Placer Gold Grains Research Articles

Internal Structures of Placer Gold Grains As Prospecting Indicators of Primary Gold Sources in the Northeast of the Siberian Platform

Internal Structures of Placer Gold Grains As Prospecting Indicators of Primary Gold Sources in the Northeast of the Siberian Platform

Characteristics, provenance, ore genesis and exploration of the Shimshal Valley placer gold deposits in the Karakoram Block, North Pakistan

The Karakoram Block in North Pakistan formed during the late Permian-Triassic on the southern margin of Asian Plate with a topography characterized by glaciated high mountains. Several rivers and streams draining from these mountain peaks carry detritus with precious and base metals. However, the primary source deposits and their ore genesis remain unknown due to the rugged glaciated topography and inaccessibility for prospecting and in situ sampling. Here, we present a multi-parametric analytical approach including major and trace element characteristics, thermoelectric analysis, EPMA elemental mapping, XRD analysis, and sulfur and lead isotopic studies on the detrital auriferous pyrite from Shimshal Valley placer gold deposits in the Karakoram Block of the north Pakistan to gain insights into the ore genesis, provenance and current erosion or denudation rate of the source primary deposits found in the hinterlands of the Shimshal Valley. The pyrite with varying forms such as {100} + {111} + {210} followed by {210} + {111} and {100} + {210}, which are highly enriched in gold (up to 1700 ppm) as revealed through EPMA analysis. The low to moderate temperature estimated for their formation (100 to 360 °C), pyrite geochemical characteristics, geochemical discrimination diagrams (Fe/S-As, Fe + S-As, As-Co-Ni) and elements contents (Au, Ag, Cu) of the placer gold grains, as well as the high (58.2 %) and low (41.8 %) thermoelectric coefficients of N-type and P-type, suggest that the auriferous pyrite belong to the epithermal type of magmatic-hydrothermal systems. The XnP value (average −48.7 %), γ value (average 62.8 %) in conjunction with the n(S)/n(Fe) values of 1.97–2.11 of pyrite indicate that the placer gold deposits are derived from the middle-upper part of the ore body. The high Mo/Ni (average 5.4) and Co/Ni (average 2.7) ratio combined with detrital zircon UPb geochronology, zircon fertility indicators such as Eu/Eu*, Ce/Nd, (Ce/Nd)/Y, Dy/Yb, (Eu/Eu*)/Y, texture and REE patterns correlate the ore mineralization with the Hunza Plutonic Unit emplaced on the catchment of the valley. The narrow variations in δ34S values, ranging from −5.3 to 3.8 ‰ with an average of −4.7 ‰, suggest that sulfur has a homogeneous magmatic source. The Pb isotopes values range 18.487 to 18.692, 15.713 to 15.745, and 38.964 to 39.416 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively. Our results suggest that metals for ore-forming fluid involved considerable contribution of crustal-derived materials.

Microchemical signatures of placer gold grains from the Gamba district, northern Cameroon: Implications for possible bedrock sources

The Central African Fold Belt (CAFB) in Cameroon hosts numerous alluvial gold deposits derived from known deposit types in the eastern and southern part of the country. In contrast, in northern Cameroon, primary source of gold remains enigmatic. Based on the morphological and geochemical signature of alluvial gold grains from northern Cameroon, this study discusses the distance-to-source and the style of gold mineralization of the Gamba district. Three hundred and fifty gold grains collected from the four locations were primarily analyzed by scanning electron microscopy (SEM) to investigate their morphological characteristics, and subsequently the composition was examined through the SEM and electron microprobe analysis (EMPA). The gold grains from the four locations have irregular to rounded morphology, with a microtextures indicating a proximal source at less than 5 km. Most of the gold grains display evidence for mechanical hammering and abrasion. EMPA results show mainly Au-Ag alloys, dominated by high Ag values in the cores from different location. Thus, Djaba is characterized by a range of 1.1–22.1 wt% Ag with 77.2–99.3 wt% Au; Bigbal includes a range of 10.3–32.9 wt% Ag with 66.1–88.7 wt% Au; Bougma is distinguished by a range of 7.3–18.3 wt% Ag with 80.8–91.9 wt% Au; and Mayo Salah is characterized by a range of 2.8–22.8 wt% Ag with 77.3–96.8 wt% Au. These range in compositions suggest multiple deposits of the same type in different locations. Gold grains from the four locations show the Te (0.1–0.3 wt%) content, Bi (0.1–0.7 wt%) content, and Cu (0–0.05 wt%) content, suggesting a source of orogenic gold mineralization. Most of the gold grains are Ag-depleted from both the cores and rims and show lack of Au-rich rims indicate no evidence of supergene precipitation, which suggests that the grains were transferred directly from hypogene source to surficial and fluvial system. EMPA data on the gold grains, plotted in the ternary diagram of Au-Ag-Cu, suggest that gold from the Gamba district is hydrothermal-orogenic in origin. Mineral inclusions include silicates (zircon, titanite, quartz, muscovites, biotite, actinolite, titanite, albite and almandine), oxides (magnetite, ilmenite and hematite), and phosphates (apatites), which suggest the magmatic and metamorphic conditions of formation for the hypogene source. This indicates the granitoids and metasediment of schists which bear multiple quartz veins as the potential bedrock sources. The microchemical signature of placer gold used in this study is a useful tool to explore the hypogene environment of an unknown primary source that can greatly help in mineral exploration in northern Cameroon.

Typomorphic features of placer gold from the Bystrinsky ore field with Fe-Cu-Au skarn and Mo-Cu-Au porphyry mineralization (Eastern Transbaikalia, Russia)

Eastern Transbaikalia is one of the oldest gold-producing regions in Siberia, and for over 300 years it has been the largest source of the most important raw minerals in Russia. A large number of gold, gold-bearing complex, antimony, mercury and other mineral deposits are known within this region. The orogenic, intrusion-related, and epithermal deposits are important in the balance of gold reserves and gold production. Au-Cu-Fe-skarn deposits are located mainly in the northeastern and southeastern parts of the area, wherein the skarn Kultuma, Lugokan, and skarn- porphyry Bystrinsky deposits are the largest deposits.Herein, we studied the the Bystraya River gold placers and tributaries, which are located to the east of the Bystrinsky massif near the Bystrinsky deposit. In this study, a microchemical characterization technique was applied to gold grains to facilitate the classification of alluvial gold localities in terms of the style or styles of source mineralization, thereby permitting a comprehensive interpretation of regional gold mineralization. In total, 55 samples were collected from four placers and processed for more than 1500 analyses of the native gold.In the alluvial placers of the Bystrinsky deposit areas, weakly rounded native gold either prevails or is significantly present. A low portion of supergenically transformed of gold grains in the placers indicates that the placers formed directly from endogenous gold mineralization. The native gold from the Bystraya River catchment can be divided into 3 types and 1 subtype: (1) Cu-bearing gold grains with a fineness of 800 − 995‰, and a copper impurity of up to 0.73%; (2) Hg-bearing gold grains with a fineness of 800 − 995‰, and an impurity of Hg up to 4.68%; (2.1) Hg-poor gold grains with a fineness of 800 − 995‰, containing the same mineral microinclusions assemblages as the Type 2 gold grains; and (3) gold with a fineness of 400 − 770‰, containing up to 8.5% Hg.The distinguished types of gold grains are manifested to varying degrees at the Bystraya, Left, and Right placers. At at the Yakovlevsky placer, there is no Hg-bearing low fineness and poorly occurring Cu-bearing native gold. Comparing of the obtained data with gold grain composition data from the hypogene deposits allows us to infer bedrock sources. The Cu-bearing high-fineness type of placer gold can be compared with the Cu-bearing native gold of the Bystrinsky, Lugokan, and Kultuma deposits. Hg-bearing low-fineness placer gold grains related to erosion of the Au-bearing base metal mineralization occurred at the Novoshirokinsky and Kultuma deposits. However, Hg-bearing high-fineness gold grains, which most widely occurring in the studied placers, are absent in the in-situ mineralization. Therefore, we suppose that the bedrock source of this gold grain type is linked to the completely eroded upper levels of the Bystrinsky deposit. Thus, our study shows that it is possible to speculate on the nature of eroded material from the gold particle signature. Hense, this generic approach informs regional metallogeny in a way that even detailed field-based classical geology/geochemistry cannot.

Native gold from the Kamenka-Barabanovsky and Kharuzovka alluvial placers (Northwest Salair Ridge, Western Siberia, Russia): Typomorphic features and possible bedrock sources

Northwest Salair Ridge, is one of the oldest alluvial gold mining regions; it is located in the western part of the Altai-Sayan folded area. The placers localized along the Kharuzovka and Kamenka-Barabanovsky creeks (the right-branch of the Suenga River) were chosen for investigation. The only bedrock deposit in the study area is the Novolushnikovsky gold deposit located in the Suenginsky placer field of the Egoryevsky district. The source of gold supply for the Kharuzovka and Kamenka-Barabanovsky placers spatially related to the hypothetical continuation of the type II’ ore zones of the Novolushnikovsky deposit. The composition of gold grains derived from placers was determined based on a representative collection. A distinctive feature of native gold from type II’ ores is unusually high content of Hg, up to 20 wt%. The composition of gold grains from studied placers shows that the average Hg content is 1.5–5 wt%. However, there is a group of Hg-rich placers (<1% of the collection), where the mercury content is as high as 16 wt%. We suggest that possible alteration of the gold composition manifested both in the rims and in the cores is represented by significant loss of Hg, while the other analyzed components underwent relative enrichment. The first time reported demercurization processes occurring under the supergenic conditions determine this alteration. The erosion of Novolushnikovsky deposit type II’ ore zones are the source of supply of Hg-rich gold in the Kharuzovka and Kamenka-Barabanovsky placers. These placers were analyzed for the existence of mineral inclusions in gold grains. The presence of placer gold grains with mineral inclusions, that partially or fully correspond to mineral association of ores of the Novolushnikovsky deposit, as well as an unusual Hg-content of the native gold suggest several sources of gold with a similar mineralization style. The undiscovered sources of gold are might be also of intrusion related type.

Microchemical characterization of placer gold grains from the Meyos-Essabikoula area, Ntem complex, southern Cameroon

Gold occurs as a native metal, usually containing silver, and in some cases mercury, copper, and palladium. It may also occur as inclusions within sulfur-rich minerals, such as pyrite and arsenopyrite. The style and variety of gold mineralization is influenced by the geological setting, chemistry of the ore fluids, and the nature of their interactions with rocks. Gold grains liberated from bedrock into surficial sediments during weathering and erosion are chemically stable and may be characterized according to their mineralogy: i.e the alloy composition and suite of mineral inclusions revealed within polished sections, characteristics faithful to gold from the hypogene source. This approach has been applied to placer gold grains from the Meyos-Essabikoula area, Cameroon, where the source of gold is not yet confirmed due to poor outcrop exposure.A total of 221 alluvial gold grains from 10 sites, tributaries of Sing and Bivele River over the Ntem Complex have been studied using Electron Probe Micro-Analysis (EMPA) to determine the concentration of minor alloying metals, (notably Au, Ag, Cu, and Hg) and Scanning Electron Microscopy (SEM) in order to evaluate the assemblage of mineral inclusions within the gold. Most of the grains are sub-rounded with pitted surfaces and inclusions of pyrrhotite, acanthite, and chalcopyrite were observed. The grains are AuAg alloys ranging from 54.4 to 99.8 wt% Au, 0.1–48.4 wt% Ag, 0.1–0.8 wt% Hg and 0–0.3 wt% Cu. The presence of Fe oxide (magnetite) inclusions containing Cr and V (to around 5 wt %) has not been reported elsewhere and suggests a strong interaction between hot reducing ore fluids and local mafic lithologies.

Secondary gold structures: Relics of past biogeochemical transformations and implications for colloidal gold dispersion in subtropical environments

Biogeochemical processes are known to drive the cycling of gold via dissolution/re-precipitation reactions that result in the transformation of gold grain in near-surface environments. In this study, placer gold grains were collected from West Coast Creek, Queensland, Australia and characterized using high-resolution electron microscopy. The outer surface of grains contains 98.1mol% Au and 1.9mol% Ag. Crevices occur on the surface of grains and are filled with organics and clay minerals in which nanometer-size gold colloids and micrometer-sized octahedral gold platelets are embedded. The formation of these secondary gold structures is attributed to gold precipitation by the availability of reducing agents such as microbiota, residual organics and clays. Bacterioform gold contains 74.7mol% Au and 25.3mol% Ag and is also embedded in clay minerals within crevices. From sonicated gold grains, grain surfaces beneath the clay minerals are striated and contain 68.5mol% Au and 31.5mol% Ag. This data suggests that dissolution processes occur at the gold grain interface. Varying sizes of gold colloids on the grains suggest that five ‘episodes’ of gold dissolution/re-precipitation processes occurred; each episode was estimated to be 7.64±4.1years. Therefore, these grains represent 17.9–58.5years of gold cycling and mobilization within this subtropical environment. Furthermore, laboratory experiments involving colloidal gold dispersion demonstrated that iron-oxides and organic material from West Coast Creek sediment adsorbed 94.5% of suspended gold colloids. In conclusion, this study highlights the value of nanophase gold characterization for the interpretation of biogeochemical processes affecting gold grain transformation and mobility in near-surface environments. Importantly, this study is the first to estimate the kinetics of biogeochemical gold cycling with regards to colloidal gold dispersion and re-concentration.

Gold and platinum group minerals in placers of the South Urals: Composition, microinclusions of ore minerals and primary sources

Gold and platinum group minerals from the gold placers of the South Urals are studied in order to identify the metal sources. In placers from the Main Uralian fault zone (MUF), the primary gold contains Ag (up to 29wt.%), Cu (up to 2wt.%) and Hg (up to 4wt.%) and its fineness ranges from 538 to 997‰. Tetra-auricupride and cupriferous gold (up to 20wt.% Cu) are common for the Nizhny Karabash placer of the MUF zone. In the eastern part of the South Urals, the placer gold is mainly characterized by high fineness of 900–1000‰ and low Cu contents (max 1.38wt.%). Most of the placer gold grains consist of the primary domains, which are rimmed by secondary high-fineness gold with diffuse and clear boundaries. The secondary gold also develops along the shear dislocations of primary gold. Gold contains microinclusions of geerite, balkanite, chalcopyrite, Se-bearing galena, sphalerite, pyrite, pyrrhotite, arsenopyrite and hematite.Twenty four (including five unnamed) platinum group minerals (PGMs) were found in 28 placers; those from the Kialim and Maly Iremel placers of the Miass placer zone were studied in details. In the Kialim placer, ruthenium is most abundant PGM, which hosts microinclusions of isoferroplatinum, ferroan platinum, laurite, cupriferous gold, a mineral similar in composition to tolovkite, heazlewoodite and unnamed RhSbS phase. The osmium contains microinclusions of erlichmanite and laurite. The iridium grains hosts various sulfides and arsenides of platinum group elements (PGEs). The inclusion-free PGMs form Ru compositional trend in contrast to Os–Ru trend of the Ir-depleted inclusion-hosted PGMs. The isoferroplatinum from the Maly Iremel placer hosts laurite, rhodarsenite, bowieite, a mineral similar in composition to miassite and unnamed sulfide of Pt (Pt1.11S2.00) and antimonide of Pd ((Pd2.41Rh0.43Fe0.17)3.01(Sb0.91Te0.09)1.00). Ruthenium is a host to isoferroplatinum, PGE sulfides and arsenides, and heazlewoodite. Osmium contains microinclusions of ferroan platinum; iridium is a host to a mineral similar in composition to hongshiite. Three types of PGM intergrowths were identified in the Maly Iremel samples: (1) the intergrowths of platy grains of ruthenium with isoferroplatinum and a mineral similar in composition to tulameenite; (2) the open-latticework intergrowths of platy crystals of ruthenium with interstitial aggregates made up of gold, isoferroplatinum and a mineral similar in composition to xingzhongite and (3) the intergrowths of osmium and irarsite and iridarsenite, which are developed along cleavage of the osmium grains. Nickel sulfides associated with some PGMs contain Ru (11.32wt.%) and Rh (2.21wt.%) in millerite and Ir (31.00wt.%), Ru (5.81wt.%) and Rh (2.87wt.%) in vaesite.The primary metal sources were determined on the basis of the mineral assemblages and composition of minerals, taking into account the nearby mineral deposits and directions of rivers. The rodingite-associated gold, gold-bearing massive sulfide and chromite deposits are major sources of gold and PGMs in placers of the Miass placer zone confined to the MUF structure of the South Urals. In the southern part of this structure, gold was mainly originated from orogenic gold–sulfide deposits associated with volcanic/volcaniclastic rocks and listvenite-associated gold deposits. The placer PGMs were derived from the adjacent ultramafic massifs of ophiolitic origin. The distance between the placers and primary deposits varies from 2 to 5km (up to 20km in the extended valley of the Miass River). Usage of ore microinclusions and associated PGMs in study of placer gold is far more advanced than an ordinary consideration of gold composition alone. This approach allowed us to identify the concrete sources for individual placers and to predict some mineralogical findings in already known primary occurrences.

Origin of placer gold and other heavy minerals from fluvial Cenozoic sediments in close proximity to Rote Fäule-related Au mineralisation in the North Sudetic Trough, SW Poland

Detailed morphological and geochemical studies of placer gold grains and other heavy minerals from Cenozoic fluvial clastic sediments in the area of the Zimnik Creek drainage basin (North Sudetic Trough, Lower Silesia) allowed the description of their specific features: shape and morphology, flatness index, internal textural features and chemical composition of the gold grains. It enables an estimation of the distance from the source area and determination of the source of the ore mineralisation, along with establishing the feeder areas for placer mineralisation including Rote Faule-related gold hosted by Rotliegend-Zechstein transitional sediments. The comparison of the heavy mineral contents of placer grains from the Zimnik Creek drainage basin makes it possible to identify the area of origin for the gold particles. The crystalline Au-Ag-Pd-Hg (polymetallic) alloy grains of fluvial gold are assumed to come from the transitional sediments between the Rotliegend and the Zechstein in the North Sudetic Basin. The morphological and geochemical features of the polymetallic grains strongly suggest that the grains from the upper course of the Zimnik Creek valley sediments represent what has eroded from the local gold mineralisation and this mineralisation is probably in the vicinity of the upstream sampling site. The bimodal chemical composition of the Au-Ag-Pd-Hg alloy grains with electrum and medium-Ag grains (<15 wt.% Ag) found in the study area is typical of the Rote Faule and Kupferschiefer-related gold mineralisation on the southern side of the North Sudetic Trough and the Sieroszowice-Polkowice copper mining district of the Fore-Sudetic Monocline. The study of the Ag contents of placer grains from the Zimnik Creek drainage basin highlights the contrast between the placer gold grains derived from Paleozoic orogenic gold mineralisation in the Sudetes (typically simple Au-Ag alloys containing between 5 and 20 wt.% of Ag) and the one formed by the oxidizing chloride hydrothermal systems. The grains identified in this manner as originating from the oxidizing hydrothermal system of the transitional zone can be distinguished from the placer gold derived from other styles of mineralisation, which are not Rote Faule-related. It is particularly important considering the ongoing reconnaissance exploration in the North Sudetic Trough (SW Poland), which employs gold grain analyses as a prospecting tool for the detection of potentially economic primary gold mineralisation.

Нанотекстуры поверхности россыпного золота

The methods of high-resolution electron microscopy were applied to study nanotextures on the surface of placer gold grains (the Urals, Siberia, Altai in Russia, Yukon Territory in Canada, etc.). Among superficial nanotextures, the cracks, furrows, pores, films, inclusions, monolayers, complicated aggregates with gold of relevant size were described. It is supposed that the nanotextures on gold surface partly depend on internal structure of metal and afterwards reflect diverse influences of supergenic factors. Some of them (nanocracks, nanopores) are connected with defects of crystal lattice. A presence of nanolayers reflects instability of gold grain growth, as well as nanoinclu-sions point to its genesis. The secondary products of weathered rocks such as individ-ual nanoparticles, their aggregates, nanofilms of Fe, Al, Si, and other compounds, coverings, and growths of “new” gold are discovered on a gold surface. There are the assumptions that gold surface nanotextures may promote remarkable changes in chemical, physical and mechanical properties of the metal, especially in the flotation activity, settling velocity and absorption capacity.

Characterization of Gold Mineralization in the Northern Cariboo Gold District, British Columbia, Canada, Through Integration of Compositional Studies of Lode and Detrital Gold with Historical Placer Production: A Template for Evaluation of Orogenic Gold Districts

There is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold, although, in many cases, the nature of the source mineralization for these placer deposits remains unclear. This study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy, followed by synthesis of this information with records of both mineral occurrences and historical placer mining. The northern Cariboo Gold District in east-central British Columbia, Canada, was chosen as a location for the study because of the gold endowment of the area (1.2 million ounces [Moz] of lode gold and between 0.5 and 3 Moz. of placer gold) and the information available from both placer and lode gold mining. Compositional analysis of 533 gold grains from 21 lode localities and 1,914 gold grains from 30 placer localities from throughout the Cariboo Gold District has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section. A distinctive low (4–7%)-Ag gold that exhibits a strong Bi association in the mineral inclusion suite is geographically limited to the Wells area, where it is recorded in both lode mineralization and its placer expression. Regionally pervasive mineralization yields gold of binary Au-Ag alloy and a simple inclusion suite of sulfides and sulfarsenides. Gold in most large placers in trunk drainages was derived from multiple (mostly small) occurrences of this type. The nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities, while others are dominated by gold deposited in a single stage. The new information from gold grain analysis has been considered in the context of other information. Classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source. Mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types. The compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness (Au-Ag ratio) to reconstruct the size and distribution of hypogene sources prior to erosion. Synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources.

Structural and Chemical Characterization of Placer Gold Grains: Implications for Bacterial Contributions to Grain Formation

Gold grains collected from the Rio Saldaña River, Colombia were hundreds of micrometers in size and discoid-ellipse in shape. Fourteen of 63 grains contained an iron oxyhydroxide coating that occurred as ca. 50 to 100 nm thick lamina while thicker coatings were comprised of colloids 200 nm to 4 μm in diameter. Bacterial-size casts were observed throughout the thicker iron oxyhydroxide coating and intuitively represent relic impressions of bacterial cells. The surface textures of gold grains were generally smooth with surficial depressions or crevices containing detrital material colonized by bacteria. Focus Ion Beam (FIB) milled cross-sections demonstrated that the detrital material contained nanophase gold particles. Biofilm attached to this detrital material contained ca. 2 to 3 nm colloidal gold attached to exopolymeric substances. Cross sections of grains revealed solid cores with vesicular voids near the grain edge including a bacterial-size cast interpreted to be a permineralized bacterial cell. Synchrotron-based elemental mapping indicated that grains contained heterogenously distributed Ag and Cu. While strong Ag and Cu signals (relative to Au) were detected in the core, a stronger Au signal occurred at the edge of grains demonstrating enriched rims of secondary gold. The preservation of bacterial casts and biofilms associated with secondary gold structures at the surface of grains suggest that bacteria may contribute to gold enrichment and growth in this placer environment. Bacteria, occurring on the surface of 13 of 25 gold grains, were enriched by “inoculating” individual grains into separate test tubes containing R2B growth medium. Enriched growth of bacteria on gold grain surfaces demonstrated preferential attachment onto detrital material within creviced regions. The dominant bacteria from these enrichments were transferred to solid R2A medium to obtain pure isolates. The isolates were identified as one of four bacterial species: Nitrobacter sp. 263, Shewanella sp. YM-8, Sediminibacterium sp. B2-10-2 and sp. I-32 based on 16S ribosomal DNA sequencing.

Placer scheelite and gold from alluvial sediments as indicators of primary mineralization – examples from SW Poland

Detailed morphologic studies of detrital scheelite and gold grains from clastic Cenozoic sediments (alluvial and eluvial-colluvial) in two selected areas of the Lower Silesia region allowed to describe their specific features (size, roundness, lustre, and lack or remnants of primary intergrowths with other minerals). It allows estimating length of hydrodynamic transport and distance from the source area. Such features are also an important tool in determining the source of ore mineralization and to establish the parent area for detrital accumulations. Moreover, electron microprobe (EMPA) investigation of detrital gold allowed to recognized internal structure and chemical composition of placer gold grains. The grains commonly revealed zonal structures due to chemical supergene gold mobility recognized nearly in all studied pan-concentrate samples.

The significance of copper concentrations in natural gold alloy for reconnaissance exploration and understanding gold-depositing hydrothermal systems

Analytical studies of placer (detrital) gold grains, intended to track bedrock sources and styles of mineralization, mainly consider alloy Ag content and inclusion mineralogy to generate ‘microchemical signatures’. Occasionally Hg and Pd help discriminate gold from different sources; Cu infrequently because electron microprobes seldom detect the low concentrations. Reporting of Cu has been largely confined to gold from porphyry–epithermal environments, and this has biased subsequent interpretations of observed Cu-bearing gold alloys. This study focuses on placer gold in the Mourne Mountains, Northern Ireland, where auriferous bedrock remains undiscovered and the complex geology is consistent with either orogenic or intrusion-related mineralization. Over 500 gold grains analysed average 0.17% Cu ranging to &gt;1% Cu and show a wide variation in Ag content. Inclusion mineralogy mainly matches that of orogenic gold elsewhere in the host terrane; however, some signatures are suggestive of zonation within an orogenic hydrothermal mineralizing system, or possibly metamorphic-hydrothermal remobilized gold associated with subsequent intrusive activity. One locality in the study area has placer gold of a distinctive Cu-rich alloy composition containing Cu sulphide inclusions, an association noted elsewhere in gold derived from alkali Cu-Au porphyries. Consideration of Cu-bearing gold alloys worldwide indicates that previously proposed compositional correlations with deposit type are of limited value. We show that Cu contents to at least 0.8% are permissible within orogenic gold – the first time that such compositions have been clearly ascribed to orogenic mineralization. The result is particularly important considering on-going exploration in northern Canada which employs gold grain analysis to help define exploration targets.

Prospection for gold and new occurrences of gold-bearing mineralization in the eastern Mongolian Altay

Geological mapping and regional geochemical prospection on a scale of 1:50,000, covered an area of about 4,000 km2 in the Zamtyn Nuruu region of the eastern Mongolian Altay. Northern part of the region is formed by the Lake Zone Terrane, the southern by the Gobi Altay Terrane; these two units are separated by the Bogd fault. The panned-concentrate heavy mineral survey was combined with the dry stream-sediment geochemistry, both being complemented by lithogeochemical sampling. The dry stream-sediment geochemistry proved to be a suitable method for regional prospection in semiarid to arid areas of Mongolia. The morphology and composition of placer gold grains were studied in order to estimate the distance from the primary source, and to constrain the nature of prospective primary mineral deposits. Based on the results of the regional geochemical survey and geological mapping, follow-up works were focused on prospective areas including the reconnaissance field works, detailed geochemical sampling, and geophysical measurements. New occurrences and signs of Au, Cu, Zn and (Ba)-As-Sb-Hg mineralizations were discovered. The ascertained goldbearing mineralizations are represented namely by Au and Au-(Ag) quartz veins in basic metavolcanic rocks of the Neoproterozoic Khan Taishir and volcanic rocks of the Permian Delger Khangay formations. Gold locally accompanies vein–disseminated base metal mineralization in some Proterozoic and Lower Palaeozoic formations. A prospective occurrence of clastic gold in Mesozoic conglomerates and sandstones was found at the northern edge of the Zamtyn Nuruu around the Samoandamba Uul. The geochemical survey has revealed a contrast in occurrence of ore indices between the two terranes formig the studied area. While the Lake Terrane and, especially, the Neoproterozoic Khan Taishir Formation are rich in gold and base metal showings, the Gobi Altay Terrane is barren.

Compositional variations and morphological evolution in platinum beach placers, southern New Zealand

Morphologies of placer platinum group minerals (PGM) are more variable and resistant to modification during transport than placer gold grains. This study documents morphological evolution of PGM placer grains during up to 120 km of transport in beach placers after river transport from inferred sources up to 200 km inland. PGM morphological changes are calibrated with changes in morphology of associated placer gold. Most of the PGM are Pt-Fe alloy and have been fed into the beach placer system from a large river at the western end of the beaches on the south coast of New Zealand. The incoming fluvial PGM suite includes Os, Ir and Ru alloys which may have been derived from distal ophiolitic sources. More proximal sources have Ural-Alaskan affinities and these contributed cooperite and braggite, or sperrylite, locally, as well as Pt-Fe alloy grains. Some PGM may have been recycled through Cretaceous-Quaternary fluvial sediments before entering the modern placer system. Recycled placer PGM grains have also been derived from elevated Quaternary beaches near the coastline. PGM grains entering beach placers have rough surfaces, with remnants of crystal faces, and these evolve to smooth flakes with progressive long-shore transport. PGM flakes have slightly thickened rims caused by impacts by saltating sand on windy beaches, and the most distal beach placers contain flakes with incipient toroidal shapes. These PGM incipient toroids are poorly developed compared to accompanying well-formed toroidal gold that has developed in nearly all beach placers, including those on elevated Quaternary beaches. Typical PGM and gold placer grain size decreases with increasing distance of transport, from fluvial grain size of 400–1,000 to ∼200 microns on the most distal beaches, accompanied by eastward loss of equant PGM grains and associated increase in proportion of flakes. Although net transport distance is ∼120 km in the beach placer complex, frequent aeolian transport of grains from beach to dunes and subsequent recycling by storm surges substantially increased total transport distance in a dynamic windy tectonic environment.

Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek, Yukon Territory, Canada as deduced from microchemical characterization of placer gold grains

Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek, which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated, exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the Klondike. Gold type 3b (8–25% Ag) is the main component of the placers in lower Dominion Creek but is augmented and eventually replaced by type 3a gold (10–40% Ag) in placers in the main Indian River valley, probably through erosion of gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%. This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides, and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly elevated (0.05–0.17%) Cu in the gold alloy, together with low (5–9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au (−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor. Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration.

Microchemical Studies of Placer and Lode Gold in the Klondike District, Yukon, Canada: 2. Constraints on the Nature and Location of Regional Lode Sources

Alloy compositions and associated suites of opaque mineral inclusions have been used to define the micro-chemical signature of 3,050 placer gold grains from 34 localities and 561 gold grains from six lode localities from the northeastern, central, and southern Klondike gold district, Yukon, Canada. These data augment those from a detailed parallel study of placer-lode gold relationships in the western part of the Klondike goldfield (see Chapman et al., 2010). Gold compositions have been considered in association with chemical controls on alloy compositions to establish whether variation between signatures represents zonation within a single mineralizing event or discrete episodes of mineralization. Three main gold types have been identified each of which exhibits subtypes. Type 1 gold is generally a simple Au-Ag alloy with low Ag (12–25%) and an inclusion suite dominated by pyrite and simple base metal sulfides. Type 2 gold is generally Ag rich (18–55%) and contains Hg (≤9%). Type 3 gold (10–40% Ag ± Hg ≤ 4%) is prevalent in the south of the region. Analysis of large sample sets has allowed us to delineate the geographic extent of each gold type and their relative importance within the placer populations. Type 1 gold is the major component of placer deposits in Bonanza, Hunker, Dominion, and upper Sulphur Creeks. Type 2 gold is relatively trivial but contributes to placer gold in Bear Creek and some of the paleoplacers in White Channel Gravel in the north of the study area. Type 3 gold is actively augmenting the placer inventory in the south of the region. This study builds upon a detailed parallel study in the western Klondike which concluded that types 1 and 2 gold were emplaced by a single hydrothermal system of limited geographic extent, centered on the Lone Star Ridge. Here we propose that small, rich hydrothermal systems were also present in the upper reaches of Bear and Last Chance Creeks and Hunker Dome. If correct, this model shows that economically important orogenic gold mineralization can be formed in smaller hydrothermal systems than previously recognized. In addition, type 3 gold is widespread in the south of the study area where it appears to be present at the surface. Consideration of the abundance of the various gold types contributing to the placers has established type 1 gold to be the most important and originally present in geographically constrained vertically persistent mineralization. We propose that this type offers the best target for exploration in the northern and central Klondike. In the southern Klondike the large placers containing only type 3b gold suggest a potentially important undiscovered source in the catchment of Gold Run Creek.

Chemical and Mineralogical Signatures of Gold Formed in Oxidizing Chloride Hydrothermal Systems and their Significance within Populations of Placer Gold Grains Collected during Reconnaissance

The close association of Pd with Au has been previously reported in highly oxidized chloride hydrothermal systems from several regions, including Australia, Brazil, Poland, and the United Kingdom. The distinctive mineralization precipitated usually as a consequence of contact with reducing environments is characterized by Au alloys containing low Ag ± Pd ± Hg ± Cu, associated with selenides ± tellurides ± antimonides ± arsenides. Placer gold grains derived from this style of mineralization inherit these chemical and physical characteristics which are shown to be distinct from those placer grains derived from other styles of mineralization. The discovery of placer gold grains of this type in areas remote from known in situ mineralization initiated the present project and provided a methodology for the study of a relatively large number of occurrences. Data describing alloy chemistry and mineralogy of polished sections of 957 placer grains from 19 localities in the United Kingdom and 1 locality in the Czech Republic have augmented previously published data for 303 gold grains from 10 localities in the United Kingdom. Six generic gold alloy types have been identified based on the relative proportions of Au, Ag, Pd, and Hg. Mineralogical descriptions of similar gold alloys from localities worldwide usually conform to this classification, but slight deviations are noted where P-T conditions differ from those previously identified in Devon. Alloy types may occur either as discrete grains or as phases within heterogeneous grains. Type 1 gold (defined as >99 mass % Au), type 2 gold (Au-Pd), and type 3 gold (Au-Ag) alloys (with selenide inclusions) are the most common compositions and occur at most localities, although in different proportions. Type 4 gold is also an (Au-Ag) alloy, but contains sulfide and sulfarsenide inclusions. Gold alloys or intermetallics containing Pd-Hg and/or Pd-Sb alloys (type 5), and Cu (type 6) are found only at some localities, as is a variety of type 3 gold (± selenide inclusions), which contains Hg. The spatial arrangement of the main alloy types within heterogeneous gold grains shows a consistent precipitation order of type 1, type 2, and type 3, independent of locality. Interpretation of the chemical controls on alloy precipitation indicates that ƒO2 controls whether type 1 or type 2 gold precipitates initially. The formation of type 4 gold (Au-Ag alloys with sulfide inclusions) is ascribed to a fall in ƒO2 to around the hematite-magnetite buffer where HS− (aq) replaces Cl−(aq) as the aqueous gold complex. Types 3 and 5 gold commonly mantle earlier alloys, and show evidence for aggressive replacement of the core. This texture is interpreted as evidence for influx of a more oxidizing fluid which also accounts for the abundance of selenide and telluride mineral inclusions within this gold type. This approach provides a plausible explanation of the complex heterogeneity within and between gold grains from the localities and accounts for the common sequence of alloy precipitation, without invoking multiple episodes of hydrothermal activity to account for different alloys. However, the absence of thermodynamic data describing the Au-Cu, Au-Ag-Cu, and Au-Ag-Pd-Hg systems prevents interpretation of the significance of some alloy types. This study has demonstrated that gold mineralization associated with oxidizing strata is more widespread than currently appreciated. The methodology described here not only permits identification of gold from this style of mineralization but allows initial evaluation of the economic potential of the source through the study of the alloy types. This information may be gained at an early stage in the exploration process from gold grains collected during routine reconnaissance.

Application of microchemical characterization of placer gold grains to exploration for epithermal gold mineralization in regions of poor exposure

The style of gold mineralization (e.g., mesothermal, porphyry, intrusion related, high-sulphidation epithermal, low sulphidation epithermal) is normally established at an early stage in the gold exploration process from the study of the mineralogical and lithological examination of outcrop, but this approach is problematic in areas of poor exposure. However, placer gold is frequently present in local drainages, and may be characterized according to the alloy composition and the suite of mineral inclusions. In the temperate areas which are the subject of this study, this microchemical signature of the cores of placer gold grains reflects that of gold from the hypogene source. Thus, informed speculation on the nature of the source bedrock mineralization is possible through comparison of the microchemical signature of placer grains with the generic characteristics of gold from different styles of mineralization. A holistic approach involving consideration of the local geology and microchemical signatures of populations of placer gold grains can be used to infer the presence of epithermal gold mineralization. Epithermal gold deposits are characterized by a wide range of Ag values in the electrum grains and a corresponding complex mineralogy of the opaque inclusion suite. Having established that the placer grains are likely to be derived from epithermal mineralization, the sulphidation style may be inferred through comparison of the sulphide mineral inclusion suite with the compositional fields which define the sulphidation state of the mineralization. In addition, suites of telluride minerals can be used to constrain the possible sulphidation state of the mineralization using log fTe 2/log fS 2 diagrams. Further useful information is available from the study of non-opaque inclusions which can be used to indicate sulphidation state and the type of wall rock alteration associated with the mineralization. Two case studies have been undertaken to illustrate the value of this approach. The Eureka Creek locality in Yukon Territory, Canada lies in an unglaciated region where bedrock is covered by mainly residual overburden derived from local bedrock sources. At Borland Glen in Scotland there have been several periods of glaciation which have influenced geomorphology in particular by forming deep thicknesses of till and rare bedrock exposures. In neither case has outcropping mineralization been identified. This study describes how consideration of compositional and morphological data of placer gold together with an understanding of the geological setting has indicated that both placers are derived from low-sulphidation epithermal mineralization. The Eureka Creek locality forms part of a goldfield with a recorded production of roughly 150,000 oz. The microchemical signature of the placer gold is consistent with the signature of the low-sulphidation epithermal gold and consequently distinct from the major gold mineralization found to the north in the Klondike District. Interpretation of the opaque mineral inclusion suite using log fS 2–temperature diagrams indicates a temperature of formation near 200 °C. A similar approach has shown gold from a highly anomalous placer occurrence in Borland Glen, Ochill Hills, Scotland to be of local origin and most probably derived from an episode of low-sulphidation epithermal mineralization unique to the region. Three phases of mineralization corresponding to different mineral inclusion/gold alloy compositions have been identified, only one of which has the potential to be of economic importance. The sequence of deposition of the three phases has been established. The mineralization from which the placers were derived is hosted at least in part by propylitically altered wall rock. The areal extent of the surface expression of the most significant gold type has been investigated through identification of gold grains which exhibit a diagnostic internal texture of different phases of alloy compositions each associated with specific inclusion assemblages. This approach has proved an efficient tool in demarking a new target area within a region where placer gold derived from sources of low economic potential is also moderately abundant. This study has highlighted the potential value of characterizing the mineral inclusion suites of placer gold grains as part of a wider array of analytical approaches including determining the alloy compositions and morphological studies.