Phyllic Stage Research Articles

Ore-forming fluid characteristics and ages of the Xinan Cu-Mo deposit at Zijinshan orefield, SE China: New insights for metallogenic mechanism and exploration

The Xinan deposit, a newly discovered porphyry Cu-Mo deposit in the Zijinshan orefield, Fujian Province, exhibits distinct alteration and porphyry mineralization stages, including the potassic, propylitic, and phyllic alteration stages. The phyllic stage is further divided into molybdenite, chalcopyrite, and pyrite sub-stages. Molybdenite Re-Os model age (108.3 ± 1.1 Ma) and concomitant sericite 40Ar/39Ar plateau age (109.0 ± 6.0 Ma) indicate that the Xinan Cu-Mo deposit was formed at 109 ∼ 108 Ma. Four types of fluid inclusions have been identified at the Xinan deposit, including vapor-rich, liquid-rich, and opaque daughter mineral-bearing subtypes with minor transparent daughter mineral-bearing subtypes. Fluid inclusion microthermometry measurements reveal that the potassic alteration stage exhibits homogenization temperatures of 375–475 °C and salinities range of 4–20 wt% NaCl eqv., suggesting slight fluid boiling. Chlorite geothermometer suggests temperatures of the propylitic alteration stage range from 261 to 336 °C, averaging at 303 °C. The molybdenite, chalcopyrite, and pyrite sub-stages have homogenization temperatures of 325–400 °C, 325–350 °C, and 300–350 °C, respectively, with corresponding salinities of 2–10 wt% NaCl eqv., 4–8 wt% NaCl eqv., and 2–4 wt% NaCl eqv.. The Laser Raman spectroscopy results show that the fluid inclusions in the Xinan deposit consist of H2O with minor CO2, hematite, chalcopyrite, and halite. The evolution of the ore-forming fluids in the Xinan porphyry Cu-Mo deposit is characterized by a transition from high-temperature, low- to moderate-salinity, oxidized, and CO2-bearing conditions during the potassic alteration stage, to moderate- to high-temperature and low-salinity conditions during the propylitic alteration stage. The subsequent phyllic alteration stage, which overprints on the potassic and partial propylitic alteration stages, is marked by moderate- to high-temperature, low-salinity, and low-oxygen fugacity environments. The primary mechanisms driving mineralization in the Xinan porphyry Cu-Mo deposit are thus identified as the reduction in temperature and oxygen fugacity, coupled with CO2 escape. In contrast to the Luoboling-Jintonghu porphyry Cu-Mo deposit, the ore-forming fluid of the Xinan deposit is characterized by lower temperatures, salinity, and CO2 contents, and reduced fluid boiling. This suggests a relatively subdued porphyry-type mineralization. The lack of a distinct potassic alteration zone in the Xinan Cu-Mo deposit suggests the potential existence of a deeper, higher temperature and salinity porphyry mineralization center beneath this deposit.

Ore-forming fluid evolution of a porphyry Cu-Mo deposit coexisting with porphyry Mo systems in a post-collisional setting, Xiaokelehe, NE China

The Xiaokelehe porphyry Cu-Mo deposit in the Great Xing’an Range contains six stages of quartz-sulfide veins (V1 to V6) including potassic (V1 to V4), chlorite-illite (V5) and phyllic (V6). Two to three types of quartz were identified within each stage, of which the V2Q1, V3Q1, V4Q2, V5Q1 and V6Q2 are spatially associated with sulfides. Three types of fluid inclusions were identified in these veins, i.e., type I aqueous inclusions homogenized to liquid, type II aqueous inclusions homogenized to vapor, and type III aqueous inclusions containing halite and homogenized to liquid. Type I and type II primary inclusions occur in all stages, and type III inclusions are developed in V2 to V4 veins. Microthermometric data show that the maximum formation temperatures and pressures of V2-V6 veins of type I inclusions are 353 to 437 °C and 19.0 to 27.6 MPa, 309 to 415 °C and 15.0 to 30.1 MPa, 330 to 365 °C and 16.7 to 27.6 MPa, 243 to 351 °C and 13.9 to 18.9 MPa and 255 to 347 °C and 13.9 to 17.8 MPa, respectively, which show a decreasing trend. Oxygen isotope data show that the δ18OH2O values of V2-V4 veins (5.3 to 10.4 ‰) are consistent with typical magmatic values, whilst the δ18OH2O values of V5-V6 veins (0.2 to 4.6 ‰) are much lower than those of the magmatic water, which is due to involvement of meteoric water. The fluid inclusion microthermometry and O isotope data suggest an evolving magmatic-hydrothermal system with decreasing temperatures, pressures from V2 to V4 veins (potassic stage) to V5 (chlorite-illite stage) and V6 veins (phyllic stage), and an increasing incorporation of meteoric water in V5 and V6. The Xiaokelehe deposit has lower CO2 contents than the adjacent porphyry Mo deposits formed in the same post-collisional setting, which is mainly due to differences of mineralization depths and magma source. The formation of porphyry deposits with different Mo/Cu ratios highlights the diversity and complexity of the mineralizing systems in post-collisional settings, which has important implications for mineral exploration in such environments.

Biotite geochemistry and its implication on the temporal and spatial difference of Cu and Mo mineralization at the Xiaokele porphyry Cu-Mo deposit, NE China

The Late Jurassic large Xiaokele porphyry Cu-Mo deposit is newly discovered in the northern Great Xing’an Range, NE China. Both Cu and Mo mineralizations have occurred in the potassic stage and are present in the potassic zone, but only Mo (Cu negligible) mineralization has been developed in the phyllic stage and is present in the phyllic zone. The cause for such temporal and spatial difference of Cu and Mo mineralization at Xiaokele remains unclear. In this study, biotite major element compositions were studied to reveal the magmatic-hydrothermal processes, and the temporal and spatial difference of Cu and Mo mineralization at Xiaokele. Biotite geochemical data show that the magmatic biotites (Bi-I), and the potassic-stage (Bi-II) and phyllic-stage (Bi-III) hydrothermal biotite are Mg-biotites, and that mineralization at Xiaokele occurred under 52–86 MPa (equivalent to 1.95–3.32 km depth). Calculated oxygen fugacity of Bi-I, Bi-II, and Bi-III biotites are all above nickel-nickel oxide (NNO) oxygen buffer, suggesting that these biotites were formed under oxidizing conditions. The decreasing Cl and largely constant F content from Bi-I, Bi-II to Bi-III result in an increasing trend of IV(F/Cl), indicating different elemental behavior between F and Cl during the magmatic-hydrothermal fluid evolution. Given that Cl content is a key Cu-transporting ligand in porphyry type mineral systems, the decreasing Cl content in Bi-III indicates lower Cu transporting capacity in the phyllic stage, resulting in the negligible Cu mineralization in the phyllic zone. Since F is important for Mo transport in hydrothermal systems, we conclude that the constant F content reflects stable Mo transporting capacity in the fluids, resulting in significant Mo mineralization in both the potassic and phyllic zones. Our results show that biotite geochemistry can be used to explain the temporal and spatial distribution of Cu and Mo mineralization in porphyry Cu-Mo systems.

Tourmaline Composition of the Kışladağ Porphyry Au Deposit, Western Turkey: Implication of Epithermal Overprint

The Kışladağ porphyry Au deposit occurs in a middle Miocene magmatic complex comprising three different intrusions and magmatic-hydrothermal brecciation related to the multiphase effects of the different intrusions. Tourmaline occurrences are common throughout the deposit, mostly as an outer alteration rim around the veins with lesser amounts disseminated in the intrusions, and are associated with every phase of mineralization. Tourmaline mineralization has developed as a tourmaline-rich matrix in brecciated zones and tourmaline-quartz and/or tourmaline-sulfide veinlets within the different intrusive rocks. Tourmaline was identified in the tourmaline-bearing breccia zone (TBZ) and intrusive rocks that had undergone potassic, phyllic, and advanced argillic alteration. The tourmaline is present as two morphological varieties, aggregates of fine crystals (rosettes, fan-shaped) and larger isolated crystals and their aggregates. Four tourmaline generations (tourmaline I to IV) have different compositions and substitutions. Tourmaline I in TBZ and INT#1 is distinguished by the highest Fetot and enriched in Fe3+. Tourmalines II and III occur as fine aggregates, accompanied by the formation of isolated crystals and are characterized by lower Fetot and Fe3+. Tourmaline IV is characterized by the lowest Fetot, enriched in Cl, and has the highest proportion of X-site vacancy among all the tourmalines. Tourmaline I may be attributed to the potassic stage in INT#1 and early tourmaline in TBZ. Tourmalines II and III from INT#1 and the TBZ could be referred to the phyllic stage. The low Fe content in tourmaline is caused by the simultaneous deposition of sulfide minerals. Tourmaline IV from the TBZ and tourmaline II from INT#3 are distinguished by the high X-site vacancy proportion up to the formation of X-site vacant species as well as enriched in Cl; they can be attributed to the argillic stage of the hydrothermal process. The textural and especially chemical data of the tourmaline from the Kışladağ Au deposit provide information on the physico-chemical conditions during the porphyry to epithermal transition and subsequent epithermal overprinting.

Mineralogy and mineral geochemistry of the Tuwu porphyry Cu deposit, Eastern Tianshan, NW China: implication for the ore-forming condition and Cu mineralization

The Tuwu deposit is one of the largest porphyry copper deposits in Eastern Tianshan, NW China. The widespread hydrothermal alteration is associated with porphyries and their wall-rocks in the porphyry deposit can be divided into potassic, chlorite–sericite, phyllic, and propylitic zones. In this study, we conducted a detailed study on the mineralogy of hydrothermal magnetite, mica, and epidote. Magnetite is closely related to the potassic alteration, and evolves from earlier disseminated magnetite (Mt-I) to the later magnetite–quartz ± biotite ± pyrite ± bornite ± chalcopyrite veins (Mt-II). The significant decrease in the Cr2O3 and V2O3 contents of magnetite from early to late period indicates the gradual increase of oxygen fugacity. The crystallization of magnetite supplies enough reduced sulfur that promotes the sulfide precipitation. The dominance of muscovite (Mc-I) in the early phyllic stage and the increasing of phengite muscovite (Mc-II) in the late phyllic stage suggest the evolution of hydrothermal fluid from a high-temperature and acidic environment to a low-temperature and less acidic environment. This process also results in the decrease of sulfide mineral deposition in the late phyllic stage. Epidote from propylitic zones at Tuwu has similar characteristic with the overprinting epidote in the Yandong porphyry Cu systems. Propylitic Ep-I have higher FeO content than Ep-II. The interaction of Fe-rich propylitic epidote with hydrothermal fluid enhanced both Ca2+ activity and pH value, and favored the precipitation of sulfides (e.g., chalcopyrite). In addition, the Fe-rich and less acidic fluids, which derived from the propylitic zones and flow though the sericitic rocks in the phyllic zones, also promote the deposition of Cu mineralization. Our results, combined with previous research on fluid inclusion, isotopes, and mass transfer, suggest that the changes in temperature, oxygen fugacity, and pH caused by the fluid–rock interaction played a critical role for the Cu mineralization at Tuwu.

Hydrothermal evolution and isotope studies of the Baghu intrusion-related gold deposit, Semnan province, north-central Iran

The middle Eocene intrusion-related Baghu gold deposit is located in the Torud-Chahshirin segment of the Neotethyan magmatic arc of north-central Iran, Semnan province, Iran. The wallrocks at the deposit include a volcano-sedimentary sequence of calc-alkaline andesitic to dacitic lavas, marls, tuffs, and sandstones, which are intruded by epizonal granodioritic stocks and mafic dikes. Zircon U-Pb secondary ion mass spectrometry dating of a number of the granodiorite and micro-granite bodies yields an age of ca. 43.4 ± 1.3 Ma. Mineralization is in the form of subparallel sheeted quartz veins that are up to ∼1 km long within extensional faults. The ore-forming process included early potassic alteration with quartz + adularia + biotite + pyrite + chalcopyrite + native gold veins (average 0.25 g/t), and late phyllic alteration with quartz + sericite + pyrite + chalcopyrite + bornite + sphalerite + native gold veins (average 1.5 g/t). Supergene kaolinitization is also extensive in the Baghu area and has locally replaced up to 50 percent of the hypogene sulfide minerals. Turquoise is a diagnostic supergene phase, and locally, the iron oxy-hydroxides replacing the sulfides contain up to 57 g/t Au. The current hypogene and supergene resource estimate for the Baghu Au-Cu deposit is approximately 3.5 million tonnes averaging 1.17 g/t Au.Fluid inclusions in quartz from phyllic stage vein samples include monophase aqueous (I), monophase gas-rich (II), aqueous liquid + vapor (III), mixed aqueous-carbonic (IV), and multi-solid (V) inclusion types. The average homogenization temperatures and salinities of fluid inclusion assemblages from type III and IV inclusions range from 180 to 382 °C, with a mode at about 330 °C, and 4.2–5.4 wt% NaCl equiv., respectively. The type IV inclusions showed clathrate melting temperatures between +3.1 and +8.5 °C, indicating salinities of 2.8–11.5 wt% NaCl equiv. The type V inclusions also show a Th range from 310 to 478 °C (average 440 °C) corresponding to high salinities of 33–45 wt% NaCl equiv. The trapping pressure of the fluid inclusions was estimated to be 94–132 MPa corresponding to depths of 2.5–3.9 km during late stage of mineralization. Sericite from the stage II veins yields an 40Ar/39Ar age of ca. 44 Ma, suggesting that the mineralization at Baghu overlap granite emplacement. The δ18O of the ore-forming fluid also indicates no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment (δ18Ofluid = +8.0 to +10.3 per mil). In addition, δ34S values in stage II pyrite and chalcopyrite sulfide minerals range from +1.5 to +3.1 per mil, which is consistent with fluid saturation from a reduced magmatic source of sulfur. Historic gold production from occurrences hosted by the Torud-Chahshirin volcano-plutonic complex has been minor compared to that from orogenic gold deposits in Iran. However, identification of intrusion-related gold deposits in the complex indicates that more thorough exploration in the Eocene arc of north-central Iran could be promising.

Overprinting mineralization in the Paleozoic Yandong porphyry copper deposit, Eastern Tianshan, NW China—Evidence from geology, fluid inclusions and geochronology

The Yandong porphyry copper deposit is located in the southern part of the Dananhu-Tousuquan Island Arc belt in Eastern Tianshan (Xinjiang, NW China). The Dananhu-Tousuquan belt contains a number of large and medium Cu deposits, among which the Yandong deposit (∼2.0 Mt Cu @ 0.67%) is the largest. The Yandong Cu deposit is hosted by plagiogranite porphyry and the Carboniferous Qi’eshan Group mafic to intermediate calc-alkaline volcanic and pyroclastic rocks. According to the crosscutting relationships and mineral assemblages, the alteration and mineralization processes at Yandong comprise three episodes: the porphyry mineralization episode (including potassic, propylitic, early phyllic and late phyllic stages), the overprinting mineralization episode (including main- and post-mineralization stages) and lastly supergene alteration episode. Fluid inclusions (FIs) in quartz from the potassic, early/late phyllic and post-mineralization stages, and anhydrite from the main-/post-mineralization stages include four types: 1) vapor-liquid two phases (W-type), 2) daughter mineral-bearing (S-type), 3) pure liquid (PW-type), and 4) pure vapor (PV-type). The potassic stage contains W-, S- and PV-type FIs, which yielded homogenization temperatures of 304–445°C (mainly 340–400°C) and salinities of 3.1–16.4 (mainly 4.0–10.0)wt% NaCl eqv. At the early phyllic stage, three types of FIs are identified, i.e., the W-, S-, and PV-type. The S-type and W-type FIs homogenized at 213–441°C (mainly 280–300°C) and 241–352°C (mainly 300–320°C), respectively, with corresponding salinities of 33.1–52.0 (mainly 34.0–40.0)wt% NaCl eqv. and 0.9–15.9 (mainly 0.9–4.0)wt% NaCl eqv., respectively. The late phyllic stage contains W-, S- and PW-type FIs, which homogenized at 146–248°C (mainly 160–180°C), with salinities ranging from 3.9 to 18.2 (mainly 4.0–8.0)wt% NaCl eqv.Mineralization depths calculated from FIs are: 0.8–1.2km (Stage I), 0.6–1.8km (Stage IIIA) and 0.4–0.7km (Stage IIIB). Fluid boiling and mixing likely occurred during the porphyry mineralization period, leading to the precipitation of pyrite and chalcopyrite. During the overprinting mineralization, the W- and PV-type FIs occur in the main-mineralization anhydrite, with homogenization temperatures and salinities being 248–421°C (mainly 300–340°C) and 5.3–13.8 (mainly 8.0–10.0)wt% NaCl eqv., respectively. The post-mineralization quartz and anhydrite homogenized at 105–195°C (mainly 120–140°C), with salinities of 0.5–13.6 (mainly 8.0–10.0)wt% NaCl eqv. The ore-forming fluids may have predominantly been entrapped at 0.8–1.6km (Stage IV) and 0.3–0.6km (Stage V). The decreasing temperature may have led to the sulfides and sulfate precipitation and enrichment.Sericite 40Ar/39Ar dating of the altered plagiogranite porphyry yielded a plateau age of 332.8±3.8Ma, consistent with the age of the early ore-forming plagiogranite porphyry (339–332Ma). Re–Os dating of molybdenite, coexisting with chalcopyrite-anhydrite-chlorite-calcite assemblage, yielded an age of 324.3±2.7Ma (weighted mean), consistent with the late quartz albite porphyry age (zircon U-Pb: 323.6±2.5Ma).Consequently, we propose that at least two mineralization episodes may have occurred at Yandong, with the second episode contributing to the majority of the Cu-Mo resources. Such overprinting mineralization style at Yandong may represent a good example of the Paleozoic Cu deposits in the Central Asian Orogenic Belt.

Geology, mineralization, stable isotope, and fluid inclusion characteristics of the Vostok-2 reduced W-Cu skarn and Au-W-Bi-As stockwork deposit, Sikhote-Alin, Russia

The large (>180KtWO3 and at least 10–15 t Au) Vostok-2 deposit is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in late to post-collisional tectonic environment after cessation of active subduction. The deposit is related to an ilmenite-series high-K calc-alkaline plutonic suite that, by its petrologic signatures, is transitional between those at W-dominant and Au-dominant reduced intrusion-related deposits. Consistently, besides large W-Cu skarns of the reduced type, the deposit incorporates quartz stockworks with significant Au-W-Bi mineralization also formed in a reduced environment. The hydrothermal stages include prograde and retrograde, essentially pyroxene skarns, hydrosilicate (amphibole, chlorite, quartz) alteration, and phyllic (quartz, sericite, albite, apatite, and carbonate) alteration assemblages. These assemblages contain abundant scheelite associated with pyrrhotite, chalcopyrite and, at the phyllic stage, also with Bi minerals, As-Bi-Sb-Te-Pb-Zn sulfides and sulfosalts, as well as Au mineralization. The fluid evolution included hot, high-pressure (420–460°C, 1.1–1.2kbar), low-salinity (5.4–6.0wt% NaCl-equiv.) aqueous fluids at the retrograde skarn stage, followed by lower temperature cyclic releases of high-carbonic, low salinity to non-carbonic moderate-salinity aqueous fluids. At the hydrosilicate stage, a high-carbonic, CH4-dominated, hot (350–380°C) low salinity fluid was followed by cooler (300–350°C) non-carbonic moderate-salinity (5.7–14.9wt% NaCl-equiv.) fluid. At the phyllic stage, a high-carbonic, CO2-dominated, moderately-hot (330–355°C, 0.9kbar) low salinity fluid was followed by cooler (230–265°C) non-carbonic moderate-salinity (6.6–12.0wt% NaCl-equiv.) fluid. A homogenized magmatic source of water (δ18OH2O=+8.3 to +8.7‰), and a sedimentary source of sulfur (δ34S=−6.9 to −6.2‰) and carbon (δ13Cfluid=−20.1 to −14.9‰) at the hydrosilicate stage are suggested. A magmatic source of water (δ18O=+8.6 to +9.2‰) and a sedimentary source of sulfur (δ34S=−9.3 to −4.1‰) but a magmatic (mantle- to crustal-derived) source of carbon (δ13Cfluid=−6.9 to −5.2‰) are envisaged for fluids that formed the early mineral assemblage of the phyllic stage. Then, the role of sedimentary carbon again increased toward the intermediate (δ13Cfluid=−16.4 to −14.5‰) and late (δ13Cfluid=−16.3 to −14.7‰) phyllic mineral assemblages. The magmatic differentiation was responsible for the fluid enrichment in W, whereas Au and Bi could also have been sourced from mafic magma. The decreasing temperatures, together with elevated Ca content in non-boiling fluids, promoted scheelite deposition at the early hydrothermal stages. The most intense scheelite deposition at the phyllic stage was caused by CO2 removal due to boiling of CO2-rich fluids; further cooling of non-boiling fluids favoured joint deposition of scheelite, Bi and Au.

Geology, mineralization, and fluid inclusion characteristics of the Skrytoe reduced-type W skarn and stockwork deposit, Sikhote-Alin, Russia

The Skrytoe deposit (>145 Kt WO3, average grade 0.449% WO3) in the Sikhote-Alin orogenic system (Eastern Russia) is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in a late to post-collisional tectonic environment after cessation of active subduction. It is localized within a mineralized district of reduced-type skarn W and veined Au (±W) deposits and occurrences related to the Early Cretaceous ilmenite-series plutonic suite. The deposit incorporates large stockworks of scheelite-bearing veinlets related to propylitic (amphibole, chlorite, quartz) and phyllic (quartz, sericite, albite, apatite, and carbonate) hydrothermal alteration. The stockwork cuts flat-lying mafic volcanic rocks and limestone partially replaced by pyroxene skarn that host the major W orebodies. Scheelite is associated with pyrrhotite and/or arsenopyrite, with minor chalcopyrite and other sulfide minerals; the late phyllic stage assemblages hosts Bi and Au mineralization. The fluid evolution included low-salinity moderate-temperature, moderate-pressure (∼370–390 °C, ∼800 bars) methane-dominated carbonic-aqueous fluids that formed post-skarn propylitic alteration assemblages. Then, at the phyllic stage, there has been an evolution from methane-dominated, moderate-temperature (330–360 °C), low-salinity (<12.3 wt% NaCl equiv.) fluids forming the early quartz-sericite-albite-arsenopyrite assemblage, through lower temperature (290–330 °C) methane-dominated, low-salinity (∼9–10 wt% NaCl equiv.) fluids forming the intermediate quartz-sericite-albite-scheelite-pyrrhotite assemblage, to yet lower temperature (245–320 °C) CO2-dominated carbonic-aqueous low-salinity (∼1–7 wt% NaCl equiv.) fluids forming the late quartz-sericite-sulfide-Bi-Au assemblage. Recurrent fluid immiscibility (phase separation) and cooling probably affected W solubility and promoted scheelite deposition. The stable isotope data support a sedimentary source of carbon (δ13Cfluid = ∼−21 to −10 ‰), a magmatic source for water (δ18OH2O = +7.4 to +7.7 ‰), and dominantly crustal-derived source of sulfur (δ34S = −4.6 to −2.9 ‰) in the hydrothermal fluids. This is consistent with the development of larger, longer crystallizing crustal intermediate to felsic magma chambers in the late to post-collisional tectonic environment, with their protracted magmatic evolution advancing magmatic differentiation and partitioning of W into magmatic-hydrothermal fluid. The dominating role of the crustal-derived magmatic water, sulfur, and carbon appears to be an important feature of reduced W skarn deposits related to ilmenite-series granitoids.

Reverse Osmosis contributing to metal zoning in porphyry type deposits: A case study

Representing the main source of copper in the world, porphyry copper deposits have been widely studied. Different models have tried to explain the observed metal zoning, but they did not completely explain it. A Permian Cu–Mo porphyry deposit in the San Rafael Massif in Argentina shows a similar metal zoning pattern to those described elsewhere. However, some particular features depart from the conventional cooling model. Based on data from this deposit and on theoretical background, we present and discuss Reverse Osmosis as a complementary process which could have contributed to porphyry metal zoning during the phyllic stage. The existence of potential-energy gradients and the different relative rejection values for distinct ions make Reverse Osmosis a natural mineral-concentration process. At temperatures corresponding to those of porphyry phyllic halo formation and in presence of a phyllic (clay) membrane, Cu, Ag, Zn and Pb would show a strong osmotic differentiation that could have led to the observed metal zoning.

Isotope geochemistry and Re–Os geochronology of the Yanjiagou Mo deposit in the central North China Craton

The Yanjiagou deposit, located in the central North China Craton (NCC), is a newly found porphyry‐type Mo deposit. The Mo mineralization here is spatially associated with the Mapeng batholith. In this study, we identify four stages of ore formation in this deposit: pyrite phyllic stage (I), quartz–pyrite stage (II), quartz–pyrite–molybdenite stage (III), which is the main mineralization stage, and quartz–carbonate stage (IV). We present sulphur and lead isotope data on pyrite, and rhenium and osmium isotopes of molybdenite from the porphyry deposit and evaluate the timing and origin of ore formation. The δ34S values of the pyrite range from ‐1.1‰ to −0.6‰, with an average of −0.875‰, suggesting origin from a mixture of magmatic/mantle sources and the basement rocks. The Pb isotope compositions of the pyrite show a range of 16.369 to 17.079 for 206Pb/204Pb, 15.201 to 15.355 for 207Pb/204Pb, and 36.696 to 37.380 for 208Pb/204Pb, indicating that the ore‐forming materials were derived from a mixture of lower crust (or basement rocks) and mantle. Rhenium contents in molybdenite samples from the main ore stage are between 74.73 to 254.43 ppm, with an average of 147.9 ppm, indicating a mixed crustal‐mantle source for the metal. Eight molybdenite separates yield model ages ranging from 124.17 to 130.80 Ma and a mean model age of 128.46 Ma. An isochron age of 126.7 ± 1.1 Ma (MSWD = 2.1, initial 187Os = 0.0032 ± 0.0012 ppb) is computed, which reveals a close link between the Mo mineralization and the magmatism that generated the Mapeng batholith. The age is close to the zircon U–Pb age of ca. 130 Ma from the batholith reported in a recent study. The age is also consistent with the timing of mineralization in the Fuping ore cluster in the central NCC, as well as the peak time of lithosphere thinning and destruction of the NCC. We evaluate the spatio‐temporal distribution of the Mo deposits in the NCC and identify three important molybdenum provinces along the northern and southern margins of the craton formed during three distinct episodes: Middle to Late Triassic (240–220 Ma), Early Jurassic (190–175 Ma), and Late Jurassic to Early Cretaceous (150–125 Ma). The third period is considered to mark the most important metallogenic event, coinciding with the peak of lithosphere thinning and craton destruction in the NCC. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Source characteristics and fluid evolution of the Beiyingxigou Pb–Zn–Ag deposit, central North China Craton: An integrated stable isotope investigation

The Beiyingxigou quartz vein and alteration type Pb–Zn–Ag deposit forms part of the metallogenic region southwest of the Mapeng batholith in the Central North China Craton. The quartz vein-type polymetallic deposit is associated with intermediate-felsic intrusives emplaced with Mesoarchean metamorphic basement. The mineralization is dominantly controlled by N–S and NW–SE trending faults. We identify four distinct stages of mineralization associated with the hydrothermal activity: pyrite phyllic stage, quartz–pyrite stage, quartz–polymetallic sulfide stage, and quartz–carbonate stage. Among these, the first two stages and the quartz–polymetallic sulfide stage represent the early and main metallogenic stages, respectively. Here we present results from integrated stable isotope studies (S, Pb, He–Ar and H–O) to deduce the possible sources of the mineralizing fluids and metals. The δ34S values recorded in the sulfide minerals of the early and main metallogenic stages show ranges of +0.4‰ to +2.2‰, and −11.4‰ to −6.8‰, respectively. The data illustrate two distinct pulses of hydrothermal fluids with δ34S mean values of 1.1‰ and −8.7‰. The S isotopes of pyrite and galena, sphalerite from the early and main metallogenic stages suggest a mixed source from the Mesozoic magmatism and the Precambrian basement. We infer that the slow cooling magma at the main metallogenic stage led to a larger interaction with the country rocks and resultant contamination, as compared to the early–middle metallogenic stage. The lead isotopic composition of pyrite, galena and sphalerite from the early to main stages shows restricted ranges with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 15.720–16.275, 15.068–15.140 and 38.688–41.316, respectively. The data are all characterized by lower crustal nature, suggesting the contribution of Pb from both the Mesozoic intrusions and the basement rocks in the region. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions from the Beiyinxigou Pb–Zn–Ag deposit show a range of 0.01–0.23 Ra and 270.2–298.1, respectively. The 4He and 40Ar concentrations vary from 26.6 to 340.6 (10−7cm3STP/g) and 165.6 to 357.9 (10−7cm3STP/g), respectively, suggesting dominantly crustal contribution. The δ18OQ values of quartz and the δ18Dw values show ranges of 10.9‰ to 13.6‰ and −85.2‰ to −57.8‰, respectively. The calculated δ18OW values of the mineralizing fluids show a range of 3.7‰ to 7.5‰. Our data suggest that the hydrothermal fluids were a mixture of both magmatic and meteoric water. The results presented in this study link the Beiyinxigou Pb–Zn–Ag deposit to metallogeny associated with Mesozoic magmatism in the North China Craton.

Evidence for a nonmagmatic component in potassic hydrothermal fluids of porphyry cu-Au-Mo systems, Yukon, Canada

Isotopic (H, Sr, Pb, Ar) and fluid inclusion data for hydrothermal fluids associated with potassic alteration from three Late Cretaceous porphyry Cu occurrences, west central Yukon, suggest a nonmagmatic fluid component was present in these hydrothermal fluids. Potassic stage quartz veins contain a dominant assemblage of saline and vapor-rich fluid inclusions that have δD values between −120 and −180‰. Phyllic stage quartz veins are dominated by vapor-rich fluid inclusions and have δD values that overlap with but are, on average, heavier (−117 to −132‰) than those in potassic stage quartz veins. These δD values are significantly lower than those from plutonic quartz phenocrysts (−91 to −113‰), and from values typically reported for primary fluids from porphyry-style mineralization (−40 to −100‰). The initial Sr ( 87Sr/ 86Sr i) isotopic values for the plutons are 0.7055 (Casino), 0.7048 (Mt. Nansen), and 0.7055 (Cash). The 87Sr/ 86Sr i compositions of hydrothermal K-feldspar ranges from magmatic Sr i values to more radiogenic compositions (Casino: 0.70551–0.70834, n = 8; Mt. Nansen: 0.7063–0.7070, n = 4; Cash: 0.7058, n = 1). The fluid inclusion waters from potassic quartz veins have 87Sr/ 86Sr i values that are similar to those of co-existing hydrothermal K-feldspar. The Pb isotopic compositions of hydrothermal K-feldspar show a weak positive correlation with Sr i for identical samples. Fluid inclusion waters of phyllic quartz veins also have Sr i compositions more radiogenic than the plutons. The Pb isotopic composition of pyrite and bornite from phyllic alteration veins are similar to, or more radiogenic than, hydrothermal K-feldspar Pb isotopic values. Hydrothermal K-feldspar samples yield 40Ar/ 39Ar ages (Casino = 71.9 ± 0.7 to 73.4 ± 0.8 Ma; Mt. Nansen = 68.2 ± 0.7 and 69.5 ± 0.6 Ma; Cash = 68.3 ± 0.8 Ma) similar to the U-Pb zircon, K-Ar biotite and Re-Os molybdenite ages of the Late Cretaceous plutons, with the age spectra indicating no excess 40Ar or disturbance. The 40Ar/ 36Ar values (285–292) of the K-feldspar samples are similar to the atmospheric compositions (295 ± 5) during Late Cretaceous time. The H, Sr, Pb, and Ar isotopic compositions of hydrothermal K-feldspar and quartz vein fluid inclusion waters that characterize the potassic hydrothermal fluids show evidence for an exotic component in addition to magmatic water (fluid). This component has a low δD, radiogenic Sr and Pb, and an atmospheric Ar composition. The inheritance of pre-existing isotope compositions from the host rocks, postpotassic alteration isotope exchange, or the replenishment of the magma chamber with magma of different isotopic composition cannot explain the isotope data. We suggest that to generate the observed H, Sr, Pb, and Ar isotope compositions, crustal fluids must be a component (15–94%) of potassic hydrothermal fluids in porphyry mineralization in the deposits studied.