Au Prospect Research Articles

Geological characteristics and ore‐forming conditions of the Tasikmadu porphyry Cu–Au prospect in Trenggalek, East Java, Indonesia

Tasikmadu is a newly discovered porphyry Cu–Au prospect in the eastern Sunda arc, Indonesia. This study aimed to elucidate salient diagnostic characteristics and ore‐forming conditions of the prospect. Fieldwork and various laboratory analyses for a suite of representative samples were performed for mineralogy, bulk‐geochemistry, mineral chemistry and ore fluid characterization. The study area is composed of three diorite porphyries, that is, fine‐grained, medium‐grained and coarse‐grained diorite porphyry, respectively. The intrusions are calc‐alkaline with a high Sr/Y value, which is similar to many ore‐bearing intrusions in the eastern Sunda arc. Ore mineralization occurs in quartz veins and veinlet stockwork, centred in the potassic zone, and dominated by chalcopyrite and bornite occurring in A and B veins, which cut earlier barren (EB) and M veins. The mineralization core has an average grade of 0.63 wt% Cu and 0.25 ppm Au, respectively. Outwardly, the potassic zone changes to the propylitic zone, which still bears copper in the quartz and pyrite veinlets, although the grade is low. Fluid inclusion microthermometry revealed that the A and B veins in the potassic zone formed at 464 and 390°C by hypersaline boiling fluids, respectively. The temperature temporally and spatially decreased, that is, in the propylitic zone, the quartz veinlets formed at 260–400°C. Hypogene mineralization that formed the A veins occurred at 1.5 km below the palaeosurface, indicating a relatively shallow depth as a porphyry deposit. Nevertheless, the δ34SCDT values of sulphides range from −2.0 to −0.1‰, inferring a magmatic origin. The Tasikmadu prospect shares some similarities compared with other porphyry deposits worldwide, but it also reveals unique characteristics that differ from others, for example, potassic‐altered rocks are only typified by secondary biotite without/rare secondary K‐feldspar reflecting the lack of magma contamination by continental crustal components. In addition, current surface geological features and shallow depth erosion level of the prospect may imply that the potential of Cu–Au mineralization underneath is still open to depth.

Sources, transport, and deposition of metal(loid)s recorded by sulfide and rock geochemistry: constraints from a vertical profile through the epithermal Profitis Ilias Au prospect, Milos Island, Greece

Drill core samples from the Profitis Ilias Pb-Zn-Cu-Ag-Au vein mineralization on Milos Island, Greece provide new insights into (i) the metal sources, (ii) the primary vertical metal(loid) distribution, and (iii) the supergene enrichment processes in a transitional shallow-marine to subaerial hydrothermal environment. Metal contents of unaltered and altered host rocks combined with Pb isotope analyses of hydrothermal sulfides suggest that most metal(loid)s were derived by leaching of basement rocks, whereas the distinct enrichment of Te is related to the addition of Te by a magmatic fluid. The trace element contents of base metal sulfides record decreasing Au, Te, Se, and Co, but increasing Ag, Sb, and Tl concentrations with increasing elevation that can be related to progressive cooling and fluid boiling during the hypogene stage. The formation of base metal veins with porous pyrite hosting hessite inclusions at ~ 400 m below the surface was triggered by vigorous fluid boiling. By contrast, the enrichment of native Au associated with oxidized Fe and Cu phases in the shallower part of the hydrothermal system resulted from supergene remobilization of trace Au by oxidizing meteoric water after tectonic exhumation to subaerial levels. Disseminated pyrite with higher Tl/Pb ratios and locally elevated Hg concentrations relative to vein pyrite reflects infiltration of the host rocks by boiled liquids and condensed vapor fluids. The vertical and temporal evolution of the Profitis Ilias mineralization, therefore, provides unique insights into the transport and precipitation of Au, Ag, Te, and related metal(loid)s by multiple fluid processes.

Geophysical exploration and geological appraisal of the Siah Diq porphyry Cu–Au prospect: A recent discovery in the Chagai volcano magmatic arc, SW Pakistan

AbstractDiscovery of the Siah Diq porphyry (Cu–Au) prospect in the foothill of Dam Koh volcano is a recent exploration success story of mineralization buried under a 46 m alluvium cover in an exploratory mature Chagai belt. Acquisition of geophysical data followed by drilling and logging was key in the discovery. Integrated magnetics and induced polarization (IP) surveys in an area of 7.5 km2, pointed out magnetic-low, IP-high, and resistivity-low anomalies corresponding to porphyry Cu–Au type sulfide mineralization. Three bore holes were drilled to test the geophysical anomalies. After careful observation and geoscientific logging of core, porphyry style Cu–Au mineralization was revealed. The porphyry prospect was further characterized based on host rock lithology, petrography, alteration mineralogy, ore vein characterization, and Cu/Au geochemical assays based on core samples. Rocks hosting the mineralization include andesite, granodiorite, coarse, as well as fine grained diorite and pink granite, all highly altered, mineralized and porphyritic. Propylitic alteration was dominant in all the three bore holes and developed earlier followed by phyllic, potassic, and argillic alterations. Sulfide mineralization is present as cross-cutting stockwork veins and disseminations. Average copper and gold assays of drill core are 0.17% Cu and 0.78 ppm Au, respectively. Economically insignificant values of molybdenum and silver have been noted in some samples.

The mineral deposits of Panama: Arc metallogenesis on the trailing edge of the Caribbean large igneous province

The mineral deposits of the Panama microplate are hosted by a composite volcano-plutonic island arc of Late Cretaceous to Quaternary age developed on an oceanic plateau, the Caribbean Large Igneous Province (CLIP), on the western, trailing edge of the Caribbean Plate. Large igneous provinces are formed by gold and chalcophile element-enriched, mantle plume-related magmas, which may explain the strong metal endowment of Panama of about 32 Mt copper and >984 t gold. The progressive collision of the Panama arc with South America since the middle Eocene resulted in deformation and oroclinal bending of the arc and metallogenic belts. In western Panama, the copper-gold belts young from south to north away from the subduction zone, whereas in eastern Panama the belts young from north to south. An Early Arc of late Campanian to Eocene age (71-34 Ma) developed related to northerly subduction of the Farallon Plate. It has submarine Si-Mn-Fe exhalite deposits in the Nombre de Dios and Montijo belts with showings of Au-rich VMS Cu-Zn mineralization. The San Blas porphyry Cu-Au belt formed in eastern Panama (Rio Pito and other porphyry Cu-Au prospects), while the western continuation, offset by 200 km sinistrally to the Azuero Belt, hosts high sulphidation epithermal Au-Cu deposits (Cerro Quema and others) in porphyry lithocaps.The arc migrated to the northern Azuero-Soná belt in the middle to late Eocene with the formation of epithermal Au-Ag-Pb-Cu mineralization, and a porphyry Au prospect (Soná). A Middle Arc developed in the Oligocene to lower Miocene (31-18 Ma) related to the NE-dipping subduction of the Farallon Plate followed by the Nazca Plate. In eastern Panama, the arc formed intermediate sulphidation epithermal Au deposits in high grade breccias (Espiritu Santo de Cana mine) and carbonates (Rio Mogue prospect), and porphyry Cu mineralization (Ipeti). The eastern Panama arc shut down in the lower Miocene as a result of the change of the Nazca Plate convergence to strike slip. The Middle Arc in western Panama, on the northern side of the Central Cordillera, hosts the Petaquilla belt of porphyry Cu (the supergiant Cobre Panama porphyry Cu-Mo-Au-Ag deposit) and epithermal Au (Molejon) deposits, and probably the epithermal Au deposits of the Veraguas belt. The Later Arc of Miocene to Quaternary age (18-0 Ma) of the Central Cordillera of western Panama hosts low sulphidation epithermal Au vein and breccia-hosted deposits (Capira, Remance, Santa Rosa), high sulphidation epithermal Au deposits (Cerro Lloron, Rio Liri), and porphyry Cu deposits (the supergiant Cerro Colorado porphyry Cu-Mo-Au-Ag deposit and the Cerro Chorcha porphyry Cu-Au deposit). Uplift of the young porphyries in the western part of the belt is related to the subduction of the Cocos aseismic oceanic ridge. Deposits formed by Quaternary weathering include lateritic bauxite and iron ore in the Chiriqui-Veraguas belt; heavy mineral Fe-Ti sands in beach/marine placer deposits in the Gulf of Panama; and extensive placer Au deposits in Northern Darien, Darien, Chepo, Coclé, Veraguas, western Azuero-Soná, and many other small deposits.

Stable C, O, and S Isotope Record of Magmatic-Hydrothermal Interactions Between the Falémé Fe Skarn and the Loulo Au Systems in Western Mali

Abstract The Gara, Yalea, and Gounkoto Au deposits of the >17 Moz Loulo mining district, largely hosted by the Kofi series metasediments, are located several kilometers to the east of the 650-Mt Fe skarn deposits in the adjacent Falémé batholith. The Au deposits are interpreted to have formed through phase separation of an aqueous-carbonic fluid, which locally mixed with a hypersaline brine of metaevaporite origin. Recognition of an intrusive relationship between the Falémé batholith and Kofi series opens the possibility that the Fe skarns and Au deposits are part of the same mineral system. In this paper, we combine new δ13C, δ18O, and δ34S data from the Karakaene Ndi skarn, Au occurrences along the western margin of the Kofi series, and zircons within plutonic rocks of the Falémé batholith. We combine these with existing data from the Loulo Au deposits to model the contribution of magmatic volatiles to Au mineralization. C and O isotope compositions of auriferous carbonate-quartz-sulfide veins from the Loulo Au deposits have wide ranges (δ13C: –21.7 to –4.5‰ and δ18O: 11.8 to 23.2‰), whereas values from carbonate veins in Kofi series Au prospects close to the Falémé batholith and the Karakaene Ndi Fe skarn deposit have more restricted ranges (δ13C: –16.8 to –3.7‰, δ18O: 11.4 to 17.2‰, and δ13C: –3.0 ± 1‰, δ18O: 12.6 ± 1‰, respectively). Kofi series dolostones have generally higher isotopic values (δ13C: –3.1 to 1.3‰ and δ18O: 19.1 to 23.3‰). Pyrite from Kofi series Au prospects adjacent to the Falémé batholith have a wide range of δ34S values (–4.6 to 14.2‰), similar to pyrite from the Karakaene Ndi skarn (2.8 to 11.9‰), whereas δ34S values of pyrite and arsenopyrite from the Loulo deposits are consistently >6‰. Comparison of the C and O isotope data with water-rock reaction models indicates the Loulo Au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilization of sedimentary rocks with an organic carbon component. Isotopic data are permissive of the hypersaline brine that enhanced this phase separation including components derived from both Kofi series evaporite horizons interlayered with the dolostones and a magmatic-hydrothermal brine. This magmatic-hydrothermal component is particularly apparent in O, C, and S isotope data from the Gara deposit and Au prospects immediately adjacent to the Falémé batholith.

Epithermal Mineralization in the Busang Southeast Zone, Indonesia: New Insight into the Au Prospect at the Center of the Bre-X Fraud

The Busang mineral prospect in Kalimantan, Indonesia, was reported to host a large Au resource until 1997 when it was revealed that drill core samples had been deliberately and systematically contaminated (“salted”) with extraneous Au to falsify resource estimates. One month before the fraud was uncovered, Dr. G. Milligan, then professor emeritus of geology, visited the site to collect a suite of core samples for academic study that was deemed representative of the host rocks, alteration, and mineralization of the Busang Southeast Zone. These samples were re-examined here by optical microscopy, electron microprobe (EMPA), whole-rock geochemistry, and fluid inclusion microthermometry to characterize the subsurface geology and hydrothermal mineralization, and to assess reasons why the system is of uneconomic character. The host rocks were variably altered calc-alkaline porphyritic subvolcanic diorites, typical of the lithological units along the mineralized trend in the Kalimantan Gold Belt. Early hydrothermal mineralization with quartz-sulfide (pyrite, chalcopyrite, Cu-sulfosalts) stockwork veinlets associated with pervasive phyllic and propylitic alteration was overprinted by crudely banded quartz-carbonate-sulfide/sulfosalt (pyrite, sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bournonite-seligmannite) veins. The stockwork veins were associated with up to 140 ppb bulk rock Au, some of which was hosted by Cu-sulfosalts. Microthermometry on quartz-hosted aqueous fluid inclusion assemblages (FIA; n = 13) and single inclusions (non-FIA; n = 20) in quartz-carbonate-sulfide/sulfosalt veins yielded an overall range in homogenization temperatures (Th) between 179 °C and 366 °C and bulk salinities between 1.1 wt.% to 8.6 wt.% NaCl equivalent, with much smaller data ranges for individual FIA (e.g., FIA 3; 239.1 °C to 240.5 °C and 0.5 wt.% to 1.4 wt.% NaCl equivalent). Primary FIA along growth zones in quartz were identified, providing constraints on fluid characteristics at the time of quartz growth. Carbonate-hosted FIA (n = 3) and single inclusions (non-FIA; n = 3) in the same veins yielded Th between 254 °C and 343 °C and bulk salinities of 1.1 wt.% to 11.6 wt.% NaCl equivalent. Likewise, data ranges for individual FIA were much smaller. Many of the geological characteristics of the Busang Southeast Zone were compatible with a telescoped, intermediate-sulfidation epithermal system, having formed from diluted magmatic fluids that precipitated weak base metal mineralization. However, the system was unproductive with respect to Au and Ag, at least within the studied area. Of note, vein textures and fluid inclusion characteristics indicative of boiling or efficient fluid mixing—processes both considered critical for the formation of economic lode gold deposits—were absent in the samples.

A shear-hosted Au-Cu-Bi metallogenic event at ~1660 Ma in the Tennant Creek goldfield (northern Australia) defined by in-situ monazite U-Pb-Th dating

High-grade Au, Cu and Bi ores in the Tennant Creek goldfield have been mined from hydrothermal magnetite- and/or hematite-rich ironstone bodies. Less well known is a style of Au-Cu-Bi mineralisation hosted by quartz vein systems within shear zones outside ironstones. We report ion probe (SHRIMP) U-Pb-Th ages of 1659 ± 13 Ma and 1659 ± 15 Ma for hydrothermal monazite associated with this mineralisation style at the Orlando East Au-Cu-Bi deposit and Navigator 6 Au prospect, respectively. The dated monazite at Orlando East is intergrown with Cu and Bi sulfides and Au in quartz-chlorite veins within a shear zone. At Navigator 6 the monazite occurs both in late-stage chalcopyrite-bearing quartz-hematite veins and within earlier-formed sheared chloritic breccia that also contains anomalous Au.Although the full significance of the ~1660 Ma monazite ages has yet to be determined, the results indicate the introduction of Au, Cu and Bi in quartz vein systems at ~1660 Ma, or remobilisation of earlier, ~1850 Ma, main-stage Au-Cu-Bi from mineralised ironstones and into quartz veins 10 s–100 s of metres from the initial mineralisation. In either case, hydrothermal fluids capable of transporting significant concentrations of ore metals were present in the Tennant Creek goldfield at ~1660 Ma, synchronous with deformation. A key implication is that rock sequences up to ~200 million years younger than the ironstone-hosted Au-Cu-Bi deposits may now be considered prospective for a second stage of Au ± Cu ± Bi mineralisation. The results also validate non-ironstone shear-hosted deposits in the Tennant Creek region as an alternative exploration target to the well known high-grade but low-tonnage Tennant Creek ironstone-hosted deposits.

Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji

The Navilawa caldera is the remnant of a shoshonitic volcano on Viti Levu, Fiji, and sits adjacent to the low-sulfidation Tuvatu epithermal Au–Te deposit. The caldera occurs along the Viti Levu lineament, approximately 50 km SW of the Tavua caldera, which hosts the giant low-sulfidation Emperor epithermal Au–Te deposit. Both calderas host alkaline rocks of nearly identical age (~5.4–4.6 Ma) and mineralization that occurred in multiple stages. The gold mineralization in these locations is spatially and genetically related to monzonite intrusions and low-grade porphyry Cu-style mineralization. Potassic, propylitic, phyllic, and argillic alteration extends from the Tuvatu Au–Te deposit towards the central, northern, and eastern parts of the Navilawa caldera where it is spatially associated with low-grade porphyry Cu–Au mineralization at the Kingston prospect and various epithermal Au–(Te) vein systems, including the Banana Creek and Tuvatu North prospects. Chalcopyrite, and minor bornite, occurs in quartz–calcite–(adularia) veins in the Kingston deposit associated with weak propylitic and phyllic alteration, whereas NE-trending epithermal gold veins at the Banana Creek and Tuvatu North prospects are associated with weak potassic alteration that is overprinted by propylitic and phyllic alteration. Gold is accompanied by chalcopyrite, galena, and sphalerite in quartz–pyrite veins that also have a Ag–As–Hg–Te signature. The temperature range for phyllosilicates in the phyllic alteration (chlorite ± smectite ± corrensite ± illite) is in good agreement with temperatures recorded from previous fluid inclusion studies of quartz at the Banana Creek Au prospect (~260 °C) and the nearby Tuvatu Au–Te deposit (205 to 382 °C). Sulfur isotope compositions of pyrite (−6.2 to +0.4‰) from the Banana Creek prospect indicate a likely magmatic source of sulfur. Oxidation of the ore fluids or a direct addition of volatiles to the hydrothermal fluids may account for the lighter isotopic values. The similarities of the igneous rock types and compositions, transition from porphyry- to epithermal-style mineralization, alteration assemblages, paragenetic relationships, and stable isotope data suggest a common origin for the porphyry- and epithermal-style mineralization within the Navilawa and between the Navilawa and Tavua calderas.

Dispersion of gold and other metals by trees, gravels and soils near Boddington Gold Deposit, Western Australia

South west Western Australia is host to some of the world's largest mineral deposits including bauxite, Ta and Au. The giant Boddington Gold Mine exploits one such deposit and is located in forested areas south east of Perth. Exploration for Au in this area has concentrated on sampling surficial Fe-rich lateritic residuum and its degradation products and was how Boddington was originally discovered; it has a multi-element signature. Despite its location in forest there has been little biogeochemistry undertaken in this area or consideration of whether trees (including eucalypts) are agents of dispersion of metals in this terrain. Eucalyptus trees have been shown to be responsible for forming anomalies above gold deposits in semi-arid areas of Australia and the location of Boddington in a humid environment provided an important comparison in which to investigate this possibility.The Golden Triangle Au prospect near Boddington Gold Deposit is located on the flank of a small lateritic hill. Limited shallow drilling has identified sub-economic mineralisation with some metre composite cuttings grading >1ppm Au. A selection of different trees and shrubs, organic soil and ferruginous pisoliths were collected over a surface traverse of 800m at 50m intervals from across the prospect. Samples were analysed for major elements, Au and several pathfinder metals. Additional foliage samples were collected from the same trees over mineralisation and background to test for sample heterogeneity.Golden Triangle vegetation samples were generally anomalous in Au and pathfinder elements (e.g. Ag, Bi, W and Sn). For example, Banksia and Macrozamia had elemental anomalies in Bi and Ag located directly above or down slope of mineralisation although for Eucalyptus the anomaly over mineralisation itself was poorly defined. Topsoil containing organic matter overlying the ferruginous pisoliths was particularly anomalous in Au (mean of 47ppb against of background of <5ppb). Pathfinder element anomalies were present in all sample media (soil, vegetation and pisoliths) although the specific elements and tenor of the anomaly varied between sample media.The results demonstrate a biotic (plant) influence on the dispersion of metallic elements at Golden Triangle prospect. This process of dispersion may serve to not only disperse and dilute metals in the regolith but potentially create a larger exploration target to detect the presence of the mineralisation. The anomalous metal content of the pisoliths themselves may be due to an earlier biotic mechanism of metal mobilisation from the deeper regolith or be solely a result of inorganic processes as previously thought. The sampling of deep-rooted trees and soil beneath them may assist in the discovery of mineral deposits in areas where there is transported material or leached regolith.

Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: Examples at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and other localities

The felsic-dominant Gawler Range Volcanics and cogenetic Hiltaba Suite granitoids constitute the ca. 1590Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. The province includes minor occurrences of olivine-phyric basalt at Kokatha and Mount Gunson. In this study, we describe additional olivine-phyric basalts and dykes intersected by drill holes at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and the Wirrda Well Cu–Au prospect. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb dating results (un-anchored) of apatite in the basalts and dykes at Mount Gunson (1576±33Ma), Wirrda Well (1596±17Ma), and Olympic Dam (1621±20Ma) confirm their effective temporal correlation with the ca. 1590Ma Gawler silicic large igneous province.Compositions of Cr-spinel inclusions enclosed in former olivine phenocrysts (forsterite number >80) in basalts at Mount Gunson, Wirrda Well and Olympic Dam imply derivation from a heterogeneous mantle source that may have been modified by subduction. Least-altered olivine-phyric basalts at Mount Gunson and Kokatha are characterized by negative Nb and Ta anomalies, which are typical of arc basalts but are also common in back-arc basin basalts, in accordance with the result suggested by the Cr-spinel source indicator. The mafic components of the Gawler Range Volcanics generally have higher Zr contents and Zr/TiO2 ratios than those of high-Mg basalts and picrites produced in variable tectonic settings worldwide, possibly reflecting continental crustal components involved in their mantle source.High abundances (typically ∼20vol.%) of former olivine phenocrysts in the basalt at Olympic Dam imply high-Mg whole-rock compositions and a high temperature of the primary magma. This result is consistent with previous models in which the heat flux from mantle magmas caused large-scale partial melting of crustal rocks that subsequently gave rise to silicic magmas erupted and intruded to form the Gawler silicic large igneous province.

The Chahnaly Low-Sulfidation Epithermal Gold Deposit, Western Makran Volcanic Arc, Southeast Iran

The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

Kuh-e Dom Fe–Cu–Au prospect, Anarak Metallogenic Complex, Central Iran: a geological, mineralogical and fluid inclusion study

The Kuh-e Dom Fe–Cu–Au prospect is located in the Urumieh-Dokhtar Magmatic Belt, and is characterized by copper–iron oxide and gold veins, stockworks and breccias hosted by the Eocene Kuh-e Dom arc intrusion. Mineralization is located within NE–SW to WNW–ESE sinistral faults and likely formed in a subduction-related continental margin that is typical of IOCG deposit systems. The deposits have a distinct metal composition of Fe, Cu, Bi, Co, Mo and LREE with gold (up to 3 g/t), and the mineral assemblages are quartz, hematite, pyrite, chalcopyrite, emplectite, magnetite, free gold, calcite, barite, chlorite, and tourmaline. Three paragenetic stages of mineralized quartz veins are distinguished in the Kuh-e Dom prospect, including: (i) hematite-bearing quartz veins, (ii) quartz-sulfide stockwork and breccia veins, and (iii) quartz-calcite±sulfide infilling veins. Sodic (albitization), potassic, and quartz–calcite±chlorite pervasive alterations are commonly associated with these three mineralization stages. Three types of fluid inclusions have been identified at Kuh-e Dom, including: aqueous two-phase (H2O−NaCl−CaCl2±FeCl2), halite-saturated aqueous (H2O−NaCl±KCl), and CO2-bearing (H2O–CO2±CH4 and CO2±CH4) fluid inclusions. A hypersaline (~35 wt% NaCl equiv.), aqueous magmatic fluid was released at about 400 °C and a pressure of nearly 4 kbar, forming early hematite-bearing quartz veins. These high salinity fluids were progressively diluted further away from Kuh-e Dom intrusion due to substantial input of meteoric water and mixing with the magmatic components during the middle and late stages of mineralization. The mineralogy, alteration, and fluid composition of the Kuh-e Dom Fe–Cu–Au prospect compared well with Fe oxide Cu–Au (IOCG) deposits worldwide.

Regional setting and characteristics of the Neoproterozoic Wadi Hamama Zn–Cu–Ag–Au prospect: evidence for an intra-oceanic island arc-hosted volcanogenic hydrothermal system

The Wadi Hamama area is a volcanogenic Zn–Cu–Au–Ag prospect. It is hosted by a Neoproterozoic bimodal-mafic sequence, which comprises basalt, dacite and rhyolite along with volcaniclastic rocks. The rocks have a low-K tholeiitic affinity and are enriched in large ion lithophile elements over high field strength elements, which indicated their formation in an intra-oceanic island arc tectonic setting. The area was intruded by a tonalite–trondhjemite body, which has an intra-oceanic island arc affinity and later by diorite, which has a cordilleran-margin geochemical affinity. These rock units were intruded by post-tectonic granite dykes, which have a within-plate geochemical signature. There is a quartz-carbonate horizon extending along the contact between the basalt and the volcaniclastic rocks, mainly banded and lapilli tuffs. This horizon is of exhalative origin and is underlain by a mushroom-shaped alteration zone extending from the horizon down to the massive basalt. The footwall alteration is characterized by a silica-rich core surrounded by a thick chlorite sheath. Both the quartz-carbonate horizon and the footwall-altered rocks enclose historical trenches and pits. Sulfide-rich core samples are enriched in Zn, relative to Cu, and in Ag, which indicates the low-temperature nature of the hydrothermal system. The prospect was affected by supergene processes, which led to the widespread occurrence of secondary copper minerals and gold enrichment relative to the leached base metals, especially Zn. The prospect formed through a limited rifting of an intra-oceanic island arc which resulted in the formation of a small-scale volcanogenic Zn–Cu–Ag–Au prospect.

Epithermal Au and polymetallic mineralization in the Tulasu Basin, western Tianshan, NW China: Potential for the discovery of porphyry Cu[sbnd]Au deposits

The Tulasu basin is an important epithermal Au mineralization district in western Tianshan in northwestern China. Geochemical and geochronological studies of the volcanic host rocks (the Dahalajunshan Formation) and associated intrusive rocks, particularly the enrichment of large-ion lithophile elements and depletion of high field strength elements, suggest that the Tulasu basin was located in a magmatic-arc setting from Late-Devonian to Early-Carboniferous, associated with the southward subduction of the North Tianshan Ocean beneath the Kazakhstan–Yili plate. Geologic and geochemical characteristics of the AuCu–polymetallic deposits in the Tulasu basin suggest that the mineralization is related to the Late-Devonian to Early-Carboniferous arc magmatism, and is mainly of epithermal nature, including both the adularia–sericite type (Axi, Tawuerbieke and Tabei) and alunite–kaolinite or acid–sulfate type (Jingxi–Yelmend and Tieliekesayi). However, some evidence of porphyry-type mineralization has also been found, including the presence of mineralized porphyry enclaves in volcanic rocks hosting the Tawuerbieke Au prospect, and the development of porphyry-style Cu mineralization overprinted by vein-type mineralization at Kexiaxi. Considering the magmatic-arc setting that is favorable for both epithermal and porphyry types of mineralization, and the development of such mineralization systems in other parts of western Tianshan and the Altaid Belt (e.g., Uzbekistan, Kazakhstan and Kyrgyzstan), we suggest that major porphyry CuAu deposits may have been developed in the Tulasu basin, probably beneath or adjacent to known epithermal Au mineralization.

Supergene and Hypogene Halloysite in a Porphyry-Epithermal Environment at Cerro la Mina, Chiapas, Mexico

Abstract Halloysite, a phyllosilicate clay mineral chemically similar to but structurally different from kaolinite, occurs in a variety of mineral deposit types. It is, however, difficult to identify and dehydrates quite readily. When identified in a porphyry or epithermal environment, halloysite is often interpreted to be a low-temperature polymorph of kaolinite and an indication of supergene processes. The occurrence of halloysite in the Cerro la Mina Au (Cu-Mo) prospect, Chiapas, Mexico, was investigated to determine if halloysite in porphyry-epithermal deposits might also be the product of hypogene processes. The prospect consists of Quaternary volcaniclastic breccias intruded by monzodiorites and trachyandesites crosscut by a volcanic-hydrothermal breccia pipe. Gold-copper-molybdenum mineralization at Cerro la Mina is structurally and lithologically controlled by matrix-rich breccia pipes hosting all of the significant alteration and mineralization drilled to date. Average grades within the prospect are 0.4 ppm Au, 0.16% Cu, and 131 ppm Mo. A total of 100 samples were obtained from 22 diamond drill holes over a range of stratigraphic levels. X-ray diffraction (XRD) and SWIR techniques were employed to identify the different clay minerals found within these samples, including halloysite, kaolinite, dickite, and illite. Identification of halloysite and kaolinite were facilitated by the use of formamide intercalation to produce distinctive peaks for the two minerals in X-ray diffractograms. Halloysite and kaolinite were found to occur from the current erosional surface to depths of over 800 m. Halloysite occurs as both the hydrated 10Å and dehydrated 7Å forms within the deposit. The morphology of the halloysite present is that of well-developed tubes and spheroids. Results of the study show that halloysite associated with gypsum and jarosite is of supergene origin, whereas halloysite occurring with quartz, alunite, dickite, kaolinite, and pyrite is of hypogene origin.

Calcrete and plant inter-relationships for the expression of concealed mineralization at the Tunkillia gold prospect, central Gawler Craton, Australia

Transported and typically weathered material that extends over much of Australia presents a challenge for the successful application of surficial geochemical exploration methods. The Tunkillia Au-prospect in the Gawler Craton, South Australia provides a case study where unconsolidated and potentially laterally transported aeolian and alluvial sediments overlie a deeply weathered profile with variable supergene and primary Au mineralization at depths typically exceeding 40 m. The Tunkillia Au prospect was discovered through a regional calcrete sampling program, but elevated Au concentrations in this material do not always express underlying mineralization, such is the case at Tomahawk, an area of elevated Au in calcrete results overlying limited, low grade mineralization. The distribution and sampling accessibility of calcrete is also not continuous across the landscape, where in this region young siliciclastic linear dune ridges bury older calcrete that are otherwise exposed or only at shallow depths in dune swales or across sheetwash plains. Vegetation cover is more continuous across this landscape and is dominated by several main species. Sampling of dominant plant species: Casuarina pauper , and Eucalyptus concinna , has been conducted across Tomahawk where some sites of elevated Au in calcrete have been drilled. Results show a limited spatial association between calcrete geochemistry and plant biogeochemistry. This demonstrates that plant sampling is able to express low grade, discrete areas of buried Au mineralization typically on the scale of tens of metres, whereas the expression provided from calcrete is more levelled, and typically representative of larger areas, especially when they have formed within transported detritus host materials. It is recommended that although plant biogeochemistry and calcrete geochemistry may have their strengths and weaknesses, such as scale constraints, in regional geochemical exploration programs when these methods are integrated they potentially provide a more comprehensive and widely available geochemical exploration assessment across a wider range of regolith-landform settings.

Experimental studies on the gold-in-calcrete anomaly at Edoldeh Tank Gold Prospect, Gawler Craton, South Australia

Calcrete sampling is the near-surface exploration method of choice for Au in many drier parts of the world, particularly southern Australia. Edoldeh Tank is a weakly mineralised Au prospect in calcrete terrain that lies at the eastern edge of the Great Victoria Desert dunefield (South Australia). At Edoldeh Tank a variety of calcretes occur but the dominant form is a laminated calcrete horizon (LCH). The mineralogy of the near-surface soil is relatively simple and consists of calcite, dolomite, quartz, kaolinite and minor smectite; quartz dominates the unconsolidated overlying sandy soil, and carbonate minerals dominate the LCH. We determine the distribution and nature of the Au at a small scale using a variety of techniques, including SEM, LA-ICP-MS and SXRF and dated sediments to understand calcrete genesis. In a series of thirty excavated soil pits, Ca and Au concentrations increased with depth, markedly so at the LCH. We provide multiple lines of evidence to show there is a general association of Au with calcrete but not a strong correlation as seen with soil profiles elsewhere that have younger, recently formed powdery calcrete. Experiments suggest Au and Ag are currently mobile in this environment despite the low rainfall and that Au occurs in two forms: Au (possibly ionic) occurs throughout the sample with some regions having higher concentrations than others; particulate Au occurs randomly but is more common where the general level of Au is higher. The laminated nature of the calcrete suggests it has formed episodically. An association of Ag with Au in calcrete suggests a means to distinguish anomalies that have developed in residual regolith from those that have dispersed into adjacent sediments. Laminated calcrete is just as effective an exploration sample medium as powdery calcrete. Mobilised Ca, Au and Ag in calcrete can extend the lateral extent and distance from the source of the geochemical anomaly thus providing an effective vector to target for sampling. A landscape dispersion model of Au in calcrete is presented, which requires further testing, to assist the mineral explorer in covered terrains.

The gold-in-calcrete anomaly at the ET gold prospect, Gawler Craton, South Australia

Abstract Much of Australia has an extensive regolith cover that conceals basement rocks and hinders mineral exploration and this situation is particularly acute in the Western Gawler Craton (South Australia), where, in addition to fluvial, marine and colluvial sediments, the land surface is extensively cloaked by sand dunes. This study documents the Au distribution at the ET Au prospect (Great Victoria Desert) in the Western Gawler Craton (South Australia). Although no economic Au mineralisation has yet been found at ET, the prospect hosts one of the larger Au-in-calcrete anomalies in Australia and is typical of many such prospects identified in the region. In addition to calcrete, the distribution of Au in regolith and biotic sample media was also examined. The study at ET shows that: (i) For in situ regolith, Au is concentrated in surficial calcrete and above an upper saprolite zone depleted in Au. (ii) The Au anomaly extends from a ridge of weathered Archaean basement into adjacent and transported regolith dominated by thin (∼5 m) aeolian sand cover. The anomaly appears to be locally broadened by lateral dispersion in the transported overburden, or, possibly, Au additions to the surface from underlying buried mineralisation. (iii) Calcrete appears to be the most consistent sample medium for Au providing coherent anomalies compared with soil, vegetation, near surface drill cuttings and Bacillus cereus . (iv) Gold appears to be the best target element in upper regolith or calcrete, although As may provide supplementary information on the location of prospective mineralisation. Calcrete sampling has been a successful exploration technique to reveal cohesive Au anomalies within in situ regolith. Where transported regolith dominates and landforms are favourable e.g. sloping, calcrete (containing Au) will disperse (in solution and/or mechanically eroding) and thus provide a spatially larger target area for mineral exploration purposes. However, in these settings, the actual source of mineralisation may be difficult to locate due to various factors related to the dispersion processes and weathering history. At ET, other sampling media such as bacteria ( Bacillus cereus ) and vegetation have a greater uncertainty associated with them and their anomalies are not as cohesive.

Andean uplift and climate evolution in the southern Atacama Desert deduced from geomorphology and supergene alunite-group minerals

Supergene alunite group minerals from the Late Eocene El Salvador porphyry Cu district, the El Hueso epithermal gold deposit and the Coya porphyry Au prospect located in the Precordillera of Northern Chile (~ 26 to 26° 30´ Lat. S) have been dated by the 40Ar/ 39Ar method and analyzed for stable isotopes. These data support published geomorphologic and sedimentologic evidence suggesting that the Precordillera in the Southern Atacama Desert had been uplifted as early as the late Eocene and, thus, significantly prior to the Altiplano which attained its high elevation only in the late Miocene. The oldest supergene alunite from the Damiana exotic deposit at El Salvador was dated at 35.8 ± 1 Ma and yielded a δD (VSMOW) value of −74‰ which indicates elevations of the Precordillera near El Salvador of at least 3000 m in the Late Eocene. In contrast, Miocene supergene alunite from El Salvador, El Hueso, and Coya have less negative δD signatures reaching values as high as −23 to −25‰ at El Hueso and El Salvador between about 8.2 and 14 Ma. Late Miocene to Holocene supergene alunite, jarosite and natroalunite ages are restricted to El Hueso and Coya located near 4000 m above sea level in the Precordillera, roughly 1000 m higher than the present elevation of El Salvador. The δD values of samples younger than ~ 5 Ma vary between −57 and −97‰. The complex evolution of the δD signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the Central Depression of the southern Atacama Desert after 15 Ma, which agrees well with published constraints from the Atacama Desert at 23–24° Lat. S. Our data also suggest that wetter climatic conditions than at present prevailed in the latest Miocene and early Pliocene in the Precordillera. The new and previously published age constraints for El Salvador indicate that supergene mineralization at the Damiana exotic Cu deposit occurred periodically over 23 Ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration.

RHENIUM-RICH MOLYBDENITE AND RHENIITE IN THE PAGONI RACHI Mo-Cu-Te-Ag-Au PROSPECT, NORTHERN GREECE: IMPLICATIONS FOR THE Re GEOCHEMISTRY OF PORPHYRY-STYLE Cu-Mo AND Mo MINERALIZATION

The porphyry-type Pagoni Rachi Mo–Cu–Te–Ag–Au prospect, in northern Greece, is a porphyry-epithermal system hosted by an Oligocene dacite porphyry and quartz–feldspar porphyry dikes. The rare mineral rheniite (ReS2) and molybdenite, with very high contents of Re (up to 4.7 wt% Re), occur in quartz veins along with Fe–Cu sulfides, Pb-, Sn-, and Cl-bearing oxides, hematite, ilmenite and tellurides of Bi; these veins are spatially related to sericitic and transitional sericitic–sodic–potassic alteration. Earlier-formed gold-bearing quartz and magnetite veins with sodic–potassic–calcic alteration and late precious-metal telluride-rich carbonate–quartz veins with argillic alteration contain only minor amounts of molybdenite. The composition of rheniite ranges from almost pure, stoichiometric rheniite with low Mo content to rheniite with up to 5.99 wt% Mo. Petrographic, scanning electron microscope, and structural studies suggest that the high Re content of molybdenite from Pagoni Rachi is the result of the isovalent Re-for-Mo substitution in molybdenite. This fact is corroborated by the progressive shortening of the Mo–S mean bond-distance (from 2.414 A in pure molybdenite to 2.355 A), as well as by the isotropic decrease of the unit-cell values with the increase of the Re:Mo ratio. The structural analysis of four molybdenite crystals, with the highest Re content ever reported in nature, demonstrates that they crystallize as the 2 H polytype and not the 3 R polytype, as previously hypothesized, thus suggesting that Re concentration does not correlate with a specific polytype. The fluid inclusions in quartz in the Re-bearing molybdenite–rheniite veins at Pagoni Rachi show that they homogenize to either the liquid or vapor phases (354 to 428°C) or by halite dissolution at 317 to 585°C, which equates to salinities of 40 to 59 wt% NaCl equiv. Rheniite and molybdenite likely precipitated as temperatures fell below 400°C during phase separation under relatively oxidizing conditions, at elevated chlorine activity, and from relatively acid hydrothermal solutions. However, the presence of Pb oxides, Sn-bearing minerals and tellurides is compelling evidence that rheniite and Re-rich molybdenite may have formed directly from the vapor as sublimates, in a manner similar to the way they are deposited at the Kudriavy volcano, Kurile Islands.