White Mica Composition Research Articles

Dating metamorphic processes and identifying 87Sr/86Sr inheritance using volume-coupled Rb/Sr geochronology and geochemistry of in situ white mica: A demonstration with HP/LT rocks from Syros, Greece

High-pressure/low-temperature rocks from two well-studied outcrops of the Cycladic Blueschist Unit on Syros are used to demonstrate the advantages of volume-coupled 87Rb/87Sr geochronology and geochemistry of in situ white mica. Single-spot Rb/Sr dates are interpreted based on the microstructural position of white mica combined with its chemical information, and the results are discussed within the tectonic framework of the island. The first outcrop on Katerghaki cape exposes rigid blocks of foliated eclogite within well-foliated blueschist, whereas the second outcrop on Delfini peninsula comprises massive eclogitic blocks, likely of metasomatic origin, hosted within blueschist that are interlayered with white mica schists. The rheological contrasts exhibited in both outcrops preserve (near-)peak HP/LT metamorphism/metasomatism (eclogites) followed by retrogression and deformation in lower P-T conditions (blueschist, micaschist). The white mica in eclogite from both outcrops are in equilibrium with the HP/LT mineral assemblages, do not show chemical zoning nor systematic chemical variations, and yield uniform single-spot Rb/Sr dates. Weighted averages of the Rb/Sr dates are 48.2 ± 3.7 Ma (MSWD: 0.9, n: 34; Katerghaki) and 44.5 ± 3.1 Ma (MSWD: 0.7, n: 28; Delfini), interpreted as (near-)peak HP/LT metamorphism and metasomatism, respectively, overlapping with published garnet LuHf and SmNd geochronology of the corresponding outcrops. White mica in the Katerghaki blueschist and the Delfini micaschist exhibit significant chemical zoning and correlations of Ti with major and trace elements (Al, Fe, Mg, Rb, Sr, Zn, Ba, Nb). Weighted age averages were calculated from Rb/Sr dates corresponding to high-Ti and low-Ti white mica compositions. The blueschist yields ages of 56.0 ± 10.6 Ma (MSWD: 2.2, n: 20/80) and 36.3 ± 2.1 Ma (MSWD: 0.9, n: 20/80), whereas the micaschist produced ages of 43.7 ± 3.5 Ma (MSWD: 0.5, n: 14/53) and 31.9 ± 2.7 Ma (MSWD: 1.0, n: 13/53), respectively. These results are interpreted to resolve the timing of fluid-mediated isotopic re-equilibration of white mica during deformation and retrogression (low-Ti), and reflect 87Sr/86Sr inheritance from the earlier HP/LT history (high-Ti). Overall, the results demonstrate that coupling 87Rb/87Sr geochronology with geochemistry of white mica in situ is advantageous for evaluating Rb/Sr datasets that may possess a single mica population (eclogites) or are affected by 87Sr/86Sr inheritance (blueschist, micaschist) in order to interpret geologically meaningful ages.

Fractionation and Enrichment Patterns in White Mica from Li Pegmatites of the Wekusko Lake Pegmatite Field, Manitoba, Canada

ABSTRACTThe Wekusko Lake pegmatite field in central Manitoba, Canada, is known for its multiple pegmatite dike occurrences, most remarkably its Li-rich pegmatites of economic importance. The Li-rich pegmatites from Wekusko Lake are the focus of this study and belong to the Green Bay group of the Wekusko Lake pegmatite field. These dikes were dated at ca. 1.78 Ga and were emplaced early during the D4 brittle–ductile deformational event. The results presented here describe in detail the pegmatite mineralogy, textures, and zonation of the Li-rich dikes of the Green Bay group, with emphasis on white mica chemistry. The aim of this study is to establish the nature and evolution of white micas from the Li-rich pegmatites of the Wekusko Lake pegmatite field in Manitoba. We aim to understand the differentiation mechanisms that allowed the high level of trace element enrichment observed in the white micas of the studied Li-rich pegmatites. Major and trace elements in white micas from a representative and well studied pegmatite dike were analyzed by electron microprobe and LA-ICP-MS. White mica compositions and textural evidence were used to define two different populations that seem to have been affected by magmatic processes (fractional crystallization) and a secondary episode of metasomatism. Fractional crystallization modeling of a granitic melt can explain some of the trace element enrichment, but extreme Cs enrichment cannot be explained via this mechanism. We interpret that many metasomatic white micas crystallized in boundary layers. Their compositions are controlled by the local melt composition, but aqueous fluids likely contributed to the development of the white mica textures. The substitution mechanisms at play depend on the type of trend and on the stage of evolution. Li enrichment without M2+ enrichment in metasomatic white mica grains is observed, and it is postulated that Fe3+ in white mica explains this behavior. The K/Rb ratio decreases in white micas with fractionation, whereas the concentrations of incompatible elements, such as Cs, Rb, Tl, Ta, and Li, increase. At Wekusko Lake, the Nb and Ta contents seem to be controlled in part by the presence of columbite group minerals and in part by crystallization in boundary layers.

Material fingerprinting as a tool to investigate between and within material type variability with a focus on material hardness

Geochemical and mineralogical datasets from Tropicana Gold Mine, Australia, have been used to define Au-mineralised fingerprints. VNIR-SWIR spectral data were represented by four normalised wavelength regions and were clustered to form spectral classes. Sequentially, these spectral class proportions within a block and co-located pXRF data were clustered to discriminate material types (fingerprints). The hardness of each type was further explored using collocated BWi, Axb, Equotip rebound hardness and penetration rate datasets, but also by considering spatial contextual relationships and the within material type variability. The Tropicana orebody example gave a good illustration of how a phengitic-epidote K-feldspar rich domain (schistosity and softer, ∼15–18 kWh/t) separated from a harder (>20 kWh/t), shorter wavelength phengitic plagioclase-rich feldspar dominated domain. Exploring the within material type differences using the white mica composition (wAlOH) and a new w605 spectral feature demonstrated how the effects of shearing were captured within material types. Such findings will ultimately improve the understanding of the constitutive material hardness and have significance for process optimisation and blending strategy design.

Qualitative barometry of high P/T rocks with field based NIR spectroscopy of white mica

White micas (muscovite, phengite, paragonite) are widespread in rocks of high-pressure, low-temperature (high P/T) terranes. Metamorphism in these settings occurs over a wide range in pressure and a relatively narrow range in temperature. Pressure increase results in forward progress of the aluminoceladonite exchange (Al-Cel, AlIV + AlVI=SiIV+[Fe2+,Mg]VI) in the muscovite-phengite series, ultimately producing very high Si content in white micas in blueschists and eclogites. Visible to near-infrared spectroscopy (Vis–NIR, 350–2500 nm) provides an effective monitor of the Al-Cel exchange because the substitution of Fe and Mg for Al in the octahedral site shifts the frequency of the Al-OH absorption band near 2200 nm to longer wavelengths. Importantly, Vis–NIR measurements are readily acquired with field spectrometers and similar data can be acquired with airborne and spaceborne imaging spectrometers. Here we present results of a field-based Vis–NIR study into variation of white mica Al-OH wavelength values and the potential to perform qualitative field-based barometry in high P/T terranes. Field spectra and samples were collected in five regions in northwest Turkey in which previous studies documented metamorphism over a wide range of pressure at relatively constant temperature. Spectra were collected from 186 samples and 20 samples were prepared for electron microprobe analysis (EMPA) and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) to determine mineral assemblages and white mica compositions. To address possible bulk composition effects, different lithologies were evaluated, broadly categorized as metabasite, metacarbonate, and metapelite. The field-based Al-OH wavelength values range from 2190 nm to 2235 nm, which is equal to the full range of published values for metamorphic or hydrothermal white micas. White mica Si contents range from 2.97 a.p.f.u. (on an 11‑oxygen basis) in paragonite in retrograded calc-schist to 3.66 a.p.f.u. in lawsonite blueschist. In each high P/T region the field-based Al-OH wavelength values and white mica compositions are highly variable at scales of 100 s of meters to thin section scale. We interpret this to record variable retrograde re-equilibration at pressures lower than the peak pressure. Despite the variability, the maximum Al-OH wavelength values in each region, taken to represent the highest Al-Cel content in white mica and the most pristine high-pressure assemblage, correlate well with estimated peak pressure for the region. The maximum wavelength increases from ~2202 nm at P ≈ 6.5 kbar to ~2228 nm at P ≈ 11 kbar to ~2235 nm at P ≈ 24 kbar. Moreover, different lithologies in each region exhibit similar Al-OH wavelength variability and maximum Al-OH wavelength values. This suggests that bulk composition effects are minor compared to pressure effects on Al-OH wavelength variation. This study shows that field-based Vis–NIR spectroscopy can provide valuable information regarding peak metamorphic pressures in high P/T terranes as well as the extent and distribution of post-peak re-equilibration. This can help identify the most pristine, peak-pressure samples as well as those that may yield important information regarding patterns of hydration and fluid flow along the retrograde P-T path at scales ranging from hundreds of meters to centimeters.

Raman Spectroscopy Coupled with Reflectance Spectroscopy as a Tool for the Characterization of Key Hydrothermal Alteration Minerals in Epithermal Au-Ag Systems: Utility and Implications for Mineral Exploration.

Raman spectroscopy of fine-grained hydrothermal alteration minerals, and phyllosilicates in particular, presents certain challenges. However, given the increasingly widespread recognition of field portable visible–near infrared–shortwave infrared (Vis-NIR-SWIR) spectroscopy as a valuable tool in the mineral exploration industry, Raman microspectroscopy has promise as an approach for developing detailed complementary information on hydrothermal alteration phases in ore-forming systems. Here we present exemplar high-quality Raman and Vis-NIR-SWIR spectra of four key hydrothermal alteration minerals (pyrophyllite, white mica, chlorite, and alunite) that are common in precious metal epithermal systems, from deposits on the island of Newfoundland, Canada. The results reported here demonstrate that Raman microspectroscopy can accurately characterize pyrophyllite, white mica, chlorite, and alunite and provide details on their compositional variation at the microscale. In particular, spectral differences in the 1000–1150 cm−1 white mica Raman band allows the distinction between low-Tschermak phases (muscovite, paragonite) and phases with higher degrees of Tschermak substitution (phengitic white mica composition). The peak position of the main chlorite Raman band shifts between 683 cm−1 for Mg-rich chlorite and 665 cm−1 for Fe-rich chlorite and can be therefore used for semiquantitative estimation of the Fe2+ content in chlorite. Furthermore, while Vis-NIR-SWIR macrospectroscopy allows the rapid identification of the overall composition of the most abundant hydrothermal alteration mineral in a given sample, Raman microspectroscopy provides an in-depth spectral and chemical characterization of individual mineral grains, preserving the spatial and paragenetic context of each mineral and allowing for the distinction of chemical variation between (and within) different mineral grains. This is particularly useful in the case of alunite, white mica, and chlorite, minerals with extensive solid solution, where microscale characterization can provide information on the alteration zonation useful for mineral exploration and provide insight into mineral deposit genesis.

Lithogeochemical and Hyperspectral Halos to Ag-Zn-Au Mineralization at Nimbus in the Eastern Goldfields Superterrane, Western Australia

With new advances in rapid-acquisition geochemical and hyperspectral techniques, exploration companies are now able to detect subtle halos surrounding orebodies at minimal expense. The Nimbus Ag-Zn-(Au) deposit is unique in the Archean Yilgarn Craton of Western Australia. Due to its mineralogy, alteration assemblages, geochemical affinity, and tectonic setting, it is interpreted to represent a shallow water (~650 mbsl) and low-temperature (<250 °C) volcanogenic massive sulfide (VMS) deposit with epithermal characteristics (i.e., a hybrid bimodal felsic deposit). We present a detailed paragenetic account of the Nimbus deposit, and establish lithogeochemical and hyperspectral halos to mineralization to aid exploration. Mineralization at Nimbus is characterized by early units of barren massive pyrite that replace glassy dacitic lavas, and underlying zones of polymetallic sulfides that replace autoclastic monomict dacite breccias. The latter are dominated by pyrite-sphalerite-galena, a diverse suite of Ag-Sb ± Pb ± As ± (Cu)-bearing sulfosalts, minor pyrrhotite, arsenopyrite, and rare chalcopyrite. The main sulfosalt suite is characterized by pyrargyrite, and Ag-rich varieties of boulangerite, tetrahedrite, and bournonite. Zones of sulfide mineralization in quartz-sericite(±carbonate)-altered dacite are marked by significant mass gains in Fe, S, Zn, Pb, Sb, Ag, As, Cd, Ni, Cu, Ba, Co, Cr, Tl, Bi, and Au. Basaltic rocks show reduced mass gains in most elements, with zones of intense quartz-chlorite-carbonate±fuchsite alteration restricted to thick sequences of hyaloclastite, and near contacts with dacitic rocks. Broad zones of intense silica-sericite alteration surround mineralization in dacite, and are marked by high Alteration Index and Chlorite-Carbonate-Pyrite Index (CCPI) values, strong Na-Ca depletion, and an absence of feldspar (albite) in thermal infrared (TIR) data. White mica compositions are predominantly muscovitic in weakly altered sections of the dacitic footwall sequence. More paragonitic compositions are associated with zones of increased sericitization and high-grade polymetallic sulfide mineralization. Chlorite in dacitic rocks often occurs adjacent to zones of sulfide mineralization and is restricted to narrow intervals. Carbonate abundance is sporadic in dacite, but is most abundant outside the main zones of Na-Ca depletion. Basaltic rocks are characterized by strongly paragonitic white mica compositions, and abundant chlorite and carbonate. Shifts from Ca carbonates and Fe-rich chlorites to more Mg-rich compositions of both minerals occur in more intensely hydrothermally altered basaltic hyaloclastite, and near contacts with dacitic rocks. Hanging-wall polymict conglomerates are characterized by minor amounts of muscovitic to phengitic white mica (2205–2220 nm), and an absence of chlorite and carbonate alteration.

The pressure–temperature–time–deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono‐metamorphic evolution of the Alboran Domain of the western Mediterranean

AbstractThe structural and thermal relaxation overprint associated with the Neogene Alboran rifting have obscured the early Alpine tectono‐metamorphic evolution of the Alboran Domain, representing the metamorphic core of the Betic–Rif orogen of the western Mediterranean region. This study focuses on the Beni Mzala unit, forming the lower and deeper structural level of the Alpine metamorphic nappe stack (Upper Sebtides) in the Moroccan Rif. Meso‐ and micro‐scale structural investigations are carried out on high‐P aluminum silicate (Ky‐bearing)‐quartz segregations that occur as boudins within the main retrogressive syn‐greenschist foliation (S2/D2) and assumed to preserve the early M1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high‐P foliation (S1, D1/M1) made of quartz–kyanite–white mica–rutile. A large spread in white mica composition is documented, with the highest Si content per formula unit (up to 3.18 apfu) preserved along the S1 foliation and the lower Si content observed in the white mica marking the S2 retrogressive foliation (D2/M2) and the rim of S1 mica. Microtextural evidence documents post‐tectonic andalusite growth and static recrystallization of the quartz microlithons. Inverse (Zr‐in‐Rt thermometry) and forward modelling thermobarometry are integrated with Ar–Ar white mica geochronology to define the peak and exhumation pressure–temperature–time (P–T–t) path of the Beni Mzala unit. Minimum thermo‐baric estimates for the M1 event are ~1.4 GPa and 600°C, corresponding to a metamorphic gradient of ~11°/km, consistent with subduction zone metamorphism. Exhumation is constrained by re‐equilibration of the white mica composition (from high to low celadonite) between c. 29 and 22 Ma, during a nearly isothermal retrogressive path, with final equilibration at high‐T/low‐P conditions within the andalusite stability field (~0.2–0.3 GPa and 500°C). A minimum late Oligocene age is proposed for the Alpine D1 tectono‐metamorphic stage in the Rif, suggesting as feasible the previously proposed Eocene timing for the subduction‐zone metamorphism of the Alboran Domain. Conclusive evidence is provided to link the early Miocene tectono‐metamorphic event to a late thermal perturbation that affected the Alboran Domain at shallow crustal conditions, post‐dating the almost complete exhumation of the deep roots of the Alpine belt in the western Mediterranean.

Using hyperspectral imaging to vector towards mineralization at the Canadian Malartic gold deposit, Québec, Canada

Canadian Malartic is a large-tonnage, low-grade Archean gold deposit (16.3 Moz, 1.08 g/t Au) located in the Abitibi region of Québec, Canada. A large part of the mineralization is hosted in the Pontiac Group metasedimentary rocks, which consist of mudstones to greywackes at upper greenschist to amphibolite facies. In exploration and production environments, these lithologies are challenging to characterize by conventional core logging methods, while in a research setting thin section-sized samples (2 cm × 4 cm) do not capture the full extent of mineralogical variability, which can extend from centimeters to meters away from mineralized zones. Here, high-resolution hyperspectral imagery (0.2–1.0 mm/pixel) in both shortwave infrared (SWIR, 1000–2500 nm) and longwave infrared (LWIR, 8000–12000 nm) is acquired for over two thousand meters of drill core, and is used to visualize changes in mineralogy and mineral chemistry related to metamorphism and hydrothermal alteration. Unaltered metasedimentary rocks contain metamorphic white mica with AlVI contents varying between 1.90 and 1.75 apfu (2195 to 2203 nm), depending on metamorphic grade. Hydrothermal alteration is characterized by white mica which becomes progressively more phengitic with increasing alteration intensity, with AlVI contents ranging from 1.70 to 1.50 apfu (2204 to 2212 nm). Phengitic white mica extends from meters to tens of meters away from major mineralized zones, and can be used as a vector towards mineralization in an exploration setting. White mica composition is correlated to Au content, and can be used to discriminate between unmineralized (<0.3 g/t Au, 2195–2204 nm), weakly mineralized (>0.3 g/t Au, 2205–2208 nm), and highly mineralized (>1.0 g/t Au, >2208 nm) samples, which is a simple metric that can directly be applied to sort ore in a production environment. The Mg# (molar Mg/[Mg + Fe]) of biotite, on the other hand, is unaffected by metamorphic grade, and consistently is Mg# 55–60 (2251–2250 nm) in unaltered samples. In mineralized samples, biotite is Mg-rich (Mg# >65, <2249 nm), but rapidly grades into background compositions outside of the ore zones. LWIR hyperspectral data is used in drill core to estimate silicification abundance from the strength of the 9200 nm quartz reflectance peak. Peak strength was generally in good agreement with the silicification abundance estimated by core logging, and is therefore a metric that can directly be used for characterization of this parameter in exploration and production environments. Over 1500 SWIR point measurements were also acquired in a 50 km by 15 km area around the deposit, and were used to delineate metamorphic isograds on regional scales, and a large 12 km by 3 km hydrothermal alteration halo around the Canadian Malartic deposit. Similar measurements, if performed along the over 200 km long Cadillac-Larder Lake Deformation zone, could be used to rapidly delineate other potentially mineralized areas.

Phyllic alteration and the implications of fluid composition at the Copper Flat hydrothermal System, New Mexico, USA

The Copper Flat hydrothermal system in Hillsboro, New Mexico, USA, is hosted by a partially brecciated porphyritic quartz monzonite intrusive body. Igneous rocks are alkaline to calc-alkaline, and chondrite-normalized rare earth element (REE) plots show enrichment in REEs that is consistent with other regional Laramide igneous rocks. Five assemblages of weak to moderate intensity phyllic alteration with distinct paragenetic relationships exist at the deposit and are classified as: early phyllic, quartz-sericite-pyrite (QSP), sericite-quartz, sericite-carbonate and late QSP. Carbonate is common and is part of some phyllic and the potassic alteration assemblages. Carbonates and phyllosilicates were analyzed by electron microprobe. White micas show substantial substitution of Al by Fe and Mg, and secondary biotite is Mg-rich. Relative to other porphyry Cu systems, the micas from Copper Flat contain, on average, greater F. Carbonates from veins and the replacement of mafic silicates and feldspars were determined to be mostly calcite, but reduced metal carbonates such as ankerite and rhodochrosite are also present in the alteration assemblages. The fluid(s) responsible for alteration are inferred to be CO2-rich and pH neutral as interpreted from the widespread presence of carbonates in both the potassic and phyllic alteration zones, and the white mica composition. Carbonate mineral compositions and absence of sulfates in the potassic alteration suggest that the hydrothermal fluids were not highly oxidized and were sulfur-poor. These features of alteration and geochemistry at Copper Flat are atypical of most porphyry Cu-Mo deposits in the southwestern U.S. and may be more characteristic of porphyry Cu-Au and porphyry Mo deposits.

Alteration and mineral zonation at the Mt Lyell copper–gold deposit, Tasmania

ABSTRACTThe Mount Lyell copper deposits are located in the middle Cambrian Mount Read volcanic belt of western Tasmania and consist of more than 24 separate copper–gold–silver orebodies. The dominant copper mineralisation style is disseminated pyrite–chalcopyrite subvertical pipes with subordinate chalcopyrite–bornite ± other copper phases, massive pyrite and base metal sulfides. A zonation in mineralisation style within the pipes is defined from chalcopyrite–magnetite at depth to chalcopyrite–pyrite at intermediate levels, to chalcopyrite–bornite at the shallowest level. Alteration is developed broadly symmetrically around the ore zones and zoned from quartz–chlorite–phengite ± biotite at depth to quartz–muscovite at intermediate levels, and a quartz–muscovite–pyrophyllite–zunyite assemblage at the shallowest levels. This is interpreted to be a result of a fluid that evolved from hot, reduced and neutral conditions at depth to cool, oxidised and acidic conditions at the shallowest level. The chalcopyrite–bornite deposits occur at the top of the hydrothermal system and are associated with intensely silicified rock and muscovite/pyrophyllite alteration. The close relationship of these deposits with the top of the pipes suggests they are part of a single mineralising event. Where the chalcopyrite–bornite deposits are juxtaposed with the Owen Group, rather than a simple chalcopyrite–bornite mineralogy, there are numerous other copper phases, which represent higher oxidation states and collectively suggest variable and fluctuating fluid conditions during deposition. It is proposed that these deposits are formed by an interaction of the reduced hydrothermal fluid with an oxidised fluid generated at very shallow levels within and during deposition of the Owen Group. Mineralisation within the middle Owen Group sandstones and clasts of altered rock within the middle and upper Owen Group sediments marks the end of the hydrothermal system. Around the entire edge of the Mt Lyell field, there is a variation in the white mica composition from proximal muscovite to distal phengite that represents the neutralisation of the hydrothermal fluid by fluid–wall rock interaction.

Mica composition as a vector to gold mineralization: Deciphering hydrothermal and metamorphic effects in the Malartic district, Quebec

Canadian Malartic, with a total endowment of 16.3 Moz Au, is an important example of a large-tonnage, low-grade Archean gold deposit (current reserves of 204 Mt @ 1.08 g/t Au). It is located in the southern Superior Province in contact with, and immediately south of the east-west trending Cadillac-Larder Lake fault zone, which delineates the boundary between the Pontiac and Abitibi subprovinces. The deposit is hosted by Pontiac Group metaturbidites, Piché Group mafic-ultramafic metavolcanics and by porphyritic quartz monzodiorite to granodiorite that intruded these lithologies. The metamorphic grade increases southward from upper greenschist facies, which characterizes the immediate metamorphic environment of the Canadian Malartic deposit, to mid-amphibolite facies; the garnet isograd is crossed 1.5 km south of the deposit, and the staurolite isograd about one kilometer further south. Textural, structural and geochronological observations are consistent with a syn-kinematic (D2), early- to syn-peak metamorphic (M2) timing for the main ore-forming event.Mineralization in the metasedimentary rocks and porphyritic intrusions consists of disseminated native gold and minor gold-tellurides within stockworks of quartz-biotite-microcline-carbonate±pyrite veinlets (v2) and associated pervasive microcline-albite-biotite±(white mica)-carbonate-pyrite alteration. A zonal distribution of alteration is centered on structures that acted as preferential pathways for the hydrothermal fluids (e.g., Sladen Fault, NW-SE deformation zones). In the metasedimentary rocks, the proximal potassic alteration zone is dominated by microcline-albite±quartz with variable proportions of phlogopite (±white mica), carbonate minerals (calcite-ankerite±Fe-dolomite), quartz, pyrite and rutile. It grades outwards into a distal potassic-sericitic alteration zone characterized by relatively abundant Mg-rich biotite and phengitic white mica, as well as microcline, albite, quartz, calcite, pyrite and rutile. The zonal distribution of alteration features was due in part to a decrease in the total activity of sulfur species (∑aS) and oxygen fugacity (fO2) away from the main hydrothermal corridors, which is manifested by systematic changes in Fe-sulfide and (Fe-Ti)-oxide mineralogy.The effects of sulfidation (∑aS), oxidation (fO2) as well as K+, Fe2+ and H+ activity on biotite and white mica compositions were assessed from silicate-oxide-sulfide equilibria. Increasing ∑aS-fO2 conditions proximal to the hydrothermal fluid pathways caused pyrite to be stabilized over biotite, and iron sequestration in the sulfide phase in turn promoted the stability of magnesian biotite. Increasing sulfur metasomatism towards the hydrothermal centers was associated with systematic increases in biotite Mg# [Mg/(Fetotal+Mg)] and fluorine concentration, in agreement with the Fe-F avoidance principle. Alteration was also associated with a decrease in Al in biotite and white mica, coincident with increases in Si and Fe + Mg concentrations. These compositional trends were controlled by a Tschermak exchange reaction and are consistent with a gradual decrease in aK+ and/or pH upon progressive rock-buffering of a mildly alkaline, potassium-rich ore-forming fluid.Multicomponent phase equilibria (pseudosections), constrained for specified bulk rock compositions typical of non-mineralized Pontiac Group metasedimentary rocks, are used to model the expected composition of mica solid solutions under varying metamorphic P-T conditions. The thermodynamic modelling of biotite and white mica compositions suggests that the district-scale compositional trends documented for biotite and white mica along the metamorphic gradient, including the progression of the Tschermak substitution towards more aluminous compositions southwards, were likely the product of increasing metamorphic P-T conditions, rather than a distal effect of hydrothermal alteration.Mica mineral chemistry is clearly a sensitive indicator of hydrothermal and metamorphic processes at Canadian Malartic. Our results show that biotite and white mica compositions provide valuable tools that can be used to define hydrothermal fluid pathways in and around the deposit. These factors should be particularly useful for mapping the zonation of alteration features that characterize the footprint of major gold deposits in metamorphic terranes where micas typically display a spatial distribution ideal for defining mineral-chemical vectors to ore.

Mapping white mica alteration associated with the Jiama porphyry-skarn Cu deposit, central Tibet using field SWIR spectrometry

Field measurement of the short-wave infrared (SWIR) reflectance from drill hole samples proved to be an effective method to map variations in white mica composition (Tschermak substitution) associated with hydrothermal alteration zonation of the Jiama porphyry-skarn Cu deposit, Tibetan Plateau, China. This SWIR mineral spectral information provides geological insights into: (a) the possible nature of the related magmatic intrusions. (b) the physicochemistry of the associated hydrothermal fluids; (c) the interactions between these fluids and country rocks; and (d) potential vectors to economic mineralization. The results suggest that the Al-OH band wavelength of white mica can used as an indicator in the subsequent steps of prospecting.

Discriminating ore and waste in a porphyry copper deposit using short-wavelength infrared (SWIR) hyperspectral imagery

A recent study by Dalm et al. (2014) showed that alteration mineralogy acquired using SWIR point spectrometry could be linked to copper grade distribution for a group of samples from a South American copper mine. Since it was expected that SWIR hyperspectral imagery can provide more detailed information about the alteration mineralogy of these ores, we investigated whether this technique can be used to improve upon the indirect characterization of copper grades. Maps showing the distributions of SWIR-active minerals, white mica crystallinity, white mica composition, and chlorite composition were produced from SWIR hyperspectral images of 43 samples from the Dalm et al. (2014) study. Subsequently, a principle component analysis (PCA) was applied to the relative mineral abundances and the average white mica crystallinity and composition that were extracted from these maps. The PCA showed that this mineralogical data could be used to discriminate a significant portion of the samples with sub-economic copper grades. Furthermore, the study showed that SWIR hyperspectral imaging has the following advantages over SWIR point spectrometry: minerals that are present in relatively low quantities can be detected, the SWIR-active mineralogical composition at the surface of a sample can be quantified, and the texture of samples, such as grain sizes and cross-cutting vein structures, can be characterized. However, these advantages did not improve upon the indirect characterization of copper grades that was achieved using SWIR point spectrometry. This was attributed to the relatively small size of the sample set and the high textural variability between samples.

Hyperspectral interpretation of selected drill cores from orogenic gold deposits in central Victoria, Australia

ABSTRACTHyLogger hyperspectral data obtained from seven orogenic gold deposits in central Victoria, including Bendigo, Ballarat, Maldon, Fosterville, Costerfield, Castlemaine and Wildwood, are presented. The data demonstrate that fresh diamond drill core displays substantial mineralogical variation that can be attributed to the effects of cryptic hydrothermal alteration that might not otherwise be recognised. The most significant hyperspectral response lies in the white mica compositions, which vary in a systematic manner between high-Al muscovitic zones (Al–OH absorption around 2208 nm) that define a phyllic alteration halo around mineralised structures, and low-Al phengitic–chlorite zones (Al–OH absorption >2014 nm) inferred to represent either more distal alteration or possibly regional metamorphic background. An extensive ferroan dolomite alteration halo overlaps the phyllic and sulfidic alteration zones and extends beyond the sampled core in most instances. This ferroan dolomite halo has previously been defined petrographically, geochemically and using carbonate staining techniques, and is further characterised using thermal infrared hyperspectral data in drill core from the Ballarat goldfield. The mineralogical trends identified by the hyperspectral data are best developed in diamond drill core from the Costerfield, Fosterville and Ballarat goldfields, and are less pronounced at the other deposits. At Bendigo and Castlemaine the reasons for this are not immediately clear, but may be related to the close timing of gold mineralisation relative to peak metamorphism. The Maldon area lies within the contact aureole of the Harcourt Batholith and so has been thermally overprinted leading to the recrystallisation of earlier hydrothermal assemblages. The Wildwood deposit is similar to the Magdala deposit at Stawell and differs from the other goldfields in its geological setting, host rock lithologies and style of hydrothermal alteration, with the development of Fe-rich chlorite closely associated with gold mineralisation. The results demonstrate how hyperspectral data can be used to define large hydrothermal alteration footprints associated with orogenic gold mineralisation in central Victoria that are of direct benefit to mineral explorers, as well as independently characterising lithological variations in drill core.

Mapping the alteration footprint and structural control of Taknar IOCG deposit in east of Iran, using ASTER satellite data

Taknar Fe+Cu±Zn±Pb±Au±Ag deposit in northeast of Iran is studied by Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) reflectance and emittance data. Structural and mineralogical evidences of IOCG mineralization is mapped by visual image interpretation and spectral processing techniques. The tectonic model is consistent with an extensional zone associated with a releasing bend of right-lateral regional faults, extending about 7km2 and encompassing all the known orebodies of Taknar. A combination of band ratio logical operator and matched filtering were used for spectral mapping, which lead to a series of mineral content and crystallinity maps including ferric oxide, ferrous, white mica, chlorite, silica and opaque minerals. The channel way in which hydrothermal fluids were migrating is accurately defined by abundance of white mica and ferric iron oxide maps. Rhythmic sediments of Taknar formation which was characterized by chlorite mineral map is a “reducing” environment that hosts the mineralization. This REDOX environment is also marked by a sudden change in white mica composition from acidic phases to neutral/alkaline. Subsequent field check and microscopic study indicated the accuracy of these remotely mapped minerals. Based on this finding, several new prospects for further exploration was proposed. These results indicates that ASTER data is capable of delineating alteration footprints of an IOCG mineral system in deposit scale exploration.

Hydrothermal mineral alteration patterns in the Mount Isa Inlier revealed by airborne hyperspectral data

High-resolution mineral maps derived from hyperspectral imaging (4.5 m pixel) enable the recognition of various types of hydrothermal alteration and the identification of fluid pathways. Airborne hyperspectral images from the Eastern Fold Belt of the Mount Isa Inlier were tested as a new tool for the detection of Fe-oxide Cu–Au (IOCG) related alteration. Four different types of hydrothermal alteration were identified with the hyperspectral mineral maps: (1) Metasomatic 1: white mica mineral maps show the spatial distribution of regional sodic–calcic alteration in metasedimentary successions of the Soldiers Cap Group in the Snake Creek Anticline. (2) Metasomatic 2: alteration zonation is evident from albitised granites assigned to the Williams–Naraku Suite along the Cloncurry Fault. These show characteristic absorption features in the shortwave infrared range (SWIR) which are depicted on the white mica mineral maps (white mica composition, white mica content, white mica crystallinity index). Alteration zonation in gabbros of the Cloncurry District was detected by a combination of MgOH and Fe2+ mineral maps (MgOH content, MgOH composition, amphibole/chlorite and Fe2+ and MgOH) combined with white mica mineral maps (white mica composition and white mica content). (3) Fluid channels 1: major fault zones, such as the Mt Dore fault zone in the Selwyn Corridor, are interpreted as important fluid pathways, where gradual changes in the mineral chemistry are highlighted with mineral maps (e.g. white mica content, white mica composition, white mica crystallinity index). (4) Fluid channels 2: MgOH and Fe2+ mineral maps were used to map breccia pipes in the northern Cloncurry District north of the Saxby Granite (Suicide Ridge). The MgOH and Fe2+ mineral maps were also used to distinguish various mafic rocks from amphibolites, which are host rocks for some of the IOCG deposits in the Eastern Fold Belt (e.g. Mount Elliott), and calcsilicate breccias pipes (e.g. Suicide Ridge).

Compositional variations of syntectonic white-mica in low-grade ignimbritic mylonite

The Silla del Gobernador Shear Zone (SGSZ), Coastal Range of central Chile (32°S), is a low-grade mylonite generated from a Triassic acidic ignimbritic protolith. Different microstructures (e.g. quarter mats and pressure shadow structures) associated with pressure solution and solution transfer processes and well-preserved porphyroclasts can be observed. White-mica generation appears to be favoured by reaction-softening from dissolved quartz and feldspar porphyroclasts. According to the mylonite fabrics three different types of white-mica can be established (A, B and C) representing mylonites with evidences of relatively low, intermediate and high strain conditions. Deformation mechanisms suggest low-grade (350–400 °C) conditions for mylonitic deformation in the presence of fluids, consistent with the temperature estimations based on chlorite chemistry. The geological setting of the SGSZ also suggests low-pressure (<3 kbar) conditions. No major element mobility with respect to protolith composition was observed in low-strained mylonites, whereas for the highly strained mylonites, a marked K 2O secondary input and higher fO 2 conditions during deformation are suggested. The chemical composition of the neoformed white-mica correlates with strain intensity: the high-strained mylonites contain phengitic–celadonitic micas, whereas low-strained mylonites have paragonitic and muscovitic micas. White-mica geobarometry in highly deformed rocks yields pressure values with no geological significance, because the metasomatic K-input favour their high K content, independent of the confining pressure. Thus, the white-mica composition in the mylonites of the SGSZ is controlled by the whole-rock geochemistry and is related to strain conditions.

Short Wavelength Infrared Spectral Characteristics of the HW Horizon: Implications for Exploration in the Myra Falls Volcanic-Hosted Massive Sulfide Camp, Vancouver Island, British Columbia, Canada

Short wavelength infrared (SWIR) spectrometry has been used to identify previously unmapped hydrothermal alteration zones around volcanic-hosted massive sulfide (VHMS) orebodies at Myra Falls, Vancouver Island, British Columbia. Hydrothermal alteration assemblages are uniformly dominated by fine-grained white mica, with poor development of mineralogical zonation. SWIR spectrometry is an ideal exploration tool for characterizing this fine-grained hydrothermal alteration. At Myra Falls, SWIR spectrometry has identified subtle shifts in the wavelengths of the AlOH absorption feature of white mica, corresponding to compositional changes in altered rhyolite distal and proximal to ore. AlOH absorption occurs at shorter wavelengths (<2,198 nm) and corresponds to lower Fe, Fe + Mg, and Si/Al and higher Na/(Na + K) in strongly altered samples proximal to ore (slightly sodic muscovites). AlOH absorption occurs at longer wavelengths (>2,206 nm) and corresponds to higher Fe, Fe + Mg, and Si/Al and lower Na/(Na + K) in samples distal to ore (nonsodic slightly phengitic muscovites). White mica in siltstone within a meter of VHMS ore has higher Zn, V, Fe, and Mg contents than white mica distal to these altered samples. Chlorite compositions, identified by SWIR, also show systematic changes with intensity of alteration and distance from ore. The average wavelength of the FeOH absorption feature for chlorite in rhyolitic samples proximal to ore is 2,241 nm (intermediate Mg chlorite), whereas wavelengths in background samples average 2,247 nm (intermediate Fe chlorite). Similar changes are observed in footwall and hanging-wall andesites, with samples near the Battle mine containing muscovite to phengitic muscovite (average wavelength of the AlOH absorption feature of 2,200 nm) and Mg-rich chlorite (average wavelength of the FeOH absorption feature of 2,245 nm) to regional andesite samples with phengitic muscovite (average wavelengths of the AlOH absorption feature of 2,209 nm) and Fe-rich chlorite (average wavelength of the FeOH absorption feature of 2,249 nm). In weakly altered rocks white mica compositions also vary with host lithology. The AlOH absorption feature occurs at longer wavelengths in white mica in dacite and andesite compared to adjacent rhyolitic rocks, suggesting that higher Fe and Mg in the host lithology affects the composition of white mica. Two zones of intense hydrothermal alteration above the Battle and HW orebodies have distinctive SWIR spectral characteristics, with the AlOH and FeOH features occurring at shorter wavelengths (<2,197 and <2,240 nm, respectively). Small anomalous zones of alteration were also identified in the Thelwood Valley area, where minor mineralized zones are present. As broad zones of fine-grained white mica (sericite) alteration are ubiquitous throughout the Myra Falls property, alteration proximal to ore cannot be identified simply by visual logging of drill core. Alteration zonation may be determined by subtle shifts in white mica spectral characteristics. This study indicates that SWIR analysis may be an effective field-based exploration tool for quantifying the intensity of alteration associated with VHMS orebodies, and that trends in mineral compositions, even in very fine grained rocks, can be used as mine-scale vectors to ore.

Growth of myrmekite coronas by contact metamorphism of granitic mylonites in the aureole of Cima di Vila, Eastern Alps, Italy

AbstractIn the contact aureole of the Oligocene granodiorite of Cima di Vila, granitic pegmatites of Variscan age were strongly deformed during eo‐Alpine regional metamorphism, with local development of ultramylonites. In the ultramylonite matrix, consisting of quartz, plagioclase, muscovite and biotite, microstructures show grain growth of quartz within quartz ribbons, and development of decussate arrangements of mica. These features indicate that dynamic recrystallization related to mylonite development was followed by extensive static growth during contact metamorphism. K‐feldspar porphyroclasts up to 1.5 cm are mantled by myrmekite that forms a continuous corona with thickness of about 1 mm. In both XZ and YZ sections, myrmekite tubules are undeformed, and symmetrically distributed in the corona, and oligoclase‐andesine hosts have random crystallographic orientation. Myrmekite development has been modelled from the P–T–t evolution of the ultramylonites, assuming that the development of the ultramylonites occurred during eo‐Alpine metamorphism at c. 450 °C, 7.5 kbar, followed by contact metamorphism at c. 530 °C, 2.75 kbar. Phase diagram pseudosections calculated from the measured bulk composition of granitic pegmatite protolith indicate that the equilibrium assemblage changes from Qtz–Phe–Ab ± Zo ± Cpx ± Kfs during the ultramylonite stage to Qtz–Pl(An30–40)–Ms–Kfs–Bt(Ann55) during the contact metamorphic stage. The thermodynamic prediction of increasing plagioclase mode and anorthite content, change of white mica composition and growth of biotite, occurring during the end of the heating path, are in agreement with the observed microstructures and analysed phase compositions of ultramylonites. Along with microstructural evidence, this supports the model that K‐feldspar replacement by myrmekite took place under static conditions, and was coeval with the static growth accompanying contact metamorphism. Myrmekite associated with muscovite can develop under prograde (up‐temperature) conditions in granites involved in polymetamorphism.

Hydrothermal Alteration and Volatile Element Halos for the Rosebery K Lens Volcanic-Hosted Massive Sulfide Deposit, Western Tasmania

A detailed study of alteration mineralogy, mineral chemistry, and lithogeochemistry in the host rocks surrounding the A-B and K lenses at the north end of the Rosebery mine has revealed a series of overlapping alteration halos with characteristic mineralogy and geochemistry. The study involved logging and sampling from nine drill holes spaced at varying distances from the A-B and K lenses. The stratiform Zn-Pb-Cu ore lenses have a sheetlike morphology and are hosted by a mixed sequence of rhyolitic to dacitic massive mediumgrained quartz-porphyritic pumice breccia, black mudstone, and crystal-rich volcaniclastic sandstone, overlying a thick homogeneous sequence of rhyolitic pumice breccia. The major alteration minerals at Rosebery are arranged in a complex series of zones passing away from the deposit-quartz-sericite zone, Mn carbonate zone, chlorite zone, and outer sericite zone. The chlorite zone is best developed in the immediate footwall below the copper-pyrite-rich ore lenses, whereas the strongest Mn carbonate alteration occurs in the immediate hanging-wall volcanics or lateral to the ore lenses. The outermost visible sericitic alteration extends about 60 to 100 m into the footwall, 10 to 20 m into the hanging wall, and over 500 m along the upper contact of the footwall pumice breccias. Thallium and antimony form the most extensive trace element halos related to the mineralization. Thallium forms a halo that extends 200 to 300 m into the overlying volcanics and 60 to 100 m into the footwall. Anomalously high thallium also occurs over 500 m along strike marking the contact between the footwall pumice breccias and the overlying volcaniclastic sandstones. Proximal to the ore lenses Tl and Sb values range from 10 to 100 ppm, compared to the halo zone where they vary from around 1 to 10 ppm. Studies of alteration mineral chemistry at Rosebery have revealed some important relationships that may assist exploration. The Mn content of alteration carbonate increases toward ore, both along strike and across strike. Close to ore, alteration carbonates contain >20 mole percent MnCO3 (kutnahorite, rhodochrosite, Mn siderite, and Mn ankerite), whereas at distances of 40 to 60 m across strike the mole percent MnCO3 in carbonate drops to below 10 percent. At greater than 80 m, the carbonates are Mn-poor calcites and are commonly located in synmetamorphic structures. White mica composition changes with stratigraphy and alteration assemblages and may be related to the mineralizing event, although this has not been convincingly demonstrated. Proximal white mica contains minor Ba substituting for octahedral Al. However, except for their Ba content, these phengetic white micas are similar to those found in nonmineralized areas of the Mount Read Volcanics. Sodic white mica with up to 0.35 Na/(Na + K) and a low phengite content (<0.5 Fe + Mg cations) occurs in a zone of volcanic sandstones and black slates overlying the ore deposit. This research has lead to the development of a series of proximal, medial, and distal vectors useful for both regional and mine-scale exploration. The most useful vectors, listed from proximal to distal, include Zn, Ba content of white mica, Na2O, K2O, Ishikawa alteration index (AI), S/Na2O, Ba/Sr, Mn content of carbonate, Tl, and Sb.