Statistical evaluation of the spatial relationship of intrusions and faults to Fe-Oxide Cu-Au systems, Cloncurry district
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Abstract:
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
Stockwork
Breccia
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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 Bajo de la Alumbrera porphyry ore deposit, NW Argentina occurs in the Late Miocene Farallon Negro Volcanic Complex. Dacitic intrusions associated with Cu-Fe sulfide and Au mineralization are cotemporal with part of the host volcanic rocks. Observations made at Bajo de la Alumbrera, including those documenting the concentric zonal pattern of hydrothermal alteration, are included in the accepted genetic models of porphyry ore deposits. Geological mapping and field observations (~18 km of measured stratigraphic sections, and ~75 km2 detailed structural mapping), combined with new U-Pb zircon geochronology and whole-rock geochemistry of the Farallon Negro Volcanics have reinterpreted the structure, architecture, and development of the volcanic complex that hosts the Bajo de la Alumbrera deposit. The Late Miocene Farallon Negro Volcanic, which conformably overlie a generally thin Miocene red-bed sequence, has been dismembered by a series of major younger steep reverse faults, such that crystalline basement is now exposed on three sides of the complex. Broadly a volcanic complex, the district is dominated (particularly in the basal parts) by thick volcaniclastic accumulations with lesser lavas and associated breccias. This volcanosedimentary pile (~1.5 km exposed above the Miocene basement) is characterized by strong lateral facies changes. The lower sedimentary breccia dominated sequence is principally basaltic andesite to andesite in clast composition and associated coherent facies (e.g., lavas and sills). A prominent disconformity with strong local relief separates this lower mafic-volcanic dominated sequence from an upper silicic and more lava dominated sequence. A thin conformable red-bed formation overlies the andesitic volcanics and this is in turn disconformably overlain by Pliocene and younger sediments. This later disconformity has removed the tops of major intrusions across the district and possibly a large part of the most recent volcanic and volcaniclastic deposits. The occurrence of thick sedimentary debris and hyperconcentrated flood flow breccias along with peperitic intrusive contacts throughout the andesite volcanic stratigraphy are inconsistent with the widely accepted interpretation of a single large (25 km in diameter and up to 4 km high) stratovolcano. Rather, sedimentation occurred into an intermontane basin that formed during the Early to Middle Miocene, synchronous with the uplift of the Puna-Altiplano of NW Argentina and NE Chile. Porphyry-related mineralization occurred beneath a multi-vent volcanic complex, a setting common to many Au-rich, porphyry Cu deposits of the circum-Pacific. The results of ELA-ICP-MS, U-Pb zircon geochronology show that volcanism in the complex began at about 8.5 Ma and persisted for a little over 1.5 m.y. Porphyry-related Cu- Au mineralisation began during the early volcanism. This differs from most other Andean porphyry districts where ore-related intrusions are emplaced at the culmination of a protracted history of tens of millions of years. At Bajo de la Alumbrera, hydrothermal alteration developed in and around multiphase porphyries. These intrusions are high-K, calc-alkaline, biotite(hornblende)-phyric dacites, which commonly contain mafic inclusions rich in clinopyroxene-hornblende-magnetite-plagioclase-apatite and some biotite. New petrologic and geochemical studies of the intrusions and volcanic rocks of the Farallon Negro Volcanic Complex reveal that at the time of porphyry mineralization the associated dacitic magmas became dramatically enriched in sulfur. This is corroborated by unusually high sulfur contents (up to 0.6 wt% as SO3 ) in apatite phenocrysts. In addition, geochemical evidence shows that mingling and/or incomplete mixing was important in the petrogenesis of the Farallon Negro Volcanics. The abundance of mafic inclusion in dacite intrusions associated with porphyry ore deposits (e.g., Bajo de la Alumbrera and El Durazno), combined with sieve textures and the presence of internal resorption discontinuities in plagioclase are, in part, evidence for magma mixing and/or mingling. The importance of the mafic magma mixing in a compositionally evolved magma chamber is as a source of sulfur and metals, as previously described in a number of porphyry ore deposits. In the Bajo de la Alumbrera porphyry Cu-Au deposit, intrusions are multiphase, occurring at two discrete intervals. The earliest at 8 Ma (7.99±0.12 Ma) is believed to be associated with minor Cu-Au ore while the later intrusions at 7 Ma (7.12±0.04 Ma) are emplaced synchronously with the bulk of the mineralization and hydrothermal alteration. Comparison of these intrusion ages with previously published40Ar/39Ar ages for potassic and phyllic alteration shows that the magmatic-hydrothermal system was probably sustained for a few to several hundred thousand years. A prolonged period of formation is inconsistent with accepted mechanisms of formation as the small intrusions typically associated with porphyry Cu deposits cannot solely sustain a hydrothermal system more than 100,000 yr. before the invasion of meteoric water occurs, as shown by numerical modelling. Consequently, the protracted development of hydrothermal alteration is cause by continued fluid exsolution and/or magma degassing of an underlying batholith. Hydrothermal alteration at Bajo de la Alumbrera is concentrically zoned from a central quartz-magnetite barren core outwards through potassic (biotite-K-feldspar±quartz) and propylitic (chlorite-illite-epidote-calcite) assemblages. Potassic alteration developed during two stages: the first was a K-feldspar-dominant assemblage associated with only minor mineralization. The bulk of the Cu-Fe sulfide and Au ore was emplaced during the second biotite-dominant phase. Intermediate argillic alteration (chlorite-illite±pyrite), which also accompanies significant mineralization, overprints the later biotite alteration and is zoned outwards into phyllic (quartz-muscovite-illite±pyrite) alteration across the top of the deposit and extends downwards along the periphery of the potassic zone. Significant amounts of Cu-Fe sulfides are found in these zones. Late basemetal-rich fault and fracture-fills (carbonate-galena-sphalerite±quartz-anhydrite-montmorillonite-nontronite) occur as the final phase in the deposit. Fluid inclusion studies and stable isotope geochemistry on material from Bajo de la Alumbrera show that the earliest potassic alteration developed from high-temperature (300 to 700°C), saline (32 to 68 wt.% NaCl equiv.) fluids, whereas the successive biotite-dominant assemblages decline in temperature (300 to 550°C) and overall salinity (32 to 53 wt.% NaCl equiv.). Calculated and measured δ18O and δD compositions of fluids (+3.8 to +10.4‰ δ18O and –37.8 to –76.2 ‰ δD) confirm a primary magmatic origin for the potassic alteration phase. The depleted D compositions measured in the late potassic alteration are similar to those measured in other porphyry ore deposits (e.g., Butte and Copper Canyon, North America; and Granisle and Bell, British Columbia). It is often thought the depleted D compositions of potassic alteration implies a component of meteoric water in its formation; however, at Bajo de la Alumbrera, the isotopic composition of the potassic alteration is consistent with late degassing and volatile exsolution of the underlying batholith. Similar mechanism can explain the isotopic composition (+5.1 to +8.4‰ δ18O and –23.6 to –81.4‰ δD) of the lower temperature (300 to 390°C) and less saline (less than 15 wt.%, but averaging 4.1 wt.% NaCl equiv.) phyllic alteration, whereby the alteration developed from magmatic fluids, not meteoric water mixed with magmatic aqueous fluids. The Bajo de la Alumbrera deposit is the result of the conjunction of several critical processes, including the protracted period of hydrothermal alteration formation (probably 2 to 4 m.y.) developed on multiple dacitic intrusions emplaced one million years apart. Volatiles (along with sulfur) were exsolved from a huge underlying pluton, which was being replenished by new batches of mafic magma. Magmatic aqueous fluids with initial high temperatures (up to 700°C) and salinities (greater than 32 wt. % NaCl equiv.) formed potassic (K-feldspar-biotite) alteration associated with the bulk of the Cu-Au. Lower temperature (below 400°C) and less saline (less than 4.1 wt.% NaCl equiv.) magmatic fluids formed the overprinting mineralized phyllic (illite-muscovite-pyrite-quartz) alteration. Although this differs from the traditional porphyry model, where phyllic alteration is peripheral to mineralized potassic alteration, it is not unique. Cu-Fe sulfide- and Au-bearing phyllic (and intermediate argillic) alteration within and not peripheral to the potassic altered core, has been documented at a number of porphyry ore deposits. These observations challenge the widely used genetic models in which phyllic alteration in porphyry ore deposits results from the ingress of meteoric fluids and refocuses attention on the evolution of the underlying batholith, from which the exsolved fluids and metal are derived.
Breccia
Basaltic andesite
Silicic
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The Ossa Morena Zone (SW Iberia) hosts an unusual suite of ore deposits, including
magmatic Ni-(Cu) and IOCG mineralization. These deposits are interpreted to have a relationship to
a deep mafic sill intruded in the middle crust. Interaction of mafic magmas with crustal rocks produced
immiscible sulphide-rich melts and water-rich melts. The latter exsolved large amounts of
Fe- and CO2-rich brines that were responsible for widespread albite-actinolite alteration and IOCG
mineralization.
Sill
Metallogeny
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Felsic
Batholith
Metallogeny
Geochronology
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Lithophile
Fractional crystallization (geology)
Lile
Adakite
Petrogenesis
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Porphyritic
Geochronology
Pyroxene
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The Rakkurijarvi prospect consists of a group of mineralized magnetite and lithic breccias within the ca. 2.05- to 1.90-Ga Proterozoic supracrustal sequence of the Kiruna district, northern Sweden. Potentially economic grades of Cu and Au, largely in the form of chalcopyrite and other sulfide assemblages, are hosted in brecciated magnetite and metavolcanic rocks. The extent of the mineralization is currently open, both downdip and along strike. The deposit was discovered through an integrated geophysical and geochemical program focused on iron oxide-copper-gold (IOCG)–style mineralization. It is hosted by brecciated greenschist facies metavolcanic rocks within and adjacent to an east-northeast–trending shear zone. The dominant characteristics of the deposit are consistent with the IOCG class and include magnetite and lithic breccias hosted in a metavolcanic sequence, with matrices of albite, actinolite, and calcite surrounded by halos of sodic (albitescapolite) and potassic (scapolite-K-feldspar-biotite) alteration. A distinctive accessory mineral assemblage includes apatite, titanite, and allanite. The paragenesis and textural evolution of the deposit includes early Narich alteration accompanying massive magnetite alteration. The Na-rich alteration is overprinted by potassic alteration (also associated with magnetite), although the paragenesis is complex and multiple generations of both sodic and potassic alteration are recognized. Alteration of lithic clasts to magnetite confirms a metasomatic origin, as opposed to an orthomagmatic origin, for the magnetite mineralization. Re-Os analyses of two separates of molybdenite intergrown with magnetite, interpreted as cogenetic with the sulfide assemblage, yield mineralization ages of 1853 ± 6 and 1862 ± 6 Ma. Reconnaissance bulk-rock chemistry of the host volcanic rocks is consistent with an intermediate volcanic
protolith, but much of the original character of the rocks is masked by albitization and incipient iron, sodic, and potassic alteration. The data also indicate significant element mobility during metasomatism and, in particular, the addition of Ti to the rock mass in biotite and as titanite. The compositions of secondary minerals are consistent with alteration and mineralization caused by highly saline fluids of relatively low F activity. The stable isotope characteristics of calcite, with δ18OSMOW ranging from 9.43 to 19.89 per mil and δ13CPDB ranging from –11.69 to +4.88 per mil, suggest that the fluids of the calcite and sulfide stage were derived from a magmatic
source but had interacted extensively with local sedimentary and volcanic rocks.
Greenschist
Paragenesis
Sericite
Actinolite
Breccia
Metasomatism
Allanite
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The Holloway mine, a mesothermal gold deposit, is located within the western Abitibi subprovince, Ontario. Mineralized zones that contain microscopic gold within pyrite are hosted by a sequence of metakomatiites, mafic to felsic metavolcanic, and metasedimentary rocks. There were two mineralization events, however the early one was the main event in terms of the amount of gold mineralization. This event was associated with albitization, silicification, chloritization, and sericitization in addition to pyritization. The later mineralization is associated with sericitization and pyritization. Carbonate and sericite alterations are associated with both mineralizing events. Results of U/Pb zircon age dating of rocks within the sequence constrains the main mineralization event to ~2672 Ma. Ore lenses of the Holloway deposit are enveloped by 080°-striking, steeply south dipping zones of intense deformation and fabric development within the Porcupine-Destor deformation zone. The shear zones are characterized by a strong foliation and, locally, an associated extension lineation. The ore lenses are interpreted to represent lithons that were formed by boudinage of the more competent mineralized zones, which are pervasively albitized. Analysis of geochemical data demonstrates that the REE, Th, Nb, Hf, Ti, Fe, and Al were largely immobile with respect to the albite, quartz, chlorite, sericite, and hematite alterations. Locally preserved textures such as abundant varioles and spherulites indicate that the host metavolcanics were relatively evolved rocks. Geochemical data show they have an iron enrichment trend typical of tholeiitic mafic to felsic suites and that they were derived through fractional crystallization of parent basalt. Because of their high Fe/Mg ratios the rocks reacted with hydrothermal solutions carrying gold, presumably as thio complexes, to form pyrite, which scavenged the gold.
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