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Holly Stein

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Papers by Holly Stein

Molybdenite under Stress: Dating the Contrarian Bou Azzer Co-Ni-As Deposit, Morocco

Goldschmidt2021 abstracts, 2021

The Co-Ni-As deposits of Bou Azzer floored by the Anti-Atlas Supergroup have a rich orogenic hist... more The Co-Ni-As deposits of Bou Azzer floored by the Anti-Atlas Supergroup have a rich orogenic history reaching back to the Mesoproterozoic 1 . Relicts of a rift-generated, then obducted oceanic crust hosting the Bou Azzer mineralization present as highly sheared ~760 Ma allochthonous ophiolite complexes intruded by younger Neoproterozoic quartz diorites. The Bou Azzer ore bodies are highly deformed and hosted either within serpentinite or more commonly along the contact with the intrusions 2 . Many disparate age models exist for Bou Azzer mineralization, and prevailing models advocate Neoproterozoic, Variscan, and/or Late Triassic ore genesis. Re-Os dating of sulfides in poly-deformed environments is fraught with challenges as multiple, temporally distinct generations of sulfides may be present. If more than one generation carries significant Re, a range of ages may result as documented in the first Re-Os molybdenite dating of Bou Azzer (350-400 Ma) 3 . Here, we present new Re-Os ages derived from unusual mirror-like, woody-textured, molybdenite-cryptocrystalline quartz occurrences syn-kinematically deformed with widespread chlorite. Our results clearly outline a 385-390 Ma event associated with molybdenite-chalcopyrite deposition with an Os isotope ratio that may reflect influx of primitive fluids in the late Devonian. Further, one set of samples contains significant common Os, whereas the other set displays Os isotopic compositions as expected for molybdenite (no common Os). Both sets have similar, but distinct ages with molybdenites containing common Os being slightly younger (375 Ma). Proterozoic shales as source rocks are difficult to reconcile with our new Re-Os data. Rather, Late Devonian mineralization appears to have involved fluids from the mantle and potentially Neoproterozoic oceanic crust. Brannerite, as pristine inclusions in the Co-Ni arsenide ore mineral skutterudite, also gives Late Devonian U-Th-Pb ages (385-375 Ma) 4 . Thus, a significant ore-forming event at Bou Azzer is Late Devonian, calling for a significant revisit of tectonic and Co-Ni-As ore events in this region of Morocco. This project was initiated and funded by the Managem Group.

Geology, Re-Os age, S and O isotopic composition of the Lar porphyry Cu-Mo deposit, southeast Iran

Ore Geology Reviews, 2019

The Lar porphyry Cu-Mo deposit is located 20 km northeast of Zahedan in the Sistan Suture Zone (S... more The Lar porphyry Cu-Mo deposit is located 20 km northeast of Zahedan in the Sistan Suture Zone (SSZ), in the southeastern part of Iran. Lar is a sub-economic porphyry Cu-Mo deposit with average grades of 0.16% Cu, 0.01% Mo, and 0.66 ppb Au. The Cu-Mo mineralization developed in post-collisional shoshonitic syenitic to monzonitic intrusions in an Oligocene igneous complex known as the Lar Igneous Complex (LIC). The host and country rocks have undergone variable degrees of supergene and hypogene alteration including silicic, potassic, propylitic, phyllic, and argillic alteration. The Cu-Mo mineralization is mainly controlled by NNW striking faults and fractures, and occurs as quartz veins and veinlets, disseminated sulfides, and stockworks. The mineralization can be divided into two types: hypogene and supergene. Hypogene mineralization is characterized by chalcopyrite, pyrite, bornite, molybdenite and magnetite, whereas supergene mineralization is characterized by enargite, covellite, chalcocite, digenite, malachite, azurite, and iron hydroxides. Sulfur isotope values for hypogene sulfides range from 0.28 to -1.76‰ and suggest an igneous source/or mantle-derived origin for sulfur. The δ 18 O values for quartz in mineralized quartz veins range from 9.6 to 11.5‰. These values would be in equilibrium with waters having calculated δ 18 O H2O values of 9.3 to 11.2‰, suggesting magmatic water. Re-Os dating of two molybdenite samples from two different drill holes yields dates of 29.72 ± 0.11 and 31.95 ± 0.11, suggesting that the system was active for at least 2.0 m.y.

Effects of frictional–viscous oscillations and fluid flow events on the structural evolution and Re–Os pyrite–chalcopyrite systematics of Cu-rich carbonate veins in northern Norway

Tectonophysics, 2015

Mesothermal chalcopyrite + pyrite + magnetite-bearing calcite-dominated vein deposits in the Repp... more Mesothermal chalcopyrite + pyrite + magnetite-bearing calcite-dominated vein deposits in the Repparfjord Tectonic Window, northern Norway, have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation. The veins cut across Paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured, coarse-grained margins, and the fine-grained mylonitic cores. They formed under overall viscous conditions, although cyclically increased fluid pressures caused transient embrittlement. As mineral precipitation sealed the fractures, deformation was accommodated again viscously leading to mylonitization of the vein's core. Local brecciation of the calc-mylonite demonstrates the cyclicity of this process. Re-Os chalcopyrite-pyrite and K-Ar fault gouge dates define an almost continuous age range from ~2540 Ma to ~460 Ma. Regression of three Re-Os analyses yields a 2069 ± 14 Ma age ( 187 Os/ 188 Os = 0.18 ± 0.04), interpreted as the age of vein emplacement, sulfide precipitation, and initial frictional-viscous deformation. K-Ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 Ma, hence indicating structural reactivation connected with Silurian Caledonian orogenic compression. The spread in Re-Os model ages reflects this reactivation, wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing, dynamic recrystallization, and isotopic disturbance of the sulfides. The study provides evidence for fluid flow during viscous deformation and demonstrates that strain, and flow of oxidizing fluids, can have a significant yet localized control on the integrity of the Re-Os systematic in pyrite and chalcopyrite.

Low-rhenium molybdenite by metamorphism in northern Sweden: Recognition, genesis, and global implications

Lithos, 2006

Re Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore dep... more Re Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore deposits using the model age or isochron approach. But, molybdenite has a new niche in the greenschist- to granulite-facies metamorphic environment. Re Os ages for metamorphic molybdenite may be used to construct regional metamorphic histories. Age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single, petrographically-characterized molybdenite occurrences. In this study, twelve geologically distinct molybdenite-bearing samples from two small Mo districts in northern Sweden trace a 150 m.y. Paleoproterozoic Svecofennian metamorphic history from ˜1900 to 1750 Ma. These data reveal a little-known, widespread and protracted, Late Svecofennian anatexis in northern Sweden. The Kåtaberget Mo (Cu, F) deposit is located in the Moskosel granite batholith north of the economically-renown Skellefte district. Four different molybdenite samples from outcrop at Kåtaberget indicate an intrusion age of 1895 ± 6 Ma with the formation of later pegmatite aplite at 1875 ± 6 Ma. The Allebuoda (Björntjärn) and Munka Mo (W) deposits in the Rappen district are represented by three outcrop and five drill core samples of molybdenite-bearing aplite pegmatite granite. These two deposits were previously described as intrusion-related Climax-type Mo mineralization. Re Os ages for molybdenites from these deposits range from 1865 to 1750 Ma and, significantly, Re concentrations are markedly low, extending to the sub-ppm level. Age agreement within the deposits is conspicuously lacking, whereas, with one exception, age agreement within any single sample (geologic occurrence), as established by analysis of additional molybdenite separates, is very good. These data, together with fundamental geologic observations discussed in this paper, suggest that Mo (W) mineralization in northern Sweden is not intrusion-related, but the local product of episodic melting of Archean Paleoproterozoic supracrustal gneisses related to the Svecofennian orogeny. Petrographic traverses across the boundary between widespread, foliation-parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation. Dehydration breakdown of zircon-rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine, post-deformational biotite plus sulfides, oxides, hydrothermal zircon and fluorite, all associated with microcline-dominant leucosomes. This process has profound implication for the traditional leucogranite, intrusion-related genesis attributed to the broad classification of Mo W Sn base and precious metal mineralization (e.g., South Mountain Batholith, Nova Scotia; Okiep, Namaqualand, South Africa; Mactung, Yukon; Pogo Liese, Tintina, Alaska; Carajás and Goiás Rio Tocantins, Brazil; New England Batholith, NSW, Australia; Bergslagen, Sweden; Nevoria, Western Australia; Alpeinerscharte, Austria; Erzgebirge, Germany; Sardinia Corsica Batholith). In addition to biotite, metallogenic contributions (e.g., Mo, W, Sn, U, Bi, Cu, Pb, Zn, Fe, Ni, Co, Au, Ag, Te, As, Sb, REE) in various combinations may also be controlled by breakdown of amphibole. In effect, the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite. In the absence of focusing structures (e.g., shear zones, sheeted vein development), this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades. Low Re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss (Mo W) or muscovite schist (Sn W).

The Emerging Potential of Re-Os Isotope Geochemistry for Source Rocks and Maturation-Migration Histories

International Petroleum Technology Conference, 2014

Re-Os age for molybdenite from SW Sardinia (Italy): A comparison with 40Ar/39Ar dating of Variscan granitoids

ABSTRACT

Timing of porphyry (Cu-Mo) and base metal (Zn-Pb-Ag-Cu) mineralisation in a magmatic-hydrothermal system—Morococha district, Peru

Mineralium Deposita, 2015

The Morococha district in central Peru is characterised by economically important Cordilleran pol... more The Morococha district in central Peru is characterised by economically important Cordilleran polymetallic (Zn-Pb-Ag-Cu) vein and replacement bodies and the large Toromocho porphyry Cu-Mo deposit in its centre. U-Pb, Re-Os, and 40 Ar/ 39 Ar geochronology data for various porphyry-related hydrothermal mineralisation styles record a 3.5-Ma multi-stage history of magmatichydrothermal activity in the district. In the late Miocene, three individual magmatic-hydrothermal centres were active: the Codiciada, Toromocho, and Ticlio centres, each separated in time and space. The Codiciada centre is the oldest magmatichydrothermal system in the district and consists of a composite porphyry stock associated with anhydrous skarn and quartz-molybdenite veins. The hydrothermal events are recorded by a titanite U-Pb age at 9.3±0.2 Ma and a molybdenite Re-Os age at 9.26 ± 0.03 Ma. These ages are indistinguishable from zircon U-Pb ages for porphyry intrusions of the composite stock and indicate a time span of 0.2 Ma for magmatic-hydrothermal activity. The small Ticlio magmatic-hydrothermal centre in the west of the district has a maximum duration of 0.3 Ma, ranging from porphyry emplacement to porphyry mineralisation at 8.04 ±0.14 Ma ( 40 Ar/ 39 Ar muscovite cooling age). The Toromocho magmatic-hydrothermal centre has a minimum of five recorded porphyry intrusions that span a total of 1.3 Ma and is responsible for the formation of the giant Toromocho Cu-Mo deposit. At least two hydrothermal pulses are identified. Post-dating a first pulse of molybdenite mineralisation, widespread hydrous skarn covers an area of over 6 km 2 and is recorded by five 40 Ar/ 39 Ar cooling ages at 7.2-6.8 Ma. These ages mark the end of the slowly cooling and long-lived Toromocho magmatic-hydrothermal centre soon after last

Geochemical and isotopic constraints on Palaeozoic orogenic gold endowment and crustal evolution of the south central Andes, NW Argentina

Mineral Deposit Research: Meeting the Global Challenge, 2005

ABSTRACT Major and trace element concentrations, and Nd isotope compositions of Ordovician volcan... more ABSTRACT Major and trace element concentrations, and Nd isotope compositions of Ordovician volcano-intrusive rocks in the Puna of northwestern Argentina point to their formation in a pericratonic setting above thickened and evolved continental crust. Input from juvenile sources was limited and there is no evidence for oceanic crust or the presence of a terrane-bounding suture in the region. In contrast to analogous terrains elsewhere in the Central Andes (e.g., in Peru and Bolivia), relatively minor orogenic gold mineralisation is known to occur in the Palaeozoic succession in northwestern Argentina. Possible explanations for the lack of more substantial orogenic gold endowment include the absence of hydrated oceanic substrate in the Puna, lack of 1st-order conduits and the limited extent of crustal shortening and thickening during Ordovician orogenic events. KeywordsOrogenic gold–Puna Argentina–Re-Os–endowment–petrogenesis

A Hidden Alkaline and Carbonatite Province of Early Carboniferous Age in Northeast Poland: Zircon U-Pb and Pyrrhotite Re-Os Geochronology

The Journal of Geology, 2013

Extensive geophysical investigations in NE Poland in the 1950s and 1960s led to the discovery of ... more Extensive geophysical investigations in NE Poland in the 1950s and 1960s led to the discovery of an alkaline and carbonatite magmatic province buried under thick (600-800 m) Meso-Cenozoic cover north of the Trans-European Suture Zone, or Tornquist Line. Drilling focused on geophysical anomalies identified three intrusions in the Paleoproterozoic metasedimentary and metavolcanic rocks of the Mazowsze Domain: the Pisz gabbro-syenite massif, the Ełk syenite massif, and the small, differentiated Tajno body consisting of clinopyroxenite cumulates and syenites crosscut by carbonatite veins. Emplacement ages for these intrusions have been obtained by (1) zircon U-Pb geochronology on a gabbro from Pisz, a syenite from Ełk, and an albitite from Tajno and (2) a Re-Os model age for pyrrhotite from a Tajno carbonatite. The ages measured by both methods fall in the narrow range 354-345 Ma (Early Carboniferous: Tournaisian). This is slightly younger than the Late Devonian (380-360 Ma) Kola Peninsula alkaline and carbonatite province (20 intrusions) of NW Russia and Karelia but is of comparable age to the first manifestations of the long-lasting (∼100 m.yr.) Carboniferous to Permian magmatic event (360-250 Ma) manifest in northern Europe (from the British Isles to southern Scandinavia, the North Sea, and northern Germany) in the foreland of the Variscan orogeny (in the so-called West European Carboniferous Basin) and the East European Craton.

The north-eastern Polish anorthosite massifs: petrological, geochemical and isotopic evidence for a crustal derivation

Terra Nova, 2002

Corrigendum to “Results of pilot Re–Os dating of sulfides from the Sukhoi Log and Olympiada orogenic gold deposits, Russia”[Ore Geol. Rev. (2014) 21–28]

Ore Geology Reviews, 2014

We regretfully acknowledge three errors of omission and incorrect figures. First, a critical refe... more We regretfully acknowledge three errors of omission and incorrect figures. First, a critical reference was inadvertently omitted . The U-Pb monazite and zircon data from their Sukhoi Log study support the Re-Os modeling we performed in our pilot study. That is, a U-Pb age of 486 ± 18 for monazite recrystallization (see their Fig. ), provides credence to calculated Re-Os ages. Given the data we acquired in our study, it is not possible to surmise the initial Os isotope ratio. We emphasize that a small change in the initial Os ratio creates a large change in the calculated Re-Os age. The initial 187 Os/ 188 Os ratios for both Sukhoi Log and Olympiada, 0.55 and 0.75, respectively, are based only on convergence of age results on the initial value and the likely preservation of Re-Os systematics through sulfide-stable metamorphism. These two different 187 Os/ 188 Os ratios would reflect 100-200 m.y. of radiogenic Os in-growth during ocean basin closure. In this case, the two 187 Ore Geology Reviews 59 (2014) 152-153

Results of pilot Re–Os dating of sulfides from the Sukhoi Log and Olympiada orogenic gold deposits, Russia

Ore Geology Reviews, 2014

The pilot study with Re-Os dating of sulfides from Sukhoi Log and Olympiada gold deposits reveale... more The pilot study with Re-Os dating of sulfides from Sukhoi Log and Olympiada gold deposits revealed early Paleozoic ages of the auriferous sulfides from the two largest orogenic gold systems in the Neoproterozoic orogens of the Baikalides framing the Siberian craton. The age-dating results indicate that gold mineralization is therefore epigenetic. The formation of the dated orogenic gold deposits is synchronous with some regional metamorphic events in the Baikalides, at least in case of the Sukhoi Log deposit. The metamorphic events occurred in the rear parts of the early Paleozoic magmatic arcs, where coeval subduction-related magmatism produced porphyry copper-(molybdenum) mineralization.

Re–Os molybdenite dating of granite-related Sn–W–Mo mineralisation at Hnilec, Gemeric Superunit, Slovakia

Mineralogy and Petrology, 2005

Re-Os molybdenite ages from the exocontact of the Hnilec granite-greisen body provide temporal co... more Re-Os molybdenite ages from the exocontact of the Hnilec granite-greisen body provide temporal constraints for tin, tungsten and molybdenite mineralisation in the Gemeric Superunit, Slovakia. Two molybdenite separates were taken from a representative sample of the Sn-W-Mo mineralisation at Hnilec and their Re-Os ages of 262.2 AE 0.9 and 263.8 AE 0.8 Ma (2-sigma) are in excellent agreement. The obtained Re-Os molybdenite ages are similar to recent but less precise electron microprobe monazite (276 AE 13 Ma) and U-Pb single zircon (250 AE 18 Ma) ages from the Hnilec granite intrusion, supporting a granite-related greisen origin for the Sn-W-Mo mineralisation. Our precise Re-Os molybdenite ages resolve the long time controversy over the timing of high-temperature mineralisation in the Gemeric Superunit. These Permian ages eliminate suggestions of an Alpine age. The sulphur isotope composition of the studied molybdenite is 34 S (CDT) ¼ 1.71 AE 0.2ø and is consistent with a magmatic sulphur source. Field observations indicate the lack of a broad contact aureole in the vicinity of the Hnilec granite body. Shallow level granite emplacement in schistose host rocks was accompanied by alteration and formation of tin-tungsten greisen in the upper part of the granite and exocontact molybdenite mineralisation, both commonly lacking in other granite bodies within the Gemeric Superunit.

Geology and genesis of Variscan porphyry-style gold mineralization, Petr�?kova hora deposit, Bohemian Massif, Czech Republic

Mineralium Deposita, 2001

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohem... more A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The PetraÂ cÏ kova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the PetraÂ cÏ kova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small PetraÂ cÏ kova hora granodiorite stock. Gold is found mainly as free grains of >900 ®neness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the PetraÂ cÏ kova hora deposit has been dated by the Re±Os method at 344.42.8 Ma. Hydrothermal alteration in the PetraÂ cÏ kova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silici®cation are the oldest hydrothermal phases. These were followed by early quartz veins (Q 1 to Q 4 ) that contain most of the gold mineralization. Late quartz veins (Q 5 ) and fracture-controlled silici®cation are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower d 18 O whole rock values for PetraÂ cÏ kova hora

The geology and Re–Os geochronology of the Palaeoproterozoic Vaikijaur Cu–Au–(Mo) porphyry-style deposit in the Jokkmokk granitoid, northern Sweden

Mineralium Deposita, 2005

The Vaikijaur Cu-Au-(Mo) deposit is located in the ca. 1.88 Ga calc-alkaline Jokkmokk granitoid n... more The Vaikijaur Cu-Au-(Mo) deposit is located in the ca. 1.88 Ga calc-alkaline Jokkmokk granitoid near the Archaean-Proterozoic palaeoboundary within the Fennoscandian shield of northern Sweden. The Skellefte VMS district lies immediately to the south and the northern Norrbotten Fe-oxide-Cu-Au deposits to the north. The Vaikijaur deposit occupies an area of 2•3 km within the Jokkmokk granitoid and includes stockwork quartz-sulphide veinlets and disseminated chalcopyrite, pyrite, gold, molybdenite, magnetite, and pyrrhotite. Porphyritic mafic dykes were emplaced along fractures in a ring dyke pattern. The Jokkmokk granitoid, dykes, and the mineralized area are foliated, indicating that mineralization predated the main regional deformation. The mineralized area is characterized by strong potassic alteration. Phyllic and propylitic alteration zones are also present. A pyrite-rich inner core is surrounded by a concentric zone with pyrite, chalcopyrite, and gold. Molybdenite is distributed irregularly throughout the chalcopyrite zone. Geophysical data indicate a strongly conductive central zone in the mineralized area bordered by conductive and high magnetic zones. Five high precision Re-Os age determinations for three molybdenite occurrences from outcrop and drill core samples constrain the age of porphyry-style Cu-Au-(Mo) mineralization to between 1889±10 and 1868±6 Ma. A younger molybdenite is associated with a much later metamorphic event at about 1750 Ma. These data suggest that primary porphyry-style mineralization was associated with calcalkaline magmatism within the Archaean-Proterozoic boundary zone at ca. 1.89-1.87 Ga.

Re?Os and Pb?Pb geochronology of the Archean Salobo iron oxide copper?gold deposit, Caraj�s mineral province, northern Brazil

Mineralium Deposita, 2003

Rhenium-osmium ages were determined for two molybdenite samples and a Pb-Pb age was derived from ... more Rhenium-osmium ages were determined for two molybdenite samples and a Pb-Pb age was derived from bornite-chalcopyrite-magnetite at the Salobo iron oxide copper-gold deposit to determine the timing of mineralization and its relation to the nearby Old Salobo Granite. Rhenium-osmium dating of molybdenite spatially associated with copper sulfide minerals yields ages with weighted means of 2576±8 and 2562±8 Ma. Removing the error multiplier introduced by the decay constant uncertainty, appropriate for comparing ages from the same isotopic system, these data convincingly argue for two temporally separated pulses of molybdenite deposition at 2576.1±1.4 Ma (n=2) and 2561.7±3.1 Ma (n=3). The 2576±8 Ma age coincides with a previously published U-Pb age of 2573±2 Ma for the Old Salobo Granite, suggesting that main stage ore formation may have been contemporaneous with granite magmatism. The slightly younger 2562 Ma age most likely represents new molybdenite precipitation associated with the development or reactivation of local shear zones. Lead-lead stepwise leaching of copper sulfide minerals yields a less precise isochron age of 2579±71 Ma, and supports an Archean age for the Salobo ores. This is the first documentation of an Archean iron oxide copper-gold deposit, and the Re-Os and Pb-Pb geochronology herein support 2580-2550 Ma estimates for basement reactivation and regional granite magmatism associated with the development of brittle-ductile shear zones.

Mineralization and deformation of the Malanjkhand terrane (2,490–2,440 Ma) along the southern margin of the Central Indian Tectonic Zone

Mineralium Deposita, 2005

The Malanjkhand Cu-Mo-Au deposit, located near the northwest margin of the Malanjkhand batholith ... more The Malanjkhand Cu-Mo-Au deposit, located near the northwest margin of the Malanjkhand batholith (terrane), is a strategic and significant porphyry-style deposit that experienced a protracted 50 m.y. deformational history shortly after its formation at 2,490±8 Ma ). In a recent study, averaged U-Pb SHRIMP zircon data from a pooled set of samples from the Malanjkhand batholith to advocate a meaningless intermediate age of $2,476 Ma for the Malanjkhand granitoid and its Cu-Mo-Au deposit. In the northwest part of the Malanjkhand batholith, Re-Os dating of occurrence-specific molybdenite captures not only the age of porphyry-style mineralization and associated magmatism, but also elucidates a complex deformational history that extends to $2,450 Ma. In the central part of the Malanjkhand batholith, Re-Os dating of delicate spindles of accessory molybdenite occurring with pristine muscovite in miarolitic cavities within the undeformed microgranitoid at the Devgaon Mo prospect unequivocally shows that deformation ceased at this location no later than 2,470-2,465 Ma. The deformational history recorded at the Malanjkhand deposit in the northwest most likely reflects prolonged transpressive convergence and docking of the Malanjkhand terrane with units in the poorly understood (proto) Central Indian Tectonic Zone (CITZ) along its southern margin, the Central Indian shear zone. The timing for this convergence is Late Archean-Early Paleoproterozoic.

U–Pb, Re–Os, and 40Ar/39Ar geochronology of the Nambija Au-skarn and Pangui porphyry Cu deposits, Ecuador: implications for the Jurassic metallogenic belt of the Northern Andes

Mineralium Deposita, 2008

New U-Pb, Re-Os, and 40 Ar/ 39 Ar dates are presented for magmatic and hydrothermal mineral phase... more New U-Pb, Re-Os, and 40 Ar/ 39 Ar dates are presented for magmatic and hydrothermal mineral phases in skarn-and porphyry-related ores from the Nambija and Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently discovered porphyry Cu-Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type, subduction-related, Jurassic Zamora batholith, which intrudes Triassic volca-nosedimentary rocks. The Zamora batholith is in turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High precision U-Pb and Re-Os ages for porphyritic stocks (U-Pb, zircon), associated prograde skarn (U-Pb, hydrothermal titanite), and retrograde stage skarn (Re-Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization at Nambija was considered to be Early Tertiary based on K-Ar ages

Regional setting and geochronology of the Late Cretaceous Banatitic Magmatic and Metallogenetic Belt

Mineralium Deposita, 2002

The 1,500-km-long Banatitic Magmatic and Metallogenetic Belt (BMMB) of Romania, Serbia and Bulgar... more The 1,500-km-long Banatitic Magmatic and Metallogenetic Belt (BMMB) of Romania, Serbia and Bulgaria is a complex calc-alkaline magmatic arc of Late Cretaceous age. It hosts a variety of magmatic-hydrothermal Cu, Au, Mo, Zn, Pb and Fe deposits, including Europe's only world-class porphyry-copper deposits. Regional metallogeny can be linked to subduction of the Vardar Ocean during the Late Cretaceous, as part of the closure of the Neotethys Ocean that had separated Europe and Africa in the Mesozoic. Porphyry Cu-(Au)-(Mo) and intimately associated epithermal massive sulphides dominate in the central segments of the belt in southernmost Banat (Romania), Serbia and north-west Bulgaria. These districts are the economically most important today, including major active Cu-Au mines at Moldova Noua˘in Romania, Majdanpek, Veliki Krivelj and Bor in Serbia, and Elatsite, Assarel and Chelopech in Bulgaria. More numerous (and mostly mined in the past) are Fe, Cu and Zn-Pb skarns, which occur mainly at the two ends of the belt, in Eastern Bulgaria and in Romania. This paper summarises some of the deposit characteristics within the geodynamic framework of terminal Vardar subduction. Heterogeneous terranes of the belt, including the Apuseni Mountains at the western end, are aligned parallel to the Vardar front following continental collision of the Dacia and Tisza blocks. All available geochronological data (numerous K-Ar and some U-Pb and Re-Os ages) are compiled, and are complemented by a new high-precision Re-Os date for the Dognecea skarn deposit, south-west Romania (76.6±0.3 Ma). These data indicate that magmatism extended over at least 25 million years, from about 90 to 65 Ma in each segment of the belt. Within Apuseni Mountains and Banat, where magma emplacement was related to syn-collisional extension in the orogenic belt of Carpathians, ore formation seems to be restricted in time and maybe constrained by a shared tectonic event.

Comment on the paper by Bernd Lehmann: Molybdenum distribution in Precambrian rocks of the Colorado mineral belt

Mineralium Deposita, 1988

Lehmann is to be congratulated on producing an extensive data set showing molybdenum concentratio... more Lehmann is to be congratulated on producing an extensive data set showing molybdenum concentrations for a wide compositional range of Proterozoic rock samp!es from the Colorado Mineral Belt . Although some Precambrian batholithic and pegmatitic rocks have molybdenum concentrations greater than average crustal values of 1 to 2 ppm, these are balanced by molybdenum concentrations of less than 1 ppm in metasedimentary and metavolcanic units. Lehmann correctly points out that these molybdenum concentrations cannot be used to argue for an initially Mo-enriched Proterozoic basement in Colorado. We disagree, however, with Lehmann's conclusions relating molybdenum concentrations in surface outcrops of Precambrian rocks to the origin of Climax-type granites and their associated porphyry molybdenum deposits in the Colorado Mineral Belt. Lehmann attributes economic molybdenum concentrations in Climax-type granites solely to magmatic differentiation and late-stage vapor evolution following "partial melting of average lower crustal rocks." Presumably, Lehmann's use of the word "average" refers to the lack of a molybdenum enrichment in the source for Climax-type granites. While his proposed "average" lower crustal source may be quite reasonable, we do not feel that Lehmann's data necessarily characterize unexposed crustal rocks at depth beneath the Colorado Mineral Belt. Furthermore, his data do not answer the question, "Are Climax-type porphyry molybdenum deposits derived from a source which is enriched in molybdenum?" Lehmann bases his conclusions on the assumption that the exposed Proterozoic crust in the Colorado Mineral Belt offers a "window into the unexposed portions of the crust/mantle segment." He contends that the exposed Precambrian rocks "are probably representative of the substratum of the mineral belt." We disagree strongly with these assumptions. The base of the 1750 to 1950 Ma Idaho Springs sequence of metasedimentary and metavolcanic rocks is nowhere exposed in the Colorado Mineral Belt or adjacent areas. Furthermore, at least two Proterozoic orogenic events produced copious, crustally derived melts (Silver Plume and Pikes Peak Batholiths), and each must have left a refractory residue in the lower crust . Low initial ~Sr/86Sr for both the Silver Plume and Pikes Peak Batholiths preclude their derivation from the more radiogenic Idaho Springs Formation (Peterman and Hedge 1968). Rather, Protero-