Statistical evaluation of the spatial relationship of intrusions and faults to Fe-Oxide Cu-Au systems, Cloncurry district
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The origin of Fe oxide-(Cu-Au) deposits and the relative role of played by magmas (both felsic and mafic) versus evaporite-rich country rocks as a source of fluids and/or metals remains controversial. A popular model for the formation of IOCG deposits in the Mt Isa Eastern Succession involves fluids derived from the late orogenic granites mixing with a second external fluid source forming Fe(commonly magnetite-) rich alteration zones that contain vein stockwork, breccia, dissemination or replacement style mineralization (Oliver et al., 2000). This is assumed to be commonly spatially and temporally associated with felsic pluton emplacement and cooling around 1540-1500 Ma. This contrasts with an alternative model in which the fluids are entirely intra-basinal and amagmatic in origin (Barton and Johnson, 1996). Recent dating studies at Osborne have highlighted a potential syn-peak metamorphic timing to mineralization (based on 1595 Ma Re-Os age dates on molybdenite and a 1595 ± 6 Ma U-Pb age date on hydrothermal titanite), with no apparent proximal major intrusion (Gauthier et al., 2001). There is also a potential link between mineralization and widespread mafic intrusive activity that occurs in the Eastern Succession for the entire range of known mineralization ages. Futhermore, at some deposits (276 orebody at Starra) intra-ore mafic intrusives have been recorded.Keywords:
Felsic
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Breccia
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The Eastern Succession of the Proterozoic Mt Isa Block, including the
Cloncurry District and the Mary Kathleen Fold Belt (MKFB), contains numerous
examples of Fe-oxide-Cu-Au mineralisation. Most deposits, including Ernest
Henry, Eloise, Starra and Mt Elliott formed after the peak of ca. 1600-1575 Ma
upper greenschist to amphibolite facies metamorphism, and during the waning
phases of the Isan Orogeny. Mineralisation was broadly synchronous with
emplacement of voluminous phases of the Williams and Naraku batholiths (ca. 1550
- 1500 Ma) and widespread brecciation and accompanying metasomatism.
Brecciation and metasomatism were best developed within Cover Sequence 2
stratigraphy, and in particular within calc-silicate rock and meta-siltstone
stratigraphy of the Corella Formation, the predominant rocks of the Mary Kathleen
Group.
The geometry and distribution of brecciation in the Corella Formation was
in part controlled by retrograde buckle folding imposed on a heterogeneous rock
sequence that was fractured and boudinaged both pre- and syn-buckle folding.
Brecciation is far more widespread in the Cloncurry District relative to the MKFB,
reflecting in part a larger proportion of stratigraphy in the Cloncurry District that
was at low angles to the shortening direction during the waning phases of the Isan
Orogeny, favoring refolding and consequent fracturing. Variations in regional
structural trends reflect strain partitioning around competent intrusive bodies, fault
reactivation and refolding. Other contributing factors for brecciation include low
temperature conditions during late deformation, and the proximity to voluminous
intrusions, the emplacement of which likely resulted in transient elevated fluid
pressure and strain rates, favoring fracturing and brecciation. The relative paucity of
brecciation in most stratigraphic units outside of the Corella Formation reflects a
high proportion of incompetent stratigraphy in these sequences (e.g. voluminous
micaceous schists within the Soldiers Cap Group), which were able to• accommodate
strain by plastic flow.
The broad-scale geometry of the Cloncurry District reflects Cover Sequence
3 rocks overlying Cover Sequence 2 rocks, the two sequences being separated by
early faults. Marbles within the Corella Formation, and schists in other stratigraphic
sequences were not prone to brittle failure, and acted as low permeability barriers to
fluid flow. These horizons allowed for the attainment of elevated fluid pressures
within large volumes of brecciated rock. During the final stages of brecciation, these
low competence marbles and schists were fractured and brecciated, predominantly
within discrete fault zones. This shift from widespread brittle-ductile to purely
brittle deformation likely reflects progressive cooling, as well as locally elevated
fluid pressure and/or strain rate associated with pluton emplacement and degassing.
A synchronous district-scale shift from compression to transtension facilitated the
development of vertically continuous zones of dilation within faults, resulting in
very large fluid pressure gradients and catastrophic fault valving.
Brecciation was accompanied by widespread metasomatism that ranges from
high temperature (400° - 600°C) Na-(Ca)-rich assemblages (e.g. albite ± actinolite,
clinopyroxene, scapolite, magnetite, titanite, etc.) to retrograde (<400°C) chloritic
assemblages. Interpretation of stable (0 and C) and radiogenic (Sr) isotopes and
mineral chemistry is consistent with this spectrum of alteration assemblages
reflecting metasomatic fluids of two predominant origins. The oxygen and carbon
isotopic signature of carbonates from Na-(Ca) assemblages indicates that fluids
responsible for this style of alteration were not simply equilibrated with magmatic
rocks, but were exsolved from crystallizing plutons. Low temperature, low salinity
fluids of inferred meteoric origin were introduced late in the paragenesis, and do not
appear to have contributed significantly to the mass budgets of eu-Au ore systems
in the district.
Extensive fluid-wallrock interaction prior to mineralisation appears to have
been important in the genesis of some deposits that record K- and Fe-rich alteration
haloes, including for example the Ernest Henry deposit. However, the occurrence of skarn-like, intrusion-proximal mineralisation that lacks significant K- and Fe enrichment
at for example Mt Elliott, indicates that fluid-wallrock interaction was
not a necessary precursor for all styles of Cu-Au mineralisation in the Cloncurry
District.
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gabbro and diorite with an associated magmatic magnetite enrichment support the idea of a close genetic link between such intrusives and IOA hydrothermal mineralization.All mineralization events at Peña Colorada are associated with pervasive potassic to propylitic alteration, whereas at Arrayanes they are associated with dominant sodic alteration instead.Alteration features are suggestive of relatively shallow and deep formation of these deposits, respectively .Event 3 thermally reset fluorapatite in fragments of pegmatoid magnetite + fluorapatite + diopside associations (dated at 59 ± 2 Ma, AFT) within the polymictic breccia, which were sampled from a deep orebody (still to be found) and that would be likely associated with event 1 or 2. Consequently, exploration endeavors at depth at Peña Colorada may be considered promising.In this study, we use numerous geological and geochemical proxies to constrain the likeliest genetic model for the Peña Colorada and neighboring deposits: (a) the nearness in time and space between hydrothermal mineralization and magnetite-rich, tholeiitic, relatively oxidized intrusive rocks; (b) the occurrence of key mineral associations (i.e.magnetite + fluorapatite ± diopside veins); (c) the exclusive occurrence of fluorapatite in lieu of other apatites; (d) the composition in key major cations (Ca, Fe, Na, Mn) in fluorapatite; (e) the correlations between Ni/Cr vs. Ti values, between Ti+V vs. Ni/(Cr+Mn) values, between Ti+V vs. Al+Mn values, and Mg contents in magnetite; (f) pyroxene thermometry; (g) log f(O 2 ) values calculated from Mn contents in fluorapatite; and (h) normalized REE patterns, and ΣLREE and ΣHREE contents in fluorapatite.These proxies indicate that IOA deposits in the Peña Colorada area have a hydrothermal origin with a strong magmatic influence (magmatic-hydrothermal iron oxide, or MHIO, deposits) that formed under high oxygen fugacities and "mo derate" temperatures, and with a high geochemical affinity with IOCG and Kiruna-type deposits or the general IOCG "clan" (for both hydrothermal minerals and associated hypabyssal rocks).Relatively high Ti contents in magnetite, and high Ce and low Eu contents in fluorapatite in these deposits (with respect to typical compositions in IOCG "clan" deposits) are geochemical features still in need of further explanation.The correlation between regional and local structural domains and the geochronologic study in this paper constrain the possible ages of such domains as follows: (1) the N-S to NNW-SSE domain can be bracketed between 67.6 and 63.26 Ma, (2) the WNW-ESE domain between 63.26 and 59.39 Ma, (3) the E-W domain between 54.84 and 50.70 Ma, (4) the WNW-ESE to NW-SE domain is younger than 48.18 Ma, and (5) the NE-SW domain is still active.The structural analysis also shows that the massive orebody at Peña Colorada is partially stratabound but its emplacement was also controlled by low-angle Laramide faults, and that hydrothermal fluids were preferentially driven through volcanosedimentary rocks.The latter characteristic is not only a matter of the stratigraphic distribution of relatively pervasive versus impervious rocks but also of the lateral distribution of such rocks due to N-S strikeslip faults.As additional results of this study, we determined that the conglomerates atop the host volcano-sedimentary sequence that were initially attributed to the Cerro de la Vieja Formation cannot be older than 67.6 Ma, and that the IOA deposits at Peña Colorada would be formed at depths of only a few hundred meters.
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Abstract The Sudbury Breccia is an impactite that formed in the target rocks of the 1850 +1.3/–2.4 Ma Sudbury impact structure in Ontario, Canada. The breccia is interpreted to have formed during crater excavation or modification at the time of the impact event. Copper-Nickel-Platinum Group Elements sulfide melts moved away from contact mineralization at the base of the melt sheet to form veins and stockworks in the footwall. The distribution of both Sudbury Breccia and the later sulfide melts is partially controlled by pre-existing weaknesses in the footwall, such as lithological contacts; as a result, the two are often spatially associated. A combination of early magmatic-hydrothermal and late metamorphic fluids modified the sulfide mineralization to create broad haloes of metalliferous hydrous silicate minerals proximal to the Sudbury Breccia and in the footwall rocks. This study examines variations in the trace-element geochemistry of the breccia-matrix mineral assemblage developed adjacent to footwall mineralization in the North Range (Coleman Mine) and the South Range (Creighton Mine). The Sudbury Breccia is widely considered to have formed in a single event, contemporaneous with the meteorite impact, therefore it provides a relatively consistent baseline for studying subsequent metamorphic and hydrothermal processes, unlike the Archean Superior Province and Paleoproterozoic Southern Province, which experienced pre-impact alteration associated with tectonic and igneous activity. This study focused on examining the textural, chemical, and petrological changes recorded in biotites, amphiboles, chlorites, and Fe-oxides produced by late magmatic and post-magmatic sulfide mineralization and U-Pb age dates of titanite. Titanite-chlorite assemblages in the North Range footwall of the SIC overprint metalliferous assemblages adjacent to the McCreedy East 153 footwall ore body. They yielded an age of 1358 ± 78 Ma, which is interpreted to be associated with the waning effects of the 1450 ± 0.15 Ma Chieflakian orogeny. In the South Range at the Creighton Mine, the Sudbury Breccia hosting the Creighton Deep footwall ore body records a shift from ferro-hornblende to ferro-tschermakite amphibole. When coupled with variations in Ca/Ti in titanite and a U-Pb titanite age of 1616 ± 33 Ma, the assemblage is interpreted to reflect increasing temperature-pressure gradient towards shear zones that were active during the Mazatzalian orogeny, during which time the sulfide mineralization was remobilized. At both locations, biotite exhibits an increase in Tl content that relates to the Ni content of the host rocks and proximity to sulfide mineralization. This relationship may be produced by remobilization of metals during interaction between sulfide mineralization and hydrothermal fluids. Although not directly associated with sulfide emplacement, these signatures provide evidence for a larger geochemical halo around the mineral zone that may provide the basis for ranking the prospectivity of the footwall and possibly vectoring towards mineral zones.
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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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The Cannington Ag-Pb-Zn deposit represents an important new discovery of Ag-rich Broken Hill-type mineralization in the Eastern succession of the Mount Isa inlier. The deposit is concealed beneath 10 to 60 m of Recent and Cretaceous cover, and there is no oxidation profile preserved at the basement subcrop. Mineralization is hosted by amphibolite facies migmatitic quartzofeldspathic gneisses, and is characterized by intense deformation and metamorphism, with complex metasomatic and retrograde overprints. Lithostratigraphic correlations of the host lithologies with other units in the Eastern succession are unclear. Limited dating of probable stratigraphic equivalents has given an age of 1677 + or - 9 Ma, which is broadly coeval with host depositional ages for Pb-Zn-Ag mineralization at Mount Isa, HYC and Broken Hill.The orebody is divided on the basis of late structural displacement into Northern and Southern zones. The Southern zone is the focus of current development, and mineralization occurs as crudely strata-bound massive sulfide lenses that display complex brittle and ductile disruption. A large-scale isoclinal D 2 synform within the Southern zone appears to control broad repetition patterns between ore lenses. Grade control within individual ore zones can also be related to zones of ductile strain and metasomatism influenced by strain partitioning around the termination of the Core Amphibolite.Mineralization within the Cannington Southern zone is divided into five main economic lode horizons that incorporate 10 mineralization types. These types are defined on the basis of distinctive zonations in Pb/Zn ratios, and Fe-rich versus siliceous gangue lithologies. Fe-rich mineralization types are characterized by coarse-grained, equigranular hedenbergite, Mn-Fe pyroxenoid, magnetite, olivine, and fluorite mineralogies, zones of amphibole, almandine, ilvaite, pyrosmalite-dominant mineralogies with sulfide- and fluorite-rich ductile breccias are associated with extensive postpeak metamorphic metasomatism and retrogression. Siliceous mineralization types represent late-stage metasomatism, and are associated with further modification of mineralization and retrogression of Fe silicates. Siliceous mineralization types exhibit a distinctive low abundance of magnetite and fluorite.Dominant sulfides are galena and sphalerite, which show multiple generations and variable intergrowths. Subordinate magnetite-pyrrhotite with minor arsenopyrite-lollingite-chalcopyrite are characteristic of Fe-rich mineralization types. Pyrite is generally absent and is only locally associated with late structural and low-temperature metasomatic overprints. Extreme Ag enrichment is a consistent association of all mineralization types in the Cannington deposit, and is related to argentiferous galena with freibergite inclusions. High levels of Sb, Cd, As, Cu, and F are also a feature of specific mineralization types. When in full production, Cannington will be one of the world9s largest Ag producers.Cannington shows many similarities with the Broken Hill Main lode (New South Wales), and represents an important new example of a Broken Hill-type classification. However, the Ag enrichment that characterizes Cannington is unusual even for previously considered Ag-rich members of the classification. A genetic model is proposed that involves high-temperature metasomatic zone refining of a preexisting Fe-Ca-Mn-Pb-Zn-Ag-rich mineralized system.
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The large scale Wernecke Breccia system occurs throughout the 13 km-thick Early Proterozoic Wernecke Supergroup (WSG) and is spatially associated with regional-scale faults. Breccia emplacement made use of pre-existing crustal weaknesses and permeable zones; metaevaporitic rocks in the lower WSG may be intimately related to breccia formation. The breccia bodies host vein and-disseminated iron oxide-copper-gold ± uranium ± cobalt mineralisation and are associated with extensive sodic and/or potassic metasomatic alteration overprinted by pervasive carbonate alteration. Multiple phases of brecciation, alteration and mineralisation are evident. Six widely spaced breccia bodies that occur in different part of the WSG were examined in this study (i.e. Slab, Hoover, Slats-Frosty, Slats-Wallbanger, Igor and Olympic). New information includes geological, paragenetic, geochronological, isotopic, fluid inclusion thermometric and compositional data.
Re-0s analyses of molybdenite from a late-stage vein that cross-cuts breccia gave model ages of 1601 ± 6 and 1609 ± 6 Ma. These ages range from older than to within error of the ca. 1594.8 ± 4.6 Ma published U-Pb (titanite) date for breccia in the same area. A second molybdenite sample from a late-stage vein gave a Re-0s model age of 1648 ± 5.97 Ma. This date is considered analytically sound but the significance of it is not clear as it is believed to cut the ca. 1595 Ma breccia. Step heating ⁴⁰Ar-³⁹Ar analyses carried out on muscovite from Wernecke Breccia matrix, a syn-breccia vein and two late-stage veins yielded dates of 1178.0 ± 6.1, 1135.0 ± 5.5, 1052 ± 10 and 996.7 ± 8 Ma respectively. These dates are significantly younger than the minimum age (ca. 1380 Ma) of Wernecke Breccia indicated by cross-cutting relationships and must have been reset. Samples submitted for U-Pb and Pb-Pb analyses gave discordant results that cannot be used to constrain the age of Wernecke Breccia or Wemecke Supergroup.
Fluids that formed Wemecke Breccia were hot (185-350 °C), saline (24-42 wt. % NaCl eq.) NaC1-CaC1₂ brines. Isotopic compositions for hydrothermal minerals range from: δ¹³C carbonate ≈-7 to +l ‰ (PDB), δ¹⁸O carbonate ≈ -2 and 20 ‰ (SMOW), δ³⁴Spyrite/chalcopyrite ≈-13 to +14 ‰ (CDT) and δ³⁴Sbarite ≈7 to 18 ‰. Calculated δ¹⁸Ofluid ≈-8 to +14 ‰. The isotopic compositions indicate fluids were likely derived from formation/metamorphic water mixed with variable amounts of organic water ± evolved meteoric and/or evolved seawater. Metals and sulphur were probably derived from host strata and fluids circulated via tectonic (and/or gravity) processes. Magmatic waters are considered less likely as a fluid source because the isotopic data do not have a magmatic signature and mafic to igneous rocks spatially associated with the breccia are significantly older (i.e. ca. 1710 vs. 1600 Ma) thus ruling out a genetic connection. This suggests IOCG mineralisation can occur in non-magmatic environments and a division of the broad IOCG class into magmatic and non-magmatic end-members, with hybrid types in between, is suggested that reflects the involvement of magmatic and nonmagmatic fluids. Wernecke Breccia and Redbank are representative of non-magmatic end-members, Lightning Creek is a magmatic end-member and hybrid types include Ernest Henry and Olympic Dam.
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The Cripple Creek district (653 metric tons (t) of Au) consists of Au-Te veins and disseminated gold deposits that are spatially related to alkaline igneous rocks in an Oligocene intrusive complex. Vein paragenesis includes quartz-biotite-K feldspar-fluorite-pyrite followed by base metal sulfides and telluride minerals. Disseminated deposits consist of microcrystalline native gold with pyrite that are associated with zones of pervasive adularia.New 40 Ar/ 39 Ar dates indicate that there was a complex magmatic and hydrothermal history. Relatively felsic rocks (tephriphonolite, trachyandesite, and phonolite) were emplaced into the complex over about 1 m.y., from 32.5 + or - 0.1 (1Sigma ) to 31.5 + or - 0.1 Ma. A younger episode of phonolite emplacement outside of the complex is indicated by an age of 30.9 + or - 0.1 Ma. Field relationships suggest that at least one episode of mafic and ultramafic dike emplacement occurred after relatively more felsic rocks and prior to the main gold mineralizing event. Only a single whole-rock date for mafic phonolite (which indicated a maximum age of 28.7 Ma) was obtained. However, constraints on the timing of mineralization are provided by paragenetically early vein minerals and K feldspar from the disseminated gold pyrite deposits. Early vein minerals (31.3 + or - 0.1-29.6 + or - 0.1 Ma) and K feldspar (29.8 + or - 0.1 Ma) from the Cresson disseminated deposit, together with potassically altered phonolite adjacent to the Pharmacist vein (28.8 and 28.2 + 0.1 Ma), suggest there was a protracted history of hydrothermal activity that began during the waning stages of phonolite and early mafic-ultramafic activity and continued, perhaps intermittently, for at least 2 m.y.Estimated whole-rock delta 18 O values of the alkaline igneous rocks range from 6.4 to 8.2 per mil. K feldspar and albite separates from igneous rocks have lead isotope compositions of 206 Pb/ 204 Pb = 17.90 to 18.10, 207 Pb/ 204 Pb = 15.51 to 15.53, and 208 Pb/ 204 Pb = 38.35 to 38.56. These isotopic compositions, together with major and trace element data, indicate that the phonolitic magmas probably evolved by fractional crystallization of an alkali basalt that assimilated lower crustal material. Upper crustal contamination of the magmas was not significant The 206 Pb/ 204 Pb compositions of vein galenas almost entirely overlap those of phonolites suggesting a genetic relationship between alkaline magmatism and mineralization. However, a trend toward higher 207 Pb/ 204 Pb (15.57-15.60) and a 208 Pb/ 204 Pb ratios (38.94-39.48) of some galenas suggests a contribution to the ore fluid from surrounding Early Proterozoic rocks, probably through leaching by mineralizing fluids. Limited stable isotope compositions of quartz, K feldspar, and biotite from this and previous studies support a largely magmatic origin for the early vein fluids.It is suggested that three features were collectively responsible for generating alkaline magmas and associated mineral deposits: (1) the timing of magmatism and mineralization, which coincided with the transition between subduction-related compression and extension related to continental rifting; (2) the location of Cripple Creek at the junction of four major Precambrian units and at the intersection of major northeast-trending regional structures with northwest-trending faults, which served as conduits for magmas and subsequent hydrothermal fluids; and (3) the complex magmatic history which included emplacement of relatively felsic magmas followed by successively more mafic magmas with time.
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We recognise two distinctive styles of IOCG deposits in the Cloncurry District of the Proterozoic Mount Isa Block. The earliest (Osborne) type (1680 to 1600 Ma) is dominated by basinal and/or metamorphic fluids, deformed ores in shear zones, sodic-calcic alteration, intra-basinal metal and sulphur sources, and chemically favourable ore hosts. Fluid circulation was likely driven by extensional shearing or convection and magmatic heat inputs, and there a genetic connection to the region's large Pb-Zn ± Ag deposits (Cannington, Mt Isa Pb-Zn) is possible. The later (Ernest Henry) type (c. 1530 Ma) shows potassic alteration, and co-precipitated magnetite and calcite in milled breccia. Geochemical data indicate mixtures of mantle or magmatic fluids with sedimentary formation waters (or their metamorphosed equivalents), and an abundance of CO₂₋ and Cl+F-bearing fluids. These formed primarily by release of fluids, S and probably metals from crystallizing A-type granitoids and tholeiitic gabbros via fluidised breccia pipes, and have likely parallels with Olympic Dam and other large to giant IOCGs worldwide hosted in milled hydrothermal breccias. Fluidisation occurred by violent release of overpressured fluids across permeability barriers near intrusions, carrying volatiles and cm- to m-scale rock fragments upwards, and forming ore deposits in cooling and de-pressurising dilatant structural traps.
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Research Article| January 01, 2018 Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines David P. Braxton; David P. Braxton 1 Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Private Bag 79, Hobart, 7001, Tasmania, Australia2 Anglo American, Group Exploration and Geosciences, 20 Carlton House Terrace, London SW1Y 5AN, United Kingdom †Corresponding author: e-mail, dave.braxton@angloamerican.com Search for other works by this author on: GSW Google Scholar David R. Cooke; David R. Cooke 1 Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Private Bag 79, Hobart, 7001, Tasmania, Australia3 Transforming the Mining Value Chain, an Australian Research Council Industrial Transformation Research Hub, University of Tasmania, Hobart, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Allan M. Ignacio; Allan M. Ignacio 1 Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Private Bag 79, Hobart, 7001, Tasmania, Australia ***Present address: Aranz Geo (AUS) Pty Ltd. 2F, 25 Cantonment St, Fremantle WA 6160, Australia. Search for other works by this author on: GSW Google Scholar Patrick J. Waters Patrick J. Waters 4 Anglo American Exploration (Philippines) Inc., 21 Outlook Drive, Baguio City 2600, Philippines ****Present address: Locrian Resources Inc. 400, 255-17 Avenue SW-Calgary, Alberta T2S 2T8, Canada. Search for other works by this author on: GSW Google Scholar Economic Geology (2018) 113 (1): 83–131. https://doi.org/10.5382/econgeo.2018.4545 Article history first online: 27 Feb 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation David P. Braxton, David R. Cooke, Allan M. Ignacio, Patrick J. Waters; Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines. Economic Geology 2018;; 113 (1): 83–131. doi: https://doi.org/10.5382/econgeo.2018.4545 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Boyongan and Bayugo porphyry Cu-Au mineral deposits, discovered under postmineralization cover during the previous decade, are part of an emerging belt of intrusion-centered Au-rich Cu mineral deposits and prospects in the Surigao district of northeast Mindanao, Philippines. Since their formation in the early Pleistocene, exhumation and weathering of these deposits have led to the development of a 600-m-thick oxidation profile at Boyongan and a modest (30–70 m) oxidation profile at Bayugo. Debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the weathered deposits, preserving their supergene profiles.The mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages. Three premineralization diorite porphyry stocks and a silt-sand matrix breccia complex represent early stages of magmatism and brecciation. Significant Cu and Au introduction followed these events and occurred in association with small early-mineralization diorite porphyry stocks at Boyongan and Bayugo. Within the diorite complex, the two mineral deposits are spatially distinct, separated by approximately 1 km of premineralization diorites. Inter- and late-mineralization intrusions were emplaced as the magmatic-hydrothermal system waned.A characteristic progression of vein and K silicate alteration styles affected each of the synmineralization intrusions. Rare comb quartz unidirectional solidification textures (stage 0) mark the transition from magmatic to hydrothermal conditions. Quartz-poor wispy magnetite-biotite-K-feldspar veinlets characterize stage 1. Stage 2 veins consist of quartz with selvage and/or disseminated magnetite or biotite and K-feldspar halos. Stage 3 quartz veins have K-feldspar halos but generally lack magnetite and biotite. Stage 4 veins consist of massive bornite-chalcopyrite and chalcopyrite-pyrite with K-feldspar halos. Stage 3 quartz veins and stage 4 sulfide veins host Cu-Au mineralization of the greatest volumetric significance, reflecting the general paucity of sulfide in the earlier vein stages. Despite the simplicity of this sequence, detailed paragenetic reconstructions reveal that this characteristic progression of veining and K silicate alteration was repeated with the emplacement of each synmineralization intrusive event, revealing multiple magmatic-hydrothermal cycles of alteration and mineralization.At Boyongan and Bayugo, intense and pervasive illite alteration, in association with pyrite, chalcopyrite, and tetrahedrite-tennantite, developed in narrow structures crosscutting quartz-K-feldspar veins. Debris flows in the burial sequence above and adjacent to the Boyongan/Bayugo complex also contain abundant clasts with intense, pervasive illite and alunite-pyrophyllite-dickite-kaolinite alteration assemblages that have overprinted K silicate-style quartz veins.In both mineral deposits, Cu and Au are associated with intense quartz-vein stockworks composed primarily of K silicate stage 3 veins. Despite this association, not all of these quartz-vein stockworks contain Cu and Au to the same tenor. Quartz-vein stockworks in the eastern high grade of Boyongan have been intersected over a vertical interval of 800 m, having affected much of the early-mineralization stock. However, hypogene Cu grades exceed 0.5% by weight only in the upper 300 m of the stockwork (in the cupola of the early-mineralization stock). Superior grade development in high-grade zones at Boyongan, locally exceeding 2% Cu and 3 g/t Au, developed where fertile vein stages from two or more magmatic-hydrothermal cycles affected the same wall rock.The documented paragenetic relationships demonstrate multiple discrete cycles of K silicate-stage veining and alteration associated with each synmineralization intrusive event. All such events predate formation of feldspar-destructive illite-smectite-chlorite, illite-pyrite, and quartz-alunite-clay assemblages. Existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly, in a period of less than 200,000 years (2.3–2.1 Ma). Geologic and geochronological constraints on the life cycle of Boyongan and Bayugo describe an extremely dynamic history of emplacement, exhumation, weathering, and burial over a period of 2.3 m.y. The study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting. You do not currently have access to this article.
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The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated.
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