Pyritic Ore Bodies Research Articles

Using Hg isotopes and trace elements to constrain the origin of the Ordovician Xiangquan Tl deposit, South China

Sediment-hosted stratiform sulfide (SHSS) deposits commonly contain large amounts of strategic metals, such as Tl, Hg, Cd, Ge and Ga, however, sources of these metals remain debate. Here we present new elemental and Hg isotopic data to constrain the metal sources for the Xiangquan Tl deposit (South China), a unique SHSS deposit with 250 t Tl at an average grade of 928 ppm. Stratiform pyrite orebodies are hosted in the Ordovician marine sedimentary succession of mostly limestone with minor mudstone. Pyrite, the dominant ore mineral, is rich in Hg (up to 1360 ppm). Bulk ores and pyrite separates from this deposit have significant variations of δ202Hg (−5.48 to −0.65 ‰) and Δ199Hg (−0.01 to 0.36 ‰). The Δ199Hg values are mostly positive, similar to those of seawater and marine sediments. Ore petrography, trace element enrichment coefficients, rare earth element patterns, Re-Os ages (478 ± 33 Ma) and S isotope compositions of pyrite indicate that Hg and other metals were derived from hydrothermal fluids venting at the seafloor. We suggest that the ore-forming fluids of the Xiangquan deposit were predominantly originated from large-scale circulation of evolved seawater, which leached Hg and Tl from the marine country rocks. These metal-rich fluids migrated upward, vented to the seafloor, and mixed with anoxic H2S-rich seawater to precipitate Hg- and Tl-rich pyrite. Our study highlights that the combined application of Hg isotopes and trace elements could be used to trace the sources of metals in hydrothermal ore deposits such as SHSS deposits.

Sulfur melt in Golovnin Caldera, Kunashir Island, Russia

The paper considers the formation of native sulfur and sulfides in the bottom sediments of volcanic lakes in the Golovnin Caldera. Our observations and microstructure studies indicate the presence of sulfur melt at the bottom of Kipyaschee Lake. Drops of this melt are carried to the surface of the lake as part of a light gray foam. The significant differences of sulfur spherules in the concentration of sulfide mineralization, in its composition, as well as in the presence or absence of numerous opal inclusions are most simply explained by the capture of droplets in various parts of the sulfur melt and their subsequent movement by a gas stream passing through the melt. The effusions of sulfur melt in 1979 are direct evidence of its existence. Molecular chemical modeling shows that the stability of sulfur condensate in the bottom sediments is controlled by two thermobaric surfaces. The upper is controlled by the beginning of water condensation and a wave of acidity, the lower by the beginning of sulfur condensation. Both surfaces are a consequence of the cooling effect of lake water, and when bottom sediments accumulate, they move following changes in the surface of the lake bottom. Elemental sulfur condensate is formed between these surfaces as a result of forced cooling of endogenous gas flows. This process is most active in crater depressions at the bottom of Golovnin Caldera lakes, where sulfidization of the sulfur melt occurs simultaneously with the condensation of sulfur itself. Gravitational deposition of sulfides in the sulfur melt leads to their enrichment of the root parts of crater depressions, where pyrite ore bodies are formed in real time.

Zun-Kholba Orogenic Gold Deposit, Eastern Sayan, Russia: Geology and Genesis

In this paper, we present a new point of view on the deposit geology and genesis of the largest gold deposit in Eastern Sayan, Zun-Kholba. Quartz–sulfide replacement ores and shallow quartz veins are of great economic importance. At the deposit, quartz–pyrite ore bodies are dominant, whereas paragenetically late base metal-bearing quartz veins occur only at shallow levels. The study of the fluid inclusions, stable isotopes (C–O–S), and mineral geothermometry allowed us to determine the P–T conditions of ore deposition. It is established that the temperature of ore formation changed from 380 to 433 °C at the deepest levels of the deposit to 316 °C at the shallowest levels. The pressure estimates for gold deposition at 570–950 bar were calculated for the 1490-m level in the center of the deposit. The determined P–T conditions can help estimate the average depth of deposit formation at about 3.6 km. Ore mineral assemblages were formed from homogeneous fluids of low to moderate salinity (2.4–7.9 wt.% eq. NaCl). The sulfur isotope composition of sulfide minerals (δ34S = 0–4.6‰), as well as pyrite geochemistry, corresponds to that of older volcanogenic massive sulfide (VMS) occurrences, which are widespread within the Eastern Sayan ophiolitic belts. Lens-like fragments of metamorphosed VMS-type sulfide ores are also present in the Zun-Kholba deposit. The oxygen isotope data indicate a metamorphic origin for the ore-forming fluids. Migrating metamorphic fluids may have become enriched in gold, sulfur, and other metals during the flow through the complex ore-hosting lithologies, including ophiolitic bodies with sulfide-rich VMS fragments that are characterized by a relatively high content of gold and related ore elements. The obtained data allow us to suggest a metamorphic devolatilization model for the genesis of the Zun-Kholba gold deposit.

Preservation of Underground Microbial Diversity in Ancient Subsurface Deposits (>6 Ma) of the Rio Tinto Basement.

The drilling of the Rio Tinto basement has provided evidence of an underground microbial community primarily sustained by the Fe and S metabolism through the biooxidation of pyrite orebodies. Although the gossan is the microbial activity product, which dates back to the Oligocene (25 Ma), no molecular evidence of such activity in the past has been reported yet. A Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) molecular analysis of a subsurface sample in the Peña de Hierro basement has provided novel data of the ancient underground microbial community. It shows that the microbial remains are preserved in a mineral matrix composed of laminated Fe-oxysulfates and K- and Na-bearing sulfates alternating with secondary silica. In such a mineral substrate, the biomolecule traces are found in five different microstructure associations, (1) <15 micron-sized nodular microstructures composed of POn(2≤n≤4)−, (2) <30 micron-size micronodules containing fatty acids, acylglycerides, and alkanol chains, (3) <20 micro-sized nodules containing NOn−(2≤n≤3) ions, (4) 40-micron size nodules with NH4+ and traces of peptides, and (5) >200-micron thick layer with N-bearing adducts, and sphingolipid and/or peptide traces. It suggests the mineralization of at least five microbial preserved entities with different metabolic capabilities, including: (1) Acidiphilium/Tessaracoccus-like phosphate mineralizers, (2) microbial patches preserving phosphate-free acylglycerides bacteria, (3) nitrogen oxidizing bacteria (e.g., Acidovorax sp.), (4) traces of heterotrophic ammonifying bacteria, and (5) sphingolipid bearing bacteria (e.g., Sphingomonadales, and δ-Proteobacteria) and/or mineralized biofilms. The primary biooxidation process acted as a preservation mechanism to release the inorganic ions that ultimately mineralized the microbial structures.

Textural and trace element evolution of pyrite during greenschist facies metamorphic recrystallization in the southern Apuan Alps (Tuscany, Italy): Influence on the formation of Tl-rich sulfosalt melt

Several small pyrite ± baryte ± iron-oxide orebodies (Buca della Vena, Canale della Radice, Fornovolasco, Monte Arsiccio and Pollone) are hosted in the metamorphic rocks of the southern Apuan Alps, northern Tuscany, Italy. These deposits are exceptionally thallium-rich, expressed in part by assemblages comprising rare Tl-sulfosalts. Using a variety of techniques including laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on fifty-four samples, the textural and trace element evolution of pyrite during greenschist facies metamorphism has been described. Five generations of pyrite have been recognised. Pyrite-1 (py1) and pyrite-2 (py2) (framboidal and colloform pyrite, respectively) likely represent pristine generations and are rich in trace elements. Pyrite-3 (py3) forms fine-grained disseminations often aligned with the metamorphic fabric and likely formed during the earliest stages of deformation associated with metamorphism. Pyrite-4 (py4) forms coarse, euhedral, inclusion-rich crystal clusters and likely formed as pristine pyrite generations began to recrystallize. Pyrite-5 (py5) is typically coarse, relatively ‘clean’ and often euhedral. It is interpreted to be the end-product of pyrite recrystallization, with grain size increasing and micro-inclusions being expelled late on the prograde path, then granoblastic annealed textures forming during retrograde cooling. Pyrite (especially primary pyrites; py1 and py2) is particularly rich in As (up to 17,400 ppm), Sb (up to 5100 ppm) and Tl (up to 4200 ppm), while Ni, Pb and Mn may be present above 1000 ppm, and Hg, Co, Cu, Zn, Mo, Bi and Ag may all be present above 100 ppm. Some trace elements, particularly As, Ni and Co, are commonly zoned in late metamorphic pyrite overgrowths. The incorporation of some trace elements into pyrite seems to have been facilitated by the presence of Sb in a similar way that As generally facilitates Au incorporation. Concentrations of most trace elements decrease from py3 to py4 at Fornovolasco and Monte Arsiccio, while in the other deposits the opposite trend is observed. Concentrations of most trace elements drop significantly from py4 to py5, where recrystallization of pyrite liberates a large proportion of As, Sb, Tl, Pb, Hg, Cu, Zn, Ag and Mn, likely during retrograde cooling and annealing. The release of these elements from pyrite during metamorphic recrystallization has directly facilitated the formation of late-stage sulfosalts, especially at Monte Arsiccio, where textural evidence suggests sulfosalt assemblages intimately associated with py5 were mobilised as melts during greenschist facies metamorphism. At Monte Arsiccio, around 75% of Tl hosted in early recrystallized pyrite is released upon complete recrystallization. As such, mass balance calculations show that all Tl contained in Tl-sulfosalts could be supplied from locally recrystallizing pyrite, and that in total, more than 250 tons of Tl could potentially have been liberated from the pyrite orebody at Monte Arsiccio during metamorphism. This study highlights the significant quantities of Tl and other metals that may be hosted in pyrite, and also the potential role that metamorphic recrystallization may play in mobilising and (re)-concentrating (or indeed dispersing) metals in a pyrite-dominant ore system.

Lead-Antimony Sulfosalts from Tuscany (Italy). XX. Members of the Jordanite–Geocronite Series from the Pollone Mine, Valdicastello Carducci: Occurrence and Crystal Structures

A crystal-chemical study of historical specimens as well as new ones belonging to the jordanite–geocronite series from the Pollone baryte + pyrite ± (Pb-Zn-Ag) ore deposit (Valdicastello Carducci, Apuan Alps, Tuscany, Italy) has been performed. These crystals were collected in quartz extension veins embedded in three different occurrences: (i) baryte + pyrite orebodies; (ii) schist layers interbedded between baryte + pyrite orebodies; and (iii) schists at the contact with pyrite-poor baryte orebodies. Electron-microprobe data indicated the occurrence of three distinct groups of compositions within the sample suite. These correspond to As-bearing geocronite, Sb-rich jordanite, and Sb-bearing jordanite, with mean compositions Pb14Sb3.8As2.2S23, Pb14Sb2.9As3.1S23, and Pb14Sb2.6As3.4S23, respectively. Crystals representative of these different compositions have been investigated through single-crystal X-Ray diffraction studies and their crystal structures have been solved to R1 = 0.078, 0.069, and 0.033, respectively. The unit-cell volume decreases passing through As-bearing geocronite (V = 2149.5(3) Å3) to Sb-bearing jordanite (V = 2132.3(3) Å3). The As-to-Sb substitution takes place preferentially at the Sb4 site; through the increasing of the Sb content, Sb can substitute As also at the As6 site. According to the structural study of the ore deposit, formation of jordanite–geocronite is subordinated to a late Alpine deformative D2 stage, which permitted in situ remobilization of preexisting sulfide ore in small quartz extension veins. Such a local recrystallization would explain the variability of the As/(As + Sb) ratio of the members of the jordanite series, reflecting the heterogeneity of the orebody.

Cretaceous Cu–Au, pyrite, and Fe-oxide–apatite deposits in the Ningwu basin, Lower Yangtze Area, Eastern China

The Cretaceous Ningwu volcanic basin of the Middle and Lower Yangtze River Valley metallogenic belt of eastern China hosts numerous Fe-oxide–apatite, Cu–Au, and pyrite deposits. The mineralization in the Ningwu basin is associated with subvolcanic rocks, consisting of gabbro–diorite porphyry and/or pyroxene diorite. However, the mineralization is associated with subvolcanic and volcanic rock suite belonging to the Niangniangshan Formation in the Tongjing Cu–Au deposit, including nosean-bearing aegirine–augite syenites, quartz syenites, and quartz monzonites. The zoning displayed by the alteration and mineralization comprises: (1) an upper light-colored zone of argillic, carbonate, and pyrite alteration and silicification that is locally associated with pyrite and gold mineralization, (2) a central dark-colored zone of diopside, fluorapatite–magnetite, phlogopite, and garnet alteration associated with fluorapatite–magnetite mineralization, and (3) a lowermost light-colored zone of extensive albite alteration. The Cu–Au and pyrite orebodies are peripheral to the Fe-oxide–apatite deposits in this area and overlie the iron orebodies, including the Meishan Cu–Au deposit in the northern Ningwu basin and the pyrite deposits in the central Ningwu basin. The δ34S values of sulfides from the Fe-oxide–apatite, Cu–Au, and pyrite deposits in the Ningwu basin show large variation, with a mixed sulfur source, including magmatic sulfur and/or a mixture of sulfur derived from a magmatic component, country rock, and thermochemical reduction of sulfate at 200–300°C. The ore-forming fluids associated with iron mineralization were derived mainly from magmatic fluids, and the late-stage ore-forming fluids related to Cu–Au and pyrite mineralization may have formed by the introduction of cooler meteoric water to the system. The Fe-oxide–apatite, Cu–Au, and pyrite deposits of the Ningwu basin formed in an extensional environment and are associated with a large-scale magmatic–hydrothermal system.

VOLASCHIOITE, Fe3+4(SO4)O2(OH)6{middle dot}2H2O, A NEW MINERAL SPECIES FROM FORNOVOLASCO, APUAN ALPS, TUSCANY, ITALY

Volaschioite, ideally Fe 3+ 4 (SO 4 )O 2 (OH) 6 ·2H 2 O, occurs in a small magnetite–pyrite orebody near Fornovolasco, in the Apuan Alps, Tuscany, Italy. The new mineral species is associated with pyrite and iron oxyhydroxides. In the same occurrence, other iron sulfates were identified, including alum-(K), “copiapite”, fibroferrite, halotrichite, melanterite, romerite, and voltaite. Volaschioite occurs as radial aggregates of bladed crystals, up to 100 μm in length and less than 5 μm across. The color is yellowish orange with an orange streak; the luster is vitreous to resinous. The mineral is transparent and strongly pleochroic. Electron-microprobe analyses lead to the empirical formula Fe 4.16 (SO 4 ) 0.92 O 2.32 (OH) 6 ·2H 2 O; the calculated density is 3.03 g/cm 3 . Volaschioite is monoclinic, with a 16.068(4), b 3.058(1), c 10.929(2) A, β 93.82(3)°, V 535.8(2) A 3 , Z = 2, space group C 2/ m . The strongest three X-ray powder-diffraction lines [ d in A( I )( hkl )] are: 8.03(100)(200), 4.37(24)(202), 3.989(22)(400). The single-crystal X-ray diffraction pattern shows the presence of additional very weak and streaked reflections indicating a doubling of the b parameter. The average structure of volaschioite is composed of ribbons of edge-sharing Fe-centered octahedra running along b , linked together by corner-sharing to form corrugated layers, decorated on both sides by sulfate groups. Additional H 2 O molecules are located between the layers. Volaschioite is the product of pyrite alteration in an oxidizing environment. The name of this new species refers to the ancient name of the type locality, Forno de Volaschio . This mineral and its name have been approved by the IMA–CNMNC (IMA2010–005).

Biodiversity and geochemistry of an extremely acidic, low‐temperature subterranean environment sustained by chemolithotrophy

The geochemical dynamics and composition of microbial communities within a low-temperature (≈ 8.5°C), long-abandoned (> 90 years) underground pyrite mine (Cae Coch, located in north Wales) were investigated. Surface water percolating through fractures in the residual pyrite ore body that forms the roof of the mine becomes extremely acidic and iron-enriched due to microbially accelerated oxidative dissolution of the sulfide mineral. Water droplets on the mine roof were found to host a very limited diversity of exclusively autotrophic microorganisms, dominated by the recently described psychrotolerant iron/sulfur-oxidizing acidophile Acidithiobacillus ferrivorans, and smaller numbers of iron-oxidizing Leptospirillum ferrooxidans. In contrast, flowing water within the mine chamber was colonized with vast macroscopic microbial growths, in the form of acid streamers and microbial stalactites, where the dominant microorganisms were Betaproteobacteria (autotrophic iron oxidizers such as 'Ferrovum myxofaciens' and a bacterium related to Gallionella ferruginea). An isolated pool within the mine showed some similarity (although greater biodiversity) to the roof droplets, and was the only site where archaea were relatively abundant. Bacteria not previously associated with extremely acidic, metal-rich environments (a Sphingomonas sp. and Ralstonia pickettii) were found within the abandoned mine. Data supported the hypothesis that the Cae Coch ecosystem is underpinned by acidophilic, mostly autotrophic, bacteria that use ferrous iron present in the pyrite ore body as their source of energy, with a limited role for sulfur-based autotrophy. Results of this study highlight the importance of novel bacterial species (At. ferrivorans and acidophilic iron-oxidizing Betaproteobacteria) in mediating mineral oxidation and redox transformations of iron in acidic, low-temperature environments.

Magnetite Veins in the Pyritic Ore Bodies in the Yanahara Mine

Magnetite Veins in the Pyritic Ore Bodies in the Yanahara Mine

The Río Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars

Exploration by the NASA rover Opportunity has revealed sulfate- and hematite-rich sedimentary rocks exposed in craters and other surface features of Meridiani Planum, Mars. Modern, Holocene, and Plio-Pleistocene deposits of the Rio Tinto, southwestern Spain, provide at least a partial environmental analog to Meridiani Planum rocks, facilitating our understanding of Meridiani mineral precipitation and diagenesis, while informing considerations of martian astrobiology. Oxidation, thought to be biologically mediated, of pyritic ore bodies by groundwaters in the source area of the Rio Tinto generates headwaters enriched in sulfuric acid and ferric iron. Seasonal evaporation of river water drives precipitation of hydronium jarosite and schwertmannite, while (Mg,Al,Fe(sup 3+))-copiapite, coquimbite, gypsum, and other sulfate minerals precipitate nearby as efflorescences where locally variable source waters are brought to the surface by capillary action. During the wet season, hydrolysis of sulfate salts results in the precipitation of nanophase goethite. Holocene and Plio-Pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time. Hematite in Meridiani spherules also formed during diagenesis, although whether these replaced precursor goethite or precipitated directly from groundwaters is not known. The retention of jarosite and other soluble sulfate salts suggests that water limited the diagenesis of Meridiani rocks. Diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry Rio Tinto environments. Organic matter does not persist in Rio Tinto sediments, but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers, surface blisters formed by biogenic gas, and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiological investigation of Meridiani outcrops.

Geology and geophysics as successful tools in the discovery of the Lagoa Salgada Orebody (Sado Tertiary Basin - Iberian Pyrite Belt), Gr�ndola, Portugal

Gravimetry was the main exploration method used in this high risk area to the north of the Grândola fault, in the Iberian Pyrite Belt (IPB). It showed several targets, among which the Valverde-Lagoa Salgada area stood out. Here, in August 1992, a polymetallic massive sulphide orebody was found, named Lagoa Salgada. According to the geological characteristics of the area and to the nature of the surveyed targets, other exploration techniques were used, namely: geology, magnetometry, resistivity, electrical sounding, magneto-telluric, seismic and mechanical drilling. The deposit was found under a Tertiary cover, 128 m in depth, complete with a 15 m thick gossan, caused by paleoalteration, with a supergene enrichment zone. This gossan gave way to massive sulphides, with intersections, at times, exceeding 60 m. They are limited towards the footwall by an important fault with strong associated kaolinization, which is interpreted as a reverse fault, placed on the inverse limb of an anticlinal structure. Throughout the deposit, the chemical composition of the mineralization showed great variation reaching significant values of Zn, Pb, Sn, Cu, As, Hg, Sb, Cd, Au and Ag. In the initial phase the deposit has an estimated tonnage of 5 Mt. It is related to a large gravity anomaly with the general orientation NW-SE, which was investigated by 15 boreholes made by the Instituto Geologico e Mineiro. The drilling cut an important volcanic centre of an acid to intermediate nature, around which intense hydrothermal activity was associated with contemporaneous sulphide mineralization. The gravity anomaly has two nuclei, a NW and a central one, about 450 m apart, where a pyrite orebody with polymetallic sulphides and stockwork sulphide mineralization are developed. This discovery led to an extensive exploration project that confirmed the mining potentialities of the NW sector of the IPB, to the north of Grândola fault, where previously no polymetallic sulphide occurrences were known.

Gold Enrichment in the Supergene Zone of a Southern Urals Pyrite Deposit, Russia

The Gay deposit, situated in the Orenburg region, is identified with one of Russia's principal occurrences of pyrite (pyrite deposits are an important source of Russia's gold). It belongs to the west subzone of the Magnitogorsk synclinorium and occurs in Devonian rhyolite-basaltic volcanic rocks. The deposit comprises five large pyrite-chalcopyrite, pyrite-chalcopyrite- sphalerite, and pyrite orebodies. The supergene zone extends to 120-240 m below surface and consists of the following three subhorizontal zones (from bottom to top): the secondary sulfide enrichment, the leaching, and the oxidation zone (where ores are enriched in gold). There are two levels of secondary gold enrichment in the weathering profile. The lower level, located in the leaching zone, corresponds with the level of water table fluctuations. The rich, flat-lying horizon (1.5-10.0 m) is composed of bedded, friable native sulfur-quartz ores; it contains 19.0-52.2 ppm Au and up to 389 ppm Ag. Native gold and silver halides (chlorargyrit...

8. Increasing the extraction ratio at Yanahara mine and an associated programme of field measurements

In Yanahara Mine, Japan, a large pyrite ore body has been mined for the last 25 yr. The natural conditions of the ore body and country rocks are favourable, but surface subsidence must be avoided. It was considered that to achieve a high extraction ratio of mining without causing any surface damage would be difficult because the ore body is massing and large. Towards the end of primary mining, fracture of rock occurred in the walls of excavations at the pillars. Thereafter, to investigate the effects of secondary mining on the stability of the remaining pillars and the ground, a programme of field measurements has been carried out. This Paper describes the progress of pillar recovery and presents details of following measurements: deformation of roof rock; tilt of the floor of a mine level; rock stress states; subsidence of the roof rock; and average horizontal strain in a pillar. By the end of 1984, about 70% of the mineable ore had been successfully mined without any serious trouble.

Mineralization and paragenesis of the Huanzala Mine, central Peru

The Huanzala mine is a zinc and lead mineral deposit found in the Lower Cretaceous Santa Formation limestone and has associated copper and accessory polymetallic minerals. Early-stage mineralization began with pyritization and was followed by middle-stage zinc and lead mineralization, occasionally accompanied by skarnization. The lead and zinc mineralization stage was followed by late-stage copper ore deposition, together with small amounts of silver-tin-tungsten and miscellaneous minerals.The pyrite orebodies can be grouped by texture into two kinds: compact and fine-grained, and loose and coarse-grained ore. The zinc ores are composed of two types: iron-rich sphalerite and iron-poor sphalerite. The iron-rich sphalerite contains only a few blebs of copper minerals, whereas the iron-poor sphalerite is characterized by many inclusions of copper minerals.The copper-bearing hydrothermal solution of the late-stage mineralization permeated the grain boundaries and cracks of the iron-rich sphalerite and converted it into iron-poor sphalerite. Petrographic observations suggest that iron-poor sphalerite was produced by a process involving extraction of iron from iron-rich sphalerite, and the combination of this iron with copper in the hydrothermal solution, resulting in precipitation of chalcopyrite and bornite.The compact, fine-grained pyrite ore was changed into the loose, coarse-grained type by the same hydrothermal solution which transformed the iron-rich sphalerite into the ironpoor type.Late-stage copper ores were produced by mineralization ofchalcopyrite, bornite, tennantite, and enargite, followed by scheelite, cassiterite, stannite, canfieldite, hocartite, argentite, and other miscellaneous minerals. Mineralization of enargite was accompanied by bornite and was followed by chalcopyrite and tennantite. In several stages tennantite was deposited simultaneously with bornite and also with chalcopyrite.Studies of these paragenetic relationships, together with consideration of the orebody formation temperature and thermochemical treatment of the ore-forming minerals, suggest that with a decline in temperature the mineralization proceeded toward low f (sub S 2 ) and low f (sub O 2 ) .

Chemostratigraphy and origin of the late Archean siderite–pyrite-rich Helen Iron Formation, Michipicoten belt, Canada

The 2.7 Ga old Helen Iron Formation (HIF) with its uniquely large siderite–Pyrite orebody conformably overlies chemically altered rhyolite–dacite pyroclastic rocks, including an ottrelite-bearing alteration pipe, the product of exhalative venting in freshly accumulated pyroclastics. HIF, in turn, is conformably overlain by pillowed mafic lava flows.HIF internal stratigraphy is conformable from lower massive siderite–sulfide through middle sulfidic, carbonaceous chert, to upper magnetitic chert. Major- and trace-element and carbon- and sulfur-isotope data demonstrate cyclic fluctuations in chemical precipitation from early chemically complex to later chemically "simple" components.HIF development is attributed to seawater-charged volcanic exhalations in a transitory subaqueous cauldron subsidence basin. Volcanic exhalations contained (1) the main chemical components of HIF; (2) nutrients that triggered intense bacterial activity, the principal low-Eh-inducing agent; and (3) acidic components (e.g., HCI, HF), the principal low-pH-inducing agent. HIF carbonates (predominantly siderite) and sulfides (mainly pyrite) are of marine carbonate and marine sulfate origin, respectively. Organic activity was a vital catalyst in carbonate–sulfide precipitation. HIF is notably deficient in base and noble metals, thereby indicating low circulation–exhalation temperatures, in sharp contrast to contemporary hot, metal-rich geothermal brines at modern spreading ridge axes. This transient subaqueous late Archean volcanic environment, then, was unique in scale but not in kind.

Volcanogenic mineralization at Avoca, Co. Wicklow, Ireland, and its regional implications

Abstract A description is given to the geological setting of the stratiform pyritic orebodies at Avoca in Ireland and a volcanogenic origin is proposed on the basis of comparison with stratiform orebodies located in similar volcanic host rocks in other parts of the Caledonian orogen and elsewhere. The Ordovician volcanic sequence in southeast Ireland is interpreted as a fossil island arc complex, which was formed above a contemporary margin of southeasterly subduction, which locally defined the boundary of the evolving Caledonian orogenic belt in southeast Ireland. The Avoca ore deposits are located within a sequence of calc-alkaline lavas, pyroclastics and sediments at a level marked by the predominance of felsic eruptives. The stratigraphy of the host rocks is described in some detail and related to the general geology of the Avoca belt. Massive pyritic ores and disseminated sulphide ores of ‘stringer type’ can both be recognized at Avoca. The massive ore shows a compositional gradation suggestive of a primary stratigraphy; a Cu-rich pyritic zone is overlain by a zone of Pb-Zn-Cu pyritic ore. The interpretation of the genesis of the orebody is complicated by its deformed state, which is typical of other volcanogenic orebodies within the various Caledonian provinces. A genetic model can be proposed, however, by using the descriptions of the undeformed ‘Kuroko’ deposits in Japan as a standard for comparison. The individual orebodies of Avoca type contain 5 000 000–6 000 000 tons of ore, and these are associated with extensive zones of low-grade disseminated ore in acid pyroclastic host rocks. Sequences of manganiferous and dolomitized sediments are stratigraphically closely related to the ores. The principal geological features of the Avoca orebodies having been defined, it is deduced that there is a strong possibility that similar orebodies lie hidden in the volcanic rocks of the southeast Irish Caledonides and that an aggressive exploration programme could lead to new discoveries.

Observations on the pyrite deposits of the Huelva District, Spain, and their relation to volcanism

The pyrite ore bodies are essentially conformable with the bedding of enclosing rhyolitic pyroclastic rocks, which are underlain by rhyolitic flows and conformably overlain by lower Carboniferous shales. The occurrence of the deposits at the same stratigraphic horizon in a west-trending belt over 100 km long and the character of the host rocks suggest that the metals were supplied by submarine volcanic activity during early stages of marine sedimentation. The ore bodies were subsequently buried beneath a thick sequence of shale and were subjected to fracturing and recrystallization, probably accompanied by some redistribution of metals, during metamorphism associated with Hercynian folding and granitic intrusion.

The gossans of Minas Carlota, Cuba

The indigenous gossans of Minas Carlota formed in situ by the oxidation of steeply-dipping massive, banded pyritic ore bodies from 8 to 20 ft. thick and 100 to 1200 ft. long. The gossans are apparently younger than the dissected summit peneplain of the Trinidad Mountains and probably younger than the laterites of eastern Cuba. They are pseudomorphic with a banding inherited from the primary ore and are divisible into 3 gradational types by means of color, weight, and porosity. An layer separates gossan from sulfide. Locally, ground water has removed the earthy layer and formed caverns. The assumption that earthy layers are transitional products and that most gossans have collapsed has led to a neglect of relict structures in gossans. At Carlota the earthy layer is not transitional and the relict structures show that the ore bodies are commonly en echelon. Specific gravities of limonite were determined to ascertain whether the conversion pyrite-->limonite was accompanied by expansion or contraction (collapse). Results were not consistent. Discrepancies are attributed to the presence of unoxidized pyrite, entrapped air, hematite, or extraneous material. In general a direct relationship exists between gossan porosity and the percentage of original sulfide. Exotic gossan (limonite) may coat indigenous gossan or occur as float. Varieties include: breccia, boxwork, stalactitic drip, and spongy cinder. Indigenous gossan, except where recently broken, does not occur as float. Float gossan neither indicates the type of indigenous gossan present nor the type or grade of ore to be expected. Chemical analyses indicate a general relationship between a high Fe and a low S content, a possible relationship between high Fe and low P, and a general relationship between high S and low P. The Se and precious metal content could be used to calculate the erosion the region has undergone. Differential thermal and X-ray analyses show that goethite, with or without hematite, is the dominant iron oxide. The hematite may have formed by dehydration of limonite at the surface or by reaction between bacterially-produced ferric nitrate and ammonia. Absence of siderite bears out the Eh-pH calculations of Kelly and others. Related problems are reviewed and some hypothetical gossans outlined.

Genesis of sulphur deposits in Japan, with special reference to the mineralization of sulphur and pyrite(I)

H2S is carried upward by the superheated vapour, probably generated by the contamination of acidic plutonic rocks. Variable types of sulphur deposits have been formed by the difference in the location of the deposits. According to the alteration and mineralization, the physical and chemical features of the environments during the formation of sulphur and pyrite are summarized as follows: sulphur mineralization is caused by strongly acidic gases at temperatures between 130-100°C, which is transported by saturated vapour in ore bodies. The pyrite ore bodies are formed in acidic hydrothermal solutions ranging from pH 1.O to 5.O at temperatures between 120-100°C. Most of the iron in pyrite has teen derived from the country rocks.