Arsenian pyrite is the major host mineral of gold in Carlin type gold deposit (CTGD) and forms during the interaction of H2S bearing hydrothermal fluids with Fe-bearing carbonates (e.g., siderite, ...ankerite, Fe-rich dolomite) in the wall rock. Experimental replacement of siderite at hydrothermal conditions (i.e., 200 °C, Psat) was used to study the parameters that control the formation of CTGD pyrite (i.e., pH, time, aH2S) and the partitioning of As between the hydrothermal fluid and newly formed pyrite.
Hydrothermal pyrite formed via replacement of siderite in experiments mainly occurs as two types: 1) porous, massive pyrite at alkaline pH conditions or 2) clusters of dense euhedral to anhedral pyrite grains at slightly acidic conditions (pH <6). Longer experimental run times (2 weeks) lead to formation of type 2 pyrite at alkaline conditions and an increase in euhedral grain sizes in all conditions.
Concentration of As in newly formed pyrite depends strongly on the As concentration of the hydrothermal fluid used in the experiment. Calculated partition coefficients vary between 300 and 1700 with lowest D values at high As concentrations of the fluid. In the studied system As−1 substitutes for S−2 as indicated by the correlation of high As with low S concentrations. Experimental findings agree well with previous studies in natural CTGD on As substitution and the partitioning of As between ore stage pyrite and hydrothermal fluids trapped as fluid inclusions.
The newly constrained partition coefficients for As help to explain the large heterogeneity in natural CTGD pyrite compositions which is caused by changes in the fluid composition due to the strong partitioning of As into the forming pyrite.
•Siderite interacting with H2S bearing fluids is effectively replaced by pyrite under hydrothermal experimental conditions•Highly porous and/or euhedral pyrite grains form depending on time and pH•Replacement pyrite shows textural features similar to pyrite from Carlin-type gold deposits•Partition coefficients for As between pyrite and fluid range from 300–1700•High D-values quantified in our experiments can explain compositional evolution of natural fluids
As society transitions to low-carbon, renewable energy resources, the demand for metals and minerals is set to increase. Massive quantities of base metals and mineral materials (for example, silica ...and concrete) along with smaller quantities of precious metals will be required for the construction of wind turbines, solar panels and battery storage facilities to meet the demands of the ‘Electric Planet’ of the future. Harnessing microbe-mineral-metal interactions may offer many opportunities to improve some mining practises and support the long-term sustainability of mining. As easily exploitable, high-grade deposits are becoming increasingly depleted there is a need for new technologies to improve exploration and mining strategies. Microorganisms are ubiquitous and diverse, surviving in almost all environments in the Earth's crust and recent advances in molecular techniques have enabled scientists to study these communities is extraordinary detail. Microorganisms also interact directly with their environment; both responding to and changing the environment around them. These responses and their influences on the surrounding environment are preserved within their genome (a complete set of the DNA of the microorganism). Here, we discuss using state-of-the-art sequencing techniques to identify key microbial genes that have been demonstrated to correlate with metal concentrations. These genetic-based bioindicators may provide additional tools to guide and improve the success rate of mineral exploration programmes. Advances in molecular techniques will also improve existing biohydrometallurgical techniques and expand the commodity range for which biohydrometallurgy are currently economically viable. Finally, microorganisms may be used in a number of strategies for mine remediation; specifically, we review in detail microbially accelerated carbon capture and storage strategies and mine waste stabilisation.
The clastic‐dominant (CD‐type) deposits that are contained within sedimentary basins are major resources of Zn, Pb and Ag, but their formation by basin‐scale hydrothermal mass and energy transport ...processes is still poorly understood. Using geological constraints from the Late Devonian Selwyn Basin (Canada), we apply quantitative numerical fluid flow modeling to explore the effect of strata permeability, timing of fault opening and increased heat flow in controlling fluid migration, metal leaching and ore formation during an extensional tectonic event. The results indicate that tapping hot fluids from a confined and permeable aquifer at several km depths by means of permeable normal faults is a key factor for the formation of large Zn‐Pb deposits. The hot (282°C) ore‐forming fluids are transported to the shallow subsurface shortly after the initiation of a rifting event (within 100 kyr), before the development of extensive basin‐scale convection patterns that lead to stronger cooling and a reduction in the capacity of the hydrothermal system to make an economic deposit. Such a hydrothermal event can result in metal endowments comparable to the deposits of the Selwyn Basin.
Key Points
Modeling of basin‐scale fluid flow shows that tapping hot fluids from deep confined aquifers by faults can produce large Zn deposits
Heat anomaly that drives the hydrothermal system can form at magma‐poor hyperextended continental margins as inferred for the Selwyn Basin
Modeled metal enrichment agrees with observations from the deposits, but the timescale is much shorter than assumed in classical models
Recent studies have identified gold nanoparticles in ores in a range of deposit types, but little is known about their formation processes. In this contribution, gold-bearing magnetite from the ...well-documented, world-class Beiya Au deposit, China, was investigated in terms of microstructure and crystallography at the nanoscale. We present the first three-dimensional (3D) focused ion beam/scanning electron microscopy (FIB/SEM) tomography of the distribution of gold nanoparticles in nanopores in the low-Si magnetite. The porous low-Si magnetite, which overprints an earlier generation of silician magnetite, was formed by a coupled dissolution-reprecipitation reaction (CDRR). The extrinsic changes in thermodynamic conditions (e.g., S content and temperature) of the hydrothermal fluids resulted in the CDRR in magnetite and the disequilibrium of Au-Bi melts. The gold nanoparticles crystallized from Au-supersaturated fluids originating from the disequilibrium of Au-Bi melts and grew in two ways depending on the intrinsic crystal structure and pore textures: (1) heteroepitaxial growth utilizing the (111) lattice planes of magnetite, and (2) randomly oriented nucleation and growth. Therefore, this study unravels how intrinsic and extrinsic factors drove the formation of gold nanoparticles at fluid-mineral interfaces.
In situ sulfur (S) isotope ratios and trace element chemistry were simultaneously determined in a wide selection of different (natural) sulfides and sulfates using femtosecond laser ablation split ...stream (fsLASS) inductively coupled plasma mass spectrometry (ICP-MS). The laser aerosol is split between a sequential quadrupole ICP-MS for trace element quantification and a multi-collector ICP-MS (MC-ICP-MS) for stable sulfur isotope ratio measurements. This LASS method is able to simultaneously determine S isotope ratios and element chemistry without a compromise in the measurement precision and measurement accuracy of the S isotope ratios. The quantification of major and trace elements in sulfide minerals down to the μg g −1 level was achieved. LASS shows overall lower sensitivity and higher limits of detection in comparison to direct trace element determination using LA-ICP-MS only, due to the lower amounts of sample introduced into the ICP-qMS. This compromise is acceptable given the additional information gained from obtaining simultaneously both the isotopic and elemental compositions.
The Late Triassic Guichon Creek batholith is a large (~ 1800 km
2
), composite, zoned batholith that hosts several large porphyry Cu-Mo deposits of the Highland Valley Copper district. The batholith ...consists of intrusive rocks that range in composition from gabbro to quartz monzonite. Adjacent to the mafic margin of the batholith is the Gump Lake granodiorite to quartz monzonite stock. A new U-Pb zircon age of 218 ± 0.18 for the Gump Lake stock indicates that magmatism in the region began at least seven million years prior to the emplacement of the main Guichon Creek batholith rocks at 211 Ma. Zircons from fifteen samples from the Guichon Creek batholith were analyzed by laser ablation ICP-MS to characterize the magmatic evolution and ore fertility of the batholith. The trace element composition of zircon record early, lower crustal, fractional crystallization followed by five pulses of magma recharge and mixing in an upper-crustal, oxidized, magma chamber as well as degassing of the magmatic-hydrothermal fluids that formed the porphyry copper deposits. Zircons from the early barren rocks have chondrite-normalized Eu/Eu
N
* values of 0.19 to 0.56 and estimated temperatures of 850 to 750 °C. The middle to late intrusions that host porphyry copper mineralization have zircon Eu/Eu
N
* values of 0.30 to 0.74 and slightly lower estimated temperatures of 800 to 600 °C. Late porphyritic stocks and dikes from the mineralized centers contain zircon crystals elevated in Y, Nb, Ta, and REE concentration relative to zircon from the earlier intrusions. This distinct change in zircon composition coincides with the copper mineralization, suggesting that zircon chemistry can be used as a tool to identify the genetic evolution of a crystallizing magma chamber and potential for mineralization.
Forty-seven samples of deep geothermal well fluids and hot springs from 14 geothermal sites in the Taupo Volcanic Zone (TVZ), North Island, New Zealand, were analyzed for Cl, Br and Li concentrations ...and δ37Cl, δ7Li, δ18O and δD values. The main purpose of this study was to identify the sources of salinity in the TVZ fluids and to assess the processes that control stable chlorine isotopic fractionation in the geothermal waters. The samples were obtained from deep wells in the Mokai, Ohaaki, Wairakei, Rotokawa, Kawerau, Ngatamariki and Tauhara geothermal fields and from the hot springs at Rotorua, Waiotapu, Lake Taupo, Waimangu, Waikite, Orakei Korako and Tokaanu.
Two main meteoric recharge areas were identified in the central TVZ, one located to the NE with δ18O and δD values of −5.5‰ and −35‰, respectively and the other to the SW with a δ18O value of −6.7‰ and δD value of −43‰. Water rock interactions in most of the geothermal fluids resulted in a δ18O shift to more positive values; up to 1‰ for Mokai and over 3‰ for Kawerau. The δ7Li values range from −2.9‰ to 2.0‰ and suggest that the deep geothermal fluids have interacted with rocks with low δ7Li values. The highest δ7Li values (1.4‰) in the reservoir fluids were found at Kawerau where the low quartz content of the greywacke may result in the depletion of 6Li.
The δ37Cl values of the samples ranged from −1.0‰ to 0.8‰. The δ37Cl values of 0.2‰, 0.1‰ and 0.0‰ were measured in samples from Orakei Korako, Waiotapu and Waimangu respectively, and these are interpreted to have been affected by surface evaporation and distillation processes. Excluding these data, most waters with high Cl/Br ratios had positive δ37Cl values. The Cl/Br molar ratios in well samples ranged from 659 to 1664 and the δ37Cl values from −1.0‰ to 0.8‰. The hot springs had Cl/Br molar ratios ranging from 858 to 1611and δ37Cl values from −0.8‰ to 0.7‰.
Overall, the Cl/Br molar ratios of the fluids are comparable with published data on basaltic magmas, and these magmas are inferred to be the main source of halogens in the geothermal fluids. However, the geothermal fluids can be separated into two main groups: one associated with rhyolitic magmas characterized by positive δ37Cl values and high Cl/Br molar ratios, and other related to andesitic magmas with negative δ37Cl values and lower Cl/Br molar ratios. We suggest therefore, that the isotopic fractionation of the stable Cl isotopes is related to the difference in water contents of andesitic and rhyolitic magmas and that the fluid phase is enriched in 37Cl during magmatic evolution. There is no evidence of isotopic fractionation during the transit of the geothermal fluids from the reservoir to the surface, but evaporation and distillation processes at the surface can affect the δ37Cl values.
PDF
