Quartz Eyes Research Articles

Machine Learning-Based Lithological Mapping from ASTER Remote-Sensing Imagery

Accurately mapping lithological features is essential for geological surveys and the exploration of mineral resources. Remote-sensing images have been widely used to extract information about mineralized alteration zones due to their cost-effectiveness and potential for being widely applied. Automated methods, such as machine-learning algorithms, for lithological mapping using satellite imagery have also received attention. This study aims to map lithologies and minerals indirectly through machine-learning algorithms using advanced spaceborne thermal emission and reflection radiometer (ASTER) remote-sensing data. The capabilities of several machine-learning (ML) algorithms were evaluated for lithological mapping, including random forest (RF), support vector machine (SVM), gradient boosting (GB), extreme gradient boosting (XGB), and a deep-learning artificial neural network (ANN). These methods were applied to ASTER imagery of the Sar-Cheshmeh copper mining region of Kerman Province, in southern Iran. First, several spectral features that were extracted from ASTER bands were used as input data. Second, correlation coefficients between the original spectral bands and features were extracted. The importance of the random forest features (RF’s feature importance) was subsequently computed, and features with less importance were removed. Finally, the remained features were given to the models as input data in the second scenario. Accuracy assessments were performed for lithological classes in the study region, including Sar-Cheshmeh porphyry, quartz eye, late fine porphyry, hornblende dike, granodiorite, feldspar dike, biotite dike, andesite, and alluvium. The overall accuracy results of lithological mapping showed that ML-based algorithms without feature extraction have the highest accuracy. The overall accuracy percentages for ML-based algorithms without conducting feature extraction were 84%, 85%, 80%, 82%, and 80% for RF, SVM, GB, XGB, and ANN, respectively. The results of this study would be of great interest to geologists for lithological mapping and mineral exploration, particularly for selecting appropriate ML-based techniques to be implemented in similar regions.

Microthermometry and noble gas isotope analysis of magmatic fluid inclusions in the Kerman porphyry Cu deposits, Iran: constraints on the source of ore-forming fluids

This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly > 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.

Geochronology, Oxidization State and Source of the Daocheng Batholith, Yidun Arc: Implications for Regional Metallogenesis

The Daocheng batholith consists of granite, granodiorite and K-feldspar megacrystic granite, which is located in the north Yidun Arc. It is a barren batholith in contrast to plutons of the same age that contain major copper deposits, such as Pulang to the south. In the Daocheng, abundant mafic microgranular enclaves (MMEs) mainly developed within granodiorite and K-feldspar megacrystic granite, which are characterized by quenched apatite, quartz eyes and plagioclase phenocrysts. LA-ICP-MS zircon U–Pb dating of host granodiorite yielded ages ranging from 223 Ma to 210 Ma, with a weighted mean of 215.3 ± 1.8 Ma. Zircons from MMEs yielded ages ranging from 218 Ma to 209 Ma, with a weighted mean of 214.2 ± 1.4 Ma. Geochemical analyses show that granodiorite is high-K, calc-alkaline and I-type, with SiO2 contents ranging from 67.90% to 70.54%. These rocks are metaluminous to marginally peraluminous (A/CNK = 0.98–1.00) and moderately rich in alkalis with K2O ranging from 3.28% to 4.59% and Na2O ranging from 3.18% to 3.20%, with low MgO (1.08%–1.29%), Cr (12.7 ppm–16.8 ppm), Ni (5.19 ppm–6.16 ppm) and Mg# (35–49). The MMEs have relatively low SiO2 contents (56.34%–60.91%), higher Al2O3 contents (16.06%–17.98%), higher MgO and FeO abundances and are metaluminous (A/CNK = 0.82–0.83). The MMEs and host granodiorite are enriched in light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with slightly negative Eu anomalies, and enriched in Th, U and large ion lithophile elements (LILEs; e.g., K, Rb and Pb), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P and Ti), showing affinities typical of arc magmas. The zircon εHf(t) values (−6.28 to −2.33) and ancient two-stage Hf model ages of 1.92 to 1.25 Ga, indicating that the magmas are generally melts that incorporated significant portions of Precambrian crust. The relatively low silica contents and high Mg# values of the MMEs, and the linear patterns of MgO, Al2O3 and Fe2O3 with SiO2 between the MMEs and host granodiorite, showing the formation of MMEs are genetically related to magma mixing. The Daocheng granodiorite is characterized by much lower zircon Ce4+/Ce3+ (average of 3.53) and low fO2 value (average of ∆FMQ = –10.84), whereas the ore-bearing quartz monzonite porphyries in the Pulang copper deposit are characterized by much higher zircon Ce4+/Ce3+ (average of 52.10) and high fO2 value (average of ∆FMQ = 2.8), indicating the ore-bearing porphyry intrusions had much higher fO2 of magma than the ore-barren intrusions considering that the high oxygen fugacity of the magma is conducive to mineralization.

Apatite and zircon geochemistry of Jurassic porphyries in the Xiongcun district, southern Gangdese porphyry copper belt: Implications for petrogenesis and mineralization

The Xiongcun district, a unique area where oxidized porphyry copper-gold deposits (OPCD) and reduced porphyry copper-gold deposits (RPCD) coexist, is located in the central part of the Gangdese porphyry copper belt (GPCB) in Tibet, China, and hosts the only known mineralization related to the Neo-Tethyan oceanic subduction. Apatite and zircon major and trace elements, in situ oxygen isotopes in zircon from fertile quartz diorite porphyry (QDP) of the No. I deposit, fertile hornblende quartz diorite porphyry (HQDP) of the No. II deposit, and barren hornblende quartz diorite porphyry with coarse quartz eyes (QQDP) in the Xiongcun district were determined by EPMA, LA-ICP-MS, and SHRIMP II ion microprobe to investigate the petrogenesis, magma oxidation state, and mineralization differences between the No. I and No. II deposits, as well as the potential of apatite as a metallogenic indicator. The results from this study show that a combination of REE, Ti, oxygen isotopes, and the Th/U ratios in zircon and REE as well as Sr in apatite can be used to track the magma compositions and crystallization histories in porphyry systems. Multiple indicators, including the apatite SO3 content, apatite Mn oxybarometer, and zircon Ce4+/Ce3+ ratio, indicate that (1) the parental magmas of fertile QDP and HQDP containing Cu and Au mineralization were more reduced than that of barren QQDP; (2) despite barren QQDP having higher oxygen fugacity than QDP and HQDP, the lower Cu and water contents in the magma result in a lower potential for ore formation; and (3) the similar oxidation states of QDP and HQDP show that the mineralization differences between RPCD (No. I deposit) and OPCD (No. II deposit) were not caused by the oxidation states of the magmas but were likely caused by the different processes of hydrothermal fluid evolution. Apatite grains from fertile porphyries (QDP and HQDP) have much higher Cl/F ratios than those from barren porphyry (QQDP), indicating that the Cl/F ratio of apatite can be used as a proxy for porphyry Cu-Au mineralization related to the Jurassic porphyry intrusions in the Gangdese porphyry copper belt.

U–Pb and Re–Os geochronological evidence for the Jurassic porphyry metallogenic event of the Xiongcun district in the Gangdese porphyry copper belt, southern Tibet, PRC

Two large copper–gold porphyry deposits (No.I and No.II) were discovered in the last decade within the Xiongcun district, which is located along the south margin of the Gangdese porphyry copper belt. These two deposits are hosted by two porphyries that were emplaced into the Early Jurassic volcano-sedimentary rock sequences of the Xiongcun Formation. The porphyries are both quartz diorite bodies, with one of these distinguished by its large quartz eyes. Our new molybdenite Re–Os and zircon U–Pb dates for No.I and No.II deposits and host intrusions reveal the presence of two Middle Jurassic ore-forming episodes in the Xiongcun district. The No.II deposit is related to the 181–175Ma quartz diorite porphyry and formed ca. 172.6±2.1Ma. The No.I deposit is related to the 167–161Ma quartz diorite porphyry with large quartz eyes and formed ca. 161.5±2.7Ma. Geochronological and geochemical features of the Jurassic ore-bearing porphyries in the Xiongcun district indicate that the porphyry copper–gold mineralization formed in an island arc setting, which is related to the northward subduction of the Neo-Tethys oceanic plate. These observations, in combination with geochronological data reported at Gangdese porphyry copper belt, indicated four major metallogenic events existed in the Gangdese porphyry copper belt at 172–161Ma, 65–48Ma, 40–23Ma, and 20–12Ma. We have identified two mineralization episodes comprise Middle Jurassic metallogenic event (172–161Ma) in the Xiongcun district, which provides new constrains for the Middle Jurassic metallogenic event in the Gangdese porphyry copper belt.

Diversity of primary CL textures in quartz from porphyry environments: implication for origin of quartz eyes

Porphyry-style mineralization is related to the intrusion and crystallization of small stocks, which can be of different compositions (from intermediate to felsic) and can intrude into different host rocks (from magmatic to sedimentary). We used cathodoluminescence and electron probe microanalysis to study the internal textures of more than 300 quartz eyes from six porphyry deposits, Panguna (Papua New Guinea), Far Southeast porphyry (Philippines), Batu Hijau (Indonesia), Antapaccay (Peru), Rio Blanco (Chile) and Climax (USA). Significant diversity of the internal textures in quartz eyes was revealed, sometimes even within a single sample. Quartz grains with Ti-rich cores surrounded by Ti-poor mantles were found next to the grains showing the opposite Ti distribution or only slight Ti fluctuations.We propose that diversity of the internal patterns in quartz eyes can actually reflect in situ crystallization history, and that prolonged crystallization after magma emplacement under conditions of continuous cooling can account for the observed features of internal textures. Formation of quartz eyes begins at high temperatures with crystallization of high titanium Quartz 1, which as the melt becomes more and more evolved and cooler, is overgrown by low Ti Quartz 2. Subsequent fluid exsolution brings about dramatic change in the melt composition: OH − , alkalis and other Cl-complexed elements partition into the fluid phase, whereas Ti stays in the melt, contributing to a rapid increase in Ti activity. Separation of the fluid and its further cooling causes disequilibrium in the system, and the Quartz 2 becomes partially resorbed. Exsolution of the fluid gradually builds up the pressure until it exceeds the yield strength of the host rocks and they then fracture. This pressure release most likely triggers crystallization of Quartz 3, which is higher in Ti than Quartz 2 because Ti activity in the melt is higher and pressure of crystallization is lower. As a result of the reaction between the exsolved fluid and quartz a new phase, a so called ‘heavy fluid’ forms. From this phase Quartz 4 crystallizes. This phase has extremely high metal-carrying capacity, and may give a rise to mineralizing fluids. Finally, on the brink of the subsolidus stage, groundmass quartz crystallizes. Prolonged crystallization under conditions of continuous cooling accounts better for the diversity of CL textures than crystallization in different parts of a deep magma chamber. It is also in a better agreement with the existing model for formation of porphyry-style deposits.

Cathodoluminescence properties of quartz eyes from porphyry-type deposits: Implications for the origin of quartz

Hyperspectral cathodoluminescence (CL) mapping, combined with electron probe microanalysis (EPMA) and Fourier transform infrared spectroscopy, was used for the reconstruction ofcrystallization conditions of quartz from porphyry environments. Quartz eyes from the two porphyry deposits Rio Blanco (Chile) and Climax (U.S.A.) were studied. Three peaks are found to be responsible for the total CL emission: 1.93, 2.05, and 2.72 eV. The first two peaks are assigned to O-M (with M being an alkali ion) and oxygen vacancies, respectively. The 2.72 eV peak shows a linear correlation with the Ti concentration determined by EPMA point measurements. In addition, a negative correlation between the 1.93 eV emission and the Al concentration was observed. Quartz grains often form clusters in which adjacent grains show identical CL patterns, indicating that they crystallized attached to each other and were not disturbed later. Quartz cores display sector zoning and enrichment in Li, OH, and sometimes Al, which points to rapid crystallization from an extremely evolved melt. Quartz rims show high-Ti, and low-Li and -OH contents, indicating crystallization from a less evolved melt either at higher temperatures or at higher titanium activities. The Al and Ti distribution patterns are frequently not correlated and both show uneven distribution indicating fast growth from inhomogeneous melts. Only Ti displays sharp transitions and fine oscillatory zoning, which can be explained by the higher mobility of Al in the quartz lattice. The quartz eyes crystallized after magma emplacement under non-equilibrium conditions. It is likely that the crystallization occurred from the melt enriched in Al, Li, and OH and probably other metals and/or volatiles on the brink of fluid exsolution. Subsequent fluid exsolution brought about disequilibrium to the system, resulting in dissolution of quartz and redistribution of elements between the melt and the fluid. The OH, Li, and other alkali metals and volatiles partitioned into the fluid, whereas Ti and Al remained in the melt. Resorption of quartz caused by the fluid exsolution continued until equilibrium was reached again, after which crystallization of quartz rims began from the water-, alkali-, and volatile-poor melt with higher Ti activity. Furthermore accumulation of Al and Ti in the residual melt led to crystallization of extremely Al- and Ti-rich quartz.

The nature of “quartz eyes” hosted by dykes associated with Au-Bi-As-Cu, Mo-Cu, and Base-metal-Au-Ag mineral occurrences in the Mountain Freegold region (Dawson Range), Yukon, Canada

*Corresponding author Various origins have been assigned to rounded to subrounded and elliptical quartz megacrysts (“quartz eyes”) in dyke rocks associated with mineral deposits/occurrences worldwide. An exact interpretation of their nature is likely to tightly constrain the petrogenesis of the host rocks, and by association may be critical in evaluating genetic models for spatially associated ore minerals. ChromaSEM-CL imaging and electron probe microanalysis (EPMA) of “quartz eyes” within porphyry dykes associated with Au–Bi–Cu–As, Mo–Cu, and base-metal–Au–Ag mineral occurrences across the Northern Freegold Resources (NFR) property in the Dawson Range of Yukon Territory, Canada, reveals that the cathodoluminescence (CL) response of quartz is a function of its trace-element abundance(s). Bright blue luminescent growth zones are in most cases richer in Fe (up to 8839 ppm) and Ti (up to 229 ppm) relative to CL dark growth zones, with up to 41 times lower concentration of these elements. Assuming a TiO 2 = 1, the Ti-poor dull cores consistently recorded lower temperatures (mostly < 600 °C) compared to Ti-rich brighter blue rims (up to 860 °C). This suggests either overgrowth on xenocrystic cores or an increase in crystallization temperature. The temperature rise likely reflects magma mixing, and is therefore consistent with the phenocryst/phenoclasts having formed in a magma chamber rather than by secondary processes. Also, the great variability in composition and temperature of crystallization and/or reequilibration of brighter blue growth zones of two quartz crystals (660 °C and 855 °C) from a single sample suggests that multiple episodes of magma mixing and incremental growth of parental magma chambers occurred. Some “quartz eyes” are overprinted by variably oriented, bifurcating, and anastomosing fluid migration trails (“splatter and cobweb textu res”) of red to reddish-brown CL quartz that is in most cases of low-temperature origin, and trace-elements poor, thus implying interaction of deuteric fluids with quartz phenocrysts/phenoclasts. The presence of “quartz eye” crystals with broken and angular blue cores, overgrown by oscillatory-zoned rims in which the zoning pattern does not correspond with the crystal boundaries, further suggests that some quartz crystals had been explosively fragmented (phenocrysts) and are now hosted in a recrystallized tuffisitic groundmass. The volatile exsolution that likely accompanied both magma mixing and decompression (as suggested by dendritic quartz, fine-grained recrystallized tuffisitic groundmass, and corroded quartz grains) was probably an important process that could have favoured the ore formation.

Domos volcánicos y mineralización de oro en el distrito Pueblo Viejo, República Dominicana

La mineralización de oro en el Distrito PuebloViejo de la República Dominicana se encuentra re-lacionada espacial y temporalmente con una serie de domos volcánicos del Cretácico Inferior. Las celdas hidro-termales separadas, pero solapadas, centradas en los domos, depositaron en conjunto más de 40 millones de onzas de oro, 240 millones de onzas de plata, 3 millones de toneladas métricas de cinc y 0,4 millones de toneladas métricas de cobre. Los dos depósitos principales (Moore y Monte Negro) y varios depósitos más pequeños(Cumba, Mejita, Mejita Arriba, Banco 5, Arroyo Hondo I y II) han producido mena desde que comenzó la mina en 1975.Cartografía geológica del Distrito Pueblo Viejo ha identificado por lo menos siete centros volcanicos los cuales seinterdigitan, solapan y se encuentran interestratificados con sedimentos epiclásticos derivados localmente. Éstos centros se localizan sobre domos volcánicos que varían en composición entre andesita y dacita. Los sedimentos volcánicos epiclásticos que rodean cada domo reflejan la composición de la roca fuente local. Un domo dacítico en el depósito de Moore se encuentra rodeado de sedimentos epiclásticos con ojos de cuarzo detrítico y res-tos de flujo de pórfido dacítico. Domos andesíticos en los depósitos de Monte Negro, Cumba, y Mejita se encuentran rodeado y parcialmente cubiertos por brechas de origen mixto, epiclástica y piroclástica, y por sedi-mentos volcánicos andesíticos.Las relaciones de campo indican que el emplazamiento de domos volcánicos, la acumulación de sedimentos epiclásticos, la alteración hidrotermal y la mineralización de oro fueron eventos contemporáneos. Los domos fueron emplazados en un ambiente subacuático de poca profundidad en los flancos de un emergente edificio volcánico. Las celdas hidrotermales responsables de la mineralización del oro están controladas por fallas de ánguloempinado. Estas mismas fallas influenciaron en el emplazamiento de los domos, los cuales son considerados como una etapa esencial en el desarrollo de la mena de oro del Distrito de Pueblo Viejo.

Palaeoproterozoic Ni–Cu–PGE mineralisation in the Shakespeare intrusion, Ontario, Canada: a new style of Nipissing gabbro-hosted mineralisation

The 2217 Ma Shakespeare intrusion is part of the extensive 2˙2 Ga Nipissing gabbro suite and is hosted within 2˙45–2˙2 Ga Huronian Supergroup metasediments in the Southern Province of the Canadian Shield, close to the border with the Superior Province. The Shakespeare intrusion is a complex differentiated sill approximately 14 km in strike length and approximately 300 to 430 in thickness. It comprises two distinct magmatic packages: (a) a Lower Group composed of unmineralised pyroxenite and gabbro, and (b) an Upper Group composed of Ni–Cu–PGE mineralised melagabbro, quartz gabbro, and biotite quartz gabbro-diorite. The Shakespeare intrusion formed from a tholeiitic parental magma. All of the rocks in the intrusion are enriched in highly incompatible lithophile elements (HILE: Cs, Rb, U, Th, Nb, Ta, LREE) relative to moderately incompatible lithophile elements (MILE: Zr, Ti, HREE) and are strongly depleted in Nb and Ti relative to elements of similar incompatibility. These geochemical characteristics suggest that the Shakespeare magma underwent extensive degrees of upper crustal contamination before emplacement. Although other parts of the Nipissing gabbro suite exhibit similar geochemical characteristics, the Shakespeare intrusion is more enriched in HILE and more strongly depleted in Nb–Ti, and therefore appears to have undergone greater degrees of crustal contamination. Heavily disseminated to patchy net-textured (10–15%) Fe–Cu–Ni sulphides (pyrrhotite–chalcopyrite–pentlandite) occur in the upper portion of the melagabbro of the Upper Group, near and at the contact with the overlying quartz gabbro, and in melagabbro dykes. This is a new style of magmatic Ni–Cu–PGE mineralisation in the Nipissing gabbro suite. The mineralised zone contains abundant inclusions of quartzite, blue quartz eyes and rare diorite. The ores have compositions consistent with having been derived from the Shakespeare magma and to have equilibrated at moderate magma/sulphide ratios (R=500–1000). The Ni–Cu–PGE mineralisation in the Shakespeare deposit appears to have resulted from the following processes: (a) generation of Nipissing magmas via partial melting of mantle peridotite, (b) contamination of Nipissing magmas by continental crust during ascent, (c) introduction and crystallisation of contaminated but sulphide–undersaturated magmas into the Shakespeare intrusion, forming the Lower Group, (d) additional, apparently relatively local, crustal contamination and sulphide saturation resulting in the incorporation of abundant xenoliths of country rocks and the generation of moderate amounts of Ni–Cu–(PGE) sulphide melt, and, (e) introduction of the xenoliths and sulphide-bearing magma into the Shakespeare intrusion, forming the Upper Group, with heavier sulphides settling at the base of the new crystallisation floor, resulting in low-moderate R factor values.

Volcanic domes and gold mineralization in the Pueblo Viejo district, Dominican Republic

Gold mineralization in the Pueblo Viejo district, Dominican Republic, is spatially and temporally related to a series of Early Cretaceous volcanic domes. Separate but overlapping hydrothermal cells, centered on the domes, together deposited more than 40 million oz. of gold, 240 million oz. of silver, 3 million tonnes of zinc, and 0.4 million tonnes of copper. Two principal deposits (Moore and Monte Negro) and a number of smaller deposits (Cumba, Mejita, Upper Mejita, Banco V, Arroyo Hondo I and II) have contributed ore since mining commenced in 1975. New geologic mapping has identified a series of previously unrecognized volcanic domes that vary from andesite to dacite in composition. A dacite porphyry dome intrudes epiclastic sediments in the Moore deposit and is surrounded by a baked contact metamorphic aureole. Crumble breccias of mixed epiclastic and pyroclastic origin mantle andesite domes in the Monte Negro, Cumba, and Mejita deposits. Epiclastic volcanic sediments surrounding each of the domes reflect the composition of the local source rock. Andesite domes of the Monte Negro deposit are surrounded by andesitic volcaniclastic sediments. Epiclastic sediments surrounding a dacite porphyry dome in the Moore deposit contain detrital quartz eyes and debris flows of dacite porphyry. A series of at least seven volcanic centers interfinger, overlap, and are interbedded with locally derived epiclastic sediments. Field relations indicate that volcanic dome emplacement, epiclastic sediment accumulation, hydrothermal alteration, and gold mineralization were coeval events. Domes were emplaced in a shallow subaqueous environment on the flanks of an emergent volcanic edifice. Hydrothermal cells responsible for gold mineralization are controlled by high-angle faults. These same faults influenced the emplacement of volcanic domes, an essential step in the development of gold ore in the Pueblo Viejo district.

Volcanic textures in quartzo‐feldspathic gneiss of the Willyama Supergroup, Olary Domain, South Australia

Several occurrences of quartzo‐feldspathic gneiss forming part of the Palaeoproterozoic Willyama Supergroup in the Olary Domain, South Australia, retain textural indications for a felsic volcanic origin. These examples preserve relict quartz phenocrysts ('quartz eyes') and, at a few sites, relict feldspar phenocrysts and fragmental textures are retained. The rocks contain an assemblage of albite and quartz, with minor muscovite, biotite, K‐feldspar, magnetite, hematite, pyrite, zircon and xenotime. They share chemical characteristics with other felsic gneisses in the Willyama Supergroup with U‐Pb zircon ages of ca 1700 Ma which have been inferred to have formed from A‐type rhyolite precursors. It is interpreted that the rocks represent rhyolitic pyroclastic or epiclastic deposits (with associated minor iron‐formation and cherty exhalites), which subsequently underwent pre‐ and/or syn‐metamorphic Na (‐Fe) metasomatism and amphibolite‐grade metamorphism.

Preservation and evolution of quartz phenocrysts in deformed rhyolites from the proterozoic of southwestern North America

Euhedral quartz phenocrysts (quartz eyes) are well preserved in ductilely deformed Proterozoic metarhyolites in southwestern North America. In a single sample, the phenocrysts can range from undeformed to highly deformed and recrystallized. Matrix fabrics are extremely similar to those in undeformed ash-flow tuffs. The preservation of the phenocrysts reflects an efficient partitioning of strain from the phenocrysts into the finegrained, wet and possibly glassy matrix. Variations in the degree of phenocryst deformation involve: (1) the chemical and mechanical character of the original phenocryst; (2) the degree to which matrix water has access to the quartz crystal: and (3) crystallographic orientation. Phenocryst-bearing metarhyolites are relatively strain-insensitive, but they contain important kinematic information even in recrystallized rocks where constraints on early structural events are rare. Most phenocrysts have asymmetrical tails, shadows, or deflected matrix foliations that indicate a consistent sense of shear throughout a sample or outcrop. These deformation-induced fabrics are readily distinguishable from primary fabrics in all but the lowest grade samples. In the Tusas Mountains of northern New Mexico and in the Shylock fault zone of central Arizona, quartz-eye metarhyolites have been useful in differentiating early thrusting and shortening events from later syn-metamorphic events that dominate the microstructures and obscure the early tectonic history.

Evaluation of the "quartz eye" hypothesis; a reply

Evaluation of the "quartz eye" hypothesis; a reply

Evaluation of the "quartz eye" hypothesis; a discussion

Evaluation of the "quartz eye" hypothesis; a discussion

Evaluation of the "quartz-eye" hypothesis

Deformed rocks containing are common in mining areas and a correct interpretation of the origin of the quartz eyes is important for evaluating models for ore genesis, especially volcanogenic models. Reexamination of the literature provides little or no support for the hypothesis that large, embayed quartz grains (quartz eyes) that resemble phenocrysts and that occur in fine-grained, generally foliated aggregates are porphyroblasts. Quartz phenocrysts in deformed volcanic or volcaniclastic rocks can remain undeformed, even though the surrounding fine-grained matrix has undergone extensive deformation and recrystallization. Therefore, most, if not all, quartz eyes should be interpreted as residual phenocrysts.

High 18 O ore-forming fluids in volcanic-hosted base metal massive sulfide deposits; geologic, 18 O/ 16 O, and D/H evidence from the Iberian pyrite belt; Crandon, Wisconsin; and Blue Hill, Maine

High 18 O ore-forming fluids in volcanic-hosted base metal massive sulfide deposits; geologic, 18 O/ 16 O, and D/H evidence from the Iberian pyrite belt; Crandon, Wisconsin; and Blue Hill, Maine

Extreme 18O-enriched volcanics and 18O-evolved marine water, Aljustrel, Iberian Pyrite Belt: transition from high to low Rayleigh number convective regimes

Quartz-eye keratophyric tuffs at Aljustrel, S. Portugal, Iberian Pyrite Belt possess unusually high, uniform whole rock δ-values up to 18.1%. with a mean of 16.7 ± 0.7%.. Because the quartz eye cores have δ 18O 12.0 to 13.5%. the felsic tuffs may have had an original whole rock oxygen isotope composition of 10 to 11%. The deduced enrichments of up to +8%. in keratophyric derivatives is attributed to isotope exchange with abundant marine water under temperatures which diminished to ≤ 100°C, during thermally driven convective cooling. The isotopic uniformity requires elevated permeability of ∼ 10 −8 cm 2, such that the second critical Rayleigh number was exceeded, and drifting, non steady state convection cells dominated, with relatively smooth thermal structures. In mineralised counterparts of the tuffs beneath the Feitais-Estacao Zn-Pb-Cu massive sulphide orebodies, whole rock δ-values are ~12%. and quartz (13.3–15.4%.)-chlorite (3.2–6.3%.) fractionations correspond to temperatures of 220–270°C, and a calculated fluid δ 18O of 1.4 to 5.7%. This local isotopic overprinting in vent domains of the regionally high 18O was induced by a stable, second stage convective regime imposed by drastic reduction of permeability accompanying capping of the geothermal discharge by hydrothermal cherts, and/or progressive spilitisation of the tuffs. At lower water/rock coupled with higher temperatures, the recirculating marine water underwent variable 18O enrichment up to 5.7%. by exchange with the high 18O tuffs. An upwards increase in δ 18O quartz and δ-quartz reflects progressive cooling of the geothermal discharge from 15.4%. 270°C in the stockwork, through 18.3%. 220–240°C in the orebody, to 20.1%. 110–130°C in overlying hydrothermal cherts, probably induced by entrainment of ambient marine bottom water in sub-seafloor aquifers.

Recent investigations of two quartz eye porphyroidal rock-volcanogenic massive sulfide occurrences

Recent investigations of two quartz eye porphyroidal rock-volcanogenic massive sulfide occurrences

Emplacement and deformation of the archean Saganaga batholith, vermilion district, northeastern Minnesota

The Saganaga “quartz eye” tonalite forms a crescent-shaped batholith which was intruded 2.7 b.y. ago, rapidly unroofed, and subsequently deformed. It contains four major petrologic phases and displays intrusive relations along the eastern and southern borders and fault-bounded margins on the west and north. Penetrative structures are developed to varying degrees throughout the body with lineation commonly aligned in the plane of foliation. Ductile zones of cataclasis occur locally within the pluton. Cumulate, homogeneous strain was analyzed using deformed quartz phenocrysts in samples from throughout the batholith by measuring particle dimensions and angular relationships of particle axes to principal planes. Strain parameters were calculated using the shape factor grid method although other methods were tested. Strain varies both in pattern and magnitude throughout the body. Flattening predominates near the northern, western and southern boundaries and is well developed ( E s = 0.3–3.7). Lineations generally plunge eastward from 10–20° along the western margin to 60° on the east and are moderately developed ( E s = 0.6–1.5). A zone in the interior of the pluton contains north-trending shallow lineations which are weakly developed ( E s = 0.2–0.3). The neutral zone does not coincide precisely with the change in attitude of penetrative structures. The overall strain pattern of the body indicates that it was emplaced as a crescent-shaped synform along the interface between older gneiss and greenstone units. Field evidence indicates that the body was at least partly molten during emplacement. The magnitude, pattern and geometry of strain suggest that either the body was restricted by boundary conditions during emplacement or that it was deformed after emplacement and erosion. Diapirism of the pluton after erosion appears most compatible with the strains observed as well as the regional pattern of deformation. Late-stage faulting displays right-lateral displacement along the northern and western margins of the batholith and development of cataclastic zones within the batholith postdate the penetrative deformation. Faulting and cataclasis occurred prior to 1.7 b.y. and two periods of faulting are evident.