ALBERT

Description: To reconstruct the magmatic–hydrothermal processes leading to porphyry Mo ore formation at the Climax Mo mine, Colorado, four magma units that were emplaced before, during and shortly after the mineralization events were investigated: (1) a pre-mineralization white dike of the Alma district; (2) the syn-mineralization Chalk Mountain Rhyolite; (3) a late- to post-mineralization rhyolite porphyry dyke; (4) a mafic enclave within the productive Bartlett stock. Melt inclusions, mineral inclusions and fluid inclusions in quartz phenocrysts were investigated by means of laser ablation inductively coupled plasma mass spectrometry, electron microprobe and microthermometry. Based on melt inclusion data both the Chalk Mountain Rhyolite and the rhyolite porphyry were ~10 times more fractionated than average granite and show geochemical characteristics of topaz rhyolites. They were saturated in magnetite, Mn-rich ilmenite, fluorite, aeschynite, monazite, pyrrhotite and thorite, and crystallized predominantly at 710–730°C, 1·2–2·6 kbar and log f O 2 FMQ + 2·2 (where FMQ is fayalite–magnetite–quartz). The silicate melt of the Chalk Mountain Rhyolite contained 3·5 ± 0·4 wt % F, 0·09 ± 0·03 wt % Cl, ≥ 3·0 wt % H 2 O, 15–90 µg g –1 Cs, 500–1500 µg g –1 Rb and 5–7 µg g –1 Mo, whereas that of the rhyolite porphyry contained 1·1 ± 0·3 wt % F and 4·9 ± 1·2 wt % H 2 O, but otherwise had a virtually identical major and trace element composition. The fluid exsolving from the latter melt had a bulk salinity of 10 ± 2 wt % NaCl equiv and contained of the order of 100 µg g –1 Mo. After emplacement of the Chalk Mountain Rhyolite magma at subvolcanic levels, extremely fractionated silicate melts coexisting with hypersaline brines (salt melts) and low-density vapor percolated at near-solidus conditions through the rock. These silicate melts contained 6·6 ± 0·4 wt % F, ≥ 7·5 ± 0·6 wt % H 2 O, 0·51 ± 0·05 wt % Cl, and up to 0·5 wt % Cs and 100 µg g –1 Mo, whereas the hypersaline brines contained 1–2 wt % Cs and 0·3–0·6 wt % Mo. However, owing to their negligible masses these liquids are unlikely to have played a major role in the mineralization process. The majority of Mo in the Climax deposit appears to have been derived from melts containing 5–7 µg g –1 Mo and bulk fluids containing ~100 µg g –1 Mo. These concentrations are similar to those found in similarly fractionated melts and fluids in barren and sub-economically mineralized intrusions. However, whereas in the latter intrusions fractionated melts occurred in a rather dispersed state, they seem to have been present as large, coherent masses in the apical parts of Climax-type porphyry Mo-forming magma systems. Efficient segregation of fractionated melts and fluids into the top of mineralizing magma chambers appears to have been promoted by high fluorine concentrations in the silicate melt, which was partly a primary feature, and partly an indirect consequence of other characteristics of within-plate magmatism.

Description: We identified molybdenite (MoS 2 ) as an accessory magmatic phase in 13 out of 27 felsic magma systems examined worldwide. The molybdenite occurs as small (〈 20 µm) triangular or hexagonal platelets included in quartz phenocrysts. Laser-ablation inductively coupled plasma mass spectrometry analyses of melt inclusions in molybdenite-saturated samples reveal 1–13 ppm Mo in the melt and geochemical signatures that imply a strong link to continental rift basalt–rhyolite associations. In contrast, arc-associated rhyolites are rarely molybdenite-saturated, despite similar Mo concentrations. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared with within-plate magmas. A thermodynamic model devised to investigate the effects of T , f O 2 and f S 2 on molybdenite solubility reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite. Whereas pyrrhotite microphenocrysts have been observed in some of these samples, they have not been observed from other molybdenite-bearing magmas. Based on the strong influence of f S 2 on molybdenite solubility we calculate that also these latter magmas must have been at (or very close to) pyrrhotite saturation. In this case the Mo concentration of molybdenite-saturated melts can be used to constrain both magmatic f O 2 and f S 2 if temperature is known independently (e.g. by zircon saturation thermometry). Our model thus permits evaluation of magmatic f S 2 , which is an important variable but is difficult to estimate otherwise, particularly in slowly cooled rocks.

Description: To better understand the factors leading to porphyry Mo mineralization, we studied melt and fluid inclusions in three subeconomically Mo mineralized granites in well-known Mo provinces: the Treasure Mountain dome in the Colorado mineral belt (USA), and the Drammen and Glitrevann granites in the Oslo rift (Norway). Melt and fluid inclusions were investigated in samples ranging from coarsely crystallized whole rocks to euhedral quartz crystals within miarolitic cavities. The major and trace element chemistry of individual inclusions was determined by laser ablation-inductively coupled plasma-mass spectrometry. Melt inclusions are rhyolitic in composition and record a clear trend of increasing Mo concentrations with increasing degree of melt differentiation as monitored by Cs, extending from ~5 to 10 ppm Mo at 5 ppm Cs to ~17 to 40 ppm Mo at 100 ppm Cs. Coexisting magmatic fluids were single phase, had a salinity of 4 to 6 wt % NaCl equiv and a density of 0.6 to 0.7 g/cm 3 , and contained ~0.5 wt % S and up to 6 mol % CO 2 . Molybdenum concentrations in these fluids ranged from ~20 to ~200 ppm Mo, except for some highly evolved fluids that had lower Mo contents. Comparison of our data with published fluid and melt inclusion data from porphyry Mo deposits, porphyry Cu (Mo, Au) deposits, and barren intrusions reveals that most subduction-related magmas have lower Mo/Cs ratios than within-plate magmas, but that within these two groups there are no systematic differences between barren and productive intrusions. This suggests that the mineralization potential was not primarily controlled by the metal content of the melts and fluids, but rather by other factors such as size of the magma chamber and the efficiency of residual melt and fluid extraction from the magma chamber and their focusing into a small apophysis at its top. Based on our data, it can be calculated that at least several tens of km 3 of magma were necessary to form intermediate-sized Mo deposits, and at least several hundred km 3 to form giant (≥1 Mt Mo) deposits. All three granites investigated in this study would have been large enough to produce at least an intermediate-sized Mo deposit, but they nevertheless are only subeconomically mineralized. Their low productivity thus appears to be the result of poor fluid focusing. Factors promoting a high degree of fluid focusing include (1) accumulation of major volumes of fractionated, crystal-poor melts at the top of the magma chamber, (2) formation of an apophysis, and (3) development of convection cells, leading to an efficient circulation of these fractionated melts through the apophysis.

Description: Porphyry Cu deposits are commonly thought to have formed by magmas that were unusually rich in metal and/or sulfur. In this study, we test this assumption by reconstructing the metal and sulfur content of an ore-related latite magma at Bingham Canyon and comparing it with that of intermediate magmas in several other arc magma systems. The ore-related latite magma at Bingham Canyon records strong evidence for magma mixing and has a major to trace element composition that can successfully be modeled by a mixture of ~40 wt % mafic magma, which was similar to the most mafic rock found at Bingham Canyon (a melanephelinite containing 45 wt % SiO 2 ), and ~60 wt % felsic magma of rhyolitic composition. Based on the modal abundance of 0.19 ± 0.01 vol % sulfides and laser ablation-inductively coupled plasma-mass spectrometry analyses of unaltered sulfide inclusions preserved within hornblende and plagioclase phenocrysts, the latite magma contained 50 to 90 ppm Cu, 0.8 to 2.0 ppb Au, 2 to 3 ppm Mo, and ≥0.12 to 0.14 wt % S. Whole-rock and melt and sulfide inclusion data suggest that the bulk of copper and Au in the latite magma was derived from the mafic end member, whereas significant amounts of sulfur were also provided by the felsic end member. A rough, independent estimate of the amount of Cu present in the mixed magma can be obtained by taking the Cu content of mafic, sulfide-undersaturated silicate melt inclusions and multiplying it with the mass fraction of mafic magma involved in the magma mixing. Applying this latter approach to two other porphyry Cu-mineralized magma systems (Santa Rita, USA; Bajo de la Alumbrera, Argentina) and several modern arc magma systems suggests that ore-forming intermediate magmas in mineralized systems were not unusually Cu rich. Whether or not they were unusually sulfur rich could not be answered with the available data. If the sulfur contents of mineralizing magmas prove to be normal, then the most distinctive feature of fertile magma systems may be the formation of large, long-lived magma chambers at 5- to 15-km depth and the development of vent structures that enable focused fluid flow.

Description: A specially designed quartz liner effectively prevents alloying problems with noble-metal capsules during hydrothermal experiments at elevated pressures and temperatures. The liner consists of three pieces: a cylindrical container made of a single quartz crystal, a SiO 2 glass plug, and a rounded closure lid made of a single quartz crystal. Soon after the final pressure and temperature have been reached, recrystallisation of the SiO 2 glass plug causes the quartz cap to be tightly sealed onto the quartz container, thus producing a quartz capsule that fully isolates the charge from the surrounding noble-metal capsule. The efficiency of the method is demonstrated on experiments with Cu and S (±Fe, Ag)-bearing, NaCl-H 2 O dominated fluids, which are complicated by the fact that all commonly used noble metals (and -alloys) react with either of these elements. Even small amounts of added Ag are still present after the run, showing that interaction with outer gold capsules was minimal. The new method makes it also possible to study chemical systems involving compounds with low melting points, such as Sn, Bi, Te, Tl, and Hg, and it allows the surrounding noble-metal capsules to be replaced by capsules made of non-noble materials if necessary. A disadvantage of the method is that it cannot be used in conjunction with external f O 2 buffers, as the fluid inclusions form before equilibrium with respect to an external f O 2 buffer is reached.

Description: 〈span〉〈div〉Abstract〈/div〉A fundamental question in the study of magmatic-hydrothermal ore deposits is whether the mineralization potential of intrusions was already predetermined by the metal content of the exsolving fluids. The present study aims at addressing this question by reviewing the large number of microanalytical data (mostly laser-ablation ICP-MS data) obtained on fluid inclusions from this type of ore deposits over the last 20 years. Published data sets were screened for analyses of high-temperature fluid inclusions that are representative of premineralization fluids. A set of criteria was developed to distinguish such fluids from later, lower temperature fluids. In order to compensate differences in absolute metal concentrations caused by fluid immiscibility, all element concentrations were normalized to Na. A numerical model was developed to explore at which stage different metals are most efficiently extracted from a cooling pluton. The results suggest that the timing of most efficient metal extraction varies from metal to metal and strongly depends on pressure, the fluid/melt partition coefficient and the bulk mineral-melt partition coefficient. As a consequence, fluid compositions were chosen over the entire range of Cs/Na ratios recorded from a given pluton, as this ratio gives an indication of the fractionation degree of the silicate melts from which a fluid exsolved. In order to avoid bias toward occurrences from which a large amount of data are available, maximum four intermediate-density (ID)-type fluid inclusion assemblages plus four brines assemblages were chosen from each occurrence.Using the above-mentioned criteria, 169 fluid compositions from 12 Cu (Mo, Au) mineralized intrusions, 10 Sn/W mineralized intrusions, two Mo mineralized intrusions, and one U-Th-REE mineralized intrusion were finally chosen and plotted in graphs of X/Na versus Cs/Na. The results reveal that Sn- and Cu-mineralizing fluids contained more Sn and Cu, respectively, than the fluids analyzed from barren and Mo or U-Th REE mineralized intrusions. Positive correlations between fluid metal content and mineralization potential may exist also for W and REEs, whereas for Mo no such trend is evident. Therefore, at least for certain metals, the metal content of high-temperature fluid inclusions can be used as an indicator of the type and extent of mineralization. However, elevated metal concentrations are present also in some fluids from barren intrusions, which implies that the mineralization potential additionally depends on other factors such as the size of the intrusion and the development of structures that promote focused fluid flow.〈/span〉