Mineralogy, Fluid Inclusions, and Oxygen Isotope Geochemistry Signature of Wolframite to Scheelite and Fe,Mn Chlorite Veins from the W, (Cu,Mo) Ore Deposit of Borralha, Portugal

Scheelitization of Mn-bearing wolframite, scheelite, quartz, and Fe,Mn-chlorite veins was identified in the W, (Cu,Mo) ore deposits of Borralha, by optical microscopy, electron-microprobe analysis, and stable isotope geochemistry. Fluid inclusions derived scheelite crystallization temperature was compared with the oxygen isotope temperature estimated. Scheelite was formed mainly during stage I from a low salinity aqueous-carbonic fluid dominated by CO2, where the homogenization temperature (Th) decreased from 380 °C to 200 °C (average of 284 °C). As temperature decreased further, the aqueous-carbonic fluid became dominated by CH4 (Stage II; (average Th = 262 °C)). The final stage III corresponds to lower temperature mineralizing aqueous fluid (average Th = 218 °C). In addition, salinity gradually decreased from 4.8 wt.% to 1.12 wt.%. The δ18OFluid values calculated for quartz-water and wolframite-water fractionation fall within the calculated magmatic water range. The ∆quartz-scheelite fractionation occurred at about 350–400 °C. The ∆chlorite-water fractionation factor calculated is about +0.05‰ for 330 °C, dropping to −0.68‰ and −1.26‰ at 380 °C and 450 °C, respectively. Estimated crystallizing temperatures based on semi-empirical chlorite geothermometers range from 373 °C to 458 °C and 435 °C to 519 °C. A narrower temperature range of 375 °C to 410 °C was estimated for Fe,Mn-chlorite crystallization.

Stable C, O, and S Isotope Record of Magmatic-Hydrothermal Interactions Between the Falémé Fe Skarn and the Loulo Au Systems in Western Mali

The Gara, Yalea, and Gounkoto Au deposits of the >17 Moz Loulo mining district, largely hosted by the Kofi series metasediments, are located several kilometers to the east of the 650-Mt Fe skarn deposits in the adjacent Falémé batholith. The Au deposits are interpreted to have formed through phase separation of an aqueous-carbonic fluid, which locally mixed with a hypersaline brine of metaevaporite origin. Recognition of an intrusive relationship between the Falémé batholith and Kofi series opens the possibility that the Fe skarns and Au deposits are part of the same mineral system. In this paper, we combine new δ13C, δ18O, and δ34S data from the Karakaene Ndi skarn, Au occurrences along the western margin of the Kofi series, and zircons within plutonic rocks of the Falémé batholith. We combine these with existing data from the Loulo Au deposits to model the contribution of magmatic volatiles to Au mineralization. C and O isotope compositions of auriferous carbonate-quartz-sulfide veins from the Loulo Au deposits have wide ranges (δ13C: –21.7 to –4.5‰ and δ18O: 11.8 to 23.2‰), whereas values from carbonate veins in Kofi series Au prospects close to the Falémé batholith and the Karakaene Ndi Fe skarn deposit have more restricted ranges (δ13C: –16.8 to –3.7‰, δ18O: 11.4 to 17.2‰, and δ13C: –3.0 ± 1‰, δ18O: 12.6 ± 1‰, respectively). Kofi series dolostones have generally higher isotopic values (δ13C: –3.1 to 1.3‰ and δ18O: 19.1 to 23.3‰). Pyrite from Kofi series Au prospects adjacent to the Falémé batholith have a wide range of δ34S values (–4.6 to 14.2‰), similar to pyrite from the Karakaene Ndi skarn (2.8 to 11.9‰), whereas δ34S values of pyrite and arsenopyrite from the Loulo deposits are consistently >6‰. Comparison of the C and O isotope data with water-rock reaction models indicates the Loulo Au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilization of sedimentary rocks with an organic carbon component. Isotopic data are permissive of the hypersaline brine that enhanced this phase separation including components derived from both Kofi series evaporite horizons interlayered with the dolostones and a magmatic-hydrothermal brine. This magmatic-hydrothermal component is particularly apparent in O, C, and S isotope data from the Gara deposit and Au prospects immediately adjacent to the Falémé batholith.

Microthermometric behavior of crystal-rich inclusions in spodumene under confining pressure

A hydrothermal diamond anvil cell (HDAC) was used to observe the microthermometric behavior of solid + liquid + vapor inclusions in spodumene from the Tanco pegmatite, Manitoba, under confining pressure. At 25 °C, these inclusions commonly contain a carbonate mineral (zabuyelite, rarely calcite or nahcolite), quartz, a phyllosilicate (cookeite), and an aqueous carbonic fluid phase. Heating spodumene-hosted inclusions to temperatures between 600 and 680 °C in a HDAC resulted in total or partial dissolution of the contained solid phases, followed by homoepitaxial growth of new spodumene on the inclusion walls, which reduced the inclusion volume by up to 31%. At room temperature, the homogenized inclusions contain only an aqueous fluid phase, CO2 liquid, and CO2 vapor. Inclusions that failed to homogenize at 680 °C, or leaked during heating, contain partially dissolved minerals with or without an aqueous carbonic fluid. The volume of spodumene formed within an inclusion during experimental re-heating, as determined by the difference in inclusion size before and after total dissolution of the contained solid phases, was used to estimate the volume of zabuyelite, quartz/cristobalite, and cookeite produced by the reaction The relative volumes of the calculated reaction products approximate the proportions of zabuyelite, quartz/cristobalite, and cookeite in inclusions prior to heating. The absence of silicate glass in the quenched homogenized inclusions indicates that they do not represent the crystallized products of an entrapped hydrous silicate melt that wetted the surface of spodumene during its growth. Large changes in inclusion volume and composition during experimental re-heating shows that the inclusions are neither isochoric nor isoplethic systems and as such are unsuitable for estimating the P-T conditions of trapping. Readers should therefore exercise caution when using thermobaric estimates of pegmatite crystallization inferred from microthermometric measurements of presumably primary melt inclusions in spodumene.

Solubility of gold in the reduced carbonic fluid

The first experimental data on the gold solubility in the fluid with composition CO-CO2 and C-O-S with small water content at the pressure 200 MPa and temperature 950°С are reported. Solubility in the fluid C-O-S is about 27 ppm. Estimation of the solubility of gold in fluid CO-CO2 with 10–15 mol.% CO is less accurate: its value is at least 2–3 ppm, but it probably can reach 200–300 ppm. The high solubility of gold found in this work, and previously Pt (Simakin et al., 2016), in the reduced carbon dioxide can explain the formation of these noble metal ore occurrence in the Gluli intrusion (Polar Siberia) by their fluid extraction and re-deposition at temperatures below the solidus. The reducing of a substantially oxidized carbon dioxide fluid, established by mineralogical sensors, was probably caused by subsolidus oxidation of olivine.

On the possible mechanism of the diamonds formation in Kamchatka mantle wedge.

Various geodynamic mechanisms can lead to the penetration of siliceous carbonates into the mantle wedge. Their thermal decomposition in the “mantle olivine autoclave” can be a mechanism for the formation of diamond erupted in subduction zone of Kamchatka. Using the theory of poro-elasticity, we showed that rapid heating of a mixture of sideritic dolomite and silica at 150-200oC can lead to an increase in pressure by 2-3 GPa. With the initial parameters P = 2 GPa and T = 830oC, the carbonic fluid produced during the reaction would get into the PT stability field of the diamond. The growth of diamond in the PT field of metastable graphite can be enhanced by microparticles of native Ni and Mn formed by the thermal decomposition of gaseous carbonyls. The corresponding abundant micro-inclusions of Ni and Mn are found in Kamchatka diamonds.