High-grade Gold Research Articles

Major, trace and REE characteristics of the high grade gold geochemical reference material from Middle Reef, Hutti gold mines, Karnataka

The geological, petrological, mineralogical and geochemical characteristics of the Hutti high-grade gold reference material representing the composite sample of sheared gold-sulphide bearing smoky-grey quartz vein hosted in felsic volcanics (Middle Reef) and hydrothermal altered wall rocks are investigated in the present work. Petrological studies on these rocks reveal the presence of deformed quartz ribbons, plagioclase, chlorite, biotite, stipnomelane, actinolite and abundant native gold, arsenopyrite, pyrite and minor scheelite, monazite and xenotime. The major, trace and REE geochemistry of the host barren rhyolites are interpreted as low Yb adakitic variety which show high Ba, Cr, V and Ni concentrations. REE patterns of the gold-sulphide bearing quartz reef is characterized by a pronounced positive Eu anomaly, LREE depletion and nearly flat HREE with a low total REE which indicate possible contribution from deep-seated fluids to the ore-forming process. These patterns also suggest a source which is enriched in Eu and depleted in LREE either due to crystal fractionation or the formation of scheelite. The associated hydrothermally altered wall rocks from proximal zone, exhibit flat REE patterns coupled with high concentrations of Cu, Zn, Cr, V, Sc and Ni indicating their derivation from secondary epigenetic fluids responsible for metasomatic alteration. The abundant barren schistose amphibolites from distal zone represent enriched type of island arc basalts (IAB). The trace, REE-HFSE data on the rhyolite (adakite) and IAB of Middle Reef in the Hutti schist belt substantiates the inference that Archaean subduction processes in an island arc/convergent margin setting are responsible for adakitic magmatism.

Manufacture of Au–Si alloys for use in the soldering of gold alloys

In their eutectic compositions, Au–Si alloys have a melting point of 369°C, and a colour similar to that of high-grade gold. Results of the manufacture of gold alloys with 2.5%, 2.8% and 3.0% silicon, by means of melting in a plasma furnace with an inert argon atmosphere, are presented in this study. Chemical composition characterisation was carried out by energy dispersion spectroscopy and wave dispersion spectrometry, and metallographic and microstructural analysis by optical microscopy, SEM and DRX, Vickers hardness and mircohardness testing, melting point evaluation by DSC and determination of the SCIELab colorimetric coordinates. The results of the hardness and micro-hardness testing gave values around 110HV, presenting an important increase in the mechanical properties with respect to the traditional high purity gold alloys. The melting temperature for the alloys was around 374°C as well as being a heat very close to the values of pure gold.

Sulfur isotope and trace element data from ore sulfides in the Noranda district (Abitibi, Canada): implications for volcanogenic massive sulfide deposit genesis

We examine models for volcanogenic massive sulfide (VMS) mineralization in the ~2.7-Ga Noranda camp, Abitibi subprovince, Superior Province, Canada, using a combination of multiple sulfur isotope and trace element data from ore sulfide minerals. The Noranda camp is a well-preserved, VMS deposit-rich area that is thought to represent a collapsed volcanic caldera. Due to its economic value, the camp has been studied extensively, providing a robust geological framework within which to assess the new data presented in this study. We explore previously proposed controls on mineralization within the Noranda camp and, in particular, the exceptional Au-rich Horne and Quemont deposits. We present multiple sulfur isotope and trace element compositional data for sulfide separates representing 25 different VMS deposits and “showings” within the Noranda camp. Multiple sulfur isotope data for this study have δ34SV-CDT values of between −1.9 and +2.5 ‰, and Δ33SV-CDT values of between −0.59 and −0.03 ‰. We interpret the negative Δ33S values to be due to a contribution of sulfur that originated as seawater sulfate to form the ore sulfides of the Noranda camp VMS deposits. The contribution of seawater sulfate increased with the collapse and subsequent evolution of the Noranda caldera, an inference supported by select trace and major element analyses. In particular, higher concentrations of Se occur in samples with Δ33S values closer to 0 ‰, as well as lower Fe/Zn ratios in sphalerite, suggesting lower pressures and temperatures of formation. We also report a relationship between average Au grade and Δ33S values within Au-rich VMS deposits of the Noranda camp, whereby higher gold grades are associated with near-zero Δ33S values. From this, we infer a dominance of igneous sulfur in the gold-rich deposits, either leached from the volcanic pile and/or directly degassed from an associated intrusion.

Iron Recovery by Two-Stage Roasting the Gold-Bearing Pyrite Concentrate and Gold-Leaching from the Pyrite Cinder by Thiourea

Pyrite cinders, which are the waste products of sulphuric acid manufacturing plants, contain hazardous but valuable heavy metals. This passage introduced that iron recovery by two-stage roasting the pyrite concentrate and gold-leaching from the pyrite cinder by thiourea. After re-floating and two-stage roasting of the gold-bearing low-grade pyrite concentrate, the final cinder contains about 0.056% As and 61% Fe and high grade gold and silver. The thiourea gold-leaching experiment for the pyrite cinder shows the gold-leaching rate is up to 90.4% in the case of grinding fineness of 60%-0.074mm, pH = 1~2, R=1:2, thiourea dosage of 10g / L, ferric sulfate dosage of 3g / L, leaching time of 6h.

Linking high-grade gold mineralization to earthquake-induced fault-valve processes in the Porgera gold deposit, Papua New Guinea

Processes responsible for high-grade gold enrichment are of significant economic interest and have long been elusive. Structural and geophysical models postulate that earthquake-induced fault-valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems, although a comprehensive geochemical record of these processes has been lacking. The Porgera gold deposit (Papua New Guinea) provides an opportunity to examine the complex processes surrounding high-grade Au mineralization in structurally controlled hydrothermal systems. New high-resolution trace element and sulfur isotope data obtained in situ by laser ablation–inductively coupled plasma–mass spectrometry and sensitive high-resolution ion microprobe–stable isotope analysis of pyrite across a well-constrained paragenetic sequence reveal a temporal evolution in fluid chemistry. The data uncover a stratigraphy of repeated high-Au negative δ 34 S and low-Au positive δ 34 S zones within individual pyrite crystals present in the highest grade gold event (stage II). Results provide the first clear geochemical documentation of rapid, complete switching in ore-forming processes, preserved in individual pyrite crystals throughout an evolving hydrothermal system. It is postulated that geochemical variations are attributable to rapid Au deposition during pressure release due to fault failure followed by switching to more reduced rock-buffered (metal poor) fluids from adjacent sediments as the rupture seals. The cyclicity of this process is reflected in the pyrite zonation seen at the Porgera deposit. This study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways, revealing that deposition of exceptionally high grade ore at the Porgera deposit was the result of multiple events in a fluctuating ore-forming environment.

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

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

Genesis and Exploration Implications of Epithermal Gold Mineralization and Porphyry-Style Alteration at the Endeavour 41 Prospect, Cowal District, New South Wales, Australia

Endeavour 41 is a deep-level, structurally controlled epithermal gold deposit hosted by Early Ordovician subaqueous volcanosedimentary rocks. Calc-alkalic to shoshonitic, mafic to intermediate sills, dikes, and stocks intruded the volcanosedimentary units in the Middle Ordovician. The most felsic intrusions, together with pyroxene-bearing dikes, are temporally related to gold mineralization. Postmineralization intrusions are exclusively of mafic character. Endeavour 41 evolved from early, high-temperature porphyry-style veins and alteration to lower-temperature epithermal-style gold mineralization. Early magnetite and garnet-bearing veins (stage 1 and 2, respectively) associated with actinolite, magnetite, and biotite-bearing alteration assemblages have been cut by gold-bearing veins and associated alteration assemblages. There were two main epithermal-style gold mineralizing events: (1) quartz-pyrite ± calcite ± adularia ± chlorite veins (stage 3) and (2) carbonate-base metal sulfide veins (calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite, Ag tellurides, arsenopyrite, apatite, hematite, illite-muscovite, and chlorite [stage 4]). Gold occurs principally as a refractory phase in pyrite. It also occurs as grains of Au-Ag tellurides and as inclusions of free gold in pyrite, sphalerite, and chalcopyrite. Hydrothermal alteration associated with gold-mineralized veins produced early epidote and K-feldspar-epidote–bearing alteration halos and later-stage illite-muscovite-K-feldspar and calcite-rich alteration halos. The highest gold grades are associated with muscovite and illite alteration. Stable isotope analyses and fluid inclusion data provide evidence of a magmatic-hydrothermal component to the mineralizing fluids. Fluid inclusion data suggest that gold precipitated from boiling saline waters (~9.0 wt % NaCl) at temperatures of about 310°C. Stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars). Stage 4 illite formed at temperatures below ~280°C. Stage 3 calcite has δ 13 C calcite and δ 18 O calcite values that range from −5.2 to −4.6 and from 11.6 to 12.1‰, respectively. Calculated fluids for these mineral values at 300°C (δ 13 C fluid = −3‰; δ 18 O fluid = 6‰) are consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3. A component of meteoric waters is inferred for stage 4, because δ 13 C carbonate and δ 18 O carbonate values range from −6.9 to −0.5 and from 10.9 to 30.1‰, respectively, corresponding to δ 13 C fluid and δ 18 O fluid values of −5 and −2‰ at 200° to 250°C. The δ 34 S sulfide values for early vein stages range between −4.9 and −0.5‰. Stage 3 has δ 34 S sulfide values ranging from −5.2 to +0.8‰ with the most 34 S enriched values deposited away from the mineralized center. Stage 4 sulfides have isotopic compositions from −7.5 to +2.5‰. The negative isotopic values are consistent with oxidized (sulfate-predominant) magmatic-hydrothermal fluids. Sulfur isotopic zonation patterns show that the most negative δ 34 S values correlate with gold-enriched domains and also with areas that contain high-temperature, porphyry-style alteration facies. The negative sulfur isotope values define zones of upflow for the mineralizing magmatic-hydrothermal fluids. The paragenetic history of Endeavour 41 records a transition from deep-level to shallow-level magmatic-hydrothermal activity. This transition implies erosion and unroofing of the system synchronous with mineralization. High-temperature assemblages (e.g., actinolite-magnetite, biotite, and K-feldspar-epidote) indicate that epithermal mineralization occurred proximal to a magmatic-hydrothermal center and that there is potential for the discovery of porphyry copper-gold mineralization below the current level of diamond drilling.

Sedimentary-rock-hosted epithermal systems of the Tertiary Eastern Rhodopes, Bulgaria: new constraints from the Stremtsi gold prospect

Abstract Precious metal epithermal, sedimentary-rock-hosted prospects constitute a new class of ore deposits recently described in the Tertiary Eastern Rhodopes of southeastern Bulgaria. The Stremtsi prospect investigated in this contribution is located in a distal location with respect to the main cluster of sedimentary-rock-hosted Ada Tepe and Rosino gold prospects of the Eastern Rhodopes. The Stremtsi prospect is hosted by a Priabonian clastic sedimentary rock sequence, overlying metamorphic rocks of the Central Rhodopean dome. The eastern part of the Stremtsi prospect contains high gold grades, and is characterized by a strongly silicified zone, including adularia and silicified dolomite blades, diagnostic for boiling conditions during ore formation in such low-sulphidation epithermal systems. The western part of the Stremtsi prospect consists of a barite, sphalerite and galena mineralization, associated with silicification, and illite and carbonate alteration. Both parts are underlain by subvertical quartz-carbonate-pyrite veins. Primary and secondary fluid inclusions, respectively, in dolomite and barite yield homogenization temperatures ranging between 90 and 247 °C. The salinity of primary inclusions in dolomite falls between 1.9 and 5.1 wt% NaCl equivalent, whereas the one of secondary fluid inclusions in barite ranges between 0.0 and 3.1 wt% NaCl equivalent. The variable homogenization temperatures reflect post-entrapment re-equilibration of the fluid inclusions, whereas the salinities were preserved and the inclusions in dolomite are interpreted in terms of dilution of a saline fluid in the western part of the Stremtsi prospect. The sulphur isotope compositions of sulphides from Stremtsi range mainly between −4 and +4‰. They are not diagnostic and can be attributed to magmatic, metamorphic, and sedimentary sources. They overlap with the main compositional range of sulphides from other sedimentary-rock-hosted epithermal systems and reveal the existence of hydrothermal fluids with common characteristics during ore formation throughout the Eastern Rhodopes. In addition, at Stremtsi, negative δ 34 S values between −42.6‰ and −8.8‰ combined with framboidal pyrite and elevated δ 34 S values of +7.0‰ to +19.5‰ support locally derived sulphur generated, respectively, by bacterial and thermochemical sulphate reduction. Modelling of O, C, and Sr isotope data of dolomite support the above described ore-forming processes. A positive correlation between δ 18 O (+12.7‰ to +19.7‰ V-SMOW) and δ 13 C (−2.8‰ to +1.5‰ V-PDB) values for dolomite from the eastern, silicified and gold-enriched zone of the Stremtsi prospect is satisfactorily modelled by boiling between 140 and 180 °C of a deeply circulating fluid characterized by δ 18 O and δ 13 C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively, and radiogenic strontium leached from the metamorphic basement rocks or its clastic counterparts in the Priabonian host rocks. By contrast, negative correlations of δ 18 O values (+13.4‰ to +23.3‰ V-SMOW) with δ 13 C values (−0.6‰ to −3.9‰ V-PDB) and 87 Sr/ 86 Sr ratios of dolomite from the western, barite and base metal-rich zone are adequately modelled by a shallow, low temperature (70 °C), intra-formational fluid recharged by meteoric water, which interacted with organic matter, that is, coal layers, and carbonate rocks from the Priabonian host sequence, mixing with a deep, moderate temperature (190 °C), 87 Sr-enriched fluid characterized by δ 18 O and δ 13 C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively. Disequilibrium conditions revealed by sulphur isotope thermometry of two galena-barite pairs yielding discrepant temperatures of 190 and 306 °C are consistent with fluid mixing. A plateau age of 37.57±0.31 Ma obtained by 40 Ar/ 39 Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with 40 Ar/ 39 Ar age data from previous studies, it reveals that the sedimentary-rock-hosted epithermal prospects constitute an independent, regional and older ore-forming hydrothermal system, distinct from the younger volcanic-rock-hosted epithermal deposits of the Bulgarian and Greek Eastern Rhodopes.

Characterisation and 3D modelling of a nuggety, vein-hosted gold ore body, Sunrise Dam, Western Australia

In high-nugget gold ore bodies, samples taken from drill core for gold assay are typically too small to allow for the extreme spatial variability in grade and are a poor representation of the underlying distribution of mineralisation. The GQ North lode at the Sunrise Dam Gold Mine in Western Australia is a good example of an orebody which has a very strong nugget effect (coefficient of variation >20) and that has proved very problematic to model. Gold is hosted in vein stockworks and shear zones and although there is a clear spatial relationship between mineralisation and alteration, high vein density and well-developed foliations, the relationship is best defined statistically because the association between high gold grades and various combinations of these features is non-trivial. We present a method for automating the inclusion of geological data (proxies for gold mineralisation) into the prediction of mineralised rocks, using conditional probability. The method uses the gold assays and the logged geological data to calculate the probability that rocks with particular geological features will be mineralised. The ore body can then be modelled automatically using interpolation software with isosurfaces indicating the regions with highest probability of gold mineralisation. A good understanding of the geological features associated with mineralisation and consistent geological logging are important prerequisites for successful conditional probability modelling of drill hole data.

THE PAGONI RACHI/KIRKI Cu-Mo Re Au DEPOSIT, NORTHERN GREECE: MINERALOGICAL AND FLUID INCLUSION CONSTRAINTS ON THE EVOLUTION OF A TELESCOPED PORPHYRY-EPITHERMAL SYSTEM

The Pagoni Rachi/Kirki Cu-Mo ± Re ± Au deposit is a telescoped porphyry-epithermal system that contains molybdenite with up to 4.7 wt.% Re and the rare mineral rheniite (ReS2). The deposit is spatially related to an Oligocene, calc-alkaline hornblende-biotite- bearing granodiorite-tonalite porphyry stock; however, a genetic link to a buried stock of quartz-feldspar porphyry magma beneath Pagoni Rachi cannot be excluded. Alteration types and vein relationships suggest that mineralization can be divided into four paragenetic stages, which are, from early to late: (1) sodic/potassic-calcic alteration with quartz- and magnetite-bearing veins (A- and M-type) and distal propylitic alteration, (2) sodic/potassic alteration with quartz-pyrite-chalcopyrite-molybdenite veins (B-type), (3) sericitic alteration with “transitional” porphyry to epithermal pyrite-chalcopyrite-molybdenite veins (D-type), and (4) argillic alteration with quartz-calcite base metal and precious metal rich veins (E-type) with epithermal affinity. Bulk ore analyses and mineralogical data are consistent with deposition of precious metals in all mineralization stages: electrum in A- and M-type veins is spatially associated with bornite and chalcopyrite, whereas tellurides (cuprian hessite, stutzite, petzite) and Ag-Au alloy occur in the epithermal veins. The D-type veins contain the highest gold grades (up to 5 g/t) and include Ag-, Bi-, Te-, and Se-bearing minerals indicating a possible genetic relationship with E-type epithermal veins. Fluid inclusion data indicate that A- and B-type veins (porphyry style mineralization) were deposited at 360–510 °C and at pressures up to 690 bars ( 74 wt.% NaCl equiv.) fluids. Subsequent dilution of the moderately saline fluid formed a lower temperature (210–340 °C), less saline (1.4–2.9 wt.% NaCl equiv.) fluid in late epithermal E-veins. Based on the Au and Re grades, Cu/Mo ratios, presence of rheniite and the extremely high Re content of molybdenite, Pagoni Rachi may be considered as a distinct class of porphyry deposits, a Cu-Mo±Re±Au system that is derived from rocks transitional between calc-alkalic and alkalic porphyries. Its similarity with other Re-Au enriched porphyry-style deposits in northeastern Greece indicates that this area may represent a new Au-Re-Te metallogenetic province.

Structural Constraints and Numerical Simulation of Strain Localization in the Bendigo Goldfield, Victoria, Australia

Strain localization and fluid flow patterns adjacent to the massive quartz-hosted reefs of the Bendigo gold-field have been investigated using new structural observations, three-dimensional geometry of fold-fault relationships, and gold distribution. Quartz reef and vein formation in the Bendigo goldfield is associated with an initial E-W compression event that produced a penetrative upright cleavage and N-S–trending fold hinges. In the immediate hanging wall of quartz reefs with high gold grades, the S1 cleavage is locally deformed by an E-W–trending S2 cleavage that is attributed to a low-strain N-S–oriented compressive event. Further evidence of this low-strain event is provided by gently S plunging quartz-carbonate slickenline lineations on S1 cleavage surfaces and the reef margin surfaces and visible gold within the outer portions of the reef. The reefs are commonly situated on or immediately to the south of a left-flex (sinistral) change in strike of the axial surface of the folds, and the flexure occurs close to an anticlinal culmination defined by the double-plunging N-S–striking folds. The folds become deformed with meter to decimeter wavelengths, which generally affect the stiff units, in a disharmonic style. Accompanying this is a reactivation of reef margins by oblique slip and the formation of dilational sites and tensional vein arrays that provide the conduits for fluid flow and gold deposition. Numerical models that best simulate the observed distribution of strain, based upon the location of quartz reefs, involve NW-SE–oriented contraction with a significant sinistral strike-slip component. This contraction event has an overall N-S–sense orientation and is therefore consistent with deformation features in the immediate hanging wall of quartz reefs. The models also demonstrate a geometrical dependence on the localization of strain and, thus, the potential localization of fluid flow. Volume and shear strain increase with the flex angle, and variations in the magnitude of strain are also associated with variations in the strike angle of the flexure. This analysis highlights that the modification of preexisting fold structures and quartz veins by a low-strain event may enhance the localization of dilation sites within individual quartz reefs where there are subtle changes in the orientation of the reef margins. Such numerical modeling can also improve ranking of exploration targets based on fold geometry and provide a better understanding of fluid flow in hydrothermal mineral systems.

Eucalyptus oils as green collectors in gold flotation

In this study, the potential of Eucalyptus oils as green collectors in gold flotation was examined. Flotation tests were conducted on a sample of Boddington’s copper–gold sulfide ores. Eucalyptus oil compositions were determined by gas chromatography–mass spectrometry. Gold association of flotation products was investigated by Mineral Liberation Analyzer. It was found that Eucalyptus oils have the potential to replace potassium amyl xanthate as a collector in free gold flotation. The collecting power of Eucalyptus oils is correlated to the percentage of non-polar and weakly polar constituents, and in particular 1,8-cineole, phellendrene and pipertone. Eucalyptus oils possess strong selectivity for free gold rather than gold carriers, and therefore achieve high gold grades at the head of the flotation circuit.

Evolutionary historical model of the Dukat silver giant

There are good grounds to believe that the inherited development of ore bodies controls the most important regularities in the location of deposits, especially of large ones, and also explains quite clearly the relatively selective ore occurrence in different magmas. Giant silver deposits were identified among both Paleozoic bodies and their regenerated Late Mesozoic vein-disseminated analogues. Jointly they represent a unified polychronic ore formation range. The performed investigations of mineral composition and the isotopic Sm-Nd system of granitoids in the Dukat ore field and their gold- and silver-bearing melanocratic inclusions established the possible occurrence of the Paleozoic juvenile continental crust of the Siberian Craton in their composition and confirmed our complicated scheme of oregenesis at the Dukat silver giant. The occurrence of high-grade gold in the melanocratic mineral aggregates of granitoids in the Dukat ore field should be regarded as a feature of hypabyssal gold-bearing zones being of high commercial interest.

Gangue mineral textures and fluid inclusion characteristics of the Santa Margarita Vein in the Guanajuato Mining District, Mexico

Abstract Successful exploration for mineral deposits requires tools that the explorationist can use to distinguish between targets with high potential for mineralization and those with lower economic potential. In this study, we describe a technique based on gangue mineral textures and fluid inclusion characteristics that has been applied to identify an area of high potential for gold-silver mineralization in the epithermal Ag-Au deposits at Guanajuato, Mexico. The Guanajuato mining district in Mexico is one of the largest silver producing districts in the world with continuous mining activity for nearly 500 years. Previous work conducted on the Veta Madre vein system that is located in the central part of this district identified favorable areas for further exploration in the deepest levels that have been developed and explored. The resulting exploration program discovered one of the richest gold-silver veins ever found in the district. This newly discovered vein that runs parallel to the Veta Madre was named the Santa Margarita vein. Selected mineralized samples from this vein contain up to 249 g/t of Au and up to 2,280 g/t Ag. Fluid inclusions in these samples show homogenization temperatures that range from 184 to 300°C and salinities ranging from 0 to 5 wt.% NaCl. Barren samples show the same range in homogenization temperature, but salinities range only up to 3 wt.% NaCl. Evidence of boiling was observed in most of the samples based on fluid inclusions and/or quartz and calcite textures. Liquid-rich inclusions with trapped illite are closely associated with high silver grades. The presence of assemblages of vapor-rich-only fluid inclusions, indicative of intense boiling or “flashing”, shows the best correlation with high gold grades.

Geology of the Didipio Region and Genesis of the Dinkidi Alkalic Porphyry Cu-Au Deposit and Related Pegmatites, Northern Luzon, Philippines

The Dinkidi Cu-Au porphyry deposit, northern Luzon, Philippines, formed at the tip of the southward-propagating Cagayan Valley basin in a late Oligocene to early Miocene back-arc setting. The 110 Mt ore deposit is characterized by elevated hypogene grades (avg 1.2 g/t Au and 0.5% Cu) and is one of only a few alkalic porphyry deposits to have been discovered outside of British Columbia and eastern Australia. Dinkidi is hosted by the Didipio intrusive complex, which intruded calc-alkalic to shoshonitic volcanic rocks of the Mamparang and Upper Mamparang formations in the late Oligocene. An early, composite diorite-monzodiorite pluton was intruded by the Surong monzonite and the Dinkidi stock. The initial phases of the Cu-Au mineralized Dinkidi stock were the equigranular biotite-amphibole–bearing Tunja monzonite and the Balut dike—a thin, variably textured and strongly mineralized clinopyroxene syenite pegmatite dike. The youngest phases of the Dinkidi stock are a plagioclase-phyric monzonite-syenite (Quan porphyry) and the Bufu syenite, a crystal-crowded leucocratic quartz syenite. Postmineralization andesite dikes cut the intrusive complex. Emplacement of the Tunja monzonite was temporally and spatially associated with pervasive stage 1 biotitemagnetite- K silicate alteration, which primarily affected the diorite-monzodiorite pluton. Emplacement of the diopside-phyric Balut dike produced the stage 2 calc-potassic diopside-actinolite-K feldspar-bornite vein stockwork and a calc-potassic alteration assemblage typical of silica-undersaturated alkalic porphyry deposits. Stage 2 lacks quartz, contains high gold grades (2–8 g/t Au) and its sulfides have a34S values of –3.5 to –0.7 per mil. The stage 2 calc-potassic assemblage is inferred to have formed at temperatures in excess of 600°C from an oxidized (sulfate-predominant) Na-K-Ca-Fe–rich brine. Intrusion of the quartz-saturated Quan porphyry and Bufu syenite led to the formation of the stage 3 quartzillite- calcite-chalcopyrite stockwork vein and alteration assemblage. The quartz stockwork hosts most of the lower-grade (1–2 g/t Au) mineralization at Dinkidi and is typical of silica-saturated alkalic porphyry systems. A coarse-grained assemblage of quartz-actinolite-perthite (the Bugoy pegmatite) formed as an apophysis on the Bufu syenite and was subsequently brecciated by faulting late in stage 3. The stage 3 quartz stockwork was emplaced at high temperatures (mostly >600°C) from a quartz-saturated, oxidized (sulfate-predominant) Na- K-Fe brine (>68 wt % NaCl equiv) that contained up to 0.6 wt percent Cu and 4 wt percent Fe. Cooling to ~430°C and sulfate reduction by wall-rock interaction led to the precipitation of stage 3 sulfides with a34S values of –4.2 to –0.2 per mil in the quartz stockwork. The quartz-bearing assemblage formed at paleodepths of 2.9 to 3.5 km. Periods of quartz growth from overpressured brines were interrupted episodically by brittle failure events that caused the system to depressurize to near-hydrostatic conditions, triggering vapor generation via boiling. Mineralization was followed by intermediate argillic and high-level advanced argillic alteration (stage 4), and by late-stage, fault-related zeolite-calcite alteration and veins (stage 5). The hydrothermal mineral assemblages at Dinkidi reflect the composition and degree of fractionation of the associated intrusions. Extensive fractionation in a silica-undersaturated dioritic magma chamber is interpreted to have ultimately caused quartz saturation and the development of the late-stage syenite intrusions and related quartz stockwork mineralization. The calcic, silica-undersaturated pegmatitic Balut dike, which is associated with the calc-potassic stockwork, does not fit this fractionation trend and is interpreted to have formed by interaction between the late-stage syenitic melt and a comagmatic mafic melt that underplated the siliceous magma chamber prior to formation of the Balut dike. A reversion to fractionation-dominated magmatic processes in the silicic magma chamber then led to the intrusion of the quartz-saturated Quan porphyry and Bufu syenite. Ultimately, the residual mafic melt was emplaced as a series of late-stage andesite dikes.

Targeting of Gold Deposits in Amazonian Exploration Frontiers using Knowledge- and Data-Driven Spatial Modeling of Geophysical, Geochemical, and Geological Data

This paper reports the application of weights-of-evidence, artificial neural networks, and fuzzy logic spatial modeling techniques to generate prospectivity maps for gold mineralization in the neighborhood of the Amapari Au mine, Brazil. The study area comprises one of the last Brazilian mineral exploration frontiers. The Amapari mine is located in the Maroni-Itaicaiunas Province, which regionally hosts important gold, iron, manganese, chromite, diamond, bauxite, kaolinite, and cassiterite deposits. The Amapari Au mine is characterized as of the orogenic gold deposit type. The highest gold grades are associated with highly deformed rocks and are concentrated in sulfide-rich veins mainly composed of pyrrhotite. The data used for the generation of gold prospectivity models include aerogeophysical and geological maps as well as the gold content of stream sediment samples. The prospectivity maps provided by these three methods showed that the Amapari mine stands out as an area of high potential for gold mineralization. The prospectivity maps also highlight new targets for gold exploration. These new targets were validated by means of detailed maps of gold geochemical anomalies in soil and by fieldwork. The identified target areas exhibit good spatial coincidence with the main soil geochemical anomalies and prospects, thus demonstrating that the delineation of exploration targets by analysis and integration of indirect datasets in a geographic information system (GIS) is consistent with direct prospecting. Considering that work of this nature has never been developed in the Amazonian region, this is an important example of the applicability and functionality of geophysical data and prospectivity analysis in regions where geologic and metallogenetic information is scarce.

Fluid sources and the role of abiogenic-CH 4 in Archean gold mineralization: Constraints from noble gases and halogens

The St Ives Goldfield of Western Australia hosts high grade gold mineralization in a variety of lithologies within ∼2.7 Ga greenstone belts. The low salinity CO 2-bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones, or from external magmatic sources, and independently sourced abiogenic-CH 4 has been suggested as an important control on gold mineralization. In order to better constrain possible fluid sources, we applied a combination of He–Ne–Ar–Kr–Xe isotope and Cl–Br–I analyses to quartz and carbonate veins containing H 2O–CO 2 fluid inclusions, pyrite ± magnetite/hematite; and quartz veins containing CH 4-rich fluid inclusions, pyrrhotite ± pyrite. Samples containing H 2O–CO 2 fluid inclusions have higher Br/Cl and I/Cl values and less radiogenic noble gas isotope signatures than suggested for CH 4-rich fluid inclusions. The samples containing CH 4 fluid inclusions have maximum 40Ar/ 36Ar of ∼50,000; 20Ne/ 22Ne of ≤9.8 and maximum 21Ne/ 22Ne of ∼0.56. In contrast, H 2O–CO 2 fluid inclusions have maximum 40Ar/ 36Ar of only ∼20,000 and maximum 21Ne/ 22Ne of ∼0.14. The use of 40Ar– 39Ar methodology to measure K simultaneously with 40Ar/ 36Ar, enables us to show the highest 40Ar/ 36Ar ratios are representative of the fluids trapped at ∼2.65 Ga, and have not been significantly modified by post-entrapment 40Ar R production ( 40Ar R = radiogenic 40Ar). Furthermore, the 40Ar/ 36Ar ratio is correlated with the 21Ne/ 22Ne and 136Xe/ 130Xe ratios, demonstrating that other heavy noble gas isotope ratios are also close to their original composition. The highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host-rocks, because seawater altered-volcanic rocks have high abundances of non-radiogenic atmospheric noble gas isotopes. In contrast, the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at ∼2.65 Ga. The combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced H 2O–CO 2 and CH 4 localized high-grade gold mineralization. We suggest that the fluids were intimately associated with crustal magmatism in the St Ives Goldfield and that abiogenic CH 4 could be an important moderator of redox processes in other crustal environments.

Shkol’noe gold deposit, the Russian Northeast

The Shkol’noe deposit is localized in a small granitoid stock, the root portion of which is traced using geophysical data to a depth of 5–8 km. The high-grade gold ore (33 gpt Au) is enriched in silver and principally differs in ore composition from the previously studied mesothermal gold-quartz and epithermal gold-silver deposits in the Russian Northeast. The main reserves of the Shkol’noe deposit concentrate in bonanzas (20% of the total volume of orebodies). The internal deformation is related to the rearrangement of matter in freibergite; exsolution structures in fahlore and native gold are related to postmineral metamorphism. It is suggested that the ore of the Shkol’noe deposit occupies a transitional position between porphyrytype and epithermal levels of ore deposition.

The role of early formed structures on the development of the world class St Ives Goldfield, Yilgarn, WA

A revised structural interpretation for the Victory to Kambalda area of the world class St Ives Goldfield in the Archean Yilgarn Craton has mapped out the distribution of WNW-trending faults within this area of the field. These previously cryptic WNW-trending structures had been identified in gravity data, and also by isopach thickness variations. The WNW-trending faults acted as transfers syn-gold mineralization, although only discrete segments of these faults were active during the main stage of gold mineralization. Where mineralized, the faults transferred strain from a complex combination of block-on-block movement associated with thrusting and strike-slip movement on NW- and N-trending faults. Along some segments N-trending mineralized faults terminate against the WNW-trending faults. Many of the WNW-trending faults correlate with major strike changes on regional and camp-scale faults and they are domain boundaries for the critical N-trending fault segments that host high-grade gold within contractional jogs. The WNW-trending faults also show evidence for an older deformation history prior to main-stage gold, which may extend back to early basin development associated with ultramafic and mafic volcanism. They are inferred to have been a series of early WNW-trending normal faults and breached relay ramps associated with oblique rifting along an older NNW-trending basement boundary.

P-T-X Conditions of Fluids in the Sunrise Dam Gold Deposit, Western Australia, and Implications for the Interplay between Deformation and Fluids

The Late Archean Sunrise Dam gold deposit (~10 Moz) is hosted within greenschist-facies rocks and is characterized by extreme structural complexity, resulting from a protracted deformation history with evidence of structural reactivation and multiple phases of gold mineralization. Early Group I orebodies are hosted within shallow- to moderately dipping northwest-trending shear zones and occur in foliation parallel veins within a strong penetrative fabric. Group II orebodies occur within steeply dipping shear zones and are characterized by veins and breccias, and Group III and IV orebodies comprise dominantly stockwork veins. The orebodies formed in response to multiple episodes of deformation. Many structures were created during D1 and D2 shortening whereas the bulk of the gold was deposited during D3 and D4 that marks the transition from a dominantly lithostatic pore fluid regime to hydrostatic conditions. Two main fluid inclusion types were recognized at Sunrise Dam: low-salinity CO2-H2O inclusions and CO2 inclusions. CO2-H2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D3 (minimum estimates suggest conditions were likely >~300°C and >1 to 3 kbars predominantly within the one-phase field). CO2-H2O inclusions that occur in later Group II and III orebodies were trapped predominantly within the two-phase field at temperatures 1–3 kbars) at a temperature >300°C. Shear failure along these structures resulted in widespread precipitation of moderate-grade gold mineralization from the CO2-H2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO2-H2O fluid entering the two-phase field. A combination of temperature decrease, a transition from lithostatic to suprahydrostatic and/or hydrostatic pressure conditions, fluid immiscibility, and the influx of a second CO2-rich fluid, resulted in fracture during D4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks.