Gold Occurrences Research Articles

Genesis of the Selinsing gold deposit, Peninsular Malaysia: Constraints from mineralogy, geochemistry and in situ sulfur isotope compositions of sulfides

As a renowned orogenic lode gold deposit, the Selinsing gold deposit is located in the Central Belt of Peninsular Malaysia. Geologically, the gold deposit is characterized by auriferous quartz veins and fine-grained gold minerals coexisted with sulfides, and the ore body is hosted by low-grade metamorphic sedimentary rocks and volcanic rocks. Furthermore, gold mineralization is closely related to tectonic deformation. Four main stages of vein formation have been identified, namely, (1) quartz-pyrite-arsenopyrite vein, (2) quartz-pyrite-gold vein, (3) quartz-stibnite-pyrite vein, and (4) quartz-chlorite vein. The occurrence of gold includes two types, i.e., visible gold and invisible gold. The former is featured by coarse-grained gold (millimeter-scale, such as auriferous quartz veins) and fine-grained gold (micron-scale, such as native gold, calaverite, and electrum), the latter is contained in pyrite and arsenopyrite in the form of nano-scale native gold particles (Au0) and lattice gold (Au1+). The geothermometer of arsenopyrite and chlorite as well as temperature-sulfur fugacity diagram indicate that the temperature and sulfur fugacity of hydrothermal fluid decrease gradually. Considering that the sulfur fugacity drops gradually and aqueous inclusions coexist with carbonic inclusions, it is inferred that sulfuration reaction in which sulfur is consumed to produce sulfides and fluid immiscibility or phase separation may be responsible for gold precipitation. Tested by LA-MC-ICP-MS, the in-situ sulfur isotope compositions of sulfides formed in different stages are homogenous (about −1 to 1‰) and the calculated H2S-dominated hydrothermal fluid is also consistent (about −2 to 0‰) in sulfur isotope compositions, which reflect the mantle or ultrabasic rock and basic rock or magmatic rocks may be a potential source of sulfur. Combined with possible source and tectonic evolution of Peninsular Malaysia, possible metallogenic processes are proposed to explain genesis of the Selinsing gold deposit and other gold deposits in the Central Belt, i.e., ore-forming fluid was differentiated from oceanic crust and/or fertilized mantle wedge, or ore-forming fluid may be sourced from concealed magmatic rocks that formed by subduction of the Palaeo-Tethys toward the East Malaysia block.

A Nanoscale Investigation of Carlin-Type Gold Deposits: An Atom-Scale Elemental and Isotopic Perspective

Abstract Carlin-type gold deposits are one of the most important gold mineralization styles in the world. Despite their economic importance and the large volume of work that has been published, there remain crucial questions regarding their metallogenesis. Much of this uncertainty is due to the cryptic nature of the gold occurrence, with gold occurring as dispersed nanoscale inclusions within host pyrite rims that formed on earlier formed barren pyrite cores. The small size of the gold inclusions has made determining their nature within the host sulfides and the mechanisms by which they precipitated from the ore fluids particularly problematic. This study combines high-resolution electron probe microanalysis (EPMA) with atom probe tomography (APT) to constrain whether the gold occurs as nanospheres or is dispersed within the Carlin pyrites. APT offers the unique capability of obtaining major, minor, trace, and isotopic chemical information at near-atomic spatial resolution. We use this capability to investigate the atomic-scale distribution of trace elements within Carlin-type pyrite rims, as well as the relative differences of sulfur isotopes within the rim and core of gold-hosting pyrite. We show that gold within a sample from the Turquoise Ridge deposit (Nevada) occurs within arsenian pyrite overgrowth (rims) that formed on a pyrite core. Furthermore, this As-rich rim does not contain nanonuggets of gold and instead contains dispersed lattice-bound Au within the pyrite crystal structure. The spatial correlation of gold and arsenic within our samples is consistent with increased local arsenic concentrations that enhanced the ability of arsenian pyrite to host dispersed gold (Kusebauch et al., 2019). We hypothesize that point defects in the lattice induced by the addition of arsenic to the pyrite structure facilitate the dissemination of gold. The lack of gold nanospheres in our study is consistent with previous work showing that dispersed gold in arsenian pyrite can occur in concentrations up to ~1:200 (gold/arsenic). We also report a method for determining the sulfur isotope ratios from atom probe data sets of pyrite (±As) that illustrates a relative change between the pyrite core and its Au and arsenian pyrite rim. This spatial variation confirms that the observed pyrite core-rim structure is due to two-stage growth involving a sedimentary or magmatic-hydrothermal core and hydrothermal rim, as opposed to precipitation from an evolving hydrothermal fluid.

Trace-element systematics of pyrite and its implications for refractory gold mineralisation within the carbonaceous metasedimentary units of Palaeoproterozoic South Purulia shear zone, eastern India

Globally, refractory gold occurs in significant proportions in many types of gold deposits. The present work reports the occurrence of sulphide-hosted refractory gold within the carbonaceous phyllites of the South Purulia shear zone in the Singhbhum crustal province, eastern India. Detailed textural characteristics, paragenesis and trace-element concentrations of different generations of pyrites were studied to understand their evolutionary stages and the mechanism of gold incorporation in pyrites. Four types of pyrites, which are closely associated with gold mineralisation were identified in the host rock, i.e., carbonaceous phyllite. Py I is of diagenetic origin, whereas Py II, Py III and Py IV are of hydrothermal origin. Laser ablation inductively coupled plasma mass spectrometry studies confirm the presence of invisible/refractory gold concentration up to 110.55 ppm within these pyrites. The positive correlation between Au and As in pyrite indicates the significant role of As in the incorporation of gold in pyrite. Fourier transform infrared spectroscopy confirms the presence of organic matter that provided suitable redox conditions for the precipitation of auriferous pyrites. The refractory gold mineralisation is attributed to widespread sulphidation during both sedimentation and hydrothermal ore-forming processes. Transformation of diagenetic pyrite to pyrrhotite during prograde metamorphism of carbonaceous rocks promoted the liberation of sulphur and Au from the lattice of abundant diagenetic pyrites to the hydrothermal fluid which later precipitated sulphides in the quartz ± carbonate veins.

Optical image and microchemical analysis of gold grains from a weathered profile of the Minvoul greenstone belt, northern Gabon

AbstractMorphological characterization and quantification of gold particles by optical image analysis (OIA) and by compositional analysis of microprobes using scanning electron microscope and electron microprobe analysis techniques were carried out on gold grains from the Minvoul area (Archean greenstone belt in Gabon). Large grains of almost pure gold were found throughout a weathering profile, which consisted of saprolite, mottled clay zone, iron duricrust, pisolitic gravels and yellow latosol. In the deeper horizons, gold was dissolved as shown by corrosion features on the surface of particles with average sizes of 2.6Φ and 2.35Φ in the saprolite and mottled clay zones, respectively. The occurrence of secondary gold in the duricrust was indicated by the larger size of the nuggets (average, 1.8Φ), the high fineness (> 995 in average) and the close textural relationship between gold and neoformed iron oxyhydroxides. The uppermost horizons composed of yellow latosol and pisolitic gravels were interpreted as transported materials based on their size distribution (average 0.85Φ and 1.34Φ), sorting and shape parameters. The best morphological parameter to describe the whole weathering profile was found to be the perimeter/area ratio. The highest ratios were recorded in the saprolite (average 0.192 μm−1), and decreased towards the surface (average 0.057 μm−1). The combination of the OIA technique and the microchemical analysis of gold grains allowed us to define specific morphological and compositional characteristics of the gold particles for each horizon. Both methods proved to be of great utility to understand gold concentration, dissolution and dispersion processes in supergene environments.

The gold occurrence in pyrite and Te–Bi mineralogy of the Fancha gold deposit, Xiaoqinling gold field, southern margin of the North China Craton: Implication for ore genesis

The Fancha lode gold deposit is hosted in the Neoarchean Taihua Group metamorphic rocks in the Xiaoqinling gold field, on the southern margin of the North China Craton. Pyrite, as the predominant sulphide mineral and Au‐hosting mineral, was investigated using a combination of ore microscopy and in‐situ laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Three types of pyrite were identified during the ore‐forming process, that is, the coarse euhedral grains in milky quartz veins (Py1), the fine to medium, euhedral to subhedral grains in light grey quartz (Py2), and the fine grains intergrown with abundant sulphide minerals in light grey quartz (Py3). The trace elements exhibit systematic different concentrations among three types of pyrite, implying different crystallization processes. The Py1 grains have the lowest concentrations of Au and other accompanying elements such as Ag, Te, Bi, Cu, Pb, and Zn, and the trace elements of pyrite in wall rock are similar to those of Py1. The Py2 and Py3 grains show consistently high contents of Au, Ag, Te, Bi, Cu, Pb, and Zn. The uneven distribution of gold and positive correlation between Au, Ag, Te, and Bi within Py2 and Py3 grains, indicating that gold mainly occurs as submicroscopic inclusions of Au–Ag–Te–Bi assemblages in the pyrite. This interpretation is supported by the microscopic observation that large quantities of telluride and Bi‐sulfosalt minerals contact directly with gold. Observed Te–Bi minerals include petzite, sylvanite, hessite, calaverite, rucklidgeite, altaite, volynskite, tellurobismuthite, buckhornite, tetradymite, krupkaite, and bismuthinite, and native bismuth is also found. Bi (Te) melts, as an excellent gold scavenger, have played a certain role in governing gold distribution patterns and leading to high concentrations during the mineralization process in the Fancha gold deposit. The Te–Bi mineral assemblages also reveal that the gold was precipitated under variable redox conditions with logfTe2ranged from <−11.0 to −6.4, and logfS2ranged from <−11.8 to −8.6 at 300°C, vertically upward from the deep fluid source. Combined with available geochronological and geochemical data, enrichment of Te and Bi indicates that the ore‐forming fluids and metals might be stemed from a mantle‐derived magmatic system.

Vapor transport of silver and gold in basaltic lava flows

Abstract We documented occurrences of native copper (Cu), silver (Ag), and gold (Au) in a pāhoehoe flow from Kīlauea volcano (Hawaii, USA), an a‘ā flow from Mauna Loa volcano (Hawaii), and a mid-oceanic-ridge basalt (MORB) from the Chile Ridge (southeastern Pacific Ocean). Native Ag in Kīlauea and MORB samples consistently contained minor Cl (<1 wt%). Native Ag in Hawaiian basalts can occur at the center of nearly circular patches of relatively evolved minerals, which presumably formed after late-stage silicate liquid infilled pipe vesicles. Sulfur loss and oxidation of a Cu-sulfide phase can explain the native Cu, but not Au and Ag deposition. The rare occurrence of native Cu-Au-Ag alloys and the large native Au and Ag grain size suggest separate metal precipitation mechanisms. A fractional crystallization and degassing model envisions initial Au and Ag enrichment in crystallizing interstitial liquid and further enrichment in a separating vapor phase. From the flow interior, the metals ascend through ephemeral pipe vesicles as bisulfide (Au) or chloride (Ag) vapor complexes and precipitate in the transition zone below the upper vesicular zone, owing to temperature and oxidation state changes. Our results support igneous vapor transport of ore elements in mafic plutonic systems and imply preconcentration of gold during lava solidification before later hydrothermal remobilization.

Occurrence of Placer Gold in the Marlagalla-Allapatna Areas, Mandya District, Karnataka

ABSTRACT Occurrence of placer gold is reported for the first time in the insitu and colluvial gravels derived from amphibolite/hornblende schist, associated pegmatite and quartz veins of Nagamangala schist belt in Marlagalla –Allapatna areas, Mandya district, Karnataka. Gold occurs in the form of small nuggets, measured up to 3mmx4mm in size and 0.2 gram in weight. The host rock is amphibolite which are intruded by numerous pegmatite and quartz veins. The pegmatites are complex, highly evolved type and contain economic concentration of rare metal bearing minerals viz., columbite-tantalite, beryl and spodumene. Prolonged weathering of these rocks resulted in liberation of gold from host rocks and concentrated in the insitu/eluvial and colluvial gravels. The present finding of gold opens up a new vista for further detailed geological/geochemical exploration to establish and evaluate gold potential in the eluvial, colluvial gravels as well as in the other lithounits of Nagamangala schist belt in Marlagalla-Allapatna areas.

Multispectral and Radar Data for the Setting of Gold Mineralization in the South Eastern Desert, Egypt

Satellite-based multi-sensor data coupled with field and microscopic investigations are used to unravel the setting and controls of gold mineralization in the Wadi Beitan–Wadi Rahaba area in the South Eastern Desert of Egypt. The satellite-based multispectral and Synthetic Aperture Radar (SAR) data promoted a vibrant litho-tectonic understanding and abetted in assessing the regional structural control of the scattered gold occurrences in the study area. The herein detailed approach includes band rationing, principal component and independent component analyses, directional filtering, and automated and semi-automated lineament extraction techniques to Landsat 8- Operational Land Imager (OLI), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Phased Array L-band Synthetic Aperture Radar (PALSAR), and Sentinel-1B data. Results of optical and SAR data processed as grayscale raster images of band ratios, Relative Absorption Band Depth (RBD), and (mafic–carbonate–hydrous) mineralogical indices are used to extract the representative pixels (regions of interest). The extracted pixels are then converted to vector shape files and are finally imported into the ArcMap environment. Similarly, manually and automatically extracted lineaments are merged with the band ratios and mineralogical indices vector layers. The data fusion approach used herein reveals no particular spatial association between gold occurrences and certain lithological units, but shows a preferential distribution of gold–quartz veins in zones of chlorite–epidote alteration overlapping with high-density intersections of lineaments. Structural features including en-echelon arrays of quartz veins and intense recrystallization and sub-grain development textures are consistent with vein formation and gold deposition syn-kinematic with the host shear zones. The mineralized, central-shear quartz veins, and the associated strong stretching lineation affirm vein formation amid stress build-up and stress relaxation of an enduring oblique convergence (assigned as Najd-related sinistral transpression; ~640–610 Ma). As the main outcome of this research, we present a priority map with zones defined as high potential targets for undiscovered gold resources.

Refractory gold ores and concentrates part 2: gold mineralisation and deportment in flotation concentrates and bio-oxidised products

ABSTRACT Gold mineralisation and deportment in two refractory flotation concentrates and their corresponding bio-oxidised products were investigated using several analytical techniques. One concentrate and its product show higher gold grades (28 and 38 g/t, respectively) than the other (17 and 23 g/t, respectively). The results showed that both visible, polydispersed and invisible gold particles were present in all four samples. The invisible gold was disseminated in arsenian pyrite and arsenopyrite mineral phases. Unliberated, visible gold particles were in complex gold associations with sulphides, silicates, and oxides. In addition, visible gold-sulphate mineral associations were also noticeable in the bio-oxidised products. Marked differences in gold mineralisation and association such as occurrence of gold in muscovite and jarosite-cemented gangue mineral agglomerates were notable between the two bio-oxidised products. Furthermore, combined jarosite and gypsum mineral encapsulation accounted for ≈ 43 wt.% Gold refractoriness in the lower gold-bearing bio-oxidised product.

Chemostratigraphic constraints on the nature and origin of felsic schist units hosting stratabound and orogenic vein gold on Lone Star Ridge, Klondike Gold District, Yukon

Felsic schist units in the Lone Star Ridge area of the northern Klondike Gold District, Yukon, Canada, host both locally gold-bearing orogenic quartz veins and much more extensive zones of disseminated gold mineralization that is stratabound within specific schist units and is interpreted to be syngenetic in origin. Scarcity of outcrop and the difficulty in distinguishing between subtly different schist units has hampered correlation between surface exposures and drill core, and production of a detailed geological map of the area. Lithogeochemistry offers an alternative, unbiased way to map and interpret the various subunits of the felsic schist package that host disseminated gold mineralization and gold-bearing veins in parts of the Klondike, and that can potentially form the basis for correlations between various drill holes, and between drill holes and surface exposures. A detailed investigation of 4590 multi-element geochemical analyses (by 4-acid digestion) from a set of 27 diamond drill holes in the vicinity of the Boulder Lode open cut (Lone Star gold occurrence) on Lone Star Ridge has revealed several lithogeochemical signatures that correlate closely with gold content and to some extent with lithological units distinguished during initial logging of the drill core. Research quality geochemical analyses for a suite of representative rock units selected from the 2005 drill holes have been used to verify the multi-element geochemical analyses, and to determine the petrochemical character of some of the individual rock units. Results of this study demonstrate that it is possible to construct a chemostratigraphic framework for the felsic schist units in the vicinity of the Boulder Lode. Distinctive rock units can be broken out that have very consistent geochemical signatures, and elevated gold values related to the presence of disseminated gold appear to be consistently associated with specific rock units. These chemostratigraphic marker units have been utilized to constrain the geology and structure of the immediate study area, and in particular to define the geometry of the schist units that contain disseminated gold. The same approach has also been applied to a very extensive set of geochemical data (comprising nearly 34,000 individual samples) from chip samples, diamond drill core, reverse circulation drill samples, and conventional soil samples obtained during other historical and more recent exploration programs, to better resolve the geological interpretation of entire Lone Star Ridge area.

Trace Element Geochemistry in Quartz in the Jinqingding Gold Deposit, Jiaodong Peninsula, China: Implications for the Gold Precipitation Mechanism

Lots of studies on gold precipitation mechanisms have focused on fluid inclusions within quartz. However, the trace elements in quartz reflect the properties of the ore fluid, and a comparison of the trace element content in different types of quartz can reveal the precipitation mechanism. The Jinqingding gold deposit is the largest gold deposit in the Muping–Rushan gold belt and contains the largest single sulfide–quartz vein type orebody in the gold belt. This study distinguished four types of quartz in this orebody through field work and investigations of the mineralogy and cathodoluminescence (CL) of the quartz and crosscutting relationships as seen under a microscope. In situ studies via electron probe micro-analyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were used to determine the trace element content of the different quartz types. Type Qa displayed a comb structure in the field and zoning under the microscope and in CL. Milky white and smoke grey Qb was the most common quartz type and hosted the most sulfide and gold. Qc was Qa and Qb quartz that recrystallized around pyrite or overgrew and appeared different from Qa and Qb in CL images. Qd occurred within fractures in pyrite. Qa formed prior to the mineralization of gold, and Qd formed post-mineralization. Qb and Qc provided information regarding the ore fluid during mineralization. Sericites occurred with pyrite in fractures in the quartz, and some, along with free gold, filled in fractures in pyrite. Free gold occurred within Qa, Qb, Qc, and in brittle fractures in pyrite. Qc had the lowest Al content of all of the quartz types. As Al content is related to the acidity of the ore fluid in previous study, this indicated an acidity decrease during mineralization, which could be attributed to the sericitization. Sericitization could indicate a potential gold occurrence. The Ti content decreased from Qb to Qc, indicating a decrease in temperature during quartz overgrowth formation. Change in acidity and cooling can therefore be identified as possible causes of gold precipitation in the sulfide–quartz vein type in the Jinqingding gold deposit.

Gold endowment in the evolution of the Allaqi-Heiani suture, Egypt: A synthesis of geological, structural, and space-borne imagery data

Field and structural studies supported by multi-sensor satellite-based imagery data are attempted to decipher the setting and controls of gold-bearing quartz veins in the central part of the greater Wadi Allaqi district in the South Eastern Desert of Egypt. Here, we assess the evolution of a complex part of the Neoproterozoic Allaqi-Heiani suture (∼730–700 Ma) with ample structures demonstrating the final Gondwanan collision. Abundant extensive N-trending folds, sinistral shear zones and oblate fold patterns and strain fabrics exemplifying an oblique convergence and torsion related to a regional ∼E-W compressional regime.In the Wadi Seiga area, the first order upright and overturned folds and thrusts (D1 structures) are mostly obliterated by intense NW-ward transpression, N-ward folding and transcurrent shearing assigned to as D2 and D3 structures. The domal crescentic and mushroom superimposed fold patterns suggest that Type 2 and Type 3 fold interference patterns were synchronous at some time during D2 or D3. Shear zones with abundant signs of plastic deformation and associated with quartz veins, within the axial planes of NNW- and NNE-trending F3 folds may imply gold mineralization during the third deformation stage (D3). The latter was supplemented or terminated by an episode of northward tectonic escape and slip reactivation of the pre-existing NW-trending faults.Integrated with field measurements, satellite-based radar and radiometer data abetted untangling the superimposed structures. Gold occurrences in the area are confined to domains of superimposed foliations, foliation deflection/steepening, foliation refraction and substantial strain partitioning, or at oppositely interfered plunging folds. The scattered small scale occurrences of ≤10 s g/t Au-bearing gold-quartz veins in such high strain zones, together with the lack of spatial association with a particular intrusive phase weigh with the metamorphic model for gold-only endowment in the area. The peak metamorphic conditions (amphibolite facies) in the central Allaqi-Heiani suture were attained not earlier than 630 Ma, during the collision of the Arabian-Nubian Shield with the Nile Craton to the west (provoked at 670–610 Ma). Gold-only endowment was complementing to the latest stages of deformation and terrane cooling between 610 and 585 Ma. Listvenitization of tectonized ophiolites and widespread sericite alteration of the island arc metavolcaniclastic rocks could have been promoted by the late-orogenic felsic intrusions (∼603 Ma) that filled oblate dilation spaces developed by structural transport to the north. The late orogenic timing of mineralization is one of many features making the Allaqi-Heiani suture comparable to well-documented global examples of accretionary metallogenic belts.

Mineral Occurrence Target Mapping: A General Iterative Strategy in Prediction Modeling for Mineral Exploration

Procedures are discussed to construct target maps ranking the likelihood of future discoveries: for instance, of gold occurrences, knowing location and spatial context, of a set of genetically related gold vein deposits. A favorability modeling process is iterated with a subset of the known occurrences, i.e., the locations of the deposits. The resulting prediction patterns are cross-validated with the distribution of the left-out occurrences, considered as representing the future discoveries. The target map originates from integration of all prediction patterns from the iterations. Rank-based statistics related to the target maps provides measures of quality, robustness and uncertainty of the classification of a study area into likelihood of discovery. Much of this is a relatively new area of research, so that to interpret such uncertainty is still a challenge. Four critical questions are formulated that identify areas in need of extensive research for any modeling procedure. They relate to the quality of prediction patterns, their associated uncertainty of class membership, their sensitivity to redundancy and to congruity within the database. A spatial database developed for advanced training is used to generate target maps. It comes from a study in the Red Lake area in northern Ontario, Canada. It contains information on 37 gold vein deposits. Their neighborhood distribution is instrumental to establishing spatial relationships with the units of categorical thematic maps and continuous field maps. The experimental results point at the extraction of significant properties of the spatial data that cannot be ignored but that we have yet to master to substantiate the reliability of prediction patterns.

Combining a robust PCA of logratio transformed data and geostatistical sequential Gaussian simulation approach for geochemical characterization of orogenic gold deposits: a case study from the Alut area, NW of Iran

Robust principal component analysis (PCA) has been proved to be an efficient multivariate statistical method for extraction of a multivariate structure of geochemical data containing outlier values. Furthermore, because of the compositional characteristics of geochemical data, logratio transformation approaches also have to be implemented to transform the data from the simplex space into real space. In order to model the extracted principal component of transformed data, we used sequential Gaussian simulation (SGS) to overcome the drawbacks associated with the traditional kriging method, e.g. bias conditionally relevant to underestimation and overestimation, the smoothing effect and so on. The results of implementing robust PCA on both untransformed and logratio transformed data, which have been acquired from an orogenic gold deposit, indicate that robust PCA analysis of untransformed data is not capable of certifying element association of gold occurrences. However, in robust PCA analysis of transformed data, both varieties of gold mineralization, associated with pyrite mineral and quartz veins, have been revealed. The first variety of gold occurrence is detected in an integrated PC obtained from PC1, PC2 and PC3, which are associated with Pb, As, Hg, Mo, S and Sb elements in quartz–sulphide veins. The second one, PC4, could be correlated with quartz veins. Furthermore, the results of the geostatistical SGS approach are compatible with the detected anomaly in the Alut gold deposit.

The Occurrences of Gold and Its Dissolution Method of Geochemical Mapping Sample of Swamp-Forest Area, China

The Au value of stream sediments in swamp-forest areas in Northeast China is lower than in other areas, according to results from the Regional Geochemistry–National Reconnaissance Program. Two dissolution methods for measurement of Au levels were compared in this study. The Au values in samples dissolved by using four acids (HClO4 + HF + HNO3 + HCl) are larger than those dissolved by using aqua regia, especially when the Au content is greater than 1.25 ng/g. We found that a large amount of Au may be absorbed to or/and included in SiO2 colloids, apart from that occurring in organically bound forms in organic-rich sediments of swamp-forest areas. The Au absorbed to or/and included in SiO2 colloids is related to plant metabolism. This Au can migrate for long distances protected by high-concentration organic acids in water solution, but cannot be dissolved by aqua regia. The results of a pilot survey with an ultra-low sampling density in the Jiayin area, Heilongjiang Province, show that the Au anomalies obtained by using four-acid dissolution are not only larger in scale but also more coincident with corresponding metallogenic belts than those using aqua regia dissolution.

Study on clues for gold prospecting in the Maizijing-Shulonggou area, Ningxia Hui autonomous region, China, using ALI, ASTER and WorldView-2 imagery

Several gold occurrences have been discovered in the Maizijing-Shulonggou area, Ningxia Hui autonomous region, China. The clues for gold prospecting in this area may relate to fractures and hydrothermal alterations, but it is very difficult to conduct a field investigation on the clues because of the alpine valleys. To explore the clues for gold prospecting, this study extracted geological information related to fracture zones and hydrothermal alterations using ALI, ASTER, and WorldView-2 data, and subsequently, explored potential relationships between extracted geological characteristics and gold mineralization. The WorldView-2 image in the study area was used for fracture interpretation. The azimuth and density of linear fractures were investigated, and the results showed that linear fractures with NE direction were most common, the gold occurrences except Au2 were all located in NE fracture zones, and all gold occurrences located in high fracture density areas (level 1–3 areas) where NE and NW fractures were the most common observations. This suggests that the high fracture density and NE and NW fractures are important indicators of potential gold mineralization. Hydrothermal alteration minerals in exposed bedrocks were mapped using ALI and ASTER imagery. Based on the spectral analysis, ASTER image in the study area were used to map quartz, illite and chlorite, while ALI image were used to map limonite. The spatial distribution of alteration minerals revealed regional specific alteration mineral in gold mineralization. In the northern Maizijing region, the composition of alteration mineral in the surrounding of Au1 was overwhelmed by quartz, while the alteration minerals in the surrounding of Au2-4 were dominated by limonite in the southern Shulonggou region, which showed different indicator minerals for gold prospecting in the two regions. As a result, the key areas for prospecting are the NE and NW fracture zones with quartz veins in the Maizijing region and the NE and NW fracture zones with limonitization in the Shulonggou region. In the Shulonggou region, pyrite in buried quartz-pyritization belt and illitization-pyritization belt is likely to be the precursor of limonite. Heavy mineral assemblages were analyzed for native gold content, and the results suggested high native gold content in the illitization-pyritization belt, followed by quartz-pyritization belt, which was consistent with the strong association of limonite and gold mineralization, thus the two belts shall be prioritized for gold prospecting in the Shulonggou region.

Controls on the Distribution of Invisible and Visible Gold in the Orogenic Gold Deposits of the Yangshan Gold Belt, West Qinling Orogen, China

Six orogenic gold deposits constitute the Yangshan gold belt in the West Qinling Orogen. Gold is mostly invisible in solid solution or in the sulfide lattice, with minor visible gold associated with stibnite and in quartz-calcite veins. Detailed textural and trace-element analysis of sulfides in terms of a newly-erected paragenetic sequence for these deposits, together with previously published data, demonstrate that early magmatic-hydrothermal pyrite in granitic dike host-rocks has much higher Au contents than diagenetic pyrite in metasedimentary host rocks, but lower contents of As, Au, and Cu than ore-stage pyrite. Combined with sulfur isotope data, replacement textures in the gold ores indicate that the auriferous ore-fluids post-dated the granitic dikes and were not magmatic-hydrothermal in origin. There is a strong correlation between the relative activities of S and As and their total abundances in the ore fluid and the siting of gold in the Yangshan gold ores. Mass balance calculations indicate that there is no necessity to invoke remobilization processes to explain the occurrence of gold in the ores. The only exception is the Py1-2 replacement of Py1m, where fluid-mediated coupled dissolution-reprecipitation reactions may have occurred to exchange Au between the two pyrite phases.

Mapping hydrothermal alteration zones and lineaments associated with orogenic gold mineralization using ASTER data: A case study from the Sanandaj-Sirjan Zone, Iran

The Sanandaj-Sirjan Zone (SSZ) is considered as an important region for gold exploration in the western sector of Iran. Its mountainous topography and unpaved routes make its study challenging for researchers and raise the costs for mining companies strating new exploration plans. Gold mineralization mainly occurs as irregular to lenticular sulfide-bearing quartz veins along shear zones in deformed mafic to intermediate metavolcanic and metasedimentary rocks. In this investigation, ASTER data are used for mapping hydrothermal alteration minerals and to better discriminate geological structural features associated with orogenic gold occurrences in the area. Image transformation techniques such as specialized band ratioing and Principal Component Analysis are used to delineate lithological units and alteration minerals. Supervised classification techniques, namely Spectral Angle Mapper (SAM) and Spectral Information Divergence (SID) are applied to detect subtle differences between indicator alteration minerals associated with ground-truth gold locations in the area. The directional filtering technique is applied to help in tracing along the strike the different linear structures. Results demonstrate that the integration of image transformation techniques and supervised classification of ASTER data with fieldwork and geochemical exploration studies has a great efficiency in targeting new prospects of gold mineralization in the SSZ. The approach used in this research provides a fast, cost-efficient means to start a comprehensive geological and geochemical exploration programs in the study area and elsewhere in similar regions.

Imaging Regional Geology and Au – Sulphide mineralization over Dhanjori greenstone belt: Implications from 3-D Inversion of Audio Magnetotelluric data and Petrophysical Characterization

The 2.1-Ga-old Paleoproterozoic Dhanjori Group of rocks covers 800 sq. km area within the Singhbhum Crustal province of eastern India, and is composed of siliciclastic metasedimentary sequence interlayered with mafic to ultramafic rocks. The meta-volcanosedimentary sequence is deformed and is of low-grade greenschist metamorphic facies. The gold occurrences within the study area are confined to the sulphide hosted mafic/ultramafic volcanic rocks and also in the basal quartz pebble conglomerates (QPC) within the Dhanjori Formation. Gold mineralization within the mafic/ultramafic meta-volcanics formations occurs within the quartz ± carbonate vein i.e. lode gold and as detrital gold within the QPC (0.5−7 ppm) of Phuljhari Formations. Regional Audio Magnetotelluric (AMT) measurements across the geological domains of Dhanjori Group and Iron Ore Group (IOG) were carried out in the frequency range of 10 kHz–10 Hz. This work discusses the conductivity model from 3-D inversion along with geology and petrophysical properties in mapping the regional geological features, associated sulphide mineralization and influence of texture, grain and bulk properties on mineralization. Dimensionality and strike analysis of the AMT data indicate complex conductivity structures. 3-D inversion of off-diagonal component of impedance was performed for obtaining an optimum value of uniform half space initial resistivity and error floor for off-diagonal component of impedance data. These were used to perform 3-D inversion of full impedance to produce a conductivity model of the study area. The conductivity model delineated major geological boundaries and the sulphide bearing mineralized zone. A major conductor is observed which extends to a depth of ∼250 m within meta-volcanics. On comparison with the available borehole adjacent to this, it is confirmed that the conductor corresponds to sulphide mineralization. The resistivity low at a depth of 150 m coincides with metabasalts (pyrite present which extends to 159 m with arsenopyrite). Another mineralized zone is observed at the boundary of iron ore group volcanics and Proterozoic-gabbro-anorthosite-ultramafics (PGAU), it starts appearing from a depth of ∼400 m and can be seen which extends to a depth of ∼800 m. It is worth mentioning that there lies Kundarkocha gold mine in the same geological domain. In addition, near surface conductor to a maximum depth of ∼50 m is observed all along the AMT profile. The sediments and fluids could account for this conducting feature. This is in agreement with available Electrical Resistivity Tomography section. The borehole within Dhanjori Volcanics also detected sulphide mineralization for ∼50 m. Thus, it can be inferred that the near surface conductor indicates the presence of gold in association with other sulphide minerals. The petrophysical studies helped to estimate the concentration of minerals within the rocks. High density and low susceptibilities in the area is an indicative of low-grade metamorphism and low level of serpentenization. Combined density and susceptibility scatter diagram and component proportion phase diagram suggests 20 vol%, 78 vol% and 2 vol% of haematite/sulphides, barren rocks and magnetite respectively. The high value of chargeability and negligible value of susceptibility indicates the presence of disseminated sulphides rather than haematites and/or massive sulphides in the rocks. Ternary plot suggest that the mineralization is controlled by texture and grain properties. Petrophysical analysis suggests that the lithotypes of the study area are similar to those of greenstone terrain in the Abitibi Subprovince. Integrating inverted section, geology and petrophysical properties has a substantial impact on the study area in terms of predicting the mineralization prospect and at the same time substantially reduces the drilling cost. Thus, the study provides a new orientation and advancement to the exploration strategy for deeper deposits.

New orogenic type gold occurrences in the Uyanga ore knot (Central Mongolia)

The Khangay-Khentey belt is located in central Mongolia (Central Asian Orogenic Belt). The Uyanga ore knot district of the Khangay metallogenic zone are hosted by the lower-middle Devonian volcanogenic-sedimentaryErdenetsogtformation. The new Burgetei, Ult and Senjit gold occurrences were studied. The rocks of theErdenetsogtformation have an irregular gold content: 0.96 g/t Au is determined in quartz vein (BG-7/16), Au content is highest up to 3.5 g/t in the quartzite-jasper (Ult-7/16 and Ult-9/16) cut by quartz veins in the Ult occurrence. The Senjit occurrence represents Au-Hg-Sb epizonal level of orogenic gold deposits structure with highest Hgand Sb content up to 8.5 ppm and 39 ppm respectively. The Au content of arsenic pyrite of the Burgetei and Ult is below the detection limit by electron microprobe analysis. The Au content of arsenopyrite of the Ult occurrence is highest (up to 238 ppm). The ore-mineral assemblages in the new gold occurrences reflect the differences between three explored sites, formed in the course of fluid evolution during the water-rock interaction. Variable concentrations of indicative elements (As, Te, Sb, Hg) and their ratios confirm this fact.