Gold Veins Research Articles

Trace element geochemistry of scheelite and rutile from metaturbidite-hosted quartz vein gold deposits, Meguma Terrane, Nova Scotia, Canada: genetic implications

Scheelite and rutile from several metaturbidite-hosted gold-bearing quartz vein deposits of the Meguma Terrane of Nova Scotia were analyzed for trace elements including rare earth elements, niobium and tantalum. Scheelites have high concentrations of Sr, Nb, Y and rare earth elements (REE) with bell-shaped chondrite-normalized REE patterns accompanied by both positive and negative Eu anomalies. They also have high Nb/Ta ratios (80–300). Three distinct trace element types of the scheelites are interpreted to reflect chemical differences in the pulses of hydrothermal fluids. Hydrothermal rutiles have high contents of W (up to 4.2 wt.% WO3), are rich in Ta compared to Nb and have a very low Nb/Ta ratio (~0.3). Hydrothermal fluids which produced both scheelite with a high Nb/Ta and rutile with a low Nb/Ta ratio are an efficient medium for fractionation of this ratio although these two minerals play an important role during the process.

Geology and Fluid Characteristics of the Mina Velha and Mandiocal Orebodies and Implications for the Genesis of the Orogenic Chega Tudo Gold Deposit, Gurupi Belt, Brazil

The Chega Tudo gold deposit is located in the Gurupi belt of northern Brazil. Gold mineralization in the Mina Velha (upper level) and Mandiocal (intermediate and lower levels) orebodies is hosted in highly strained Paleoproterozoic (2148‐2160 Ma) felsic to intermediate metavolcanic rocks and schists that record greenschist facies metamorphism. The orebodies are broadly concordant with the regional northwest-oriented foliation, and mineralization comprises thick quartz vein sets and small quartz-carbonate-sulfide veinlets and enclosing hydrothermally altered host rocks. The hydrothermal mineral assemblage is postpeak metamorphism, syn- to late tectonic, and includes quartz, calcite, chlorite, white mica, pyrite, subordinate chalcopyrite, and traces of sphalerite and galena. Free-milling gold occurs in sulfide-poor veins in the upper and lower zones of the deposit, whereas refractory gold was deposited in fractures of pyrite and quartz in the more gold-enriched intermediate (±lower) level. Fluid inclusion and stable isotope (O, H, C, and S) data indicate that the two styles of gold deposition were produced by a combination of fluid immiscibility, fluid-rock reactions (sulfidation, carbonatization), and probably mixing, occurring in two discrete stages within a single mineralizing event associated with fluid flow within and around an active shear zone under fluctuating pressure conditions. The refractory gold was deposited at 340° to 370°C from a CO2-CH4-H2O‐NaCl fluid having salinity of 5.8 ± 2.7 wt percent NaCl equiv, variable CO2 contents (typically 12‐22 mol %), and up to 6 mol percent CH4. The δ18O and δD values of this fluid are 7.9 to 9.4 per mil and ‐29 to ‐37 per mil, respectively, indicating a metamorphic origin. Carbon isotopes show contrasting values in fluid inclusion CO2 (δ13C = ‐24.1‰) and calcite (δ13C = of ‐3.8‰). The strongly negative value of fluid inclusion CO2 is interpreted to be an organic signature acquired at the site of deposition by reaction of the deeply sourced ore-forming fluid with carbonaceous schists present in the deepest part of the deposit. The higher value for calcite probably records a crustal source. Oxygen fugacities calculated for the whole range of T-P-XCO2 conditions yielded log fO 2 between ‐28.7 and ‐30.5, indicating reduced conditions for the fluid, which is in agreement with the mineral paragenesis, fluid inclusion compositions, and sulfur isotope values. The second fluid, responsible for the deposition of the free-milling gold in veins and probably part of the gold of the lower zone of the deposit, had a lower temperature (330°‐340°C). This CO2-H2O‐NaCl fluid had a salinity of 1.6 to 2.5 wt percent NaCl equiv and contained 11 to 13 mol percent CO2. The δ18O and δD values of this fluid are 5.3 to 7.2 per mil and ‐12 to ‐30 per mil, respectively, also indicating a metamorphic source. The δ13C value of fluid inclusion CO2 is ‐6.9 per mil, likely representing a mantle-derived carbon source. Log fO 2 values between ‐30.5 and ‐31.5 also indicate slightly more reduced conditions for this fluid. Both fluids had near-neutral pH, between 5 and 6.2, and δ34S values of H2S between ‐0.2 and ‐1.0 per mil, interpreted to reflect derivation of sulfur from magmatic sulfides.

Borehole magnetics navigation: An example from the Stratmat Deposit, Bathurst, New Brunswick

Information acquired from studies of borehole core or from borehole geophysical logs form an integral part of all mineral and oil exploration programs. Yet, as is the case of any survey, the value of that information is dependent upon how well the location of each observation point is known. Location information becomes especially critical when the resource target has limited depth extent. For example, a location error of 10 m when evaluating a 5-m thick gold vein can make the difference between an economic and noneconomic deposit. The trajectory of a borehole is commonly computed using an array of data points that are acquired progressively with increasing distance along the borehole. Depending on the type of survey employed, observation points may be sparse (static readings at a limited number of points) or redundant (dynamic surveys where the distance between observation points is less than the length of the rigid probe). At each observation point, three parameters are normally acquired: (a) inclinatio...

Geologia do depósito de Au Cavalo Branco (Botuverá - SC)

Cavalo Branco is a quartzvein gold deposit located in the Botuvera region (SC, Brazil). The deposit is genetically related to the emplacement of a small diorite pluton in a conjugated fracture of a shear zone affecting metapelitic hornfels from the Brusque Group, formed by granitic plutons emplaced around the deposit. The mineralizing fluid that formed the gold quartz vein has generated potassic, phyllic and propilithic hydrothermal zones around the vein. Gold, with small amounts of pyrite, chalcopyrite and galena, has crystallized during the development of the phyllic zone. A shear zone reactivation has deformed part of the gold bearing quartz vein and provided the rising of pink granite veins. New hydrothermal fluids related with the reactivation displaced gold and sulfides from the sheared portion of the deposit and promote carbonatization and chloritization of the host rocks. Recrystallized gold shows silver (23,20%) and copper (0,51%) grades a little higher than primary crystallized gold (19,95% e 0,01%, respectivelly). Recrystallization has not changed the sulfide compositions. Primary mineralization has the same characteristics of periplutonic granitogenic gold veins. Although the shear reactivation has changed gold composition and the alteration style of the host rocks, these changes are not considered enough to justify the classification of the deposit as “orogenic”. The facies of the diorite that formed the deposit and the pink granite dykes emplaced during the shear reactivation were not yet known in the Botuvera region.

Cretaceous reduced granitoids in the Goodpaster Mining District, east central Alaska

The >5 Moz (1 oz (troy) = 31.103 g) gold veins of the Liese Zone and nearby prospects in the Goodpaster Mining District of east central Alaska are spatially and temporally associated with late-Early to Late Cretaceous reduced granitoids that are divided into a granite suite, tonalite suite, and diorite suite in decreasing age. Synkinematic to postkinematic biotite ± hornblende granite, granodiorite, pegmatite, and two-mica granite with accessory garnet compose the granite suite (109–107 Ma). The tonalite suite (107–103 Ma) forms small to large bodies of postkinematic hornblende–biotite granodiorite to tonalite with rare granite. Intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike. The diorite suite (95.4–93.7 Ma) consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to intrude the shallowly dipping auriferous quartz veins. Limited data indicate granitoid emplacement at 5–9 km depths, consistent with formation of the auriferous quartz veins based upon published fluid inclusion data. The weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous, contains <5% magnetite, and lacks monazite. All suites are subalkalic, calc-alkaline and have low magnetic susceptibilities, high large-ion lithophile element/high field-strength element (LILE/HFSE), and depleted Nb and Ti. The granite suite has higher206Pb/204Pb values (19.4–19.6) than the diorite (19.1). Overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhumation, whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust.

Geology, Geochemistry, Stable Isotope, and Fluid Inclusion Investigation of the Iron Oxide-Gold Mineralization in Bakir Tepe, Kangal-Sivas, East-Central Turkey

The Bakir Tepe area is characterized by four different lithologic units of Devonian-Carboniferous age. These units, from bottom to top, are metasiltstone, metacarbonate, metasandstone, and metaquartz sandstone. Metamorphic rocks in the study area are tectonically overlain by the recrystallized Mesozoic Munzur limestone. Iron oxide-gold mineralization in the Bakir Tepe area occurs in two different localities. The first is an alteration zone, which was formed at the thrust contact of metasiltstone with metacarbonate and metasandstone; the second locality is in metaquartz sandstone. The dominant minerals are quartz + hematite/magnetite ± pyrite ± gold in veins of various lengths and widths. Magnetite occurred after hematite, along fractures in hematite. Oxygen isotope data (δ18O = 6.2-13.3‰) from fluids in equilibrium with quartz, and hydrogen isotope data (δD = -55 to -75‰) from fluid inclusions trapped in quartz, indicate that the hydrothermal fluids responsible for formation of the gold mineralization were dominated by metamorphicderived waters. Hydrothermal solutions were of the NaCl-CO2-H2O type. Salinity values as NaCl equivalent wt% range between 34 and 39. Total homogenization temperatures range between 132 and 382°C in secondary L+V inclusions, and 237 to 324°C in primary L+V+H inclusions. Calculated temperatures of vein formation using oxygen isotopes in quartz-hematite mineral pairs yields formation temperatures between 200 and 330°C, consistent with the fluid inclusion measurements.

Syngenetic gold in western Victoria: occurrence, age and dimensions

In western Victoria, a widespread stratiform style of gold enrichment in Palaeozoic black mudstone and chert—clearly different from the classic mesothermal quartz vein deposits of the Victorian goldfields—has been confirmed by whole-rock geochemistry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This enrichment pre-dates compaction, deformation and low-grade metamorphism of the sedimentary host-rocks, and therefore possibly developed diagenetically during slow deposition of the thin carbonaceous black mudstone beds and the thinner layers of chert. These paired strata have been documented at many locations in three regional outcrop areas of chevron-folded Cambrian and Lower Ordovician metasediments in the Stawell and Bendigo Zones, where they are interbedded with quartz-rich turbidites. The layers were named ‘indicators’ by the early miners, who found locally rich nuggety gold deposits at intersections between these layers and mesothermal quartz veins. Gold is present in euhedral pyrite crystals in both black shale and chert. LA-ICPMS analysis of individual pyrite crystals in the indicator beds shows that pyrite is enriched in Au, As, Sb, Se, Te and Bi. The Au content of pyrite varies from 0.03 to 2.69 ppm with a mean of 0.58 ppm and shows a positive correlation with As, which varies from 1000 to 6000 ppm. Many pyrite crystals show enrichment of gold in their cores and depletion in their rims, confirming the likely syngenetic or syndiagenetic accumulation of gold during pyrite formation in the sediments. Prior to regional metamorphism, folding and faulting, the many indicator strata in the outcrop areas were parts of an extensive marine sequence of Late Cambrian and Early Ordovician age. The former primary source of this mineralisation is considered to have been one or more contemporaneously exhalative submarine hydrothermal systems. Thus, the older Palaeozoic sediments of the western Lachlan Fold Belt were significantly enriched in syngenetic gold in the Early Palaeozoic, at least 40 million years before emplacement of the quartz – gold vein deposits of the goldfields.

The Bisha Volcanic-Associated Massive Sulfide Deposit, Western Nakfa Terrane, Eritrea

The Neoproterozoic Bisha volcanic-associated massive sulfide deposit (VMS) is a large (>39 million metric tons (Mt)) relatively high grade Zn-Cu-Au-Ag deposit in a newly discovered VMS district in western Eritrea. The host stratigraphic section comprises a large, possibly coeval and cogenetic intrusion known as the Bisha Gabbroic Complex, ~3 km downsection, a series of mafic to felsic tuffs with minor flows and minor sedimentary rocks in the proximal footwall, and felsic flows, fine tuffs, and volcanic-derived siliciclastic rocks in the proximal hanging wall. Minor siliceous exhalite units extend for several kilometers along strike and are present upsection. Two other VMS deposits are present along strike: Bisha Northwest, 1.5 km to the north-northwest, and Harena, 9.5 km to the south-southwest; additionally, a large, lower grade VMS deposit, Hambok, is present ~15 km to the southwest. The Bisha deposit is a single, nearly inclusion and dike-free lens, 1.2 km along strike, that has been folded and affected by significant, near-surface oxidation and supergene processes. The deposit and host strata form an overturned synform within a probable, west-over east, kilometer-scale nappe structure. It would appear that the strata have been folded and thrust against the gabbroic complex which behaved as a competent buttress during deformation. The oxidation and supergene enrichment have led to the development of four mineralogically and spatially distinct ore types or zones. The hematite-goethite-quartz gossan, from 0- to ~30-m depth, has high gold, silver, and lead contents, with native gold, argentiferous gold, electrum, anglesite, and lesser cerrusite. The kaolinite-quartz-sulfate zone occurs at ~25- to 35-m depth, and also is enriched in gold, silver, and lead, and has the same ore mineralogy as the gossan; additionally it has minor chalcocite and minor to trace amounts of unusual Zn-Pb-Fe carbonate and sulfate minerals. The supergene sulfide zone is present from ~35- to 65-m depth and has significant copper enrichment, with predominant chalcocite, and lesser digenite, covellite, and bornite, along with pyrite, minor pyrrhotite, anglesite, and gangue minerals. The primary massive sulfide, from ~65- to more than 450-m depth, has a significant volume of zinc-rich ore and a typical VMS sulfide assemblage of pyrite, sphalerite, chalcopyrite, and minor to trace galena, pyrrhotite, tetrahedrite, tennantite, arsenopyrite, barite, and gangue. The average ore and trace metal contents of the four ore types are calculated from an extensive assay and whole-rock geochemical database. The average metal contents for primary massive sulfide are comparable to that of other bimodal-siliciclastic deposits using an extended, primitive mantle-normalized plot. The ore metals, Cu, Pb, Zn, Au, and Ag, are 2.5 to 3 orders of magnitude higher than average continental crust, whereas the trace metals are more variable. Manganese is significantly depleted in comparison to the average crustal value. Moderate to strong chloritic alteration and stringer sulfide veins are present in footwall felsic and intermediate tuffs along the entire strike length, immediately east of the eastern massive sulfide lens. This is high-lighted using a chlorite alteration index, in wt percent: (MgO + FeO)*100/(MgO + FeO + CaO + K2O + Na2O). In addition, the chloritic footwall is depleted in silica, alkali, and alkaline earth elements, and europium. In contrast, these elements are enriched in the hanging-wall tuffs, indicating that the Bisha hydrothermal system continued after deposition of the tuffs. Primary geochemistry of mafic to felsic volcanic and nearby intrusive rocks, including the Bisha Gabbroic Complex, indicate a bimodal igneous suite, with evidence for cogenetic and possibly coeval volcanic and intrusive rocks. The suite has tholeiitic to transitional calc-alkalic affinities, with mantle plume-influenced, primitive arc signatures for least altered basalts. Lead isotope ratios of galena and anglesite mineral separates from the Bisha, Bisha Northwest, and Harena deposits have model ages that range from 790 to 770 Ma. This is in contrast to a lead model age of ~720 Ma for the Adi Nefas VMS deposit in the eastern Nakfa terrane and to galena from a nearby gold vein deposit at ~490 Ma. The model ages for the Bisha area VMS deposits indicate that these deposits and their host rocks formed early during the development of the 850 to 550 Ma Nakfa terrane.

Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ∼625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632–616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689–617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607–583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au–Ag–Zn–Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn–Cu–Fe–Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu–Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620–600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen.

An epigenetic origin for the Passagem de Mariana gold deposit, Quadrilátero Ferrífero, Minas Gerais, Brazil

The gold–tourmaline quartz–vein deposit of Passagem de Mariana, in the southeastern part of the Quadrilátero Ferrífero, produced more than 60 tonne of gold, from the end of the 17th Century until 1954. The mine has not operated since 1985. Orebodies are veins composed of white quartz (> 60 vol.%), carbonate (ankerite), tourmaline, sericite and sulfides. Tourmaline (dravite), up to 10 vol.% of the vein, occurs as subhedral, coarse, commonly zoned crystals, and is concentrated along vein boundaries and on the edges of host rock inclusions in the veins. Tourmaline is present in all rock types in the mine, but the chemical composition of the host rocks determinates the intensity of tourmalinization, with the alteration being greater in sericitic phyllites, graphite–sericite phyllites, and calcareous rocks. The most abundant sulfide is arsenopyrite, which is normally associated with pyrite and pyrrhotite. Minor amounts of chalcopyrite, galena, löllingite, berthierite, and maldonite are present throughout the deposit. Sulfides are concentrated at veins boundary or are dispersed in the veins. Arsenopyrite is associated, most commonly, with calcareous rocks, and graphite–sericite phyllite. Pyrrhotite is usually found at the base of itabirites. Gold abundance is directly proportional to sulfide concentration. Hydrothermal alteration associated with the veins includes silicification, tourmalinization, and sulfidation. The mineralized zone is a shear zone associated with a bedding-parallel thrust fault that juxtaposes the itabirite (Lower Proterozoic Minas Supergroup) over other units. This shear zone/thrust fault extends for tens of km beyond the Passagem mine and hosts numerous gold deposits. The richest orebodies are along the itabirite footwall contact and within the graphite–sericite phyllite (Main orebody). Although many lithologic units were mineralized the graphite–sericite phyllite appears to have been most favorable for gold deposition. The area underwent three phases of deformation, D 1, D 2 and D 3. Mineral assemblages indicate upper-greenschist to lower-amphibolite conditions of regional metamorphism. Retrograde metamorphism, characterized by chloritization of biotite and chloritization and biotitization of garnet, developed locally. The gold-bearing veins crosscut the main foliation and lithologic contacts at low angle and occur within, or are in contact with, all lithotypes. Field and laboratory data indicate that gold mineralization at Passagem de Mariana is epigenetic. Gold deposition occurred after the peak of metamorphism, within the late- to post-D 2 period of deformation, which is correlated with second set of structures of Trasamazonian age of Alkmim and Marshak [Alkmim, F.F., Marshak, S., 1998. Transamazonian Orogeny in the Southern São Francisco Craton Region, Minas Gerais, Brazil: evidence for Paleoproterozoic collision and collapse in the Quadrilátero Ferrífero. Precambrian Research 90, 29-58.], indicating that the gold mineralization occurred between 2.124 and 2.04 Ga. We choose to regard Passagem de Marina as an orogenic gold deposit as defined by Groves et al. [Groves, D. I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., Robert, F., 1998. Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geology Reviews 13, 7-27.], i.e., an epigenetic, structurally-hosted lode–gold vein system in a deformed metamorphic terrane.

Eocene Gold Ore Formation at Muteh, Sanandaj-Sirjan Tectonic Zone, Western Iran: A Result of Late-Stage Extension and Exhumation of Metamorphic Basement Rocks within the Zagros Orogen

A multidisciplinary study including field geology, microstructure analysis, 40Ar/39Ar dating, and fluid inclusion microthermometry and Raman spectrometry has been carried out on the Muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the Sanandaj-Sirjan tectonic zone, Zagros orogen, Iran. The Muteh gold deposit has been previously interpreted as genetically related to Precambrian granitic intrusions, as an exhalative hot-spring deposit related to Paleozoic rhyolitic-acidic tuffs, and to local metamorphic processes. Host rocks of the gold deposit are predominantly schist and gneiss, and subsidiary amphibolite and quartzite, and are intruded by leucogranites. In the vicinity of the gold deposit, the metamorphic and granitic rocks display a subhorizontal mylonitic foliation, containing a northeast-oriented stretching lineation. Field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest-oriented normal faults and joints, dipping to the northeast and the southwest, and crosscutting the ductile fabric of the host rocks. Both the ductile fabric of the host rocks and the gold ore-controlling brittle structures are interpreted to have formed within a single, continuous extensional event, which started with ductile deformation and gradually changed into brittle deformation. Hydrothermal alteration associated with ore formation consists of quartz, muscovite, pyrite, dolomite-ankerite, and albite, which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks. Pyrite is the dominant opaque mineral and is the major phase associated with gold. Chalcopyrite, marcasite, bismuth, galena, sphalerite, and pyrrhotite are subsidiary to rare phases. 40Ar/39Ar incremental-heating and in situ laser-ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the Muteh gold deposit. Muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40Ar/39Ar ages between 55.7 and 38.5 Ma and show that gold mineralization is the youngest among the different dated events. This Eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults, which can be correlated with Tertiary extensional tectonic events reported by previous field investigations. The data also indicate that gold ore formation is coeval with magmatism in the adjacent Tertiary Urumieh-Dokhtar magmatic belt and with intrusive activity within the Sanandaj-Sirjan zone, with a 40Ar/39Ar age of 54 Ma near Muteh according to our study. The remaining 40Ar/39Ar data reveal a Cretaceous to early Tertiary metamorphic, magmatic, and deformation history of the complex hosting the Muteh gold deposit. The fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later Eocene gold ore formation. Early regional fluids consist of an H2O-CO2-NaCl fluid with subsidiary N2 and CH4 that underwent local unmixing. The salinities of the liquid-rich inclusions resulting from unmixing range between 2.2 and 17.5 wt percent NaCl equiv, and those of the CO2-rich, vaporrich inclusions range between 2.8 and 7.2 wt percent NaCl equiv. The high salinity of the regional liquid-rich fluids is attributed to metamorphism of evaporite-bearing sedimentary rocks, magmatic intrusions, retrograde hydration reactions, metamorphism of impure marbles, and migmatization. The fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low-salinity aqueous fluid with a dilute, low-density CO2 component, and maximum salinities between 2.6 and 5.8 wt percent NaCl equiv. Gold deposition at Muteh occurred during or after late stages of Eocene brittle extension as a result of exhumation of the host metamorphic complex, possibly during magmatic activity in the Sanandaj-Sirjam zone. It is most likely linked to the introduction of a distinct low-salinity, low CO2 content aqueous fluid during crustal extension. These fluids are younger than, and therefore unrelated to, the highly saline and CO2-rich, regional metamorphic-magmatic fluids. Our results refute previous models advocating Precambrian to Paleozoic ages of the Muteh gold deposit.

Geological and structural controls on gold mineralization in the Tanami District, Northern Territory

Gold mineralization in the Tanami district is hosted within moderately northwest dipping turbiditic sedimentary and basaltic volcanic rocks of the Paleoproterozoic Mt. Charles Formation. The gold occurs within a complex sinistral wrench-fault array and associated veins and alteration haloes. The main mineralized faults have a northerly trend and dip steeply east. Subsidiary structures trend at 030° and 070° and dip towards the southeast. Paleostress calculations based on fault striation populations and geometry (strike and dip) of faults indicate that at the time of the mineralizing event, σ 1 was sub-horizontal and SE–NW directed with σ 2 subvertical. Structural studies indicate that the mineralization occurred after the regional folding event and synchronous with the emplacement of felsic dykes into the mine sequence. Gold veins in the Tanami district are interpreted to be part of an outer thermal aureole gold system that formed during the emplacement of granitoids in the nearby ∼1,815 to ∼1,799 Ma Frankenia and/or Coomarie domes. Economic gold mineralization occurred late in the paragenetic history of the district. Gold is hosted by quartz-carbonate veins within shear zones, and also in the surrounding sericite- quartz- pyrite ± carbonate-altered wallrocks. Gold-mineralized veins precipitated at depths of 3 to 6 km from high temperature (∼300°C), low salinity (∼5 wt% NaCl equivalent) fluids with low CO2 contents. Barren quartz, dolomite and calcite veins that occur in pre- and post-mineralization thrust faults formed from high salinity (∼20 wt% NaCl equivalent), low temperature (∼120–150°C) basinal brines. Pyrite in the gold mineralized veins and alteration halos has lower δ 34S values (6.8 to 12.5‰) than local diagenetic pyrite (17.8 to 19.2‰) or pyrite in pre-mineralization thrust faults (31.7 to 37.1‰). The mineralizing fluids are inferred to have contained a well-homogenized mixture of magmatic and sedimentary-derived sulfur.

Textural and chemical zonation of pyrite at Pajingo: a potential vector to epithermal gold veins

The Carboniferous Pajingo Epithermal System (PES) comprises several low sulphidation Au–Ag ore zones that have a total resource of c . 3 Moz (million ounces). The main Vera–Nancy vein is hosted by andesite that contains alteration which zones from inner silica–pyrite to argillic to distal propylitic alteration. Pyrite is ubiquitous in both alteration and vein. In the latter, pyrite is a minor component (<1%), but the dominant sulphide, and occurs as fine-grained (<50 μm) bands or disseminations. Coarse (up to 0.5 mm) euhedral pyrite is most abundant (up to 10%) in the silica–pyrite alteration whereas in argillic alteration pyrite (<50 to 250 μm) occurs as round to subhedral forms either as infill in narrow veinlets or as replacement. Fine-grained (<50 μm) pyrite occurs in propylitic alteration along rims of earlier mafic phenocrysts and has a low abundance (<1%). In-situ laser ablation inductively coupled plasma mass spectrometry was carried out on the pyrite in order to test the chemical variations between pyrite in different alteration zones. Results indicate that Mo and Ag concentrations vary over two orders of magnitude from the outer alteration zones (Mo c . 0.1 ppm; Ag c . 0.5 ppm) to the proximal vein alteration (Mo c . 30 ppm; Ag c . 20 ppm) whereas Pb displays an overall decrease from the hanging wall (a few hundred ppm) to the footwall (<100 ppm). Pyrite in the hanging- wall argillic alteration zone has Pb/Mo and Pb/Ag ratios of >100 whereas pyrite in silica–pyrite alteration zones has ratios of <100, and pyrite in silica–pyrite alteration zones immediately adjacent to the vein has Pb/Mo ratios of <30 and Pb/Ag ratios of <10. Thus, Pb/Mo and Pb/Ag ratios may provide a potentially powerful vector to epithermal gold veins in the PES and elsewhere.

Structural and lithological controls on the high-grade Hangingwall Reef quartz – gold veins, Stawell, Victoria

The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites and jogs in which the auriferous quartz veins formed. Results from 3D numerical modelling show that dilation along the Stawell Fault provided the conduit for fluid flow.

The assessment of the radiation hazard indices due to uranium and thorium in some Egyptian environmental matrices

The estimation of exposures of humans to the various sources of radiation is important. Instrumental neutron activation analysis has been used for the determination of uranium and thorium in environmental samples such as Toshki soil, Aswan iron ore, gold vein deposits and phosphate samples from Upper Egypt. The specific activities due to 238U and 232Th have been measured and the radiation hazard to the workers, the Radium equivalent activities, representative level index and dose rates were also estimated. The lowest external γ-radiation dose received by the workers is 33.15 ± 6.06 µSv y−1 which comes from gold samples and the highest one was 315.65 ± 7.98 µSv y−1 which comes from phosphate samples which is far below the worldwide allowed dose of 20 mSv y−1 for workers (ICRP-60 1990).

Trace element geochemistry of the 1991 Mt. Pinatubo silicic melts, Philippines: Implications for ore-forming potential of adakitic magmatism

The dacite pumice erupted from Mt. Pinatubo on June 15, 1991 (whole-rock, rhyolitic groundmass glasses and homogenized melt inclusions) has been analyzed using inductively coupled plasma-mass spectrometry (ICP-MS), nanosecond and femtosecond laser ablation ICP-MS and secondary ion mass spectrometry (SIMS) to evaluate its ore-forming potential. Data suggest that adakite magmas are metal-rich and concentrate ore metals during magmatic differentiation. Sulfides segregate in limited amounts under the hydrous, oxidizing conditions typical of adakitic magmas resulting in incompatible behavior for Au (6–22 ppb), Cu (26–77 ppm), and Pb, Mo, As, and Sb in melts of dacitic to rhyolitic compositions. Metal transfer from this adakite magma to the coexisting aqueous phase was favored by the peraluminous composition of the rhyolitic melt and high aqueous chloride concentrations. Mass balance calculations suggest that the pre-eruptive aqueous phase could have extracted a minimum of 100 t Au and 5 × 10 5 t Cu from the Mt. Pinatubo magma. Our data suggest that intrusives having adakitic signatures are genetically associated with Au–Cu and Cu–Mo mineralization, auriferous porphyry copper deposits, and epithermal gold veins. High H 2O, Cl, Sr/Y, Pb/Ce, Mo/Ce, As/Ce and Sb/Ce in Mt. Pinatubo melts reflect the contribution of deep fluids derived from subducted sediments and altered MORBs in the dacite genesis. The slab-derived fluids carrying mobile elements are likely responsible for the enrichment of adakite magmas in gold, associated metals and H 2O, and may explain the exceptional ore-forming potential of adakite magmatism.

‘Radiomarathon’ in the Guinness World of Records: A tool for building a strong corporate reputation through corporate social responsibility

Is it in corporations' long‐term interests to exceed their social and environmental obligations and deliver a superior ethical performance? This is one of the key questions raised in the debate launched by the European Commission's green paper last year regarding the concept of corporate social responsibility (CSR). It is clear that the commercial world must rise to the challenge of building the confidence and trust demanded by stakeholders worldwide. There is a broad consensus on the need for businesses to take the social, economic, and environmental impact of their actions — the ‘triple bottom line’ — into account. Businesses are an integral part of the society (local and international) and they have to consider the impact their behaviour has on it. Many businesses highlight the links between CSR and their wider sustainable development agenda. Corporate reputation or image depends on how the company conducts or is perceived as conducting its business. Today the ability to build a sustainable corporate reputation is more important than ever before as stakeholders are more educated, more knowledgeable, and more demanding. The Cyprus Popular Bank, the second largest banking organisation in Cyprus, has developed and launched ‘Radiomarathon’ in support of children with special needs, which has won a place in the Guinness World of Records as the most successful charitable event in the world on the basis of per capita contribution, and was chosen among the top five charity events worldwide by the Chartered Institute of Bankers for 2003. Radiomarathon has been used in order to build a strong corporate identity and corporate reputation: “With the Radiomarathon we have hit a vein of gold…our corporate reputation is stronger than ever before! In such a turbulent climate, a positive corporate reputation can play a vital role in ensuring that the organisation is on a solid footing. ” (Yiannos Pissourios, Cyprus Popular Bank). The bank realised the importance and need for corporate social responsibility in their efforts to build a good corporate reputation and achieve competitive differentiation.

Mass transfer during gold precipitation within a vertically extensive vein network (Sigma deposit - Abitibi greenstone belt - Canada). Part I. Patterns of hydrothermal alteration haloes

The Sigma deposit is a large network of Au-mineralized veins that crosscut a sequence of andesitic volcanics and volcanoclastics intruded by porphyritic diorite and feldspar porphyry dikes of the Archean Abitibi greenstone belt of Canada. The network is mainly made of shear zone-hosted, steeply dipping veins and subhorizontal extension veins filled for the most part by quartz and tourmaline. Hydrothermal alteration around these veins is made of coeval albite-rich and tourmaline-rich visible haloes, and formed during multiple stages of shearing of the host rocks, opening, growth, and interconnection of vein segments, and during gold precipitation within veins and wall rocks. Appropriate scale mapping of vein outcrops located within a 1.3 km-deep section of the deposit is used here to gather quantitative geometrical data on the vein network and the alteration haloes developed at the vein walls. These data show that the albite- and tourmaline-rich haloes have no preferential distribution within the deposit, form patterns that are asymmetrical along and across vein strike, and are mainly controlled by the local three-dimensional arrangement of the veins. Vein gold grades do not correlate with metric properties of veins like thickness and depth. Results suggest that the hydrothermal alteration at Sigma developed mainly from diffusive transport of mass through the wall rocks. They also suggest that the control of local topology of vein arrays on alteration patterns, underestimated by previous studies of Sigma, may be important in other structurally complex vein network deposits. Scale mapping of both mineralized veins and hydrothermal alteration envelopes has proved useful in constraining the structural relations between vein growth and fluid-rock interaction and in determining locations of most intense wall-rock metasomatism, which provides an objective guide to the choice of samples for geochemical studies.

The Gandy and Abolhassani Epithermal Prospects in the Alborz Magmatic Arc, Semnan Province, Northern Iran

The Gandy and Abolhassani epithermal precious and base metal deposits occur in the Torud-Chah Shirin mountain range in the Alborz magmatic belt of northern Iran. The mountain range is considered to be part of the Paleogene Alborz volcanic arc. The exposed rocks in the study area consist of a volcaniclastic sequence of thin-bedded siltstones and sandstones, lapilli tuffs, volcanic breccias, and intermediate lava flows at Gandy, and mostly andesitic flows at Abolhassani. The flows are middle to upper middle Eocene age and they show a typical arc geochemical signature, with low concentrations of Nb, Ta, Zr, Hf, and Ti. Variable hydrothermal alteration occurs in scattered outcrops, covering about 4 km^2 at Gandy and 1 km^2 at Abolhassani. The Gandy and Abolhassani areas are about 3 km apart, and each contains a small abandoned Pb-Zn mine. Mineralization at Gandy occurs in quartz sulfide veins and breccias and is accompanied by alteration halos of quartz, illite, and calcite up to 2 m wide. The mineralization is divided into three main stages: brecciation (I), fracture filling (II), and crustiform banding (III). Stage I is economically important in terms of precious metal content. Stage II consists of four substages and contains the majority of base metal ore with quartz, calcite, and barite. Native gold is commonly found within partially oxidized pyrite and secondary iron (hydr)oxides such as goethite in stage I and coexists with galena and chalcopyrite in stage II. The final stage is dominated by quartz and calcite. Mineralization in the Abolhassani veins occurred in three main stages. The first two stages, which are economically important, contain similar mineral assemblages, including quartz, calcite, barite, galena, sphalerite, pyrite, and chalcopyrite, whereas the final stage is dominated by quartz and calcite. No gold grains were found in the Abolhassani samples. The average (max) assays from 14 channel samples of Gandy veins are 14.5 (68.3) g/t Au, 30.6 (161) g/t Ag, 3.1 (13) wt percent Pb, 0.84 (3.8) wt percent Zn, and 1.0 (6.3) wt percent Cu. For comparison, the values from 19 channel samples of Abolhassani veins are 0.85 (6.0) g/t Au, 29.5 (115) g/t Ag, 6.4 (16.5) wt percent Pb, 1.2 (5.2) wt percent Zn, and 0.83 (7.7) wt percent Cu. Fluid inclusion and sulfur isotope compositions were analyzed for the sulfide-sulfate assemblages of stage II at Gandy and stages I and II at Abolhassani. In both cases fluid inclusion assemblages were examined mostly in subhedral crystals of sphalerite. The average homogenization temperatures (T_h) and salinities of fluid inclusion assemblages from Gandy range from 234° to 285°C, with a peak at about 250°C and 4.2 to 5.4 wt percent NaCl equiv. These T_h values are in good agreement with isotopic temperatures from two sphalerite and galena pairs (236° and 245°C). The temperature and salinity values in fluid inclusion assemblages from the Abolhassani deposit range from 234° to 340°C and 6.7 to 18.7 wt percent NaCl equiv. Sulfide pairs of sphalerite-galena do not give reasonable isotopic equibrium temperatures at Abolhassani. Comparison of Th versus ice melting (T_(m(ice))) values for the two deposits indicates the presence of a moderate-salinity fluid (5–6 wt % NaCl equiv) of similar temperature (~250°C) in each deposit but with a higher temperature and salinity component also present at Abolhassani. The base metal-rich mineralization at Abolhassani may thus have been caused by the periodic injection of this higher salinity fluid. The Abolhassani deposit has a higher average Ag/Au ratio (~35) and Pb + Zn concentration (up to 7.6 wt %) than Gandy (Ag/Au ~ 2 and 3.9 wt %), consistent with this interpretation. The minimum depth of formation was at least 430 m below the paleowater table for Gandy and possibly as much as 600 m at Abolhassani. The lower grades of gold and the presence of higher salinity fluids at Abolhassani suggest that the occurrence of higher grade precious metal zones is unlikely at greater depth. By contrast, Gandy may have potential at depth for extensions of the high-grade gold veins. Exploration in the region should focus on areas with geologic evidence for relatively little posthydrothermal erosion, <200 to 300 m, thus increasing the preservation potential of epithermal veins with high gold grades, similar to those at Gandy. Results from Gandy and Abolhassani may aid exploration and assessment of the numerous, untested epithermal and related prospects along this 1,800-km-long volcanic belt in northern Iran.

Strong arsenic enrichment in sediments from the Elqui watershed, Northern Chile: industrial (gold mining at El Indio–Tambo district) vs. geologic processes

Sediments from the Elqui watershed river system (9800 km 2; northern Chile) are highly enriched in arsenic. The river system initiates in the high altitude domain of the Andes (3500–4000 m) and drains important hydrothermal alteration zones and epithermal deposits, including the copper-, and arsenic-rich gold veins of the well-known El Indio–Tambo district. In order to study the extent, source, and age of contamination, we carried out a geochemical survey of stream sediments and older, Early Holocene lacustrine sediments. The results reveal very high contents of arsenic in both, the stream (55–485 ppm As) and lacustrine sediments (119–2344 ppm As). As shown by the ESEM-EDX study, arsenic is associated with the iron oxide mineral phase. Given that arsenic is enriched in both the stream sediments and the Early Holocene sequence, we suggest that contamination is not only industrial derived (mining operations), but also a major geological process, related to long-lived erosion of the As-rich epithermal ores and alteration zones. Erosion in this realm is particularly important during El Niño years, when increased activity of the Westerly winds brings intense rains, with catastrophic consequences in the Elqui watershed, including the massive removal of sediments in the form of large debris flows.