Preliminary findings from field mapping, core logging, assaying, and petrographic analysis of the large (400m by 1500m), deep seated (>1500m) New York breccia pipe adjacent to the Ok Tedi porphyry/skarn deposit indicate that: 1) the breccia itself and gold mineralisation display a significant vertical extension (>1500m); 2) early igneous breccias are overprinted by hydrothermal breccias, which are characterised by abundant hydrothermal infill and late brecciation affecting previously formed breccias (i.e. evidence of multiple brecciation events); 3) there is a strong vertical zonation, which is reflected in the presence of shallow base metal rich sulphides (galena-sphalerite) and gold with crosscutting epithermal-style quartz-rhodochrosite veins, and deep chalcopyrite-magnetite-pyrrhotite mineralisation in veins and breccias with carbonate-adularia alteration; 4) there is a distinct zonation in sulphide and oxide mineral assemblages, which is reflected in the occurrence of shallow pyrite-marcasite-hematite assemblages compared to pyrrhotite-magnetite at depth; and 5) there is evidence for long distance transportation of brecciated clasts, reflected in the presence of fragments of a mapped sandstone formation that underlies a thick (1.5-2.0km) siltstone unit which hosts the breccia. The hydrothermally zoned breccia pipe at New York, Ok Tedi, could represent the continuation of deeper porphyry mineralisation into epithermal environments.

The Yichun Ta-Sn-Li deposit occurs within the 9.5-km(2) Yashan Igneous Complex, which is made up of felsic, peraluminous intrusions including two-mica, Li-mica, and topaz-lepidolite granites. The topaz-lepidolite granite forms a thin sheet confined to the upper part of the Li-mica granite and is separated from the host metasediments by a marginal pegmatite. The topaz-lepidolite granite is characterized by the presence of snowball textures in which quartz, K-feldspar, and topaz phenocrysts contain albite laths and rare lepidolite and columbite arranged along growth zones. These textures indicate simultaneous crystallization of these phases and attest to the subsolvus character of the topaz-lepidolite granite. Minor and accessory phases in the topazlepidolite granite include amblygonite, cassiterite, columbite-tantalite, microlite, wodginite, zircon, monazite, and pollucite. The topaz-lepidolite granite is characterized by low SiO2 (68.6-69.7 wt %), TiO2 (0.01 wt %), Fe2O3(total) (0.15-0.33 wt %), MgO (0.01-0.05 wt %), and CaO (0.15-0.19 wt %), whereas Al2O3 (17.97-18.26 wt %), Li2O (0.90-2.09 wt %), P2O5 (0.43-0.54 wt %), and F (1.09-2.30 wt %) are all very high. Na2O >> K2O due to both high Na2O (5.18-6.30 wt %) and low K2O (2.63-3.05 wt %). The topaz-lepidolite granite contains extremely high concentrations of Rb (>3,300 ppm), Cs (>340 ppm), Sn (>140 ppm), and Ta (>120 ppm), with very low Zr/Hf and Nb/Ta. Gaps of >3.5 wt % SiO2 and >2.5 wt % Al2O3 together with large increases in F, Li2O, and P2O5 between the Li-mica and topaz-lepidolite granites are not compatible with an evolution by Rayleigh fractionation processes. It is proposed instead that early crystallization of the Li-mica granite led to the formation of an Si-poor, Al-, Na-, and flux-rich boundary layer whose low viscosity and density allowed effective separation from the remaining melt and crystals. This melt concentrated incompatible elements including tin and tantalum and accumulated in the upper part of the magma chamber where it crystallized the sheet-like bodies of marginal pegmatite and topaz-lepidolite granite. The uppermost part of the topaz-lepidolite granite consists of aplite and overlying quartz lepidolite rock, which is composed mainly of quartz, lepidolite, albite, K-feldspar, topaz, amblygonite, cassiterite, and columbite. Compared to the topaz-lepidolite granite the aplite and quartz-lepidolite rock shows a number of complementary element enrichments and depletions. The quartz-lepidolite rock is enriched in Fe, Mn, K, Rb, Cs, Li, F, Ba, Sr, Ti, Zr, Nb, W, Th, and Sm. The formation of the quartz-lepidolite rock is interpreted to result from separation of an F- and Li-rich alkaline phase, which accumulated beneath the already crystallized marginal pegmatite. Complementary to this was crystallization of the strongly peraluminous sodic aplite with the highest tantalum contents measured in this study.

The Ok Tedi copper-gold mine in Western Province, Papua New Guinea, is situated in the western part of the Ok Tedi Complex where monzodiorite to quartz monzonite intrusions are associated with porphyry- and skarn-style copper-gold mineralization. The Pleistocene age of the intrusive rocks and mineralization provides an opportunity to study the longevity of the magmatic and hydrothermal evolution at Ok Tedi through U-Pb dating of zircon and high-precision Re-Os dating of molybdenite. Six main phases of intrusive rocks can be recognized within the mine area, with the sequence of intrusion indicated by contact relationships. Each has been dated by the SHRIMP U-Pb technique with correction for Th-U disequilibrium based on the U and Th content of each sample. In order of intrusion from oldest to youngest these include: Sydney Monzodiorite (1.368 ± 0.045 Ma), Warsaw Monzodiorite (1.269 ± 0.039 Ma), Kalgoorlie Monzodiorite (1.261 ± 0.050 Ma), Ningi Quartz Monzonite Porphyry (QMP)(1.229 ± 0.051 Ma), Bonn Quartz Monzonite (1.219 ± 0.040 Ma), and Fubilan QMP (1.213 ± 0.049 Ma). The intrusions are alkaline, high K to shoshonitic rocks with high Sr/Y ratios typical of Cu-fertile arc magmas. Chondrite-normalized REE patterns have minor or no negative Eu anomalies and downward sloping to listric-shaped HREE patterns typical of arc magmas in which high water contents supress plagioclase fractionation in favor of an evolution by hornblende ± garnet ± titanite fractionation. Cu-Au mineralization at Ok Tedi can be divided into four main stages based on crosscutting relationships: (1) skarn-endoskarn and associated vein-style mineralization in the Darai Limestone, Ieru siltstone, and Sydney Monzodiorite; (2) porphyry-style veins and breccias within the Ningi QMP and older intrusions, and at Siltstone Ridge: (3) porphyry-style veins and breccias in the Fubilan QMP and older intrusions: and (4) skarn-style mineralization in the lower part of the Darai Limestone along the Taranaki thrust. High-precision Re-Os dating of molybdenite has enabled a chronology to be established for the first three stages. Molybdenite from a quartz-mushketovite-epidote-carbonate-pyrite-chalcopyrite-molybdenite vein in clinopyroxene- and garnet-altered Sydney Monzodiorite has an age of 1.3206 ± 0.0020 Ma, and this dates the formation of the Gold Coast and Berlin skarns. Molybdenite from a quartz-pyrite-chalcopyrite-molybdenite vein in the sericite-altered Sydney Monzodiorite yields an age of 1.3166 ± 0.0043 Ma, and a quartz-pyrite-chalcopyrite-molybdenite vein with K-feldspar alteration selvages hosted in Ieru siltstone beneath the Gold Coast skarn has an age of 1.3031 ± 0.0015 Ma. Samples of molybdenite from quartz-sulfide veins from Siltstone Ridge have ages of 1.2116 ± 0.0029 and 1.2078 ± 0.0031 Ma. Molybdenite from a quartz-K-feldspar-pyrite-molybdenite vein, which overprints propylitic alteration in the Sydney Monzodiorite, has an age of 1.2120 ± 0.0024 Ma. These samples date porphyry-style mineralization in and around the Ningi QMP and at Siltstone Ridge. A sample of molybdenite from the matrix of hydrothermal intrusive breccia in the Fubilan QMP has an age of 1.2146 ± 0.0020 Ma, similar to the age of the adjacent Siltstone Ridge mineralization, and is interpreted to have been mechanically incorporated into the breccia during its formation. Several samples have been dated from the Fubilan porphyry system, including molybdenite from the matrix of a hydrothermal intrusive breccia (1.1648 ± 0.0020 Ma) and three samples from veins which postdate the breccias: a vuggy quartz-sulfide vein (1.1532 ± 0.0027 Ma), chalcopyrite-pyrite-molybdenite vein (1.1446 ± 0.0028 Ma), and duplicate analyses of a molybdenite-only vein (1.1326 ± 0.0034 and 1.1297 ± 0.0026 Ma) in agreement at 2σ. Molybdenite from a quartz-K-feldspar-biotite-magnetite-pyrite-chalcopyrite-molybdenite vein in endoskarn-altered Sydney Monzodiorite (beneath the Gold Coast skarn) has an age of 1.1459 ± 0.0012 Ma, and a similar vein without magnetite hosted in Warsaw Monzodiorite has an age of 1.1438 ± 0.0042 Ma, both within error of the chalcopyrite-pyrite-molybdenite vein in Fubilan QMP. Intrusive rocks in the Ok Tedi mine were emplaced over a period of approximately 200,000 years, with Cu-Au mineralization formed in discrete episodes of much shorter duration. The Gold Coast skarn and associated porphyry-style veins in Sydney Monzodiorite and Ieru siltstone formed in 14,000 to 21,000 years (n = 3), the Siltstone Ridge porphyry system in 2,000 to 12,000 years (n = 4), and the Fubilan porphyry system in 31,000 to 40,000 years (n = 6). The Taranaki skarn has not been dated in the mine area due to a lack of molybdenite, but geologic relationships indicate it is younger than the Fubilan QMP.

40 Ar- 39 Ar dating of biotite from IOCG and granite-related Cu-Au deposits in the Carajás Mineral Province provides evidence for the timing of mineralization and constraints on genetic models of ore formation. Ages of biotite from greisen and quartz-rich vein and breccia deposits, Alvo 118—1885 ± 4 Ma, Breves—1886 ± 5 Ma, Estrela—1896 ± 7 Ma, and Gameleira—1908 ± 7 Ma, demonstrate the close temporal relationship between Cu-Au mineralization and subjacent A-type granites. Mineralization is hosted within granite cupolas (Breves) or in vein/breccia systems emanating from the cupolas (Estrela and Gameleira), consistent with a genetic relationship of mineralization to the B-Li-F-rich granites. Plateau and minimum ages of biotite from IOCG deposits, including Igarapé Bahia, Cristalino, Corta Goela, and GT34, range from 2537 ± 6 Ma to 2193 ± 4 Ma. The 40 Ar- 39 Ar age of biotite from Igarapé Bahia (2537 ± 6 Ma) is similar to a previous SHRIMP 207 Pb- 206 Pb age for monazite of 2575 ± 12 Ma when the uncertainties in the respective analyses and standards are taken into account. The age spectrum for biotite from Cristalino shows increasing ages for successive steps, consistent with post-crystallization Ar loss, and the age of 2388 ± 5 Ma for the last three steps is considered a minimum age for Cu-Au mineralization. The age of biotite from the GT34 prospect (2512 ± 7 Ma) coincides with a previously identified period of basement reactivation and may indicate the formation of Cu-Au mineralization at this time or resetting of biotite from an older mineralization event at this time. At Corta Goela, within the Canaã Shear Zone, the biotite age of 2193 ± 4 Ma lies between the ages of IOCG (2.57–2.76 Ga) and granite-related Cu-Au (~ 1.88 Ga) deposits elsewhere in the Carajás district but is similar to previously reported 40 Ar- 39 Ar ages for amphibole from Sossego, possibly indicating that mineralization at both Sossego and Corta Goela was affected by a thermal event at this time. The Paleoproterozoic Cu-Au deposits are commonly hosted within Neoarchean IOCG alteration systems and the common occurrence of potassic alteration (especially biotite) in both types of deposits means that special care is required in interpreting the paragenesis of alteration in both types of deposits. The Paleoproterozoic Cu-Au deposits are reduced, and sulfur- and quartz-rich deposits lacking in major amounts of iron oxides and are therefore unlike IOCG deposits. Instead, they share many characteristics in common with widespread Paleoproterozoic Sn-W deposits in the Amazon Craton, including close spatial and temporal relationships with reduced A-type B-Li-F granites, and the occurrence of greisen and quartz-rich vein/breccia systems within and above granite cupolas. The occurrence of sericitic alteration in the Paleoproterozoic Cu-Au deposits is not evidence for an upward transition to sericitic alteration in IOCG deposits in the Carajás Mineral Province.

The Ak-Sug porphyry Cu-Au-Mo deposit is situated within the Altai Sayan region that forms part of the Tuva-Mongol arc of the Central Asian orogenic belt. The Ak-Sug intrusive complex is made up of a series of nested intrusions, including diorite, feldspar porphyry, quartz-feldspar porphyry, tonalite I, and tonalite II. Cu-Au-Mo mineralization occurs mainly within quartz-carbonate ± K-feldspar sulfide veins and later sericite-sulfide veins and alteration. In the northern zone, sulfide zoning consists of a central bornite core flanked by chalcopyrite and pyrite-chalcopyrite zones, with a pyrite zone on the hanging-wall side. Re-Os molybdenite dating of an early-stage quartz-carbonate ± K-feldspar sulfide vein and a late-stage molybdenite-only vein gives ages of 517.3 ± 3 and 517.4 ± 3 Ma, respectively. This indicates an Early Cambrian age for mineralization at Ak-Sug, making it one of the oldest porphyry copper deposits in the Central Asian orogenic belt. Published whole-rock geochemical data indicate that intrusive rocks at Ak-Sug and other Cambrian to Early Ordovician intrusive complexes in the Tuva-Mongol arc evolved by hornblende ± titanite fractionation due to high magmatic water contents, leading to adakite-like compositions with high Sr/Y and flat to listric-shaped rare earth element patterns that characterize many fertile arc magmas.

Major Cu–Au deposits of iron oxide–copper–gold (IOCG) style are temporally associated with oxidized, potassic granitoids similar to those linked to major porphyry Cu–Au deposits. Stable and radiogenic isotope evidence indicates fluids and ore components were likely sourced from the intrusions. IOCG deposits form over a range of crustal levels because CO2-rich fluids separate from the magmas at higher pressures than in CO2-poor systems, thereby, promoting partitioning of H2O, Cl and metals to the fluid phase. At deep levels, the magma–fluid system cannot generate sufficient mechanical energy to fracture the host rocks as in porphyry systems and the IOCG deposits therefore form in a variety of fault-related structural traps where the magmatic fluids may mix with other fluids to promote ore formation. At shallow levels, the IOCG deposits form breccia and fracture-hosted mineralization styles similar to the hydrothermal intrusive breccias and sulphide vein systems that characterize many porphyry Cu–Au deposits. The fluids associated with IOCG deposits are typically H2O–CO2–salt fluids that evolve by unmixing of the carbonic phase and by mixing with fluids from other sources. In contrast, fluids in porphyry systems typically evolve by boiling of moderate salinity fluid to produce high salinity brine and a vapor phase commonly with input of externally derived fluids. These different fluid compositions and mechanisms of evolution lead to different alteration types and parageneses in porphyry and IOCG deposits. Porphyry Cu–Au deposits typically evolve through potassic, sericitic and (intermediate and/or advanced) argillic stages, while IOCG deposits typically evolve through sodic(–calcic), potassic and carbonate-rich stages, and at deeper levels, generally lack sericitic and argillic alteration. The common association of porphyry and IOCG Cu–Au deposits with potassic, oxidized intermediate to felsic granitoids, together with their contrasting fluid compositions, alteration styles and parageneses suggest that they should be considered as part of the broad family of intrusion-related systems but that they are typically not directly related to each other.

The 40Ar/39Ar ages of 10 magmatic and hydrothermal micas from the Grasberg Igneous Complex range from 3.33 ± 0.12 to 3.01 ± 0.06 Ma. The ages of intrusive rocks and the paragenetic relationships between intrusive rocks and hydrothermal alteration and mineralization indicate that the Grasberg Igneous Complex formed during several cycles of intrusion and hydrothermal alteration. These include the Dalam and Main Grasberg intrusion and alteration cycles (3.33 ± 0.12–3.19 ± 0.05 Ma), a Kali intrusion and alteration cycle (3.16 ± 0.06–3.06 ± 0.03 Ma), and a post-Kali intrusion and Grasberg mineralization cycle (3.06 ± 0.03 and 3.01 ± 0.06 Ma). Each cycle of intrusion and alteration appears to have lasted around 0.1 m.y. or less and indicates that the huge size and high grade of Grasberg did not result from an unusually prolonged period of hydrothermal activity. A sample of phlogopite predating magnetite from the Kucing Liar Cu-Au deposit adjacent to Grasberg has an age of 3.41 ± 0.03 Ma. This is within error of the age of a Dalam intrusive rock from the Grasberg Igneous Complex and suggests formation of the calc-silicate skarn part of Kucing Liar at an early stage in the development of the complex. The ages of the equigranular diorite from the Ertsberg intrusion (2.67 ± 0.03 Ma), phlogopite from an endoskarn vein in the intrusion (2.71 ± 0.04 Ma), and phlogopite from the Ertsberg Cu-Au deposit (2.59 ± 0.15 Ma) indicate that intrusion and alteration and/or mineralization at Ertsberg are younger than the intrusions and mineralization in the Grasberg Igneous Complex. The Ertsberg therefore represents at least one additional cycle of approximately 0.1 m.y. of intrusion and alteration and/or mineralization in the district. The intrusions that make up the Grasberg Igneous Complex and Ertsberg and the hydrothermal fluids responsible for much of the alteration and mineralization appear to have been derived from a deeper level magma chamber. The youngest dated intrusive phase in the Grasberg Igneous Complex is a post-Kali diorite dike that is more basic than the preceding Kali quartz monzodiorite intrusions. This, together with the presence of mafic xenoliths in the Kali and Ertsberg intrusions, suggests that the magma chamber from which the intrusions and fluids were sourced was periodically replenished by basic magma. This process may also have triggered release of magma to form the shallow-level intrusions now exposed at the surface. The basic magmas also may have contributed components including fluids, metals, and/or sulfur to the Cu-Au deposits in the Ertsberg district.

Research Article| April 01, 2005 A Comparison of Granite-Related Tin, Tungsten, and Gold-Bismuth Deposits: Implications for Exploration T. Baker; T. Baker (SEG 1996 F) †Corresponding author: e-mail, Timothy. Baker@jcu.edu.au; Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland, Australia Search for other works by this author on: GSW Google Scholar P. J. Pollard; P. J. Pollard (SEG 1985 F) Search for other works by this author on: GSW Google Scholar R. Mustard; R. Mustard (SEG 1999) Search for other works by this author on: GSW Google Scholar G. Mark; G. Mark (SEG 1999) Search for other works by this author on: GSW Google Scholar J. L. Graham J. L. Graham (SEG 2005 S) Search for other works by this author on: GSW Google Scholar SEG Discovery (2005) (61): 5–17. https://doi.org/10.5382/SEGnews.2005-61.fea Article history first online: 13 Sep 2021 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation T. Baker, P. J. Pollard, R. Mustard, G. Mark, J. L. Graham; A Comparison of Granite-Related Tin, Tungsten, and Gold-Bismuth Deposits: Implications for Exploration. SEG Discovery 2005;; (61): 5–17. doi: https://doi.org/10.5382/SEGnews.2005-61.fea 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 SocietySEG Discovery Search Advanced Search Granite-related Au deposits associated with Sn-W provinces have been the subject of extensive debate over the past five years, since Thompson et al. (1999) defined their broad characteristics. The majority of these arguments have centered upon comparisons between granite-related Au deposits and orogenic Au deposits, and in particular, on how the deposits differ (e.g., Sillitoe and Thompson, 1998; Groves et al., 2003). This discussion is ongoing but there is a growing body of evidence that many of these Au deposits, particularly those with a strong Au-Bi association (herein described as granite-related Au-Bi deposits), are indeed magmatic in origin.... You do not currently have access to this article.

Liberation of iron and potassium by widespread postmetamorphic albitization of country rocks was one of the likely contributing processes in the formation of both barren and mineralized magnetite +/- chalcopyrite + biotite + gold + hematite + clinopyroxene + actinolite + apatite ironstones in the Cloncurry district of the Proterozoic Mount Isa block. Whole-rock geochemical data indicate nearly immobile Al, Ga, +/-Ti, Zr during transformation of a variety of least altered rocks toward albitite. The data indicate that the addition of Na from a brine to the rock accompanied the loss of Fe, K, Ba, Rb +/- Ca, Sr, Co, V, Mn, Pb, and Zn from those altered rocks and enrichment in the brine, but that Cu was not systematically stripped from a variety of wall rocks during albitization. Conversely, the formation of metasomatic ironstones, the immediate hosts to sonic Cu-Au ores, involves addition of most of the same elements that were lost during albitization. The correlation between intensity of alteration, its distribution and timing (e,g., in breccias cored by ca. 1530-4500 Ma granitoids), and convergence of all rock types toward magmatic stable isotope values provides strong evidence for a substantial component of igneous-derived fluid. Simulations of the albitization process were carried out isothermally at 550degreesC and 350 MPa and polythermally from these conditions down to 400degreesC and 200 MPa, using the Gibbs minimization method with HCh software and the UNITHERM database. Both simple fluid-rock mixing models and more complicated reactor-style algorithms used a range of input fluids (from fluids equilibrated fluid with two-feldspar granite through to Na/K ratios consistent with fluid inclusion results) and geochemical data for initially unaltered wall rocks. The apparent paradox of widespread albitization resulting from fluid released by two-feldspar granites can be explained by relatively small shifts away from the K-feldspar-albite equilibrium carve, and even isothermal model fluids derived from two-feldspar granites produce albitites in calc-silicate rocks outboard of granite-proximal K-feldspar-clinopyroxene skarns, matching field patterns. Those models with fluid Na/K ratios similar to those of fluid inclusions produced the most realistic alteration assemblages, dominated by albite, for both isothermal conditions and decreasing temperature, which approximate those observed in the field. PIXE and microthermometric data on fluid inclusions from quartz in two-feldspar quartz monzonite and pegmatite at the top of the Mount Angelay pluton indicate bulk Na/K molar ratios in the fluid of between 10 and 20, considerably higher than our thermodynamically calculated values for fluid in equilibrium with two-feldspar granite of around 3 (at 550degreesC, 350 MPa). Such shifts may have been attained by admixture of magmatic-hydrothermal fluid with small amounts of NaCl brines trapped along grain boundaries in scapolite-bearing calc-silicate wall rocks, by fluid immiscibility due to high initial CO2 contents in the felsic illtrusions, by contributions from mafic magmas, or from dissolution of salt-rich layers into the intrusions prior to crystallization and fluid release. With increasing amounts of fluid-rock interaction in the models, the fluids were enriched in K, Fe, and Ca, approaching compositions observed in fluid inclusions ill the ore deposits. These fluids, reacted with pelitic rocks (which are common ore hosts), would produce magnetite-clinopyroxene biotite-actinolite alteration at high temperature, similar to the proximal alteration around ore deposits.We infer that precipitation of sulfides in the Cu-Au deposits was the result of mixing of Cu-bearing brine, of ultimately magmatic origin, but modified extensively via albitization, with sulfur-bearing fluids or reaction of the brine with sulfur-bearing rocks. When Cu was absent from the initial magmatic fluid, barren ironstones may have been the result.

Fluid inclusions from two key iron oxide-copper-gold ore provinces (including data from mined deposits at Aitik and Pahtohavare in Norrbotten; and Ernest Henry, Mount Elliott, Osborne and Starra near Cloncurry) reveal distinctive features compared to other types of hydrothermal ore deposit. Many different types of fluid inclusions occur but the most common are mixed populations of a) liquid-rich carbonic (CO2 +/- CH4 +/- H2O), b) complex aqueous brine with several solid phases, commonly including carbonates, and c) liquid-dominated CaCl2-bearing, aqueous L-V halite inclusions. Varying compositions within these groups result from a combination of source-related factors, host rock interactions, fluid mixing and phase separation. Coupled with the consistent and distinctive alteration and mineralization parageneses, the fluid inclusion data support the classification of the deposits as a distinct genetic group that may be linked to CO2-bearing magmas.

The Água Boa and Madeira igneous complexes at the Pitinga mine were emplaced into acid volcanic rocks of the Paleoproterozoic Iricoumé Group, and host major tin, rare-metal (Zr, Nb, Ta, Y, REE) and cryolite mineralization. The igneous complexes are elongate NE–SW and each is composed of three major facies that, in order of emplacement, include porphyritic and equigranular rapakivi granite and biotite granite in both igneous complexes, followed by topaz granite in the Água Boa igneous complex (ABIC) and albite granite in the Madeira igneous complex (MIC).

Fluid mixing and/or unmixing (including boiling) are thought to be important mechanisms of mineralisation in copper-golddeposits. Detailed fluid-inclusion studies of regional sodic (-calcic) alteration and local mineralisation in the Cloncurry Fe-oxide-CuAu District, NW Queensland, suggest that both fluid mixing and unmixing occurred in these giant mineralised hydrothermal systems. In some cases, the primary character of coexisting multisolid, hypersaline brine inclusions and CO2- or vapour-rich inclusions, the latter crosscut by late Ca- and Na-rich fluid inclusions, indicate that fluid mixing probably occurred subsequent to fluid unmixing and finally resulted in CuAu mineralisation. However, the relationship between hypersaline brines and CO2, which was believed to result from an unmixing of a magma-derived H2OCO2NaCl ± CaCl2 fluid (see [Miner. Depos. 36 (2001) 93] and references therein), is rather complex as some hypersaline brine inclusions obviously predate CO2 inclusions.

The Grasberg Cu–Au deposit is hosted within the Grasberg Igneous Complex (GIC), a Pliocene volcanic and intrusive complex situated in the highlands of Irian Jaya, Indonesia. The GIC is composed of intrusive and volcanic rocks that were disrupted by formation of the Dalam Diatreme and intruded by later, multistaged Grasberg and Kali intrusions. Each intrusive phase is overprinted by extensive hydrothermal infill and alteration. Based on drillcore logging on section 13, 35 separate stages of alteration and infill have been recognized, and their spatial distribution mapped in 14 drillholes that represent approximately 1.8 km of vertical section. Using intrusions as timelines, the hydrothermal stages can be timed as post-Dalam–pre-MGI (Main Grasberg Intrusion), post-MGI–pre-Kali, and post-Kali, and linked into seven groups that are interpreted as separate hydrothermal systems. Pre-Kali systems include ten of the recognized stages, and are mostly high-temperature alteration (K-feldspar and/or biotite) devoid of sulfide mineralization. Sulfides are restricted to post-Kali time and, excluding early quartz–anhydrite±sulfide and molybdenite veins, can be grouped into three main stages: (1) Heavy Sulfide Zone (HSZ) mineralization, (2) Grasberg copper–gold stage, and (3) late copper mineralization (mixed copper sulfides, covellite–enargite–pyrite and pyrite–covellite–marcasite). The HSZ is dominated by fine-grained replacement pyrite and distributed mainly towards the periphery of the GIC, with only minor occurrences towards the central zones. It is suspected that a high proportion of the copper and gold content of the HSZ is due to overprinting by the Grasberg copper–gold stage and late copper mineralization.

The Lightning creek Fe-oxide Cu–Au prospect is hosted within a Mesoproterozoic granitoid batholith, composed dominantly of high-K, calc-alkaline, porphyritic quartz monzodiorite. The quartz monzodiorite contains enclaves of quartz diorite and has been intruded by more felsic granitoids (monzogranite and alkali-feldspar granite) and a series of subhorizontal sills. The quartz monzodiorite crystallised at crustal depths in excess of 10 km (P∼4 kb, T=800–850°C, melt XH2O=>4 wt.%, fO2∼NNO buffer), and was probably fluid-saturated (XH2O:XCO2 of >0.5). As might be expected, the more fractionated granitoids crystallised at progressively lower temperatures (quartz monzonite ∼800°C, alkali-feldspar granite <800°C). Subsolidus re-equilibration took place at temperatures between 700 and 600°C under increasingly oxidising conditions (amphiboles become increasingly TiO2-poor and MgO-rich). Although there is abundant petrographic evidence for magma mingling in the petrogenesis of this granitoid suite, the curvilinear nature of many of the chemical trends indicates an additional contribution from crystal fractionation. Sm–Nd isotopic characteristics (εNd=−2.7 to −3.3) indicate an older crustal component in the source region. Sill emplacement was contemporaneous with the development of a large magnetite-rich vein system. The sills are remarkable for their mineralogical and textural complexity. In addition to aplitic rocks, there are zones of albite–magnetite–quartz rock, which display a variety of unusual spherulitic textures. These Fe-rich portions of the sills are marginally younger than the aplites and are mineralogically similar to the magnetite-rich veins. The aplites are interpreted as late-stage differentiates of the plutonic suite, with which they share many mineralogical and compositional characteristics. It is argued that the textural and compositional variation within the sills reflect crystallisation under different degrees of melt saturation, and that the Fe-rich spherulitic rocks reflect the transition from magmatic to hydrothermal conditions within the crystallising pluton. Fluid inclusion studies indicate that this transition occurred at (hydrostatic?) pressures in excess of 1.5 kb (and possibly >2.5 kb) and temperatures greater than 500°C. Phase separation occurred in the fluid phase, producing in a CO2-rich vapour and a metal-rich (Fe, Cu) brine.

Plutons of the Naraku Batholith were emplaced into Proterozoic metasediments of the northern portion of the Eastern Fold Belt of the Mt Isa inlier during two intrusive episodes approximately 200 million years apart. Structural relationships and geochronological data suggest that the older plutons (ca 1750 Ma) are contemporaneous with granites of the Wonga Batholith to the west. The Dipvale Granodiorite and the Levian Granite represent these older intrusive phases of the Naraku Batholith. and both contain an intense tectonic foliation, S-1. which is interpreted to have formed during the north-south shortening associated with D-1 of the isan Orogeny. The geometry of S1 form surfaces at the southern end of the Dipvale Granodiorite, and of the previously unrecognised sheeted contact, defines a macroscopic, steeply south-southwest-plunging antiform. which was produced by the regional D-2 Of the Isan Orogeny. Si form surfaces in the Levian Granite define open F-2 folds With wavelengths of several hundred metres. The structural age of emplacement of the Dipvale Granodiorite and the Levian Granite is interpreted to be pre- or syn- the regional Di. An intense foliation present in some of the younger (ca 1505 Ma) granites that comprise the bulk of the Naraku Batholith is interpreted to represent S-3 of the Isan Orogeny. Foliations commonly have similar styles and orientations in both the pre-D-1 and younger plutons. This emphasises the simplicity with which regional fabrics can be, and probably have been, miscorrelated in the Eastern Fold Belt, and that the classification of granites in general on the basis of structural and geometric criteria alone is fraught with danger.

Iron-oxide–Cu–Au deposits, particularly those formed in deeper level (plutonic) environments, are commonly characterized by regional scale sodic(–calcic) alteration, which typically formed pre- or syn-Cu–Au mineralization. The sodic(–calcic) assemblages include albite, scapolite, pyroxene, actinolite, apatite, titanite, epidote and calcite. The consistent presence of coexisting hypersaline aqueous and CO2-rich fluids in minerals from sodic(–calcic) alteration and associated Fe-oxide–Cu–Au deposits is the result of unmixing of H2O–CO2–NaCl ± CaCl2–KCl magmatic fluids. Experimental evidence indicates that the Na/(Na + K) ratio of fluids in equilibrium with two alkali feldspars in CO3 2−-bearing parent fluids would be significantly higher than in unmixed chloride-bearing aqueous fluids. Therefore, fluid unmixing caused by decreases in temperature and/or pressure, will result in albitization of wall rocks, as is observed in most deeper level Fe-oxide–Cu–Au deposits. This alteration style may be succeeded by K-feldspathization with decreasing temperature because of the increase in equilibrium Na/(Na + K) in chloride-bearing fluids buffered by alkali feldspars.