Genesis of the Heiyanshan Tungsten Skarn Deposit in the East Tianshan, NW China: Insights From Geology, Fluid Inclusion, Isotopic Geochemistry and Geochronology

The eastern Tianshan Terrane is a highly prospective zone that contains several porphyry Cu–Mo, VMS Cu–Zn, magmatic Cu–Ni, epithermal and orogenic Au deposits. However, few attention has been paid to tungsten deposits. Of these, the source and evolution of the mineralising fluids related to the skarn W deposits are poorly understood. The Heiyanshan W deposit is hosted by metamorphosed clastic and carbonate beds in the Mesoproterozoic Jianshanzi Formation deposited on a continental margin tectonic setting. The Jianshanzi Formation is intruded by biotite monzogranite that yield weighted 206Pb/238U age of 326.9 ± 1.6 Ma, which suggest that the Heiyanshan W deposit was formed in the Carboniferous. The mineralisation is hosted by a prograde hydrothermal altered zone represented by a garnet (–pyroxene) skarn, and retrograde skarn characterised by fine-grained scheelite. The paragenesis of the Heiyanshan mineralisation can be subdivided into prograde skarn stage, retrograde skarn stage, quartz-sulphide stage and quartz-calcite vein stage. The types of fluid inclusions recognised in the various minerals in the deposits are liquid-rich aqueous, vapour-rich aqueous, and daughter mineral-bearing. The homogenisation temperatures of fluid inclusions from the Heiyanshan deposit decrease from 290 ± 28°C in garnet, through 232 ± 31°C in scheelite, to 232 ± 36°C in quartz and 158 ± 15°C in non-mineralised calcite, which is typical of W-bearing skarn deposits worldwide. The δ18Owater values from the Heiyanshan deposit range from +4.7 to +6.6‰ in garnet, +1.3 to +1.9‰ in quartz and −6.1 to −4.4‰ in calcite. We have measured δD in fluid inclusions from different minerals, although these bulk analyses are just a mixture of the different FIA’s present in the sample. The δD values of fluid inclusions in garnet, quartz, and calcite are from −121 to −71‰, −84 to −75‰ and −101 to −82‰, respectively, also indicative of deep-sourced magmatic fluids mixed with meteoric water. The decrease in the homogenisation temperatures for the fluid inclusions at the Heiyanshan deposit is accompanied by a drop in salinity indicating that tungsten-bearing minerals precipitated during fluid mixing between magmatic fluids and meteoric water. We conclude that eastern Tianshan Terrane contains two pulse of tungsten metallogenic events of Late Carboniferous and Early Triassic.

Geophysical prospecting for epithermal gold deposits (a case study from the Maletoivayam gold ore field, Kamchatka Peninsula)

Research subject. The high-sulfidation (HS-type) epithermal gold deposits of the Maletoyvayam ore field, which is located in the volcanic belts of the Kamchatka Peninsula. Materials and methods. The paper presents the results of geophysical and petrophysical studies carried out by “NPP VIRG-Rudgeofizika” (2016–2017 years) in the central part of the Maletoyvayam ore field hosting HS-type gold deposits. Results. The presented case study shows the potential of geophysical prospecting for HS-type epithermal gold deposits located within altered rock areas. On the basis of the conducted magnetic surveys, fragments of a ring structure broken by faults and the hydrothermally altered bodies within the Neogene volcanogenic-sedimentary sequence of the andesite composition were identified. Using the 3D inversion of magnetic data, the shape, size and structure of the hydrothermally altered bodies were estimated. The electrical survey data allowed the hydrothermally altered areas to be differentiated in terms of composition. As a result, zoning of hydrothermally altered areas was conducted, and ore-bearing sites and gold-bearing bodies of monoquartzites in the plan and cross-section were determined. Conclusions. Geophysical techniques for prospecting HS-type epithermal gold deposits are effective in distinguishing important gold-related features within the altered zone thereby facilitating identification of specific drill targets.

The Role of Manganese as a Controller for Gold Mineralization

The San José de Las Malezas quartz-gold deposit is located on the North side of the Cuban Ophiolitic Complex, within the Structural-Facial Zone "Zaza", Province of Villa Clara, Cuba. In the area of the deposit there is a well developed listwanite zone that begins with (i) massive and relatively little altered serpentinites that pass into (ii) iron-altered serpentinites which are in gradational contact with (iii) completely carbonatized rocks and (iv) a core of silicified rocks and quartz veins. Copper and gold mineralization are associated with these quartz bodies, but binary correlation analysis of geochemical data suggests that the ores are genetically independent of one another. In concordance with the proposed genetic model supported by thermodynamic calculations and field observations, copper is thought to be introduced by a hydrothermal process, whereas gold was leached from the serpentinites. The evolution of manganese minerals from anaerobic to aerobic species provoked a reducing environment which difficulted the precipitation of gold and other ores inside and near the serpentinites. Accordingly, a significant negative correlation between gold and manganese in similar areas of hydrothermally altered serpentinites, may be good indicators of possible gold concentrations. The mechanical transportation of very small and thin scales of native gold from the serpentinites by the fluids is also suggested as a possible mechanism to explain the presence of this kind of scales inside the iron-altered zone.

Chemical Characteristics of Zircon from Khaldzan Burgedei Peralkaline Complex, Western Mongolia

The Khaldzan Burgedei peralkaline complex is one of the potential rare metal (Zr–Nb–REE) deposits in Mongolia. The complex consists mainly of quartz syenite and granite, and zircon is the most common accessory mineral in the rocks. Based on texture and mineral paragenesis, zircon is classified into three types. Type-I zircons in the quartz syenite and granite are generally isolated and euhedral to subhedral, 25–100 μm in size, enclosed by albite, K-feldspar, and quartz. Type-II zircons occur as subhedral to euhedral 20–150 μm grains, with quartz, and fluorite in the metasomatized zone in the quartz syenite as well as an upper part of the granite near the contact with the quartz syenite. These zircons contain porous core parts (Type-I) or remnants of corroded xenotime-(Y) and synchysite-(Ce). Type-III zircons are observed in the hydrothermally altered zone in quartz syenite and pegmatite. These zircons are anhedral, fine-grained, 10–30 μm in size, and occur in amphibole pseudomorphs which were replaced by quartz, fluorite, chlorite, and hematite. Laser Raman spectra show that Type-I and Type-II zircons contain high amounts of water. Among these, three types of zircons, Type-II zircons are most enriched in REE, Nb, and Th. The texture and composition of the three types of zircons indicate that Type-I, Type-II, and Type-III zircons are magmatic, metasomatic and late hydrothermal in origin, respectively, and they experienced remobilization and recrystallization during the transition from a magmatic to a hydrothermal system.

Superficial structures cartography of the Essaouira basin under ground (Morocco), by small refraction seismic: contribution of the static corrections in the reinterpretation of the speeds variations.

The static corrections are a necessary step in the sequence of the seismic processing. This paper presents a study of these corrections in the Essaouira basin. The main objective of this study is to calculate the static corrections by exploiting the seismic data acquired in the field to improve the deep structures imaging. It is to determine the roof and the basis of the superficial layers which constitute the weathered zone while calculating the delays of seismic wave’s arrivals in these layers. The purpose is to cancel the effect of the topography and the weathered zone, in order to avoid any confusion when the seismic and geological interpretation. The results obtained show the average values of the static corrections varying between - 127 and 282 ms (double time), with existence of high values by location, particularly in the Eastern and North-Eastern of the basin, which meant the presence of altered zone with irregular topography and whose thickness and speeds vary laterally. In effect the variations of velocities in the fifty meters from the surface may introduce significant anomalies in seismic refraction, with heavy consequences when the interpretation or the drilling establishment. These variations are mainly due to lateral changes in facies and variations in the formations thickness. The calculation of the static corrections, revealed high values at certain areas (East and North-East), which will enable us to better orient the future campaigns in these zones. It is therefore necessary to concentrate the seismic cores drillings and the small refraction seismic profiles by tightening the seismic lines meshes in order to have the maximum values of static corrections and thereafter a better imaging of the reflectors.

CROSTAS BIOLÓGICAS DE SAPRÓLITOS DA REGIÃO DO QUADRILÁTERO FERRÍFERO, MG: CICLAGEM BIOGEOQUÍMICA E MICROMORFOLÓGICA

The role of biological crusts in nutrient cycling and soil structure genesis in saprolites, in tropicalconditions, is little known. This work reports on the biochemical weathering and structuralreorganization resulting from the biological crusts action in different saprolites of the QuadriláteroFerrífero and neighbor areas, MG. These road exposures were oriented to the south, encompassing adiabase (DB), gneiss (GN), itabirite (IT), schist (XT) and gold-mine spoil-pile from Morro Velho(MV), comprising metasediments of the Rio das Velhas supergroup. There were collected and identifiedcyanobacteria, fungi, lichens and mosses. Based on the biogeochemical study, the exchangeable levelsof K, Mn and Zn were preferably concentrated in the crust. The exchangeable Fe levels were higherin the altered under zone, constituted possibly by secondary Fe-migration and precipitation throughlateral fluxes. There were no clear trend of Ca and Mg distribution, although they were apparentlyconcentrated in the crust in the IT and XT substrates, and in the altered zone in the DB. In MV spoil,the Ca dynamic was affected by sulfide oxidation, following acid drainage. In terms of total levels Mn,Fe, Mg, Cu and Zn were higher in the diabase saprolite, due to its maficity, in a clear contrast with thehost gneiss, represented by the GN saprolite, with low levels of Fe, Mn, Mg and Cu. The richersubstrates (DB, MV e XT) showed a trend of increasing total levels with depth, whilst the nutrientpoor substrates (GN e IT) had higher levels of some elements in the crust, compared with thesaprolito. The micropedological features are notably rich and varied, showed buried micropedogenetichorizons, sapric and histic micro-horizons, micro-stratification, ferruginous zones similar to Fe/Mnmicropans, amongst others. The biological crusts efficiently covered the saprolites, being responsibleto micropedogenetic features and a clear process of biological microstructuration of the exposedsaprolites. This process is capable of altering the moisture balance and erosion dynamic of the slope,with important geomorphological and environmental implications.

Anatomical changes induced by fire-damaged cambium in two native tree species of the Chaco Region, Argentina

This study examined anatomical responses to fire damage of the cambium in Schinopsis lorentzii and Aspidosperma quebracho-blanco. Bole cross sections were extracted from specimens with external signs of fire damage. Samples were taken from zones designated normal, discoloured and wound altered. The vessel, fibre, axial and ray parenchyma percentages, tangential vessel diameter, vessels per mm2, rays per mm, and ray width and height of these zones were compared. Fire scars and fire marks were identified on cross sections of S. lorentzii and A. quebracho-blanco. The fire marks reflect minor wounds that did not affect wood formation. The fire scars, on the other hand, are the result of wounds that interrupted cambial activity thus affecting the shape of the bole and causing discolouration of pre-existing wood adjacent to wounds. The wood formed after fire damage included callus, barrier zones at fire scar edges and the formation of ribs of wound wood. The wound altered zone was characterised by a decrease in the percentage of vessels and fibres, an increase in the percentage of axial parenchyma, the formation of grouped rays, a decrease in vessel tangential diameter, and occurrence of fibres with atypical structure. Disorientation in the axial xylem system was observed in the barrier zone. The anatomical responses to cambium damage and formation of discoloured wood and woundwood ribs suggest that wood quality and utilisable volume of bole in the studied species is affected by fire.

Gold mineralisation at Eqi, north-east Disko Bugt, West Greenland

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Stendal, H., Knudsen, C., Marker, M., & Thomassen, B. (1999). Gold mineralisation at Eqi, north-east Disko Bugt, West Greenland. Geology of Greenland Survey Bulletin, 181, 129-140. https://doi.org/10.34194/ggub.v181.5121 _______________ Gold mineralisation at Eqi, north-east Disko Bugt, West Greenland, is hosted in Archaean (c. 2800 Ma old) supracrustal rocks; the latter are divided by a thrust into a lower volcanic unit and an upper sedimentary and volcaniclastic unit. The lower volcanic unit comprises three parts: a basal pillowed greenstone sequence, an acid volcanic complex, and an upper mafic igneous complex. Intensive hydrothermal activity resulted in extensive carbonatisation and sericitisation, which is most intense just above a system of acid feeder dykes within the basal greenstone sequence. Primary enrichment in gold took place during pervasive hydrothermal alteration, and the gold is mainly located in carbonate-altered rocks. Remobilisation of gold occurred during formation of later quartz veins in the altered zone; these quartz veins have gold contents of up to 60 ppm. The geological setting, geochemistry and formation of the gold mineralisation at Eqi is similar to many Archaean gold deposits in the Abitibi belt of Canada.