Hypozonal gold mineralisation in shear zone hosted deposits driven by fault valve action and fluid mixing: the Nalunaq deposit, Greenland

The Nalunaq deposit, Greenland, is a hypozonal, shear zone-hosted, Au deposit. The shear zone has previously been interpreted to have undergone 4 stages of deformation, accompanied by fluid flow,and vein formation. Coupled with previous trapping T estimates, fluid inclusion data are consistent with trapping of fluids with salinities between 28-45 wt. % NaCl eq., from 300-475°C during D2 and D3, with pressure varying between ∼800 and 100Mpa. The range reflects pressure cycling during seismic slip related depressurisation events. D4 fluids were lower salinity and trapped from 200-300°C, at ∼50-200Mpa during late stage normal faulting. The variation in major element chemistry is consistent with ingress of hypersaline, granitoid equilibrated fluids into the shear zone system and mixing with fluids that had reacted with the host metamorphic rocks. D4 stage fluids represent ingress of meteoric fluids into the system. Gold contents in inclusion fluids range from ∼300-10mg/kg. These data are consistent with the high P-T solubility of Au as AuHS(H2S)30 complexes, and Au deposition by decompression and cooling. The high salinities also suggest Au transport as chloride complexes may have been possible. Gold distribution was modified by the release of chemically bound or nanoscale Au during sulphide oxidation at the D4 stage.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5635812

Recurrence of Fault Valve Behavior in a Continental Collision Area: Evidence From Tilt/Strain Measurements in Northern Adria

We analyzed the data recorded by the NE-Italy subsurface tilt and strainmeter network evidencing a coherent transient signal in the recordings of four tiltmeter sites in the 1984–1990 period that produced a tilt along the main fractures. Borrowing from classical seismology techniques, we used the uprise times to locate the transient signal source. The propagation velocity is compatible with a fluid diffusion process that starts from a source located close to the hypocenter of the February 10, 1983 Uccea earthquake, MD = 4.2 at the Italy-Slovenia border, at an estimated depth of 10.8 km. Our results add to the previous interpretation of a transient signal recorded by several global navigation satellite system (GNSS) stations in the 2006–2009 period in terms of fluid diffusion below the Bovec basin (Slovenia). That source was located upon continuation to the northwest of the Ravne fault, few kilometers to the northeast from the present one, and about 6 km from the July 12, 2004 Bovec–Krn earthquake, Mw = 5.1, depth ~6.1 km. These observations suggest that the area is subject to fault valve behavior episodes that released fluids trapped at depth to the surrounding region as pore-pressure bulges. The convergence between Alpine and External Dinarides structures in this area puts highly permeable dolomitic limestones in contact with low-permeable fine-grained limestones and flysch formations. Therefore, the conditions for overpressure generation can be created, whereas fault movements, from time to time, in close relation with seismic events, can enable fluid diffusion in the surroundings. We also estimated the possible fluid influx needed to maintain overpressure and possible discharge across both the faults. The study provides insights on pore–fluid pressure variations related to slow slip events from a context different from subduction or transform margins, i.e., in a continental collision area.

Fluid flow and syntectonic veining in an Ediacaran-Cambrian foreland fold–thrust zone, western margin of the São Francisco Craton, Brazil

The western margin of the São Francisco Craton, central Brazil presents a 1300 km long foreland fold–thrust belt where Ediacaran-Cambrian (560–520 Ma) metasedimentary rocks from the Bambuí Group were subsequently deformed during post-collisional stages (520–495 Ma) related to Gondwana assembly. This scenario provides an opportunity to quantify fluid flow regimes and fault-related processes that were active in exhumed foreland fold–thrust zones, which were estimated based on structural, microstructural and fluid inclusion studies of syntectonic veins and host rocks. Kaolinite-bearing synkinematic mineral assemblages from metasedimentary rocks, thermodynamic models and grain-scale deformation accommodated by dissolution–precipitation creep and intracrystalline deformation indicate metamorphic and deformational conditions of 250–270°C. Subhorizontal extensional veins formed under subhorizontal shortening and subvertical extension, supporting vein development under a fold–thrust regime that formed regional NW–SE-trending thrust fault zones and megafolds with NW–SE-trending axes. Orientation and growth microstructures indicate that NW–SE-trending subvertical cleavage-parallel veins formed under subhorizontal NE–SW extension, compatible with those inferred to produce mapped kilometre-scale gentle folds with NE–SW-trending traces. Two primary aqueous fluid inclusion assemblages (FIA) are distinguished by salinity variation: 2–21 wt% NaCleq. in subhorizontal veins and 6–0 wt% NaCleq. in cleavage-parallel subvertical veins. Fluid inclusion thermometry and microstructural analysis suggest that veins crystallized between 250 and 270°C under fluid pressure fluctuating within a range of 50–500 MPa (subhorizontal veins) and 80–320 MPa (cleavage-parallel subvertical veins), evidencing fault-valve behaviour. Trends of coupled decreases in salinity and homogenization temperatures in both FIA indicate downward mixing of meteoric fluids, which was more effective in subvertical veins and was in both cases enhanced by fault-valve behaviour. Dominance of moderate salinity and absence of CO2 and CH4 indicate that the fluids are dominated by formation waters. The salinity signature is similar to those of formation waters and metamorphic fluids derived from rocks of shallow marine environments worldwide.Supplementary material: Details of samples and analytical data are available at https://doi.org/10.6084/m9.figshare.c.5275031

Mineralization constraints on the origin of polymetallic (Pb, Ag, Zn, Cu, Au) deposits hosted in the metasedimentary Lajeado Group, Southern Ribeira Belt, Brazil

The sediment-hosted polymetallic (Pb, Zn, Ag, Au-Cu) mineralization of the Vale do Ribeira Mineral District has been known since the beginning of the 20th century, but exploration was interrupted just before the turn of the century. The Vale do Ribeira Mineral District is part of the Southern Ribeira Belt, developed during the Brasiliano-Pan African orogeny. Polymetallic mineralization is mainly hosted in metalimestones of the Lajeado Group, a typical platform carbonate sequence of a passive margin, which has been deformed during the Gondwana assembly. The region has a gap of research since the mines were closed, which justifies new projects on their mineral economic potential. Fieldwork, petrographic and geochemical analyses were developed in five currently inactive mines (Panelas, Barrinha, Rocha, Lajeado, and Furnas) and their surroundings, along with the description of a drill core executed in the 1980s. The main type of ore consists of polymetallic fault-fill veins of massive sulfide, which are composed essentially by argentiferous galena, sphalerite, pyrrhotite, arsenopyrite, and chalcopyrite. The highest ore grades were obtained from samples in the Panelas Mine, with contents of up to 35% lead, 5% zinc, > 1% copper and 564 ppm silver, as well as 23% iron. New ore occurrences were described in a secondary gallery of the Barrinha Mine, whose gold grades reached up to 5,630 ppb. The main controls of the mineralization are lithological - since the ore occurs exclusively in carbonate rocks, irrespective of the geological unit - and structural, related to NE high-angle strike-slip fault zones, including evidence of fault-valve behavior. Fault zones as the main control of the polymetallic veins is an innovative interpretation, increasing the perspectives for mineral exploration in the area. Although they are small deposits, the presented data indicate that the region has potential for new discoveries and that the mined deposits are probably not exhausted.

Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods

The timing of slip on brittle faults in Earth’s upper crust is difficult to constrain, and direct radiometric dating of fault-generated materials is the most explicit approach. Here we make a direct comparison between K-Ar dating of fault gouge clay (authigenic illite) and U-Pb dating of carbonate slickenfibers and veins from the same fault. We have dated fault generated materials from the Big Creek fault, a northwest-striking, dextral strike-slip fault system in Yukon Territory, Canadian Cordillera. Both methods yielded dates at ca. 73 Ma and ca. 60–57 Ma, representing at least two periods of fault slip that form part of a complex fault and fluid-flow history. The Cretaceous result lies within previous indirect estimates for major slip on the fault. The Paleocene–Eocene result coincides with the estimated timing of slip of the nearby Tintina and Denali faults, which are crustal-scale, northwest-striking dextral faults, indicating Big Creek fault reactivation during regional faulting. The coincidence of periods of carbonate-crystallizing fracturing and fluid flow with intervals of seismic, gouge-generating slip supports the fault valve model, where fault strength is mediated by fluid pressures, and fluid emplacement requires seismic pumping in otherwise impermeable aseismic fault zones. The reproducibility of slip periods for distinct fault-generated materials using different decay systems indicates that these methods provide complimentary results and can be reliably applied to date brittle fault slip, opening new opportunities for investigating fault conditions with associated mineralizing fluid events.

Building a giant quartz reef at the Heyuan fault, South China: observations and multiphysics model

<p>A paleohydrothermal giant quartz reef (at least 75 m wide, 40 km long) and abundant hot springs at the Heyuan fault, South China, provide an excellent opportunity to investigate hydrothermal flows from the Mesozoic through to present-day.</p><p>The giant quartz reef has formed in the extensional regime initiated in the Mesozoic, while a change to  compressional stress on the Heyuan in the Cenozoic led to the development of cross-cutting strike-slip faults and associated vertical fracture network. Here, we present multiscale observations and analyses from the earlier long-term extensional phase.</p><p>Detailed microstructural analyses identified a 'quartz-reef window' of formation occurring between ~200-350˚C, linking in both quasi-static criteria (accommodation space; massive fluid sources; and a cap rock/seal)  and dynamic mechanisms (episodic-dynamic permeability; brittle-ductile cycles; and fluid injection though brittle-ductile equivalent of Sibson's 'fault-valve' behaviour.</p><p>This oscillatory brittle-ductile fault-valve is recorded in the field through its apparent contradiction between idiomorphic 5 cm long quartz crystal growth in mode-I fractures, embedded at large-scale inside far from equilibrium fault zones with mylonitic and cataclastic microstructures. Another characteristic feature is the increasing quartz vein frequency towards the core shown by enrichment of SiO<sub>2</sub>, with depletion of K<sub>2</sub>O and  Na<sub>2</sub>O in tectonites during alteration from the host granite; a reaction partly sourcing the SiO<sub>2</sub> for the quartz reef.<br><br>We present a first theoretical model compatible with the observation of oscillatory macroscale far from equilibrium conditions, followed by long periods of micro-scale local equilibrium. The model can in particular describe mechanisms of abundant SiO<sub>2</sub> dominated fluid release reaching episodically above hydrostatic pressures followed by long periods of SiO<sub>2</sub> precipitation, allowing growth of up to 5 cm long idiomorphic quartz  crystals in subparallel open channels, which presumably were held open by high fluid pressures. In this interpretation, the observations instabilities are seen to stem from the multiscale and multiphysics of the mineral reactions at the brittle-ductile transition, promoted by a slow extensional geodynamic driver at the Heyuan fault.<br><br>The new approach allows interpretation of rock physics properties in terms of recently discovered Thermo-Hydro-Mechanical-Chemical (THMC) multiscale wave-like instabilities. In the model short wavelength chemical dissolution-precipitation reaction waves are bouncing between the phyllonitic cap rock and the mylonitic shear zone below. A resonance phenomenon of constructive interference in a finite width around the future quartz-reef triggers the long-time scale steady-state attractor allowing quartz reef growth over geodynamic time scales. We show that this solitary wave limit forms a standing wave matching the characteristic periodic pattern of mode-I quartz veining around the reef and also explaining the fluid overpressures leading to local hydro-fracturing.</p>