Magmatic-hydrothermal Event Research Articles

The giant Dexing porphyry Cu–Mo–Au deposit in east China: product of melting of juvenile lower crust in an intracontinental setting

The Dexing porphyry Cu–Mo–Au deposit in east China (1,168 Mt at 0.45 % Cu) is located in the interior of the South China Craton (SCC), made up of two lithospheric blocks, the Yangtze and Cathaysia blocks. The Cu–Mo–Au mineralization is associated with mid-Jurassic granodioritic porphyries with three high-level intrusive centers, controlled by a series of lineaments at the southeastern edge of the Yangtze block. Available age data define a short duration (172–170 Ma) of the felsic magmatism and the mineralization (171 ± 1 Ma). The deposit shows broad similarities with deposits in volcanoplutonic arcs, although it was formed in an intracontinental setting. Porphyries associated with mineralization are mainly granodiorites, which contain abundant phenocrysts (40–60 %) and carry contemporaneous microgranular mafic enclaves (MMEs). They are mainly high-K calc-alkaline and show geochemical affinities with adakite, characterized by relatively high MgO, Cr, Ni, Th, and Th/Ce ratios. The least-altered porphyries yielded relatively uniform e Nd(t) values from −0.9 to +0.6, and wide (87Sr/86Sr)i range between 0.7046 and 0.7058 partially overlapping with the Sr–Nd isotopic compositions of the MMEs and mid-Jurassic mafic rocks in the SCC. Zircons from the porphyries have positive e Hf(t) values (3.4 to 6.9), and low δ18O values (4.7 to 6.3 ‰), generally close to those of depleted mantle. All data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components (e.g., Neoproterozoic mafic arc magmas), triggered by invasion of contemporaneous mafic melts at Dexing. The MMEs show textural, mineralogical, and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas. These MMEs usually contain magmatic chalcopyrite, and have original, variable contents of Cu (up to 500 ppm). Their geochemical characteristics suggest that they were derived from an enriched mantle source, metasomatized by Proterozoic slab-derived fluids, and supplied a part of Cu, Au, and S for the Dexing porphyry system during their injection into the felsic magmas. The 171 ± 1 Ma magmatic-hydrothermal event at Dexing is contemporaneous with the mid-Jurassic extension in the SCC, followed by 160–90 Ma arc-like magmatism in southeastern China. With respect to the tectono-magmatic evolution of the SCC, the emplacement of Cu-bearing porphyries and the associated Cu mineralization occurred in response to the transformation from a tensional regime, related to mid-Jurassic extension, to a transpressional regime, related to the subduction of the Paleo-Pacific oceanic lithosphere.

How Accurately Can We Date the Duration of Magmatic-Hydrothermal Events in Porphyry Systems?--An Invited Paper

Determining the absolute duration of magmatic-hydrothermal events leading to the formation of porphyry systems (i.e., including porphyry copper, skarn, and epithermal deposits) is one of the key questions in ore geology. This is so because the duration of magmatic-hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy. The problem of determining the absolute duration of magmatic-hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes. Both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo-annum (ka). Isotopic dating in contrast is the only possible way to determine the life span of magmatic-hydrothermal events in fossil porphyry systems. Analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating (U-Pb, 40Ar/39Ar, Re-Os) allow us to date minerals with internal precisions <0.2% (2σ). For a 10-Ma-old mineral this corresponds to a <20-ka uncertainty, which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo-annum scale. However, many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events, which extend the overall life of the porphyry systems to a few 0.X and up to ~2 Ma in some cases. Internal precisions of the above mentioned dating methods allow us, in theory, to comfortably discriminate events at the 0.X scale and the combination of U-Pb, 40Ar/39Ar, and ReOs geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic-hydrothermal events in porphyry systems. In this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems. We show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated. We also provide basic guidelines to follow when trying to resolve the duration of magmatic-hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable.

Geometallurgy of the Pebble Porphyry Copper-Gold-Molybdenum Deposit, Alaska: Implications for Gold Distribution and Paragenesis

The Pebble Cu-Au-Mo deposit in southwest Alaska is one of the world's largest porphyry deposits. The deposit contains over 80 billion pounds (Blbs) Cu, 107 million ounces (Moz) Au, and 5.6 Blbs Mo in all resource categories. Copper and gold grades vary across diverse hydrothermal alteration assemblages with higher grades associated with sericite- and pyrophyllite-rich assemblages in an advanced argillic alteration zone. Moderate grades are associated with the potassic and sodic-potassic alteration assemblages that dominate the deposit and low grades are found in quartz-illite-pyrite assemblages. These variations reflect fluctuations in both the temperature and composition of the magmatic-hydrothermal fluids in time and space. QEMSCAN scanning electron microscope-based mineral mapping and in situ laser ablation inductively coupled plasmamass spectrometry methods designed to document the spatial and temporal variations in metal deportment across alteration zones show that gold occurs as inclusions of electrum and high-fineness gold in chalcopyrite and pyrite and as free grains hosted by silicate minerals, with the proportion of each related to the principal host alteration assemblage. Gold associated with the earliest high temperature sodic-potassic and potassic assemblages occurs as electrum inclusions in chalcopyrite and to a lesser extent pyrite, and pyrite trace element signatures have high gold, silver, and copper concentrations (pyrite-1). During cooling of the hydrothermal system lower temperature illite and illite-kaolinite alteration overprinted the earlier assemblages with varying degrees of recrystallization and precipitation of more pyrite-rich assemblages with pyrite having a gold-poor trace element composition (pyrite-2). Gold released during recrystallization formed high-fineness gold inclusions in newly formed pyrite and narrow gold-rich pyrite rims (pyrite-3). A second magmatic-hydrothermal event resulted in a sericite- and pyrophyllite-rich advanced argillic overprint, which introduced more copper and gold to the system and recrystallized preexisting sulfides into both high and low sulfidation assemblages with high-fineness gold inclusions in pyrite and chalcopyrite and solid solution gold in bornite. A second generation of pyrite-1 with elevated palladium concentrations is associated with this hydrothermal fluid pulse and also possibly high arsenic pyrite (pyrite-4). Understanding the controls on gold deportment provides genetic constraints on ore deposit genesis. Alteration and sulfide assemblages and gold compositions provide information on the hydrothermal fluid compositions, pH, and temperature evolution of the magmatic-hydrothermal system. The presence of electrum inclusions in chalcopyrite and pyrite in potassic and sodic-potassic alteration is consistent with these zones forming from early high-temperature magmatic fluids because both gold and silver are transported together in such fluids. High-fineness gold inclusions in younger pyrite related to low-temperature clay alteration are evidence that the gold in early electrum was partially remobilized and reprecipitated. The low-temperature clay alteration most likely formed from a mixture of H2S-rich vapor with meteoric waters, conditions where gold is transported as a bisulfide complex and silver is not, resulting in the separation of the two metals. A second, structurally controlled pulse of magmatic fluids formed a well-mineralized advanced argillic alteration assemblage containing high-fineness gold inclusions in chalcopyrite and pyrite, the only part of the deposit where high-fineness gold is hosted by chalcopyrite. In the pyrophyllite stability field acidic fluid compositions transport gold and silver as different complexes and gold solubility is significantly lower than silver leading to the precipitation of highfineness gold inclusions. Palladium is transported by the same complex as gold under these conditions, consistent with the elevated palladium content of pyrite in the advanced argillic alteration assemblage. The temporal and spatial studies of variations in gold deportment across the Pebble deposit provide critical inputs to optimization of mineral processing design. The greatest influence on metallurgical gold recovery at Pebble is the proportion of gold that is hosted by pyrite. Pyrite-hosted gold may require different mineral processing methodologies compared with gold hosted by chalcopyrite. Therefore, defining domains of consistent hydrothermal alteration, and sulfide mineralogy and gold deportment is the key to the geometallurgical characterization of the deposit.

Emplacement ages and exhumation rates for intrusion-hosted Cu–Mo–Sb–Au mineral systems at Freegold Mountain (Yukon, Canada): assessment from U–Pb, Ar–Ar, and (U–Th)/He geochronometers

To decipher the thermal history of mineralized systems across the Freegold Mountain area (Yukon, Canada), a combined geochronology (zircon U–Pb and hornblende, biotite, and whole rock Ar–Ar) and thermochronology (apatite and zircon (U–Th)/He) study was carried out. Previous U–Pb data combined with new U–Pb and Ar–Ar data show that intrusive bodies across the Freegold Mountain were emplaced during two protracted episodes, the first spanning from 109.6 to 98 Ma and the second between 79 and 68 Ma. Overprinting of the first intrusive event by a second magmatic hydrothermal event is suggested by a zircon U–Pb age of 108.7 ± 0.4 Ma for a chlorite-altered dyke and a whole rock Ar–Ar plateau age of 76.25 ± 0.53 Ma. Zircon (U–Th)/He data are between 66 and 89 Ma, whereas apatite (U–Th)/He data are scattered (38.7–109.9 Ma) and bracket the two magmatic emplacement events. Our combined data reveal a complex history of reheating that led to resetting of numerous chronometers. In most of the investigated magmatic hydrothermal systems, early fast cooling from igneous emplacement through hydrothermal alteration (between 900 and 200 °C) was followed by later and slower cooling accompanying post mineralization uplift and erosion (between 200 and 70 °C). Preliminary models indicate intrusive bodies associated with the Stoddart Cu–Mo ± W prospect cooled slowly (23 °C/Ma) compared with the ones spatially associated with the Revenue Au–Cu prospect (43 °C/Ma), and the similarity of the zircon U–Pb and (U–Th)/He ages from Revenue dyke further supports a rapid cooling from 700 to 180 °C. Erosion rates of 0.035–0.045 mm/year are consistent with tectonic quiescence during the Late Tertiary combined with the lack of Pleistocene glaciation in central Yukon. Such low rates of exhumation favour the formation and preservation of supergene mineralization, such as that found north of Freegold Mountain.

Ages and Sources of Components of Zn-Pb, Cu, Precious Metal, and Platinum Group Element Deposits in the Goodsprings District, Clark County, Nevada

The Goodsprings district, Clark County, Nevada, includes zinc-dominant carbonate replacement deposits of probable late Paleozoic age, and lead-dominant carbonate replacement deposits, copper ± precious metal-platinum group element (PGE) deposits, and gold ± silver deposits that are spatially associated with Late Triassic porphyritic intrusions. The district encompasses ~500 km2 although the distribution of all deposits has been laterally condensed by late Mesozoic crustal contraction. Zinc, Pb, and Cu production from about 90 deposits was ~160,000 metric tons (t) (Zn > Pb >> Cu), 2.1 million ounces (Moz) Ag, 0.09 Moz Au, and small amounts of PGEs—Co, V, Hg, Sb, Ni, Mo, Mn, Ir, and U—were also recovered. Zinc-dominant carbonate replacement deposits (Zn > Pb; Ag ± Cu) resemble Mississippi Valley Type (MVT) Zn-Pb deposits in that they occur in karst and fault breccias in Mississippian limestone where the southern margin of the regional late Paleozoic foreland basin adjoins Proterozoic crystalline rocks of the craton. They consist of calcite, dolomite, sphalerite, and galena with variably positive S isotope compositions ( δ 34S values range from 2.5–13‰), and highly radiogenic Pb isotope compositions (206Pb/204Pb >19), typical of MVT deposits above crystalline Precambrian basement. These deposits may have formed when southward flow of saline fluids, derived from basinal and older sedimentary rocks, encountered thinner strata and pinch-outs against the craton, forcing fluid mixing and mineral precipitation in karst and fault breccias. Lead-dominant carbonate replacement deposits (Pb > Zn, Ag ± Cu ± Au) occur among other deposit types, often near porphyritic intrusions. They generally contain higher concentrations of precious metals than zinc-dominant deposits and relatively abundant iron oxides after pyrite. They share characteristics with copper ± precious metal-PGE and gold ± silver deposits including fine-grained quartz replacement of carbonate minerals in ore breccias and relatively low S and Pb isotope values ( δ 34S values vary from 0–~4‰; 206Pb/204Pb <18.5). Copper ± precious metal-PGE deposits (Cu, Co, Ag, Au, Pd, and Pt) consist of Cu carbonate minerals (after chalcocite and chalcopyrite) and fine-grained quartz that have replaced breccia clasts and margins of fissures in Paleozoic limestones and dolomites near porphyritic intrusions. Gold ± silver deposits occur along contacts and within small-volume stocks and dikes of feldspar porphyry, one textural variety of porphyritic intrusions. Lead isotope compositions of copper ± precious metal-PGE, gold ± silver, and lead-dominant carbonate replacement deposits are similar to those of Mojave crust plutons, indicating derivation of Pb from 1.7 Ga crystalline basement or from Late Proterozoic siliciclastic sedimentary rocks derived from 1.7 Ga crystalline basement. Four texturally and modally distinctive porphyritic intrusions are exposed largely in the central part of the district: feldspar quartz porphyry, plagioclase quartz porphyry, feldspar biotite quartz porphyry, and feldspar porphyry. Intrusions consist of 64 to 70 percent SiO2 and variable K2O/Na2O (0.14–5.33) that reflect proportions of K-feldspar and albite phenocrysts and megacrysts as well as partial alteration to K-mica; quartz and biotite phenocrysts are present in several subtypes. Albite may have formed during emplacement of magma in brine-saturated basinal strata, whereas hydrothermal alteration of matrix, phenocrystic, and megacrystic feldspar and biotite to K-mica, pyrite, and other hydrothermal minerals occurred during and after intrusion emplacement. Small volumes of garnet-diopside-quartz and retrograde epidote-mica-amphibole skarn have replaced carbonate rocks adjacent to one intrusion subtype (feldspar-quartz porphyry), but alteration of carbonate rocks at intrusion contacts elsewhere is inconspicuous. Uranium-lead ages of igneous zircons vary inconsistently from ~ 180 to 230 Ma and are too imprecise to distinguish age differences among intrusion subtypes; most ages are 210 to 225 Ma, yielding a mean of 217 ± 1 Ma. K-Ar and 40Ar/39Ar ages of magmatic (plagioclase, biotite) and hydrothermal (K-mica) minerals span a similar range (183–227 Ma), demonstrating broadly contemporaneous intrusion emplacement and hydrothermal alteration but allowing for multiple Late Triassic magmatic-hydrothermal events. Imprecision and range of isotopic ages may have resulted from burial beneath Mesozoic and Tertiary strata and multiple intrusion of magmas, causing thermal disturbance to Ar systems and Pb loss from zircons in intrusions. Separate late Paleozoic (zinc-dominant carbonate replacement deposits) and Late Triassic (all other deposits) mineralizing events are supported by form, distribution, and host rocks of metal deposits, by hydrothermal mineral assemblages, isotope compositions, metal abundances, and metal diversity, and by small intrusion volumes. These characteristics collectively distinguish the Goodsprings district from larger intrusion related carbonate replacement districts in the western United States. They can be used to evaluate proximity to unexposed porphyritic intrusions associated with PGE and gold ± silver mineralization.

Geochronological and geochemical characterization of magmatic-hydrothermal events within the Southern Variscan external domain (Cévennes area, France)

Geochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Loze're- Borne plutonic complex and surrounding rocks (Ce'vennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic- hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures.

Mineralogical and fluid inclusion constraints on the evolution of the Plaka intrusion-related ore system, Lavrion, Greece

The intrusion-centered, polymetallic Plaka ore system, Lavrion, Greece, is genetically related to the emplacement of a Miocene biotite ± hornblende granodioritic body and granodiorite porphyry dikes/sills within metamorphic Mesozoic rocks of the Attic-Cycladic Crystalline Belt. The ore system includes porphyry, breccia-type, skarn, skarn-free carbonate replacement and vein-type ores enriched in Mo, W, Pb, Zn, Cu, As, Sb, Bi and Ag. Early pyrite-molybdenite (±chalcopyrite ± pyrrhotite) mineralization was deposited in multidirectional and sheeted quartz veins within the silicically and sericitically altered granodiorite intrusive. The emplacement of granodiorite porphyry dikes/sills was followed by the introduction of base metal sulphides during hydrothermal brecciation of the porphyritic rocks. Skarn- and skarn-free carbonate replacement ores developed contemporaneously with these magmatic-hydrothermal events and are characterized by the presence of pyrrhotite, pyrite, Fe-rich sphalerite and Bi-Ag-rich galena. A late Pb-As-Sb-Cu-Ag-rich banded vein (Filoni 80) is composed of pyrrhotite, chalcopyrite, arsenopyrite, sphalerite (both Fe-rich and Fe-poor), lollingite, argentian tetrahedrite, bournonite, pyrargyrite, miargyrite, native arsenic, semseyite, heteromorphite, veenite, falkmanite and fluorite, quartz and siderite. The ore mineralogy of the Plaka deposit is generally consistent with reducing and low sulphidation fluid conditions during metal deposition. Fluid inclusion data indicate that molybdenite deposition was associated with boiling between 270 and 360 °C and at a pressure of 50–200 bars, corresponding to a depth of approximately 1 km. Dilution of a saline fluid by mixing with less saline water resulted in the formation of a moderately saline (14–18 wt% NaCl equiv.), low-temperature (190–220 °C) fluid and the precipitation of sulphides and sulphosalts in the Filoni 80 vein. The evolution from early porphyry-style mineralization to the later vein ores likely formed either as a result of continuous syn-mineralization uplift of the Plaka granodiorite in relation to post-orogenic regional uplift during the Upper Miocene, or as a simple cooling of the high-temperature hydrothermal system.

Re?Os isotopic dating of molybdenite and pyrite in the Baishan Mo?Re deposit, eastern Tianshan, NW China, and its geological significance

The Baishan Mo–Re deposit is located in the eastern section of the eastern Tianshan orogenic belt, NW China. The deposit has a grade of 0.06% Mo and a high content of rhenium of 1.4 g/t. Rhenium and osmium isotopes in sulfide minerals from the Baishan deposit are used to determine the age of mineralization. Rhenium concentrations in molybdenite samples are between 74 and 250 μg/g. Analysis of eight molybdenite samples yields an isochron age of 224.8±4.5 Ma (2σ). Pyrite samples have rhenium and osmium concentrations varying in the range 33.4–330.6 ng/g and 0.08–0.81 ng/g, respectively. Isotope data on seven pyrite samples yield an isochron age of 225±12 Ma (2σ) on the 187Re/188Os versus 187Os/188Os plot and an age of 233±14 Ma (2σ) on the 187Os versus 187Re correlation diagram. The ages of molybdenite and pyrite are consistent within the analytical errors. Combined with field observations, the data indicate that Mo–Re mineralization in the Baishan deposit is produced by a magmatic-hydrothermal event in an intracontinental extensional setting after late Paleozoic orogeny. The initial 187Os/188Os ratio of pyrite is 0.3±0.07. The δ34S values of molybdenite vary from +0.5 to +3.6‰. Both data indicate that mineralization is derived mainly from a mantle source.

From andesitic volcanism to the formation of a porphyry Cu-Au mineralizing magma chamber: the Farallón Negro Volcanic Complex, northwestern Argentina

The geological relations and geochemical composition of extrusive and subvolcanic intrusive rocks of the Upper Miocene Farallón Negro Volcanic Complex (NW Argentina) were studied to reconstruct the evolution of a deeply eroded andesitic volcano over most its life history of some 3.5 my. This paper is part of an integrated study of the processes that generate magmatic-hydrothermal fluids responsible for the formation of porphyry-Cu-Au deposits. Here we present new data on bulk rock chemical and Nd, Sr and Pb isotopic compositions of unaltered volcanic and subvolcanic intrusive rocks, for which geological relations and ages are constrained by field-based stratigraphy and extensive Ar–Ar geochronology. The combined results indicate that voluminous volcanism started at ca. 9.7 Ma with the eruption of basaltic andesite to andesite lava flows and pyroclastic deposits. This basic to intermediate volcanism persisted until 7.5 Ma and was followed at 7.35 Ma by the eruption of dacites as the last recorded major extrusive event. The oldest andesitic intrusions, emplaced at 9.0 Ma, were followed by hypabyssal intrusions of basaltic andesite, andesite and dacite composition, which intruded the volcanic sequence between 8.3 and 6.1 Ma. Intrusions at 6.8±0.2 Ma led to the formation of over 800 Mt of porphyry-Cu-Au ore at Bajo de la Alumbrera. It was formed by a large flux of magmatic-hydrothermal fluids channeled through several phases of fractured dacitic porphyry, which together with immediately following barren andesitic intrusions make up a composite stock. After this magmatic-hydrothermal event, only minor rhyodacitic to rhyolitic intrusions were emplaced until magmatic activity ceased at 6.1 Ma. Geochemical data for the magmatic rocks indicate that the evolution of the system was dominated by mixing of a mafic magma with a felsic magma that contained a component of crustal material, as indicated by linear compositional correlations between compatible and incompatible elements. Initial strontium isotopic ratios vary between 0.7047 and 0.7118 while 143Nd/ 144Nd range from 0.5127 to 0.5124, and both isotopic variations correlate with each other and with SiO 2 content of the rocks. After 8.5 Ma, a continuous increase in the silica content of intrusive rocks suggests the gradual formation of a magma chamber. A substantial magma reservoir (>7 km 3) must have been established by the time of porphyry-Cu-Au mineralization, to allow rapid release of a large volume of ore-forming magmatic brine as a result of wholesale crystallization and de-volatilization of the magma chamber.

Origin of ore-forming fluids of the Dachang Sn-polymetallic ore deposit: Evidence from helium isotopes

The ore genesis model for the Dachang Snpolymetallic ore deposit has long been in dispute, and the major debate focuses on whether the stratiform and massive orebodies formed during the Yanshanian magmatic-hydrothermal event or they were products of Devonian syn-sedimentary exhalative-hydrothermal event. This note presents new helium isotope data from fluid inclusions of four pyrites and one fluorite. The pyrites were collected from the stratiform and massive orebodies in the deposit, and their3He/4He ratios are significantly higher than 1, ranging from 1.7 to 2.5 Ra, which indicates a mantle component in the responsible hydrothermal fluids. It is suggested that the ore-forming fluids were a mixture of deep circulating seawater and a mantle-derived fluid, which are similar to many of those modern submarine hydrothermal fluids. In contrast, the fluorite, collected from a granite-related hydrothermal vein in the deposit, shows a low3He/4He ratio of 0.7R a, which indicates no mantle component involvement in the fluids. Hence, the helium isotope data indicate that the two kinds of fluids from the Dachang deposit may have different origins. Together with other geological and geochemical evidence, it is suggested that the stratiform and massive ore formation has no genetic relation to the Yanshanian granite. Rather, they were likely formed during Devonian submarine exhalative-hydrothermal processes.

Chemical and Rb–Sr, Sm–Nd isotopic systematics of tourmaline from the Dachang Sn-polymetallic ore deposit, Guangxi Province, P.R. China

The Dachang Sn-polymetallic ore deposit (Guangxi, China) is one of the largest Sn deposits in the world, which occurs in Devonian siliceous rocks and limestones as stratiform, massive and vein orebodies. Various models for the ore genesis of the deposit have been proposed, and the major debate focuses on whether the stratiform Sn–Pb–Zn orebodies are syn-sedimentary in origin or they were products of the Yanshanian magmatic-hydrothermal event (∼100 Ma) like the vein-type ores. In this paper, we present a chemical and Rb–Sr, Sm–Nd isotopic study of tourmaline, which occurs both in the stratiform orebodies and in veins associated with the granite in the Dachang deposit, and use these data to constrain the origin of the tourmaline and associated mineralisation. Two types of tourmaline occur in the Dachang deposit: (I) tourmaline from the stratiform Sn–Pb–Zn ores and their host siliceous rocks; (II) quartz-tourmaline veins in or near the granite. Group I tourmalines are Mg-rich dravites with low Fe/(Fe+Mg) ratios (0.01–0.25) and variable Na/(Na+Ca) ratios (0.62–1.0); group II tourmalines are Fe-rich schorls with high Fe/(Fe+Mg) and Na/(Na+Ca) ratios (0.81–0.88 and 0.92–0.97, respectively). Group II tourmalines have low REE contents and LREE-enriched patterns with negative Eu anomalies (Eu/Eu*=0.06 to 0.35). In contrast, group I tourmalines have higher and more variable REE contents with HREE-enriched patterns and negative to positive Eu anomalies (Eu/Eu*=0.18 to 3.64). They also have different contents for other trace elements such as U, Th, Zr, Hf, and Sn, which may reflect the nature of the hydrothermal fluids that circulated deep into the footwall lithologies. Group I tourmalines have 87 Sr/ 86 Sr ratios of 0.71339–0.71818 and 143 Nd/ 144 Nd ratios of 0.51201–0.51210; while the group II tourmalines have more variable 87 Sr/ 86 Sr ratios (0.71187–0.72735) and slightly higher 143 Nd/ 144 Nd ratios (0.51210–0.51224). The Sr and Nd isotopic data display different mixing trends, suggesting different origins for the two types of tourmalines. Group I tourmalines are thought to have formed during the Devonian from deeply circulating submarine hydrothermal fluids. The fluids leached Sr and Nd from the footwall lithologies (with high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd ) and mixed Sr and Nd derived from the overlying Devonian host sedimentary rocks (with low 87 Sr/ 86 Sr and intermediate 143 Nd/ 144 Nd ). Group II tourmalines formed from Yanshanian magmatic-hydrothermal fluids, with their Sr and Nd derived from a granitic source (remelted crustal rocks with high 87 Sr/ 86 Sr and relatively high 143 Nd/ 144 Nd ) and again mixing with Sr and Nd derived from host Devonian sedimentary rocks.

Facies analysis of a 1.9 Ga, continental margin, back-arc, felsic caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen region, Sweden

The Bergslagen mining district in south-central Sweden has been a major metal producer for more than 1,000 years. In geologic terms, Bergslagen is the intensely mineralized part of a large, Early Proterozoic (mainly 1.90-1.87 Ga), felsic magmatic region of mainly medium to high metamorphic grade in the Baltic Shield. The district contains a diverse range of ore deposit types, including banded iron-formation, magnetitecalc-silicate skarn, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and strata-bound Zn-Pb-Ag-(Cu-Au) sulfide ores, and W skarn. Most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns.This study is a field-oriented, regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration, deformation, and metamorphism. The region is interpreted as an extensional, probably back-arc, active continental margin magmatic region. It evolved through stages of intense magmatism, thermal doming, and crustal extension, followed by waning extension, waning volcanism, thermal subsidence, reversal from extension to compressional deformation, regional metamorphism, and structural inversion. The volcanic successions comprise interfingering proximal (near vent), medial (volcano flanks), and distal (volcano margin) facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite-rhyolite complexes. Nonwelded to poorly welded pyroclastic flow and fallout units, and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies. Subvolcanic porphyritic intrusions and cryptodomes are also abundant. The pyroclastic debris was shed from mainly shallow-water and subaerial proximal areas to shallow- and moderately deep-water (above and below storm wave base) distal areas.Most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash-siltstone, limestone, and vitric crystal sandstone and breccia. These mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base, but locally above the storm wave base. Water depths are interpreted to have been mainly 10 to 500 m, with local deeper areas. Although the ore deposits occur in medial to distal facies associations, many occur above, or laterally adjacent to, subsided volcanic vent complexes. This suggests a spatial, temporal, and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers. Except for the apatite-bearing iron deposits, it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes. This is possible, especially for the base metal sulfide deposits, which have intense localized footwall alteration in felsic volcanic rocks. However, an important role of the volcanic centers may have been to provide strong positive pertubations (+ or - magmatic hydrothermal input) to the regional convective geothermal system.The main Zn-Pb-Ag-(Cu-Au) ore deposits span a range between two end-member types: stratiform ash-siltstone-hosted sea-floor deposits that have similarities to some stratiform sediment-hosted Zn-Pb-Ag deposits, especially the Broken Hill-type deposits; and strata-bound volcanic-associated limestone-skarn subsea-floor replacement deposits that more closely resemble felsic volcanic-associated massive sulfide deposits. Facies models are presented for both. The banded iron-formations are interpreted as chemical suspension deposits, rhythmically interbedded with rhyolitic ash-siltstone. The skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements. Apatite-bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite-rhyolite volcanic center.

Geology of epithermal silver-gold bulk-mining targets, bodie district, Mono County, California

The Bodie mining district in Mono County, California, is zoned with a core polymetallic-quartz vein system and silver- and gold-bearing quartz-adularia veins north and south of the core. The veins formed as a result of repeated normal faulting during doming shortly after extrusion of felsic flows and tuffs, and the magmatic-hydrothermal event seems to span at least 2 Ma.

Origin and mode of copper enrichment in biotite from rocks associated with porphyry copper deposits; a transmission electron microscopy investigation

This paper presents results of a systematic transmission electron microscopy (TEM) study of biotites from a suite of core samples from the Cyprus Casa Grande porphyry copper deposit, Arizona. The primary purpose of this study was to determine the mode and timing of copper enrichment in biotite and to compare the results to TEM observations of biotites from other porphyry copper deposits. The core samples represent both oxidized and unoxidized rock. Biotites and chlorites from unoxidized protore contain less than 200 ppm copper as measured by electron microprobe and are free of copper-enriched expanded interlayers and the native copper inclusions described by Ilton and Veblen (1988); biotites and chlorites from oxidized ore are commonly copper enriched. The mode of copper incorporation in biotites and chlorites in oxidized rock is complex and varies with depth. Biotites from the goethite zone contain submicroscopic native copper inclusions and both copper-enriched expanded interlayers and submicroscopic iron oxide inclusions. Biotites and chlorites from the chrysocolla zone contain copper-enriched expanded interlayers, amorphous Al-Si intergrowths with dense arrays of copper- and iron-rich inclusions, and copper-rich inclusions that rapidly evaporate under the electron beam and hence have yet to be identified. It is possible that copper is incorporated into the brucite layer in chlorite. Biotites from the native copper zone contain native copper inclusions, as expected, but they also contain copper-rich expanded interlayers. Biotites associated with atacamite mineralization contain high concentrations of copper that are associated with expanded interlayers. In the chalcocite zone, biotites appear relatively unweathered and contain minor concentrations of copper that are associated with infrequent expanded interlayers.A comparison of observations from oxidized and unoxidized rock indicates that the occurrence of copper-rich biotites and both copper-enriched expanded interlayers and native copper inclusions in biotite are restricted to oxidized rock. We conclude that the native copper inclusions and copper-rich expanded interlayers in biotite from Cyprus Casa Grande formed during weathering and not during the hydrothermal event.Native copper inclusions and copper-rich expanded interlayers in biotites have been documented from a variety of porphyry copper deposits. Earlier studies of the same samples have suggested that copper in copper-rich biotites was representative of magmatic hydrothermal events and that the difference between mineralizing and barren intrusions was recorded by differences in copper concentrations in biotite. The present study, together with previous experimental work, suggests that the copper-rich biotites became copper enriched during weathering and that copper concentrations in copper-rich biotites do not reflect magmatic hydrothermal processes.

The role of fluids in the formation of platinum-group minerals, Duluth Complex, Minnesota; mineralogic, textural, and chemical evidence

Several new occurrences of platinum-group minerals (PGM), including sperrylite (PtAs 2 ), taimyrite ((Pd, Cu) 3 Sn), froodite (PdBi 2 ), michenerite (PdBiTe), and moncheite ((Pt,Bi) 2 Te 3 ) are recorded from drill core samples of the basal troctolites of the Duluth Complex. The first documentation of a vanadium-chromium hercynite (V 2 O 3 ranging from 5.14-5.23 wt %) in graphite-rich samples of the mineralized zone of the Duluth Complex is also reported.The PGM are associated predominantly with serpentinized olivine and secondary magnetite or with altered, calcic plagioclase ( approximately An 75 ), prehnite, actinolite, hornblende, and chlorite. The Pd bismutho-tellurides are associated with parkerite (Ni 3 Bi 2 S 2 ), tellurobismuthite (Bi 2 Te 3 ), bismuthinite (Bi 2 S 3 ), hessite (Ag 2 Te), native gold, and clausthalite (Pb(S,Se)) with low-temperature sulfides like valleriite (2(Fe,Cu)S . 3(Mg,Fe)(OH) 2 ), violarite (FeNi 2 S 4 ), heazlewoodite (Ni 3 S 2 ), and in some instances, graphite. These associations, especially the occurrence of the PGM in altered silicates, suggest that the PGM reported in this study were transported and deposited during a late magmatic hydrothermal event which could be related to the introduction of volatiles from the metasedimentary Virginia Formation and the Biwabik Iron-Formation. The high Cl contents of the serpentine, biotite, apatite, and graphite as well as the high An contents of the plagioclase in contact with the sulfides and PGM further suggest that chloride complexing was a possible mechanism for the transport and deposition of the PGM during the hydrothermal event. The occurrence of high vanadium-chromium hercynite is also consistent with this interpretation.

The Kelian disseminated gold deposit, East Kalimantan, Indonesia

Kelian is one of a number of recently discovered Tertiary volcanic-hosted gold deposits which occur within a 400-km-long, northeast-trending belt in the interior of Kalimantan, the Indonesian part of Borneo. It consists of two main orebodies, namely West and East Prampus, and four smaller mineralized ore zones. These have a combined resource potential of +75 Mt at 1.8 g/t Au, making Kelian the largest known gold deposit in Indonesia. The deposit was discovered in 1976 during follow up work of alluvial gold occurrences within the Kelian River, using traditional exploration techniques of stream sediment, pan concentrate, rock float and outcrop sampling. Subsequent detailed investigations included soil sampling, trenching, deep augering, ground magnetics, induced polarization and 60,000 m of diamond drilling. The geology of the deposit consists of a pile of silicic pyroclastics grading upwards into a series of sediments of Late Eocene age. The sequence was folded and faulted along northerly and northeasterly trends and intruded by a number of subvolcanic andesitic to trachyandesitic bodies in the Early Miocene. Some time after the emplacement of the andesites a hydrothermal system was established. It brought about extensive alteration, mineralization and hydrothermal brecciation. The preserved part of this system is 1 km 2 in area and has a vertical extent of at least 600 m. Limited radiometric age data suggest an age of around 20 Ma for the alteration. Four partly overlapping stages of alteration/mineralization are recognized: 1. Stage I: chlorite+carbonate+sericite+pyrite±epidote alteration in the cores of the larger andesite bodies. 2. Stage IIA: localized fracturing and sericite±quartz deposition. Stage IIB: a major period of polyphasal fracturing and hydrothermal brecciation with associated boiling of hot (290–330°C), dilute (0.5–4.2 eq. wt.% NaCl) and CO 2-rich fluids, and development of replacement/vein alteration of adularia+sericite+quartz+pyrite accompanied by subordinate As-Ag mineralization and possibly some gold mineralization. Progressive loss of CO 2 upon boiling caused a progressive increase in adularia and a corresponding decrease in sericite deposition. Deeper in the system a quartz-sericite-carbonate assemblage was formed without adularia. 3. Stage III: renewed magmatic activity, focused on East Prampus, released volatiles to the hydrothermal system, producing a hot (300–330°C), relatively saline (> 10 wt.% NaCl), CO 2-rich brine, and extensive fracturing and hydrothermal brecciation. This was accompanied by boiling, deposition of various carbonates±quartz±sericite and gold-base metal mineralization; preliminary stable isotope data on the carbonates support the presence of a magmatic component. Mixing of the upwelling hot, saline fluids with descending cooler, relatively low pH, steam-heated fluids has resulted in a complex zonation in carbonate mineralogy and a wide range of fluid salinities and homogenization temperatures. Away from the centre of brecciation and boiling, gold and base metals were deposited in a more passive environment together with carbonate and/or quartz. 4. Stage IV: late-stage widespread influx of relatively cool (<200°C), low pH (3–4), CO 2-rich, steam-heated fluids from the top and margins of the system forming a kaolinite+Fe/Mn carbonate+pyrite ±hematite±cinnabar assemblage. The magmatic-hydrothermal event was followed by uplift and erosion of more than 900 m during the Midddle Late Miocene and basaltic volcanism in the Plio-Pleistocene. A second cycle of erosion, which began in the Pleistocene, has removed most of the young volcanic cover exposing the deposit as known today.

Scandian ixiolite, pyrochlore and bazzite in granite pegmatite in terdal, Telemark, Norway. A contribution to the mineralogy and geochemistry of scandium and tin

In Tordal granite pegmatite veins cut through a volcano-sedimentary sequence which overlies an older, gneissic basement and younger granite from which they originate. Positive co-variations of scandium, lithium, beryllium and tin, which were documented through a recent prospecting program for tin, indicate a possible enrichment of scandium and tin during the passage of the pegmatitic material through the volcanic sedimentary sequence. A description is given of scandium-rich ixiolite, (Ta,Nb,Sc,Sn,Fe,Mn,Ti)2O4 in association with pyrochlore, (Ca,Sc,Y,Sn,U)2(Ta,Nb,Ti)2O6(O,OH,F) and bazzite, Be3Sc2Si6O18 from a cleavelandite-amazonite pegmatite occurring as a part of a larger network of granitic pegmatite veins. Alteration from ixiolite to pyrochlore appears as a late magmatic-hydrothermal event.

Identification of the origin of oreforming solutions by the use of stable isotopes

Synopsis The four major different types of water — magmatic, metamorphic, sea water and/or connate, and meteoric water — have characteristic hydrogen (D/H) and oxygen ( 18 O/ 16 O) isotope ratios. Applied to the analysis of isotopic data on hydrothermal minerals, fluid inclusions and waters from active geothermal systems, these ratios indicate that waters of several origins are involved with ore deposition in the volcanic and epizonal intrusive environment. Water of a single origin dominates main-stage mineralization in some deposits: magmatic — Casapalca, Peru (Ag-Pb-Zn-Cu); meteoric — Butte, Montana (Cu-Zn-Mn), epithermal deposits, e.g. Goldfield, Tonopah, Nevada (Ag-Au), Pachuca, Mexico (Ag-Au), San Juan Mountains District, Colorado (Ag-Au-Pb-Zn); sea water — Troodos, Cyprus (Fe-Cu), Kuroko, Japan (Fe-Cu-Pb-Zn). Solutions of more than one origin are important in certain deposits (magmatic and meteoric — porphyry copper and molybdenum deposits) and are present in many. In the porphyry Cu-Mo deposits the initial major ore transportation and alteration processes (K-feldspar-biotite alteration) are magmatic-hydrothermal events that occur at 750–500°C. These fluids are typically highly saline Na-K-Ca-Cl-rich brines (more than 15 wt % equivalent NaCl). The convecting meteoric-hydrothermal system that develops in the surrounding country rocks with relatively low integrated water/rock ratios (less than 0.5 atom % oxygen) subsequently collapses in on a waning magmatic-hydrothermal system at about 350–200°C. These fluids generally have moderate to low salinities (less than 15 wt % equivalent NaCl). Differences among these deposits are probably in part related to variations in the relative importance of the meteoric-hydrothermal versus the magmatic-hydrothermal events. The sulphur comes from the intrusion and possibly also from the country rocks. Deposits in which meteoric or sea water is the dominant constituent of the hydrothermal fluids come from epizonal intrusive and sub-oceanic environments where the volcanic country rocks are fractured or well jointed and highly permeable. Integrated water/rock ratios are typically high, with minimum values of 0.5 or higher (atom % oxygen) — the magmatic water contribution is often ‘drowned out’. Salinities are low to very low (less than 10 wt % equivalent NaCl), and temperatures are usually in the range 350–150°C. The intrusion supplies the energy to drive the large-scale convective circulation system. The sulphur comes from the intrusion, the country rocks and/or the sea water. Argillic alteration, which occurs to depths of several hundred metres, generated during supergene weathering in many of these deposits is isotopically distinguishable from hydrothermal clays.