The Vulcan prospect is a newly discovered Cu-Au prospect located within the eastern Gawler craton, South Australia. The prospect is entirely subsurface and defined by a geophysical anomaly intersected by diamond drilling. Eight drill holes have all intersected hematite-rich alteration and Cu-Au mineralization within a variety of breccia types ranging from those composed entirely of hydrothermal hematite + sericite + chlorite to those dominated by altered rock in which clasts of protolith rock are preserved. The highest-grade zones of mineralization at Vulcan occur within hematite-dominant breccias. Molybdenite within hematite-rich brecciated granite samples has been dated via the Re-Os method and yields a crystallization age of 1586 +/- 8 Ma (including decay constant uncertainties). SHRIMP zircon U-Pb analyses show that a sample of hematite-sericite-chlorite altered and brecciated granite was emplaced at 1743 +/- 7 Ma. The new data from Vulcan confirm that hematite breccia-related Cu-Au mineralization formed during a widespread alteration and mineralization event between ca. 1600 and 1570 Ma, broadly coeval with the voluminous felsic large igneous province of the Gawler Range Volcanics/Hiltaba Suite, and are consistent with most interpretations of the geology of the Olympic Cu-Au province.

Iron oxide – Cu ± Au ± U ± Co (IOCG) mineralization is associated with numerous Proterozoic breccia bodies, collectively known as Wernecke Breccia, in Yukon Territory, Canada. Multiphase breccia zones occur in areas underlain by Paleoproterozoic Wernecke Supergroup metasedimentary rocks and are associated with widespread sodic, potassic, and carbonate alteration assemblages. Fluid inclusion data indicate syn-breccia fluids were hot (185–350 °C) saline (24–42 wt.% NaCl equivalent) NaCl–CaCl2–H2O brines. Estimates of fluid pressure vary from 0.4 to 2.4 kbar (1 kbar = 100 MPa). Carbon and oxygen isotopic compositions of breccia-related carbonates range from ~–11‰ to +1.5‰ (Pee Dee belemnite (PDB)) and –2‰ to 20‰ (Vienna standard mean ocean water (V-SMOW); δ18Owater ~–8‰ to +15‰), respectively. δ13C and δ18O values for host Wernecke Supergroup limestone/dolostone vary from ~–2‰ to 1.6‰ and 12‰ to 25‰, respectively. Sulfur isotopic compositions of hydrothermal sulfides and sulfate vary from ~–12‰ to +13‰ and +8‰ to +17‰ (Cañon Diablo Troilite (CDT)), respectively. Syn-breccia biotite, muscovite, and actinolite have δD and δ18O values of ~–141‰ to –18‰ and +7‰ to +12‰ (V-SMOW; δ18Owater ~7‰ to 11‰), respectively. The Wernecke Breccias and the associated IOCG mineralization appear to have formed from largely nonmagmatic fluids — based on isotopic, fluid inclusion, and geological data. The emerging hypothesis is that periodic overpressuring of dominantly formational/metamorphic water led to repeated brecciation and mineral precipitation. The weight of overlying sedimentary rocks led to elevated fluid temperatures and pressures; fluid flow may have been driven by tectonics and (or) gravity with metals scavenged from host strata.

In this paper we explore the origin and timing of IOCG mineral systems by evaluating one of the most significant IOCG ore forming events in Earth's history; the period between 1.5 and 1.6 Ga. Major economic examples of IOCG deposits are found in the Gawler Craton, South Australia, and the Cloncurry district, Queensland, and include the giant Olympic Dam deposit, and world class Prominent Hill and Ernest Henry deposits. The most economically significant deposits are characterized by hematite-magnetite breccias but other styles are also mined including skarn-like (Eloise) and ironstone-hosted examples (Osborne and Starra). Economic IOCG breccia systems in the Gawler Craton and Cloncurry district appear to have the most strongest connection to magmatism based on their timing and fluid chemistries. Recently dated IOCG skarn-like systems in both terranes also coincide with magmatism. However, ironstone-hosted deposits such as Osborne and Starra in the Cloncurry district pre-date magmatism and their fluid chemistries suggest a non-magmatic source. The sub-economic Wernecke breccias post-date magmatism in that terrane and have the strongest basinal fluid signature of all IOCG systems. This period in Earth's history likely coincided with the formation of two major salt sources; (1) ultrasaline, anhydrous metal-charged F, Cl and CO2 rich fluids derived from A-type magmas and (2) high salinity, hydrous basinal fluids derived from one of the first supercontinents that sequestered salt in giant halite beds (Columbia). The combination of these two exceptionally saline reservoirs likely resulted in the generation of the most significant IOCG systems globally.

The Sunrise Dam Gold Mine is the largest gold deposit in the eastern part of the Yilgarn craton. In several ways it is a typical Archean lode gold deposit, other than its exceptional size. Mineralization is hosted in intermediate-mafic metavolcanics and metasedimentary rocks. The major structures are gently NW to NNW dipping shear zones, which are connected by sub-vertical NNE and NE trending fault and breccia zones. Ore bodies occur parallel to the shear zones and faults. At least 6 deformation events can be distinguished. The majority of the gold mineralisation occurred in D3 and D4, with some also possibly occurring in the first event. The two major deformation/ mineralizing events had strike slip kinematics, reflecting SE and NE shortening respectively. Two key mechanical factors in the formation of the Sunrise Dam deposit were reactivation, and the existence of structures and competent bodies that caused stress perturbations. The identification of these factors could have useful exploration implications.

The Suicide Ridge Barren Breccia Pipe (∼1527±4Ma) is one of numerous breccia occurrences that characterize the Cloncurry district, Australia. Interest in breccia systems in the Cloncurry district is related to their possible link with iron oxide copper–gold deposits. The Suicide Ridge Barren Breccia Pipe is temporally and spatially associated with a granite that outcrops at its southwestern boundary and which is ascribed to the Williams and Naraku magmatic event (ca. 1550–1500Ma). The breccia is characterized by clasts of variable dimensions, degree of rounding and composition that includes dominant calc-silicate clasts of the Corella Formation that underlie rocks of the Soldiers Cap Group that host the pipe, indicating a dominant upward transport of the fragments. Other breccia fragments include granite bodies characterized by albite–quartz mineralogy and textures typical of the magmatic-hydrothermal transition. These rocks are interpreted to have formed in a fractionated granite carapace during the exsolution and accumulation of volatiles associated with the cooling magma. Two dominant types of inclusions have been identified at the Suicide Ridge Barren Breccia Pipe. These include early primary and pseudosecondary carbonic fluid inclusions, observed only in magmatic-hydrothermal transitional textures of granite breccia clasts, and secondary sodic (-calcic) brine inclusions that occur in all the collected samples. Variable homogenization temperatures in the primary and pseudosecondary pure CO2 fluid inclusions indicate that they were trapped under different pressure conditions. Pressures estimated from the primary CO2 fluid inclusions suggest that they were trapped at 4.2kbar, at least 1kbar above the inferred lithostatic pressure. This likely represents the overpressuring produced by release of CO2-rich volatiles during the crystallization of magma and which ultimately caused the subsequent brecciation. Estimated pressures for pseudosecondary CO2 fluid inclusions indicate pressure entrapment conditions around 2.7kbar, similar to the lithostatic pressure and 1.5kbar lower than pressures recorded by type Ia fluid inclusions. This significant drop of pressure likely represents the sudden pressure depression occurring in response to brecciation. Evidence from field relationships, breccia characteristics, mineralogy and textures of granite clasts within the pipe and fluid inclusion studies support an origin of the Suicide Ridge Barren Breccia Pipe in response to overpressuring generated by the release, expansion and possibly phase separation of fluids of dominant carbonic composition from a crystallizing magma beneath and adjacent to the pipe. Sodic (-calcic) fluid inclusions represent a later influx of fluids in the breccia system that produced widespread albitization. Significantly, high temperature, ultrasaline multisolid fluid inclusions abundant in IOCG deposits and considered to be samples of the mineralizing fluid were not observed at the Suicide Ridge Breccia Pipe. Their absence in the barren systems suggests that they were critical for the formation of IOCG deposits.

A study of fluid inclusion and alteration chemistry has revealed new information regarding the source and evolution of IOCG-related hydrothermal fluids in the Wernecke Mountains, Canada. The evidence suggests that the dominant hydrothermal fluids originated as seawater-derived bittern brines that variably dissolved evaporitic halite. A minor basement-derived fluid has also been detected in at least one location. Sampling of fluids from different points along fluid flow paths highlight an evolving fluid chemistry that was strongly controlled by temperature, salinity and redox conditions. Generally low metal concentrations in the fluids were probably a key factor limiting the formation of economic IOCG deposits in the area.

The Bismark deposit (northern Chihuahua, Mexico) is one of several base metal-rich high-temperature, carbonate-replacement deposits hosted in northern Mexico. Previous fluid inclusion studies based on microthermometry and PIXE have shown that the Zn-rich, Pb-poor Bismark deposit formed from a moderate salinity magmatic fluid [Baker, T. and Lang, J.R., 2003. Reconciling fluid inclusion types, fluid processes, and fluid sources in skarns: an example from the Bismark Deposit, Mexico. Mineralium Deposita 38(4), 474–495; Baker, T., van Achterberg, E., Ryan, C.G. and Lang, J.R., 2004. Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit. Geology 32(2), 117–120]. The exact precipitation mechanisms are unclear and may have due to cooling, salinity decrease and wall rock reaction. Furthermore, PIXE data suggested that Pb and Zn concentrations were comparable and inconsistent with the Zn-rich nature of the ore. However, Pb was commonly below the limit of detection for PIXE and the data presented by Baker et al. [Baker, T., van Achterberg, E., Ryan, C.G. and Lang, J.R., 2004. Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit. Geology 32(2), 117–120] are regarded as the maximum concentrations of Pb in the fluid. In this study new LA ICP MS analysis was carried out on the same fluid inclusion population to compare with the PIXE data in order to constrain the uncertainty related to the Pb data and the new results are used to model possible ore deposition mechanisms. The new laser ablation data reveal overall lower concentrations of Pb in the ore fluid (average value ~285 ppm) than previously indicated by PIXE analysis (average value ~713 ppm). Chemical modelling using the new laser ablation data tested the following ore deposition processes: 1) cooling; 2) fluid–rock reaction at constant temperature; 3) cooling and simultaneous fluid–rock interaction. Modelling results show that the gangue and ore minerals observed at Bismark are best reproduced by fluid–rock interaction and simultaneous cooling. Results from the simulations strongly indicate that ore deposition was mainly driven by a pH increase due to the neutralization of the acidic ore fluid (pH=3.9) as the result of the reaction with the limestone. Modelling results also suggest that the deposit likely formed under cooling conditions, but do not support the hypothesis of a temperature decrease as the principal ore-forming process.

Proterozoic rocks of the Cloncurry district in NW Queensland, Australia, are host to giant (tens to hundreds of square kilometers) hydrothermal systems that include (1) barren regional sodic–calcic alteration, (2) granite-hosted hydrothermal complexes with magmatic–hydrothermal transition features, and (3) iron oxide–copper–gold (IOCG) deposits. Fluid inclusion microthermometry and proton-induced X-ray emission (PIXE) show that IOCG deposits and the granite-hosted hydrothermal complexes contain abundant high temperature, ultrasaline, complex multisolid (type 1) inclusions that are less common in the regional sodic–calcic alteration. The latter is characterized by lower salinity three-phase halite-bearing (type 2) and two-phase (type 3) aqueous inclusions. Copper contents of the type 1 inclusions (>300 ppm) is higher than in type 2 and 3 inclusions (<300 ppm), and the highest copper concentrations (>1,000 ppm) are found both in the granite-hosted systems and in inclusions with Br/Cl ratios that are consistent with a magmatic source. The Br/Cl ratios of the inclusions with lower Cu contents are consistent with an evaporite-related origin. Wide ranges in salinity and homogenization temperatures for fluid inclusions in IOCG deposits and evidence for multiple fluid sources, as suggested by halogen ratios, indicate fluid mixing as an important process in IOCG genesis. The data support both leaching of Cu by voluminous nonmagmatic fluids from crustal rocks, as well as the direct exsolution of Cu-rich fluids from magmas. However, larger IOCG deposits may form from magmatic-derived fluids based on their higher Cu content.

We present a 3D geological model that integrates different datasets and incorporates geophysical inversion of airborne gravimetric and magnetic surveys of the northern part of the Drummond and Bowen basins. These basins are known for their endowment of low-sulphidation, epithermal Au-Ag mineralisation. The objective Of this Computer based reconstruction is to empirically evaluate the key controlling variables that contributed to the spatial localization of 147 Palaeozoic and Mesozoic shallow hydrothermal gold systems, found predominantly in veins and breccia hosted in basal volcanics and volcaniclastic intervals developed in a back-arc rift environment. The model provides a three-dimensional, regional scale (100,000 km(2)) perspective on the spatial associations between geology, structure, magmatism and known mineral occurrences, representing a 3D framework for precious-metals exploration. Results of 3D visualisation of geological and geophysical data Suggest that magmatic intrusions, and correlative volcanic centres localized most of the major deposits and also controlled the arrangement of clusters of uneconomic Au-Ag occurrences. The empirical modelling supports a model for the genesis of low-sulphidation epithermal Au-Ag mineralisation that favours a strong spatial association with shallow felsic to intermediate magmatic intrusions, similarly to that proposed for high-sulphidation systems. Geophysical inversion estimates for the depth of magmatic bodies suggest also that they may have controlled the efficiency of the hydrothermal, mineralising systems - shallow intrusions tend to be associated with greater tonnages of Au-Ag and are spatially associated with the larger clusters of occurrences. However, the composition of magmatic intrusions (mafic, intermediate, felsic) may have been important in regulating the amount of available bisulphide in the volatile phase, exerting a control on An grade/tonnage independent of the depth of emplacement of intrusions.

Iron oxide-copper-gold (IOCG) minera-lization occurs within and peripheral to a large-scale Proterozoic breccia system known as Wernecke Breccia. The breccia and related mineralization appear to have formed, independent of a magmatic cycle, during the expansion of over-pressured, hot, highly saline, basinal fluids into weak and/or permeable regions of the strata during the temporal evolution of the Wernecke Basin. The mineralization contains abundant magnetite and hematite and is similar to typical IOCG deposits. However, the formation of the Wernecke Breccia IOCG system in a thick evaporite-bearing sedimentary package also has similarities to the formation of sediment-hosted copper deposits such as those in the Mount Isa area and the Zambian copper belt.

Fluid plays a key role in the formation of giant iron-oxide-copper-gold (IOCG) deposits according to either fluid mixing or unmixing models (e.g. Barton & Johnson 1996, Pollard 2001). Our previous work shows some evidence for fluid mixing processes in the formation of both regional alteration assemblages and deposits from the Mount Isa Eastern Succession, NW Queensland, Australia (Fu et a1. 2003). In this study, we investigated aqueous inclusions in regional qUaliz veins associated with albitisation using state-of-the-art microanalytical techniques.

The Bismark deposit (8.5 Mt at 8% Zn, 0.5% Pb, 0.2% Cu, and 50 g/t Ag) located in northern Mexico is an example of a stock-contact skarn end member of a continuum of deposit types collectively called high-temperature, carbonate-replacement deposits. The deposit is hosted by massive sulfide within altered limestone adjacent to the Bismark quartz monzonite stock (~42 Ma) and the Bismark fault. Alteration concurrently developed in both the intrusion and limestone. The former contains early potassic alteration comprising K-feldspar and biotite, which was overprinted by kaolinite-rich veins and alteration and later quartz, sericite, and pyrite with minor sphalerite and chalcopyrite. Prograde exoskarn alteration in the limestone consists of green andradite and diopside, and transitional skarn comprising red-brown andradite, green hedenbergite and minor vesuvinite, calcite, fluorite, and quartz. The main ore stage post-dates calc-silicate minerals and comprises sphalerite and galena with gangue pyrite, pyrrhotite, calcite, fluorite, and quartz. The entire hydrothermal system developed synchronously with faulting. Fluid inclusion studies reveal several distinct temporal, compositional, and thermal populations in pre-, syn- and post-ore quartz, fluorite, and calcite. The earliest primary fluid inclusions are coexisting vapor-rich (type 2A) and halite-bearing (type 3A; type 3B contain sylvite) brine inclusions (32 to >60 total wt% salts) that occur in pre-ore fluorite. Trapping temperatures are estimated to have been in excess of 400 °C under lithostatic pressures of ~450 bar (~1.5 km depth). Primary fluid inclusions trapped in syn-ore quartz display critical to near critical behavior (type 1C), have moderate salinity (8.4 to 10.9 wt% NaCl equiv.) and homogenization temperatures (Th) ranging from 351 to 438 °C. Liquid-rich type 1A and 1B (calcite-bearing) inclusions occur as primary to secondary inclusions predominantly in fluorite and show a range in Th (104–336 °C) and salinity (2.7–11.8 wt% NaCl equiv.), which at the higher Th and salinity ranges overlap with type 1C inclusions. Oxygen isotope analysis was carried out on garnet, quartz, and calcite (plus carbon isotopes) in pre-, syn-, post-ore, and peripheral veins. Pre-ore skarn related garnets have a δ18Omineral range between 3.9 and 8.4‰. Quartz from the main ore stage range between 13.6 and 16.0‰. Calcite from the main ore stage has δ13C values of –2.9 to –5.1‰ and δ18O values of 12.3 to 14.1‰, which are clearly distinct from post-ore veins and peripheral prospects that have much higher δ18O (16.6–27.3‰) and δ13C (1.3–3.1‰) values. Despite the numerous fluid inclusion types, only two fluid sources can be inferred, namely a magmatic fluid and an external fluid that equilibrated with limestone. Furthermore, isotopic data does not indicate any significant mixing between the two fluids, although fluid inclusion data may be interpreted otherwise. Thus, the various fluid types were likely to have formed from varying pressure–temperature conditions through faulting during exsolution of magmatic fluids. Late-stage hydrothermal fluid activity was dominated by the non-magmatic fluids and was post-ore.

The Dublin Gulch intrusion is a member of the Tombstone plutonic suite, a linear belt of middle Cretaceous intrusions that extend across the Yukon Territory. Like many of the intrusions in this suite, the Dublin Gulch intrusion is associated with several different zones of gold and tungsten mineralization, within and immediately adjacent to the intrusion. The Eagle zone (50.3 Mt @ 0.93 g/t gold), located in the southwestern part of the Dublin Gulch intrusion, hosts the most significant concentration of gold in the area. The gold occurs in a broadly east–west-striking, steeply south-dipping series of sheeted veins. The veins consist of early quartz–scheelite±pyrrhotite±pyrite±arsenopyrite, and are associated with K-feldspar±albite alteration envelopes. These grade out to and are overprinted by sericite–carbonate±chlorite alteration. The same assemblage also occurs in veinlets that refracture sheeted quartz veins and contain the majority of the gold. The gold occurs with molybdenite, lead–bismuth±antimony sulfosalts, galena, and bismuthinite. Gold correlates strongly with bismuth (r 2=0.9), a relationship common to several intrusion-related gold deposits, but has a poor correlation with all other elements. Tungsten and molybdenum have a weak inter-element correlation (r 2=0.55) and paragenetically pre-date the majority of gold precipitation. Lead, zinc, copper, silver, antimony, and arsenic have moderate to strong inter-element correlations (0.58 to 0.93). The change from tungsten-bearing mineralization through to gold–bismuth-rich ores with elevated syn- to post-ore lead, zinc, copper, silver, antimony, and arsenic can be grossly correlated with a change in hydrothermal fluid composition. Early scheelite-bearing quartz contains primary CO2-rich fluid inclusions, which are post-dated by secondary inclusions with higher salinities (up to 15 wt% NaCl equiv.) and less CO2. These latter inclusions are interpreted to coincide with the later gold–bismuth and base metal mineralization. The favored genetic model is one in which early CO2-rich fluids exsolved from a magma with an initially high CO2 content, but progressively became more saline and H2O-rich as the system evolved.

Intrusion-hosted, low sulfide, sheeted vein systems are common within many plutons and stocks of the middle Cretaceous Tombstone–Tungsten magmatic belt, Yukon Territory, and host significant gold mineralization. Fluid inclusion characteristics of five such systems, namely Emerald Lake, Dublin Gulch, Scheelite Dome, Mike Lake, and MacTung, constrain the vein-forming fluid composition, formation temperatures and pressures, hydrothermal fluid processes, and potential fluid sources. The veins contain a wide range of fluid inclusion types. Ubiquitous type 1A and 1B inclusions are low salinity (1A: XNaCl<0.02; 1B: XNaCl<0.03), CO2-rich (1A: XCO2=0.18–1.00; 1B: XCO2=0.02–0.33). Laser Raman studies indicate that type 1A and 1B inclusions commonly contain minor CH4 (XCH4<0.09) and N2 (XN2<0.12). Type 2 inclusions are H2O-rich (XH2O=0.94–0.99), of low to moderate salinity (XNaCl=0.01–0.06), and were common at Emerald Lake and Dublin Gulch, in addition to localized type 3A halite-bearing inclusions (XNaCl=0.12–0.16). Both inclusion types post-dated the CO2-rich inclusions. Sheeted veins in the Mike Lake pluton contained coexisting type 1A and 3A and 3B (halite + sylvite) inclusions. Type 1A inclusions in all studied systems homogenized between 208 and 362 °C, type 1B ranged between 205 and 329 °C, and type 2 between 154 and 261 °C. Type 3A and 3B inclusions homogenized between 217 and 355 °C. A predominantly magmatic source for both CO2-rich and saline H2O-rich fluids is favored, with variations in trapping pressure (<1 kbar at Mike Lake, >1 kbar at Emerald Lake and Dublin Gulch, and >2 kbar at MacTung and Scheelite Dome) that potentially control fluid composition and evolution. A variety of fluid processes may have been responsible for gold precipitation including immiscibility and/or release of an evolving magmatic fluid.