Greisen Zones Research Articles

Computer-assisted regional-residual gravity anomaly separation with regularized first- and second-order derivatives

The regional-residual separation of gravity anomalies in crustal and mineral exploration was a graphical-based procedure before the advent of fast digital computers and the need for more efficient algorithms to process large data sets. However, since requiring the supervision of an experienced interpreter, the results, once obtained with graphical procedures, are often accepted as second to none in producing anomalies with geologic significance. Numerical methods based on spectral filtering and robust polynomial fitting have worked in many scenarios but seem not fully effective in replicating in algorithms the kind of results once obtained with interpreter-assisted graphical methods. We develop a computer-assisted regional residual separation (CARRS) procedure implemented by a set of short MATLAB scripts; in many aspects, CARRS simulates the operations of former graphical methods but requires few decisions and minor handwork from the interpreter. CARRS applies robust polynomial fitting to points with a low horizontal gradient and a vertical second-order derivative; thus, selecting fitting points as a real interpreter would do with graphical approaches in outlining the regional field. A regularized procedure is used to calculate the stable first- and second-order derivatives. Companion MATLAB codes allow for the replication of our results and further exploration with different threshold levels to identify flat domain regions. CARRS is illustrated with airborne gravity data CPRM-1123, freely available to download. A data window of the residual field is used to analyze the distribution of cassiterite deposits in greisen zones of the Paleoproterozoic Velho Guilherme granites in the Amazon Craton, as their distribution appears in the observed gravity anomaly and its corresponding residual field.

Formation of Topaz-Greisen by a Boiling Fluid: A Case Study from the Sn-W-Li Deposit, Zinnwald/Cínovec

Abstract Zinnwald/Cínovec is one of the largest Li-Sn-W greisen deposits in Europe. Greisens in general are important hosts for many major ore deposits of several high-tech elements—most prominently Sn, W, and Li. However, the mechanisms of greisenization in relationship to ore formation are still not well understood, especially for the important case of fluoride-rich greisen. Here, we investigate the sequence and formation mechanism of the topaz-greisen in the Zinnwald/Cínovec deposit. Based on the mineral abundances, the alteration profile can be divided into four distinct zones: (1) rhyolite, (2) albitite, (3) low-degree greisen, and (4) high-degree greisen. In the greisen zones, almost all the feldspar has been replaced by topaz (up to 12 vol %) + quartz (up to 78 vol %), and the abundance of mica gradually decreases from 11 to 3 vol % with increasing degree of greisenization. Mass balance calculations indicate a net influx of F and silica during greisenization. Our observations are best explained by a two-stage greisenization process involving phase separation by boiling in the pore space of the sample: first an acidic HF-rich phase, likely a vapor, reacted with feldspar to form topaz and quartz in a dissolution-precipitation reaction. This reaction created substantial transient porosity, which was subsequently sealed by the precipitation of quartz and fluorite from a boiling liquid. We interpret the vapor and liquid as emerging from a common supercritical aqueous parental fluid. The characteristic sequence of creation of pore space by a vapor and the subsequent reduction of porosity by precipitation from the associated boiling liquid constrain the conditions and time available for ore formation. This study evaluates the mechanism of topaz greisenization and the controls on porosity evolution, which are crucial to ore element redistribution.

Prognostic Exploration of U-F-Au-Mo-W Younger Granites for Geochemical Pathfinders, Genetic Affiliations, and Tectonic Setting in El-Erediya-El-Missikat Province, Eastern Desert, Egypt

Younger granite bodies form two arches, the western and the eastern (WA, EA), which extend from the south northwards from the Meatique, ophiolitic group-island arc rocks, to the large older granite outcrop to the north. This paper concerns the feasibility of exploration in the El-Erediya-Ria El-Garah-El-Gidami-El-Missikat Y Gr regions. Fieldwork and remote sensing, together with geochemical, petrochemical, and mineralogical studies, are used to show the controlling factors, routes, and the origins of the deposits. Remote sensing is used to delineate the different rock units. Normal and strike–slip NW, NNE faults, veins, fractured ENE shear zones, and alteration zones of magmatic-hydrothermal fluids are discussed. Granites are considered using petrochemical diagrams as resources. These rocks are categorized as syeno- and alkali feldspar granites. Geochemical binary relationships recognized the granites are highly fractionated calc-alkaline-altered Monzo-, syeno-, and alkali feldspar granites formed in the active continental margin. The observed positive Ga vs. Cu, Zn, and Ni correlations are used for epithermal-magmatic-hydrothermal polymetallic veins and mineralized greisen zones. Negative Cu vs. Mo correlation patterns show probable Mo-porphyry deposits in the deeper zones at the contact point between porphyritic perthite and perthite granitic El-Erediya mass. The Zr/Sr between 1.65 to 2.93 plus fluorites in El-Missikat and up to 5.48 plus fluorites in El-Erediya show both U-poor at El-Missikat and U-rich deposits at El-Erediya. The recorded U, Th, Cu, and Pb vertical zoning sequence of deposition differentiates U aureole and deposit zones. The estimated lateral zoning sequences of deposition of these elements define the centers of U deposits. Pathfinders for the deposit of the examined area include the positive Fe2O3 vs. Mg O and Fe2O3 vs. Ca O correlations, and also negative Rb/Sr vs. K/Na and Rb vs. Sr ones, can be applied to future prospecting for similar U-F-Au-W-Mo deposits in the Eastern Desert of Egypt.

Lithium-mica composition as pathfinder and recorder of Grenvillian-age greisenization, Rondonia Tin Province, Brazil

The tin-greisens of the Rondonia Tin Province, Brazil, are related with the intrusion of a 995−975 Ma evolved rapakivi granite suite interpreted as post-collisional with respect to the Grenvillian orogeny during assembly of Rodinia. Lithium-iron mica (‘zinnwaldite’) is the main mineral in late- to post-magmatic and ore stages of such greisens, and has the potential of being a recorder of the mineralization processes. We provide bulk rock geochemistry of granite, greisen, and greisenized granite, coupled with in-situ major and trace element analyses in mica. Trace element and Li contents in mica were assessed via LA-ICP-MS analysis to avoid interference from ore-mineral inclusions. There is a large-scale zoning (hundreds of meters) of the composition of magmatic mica within the massif. Within 200 m of greisen zones, the mica composition in granite becomes similar to hydrothermal greisen mica, i.e. mica composition is suggested as a proximity indicator for greisen. Mica records the evolution of the system from magmatic to hydrothermal. Early-magmatic mica is Li, Rb and F poor and Mg, Ti and Fe rich, as opposed to greisen mica. Rare metals (e.g. Sn, Ta, W) display complex behavior, as their content in mica increases from magmatic to transitional stages, but decreases from transitional to ore (greisen and vein) stages. This can be explained by a complex interaction between enrichment of metals in the fluid, crystallization order of HFSE-bearing minerals, a decrease in the acceptance of HFSE in mica due to Ti depletion, and a change in the system from melt-dominated to fluid-dominated. Depletion of rare metals in mica can be an important factor for mineralization, since binding these metals to silicates reduces the amount of ore minerals. In granite, up to 86 % of Sn is bound to mica, while in greisen, up to 95 % of it is available to form cassiterite. Niobium behaves differently than other rare metals, likely due to its very high initial partition coefficient in mica and its lower solubility in fluids when compared to Sn and Ta. As such, changes in the Nb/Sn ratio in mica can be used as a proxy for the rock/fluid ratios. Mica pseudomorphs after feldspar in greisenized granite have anomalously high Sr contents inherited from their albite precursor.

Tin-tantalum-niobium mineralization in the Penouta deposit (NW Spain): Textural features and mineral chemistry to unravel the genesis and evolution of cassiterite and columbite group minerals in a peraluminous system

The Sn-Ta-Nb Penouta granite is a highly evolved rare-metal post-kinematic leucogranite located in north-western Spain, which was intermittently mined from Roman times until 1985, when falling metal prices led to the closure of most mines in the country.Mineralization consists mainly of cassiterite (cassiterite I) and columbite group minerals (CGM) disseminated in the leucogranite, that increase in abundance and crystal size towards the upper part of the granitic cupola, especially in banded pegmatite-like dikes. Upwards quartz veins and greisen zones associated with the wall rock (mainly augen gneisses) were developed, containing coarse-grained cassiterite (cassiterite II). In contrast to nearly homogenous cassiterite chemistry, CGM may show complex compositional zonation patterns in relation with compositional variations of the leucogranite. Taking into account the mineral chemistry and textural features of CGM, two main groups were distinguished: (i) the first one (Group I) follows a general main trend from columbite-Fe to columbite-Mn, and finally to tantalite-Mn, which is in accordance with the evolution of the magma observed from the less to the more evolved zones of the leucogranite and from bottom to top of the body, and responds to common fractionation patterns of decreasing Nb/Ta ratios in the melt; and (ii) a second group (Group II), with intermediate values of Mn/(Mn+Fe) and Ta/(Ta+Nb) due to local supersaturation effects of the melt during crystallization, resulting in complex oscillatory zoned CGM crystals.Textural and chemical features of cassiterite and CGM support the magmatic origin for both minerals, although cassiterite crystallized later in the sequence in both generations. Mineral chemistry evolution of CGM is in good agreement with a greater solubility of tantalite in the melt at the upper levels, where, besides, the fluorine contents of apatite and muscovite were increased and the onset of fluorite crystallization occurred. So, the fluorine enrichment in the upper levels seems likely to be an important component to explain the mineral chemistry of the Penouta granite, although the same mineral chemistry scheme could also be explained in terms of white mica fractionation upwards. Furthermore, the absence of typical textures originated by fluids in the CGM as well as the lack of these minerals in the quartz veins point to a low influence of the hydrothermal fluids in dissolution and recrystallization processes of the primary oxides.

THE PUENTEMOCHA BERYL-PHOSPHATE GRANITIC PEGMATITE, SALAMANCA, SPAIN: INTERNAL STRUCTURE, PETROGRAPHY AND MINERALOGY

The Puentemocha pegmatite, located in the Tormes Dome of the Central Iberian Zone (Salamanca, Spain), is a beryl–phosphate granitic pegmatite hosted by a Variscan anatectic leucogranite. It shows a zonal internal structure with a wall zone, an intermediate zone, and a prominent quartz core. The essential minerals of the pegmatite are quartz, K-feldspar, plagioclase, muscovite, biotite and beryl. Phosphates, pyrite, arsenopyrite and rutile occur as accessory minerals associated with greisen zones developed in the intermediate zone. The gradational contact and mineral association attest to the intimate petrogenetic relation between the Puentemocha pegmatite and its host granite. Petrographic, mineralogical and chemical data suggest a crystallization from the border inward at temperatures in the range of 700–500°C, under f (O2) conditions at ΔFMQ = ~−0.5, and H2O contents of 6–7.5 wt% at the level of emplacement. As there is no evidence of residual melt extraction, the in situ fractionation of a peraluminous granitic melt is presumed to be sufficient to attain beryl saturation. The greisen bodies developed under subsolidus conditions, at the expense of the intermediate zone. A later episode of Na-metasomatism affected unevenly the primary zones of the pegmatite and the greisen bodies.

Micro- and nanoparticles of zincite and native zinc from disseminated mineralization of metasomatic rocks in the Dukat ore field

Data on the morphology and crystal structure of micro- and nanoparticles of zincite and native zinc that occur among the newly formed minerals of metasomatic rocks in the Dukat ore field are presented. The origin of minute particles in the discharge areas of hydrothermal fluids is caused by the specific dynamics of the heterogeneous mineral-forming environment, resulting in the formation of ultralocal zinc concentrations. In propylite and greisen, the minute particles, which are products of the earliest stages of ore deposition, arise at the front of growing Fe-chlorite and apophyllite crystals. Aggregation of zincite nanoparticles with formation of metalliferous inclusions and fibrous native zinc proceeds in the greisen zones during aggradation recrystallization of a finely dispersed mineral system under the effect of additional portions of reduced fluids. In the zone of argillic alteration, dendritic zincite crystals result from crystallization of gels formed during coagulation of colloid solutions.

Petrographic Characteristics and Alteration Geochemistry of Granite-hosted Tungsten Mineralization at Degana, NW India

Abstract. Vein type tungsten mineralization at Degana is genetically and spatially associated with the Degana Granite. The deposit is characterized by pervasive wall rock alteration around the mineralized quartz veins. Laterally three different alteration zones, greisen, silicification and potassic zones, are marked based on the field features, mineral assemblages and geo-chemical characteristics. In the present paper, systematic mineralogical and chemical variation in these alteration zones is reported. Thick mono-mineralic (zinnwaldite) selvages around the veins characterize the deposit. Plagioclase and alkali feldspar are low in the greisen zones while K-feldspar shows more increase than plagioclase in the potassic zone. Quartz is uniformly high in all the alteration zones, but it shows an anomalous value in the silicification zone. Al2O3 concentration shows initial depletion in greisen zone with gradual increase away from the contact. MgO and FeO are higher in greisen zone than silicification and potassic zones. The potassic zone is characterized by the depletion of Na2O and higher value of K2O. The common presence of topaz and fluorite as both primary and secondary minerals and fluorine-bearing micas suggest fluorine partitioning in substantial amount between granitic melt and coexisting aqueous fluid phase and higher HF activity during the evolution of hydrothermal fluid. The mutual relationship of the fluorine minerals (topaz and fluorite) in the different alteration zones suggests an increase in the Ca2+ activity and decrease of H+ activity during the fluid evolution from greisenization towards alkali-metasomatised granite and the fluid is assumed to change from low to high activity ratio of Ca2+/H+.

DATAÇÃO U-Th-Pb DE MONAZITA HIDROTERMAL E SUA APLICAÇÃO NA GEOCRONOLOGIA DA MINERALIZAÇÃO DE ESTANHO EM ZONAS DE GREISEN DO SISTEMA GRANÍTICO PALANQUETA, DEPÓSITO DO BOM FUTURO (RO)

The ages of hydrothermal events responsible for tin mineralization in the granitic systems from the Rondonia Tin Province are poorly defined so far. Available K-Ar dating in Li-micas of greisens and veins yielded values around 965 Ma, younger than the late mineralized granites ages, around 990 Ma. Electron microprobe U, Th and Pb dating of hydrothermal monazite from the greisen zones of the Palanqueta plutonic system, Bom Futuro tin deposit, yielded an age of 997±48 Ma. Considering that: i) the K-Ar method is influenced by increasing temperature; ii) critical temperature of Ar retention in micas is 300oC; and iii) fluid inclusions and a18O studies indicate, respectively, trapping temperature above 300oC and cassiterite crystallization temperature above 400oC, we suggest that the K-Ar dating in micas from Rondonia tin deposits are indicative of cooling and closure of the hydrothermal systems. Therefore, ages obtained from U-Th-Pb data in hydrothermal monazite from greisen can be considered a good reference for the tin ore formation in the Bom Futuro deposit, associated with late to post-magmatic processes, and coeval with the end of crystallization of albite granite in the Palanqueta granitic system.

Mineralization and Fluid Inclusion Study of the Shizhuyuan W-Sn-Bi-Mo-F Skarn Deposit, Hunan Province, China

The Shizhuyuan deposit, China, is a world-class W-Sn-Bi-Mo-F skarn deposit hosted by Devonian limestone in the thermal aureole of the Qianlishan granite complex. The Qianlishan complex comprises five separate intrusions, including fine-grained porphyritic biotite granite (182–187 Ma, granite 1), medium-grained biotite-K feldspar granite (158–163 Ma, granite 2), fine-grained biotite and K feldspar granite (granite 3), granitic porphyry (144–146 Ma, granite 4), and diabase (142 Ma). The upper part of the granite complex is characterized by extensive greisen alteration. Skarn zones distributed around the intrusions are mainly calcic, consisting of garnet, garnet-pyroxene, vesuvianite-garnet, and wollastonite-vesuvianite, progressively outward from the intrusion. Following the primary skarn formation, some of the skarn zones underwent retrograde alteration. The high garnet/pyroxene ratio and the diopside-rich and andradite-rich compositions of the pyroxene and garnet indicate that the skarn belongs to the oxidized type. Mineralization consists of Sn-Be veinlet ore (type I) in marble and porphyry, massive W-Bi-Mo-Sn skarn ore (type II), stockwork W-Sn-Bi-Mo-F ore (type III), and W-Sn-Mo-Bi greisen ore (type IV), mainly associated with granite 2. Emplacement of granite 2 was accompanied by late, intense fracturing characterized by stockwork mineralization (type III), which was superimposed on massive skarn and greisen zones. The stockwork ore consists mainly of greisen and skarn veins and veinlets with scheelite, wolframite, molybdenite, cassiterite, bismuthinite, and fluorite. The Sm-Nd method has been used to analyze the Sm-Nd concentration, and data for pyroxene and garnet in the massive skarn (type II ore) associated with granite 2 yields an isochron age of 157 Ma. This age corresponds closely to the age of granite 2. Fluid inclusion studies of four types of ore samples reveal four types of inclusions: aqueous two-phase inclusions, H2O-CO2 inclusions, gas-rich inclusions, and daughter mineral-bearing inclusions containing halite or calcite. Homogenization temperatures of fluid inclusions in skarn minerals range from 350° to 535°C. The homogenization temperatures of fluid inclusions in greisen and stockwork are lower than that in skarn, and range from 200° to 360°C. The fluid inclusion data indicate that there are two types of fluids associated with the massive skarn and greisen, having salinities from 26 to 41 wt percent NaCl equiv, and from 1 to 21 wt percent NaCl equiv, respectively. The high-salinity inclusions are daughter mineral bearing, whereas the low-salinity inclusions are dominantly aqueous with a few gas-rich inclusions. The results suggest the evolution of the ore-forming fluids either by immiscibility or by mixing between a high-temperature, high-salinity magmatic water and a low-temperature, low-salinity fluid such as meteoric water, consistent with previously published isotopic studies. The Shizhuyuan deposit is a multi-element skarn, highly enriched in W, Sn, Bi, Mo, and F. This is interpreted to reflect multiple sources and stages of mineralization, which, in turn, may account for the large size of the Shizhuyuan deposit.

Fluid inclusion microthermometry and the P-T evolution of gold-bearing hydrothermal fluids in the Niuxinshan gold deposit, eastern Hebei province, NE China

Niuxinshan is a typical example of the numerous mesothermal gold deposits formed during Mesozoic tectono-magmatic reactivation of the Archean North China Craton in eastern Hebei province. Gold occurs in quartz-sulfide lodes in Archean amphibolites and also in greisen zones in the Mesozoic Niuxinshan granite stock. Four mineralization stages can be recognized from early to late: (1) quartz-K-feldspar, (2) quartz-pyrite, (3) quartz-polysulfide, and (4) quartz-carbonate. Gold mineralization mainly occurs in stages 2 and 3. Fluid inclusions in quartz and fluorite from greisen zones in the Niuxinshan granite, and inclusions in vein quartz and sphalerite from stages 1 to 3 in the amphibolites, have been studied by microthermometry. Three compositional types of inclusions are recognized: type 1 (Tp1) are H2O-CO2-bearing inclusions and include primary (Tp1-P) and secondary (Tp1-S) inclusions. These are found in quartz and fluorite from the greisen zones as well as in vein quartz and sphalerite from stages 1 to 3. The Tp1-P inclusions are considered to represent the gold-bearing hydrothermal fluids. Type 2 (Tp2-S) are secondary H2O-CO2 + solid phase inclusions in fluorite from the greisen zones. Type 3 (Tp3-S) are secondary aqueous inclusions with a solid phase which coexist with the Tp2-S in fluorite from the greisen zones. The Tp1-P inclusions show variable VCO2 (commonly 0.3 to 0.6) and XCO2 values (mainly 0.1 to 0.4). The salinities of inclusions cluster around 3 to 11 wt.% NaCl equivalent and their homogenization temperatures to the liquid phase (Th(L)) fall dominantly in the range of 260 to 360 °C. The compositional variations of inclusions in stage 1 probably result from exsolution of magmatic fluids at various stages; immiscibility or boiling of the fluids can be ruled out. The compositional variations of inclusions in the greisen zones and in vein stages 2 and 3 are attributed to cooling, mixing (dilution), and necking-down of the fluids. The Tp1-S and Tp2-S inclusions show salinities of 3 to 6 wt.% NaCl equivalent and XCO2 values of 0.04 to 0.17. Th(L) clusters at 240 to 260 °C. The Tp3-S inclusions have salinities of 3 to 6 wt.% NaCl equivalent and Th(L) of 170 to 240 °C. Isochoric reconstructions, combined with oxygen and sulfur isotope geothermometry of mineral pairs, give trapping P-T conditions for the gold-bearing fluids. The greisen zones formed at 310 to 460 °C and 1.3 to 3.7 kbar; stage 1 veins at 300 to 430 °C and 1.2 to 3.7 kbar; stage 2 veins at 290 to 380 °C and 1 to 3 kbar; stage 3 veins at 250 to 350 °C and 1 to 3 kbar. H2O-CO2 fluids with low to moderate salinities and moderate to high densities (0.66 to 1.01 g/cm3) dominated at early mineralization stages, and evolved towards H2O-richer and CO2- and less saline fluids through time. The retrograde P-T evolution probably resulted from regional uplift and cooling of gold-bearing hydrothermal fluids. The gold bisulfide complex was dominant in the fluids during mineralization and gold deposition was mainly induced by decreases of temperature and pressure, as well as destabilization of the bisulfide complex during sulfidization of wall rocks.

Greisenization and albitization at the Tikus tin-tungsten deposit, Belitung, Indonesia

Tikus is the most important granite-hosted primary tin-tungsten deposit on the tin island, Belitung. The bulk of the ore is located in greisen zones which enclose very irregularly shaped bodies composed almost exclusively of quartz + or - cassiterite + or - wolframite. A small volume of ore is also located in albitized granite which grades into albitite.Detailed geochemical studies indicate that the greisen and albitite were formed by alteration of a K feldspar megacrystic medium-grained biotite granite. Greisenization produced an enrichment in Sn (avg 3,000 ppm), W (1,110 ppm), Fe 2 O 3 , MnO, Bi, Cs, Cu, F, Li, Pb, Rb, and Zn, as well as a depletion of Na 2 O, CaO, and Sr. Albitization is characterized by an enrichment in Na 2 O, CaO, Al 2 O 3 , Cu, F, Pb, Sr, and Zn, as well as by a depletion of K 2 O, MgO, SiO 2 , and Ba. However, Sn (2,100 ppm) and W (914 ppm) were concentrated only with moderate albitization (albitized granite) whereas extreme albitization (albitite) is characterized by low concentrations of these metals.CO 2 -H 2 O(-CH 4 -salt) fluid inclusions with highly variable CO 2 /H 2 O ratios are the most abundant type in the quartz of the greisen as well as in the coarse-grained quartz + or - cassiterite + or - wolframite bodies enclosed in it. They homogenize in the 200 degrees to 340 degrees C range and indicate minimum trapping pressures of 0.7 to 2 kbars. CO 2 -(CH 4 ) inclusions with densities of 0.53 to 0.99 g/cm 3 without a visible H 2 O phase are less common. Aqueous inclusions without a visible CO 2 phase are subordinate in greisen and enclosed quartz bodies (0.5-8 equiv wt % NaCl) but are the only type observed in albitized granite (1-18 equiv wt % NaCl). The CaCl 2 /NaCl ratios are high in the albitized granite (up to 1:1); the homogenization temperatures are in the 150 degrees to 355 degrees C range.It is assumed that an evolutionary relationship exists between greisenization and albitization. Na and Ca were released through fluid-silicate reactions during greisenization and the fluids lost CO 2 (-CH 4 ) with falling temperature. The resulting Na-Ca-enriched aqueous fluids deposited albite and fluorite by replacement reactions in the granite surrounding the greisen. The major deposition mechanism for cassiterite and wolframite was pH increase and concurrent temperature decrease in both greisen and moderately albitized granite (with secondary muscovite and topaz). Strong albitization (albitite) did not increase the fluid pH, and therefore, did not precipitate cassiterite or wolframite.

Plutonic and hydrothermal events in the Ackley Granite, southeast Newfoundland, as indicated by total-fusion 40Ar/39Ar geochronology

Results of total-fusion 40Ar/39Ar dating (eight biotites, four muscovites, and one hornblende) of magmatic and hydrothermal stages of the high-silica, chemically zoned Ackley Granite indicate three distinct episodes of magmatic activity, viz. ≥ 410, 378–374, and 355 Ma, and that the metallogenic system (W–Sn, Mo) evolved synchronously with only one of the magmatic phases.The oldest suite is indicated by two biotite dates at 410.4 ± 4.4 and 392.5 ± 7.6 Ma, which are themselves considered as representing partial resetting related to a tectonothermal event of Acadian age. Thus, the 410 Ma age offers the best estimate of intrusion, although it must be considered a minimum. The main magmatic pulse for the Ackley Granite is indicated by dates between 378 and 374 Ma, with most of these phases occurring in the southern part of the complex. Concordant hornblende–biotite ages (374.8 ± 3.8 and 372.3 ± 4.8 Ma, respectively) for one of the southern phases (Rencontre Lake granite) suggests rapid cooling for this part of the complex. In contrast, the data for a contemporaneous intrusion (Kepenkeck granite) in the northern part of the Ackley Granite are discordant, with a muscovite–biotite pair yielding ages of 378.4 ± 4.8 and 367.7 ± 4.3 Ma, respectively. Four remaining biotite dates, widely distributed within the southern part of the Ackley Granite, gave similar ages of ca. 368 Ma.Three hydrothermal muscovites from mineralized greisen zones along the soumern margin of the granite are dated at 371.3 ± 4.5, 371.3 ± 5.4, and 373.5 ± 4.0 Ma, essentially coeval with the main magmatic event of the Ackley Granite. Since these paragenetically late muscovites give slightly older ages than the magmatic biotites of the second pulse, it is suggested that the ambient temperature for a large part of the Ackley Granite remained in excess of the 250–300 °C biotite blocking temperature for several million years after initial intrusion.A third and presumably magmatic event, presently of unknown dimensions, occurred at 355 Ma. The coincidence of this age with previously published Rb–Sr whole-rock isochron dates of 355 Ma for the Ackley Granite might indicate that this younger magmatic event was in part responsible for resetting the Rb–Sr systematics.

Petrogenesis of the Ear Mountain tin granite, Seward Peninsula, Alaska

Ear Mountain is part of a belt of Cretaceous tin on the Seward Peninsula of western Alaska. Porphyritic and seriate biotite quartz monzonite and quartz syenite represent early pluton units. These were followed by equigranular and fine-grained two-mica tourmaline-bearing granite. Late units are enriched in B, F, and Na/K and are depleted in Ti, A greisen alteration assemblage of tourmaline-quartz-cassiterite-white mica + or - magnetite + or - pyrite + or - pyrrhotite + or - rutile + or - arsenopyrite is found in the granite and skarns surrounding the pluton.Granite compositions approximate the minimum in the synthetic granite system at a pressure of 0.5 kb. Compositions of fine-grained granite are shifted toward the F-enriched minimum in the synthetic system. Temperature estimates based on feldspar geothermometry range from 624 degrees to 501 degrees C and are close to the solidus temperature in the F- and B-enriched systems. Coexisting Fe-Ti oxide mineral data give temperature estimates of 625 degrees to 675 degrees C and indicate cooling along a buffered path intermediate between the quartz-fayalite-magnetite and nickel-nickel-oxide curves. Biotite compositions (Fe/Fe + Mg = 0.69-0.99) combined with the feldspar geothermometry also indicate that crystallization of the pluton followed a buffer near the nickel-nickel-oxide curve.The tin contents of Ear Mountain granites largely correlate with degree of greisen alteration. Unaltered granites contain less than 80 ppm Sn, whereas thoroughly altered granites contain over 950 ppm Sn. Tin concentrations in unaltered granitic rocks increase progressively from porphyritic to seriate to some equigranular rocks, indicating a progressive concentration of Sn with fractional crystallization. Most unaltered fine-grained granites contain low Sn values, interpreted as results from Sn lost to the evolved vapor phase.The moderately reducing, weakly peraluminous and low calcium conditions during crystallization of the Ear Mountain pluton allowed for concentration of Sn into the melt. High B and F contents in the melt promoted prolonged fractionation of the system and increased concentration of Sn. Evolution of a vapor phase removed Sn from the granitic melt and reaction of this vapor with the granite and the skarn produced the Sn-enriched greisen alteration. There is no evidence that Sn was remobilized from Sn-enriched biotites into greisen.Greisen development is not extensive at present levels of erosion. Abundant cassiterite in placers surrounding the pluton suggest much of the greisen zone has been removed by erosion. The apical parts of other tin granites on the Seward Peninsula, such as Lost River and Kougarok, have well-developed greisen zones. Ear Mountain probably resembled these systems prior to erosion.

Porphyry tungsten-molybdenum orebodies, polymetallic veins and replacement bodies, and tin-bearing greisen zones in the Fire Tower Zone, Mount Pleasant, New Brunswick

The Fire Tower zone at Mount Pleasant contains geologically complex deposits that are associated with two distinct phases of granite; an older fine-grained granite associated with tungsten-molybdenum orebodies and a younger granite porphyry to which tin and polymetallic zones appear to be related. Based on potassium-argon and rubidium-strontium isotope analyses, both phases of granite and their associated deposits were emplaced at approximately 340 to 330 m.y. ago. Geochemically, both phases are silica rich, aluminous, potassic, and fluorine rich.Porphyry tungsten-molybdenum orebodies occur mainly in Fire Tower breccia and, to a lesser extent, in quartz-feldspar porphyry country rock and the fine-grained granite which underlies the breccia. The orebodies consist of mineralized fractures, quartz veinlets, and disseminations in breccia matrix. Wolframite and molybdenite, the principal ore minerals, occur together with abundant fluorite and arsenopyrite and minor amounts of bismuth and bismuthinite. Associated greisen-type alteration consists of quartz + topaz + sericite + fluorite, which grades outward through an assemblage of quartz + biotite + chlorite + topaz into propylitic alteration, mainly chlorite + sericite. Crosscutting relationships between mineralized fractures and veinlets indicate that mineralization occurred in multiple stages, the earlier stages being more tungsten-rich and the later stages more molybdenum-rich.Tin-bearing polymetallic veins and replacement bodies, in many places spatially related to granite porphyry, are superimposed on the porphyry tungsten-molybdenum orebodies and surrounding rocks. They consist mainly of chlorite, abundant fluorite, and complex assemblages of disseminated to massive sulfide and oxide minerals. Some cassiterite-rich, relatively sulfide-poor zones occur also within greisen-altered granite porphyry. The relationship of these zones to the polymetallic veins and replacement bodies is not clear. Cassiterite-rich greisen occurs typically at deeper levels and may represent either the roots of the higher level polymetallic zones or an entirely separate stage of mineralization.The contrast in the styles of mineralization between porphyry tungsten-molybdenum ore-bodies on the one hand, and tin and polymetallic zones on the other, is striking. Formation of the porphyry deposits appears to be related to the development of high fluid pressures associated with crystallization of the underlying fine-grained granite magma and subsequent fracturing and breccia formation. The tin and polymetallic zones indicate not only a significant change in the composition of the ore-forming fluids but also formation at fluid pressures that were either too low or too localized to cause extensive fracturing and brecciation.

Lithogeochemical dispersion associated with Ririwai zinc-tin lode, northern Nigeria

The Ririwai complex forms a classic younger granite ring complex, 180 km 2 in size within the anorogenic younger granite province of Nigeria. An outer ring dyke of granite porphyry defines the caldera ring fracture in which fragmental rhyolite units, largely ignimbritic and intercalated thin basalt flows are preserved. Into this volcanic cover a biotite granite with widely varying texture and degree of mineralization has been centrally emplaced. Related to this biotite granite are two phases of mineralization, an early dispersed phase in which the granite is texturally modified and recrystallized (by intense albitization) and columbite, xenotime and thorite are introduced and a later phase of fracture-controled lode mineralization associated with acidic hydrothermal solutions in which sulphides are common. The Ririwai tin-zinc deposit is one of the major lode mineralizations of considerable economic importance so far located in the Younger granite province of Nigeria. It is localized within a major fracture zone approximately 5 m wide and extending east west for 5 km in the Ririwai biotite granite. Principal ore minerals are sphalerite, cassiterite and minor galena, chalcopyrite and wolframite, concentrated within quartz veins and greisens. Studies in wall-rock alteration show that the central quartz veins are bordered by zones of greisen and greisenized granite that are characterized by zinnwaldite, quartz, fluorite and minor topaz. Inward from the greisen zones are envelopes of K-feldspathized and reddened granite (orthoclase + chlorite + sericite + biotite) which locally pass outwards into kaolinized or unaltered granite. The mineralogy and chemistry of alteration products indicate that they were produced by hydrogen and potassium metasomatism. Geochemical analysis of unaltered bedrock samples indicate that the Ririwai biotite granite is characterized by an enrichment in Sn, Li, U, Th, Rb, F, Nb, W, Zn and Y and a depletion in Ba, and Sr relative to barren rocks of similar composition. It is also characterized by high values for the following whole-rock element ratios, Rb/Zr, Rb/Sr, Y/Sr and low Ba/Rb and K/Rb. These anomalous element concentrations are interpreted as reflecting geochemical specializations and an indication of the ore-bearing potential of the granite. The above mineralogical and chemical changes are probably the result of extreme fractionation with superimposed autometasomatic alteration.

AN OUTLINE OF METASOMATIC PROCESSES (PART 1 OF 3)

The present outline presents the physical chemistry of metasomatism in a form understood readily by the largest possible segment of the geological profession, in the first instance; this section is concerned especially with the theory of metasornatic zones. Next we discuss the fundamental problems in metasomatism, their relationships to postmagmatic processes and - in particular - to the acid-base relationships in the solutions segregated from the intruded magmatic pool. The body of the paper, however, is concerned with a survey of the several stages of the metasomatic process consequent upon the stages of magmatic evolution; special emphasis is placed upon contact-metamorphic processes, leaching processes (the origin of secondary quartzites, greisen zones, ore enrichment in the contact-metamorphic zone). All the relationships are considered from the standpoint of metasomatic zone distribution. [Metasomatic zonation is shown to be a consequence of the fact that aqueous systems become increasingly acid (the positive charge increases) directly with temperature increase in the hypothermal range, and increasingly alkaline (the negative charge increases) as temperatures decrease in the epithermal range. Two classes of metasomatic reactions are distinguished: those which occur between ascending post-magmatic solutions and porous rock, and those which occur between such solutions and the wall rock of fissures. The extent of a solution-rock reaction in every temperature range is shown to depend upon the mobility of the oxide present in the system. The most mobile oxide is water, in all temperature ranges, and the least mobile is titanium oxide. The simplest reaction systems occur in fissures; here the most strongly acid solutions, at the highest temperatures, make for the strongest leaching reactions and leave only quartz as vein fillings; the mineral association becomes increasingly complex as temperature and acidity decrease, and therefore, leaching action is weaker. The analysis of propylitization is an interesting feature of the analysis. The mineralogical complexity of the propylitic associations shows it to be the product of reactions between extrusives and - in the main - epithermal solutions at shallow depths. A thermal control of feldspar chemistry is strongly indicated; the plagioclases tend to be hypothermal, while the soda-potash feldspars tend to be epithermal. M. E. Burgunker