Quartz Stockwork Veins Research Articles

Migration and focusing of porphyry deposit-forming fluids through aplitic mush of the Saginaw Hill cupola, Arizona, United States

Porphyry-type Cu ± Au ± Mo deposits form in the upper (ca. 2–5 km deep) parts of large, long-lived magmatic-hydrothermal systems in which mineralising fluids are thought to be derived from mid-to shallow-crustal magma chambers. Increasingly, however, magmatic systems are viewed as consisting of mush with minor and transient lenses of magma, with mush being a variably packed framework of crystals with interstitial melt and magmatic volatile phase (MVP). In this context, questions remain as to the source (mainly depth) and mechanisms of transport and focussing of the vast volumes of fluids required for shallow level porphyry-type mineralisation. Even more problematic is a paucity of first-order textural evidence for the presence of mush in magmatic-hydrothermal systems, including those which host porphyry-type deposits. To address this, we have studied the aplitic porphyry cupola of the Saginaw Hill magmatic system, Tuscon, Arizona, United States, where magmatic-hydrothermal features are exceptionally well exposed, including a massive silica cap, quartz unidirectional solidification textures (USTs), stockworks of multiple generations of variably mineralised quartz veins and mineralised miarolitic cavities. From field-to micro-scale textural and geochemical studies, particularly observations of vermiform quartz between earlier generations of magmatic quartz and feldspar, we evidence the development of fluid pathways through mush at the magmatic-hydrothermal transition. These are shown to connect and provide fluids and ore constituents to the mineralised miarolitic cavities and early quartz vein stockworks. We suggest that this process should be considered in all new genetic, exploration and numerical models for porphyry and similar types of magmatic-hydrothermal ore-deposits.

Fluid Evolution at the Batu Hijau Porphyry Cu-Au Deposit, Indonesia: Hypogene Sulfide Precipitation from a Single-Phase Aqueous Magmatic Fluid During Chlorite–White-Mica Alteration

Abstract Mineralization at the Cu-Au porphyry deposit of Batu Hijau, Indonesia, was previously reported to be associated mainly within stockwork quartz veins accompanied by pervasive biotite-magnetite alteration. We use cathodoluminescence imaging of vein textures followed by microthermometry and laser ablation-inductively coupled plasma-mass spectrometry microanalysis of fluid inclusions to decipher the spatial-temporal evolution of the hydrothermal system. Our results indicate that sulfide precipitation largely postdated the main stockwork quartz veining. Chalcopyrite and bornite were found in three textural positions: (1) within conspicuous quartzpoor veinlets (“paint veins”) that postdate quartz stockwork veins and that also appear to account for the bulk of seemingly disseminated sulfides, (2) as centerlines in B-type veins, and (3) as interstitial grains in A-type veins. In all three textural positions, the sulfides occur together with a volumetrically minor, dull-luminescent quartz generation after local dissolution of the granular quartz dominating the stockwork veins. All three positions are associated with chlorite ± variable phengitic white mica with 3–6 wt % FeO + MgO. In the barren core of the deposit, quartz veins host, almost exclusively, fluid inclusions of intermediate density (~0.6 g/cm3) and near-constant salinity of ~3.7 wt % NaCl equiv, representing the input magmatic fluid. This fluid subsequently separated into a highly saline brine1 and low-density vapor during quartz vein formation in the mineralized parts of the deposit, but we found no textural or fluid-chemical evidence that brine + vapor already reached saturation in sulfides. Within the studied samples, Cu-Fe sulfides are invariably associated with the dull-luminescent quartz hosting only low-salinity (~2–8 wt % NaCl equiv) aqueous fluid inclusions with a density of ~0.8 g/cm3 and minimum formation temperatures of 300°–360°C, in agreement with Ti-inquartz and chlorite thermometry indicating trapping conditions only slightly above the boiling pressure of these liquids. On average, this mineralizing aqueous fluid is compositionally similar to the initial magmatic fluid, suggesting a common source, but some inclusion assemblages deviate to significantly lower or higher salinities (0.5–25 wt % NaCl equiv). We propose a formation model for the Batu Hijau porphyry Cu-Au deposit in which mostly barren quartz veins formed at high temperature (>400°C) in the central part of the system, while sulfide mineralization commences to form peripheral to this zone. The economic ore shell was growing inward and downward as a zone of active sulfide precipitation at 300°–360°C shifted in response to progressive retraction of isotherms, while barren quartz vein formation continued in the system’s core at higher temperature. The aqueous ore-forming liquid is interpreted to have formed by rehomogenization of magmatic brine and vapor that previously formed by phase separation and later became miscible again after cooling over a narrow temperature interval. Vapor condensation into the highly saline brine phase at low pressure and subcritical temperature led to partial dissolution of earlier formed quartz veins and created secondary porosity for subsequent sulfide deposition. We propose that Cu-Fe sulfide precipitation by the low-temperature aqueous fluid was driven by the rehomogenization of S-rich vapor with Cu-rich brine originating from the same input fluid. The selective dissolution of earlier quartz veins in an inward- and downward-growing ore shell explains the positive correlation of ore grades with the density of earlier quartz veining in the ore shell, even though copper mineralization postdates quartz vein formation at any location in the deposit. Late-stage sulfide deposition in paint veins has been noted at other porphyry Cu-(Au-Mo) deposits worldwide, indicating that the proposed fluid evolution model may be applicable to many other porphyry systems.

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.

Microthermometry and noble gas isotope analysis of magmatic fluid inclusions in the Kerman porphyry Cu deposits, Iran: constraints on the source of ore-forming fluids

This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly > 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.

The Onto Cu-Au Discovery, Eastern Sumbawa, Indonesia: A Large, Middle Pleistocene Lithocap-Hosted High-Sulfidation Covellite-Pyrite Porphyry Deposit

Abstract In 2013, a diamond drill program tested an extensive advanced argillic alteration lithocap within the Hu’u project on eastern Sumbawa Island, Indonesia. A very large and blind copper-gold deposit (Onto) was discovered, in which copper occurs largely as disseminated covellite with pyrite, and as pyrite-covellite veinlets in a tabular block measuring at least 1.5 × 1 km, with a vertical thickness of ≥1 km. Copper and gold are spatially related with a series of coalesced porphyry stocks that intrude a polymictic diatreme breccia capped by a sequence of intramaar laminated siltstones, volcaniclastic and pyroclastic rocks, and overlain by andesite flows and domes. The porphyry intrusions were emplaced at shallow depth (≤1.3 km), with A-B–type quartz veinlet stockworks developed over a vertical interval of 300 to 400 m between ~100 and 500 m below sea level (bsl), 600 to 1,000 m below the present surface, which is at 400 to 600 m above sea level. In the area drilled at Onto, the diatreme breccia, all porphyry intrusions and, to a lesser extent, the surrounding older andesite sequence have all been overprinted by intense subhorizontal advanced argillic alteration, zoned downward from illite-smectite, quartz-dickite to quartz-alunite and quartz-pyrophyllite ± diaspore alteration. The alteration package includes two particularly well-developed zones of residual quartz with vuggy texture in subhorizontal zones at shallow depth, the upper one is still porous but the lower horizon, ~100 m thick, is largely silicified and is located at or near the top of the quartz-alunite alteration. Mineralization starts below the lowermost silicic horizon with more than 90% of the current resource in quartz-pyrophyllite-alunite and quartz-alunite alteration. Mineralization is dominated by a high-sulfidation assemblage of covellite-pyrite ± native sulfur largely in open-space fillings and replacements, but also as discrete pyrite-covellite and covellite only veins down to at least 1 km. Although the greatest amount of copper occurs as paragenetically late covellite deposited during formation of the advanced argillic alteration, approximately 60% of resource at 0.3% Cu cutoff still occurs within the porphyry stocks, indicating the porphyry stocks are a fundamental control on mineralization. There is considerable remobilization and dispersion of copper and, to a lesser extent, gold into the surrounding pre-mineral breccia and the late intermineral intrusions from the two earliest porphyry phases, resulting in quite consistent copper and gold grades throughout the currently delineated mineral resource. The very high sulfidation state of the mineralization is thought to be a consequence of the metal-bearing ore fluids cooling in the advanced argillic-altered host rocks in the absence of a rock buffer. Early chalcopyrite-bornite ± pyrite mineralization with potassic ± chloritic and sericitic alteration is only preserved on the margins of the system and more rarely at depth in a few holes 600 m bsl (~1,100 m below surface) but makes up only a small proportion (~8%) of the current resource. The Onto system is exceptionally young and formed rapidly in the middle Pleistocene and is not significantly eroded. A U-Pb zircon age for the andesite that caps the volcanosedimentary host rocks provides a maximum age of 0.838 ± 0.039 Ma, with a slightly younger porphyry zircon crystallization age of 0.688 ± 0.053 Ma. Re-Os dating of molybdenite that is associated with both the quartz vein stockwork and high-sulfidation assemblage copper mineralization shows overlap between 0.44 ± 0.02 and 0.35 ± 0.0011 Ma. 40Ar/39Ar ages for alunite within the advanced argillic alteration block ranges from 0.98 ± 0.22 to 0.284 ± 0.080 Ma, and alunite closely associated with covellite spans a period from 0.537 ± 0.064 to 0.038 ± 0.018 Ma.

Laser ablation ICP-MS trace element systematics of hydrothermal pyrite in gold deposits of the Kalgoorlie district, Western Australia

The Kalgoorlie district in the Archean Yilgarn Craton of Western Australia contains two world-class gold deposits: the giant Golden Mile shear-zone system and the Mt Charlotte quartz-vein stockworks. Mineralization occurs in three styles: (a) Fimiston style is characterized by ankerite-pyrite ± hematite-magnetite-gold replacement, (b) Oroya style overprints Fimiston ore in the shear zones and is characterized by silica-ankerite-V-muscovite-pyrite ± pyrrhotite-gold-telluride replacement and (c) Mt. Charlotte style is characterized by veins with ankerite-sericite ± albite-pyrite-pyrrhotite-gold selvages. Hydrothermal pyrite is ubiquitous in all styles and occurs in several stages. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analyses (n = 652) were collected on 54 representative samples of pyrite from three deposits. Smooth sections in the ablation spectra were selected for quantitative analysis excluding peaks caused by micron-sized inclusions. Linear mixed effects (LME) modeling of the analytical results indicates no systematic differences between the Fimiston, Oroya and Mt Charlotte styles. The variance introduced to the dataset by geological variability reflected in random differences between samples and deposits is large. This may be a major reason for difficulties in distinguishing the differences due to mineralization style. However, there are clear differences between pyrites co-existing with different mineral assemblages. These indicate a strong control on pyrite chemistry by the composition of the hydrothermal fluids. Finally, Au-Te-As systematics show that a substantial proportion of the analyzed pyrites in all deposits fall into the field of gold saturation consistent with the known metallurgical character of the ores. Mineralogical studies, ultra-fine grinding and recovery by cyanide leach show that > 82% of all gold is present in native grains or in Au-Ag-tellurides. The refractory nature of the Fimiston pyrite concentrates is due to clusters of micron- to nano-sized inclusions rather than due to abundant lattice-bound gold.

The granite-related, high-tonnage Sn-Li deposit of Argemela, central Portugal

The Argemela Sn-Li quartz vein stockwork, east of the Panasqueira tungsten mine, is hosted in Cambrian slates and greywacke. An underlying Variscan granitic cupola is inferred from spotted slates and albitic microgranite dykes, the last including a thick modified albitic microgranite, enriched in F and Li, exposed some 500 m away, on the hill top. Inferred resources for shallowest deposit portion are 20.1 million tonnes (Mt) at 0.1-0.2% Sn, 0.2% Li and 0.1% (estimate) Rb, but the 650-m vertical-extent of the deposit suggests a resource of >200 Mt (with identical grades). The hydrothermal paragenetic sequence is amblygonite-montebrasite (mostly montebrasite) (Stage I)–quartz I-II–cassiterite (with columbite-tantalite inclusions)─arsenopyrite I─carbonate I-white mica I ─chlorite I–fluorite–apatite–rutile (Stage II)– white mica II–molybdenite– tourmaline– carbonate II-quartz III–arsenopyrite II–sphalerite–stannite–chalcopyrite–pyrite– pyrrhotite-chlorite II (Stage III)–covellite– vivianite–goethite/lepidocrocite (Stage IV). Amblygonite-montebrasite is the main Li carrier; Sn is evenly distributed between cassiterite and stannite; Rb is mostly in white mica (with 0.25-1.23 wt % Rb2O in the hill-top albitic microgranite). Primary aqueous, 1-3 um-wide fluid inclusions in the deposit in quartz I, carbonate I, apatite and cassiterite growth zones yield overall salinities and homogenisation temperatures of 7.2-19.1 wt % NaCl equiv. and 290-360ºC, respectively. The trace concentrations (electronic microprobe analysis) in quartz vein cassiterite reach 1.95 wt % Nb, 0.39 wt % Fe, 0.13 wt % Ti, and low/negligible values of Sb, Zn, As, Ag and Bi showing its granite-hydrothermal affiliation. Fe-poor and Fe-rich sphalerite (lower-intermediate and upper deposit parts) contain 1.0-1.6 and 7.9-9.4 wt % Fe, 64.3-66.0 and 55.9-57.2 wt % Zn, 0.4-0.5 and 0.9-1.1 wt % Cd, respectively. The sphalerite-stannite geothermometer yields temperatures of 245-297ºC. Following higher temperature amblygonite-montebrasite deposition (Stage I), hydrothermal fluids (aCl-=0.25 m), related to the hidden granitic cupola, at a mean pressure-corrected (50 MPa) temperature of 350ºC, were responsible for Stage II minerals deposition. Calculated cassiterite deposition from Sn chloride complexes occurred likewise, from probable magmatic-hydrothermal fluids, at fO2 = 10-34 -10-32 atm and pH=3.5-4. Cassiterite deposition mechanisms were oxidation, mixing, neutralisation, possible aCl- increase, and cooling. Later Fe-poor sphalerite (+kesterite/ferrokesterite) and Fe-rich sphalerite (+stannite) deposited at higher and lower fS2, respectively, the latter probably at a higher fO2 (Stage III). The uniqueness of Argemela system with abundant amblygonite-montebrasite in hydrothermal quartz tin veins may be related to an extreme fractionated F-, Li- and P-rich granitic magma. After the emplacement of the granite/albitic microgranite dykes, an emerging pegmatitic fluid was unable, possibly due to insufficient depth, to form pegmatites but only modified the hill-top albitic microgranite. As a consequence, the system, where mixing of high- and low-salinity magmatic fluids probably occurred, remained very enriched in F, Li and P and by the time the hydrothermal stockwork developed amblygonite-montebrasite (Stage I) was the first mineral to deposit abundantly before Stage II minerals deposition in those Argemela Sn-Li quartz veins.

Mineralogical Study of the Advanced Argillic Alteration Zone at the Konos Hill Mo–Cu–Re–Au Porphyry Prospect, NE Greece

The Konos Hill prospect in NE Greece represents a telescoped Mo–Cu–Re–Au porphyry occurrence overprinted by deep-level high-sulfidation mineralization. Porphyry-style mineralization is exposed in the deeper parts of the system and comprises quartz stockwork veins hosted in subvolcanic intrusions of granodioritic composition. Ore minerals include pyrite, molybdenite, chalcopyrite, and rheniite. In the upper part of the system, intense hydrothermal alteration resulted in the formation of a silicified zone and the development of various advanced argillic alteration assemblages, which are spatially related to N–S, NNW–SSE, and E–W trending faults. More distal and downwards, advanced argillic alteration gradually evolves into phyllic assemblages dominated by quartz and sericite. Zunyite, along with various amounts of quartz, alunite, aluminum phosphate–sulfate minerals (APS), diaspore, kaolinite, and minor pyrophyllite, are the main minerals in the advanced argillic alteration. Mineral-chemical analyses reveal significant variance in the SiO2, F, and Cl content of zunyite. Alunite supergroup minerals display a wide compositional range corresponding to members of the alunite, beudantite, and plumbogummite subgroups. Diaspore displays an almost stoichiometric composition. Mineralization in the lithocap consists of pyrite, enargite, tetrahedrite/tennantite, and colusite. Bulk ore analyses of mineralized samples show a relative enrichment in elements such as Se, Mo, and Bi, which supports a genetic link between the studied lithocap and the underlying Konos Hill porphyry-style mineralization. The occurrence of advanced argillic alteration assemblages along the N–S, NNW–SSE, and E–W trending faults suggests that highly acidic hydrothermal fluids were ascending into the lithocap environment. Zunyite, along with diaspore, pyrophyllite, and Sr- and Rare Earth Elements-bearing APS minerals, mark the proximity of the hypogene advanced argillic alteration zone to the porphyry environment.

Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines

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.

Archean Sanukitoid Gold Porphyry Deposits: A New Understanding and Genetic Model from the Lac Bachelor Gold Deposit, Abitibi, Canada

In the Abitibi subprovince of the Superior craton, Canada, recognition of intrusion-related gold deposits is improving, but their formation remains poorly understood. The recently reopened Lac Bachelor deposit in the northeastern part of the Abitibi subprovince is located on the edges of a syenitic complex, and this deposit is an example of a highly fractionated, Fe-Mg–poor, quartz-syenite to alkali-granite centered deposit. Gold is associated with pyrite in localized alkaline, fluorine, and hematite-rich metasomatized zones in magnetite-rich host rock. Minor mineralization is present in a quartz vein stockwork on the edges of the syenitic complex. A genetic model is proposed, wherein alkaline-oxidized magma, which favors sulfur and gold solubility and transport, migrates through the crust via major and subsidiary faults. The presence of miaroles, transolvus, and low-temperature crystallization conditions confirms the shallow emplacement of the magma. The high fluorine content of the magma, which lowers both viscosity and solidus temperature, favored the shallow emplacement of the magma and permitted the development of a long-lived magmatic-hydrothermal system. Oxygen and hydrogen isotopes, miaroles filling, and mineralized zone paragenesis confirm that oxidized hydrothermal fluids were exsolved from the magma. The planar geometry of the metasomatized ore zones indicates that hydrothermal fluid circulation and metasomatism reactions occurred preferentially in high permeability pathways, such as preexisting faults in the host rock. The redox contrast between the oxidized magmatic-hydrothermal fluid and the intermediate volcanic host rock caused sulfide precipitation and gold deposition. This genetic model is applied to the Abitibi subprovince and other Archean cratons.

Structural controls of fluid flow and gold mineralization in the easternmost parts of the Karagwe-Ankole Belt of north-western Tanzania

Gold mineralization in the Biharamulo region of western Tanzania is confined to the sheared, low-angle basement-cover contact between Archaean basement gneisses of the Tanzania Craton and the structurally overlying, low-grade metamorphic metasediments of the Mesoproterozoic Karagwe-Ankole Belt. Regional-scale fluid flow along this detachment is indicated by the pervasive silicification and retrogression of wall rocks to pervasively foliated phyllonites and pyritization of particularly metasediments, commonly graphite-rich, in the hanging wall of the shear zone. Gold mining centres on specific structural sites along the detachment, but also in stratigraphically higher sections in the structurally overlying metasediments. Zones of gold mineralization along the detachment correlate with NE trending ramp structures (dip angles 20°–35°) that are most ideally orientated for slip and reactivation within the low-angle phyllonitic detachment. Repeatedly overprinted auriferous quartz-vein stockworks in quartzofeldspathic gneisses immediately below the detachment indicate brittle fracturing of the competent footwall lithotypes during slip along the weaker detachment. In cases of massive silicification, up to 50m thick quartz blows are formed along the contacts between detachment phyllonites and footwall gneisses. The multiple overprinting relationships of successive quartz-vein generations in these zones of massive silicification suggests that the quartz blows acted as competent blocks in the weak detachment, causing the repeated overprint of earlier silicification by later fracturing and quartz-veining events. Gold mineralization above the detachment and in stratigraphically higher metasediments is closely associated with fold structures that form part of the low-grade metamorphic fold-and-thrust belt. Veining is particularly abundant in competent lithotypes, such as quartzite and chemically reactive ferruginous mafic sills. Overprinting relationships between quartz vein sets illustrate fluid flow during fold amplification and, importantly, the final lock-up stage of folds, during which much of the mineralization was introduced. Oxygen isotope values for quartz veins indicate fluids were likely derived from clastic, mainly metapelitic sedimentary sequences of the Karagwe-Ankole Fold Belt. The data also implies that the partially reworked Archaean granitoid-greenstone basement of the Tanzania Craton has not contributed to the fluid evolution and possibly gold mineralization. The extent (>100km) of the basement-cover detachment and associated alteration is indicative for a regional-scale fluid system. Gold mineralization is, however, controlled by local structures and lithological contrasts that require the detailed mapping and sampling of the regional structure.

Pressure, temperature, and timing of mineralization of the sedimentary rock-hosted orogenic gold deposit at Klipwal, southeastern Kaapvaal Craton, South Africa

Gold mineralization in the Klipwal Shear Zone (KSZ) at the Klipwal Gold Mine is confined to laminated quartz-carbonate lodes, stringers, and associated alteration in sandstone and siltstone of the Delfkom Formation in the upper Mozaan Group of the Mesoarchaean Pongola Supergroup. The moderately dipping brittle-ductile KSZ strikes N-S with an oblique-reverse, sinistral sense of shear. The deformational events that are recognized include an early phase that produced anastomosing shears defined by shear fabrics with numerous shear-parallel laminated quartz-carbonate fault-fill veins and, in places, extensional quartz vein stockworks, and a late brittle reactivation phase that produced fault breccias, displacing earlier extensional veins. Three closely spaced economic reefs (lodes) are developed: the main R-reef constitutes the KSZ, while the J- and H-reefs represent footwall splays. Alteration comprises chlorite, muscovite, epidote, feldspar, and carbonates along with pyrite, arsenopyrite, and chalcopyrite +/- pyrrhotite. An inner alteration zone is dominated by laminated quartz-carbonate veins with alternating quartz-carbonate-rich and muscovite-chlorite-rich laminae, whereas the proximal zone is characterized by alteration halos of K-feldspar, albite, epidote, chlorite, and muscovite along with carbonates and associated quartz veins. Chlorite thermometry from the inner and proximal zones yielded temperatures of 267 to 312 A degrees C. Arsenopyrite compositions provide temperatures in the same range, 255 to 318 A degrees C. Fluid inclusion microthermometry and Raman spectrometry of quartz veins in the mineralized reefs reveal the presence of metamorphogenic aqueous-gaseous fluid with an average salinity of 6.5 wt% NaCl equiv. Fluid compositions and estimated pressure-temperature (P-T) range (1.1 to 2.5 kbar at 255 to 318 A degrees C) are typical of orogenic gold deposits. Devolatilization during the regional facies metamorphism of the Pongola Supergroup is considered the likely fluid-forming event with fluid flow focused into a compressional jog of the KSZ. Shear-induced pressure fluctuations generated a phase separation of the initial aqueous-gaseous fluid, producing a gaseous and low-salinity aqueous fluid. This, together with fluid-rock interaction, and a decrease in fO(2) lead to sulfide and gold precipitation at Klipwal. Re-Os data from six sulfide samples constrain the age of sulfide precipitation and, by inference, gold mineralization, to 2563 A +/- 84 Ma, with an initial Os-187/Os-188 = 0.29 A +/- 0.08 (MSWD = 0.38). This age is distinctly younger than the post-Pongola granites (2863-2721 Ma), ruling out the association of granite emplacement with mineralization. This would suggest that mineralization is linked to the regional D-3 folding event which reactivated the KSZ after emplacement of the post-Pongola granites and that final brittle, post-mineralization reactivation is related to Karoo age faulting. Low initial Os values suggest that ore fluid interacted with mafic rocks, leaching non-radiogenic Os, the likely source being the deeper seated Nsuze Group volcanics and/or the greenstone belts that underlie the Pongola Supergroup.

Some Features of Fluid Inclusions in Garnets from the Seleteguole Skarn Cu‐Mo Deposit, Jinghe County, Xinjiang, China

the deposit. Light brownish red garnet occurred as 3-10 mm sized disseminated spots in white hornfelsed felsic tuff which is directly in contact with the Cu-mineralized granite porphyry stock. Brown garnet occurred as massive aggregates of 2-6 mm sized tetragonal trisoctahedron crystals in the garnet skarn, which is close to the Cumineralized granite porphyry stock, with irregular quartz stockwork veins and local flaky molybdenite. Yellowish green garnet occurred as aggregates of 3-5 mm sized rhombic dodecahedron crystals in greyish white hornfelsed siltstone which is relatively far away from the

Structure, alteration, and geochemistry of the Charlotte quartz vein stockwork, Mt Charlotte gold mine, Kalgoorlie, Australia: time constraints, down-plunge zonation, and fluid source

The Kalgoorlie district in the Archean Yilgarn Craton, Western Australia, comprises two world-class gold deposits: Mt Charlotte (144 t Au produced to 2013) in the northwest and the Golden Mile (1,670 t Au) in the southeast. Both occur in a folded greenschist-facies gabbro sill adjacent to the Golden Mile Fault (D2) in propylitic alteration associated with porphyry dikes. At Mt Charlotte, a shear array of fault-fill veins within the Golden Mile Fault indicates sinistral strike-slip during Golden Mile-type pyrite–telluride mineralization. The pipe-shaped Charlotte quartz vein stockwork, mined in bulk more than 1 km down plunge, is separated in time by barren D3 thrusts from Golden Mile mineralization and alteration, and occurs between two dextral strike-slip faults (D4). Movement on these faults generated an organized network of extension and shear fractures opened during the subsequent infiltration of high-pressure H2S-rich fluid at 2,655 ± 13 Ma (U–Pb xenotime). Gold was deposited during wall rock sulphidation in overlapping vein selvages zoned from deep albite–pyrrhotite (3 g/t Au) to upper muscovite–pyrite assemblages (5 g/t Au bulk grade). Chlorite and fluid inclusion thermometry indicate that this kilometre-scale zonation is due to fluid cooling from 410–440 °C at the base to 350–360 °C at the top of the orebody, while the greenstone terrane remained at 250 °C ambient temperature and at 300 MPa lithostatic pressure. The opened fractures filled with barren quartz and scheelite during the retrograde stage (300 °C) of the hydrothermal event. During fracture sealing, fluid flux was periodically restricted at the lower D3 thrust. Cycles of high and low up-flow, represented by juvenile H2O–CO2 and evolved H2O–CO2–CH4 fluid, respectively, are recorded by the REE and Sr isotope compositions of scheelite oscillatory zones. The temperature gradient measured in the vein stockwork points to a hot (>600 °C) fluid source 2–4 km below the mine workings, and several kilometres above the base of the greenstone belt. Mass balance calculations involving bulk ore indicate enrichment of both felsic (K, Rb, Cs, Li, Ba, W) and mafic elements (Ca, Sr, Mg, Ni, V, Cr, Te), a source signature compatible with the local high-Mg porphyry suite but not with the meta-gabbro host rock. The initial 87Sr/86Sr ratios of the vein scheelites (0.7014–0.7016) are higher than the mantle ratio of the meta-gabbro (0.7009–0.7011) and overlap those of high-Mg monzodiorite intrusions (0.7016–0.7018) emplaced along the Golden Mile Fault at 2,662 ± 6 Ma to 2,658 ± 3 Ma.

Silver-rich telluride mineralization at Mount Charlotte and Au–Ag zonation in the giant Golden Mile deposit, Kalgoorlie, Western Australia

The gold deposits at Kalgoorlie in the 2.7-Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, occur adjacent to the D2 Golden Mile Fault over a strike of 8 km within a district-scale zone marked by porphyry dykes and chloritic alteration. The late Golden Pike Fault separates the older (D2) shear zone system of the Golden Mile (1,500 t Au) in the southeast from the younger (D4) quartz vein stockworks at Mt Charlotte (126 t Au) in the northwest. Both deposits occur in the Golden Mile Dolerite sill and display inner sericite–ankerite alteration and early-stage gold–pyrite mineralization replacing the wall rocks. Late-stage tellurides account for 20 % of the total gold in the first, but for 30 g/t Au) is characterized by Au/Ag = 2.54 and As/Sb = 2.6–30, the latter ratio caused by arsenical pyrite. Golden Mile-type D2 lodes occur northwest of the Golden Pike Fault, but the Hidden Secret orebody, the only telluride bonanza mined (10,815 t at 44 g/t Au), was unusually rich in silver (Au/Ag = 0.12–0.35) due to abundant hessite. We describe another array of silver-rich D2 shear zones which are part of the Golden Mile Fault exposed on the Mt Charlotte mine 22 level. They are filled with crack-seal and pinch-and-swell quartz–carbonate veins and are surrounded by early-stage pyrite + pyrrhotite disseminated in a sericite–ankerite zone more than 6 m wide. Gold grade (0.5–0.8 g/t) varies little across the zone, but Au/Ag (0.37–2.40) and As/Sb (1.54–13.9) increase away from the veins. Late-stage telluride mineralization (23 g/t Au) sampled in one vein has a much lower Au/Ag (0.13) and As/Sb (0.48) and comprises scheelite, pyrite, native gold (830–854 fine), hessite, and minor pyrrhotite, altaite, bournonite, and boulangerite. Assuming 250–300 °C, gold–hessite compositions indicate a fluid log f Te2 of −11.5 to −10, values well below the stability of calaverite. The absence of calaverite and the dominance of hessite in the D2 lodes of the Mt Charlotte area point to a kilometer-scale mineral and Au/Ag zonation along the Golden Mile master fault, which is attributed to a lateral decrease in peak tellurium fugacity of the late-stage hydrothermal fluid. The As/Sb ratio may be similarly zoned to lower values at the periphery. The D4 gold–quartz veins constituting the Mt Charlotte orebodies represent a younger hydrothermal system, which did not contribute to metal zonation in the older one.

Modification of a Palaeoproterozoic porphyry-like system: Integration of structural, geochemical, petrographic, and fluid inclusion data from the Aitik Cu–Au–Ag deposit, northern Sweden

The Aitik Cu–Au–Ag deposit in the Gällivare area in northern Sweden is Sweden's largest sulphide mine with an annual production of 35Mt of ore, and the biggest open pit operation in northern Europe. It is proposed in the present study that the Aitik deposit represents a Palaeoproterozoic, strongly metamorphosed porphyry copper deposit that was affected ca. 100Ma later by a regional IOCG-type hydrothermal event. Consequently, the Aitik deposit might represent a mixed ore system where an early copper mineralisation of porphyry type has been overprinted by later regional IOCG mineralisation.Several attempts have previously been made to genetically classify the Aitik Cu–Au–Ag deposit as a distinct ore type. New geochemical, petrographic, structural, and fluid inclusion results combined with published data have provided the opportunity to present new ideas on the genesis and evolution of the Aitik Cu–Au–Ag deposit. The emplacement of a ca. 1.9Ga quartz monzodiorite that host the ore at Aitik was related to subduction processes and volcanic arc formation, and synchronous with quartz vein stockwork formation and porphyry copper mineralisation. Highly saline aqueous (38wt.% NaCl) fluid inclusions in the stockwork veins suggest entrapment at 300°C and a pressure of nearly 3kbar, a high pressure for a typical porphyry copper ore, but consistent with conditions at associated deep root zones of intrusion-related magmatic–hydrothermal systems. The highly saline fluid formed disseminated and vein-type ore of mainly chalcopyrite and pyrite within comagmatic volcaniclastic rocks, and caused potassic alteration (biotite, microcline) of the host rocks. The early porphyry copper mineralising event was followed, and largely overprinted, by CO2 and aqueous medium- to high-salinity (16–57wt.% salts) fluids related to a ca. 1.8Ga tectonic and metamorphic event (peak conditions 500–600°C and 4–5kbar). Extensive deformation of rocks and redistribution of metals occurred. Magnetite enrichment locally found within late veins, and late amphibole–scapolite and K feldspar alterations within the deposit, are some of the features at Aitik implying that aqueous fluids responsible for IOCG-mineralisation (200–500°C and ~1kbar) and extensive Na–Ca alteration in the region during the 1.8Ga tectonic event also affected the Aitik rocks, possibly leading to addition of copper±gold.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. II. Vein Geometry and Ore Shell Formation by Pressure-Driven Rock Extension

Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system.

Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence

The Summitville Au–Ag–Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5±0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite–quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz–alunite, quartz–kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite–base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 °C (90% between 190 and 310 °C), consistent with the range (200–250 °C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite–pyrite pair yielded a sulfur-isotope temperature of 390 °C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The δ 34S values of sulfides in magnetite (−2.3‰) and of sulfate in apatite (5.4‰) in unaltered quartz latite indicate that δ 34S ΣS was near 0‰. The δ 34S values of coexisting alteration alunite and pyrite are 18.2‰ to 24.5‰ and −8.1‰ to −2.2‰, respectively. Deep in the system, most of the change in δ 34S values occurs in the sulfates, indicating that the fluids were initially H 2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in δ 34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO 4 2−-dominant as pyrite precipitated and SO 2 disproportionation produced the sulfuric acid requisite for acid leaching. The δ 34S values of the late-stage barite and sulfides indicate that the system returned to high H 2S/SO 4 2− ratios typical of the original rock-buffered fluid. The δD H2O of alunite parent fluids was near −45‰ and their δ 18O ranged from 7‰ to −1‰, depending on the degree of exchange in the alteration zone at low water–rock ratio, or mixing with unexchanged meteoric water. The low δD values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved from the magma at depth had δD H2O and δ 18O H2O values near −70‰ and 10‰, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had δD H2O and δ 18O H2O values close to −40‰ and 5‰, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separated into a low-density phase and hypersaline brine. The low-density phase rose to the level of acid-sulfate alteration, where disproportionation of SO 2 commenced upon condensation of the vapor plume causing the acid-sulfate alteration. Hydrogen and oxygen isotopic compositions estimated from deep kaolinitic wallrock alteration and fluid inclusions in quartz stockwork veins indicate the brine mixed with highly exchanged meteoric water at depth and rose to the ore zone, producing the Cu–Au–Ag mineralization. The composition of inclusion fluids in a single sample of enargite also indicates mixing between ore fluids and unexchanged meteoric water at shallow depths. The sulfate in fluids responsible for precipitation of most barite samples was of magmatic origin, but the δ 34S of a single sample suggests mixing with sulfate derived from the atmospheric oxidation of H 2S in an overlying steam-heated zone.

TELLURIDE MINERALOGY OF THE GOLDEN MILE DEPOSIT, KALGOORLIE, WESTERN AUSTRALIA

The Golden Mile, Kalgoorlie, Western Australia, is a mesozonal gold deposit unusual because of its enormous size (>1,457 tonnes of gold) and because tellurides are responsible for approximately 20% of gold production. Gold mineralization is hosted primarily by Archean-aged dolerites and basalts that have been metamorphosed to the greenschist facies. This mineralization occurs in hundreds of auriferous and telluride-bearing lodes, which have been classified into three structural types on the basis of orientation: main (trending NNW), caunter (trending NW) and cross lodes (trending NE). Three distinct styles of hydrothermal mineralization are present in the deposit. The early Fimiston-style lodes are characterized by main, caunter and cross lodes. The Oroya-style lodes (V- and Te-rich) occur in later dilational jogs and veinlets within a shear zone. Both of these are overprinted by the younger Mount-Charlotte-style mineralization (quartz-vein stockwork). Nineteen tellurium-bearing minerals occur in the Golden Mile deposit. Calaverite, petzite, coloradoite, altaite and native gold are most common throughout the deposit, whereas tellurantimony, melonite, hessite, stutzite, krennerite and sylvanite are rare in the main and caunter lodes. In addition, hessite and montbrayite are found in trace amounts in the cross and main lodes, respectively. There is no systematic distribution of tellurides within a given lode. Tellurides in Fimiston-style ores formed at ca. 300° to 170°C during cooling of the deposit. Values of f (Te2) and f (S2) determined at 300°C on the basis of the stability of native elements, tellurides and sulfides, are ca. log f (Te2) in the range −11.4 to −6.8 and log f (S2) in the range −12.6 to −5.5. No spatial variations in the composition of native gold, tetrahedrite-group minerals and tellurides occur in the Golden Mile deposit, although there is a weak to moderate decrease in gold fineness with depth. Marked differences in telluride mineralogy in and between individual lodes likely reflect variations in physicochemical conditions during ore formation or mechanisms of precious-metal deposition.

Geology and Alteration‐Mineralization Characteristics of the Cibaliung Epithermal Gold Deposit, Banten, Indonesia

Abstract. The Cibaliung gold project is located at the central portion of the Neogene Sunda‐Banda magmatic arc. Gold‐silver mineralization in the area is hosted in a thick sequence of sub‐aqueous basaltic andesite volcanics with intercalated sediments intruded by sub‐volcanic andesite to diorite plugs and dykes, and subsequently cut by a cluster of diatreme breccias. These host rocks are unconformably overlain by dacitic tuffs, younger sediments and basalt flows.The gold prospects in Cibaliung occur within a NW‐trending structural corridor that is 3.5 km wide by at least 6 km long. It is fault‐bounded and is considered to be a graben. Two aligned NNW‐trending sub‐vertical shoots, Cikoneng and Cibitung, host the currently defined resource within the steeply dipping vein system with a minimum strike length of 1,300 m. As of July 2001, exploration has defined an inferred + indicated mineral resource of approximately 1.3 million tonnes at 10.42 g/t gold and 60.7 g/t silver at a 3 g/t Au cut‐off. This equates to approximately 435,000 ounces of gold and 2.54 million ounces of silver.Gold‐silver mineralization occurs as quartz veins characteristic of the low‐sulphidation epithermal adularia‐sericite type. Progressive dilation with a general increase in gold grade has produced multi‐stage veining and brecciation that grades from early to late stages as: pre‐mineral fluidized breccia, quartz vein stockwork, massive vein, crustiform vein, colloform‐crustiform vein with progressive increase in chloritic clay bands, clay‐quartz milled matrix breccias with a progressive increase in clay content, and synto post‐mineral fault gouge with vein clasts. Wall rock alteration is characterized by pro‐grade chlorite+adularia flooding that is locally overprinted by a low temperature argillic alteration (smectite, illite and mixed layered clays). Generally, the argillic alteration becomes weak with depth. The major mineral constituents of the veins are quartz, adularia and clay. In the early gold‐poor hydrothermal stages, quartz and adularia dominate with minor calcite and clay (smectite, poorly crystalline chlorite, interlayered chlorite‐smectite and illite‐smectite). In the later gold‐rich hydrothermal stages, clay with variable amounts of carbonate increases whereas the abundance of quartz and adularia decreases. Gold occurs mainly as electrum while silver occurs as argentite‐aguilarite‐naumannite and electrum, and rarely as native silver, sulphosalts and tellurides. Sulphides generally comprise <1 vol % of the vein, with pyrite as the most common species. Together with pyrite, traces of very fine‐grained base metal sulphides dominated by chalcopyrite, sphalerite and galena are in most cases intimately associated with electrum and silver minerals. Partial supergene oxidation generally extends down to about 200 m below the surface at Cikoneng and further down to more than 300 m at Cibitung.The hydrothermal system responsible for the gold‐silver mineralization in the area may be related to rhyolitic magmatism focused on a volcanic intrusive center during back arc rifting that formed a graben or pull‐apart basin. The dominant mechanism for the higher grade gold deposition is fluid mixing of up welling metal‐bearing hydrothermal solutions with relatively near surface cool, oxygenated condensate and/or steam‐heated meteoric fluids, as opposed to retrograde boiling. The strongly focused dilational structural environment is thought to have been the mechanism for focusing fluid flows, both up welling and descending, forming pipe‐like mineralized bodies in the rhomboidal dilation zones. It is interpreted that mineralization took place under low temperature conditions (<150–220d̀C) at a minimum depth of around 200–250 m below the palaeo‐water table.