Adjacent Wall Rocks Research Articles

Elbaite-liddicoatite from Black Rapids glacier, Alaska

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 57-73 - DOI: 10.2451/2011PM0005 Special Issue in memory of Sergio Lucchesi Elbaite-liddicoatite from Black Rapids glacier, Alaska Aaron J. Lussier 1 , Frank C. Hawthorne 1,* , Vladimir K. Michaelis 2 , Pedro M. Aguiar 2 and Scott Kroeker 2 1 Department of Geological Sciences, University of Manitoba, Winnipeg, Canada 2 Department of Chemistry, University of Manitoba, Winnipeg, Canada *Corresponding author: frank_hawthorne@umanitoba.ca Abstract Liddicoatite, ideally Ca(AlLi 2 )Al 6 (SiO 6 )(BO 3 ) 3 (OH) 3 F, is an extremely rare species of tourmaline, found in very few localities worldwide. A large (~ 2 cm in cross section), euhedral sample of tourmaline retrieved from atop the Black Rapids glacier, Alaska, is shown to vary from a light pink elbaite in the core region, average composition (Na 0.4 Ca 0.3□0.3 )(Al 1.75 Li 1.25 ) Al 6 (BO 3 ) 3 (Si 6 O 18 )F 0.4 (OH) 3.6 , to a pale green liddicoatite at the edge of the crystal, (Na 0.3 Ca 0.6 □ 0.1 )(Al 1.0 Li 1.6 Fe 0.2 Mn 0.2 )Al 6 (BO 3 ) 3 (Si 6 O 18 )F 1.0 (OH) 3.0 . Detailed electron-microprobe analysis and 11 B and 27 Al Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopy show that several substitutions were active during growth, with X □ + Y Al → X Ca + Y Li (liddicoatite-rossmanite solid-solution) and 2 Y Al + X □ → 2 Y M* + X Ca accounting for most of the compositional variation. Throughout the tourmaline, there are instances of reversals in the trends of all major constituents, although few compositional gaps are observed. Most notably, a sharp decline in Ca content from ~ 0.35 to ~ 0.05 apfu (atoms per formula unit) with increasing distance from the core at ~ 2 mm from the crystal edge is followed by a sharp rise in Ca content (to 0.55 apfu), along with (Fe + Mn) content (from 0.01 to 0.35 apfu). In the core region, the origin of the Ca in the tourmaline is not clear; the correlation of Ca and F is consistent with both (1) a melt in which Ca was held as complexes with F, or (2) earlier contamination of the melt by a (Ca, F)-rich fluid. Close to the rim, a dramatic increase in Ca, F, Mn and Fe is probably due to late-stage contamination by fluids that have removed these components from adjacent wallrocks. Key words: liddicoatite; elbaite; tourmaline; late-stage Ca enrichment; pegmatite; zoning; electron-microprobe analysis; Black Rapids glacier, Alaska; 11 B MAS NMR; 27 Al MAS NMR.

Mineral chemistry of polymetallic mineralization associated with altered granite, Hangaliya area, South Eastern Desert, Egypt

The Hangaliya gold deposit, located in the South Eastern Desert of Egypt, comprises a series of milky quartz veins along NW-trending Hangaliya shear zone, cutting through granitic rocks of Gabal Nugrus monzogranite. This shear zone, along with a system of discrete shear and fault zones, formed in the late deformation history of the area. The quartz vein emplacement took place under a brittle-ductile shear regime. Auriferous quartz veins are slightly sheared and boudinaged within the shear zone, especially in the hematized granite. Hydrothermal alteration is pervasive in the granitic wall rocks including sericitization, chloritization, fluoritization, sulphidization and minor carbonatization. The altered zones and associating quartz veins contain sulphides, gold, silver, cobalt, bismuth, and uraninite minerals. The Hangaliya gold veins include gold, silver, cobaltite, native bismuth, chalcopyrite, pyrite, galena, ferrocolumbite, fergusonite and uraninite. The Au-ore occurs in the quartz veins and adjacent wall rocks as dissemination in chalcopyrite and pyrite. Presence of refractory native silver, bismuth and cobalt in chalcopyrite is inferred from microprobe analyses. Wall rock sulphidization also likely contributed to destabilising the gold-silver, cobalt, bismuth assemblages and precipitation of the minerals in the hydrothermal alteration zone adjacent to the quartz veins. Gold occurs in two main modes: "invisible gold" in sulphides and native gold. Most of the "invisible gold" occurs in chalcopyrite and bismuth. The altered granites in the Hangaliya shear zone are enriched in Au, Ag, Bi, Co, and Ni with chalcopyrite, which suggests derivation of these metals from serpentinites due to interaction with the felsic Nugrus granite.

Listvenite–lode association at the Barramiya gold mine, Eastern Desert, Egypt

Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses. The mineralized quartz veins are characterized by abundant carbonic (CO 2 ± CH 4 ± H 2O) and aqueous-carbonic (H 2O–NaCl–CO 2 ± CH 4) inclusions along intragranular trails, whereas aqueous inclusions (H 2O–NaCl ± CO 2) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ 34S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ 34S Σs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO 2 via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites.

Initiation and development of the Twelve Mile Bay shear zone: the low viscosity sole of a granulite nappe

Structural, microstructural and petrological data have enabled determination of the mechanical and geochemical processes involved in dynamic weakening and fabric transposition along the margins of a granulite nappe (the Parry Sound domain (PSD)) during transport to mid-crustal levels of the Grenville Orogen. The data establish a genetic link between outcrop-scale structures in the southern PSD and the development of the underlying Twelve Mile Bay shear zone (TMBSZ). Following granulite facies metamorphism (� 11 kbar ⁄ � 850 � C) in the southern PSD, the emplacement of pegmatite dykes resulted in hydration reactions within adjacent wall rocks and the development of thin (<1 m) amphibolite facies (� 6.5 kbar ⁄ � 700 � C) shear zones. The shear zones exhibit bulk H2O and K2O enrichment and oxygen isotope values similar to the adjacent pegmatites, suggesting metasomatic alteration by pegmatite-derived fluids. Phase-equilibrium models indicate that the destabilization of the pre-existing pyroxene and garnet-bearing assemblages, as observed within discrete shear zones in the southern PSD and the TMBSZ, requires H2O-saturated conditions at these (amphibolite facies) P-T conditions. The spacing between discrete shear zones and the depth of hydration into the adjacent wall rock are of comparable length-scales (� metres), suggesting that this type of reworking process can be an effective means of hydrating kilometre-scale areas of crust relatively rapidly. Furthermore, considering the well- established effects of hydrous fluids on the creep strength of anhydrous minerals, a fracture-initiated, localized hydration-and-shearing process may be an efficient mechanism for weakening strong, dry rocks (e.g. granulites) in the middle to lower orogenic crust.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. I. Sequence of Intrusions, Vein Formation, and Sulfide Deposition

The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz mon-zonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast–trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast– and northwest-striking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber.

Structural and lithological controls on gold mineralisation at Oturehua on the northeastern margin of the Otago Schist, New Zealand

A set of more than 50 mineralised normal faults cuts lower greenschist facies rocks at the northeastern margin of the Otago Schist belt near Oturehua, central Otago. The faults strike northwest and dip steeply (>60°) northeast, and individual mineralised zones are traceable for up to 700 m along strike. The mineralised faults are closely associated with, and are generally parallel to, a swarm of northwest striking, steeply dipping extensional quartz ± ankerite veins (centimetre scale) that occur over an area of at least 25 km2in the host schist. Mineralised faults contain 10–100 cm thick quartz veins that are traceable as interlinked structures for tens of metres along strike. Where faults cut massive quartz‐feldspar‐rich schist, the veins are planar, contain scattered sulfide minerals and visible gold, and there has been little alteration of essentially undeformed adjacent wall rock (centimetre scale). In contrast, muscovite‐rich laminated schist host rock is extensively deformed and altered adjacent to mineralised faults (metre scale), with addition of quartz (silicification) and ankeritic replacement of metamorphic chlorite by kaolinite and/or sericitic muscovite. Veins adjacent to laminated (micaceous) schist contain abundant breccias with variably altered clasts. Widespread addition of auriferous sulfide minerals, mainly arsenopyrite, has occurred in the laminated schist alteration zones and breccias. The mineralised faults formed in the hinge of a northwest‐trending post‐metamorphic antiform, and the faults may have been localised by zones of weakness associated with kink folding in the hinge of the antiform. Hydrothermal fluids responsible for mineralisation were channelled by the normal faults. These faults developed by coalescence of extensional fractures in the schist rock mass during middle Cretaceous regional extensional unroofing of the Otago Schist belt. The location of these mineralised faults near the margin of the Otago Schist belt suggests a genetic relationship between mineralisation and the regional northwest‐striking normal fault systems that cut and define the schist margin.

The problem of gravitational redistribution mechanisms

The theory of gravitational redistribution suggests that ascending rocks must have structural features that indicate upward movement, while adjacent wall rocks must have structural features of downward movement. In metamorphic rocks, these features are represented by lineations, drag folds, orientation of fold hinges, etc. Gravitational redistribution is the main mechanism that forms granulite facies terrains, which has been proven by data on general geology, tectonics, petrology, geochemistry, isotopic geology, geophysics, and numerical modeling. Apart from “straight gneisses,” no direct evidence for the internal dynamics of the formation of high-grade terrains has ever been considered. In this paper, we formulate a rule allowing discrimination between cylindrical metamorphogenic and magmatogenic structures and demonstrate a model of their formation. Two examples of ring structures are considered as indicators of ascending granulites toward the surface, i.e., cylindrical folds and stocks. Despite the fact that these structures are located more than 100 km apart, they are characterized by similar orientation of foliation and lineations, which is evidence for their simultaneous formation. This conclusion is well supported by isotopic geochronological data.

Hydrothermal alteration geochemistry of the Betam gold deposit, south Eastern Desert, Egypt: mass-volume-mineralogical changes and stable isotope systematics

Although there is relatively minor gold production from the Arabian–Nubian shield at present, extensive alluvial and lode fields along the western side of the Red Sea in Upper Egypt and northern Sudan were worked out by the ancient Egyptians for thousands of years. In the Eastern Desert of Egypt, numerous but small gold deposits are generally related to auriferous quartz veins commonly associated with brittle-ductile shear zones, generally cutting through the Neoproterozoic crystalline basement rocks and trending in different directions. Gold mineralisation at the Betam mine area, south Eastern Desert, is related to a series of milky quartz veins along a NNW- trending brittle-ductile shear zone cutting through successions of pelitic schists, next to a small granite intrusion. Gold-sulphide mineralisation (pyrite, arsenopyrite, galena, subordinate chalcopyrite and gold) is closely associated with a conspicuous hydrothermal alteration halo. Textural relationships, including replacement and crosscutting of mineral phases and quartz veins record a post-foliation alteration assemblage of quartz+sericite+chlorite+calcite±albite±epidote. The hydrothermal alteration halo alongside the auriferous quartz veins comprises three distinct zones, namely distal chlorite-calcite zone (chlorite+calcite±biotite±pyrite±sericite±epidote), an intermediate sericite-chlorite zone (sericite+chlorite+pyrite±biotite), and a proximal pyrite-sericite zone (quartz+pyrite+sericite±albite). These zones merge to each other gradually, ending outwards into the unaltered metasediments. The pyrite-sericite zone contains the highest gold grades, especially in zones thickly seamed with sulphide-rich quartz veinlets. Mass balance calculations have revealed that the pyrite-sericite zone experienced significant metasomatic changes relative to limited mass and volume changes for the chlorite-calcite zone. The overall picture of chemical gains and losses with increasing the intensity of hydrothermal alteration is indicative of addition of SiO2, K2O, Na2O, and volatile elements (S, CO2), removal of MgO, and relatively inert behaviour of Al2O3, TiO2, MnO, Fe2O3. CaO is variably mobile; slightly enriched in the chlorite-calcite and sericite-chlorite zones but depleted in the pyrite-sericite zone. Concentrations of the trace elements are variable in the different alteration types, but a notable increase in Au, As, Ba, Sr, Rb, V, and Ni in the intensively altered rocks is apparent. Investigation of the rare earth element (REE) behaviour reveals a little modification; heavy REE are more or less unchanged, whereas light REE are significantly mobile in all alteration types. New geochemical data provide evidence for progressive silicification, sericitisation, and sulphidation as a function of gold mineralisation. A proposed model for the hydrothermal alteration system for the Betam deposit includes fluctuation in pH and redox state (fO2), mainly during the wallrock sulphidation. This might have destabilised gold complexes and lowered gold solubility in the ore solution, and hence contributed at least partly in gold deposition in the study area. A decrease in the whole rock δ18O values from un-altered country metasediments to the distal, intermediate and proximal alteration zones, respectively, might be attributed to rock interaction with an isotopically lighter fluid. In addition, sulphur isotope data of pyrite-arsenopyrite pairs in the mineralised quartz veins and adjacent wallrock along with the calculated δ34S∑s values for the ore fluids suggest derivation from non-homogenous (mixed magmatic and metamorphic) fluids.

The Role of Sulfate-Sulfide-Oxide-Silicate Equilibria in the Metamorphism of Hydrothermal Alteration at the Hemlo Gold Deposit, Ontario

The Hemlo Au (-Mo) deposit is an example of atypical, greenstone-hosted, mesothermal mineralization from the Superior province of north-central Ontario. The ore is shear hosted and consists primarily of disseminated native gold in several mineralized zones near the contact between quartz-feldspar porphyry and metasedimentary rocks. Metsasedimentary rocks (i.e., pelite, graywacke, arenite, marl, mafic fragmental rocks, and baritic sediment) host most of the ore. The dominant alteration is potassic and is characterized by a central zone of granoblastic microcline-quartz-(barite-muscovite-biotite-pyrite) rocks, surrounded by muscovite-quartz-pyrite schists. Regional, amphibolite-facies metamorphism (~630°C, 5–7 kbars) overprinted the ore zones and recrystallized the alteration assemblages. Unaltered metasedimentary wall rocks contain the assemblages biotitegarnet-staurolite-kyanite and/or sillimanite (metapelite) and amphibole-plagioclase (mafic fragmental rocks and marly sediment). As the ore zones are approached, the proportions of muscovite and K-feldspar and the mole fraction of pyrope in garnet increase, garnet, staurolite, and kyanite gradually disappear, biotite becomes Mg rich, ilmenite is replaced by rutile, and pyrrhotite is replaced by pyrite. In altered and mineralized mafic fragmental rocks, biotite replaced amphibole, which indicates that amphibole was present prior to alteration and that gold mineralization occurred at pressure-temperature conditions equivalent to those of the greenschist facies (400°–500°C, 3–5 kbars). The presence of garnet and staurolite in unaltered metasedimentary rocks, but not in the ore zones, is consistent with phase equilibria which indicate that only Mg-rich silicates can coexist with pyrite, and that Mg-rich end members of garnet and staurolite are only stable at temperatures and pressures considerably higher than those of peak metamorphism. This change in mineral assemblage is due primarily to the very high f S 2 during alteration and gold mineralization, which caused iron to partition preferentially into pyrite, and is supported by the decrease in the Fe/Mg ratio of garnet and biotite toward the ore zones. Similarly, the absence of kyanite in the ore zones, despite its common occurrence in the immediately adjacent wall rocks, can be attributed directly to the high activity of potassium, which stabilized the assemblage muscovite-microcline in favor of the assemblage kyanite-muscovite. The combined effects of sulfidation and potassic alteration ensured that all phases in the ore zones participated directly or indirectly in metamorphic reactions. The evolution of such rocks therefore differs significantly from those of normal metasedimentary rocks. In general, typical peak metamorphic assemblages do not develop, and thus timing relationships of mineralization hosted by amphibolite-grade rocks are obscured. However, the disappearance of metamorphic index minerals, modification of mineral assemblages, and mineralogical changes in sulfides and oxides that are recognizable on an outcrop scale at Hemlo serve as important vectors to economic mineralization.

Element mobility and scale of mass transport in the formation of quartz veins during regional metamorphism of the Waits River Formation, east-central Vermont

Veins and adjacent alteration selvages in the Waits River Formation were investigated to determine whether associated mass transfer was due primarily to large-scale advection or small-scale diffusion. Samples of the vein, selvage, and adjacent wall rock were collected from the earliest and most numerous generation of veins in pelite and carbonate hosts from outcrops in the chlorite and the kyanite zones. Bulk compositions of selvages and unaltered wall rocks were compared using a reference frame defined by a combination of Zr, Ti, REEs, and U. Selvages from both outcrops typically exhibit losses of Si, K, Ba, and Rb relative to unaltered wall rock. Kyanite zone selvages show losses in Mg and Cs in addition. Differences in K, Ba, Rb, Cs, and Mg between the vein-selvage system as a whole and adjacent unaltered wall rock are possibly accounted for by development of micas in veins. The addition of Si and Ca to veins is not balanced by removal of Si and Ca from the selvages. Vein-selvage systems contain more Si than wall rock, with an overall addition of ≈40 mg Si/cm3 to the chlorite zone and ≈55 mg Si/cm3 to the kyanite zone. Mass balance of Si at the outcrop scale requires that >90% of the Si in quartz veins was derived externally. Quartz veins studied formed primarily by fluid flow and large-scale advective mass transfer with a relatively minor component of local mass transport by diffusion. The estimated time-integrated fluid flux necessary to produce the observed amount of quartz in veins in an entire outcrop is ≈2-6·106 cm3 fluid/cm2 rock. Mineral inclusions in garnet and fracturing of garnets adjacent to veins indicate that formation of selvages and veins initiated prior to formation of garnet and continued after the end of garnet growth.

Radionuclide transport in fractured granite interface zones

In situ radionuclide migration experiments, followed by excavation and sample characterization, were conducted in a water-conducting shear zone at the Grimsel Test Site (GTS) in Switzerland to study migration paths of radionuclides in fractured granite. In this work, a micro-scale mapping technique was applied by interfacing laser ablation sampling with inductively coupled plasma-mass spectrometry (LA-ICP-MS) to detect the small scale (micron-range) distribution of actinides in the interface zones between fractures and the granitic rock matrix. Long-lived 234U, 235U, and 237Np were detected in flow channels, as well as in the diffusion accessible rock matrix, using the sensitive, feature-based mapping of the LA-ICP-MS technique. The retarded actinides are mainly located at the fracture walls and in the fine grained fracture filling material as well as within the immediately adjacent wallrock. The water-conducting fracture studied in this work is bounded on one side by mylonite and the other by granitic matrix regions. Actinides studied in this work did not penetrate into the mylonite side as much as into the granite matrix, most likely due to the lower porosity, the enhanced sorption capacity and the disturbed diffusion paths of the mylonite region itself. Overall, the maximum penetration depth detected with this technique for 237Np and uranium isotopes over the field experimental time scale of about 60 days was about 10 mm in the granitic matrix, illustrating the importance of matrix diffusion in retarding radionuclide transport from the advective fractures. Laboratory tests and numerical modelling of radionuclide diffusion into granitic matrix was conducted to complement and help interpret the field results.

Numerical simulation and a geochemical model of supergene carbonate-hosted non-sulphide zinc deposits

The proposed metallogenetic model presented in this paper is based on observations from the Iranian non-sulphide zinc deposits Mehdi-Abad, Iran-Kuh, and Kuh-e-Surmeh. The emplacement of non-sulphide ore can be generally subdivided into an ‘oxidation stage’ followed by a ‘post-oxidation stage’. The formation of carbonate-hosted non-sulphide zinc deposits is primarily controlled by three factors: climate, protore composition, and geology (lithology and structure) of the wall rocks. The climate controls the oxidation and metal transport, predominantly through the amount of meteoric water and the availability of oxygen, which indirectly is controlled through the thickness of the soil cover and biogenic oxygen consumption. Favourable conditions for oxidation are an arid climate with minimal biogenic activity within the soil. As a consequence, oxygen can reach the sulphides easily, resulting in higher metal concentrations of the supergene aqueous solutions. Dry climates are associated with a low water table, preventing descending Zn-bearing fluids from contact with the aquifer and thus from dilution, dispersion and removal of metals. Commonly, non-sulphide zinc deposits consist of two types of ore: red zinc ore, rich in Zn (> 20%), Fe (> 7%), Pb-(As) and white zinc ore, typically with very high zinc grades (up to 40%) but low concentrations of iron (< 7%) and lead. Typical minerals of the red zinc ore are Fe-oxyhydroxides, goethite, hematite, hemimorphite, smithsonite, and/or hydrozincite and cerrusite. Common minerals of the ‘white zinc ore’ are smithsonite or hydrozincite and only minor amounts of Fe-oxyhydroxides. Metal separation is caused by a gradual change from an acidic oxidation zone to alkaline conditions in the adjacent carbonate wall rock. The formation of an acidic oxidation zone within carbonate host rocks is facilitated by the “armouring” of calcite by gypsum and hydrous ferric oxides (HFO) and by several pH-buffering reactions. The HFO within the oxide zone additionally adsorb various amounts of Pb and Zn, depending to the pH. The high activity of sulphuric acid-related SO 4 2− ions during the oxidation stage leads to the precipitation of highly insoluble anglesite, which results in low Pb 2+ concentration within the fluid. Most of the zinc (up to 80% at pH 6 or 97% at pH 5) leaves the oxidation zone and precipitates as zinc carbonates in adjacent parts of the carbonate host rock. The neutralisation processes of the sulphuric acid with the carbonates of the host rock during the ‘oxidation stage’ lead to the formation of CO 2(g). Consequently, the CO 2 partial pressure (P CO2(g)) increases drastically and forms a “CO 2(g)-halo” around the active oxidation zone. Thus, most of the zinc precipitates as smithsonite, which is stable and occurs exclusively in a high P CO2(g) environment. The P CO2(g) decreases during the ‘post-oxidation stage’ and reaches the level of atmospheric P CO2(g). Under these conditions, hydrozincite becomes stable and starts to replace smithsonite. Most of the stage-I smithsonite becomes corroded and is altered to stage-II hydrozincite. The ‘post-oxidation stage’ is also associated with the successive formation of local zinc (hydro-) silicates, depending on the availability of SiO 2 within the solution. Generally, arid climates provide the best conditions for the preservation of non-sulphide deposits. The limited availability of meteoric water and deep to very deep water tables protect the non-sulphide ore from subsequent dissolution.

Structural controls, temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine, south Eastern Desert, Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (LCO2 + Laq ± VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2 ± VCO2) and aqueous H2O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H2O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f S2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f O2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.

The Trawenagh Bay Granite and a new model for the emplacement of the Donegal Batholith

ABSTRACTThe Trawenagh Bay Granite (TBG) is shown to be a tabular pluton with gently inclined contacts that, from anisotropy of magnetic susceptibility (AMS) studies, was emplaced as a series of flow lobes whose geometries indicate that it flowed horizontally towards the W out of late stage adjacent steeply inclined monzogranite sheets of the Main Donegal Granite (MDG). We thus confirm in detail the central broad idea of the Pitcher &amp; Read (1959) model that the Main Donegal Granite fed the Trawenagh Bay Granite. Early TBG flow lobes cut and are cut by deformation associated with the sinistral shear zone in which the MDG lies, thus demonstrating synchronicity of shearing and magmatism. The TBG magma leaked out of the shear zone and emplaced into undeformed country rocks and was probably guided by shear zone splays that die out along its northern and southern margins. At a late stage in the development of MDG, the splays developed from the NNE-trending SW boundary of the shear zone and caused a gap in this structure through which TBG magma was channelled out of the MDG. A review is presented of the last twenty-five years of published and unpublished work on the batholith, showing that the MDG shear zone was a long-lived structure almost certainly in existence before the emplacement of that body, and that four of the contiguous granitiods (Thorr, Ardara, and Rosses, as well as Trawenagh Bay) were all sourced within the shear zone. A new model is presented for the development of the batholith. The pre-existing crustal structure was a deep-seated N12°E fault in the basement to the Dalradian wall rocks of the granites, that was coupled to up to six other more minor WNW–ESE basement faults in the W. A NE–SW-trending sinistral shear zone was initiated at the end of the Caledonian orogeny, as calc-alkaline and deep-seated appinites were generated in the area. This shearing activated the pre-existing structures at the current crustal level, and the N12°E structure acted as a continental transform fault which allowed the dilation needed to facilitate the wedging space requirements of the MDG and the other units in the shear zone, as well as transferring regional sinistral shear through the system. The Thorr and Ardara plutons were emplaced first into the shear zone and then those magmas leaked out into the adjacent wall rocks: one to form a large laccolith, the other to form a balloon. Steep early MDG complex sheets (granodiorites and tonalities) were emplaced in the shear zone between the Thorr and Ardara emplacement sites. Dilation continued until late stage extensive monzogranite sheets were intruded in the NW and SE of the pluton. One of these probably leaked material westward to form the Rosses laccolith and southwestwards to form the TBG in the final stages of shear zone movement.

Granite‐related overpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry District, Australia

AbstractThe source and transport regions of fluidized (transported) breccias outcrop in the Cloncurry Fe‐oxide–Cu–Au district. Discordant dykes and pipes with rounded clasts of metasedimentary calc–silicate rocks and minor felsic and mafic intrusions extend several kilometres upwards and outwards from the contact aureole of the 1530 Ma Williams Batholith into overlying schists and amphibolites. We used analytical equations for particle transport to estimate clast velocities (≥20 m sec−1), approaching volcanic ejecta rates. An abrupt release of overpressured magmatic‐hydrothermal fluid is suggested by the localization of the base of the breccias in intensely veined contact aureoles (at around 10 km, constrained by mineral equilibria), incorporation of juvenile magmatic clasts, the scale and discordancy of the bodies, and the wide range of pressure variation (up to 150 MPa) inferred from CO2 fluid inclusion densities and related decrepitation textures. The abundance of clasts derived from depth, rather than from the adjacent wallrocks, suggests that the pressure in the pipes was sufficient to restrict the inwards spalling of fragments from breccia walls; that is, the breccias were explosive rather than implosive, and some may have vented to the surface. At these depths, such extreme behaviour may have been achieved by release of dissolved fluids from crystallizing magma, in combination with a strongly fractured and fluid‐laden carapace, sitting under a strong, low permeability barrier. The relationship of these breccias to the Ernest Henry iron‐oxide–Cu–Au deposit suggests they may have been sources of fluids or mechanical energy for ore genesis, or alternately provided permeable pathways for later ore fluids.

Layered mafic sill complex beneath the eastern Snake River Plain: Evidence from cyclic geochemical variations in basalt

The eastern Snake River Plain in southern Idaho, western United States, is characterized by 1–2 km of Pleistocene to late Pliocene basalt overlying rhyolite caldera complexes. Cyclic variations in the chemical composition of basalts from 1136 m of scientific drill core show that the parent magmas of these lavas evolved by crystal fractionation at shallow to intermediate crustal depths, punctuated by episodic recharge with more primitive compositions and assimilation of adjacent wall rock. We have identified 10 upward fractionation cycles and four reversed cycles; assimilation of sialic crust was limited and mainly affects the oldest basalts, which directly overlie rhyolites. We infer that the crystal fractionation and/or recharge cycles took place in a series of sill-like intrusions at intermediate crustal depths that now form a layered mafic intrusion that underlies the eastern Snake River Plain at depth. This layered sill complex is represented by the ∼10-km-thick “basaltic sill” that has been imaged seismically at ∼12–22 km depth. The association of this mid-crustal sill complex with geochemical fractionation cycles in basalt supports the concept that exposed layered mafic intrusions may be linked to overlying basalt provinces that have since been removed by erosion.

Textural and Thermal History of Partial Melting in Tonalitic Wallrock at the Margin of a Basalt Dike, Wallowa Mountains, Oregon

Columbia River Basalt Group dikes invade biotite---hornblende tonalite to granodiorite rocks of the Wallowa Mountains. Most dikes are strongly quenched against wallrock, but rare dike segments have preserved zones of partial melt in adjacent wallrock and provide an opportunity to examine shallow crustal melting. At Maxwell Lake, the 4 m thick wallrock partial melt zone contains as much as 47 vol. % melt (glass plus quench crystals) around mineral reaction sites and along quartz--feldspar boundaries. Bulk compositional data indicate that melting took place under closed conditions (excepting volatiles). With progressive melting, hornblende, biotite, and orthoclase were consumed but plagioclase, quartz, and magnetite persisted in the restite. Clinopyroxene, orthopyroxene, plagioclase, and Fe---Ti oxides were produced during dehydration-melting reactions involving hornblende and biotite. Reacting phases became more heterogeneous with progressive melting; crystallizing phases were relatively homogeneous. Progressive melting produced an early clear glass, followed by light (high-K) and dark (high-Ca) brown glass domains overprinted by devitrification. Melts were metaluminous and granitic to granodioritic. Thermal modeling of the partial melt zone suggests that melting took place over a period of about 4 years. Thus, rare dikes with melted margins represent long-lived portions of the Columbia River Basalt dike system and may have sustained large flows.

Geology of the Melas Chasma landing site for the Mars Exploration Rover mission

The Melas Chasma landing site was considered a high‐priority site for the Mars Exploration Rover (MER) mission because of the opportunity to land on and study potential layered sedimentary deposits. Though no longer considered a candidate site because of safety concerns, the site remains a scientifically interesting area that provides insight into the geologic history of Valles Marineris. Within the landing ellipse are dunes, landslide material, and unusual blocky deposits. The blocky deposits are composed of rounded blocks, some of which have meter‐scale layering, and they show evidence of ductile deformation, including bending and distortion of coherent blocks around each other. The morphologic characteristics of the blocks are unique, and they appear to have no terrestrial analogue. However, the gross morphology of these blocky deposits and their superposition on adjacent wallrock is consistent with the blocks having been transported downslope. Given the existence of other large mass failures in the area, we propose the blocky deposits may also have originated from mass movement events. The size of the blocks coupled with the distances they traveled indicates high mobility. The distances the blocks were transported and their rounded, irregular shapes suggest either water in the source material or deposition in a subaqueous environment. The source for the main blocky deposit inside the landing ellipse is considered to be the wallrock to the south, while the other two blocky deposits have source regions along the northern canyon walls. The southern wallrock of Melas Chasma contains numerous valleys not seen elsewhere in Valles Marineris. The identification of valley networks along the southern wallrock suggests that a source of water existed below the surface of the plateau and produced the valleys after intersecting the edge of the exposed canyon walls.

Evaluation of the Role of Sulfidation in Deposition of Gold, Screamer Section of the Betze-Post Carlin-Type Deposit, Nevada

This study was undertaken to evaluate the relation between and gold deposition in the Screamer section of the Betze-Post Carlin-type deposit. We also attempted to determine the source of sulfur in the deposit and the possibility that more than one gold-depositing event contributed metal to the Screamer system. Gold ore at Screamer forms a generally stratiform body hosted largely by the Wispy member of the Devonian Popovich Formation and lacks obvious alteration of wall-rock gangue minerals. Gold at Screamer is hosted by arsenian pyrite that forms disseminated grains and overgrowths on diagenetic pyrite. A strong correlation is observed between the gold content of rock samples and their proportion of ore-related (arsenian) pyrite, as determined by point counts. Isocon plots show that mineralization at Screamer involved addition of both sulfur and iron, along with gold, arsenic, antimony, tungsten, and local silica, barium, and phosphorus. Fe/Al vs. S/Al plots show that most ore samples at Screamer do not contain enough sulfur to account for all of their iron as pyrite; petrographic examination shows that the iron occurs in pyrite, ferroan dolomite, and iron-bearing micas in order of decreasing abundance. The Fe/Al vs. S/Al plots also show that Screamer samples with high gold contents contain more pyrite than samples with low gold contents and that samples inside the ore zone have more pyrite than those in the surrounding area. These relations are interpreted to indicate that Screamer has undergone at least two pyrite-forming events. The first event, which probably took place during diagenesis, involved incomplete that left some iron in carbonate and silicate minerals. The second event, which probably took place during gold mineralization, sulfidized most remaining iron and added pyrite in the Screamer ore zone. The δ 34 S values of chemically extracted sulfur from disseminated pyrite at Screamer range from –13.8 to 16.5 per mil, and δ 34 S values for hand-picked separates of pyrite and other sulfides in veins cutting these rocks range from –21.2 to 11.7 per mil. A significant fraction of samples with high gold contents and of samples in the ore zone regardless of gold content have δ 34 S values between –1 and 5 per mil. These data suggest that early diagenetic sulfur with a wide range of δ 34 S values was overprinted by sulfur with isotopic compositions in the –1 to 5 per mil range that was associated with gold mineralization. The δ 34 S values for gold-related sulfur at Screamer are lower than those reported for bulk mineral separates from most other Carlin-type deposits and from SIMS analyses of sulfides from the proximal Post part of the Betze-Post system and could be magmatic. Limited evidence can be found for multiple gold-forming events in the Screamer zone. Tungsten, which might have been introduced by the Jurassic-age Goldstrike stock, is widespread in the deposit and correlates closely with gold. A few samples with high tungsten/gold ratios found along fault zones might be part of an earlier phase of mineralization related to the Goldstrike stock. A δ 34 S value for pyrite in one of these samples is similar to the high values reported for sulfides in auriferous, polymetallic mineralization in the Post section of the deposit, but this mineralization is not reported to contain tungsten. Other veins containing sphalerite have lower δ 34 S values and lack consistent gold values. Whereas there is no correlation between gold values and the degree to which the host rocks have undergone sulfidation, there is a strong and highly significant correlation between gold values and the abundance of ore-related pyrite. This shows that simple of immediately adjacent wall rock cannot account alone for gold deposition at Screamer and it might not be the only ore-depositing process in some other Carlin deposits. A more general, extended sulfidation process could be important, however. One likely process involves mixing of an invading, mineralizing fluid containing sulfur and gold with a wall-rock fluid containing iron derived from adjacent or distal wall rocks. Recognition of the source(s) for this iron and fluid flow pathways responsible for introducing it to the ore zone could provide useful guidance in exploration for the next Carlin trend.

(Au-Fe) Skarn Deposits of the Mezcala District, South-Central Mexico: Adakite Association of the Mineralizing Fluids

Geochemical evidence is presented that the auro-ferriferous (Au-Fe) mineralized belt of southcentral Mexico (Sierra Madre del Sur-Mezcala region) is directly linked to a regional post-Laramidic tectono-magmatic event, which was the product of interaction between the Farallon oceanic plate and the North America continental plate during the Middle to Late Tertiary. The Fe-rich skarn-type ore deposits are located near to the coast (80 to 100 km from the paleo-trench), whereas the Au-iron rich deposits of the Mezcala district are located 270 km from the paleo-trench where the crustal thickness is approximately 45 km. The magmatic record of the Tertiary rocks from the "Sierra Madre del Sur" extends from the Paleocene to Miocene, constituting an extended calc-alkaline magmatic province. Adakite-type intrusive rocks are reported in the area, and their relationship with the Au-Fe mineralization in Mezcala is well established. The geochemical anomalies of MgO, Cr, and Ni observed in the intrusive adakites are higher than the experimentally generated adakites; these geochemical features are interpreted to represent an adakite magma that reacted with a mantle wedge. The Sr/Y vs. Y plot shows a clear overlapping relationship between intrusions associated with Au and Cu ore deposits and adakites worldwide. The high La/Sm and Sm/Yb ratios are consistent with an amphibolite-garnet source for these Mezcala adakite rocks. The Au mineralization at Mezcala is spatially related to quartz porphyritic granodiorite (adakite) sills, and developed peripherally on adjacent wall rocks. Gold abundance in wall rock correlates very well with the composite thickness of the adjacent sills. The fluid inclusion analyses reveal that homogenization temperatures (Th) and exo-oxidizing high-salinity magmatic fluids are related to the gold- mineralizing event. The fluid inclusions in late magmatic quartz phenocrysts are multi-component (L + V + S1-5). RAMAN microspectrometric results indicate the presence of halite (S1), sylvite (S2), hematite (S3), anhydrite (S4), and an unidentified crystal (S5). The late fluids have L + V and L-rich fluid inclusions. The earliest fluid identified has the higher salinity (63% total salinity, NaCl + KCl) and Th between 480 and 675°C. This early fluid exhibits a large Th variation from 420°C (decompression-boiling) to final dilution processes at 100-127°C and salinities of 0.25 wt% NaCl equivalent. Most likely, this late fluid dilution is the product of mixing of nearly neutral meteoric water (pH = 7) with hot NaCl-rich fluids transporting gold in solution. The main mineralization in the area (hematite-gold) is the product of these late hydrothermal processes. The original magmatic fluids are dense and oxidizing, with boiling and late dilution linked to the retrograde evolution of the Au-Fe-rich skarn.