Chlorite Zone Research Articles

Volcanic Influences on Hydrothermal and Diagenetic Alteration: Evidence from Highway-Reward, Mount Windsor Subprovince, Australia

Synvolcanic sills and cryptodomes hosting the Highway-Reward volcanic-hosted massive sulfide deposit invaded wet, poorly consolidated silt, sand, and pumice breccia. Margins of the porphyriric units display perlitic and quench fractures suggesting that they were formerly glassy. Interiors of the rhyolites, rhyodacites, and dacites were originally glassy, partly glassy, or comprised crystalline domains exhibiting spherulitic, lithophysae, and micropoikilitic textures. No glass is preserved and the sills and cryptodomes are now completely crystalline. Primary devitrification structures crystallized early above the glass transition temperature and before the formarion of perlite or quench fractures. Crystallization of residual glass proceeded by diagenetic alteration, volcanic-associated hydrothermal alteration, and ore-associated hydrothermal alteration. Massive sulfide ore is enclosed in a discordant alteration envelope that extends at least 60 m above ore and is known to a depth of 150 m in the footwall. In general, there is a progression from central quartz-sericite : pyrite zones, giving way to zones of chlorite n anhydrite n gypsum, chlorite-sericite-quartz, and finally chloritesericite distal to ore. The outermost alteration grades into !ithofacies that have altered to various background assemblages of feldspar, hematite, sericite, chlorite, quartz, epidote, and carbonate. Compared to least altered lithofacies, the quartz-sericite : pyrite zones are enriched in SiOn., K20, Fe.O3, S, and Ba, and depleted in N%O, CaO, MgO, and Sr. Elevated concentrations of MgO, Fe203. and S, coupled with low Na.O, CaO, and K.O values, characterize chlorirized zones. The pattern of low N%O, CaO, and MgO values and high K.O values passing outward through the chlorite-sericite n quartz zones is more erratic. The Ishikawa alteration index (AI) value and the chlorite-carbonate-pyrite index (CCPI) value become progressively larger passing from background-altered facies (AI <60; CCPI = 20-55) into the footwall and hanging-wall alteration zones (AI = 89-97; CCPI = 41-99). At AI values greater than 90, Cu, Bi, and Mo are elevated. The S/N%O ratio increases toward ore, whereas Sr/Ba ratios are lowest at AI values above 90. The geochemical halo patterns reflect the composite response of lithofacies to mineralogical and textural chan.es accompanying diagenetic alteration, volcanic-associated hydrothermal alteration, and ore-associated hydromermal alteration. Most alteration zones around ore display two or more overprinting alteration mineral assemblages. Within single alteration zones, the distribution of alteration minerals and assemblages varies spatially in response to contrasring lithofacies character. Formerly glassy coherent and volcaniclastic facies display mineralogical and textural changes that reflect the influence of fracture (perlitic, quench) and/or matrix permeability and porosity on fluid migration. Crystalline domains of volcanic facies generally preserve their original texture, although recrystallization has partly destroyed primary devitrification microstructures. Alteration minerals are localized along cooling joints, hydraulic fractures, and veins or form mottled, patchy, or wispy domains. In contrast, siltstone that separates intrusions and forms the matrix in peperite is relatively unaltered or was baked during magma-sediment mixing. In zones of intense quartz-sericite n pyrite, chlorite n anhydrite : gypsum, and sericite alteration, high water/rock ratios, an extended history of fluid flow, and elevated temperatures promoted large-scale mobilization of elements in all facies. Relics of the early alteration mineral assemblages are locally preserved in these zones and imply a progression from initial volcanic-determined

Paragenese a faible variance dans les metapelites de la serie de Filali (Rif interne marocain); description, interpretation et consequence geodynamique

Abstract The metamorphic series of Filali and the Beni Bousera massif represent the most metamorphic unit in the inner part of the Moroccan Rif. The Filali series is composed of micaschists, gneisses, migmatites and granulites wrapped around the ultramafic body of the Beni Bousera. The foliation is broadly coherent all over the massif although differences in lineation and fold axis have been found between the granulites and the micaschists-gneisses. Metamorphic grade increases continuously towards the ultramafic body; however, seven metamorphic zones which account for this increase can be defined: - chlorite zone: muscovite and chlorite underline the S1 schistosity refolded by S2. Small garnet is wrapped by or cut across S2; - chloritoid zone: chloritoid displays the same textures as chlorite and muscovite, underlining S1 and S2 schistosity. Chlorite reacts out to biotite; - staurolite+andalusite zone: staurolite, andalusite, cordierite and garnet have pre-, syn- and post- kinematic features with respect to S2. Garnet reacts out to muscovite, biotite and plagioclase; - staurolite+andalusite+kyanite zone: same textural relationships as in the previous zone. Kyanite is an additional phase, often displaying epitaxial textures with staurolite and andalusite; - andalusite+kyanite+sillimanite zone: fibrolitic sillimanite generally associated with biotite crystallizes from staurolite. However, sillimanite is also directly formed from andalusite and kyanite. As in the previous zone garnet reacts out to biotite, muscovite and plagioclase; - sillimanite+k-feldspar zone: muscovite reacts out to k-feldspar. Modes of biotite and sillimanite increase; - migmatites zone are composed by biotite, sillimanite, garnet, cordierite quartz and feldspars. Garnets do not display the reaction textures seen in the previous zones; - granulites are composed of a primary paragenesis with kyanite+k-feldspar+garnet+ or -biotite+plagioclase+rutile, partially obliterated by a secondary paragenesis with cordierite, sillimanite and spinel.

The Palma Volcano-Sedimentary Supersuite, Precambrian Sul-Riograndense Shield, Brazil

Komatiites are generally found in Archean greenstone belts, but have been reported in some Mesozoic volcano-sedimentary sequences. The Palma Volcano-Sedimentary Supersuite (PVSS) is a Neoproterozoic sequence composed of meta-ultramafic volcanic rocks, mafic-to-felsic metavolcanic rocks, and metasedimentary rocks ranging from quartzites to chlorite-mica-quartz schists, marbles, and marls. Original PVSS structures were overprinted by two penetrative deformational surfaces (So//Sn and Sn+1); associated metamorphic assemblages are assigned to the upper greenschist facies. Basalts predominate over andesites and rhyodacites, and their major, minor, trace, and rare-earth elements display compatible geochemical trends characteristic of calc-alkaline magmas. Meta-ultramafic rocks are variable-thickness flows in the metasedimentary sequence. Metamorphism caused the development of a metasomatic tremolite zone around the serpentinite bodies, as well as chlorite and talc schist metasomatic zones. Evaluation of the geochemical mobility of the elements showed that the ratios between major and minor elements have not changed in the serpentinites. It was then possible to investigate the lithogeochemical nature of the serpentinites by studying the minor, trace, and rare-earth elements. The lithogeochemical investigation and the structures of the serpentinite bodies enable them to be regarded as komatiitic flows extruded under shallow water. These komatiite flows and calc-alkaline basalt to rhyodacite flows associated with shallow-water sediments suggest that this volcano-sedimentary sequence was developed in a Neoproterozoic arc-related tectonic setting.

Metamorphic discontinuities in orogenic belts: example of the garnet-biotite-albite zone in the Otago Schist, New Zealand

A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (“grossalspite” with typical rim composition almandine41, spessartine25, grossular33, pyrope1) and biotite in 37 new samples, more than doubling the previously known number. A new garnet–biotite–albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet–biotite–albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.

Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand

Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (KD≈2.5) and into chlorite relative to phengite (KD≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl3Si3O10(OH)2, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe3+/Fe2+, O/OH and H2O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO2, SiO2 and H2O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO2, CaAl2O4, SiO2 and H2O, may prove useful in the evaluation of other low‐grade assemblages.

Shuffled‐cards structure and different P/T conditions in the Sanbagawa metamorphic belt, Sakuma‐Tenryu area, central Japan

AbstractThe Sanbagawa metamorphic terrain of the study area is divided into two units, the Shirakura and Sejiri units. The metamorphic thermal structure is interpreted on the basis of the degree of graphitization (GD) of carbonaceous material in pelitic schists. The areal variations of the metamorphic grade are presented by the distribution of GD calculated using the Lc and d002 of carbonaceous material. As a result, the two units are classified into four metamorphic zones, respectively: A1, A2, B1 and B2 for the Shirakura Unit; and I1, I2, II1 and II2 for the Sejiri Unit. The metamorphic grades of A1, A2, I1 and I2 are included in the chlorite zone, and that of B1, B2, II1 and II2 in the garnet zone of the Sanbagawa metamorphism. The degree of graphitization at the boundary between A2 and B1 zones is the same as that between I2 and II1 zones. Detailed study on the variation of GD suggests that the present‐day structure of the study area is best interpreted as a model of shuffled‐cards structure. An estimated minimum thickness of a stack of continuous cards is about 25 m. The compositions of garnet in pelitic schists and of amphibole in basic schists are different from those in the identical metamorphic range of the Shirakura and Sejiri units. It is suggested that rocks of the Shirakura Unit were metamorphosed under higher P/T conditions than those of the Sejiri Unit.

Geology and fluid chemistry of the Fushime geothermal field, Kyushu, Japan

The Fushime geothermal field is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>350°C), and a high salinity production fluid. Geological analysis shows that the main reservoir in this field occurs in a fractured zone developed around a dacite intrusion located in the center of the field. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW–SE trending andesite dike swarm which was encountered in some wells. Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium–magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K-feldspar–quartz, sericite–quartz, and albite–chlorite zones. Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this field ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal fluid originated from sea-water and is diluted by ground water during its ascent. Some fluids produced from geothermal wells show low pH (about 4). It is thought that sulfide mineral (PbS, ZnS) precipitation during production produces this acidic fluid.

Relationship between fluid flow and faulting in the Alpine realm (Austria, Germany, Italy)

Abstract Five carbonate cement generations in faults and associated fractures, which precipitated during two major cementation phases, were studied in Permian to Tertiary sedimentary rocks in the Drau Range, in the Southern Alps and in different nappes of the Northern Calcareous Alps (NCA). The first major cementation phase is represented by clear saddle dolomite (CSD) that can be further subdivided by cathodoluminescence (CL) into five CL-zones. A non-carbonate zone, consisting of chlinochlorite (chlorite zone), is intercalated between CL-zones II and III. CL-zones I–IV occur in rocks of Permian to Early Jurassic age, while CL-zone V is also present in Upper Jurassic strata. The second major cementation phase, characterized by another saddle dolomite and three generations of blocky calcite, is of Miocene age. Dedolomitized CSD, which occurs in a synsedimentary normal fault of Early Jurassic age contains a zone of pyrite. The trace element composition of this pyrite zone and of an Upper Hettangian Fe–Mn crust (Marmorea crust) were compared with the trace element composition of the chlorite zone. Similar trace element patterns within all three non-carbonate zones suggest the same fluid source for these zones during Upper Hettangian time. This fluid could be of hydrothermal origin, as indicated by the geochemical characteristics of the Fe–Mn crust. Hydrothermal activities started during the beginning of rifting in the Alpine realm in the Late Triassic/Early Jurassic. Fluid inclusion data reveal variable homogenization temperatures for the five CL-zones in the clear saddle dolomite. Formation temperatures between 146 and 201°C were calculated, considering the overburden pressure during the formation of the CL-zones. Calculated equilibrium oxygen isotopic fluid compositions indicate δ 18 O values between 4.0 and 9.0‰ SMOW for the fluid from which the different saddle dolomite zones precipitated. The origin of the fluid can be related to brines present in the hinterland (Vindelician high, Bohemian massif). The stratigraphic occurrence of CSD and its paragenetic relationship to the non-carbonate zones enables a more exact dating of the CL-zones. CL-zones I and II precipitated between Norian and Late Hettangian time, and CL-zones III and IV are of Early Sinemurian age. The formation period of CL-zone V is Late Jurassic to Early Cretaceous. The distribution of the carbonate cements was investigated within different fault sets and in associated subparallel fractures in the NCA, i.e. the Allgau-, Lechtal- and Inntal-nappe, which show a polyphase deformation history. CSD occurs in normal faults, N–S trending high angular faults and WNW-trending strike-slip faults, indicating minimum an Early Jurassic age for these faults. The only carbonate cements that are present within NE-trending sinistral strike-slip faults are of the second major cementation phase, implicating a Miocene age for these faults. Fibrous shaped crystals of the first calcite cement of the second major cementation phase preferentially occur within WNW-trending strike-slip faults. Reactivation of these faults as conjugated shear pairs in connection with the formation of NE-trending strike-slip faults, is assumed.

An oxygen and carbon isotopic study of multiple episodes of fluid flow in the Dalradian and Highland Border Complex, Stonehaven, Scotland

The carbon and oxygen systematics of rocks north of Stonehaven, Scotland were studied to provide new constraints on the nature and timing of metamorphic and post‐metamorphic fluid infiltration. Carbon and oxygen isotopic data were collected from carbonated spilites and carbonate‐bearing veins in the Highland Boundary Fault, from Dalradian metacarbonate layers and pervasively carbonated schists, and from three generations of carbonate‐bearing veins, and a quartz porphyry dyke within the Dalradian. Isotopic evidence for syn‐metamorphic fluid infiltration in the Dalradian is preserved in quartz–calcite veins in the upper chlorite zone and in staurolite zone metacarbonates. δ 18 O values for coexisting quartz (δ 18 O quartz = 13.6‰) and calcite (δ 18 O calcite = 11.6‰) in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c. 375°C. Oxygen isotopic data from staurolite zone metacarbonate layers (δ 18 O calcite = 10.4 to 12.9‰) suggest major isotopic exchange with an external fluid equilibrated with a quartzo‐feldspathic reservoir, such as the Dalradian metasediments, at metamorphic temperatures. Low, negative carbon isotope data from these metacarbonate layers (δ 13 C calcite = −12.4 to −17.7‰) may indicate a component of oxidized organic material in the infiltrating fluid. Post‐metamorphic fluids derived from or equilibrated with a high‐temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post‐metamorphic vein carbonate, carbonation of pelitic schists, and alteration of metamorphic feldspar porphyroblasts from Barrow’s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes. Some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow. Quartz from this generation of veins preserves a metamorphic isotopic signature (δ 18 O quartz = 11.2 to 13.2‰) whereas dolomite and calcite are considered to be post‐metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals (δ 18 O dolomite = 23.7 to 24.2‰, δ 18 O calcite = 25.3‰, δ 18 O quartz = 11.2 to 13.2‰). Variably depleted carbon isotopes from carbonate minerals in these veins (δ 13 C dolomite = −8.1 to −13.4‰, δ 13 C calcite = −12.2‰) may reflect input of oxidized organic material in the mineralizing fluid. Isotopic data from carbonate veins and variably carbonated spilites from the Highland Boundary Fault are consistent with hydrothermal alteration of basalts by seawater, which had its carbon isotope systematics modified by addition of magmatic CO 2 and/or oxidation of organic carbon. Such alteration may have occurred on the seafloor or during emplacement in the Highland Boundary Fault.

Geology and geochemistry of paleosols developed on the Hekpoort Basalt, Pretoria Group, South Africa.

The Hekpoort paleosols comprise a regional paleoweathering horizon developed on 2.224 +/- 0.021 Ga basaltic andesite lavas at the top of the Hekpoort Formation of the Pretoria Group, Transvaal Supergroup, South Africa. In five separate profiles, from outcrops along road cuts near Waterval Onder and the Daspoort Tunnel and in three drill cores from the Bank Break Area (BB3, BB8, and BB14), the top of the paleosol is a sericite-rich zone. The sericite zone grades downward into a chlorite-rich zone. In core BB8 and in the road cut at the Daspoort Tunnel, we sampled the underlying or parent basaltic andesite into which the chlorite zone grades. We did not obtain samples of the parent material at Waterval Onder and in cores BB3 and BB14, but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core BB8 and at the Daspoort Tunnel. The presence of apparent rip-up clasts of the paleosol in the overlying ironstones of the Strubenkop Formation in the cores from Bank Break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids. Desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at Waterval Onder point to a subaerial weathering origin. Very little, if any, Al, Ti, Zr, V, or Cr moved a discernible distance during weathering of any of the five profiles. The vertical distribution of Fe, Mg, Mn, Ni, and Co indicates that these elements were largely removed from the top of the soil during weathering. The overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an Fe2(+) -rich smectite. The loss of Fe from the top of the soil during weathering of the Hekpoort paleosols indicates that atmospheric PO2 was less than 8 x 10(-4) atm about 2.22 Ga. Fe2(+) -rich smectite should only precipitate during soil formation if atmospheric PCO2 is less than or equal to 2 x 10(-2) atm (Rye, Kuo, and Holland, 1995). Ca and Na were largely lost during weathering. Some Na was apparently added to the sericite zone in cores BB3, BB8, and BB14 after weathering. All five profiles are enriched in K and Rb, and most are enriched in Ba. The distribution of these elements indicates that they all were added during post-weathering hydrothermal metasomatism. Rb-Sr analysis of the paleosol at the Daspoort Tunnel indicates that metasomatism last affected that profile 1.925 +/- 0.032 Ga (Macfarlane and Holland, 1991).

An F-rich, Sn-bearing volcanic-intrusive complex in Yanbei, South China

The igneous complex that hosts the Yanbei porphyry tin deposit is situated along the eastern margin of the southeast China Caledonian fold belt. It intrudes a Late Jurassic granitic batholith and was unconformably covered by Late Cretaceous red beds. The complex consists mainly of volcanic (ignimbrites and minor rhyolitic lavas, tuffs, and subvolcanic rhyolite porphyry) and intrusive rocks (topaz-bearing granite porphyry and granite). These rocks are generally peraluminous with F- and VI Al-rich biotite as the main mafic mineral, and F-rich and OH-poor topaz as an accessory mineral in the granite porphyry and granite, and as an essential mineral in the topazite. Topaz in the granites is usually interstitial and crystallized during the late magmatic stage. The volcanic rocks and granites are rich in Si, K, F (SiO 2 > 75 wt %, Na 2 O/K 2 O = 0.03-0.72, F = 0.06-6.48 wt %) and incompatible trace elements (Rb, Zr, Nb, W, Sn ) and poor in Mg, Ti, Ca, P and compatible trace elements (Sr, Eu ). The chondrite-normalized rare earth element (REE) patterns for the granites generally have a distinct V shape, with strong Eu depletion (Eu/Eu (super [whitesunwithrays]) = 0.04-0.20). The main rocks of the complex have low epsilon Nd (T) values (-9.3 to -8.9 for volcanic rocks, -4.0 to -4.8 for granites), and high delta 18 O values of quartz from dacite (9.5ppm) and granites (9.6-10.1ppm). The difference in epsilon Nd (T) between the volcanic rocks and granites indicates that they are not comagmatic. The source of the volcanic rocks probably contains a greater metasedimentary component than that of the granites.The Yanbei porphyry tin deposit is located along the contact zone between topaz-bearing granite porphyry and rhyolite porphyry, and consists principally of one orebody which makes up 99 percent of the total reserves. Of the metallic minerals (5% by volume), cassiterite, pyrite, and chalcopyrite are dominant, sphalerite, wolframite, and galena are less important, and siderite, molybdenite, and stannite are minor. Cassiterite accounts for 96 percent of the total Sn reserves and stannite is the subordinate Sn-bearing mineral that occurs as rims around chalcopyrite. Cassiterite occurs mostly in disseminated form and as veinlets. The wall-rock alteration is intense and can be roughly grouped into three zones spatially arranged around the granite porphyry, namely: topaz-quartz, chloritic, and sericitic zones. Most of the orebody occurs in the topaz-quartz and chloritic zones. Hydrothermal alteration caused a decrease in K and Na and an increase in Al, Fe, Ca, Sn, W, and Cu. The delta 18 O values of quartz and cassiterite from the mineralized zones vary from 6.0 to 11 per mil, and the calculated delta 18 O (sub H 2 O) values of the ore-forming fluid range from 4.2 to 7.8 per mil. The altered rocks have elevated delta 18 O (sub H 2 O) values (14.5-9.7ppm) relative to fresh rocks. The pyrite from the ore has a limited range of delta 34 S values (-0.3-+1.5ppm). These data suggest a magmatic source for the ore-forming hydrothermal fluid.

Low‐pressure metamorphism of Palaeozoic pelites in the Aspromonte, southern Calabria: constraints for the thermal evolution in the Calabrian crustal cross‐section during the Hercynian orogeny

During Hercynian low‐pressure/high‐temperature metamorphism of Palaeozoic metasediments of the southern Aspromonte (Calabria), a sequence of metamorphic zones at chlorite, biotite, garnet, staurolite–andalusite and sillimanite–muscovite grade was developed. These metasediments represent the upper part of an exposed tilted cross‐section through the Hercynian continental crust. P–T information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the Calabrian crust during the late Hercynian orogenic event.Three foliations formed during deformation of the metasediments. The peak metamorphic assemblages grew mainly syntectonically (S2) during regional metamorphism, but mineral growth outlasted the deformation. This is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern Serre. Pressure conditions recorded for the base of the upper crustal metasediments are c. 2.5 kbar and estimated temperatures range from &lt;350 °C in the chlorite zone, increasing to 500 °C in the lower garnet zone, and reaching 620 °C in the sillimanite–muscovite zone. Geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust (c. 60 °C km−1) than for the granulite facies lower crust (30–35 °C km−1). The small temperature difference between the base of the upper crust (620 °C at c. 2.5 kbar) and the top of the lower crust (690 °C at 5.5 kbar) can be explained by intrusions of granitoids into the middle crust, which, in this crustal section, took place synchronously with the regional metamorphism at c. 310– 295 Ma.It is concluded that the thermal structure of the Calabrian crust during the Hercynian orogeny – as it is reflected by peak metamorphic assemblages – was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust.

Boiling and fluid mixing in the chlorite zone of the Larderello geothermal system

The geochemical features of the geothermal fluids produced within the boiling zone in the relatively shallow parts of the Larderello geothermal system (Italy) have been documented as a result of deep drilling which provided samples from 1480 to 2500 m depth. Four wells (Monteverdi 1, Monteverdi 2A, Sasso 22 and Capannoli 2B) have been sampled in the intermediate parts of the Larderello aquifer located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the shallow aquifer at Larderello. Fluid inclusions from recrystallized quartz lenses and quartz veins in samples displaying a predominant quartz–chlorite–(epidote–adularia) paragenesis have been studied by microthermometry and Raman spectroscopy. The inclusions are primary and pseudo-secondary in origin when formed in authigenic quartz, or of secondary origin, when located in fluid inclusion planes related to microfracturing of metamorphic quartz lenses. Several generations of fluids are present and include: H 2O–(CO 2)-dominated vapours and liquids, and a series of aqueous liquids, for the most part of relatively low salinity. The T m-ice of both early and late inclusions are mostly between 0.0 and −4.5°C, indicating that the salinities of the hydrothermal fluid were very low to moderate. However, rare fluid inclusions with lower T m-ice (from −4.9 to −25.0°C) were also observed. These inclusions may record the occasional input of saline fluids, which may be derived from the interaction of the hydrothermal waters with the evaporites present in the shallow part of the Larderello field. At Capannoli 2B, earliest H 2O–(CO 2) liquids were trapped under minimal pressures of 610–645 bars, which bracket the estimated present-day lithostatic pressure (640 bars). In all other samples, the main stage of quartz–chlorite crystallization occurs under boiling conditions attested by the presence of liquid and vapour-rich inclusions, that, in some instances, can be texturally interpreted to be coeval. Their trapping conditions (350–375°C, 160–215 bars) are higher than the present day temperatures at the same depth. Later fluid inclusions attest to a significant cooling of the fluids down to temperatures similar to the present-day temperatures. During this time, pressures were close to hydrostatic conditions. Most fluid inclusions were trapped within the liquid field, this indicating that a significant pressure drop has since affected the main aquifers or fractured zones which are, at present, under vaporstatic conditions.

Geology, structure and metamorphism of the Lesser Himalayan metasedimentary sequence in Pokhara region, western Nepal

The Lesser Himalaya in the Pokhara region, western Nepal, comprises low grade metamorphic rocks of the Nawakot Complex in the south and mylonitic rocks of the Main Central Thrust Zone in the north. It is delimited in the north by the Main Central Thrust (MCT), which brings the kyanite gneisses of the Higher Himalayan Crystallines over the Lesser Himalayan metasediments. Cross-cutting pegmatite veins injected prior and posterior to the main deformation event are reported for the first time from the MCT Zone. The Phalebas Thrust, Lumle Thrust and the MCT are the major structural discordances, which divide the area into various tectonic zones with different style and intensity of deformation. At least three deformation events are recognised in each tectonic zone. Sense of shearing is always top to the south in the MCT Zone.&#x0D; The Higher Himalayan Crystallines and the Lesser Himalayan metasediments show marked differences both in history and grade of metamorphism. The gneisses of the Higher Himalayan Crystallines exhibit polyphase metamorphism. The older high grade (kyanite-grade) metamorphism formed pyrope-rich cores in garnets and plagioclase with An&gt;20%. The younger retrograde metamorphism caused to recrystallise phengitic muscovite and to form retrograde chemical zoning in garnets. The Lesser Himalaya shows syn- to post-tectonic, garnet-grade, inverted prograde metarnrophism. The grade of metamorphism increases from chlorite zone in the south to the biotite zone below the Lumle Thrust and the garnet zone in the MCT Zone. The garnets show prograde zoning pattern, muscovite gradually becomes poor in celadonite component, biotite becomes rich in Ti-content, and Ca-amphiboles become rich in Na and K from south towards the MCT. Both the Lesser Himalayan metasediments and the Higher Himalayan Crystallines have experienced the late-stage weak regression due to uplift and erosion.

Geology and genesis of the Aznalcóllar massive sulphide deposits, Iberian Pyrite Belt, Spain

The Aznalcollar mining district is located on the eastern edge of the Iberian Pyrite Belt (IPB) containing complex geologic features that may help to understand the geology and metallogeny of the whole IPB. The district includes several ore deposits with total reserves of up to 130 Mt of massive sulphides. Average grades are approximately 3.6% Zn, 2% Pb, 0.4% Cu and 65 ppm Ag. Mined Cu-rich stockwork mineralizations consist of 30 Mt with an average grade of 0.6% Cu. Outcropping lithologies in the Aznalcollar district include detrital and volcanic rocks of the three main stratigraphic units identified in the IPB: Phyllite-Quartzite Group (PQ), Volcano-Sedimentary Complex (VSC) and Culm Group. Two sequences can be distinguished within the VSC. The Southern sequence (SS) is mainly detritic and includes unusual features, such as basaltic pillow-lavas and shallow-water limestone levels, the latter located in its uppermost part. In contrast, the Aznalcollar-Los Frailes sequence (AFS) contains abundant volcanics, related to the two main felsic volcanic episodies in the IPB. These distinct stratigraphic features each show a different palaegeographic evolution during Upper Devonian and Lower Carboniferous. Massive sulphides occur in association with black shales overlying the first felsic volcanic package (VA1) Palynomorph data obtained from this black shale horizon indicate a Strunian age for massive sulphides, and consequently an Upper Devonian age for the VA1 cycle. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment (VA1) to hydroclastic subvolcanic conditions for the VA2. This evolution can be related to compartmentalizing and increasing depth of the sedimentary basin, which may also be inferred from changes in the associated sediments, including black shales and massive sulphides. Despite changes in the character of volcanism, the same dacitic to rhyolitic composition is found in both pyroclastic and subvolcanic igneous series. The main igneous process controlling chemical variation of volcanics is fractional crystallization of plagioclase (+accessories). This process took place in shallow, sub-surface reservoirs giving rise to a compositional range of rocks that covers the total variation range of felsic rocks in the IPB. The Hercynian orogeny produced a complex structural evolution with a major, ductile deformation phase (F1), and development of folds that evolved to thrusts by short flank lamination. These thrusts caused tectonic repetition of massive and stockwork orebodies. In Aznalcollar, some of the stockwork mineralization overthrusts massive sulphides. These structures are cut by large brittle overthrusts and by late wrench faults. The original geometric features of massive sulphide deposits correspond to large blankets with very variable thicknesses (10 to 100 m), systematically associated with stockworks. Footwall rock alteration exhibits a zonation, with an inner chloritic zone and a peripheral sericitic zone. Silicification, sulphidization and carbonatization processes also occur. Hydrothermal alteration is considered a multi-stage process, geochemically characterized by Fe, Mg and Co enrichment and intense leaching of alkalies and Ca. REE, Zr, Y and Hf are also mobilized in the inner chloritic zones. Three ore types occur, both in stockworks and massive sulphides, named pyritic, polymetallic and Cu-pyritic. Of these, Cu-pyritic is more common in stockworks, whereas polymetallic is prevalent in massive sulphides. Zoning of sulphide masses roughly sketches a typical VHMS pattern, but many alternating polymetallic and barren pyritic zones are probably related to tectonics. Although the paragenesis is complex, several successive mineral associations can be distinguished, namely: framboidal pyritic, high-temperature pyritic (300 °C), colloform pyritic, polymetallic and a late, Cu-rich high-temperature association (350 °C). Fluid inclusion data suggest that hydrothermal fluids changed continuously in temperature and salinity, both in time and space. Highest Th and salinities correspond to inner stockworks zones and later fluids. Statistic population analysis of fluid inclusion data points to three stages of hydrothermal activity, at low (<200 °C), intermediate (200–300 °C) and high temperatures (300–400 °C). 34S values in massive sulphides are lower than in stockwork mineralization suggesting a moderate bacterial activity, favoured by the euxinoid environment prevailing during black shale deposition. The intimate relation between massive sulphides and black shales points to an origin of massive sulphides by precipitation and replacement within black shale sediments. These would have acted both as physical and chemical barriers during sulphide deposition. Hydrothermal activity started during black shale deposition, triggered by a rise in thermal gradient due to the ascent of basic magmas. We suggest a three-stage genetic model: (1) low temperature, diffuse fluid flow, producing pyrite-bearing lenses and disseminations interbedded with black shales; locally, channelized high-T fluid flow occurs; (2) hydrothermal cyclic activity at a low to intermediate temperature, producing most of the pyritic and polymetallic ores, and (3) a late high-temperature phase, yielding Cu-rich and Bi-bearing mineralization, mainly in the stockwork zone.

Metamorposed Archean epithermal Au-As-Sb-Zn-(Hg) vein mineralization at the Campbell Mine, northeastern Ontario

The Campbell mine and the adjacent Red Lake mine are developed in an Archean gold deposit hosted within volcanic rocks of the Red Lake greenstone belt, situated in the western part of the Uchi subprovince of the Superior province. Geochronological evidence suggests that the Campbell-Red Lake deposit formed after volcanism but prior to late felsic plutonism and regional metamorphism, between 2722 and 2710 Ma. The orebodies at Campbell mine are largely hosted in epithermal-style veins and vein stockworks associated with strike-slip faults.Complex, superimposed phases of wall-rock alteration occurred prior to the emplacement of main stage veins and were controlled by both primary and secondary (contact, fault, and fracture induced) permeability in mafic and ultramafic rocks as well as secondary permeability in rhyolite and diorite. Widespread pervasive carbonatization and potassic (biotite) alteration with distal chloritic alteration is well developed in basalt and ultramafic rocks. In basalt, this alteration was in turn partly overprinted by silicification and aluminous alteration consisting of zoned mineral assemblages of quartz-andalusite-sericite-chloritoid-cordierite-margarite occurring within bleached zones, and quartz-chlorite-chloritoid-garnet-cummingtonite assemblages occurring in flanking chloritic zones. Similarly, zoned aluminous mineral assemblages also occur in diorite and rhyolite. In ultramafic rocks, the aluminous alteration is represented by sericite-fuchsite-cordierite assemblages, which grade out toward chlorite-anthophyllite-cordierite assemblages. This zoning appears to reflect the bulk composition of zoned premetamorphic aluminous (argillic) alteration.Main-stage carbonate (dolomite to ankerite)-quartz veins exhibit well-developed open-space-filling textures characterized by colloform and crustiform banding and cockade breccia infill textures typical of veins which formed in a near-surface environment. Fissure veins occur within fault zones themselves, whereas large snowbank (banded carbonate-quartz) veins formed in dilatant sites associated with a releasing bend in the faults. Fissure veins and snowbank veins are best developed in basalt, whereas veinlet stockwork and sheeted veinlet zones occur in ultramafic rocks adjacent to main-stage vein sites in basalt. The main-stage veins partially sealed remaining permeable zones, and proximal early alteration assemblages were overprinted by distal chloritic alteration as the hydrothermal system collapsed inward. The main-stage veins were then overprinted by silicification + or - tourmaline within breccias and veinlet stockworks.Mineralization is associated with microstockwork sulfide veinlets, strongly disseminated sulfide, and sulfide cemented breccias which cut silicified zones in veins and wall rocks. Native gold occurs on its own or with sulfides in fractures cutting silicification. Arsenopyrite is commonly associated with gold and occurs in ore zones with pyrite, pyrrhotite, and magnetite. Stibnite and sphalerite commonly occur in high-grade zones. Mineralized zones hosted in fissure veins and wall-rock replacement zones tend to be narrow (<1 m wide) and in narrow fault zones. Mineralization associated with brecciated snowbank veins are typified by multiple brecciation and fracturing events and are formed in dilatant sites at a high angle to fault zones. Mineralized zones are cut by late hydrothermal breccia dike and pipelike bodies as well as late quartz-carbonate veins and faults.The zoned aluminous alteration, open-space-filling textures of the main-stage veins, sheeted veinlet zones and stockwork fracturing, multiple phases of hydrothermal breccias, and anomalous Au-Ag-As-Sb-Hg-Zn-K at the Campbell mine are characteristics similar to those of Phanerozoic low sulfidation epithermal deposits. The mineralized environment has been deformed (flattened and stretched) and metamorphosed to middle to upper greenschist facies during the Kenoran orogeny (ca. 2650 Ma).

Mount Isa lead-zinc orebodies: Replacement lodes in a zoned syndeformational copper-lead-zinc system?

Examination of the microstructural and textural development of the lead-zinc mineralisation at Mount Isa reveals that it formed cogenetically with the large-scale Cu orebodies by late syndeformational replacement. The constraints on mineralisation provided by the sedimentological framework (Neudert, 1984) and timing criteria place the lead-zinc and its associated pyrrhotite and layer-parallel pyrite as forming late in the last major phase of deformation affecting the mine area. This is the same event as that which formed the copper ore (Perkins, 1984; Swager, 1985a) and indicates copper-lead-zinc to be a zoned late-stage epigenetic system. A succession of stratigraphically-constrained samples traced into the orebody reveals the progressive development of an alteration system that formed from well-laminated carbonaceous siltstones and mudstones. At the periphery, alteration consists of bleaching by destruction of carbonaceous seams and enhanced growth of dolomite. Progressively further inwards, it changes to mica and albite-bearing ‘buff alteration’, commonly into siderite, magnetite and stilpnomelane, through a chloritic zone, to ultimately massive ‘silica-dolomite’ alteration. Paralleling this alteration was the formation of abrupt pyrite-pyrrhotite transitions, and a zonation from sphalerite to galena to chalcopyrite. The characteristic folded and breccia ores were not the result of ductile or fluid-assisted deformation of preexisting sulphide-siltstone interlayers, but instead formed by shear-controlled carbonate alteration that was ultimately replaced by sulphides. Layer-parallel mineralisation commonly terminates abruptly on or near extensional dolomite veins that formed progressively through the last major deformation event in the mine. Layer-parallel mineralisation formed at the same time as one in folded and brecciated zones. Evidence cited in previous studies for large-scale remobilisation and recrystallisation during deformation, which is a logical consequence if mineralisation had been an integral part of the sedimentary succession, can be more consistently interpreted as resulting from a single episode of sulphide deposition. Both bedding-parallel and cross-cutting structures that control the localisation of mineralisation existed prior to the precipitation of sulphides. Dissolution of sulphides only occurs where there is a paragenetic sequence of sulphides, with fine-grained pyrite locally overprinted by all other economic sulphides (Grondijis and Schouten, 1937). Sphalerite is overprinted by galena, and both are overprinted by chalcopyrite, which everywhere is paragenetically the youngest sulphide. Copper-lead-zinc concentrations formed from hydrothermal solutions entering the region of the ramp of the Paroo Fault and then migrating outwards to form an evolving alteration system during the formation of both the layer-parallel and cross-cutting steep cleavages. This was overprinted by a relatively short-lived mineralisation event to form the zoned copper-lead-zinc system. Implications of this interpretation for other similar deposits around the world are profound. It indicates a necessity to reexamine many other deposits using techniques that relate ore textures to associated structures at all scales. A completely different ore genesis paradigm may result and exploration concepts may need to be substantially modified.

Asymmetric X‐ray diffraction peak of metamorphosed carbonaceous material

Abstract It was revealed that the asymmetric X‐ray diffraction peak of metamorphosed carbonaceous material was a combination of two types of symmetric peaks from the coaly material and graphite crystallite. Numerical synthesis using the two symmetric peaks shows the graphitization process observed in a metamorphic sequence is accompanied by a continuous change of the peak shape and apparent interplanar spacing. Based on the analytical results and geological observations, it is suggested that graphitization of carbonaceous material consists of two processes: (i) the formation of graphite crystallites at the expense of the coaly material through the formation of transitional material and the increasing abundance of crystallites which is observed in the chlorite zone; and (ii) growth of crystallites forming well‐ordered graphite, which occurs in the higher‐grade zone.

Geochemistry of hydrothermal alteration at the Deri massive sulphide deposit, Sirohi district, Rajasthan, NW India

The 1.1 Ga volcanogenic massive sulphide deposit at Deri in the Sirohi district of south Rajasthan occurs within a bimodal volcanic suite of tholeiites and rhyolites, with minor amounts of andesites and tourmaline-bearing chert, interlayered with arkosic sediments. The ores and the enclosing rocks have undergone superposed deformation and polymetamorphism initially under amphibolite facies conditions and later under hornblende hornfels facies conditions. Metamorphism, however, has not affected the bulk composition of the rocks to any significant degree. Three distinct semiconformable alteration facies, characterized by their conspicuous magnesian mineralogy, are recognized in the host rocks: (1) hornblende-biotite-plagioclase-quartz schist (AMV); (2) cordierite-anthophyllite-chlorite hornfels (AFV); and (3) biotite-chlorite(-sericite) schist/hornfels (BCS). The first is derived from the mafic volcanics, whereas the other two represent progressive alteration of felsic volcanic protoliths. Fe, Mg and water were added and Na was removed from all the alteration facies in varying amounts. The maximum enrichment is noted in BCS for Mg and Fe, whereas the maximum depletion is seen in this facies for Si, an element which is also depleted significantly in AFV. AMV on the other hand, shows enrichment of Si, Ca and to some extent, in Al. Alumina is also enriched considerably in BCS, probably due to clayey alteration and extreme leaching of silica. Amongst the trace elements, Rb, Ba, Nb and Y are gained in most of the facies, except in BCS, where Ba and Y show distinct depletion. The LREE, from La to Sm, were enriched about 1.5- to 3.0-fold in all the facies with a maximum in AFV where the flux took place at constant inter-REE proportions: 1.0 La, 0.79 Ce, 0.48 Nd and 0.35 Sm. Eu was depleted from both felsic facies, 7-fold in BCS to 4-fold in AFV, during alteration. The HREE (Er to Lu) remained immobile in all the altered facies. The chemical and mineralogical zonation in the alteration facies are interpreted to be due to the progressive reaction of an evolving sea-water hydrothermal fluid with the bimodal volcanic protoliths during convective circulation. Fluid-rock interaction, guided by vertical and lateral thermal gradients, produced a sericite-quartz assemblage in the felsic volcanics at the expense of feldspar during the initial stages (175 °C) which formed a sericite-chlorite zone upon rising temperature (200–250 °C) by base-fixing reactions. A further temperature increase (to ~ 300 °C) and deeper circulation in the mafic pile introduced more Fe and Mg, thereby transforming the previously formed assemblage to a nearly pure chloritic zone and the most intensely altered biotite-chlorite(-sericite) facies.

Petrochemical study of regional/contact metamorphism in metaclastic strata of the central White-Inyo Range, eastern California

Sedimentary rocks of the White-Inyo Range have been metamorphosed by episodically emplaced Jurassic–Cretaceous calc-alkaline arc plutons, reflecting stages in the thermotectonic history of this evolving continental margin. A north-northwest–trending anticlinorium is manifested in strata ranging in age from latest Precambrian to middle Paleozoic. Most middle and late Mesozoic (80–180 Ma) intrusive igneous bodies crosscut or deflect the regional fold pattern and incipient axial-plane cleavage in the country rocks—hence granitoid emplacement was largely postkinematic and, accordingly, isogradic surfaces transect structural levels. Retrograde andalusite ± cordierite-bearing metapelitic assemblages and diopside + grossular-bearing calc-silicate wall-rock skarns at plutonic contacts reflect high-temperature thermal maxima in the inner aureoles. Bulk-rock chemistries, mineral parageneses, and compositions of coexisting phases for several noncalcareous, weakly foliated sequences, the uppermost Proterozoic Wyman meta-argillites, the Lower Cambrian Deep Spring and Andrews Mountain metaquartzites, and the Montenegro phyllites, demonstrate that the overall metamorphic zonation is due to advective heat transport by the postkinematic granitoids. The higher grade parageneses do not represent a synchronous thermal event, but instead constitute a composite of local recrystallization episodes attending calc-alkaline pluton emplacement over an ≈100 m.y. interval, overprinting a pervasive, weak, chloritic metamorphism. The latter may have been produced during late Paleozoic and/or early Mesozoic folding. Bulk-rock X-ray fluorescence analyses demonstrate that Wyman meta-argillites are rich in CaO, Al 2 O 3 , Cr, Th, and Sr, and low in SiO 2 , TiO 2 , P 2 O 5 , Na 2 O, and K 2 O, indicating the former presence of carbonate, abundant clays, and minor detrital feldspars and iron oxides. Deep Spring and Andrews Mountain metaquartzite analyses exhibit high concentrations of titania, iron, silica, alkalies, Zr, Nb, and phosphate, but are poor in Al 2 O 3 , CaO, Cr, Th, and Sr, reflecting original protoliths enriched in alkali feldspars, quartz, and magnetite. Montenegro phyllites have compositions intermediate between the other two investigated metasedimentary units; however, they are relatively low in CaO and Sr, and high in Al 2 O 3 , suggesting noncalcareous precursor sediments containing abundant clay minerals, quartz, and alkali feldspars. Metaquartzites and phyllites are more oxidized than the sparsely carbonaceous meta-argillites. Except for a gradual decrease in volatile contents, the analyzed rocks do not exhibit systematic chemical variation with increasing metamorphic grade. Contrasts in phase compositions and proportions reflect protolith chemistries as well as inferred conditions of recrystallization. The areal disposition of visually estimated neoblastic biotite is mapped as isopleths transecting the three investigated sequences. Titanium contents of biotite and white mica, and An contents of plagioclase increase toward the larger, more mafic Barcroft, Cottonwood- Beer Creek, and Joshua Flat plutons, whereas the cation proportions of Si in white micas decrease. Taking into account the ferric iron contents of white micas, Fe 2+ Mg −1 fractionation becomes less pronounced adjacent to these thermal highs. Metamorphic grade west of the range crest is greenschist facies chlorite zone (≈300 °), but rises abruptly on the extreme north adjacent to the Barcroft granodiorite, and gradually increases eastward toward the Beer Creek–Cottonwood and Joshua Flat plutons. An ill-defined metamorphic culmination is directly north of the Papoose Flat granite. Along contacts with plutons, hornfelsic wall rocks approached temperatures of 500–600 °C. On the basis of rare preservation of cordierite and/or andalusite, phase chemistry, and mineralogic thermobarometry, pressures attending regional and thermal recrystallization were about 3 ± 1 kbar. The activity of H 2 O was moderate (2 ± 1 kbar) during recrystallization of carbonate-poor rocks. Compositions of coexisting phases exhibit systematic, sympathetic variations, compatible with a close approach to local chemical equilibrium.