The Importance of Oxidized Brines for the Formation of Australian ProterozoicStratiform Sediment-Hosted Pb-Zn (Sedex) Deposits

David R. Cooke,† Stuart W. Bull, Ross R. Large, and Peter J. McGoldrick

Centre for Ore Deposit Research, Geology Department, University of Tasmania, GPO Box 252-79, Hobart,Tasmania 7001, Australia

Abstract

A two-fold subdivision for stratiform sediment-hosted Pb-Zn (sedimentary exhalative, sedex) deposits is proposed, based on fundamental differences in the chemistry of the mineralizing brines. The type of sedimentary basin from which the ore fluids are derived, and the lithologies contained within the basin, control these differences in fluid chemistry. 

The two discrete brine types capable of transporting Zn and Pb are oxidized brines and reduced, acidic brines. McArthur-type deposits (e.g., McArthur River, Mount Isa, Hilton) precipitate from oxidized (SO₄^2-- predominant), acidic to near-neutral brines that evolve from sedimentary basins dominated by carbonates, evaporites, and hematitic sandstones and shales. Selwyn-type deposits (e.g., Sullivan, Rammelsberg, sedex deposits of the Selwyn basin) precipitate from acidic, reduced (H₂S-predominant) connate brines that evolved in reduced siliciclastic and shale basins.

Temperature decrease and dilution (fluid mixing), addition of H₂S, and pH increase can all be effective depositional processes for Zn and Pb from reduced (Selwyn-type) brines. In contrast, sulfate reduction and/or addition of H₂S (via fluid mixing or interaction with earlier formed pyrite) may be the important processes for sphalerite and galena deposition from oxidized (McArthur-type) brines. McArthur-type sedex deposits are intimately associated with siderite or ferroan carbonate alteration halos and most likely precipitate from lower temperature brines than Selwyn-type deposits. 

The redox state of the mineralized brines (sulfate or sulfide predominant) is important for controlling minor element associations in the two classes of sedex deposits. Weakly acidic to weakly alkaline oxidized brines can precipitate siderite but are incapable of carrying significant gold, tin, and barium in solution, and as such, McArthur-type deposits do not contain anomalous concentrations of these elements. Reduced, acid brines can carry high concentrations of barium, explaining the common association with barite in these deposits. If reduced sulfur concentrations were sufficient in the mineralizing brines, individual Selwyn-type deposits may contain anomalous or ore-grade gold. If the brines were highly reduced (pyrrhotite-stable), they may have carried high concentrations of tin (e.g., Sullivan). The lack of sulfide-bearing feeder systems in McArthur-type deposits and their common occurrence in Selwyn-type deposits probably also relate to the redox state of the brines.

From a mineral exploration perspective, oxidized sedimentary brines are more likely to produce large tonnage Zn-Pb-Ag deposits that have siderite or ankerite alteration halos and commonly lack barite lenses and vent complexes. By contrast, deposits that form in reduced siliciclastic and shale-dominated basins are more likely to be lower tonnage and to contain barite, vent complexes and may have minor gold or tin credits.

Metallogenesis of Zn-Pb Carbonate-Hosted Mineralization in theSoutheastern Region of the Picos de Europa (Central Northern Spain) Province:Geologic, Fluid Inclusion, and Stable Isotope Studies

F. Gómez-Fernández,†

Departamento de Ingeniería Minera, Escuela Universitaria de Ingeniería Técnica Minera, C/ Jesús Rubio 2, 24004 León, Spain

R. A. Both,

Department of Geology and Geophysics, University of Adelaide, South Australia 5005, Australia

J. Mangas,

Departamento de Física-Geología, Universidad de Las Palmas de G.C., Aptdo. 550, 35080 Las Palmas de Gran Canaria, Spain

and A. Arribas

Departamento de Ingeniería Geológica, Escuela Técnica Superior de Ingenieros de Minas, C/ Ríos Rosas 21, 28003 Madrid, Spain

Abstract

The Zn-Pb deposits of the southeastern area of the Picos de Europa province display an epigenetic hydrothermal mineralization of moderate temperature hosted by intensely dolomitized limestones of Carboniferous age. They are structurally and lithologically controlled since the three phenomena of fracturing, dolomitization, and mineralization are interrelated. Two types of mineralization have been found: type I, granular dark brown-colored sphalerite, galena, and dolomite, and type II, toffee-colored sphalerite, galena, and calcite. Type II mineralization is later than type I. 

The Aliva and Andara deposits are the two largest in the Picos de Europa area, and both contain type I and type II styles of mineralization. Fluid inclusion studies indicate that the mineralizing solutions for both stages were similar and had trapping temperatures between 170° and 200°C and salinities around 15 wt percent NaCl equiv. However, microthermometric data from sphalerite (I and II), dolomite I, calcite II, and fluorite samples show various stages of circulation and trapping of brines with spatial and temporal variations in salinity and temperature.

Calcite and dolomite from the mineralization have d¹³C_PDB values between 1 and 2.9 per mil (d¹³C^SCO2= –0.8–1.1‰), and d¹⁸O_SMOW values between 11.5 and 16.2 per mil (d¹⁸O^H2O varied from 0.7–1.9‰). Sulfides show d³⁴S values between 12.9 and –16.8 per mil (d³⁴S_fluid for the Aliva deposit ranged between 13.4 and –2.5‰). The combined stable isotope data indicate evolved marine waters. Isotope evidence suggests that the mineralizing fluids were probably formation waters of the Palentine zone and the Picos de Europa province sediments. During the Permian, possibly during extensional tectonic phases, these brines moved toward and through the Picos de Europa province along late Hercynian fractures giving rise to the Zn-Pb mineralization.

A Brittle Failure Mode Plot Defining Conditions for High-Flux Flow

Richard H. Sibson†

Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand

Abstract

Stress and fluid-pressure conditions for the initial formation within intact rock of faults (shear fractures), extension fractures, and extensional shears, and for the reshear of existing faults, may all be represented on a generic failure plot of differential stress (s₁ – s₃) versus least effective compressive stress, s₃' = (s₃ – P_f), scaled to nominal tensile strength, T (~ half the cohesive strength). Plots of this kind may be used to define maximum sustainable overpressure in different tectonic environments and the structural conditions under which the flow of large fluid volumes may occur through fault-fracture meshes containing gaping extension and extensional shear fractures. They may also be adapted to different tectonic regimes and correlated to depth for particular fluid-pressure conditions. High-flux flow through distributed fault-fracture meshes requires the tensile overpressure condition, s₃' < 0, to be met (generally involving P_f > s₃), which can only be achieved in the absence of throughgoing cohesionless faults that are well oriented for frictional reactivation in the prevailing stress field. High-flux flow through distributed fault-fracture meshes, intrinsically a transient, pulsing phenomenon, may therefore occur as follows: (1) in effectively intact low-pemeability crust devoid of throughgoing favorably oriented faults, (2) where existing faults have become severely misoriented in the prevailing stress field, and (3) where existing faults have regained cohesive strength through hydrothermal cementation.

The Archean Amphibolite Facies Coolgardie Goldfield, Yilgarn Craton,Western Australia: Nature, Controls, and Gold Field-Scale Patterns ofHydrothermal Wall-Rock Alteration

Joseph T. Knight,†,*

BHP Minerals Discovery Group, BHP World Minerals, 3 Plain Street, East Perth, W.A. 6004, Australia

John R. Ridley,

GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia

and David I. Groves

Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands, W.A. 6907, Australia

Abstract

The late Archean Coolgardie Goldfield at the western margin of the Norseman-Wiluna belt, Yilgarn craton, comprises an arcuate belt of deformed mafic, ultramafic, and sedimentary rocks which is bounded to the west by the syntectonic Calooli monzogranite. Greenstones at Coolgardie preserve a broad metamorphic gradient, with peak metamorphic temperature varying from 480° ± 50°C at the center of the gold field to 545° ± 50°C adjacent to the western granitoid-greenstone contact, at an approximate pressure of 3 to 4 kbar.

Gold field-scale variations in the gangue and ore mineralogy of zoned wall-rock alteration assemblages around lodes, the ore geochemistry, and the isotope chemistry of vein minerals at Coolgardie are correlated with plan-view distance from the Calooli monzogranite. Evidence supporting synpeak metamorphic gold mineralization in the Coolgardie Goldfield includes the equilibrium textural relationships between gold, sulfides, and high-temperature silicate gangue; the occurrence of undeformed auriferous quartz veins, enveloped by garnet-hornblende-plagioclase-calcite alteration, which crosscut peak-metamorphic fabrics; and the siting of variably deformed gold ores in synpeak-metamorphic structures. Conditions of gold mineralization at deposits less than 1 to 2 km from the Calooli monzogranite are determined from geothermometry and barometry to be 510° ± 50°C to 590° ± 25°C at 3 to 4 kbar, whereas those at greater distances from the monzogranite are 490° to 525° ± 50°C at 3 to 4 kbar.

Alteration assemblages in mafic host rocks can be divided into garnet-bearing (garnet-hornblende-plagioclase-calcite) and garnet-absent (biotite-amphibole-plagioclase-calcite). The presence or absence of garnet is mainly controlled by the Mg number of the host mafic rocks. Ore in deposits with garnet-bearing alteration is enriched in Ag, Na, Pb, S, and W, but only weakly enriched or depleted in K₂O and other large ion lithophile elements, CO₂, As, Mo, Sb, and Te, whereas deposits with biotite-amphibole-plagioclase-calcite alteration are strongly enriched in Ag, As, S, Sb, W, CO₂, and large ion lithophile elements. Sulfide-oxide assemblages are regionally zoned from pyrite-ilmenite in deposits in granitoids and adjacent to the granitoid-greenstone contact, through pyrrhotite-ilmenite ± pyrite in garnet-bearing alteration 1 to 2 km from the contact, to arsenopyrite-pyrrhotite-ilmenite assemblages in biotite-bearing alteration >2 km from the greenstone-granitoid contact. This variation is potentially related to gradients in fluid f_O2 away from the granitoids.

Isotopic compositions of oxygen in quartz (d¹⁸O = 10.8–12.4‰), scheelite (d¹⁸O = 4.0–4.1‰), and oxygen and carbon in calcite (d¹⁸O = 8.9–13.2‰, d¹³C = –0.5 to –5.3‰) are generally more positive in deposits with garnet-bearing alteration than in those with biotite-bearing alteration (d¹⁸O_quartz = 6.6–11.8‰, d¹⁸O_scheelite = 2.3–4.6‰, d¹⁸O_calcite = 8.7–11.4‰, d¹³C_calcite = –4.3 to –8.4‰), whereas both alteration styles have dD_biotite and dD_amphibole values in the range –65 to –86 per mil. These differences are interpreted to reflect interaction of isotopically heavy ore fluids with relatively depleted greenstone host rocks during fluid migration through structurally controlled conduits.

The gold field-scale variations in alteration mineralogy and ore chemistry are considered to be related not to initial ore-fluid composition but to temperature, to host-rock composition, and to changes in fluid composition resulting from reaction with greenstone-belt rocks. The correlation between the calculated temperature of alteration and distance from the western granitoid-greenstone contact suggests that the Calooli monzogranite played some genetic role in determining the nature of hydrothermal alteration across Coolgardie.

Lithological and Structural Controls on the Form and Setting ofVein Stockwork Orebodies at the Mount Charlotte Gold Deposit, Kalgoorlie*

John Ridley†

GEMOC, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia

and Faron Mengler 

Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, Western Australia 6907, Australia

Abstract

The Mount Charlotte quartz vein gold deposit comprises a series of steeply plunging, pipelike vein stockwork orebodies in massive metagabbro. The orebodies are strata bound to the most differentiated unit of the host sill and are typically adjacent to major steeply dipping faults that cut the sill. The stockworks have two sets of veins with a dihedral angle of about 50° that developed as hydraulic fractures, filled simultaneously, and are generally approximately equally developed. Veins crosscut major faults and parallel minor faults of two sets but are cut along reactivated fault surfaces. The fault sets were inactive during mineralization and are neither veined nor are loci of zones of intense alteration. Rare faults of a third set are in part veined and are loci of zones of mineralization. Two interpretations of the stress regime during vein formation are based on different models of fracture formation. For both stress regimes, the major fault sets are relatively unfavorably oriented for slip or dilation, and predicted movement vectors do not fit fault-plane lineations. The lack of fault activity during ore fluid flow promoted formation of vein stockworks at Mount Charlotte rather than shear zone or fault-hosted veins. Fluid flow paths and orebody siting are controlled by stress-guide effects due to the rheology of the host gabbro, and by the three-dimensional geometry of impermeable faults and of fault-bounded blocks of rock.

Origin of Massive Calcite Veins in the Golden Cross Low-Sulfidation,Epithermal Au-Ag Deposit, New Zealand

Stuart F. Simmons,†

Geothermal Institute and Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

Greg Arehart,*

Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand

Mark P. Simpson,

Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

and Jeffrey L. Mauk

Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

Abstract

At Golden Cross, andesitic lavas and volcaniclastic rocks host epithermal veins that formed in the shallow part (<500 m depth) of a hydrothermal system. Calcite is a trace mineral in precious metal-bearing quartz veins and a common replacement mineral in the surrounding intensely altered host rocks. Late barren calcite veins crosscut the precious metal-bearing quartz-sulfide veins and were a significant source of dilution in the underground workings of the mine; where large, they also posed significant problems for ground control. These veins range up to 10 m in width and contain more than 99 percent calcite, predominantly as massive coarse crystals, with only trace amounts of quartz, pyrite, and clays.

Fluid inclusion data indicate that much of the late barren calcite formed between 160° and 220°C, overlapping the temperature range of fluid inclusions from the precious metal-bearing quartz-sulfide veins. Ice melting temperatures range from 0.0° to –1.1° C. Slight vapor bubble expansion during crushing of a few calcite-hosted fluid inclusions indicates the presence of dissolved carbon dioxide. These results indicate that the hydrothermal solutions responsible for late calcite deposition were very dilute (<2 NaCl wt percent equiv) and contained up to approximately 2.5 wt percent dissolved carbon dioxide. The best interpretation of the steep T_h vs. T_m cooling trend is carbon dioxide gas loss through phase separation combined with variable amounts of mixing.

The d¹⁸O composition of calcite from the altered country rock and late veins ranges from 3.4 to 15.4 per mil, with the bulk of the data corresponding to equilibrium d¹⁸O water compositions of –2 to –6 per mil; this range of compositions is 0 to = 2 per mil lower than the d¹⁸O compositions for the waters in equilibrium with quartz from precious metal bearing quartz-sulfide veins. The d¹³C composition of calcite ranges from –3.1 to –9.0 per mil. The equilibrium d¹³C compositions of carbon dioxide for most of these data fall between –7 and –9 per mil. 

Electron microprobe analyses indicate that calcite contains less than 10 mole percent combined Mn, Mg, and Fe. Replacement calcite and veinlet calcite show greater substitution by these elements compared to calcite in massive veins, which is nearly pure. The minor element compositions of calcite appear to be primarily controlled by solution composition, and these constituents may be locally derived from the country rock.

Using the knowledge from active geothermal systems of the Taupo Volcanic Zone as a framework for interpretation, we propose that the late massive calcite veins were deposited from downward-moving, CO₂-rich, steam-heated water. This water was heated and locally reached vapor saturation as it descended into the former upflow zone of the hydrothermal system during waning activity. The reverse solubility of calcite accounts for the selective deposition of calcite over all other common hydrothermal phases, and condensation of steam into local ground water accounts for the slightly lower d¹⁸O water values. From this we suggest that, for some low-sulfidation epithermal prospects, the occurrence of barren calcite veins may be indicative of CO₂-rich, steam-heated waters that formed as a result of boiling.

Layering and Precious Metals Mineralization in theRincón del Tigre Complex, Eastern Bolivia

M. D. Prendergast†

1 Carson Close, Greendale, Harare, Zimbabwe

Abstract

Geologic investigations supporting recent, technically successful, platinum exploration of the Rincón del Tigre Complex—a ca. 4.6-km-thick layered sill—have provided significant new understandings of the layering, development, and precious metals mineralization of one of South America’s largest mafic-ultramafic intrusions. Three macrocyclic units are recognized—from the base up, Basal unit, Unit 1, and Unit 2—each most likely the product of a major replenishment with mafic magma of one compositional type. The ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite, granular harzburgite (olivine bronzitite) each a few meters thick, with occasional development of bronzitite at the top. Mafic rocks, absent from Unit 1 and present only locally in the Basal unit, form a well-developed norite, gabbro, magnetite gabbro sequence in the upper half of Unit 2.

Associated with the base of the magnetite gabbro is the Precious Metals zone, a persistent zone of low-grade (subeconomic) sulfide and precious metals mineralization, 80 to 185 m thick. The Precious Metals zone comprises an upper Cu sulfide-rich portion, its base (the sulfide phase boundary) about 23 to 70 m above the magnetite phase boundary, and a lower precious metals-rich portion, the lower part straddling the magnetite phase boundary. The individual precious metals are concentrated in separate, sulfide-poor subzones, each up to many tens of meters thick, in the stratigraphic order (Rh) Pt, Pd, Au, with peak offsets at the same scale. Modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic (precious metals) layering of the sulfides in the Precious Metals zone, all at three different superimposed scales from about 3 m to several tens of meters. A possible interpretive framework for the cryptic layering (and perhaps for much of the other layering, including the meter-scale layering of the ultramafic rocks) is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and, in the magnetite gabbro, by repeated convective overturns caused by the onset of magnetite precipitation.

The characteristics of the Precious Metals zone fit an orthomagmatic model. The composition, stratigraphic relations, and modal correlation (with magnetite) of the sulfides all imply that S saturation was induced by loss of FeO to magnetite precipitation. This took place after prolonged closed-system fractionation (following the last major magma injection at the base of Unit 2) during which crystallization of olivine, pyroxene, plagioclase, and (finally) magnetite precipitation led to overall S, Cu, and precious metals enrichment, and Ni and (finally) Fe depletion of the magma. The development of the mineralization and its complex layering was controlled by sulfide segregation, Rayleigh fractionation, the different but generally very high partition coefficients (D_{silicate/sulfide}) for each precious metal, and repeated convective overturns at three different superimposed scales.

The decoupled metals distribution is analogous and similar in origin to that of the Main Sulfide zone of the Great Dyke (Zimbabwe) and of the Main Sulfide layer (offset portion) of the Munni Munni Complex (Western Australia), but the stratigraphic order of metal enrichment peaks is different (Pt, Pd, Au vs. Pd, Pt, Au). This suggests a reversal in the relative magnitudes of the partition coefficients D_Pt and D_Pd that was possibly brought about by changes in the temperature and O, S, and Fe activities of the magmas between the middle and late stages of crystallization. Constancy of D values during fractionation of mafic magma should not be assumed.

The Precious Metals zone is one of several Skaergaard-type strata-bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions, and the origin of the Skaergaard mineralized zone itself—including the potentially economic Platinova reefs—can be explained in terms of the Precious Metals zone model presented here. The very low grade, great thickness, and wide separation of individual metal subzones in the Precious Metals zone are similar to those of the subeconomic axial facies of the Main Sulfide zone in the wider parts of the Great Dyke (cf. the economic, more compact, marginal facies). On the basis of the lateral and vertical thermal model of the Great Dyke magma chamber, the subeconomic status of the Precious Metals zone may be in large part a function of the specific fluid dynamics of the Rincón del Tigre magma chamber. 

Three-Dimensional Oxygen Isotope Imaging of Convective Fluid Flow around theBig Bonanza, Comstock Lode Mining District, Nevada

R. E. Criss,† M. J. Singleton,

Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130

and D. E. Champion

U.S. Geological Survey, Mail Stop 937, 345 Middlefield Rd., Menlo Park, California 94025

Abstract

Oxygen isotope analyses of propylitized andesites from the Con Virginia and California mines allow construction of a detailed, three-dimensional image of the isotopic surfaces produced by the convective fluid flows that deposited the famous Big Bonanza orebody. On a set of intersecting maps and sections, the d¹⁸O isopleths clearly show the intricate and conformable relationship of the orebody to a deep, ~500 m gyre of meteoric-hydrothermal fluid that circulated along and above the Comstock fault, near the contact of the Davidson Granodiorite. The core of this gyre (d¹⁸O = 0 to 3.8‰) encompasses the bonanza and is almost totally surrounded by rocks having much lower d¹⁸O values (–1.0 to –4.4‰). This deep gyre may represent a convective longitudinal roll superimposed on a large unicellular meteoric-hydrothermal system, producing a complex flow field with both radial and longitudinal components that is consistent with experimentally observed patterns of fluid convection in permeable media.

The Hydrothermal Geochemistry of Tungsten in Granitoid Environments: I. Relative Solubilities of Ferberite and Scheelite as a Function 

of T, P, pH, and m_NaCl

Scott A. Wood†

Department of Geology and Geological Engineering, University of Idaho, Moscow, Idaho 83844-3022

and Iain M. Samson

Department of Earth Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4

Abstract

The characteristics of granitoid-related tungsten deposits hosted in siliceous (carbonate-free) rocks (e.g., Panasqueira, Cligga Head, Pasto Bueno) are reviewed and the ranges of physicochemical parameters of the ore-forming fluids are summarized. The two important tungsten minerals in these deposits are wolframite and scheelite, which were deposited mostly between 200º and 500ºC and 200 and 1,500 bars. The salinities of the mineralizing fluids were typically less than 15 wt percent but commonly were significantly higher (up to 55 wt %). The two predominant dissolved components are Na⁺ and Cl^– with subordinate Ca²⁺, K⁺, and carbonate species (CO₃^2-/HCO₃^-). The contents of CO₂ are highly variable, but X_CO2 values typically range from 0 to 0.1. Limited pH and f_O2 estimates indicate a moderately acidic fluid with oxygen fugacities between those of the QFM and HM buffers. These parameters were used to guide solubility and speciation modeling of W in hydrothermal fluids in granitoid environments.

Experimentally derived thermodynamic data for scheelite, ferberite, aqueous Ca, Fe, and W species, and other required aqueous species were critically evaluated and the most reliable data were adopted. Where necessary, missing data were estimated. The resultant thermodynamic database provides a basis for solubility and speciation calculations in the system Ca-Fe-W-Cl-O-H. The simultaneous solubilities of scheelite and ferberite in NaCl-HCl-H₂O solutions were calculated at temperatures from 200º to 600ºC, pressures from 500 to 1,000 bars, pH from 3 to 6, and m_NaCl from to 0.1 to 5.0 moles/kg H₂O. The solubility model takes account of the species H⁺, OH^–, Na⁺, Cl^–, NaCl⁰, HCl⁰, NaOH⁰, H₂WO₄⁰, HWO₄^-, WO₄^2-, Fe²⁺, FeCl⁺, FeCl₂⁰, FeOH⁺, FeO⁰, HFeO₂^-, Ca²⁺, CaCl⁺, CaCl₂⁰, CaOH⁺, NaHWO₄⁰, and NaWO₄^-. The calculations indicate the following: (1) solubilities of scheelite and/or ferberite can attain values as high as hundreds to thousands of parts per million as the tungstate species H₂WO₄⁰, HWO₄^-, WO₄^2-, NaHWO₄⁰, and NaWO₄^-; thus, tungsten-chloride, -fluoride, or -carbonate complexes, or more exotic species are not required to transport sufficient W to form an ore deposit; (2) the tungsten concentration in equilibrium with scheelite and ferberite increases strongly with increasing temperature, increasing NaCl concentration and decreasing pH, but is only weakly dependent on pressure; (3) the Ca/Fe ratio of a solution in equilibrium with both scheelite and ferberite decreases strongly with increasing temperature, i.e., the field of stability of scheelite expands with increasing temperature; the implication, therefore, is that simple cooling of a solution with a constant Ca/Fe ratio cannot result in the replacement of ferberite by scheelite, and that field observations of the late-stage replacement of ferberite by scheelite require an increase in the Ca/Fe ratio concomitant with cooling; (4) the Ca/Fe ratio is relatively independent of pH; and (5) the effect of NaCl concentration on this ratio changes as a function of temperature and pressure. At less than 400°C the ratio is independent of, or decreases with, increasing NaCl concentration; at higher temperatures the ratio first decreases and then increases with increasing NaCl concentration. Experimental data on the solubility of scheelite and the Ca/Fe ratio of fluids in equilibrium with scheelite + ferberite, and which are not used in parameterizing our model, generally agree with the results of calculations performed using our thermodynamic database within an order of magnitude. However, our critical examination of available thermodynamic data reveals that significant uncertainty remains in several parameters (e.g., the solubility products of scheelite and ferberite and the association constants for alkali tungstate ion pairs). This uncertainty can only be reduced via carefully conceived, executed, controlled, and interpreted experiments, taking into account the various experimental pitfalls identified in this paper. 

Hydrothermal Alteration and Fluid Chemistry of theEndako Porphyry Molybdenum Deposit, British Columbia

David Selby,† Bruce E. Nesbitt,* Karlis Muehlenbachs,

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3

and Walter Prochaska

Institut für Geowissenschaften, Montanuniversität, A-8700 Leoben, Austria

Abstract

Hydrothermal alteration and fluid chemistry data of the early Cretaceous Endako porphyry molybdenum deposit, British Columbia, provide new information on the hydrothermal fluids associated with low-fluorine molybdenite mineralization. Molybdenite mineralization and hydrothermal alteration occur as early quartz ± molydenite stockwork veins with K feldspar-bearing selvages and paragenetically later quartz-molybdenite ribbon veins with sericite-bearing selvages. Late hydrothermal alteration is associated with the development of kaolinite and postore (Tertiary age) calcite veins.

Fluid inclusions in early-formed quartz ± molybdenite stockwork veins with K feldspar-bearing alteration assemblages are dominated by moderate-salinity (5 to 15 wt % NaCl equiv), liquid-rich (type 1) and rare high-salinity (30 to 45 wt % NaCl equiv), halite-bearing (type 3) fluid inclusions. Type 1 and type 3 fluid inclusions in early veins homogenize between 390° and 430°C and 375° and 420°C, respectively. Secondary fluid inclusions (type 2) of low salinity (1 to 5 wt % NaCl equiv) in these early veins are minor, and homogenize between 130° and 285°C. Fluid inclusions in quartz-molybdenite ribbon veins with sericite-bearing alteration assemblages are dominated by moderate-salinity, liquid-rich (type 1) inclusions, with minor type 2 fluid inclusions. Type 1 fluid inclusions of ribbon veins homogenize between 360° and 400°C. Fluid inclusions in postore calcite veins are of only type 2 fluid inclusions, which homogenize at 209°C. Hydrothermal fluids recorded by type 1 and type 3 fluid inclusions in early veins were trapped under lithostatic to hydrostatic conditions between 0.3 and less than or equal to 2.0 kbar, and 360° and 560°C. Postore fluids recorded by type 2 fluid inclusions were trapped under conditions less than or equal to 0.5 kbar, and between 190° and 300°C.

Quartz stockwork and ribbon veins possessd¹⁸O values of 8.4 ± 0.2 (n = 9) and 8.4 ± 0.6 (n = 13), respectively. Hydrothermal K feldspar and biotite from K feldspar alteration assemblages possess d¹⁸O values of 6.8 ± 0.4 (n = 7) and 3.5 ± 0.8 (n = 8), respectively. Oxygen isotope geothermometry of quartz-biotite and quartz-K feldspar pairs from K feldspar alteration assemblages yield temperatures between 200° and 490°C, which is similar to the trapping temperatures of hydrothermal fluids determined from fluid inclusion studies associated with molybdenite mineralization, the development of kaolinite, and calcite veins. The oxygen isotope temperatures of the quartz-biotite and quartz-K feldspar pairs suggest that K feldspar and biotite either record the approximate ¹⁸O composition of hydrothermal fluids associated with K feldspar alteration or have undergone ¹⁸O exchange with late-stage hydrothermal fluids. Hydrogen isotope composition of quartz stockwork and ribbon veins fluid inclusion waters range between –105 and –173 per mil.

Solute chemistry studies of fluid inclusion waters indicate that ore-forming fluids from Endako have low Br/Cl and Br/Na ratios, and high I/Cl and I/Br ratios in comparison to Porgera (epithermal), Babine Lake (porphyry Cu), and St. Austell, Capitan Pluton (vein) deposits associated with magmatic processes. Na/K ratios of fluid inclusion waters yield temperatures (308° to 429°C) similar to those determined from type 1 and type 3 fluid inclusions and stable isotope thermometry.

Results from fluid inclusion and solute chemistry studies indicate the involvement of hydrothermal fluids exsolved from a crystallizing melt in the formation of the Endako molybdenum deposit. However, oxygen and hydrogen isotope values deviate from the generally accepted magmatic compositions, which suggests the early involvement of meteoric water in the ore-forming fluids and ore genesis.

Supergene Ferromanganese Wad Deposits Derived from Permian Karoo Strata along the Late Cretaceous–Mid-Tertiary African Land Surface, Ryedale, South Africa

A. Pack,†,* J. Gutzmer, N. J. Beukes, H. S. van Niekerk,

Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa

and S. Hoernes

Mineralogisch-Petrologisches Institut und Museum, Rheinische Friedrich-Wilhelms-Universität Bonn,Poppelsdorfer, Schloß, 53115 Bonn, Germany

Abstract

A previously unknown type of ferromanganese wad deposit is described at the Ryedale mine situated 110 km west of Johannesburg in the northwestern province of South Africa. The wad was derived from supergene alteration of Glossopteris-bearing Permian strata of the Karoo Supergroup that fill shallow karstic depressions in Neoarchean Malmani dolomite of the Transvaal Supergroup. The depressions contain up to several million tons of friable and highly porous ferromanganese wad with an average Mn/Fe ratio of about 0.3 and high grades of between 77 and 91 wt percent Fe₂O₃ + MnO. A detailed mineralogical, petrographical, and geochemical study of the orebody suggests that the wad is a saprolitic residue of manganiferous blackband iron ores known to be associated with Permian coal measures of the Karoo Supergroup. The protore was deposited in shallow lakes in preexisting karstic depressions in the Malmani dolomite as finely laminated mud composed of biogenic detritus and Mn-Fe oxyhydroxide precipitates. Siliciclastic detritus is conspicuously absent, suggesting that the lakes were exclusively fed by reducing and acidic ground water that leached manganese and iron from the underlying Malmani dolomite. Anaerobic early diagenesis led to the transformation of oxide precipitates into Mn-Fe carbonates. Much later, during the African cycles of erosion and weathering, the blackband ores were exhumed, partly eroded, and altered by deep lateritic weathering to form the present ferromanganese wad.

SCIENTIFIC COMMUNICATIONS

FLUID INCLUSION STUDY OF STIBNITE USING INFRARED MICROSCOPY: 

AN EXAMPLE FROM THE BROUZILS ANTIMONY DEPOSIT (VENDEE, ARMORICAN MASSIF, FRANCE)^*

Laurent Bailly,† Vincent Bouchot, Claire Bény, and Jean-Pierre Milési

Bureau de Recherches Géologiques et Minières, BP 6009-3, Avenue Claude Guillemin, 45 060 Orléans, France

Abstract

A microthermometric study using infrared microscopy was performed on fluid inclusions hosted in stibnite crystals of the Brouzils antimony deposit, in order to characterize the fluids responsible for the deposition of the antimony at the end of the Variscan metallogenic sequence. Primary fluid inclusions in stibnite were moderately saline (3.5–4.75 wt % NaCl equiv) with homogenization temperatures varying between 140° and 160°C. Two generations of later fluids are also contained in the stibnite samples. The first generation of secondary fluid inclusions is found exclusively in stibnite and is characterized by low salinities (1.35–2.2 wt % NaCl equiv), homogenization temperatures of approximately 215°C, and the presence of one or more solid mineral phases. The second generation of fluid inclusions is younger and is found in stibnite along secondary planes as well as in quartz gangue; it corresponds to H₂O-CO₂ fluid with homogenization temperatures of about 180°C. This infrared microscopy study thus shows that the quartz and stibnite of the Brouzils veins are not cogenetic and reveals the polyphase character of the vein infill in which three successive episodes of hydrothermal circulation are identified. 

THE ORIGIN OF GREISEN FLUIDS OF THE FOLEY’S ZONE,CLEVELAND TIN DEPOSIT, TASMANIA, AUSTRALIA

Peter Jackson,

Department of Geology, La Trobe University, Bundoora, Australia 3083

Amarendra Changkakoti,†

School of Earth Sciences, University of Melbourne, Parkville, Australia 3052

H. Roy Krouse,

Department of Physics, University of Calgary, Calgary, Canada T2N 1N4

and John Gray

Department of Physics, University of Alberta, Edmonton, Canada T6G 2J1

Abstract

The Cleveland deposit, located in northwest Tasmania, Australia, was a major tin-producing mine until its closure in 1986. The deposit is contained by the Cambrian Creek Formation, which comprises argillite, quartz, and lithic wacke, basalt lava flows, pyroclastic deposits, calcareous wacke, arenites, and unfossiliferous limestone.

Three styles of mineralization occur in the mine sequence. These are carbonate replacement, greisenization of a quartz porphyry dike, and fissure veins. The area in the mine encompassing the dike and the surrounding vein halo is referred to as Foley’s zone. Five major alteration facies are recognized within the dike with distinct zonation between the types. These include sericitized feldspar greisen, quartz-muscovite greisen, quartz-muscovite-topaz greisen, quartz-topaz greisen and quartz ultragreisen.

Oxygen, hydrogen, and sulfur isotope data indicate that the hydrothermal fluids producing the bulk of the mineralization in the Foley’s zone veins show narrow ranges in isotopic compositions. Measured dD (–65 to –85‰) and calculated d¹⁸O (7.7–10.3‰) values suggest two possible interpretations. First, the fluids may simply be primary magmatic fluids. Secondly, the fluids may have originated from outside an igneous intrusion and undergone isotopic exchange with a large volume of igneous rock at magmatic temperatures. Sulfur data (1.7–4.1‰) are strongly suggestive of a magmatic origin.