Porphyritic Monzonite Research Articles

Geochemistry and Geochronology of the Huangcha Pluton and Tectonic Significance

The Zanhuang Complex is situated on the eastern margin of the Trans-North China Orogen, with the Huangcha Pluton being a constituent of this complex. To ascertain the nature of the approximately 2.5-billion-year-old Huangcha Pluton, crucial evidence for understanding its extensional setting was sought through petrogenesis and dating investigations. LA-ICP-MS dating of zircon from the granite yielded an age of (2488 ± 6) Ma. Primarily composed of porphyritic monzonite with sporadic melanocratic enclaves, the Pluton’s phenocrysts are predominantly feldspar with minor quartz. The granite exhibits high SiO2 content (72.64%–74.16%) and alkali levels, with Na2O + K2O ranging from 7.59% to 9.07%, classifying it as a shoshonitic series with a slightly peraluminous feature. Enrichment in large-ion lithophile (LIL) elements (Rb, Th, and U) and depletion in Sr, V, Cr, Co, and Ni were observed, with high Rb/Sr and Ga/Al ratios ranging from 0.73 to 2.72 and 2.75 × 10−4 to 3.11 × 10−4, respectively. The rock exhibits high εNd(t) values, ranging from −0.06 to 0.88, with TDM2 ages falling between 2.79 and 2.87 billion years. Zircon grains display 176Hf/177Hf ratios ranging from 0.281266 to 0.281412 and εHf(t) values spanning from 0.96 to 6.18, calculated using the 207Pb/206Pb age. It is suggested that the Huangcha Pluton represents A-type granite formed via anatexis of the Neoarchean TTG in an extensional setting following orogenic processes. The formation of the Huangcha Pluton further corroborates the stabilization of the North China Craton towards the end of the Neoarchean. This finding supports the hypothesis that the North China Craton may belong to the Rae-family cratons, sharing similar magmatic and tectono-metamorphic records around ~2.5 billion years ago.

Formation mechanism of skarn iron deposit: Evidence from Fe-rich enclave in porphyritic monzonite

Skarn deposits constitute a significant reservoir of high-grade iron (Fe) ores in China, yet the metallogenic processes remain debated. This study focuses on the iron-rich enclaves embedded within porphyritic monzonite, aiming to unravel the metallogenic mechanisms underlying skarn Fe deposits. The host porphyritic monzonite intrudes into porphyritic diorite. Within the iron-rich enclaves three types of magnetite (Mt) are discerned. Mt-I, ranging from 50 to 100 μm in size, predominantly occupies the core of the enclave, enshrouding diopside at its center. Mt-II, with a size range of 20–40 μm, is primarily distributed in the middle of the enclave. The smallest variant, Mt-III, measuring approximately 1 μm, typically manifests at the peripheries of Mt-I and Mt-II. Comparative analysis reveals that Mt-II exhibits higher total rare earth elements (REE), Sr, Ca, alongside lower TiO2 content in contrast to Mt-I. Notably, Mt-II displays a characteristic W-type tetrad effect of REE, indicative of crystallization in a fluid-saturated environment. In contrast, Mt-III, characterized by microcrystalline magnetite, is inferred to have formed under conditions of rapid cooling.Our interpretation posits a two-stage genesis for these enclaves. The initial stage involves the formation of iron-rich magma within a magma chamber situated at depths of 8–10 km, during which Mt-I crystallizes. Subsequently, the second stage unfolds as mantle-derived fluids infiltrate the magma chamber, leading to the formation of Mt-II. The fluid overpressure within the magma chamber triggers a swift ascent of iron-bearing melt-fluid, localized at depths ranging from 1.5 to 2.6 km, resulting in the crystallization of Mt-III. Our results provide valuable insights into the metallogenic processes governing skarn Fe deposits, and the complex geological evolution of these deposits.

Zircon Petrochronology of the Kışladaǧ Porphyry Au Deposit (Turkey)

Abstract Porphyry deposits typically occur in subduction-related arcs but have more recently also been described in postsubduction, collisional to extensional back-arc settings. These different tectonic environments not only might imply different genetic processes but also seem to result in different metal endowments (e.g., Au rich versus Cu rich). It is therefore relevant, also for exploration purposes, to understand the magmatic processes involved in porphyry formation in these different tectonic environments. This study focuses on the Kışladaǧ porphyry Au (17.4 Moz) deposit in western Anatolia, which is centered on a series of porphyritic monzonite stocks of high-K calc-alkaline to shoshonitic affinity and formed in a continental rifting environment. With 17.4 Moz of Au, Kışladaǧ is of global metallogenic importance and hence a good example for studying the genetic processes associated with porphyry deposits in extensional back-arc settings. We herein combine a comprehensive set of new zircon textural observations, in situ zircon trace element and Hf isotope data, and previously published zircon geochronology to study the magmatic processes associated with porphyry deposit formation at Kışladaǧ. We show that mafic rejuvenation of a slowly crystallizing (between ~15.8 and 14.9 Ma) magma reservoir below Kışladaǧ immediately preceded porphyry deposit formation. Zircon trace elements and geochronology suggest a longer and deeper evolution for the early fertile magmas compared to the later infertile magmas. Magma evolution at Kışladaǧ was accompanied by crustal wall-rock assimilation. Whole-rock Nd and Sr radiogenic isotopes show that increasing asthenosphere-derived melt input under accelerated regional extension caused a loss in fertility of the system over time.

Platinum-group minerals in the Skouries Cu-Au (Pd, Pt, Te) porphyry deposit

The Skouries deposit is a platinum-group element (PGE) enriched Cu-Au porphyry system located in the Chalkidiki peninsula, Greece, with associated Ag, Bi and Te enrichment. The deposit is hosted by multiple porphyritic monzonite and syenite intrusions, which originated from a magma chamber at depth. An initial quartz monzonite porphyritic intrusion contains a quartz–magnetite ± chalcopyrite–pyrite vein stockwork with intense potassic alteration. The quartz monzonite intrusion is cross cut by a set of syenite and mafic porphyry dykes and quartz–chalcopyrite–bornite ± magnetite veins which host the majority of the Cu and Au mineralisation. Late stage quartz–pyrite veins, with associated phyllic alteration crosscut all previous vein generations. Electron microprobe and scanning electron microscopy shows that the PGE are hosted by platinum-group minerals (PGM) in the quartz-chalcopyrite–bornite ± magnetite veins and within potassic alteration assemblages. The PGE mineralisation in Skouries is therefore part of the main high temperature hypogene mineralisation event. Platinum-group minerals at Skouries include: sopcheite [Ag4Pd3Te4], merenskyite [(Pd,Pt)(Te,Bi)2] and kotulskite [Pd(Te,Bi)], with rare telargpalite [(Pd,Ag)3Te], isomertieite [Pd11Sb2As2], naldrettite [Pd2Sb], testibiopalladite [PdTe(Sb,Te)] and sobolevskite [PdBi]. The most common platinum-group mineral is sopcheite. The PGM in Skouries are small, 52 µm2 on average, and occur as spherical grains on the boundaries between sulphides and silicates, and as inclusions within hydrothermal quartz and sulphides. These observations support a “semi-metal collector model” whereby an immiscible Bi-Te melt acts as a collector for PGE and other precious metals in high temperature hydrothermal fluids. This mechanism would allow the formation of PGM in porphyries without Pt and Pd fluid saturation.

Petrogenesis and timing of emplacement of porphyritic monzonite, dolerite, and basalt associated with the Kuoerzhenkuola Au deposit, Western Junggar, NW China: implications for early Carboniferous tectonic setting and Cu–Au mineralization prospectivity

ABSTRACTThe Kuoerzhenkuola epithermal Au deposit is located in the northern part of the West Junggar region of NW China and is underlain by a recently discovered porphyritic monzonite intrusion that contains Cu–Au mineralization. Zircon LA-ICP-MS U–Pb dating of this intrusion yielded an age of 350 ± 4.7 Ma. The porphyritic monzonite is calc-alkaline and is characterized by high concentrations of Sr (583–892 ppm), significant depletions in the heavy rare earth elements (HREE; e.g. Yb = 0.96–2.57 ppm) and Y (10.4–23.3 ppm), and primitive mantle-normalized multi-element variation diagram patterns with positive Sr and Ba and negative Nb and Ti anomalies, all of which indicate that this intrusion is compositionally similar to adakites elsewhere. The composition of the porphyritic monzonite is indicative of the derivation from magmas generated by the melting of young subducted slab material. The area also contains Nb-enriched basalts that are enriched in sodium (Na2O/K2O = 1.20–3.90) and have higher Nb, Zr, TiO2, and P2O5 concentrations and Nb/La and Nb/U ratios than typical arc basalts. The juxtaposition of adakitic rocks, Nb-enriched basalts, and dolerites in this region suggests that the oceanic crust of the expansive oceans within the West Junggar underwent early Carboniferous subduction. Magnetite is widespread throughout the Kuoerzhenkuola Au deposit, as evidenced by the volcanic breccias cemented by late hydrothermal magnetite and pyrite. In addition, the zoned potassic, quartz-sericite alteration, and propylitic and kaolin alteration in the deeper parts of the porphyritic monzonite are similar to those found in porphyry Cu–Au deposits. These findings, coupled with the mineralogy and geochemistry of the alteration associated with the Kuoerzhenkuola Au deposit, suggest that the mineralization in this area is not purely epithermal, with the geology and geochemistry of the porphyritic monzonite in this area suggesting that a porphyry Cu–Au deposit is probably located beneath the Kuoerzhenkuola Au deposit.

Geology, geochronology, and Hf isotope geochemistry of the Longtougang skarn and hydrothermal vein Cu–Zn deposit, North Wuyi area, southeastern China

The recently discovered Longtougang skarn and hydrothermal vein Cu–Zn deposit is located in the North Wuyi area, southeastern China. The intrusions in the ore district comprise several small porphyritic biotite monzonite, porphyritic monzonite, and porphyritic granite plutons and dikes. The mineralization is zoned from a lower zone of Cu-rich veins and Cu–Zn skarns to an upper zone of banded Zn–Pb mineralization in massive epidote altered rocks. The deposit is associated with skarn, potassic, epidote, greisen, siliceous, and carbonate alteration. Molybdenite from the Cu-rich veins yielded a Re–Os isochron age of 153.6±3.9Ma, which is consistent with U–Pb zircon ages of 154.0±1.3Ma for porphyritic monzonite, 154.0±0.8Ma for porphyritic biotite monzonite, and 152.0±0.8Ma for porphyritic granite. Geological observations suggest that the Cu mineralization is genetically related to the porphyritic biotite monzonite and porphyritic monzonite. All the zircons from intrusive rocks in the ore district are characterized by εHf(t) values between −13.41 and −4.38 and Hf model ages (TDM2) between 2054 and 1482Ma, reflecting magmas derived mainly from a Proterozoic crustal source. Molybdenite grains from the deposit have Re values of 14.6–27.7ppm, indicative of a mixed mantle–crust source. The porphyry–skarn abundant Cu and hydrothermal vein type Pb–Zn–Ag deposits in the North Wuyi area are related to the Late Jurassic porphyritic granites and Early Cretaceous volcanism, respectively. The Late Jurassic mineralization-related granites were derived from the crustal anatexis with some mantle input, which was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo-Pacific plate beneath the South China block, and the Early Cretaceous mineralization-related granitoids mainly from crust material formed within a series of NNE-trending basins during margin-parallel movement of the plate.

SHRIMP zircon U–Pb and biotite and hornblende Ar–Ar geochronology of Sungun, Haftcheshmeh, Kighal, and Niaz porphyry Cu–Mo systems: evidence for an early Miocene porphyry-style mineralization in northwest Iran

The Cenozoic Urumieh–Dokhtar magmatic belt (UDMB) extends for over 2,000 km from northwest to southeast Iran and is characterized by dominantly calc-alkaline volcanic, pyroclastic, and intrusive rocks. The UDMB hosts numerous porphyry-type Cu ± Mo deposits, mostly distributed in two separate areas, one known as the Kerman copper belt (KCB) in the south, and the other, here referred to as the Arasbaran Metallogenic Zone (AMZ), in the north, of the UDMB. The two areas are represented by two world-class Cu–Mo deposits, Sarcheshmeh (1,200 Mt of ore at 0.69 % Cu and 300 ppm Mo) and Sungun (>500 Mt of ore at 0.69 % Cu and ~250 ppm Mo), respectively. Chronology data were obtained for the Sungun, Haftcheshmeh, Kighal, and Niaz deposits in the AMZ. The Sungun deposit is associated with a suite of porphyritic granodiorite to monzodiorite stocks and late dykes intruding older andesitic lavas and limestones. SHRIMP zircon U–Pb data indicate that the host andesites were emplaced at 27.65 ± 0.51 Ma (±0.2σ). The main Sungun porphyritic intrusion crystallized at 20.69 ± 0.37 (±0.2σ) Ma. The Haftcheshmeh deposit is associated with a porphyritic granodiorite body intruding an older gabbro-diorite intrusion. Primary magmatic hornblende from the gabbro-diorite host rock yielded a 40Ar/39Ar plateau age of 27.47 ± 0.17 Ma. The main porphyritic intrusion crystallized at 19.46 ± 0.39 Ma. The Kighal porphyry system is associated with a porphyritic monzonite body intruding into older andesitic and dacitic lavas, and the Niaz porphyry system is associated with a porphyritic granodiorite stock cutting through an older monzodiorite intrusion. For the Kighal and Niaz, secondary biotite concentrates collected from potassic alteration zones in the parent porphyritic bodies yielded plateau ages of 20.1 ± 1.8 and 22.14 ± 0.13 Ma, respectively. The timing of the porphyritic intrusions and the associated mineralizations in the AMZ is considerably older than that in KCB in southern UDMB (14–7 Ma). There is evidence that the AMZ formed on a separate arc related to the southern lesser Caucasus subduction zone. The arc extends from northwest Iran to Armenia and Azerbaijan to the north. Similar deposits of the same age have been discovered in Armenia, exemplified by Kajaran and Dastakert.

Zircon U–Pb geochronology, geochemistry, and Sr–Nd–Hf isotopes of granitoids in the Yulekenhalasu copper ore district, northern Junggar, China: Petrogenesis and tectonic implications

The Yulekenhalasu porphyry copper deposit is located in the Kalaxiange'er metallogenic belt in northern Junggar, China. We present the results from zircon U–Pb geochronology, and geochemical and Sr–Nd–Hf isotope analyses of the granitoids associated with the ore deposits with a view to constrain their petrogenesis and tectonic setting. The granitoids consist of quartz diorite, diorite porphyry, porphyritic monzonite, and quartz porphyry, emplaced at 382, 379, 375–374, and 348Ma, respectively, which span Late Devonian to early Carboniferous ages. The ore-bearing intrusion is mainly diorite porphyry, with subordinate porphyritic monzonite. The Late Devonian intrusions are characterized by SiO2 contents of 54.5–64.79wt.%, Na2O contents of 3.82–8.24wt.%, enrichment in Na, light rare-earth elements (LREEs), and large ion lithophile elements. They also display relative depletion in Y, Ba, P, Nb, Ta, and Ti, and weak negative Eu anomalies (δEu=0.6–0.87). The early Carboniferous quartz porphyry is characterized by high SiO2 content (72.26–73.35wt.%), enrichment in LREEs, K, and Sr, and relative depletion in Y (10.82–12.52ppm) and Yb (1.06–1.15ppm). The Late Devonian and early Carboniferous granitoids are characterized by positive εNd(t) values (5.2–10.1, one sample at −1.9), positive εHf(t) values (7.46–18.45), low (87Sr/86Sr)i values (0.70363–0.70476), and young crustal residence ages. These data indicate that the sources of the granitoids were mainly mantle-derived juvenile rocks. Geochemical and Nd–Sr–Hf isotopic data demonstrate that the Late Devonian granitoids formed in an oceanic island arc, and they were formed from different sources, among which the mineralized diorite porphyry might have originated from a mixed slab-derived and mantle wedge melt source. The early Carboniferous quartz porphyry was likely emplaced in a mature island arc environment, and was probably derived from juvenile crust.

SHRIMP U–Pb and Ar–Ar geochronology of major porphyry and skarn Cu deposits in the Balkhash Metallogenic Belt, Central Asia, and geological implications

The Balkhash Metallogenic Belt (BMB) in Kazakhstan, Central Asia, with the occurrence of the super-large Kounrad and Aktogai, the large Borly porphyry Cu–Mo deposits, and the large Sayak skarn polymetallic ore-field, is one of the central regions of the Paleozoic Central Asian metallogenic domain and orogenic belt. In this study, newly obtained SHRIMP zircon U–Pb ages of nine samples and 40Ar/39Ar ages of six mineral samples (inclding hornblende, biotite and K-feldspar) give more detailed constraints on the timing of the granitic intrusions and their metallogeny. Porphyritic monzonite granite and tonalite porphyry from the Kounrad deposit yield U–Pb zircon SHRIMP ages of 327.3±2.1Ma and 308.7±2.2Ma, respectively. Quartz diorite and porphyritic granodiorite from the Aktogai deposit yield U–Pb SHRIMP ages of 335.7±1.3Ma and 327.5±1.9Ma, respectively. Porphyritic granodiorite and granodiorite from the Borly deposit yield U–Pb SHRIMP ages of 316.3±0.8Ma and 305±3Ma, respectively. Diorite, granodiorite, and monzonite from the Sayak ore-field yield U–Pb SHRIMP ages of 335±2Ma, 308±10Ma, and 297±3Ma, respectively. Hornblende, biotite, and K-feldspar from the Aktogai deposit yield 40Ar/39Ar cooling ages of 310.6Ma, 271.5Ma, and 274.9Ma, respectively. Hornblende, biotite, and K-feldspar from the Sayak ore-field yield 40Ar/39Ar cooling ages of 287.3±2.8Ma, 307.9±1.8Ma, and 249.8±1.6Ma, respectively. The new ages constrain the timing of Late Paleozoic felsic magmatism to ∼336 to ∼297Ma. Skarn mineralization in the Sayak ore-field formed at ∼335 and ∼308Ma. Porphyry Cu–Mo mineralization in the Kounrad deposit and the Aktogai deposit formed at ∼327Ma, and in the Borly deposit at ∼316Ma. The Late Paleozoic regional cooling in the temperature range of ∼600°C to ∼150°C occurred from ∼307 to ∼257Ma.

Indosinian isotope ages of plutons and deposits in southwestern Miaoershan-Yuechengling, northeastern Guangxi and implications on Indosinian mineralization in South China

The Miaoershan-Yuechengling complex pluton is the largest granitoid complex in the western Nanling metallogenic belt with a surface exposure of >3000 km2. The complex pluton is composed of an early stage granitoid batholith and late stage small intrusions. The early stage batholith contains mainly medium-grained porphyritic mica granite and porphyritic monzonite granite, whereas the late stage intrusions are composed of muscovite granite porphyry and muscovite monzonitic granite. There are many W-Sn-Mo-Pb-Zn-Cu ores in the contact zone between the batholith and strata, forming an ore-rich belt around the batholith. Based on zircon LA-ICP-MS U-Pb ages, the southwestern part of the early stage batholith formed at 228.7 ± 4.1 Ma (MSWD = 2.49), with slightly earlier magmatic activity at 243.0 ± 5.8 Ma (MSWD = 2.62). The Yuntoujie muscovite granite was associated with W-Mo mineralization and has a zircon LA-ICP-MS U-Pb age of 216.8 ± 4.9 Ma (MSWD = 1.44). The Re-Os isochron age of molybdenite from the Yuntoujie W-Mo ore was 216.8 ± 7.5 Ma (MSWD = 11.3). Our new isotope data suggest that the late stage intrusive stocks from the southwestern Miaoershan-Yuechengling batholith were closely associated with W-Mo mineralization from the Indosinian period. These new results together with previous isotope data, suggest that South China underwent not only the well-known Yanshanian mineralization event, but also a widespread Indosinian metallogenic event during the Mesozoic period. Therefore, South China has a greater potential for Indosinian mineralization than previously thought such that more attention should be given to the Indosinian ore prospecting in South China.

Archean Karari gold deposit, Eastern Goldfields Province, Western Australia: a monzonite-associated disseminated gold deposit

The Karari gold deposit is situated in the Carosue sedimentary basin, 110 km northeast of Kalgoorlie, in the Archean Eastern Goldfields Superterrane of Western Australia. The Carosue basin is a late-tectonic accumulation of volcaniclastic sedimentary rocks that unconformably overlies a deformed granite–greenstone association. The sedimentary basin is intruded by numerous plutons and dykes of monzonite, lamprophyre and syenite and is cut by a swarm of post-intrusion faults with north–south orientations. Gold mineralisation at Karari occurs in a fault-bound zone of volcaniclastic sedimentary rocks that are intruded by monzonite porphyry and lamprophyre dykes. The hangingwall of the central mineralised zone is formed by the eastern intrusive complex, a porphyritic monzonite unit intruded by numerous dykes of monzonite porphyry, syenite porphyry and lamprophyre. The eastern intrusive complex is characterised by widespread potassic alteration and contains minor low-grade copper mineralisation. In the Karari pit, gold is associated with W and As, whereas Ag, Bi, Cu, Mo, Pb, Te and Zn form spatially distinct anomalous zones in the eastern intrusive complex and associated bounding faults. The central mineralised zone is interpreted as a downfaulted, higher-level exposure of the magmatic system represented by the eastern intrusive complex. Gold lodes are steep tabular zones of sodic alteration within a more extensive area of potassic alteration. Sodic alteration zones contain numerous veins and veinlets, which contain a variety of assemblages, several of which are mutually overprinting. Hematite occurs as a dusting in fine-grained albite and carbonate in the sodic alteration zones but is interpreted as a later (post-gold) event. Modelling using Hch software suggests that potassic alteration and low-grade copper mineralisation were caused by a high-temperature, saline fluid, probably derived from magmas of the eastern intrusive complex. The sodic alteration assemblage at Karari could not be duplicated but the results of other workers show that sodic alteration could have formed by reaction of quartzo-feldspathic rocks with a mesothermal, low-salinity H2O–CO2 fluid. The data and observations described in this paper do not permit an unequivocal distinction between orogenic and orthomagmatic models for the gold mineralisation.

Tectonic implications of 40Ar/39Ar muscovite dates from the Mt. Haley stock and Lussier River stock, near Fort Steele, British Columbia

The Mt. Haley and Lussier River stocks are located northeast of Cranbrook, B.C. near the south end of the Western Main Ranges of the Southern Canadian Rocky Mountains. Both are multiphase, potassium-feldspar porphyritic monzonite plutons that intrude lower Paleozoic miogeoclinal strata. They crosscut and thermally overprint the Lussier River fault and the thrust and fold structures in the east flank of the Purcell anticlinorium and the west limb of the Porcupine Creek anticlinorial fan structure. Muscovite from the Mt. Haley stock yielded a 40Ar/39Ar plateau age of 108.2 ± 0.7 Ma (2σ), and a single-crystal, step-heating analysis of muscovite from a skarn in the metamorphic aureole adjacent to the Lussier River stock gave a plateau date of 108.7 ± 0.6 Ma (2σ). These dates constrain the timing of thrusting and folding in this portion of the western Rocky Mountains and of the displacement along the Lussier River – St. Mary fault to pre-middle Albian.

Geochemistry and Petrogenesis of Kunduru Betta Calc-Alkaline Ring Complex in the Dharwar Craton, Southern India

The Kunduru Betta Ring Complex (KRC), at the southern margin of Dharwar craton, South India, comprises metaluminous sub-solvus syenites and quartz monzonite with a concentric disposition younging towards the center. An outer mafic syenite (of lamprophyric affinity) is followed by porphyritic monzonite, quartz monzodiorite and finally a quartz monzonitic stock at the centre. SiO 2, Al 2O 3 and Na 2O increase from the primitive lamprophyric mafic syenite to the quartz monzonite through the intermediate members, while CaO, MgO, Fe 2O 3 T, TiO 2, P 2O 5 and MnO show an opposite trend suggesting fractionation of hornblende, clinopyroxene, biotite, apatite, sphene, and iron oxide minerals. Rb, Th and U increase with a complementary decrease in Sc, V, Cr, Co, Cu, Sr and Ba from the outer mafic syenite to the inner quartz monzonite. Y, Zr and Hf decrease from lamprophyric mafic syenite to quartz monzodiorite and the trend is reversed in the final quartz monzonite phase. However, the suite is characterised by a compositional gap between quartz monzodiorite and quartz monzonite. Total REE gradually decrease from the mafic syenite to quartz monzonite and the REE distribution patterns show LREE-enriched and HREE-depleted parallel distributions with negligible Eu anomalies. The geochemical data suggest that the rock types were formed as products of progressive differentiation by crystal fractionation of calc-alkaline lamprophyric parent magma which was derived by partial melting of metasomatically enriched mantle in the Kabini lineament. Although the quartz monzonites conform to the trend of differentiated Kunduru Betta suite, the compositional gap between them and the quartz monzodiorite precludes their origin by simple differentiation. It is suggested that convective liquid fractionation might have resulted in the discrete body of quartz monzonite.

Stable isotope geochemistry of a porphyry-style hydrothermal system, West Silverton District, San Juan Mountains, Colorado, U.S.A.

A porphyritic monzonite (25.1 Ma) intrudes Tertiary volcanic rocks of pre- and post-San Juan Caldera age along the western margin of the Silverton Caldera and the northern side of the Sultan Mountain monzonite stock (25.9 Ma), within an area of strong sericitic to argillic alteration (25.0 Ma). These alteration facies, which are genetically related to the monzonite, are characterized by quartz-MoS 2 mineralization (Type 1), are surrounded by pyritic and propylitic facies alteration within which quartz/ base-metal sulfide (Type 2), quartz pyrite (Type 3), and barren quartz (Type 4) veining occurs. Whole-rock H- and O-isotope compositions are variable throughout the district (δD= − 148 to −72%; δ 18O= − 3.5 to +9.3%). The propylitized country rocks are strongly depleted in 18O and D but more highly altered samples have “normal” δD and δ 18O values. Coarse-grained sericites that also exhibit a very large range of δD values to −60%) with respect to biotites and clay-sized sericites that also exhibit a very large range of δD values (−137 to −98% and −139 to −84%, respectively). Type 1 veins have a very restricted range of 18O/ 16O ratios (+7.6 to 9.3%), whereas veins of Type 2 and Types 3 and 4 have very large, overlapping, δ 18O ranges of −2.7 to +12.0% and −2.8 to +9.0%, respectively. Type 1 veinlets are characterized by halite-bearing and vapour-rich types, whereas liquid-rich types (excluding halite-bearing types) are dominant in quartz and sphalerite of vein Types 2, 3, and 4. Homogenization and salinity data indicate higher temperatures and much higher salinities of quartz fluids in the central alteration zones (∼350°C, ∼40 ≡wt%NaCl) ompared with those of the peripheral pyritic and propylitic (∼60°C, ≦5 ≡wt%NaCl). The δD values for quartz fluid inclusion waters are quite variable: Type 1 (−125 to −79% 0); Type 2 (−130 to −67% 0); and Types 3 and 4 (−128 to −98% 0). A “magmatic” fluid component is indicated by the calculated composition of sericitic fluids. Of the quartz fluids, those associated with MoS 2-mineralization lie closest to the field of “Primary Magmatic Water”; those associated with the other vein types lie within a large field which ranges from the composition of nearly unexchanged, local meteoric water to compositions reflecting extensive hydrothermal exchange and mixing.