Steep Veins Research Articles

Bladed Texture and Exploration Implications. A Case Study from the Kuklitsa Deposit, Krumovgrad Goldfield, SE Bulgaria

This study demonstrates that the bladed texture, which is common in epithermal, low-sulfidation (adularia-sericite) precious metal deposits, can serves as exploration vector towards precious metal mineralization. The paper presents two styles of bladed texture in the Kuklitsa gold deposit (Krumovgrad goldfield, SE Bulgaria) observed at both different altitude and lateral position in respect to regional low-angle detachment fault. The first style has formed as a crackle breccia just above the detachment fault where bladed texture consists of 10–20 vol % pseudorhombic adularia, 90-80 vol % quartz, scarce pyrite, and electrum, which is often observed under optical microscope. The second style is present in steep veins which fill listric faults of sharp tectonic contacts. It is developed at a higher level relative to the detachment fault. Bladed texture there consists of 1–2 vol % pseudorhombic adularia, 99-98 vol % quartz, and scarce both electrum and pyrite. Electrum of the two styles of bladed texture comprises only gold and silver but in different proportions with a higher gold content for the first style: fineness of 765, on average, for the first style vs. fineness of 692 for the second one. In this way, it is found that the adularia abundance correlates positively with the electrum one and negatively with the quartz abundance. The author uses the proportions of adularia, quartz and electrum, the fineness of electrum, and the relative distance to the detachment fault to conclude that the first style of bladed texture has been formed at higher temperature relative to the second style. The author infers that the first style is promising for mineralization of higher grade. Methods used comprise field observations and sampling, optical and electron microscopy, powder X-ray diffraction and electron microprobe analysis.

Chemistry of quartz related to the Zinnwald/Cínovec Sn-W-Li greisen-type deposit, Eastern Erzgebirge, Germany

Magmatic and hydrothermal quartz populations related to the Late-Variscan Zinnwald/Cínovec Sn-W-Li greisen-type deposit in the Eastern Erzgebirge/Krušné Hory (German/Czech Republic) were studied in terms of trace element contents and growth structures visualized by cathodoluminescence. The sample suite represents different populations of hydrothermal greisen quartz and magmatic quartz of the granitic host collected at a 600-m long, lateral section crosscutting the northern part of the deposit at the 750-m level.The studied quartz populations are characterized by generally low concentrations and low variation of Al and Ti indicating relatively constant crystallization conditions (temperature, pressure and fluid chemistry) during ore precipitation. Magmatic and hydrothermal quartz are distinguished by their Al, Ti, Li and Ge concentrations and ratios. Magmatic quartz has commonly higher Al (>100 ppm) and Ti (>3 ppm) compared to hydrothermal quartz. The Ge/Al ratio in quartz increases systematically from high to low formation temperatures (664 to ~400 °C = temperature range including published data and our estimates) whereas Ti decreases. The Zinnwald albite granite (mineralization host), an albite-rich alkali granite, and massive quartz-zinnwaldite greisen contain both magmatic and hydrothermal quartz populations which are distinguishable by their Ge/Al weight ratio: magmatic quartz has a Ge/Al ratio of <0.008 whereas hydrothermal quartz a ratio of >0.008.Hydrothermal vein quartz related to the Sn-W mineralization (Flöze = flat veins and Morgengänge = steep veins) does not show systematic trace element variations along the sampling profile indicating that no significant chemical gradients (in lateral direction) in the ore forming fluid existed. The uptake of Li in the quartz lattice is limited by the incorporated Al although the Zinnwald/Cínovec magmatic-hydrothermal system was strongly enriched in Li. The calculated crystallization temperatures of quartz of the Sn-W mineralization applying the TitaniQ thermobarometer reveal crystallization temperatures ranging from 575 ± 48 °C (albite granite), 612 ± 12 °C (stockscheider), 610 ± 23 (massive greisen), 536 ± 77 °C (flat veins), to 516 ± 58 °C (steep veins) confirming in general early estimations based on fluid and melt inclusion studies. In economic terms, the hydrothermal quartz (flat and steep veins) with an average total trace element content of 128 ± 56 ppm has intermediate chemical quality and can serve as a by-product if colour sorting and other processing steps are applied. Rock-forming quartz of albite granite and massive greisen with an average total trace element content of 233 ± 127 ppm has medium quality and the disadvantage that quartz contains a high amount of contaminating mineral inclusion, which will be challenging to remove by processing.

Mineralogy of the epithermal precious and base metal deposit Banská Hodruša at the Rozália Mine (Slovakia)

The Au-Ag-Pb-Zn-Cu epithermal deposit Banska Hodrusa of intermediate-sulphidation type is located in the Middle Miocene Stiavnica stratovolcano on the inner side of the Carpathian arc in Slovakia. This deposit represents an unusual subhorizontal multi-stage vein system, related to processes of underground cauldron subsidence and exhumation of a subvolcanic granodiorite pluton. Veins are developed on a low-angle normal shear zone, possibly representing a detachment zone in andesitic wall rocks that formed during emplacement and exhumation of the granodiorite pluton. The deposit consists of two parts, separated by a thick sill of quartz-diorite porphyry. The eastern part is currently mined, and the western part has already been depleted. The Banska Hodrusa mineralization was formed during four stages: (1) low-grade silicified breccia at subhorizontal structures at the base of the deposit; (2) stockwork of steep veins with rhodonite-rhodochrosite, quartz-sulphide-carbonate and quartz-gold assemblages; (3) thin quartz-gold veins with medium dip in tension cracks inside the shear zone and complementary detachment hosted quartz-base metals-gold veins; (4) Post-ore veins. Gold and electrum (920–730) occur as intergrowths with base metal sulphides or hosted in quartz and carbonates, accompanied by Au-Ag tellurides (hessite, petzite). Rare Te-polybasite and Cu-cervelleite result from re-equilibration of early Te-bearing minerals during cooling. Sulphide minerals include low Fe sphalerite (~ 1.25 wt%), galena, chalcopyrite, and pyrite. The wall rock alteration is represented mostly by adularia, illite, chlorite, quartz, calcite and pyrite. Precipitation of gold, Au-Ag tellurides, Mn-bearing minerals and adularia resulted from boiling of fluids due to hydraulic fracturing, as well as opening of dilatational structures within the shear zone.

Structural Evolution of Auriferous Veins at the Endeavour 42 Gold Deposit, Cowal Mining District, NSW, Australia

Polyphase veins at the low-grade, structurally controlled epithermal gold deposit Endeavour 42 in the Junee-Narromine volcanic belt of the Ordovician to Silurian Macquarie Arc, central New South Wales (Australia), record multiple tectonically driven fluid-flow events during the mid-Paleozoic. Ordovician host rocks consist of a volcano-sedimentary succession intruded by trachyandesite and andesitic lavas and intrusions, as well as a volumetrically significant Ordovician diorite sill. The entire succession was tilted and faulted prior to the emplacement of multiple mafic to intermediate dike phases, commonly localized along faults. Gold-bearing veins cut all lithologic units at the Endeavour 42 deposit, with the exception of rare late-stage intrusions. Economic gold is restricted to faults, fault-hosted breccias, and veins with two distinct populations: (1) an older, steeply dipping conjugate vein set, cut by (2) two younger, moderately dipping vein sets. Both steep and inclined, moderately dipping vein sets contain a similar mineral assemblage, implying formation during a single epithermal system. Veins are generally 1 to 10 mm thick, commonly straight walled, but locally form overlapping veins with hard linkages. The steep veins reactivated two sets of older faults, fault-parallel fractures, and dike contacts, and accommodated subhorizontal extension. High gold grade, hydrothermally cemented breccias are associated with early gold-bearing veins and localized along deposit-scale faults. Crosscutting the early steep gold-bearing vein sets are inclined gold-bearing veins with moderate dips to the southwest and west. These opened as a new vein set and as veins localized along and at a slight angle to westward-dipping bedding planes, respectively. The geometry of the inclined, moderately dipping veins either represents a sudden change in the far-field stress state to near-vertical minimum compressive stress, thereby suggesting a change from an extensional to a contractional environment, or it may reflect local changes in the differential stress state with subhorizontal northeast-southwest tension. Late veins and thrust faults in the deposit indicate a postmineral contractional environment. The Endeavour 42 deposit underwent tectonic burial until it was exhumed in the early Carboniferous, as indicated by 40Ar/39Ar plateau ages of 362.7 ± 2.1, 356.3 ± 1.8, and 344.5 ± 2.1 Ma, toward the end of the Kanimblan Cycle.

Three‐dimensional geometry, ore distribution and time‐integrated mass transfer through the quartz–tourmaline–gold vein network of the Sigma deposit (Abitibi belt, Canada)

AbstractWe present a reconstruction of the three‐dimensional (3D) geometry and gold grade distribution of shear zone‐hosted, Au‐mineralized, quartz–tourmaline veins of the Sigma deposit (Abitibi belt). Host shears and veins form a network of anastomosing, steeply dipping structures associated with smaller subhorizontal extensional veins. Our reconstruction has been carried out using the exceptionally large geological database of the mine. From this database, we extracted the geometric position, thickness and gold grades of geometrically best‐defined steep veins contained in a representative subvolume of the deposit. These data allowed the 3D representation of 53 veins, which have been constructed by fitting surfaces through the geometrical data and by contouring thickness and gold grade. The geometry of the network is mainly characterized by: (i) a few large segmented veins, with sinuous and helicoidal shape, and typical vertical dimension of &gt;100 m; (ii) a large number of smaller vertical veins, some of which splay off the steep veins with high dip angles; (iii) subhorizontal extension veins (joints) located at, or close to, the tips of steep veins. The absolute thickness of the vertically short veins is the same as that of the large veins, suggesting that they formed simultaneously, but only a few of them interconnect to form vertically continuous bodies. Patchy, vertically elongated zones of high dilation are present in the large veins, and are poorly correlated with Au‐rich zones. They presumably represent former high‐permeability zones of the network. The highest gold grades occur at the interconnections between the large veins and small splays or subhorizontal joints. This indicates the important role of vein interconnection for fluid flow and gold precipitation within the network. Combining the calculation of the volume of the network with the estimation of tourmaline abundance in the veins, we calculate that 2.1 × 106 m3 of tourmaline and 3.2 × 106 m3 of quartz precipitated during Au deposition.

Fluid pressure and salinity variations in shear zone-related veins, central Pyrenees, France: Implications for the fault-valve model

Fluid inclusion microthermometry data is presented from quartz veins associated with high-angle reverse shear zones cutting the Néouvielle Massif in the central French Pyrenees. A complex series of veins were formed during both normal and reverse movement on brittle faults within the shear zone. Brittle structures developed contemporaneously with quasi-plastic deformation in the shear zones. Fluid inclusion data are presented from a variety of vein types and shows that a wide variation in fluid density and salinity is present in the fault system. These variations occur both within individual veins and between different vein types. Temperatures at the time of vein filling are constrained to be in the range 310–360°C by chlorite microthermometry. The variation in homogenisation temperature is too large to be accounted for by variations in the fluid temperature, and is interpreted to reflect variations in fluid pressure between lithostatic (∼ 500 MPa) and hydrostatic (∼ 200 MPa) values at the time of quartz growth in the veins. The shear zones carry modest displacements and may not themselves have been seismogenic. A model is presented in which fluid pressure variations and stress cycling within the steeply dipping shear zones is controlled by the seismogenic cycle on large underlying thrust structures. The model predicts that the highest maximum trapping pressures should occur in fluid inclusions from low-angle veins formed in a high- P fluid compressional regime immediately before earthquake rupture. Lower maximum pressures are expected in steep veins related to extension and normal fault movement across the shear zones immediately after rupture. The fluid inclusion data support this model when maximum trapping pressures are considered, but the majority of inclusions in all vein types formed at pressures well below lithostatic values. This suggests that the dominant factor causing quartz precipitation was the sudden pressure drop immediately after earthquake rupture, regardless of vein type.

Narrow vein mining 1 - steep veins: T. Brewis, Mining Magazine, 173(3), 1995, pp 116–130

Narrow vein mining 1 - steep veins: T. Brewis, Mining Magazine, 173(3), 1995, pp 116–130

Tectonic origin of fractures for fissure vein emplacement in the Boulder Batholith and adjacent rocks, Montana

In the Boulder batholith and adjacent rocks, the dominant and most productive metalliferous fissure veins trend approximately east and dip steeply. Other steep veins, generally narrower, trend northwest, northeast, and rarely north. Fissure veins also occur locally in thrust faults in volcanic rocks along the west side of the batholith. Fractures along which these veins were emplaced were probably initiated by a regional compressional stress field during Laramide (Late Cretaceous-early Tertiary) time. The batholith was emplaced during the Late Cretaceous and is synorogenic with respect to the surrounding Laramide thrust and fold belt, which is characterized by east-west compression. Postorogenic rocks (Eocene and younger) do not host major fissure vein deposits.Fractures utilized by east-trending metalliferous veins are parallel to the axis of maximum compressional stress and thus appear to be extensional; this may be the reason these veins are widest and longest. Northeast- and northwest-trending veins were probably emplaced along conjugate shears; these veins are narrower than east-trending veins. Veins that trend north are less common and probably formed along release fractures perpendicular to the principal axis of compressional stress. These veins are narrower than many of those that trend east.Mechanical behavior of various rock units during deformation had an important effect on ore deposition. The longest, widest, and most productive fissure veins occur in competent, brittle rocks (Precambrian crystalline basement, batholith intrusions, and Elkhorn Mountains Volcanics). Ductile rocks (Mesozoic shales and Belt Supergroup argillites) do not commonly contain major fissure veins, except where these rocks have been contact metamorphosed and have become more brittle. Paleozoic strata, mainly carbonates, are of intermediate ductility and are hosts to fissure veins in only a few districts adjacent to the batholith.