Au Prospect Research Articles

Termite species variations and their importance for termitaria biogeochemistry: towards a robust media approach for mineral exploration

ABSTRACT Termitaria (termite mounds) are a widespread and abundant component of tropical savannah ecosystems in northern Australia. Despite their abundance very little systematic research has been carried out on the application of termitaria sampling to mineral exploration in this region. Multi-element biogeochemistry of termitaria from species with the most abundant epigeal mounds ( Nasutitermes triodiae ( N. triodiae ), Drepanotermes rubriceps ( D. rubriceps ) and Amitermes vitiosus ( A. vitiosus )) is characterized and compared. Differences in the biogeochemistry of termitaria built by different species are identified in this biogeochemical study of the termitaria at the Titania Au Prospect, situated proximal to a palaeochannel system in a highly prospective area in northern Australia. In particular, elevated levels of Mo and Au in the coarse sand fraction of the D. rubriceps mounds are observed. Mineralization is expressed in the biogeochemical characteristics of termitaria through up to 15 m of transported regolith, particularly in the fine, silty-clay fraction of the mound samples. This is particularly obvious for Au, As, Zn and Ni, which all show relatively high analytical concentrations within material sampled from termitaria of N. triodiae . These findings show that termitaria can successfully delineate buried mineralization, reinforcing the role that soil biota, such as termites, play in forming surface geochemical anomalies above mineralization buried by transported regolith.

The sources and evolution of mineralising fluids in iron oxide–copper–gold systems, Norrbotten, Sweden: Constraints from Br/Cl ratios and stable Cl isotopes of fluid inclusion leachates

We have analysed the halogen concentrations and chlorine stable isotope composition of fluid inclusion leachates from three spatially associated Fe-oxide ± Cu ± Au mineralising systems in Norrbotten, Sweden. Fluid inclusions in late-stage veins in Fe-oxide–apatite deposits contain saline brines and have a wide range of Br/Cl molar ratios, from 0.2 to 1.1 × 10 −3 and δ 37Cl values from −3.1‰ to −1.0‰. Leachates from saline fluid inclusions from the Greenstone and Porphyry hosted Cu–Au prospects have Br/Cl ratios that range from 0.2 to 0.5 × 10 −3 and δ 37Cl values from −5.6‰ to −1.3‰. Finally, the Cu–Au deposits hosted by the Nautanen Deformation Zone (NDZ) have Br/Cl molar ratios from 0.4 to 1.1 × 10 −3 and δ 37Cl values that range from −2.4‰ to +0.5‰, although the bulk of the data fall within 0‰ ± 0.5‰. The Br/Cl ratios of leachates are consistent with the derivation of salinity from magmatic sources or from the dissolution of halite. Most of the isotopic data from the Fe-oxide–apatite and Greenstone deposits are consistent with a mantle derived source of the chlorine, with the exception of the four samples with the most negative values. The origin of the low δ 37Cl values in these samples is unknown but we suggest that there may have been some modification of the Cl-isotope signature due to fractionation between the mineralising fluids and Cl-rich silicate assemblages found in the alteration haloes around the deposits. If such a process has occurred then a modified crustal source of the chlorine for all the samples cannot be ruled out although the amount of fractionation necessary to generate the low δ 37Cl values would be significantly larger. The source of Cl in the NDZ deposits has a crustal signature, which suggests the Cl in this system may be derived from (meta-) evaporites or from input from crustal melts such as granitic pegmatites of the Lina Suite.

Relationship between CO2-dominated fluids, hydrothermal alterations and gold mineralization in the Red Lake greenstone belt, Canada

Abstract The Red Lake greenstone belt is one of the foremost Au mining camps in Canada and hosts the world-class Campbell-Red Lake Au deposit. Belt-scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with Au mineralization surrounded by distal calcite zones, both being accompanied by potassic alterations (sericite, muscovite, and biotite). At the Campbell-Red Lake and Cochenour deposits Au mineralization (in particular high-grade ore) is associated with silica and sulfides (especially arsenopyrite) that replace carbonate ± quartz veins and the host rocks. The prevalence of carbonic fluid inclusions and rare occurrence of aqueous-bearing inclusions in carbonate–quartz–Au veins in the Campbell-Red Lake deposit, and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages (FIA), have been interpreted to indicate that H2O-poor, CO2-dominated fluids were responsible for the carbonate veining and Au mineralization. Further studies of fluid inclusions in carbonate–quartz veins within and outside the deformation zone hosting the Campbell-Red Lake deposit (the Red Lake Mine trend) including the Cochenour Au deposit, the Redcon Au prospect, and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals. These studies indicate that CO2-dominated fluids were flowing through fractures during carbonate vein formation and Au mineralization both within and outside major structures. The carbonic fluid may have been initially undersaturated with water, or it may have resulted from phase separation of an H2O–CO2–NaCl fluid. In the latter case, phase separation modeling indicates that the initial fluid likely had X CO 2 values larger than 0.8. Calculations based on hydrothermal mineral assemblages indicate X CO 2 values in the host rocks from 0.025 to 0.85, reflecting a change from CO2-dominated fluids in the fractures (veins) to H2O-dominated fluids in the host rocks away from the fractures. The CO2-dominated fluids were likely advected from granulite facies in the deeper crust, whereas the H2O-dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies. Calculations based on CO2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in Au than the commonly assumed values of a few μg/L, which may explain why the Campbell-Red Lake deposit has a very high-grade of Au (average 21 g/t for the whole deposit and 81 g/t for the Goldcorp High-Grade zone). Fluid inclusion data suggest that the carbonate veining and Au mineralization likely took place at depths from 7 to 14 km. The development of crustiform–colloform structures in the carbonate ± quartz veins, which was previously interpreted to indicate relatively shallow environments, may alternatively have been related to extremely high fluid pressures and the CO2-dominated nature of the fluids, which could have enhanced the brittle properties of the rocks due to their high wetting angles.

The environmental impact of mining in the Pontides, Turkey: reconnaissance sampling and GIS-based analysis

The Pontide metallotect is the major producing area in Turkey for base metals and has significant Au producing potential. Two major developments are currently at the feasibility stage and face opposition because of their potential environmental impact. Little information exists to substantiate past impacts. This study describes reconnaissance water sampling and observations of the impact of some of the major current and disused operations. These are volcanic-hosted massive sulphide (VMS) deposits at Murgul, Kutlular, Lahanos and Çayeli. Smaller vein mines at Gümüşki Tepe and Midi Maden were also sampled. The VMS deposits have had considerable impact. At Kutlular and Lahanos acid mine drainage (AMD) is strongly developed and metals are transported into drainages. In contrast, this situation has been avoided at Murgul because tailings have been released into the local river and subsequently into the major Çoruh River resulting in Cu, Zn and Fe contamination that is transported across the border into Georgia and discharged into the Black Sea. Both As and Cd are potential contaminants at Murgul although their areal distribution requires further investigation. Remediation at the three VMS deposits is minimal. In contrast, the relatively modern mine at Çayeli has little impact, partly because tailings are disposed of at depth in the reducing Black Sea. Limited sampling at the vein mines suggests that AMD is not a major problem. Stream sediment sampling shows that As may, however, be released during ore treatment at Midi Maden. Visual examination at the vein mines shows that surface erosion is a major problem, probably due to the high altitude and thin soils. Surface conditions at the Au prospect at Mastra suggest that AMD and metal release should not be a major problem. However, the use of cyanide presents a more significant public relations issue. Examination of porphyry prospects shows that natural AMD is developed in places. Any development is unlikely as grades are sub-economic. Regional determination of potential AMD was made using the sulphide content of deposit type established in the local studies and related to measured water pH. The wide variation in rainfall was taken into account with areas of high rainfall on the Black Sea predicted as having high potential for AMD formation. Potential surface degradation was predicted using height information from a digital elevation model and deposit type.

A 2.5 Ga porphyry Cu–Mo–Au deposit at Malanjkhand, central India: implications for Late Archean continental assembly

Eight Re–Os ages from six molybdenite samples representative of the Cu–Mo–Au mineralization at the giant Malanjkhand deposit in Madhya Pradesh were obtained using ID-NTIMS. These data provide a clear Late Archean–Early Paleoproterozoic age for this deposit and by implication for its calc-alkaline granitoid host. Among other diverse models, the origin of Malanjkhand has been debated as orogenic lode-style associated with development of a quartz reef in a shear zone or as porphyry-style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle–ductile deformed quartz reef. Previous models included Middle Proterozoic ages for the Cu–Mo–Au mineralization based on poorly constrained Rb–Sr and K–Ar data. Direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 ± 8 Ma (2489.5 ± 1.4 Ma based on regression without uncertainty in the 187Re decay constant, MSWD = 0.5, n = 5). Additional molybdenite from the quartz reef dated at ∼2475 and ∼2450 Ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position. We suggest that the quartz reef is a high-silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north. Petrographic evidence coupled with high Re concentrations for molybdenites (400–650 ppm) support this scenario and suggest that Malanjkhand is a subduction-related stockwork-porphyry-style Cu–Mo–Au deposit involving mantle. Malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes. We suggest that the Malanjkhand Cu–Mo–Au deposit and the surrounding batholith containing multiple Cu–Mo–Au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern Indian craton at 2.5 Ga, as the southern craton began to converge with the northern Indian craton or other protocontinents now embedded or consumed within the central Indian Tectonic Zone (CITZ). This scenario could imply an Early Paleoproterozoic age for the earliest development of the CITZ, at least in the region of the Central Indian Shear/Suture (CIS), whose marked ENE trend imprints the northwestern part of the Malanjkhand batholith and the Cu–Mo–Au deposit. Based on a compilation of ∼2.5 Ga events in both the southern and northern Indian cratons, we suggest that some units in the CITZ linked an extensive and continuous Late Archean–Early Paleoproterozoic orogenic belt that may extend to the Moyar–Bavali, Bhavani, Attur, Palghat, and Cauvery shear zones in southern India, and include inboard units of the Eastern Ghats and the Aravalli–Delhi orogenic belts. Robust geochronology that can be tied to the early history within these orogenic belts is scarce, and extensive reworking during Grenvillian and Pan-African time creates a pervasive overprint at some localities. This Late Archean–Early Paleoproterozoic belt preserves sutures within a coalesced Late Archean continent that included East Antarctica (Napier and Vestfold Hills complexes) at ∼2.5 Ga.

Exploration at the Kuscayiri Au (Cu) prospect and its implications for porphyry-related mineralization in western Turkey

The Kuscayiri Au (Cu) deposit is localized along EW-trending linear silica-quartz structures or pods within altered Lower Miocene volcanics that are underlain by medium to high level Mesozoic granodiorite intrusions. The deposit is characterized by disseminated or fracture-controlled pyrite (up to 12%), variable amounts of silicification, hematite–pyrite matrix breccias produced by intense fracturing of siliceous rocks and moderate to weak saccharoidal to crystalline quartz stringer stockwork veining in small breccia pipes. An early quartz–sericite alteration event was focused along the EW-trending structures and resulted in the precipitation of pyrite. A later, more spatially extensive, advanced argillic alteration event overprinted the quartz–sericite event. The highest gold content is restricted to the high-temperature portion of the alteration zone characterized by stockwork or replacement quartz associated with sericite–pyrite–alunite–pyrophyllite. Trace gold and associated magnetite, arsenopyrite, realgar, specular hematite and chalcopyrite also occur within the smectite–illite/smectite–kaolinite–chlorite alteration facies. During the Tertiary, widespread subaerial to subaqueous arc volcanism developed in western Turkey, and numerous occurrences of high/low sulfidation epithermal systems are associated with major NW–SE extensional deformation. The resultant NE–SW-trending grabens appear to be highly favorable host environments for the formation of porphyry-related gold–copper systems. The Kuscayiri mineralization may represent a lithocap with Au (Cu), the top of one such system yet to be discovered.

Palladium soil geochemistry, Baronskoye Pd–Au prospect, Sverdlovsk oblast, Russia

A programme of B-horizon soil sampling coupled with low detection-limit, fire-assay/ICP-MS analysis for Pd, Au and Pt was carried out on the Baronskoye prospect in the Central Urals of Russia in 1999 and 2000. Of the three precious metals, Pd proved to be particularly effective in locating occurrences of very low-sulphide Pd-Au mineralization of inferred hydrothermal, shearzone type within pyroxenites and gabbros in fairly flat, thickly forested terrain. In contrast, potential pathfinder elements, such as Cu, Ni, Ag, As and Te, as determined by a separate aqua regia digestion/ ICP-MS analytical package, were found to be of little use.At a threshold of 55 ppb Pd the soil-sampling results clearly defined a northwest-trending linear anomaly over the pyroxenite. Although usually only about 20 m wide, this Main Anomaly, with a maximum value of 447 ppb Pd, can be traced along strike for 900 m and is well supported by Au at the 20-130 ppb level and by Pt in the range 10-45 ppb. An intermittently developed Eastern Split Anomaly was identified about 20 m east of the Main Anomaly, and a potentially more important Northeastern Anomaly, about 320 m × 150 m, was found 200 m to the east of the northern end of the Main Anomaly and just inside the gabbro surrounding the pyroxenite. Testing of the three soil anomalies by diamond drilling in 2000 confirmed the presence of sporadic Pd-Au mineralization in the bedrock below all of them.

Petrogenesis of the Squirrel Hills granite and associated magnetite-rich sill and vein complex: Lightning creek prospect, Cloncurry district, Northwest Queensland

The Lightning creek Fe-oxide Cu–Au prospect is hosted within a Mesoproterozoic granitoid batholith, composed dominantly of high-K, calc-alkaline, porphyritic quartz monzodiorite. The quartz monzodiorite contains enclaves of quartz diorite and has been intruded by more felsic granitoids (monzogranite and alkali-feldspar granite) and a series of subhorizontal sills. The quartz monzodiorite crystallised at crustal depths in excess of 10 km ( P∼4 kb, T=800–850°C, melt X H 2O =>4 wt.%, f O 2 ∼NNO buffer), and was probably fluid-saturated ( X H 2O : X CO 2 of >0.5). As might be expected, the more fractionated granitoids crystallised at progressively lower temperatures (quartz monzonite ∼800°C, alkali-feldspar granite <800°C). Subsolidus re-equilibration took place at temperatures between 700 and 600°C under increasingly oxidising conditions (amphiboles become increasingly TiO 2-poor and MgO-rich). Although there is abundant petrographic evidence for magma mingling in the petrogenesis of this granitoid suite, the curvilinear nature of many of the chemical trends indicates an additional contribution from crystal fractionation. Sm–Nd isotopic characteristics ( εNd=−2.7 to −3.3) indicate an older crustal component in the source region. Sill emplacement was contemporaneous with the development of a large magnetite-rich vein system. The sills are remarkable for their mineralogical and textural complexity. In addition to aplitic rocks, there are zones of albite–magnetite–quartz rock, which display a variety of unusual spherulitic textures. These Fe-rich portions of the sills are marginally younger than the aplites and are mineralogically similar to the magnetite-rich veins. The aplites are interpreted as late-stage differentiates of the plutonic suite, with which they share many mineralogical and compositional characteristics. It is argued that the textural and compositional variation within the sills reflect crystallisation under different degrees of melt saturation, and that the Fe-rich spherulitic rocks reflect the transition from magmatic to hydrothermal conditions within the crystallising pluton. Fluid inclusion studies indicate that this transition occurred at (hydrostatic?) pressures in excess of 1.5 kb (and possibly >2.5 kb) and temperatures greater than 500°C. Phase separation occurred in the fluid phase, producing in a CO 2-rich vapour and a metal-rich (Fe, Cu) brine.

"Extreme boiling" model for variable salinity of the Hokko low-sulfidation epithermal Au prospect, southwestern Hokkaido, Japan

The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2 wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150° to 200 °C. The results obtained from stable isotope analysis of δ18O in quartz vein material showed a gradual decrease in δ18O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ18OH2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions. We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid, before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable salinities at the same time, and in close proximity to each other. This is also reflected in the δ18OH2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within the Hokko vein system also occurred at this time as a result of boiling and gas loss.

Bulagidun prospect: a copper, gold and tourmaline bearing porphyry and breccia system in northern Sulawesi, Indonesia

The Bulagidun CuAu prospect, in a remote area of north Sulawesi, was identified during follow-up of stream sediment and panned concentrate geochemical anomalies. Soil geochemistry and drilling outlined areas of disseminated and fracture controlled CuAu mineralization. Soil sampling shows a close association of Cu and Au anomalies with three mineralized breccia zones. Numerous weak metal anomalies also correlate with peripheral quartz-sulfide veining. The three separate bodies contain a geological resource of more than 14.4 Mt at 0.68 ppm Au and 0.61 wt.% Cu. A series of intrusions into regionally-widespread andesitic volcanic rocks (ca. 9.4 Ma) vary in composition with time, from early diorite to quartz diorite to late tonalite and post-mineral andesitic dykes. Early actinolite alteration is overprinted by hydrothermal biotite + magnetite. Coarse biotite + magnetite ± quartz ± K-feldspar ± pyrite ± chalcopyrite open space filling occur within hydrothermal breccia, and postdates the pervasive biotite + magnetite alteration. This later alteration, and retrograde chlorite, co-exist with up to ten percent chalcopyrite and pyrite. A propylitic (chlorite ± epidote ± carbonate) zone surrounds the actinolite and biotite zones, and is accompanied by weak vein-controlled AuPbZn and local As mineralization. A fracture-and permeability-controlled overprinting of sericite + clay + chlorite ± quartz ± carbonate alteration (8.75 Ma) partially destroys earlier silicates. Disseminated blebs of radiating tourmaline are widespread in the breccia bodies, and only partially co-eval with visible sulfide mineralization. Bulagidun is characteristic of island-arc porphyry CuAu mineralization. However, the abundant tourmaline is unusual in the southwest Pacific; this characteristic is similar to tourmaline-bearing breccia pipes in Chile, Arizona and elsewhere that only rarely contain economic-scale disseminated Cu ores.

Metamorphosed boninitic basalts, arc tholeiites, and cryptic volcanic stratigraphy from the Elzevir Terrane of the Grenville Province, Calumet mine, Quebec

Amphibolites (hornblende–plagioclase rocks with Tpeak metamorphism = 650–700 °C) are abundant near the old Calumet Zn–Pb–Ag mine and Au prospect 90 km northwest of Ottawa. They lack primary structures and their original petrochemical character is obscured by metamorphism and profound alteration manifested by a variety of phlogopite–biotite-, garnet-, cummingtonite-, gahnite-, clinopyroxene-, carbonate scapolite-, and calcite-bearing assemblages. This alteration has modified the SiO2, alkali, and most alkaline earth distributions. Trace and minor element chemistry demonstrates that the amphibolites are metaigneous and bimodal, with two suites, one derived from differentiated arc tholeiites and the other from relatively undifferentiated rocks having transitional arc basalt – boninite chemistry. The complex rock package enclosing mineralization includes metatholeiites, whereas the boninitic metabasalts occur in a discrete interval in the structural hanging wall of the mineralization. Although these rocks represent a new geochemical assemblage in the Allochthonous Monocyclic Belt (Central Metasedimentary Belt), their implied geotectonic setting is comparable to those previously inferred for several other areas, including that near Montauban, Quebec, where there are very similar styles of mineralization.

Geochemistry of till and humus in the Kotkajärvi CuCoAu prospect, Kalvola, southern Finland

A small CuCoAu occurrence was found by means of till geochemistry. The metal anomalies in the till are highly local and the fineness of the gold grains and their association with goethite suggest that Au may first have been dissolved and then reprecipitated. The till very probably contains remnants of an older preglacial weathering crust. There are also high concentrations of Cu, Co, As, Mo and Zn in the humus horizon. Only one humus sample had a Au content above the detection limit, 0.02 ppm. The heavy-mineral samples from the near-surface part of the till proved useful in the regional prospecting phase.

Gold concentration from well water by the hydroelectrical extraction method

An experimental survey of the gold concentrated from well water by a hydroelectrical extraction (HEE) method has been carried out in the famous Au metallogenic province in the eastern part of Shandong Province in northeastern China. This method is based on the existence of ionic Au in groundwater around ore bodies. Under the action of an external electric field, ionic Au migrates toward the cathode where the metal is absorbed by a special absorbent attached to the cathode. Promising areas for further exploration can be evaluated from the amount of Au in the absorbent. Distinct Au anomalies were discovered by the experimental survey. The results from the follow-up study of some of these anomalies have shown that most of them are related to the known placer Au deposits and to the primary Au occurrences. However, two new Au prospects and several new Au sources have been found which offer new targets for Au hunting in the area. The hydroelectrical extraction method is an effective technique of looking for Au in areas covered with transported overburden. Since the Au in well water can be concentrated on the cathode with the absorbent, the sensitivity requirement for Au analysis could be reduced. The method also provides a useful tool for the exploration of blind placer Au.

Geochemical signature (bedrock and saprolite) of gold mineralization and associated hydrothermal alteration at Dorlin, French Guyana

Abstract The Dorlin Au prospect in central French Guyana is located within a Proterozoic volcano-sedimentary belt where numerous old Au workings are known. Several coincident Au anomalies in soils and stream sediments were defined in a 1975 regional reconnaissance program. These were followed up by soil and saprolite auger-sampling surveys.