U-Pb and Sm-Nd Evidence for Episodic Orogenic Gold Mineralization in the Kalgoorlie Gold Camp, Yilgarn Craton, Western Australia

Different genetic and timing models for gold mineralization in the Kalgoorlie gold camp (Yilgarn craton, Western Australia) suggest either broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma or a punctuated mineralization history from ca. 2675 to 2640 Ma with the involvement of early magmatic-hydrothermal systems (represented by the Fimiston, Hidden Secret, and Oroya gold-telluride lodes) and late metamorphic fluids (represented by the Mt. Charlotte gold stockwork veins). The results of U-Pb and Sm-Nd geochronological studies of zircon, apatite, and titanite from pre-ore dikes and syn-ore dikes constrain the absolute timing of mineralization and provide new evidence to this timing controversy. Emplacement ages constrained by U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon data are interpreted to be similar for both the pre-ore dikes (n = 10) and syn-ore dikes (n = 7) at ca. 2675 Ma. An inferred emplacement age of ca. 2675 Ma for the syn-ore dikes is supported by a Sm-Nd isochron age from apatite (laser ablation-inductively coupled plasma-mass spectrometry; LA-ICP-MS) of 2678 ± 15 Ma and by a U-Pb titanite age (LA-ICP-MS) of 2679 ± 6 Ma. The results of chemical abrasion-isotope dilution-thermal ionization mass spectrometry U-Pb zircon analysis from the pre- and syn-ore dikes are complicated by multistage Pb loss, reverse discordance, and potential inheritance. However, the data are compatible with the emplacement of Fimiston/Hidden Secret gold mineralization at ca. 2675 Ma and suggest a younger age for Oroya mineralization at ca. 2665 Ma. These results contrast with models for orogenic gold deposits that invoke broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma. The bulk of the Kalgoorlie gold camp’s estimated 2,300 t Au endowment was emplaced at ca. 2675 Ma as Fimiston/Hidden Secret Au mineralization. This early Au mineralization was deformed and overprinted twice by subordinate Au mineralization at ca. 2665 (Oroya mineralization) and ca. 2640 Ma (Mt. Charlotte mineralization). Gold mineralization in the Kalgoorlie gold camp was protracted in nature from ca. 2675 to 2640 Ma and reflects the interplay of early magmatic (Fimiston, Hidden Secret, Oroya) and late metamorphic (Mt. Charlotte) hydrothermal fluid systems in the formation of hybrid intrusion-related and metamorphic orebodies.

Identifying pathfinder elements for gold in bulk-rock geochemical data from the Cripple Creek Au–Te deposit: a statistical approach

The Cripple Creek alkaline igneous rock-related, low-sulfidation epithermal gold telluride deposit, Colorado, is hosted in the 10 km wide Oligocene alkaline volcanic Cripple Creek diatreme in Proterozoic rocks. Gold occurs as native gold, Au-tellurides, and in the structure of arsenian pyrite, in potassically altered high-grade veins, and as disseminations in the host rocks.Correlation coefficients, principal component analysis, hierarchical cluster analysis and random forests were used to analyse major and trace element compositions of 995 rock samples primarily from low-grade gold mineralization in drill core from three currently operating pits (Wild Horse Extension, Globe Hill and Schist Island) in the northwestern part of the Cripple Creek diatreme. These methods suggest that Ag, As, Bi, Te and W are the best pathfinders to gold mineralization in low-grade disseminated ore. Although Mo correlates with gold in other studies and is spatially related to gold veins, molybdenite post-dated the formation of gold and is likely related to a late-stage porphyry overprint. These elements, in conjunction with mineralogical studies, indicate that tellurides, fluorite, quartz, carbonates, roscoelite, tennantite-tetrahedrite, pyrite, sphalerite, muscovite, monazite, bastnäsite and hübnerite serve as exploration guides to ore.

STAGES OF MINERALIZATION AND LOCALIZATION FACTORS OF THE AGYURT GOLD-COPPER-MOLYBDENUM DEPOSIT (LESSER CAUCASUS, AZERBAIJAN)

The article considers the stages of mineralization of the Agyurt gold-copper-molybdenum deposit of the Lesser Caucasus. The following mineralization stages were established at the field: 1) quartz-molybdenum; 2) quartz-pyrite-chalcopyrite with gold; 3) quartz-carbonate-sphalerite; 4) quartz-carbonate. Gold ore bodies are mainly composed of aggregates of the second stage of mineralization, which is productive. Its mineral substance is represented by three paragenetic associations: 1) quartz-pyrite; 2) calcite-chalcopyrite-marcasite; 3) gold-telluride-bismuth. Chemical analyzes of pyrites, bismuthin, tellurium bismuthite are given. It has been found that native gold is found in the form of small, simple forms of gold in grains of early pyrite. In veins of chalcopyrite and grains of pyrite, it is usually confined to the marginal parts. The largest amount of gold is in close intergrowth with tellurium-bismuth minerals. It was found that the ore deposition environment (mineral composition, chemistry and structural and texture features of the host rocks) played a decisive role for various types of mineralization. It is established that, in the plan, the Agyurt deposit is localized in the contour of a rock block elongated in the northwest (submeridional) direction, bounded by tectonic zones from the north-north-west and north-east, which also bear a certain imprint of the formation of the structural plan of the ore field with near latitudinal strike of tectonic elements. These structures are most tectonically prepared for the localization of gold-copper-molybdenum mineralization (updated in the pre-ore stages and most permeable for hydrothermal structures), and were the main ore-supplying and ore-locating structural elements. The ore zones represented by hydrothermal-metasomatic formations, as well as quartz veins piercing them and numerous veinlets and sometimes mineralized dykes, are controlled by the Main Ordubad longitudinal (280°∠70–80°NE) and Agyurt-Misdag transverse (40–50°∠70° NE) with discontinuous violations and adjoin the hanging side (northeast flank) of the first. The combination of structural and petrogenetic factors not only predetermined the formation of deposits of the Agyurt type, but also determined the horizontal and vertical zonation of mineralization: an increase in the Mo content and a decrease in Cu with depth are established. The same pattern is observed in the horizontal direction: as you move away from the intrusive massif and the ore-removing channel, there is a transition from Cu-Mo-mineralization to copper and then polymetallic, i.e. the role of Cu increases, then Pb and Zn. The horizontal zoning in Agyurt is expressed in an increase in Au content and the total amount of sulfides with distance from the Main Ordubad Fault, and vertical shows an increase in Au content and decreases in Ag with depth.

Contrasting Fluids in the Svetlinsk Gold-Telluride Hydrothermal System, South Urals

The large gold-telluride Svetlinsk deposit (~135 t Au) is considered to be a nontraditional one in the Urals and its origin is debated. A specific feature of the deposit is the abundance of various tellurides, such as tellurides of Fe, Ni, Pb, Sb, Bi, Ag, and Au. The new data of microthermometry, Raman spectroscopy, LA-ICP-MS, and crush-leach analysis (gas and ion chromatography, ICP-MS) for fluid inclusions as well as O-isotope data for quartz were obtained for the construction of PTX parameters of ore-formation and fluid sources in the deposit. Mineralisation was formed at a wide range of temperature and pressure (200–400 °C, 1–4 kbar) and from contrasting fluids with multiple sources. At the early stages, the magmatic fluid evolved during its ascent and phase separation and the fluid derived from the host rock decarbonation and dehydration were involved in the hydrothermal system. In addition, mantle-derived fluid might be involved in the ore-forming process during gold-telluride precipitation as well as heated meteoric waters during the late stages. Early fluids were rich in H2S, S0, and CH4, while the Au-Te mineralisation was formed from N2-rich fluid.