydrothermal event. The source of the metals is inferred to be a highly evolved, peraluminous magma, related to the leucomonzogranitic phase of the San Rafael pluton. Preliminary fluid inclusion microthermometry suggests that ore deposition took place during the mixing of moderate and low salinity fluids, which were introduced in a series of pulses. Several large fault-jogs, created by sinistral-normal, strike-slip movement, are interpreted to have focused synkinematic magmatic fluids and permitted their effective mixing with meteoric waters. It is proposed that this mixing led to rapid oxidation of Sn (II) chloride species and caused supersaturation of the fluids in cassiterite, resulting in the development of localized, high-grade ore shoots. A favorable structural regime that promoted large-scale mixing of two fluids originating under very different physico-chemical conditions appears to have been the key factor responsible for the unusual richness of the deposit.

Characteristic Features of Tin–iron–copper Mineralization in the Anle‐Huanggangliang Mining Area, Inner Mongolia, China

Abstract: The Anle Sn‐Cu and Huanggangliang Fe‐Sn deposits have been exploited in the Linxi district, which is located 165 km northwest of Chifeng City in northern China. In this study the formation mechanisms of the tin deposits in the Anle and Huanggangliang mining area were investigated to understand the mechanisms of tin mineralization in northern China.The veins of the Anle deposit are divided into cassiterite–quartz–chlorite veins, chalcopyrite‐bearing quartz veins, cassi–terite–chalcopyrite–bearing quartz veins and sphalerite‐quartz veins. The sequence of mineralization is tin mineralization (stage I), copper mineralization (stage II), and lead‐zinc mineralization (stage III). The Huanggangliang tin deposit consists of magnetite skarn orebodies and many cassiterite‐bearing feldspar–fluorite veins and veinlets cutting the magnetite orebodies.The fluid inclusions in quartz and fluorite in ores from the Anle and Huanggangliang tin deposits are divided into two‐phase fluid inclusions, vapor‐rich fluid inclusions and poly‐phase fluid inclusions. The final homogenization temperatures of fluid inclusions of quartz in the ores of the Anle deposit and fluorite of tin‐bearing feldspar veins in the Huanggangliang tin deposit range from 195 to 425C and from 215 to 450C, respectively. The fluids responsible for the Anle and Huanggangliang tin deposits were of very high temperature and NaCl‐rich ones containing K, Ca, Al, Si, Ti, Fe and Cl in addition to ore metals such as Sn and Cu. The temperature and chemical composition of fluid in fluid inclusions of igneous rocks in the mining area are very similar to those of fluid in fluid inclusions in the ores of these deposits. The fluid for these ore deposits had a close relation with the fluid coexisting with melt of Late Jurassic granitic rocks in this mining area. Salinities of fluid inclusions from these ore deposits and granitic rocks in the mining area were estimated to range from 35 to 50 wt % NaCl equivalent. Based on arsenopy‐rite geothermometry and fluid inclusion studies, a fluid containing 40 wt% NaCl (eq.) could be formed by phase separation of fluid having 6 wt% NaCl (eq.) at a temperature of 420 to 500C and a pressure of 0.3 to 0.4 kb. The temperatures and pressures presented above indicate an NaCl‐rich magmatic fluid derived from granitic melt that had intruded into a shallow level of crust caused the Sn–Fe–Cu mineralization of the mining area. The geological relationship between these ore deposits and granitic bodies around the ore deposits, and the similarity of fluids forming these ore deposits and coexisting with granitic melt, suggest that these ore deposits were formed by the activity of fluid derived from granitic melt in Late Jurassic age.

Multiple fluid generations in the Sudbury igneous complex: fluid inclusion, Ar, O, H, Rb and Sr evidence

The elliptically zoned Sudbury Structure in northern Ontario is host to world-class nickel–copper–PGE deposits, which are generally regarded as the products of a single magmatic event. However, there is increasing evidence of multi-episodic fluid rock (including Ni–Cu sulphide) interactions in the Sudbury Structure that range from early remobilization of the cooling ores (ca. 1850 Ma), through metamorphism and deformation related to the Penokean Orogeny to neotectonic fracturing (ca. 5–13 Ma). Previous fluid inclusion studies have identified an H 2O–NaCl±CO 2±KCl fluid of highly variable phase ratios intimately associated with the ores, and a distinct population of primary two-phase fluid inclusions associated with post-intrusion metasomatic alteration. This study introduces two new distinct populations of fluid inclusions: (1) a population represented by two-phase CO 2-bearing fluid inclusions hosted along healed-fracture planes in quartz that offset ore minerals, that was trapped between 280–340°C and 1750 to 3500 bars pressure. As the fluid inclusion trails offset and therefore post-date the ore, their genesis is consistent with a late Penokean (1.83–1.89 Ga) or later (ca. 1450) contractional event; (2) a two-phase highly-saline (>16 wt.% NaCl equivalent) fluid hosted within neotectonic (5–13 Ma Rb–Sr) galena–sphalerite–calcite–quartz–chlorite veins. Inclusions of this fluid were trapped at temperatures ranging from 60 to 135°C and maximum confining pressures of 950 bars. Stable isotope data (δO,δD) from biotite, amphibole and epidote from the alteration assemblages of remobilized veins within the Sudbury Structure show distinct evidence of a mixing trend of fluids originating within or near the `magmatic water box' and trending towards a metamorphic fluid of isotopic composition similar to an evolved Sudbury Structure groundwater. Ar–Ar data from the amphiboles within Sudbury Intrusive Complex (SIC) footwall breccia from the Craig Mine are consistent with a mixed Ar–Ar spectrum. The step-heating Ar release spectrum yields ages ranging from ca. 1800 Ma at the low temperature end to approximately 2640 Ma for high temperature steps. These dates are in agreement with the ca. 1850 and 2647 Ma ages for the SIC and Levack gneisses, respectively, and are interpreted as the reinjection of country rocks partially melted by the SIC back into the Sudbury Structure. The isotopic evidence for mixed fluids derived from a combination of a magmatic fluid and a regional groundwater/metamorphic fluid suggests that the highly variable phase-ratio fluid inclusions associated with the ore may be divided into two end-member fluids represented by an exsolved SIC fluid and a regional groundwater/metamorphic fluid. This is consistent with at least five distinct fluids being present during the evolution of the Sudbury structure.