The origin and evolution of heavy rare earth element mineralisation in the Browns Range area, Northern Australia
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This thesis investigates a regional-scale heavy rare earth element (HREE) mineralisation style that appears as several structurally-controlled orebodies distributed from the Halls Creek Orogen to the Tanami Region, in an area labelled the North Australian HREE+Y (NAHREY) mineral field. The ore minerals consist only of xenotime [(Y,HREE)PO₄] and minor florencite [LREEAl₃(PO₄)₂(OH)₆], and occur mainly near a regional unconformity between the Archean metasedimentary rocks of the Browns Range Metamorphics (BRM) and overlying Paleoproterozoic Birrindudu Group sandstones in northwest of the Tanami Region. The BRM are medium- to coarse-grained arkosic metasandstones that host the bulk of the HREE mineralisation in the NAHREY mineral field.
The BRM consists mainly of detrital quartz and feldspars with minor granitic lithic fragments. Isotopic data acquired from detrital zircons from the BRM and intruding felsic igneous rocks yielded a well-defined age of ca. 3.2 to ca. 3.0 Ga, with relatively radiogenic eHf values (eHf = –1.7 to +5.1), indicating derivation from a Mesoarchean granitic basement of juvenile origin, and deposition in a continental rift basin setting. The sedimentation is constrained to between the ca. 3.0 Ga age of the source rocks and ca. 2.5 Ga age of the felsic igneous bodies that cross-cut the BRM. The ca. 2.5 Ga zircons from the felsic igneous rocks have eHf model ages comparable to those of the ca 3.2 to ca. 3.0 Ga detrital and inherited zircons (ca. 3.4 to ca. 3.1 Ga), consistent with formation via partial melting of the BRM, or the Mesoarchean granitic basement. The unconformably-overlying Gardiner Sandstone of the Birrindudu Group contains detrital zircons of ca 2.6 to ca 1.8 Ga age with no trace of Mesoarchean age, which discounts a significant contribution from the underlying BRM.
A detailed paragenetic study of the mineralisation revealed; (1) a pre-ore stage displaying mostly a greenschist-facies overprint, with the detrital/metamorphic minerals including quartz (several generations), alkali feldspar, plagioclase, and coarse-grained muscovite aligned in the pre-mineralisation foliation; (2) syn-ore quartz and white mica alteration associated with a complex multi-stage mineralisation of the ore minerals, primarily in breccias and veins; (3) a post-ore stage characterised by several generations of quartz, hematite, barite, anhydrite and pyrite veining and brecciation. Isotopic dating of xenotime ore from across the NAHREY mineral field constrained the main stage of ore formation to between ca. 1.65 Ga and ca. 1.60 Ga, which is significantly younger that the pre-ore muscovite ⁴⁰Ar/³⁹Ar age of ca. 1.72 Ga that corresponds to a regional metamorphism. The ca. 1.65-1.60 Ga timeframe does not correlate to any local magmatism or orogeny but was coincident with the collision of the North Australian Craton with the Arunta Inlier and Laurentia and subsequent initiation of the Isan and Liebig Orogenies. Far field stresses from these craton-scale events potentially acted as drivers of large-scale fluid flow and fault (re)activation that led to the HREE ore formation.
Ore petrography indicates multiple stages of xenotime and florencite crystallisation and recrystallisation. Early xenotime (up to 1 mm), coexisting with early florencite, appears in breccias (breccia-hosted) and mineralised quartz veins (vein-type). Late xenotime (<100 μm) occurs largely as pyramid-shaped overgrowths on the pre-existing xenotime and coexists with late florencite that mainly replaces early xenotime and also appears as narrow rims on early florencite. Compared with early xenotime, the late xenotime overgrowths are richer in the HREE and more depleted in P and LREE, owing to crystallisation of late florencite. Moreover, early florencite has a nearly pure florencite composition whereas the late florencite is defined by a broad chemistry including components of svanbergite, goyazite and woodhouseite. Both xenotime and florencite incorporated quantities of trace elements via a number of substitution mechanisms. High U content of xenotime and composition of early florencite potentially support a genetic association between the HREE mineralisation and the coeval U deposits of northern Australia that formed across the same basin.
Samples of the BRM are variably depleted in HREE compared to sedimentary protoliths, and also have unradiogenic Nd isotope compositions that are comparable to the orebodies, but quite distinct from the igneous rocks or other sedimentary rocks (Birrindudu Group) from across the North Australian Craton. These observations demonstrated that the ore metals were derived directly from the BRM. Moreover, investigation of a large number (ca. 550) of primary fluid inclusions from both mineralised and barren quartz veins, revealed three types of hydrothermal fluids available only in the mineralised samples including type I low salinity H₂O-NaCl (largely <5 wt.% salinity; consistent with meteoric water), type II medium salinity H₂O-NaCl (12-18 wt.% salinity) and type III high salinity H₂O-CaCl₂-NaCl (up to 25 wt.% salinity). The trapping temperature and pressure during the ore formation was between 100 to 250 °C and between 0.4 and 1.6 Kbar, respectively. Trace element analysis detected Y, Ce and Cl only in the type III fluid inclusions, which indicates that transportation of ore metals was (at least partly) by Cl complexes in the type III fluid. The P required for phosphate ore mineral formation was likely transported by the type I fluid. Moreover, mineralised quartz samples returned δ¹⁸Ofluid values in the range defined by the BRM (δ¹⁸Ofluid = +1.8 to +5.2‰) and the Birrindudu Group sandstones (δ¹⁸Ofluid = +8‰).
Combining whole-rock, fluid inclusion and isotopic data, an ore genesis model is developed that suggests mixing of at least two hydrothermal fluids, one (type III) leached HREE+Y from the BRM and moved upward along fault structures in the vicinity of the regional unconformity, and there mixed with another down-flowing P-bearing fluid (represented potentially by the type I fluid inclusions) originated from the Birrindudu Group sandstones. Leaching of ore metals was greatly enhanced by halogen (Cl, F) complexes. Introduction of P during fluid mixing/dilution and an increase in pH as recorded by the syn-ore muscovite alteration, resulted in HREE deposition.
Globally, the closest analogue to the NAHREY ore deposits is the Maw Zone, which formed in a very similar geological setting in the Athabasca Basin, Canada. Collectively, this style of REE mineralisation is unlike any other known REE ore style, and is herein labelled Unconformity-Related REE deposit. There is great potential for further unconformity-related REE deposits to be found in intercontinental basins in close proximity to regional unconformities between Archean basement rocks and overlying Proterozoic sedimentary sequences.Keywords:
Rare-earth element
Northeastern China is composed of the eastern part of the Central Asian Orogenic Belt and the northeastern margin of the North China Craton. It underwent two major sets of orogenic events, including the pre-Mesozoic amalgamation of several micro-continents and the Mesozoic subduction of the Paleo-Pacific plate. It hosts numerous ore deposits of dominantly porphyry and skarn types, most of which have Mesozoic ages. In this study, both mineralization types were studied, aiming to improve the understanding of ore genesis, hydrothermal evolution, mineralization mechanism, regional metallogeny as well as the geodynamic setting.
Systematic zircon U-Pb and/or molybdenite Re-Os dating on five porphyry deposits (i.e., Aolunhua, Haisugou, Shabutai, Banlashan, and Yangchang) in the northern Xilamulun district indicates that the timing of the magmatism and the Mo mineralization is broadly coeval, mainly at 130-140 Ma. Major and trace element geochemistry reveals the intrusions hosting Mo-only deposits (e.g., Haisugou) have stronger crystal fractionation than intrusions hosting porphyry Cu and Cu-Mo deposits (e.g., Aolunhua), indicating that such a process may have played a role in selective enrichment of Mo. A comparison of zircon Ce/Nd ratios as a proxy for the oxidation state of magmas between mineralized and barren intrusions shows that the mineralized intrusions are associated with more oxidized magmas than the cospatial barren granites, and therefore it is proposed that higher oxygen fugacity may also be important to produce economic Mo mineralization. Whole rock Sr-Nd-Pb and zircon Hf isotopes show that these mineralized granites in Xilamulun are associated with magmas generated from three different source regions (i.e., remelting of old crust material, mixing of old crust material with depleted mantle component, and juvenile mantle-derived magmas). The variation in the origin of the magmas from which the porphyry Mo systems were generated suggests that the composition of magma sources is unlikely to have played a major role in the formation of Mo deposits.
The compilation of existing geochronological data on Mo deposits in NE China, including the newly obtained data from this study, shows that Mesozoic Mo deposits (~250 to 90 Ma) widely occur in this region and are linked to three tectonic-magmatic events: (1) Triassic Mo deposits (250–220 Ma) are mainly distributed along the east-west Xilamulun fault and are related to post-collisional crustal extension following the final closure of the Paleo-Asian ocean; (2) Jurassic to Early Cretaceous Mo mineralization (200–130 Ma) displays a clear younging trend from southeast to northwest and is interpreted to be related to the northwestward flat-slab subduction of the Paleo-Pacific plate beneath the Eurasian continent that started from Early Jurassic (ca. 200 Ma); (3) Cretaceous Mo mineralization (130–90 Ma) shows a distinctly reversed migration trend from northwest to southeast, and can be explained by the coastward migration of slab rollback related lower crust delamination, asthenospheric upwelling and lithospheric thinning.
For skarns, the Baiyinnuo'er Zn-Pb deposit was selected as a representative example for detailed study in this study. It is one of the largest Zn-Pb deposits in China, with 32.74 Mt resources averaging 5.44% Zn, 2.02% Pb and 31.36 g/t Ag. Several phases of igneous rocks, including Permian, Triassic and Early Cretaceous intrusions, are exposed in the mining areas, and among them the Early Cretaceous granites, which intruded into limestone of the early Permian Huanggangliang Formation, are interpreted to be the source of ore, since their Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.25–18.35, ²⁰⁷Pb/²⁰⁴Pb = 15.50–15.56 and ²⁰⁸Pb/²⁰⁴Pb = 38.14–38.32) are highly consistent with the sulfides including sphalerite, galena and chalcopyrite (²⁰⁶Pb/²⁰⁴Pb = 18.23–18.37, ²⁰⁷Pb/²⁰⁴Pb = 15.47–15.62 and ²⁰⁸Pb/²⁰⁴Pb = 37.93–38.44). Sulfur isotope values of the sulfides fall in a narrow δ³⁴S interval of -6.1 to -4.6‰ (mean = -5.4‰, n = 15), suggesting the ore-forming fluid is of magmatic origin.
The deposit formed in three stages: the pre-ore stage (prograde skarn minerals with minor magnetite), the syn-ore stage (sulfides and retrograde skarn minerals including calcite and minor quartz), and the post-ore stage (late veins composed of calcite, quartz, fluorite and chlorite; cutting the above mineral assemblages).The pre-ore stage fluids trapped in pyroxene have higher temperatures (471 ± 31 °C), higher salinity (43.0 ± 3.1 wt. % NaCl eq.), and higher concentrations of Zn (~1.1 wt. %), Pb (~1.7 wt. %), and other elements (e.g., Na, K, Li, As, Rb, Sr, Cs, Ba, Cl and Br) than syn-ore mineralizing fluids (<400 °C, <12 wt. % NaCl eq., ~0.05 wt. % Zn and ~0.03 wt. % Pb). The post-ore fluids are much cooler (<270 °C; averaging ~210°C), with much lower salinity (<5.1 wt. % NaCl eq.), Zn (~38 ppm) and Pb (~19 ppm). Geochemically, the fluids of all paragenetic stages in Baiyinnuo'er are characterized by magmatic signatures based on the element ratios, which are distinctively different from basin brines. The inclusion fluids in pre-ore stage show little variation in composition between ~520 °C and ~420 °C, indicative of a closed cooling system. In contrast, the major components of the syn- and post-ore stage fluids including Cl, Na and K decrease with the temperature dropping from ~350 to <200 °C, indicating a dilution by mixing with groundwater. The metal contents in pre-ore fluid are significantly higher than in syn-ore fluid, but no mineralization occurred. This confirms that the early fluid was, although enriched in metal elements, not responsible for ore precipitation, likely due to its high temperature high salinity nature. The metal deposition was mostly due to mixing with groundwater, which caused temperature decrease and dilution that significantly reduced the metal solubility, thereby promoting metal deposition. The deposition was probably accompanied and facilitated by carbonate dissolution that buffered the acidity generated during the breakdown of Zn (Pb)-Cl complexes and the formation of sulfides. Boiling occurred in both pre-ore and early part of the syn-ore stages, but no evidence indicates that it was related to metal deposition. The current Baiyinnuo'er massive skarns contain both prograde and retrograde minerals (including ore minerals). Paragenetically, they were not formed at the same time, but could be attributed to two (or more) successive pluses of hydrothermal fluids released episodically from residual melts of a progressively downward crystallizing magma. The prograde alteration increased the permeability and porosity, and created sufficient spaces, which was essential for later metal deposition.
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The Las Bambas district in Southern Peru contains three major Eocene-Oligocene skarn deposits, Ferrobamba (1,257 Mt @ 0.57% Cu), Chalcobamba (325 Mt @ 0.55% Cu) and Sulfobamba (230 Mt @ 0.55% Cu), together with several skarn prospects. Southern Peru had a complex tectonic setting during the Eocene. Multiple compressional pulses related to subduction of the Farallon oceanic plate beneath the South American continental crust produced the Andean Orogeny along an active magmatic arc. In this region, the continent was bent at the Arica Elbow, and the Andes Cordillera was offset 200 km along the Abancay deflection due to differential shortening that induced block rotations and regional strike-slip displacements during the compressive Incaic tectonic phase. To the south of the Abancay deflection, the Eocene-Oligocene Andahuaylas-Yauri Batholith intruded marine sedimentary sequences, producing a wealth of porphyry and skarn Cu ± Mo ± Au deposits that constitute the 300 km long Andahuaylas-Yauri Belt. Las Bambas district is the richest Cu district in this belt.
Intrusions in the Las Bambas district were emplaced over an ~11.1 m.y. time period in the late Eocene to early Oligocene. Zircon U-Pb geochronology, whole rock geochemistry, and spatial associations with mineralisation have allowed four magmatic stages to be defined, premineralisation (41.86 ± 0.60 to 38.10 ± 1.60 Ma; gabbros, diorites, and granodiorites), earlymineralisation (36.54 ± 0.62 to 35.06 ± 0.54 Ma; diorites, quartz diorites, quartz monzodiorites and monzonites), syn-mineralisation (34.36 ± 0.40 to 32.95 ± 0.28 Ma; monzodiorites, quartz monzodiorites, quartz monzonites, and granodiorites), and late-mineralisation (33.21 ± 0.55 to 31.87 ± 0.46 Ma; quartz monzodiorites, monzonites, and monzogranites).
Whole rock trace element ratios (Sr/Y and V/Sc) and zircon geochemistry (Eu anomaly and Dy/Yb) indicate that all magmatic stages were hydrous, oxidized, and prospective for mineralisation. Lower Sr/Y and V/Sc ratios from pre-mineralisation intrusions indicate that these were the least hydrous. The early-mineralisation magmatic stage has strong indications of magma hydration and fertility (high Sr/Y and V/Sc ratios), despite a lack of any known Cu mineralisation. The high MREE/HREE ratios of early-mineralisation intrusions imply a garnetstable deeper-seated magmatic source related to a period of thickened crust, which appears to have been exhumed and partially eroded in the late Eocene, potentially resulting in the loss of any shallow level early formed Cu mineralisation.
Progressive fractionation of Las Bambas magmatism was disrupted by mafic magma injection during syn-mineralisation magmatism, reflected both in elevated whole rock Fe\(_2\)O\(_3\) and high Ni and Cr contents in disseminated magnetite. Mafic magma underplating of the felsic magma chamber is interpreted to have triggered Cu mineralisation. Tha late-mineralisation stage saw a return to a conventional fractionation trend, with only minor skarn mineralisation formed.
Syn-mineralisation intrusions from Las Bambas produced weak porphyry copperstyle mineralisation with early K-feldspar – biotite and later quartz – magnetite – Cu-sulfide vein stockworks. Intrusions in contact with limestone produced skarn alteration defined by garnet (35.0 ± 1.1 to 33.4 ± 0.6 Ma; U-Pb dates) and banded pyroxene – magnetite skarns. Retrograde skarn alteration produced massive magnetite after garnet and pyroxene skarn, epidote after grandite skarns, massive epidote endoskarns ± chalcopyrite, and quartz – calcite – epidote – specularite – Cu-sulfide mineralisation filling skarn voids, where the bulk of high grade mineralisation is located. Epidote endoskarn U-Pb dates (32.7 ± 1.0 to 31.0 ± 1.8 Ma) are younger than the intrusive protolith, with the time between emplacement of the intrusion and epidote endoskarn alteration related to the volume of the magma intruded, producing complexities in the retrograde paragenesis. Stable isotopic analyses show that magnetite (δ\(^{18}\)O: 6.6 to 13.2 ‰) and epidote (δ\(^{18}\)O: 3.3 to 8.3 ‰) precipitated from magmatic hydrothermal fluids with minimal meteoric water involvement.
Epidote has heterogeneous compositions at the grain scale, but there are systematic variations in epidote chemistry at the deposit scale. At Ferrobamba and Chalcobamba, the Fe/ Al ratios of epidotes decrease with distance outward from the centre of mineralisation. Fe-Al substitutions affect the epidote crystal lattice lengths, and have resulted in systematic changes in the SWIR 1550 nm absorption peak position across Las Bambas (Q1 = 1543.2 nm; Q3 = 1545.2 nm) with regards distance to the centres of mineralisation, providing a field exploration tool. Propylitic epidote from syn-mineralisation dykes at Chalcobamba shows systematic spatial variations in Pb, Mg, Sr, Ni, and Cr contents with distance. A 2-D Monte Carlo optimisation of multi-variable linear regression of Pb, Mg, Sr, Ni, and Cr contents for Chalcobamba epidote predicts its centre of mineralisation to be at 786681 mE, 8444007 mN. The Mn and Ga contents of hydrothermal magnetites vary systematically with respect to the centre of Ferrobamba and Chalcobamba, and a Ga proximitor based on Ferrobamba and Chalcobamba magnetite compositions predicted the hydrothermal centre of Sulfobamba to be at 781250 mE, 8443600 mN, 4200 m a.s.l.
Carbonate fluorescence and isotopic fractionation show systematic variations with distance to the mineralised centres at Las Bambas. Red-pink fluorescent calcite occurs proximal to the deposits and is related with high water/rock ratios. Decreasing water-rock interaction outboard of the mineralised centre produced a 3 x 5 km δ\(^{18}O\) depletion halo in the carbonate host rocks around Ferrobamba.
Epidote and magnetite compositions provide fertility indicators for skarn mineralisation at Las Bambas. High As and Sb contents in epidote are favourable signs of prospectivity, as are elevated Ni and Cr contents in disseminated magnetite. The Sallahue stock, a prospect located 3 km northeast of Chalcobamba, has high As and Sb concentrations in epidote and high Ni and Cr contents in disseminated magnetite, and is recommended as a priority for exploration.
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Geochronology
Orogeny
Diorite
Quartz monzonite
Devonian
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Allanite
Breccia
Fluorapatite
Rare-earth element
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The Hattu schist belt is an emerging gold-producing ore district in the western part of the Archean Karelian Province of the Fennoscandian Shield. The belt consists of 2.76 to 2.70 Ga tonalite, granodiorite, and leucogranite intruded into a mafic-felsic epiclastic-volcanic sequence of an only slightly older age. Complex and successive folding, shearing, and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism (550° ± 50°C; 3–5 kbar) at ca. 2.70 Ga. Orogenic gold deposits are hosted by the highly strained zones that developed during the Archean deformation of the belt. However, previous K-Ar and Rb-Sr geochronological studies indicated that a second tectonothermal overprint affected the Hattu schist belt between 1.7 and 1.8 Ga during the Svecofennian orogeny.
Results of our field, mineralogical, textural, and Pb isotope studies suggest that ore deposition at Pampalo was initiated by hydrothermal processes at the time of the emplacement of feldspar porphyry and tonalite intrusions and dikes, at around 2.72 Ga. Albitization and quartz-tourmaline-biotite-muscovite veining characterize this hydrothermal activity. The major stage of gold ore deposition can be confined to the subsequent development of high-strain zones in an intermediate-felsic tuffaceous unit (mafic schist) characterized by biotite-carbonate-pyrite alteration. Ar-Ar studies revealed the complete resetting of the Ar-Ar system in muscovite and biotite due to the Svecofennian orogeny, with closure of the isotopic system at 1.81 Ga. Results of Pb isotope studies of hydrothermal K-feldspar, galena, and altaite by laser ablation-inductively coupled plasma-mass spectrometry indicate that K-feldspar alteration and remobilization of metals also took place during the Svecofennian reactivation of high-strain zones. The petrographic manifestation of this process is the replacement of albite by hydrothermal K-feldspar. Fluid inclusion data from hydrothermal K-feldspar suggest that carbonic-aqueous, low-salinity fluids interacted with the Archean ore at 350° to 400°C and 1.8 to 2.4 kbar during the Svecofennian overprint. Results of mass transfer calculations indicate that potassium gain correlates with increase of gold concentrations in the mafic schist and feldspar-porphyry units. Therefore, the Svecofennian overprint locally also enhanced the grade of gold mineralization, mostly along competency differences of the mafic schist and feldspar porphyry blocks/dikes. Later percolation of relatively low temperature (<300°C) saline basin fluids in some fractures of the crystalline basement also left their Pb isotope and fluid inclusion signatures on the mineralization while further modifying the composition of the ore.
Occurrences of hydrothermal proto-ores, as well as fluid flow events along structures reactivated by overprinting orogenic processes, are not exceptional features in Archean orogenic gold provinces. This study shows that the combination of in situ Pb isotope studies of U-poor alteration minerals and Pb-rich ore minerals, together with evaluation of relationships between gold enrichment and hydrothermal alteration and fluid inclusion studies, can be very useful in determining the significance and conditions of overprinting processes, as well as their potential implications for genetic and exploration models.
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Peralkaline rock
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The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca.278 Mt of ore at 0.98 % Cu, 0.75 g/t Au, and 2.5 g/t Ag.In contrast to the predominantly granite-hosted Olympic Dam IOCG deposit, Prominent Hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate, including local massive dolomite.Essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediatecomposition lavas, inferred to be members of the lower Gawler Range Volcanics, host the economically mineralized hematite breccias.The volcanic-sedimentary package was downfaulted and tilted along a major E-W fault, north of which similar but regionally low-grade metamorphosed rocks were affected by subeconomic skarn mineralization, and (on a more regional scale of the Mount Woods domain) intruded by granitic and gabbroic bodies.Hydrothermal alteration and mineralization at Prominent Hill involved pervasive and texturally-destructive replacement of formerly calcareous, dolomitic, and siliciclastic breccia components.Hydrothermal alteration minerals comprise hematite, magnetite, siderite, ankerite, quartz, sericite, chlorite, kaolinite, fluorapatite, fluorite, barite, REE-U minerals (including monazite), uraninite, and coffinite, together with Cu sulfides including chalcopyrite, bornite, and chalcocite in the highest-grade ore.Brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies, such that sedimentary contacts became obscured.Mass-balance calculations identify Al, Ti, Si, and Zr as least-mobile components during hematite-chlorite-sericite to weak hematite-quartz alteration.Because Zr was not regularly assayed in drill cores, we use concentration ratios of Ti, Al, and Si from the deposit-scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and Cu mineralization.The resulting lithochemical model, based on one horizontal and five vertical cross sections, is used as a basis for mapping alteration patterns calculated from molar (Fe+Si)/(Fe+Si+Al), K/Na, and K/Al ratios.These chemical patterns, in conjunction with mineral stoichiometry, indicate that the spatial distribution of hematite, chlorite, variably phengitic sericite (and /or illite) ± kaolinite ± quartz-bearing alteration is superimposed on the pattern of interpreted lithological contacts.The alteration patterns confirm visual logging results showing that hematite enrichment correlates only partially with the distribution of Cu grades of >0.25 wt %.A subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a Cu-barren core in the central and eastern parts of the deposit.Zones of increasing K/Al and K/Na ratios extend upward and westward from this Cu-barren core, transgressively overprinting lithological contacts.The degree of hematite-quartz replacement can be measured by a hematite-quartz alteration index, here termed the HMSI value [(Fe+Si)/(Fe+Si+Al)], which inversely correlates with the normal probability for Cu grade.Areas of highest Cu grade (>1 wt %) spatially correlate with irregular zones having intermediate molar alteration indices: 0.34 < K/Al < 0.40, 20 < K/Na < 36, and HMSI < 0.98.Hematite breccias and Cu ore deposition developed after tilting of the host sequence into its present steep orientation, as indicated by geopetal structures within the breccia matrix.Thus, the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower Gawler Range Volcanics.The formation of the Prominent Hill orebodies occurred during or after upthrusting of deeper-seated rocks containing subeconomic Cu in skarns north of the fault.Faulting as well as ore formation may be related to orogenic processes in the central and northern part of the Mount Woods domain.Iron oxide introduction was decoupled from, and at least partly preceded, hydrothermal deposition of high-grade Cu.Geochemical and petrographic data indicate that economic Cu mineralization occurred together with mildly acidic hematite-chlorite-sericite ± siderite alteration of originally carbonate-, illite-, and feldspar-bearing sedimentary rocks.The presence of copper enrichment with an intermediate degree of cation leaching from the host rocks indicates that pH neutralization of initially highly acidic metal-transporting fluids was an essential factor causing Cu sulfide deposition.Distinct ranges in Na/K/Al ratios and low HMSI values offer potential as exploration indicators pointing towards higher ore grades.These results from Prominent Hill are consistent with recently published mineralogical studies at the giant Olympic Dam deposit, indicating similar ore-depositional controls despite lithologically different host rocks.
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The large Dahongshan Fe-Cu-(Au-Ag) deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, southwest China, contains approximately 458.3 Mt of ore at 41.0% Fe, 1.35 Mt Cu (metal) at 0.78% Cu, and significant amounts of Au (16 t), Ag (141 t), Co (18,156 t), and Pd + Pt (2.1 t). The deposit consists mainly of two types of ores: (1) lenses of massive or banded magnetite-(hematite) hosted in extensively Na metasomatized metavolcanic rocks, metaarenite, and brecciated rocks, and (2) strata-bound disseminated, stockwork, and banded magnetite-chalcopyrite-(bornite) in mica schist and marble. Both types of orebodies and country rocks underwent extensive hydrothermal alteration, resulting in a similar paragenesis. Pervasive stage I sodic alteration formed widespread albite and local scapolite. It was subsequently replaced by Ca- or K-rich minerals represented by actinolite, K-feldspar, biotite, sericite, and chlorite of stages II and III. Magnetite is slightly younger than and partly overlaps the sodic alteration assemblages. Hematite is texturally later than magnetite, is locally abundant within the massive Fe oxide orebody, and is closely associated with sericite. Copper sulfides are coeval with quartz, biotite, sericite, and chlorite in stage III assemblages. Widespread siderite and ankerite predominate in stages II and III, respectively. Quartz-calcite veins mark the result of waning stage IV hydrothermal alteration. In addition to widespread alteration during the major ore-forming event, the deposit has also undergone extensive overprinting and remobilization during post-ore magmatic and metamorphic events. The Dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore-hosting rocks. One dolerite sill that cuts a massive Fe orebody has a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age of 1661 ± 7 Ma, which is, within uncertainty, consistent with the age of 1653 ± 18 Ma determined for hydrothermal zircons from stockwork chalcopyrite-magnetite ore. The zircon U-Pb ages are thus considered to mark the timing of major mineralization that formed the Dahongshan deposit. Post-ore modification is recorded by an Re-Os isochron age of 1026 ± 22 Ma for pyrite in discordant quartz-carbonate-sulfide veins, and by younger Neoproterozoic mineralization dated at ca. 830 Ma using Re-Os isotopes on molybdenite. The former age is contemporaneous with late Mesoproterozoic magmatism in the region, whereas the latter is coeval with regional Neoproterozoic metamorphic events in southwest China. Carbon and oxygen isotope values of albitized marble are between those of mantle-derived magmatic carbon and dolostone end members. The ore-forming fluids that equilibrated with stage II magnetite have δ 18 O values of 9.1 to 9.5‰, whereas fluids linked to the deposition of quartz and ankerite during stages III and IV have lower δ 18 O values of 2.9 to 7.3‰. The oxygen isotope data indicate that the ore-forming fluids related to stage II are chiefly magmatically derived and mixed with abundant basinal brine during stages III and IV; this interpretation is consistent with sulfur isotope values of sulfides in the deposits. Pyrite and chalcopyrite from the Dahongshan deposit have a large range of δ 34 S values from −3.4 to +12.4‰, implying mixing of magmatic and external sulfur (likely from basinal brines) in sedimentary rocks. The Dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large-scale fluid circulation and pervasive sodic-calcic metasomatism in country rocks. Ore metals were derived mainly from a deep-seated magma chamber and partly from country rocks. Hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids. Magnetite and hematite precipitated early near the dolerite intrusions, whereas Cu sulfides formed later in country rocks where sulfide saturation was favored. We propose that this genetic model may be widely applicable to Precambrian IOCG deposits elsewhere that formed in intracratonic rift settings.
Sericite
Actinolite
Stockwork
Bornite
Muscovite
Ore genesis
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The intrusive rocks of the Emeksan area mainly, consist of granoaiorite, variousadamellites and granites. These rocks were intruded into the pre-Mesozoic basement and the Mesozoic to Tertiary volcanic rocks. The batholith was cut by a series of dolerite, andesite porphyry and microgranite dykes, termed the late magmatic series, which show closely similar chemistry to the Tertiary hypabyssal and volcanic rocks of the Pontids. The chemical trends of the late magmatic series are typical of calc-alkaline suites. The calc-alkaline fractionation is also thought to be responsible for the generation of the intermediate Plutonic rocks in the area although variation diagrams indicate that other processes were involved before the solidification of the granodiorite and Esenli porphyritic adamellite. Field occurrences and factor analyses of these intermediate rocks imply that their chemical compositions were affected by xenoliths of quartz-feldspar-mica schists derived from basement rocks which resulted in higher levels of Nb, Y and the lithophile elements K and Rb. An earlier leucogranite intrusion shows a distinctly different character relative to the other intrusive rocks. It has gradational, contacts to the basement rocks, is highly enriched in quartz and alkali feldspars of lower temperature form, and shows, coarser grain-size and abnormal perthite abundance. The leucogranite contains anomalous values of Na(_2)O, K(_2)O, Rb, Kb and Y relative to the normal calo-alkaline fractionation trend. The leucogranite plots in the low temperature-high-pressure fields of quartz-alkali feldspar phrase diagrams. The evidence suggests that the leucogranitic magna probably derived from the basement rocks as a result of partial fusion. N-S compressive forces were the primary causes of the structural features in the area. The E-W extension of phenocrysts and the N-S, NE-SW and NW-SE trends of dykes, veins and faults are related to these forces which were probably produced by subduction processes during the Tertiary. The variation of K(_2)O, and other indices, in the magmatic rocks across the Pontids support a northward inclined Benioff zone. The molybdenum mineralisation is related to the intermediate to acidic calo-alkaline intrusive rocks showing porphyritic or fine-grained texture. Fractures with a narrow dispersion about the N-S direction are the important structural features controlling the sulphide mineralisation. The molybdenum is introduced in K-feldspar-quartz-biotite and quartz veins and sometimes occurs as disseminations. Chalcopyrite is very limited in the molybdenum zone but relatively enriched in the surrounding areas in which pyrite is also widespread. Pb-Zn-Cu sulphide veins are occasionally found in the outermost sulphide zone. Hydrothermal alteration associated with the ore mineralisation also shows a zonal distribution. In the potassic alteration zone the host rocks were replaced by fine-grained material, including K-feldspar-quartz-biotite. Quartz, sericite and clay minerals are the main minerals of rocks in the phyllic to argillic alteration zones, which include most of the Mo-mineralisation. The development of chlorite, after mafic minerals, and day minerals after feldspars represents the outermost alteration zone, which contains pyrite, chalcopyrite and some carbonate veins. The primary distribution of elements during bydrothermal alteration was used to examine the types of host rock alteration, and to locate there mineralisation more precisely. Higher Mo, Kb, Si0(_2) and K(_2)O and lower Fe(_2)0(_3), MgO, CaO, Na(_2)0, Ti0(_2), Sr and Zn values are recorded in rocks from the pervasive hydrothermal alteration centre. Cu, Zn, S and Rb are enriched in the halo zone and may be used in exploration for the similar ore deposits in other areas. The Mo-mineralisation in the area shows a close similarity to porphyry ore deposits which are economically important. Exploration for this type of deposit should therefore be continued in the Pontids.
Leucogranite
Porphyritic
Alkali feldspar
Diorite
Batholith
Silicic
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Sm-Nd, Pb, and Sr isotope and rare-earth element (REE) analyses of quartz-carbonate veins associated with the Coeur d’Alene mining district, Idaho and western Montana, constrain the age and origin of Coeur d’Alene veining. An Sm-Nd isochron age of siderites from ore-bearing veins (1511 ± 45 Ma) and a Pb-Pb isochron age of siderites, ankerites, and calcites from ore-barren veins (1523 ± 41 Ma) are similar to the model age of Coeur d’Alene-type Pb (~1450 Ma) and constrain Mesoproterozoic events in the earliest history of the Belt-Purcell Basin. Xenotime, an accessory mineral found in all veins, is zoned with a core laser ablation age of 1420 ± 90 Ma, confirming vein emplacement early in the diagenetic and/or metamorphic history of the Belt-Purcell Basin, and an overgrowth age of 990 ± 130 Ma results from later, Grenville-age metamorphism. Pb isotope ratios of carbonates from both ore-bearing and ore-barren veins define a single linear array resulting from Mesoproterozoic vein emplacement with an initial Pb isotope ratio similar to that of Coeur d’Alene-type Pb observed in galena, while moderately radiogenic Pb isotope ratios originate from variable amounts of accompanying xenotime. This array is similar to the previously determined “more radiogenic” array of Leach et al. (1998a). Ore-barren veins are characterized by initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.5108 and 0.769, respectively, which may be partially inherited from Belt-Purcell Supergroup and/or Archean rocks. In contrast, initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.5083 and 1.146 of ore-bearing veins require a source with highly unradiogenic Nd and highly radiogenic Sr, most likely Archean crust. Ore minerals may result from interaction of hydrothermal fluids from Archean sources with potential Belt-Purcell Basin sedimentary exhalative (sedex) deposits. REEs are concentrated in carbonate fractions of veins. Siderite and ankerite are light REE depleted and heavy REE enriched, resulting in significant Sm/Nd fractionation from the dual effects of REE complexation in carbonate fluids and mineralogical control, which in the case of siderite and ankerite favors the heavy REEs. Positive Eu anomalies characterize ore-bearing veins, whereas ore-barren veins retain negative Eu anomalies. Normalization of REE patterns of ore-barren veins to those of adjacent Belt-Purcell wall rocks results in a near-zero anomaly, implying that REEs from Belt-Purcell metasediments were locally scavenged by carbonate-rich fluids responsible for ore-barren veins. In contrast, leaching of Archean gneisses followed by probable interaction with sedex deposits produced fluids that formed ore-bearing deposits. Further faulting and deposition from successive fluxes of carbonate-rich solutions derived in part from leaching Belt-Purcell metasediments generated ore-barren siderite-, ankerite-, and, finally, calcite-dominant veins.
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Breccia
Muscovite
Rare-earth element
Fluorite
Carbonatite
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