Orogenic-type Gold Deposit Research Articles

Mantle-plume model for the Zun-Kholba orogenic gold deposit (Eastern Sayan, Russia): results of mineralogical, Rb-Sr and <sup>40</sup>Ar-<sup>39</sup>Ar geochronological and Pb-Pb isotopic studies

The large Zun-Kholba deposit, belonging to a numerous group of orogenic-type gold deposits, is in the Eastern Sayan (Russia) – a segment of the Altai-Sayan fold system of the Central Asian Orogenic Belt. The paper discusses the results of mineralogical, geochemical, geochronological and Pb-isotopic studies of gold mineralization, the purpose of which was to verify the genetic model of the deposit. Mineralogical and geochemical data obtained for ore bodies located between hypsometric levels from 1290 to 2090 m indicate a complex distribution of mineral associations at the deposit. There is no vertical and horizontal zonation in the distribution of mineral associations, in the chemical composition of the main ore minerals, as well as in the content of impurity elements in them. In dating ore-forming processes at the Zun-Kholba deposit, a comprehensive approach based on the study of K-Ar and Rb-Sr isotope systems of metasomatites was applied. The combination of Rb-Sr and 40Ar-39Ar methods allowed us to determine that the age of gold mineralization is 411 ± 2 Ma, while the age of the superimposed event, which is associated with redistribution of ore, as well as disturbance of the Rb-Sr and K-Ar isotopic systems of metasomatites, is about 380 Ma. Pb-Pb isotopic study of ore mineralization at the deposit and host Precambrian rocks allowed to prove the leading contribution of the latter in ore lead supply to the mineral-forming system. The model of formation of the Zun-Kholba orogenic gold deposit assumes a genetic relationship between ore-forming processes and intraplate alkaline mafic magmatism due to the impact of the plume in the Early Devonian time on the lithosphere of the Tuva-Mongolian terrane.

Gold mineralization in the diaphthorites of the Verkhne-Timptonskiy ore district (Aldan-Stanovoy shield)

In the Verkhne-Timptonskiy gold ore district, located where the Stanovoy and Aldan blocks intersect (Aldan-Stanovoy shield), substantial placer gold deposits have been found in the Gonam, Timpton, and Iengra rivers. The exploration of certain deposits in this area began at the end of the 19th century. Despite exploration efforts conducted in the second half of the 20th century, only minor gold ore findings were made, with uncertain links to greenstones and igneous formations from various periods. After conducting a thorough analysis of both existing and new data on the geology and metallogeny of the Verkhne-Timptonskiy gold ore district, we have concluded that gold occurrences in diaphthorites of the Kholodnikan belt display characteristic features of orogenic-type gold deposits. These features include disseminated pyrite and veinlet mineralization containing iron (Fe), lead (Pb), zinc (Zn), copper (Cu) sulfides, and sulfosalts. The near-ore alterations are represented by silicification, sericitization, carbonatization, and beresitization, characteristic of mesothermal fluid systems. Moreover, we identified common features of the geological structure and metallogeny of the Verkhne-Timptonskiy gold ore district and the Jiaodong province of the North China Craton. Thus, gold mineralization in the Verkhne-Timptonskiy gold ore district and the Jiaodong province is spatially associated with suture zones. In the Verkhne-Timptonskiy gold ore district, the Amga zone of tectonic melange in the Paleoproterozoic era was one of the regions where cratonization of blocks of the Earth’s crust occurred. Later, it became a zone of permeability and concentration of tectonothermal and metallogenic events of various ages. The results obtained will help assess the metallogenic potential of the Verkhne-Timptonskiy district and the Aldan-Stanovoy shield as a whole. They make it possible to increase the efficiency of search operations and focus on localizing research objects.

Nature, origin, and evolution of carbon-rich fluids in orogenic gold deposits: Insights from fluid inclusion and C-H-O isotope studies of the Tokuzbay gold deposit, Chinese Altai

Carbon-rich or carbonic fluids are identified in some orogenic-type gold deposits worldwide. Their nature, origin, evolution, and relations with gold mineralization are still poorly understood. The Tokuzbay large-size orogenic-type gold deposit in Chinese Altai is characterized by enormous CO2-rich fluid inclusions (FIs), providing an ideal case to investigate the genetic relationship between carbon-rich hydrothermal systems and orogenic-type gold mineralization. The mineralization process in the Tokuzbay includes four stages: (1) stage I quartz-magnetite veins; (2) stage II quartz-pyrite veins; (3) stage III quartz-polymetallic sulfide veins; and (4) stage IV quartz-carbonate veins. Based on the microscopic investigation, cathodoluminescence (CL) imaging, Raman spectrum analysis, and mircothermometric measurements, three types of FIs are identified in the veins, including the N2-bearing carbonic or CO2-rich inclusions (PC-type), the CO2-H2O inclusions (C-type), and the H2O-dominated or aqueous inclusions (W-type). The FIs in the stage I minerals are dominated by PC- and C-types, with minor amount of W-type, and occur as primary and pseudosecondary within quartz grains. These inclusions show a salinity of 5.0—9.5 wt% NaCl eqv. and homogenize to liquid phase during 272℃ to 399℃, indicating their entrapment from an initially homogeneous carbon-rich CO2-H2O-NaCl metamorphic fluid system. From early to late stages, the PC- and C-type FIs are getting infertile, while the W-type FIs become more abundant, indicating the escapes of CO2 and compositional transition from CO2-rich to H2O-rich system. Moreover, the decreasing in temperature (272–399 ℃, 238—326 ℃, 173—267 ℃ and 135—182 ℃ from stages I to IV, respectively) and salinity (5.0—9.5, 2.6—9.2, 2.3—7.8 and 1.4—4.3 wt% NaCl eqv. from stages I to IV, respectively), together with the decrease of the δ18O values, suggests an input of meteoric water during mineralization. In the main mineralization stages (II and III), some FIs show divergent homogenization direction (liquid vs vapor) at similar homogeneous temperatures, yielding quite different salinities. For example, the coexisting W-type and C-type FIs yield similar homogenization temperatures, and the liquid-rich and vapor-rich FIs divergently homogenize to liquid and vapor at similar temperatures with different salinities. This indicates that the fluid effervescence drove phase separation and caused the CO2 escape from the fluid system which promoted the gold precipitation at the ductile-to-brittle transition zone that revealed by the depth (8—11 km) calculated by the C-type FIs of the immiscible inclusion assemblages.The δ13C values of the CO2 extracted from fluid inclusions range from –7.6 to –22.8‰, suggesting that the CO2 might be derived from the hydrolysis of organic matter in the source sediments during metamorphism, rather than simply sourced from magma or the mantle. This understanding is supported by the widespread observation of the low δ13C values from most of orogenic-type gold deposits hosted in sedimentary rocks worldwide.

The Peninsular Ranges orogenic gold belt: Supporting evidence from the San Pedro Mártir mining area (Baja California, Mexico)

The gold deposits in the northern Sierra San Pedro Mártir region of the Peninsular Ranges batholith (PRB) occur within a middle-Cretaceous suture zone that juxtaposes the Alisitos island arc against the continental margin of North America. Mineralization shares affinities with orogenic-type gold deposits and typically consists of shear zone-related gold-bearing quartz-carbonate veins, with minor amounts of sulfides, white mica and native gold. Host rock types encompass varied synorogenic plutonic rocks and greenschist to amphibolite facies metamorphic rocks. In this paper, we present four 40Ar/39Ar ages of hydrothermal white micas for representative gold deposits: 106.2 ± 0.2 Ma (Valladares), 100.5 ± 0.2 Ma (La Fortuna-San José), 94.2 ± 0.2 Ma (El Socorro), and 92.2 ± 0.2 Ma (Corazón de Oro de Jesús). The first two ages correspond to highly accurate determinations of the timing of the orogenic gold event, whereas we attribute the younger ages to thermal resetting caused by the emplacement of a nearby large La Posta-type pluton (∼97‒90 Ma). Additionally, in order to establish the maximum age of mineralization for the granitoid-hosted deposits, three new U–Pb zircon ages were obtained for plutonic rocks (136.7 ± 0.9 Ma, Corazón de Oro de Jesús quartz diorite orthogneiss; 107.0 ± 0.5 Ma, Valladares tonalite pluton; 105.0 ± 2.0 Ma, Santa Cruz diorite pluton) Furthermore, fluid inclusions were analyzed from six gold deposits. Four different types of ore fluids were distinguished, with likely temperatures of trapping between ∼300° and >600 °C, and salinities between ∼1 and 17 wt.% NaCl equiv. Gold mineralization formed subsequent to the initial collisional phase, occurring simultaneously with regional metamorphism and pluton emplacement. At least some synorogenic granitoid-hosted deposits were formed under a transient dextral transpressive regime, intercalated between the predominant regional orthogonal compression. A connection is established between these gold deposits and others found in historical mining districts along the exhumed axis of the PRB (e.g., El Álamo), and we propose that all form part of the ‘Peninsular Ranges orogenic gold belt’.

Metal source and ore precipitation mechanism of the Ashawayi orogenic gold deposit, southwestern Tianshan Orogen, western China: Constraints from textures and trace elements in pyrite

The metal source and ore precipitation mechanism of orogenic gold mineralization are not yet well understood, partly because ore metals may be derived from different sources. Pyrite is a dominant Au-hosting mineral in the Ashawayi orogenic gold deposit in the southwestern Tianshan orogen, western China. Petrographic features of pyrite in host rocks and orebodies define four generations: diagenetic preore (Py1), hydrothermal early-ore (Py2), main-ore (Py3), and late-ore (Py4) pyrites.Trace element abundances were analyzed in situ by femtosecond laser ablation inductively coupled plasma mass spectrometry (fs-LA-ICP-MS) to unravel the pyrite formation history. Preore Py1 contains the lowest Cu, Mo, Se, Au and As contents, consistent with a diagenetic origin. Py2 has higher Au and As contents than Py1 and may have formed by the reaction between hydrothermal fluid and preexisting Py1, as indicated by diagenetic pyrite-like As/Ni and Bi/Au ratios but lower hydrothermal pyrite-like Sb/Au ratios in Py2. Hydrothermal pyrite (Py3) contains more abundant As (1723–65182 ppm) and Au (0.32–107 ppm) but lower Co and Ni contents than Py2, suggesting a greater hydrothermal fluid contribution. Oscillatory zoning and abundant mineral inclusions (e.g., arsenopyrite and chalcopyrite) in porous Py3 indicate that fluid boiling was responsible for gold deposition during the main-ore stage. Py4 is a relict of hydrothermal pyrite (Py3) but not diagenetic pyrite, as supported by Py4 and Py3 clustering into a class based on hierarchical cluster analysis. The application of a machine learning method (i.e., artificial neural network) to the syn-ore pyrite indicates that the Ashawayi gold deposit has affinity to those from orogenic-type gold deposits worldwide. Our study, therefore, highlights that ore metals in orogenic gold deposits may originate from different sources, such as Au and As, which are largely sourced from metamorphic fluids, while Co and Ni are mainly released from preore sedimentary pyrite, fluid boiling and fluid-rock interaction triggered precipitation of Au and other metals.

Revealing the orogenic genesis of Huogeqi Cu-Pb-Zn deposit: Evidence from the machine learning-based data interpretation of pyrite geochemistry

Orogenic-type gold deposit is of great economic importance worldwide and its ore-fluid is believed to have poor ability in transporting base metals such as Cu, Pb, and Zn. However, increasing numbers of case studies have shown that base metal deposits can also form in similar geological processes as orogenic gold, and share similar characteristics with the latter. This leaves whether orogenic-type deposit is gold-only an open question for economic geologists. The shear zone-controlled Huogeqi Cu-Pb-Zn deposit is located in the north margin of the North China Craton. Although it is still debated whether this deposit is orogenic-type or of syngenetic origin, our previous geological, fluid inclusion, geochronological and stable isotopic studies have shown that it shares broad similarities with orogenic-type deposit. In this study, an alternative approach, the geochemistry of syn-ore pyrite coupled with supervised machine learning-based data interpretation, is employed to discriminate the genesis of this deposit. It is shown that the syn-ore pyrite is geochemically similar to those from orogenic-type deposits rather than VMS or sedimentary pyrite, and thus confirms the orogenic genesis and unambiguously exclude the syngenetic origin of this deposit from a different perspective. The results in this study support the view that orogenic-type ore-forming system can also form base metal deposit, which is consistent with the studies and simulation results by other researchers. In addition, since the pyrite can only record the geochemistry information in the final deposition of the mineral and eliminate the histories before the last deposition stage, the data interpretation of pyrite geochemistry by machine learning method is a powerful tool to discriminate the genesis of deposit, especially suitable for the deposits with multistage ore-forming processes.

Fluid inclusion and in situ sulfur isotope studies of the carbonaceous strata-hosted Eryiling gold deposit in the Beishan orogen, NW China

The Eryiling gold deposit in the Jinwozi orefield is a carbonaceous strata-hosted gold deposit in the Beishan Orogen, NW China. The gold orebodies are mainly layered, lenticular, and vein hosted in Jinwozi Formation, different from the Jinwozi gold deposit. In this contribution, based on a comprehensive study including field investigation of ore geology, fluid inclusion microthermometry, and in situ sulfur isotopic composition, we discuss the origin and source of the ore fluids and the gold precipitation mechanism at the Eryiling deposit. The ore-forming process includes the early milky white quartz-pyrite vein, the main stage smokey gray quartz-sulfide vein, and the late-stage quartz-calcite vein. The auriferous vein characterizes the main-stage veins. By micro-thermometry, CO2-bearing(C-), Pure CO2(PC-), Water(W), and daughter crystal(S) types of fluid inclusions were identified and investigated. The early-stage quartz vein contains C-type FIs with a salinity of 3.5 to 7.7 wt% NaCl eqv., and total homogenization temperature (Th) of 243 and 321 °C. It shows that the ore fluids possess characters of low salinity, CO2-rich, and mesothermal. These characters resemble to those of orogenic-type gold deposits. The C-type FIs in the main stage quartz show salinity of 3.5 to 8.9 wt% NaCl eqv., and Th of 200 and 350 °C. The W-type FIs are coexisting with C-type FIs which they yield salinity of 5.5 to 10.2 wt% NaCl eqv., and Th of 158 – 287 °C. A large amount of immiscibility fluid inclusion assemblages found in the main stage quartz provide clear evidence for gold precipitation caused by fluid immiscibility. Trapping pressures of 61–233 MPa for the early stage and 48–182 MPa for the main stage are estimated, equivalent to mineralization depths of ∼ 8.1 km and ∼ 6.4 km, respectively. The δ34S value of sulfide increased from early-stage (average of 3.8 ‰) to main-stage (average of 6.4 ‰), indicating sulfur contributed by the wall rocks through fluid-rock interaction as the wall-rock sulfide display value of average of 16.2 ‰. In summary, the Eryiling gold deposit is situated in the altered carbonaceous rock of the Jinwozi Formation and regulated by a fractured zone, which could be considered a typical carbonaceous strata-bound orogenic gold deposit. Moreover, the similarity of ore-forming fluids between Jinwozi and Eryiling gold deposits indicates that they are the products of the same ore-forming fluid in different wall rocks.

X-ray computed tomography study for drill core of the Drazhnoe
 orogenic-type gold deposit (Sakha (Yakutia), Russian Federation)

Orogenic gold-quartz deposits have a clear structural
 control and are accompanied by wallrock metasomatic
 alteration. However, in detailed modeling of such deposits,
 there is often a mismatch between the structural
 plans for high-grade ore zone distribution and metasomatite
 zones, and the latter are not always associated
 with faults. This is explained by the evolution of the hydrothermal
 process and the pulsating nature of the development
 of the territory. In the early stages of the mineral
 deposit study, it is very important to reliably determine
 the distribution of ore zones, since the correct targeting
 of the drilling program and the economic deposit
 assessment depend on it. The problem can be solved using
 the method of X-ray computed tomography (СТ) in the
 core study.
 This paper presents the methodology of studying fullsize
 core samples of gold deposits by using CT. A core
 sample characterizing the central part of ore body of
 Drazhnoye deposit (Tarynskoye ore field, Republic of Sakha
 (Yakutia)) was used as the study material. The sample
 studied was scanned by a SIEMENS Somatom Perspective
 tomograph at two energies (80 and 130 keV). As
 a result, a detailed three-dimensional stereological model
 of the core was obtained, which made it possible not
 only to study the distribution of ore minerals in the volume
 of the entire sample, but also to identify vein bodies
 of different ages, as well as to study their morphology
 and trace the distribution patterns of ore mineralization
 in them. Based on the study results, we can offer a
 preliminary interpretation of ore mineralization and
 vein formation sequence.

Geological and geochemical characterization of the Nanlia and Makorongo gold prospects, Mozambique Belt, northeastern Mozambique

Northern Mozambique hosts numerous occurrences of gold mineralization in metamorphic terranes, which are little known because of the insufficient studies. In this work, we describe the main geological and geochemical characteristics of the Nanlia and Makorongo gold prospects located in the Namuno district, northeastern Mozambique. We report the mode of occurrence of gold, petrography and mineralogy of the host rocks and ores, fluid inclusion characteristics and stable isotope data, to understand the genesis of gold mineralization in the study area. The gold mineralization in the Nanlia and Makorongo prospects is hosted by the Neoproterozoic Xixano Metamorphic Complex in the Mozambique Belt, which is composed of mafic granulite, amphibolite, paragneiss and marble. The host rock amphibolite consists mainly of amphibole, plagioclase, biotite and minor quartz, titanite and sulfides. The gold mineralization is associated with E-W to ENE-WSW trending quartz veins, oriented parallel to low angle to the foliation of the host rock amphibolite. The ore mineralogy consists of pyrite, pyrrhotite, chalcopyrite, magnetite, galena and sphalerite, with minor tellurides, xilingolite, electrum and native gold. Mineralized quartz veins in the Nanlia and Makorongo prospects host two types of fluid inclusions which locally coexist in the same assemblage: type 1, one-phase CO2 inclusions with final melting temperature between −58 and −52 °C; and type 2, two-phase H2O + NaCl (±CO2 ± N2 ± CH4) inclusions with homogenization temperature ranging from 246 to 370 °C and 239 to 382 °C in the Nanlia and Makorongo prospects, respectively, and salinity of 12–21 wt% NaCl eq. Pressure and temperature (P-T) conditions of ore-formation were estimated from fluid inclusions and sphalerite geobarometry at 420–620 °C and 160–280 MPa, and 330–440 °C and 150–200 MPa in the Nanlia and Makorongo prospects, respectively. The Nanlia and Makorongo gold prospects are classified as orogenic-type gold deposits. We suggest that gold mineralization was caused by an aqueous-carbonic fluid with low-moderate salinity originated by devolatilization during prograde metamorphism of underlying rocks. The gold mineralization post-dates the peak of metamorphism of the host rocks, thus formed during the retrograde metamorphism of the host rocks. The precipitation of gold in the Nanlia and Makorongo prospects was caused by fluid phase separation induced by pressure drop as the mineralizing fluid migrated to shallower depths.

Origin of ore-forming fluids of Tokuzbay gold deposit in the South Altai, northwest China: Constraints from Sr–Nd–Pb isotopes

• A low I Sr (t) fluid system is required for formation of Tokuzbay gold deposit. • Pb isotopes of sulfides are similar with the ore hosting Altai Formation. • The gold mineralization may have originated from the metamorphic dehydration of sediments of Altai Formation. Orogenic gold deposits are the most important gold resource in the world. It is controversial whether their ore-forming fluids were sourced from metamorphic dehydration of the crust or the mantle. The Tokuzbay gold deposit is a typical orogenic-type gold deposit in the Central Asian Orogenic Belt , providing a case for discussing the source of ore-forming fluids. The auriferous quartz veins are fault-controlled and occur mainly in metamorphic sediments of Devonian Altai Formation and minor in diorite dikes, forming three ore types, i.e., quartz vein, altered metamorphic sediments and altered diorite dikes. We studied the Sr–Nd–Pb isotope characteristics of the ore sulfides, and magmatic and metamorphic rocks from the Tokuzbay gold deposit, and thereby discuss the source of the ore-forming fluids. The 87 Sr/ 86 Sr ratios of meta -sediment hosted ores scatter between 0.709474 and 0.715238, and I Sr (292 Ma) values range from 0.706570 to 0.714045. The diorite dikes hosted ores show more restricted 87 Sr/ 86 Sr ratios ranging from 0.706963 to 0.711519, and I Sr (292 Ma) values ranging from 0.706777 to 0.710612. The wall rocks of the Altai Formation have I Sr (292 Ma) values of 0.711778 and 0.714409, which are higher than ore sulfides. This indicates that the I Sr (292 Ma) values of initial ore-forming fluids that interacted with the Altai Formation are no higher than 0.706777, which is the lowest I Sr (292 Ma) value for disseminated ores. The calculated ( 143 Nd/ 144 Nd) i ratios and ε Nd (292 Ma) values for meta -sediment hosted ores range from 0.511882 to 0.512464, and −7.4 to 4.0, respectively. The calculated ( 143 Nd/ 144 Nd) i ratios and ε Nd (292 Ma) values for diorite dike hosted ores vary from 0.512293 to 0.512562, and from 0.6 to 5.9, respectively. The meta -sediments of Altai Formation show ε Nd (292 Ma) values ranging from −6.8 to −4.4 with an average of −5.8, which are lower than those of ore sulfides. This indicates that the ε Nd (292 Ma) values for the fluids that interacted with the wall rocks cannot be lower than 4.5, which is the average value for disseminated ores. The calculated ( 208 Pb/ 204 Pb) i , ( 207 Pb/ 204 Pb) i , and ( 206 Pb/ 204 Pb) i values of ore sulfides are 37.796–38.989, 15.512–15.707, 17.988–18.100, respectively. The wall rocks of the Altai Formation show relatively higher ( 206 Pb/ 204 Pb) i ratios, indicating that the ore-forming fluid that interacted with the Altai Formation are not higher than 17.988, which is the lowest values of sulfides from disseminated ores. Based on the Sr–Nd–Pb isotope data, we suggest the possible ore-forming fluids should have lower I Sr (292 Ma) values, higher ε Nd (292 Ma) values, and lower ( 206 Pb/ 204 Pb) i values compared to the those of ore sulfides and were possibly sourced from a source compositionally similar to the diorite dikes in addition to wall rocks of Altai Formation. Combining with the regional metamorphism and tectonic evolution , we suggest that the ore fluids have originated from the metamorphic dehydration of sediments of Altai Formation in the South Altai during the collision between Altai Orogen and Junggar Plate.

Origin and evolution of ore fluids of the jinwozi gold deposit, beishan orogen, NW China

The Jinwozi gold deposit is one of the most economic gold deposits in the Beishan Orogen, NW China. The gold orebodies mainly occur as quartz veins hosted in Jinwozi granite. In this contribution, based on a comprehensive study including field investigation of ore geology, mineralogy, geochemistry, geochronology, and fluid inclusion microthermometry, we discuss the origin and evolution of the ore fluids and the gold precipitation mechanism at the Jinwozi deposit. The mineralization process includes three stages, i.e., the early quartz-pyrite veins, main-stage quartz-sulfide veins, and late carbonate-quartz veinlets, with gold being introduced in the main stage. Four types of fluid inclusions, i.e., PC-, C-, W-, and S-types, were observed and investigated in microthermometry. The early-stage quartz contains C-type FIs with a salinity of 3.9 to 9.0 (av. 5.8 ± 1.2) wt.% NaCl eqv., CO2 content of 10 to 85 vol%, and total homogenization temperature (Th) of 254 to 358 °C (av. 302 ± 17 °C). It shows that the ore fluids are low salinity, CO2-rich and mesothermal, and identical to those of orogenic-type gold deposits. The C-type FIs in the main stage quartz show salinity of 3.0 to 9.0 (av. 4.9 ± 1.2) wt.% NaCl eqv., CO2 contents of 8 to 90 vol% and Th of 210 to 326 °C (av. 256 ± 29 °C). The W-type FIs yield salinity of 5.5 to 10.2 (av. 8.5 ± 0.9) wt.% NaCl eqv.. and Th of 197 to 305 °C (av. 247 ± 25 °C). Gold precipitation was caused by phase separation that supported by plentiful immiscibility fluid inclusion assemblages observed in main stage quartz. Trapping pressures of C-type FIs are estimated at 90.5–286 MPa and 57.7–279 MPa for the early and main stages, corresponding to mineralization depths of ~10.4 km and ~5.8 km, respectively. Last but not least, the homogenization temperatures of the FIs in the main stage quartz gradually increase with depth (from 1590 m, through 1510 m, to 1477 m ASL) at the #3p4 orebody, but suddenly decrease at depth of 1442 m ASL. This temperature decreases possibly indicates the appearance of a new orebody beneath the currently exploited #3p4 orebody occurring in echelon formation.

Temperature-Controlled Ore Evolution in Orogenic Gold Systems Related to Synchronous Granitic Magmatism: An Example from the Iron Quadrangle Province, Brazil

Abstract Against the background of an ongoing debate on the genetic relationship between orogenic gold and granitic magmatism, we studied the evolution of a gold-mineralizing system in shear zone-hosted veins that are spatially associated with a syn- to late tectonic 2.69 Ga granite at the Satinoco deposit in the Archean Pitangui greenstone belt (Iron Quadrangle, Brazil). Detailed underground mine mapping, petrography, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and elemental mapping of sulfide grains revealed a complex polyphase history, with at least three gold-bearing stages: (1) A syntectonic arsenopyrite (Apy1)-löllingite-pyrrhotite assemblage I that formed during prograde metamorphism from ca. 475° to 650°C at 3 to 4 kbars; (2) further fluid circulation during syn- to late tectonic retrograde metamorphism from ca. 510° to 445°C, which led to a pyrrhotite-arsenopyrite (Apy2) ± löllingite ± galena assemblage II; and (3) a pyrite-pyrrhotite-galena ± chalcopyrite ± sphalerite ± ullmannite assemblage III formed during a later post-tectonic brittle deformation event from ca. 350° to 300°C at 1 to 2 kbars. Ascending granitic magma acted as a heat engine for peak metamorphic temperatures locally, that is at the immediate contact, reaching as much as 730°C and, together with the dehydration of mafic and ultramafic host rocks, generated the sulfide assemblage I. Crosscutting field relationships, together with existing S isotope data, indicate a metamorphic origin for all the sulfide stages with a contribution from granite-derived magmatogenic fluids to the mineralizing system at stage II. New X-ray element maps together with LA-ICP-MS spot analyses document oscillatory zoning of As, Fe, and Ni in Apy1 and enrichment in Ni, Mn, Zn, and V. The large amount of pyrrhotite in assemblage II is explained by granite-related Fe metasomatism. This stage led to an enrichment in Co, Sb, and Pb and the redistribution of Au. Assemblage III involved the formation of pyrite veins and precipitation of free gold and Pb-Sb-Cu-Zn sulfides in microfractures of this pyrite. This stage was likely responsible for the remobilization of Ni, Cu, Zn, and Bi-Te minerals. Based on existing geochronological data, at least most of the gold formed between ~2.72 and 2.68 Ga. This study revealed that gold mineralization took place over a wide range of temperatures—a finding that might be not only specific to the Satinoco deposit but possibly to other orogenic-type gold deposits elsewhere as well.

Caledonian (Early Paleozoic) veins overprinted by Yanshanian (Late Mesozoic) gold mineralization in the Jiangnan Orogen: A case study on gold deposits in northeastern Hunan, South China

The Jiangnan Orogen, situated between the Yangtze and Cathaysia blocks in South China, is an important gold-polymetallic metallogenic belt in China. Like many gold deposits hosted in Precambrian rocks in eastern China, the timing of gold mineralization in the Jiangnan Orogen, mostly hosted by Neoproterozoic rocks, has been controversial. A number of previous geochronological studies have suggested that mineralization ages range from Neoproterozoic to Cretaceous, with Caledonian (Early Paleozoic) and Yanshanian (Jurassic – Cretaceous) being considered most important. As Caledonian is the most important period of regional metamorphism, and Yanshanian is the most important period of granitic magmatism, the timing of mineralization is closely related to the genetic nature (orogenic versus post-orogenic) of the gold mineralization. This paper addresses these problems by studying the ages and characteristics of ore-forming fluids of two major gold deposits (Wangu and Huangjindong) in the central part of the Jiangnan Orogen, in northeastern Hunan. Muscovite Ar-Ar dating suggests that the bulk of the quartz veins in the Wangu and Huangjindong deposits were formed at ca. > 399 – 397 Ma, i.e., during the Caledonian orogeny, whereas gold mineralization (and associated veining) took place at ca. 142 – 130 Ma, i.e., during the Yanshanian orogeny. Fluid inclusions in the barren “Caledonian veins” are characterized by the H2O-NaCl-CO2 system, homogenization temperatures of 266 – 276 °C and salinities of 5 – 5.4 wt% NaCl equivalent, similar to those found in many orogenic-type gold deposits, whereas the fluid inclusions in the gold-bearing “Yanshanian veins” (with abundant sulfides) overprinting the barren veins have a H2O-NaCl composition system, homogenization temperatures of 169 to 256 °C, and salinities from 8.7 to 15.2 wt% NaCl equivalent. It is proposed that both the Caledonian and Yanshanian veining events are controlled by the same fracture systems, with the former being formed in a relatively deep, orogenic deformation-metamorphic environment, and the latter in a relatively shallow, extensional environment accompanied by extensive granitic magmatism. Reactivation of the Caledonian structures during the Yanshanian orogeny, together with availability of sulfur and gold, possibly associated with granitic intrusions, played a key role in the formation of the gold deposits in northeastern Hunan and possibly elsewhere in the Jiangnan Orogen.

Fluid Inclusion and H–O–S–Pb Isotope Geochemistry of the Yuka Orogenic Gold Deposit, Northern Qaidam, China

The Yuka gold deposit, located in the western part of northern Qaidam, contains Au orebodies hosted in early Paleozoic metamorphic basic volcaniclastic rocks. The Yuka mineralization can be divided into three stages: early quartz-pyrite (stage-I), middle quartz-gold-polymetallic sulfide (stage-II), and late quartz-carbonate (stage-III). Gold deposition is primarily contained within stage-II. Three types of fluid inclusions were identified in the vein mineral assemblages using petrography and laser Raman spectroscopy: H2O-CO2-NaCl (C-type), H2O-NaCl (W-type), and pure CO2 (PC-type). Stage-I fluids record medium temperatures (205.2°C to 285.5°C) and H2O-CO2-NaCl±CH4 fluids with variable salinities (0.6–8.5 wt.% NaCl equiv.). Stage-II fluids evolved towards a more H2O-rich composition within a H2O-CO2-NaCl±CH4 hydrothermal system at medium temperatures (193.1°C to 271.1°C), with variable salinities (0.4–11.7 wt.% NaCl equiv.). Stage-III fluids are almost pure H2O and characterized by low temperatures (188.1°C to 248.5°C) and salinities (0.4–16.1 wt.% NaCl equiv.). These data indicate that ore-forming fluids are characterized by low to medium homogenization temperatures and low salinity and are evolved from a CO2-rich metamorphogenic fluid to a CO2-poor fluid due to inputs of meteoric waters, which is similar to orogenic-type gold deposits. The average δ18OW of quartz varies from 3.3‰ in stage-I to 2.1‰ in stage-II and to 1.4‰ in stage-III, with the δD values ranging from −41.6‰ to −58.5‰, suggesting that ore-forming fluids formed from metamorphic fluids mixed with meteoric waters. Auriferous pyrite δ34S ranges from 0.5 to 7.4‰ with a mean value of 4.43‰, suggesting that fluids were partially derived from Paleozoic rocks via fluid-wall rock interactions. Auriferous pyrites have 206Pb/204Pb of 18.238–18.600 (average of 18.313), 207Pb/204Pb of 15.590–15.618 (average of 15.604), and 208Pb/204Pb of 38.039–38.775 (average of 38.1697) and stem from the upper crust. Basing on geological characteristics of the ore deposit as well as new data from the ore-forming fluids, and H-O-S-Pb isotopes, the Yuka gold deposit is best described as an orogenic-type gold deposit.

Typomorphic Characteristics of Pyrites from the Shuangwang Gold Deposit, Shaanxi, China: Index to Deep Ore Exploration

The large Shuangwang gold deposit (>80 t gold) is located in the Western Qinling Orogen (WQO) of central China. It is an orogenic-type gold deposit hosted in an NW-extending breccia belt in the Devonian Xinghongpu Formation. Gold mineralization of the Shuangwang deposit is featured by hydrothermal breccia ores with strata fragments cemented by hydrothermal minerals dominated by ankerite, quartz, and pyrite with minor amounts of calcite and albite. Pyrite is the major gold-hosting sulfide and the most abundant ore mineral. Crystal habits, thermoelectricity, and trace-element composition of pyrites from the main ore-forming stage of the Shuangwang gold deposit were studied by microbinocular, BHTE-06 thermoelectric coefficient measuring instrument, and high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). Spatial distribution of the above data for pyrites was delineated by contour maps of morphology index, P-type frequency, and primary halo elements (e.g., supraore halo elements Ba and Sb; near-ore halo elements Pb, Zn, and Cu; and subore halo elements Co, Mo, and Bi). Based on the above results, four target areas (areas between prospecting lines 0 and 1, between lines 14 and 18 below orebody KT9; areas between prospecting lines 30 and 34, between lines 44 and 46 below orebody KT8) were put forward for deep gold exploration in the future. These targets are consistent with the depth extrapolation of proven gold orebodies, indicating the practicality of typomorphic characterization of pyrites as vector to deep/concealed gold orebodies. The effectiveness of the pyrite typomorphic parameter for deep gold prediction seems to be chemical composition, crystal habits, and then thermoelectricity.

Geology and Isotope Systematics of the Jianchaling Au Deposit, Shaanxi Province, China: Implications for Mineral Genesis

The giant Jianchaling Au (52 t Au) deposit is located in the Mian-Lue-Yang Terrane in the southern part of the Qinling Orogen of central China and is hosted by metamorphosed carbonate rocks of the Late Neoproterozoic Duantouya Formation. The deposit consists of multiple generations of mineralised quartz(-carbonate) veins in WNW-trending extensional ductile-brittle shear zones. Based on the mineral assemblages and cross-cutting relationships between the quartz(-carbonate) veins, the paragenesis is characterised by an early coarse-grained pyrite-pyrrhotite-pentlandite-dolomite-quartz assemblage (I), followed by pyrite-sphalerite-galena-carbonate-arsenopyrite-fuchsite-carbonate-quartz containing gold (II), and fine-grained pyrite-dolomite-calcite-quartz-realgar (As2S2)-orpiment (As2S3) (III). The H-O-C isotope systematics for the three vein sets indicate that the mineralising fluid is probably sourced from the metamorphic dehydration of carbonate rocks in the Duantouya Formation, and gradually mixed with meteoric water during the emplacement of the third vein set. The δ34S values for sulfides (6.3–16.6‰) from the second auriferous vein set are greater than zero, indicating sulfates reduction from the Neoproterozoic metamorphic rocks (Duantouya Fm). The (206Pb/204Pb)i ratios from pyrite (17.521–18.477) from each of the vein sets overlap those of the ultramafic rocks (18.324–18.717) and the Bikou Group (17.399–18.417), indicating that the units are possible sources for the sulfides in the mineralisation. Both εNd(t) and Isr(t) of sulfide overlap with the meta-ultramafic field and Duantouya formation and dominated with mature Sr-Nd character, which indicated that the Duantouya may play an important role during the ore formation and there may exist a minor ultramafic source that is involved in the ore fluid. The S-Pb-Sr-Nd isotopic ratios are closely related to those of the Bikou Group and Duantouya Formation, which indicates that the mineralised fluid has interacted with both units. Combining the previously published data with data from this study on the mineralised area, we surmise that Jianchaling is characteristic of an orogenic-type gold deposit related to the Triassic Qinling Orogeny associated with continental collision.

Geological study and significance of typical gold deposits in eastern Qinzhou-Hangzhou metallogenic belt: Constraint from Tianjingshan gold deposit in south Anhui Province

The eastern part of Qinzhou-Hangzhou metallogenic belt, including Jiangxi, Anhui and Zhejiang province, is located in the southeastern Jiangnan Orogen. A large amount of copper‑gold polymetallic deposits are distributed in the belt, represented by Jinshan super-large gold deposit in Jiangxi province, Tianjingshan large gold deposit in Anhui province and Huangshan medium gold deposit in Zhejiang province.The LA-ICP-MS U-Pb zircon dating for Lingshan and Hanjia intrusions of Tianjingshan gold deposit yield 794.7 ± 5.2 Ma and 765.9 ± 3.7 Ma, respectively, indicating they are all formed in the Jinningian. According to the different geochemical characteristics and field features, Hanjia intrusion shouldn't be altered by Lingshan intrusion, and was not formed in Yanshanian. The Ar-Ar analyses of chlorite for gold-bearing mylonite yield a spectrum age of 331.5 ± 3.2 Ma, firstly identifying Hercynian metallogenic event. The sulfur isotopes of pyrites from gold-bearing quartz veins show that the metallogenic fluids are complex, maybe metamorphic fluids mixed with meteoric water or magmatic water. Integrating with previous studies, Tianjingshan orogenic-type gold deposit with multiple stages and large time span of mineralization is formed by the collision-amalgamation of Yangtze block and Cathaysia block, which the Jinningian is the period of initial gold enrichment, and the Hercynian is an important metallogenic period.Based on contrastive analyses, these three typical gold deposits have similar metallogenic process as follows: the gold rich materials are derived from Neoproterozoic strata and Jinningian intrusion; then the ore-bearing metamorphic fluids resulted from regional metamorphism are mixed with meteoric water or magmatic water, and results in ore-bearing fluids enriched in CO2 and depleted in salinity; finally, the gold in the fluids is enriched in the low degree thrusting brittle- ductile shear zone due to the change of temperature, press and other conditions controlled by the fault. Therefore, these typical gold deposits were directly relate to the collision- amalgamation of Yangtze block and Cathaysia block and have a large time span of mineralization, which the altered-rock type gold deposit was formed at the Jinningian while the quartz vein type gold deposit was formed in Hercynian. Comprehensively, Tianjingshan gold deposit has the typical features of orogenic gold deposit. Significantly, the orogenic type gold deposit can provide important guidance and experience on deep and regional mining exploration.

A synthesis of mineralization styles and geodynamic settings of the Paleozoic and Mesozoic metallic ore deposits in the Altay Mountains, NW China

The Altay Mountains within the Xinjiang region of northwestern China hosts major metallic ore deposits. Here we review the geological characteristics, metallogenic features and tectonic settings of these deposits. The metallic ore deposits in the Altay Mountains occur mainly within four regions: North Altay, Central Altay, South Altay and Erqis. We recognize seven types of metallic ore deposits in the Altay Mountains: VMS, submarine volcanogenic iron, magmatic, skarn, pegmatite, hydrothermal vein (Cu–Zn, Fe) and orogenic gold. Among these types, the VMS, pegmatite, orogenic gold and skarn deposits are the most common. Most of the rare metal pegmatite deposits are distributed in Central Altay, with only a few in South Altay. The VMS, submarine volcanogenic type iron and skarn-type deposits are distributed in South Altay, whereas the orogenic-type gold deposits are distributed in the Erqis Fault belt. The hydrothermal vein-type deposits occur in the Erqis Fault belt and Chonghu’er Basin in South Altay. Magmatic-type deposits are mostly in the Erqis Fault belt and Central Altay. Based on isotopic age data, the VMS, submarine volcanogenic-type Fe and skarn-type Cu, Pb, Zn, Fe mineralization occurred during Early–Middle Devonian (∼410–377Ma), orogenic-type Au, magmatic-type Cu–Ni, and a small number of skarn-type Fe, hydrothermal vein-type Cu–Zn, pegmatite-type rare-metal deposits in Early–Middle Permian (293–261Ma), pegmatite-type rare-metal deposits, few skarn-type Fe deposit in Early–Middle Triassic (248–232Ma), and dominantly represented by pegmatite-type rare-metal deposits in Late Triassic–Early Jurassic (223–180Ma). The metallic ore deposits in the Altay Mountains formed in various tectonic settings, such as the Early–Middle Devonian continental arc and oceanic island arc, Early–Middle Permian post-collisional extensional setting, and Triassic–Early Jurassic intracontinental setting.

Invisible gold distribution on pyrite and ore-forming fluid process of the Huangshan orogenic-type gold deposit of Zhejiang, SE China: implications from mineralogy, trace elements, impurity and fluid inclusion studies

The Huangshan orogenic-type gold deposit in Zhejiang of SE China occurred in quartz–pyrite veins. It is hosted by phyllonite that underwent greenschist-facies metamorphism along a large Jiangshan–Shaoxing tectonic belt with a NE–SW direction. Trace elemental characteristics, ore-forming process and invisible gold on different forms of pyrite and quartz are studied. The Au associated pyrite can be classified into two categories; recrystallized pyrite and euhedral pyrite. The precipitation of invisible Au on pyrite is mainly derived by Co and Ni with AuHS2 − complex in the mineralizing fluids in different events. The XPS results revealed that valence states of Au3+ replaced 2Fe2+ in the pyrite and Au0 replaced Si4+ in the quartz structure. The electron paramagnetic resonance and trace elemental results suggested that the element pairs of Ge–Li–Al in quartz and Mn–Co–Ni in pyrite have distinct impurities as identified. A fluid inclusion study showed that the auriferous quartz is characterized by low-saline and CO2-rich fluids. Coexistence of the type I–type III inclusions and same range of homogenization temperature with different mode are evidences of immiscible fluid process. The temperature–pressure values of ca. 250 °C/1250 bar and ca. 220 °C/780 bar for gold precipitation have been calculated by intersection of coexisting fluids during the entrapment. The Huangshan orogenic-type gold deposit may be associated with the Wuyi–Yunkai orogeny during the early Paleozoic, including an upper–mid greenschist-facies metamorphism (450–420 Ma). All the features suggest that the Huangshan gold deposit is probably a product linking with the early Paleozoic orogeny in South China.

The origin and hydrothermal mobilization of carbonaceous matter associated with Paleoproterozoic orogenic-type gold deposits of West Africa

The chemical and physical properties of carbonaceous matter (CM) were studied in Paleoproterozoic metasediment-hosted, orogenic-type gold deposits in Burkina Faso (the Inata deposit), in Mali (the Syama deposit), and in Ghana (the Obuashi and Bogoso deposits). Two types of CM occur in all the studied deposits: metamorphosed and hydrothermal. Metamorphosed CM prevails in all the deposits. Hydrothermal CM occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub-parallel to C-type cleavage within the shear zones. The origin of hydrothermal CM, which occurs in paragenesis with Au-bearing arsenopyrite or pyrite, seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels. The isotopic composition of carbon in bulk CM (−33.1 to −26.2‰, VPDB) indicates its biogenic origin. The isotopic composition of carbon in hydrothermal carbonates ranges from −14.5 to −4.4‰ (VPDB), which suggest mixing of carbon derived from a deep-seated source with carbon derived from an organic source. The interaction of hydrothermal fluids with metamorphosed CM could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization. The optical properties and Raman spectra of the metamorphosed CM particles in the individual studied mineral deposits differ considerably. The temperatures calculated on the basis of the Raman spectra of metamorphosed CM vary between 280 and 440°C, depending on the thermometer used, and correspond to temperatures of metamorphism of upper sub-grenschist and greenschist facies. The temperatures calculated for hydrothermal CM at the individual deposits, are only slightly lower compared to the metamorphosed CM at the same deposits, which indicates approximately the same temperature of the metamorphic and hydrothermal processes.