Main Mineralization Stage Research Articles

Zoning and Exploration Significance of the Maoping Pb-Zn Deposit, Northeastern Yunnan Province, China

Based on the lithofacies mapping and petrographic studies, the mineralization-alteration zoning and mineralization process of the Maoping Pb-Zn deposit are discussed. Three ore body groups of the Maoping Pb-Zn deposit have similar transversal zoning, which is characterized by different mineral assemblages from the ore body center to outside. The mineralization-alteration can be divided into diagenetic stage, massive sulfide-carbonate stage, veined sulfide-carbonate stage, and supergene stage. The massive sulfide-carbonate stage is main Pb-Zn mineralization stage. The established zoning model and indicators can be used for ore prospecting.

Structural Geologic Evolution of the Butte District, Montana

The Butte district contains two porphyry Cu-Mo deposits and the younger zoned Main stage base metal lode deposits that formed in the period from 66 to 62 Ma, synchronous with Laramide compressional thrust faulting and folding in southwest Montana. New geologic mapping of the Butte district reveals three distinct episodes of deformation: 1. Gentle (20° ± 10°) NE tilting produced during Laramide compressional deformation, uplift, and exhumation during and after ore formation in the Late Cretaceous to Paleocene (~66–~53 Ma); 2. Moderate (10°–50°) NW tilting of Eocene age produced by NE-striking normal faults that are limited to the northwest part of the district and synchronous with the eruption of the Lowland Creek Volcanics; and 3. Miocene to present gentle (15°) ESE tilting in the hanging walls of the Continental, East Ridge, and Rocker normal faults. Stratigraphic relationships, isotopic ages, hornblende barometry, and fluid inclusion data indicate that exposures of the Butte Granite, which hosts mineralization, crystallized at ~6- to 9-km depth and that early porphyry Cu-Mo deposits formed at similar depths of 5 to 9 km. A period of rapid uplift and erosion removed about 7 km of rock from above the ore deposit between 66 to 64 Ma and 53 Ma, but was mostly completed by 59 Ma. Estimated exhumation rates range from 0.5 to 2 mm/yr, and uplift was likely most rapid between ~63.5 and 62 Ma during Main stage mineralization, which occurred at depths of 2 to 6 km. The Butte Granite and Boulder batholith were emplaced at 79 to 76 Ma into thrust fault ramps, which later became inactive near Butte as Laramide crustal shortening moved eastward in the Helena salient. The accelerated period of uplift and unroofing is attributed to renewed deformation along the Montana disturbed belt south of Butte that uplifted the south end of the Boulder batholith and produced ~20° of gentle NE tilting (episode I) in the district. The exhumation history of the deposit is therefore broadly contemporaneous with ore formation, and is consistent with the general transition from lithostatic to hydrostatic conditions that is evident from the telescoping of early porphyry Cu-Mo ores cut by the Main stage lode veins. Within a 5-m.y. period, the Laramide compressional regime near Butte evolved into an extensional environment characterized by normal faulting. The normal faults were synchronous with eruption of the 53 to 48 Ma Lowland Creek Volcanics, which directly overlie the Main stage veins in the northwest part of the district. Structural reconstruction of the district at the time of mineralization produces orientations of quartz porphyry dikes, porphyry Cu-Mo veinlets, Main stage veins, and alteration zonal patterns that are steeply dipping and symmetrical, similar to observations from other upright or restored porphyry copper deposits. Early porphyry Cu-Mo veins and dikes are oriented E-W and likely originated as tensile hydrofractures above a ductile magma carapace during Laramide shortening. Additionally, during the Laramide compressional phase, two conjugate fault systems occupied by Main stage veins are normal oblique-slip faults that record minor E-W and vertical shortening and N-S extension. These normal faults with minor offset likely resulted from brittle failure caused by doming above a volatile-rich and buoyant granitic magma chamber and cupola at depth. The structural evolution of the Butte district therefore reflects the interaction of a far-field compressive stress regime with local buoyant magmatic forces.

Geology, fluid inclusion, and isotope constraints on ore genesis of the Neoproterozoic Jinshan orogenic gold deposit, South China

AbstractThe Jinshan gold deposit is located in the Neoproterozoic Jiangnan orogen between the Yangtze and Cathaysia blocks. Gold‐bearing disseminated ores are associated with the earlier stage of NWW‐trending ductile zone, and auriferous quartz vein‐type ores show an intimate relationship with the later stage of NE‐trending brittle‐ductile zones. Fluid‐inclusion studies were conducted on the quartz veins. Three types of fluid inclusions can be identified: H2O–CO2 inclusions (type I), CO2‐rich inclusions (type II), and aqueous inclusions (type III). The pre‐ore stage, quartz‐pyrite veins primarily contain type I inclusions with constant CO2 bubble volumetric proportions. The main gold mineralization‐stage veins have all three types of inclusions with variable gas‐phase ratios and CO2 contents. The post‐ore stage carbonate±chlorite veinlets only contain type III inclusions. Type I inclusions in the pre‐ore stage display homogenization temperatures (Th) of 285–340°C, with salinities of 1.4–6.1 wt.% NaCl equivalent. In the main gold mineralization stage, type II and III inclusions show similar Th at 208–277°C, but contrasting salinity values with 0.6–3.6 and 3.5–8.9 wt.% NaCl equivalent, and type I inclusions show variable CO2‐phase proportions and have Th of 241–292°C and salinities of 1.0–7.0 wt.% NaCl equivalent. In the post‐ore stage, type III inclusions yield Th of 109–201°C and salinities of 1.1–6.4 wt.% NaCl equivalent. Petrological observations and microthermometric results show that fluid immiscibility primarily occurred during the gold mineralization stages. The oxygen and hydrogen isotope compositions (δ18 O = +6.9‰ to +11.2‰, δD = −71‰ to −46‰) of inclusion water in quartz grains imply that ore fluids were principally metamorphic in origin. The sulfur and lead values of sulfide from the ores are analogous to those from the basement strata, suggesting a predominantly crustal source of the ore sulfides. The Jinshan deposit is a typical orogenic gold deposit.

Genesis and evolution of the Leimengou porphyry Mo deposit in West Henan Province, East Qinling–Dabie belt, China: Constraints from hydrothermal alteration, fluid inclusions and stable isotope data

The Leimengou Mo deposit is located in the East Qinling–Dabie metallogenic Mo belt, located at the southern margin of the North China Craton. The deposit is hosted within the Mesozoic Leimengou granite porphyry pluton and at the contact between the granite and Late Archean gneisses. In light of mineral assemblage and crosscutting relationships among veinlets, the mineralization and alteration can be classified in four distinct stages: (1) a Mo–barren pre-mineralization stage of alteration characterized by massive K-feldspar alteration of the granite porphyry; (2) a K-feldspar–quartz vein stage that formed veinlets with minor molybdenite; (3) a quartz–sulfide veinlet stage, which is the main stage of mineralization; and (4) post-mineralization quartz–calcite vein stage.Stage 1 alteration is associated with fluid inclusions of low–medium salinity (2.4–13.1wt.% NaClequiv) that homogenized at medium to high temperatures (238–476°C). Stage 2 mineralization is associated with fluid inclusions that homogenized at 201–389°C and that have salinities of 2.0–13.6wt.% NaClequiv. Fluid inclusions that formed during stage 3 have homogenization temperatures of 205–370°C and corresponding salinities of 1.0–17.6wt.% NaClequiv to 31.4–40.6wt.% NaClequiv, indicative of fluid immiscibility and mixing of hydrothermal fluids with meteoric water. Stage 4 H2O–NaCl fluids were trapped at lower temperatures (148–246°C) and have salinities of 1.7–7.9wt.% NaClequiv.Stages 1 to 4 have calculated δ18Ofluid values of 3.6–4.3‰, 2.7–2.8‰, 0.6–2.3‰, and −3.2‰ to −3.8‰, respectively, with δDfluid values of –72‰ to −80‰, −83‰ to −90‰, −68‰ to −82‰, and −81‰ to −90‰, respectively. These results are consistent with the fluid inclusion data outlined above and suggest that early fluids were exsolved from the granitic magma and underwent progressive mixing with meteoric water. Molybdenum was initially transported from the crystallizing magma in a hot, alkaline, and oxidized fluid, while decreasing temperatures and pH, water–rock interaction, and fluid immiscibility during fluid ascent resulted in its precipitation and deposition.

Fluid inclusions and He[sbnd]Ar isotopes in pyrite from the Yinjiagou deposit in the southern margin of the North China Craton: A mantle connection for poly-metallic mineralization

The Yinjiagou poly-metallic deposit is located at the southern margin of the North China Craton (SMNCC). Pyrite, the most economically important mineral, is associated with Cu, Mo, Au, Pb, Zn and Ag mineralization. Two main mineralization stages have been identified and studied: a porphyry stage, including the quartz–molybdenite and quartz–sericite–pyrite (QSP) veins, and a vein stage, including the early and the late (economically most important) pyrite stages.Three types of fluid inclusions are distinguished in quartz and pyrite, including liquid-rich, vapor-rich biphase (LV) and halite-bearing inclusions. The transmitted and infrared microthermometry produced several results. (1) Each of the three types of fluid inclusions is trapped in the quartz of the porphyry stage, with homogenization temperatures of 335–419°C and salinities of 4.7–49.6wt.% NaCl equiv. The coexistence of halite-bearing inclusions and low density, vapor-rich biphase (LV) inclusions indicates that the fluid was boiling. The boiling elevated the Fe and S levels in the hyper-saline fluid, as reflected by the development of pyrite, chalcopyrite and S daughter minerals in the halite-bearing inclusions. (2) The fluid in the early pyrite also displays features characteristic of boiling, with homogenization temperatures of 352–>400°C and salinities of 3.7–42.4wt.% NaCl equiv, whereas the late pyrite only contains liquid-rich biphase (LV) inclusions, with homogenization temperatures of 263–354°C and salinities of 6.0–21.3wt.% NaCl equiv. The relationship between the decreasing salinity and the corresponding decreasing homogenization temperature clearly indicates extensive mixing and dilution. (3) Finally, the infrared calibration indicates that the impact of infrared light intensity on the microthermometric results for the Yinjiagou pyrite-hosted fluid inclusions is limited.The HeAr isotopic analysis of the fluid inclusions in the pyrite indicates two different fluid sources that involve abundant mantle input. The 3He/4He ratios in the porphyry stage are relatively uniform, ranging from 1.39 to 1.78 Ra (Ra=1.39×10−6 for air), corresponding to 16–22% mantle 4He contribution. This relatively constant range represents the actual helium isotopic composition of the fluids emanating from the cooling intrusions at depth. The range of 3He/4He ratios in the vein stage is wide, varying from 0.80 to 5.26Ra, corresponding to 9–65% mantle 4He. The significant variation in the 3He/4He ratios in the vein stage reflects mixing between two fluids.These data indicate that there are two different fluid sources and separate evolutionary processes for the porphyry and vein stages and support a multi-stage mineralization model. The veinlet molybdenite mineralization was induced by the boiling and associated cooling of a magma-sourced fluid that emanated from intrusions at depth. The QSP veins are likely to be pressure release zones and late fluid flow channels. The early pyrite mineralization may be attributed to the influx of a dominantly mantle-derived fluid with high 3He/4He ratio (5.26Ra), which is most likely related to a later, more mafic magmatic event. The precipitation of the most economically important pyrite resulted from the mixing and diluting of this mantle-derived fluid with a surface-derived fluid. Additionally, the high 3He/4He ratios indicate a strongly extensional setting, which is most likely related to the Late Jurassic to Early Cretaceous lithospheric modification and thinning of the SMNCC.

A two-stage evolution model for the Amantaytau orogenic-type gold deposit in Uzbekistan

A lithogeochemical, mineral chemical, isotopic, and fluid inclusion study of barren, low-, and high-grade Au-mineralized samples from the shear zone-hosted Amantaytau gold deposit, Uzbekistan, shows that the local host rocks, Late Ordovician–Earlz Silurian carbonacous shales, are likely to have been an important source of Au, As, Ni, and S in the formation of the deposit. Syn-depositional pyrite in these shales contains on average 0.23 ppm Au, 1,083 ppm As, and 861 ppm Ni. The distribution of rare earth elements (REE) indicates a homogeneous source of light REE, whereas the heavy REE distribution reflects most likely primary variations in the sediments. The mineralized zone is marked by a positive Eu anomaly, which supports reducing conditions during the mineralization. A hydrothermal overprint by an aqueous–carbonic fluid is reflected in a high-grade Au-mineralized sample by δ13C values of −13.0 ‰ (V-PDB). The δ34S values in pyrite (−0.13 to +7.30 ‰ CDT) from barren and mineralized samples are consistent with marine sulfate being the principal source of the ore sulfur. Assuming a formation temperature of between 300 and 400 °C for the main stage of mineralization, as indicated by the alteration mineral assemblage, the calculated δ18Ofluid is between 9.5 and 13.4 ‰ V-SMOW, which points at a metamorphic origin of the ore fluid.

Hydrothermal alteration associated with Mesozoic granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong Gold Province, China

The Sanshandao gold deposit (reserves of more than 200t Au and average grade of 3.96g/t), located at northwestern edge of the Jiaodong Peninsula, eastern North China Craton, is one of the largest gold deposits in the Jiaodong gold province. In this deposit, disseminated- and stockwork-style ores are hosted in Mesozoic granitoids; mineralization and alteration are largely controlled by the regional Sanshandao–Cangshang fault. Host granitic rocks for the deposit display a complex paragenetic sequence of alteration and mineralization. Activities of the Sanshandao–Cangshang fault created structurally controlled permeability allowing for infiltration of hydrothermal fluids, leading to diffusive K-feldspar alteration on the two fault planes. Later, large scale diffusive sericitization symmetrically developed across the main fault, and partially overprinted the earlier K-feldspar alteration. Following the sericitization, relatively small scale silicification occurred, but now it is only retained in the hanging wall of the main fault. Subsequently, the fault gouge formed as a “barrier layer”, which is impermeable for later fluids to move upward. After that, strong pyrite–sericite–quartz alteration occurred only in the footwall of the main fault, and was accompanied by gold precipitation. The last stage carbonation and quartz-carbonate veins marked the waning of gold-related hydrothermal activity. Mass-balance calculations indicate complex behaviors of different types of elements during fluid–rock interaction. Most major elements were affected by intensive mineral replacement reactions. As expected, the fluid-mobile elements, LILE and LREE, generally show moderate to high mobility. It is notable that even the commonly assumed fluid-immobile elements, such as HREE and HFSE, tend to be changed to various degrees. In addition, Y–Ho, Zr–Hf and Nb–Ta fractionations are observed in altered domains. Studies on alteration assemblages and fluid inclusions suggest that the ore-forming fluids were characterized by low salinity (≤8.4wt.% NaCl equiv.), moderate temperature (300–400°C), weakly acidic (pH: 3–5), and relatively reducing (log fO2: ~–28) characteristics. In this type of fluids, gold was most likely transported as Au(HS)2− complex. With alteration going on, log (aK+/aH+) of fluids generally decreased due to significant formation of secondary K-bearing minerals. In addition, there might be a decrease of fO2 from pre-gold alteration stage to the main gold mineralization stage, and decrease of fO2 was probably one of the factors controlling gold precipitation. The Sr and Nd isotopic compositions of hydrothermal minerals, combined with previous H–O and He–Ar isotopic studies, indicate that the hydrothermal fluids were mainly derived from crustal sources (e.g., degassing of felsic magmas and meteoric water), but with involvement of mantle derived components. The gold mineralization event just coincided with reactivation of the North China Craton, as marked by asthenosphere upwelling, voluminous igneous rocks, and high crustal heat flow, which may have provided sufficient heat energy and fluid input required for the formation of the gold deposits.

Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru

The 13.1-Moz high-sulfidation epithermal gold deposit of Lagunas Norte, Alto Chicama District, northern Peru, is hosted in weakly metamorphosed quartzites of the Upper Jurassic to Lower Cretaceous Chimu Formation and in overlying Miocene volcanic rocks of dacitic to rhyolitic composition. The Dafne and Josefa diatremes crosscut the quartzites and are interpreted to be sources of the pyroclastic volcanic rocks. Hydrothermal activity was centered on the diatremes and four hydrothermal stages have been defined, three of which introduced Au ± Ag mineralization. The first hydrothermal stage is restricted to the quartzites of the Chimu Formation and is characterized by silice parda, a tan-colored aggregate of quartz-auriferous pyrite–rutile ± digenite infilling fractures and faults, partially replacing silty beds and forming cement of small hydraulic breccia bodies. The δ34S values for pyrite (1.7–2.2 ‰) and digenite (2.1 ‰) indicate a magmatic source for the sulfur. The second hydrothermal stage resulted in the emplacement of diatremes and the related volcanic rocks. The Dafne diatreme features a relatively impermeable core dominated by milled slate from the Chicama Formation, whereas the Josefa diatreme only contains Chimu Formation quartzite clasts. The third hydrothermal stage introduced the bulk of the mineralization and affected the volcanic rocks, the diatremes, and the Chimu Formation. In the volcanic rocks, classic high-sulfidation epithermal alteration zonation exhibiting vuggy quartz surrounded by a quartz–alunite and a quartz–alunite–kaolinite zone is observed. Company data suggest that gold is present in solid solution or micro inclusions in pyrite. In the quartzite, the alteration is subtle and is manifested by the presence of pyrophyllite or kaolinite in the silty beds, the former resulting from relatively high silica activities in the fluid. In the quartzite, gold mineralization is hosted in a fracture network filled with coarse alunite, auriferous pyrite, and enargite. Alteration and mineralization in the breccias were controlled by permeability, which depends on the type and composition of the matrix, cement, and clast abundance. Coarse alunite from the main mineralization stage in textural equilibrium with pyrite and enargite has δ34S values of 24.8–29.4 ‰ and $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ values of 6.8–13.9 ‰, consistent with H2S as the dominant sulfur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0–2) and logfO2 of −28 to −30. Alunite–pyrite sulfur isotope thermometry records temperatures of 190–260 °C; the highest temperatures corresponding to samples from near the diatremes. Alunite of the third hydrothermal stage has been dated by 40Ar/39Ar at 17.0 ± 0.22 Ma. The fourth hydrothermal stage introduced only modest amounts of gold and is characterized by the presence of massive alunite–pyrite in fractures, whereas barite, drusy quartz, and native sulfur were deposited in the volcanic rocks. The $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ values of stage IV alunite vary between 11.5 and 11.7 ‰ and indicate that the fluid was magmatic, an interpretation also supported by the isotopic composition of barite (δ34S = 27.1 to 33.8 ‰ and $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ = 8.1 to 12.7 ‰). The Δ34Spy–alu isotope thermometry records temperatures of 210 to 280 °C with the highest values concentrated around the Josefa diatreme. The Lagunas Norte deposit was oxidized to a depth of about 80 m below the current surface making exploitation by heap leach methods viable.

山东鲁东地区中生代构造–岩浆事件与金矿成矿作用

山东鲁东地区处于特提斯、古亚洲洋和太平洋三大构造域的结合部位，它们的相互作用过程决定并影响着鲁东的成矿环境，在中生代区内有印支和燕山二次造山事件。燕山造山事件在鲁东地区表现为三个造山幕，每一幕表现为挤压与伸展的旋回性，这种旋回性是鲁东金矿和多金属矿成矿的地质背景。中生代构造演化对山东的构造–岩浆事件、沉积环境和成矿作用起着制约与控制作用，岩浆–沉积–构造–成矿事件与区域构造事件具有相互耦合性，金矿、多金属矿的形成与定位同燕山期三个构造幕紧密相联，3期成矿事件160~140 Ma、130~110 Ma和105~80 Ma则与玲珑、郭家岭和伟德山岩体的构造–岩浆热事件相耦合。文章在统计区内已报道的金矿成矿年龄样品318件的基础上，划分了81~105 Ma、110~130 Ma和141~160 Ma三个年龄集中区间，按照成矿作用、矿床类型和成矿时序，认为区内成矿作用与三个构造幕同步，为第一幕Mo-W-Au、第二幕Au-Ag和第三幕Cu-Pb-Zn-Mo-Au成矿作用。Mo-W-Au成矿系列典型矿床是邢家山钼钨矿、铜矿和留村金矿；Au-Ag成矿系列是鲁东地区主成矿期的表现形式，可进一步分为与早白垩世壳幔岩浆作用有关的Au、Ag矿床成矿亚系列、与早白垩世壳幔岩浆作用有关的Au、Ag、Cu、多金属矿床成矿亚系列和与早白垩世壳幔岩浆期后热液活动有关的Cu、Au、Fe、Ag、硫铁矿矿床亚系列，其典型矿床为焦家金矿、三山岛金矿和玲珑金矿等。Cu-Pb- Zn-Mo-Au成矿系列与伟德山造山晚期闪长岩–花岗闪长岩–花岗岩组合、浅成–超浅成石英闪长玢岩–花岗闪长斑岩–石英二长玢岩组合和青山群石前庄组中酸性火山喷发建造关系密切，是区内金矿的叠加成矿期及多金属矿的主成矿期，成矿年龄81~105 Ma。 Eastern Shandong Province is situated at the intersection of three tectonic domains as Tetisi, Paleo-Asian Ocean and Pacific Ocean. Interaction process of these three global tectonic domains control and affect the metallogenic environment of eastern ShandongProvince. This area has experienced lndosinian orogenic event and Yanshanian oro- genic event during Mesozoic period. Yanshanian orogenic event represents three tectonic episodes. Each episode repre- sents cyclicity of extrusion and extension. This cyclicity is the geological background of mineralization in Ludong gold deposit and other polymetallic deposits. Mineralization geological environment of three world-class gold fields with the reserve over one thousand tons, such as Sanshaodao gold field, Jiaojia gold field and Linglong gold field have the char- acteristics of uniqueness and induplication. Mesozoic tectonic evolution controls tectonic-magmatic event, sedimentary environment and gold mineralization. There is a strong coupling relationship between magma-sedimentary-tectonism -mineralization event and regional tectonic evolution. The formation and location of gold deposits and polymetallic deposits also have close connections with three tectonic episodes of Yanshanian orogenic event. Three mineralization events (160 - 140 Ma, 130 - 110 Ma and 105 - 80 Ma) have coupling relationship with three regional tectonic-magmatic hydrothermal events of Linglong rock mass, Guojialing rock mass and Weideshan rock mass respectively. On the basis of gold mineralization ages gained from 318 samples which have been reported in this area, three age concentration periods have been divided as 81 - 105 Ma, 110 - 130 Ma and 141 - 160 Ma. According to mineralization, deposit types and ore-forming sequences, it is regarded that mineralization in this area is synchronous with three tectonic episodes. That is the first Mo-W-Au tectonic episode, the second Au-Ag tectonic episode and the third Cu-Pb-Zn-Mo-Au tectonic episode mineralization. Typical deposits of Mo-W-Au metallogenic series are Xiangjiashan molybdenum and tungsten deposit, copper deposit and Liucun gold deposit. Au-Ag metallogenic series are major representative type of main min- eralization stages in eastern Shandong Province. It can be further divided into Au-Ag metallogenic sub-series which has close relation with mantle-crust magma interaction in early Cretaceous period; Au-Ag-Cu metallogenic sub-series which has close relation with mantle-crust magma in early Cretaceous period; and Cu-Au-Fe-Ag metallogenic sub-se- ries which has close relation with post-magmatic hydrothermal in early Cretaceous period. Typical deposits are Jiaojia gold deposit, Sanshandao gold deposit and Linglong gold deposit. Cu-Pb-Zn-Mo-Au metallogenic series is the main stage of superimposition mineralization and polymetallic mineralization with a mineralization age at 81 - 105 Ma. This series is closely related to the diorite-granodiorite-granite and quartz diorite porphyry-granodiorite porphyry-monzonite porphyry of the late Weideshan orogenic stage, and acid-intermediate volcanic-sedimentary formation of Shiqianzhuang formation in Qingshan Group.

Application of number–size (N-S) fractal model for separation of mineralized zones in Dareh-Ashki gold deposit, Muteh Complex, Central Iran

The goal of this study is to separate different mineralized zones in Dareh-Ashki gold deposit located in Muteh Complex, Central Iran, by using number–size (N-S) fractal model. The N-S log–log plot shows seven geochemical populations and four Au-mineralized zones. Based on obtained results, Au thresholds are 0.17, 0.32, 6.3, and 12.6 ppm which represent weakly, moderately, highly, and extremely mineralized zones in terms of Au grades, respectively. Au values lower than 0.17 ppm illustrate wall rocks. Main mineralization stage of gold commences from 6.3 ppm in this deposit. The moderately mineralized zone with Au values between 0.32 and 6.3 ppm has occupied the biggest part of the studied deposit. However, highly (with Au values between 6.3 and 12.6 ppm) and extremely (higher than 12.6 ppm) mineralized zones have small extension. Correlation between geological model and results from N-S fractal model reveals that the gold mineralized zones specifically the moderately mineralized zone are situated in green schist units.

Mineralogy, fluid inclusion, and oxygen isotope constraints on the genesis of the Lalla Zahra W-(Cu) deposit, Alouana district, northeastern Morocco

The Lalla Zahra W-(Cu) prospect of northeastern Morocco is hosted in a Devonian volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of the 284 ± 7 Ma Alouana concentrically zoned, two micas, calc-alkaline, and post-collisional Alouana granitoid pluton has contact metamorphosed the host rocks, giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, and alusite, and cordierite. The mineralization occurs in and along subvertical, 20 to 40 cm thick, and structurally controlled tensional veins composed of quartz accompanied by molybdenite, wolframite, scheelite, base metal sulphides, carbonates, barite, and fluorite. Three main stages of mineralization (I, II, and III), each characterized by a specific mineral assemblage and/or texture, are recognized. Quartz dominates in all the veins and commonly displays multiple stages of vein filling and brecciation, and a variety of textures. The early tungsten-bearing stage consists of quartz-1, tourmaline, muscovite, wolframite, scheelite, and molybdenite. With advancing paragenetic sequence, the mineralogy of the veins shifted from stage I tungsten-bearing mineralization through stage II, dominated by base metal sulphides, to stage III with late barren carbonates and barite ± fluorite mineral assemblages. Pervasive hydrothermal alteration affected, to varying degrees, the Alouana intrusion, resulting in microclinization, albitization, episyenitization, and greisenization of all the granitic units. Fluid inclusion data yield homogenization temperatures ranging from 124°C to 447°C for calculated salinity estimates in the range of 0.4 to ~60 wt% NaCl equiv. Similarly, the δ18O values for the three generations of quartz range from 11.7‰ to 13.9‰ V-SMOW. Calculated δ18O values of the parent fluid in the range between −3‰ and +9‰ V-SMOW are consistent either with a mixture of water of different origins, including magmatic water, or an origin from seawater or meteoric water that probably exchanged oxygen with rocks at elevated temperatures. The coexistence of CO2-rich and H2O-rich fluid inclusions reflect the presence of a boiling fluid associated with the deposition of the early tungsten-bearing stage mineralization at relatively high temperature. The general temperature and salinity decrease with advancing paragenetic sequence suggest that the early high temperature, magmatic, highly saline, and boiling fluid mixed with meteoric non-boiling fluid results in the precipitation of base metal sulphide and carbonate–barite stage mineral assemblages, respectively.

Paragenesis, Stable Isotopes, and Molybdenite Re-Os Isotope Age of the Lala Iron-Copper Deposit, Southwest China

The Lala Fe-Cu deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, SW China, is hosted in the Paleoproterozoic Hekou Group. This volcanic-sedimentary sequence has undergone upper greenschist-lower amphibolite facies metamorphism at ~850 Ma. The deposit contains more than 200 million metric tons (Mt) of Fe-Cu ores with an average grade of 13 wt % Fe and 0.92 wt % Cu. Orebodies are roughly strata bound but occur along foliation, lithologic contacts, and shear zones. The paragenetic sequence of the Lala deposit includes metamorphosed (I, II, and III) and unmetamorphosed (IV and V) stages. Stage I is characterized by extensively pervasive Na alteration, forming abundant albite but minor scapolite. Stage II occurs as massive and banded replacements, consists of Ti-V–poor magnetite and apatite with minor disseminated sulfide minerals, and was associated with pervasive K-Fe alterations forming K-feldspar, chlorite, and actinolite. Stage III occurs predominantly as bands and disseminations confined along foliations with minor veinlets cross-cutting hosting rocks. This stage is characterized by abundant Fe-Cu-Mo sulfides intergrown with angular magnetite and minor titanite and allanite and is associated with alteration of carbonate, quartz, fluorite, and mica. Stage IV has an assemblage dominated by chalcopyrite with variable pyrite and bornite, occurring as veins of sulfides + quartz + calcite + biotite ± muscovite ± fluorite. Stage V occurs chiefly as veins of hematite ± calcite ± quartz and also results in extensive hematization. Stages II and III have fluids with high δ 18 O (7.2–11.7‰) and δ 34 S values (0–4‰), indicative of a magmatic-hydrothermal origin. However, the stage III fluids have δ 34 C values (~0.5‰) close to the least altered marble (0–2‰) of the hosting rocks, indicating a possible contribution of strata-sourced carbon. The stage IV fluids have δ 34 C values similar to and δ 18 O (4.1–6.7‰) lower than the stage III fluids, and a wider range of δ 34 S values (−9.0 to +10.5‰), consistent with significant contributions of strata-sourced sulfur. Fluids of the late barren vein stage (V) have relatively low δ 34 C (−4.0 to −2.4‰) and δ 18 O values (−2.9 to +0.6‰), indicative of significant contributions of meteoric and oxidized fluids. Molybdenite from stage III has an Re-Os isotope age of 1086 ± 8 Ma, providing tight constraints on the timing of the main stage of mineralization. This age indicates that the Lala deposit is coeval with the ~1100 Ma intraplate magmatism in the western Yangtze Block. We propose that the majority of the Fe-Cu deposits in the Kangdian IOCG metallogenic province may have formed in an intraplate extensional setting, although it is possible that some of the deposits may have involved in multiple events including modification by the Neoproterozoic metamorphism forming the mineral assemblages of stage IV.

Origin and evolution of hydrothermal fluids in the Taochong iron deposit, Middle–Lower Yangtze Valley, Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Middle–Lower Yangtze River Valley is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. The Taochong deposit is located in the northern part of the Fanchang iron ore district of the Middle–Lower Yangtze River metallogenic belt. The Fe-orebody is hosted by Middle Carboniferous to Lower Permian limestones. Skarns and Fe-orebodies occur as tabular bodies along interlayer-gliding faults, at some distance from the inferred granitic intrusions. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity—the skarn, iron oxide (main mineralization stage), and carbonate stages—all contributed to the formation of the Taochong iron deposit. The skarn stage is characterized by the formation of garnet and pyroxene, with high-temperature, hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids. These fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite, chlorite, quartz, calcite and hematite. The hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor-rich inclusions with anomalously low δD values (−66‰ to −94‰). The decrease in ore fluid δ18Owater with time and decreasing depth is consistent with the decreases in fluid salinity and temperature. The fluid δD values also show a decreasing trend with decreasing depth. Both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water. This process was probably induced by a drop in pressure from lithostatic to hydrostatic. The carbonate stage is represented by calcite veins that cut across the skarn and orebody, locally producing a dense stockwork. This observation indicates the veins formed during the waning stages of hydrothermal activity. The fluids from this stage are mainly represented by a variety of low-salinity fluid inclusions, as well as fewer high-salinity inclusions. These particular fluids have the lowest δ18Owater values (−2.2‰ to 0.4‰) and a wide of range of δD values (−40‰ to −81‰), which indicate that they were originated from a mixture of residual fluids from the oxide stage, various amounts of meteoric water, and possibly condensed vapor. Low-temperature boiling probably occurred during this stage.We also discuss the reasons behind the anomalously low δD values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures, and suggest that calcite data provide a possible benchmark for adjusting low δD values found in quartz intergrown with calcite.

Na-Metasomatism and Uranium Mineralization during a Two-Stage Albitization at Kitongo, Northern Cameroon: Structural and Geochemical Evidence

Mapping and documentation of lithological varieties and their corresponding geochemistry at the Kitongo uranium mineralization were concerned. The Kitongo U occurrence is hosted by granitic rocks that include interleaved sequences of metasedimentary and metavolcanic rocks of the collectively termed Poli Group. U-mineralization and Na-metasomatism are related and structurally controlled. The most promising uraniferous bodies are intimately related to intersections between the ductile ENE-trending faults and the brittle conjugate R' faults postdating the shearing event. The con- centration of uranium at fault intersections rather than along individual faults suggests that these zones that are dilatational in nature were also highly permeable and therefore the hydrothermal fluids ponded there could readily precipitate U therein. A two-stage albitization has altered the foliated granitic host rock and the second albitization that has over-printed the first one is more effective at fault intersections. Whole rock geochemistry was performed by using ICP-MS and ICP-AES respectively for major oxides, trace and REE. The U-bearing rock suite exhibits restricted range in SiO2 concentration (62.89% - 70.91%) and Al2O3 (13.16% - 18.59%) and it is poor in MgO (0.02% - 1.03%), CaO (0.24% - 1.88%) and K2O (0.08% - 5.32%). The mineralized rocks are however comparatively richer in Na2O (4.33% - 10.92%) compared to their barren counterparts. The host granite and associated granodioritic rocks in the area are weakly metaluminous, peralkaline, and are calc-alkaline. They are moderately to strongly fractionated and have tholeiitic and shoshonitic affinities with moderate to high HFSE (high field strength elements) and LILE (large ion lithophile elements) enrichment. The Rb/Sr, Rb/Ba and Sr/Ba ratios are 0.31, 0.14 and 1.48, respectively. U content in the mineralized granite is up to 651 ppm while the non-mineralized rock has only 2.4 ppm U. The REE patterns of the granite show LREE enrichment and strong Eu negative anomalies (Eu/Eu* = 0.03 to 0.48). The main mineralization stage characterized by local U, Na, Pb, Zn, Ga, Hf, Sr, Fe, Al, P and Zr enrichments is related to the second albitization event and could probably be associated in time with the calcite-uranium stage. The identification of fault segments favorable for uranium mineralization in northern Cameroon (Poli area) is important for understanding the genesis of hydrothermal ore deposits within continental strike-slip faults and therefore has great implications for exploration strategies.

LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis

The Xiaoqinling district, the second largest gold producing district in China, is located on the southern margin of the North China Craton. It consists of three ore belts, namely, the northern ore belt, the middle ore belt and the southern ore belt. Pyrite from the Dahu gold deposit in the northern ore belt and Wenyu and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). A range of trace elements was analyzed, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni, Zn, Mo, Hg, As and Si. The results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts. In general, Au concentrations in pyrite are low (from < 0.01 ppm to 2.2 ppm) but Ni concentrations are rather high (up to 8425 ppm). A four-stage mineralization process is indicated by microscopic and field observations and this can be related to the systematic trace element differences between distinct generations of pyrite. Stage I precedes the main gold mineralization stage; pyrite of this stage has the lowest Au concentrations. Stages II and III contributed most of the gold to the ore-forming system. The corresponding pyrite yielded the highest concentrations of Au and Ni. Our microscopic observations suggest that pyrite in the main gold mineralization stage precipitated simultaneously with molybdenite that has been previously dated as Indosinian (~ 218 Ma by Re–Os molybdenite dating), indicating the Indosinian as the main gold mineralization stage. The Indosinian mineralization age and the geological and geochemical features of these gold deposits (e.g., low salinity, CO 2-rich ore fluids; spatial association with large-scale compressional structures of the Qinling orogen; δ 18O and δD data suggestive of mixing between metamorphic and meteoric waters; δ 34S and Pb-isotopic data that point to a mixed crustal-mantle source) all point to typical orogenic-type gold deposits. High Ni concentrations (up to 8425 ppm) of pyrite possibly linked to deep-seated mafic/ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity, but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers. Generally low Au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits. Therefore, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is probably related to Indosinian collision between the North China Craton and the Yangtze Craton.

Ore Geology, Fluid Inclusion, and S‐ and Pb‐Isotopic Constraints on the Genesis of the Chitudian Zn‐Pb Deposit, Southern Margin of the North China Craton

AbstractThe Chitudian Zn‐Pb ore deposit, Luanchuan, Henan province, was recently discovered in the southern margin of the North China Craton. The Zn‐Pb orebodies are hosted in the Proterozoic Guandaokou and Luanchuan Groups, occurring as veins in interbedding fracture zones mainly in a WNW‐ and partially in a NS‐direction. The Zn‐Pb ores are characterized by banded, massive, and breccia structures, coarse crystal grains, and a simple mineral composition mainly of galena, sphalerite, pyrite, quartz, dolomite, and calcite. In addition to the vein type orebodies, there are Mo‐ and Zn‐bearing skarn orebodies in the northwest of the Chitudian ore field. Four types of primary fluid inclusions in quartz and calcite were recognized in the Chitudian Zn‐Pb ores, including aqueous, aqueous‐CO2, daughter‐mineral‐bearing aqueous, and daughter‐mineral‐bearing aqueous‐CO2 inclusions, with aqueous inclusion being most common. The homogenization temperatures of the fluid inclusions from the main mineralization stage are from 290°C to 340°C, and the salinities mainly from 3.7 to 14.8 wt% NaCl equivalent. In addition to CO2, CH4 and H2S were detected in the vapor phase and HS‐ in the liquid phase of the fluid inclusions by Laser Raman spectroscopy. The δ34SV‐CDT values of ore sulfides from the Chitudian deposit range from −0.32‰ to 8.30‰, and show two modal peaks in the histogram, one from 1‰ to 4‰, and the other from 5‰ to 7‰. The former peak is similar to that of porphyry‐type Mo‐W deposits in the area, whereas the latter is relatively close to the sulfur in the strata. The ore sulfur may have been derived from both the magma and the strata. The Pb‐isotopic compositions of the ore minerals from Chitudian, with 206Pb/204Pb from 17.005 to l7.953, 207Pb/204Pb from 15.414 to 15.587, and 208Pb/204Pb from 37.948 to 39.036, are similar to those of Mesozoic porphyries in the Chitudian ore field, suggesting that the ore‐forming metals were mainly derived from the Mesozoic magmatic intrusions. The Chitudian Zn‐Pb deposit is interpreted to be a distal hydrothermal vein‐type deposit, which was genetically related to the proximal, skarn‐type Mo ore deposits in the region.

Composition of Ore Fluid Inclusions from the Viburnum Trend, Southeast Missouri District, United States: Implications for Transport and Precipitation Mechanisms

Fluid inclusions hosted by sphalerite and dolomite cement from the Mississippi Valley-type (MVT) deposits of the Viburnum Trend of the Southeast Missouri district were analyzed by microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to determine the composition of mineralizing fluids and to seek insights into ore-forming processes. On the whole, fluid inclusions from throughout the paragenesis have compositions resembling deep sedimentary brines. A population of anomalously Pb-rich fluid inclusions with concentrations reaching 1,500 ppm was identified in some of the paragenetically early and main-stage sphalerite, which is interpreted to represent the influx of a Pb- and possibly overall metal-rich fluid into Southeast Missouri during the time of sulfide mineralization. This influx of Pb-rich fluid appears to have been intermittent, as at least one stage of dolomite cement that hosts Pb-poor fluid inclusions occurs between intervals of sphalerite that host Pb-rich fluid inclusions. If fluid flow into the district was principally from one direction, for example, from the Arkoma basin to the south, then the intermittency in high Pb concentrations could be explained by compositional changes in the source region or along the flow path. Alternatively, if fluid flow into the district was from two or more directions and fluid arriving from each direction had a different composition, then the intermittency in high Pb concentrations could be explained by transient variations in the relative flow rates from each direction. Lead concentrations on the order of thousands of parts per million make it unlikely that the Southeast Missouri ores were precipitated by any mechanism that involved simultaneous transport of metals and sulfide in the same fluid, as the concentrations of sulfide that could coexist with such high concentrations of Pb would be too low to allow the ores to be deposited within a geologically reasonable period of time, unless the pH was very low or groundwater flow rates were very high. Thus, some type of mixing scenario seems more likely involving either mixing of a metal- with a sulfide-rich fluid or mixing of a metal- and sulfate-rich fluid with a reductant present either in the host rock or in another fluid. Fluid inclusions hosted by sphalerite have K/Na and Ca/Mg ratios that partly resemble the compositions of both main-stage cuboctahedral galena and less abundant late-stage cubic galena, suggesting that sphalerite was precipitated from a mixture of the fluids that precipitated these two types of galena. However, the fact that aqueous Pb concentration does not correlate with these ratios or other indicators of main-stage mineralization, such as matrix Mn, Fe, Co, Cu, Ag, and Cd content in sphalerite, suggests that more than two fluids were involved in mixing.

The role of early formed structures on the development of the world class St Ives Goldfield, Yilgarn, WA

A revised structural interpretation for the Victory to Kambalda area of the world class St Ives Goldfield in the Archean Yilgarn Craton has mapped out the distribution of WNW-trending faults within this area of the field. These previously cryptic WNW-trending structures had been identified in gravity data, and also by isopach thickness variations. The WNW-trending faults acted as transfers syn-gold mineralization, although only discrete segments of these faults were active during the main stage of gold mineralization. Where mineralized, the faults transferred strain from a complex combination of block-on-block movement associated with thrusting and strike-slip movement on NW- and N-trending faults. Along some segments N-trending mineralized faults terminate against the WNW-trending faults. Many of the WNW-trending faults correlate with major strike changes on regional and camp-scale faults and they are domain boundaries for the critical N-trending fault segments that host high-grade gold within contractional jogs. The WNW-trending faults also show evidence for an older deformation history prior to main-stage gold, which may extend back to early basin development associated with ultramafic and mafic volcanism. They are inferred to have been a series of early WNW-trending normal faults and breached relay ramps associated with oblique rifting along an older NNW-trending basement boundary.

Geochemistry and geochronology of the volcanic rocks associated with the Dong'an adularia–sericite epithermal gold deposit, Lesser Hinggan Range, Heilongjiang province, NE China: Constraints on the metallogenesis

Late Mesozoic volcanism is widespread throughout NE China. On the basis of lithological associations and spatial relationships, the volcanic rocks in the Lesser Hinggan Range can be divided into two formations, i.e., felsic-dominant Fuminghe Formation and overlying mafic-dominant Ganhe Formation. The Dong'an gold deposit, a typical adularia–sericite epithermal system, is spatially closely associated with rhyolitic porphyry, which is a subvolcanic intrusion of the Fuminghe Formation. Total measured, indicated, and inferred resources for the Dong'an deposit are 70 tonnes (2.25 Moz) of gold with the grade of 5.04 g/t Au, making it one of the largest epithermal gold deposits in China. SHRIMP U–Pb zircon and 40Ar/ 39Ar geochronology applied to one rhyolitic porphyry sample and sericite separated from auriferous quartz veins of the main mineralization stage were carried out to constrain magmatic and hydrothermal events. The results suggest that the mineralization age of 107.2 ± 0.6 Ma overlaps with the age of the rhyolitic porphyry 108.1 ± 2.4 Ma. Our new age data indicate that there was a previously unrecognized mineralization event in NE China at 107–108 Ma. Systematic geochemical investigations on the volcanic rocks in the Lesser Hinggan Range show that both Fuminghe and Ganhe Formations are characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment coupled with high field strength elements (HFSE) depletion, but they have distinct Sr and Nd isotopic compositions. The Fuminghe Formation has relative high 87Sr/ 86Sr ratios of 0.707253 to 0.707373, and negative ε Nd( t) values of −2.78 to −3.05 ( t = 108 Ma), whereas the Ganhe Formation displays slightly lower 87Sr/ 86Sr range of 0.705434–0.705763 and positive ε Nd( t) values of + 0.76 to +1.83. These geochemical data suggest that the rhyolitic magmas of the Fuminghe Formation probably represent the final differentiates of parental andesitic magmas, resulted from the partial melting of mafic lower crust, whereas the volcanic rocks of the Ganhe Formation were produced by fractionation of basaltic magmas generated from partial melting of a mixture of an incompatible element depleted anhydrous lherzolite asthenospheric mantle source and a hydrous enriched lithospheric mantle source in an extensional tectonic setting, in response to upwelling of asthenospheric mantle. The rhyolite porphyries of the Fuminghe Formation are inferred to have supplied heat that drove the convective hydrothermal system at Dong'an deposit, but also provided some of the fluid sources responsible for the development of the Dong'an epithermal system.

Mineralogy and geochemistry of the sodium metasomatism-related uranium occurrence of Aricheng South, Guyana

The Aricheng South uranium occurrence is associated with Na metasomatism that affected the granitoids of the Kurupung Batholith in western Guyana. The mineral paragenesis indicates that late-magmatic albitization was followed by chlorite alteration of biotite. A minor amount of uraninite occurs in fractures in the newly formed albite crystals, often in company of calcite. The main mineralization stage occurred later than albitization and chloritization and is represented by brannerite disseminated in a groundmass of fine-grained hydrothermal zircon. Whole rock geochemistry supports the temporal dissociation of albitization from the main ore stage. Brannerite, zircon, and uraninite are often partially altered to secondary brannerite, zircon, and coffinite, respectively. Stable oxygen (chlorite, calcite) and hydrogen (chlorite) isotope compositions suggest that a highly evolved meteoric fluid, or at least one corresponding to a very high rock/fluid ratio (δ18O of approx. 3.4% to 4‰ and δD of approx. −80‰) may have caused the pre-ore alteration assemblage. The fluids in equilibrium with main ore stage zircon have δ18O of approx. 6.8‰ and appear to be of magmatic origin. The Aricheng occurrence geochemically, mineralogically, thermally, and paragenetically resembles the Valhalla U deposit in northern Australia despite differences between the deposits’ host lithologies, whereas the Lagoa Real and Espinharas U deposits in Brazil have host rock lithology that resembles that of Aricheng.