Stockwork Zones Research Articles

Effects of deep magmatic degassing and shallow seawater circulation on trace element and sulfur cycling in submarine hydrothermal systems: Insights from the Shijuligou analog, North China

Limited access to modern subseafloor sulfides hampers our understanding of the link between magmatic volatile influx and the cycling of trace elements and sulfur, as well as the effect of the subsequent shallow seawater circulation on these processes. Hence, we studied a well-preserved fossil analog of submarine hydrothermal systems – the Shijuligou volcanogenic massive sulfide (VMS) deposit from the North Qilian Mountains in North China to examine variations in elements and isotopes of subseafloor sulfides vertically.The vertical distribution of trace elements in subseafloor sulfides is strongly controlled by temperature gradients and redox states during the interaction between hot fluids and seawater beneath the paleo-seafloor. While the enrichment of elements like As, Sb, and Au (median: 1335, 43.7, and 0.30 ppm, respectively, n = 21) and negative δ34S values (mean: −3.07 ‰, n = 7) of euhedral pyrites in the jasper, along with the precipitation of high sulfidation minerals (e.g., enargite), suggest the input of magmatic volatiles into hydrothermal systems. During the shallow seawater-hydrothermal circulation, pyrites in the veined and stockwork zones exhibit distinctly elevated δ34S values (up to 15.74 ‰), accompanied by increased concentrations of wall-rock-derived elements (e.g., Cu, Ni, Si, and Ti) and low-temperature-responsive elements (e.g., Pb, Zn, and Cd). Sulfur isotopes of sulfides vary significantly from the surface to the deep ore zones, ranging from −3.36 to 19.84 ‰ (mean: 9.25 ‰, n = 37). The negative δ34S values of pyrites at the paleo-seafloor are due to the addition of H2S derived from the disproportionation of magmatic SO2. The increased δ34S values of stockwork and disseminated sulfides at depth are attributed to the progressive reduction of seawater sulfates by ferrous iron released from the alteration of fresh basalts. The trace elemental and isotopic characteristics of sulfides suggest the Shujuligou VMS deposit resembles the fossil analog of immature, subduction-related submarine hydrothermal systems.

Uranium Mineralization in the MacInnis Lake Area, Nonacho Basin, Northwest Territories: Potential Linkages to Metasomatic Iron Alkali-Calcic Systems

The intracratonic Paleoproterozoic Nonacho Basin, deposited on the western margin of the Rae craton, contains historic polymetallic (i.e., U, Cu, Fe, Pb, Zn, Ag) occurrences spatially associated with its unconformable contact with underlying crystalline basement rocks and regionally occurring faults. This study presents the paragenesis, mineral chemistry and geochemistry of uranium mineralized rocks and minerals of the MacInnis Lake sub-basin of the Nonacho Basin, to evaluate the style and relative timing of uranium mineralization. Mineralization is restricted to regionally occurring deformation zones, and post-dates widely spread and pervasive albitization and more local Ba-rich K-feldspar alteration of host rocks. Uranium mineralized rocks show elevated concentration of Cu, Ag and Au relative to variably altered host rocks. Microscopic and compositionally heterogeneous altered uraninite occurs (i) as overgrowths on partially dissolved Cu-sulphides with magnetite in chlorite ± quartz, calcite veins, and (ii) with minor uranophane in hematite-sericite-chlorite ± quartz breccia and stockwork. Both uraninite types are Th poor (<0.09 wt.% ThO2) and variably rich in SO4 (up to 2.26 wt.%), suggesting a low-temperature hydrothermal origin in a relatively oxidized environment. Rare-earth element (+Y) concentrations in type-i uraninite are high, up to 9.5 wt.% Σ(REE+Y)2O3 with CeN/YN values > 1, similar to REE compositions of uraninite in metasomatic iron and alkali-calcic systems (MIAC), including low-temperature hematite-type IOCG-deposits (e.g., Olympic Dam, Gawler Craton, Australia) and albitite-hosted uranium deposits (e.g., Southern Breccia, Great Bear Magmatic Zone, Canada, and Gunnar Deposit, Beaverlodge District, Canada). Both uraninite types are variably rich in Ba (up to 3 wt.% BaO), a geochemical marker for MIAC systems, provided by the dissolution of earlier secondary Ba-rich K-feldspar. Chemical U-Th-Pb dating yields resetting ages of <875 ± 35 Ma for type-ii uraninite-uranophane, younger than strike-slip movement along regional structures of the basin that are spatially associated with the uranium occurrences. We suggest that MacInnis Lake uranium occurrences formed from oxidized hydrothermal fluids along previously altered (albitized, potassically altered) regional-scale faults. Uranium minerals precipitated on earlier Fe-rich sulfides (chalcopyrite, bornite), which acted as a redox trap for mineralization, in low-temperature (~310–330 °C, based on Al-in-chlorite thermometry) breccias and stockwork zones, late in a metasomatic iron and alkali-calcic alteration system.

Ridge propagation, oceanic core complexes, and ultramafic-hosted hydrothermalism at Rainbow (MAR 36°N): Insights from a multi-scale magnetic exploration

A high-resolution magnetic survey conducted over the apparently inactive ultramafic-hosted hydrothermal site Clamstone reveals a positive magnetic anomaly, in accordance with the magnetic response of other ultramafic-hosted sites known to date. Here this magnetic signature is mainly the result of in-place bulk-magnetized serpentinite underlying the site rather than any concentration of magnetite within the stockwork zone, the latter being probably negligible due to the limited fluid venting. Conversely, the nearby high-temperature active ultramafic-hosted hydrothermal site Rainbow illustrates the key contribution of hydrothermal activity in producing large amounts of magnetite in stockwork zones within ultramafics. At a larger scale, we propose from the analysis of sea-surface magnetic anomalies that the underlying Rainbow Hill has been formed as an oceanic core complex associated with the Amar Minor North segment, later dismantled by the northward propagation of the Amar Minor South segment and the coeval recession of the Amar Minor North segment. This model solves the apparent paradox of a high-temperature hydrothermal site sitting on an oceanic core complex usually associated with cold, poorly magmatic spreading environments: the core complex belongs to the cold, receding Amar Minor North segment whereas the heat fueling site Rainbow comes from the hot, propagating Amar Minor South segment. It also explains why the Rainbow Hill, although interpreted as an oceanic core complex, lacks the typical corrugations of such core complexes – the latter have been dismantled by the deformation associated with the northward propagation of the non-transform offset.

The origin of the sediment-hosted Kanmantoo Cu-Au deposit, South Australia: Mineralogical considerations

Multiply deformed sediments of the Cambrian Kanmantoo Group, which were metamorphosed to the amphibolite facies, host numerous Cu-Au, Fe-S, and Pb-Zn-Ag-(Cu-Au) deposits, of which the largest Cu-Au deposit is Kanmantoo (34.5 Mt @ 0.6% Cu and 0.1 g/t Au). Mineralization at Kanmantoo is characterized by discordant and pipe-like orebodies (Kavanagh and Emily Star) along with mineralization locally concordant to bedding (Nugent) that is spatially related to meta-exhalative rocks. Previous studies have suggested a syn-sedimentary origin for the Pb-Zn-Ag-(Cu-Au) and Fe-S deposits, whereas the Kanmantoo deposit remains controversial, with syn-sedimentary, metamorphogenic, and post-peak metamorphic models having being applied. The stratiform nature of some parts of the Nugent orebody, and disseminated sulfides locally concordant to bedding along with the recognition of a zone of chalcopyrite-magnetite-rich rocks at the syn-sedimentary Wheal Ellen Pb-Zn-Ag-(Cu-Au) deposit, which shows a metallic mineral assemblage almost identical to the most common assemblage at Kanmantoo, supports a genetic link between the Pb-Zn-Ag-(Cu-Au) deposits and Cu-Au mineralization, and is consistent with a metamorphosed syn-sedimentary model for Cu-Au mineralization. The discordant nature of most orebodies at Kanmantoo is the result of the mineralization having formed as stockwork zones in sub-seafloor pipes. Varying degrees of remobilization were subsequently associated with syn- to post-peak metamorphism. Major and trace element compositions, coupled with principal component analyses, show that the compositions of garnet, biotite, staurolite, chlorite, muscovite, and magnetite in metamorphosed altered rocks spatially associated with sulfide mineralization at Kanmantoo can be distinguished from those, where present, in metamorphosed country rocks (i.e., metapsammites and metapelites). Garnet, chlorite, biotite, and muscovite in quartz garnetite within quartz mica schist associated with the Nugent orebody are elevated in Mn (up to 19.5 wt% MnO – garnet, 2,825 ppm – chlorite, 3,206 ppm – biotite, and 108 ppm – muscovite) and Zn (up to 170 ppm – garnet, 1,602 ppm – chlorite, 1,592 ppm – biotite, and 108 ppm – muscovite) relative to samples in other orebodies and in unmineralized rocks elsewhere in the Kanmantoo Group. Such enrichments in these elements mimic similar enrichments in the same minerals in metamorphosed altered rocks associated with Pb-Zn-Ag-(Cu-Au) deposits in the Kanmantoo Group. Biotite in metamorphosed altered rocks at Kanmantoo contains elevated concentrations of Pb (up to 110 ppm), and, in general, Zn (up to 841 ppm), whereas muscovite is also elevated in Pb, Zn, and Cu (up to 272 ppm Pb, 78 ppm Zn, and 173 ppm Cu). Staurolite in these same rocks contains up to 1.6 wt% ZnO (with one outlier that contains 3.2 wt% ZnO) and is considerably more enriched in Zn than in unaltered country rocks (∼0.1 wt% Zn). The trace element enrichments in silicates studied here constitute a potential pathfinder to metamorphosed Cu-Au mineralization in the Kanmantoo Group and emphasize the geochemical and genetic links between the Pb-Zn-Ag-(Cu-Au) and Cu-Au deposits.

Fluid Inclusion Investigations of the Masjed Daghi Copper-Gold Porphyry-Epithermal Mineralization, East Azerbaijan Province, NW Iran

The Masjed Daghi mineralization is located 30 km southeast of Jolfa city at the bank of Araxes River, northwest Iran. This area is situated in the Alborz-Azarbaijan structural zone of Iran. The most widespread rocks in the mineralization area are andesite and trachyandesite, while there are rock units of latite tuff, andesitic agglomerate, and hornblende porphyry basalt in eastern hills and Eocene flysch in the southern part of the area. Several intrusive bodies are present in the study area, from which the dominant intrusive rock hosting the mineralization is diorite porphyry. The mineralized rock units of the area are cut by different diorite ad mafic dikes. The most prevalent texture of mineralization is dissemination, while open space filling textures including veins and veinlets, are common as well. Diverse types of alteration including potassic, phyllic, argillic, silicification, and a little of carbonatization were recognized in the field and microscopic observations as well as by XRD. In addition to thick silica veins and stockwork zones, some silica, barite, sulfide, and calcite veins and veinlets have occurred in the Masjed Daghi mineralization area. In this research, 26 doubly polished thin sections (wafers) were prepared and investigated. Four samples were taken from surface veins, while 22 samples were chosen from core samples (of 6 boreholes) of white and grey-white silica, and silica-barite veins. The fluid inclusion studies on 105 primary fluid inclusions indicated five phases for inclusions including: 1) liquid or gas, 2) liquid and gas, 3) liquid, gas, and solid, 4) liquid, gas, halite, and solid, and 5) liquid, gas, halite, and two types of solids. The data gained from fluid inclusions approved two mineralization fluids which caused porphyry and epithermal mineralizations. The porphyry fluid inclusions were homogenized in temperatures of 122°C to 550°C with a maximum of 700°C and average salinity of 55 wt% NaCl equivalent, while the epithermal inclusions indicated an average homogenization temperature of 186°C with an average salinity of 6.23 wt% NaCl equivalent.

Sulfur and strontium isotopic study of epithermal mineralization: A case study from the SE Afar Rift, Djibouti

Epithermal mineralization was recently described in synrift volcanic rocks from the SE Afar Rift in Djibouti. To infer fluid sources in this system, coupled sulfur and strontium isotopic analyses were performed on mineralized samples from hydrothermal veins and stockwork zones. Mineralization includes gypsum and chalcedony, and/or quartz±carbonate containing gold and sulfides. Sulfur isotopic compositions of sulfides and sulfates were determined using Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC ICPMS); Laser Ablation MC ICPMS; and Continuous Flow Elemental Analyzer-Isotope Ratio Mass Spectrometry while strontium isotopes were determined by Thermal Ionisation Mass Spectrometry (TIMS) methods. Sulfur isotopic composition of sulfides (mainly pyrite) varies from −9.2 to +6.6‰. These values are classically reported for hydrothermal sulfides resulting from the mixing of variable S sources such as seawater sulfate and magmatic source. In addition, individual pyrite grains within the same sample show both positive and negative δ34S values suggesting multi-phase mineralization processes. Coupled sulfur and strontium isotopic compositions of seven sulfate samples hosted in (i) gypsum mound, (ii) evaporites, (iii) stockwork veins in sediments or in volcanic rocks were also investigated to characterize the sources of Sr and S. The δ34S values of the sulfate ranged from −0.8 to +14.3‰ while the 87Sr/86Sr ranged from 0.70389 to 0.70639 consistent with variable mixing ratios of Sr and S derived from volcanic sources and seawater-derived sulfate. This study shows that, although precious-metal deposition is related to low-sulfidation epithermal environment, both acidic fluid of magmatic and saline origins contributed to the hydrothermal system in the SE Afar Rift.

Geology and geochemistry of the Suaqui Verde deposit: A contribution to the knowledge of the Laramide porphyry copper mineralization in south central Sonora, Mexico

The Suaqui Verde porphyry copper deposit is a small ore body of about 20milliontons of oxide ore grading 0.29% copper, located ~8km E-NE of the town of Suaqui Grande in east-central Sonora, northwestern Mexico. The largest and better-known deposits occur in northern Sonora, whereas those from central and southern Sonora are smaller and poorly studied. This paper contributes with new geological and geochemical data to characterize the Suaqui Verde porphyry copper prospect, which may help to understand the porphyry copper mineralization in central Sonora. The Cu mineralization is centered on a 57±0.3Ma old quartz-feldspar porphyry stock, intruding co-magmatic Laramide volcanic and plutonic rocks. The Suaqui Verde porphyry copper system is characterized by widespread phyllic alteration dominated by quartz-sericite, propylitic and local potassic alteration, with Cu, Mo, and Fe sulfide mineralization emplaced in quartz veinlets filling stockwork zones. Quartz-tourmaline breccias, presumably pre-dating the copper sulfide mineralization, occur in the southern part of the deposit, apparently forming the apical parts of the equigranular plutons. Geochemical data of plutonic and volcanic rocks indicate derivation from subduction related calc-alkaline and metaluminous to slightly peraluminous magmas. Light REE enriched normalized patterns, with negative Eu anomalies resemble typical composition of Laramide magmatic rocks in northwestern Mexico. However, positive Eu anomalies in three of the samples indicate that some magmatic stages underwent a process that increased the Eu+2/Eu+3 ratios in the liquid, possibly associated with changes in the oxidation-reduction conditions. Chondrite-normalized multi-elemental diagrams show negative Nb-Ta, P and Ti anomalies, coupled with positive Rb, Th, U, Sr and Pb anomalies, strongly suggesting a subduction-related magmatic arc affinity. The mineralized system was segmented and tilted during the Cenozoic Basin and Range extension, which exhumed the hypogene sulfide zone, promoting supergene enrichment processes and subsequent transport of copper solutions that developed weak exotic copper mineralization.

Element mobility patterns in magnetite-group IOCG systems: The Fab IOCG system, Northwest Territories, Canada

This paper documents element mobility patterns from a magnetite-group Iron Oxide Copper–Gold (IOCG) prospect in the Northwest Territories of Canada and explores implications for space–time chemical evolution of metasomatic systems hosting IOCG deposits. The Fab system, located in the Great Bear magmatic zone (GBMZ) of the Northwest Territories, Canada, contains numerous Fe–Cu–U showings associated with high temperature (HT) potassic–iron alteration overprinting extensive zones of sodic to HT calcic–iron alteration. Each hydrothermal alteration assemblage is associated with distinct element mobility patterns that record evolving physico-chemical properties of the hydrothermal fluids. New geochronological data constrain the alteration and IOCG mineralization in the Fab system to a 3m.y. window between 1870–1867Ma, which is broadly contemporaneous with extensive high-level intrusive activity across the GBMZ. Regional- to local-scale element mobility patterns characteristic of the sodic and sodic–calcic–iron alteration type record leaching combined with weak to strong mass losses. Pure sodic alteration depleted the rocks in Ca, Co, Cu, Fe, Mg, Th, U and V. Conversely, sodic–calcic–iron alteration records significant depletions of Nb, REE, Ta, Ti, Th and U. These element mobility patterns differ from intense HT calcic–iron alteration that is enriched in Ca, Co, F, Fe, Mg, Mn, Ni and V with modest enrichments to locally significant mineralization in Th, U and REE. HT calcic–iron alteration is also characterized by substantial mass gains that translate into volume gains in stockwork zones and mass/volume gains in zones of intense host rock replacement. HT potassic–iron alteration is characterized by enrichments in Ba, K, Ni, U and V, along with locally Co and Cu. The temporal and spatial association of the Fab system alteration and the emplacement of the porphyritic dacite are indicative of the predominant involvement of magmatic–hydrothermal fluids. The high F- and Cl- contents of the porphyritic dacite and of the HT calcic–iron alteration zones as well as Nb, REE, Ta, Th, and Ti mobility provide strong evidences of high halogen activities (F and Cl) in the hydrothermal fluids. High F- and Cl-activities in the hydrothermal fluid are interpreted to have facilitated the mobilization of normally immobile (Nb, Ta, Ti, Th) or weakly mobile elements as well as some metals (e.g., V, Ni, Co). The formation of REE fluorocarbonates and calcite in the early and incipient HT calcic–iron alteration zones indicates the presence of CO2 in the hydrothermal fluids. Weaker HFSE, HREE and Ti mobility during later HT potassic–iron alteration is interpreted to reflect decreasing temperatures, pressures, F-activities and increasing fO2 as the fluids evolved and interacted with the host rocks.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

AbstractHigh‐resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back‐arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep‐sea manned submersible. A new 3‐D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic‐anomaly‐derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic‐anomaly‐derived absolute magnetization suggests that all five andesite‐hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt‐hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low‐magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off‐axis sites than at on‐axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off‐axis sites.

Shallow submarine epithermal Pb–Zn–Cu–Au–Ag–Te mineralization on western Milos Island, Aegean Volcanic Arc, Greece: Mineralogical, geological and geochemical constraints

Milos Island contains several epithermal deposits (e.g., Profitis Ilias–Chondro Vouno Pb–Zn–Ag–Au–Te–Cu, Triades–Galana–Agathia–Kondaros Pb–Zn–Ag–Bi–W–Mo±Cu–Au, and Katsimoutis–Kondaros–Vani Pb–Zn–Ag–Mn) of Late Pliocene to Early Pleistocene age. These deposits are hosted in calc-alkaline volcanic rocks emplaced as a result of three successive magma pulses in an emergent volcanic edifice: submarine rhyolitic to rhyodacitic cryptodomes at ca. 2.7.Ma (Profitis Ilias–Chondro Vouno), submarine to subaerial andesite to dacite domes at ca. 2.2 to 1.5Ma (Triades–Galana–Kondaros–Katsimouti–Vani). Hydrothermal alteration of the volcanic rocks includes advanced argillic- (both hypogene and steam-heated), argillic, phyllic, adularia-sericite and propylitic types. In the northern sector (Triades–Galana–Agathia–Kondaros), initial magma degassing derived from andesitic–dacitic intrusives along NE–SW to E–W trending faults resulted in the development of pre-ore hypogene advanced argillic alteration (dickite, alunite, ±diaspore, pyrophyllite, halite, and pyrite) in a submarine environment. Mineralogical data indicate common features among the Profitis Ilias–Chondro Vouno, Kondaros–Katsimoutis–Vani and Triades–Galana mineralized centers, all of which are characterized by the presence of galena, Fe-poor sphalerite, and chalcopyrite as well as abundant barite, adularia, sericite and, to a lesser extent, calcite, which are typical of intermediate-sulfidation epithermal type deposits. Locally, at Triades–Galana and Kondaros–Agathia, high-sulfidation conditions prevailed as suggested by the presence of coexisting enargite and covellite. The high silver and gold content of the western Milos deposits is derived from Ag-bearing sulfosalts (polybasite, pearceite, pyrargyrite, freibergite) and tellurides. Gold at Profitis Ilias, both as native gold and silver-gold tellurides, is present in base-metal precipitates within multicomponent blebs, which recrystallized to form hessite, petzite, altaite, coloradoite, and native gold. Mineralogical evidence (e.g. microchimney structures, copper sulfides, widespread occurrence of barite, aragonite) suggests that precious metal mineralization in western Milos mineralization formed in a submarine setting.We present information on the surface distribution of Au, Ag, Cu, Pb, Zn, As, Sb, Hg, Mo, Bi, W and Cd at western Milos. Gold is enriched at Profitis Ilias–Chondro Vouno deposits and to a lesser extent at Triades–Galana. Arsenic is absent from the southern sector but shows elevated concentrations together with molybdenum, bismuth and tungsten at the northern sector (Triades–Galana, Vani deposits). The differences in precious and base metal abundances may be related to the depths at which the deposits are exposed, and/or different sources of magma. The metal signatures of the Triades–Galana and Agathia–Kondaros–Katsimouti–Vani (Mo–Bi–W–As–Hg–Ag–Au) occurrences compared to Profitis Ilias (Te–Au–Ag) reflect different sources of magma (dacite–rhyodacite for Profitis Ilias, andesite–dacite for Triades–Galana, and dacite for Kondaros–Katsimoutis). The enrichment of Te, Mo, W, and Bi in the deposits is a strong indication of a direct magmatic contribution of these metals.At western Milos, precious and base-metal vein mineralization was deposited during episodic injection of magmatic volatiles and dilution of the hydrothermal fluids by seawater. The mineralization represents seafloor/sub-seafloor precipitation of sulfides that formed in stockwork zones. Base and precious metal mineralization formed from intermediate- to high-sulfidation state fluids and mostly under boiling conditions as indicated by the widespread occurrence of adularia associated with metallic mineralization. We speculate that the widespread occurrence of boiling and the shallow depth of the precious- and base-metal emplacement prevented the formation of seafloor massive sulfides.

The Himalayan Mianning–Dechang REE belt associated with carbonatite–alkaline complexes, eastern Indo-Asian collision zone, SW China

The Himalayan Mianning–Dechang REE belt, western Sichuan, SW China, is approximately 270 km long and 15 km wide, and contains total reserves of > 3 Mt of LREE, including one giant (Maoniuping), one large (Dalucao) and a number of small-medium REE deposits (Muluozhai, Lizhuang) and occurrences. The belt is tectonically located within a continental collision zone, i.e., the eastern Indo-Asian collision zone. REE mineralization is associated with Himalayan carbonatite–alkaline complexes, which consist of carbonatitic sills or dykes and associated alkaline syenite stocks. Available dating data define a Himalayan metallogenic epoch (40–10 Ma). The magmatic–hydrothermal REE systems were controlled by Cenozoic large-scale strike-slip faulting and the resultant tensional fissure zones, developed in a transform phase from a transpressional to a transtensional regime. Alteration is characterized by fenitization, which formed fenite halos enveloping the REE orebodies. Associated REE mineralization occurs as complex vein systems consisting of various veinlets, stringer and stockwork zones, and breccia-pipe systems. The REE orebodies show various shapes from plate-like, lenticular to pipe-like in different districts. Ore types are dominated by pegmatitic, carbonatitic, breccia, and stringer (stockwork), and disseminated ores, mainly composed of barite + fluorite + aegirine-augite + calcite + bastnaesite assemblages. Bastnaesite precipitation (275–325 °C) appears in three assemblages: early clastics-bearing barite + arfvedsonite + calcite + bastnaesite; intermediate quartz + fluorite + barite + bastnaesite; and late fluorite + barite + bastnaesite assemblages. These were followed by low-temperature pyrite + galena + sphalerite assemblages. Fluid inclusion studies indicate that gangue quartz and fluorite hosts numerous melt/fluid inclusions with a large proportion of sulfate (i.e., BaSO 4, K 2SO 4, and CaSO 4), in addition to CaCO 3 and CaF 2, yielding abnormally high (up to 750 °C) but wide-ranging homogenization temperatures (750–125 °C). The similarity of Sr–Nd isotopic compositions for gangue minerals with those of the host carbonatite–syenite suggests that ore-forming fluids, especially the F −, HCO 3 2−, SO 4 2− components, were derived from carbonatite–syenite magmas. The O–D–C isotopic data from gangue calcite, fluorite and quartz indicate that the fluids were of orthomagmatic origin, but also involved mixing with an external fluid. On the basis of our synthesis, we propose a possible genetic model for REE mineralization, in which the hydrothermal system temporally underwent a complicated evolution from separation of high-T sulfate-bearing NaCl–KCl brine resulting in effective precipitation of REE-fluorocarbonate and sulfate, to subsequent mixing with low-temperature meteoric water precipitating minor sulfide assemblages. Spatially, this sequence of events generated a three-tier REE mineralized architecture, in which pipe-like breccia orebodies grade downwards from shallow to deep structural levels into large veinlet ore systems, and veinlet-disseminated orebodies.

SYN- TO LATE-TECTONIC STOCKWORK EMPLACEMENT WITHIN THE SPANISH SECTION OF THE IBERIAN PYRITE BELT: STRUCTURAL, TEXTURAL, AND MINERALOGICAL CONSTRAINTS IN THE THARSIS AND LA ZARZA AREAS

In deformed massive sulfide deposits, it is commonly difficult to distinguish between primary stockwork mineralization and remobilized sulfides that occupy late cleavages and deformation structures, and the mechanism of formation of the deformation veins is not always clear. An analysis of sulfide vein networks associated with two massive sulfide deposits of the Iberian Pyrite Belt is presented as an example of this complexity. Variscan deformation has affected the massive sulfide-related stockworks at La Zarza and Tharsis and led to the formation of sulfide-bearing deformation veins. The primary feeder stockwork has a complex mineralogical composition with pyrite-rich veins rimmed by narrow Co-As assemblages (e.g., cobaltite and arsenopyrite). Interstitial galena, chalcopyrite, and sphalerite minerals are also observed. The vein set developed during deformation was emplaced within cleavage and shear planes characteristic of the regional south-verging tectonics. Away from massive sulfides, the characteristic minerals of the syntectonic veins are pyrite and quartz. However, deformation veins developed in contact with primary feeder veins and massive sulfide have Co-As–rich rims and base metal sulfides similar to the primary feeder veins. The development of the complex vein paragenesis within the stockwork zones is explained in terms of regional-scale deformation models.

Porphyry Cu–Au mineralisation associated with a multi-phase intrusion, and related replacement fronts in limestones in an island arc setting near the Gümüşhane village (Artvin) in the Eastern Black Sea Province (Turkey)

The Gümüşhane area near Artvin is highly characteristic due to a significant hydrothermal alteration zone genetically associated with a microdiorite stock and its late-stage derivatives in the form of porphyry plugs that intruded into all the pre-Middle Eocene lithologies. The porphyry intrusion is multi-stage, intermediate to felsic in composition, and divided into pre-ore feldspar porphyries and quartz–feldspar porphyries, syn-ore feldspar–amphibole porphyries, and post-ore feldspar porphyries. Sericitic alteration is dominant, but K-silicate alteration is also observed and is characterised mainly by secondary feldspars, biotites, quartz, anhydrite, magnetite and pyrite veinlets. The central part of the alteration and the porphyry system where syn-ore feldspar–amphibole porphyry outcrops coincides with intense quartz stock-working with anomalous but uneconomical Cu and Au values. In this zone, pyrite is ubiquitous and is accompanied by chalcopyrite and lesser sphalerite, sulphosalts, molybdenite, bornite and magnetite. Chemical analyses of surface and drill core samples show that overall Au and Cu values are around 0.5 ppm and 0.3%, respectively, and that they are in the ranges of 1–2 ppm and 0.3–0.4%, respectively, in the densest stock-work zones. Multi-phase intrusion of porphyries into Late Cretaceous limestone also caused replacement fronts along re-crystallised limestone and porphyry contacts. This type of mineralisation is enriched in Mn, Zn, Cu, Au and Ba, and contains on average 4.7% MnO, 3.2% FeO, 3.1% Zn, 0.95% Cu, 0.3% Pb, 200 ppb Au, 900 ppm Bi and 660 ppm Ba, present in Fe- and Mn-oxides, pyrite, chalcopyrite, sphalerite and lesser bornite, sulphosalts and gold. Supergene oxidation is well developed in these zones. Hydrothermal alteration mass change calculations reflect an intimate relationship between the two types of mineralisation. The lithologies near mineralised fronts are highly depleted in MnO and Zn, and the intensity of depletion is reduced away from the contact into the porphyry. The leached elements are added to the re-crystallised limestone block in which MnO and Zn show an 81-fold and 472-fold increase, respectively, over the least altered limestone. Au and Cu are enriched along the contact both in the porphyry and in the limestone, implying that they may have been derived either from other lithologies in the vicinity or the magma itself.

Efemçukuru B-rich epithermal gold deposit (İzmir, Turkey)

Efemçukuru, located in the Aegean Region, Turkey, is an example of vein-type epithermal gold deposit with related stockwork and replacement mineralisation. The veins are hosted by Late Cretaceous–Paleogene flysch facies rocks of the İzmir–Ankara zone, which were intruded by rhyolites from of Neogene volcanism. Gold mineralisation is associated with late pulses of magmatic hydrothermal activity and is present in zones of hydraulic brecciation adjacent to veins or stockwork zones around a dome-shaped small intrusion. The intrusion and later epithermal mineralisation are both controlled by NW–SE-trending faults, and mineral deposits occur along strike with dips 60° to 80° to the northeast. Individual quartz veins associated with sulphide minerals and mineralised hornblende facies hornfels are other important ore-bearing formations. The alteration associated with mineralisation is represented mainly by rhodonite, rhodochrosite, axinite, quartz, calcite and adularia in veins and stockwork zone and chlorite, sericite, illite and kaolinite in the wall rock. Homogenisation temperatures of 200–300 °C reflect emplacement of ore distant from inferred magmatic heat in epithermal environment. Due to wide range in salinity of the fluids (0–9 eq.wt.% NaCl), sulphide diversity in the deposit is rather rich than the low-sulphidation epithermal systems. The fluid inclusion data indicate that a complicated geothermal system existed. Both the gas data and the microthermometry data indicate that there was fluid mixing. Each type of mineralisation has been studied with the aim of clarifying the paragenetic relationships among the different minerals. Arsenopyrite, pyrite (marcasite), sphalerite, galena and chalcopyrite are the main minerals with pyrrhotite, fahlore group, sphene, rutile and gold occurring as accessories. Pyrolusite, limonite (goethite, lepidocrocite), covellite–chalcocite, malachite and azurite are of secondary origin.

Source of ore-forming fluids in El Cobre VHMSdeposit (Cuba): evidence from fluid inclusions and sulfur isotopes

The El Cobre deposit, east of Cuba, lies in the intermediate volcanosedimentary sequence of the Sierra Maestra intraoceanic island arc. The structure of the deposit corresponds to that of a volcanogenic-hosted massive sulfide (VHMS) model. It comprises (a) thick stratiform bodies (baryte and anhydrite), (b) three stratabound bodies (formed by silicification and sulfidation of limestones or sulfate strata), (c) stockwork zones, an older anhydrite stockwork and a younger quartz-pyrite stockwork grading downwards to (d) simple veins (quartz with sulfide ores). Pyrite, chalcopyrite and sphalerite are the most abundant sulfides. Fluid inclusions from this deposit have a salinity between 2.3 and 5.7 wt.% NaCl eq., homogenization temperatures range between 177 and 300 °C. Sulfur exhibits a range of δ 14S values from − 1.4 to +7.3 for sulfides and from + 16 to +21 for sulfates. Fluid inclusions and sulfur isotope data at El Cobre deposit indicate that the hydrothermal fluid from which the sulfide precipitated was seawater, modified by reaction with volcanic host rocks during hydrothermal circulation.

Characterization and modeling of illite crystal particles and growth mechanisms in a zoned hydrothermal deposit, Lake City, Colorado

Mean thickness measurements and crystal-thickness distributions (CTDs) of illite particles vary systematically with changes in hydrothermal alteration type, fracture density, and attendant mineralization in a large acid-sulfate/Mo-porphyry hydrothermal system at Red Mountain, near Lake City, Colorado. The hydrothermal illites characterize an extensive zone of quartz-sericite-pyrite alteration beneath two deeply rooted bodies of magmatic-related, quartz-alunite altered rock. Nineteen illites from a 3000 ft vertical drill hole were analyzed by XRD using the PVP-10 intercalation method and the computer program MudMaster (Bertaut-Warren-Averbach technique). Mean crystallite thicknesses, as determined from 001 reflections, range from 5-7 nanometers (nm) at depths from 0-1700 ft, then sharply increase to 10-16 nm at depths between 1800-2100 ft, and decrease again to 4-5 nm below this level. The interval of largest particle thickness correlates strongly with the zone of most intense quartz-sericite-pyrite alteration (QSP) and attendant high-density stockwork fracturing, and with the highest concentrations of Mo within the drill core. CTD shapes for the illite particles fall into two main categories: asymptotic and lognormal. The shapes of the CTDs are dependent on conditions of illite formation. The asymptotic CTDs correspond to a nucleation and growth mechanism, whereas surface-controlled growth was the dominant mechanism for the lognormal CTDs. Lognormal CTDs coincide with major through-going fractures or stockwork zones, whereas asymptotic CTDs are present in wallrock distal to these intense fracture zones. The increase in illite particle size and the associated zone of intense QSP alteration and stockwork veining was related by proximity to the dacitic magma(s), which supplied both reactants and heat to the hydrothermal system. However, no changes in illite polytype, which in other studies reflect temperature transitions, were observed within this interval.

Sulfur isotope characteristics of the Archean Cu-Zn Gossan Hill VHMS deposit, Western Australia

Gossan Hill is an Archean (∼3.0 Ga) Cu–Zn–magnetite-rich volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn Craton of Western Australia. Massive sulfide and magnetite occur within a layered succession of tuffaceous, felsic volcaniclastic rocks of the Golden Grove Formation. The Gossan Hill deposit consists of two stratigraphically separate ore zones that are stratabound and interconnected by sulfide veins. Thickly developed massive sulfide and stockwork zones in the north of the deposit are interpreted to represent a feeder zone. The deposit is broadly zoned from a Cu–Fe-rich lower ore zone, upwards through Cu–Zn to Zn–Ag–Au–Pb enrichment in the upper ore zone. New sulfur isotope studies at the Gossan Hill deposit indicate that the variation is wider than previously reported, with sulfide δ34S values varying between −1.6 and 7.8‰ with an average of 2.1 ± 1.4‰ (1σ error). Sulfur isotope values have a broad systematic stratigraphic increase of approximately 1.2‰ from the base to the top of the deposit. This variation in sulfur isotope values is significant in view of typical narrow ranges for Archean VHMS deposits. Copper-rich sulfides in the lower ore zone have a narrower range (δ34S values of −1.6 to 3.4‰, average ∼1.6 ± 0.9‰) than sulfides in the upper ore zone. The lower ore zone is interpreted to have formed from a relatively uniform reduced sulfur source dominated by leached igneous rock sulfur and minor magmatic sulfur. Towards the upper Zn-rich ore zone, an overall increase in δ34S values is accompanied by a wider range of δ34S values, with the greatest variation occurring in massive pyrite at the southern margin of the upper ore zone (−1.0 to 7.8‰). The higher average δ34S values (2.8 ± 2.1‰) and their wider range are explained by mixing of hydrothermal fluids containing leached igneous rock sulfur with Archean seawater (δ34S values of 2 to 3‰) near the paleoseafloor. The widest range of δ34S values at the southern margin of the deposit occurs away from the feeder zone and is attributed to greater seawater mixing away from the central upflow zone.

The Iberian type of volcano-sedimentary massive sulphide deposits

The Iberian Pyrite Belt, located in the SW Iberian Peninsula, contains many Paleozoic giant and supergiant massive sulphide deposits, including the largest individual massive sulphide bodies on Earth. Total ore reserves exceed 1500 Mt, distributed in eight supergiant deposits (>100 Mt) and a number of other smaller deposits, commonly with associated stockwork mineralizations and footwall alteration haloes. Massive sulphide bodies largely consist of pyrite, with subordi- nated sphalerite, galena and chalcopyrite and many other minor phases, although substantial diAerences occur between individual deposits, both in mineral abundance and spatial distribution. These deposits are considered to be volcanogenic, roughly similar to vol- canic-hosted massive sulphides (VHMS). However, our major conclusion is that the Iberian type of massive sulphides must be considered as a VHMS sub-type transitional to SHMS. This work is an assessment of the geological, geo- chemical and metallogenic data available up to date, including a number of new results. The following points are stressed; (a) ore deposits are located in three main geological sectors, with the southern one containing most of the giant and supergiant orebodies, whereas the northern one has mainly small to intermediate-sized de- posits; (b) ore deposits diAer one from another both in textures and mineral composition; (c) Co and Bi minerals are typical, especially in stockwork zones; (d) colloidal and other primary depositional textures are common in many localities; (e) a close relation has been found be- tween ore deposits and some characteristic sedimentary horizons, such as black shales. In contrast, relationships between massive sulphides and cherts or jaspers remains unclear; (f) footwall hydrothermal alterations show a rough zoning, the inner alteration haloes being charac- terized in places by a high Co/Ni ratio, as well as by mobility of Zr, Y and REE; (g) 18 O and D values indicate that fluids consist of modified seawater, whereas 34 S data strongly suggest the participation of bacterial-reduced sulphur, at least during some stages of the massive sul- phide genesis, and (h) lead isotopes suggest a single (or homogeneized) metal source, from both the volcanic piles and the underlying Devonian rocks (PQ Group). It is concluded that, although all these features can be compatible with classical VHMS interpretations, it is necessary to sketch a diAerent model to account for the IPB characteristics. A new proposal is presented, based on an alternative association between massive sulphide deposits and volcanism. We consider that most of the IPB massive orebodies, in particular the giant and su- pergiant ones, were formed during pauses in volcanic activity, when hydrothermal activity was triggered by the ascent and emplacement of late basic magmas. In these conditions, deposits formed which had magmatic activity as the heat source; however, the depositional environ- ment was not strictly volcanogenic, and many evolu- tionary stages could have occurred in conditions similar to those in sediment-hosted massive sulphides (SHMS). In addition, the greater thickness of the rock pile aAected by hydrothermal circulation would account for the enormous size of many of the deposits.

U–Pb dating of stockwork zircons from the eastern Iberian Pyrite Belt

Zircons found in the stockwork zones of the massive sulphide Los Frailes deposit, Iberian Pyrite Belt, are interpreted to have grown during the hydro-thermal alteration of host felsic volcanic rocks. Ion microprobe (SHRIMP) dating gives a 206 Pb/ 238 U age of 345.7 ± 4.6 Ma (2σ) and together with published spore data from the deposit suggest an age of uppermost Devonian to lowermost Carboniferous. Stockwork zircons offer the possibility of precisely defining the emplacement ages of these giant sulphide accumulations over the whole Pyrite Belt and from this will emerge estimates of the thermal budget necessary to generate the deposits.

Anomalous arsenic concentrations in groundwaters of an island community, Bowen Island, British Columbia

Recently, occurrences of elevated arsenic concentrations in bedrock groundwaters used for individual and municipal water supplies have been recognised along the mainland coast of southern British Columbia, Canada. An area on Bowen Island (Queen Charlotte Heights community) was chosen to investigate the source(s) of arsenic, geochemical processes controlling its aqueous uptake, the role of geology and structure in the formation of these waters, and the use of hydrogeochemical survey methods for detecting arsenious groundwater regions. Pyrite- arsenopyrite-Cu mineralisation in the Queen Charlotte Heights area is present as veins associated with NE-trending fault zones and as disseminations in marginal breccia and stockwork zones of a ryhodacite porphyry stock. Near surface mineralisation was later altered by supergene events to form minor pyrite- arsenopyrite in a matrix of limonite and clay. Geochemical factors that control the concentration and mobility of arsenic in these groundwaters include: (a) anion exchange of arsenic from clay minerals during cation divalent/univalent exchange processes (water softening) which leads to waters of high pH and high anion exchange capacities; (b) desorption of arsenic from Fe and Al oxyhydroxide minerals during pronounced shifts in pH from acid to alkaline conditions brought on by the water softening process; and (c) stepwise oxidation of arsenopyrite mineralisation to form arsenious sulphate-bearing groundwaters. The inconsistent behaviour of the As3+ and As5+ ions during the complex waterrock interaction processes described above, precludes the use of detailed chemical analyses to predict relative abundances of the more toxic As3+ species. For a full appreciation of the toxic impact of groundwaters containing total arsenic concentrations that are above regulatory guidelines an arsenic speciation analysis is required.