EOCENE MAGMA PLUMBING SYSTEM BENEATH CORTEZ HILLS CARLIN-TYPE GOLD DEPOSIT, NEVADA: IS THERE A DEEP-SEATED PLUTON?

2020 ◽  
Author(s):  
Celestine N. Mercer

Abstract The magmatic-hydrothermal conceptual model for Carlin-type gold deposit genesis calls upon deep-seated Eocene plutons as the primary source of gold-bearing fluids. However, geophysical surveys, geologic mapping, drilling, geochronology, isotopic tracers, and fluid inclusion chemistry have returned ambiguous evidence for the existence of such plutons. The high-grade Cortez Hills gold deposit in northern Nevada hosts shallow, Eocene syn- and postmineralization intrusions, offering an ideal site to investigate the existence of a deep-seated pluton beneath the district. Here, major and trace element analyses of quartz-hosted melt inclusions from four Eocene rhyolite dikes cropping out within the Cortez Hills pit and results from independent thermobarometers provide a window into the subsurface Eocene magmatic plumbing system to test the existence of a deep-seated source pluton. Dissolved volatile contents, melt inclusion entrapment pressures, and thermodynamic phase equilibria indicate that dike magmas were sourced from ~4- to ≥9-km depth from a polybaric magma reservoir residing as a physically and geochemically interconnected crystal mush with extractable or eruptible magma pockets. Magmas ascended adiabatically (nearly isothermally), exsolving fluids, evolving modestly by fractional crystallization, while trapping quartz-hosted melt inclusions steadily from depth to subvolcanic levels where they were emplaced. These data represent the first unequivocal evidence for a deep-seated magma reservoir from which fluid-saturated magma emanated and released magmatic fluids beneath the Cortez district during gold mineralization. However, further investigation into the specific metallogenic potential and metal budget of parental magmas and the partitioning of gold between silicate melt and aqueous fluids will be necessary to provide evidence that exsolved magmatic fluids may have been gold bearing.

Minerals ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 448 ◽  
Author(s):  
Shun-Da Li ◽  
Zhi-Gao Wang ◽  
Ke-Yong Wang ◽  
Wen-Yan Cai ◽  
Da-Wei Peng ◽  
...  

The Jinchang gold deposit is located in the eastern Yanji–Dongning Metallogenic Belt in Northeast China. The orebodies of the deposit are hosted within granite, diorite, and granodiorite, and are associated with gold-mineralized breccia pipes, disseminated gold in ores, and fault-controlled gold-bearing veins. Three paragenetic stages were identified: (1) early quartz–pyrite–arsenopyrite (stage 1); (2) quartz–pyrite–chalcopyrite (stage 2); and (3) late quartz–pyrite–galena–sphalerite (stage 3). Gold is hosted predominantly within pyrite. Pyrite separated from quartz–pyrite–arsenopyrite cement within the breccia-hosted ores (Py1) yield a Re–Os isochron age of 102.9 ± 2.7 Ma (MSWD = 0.17). Pyrite crystals from the quartz–pyrite–chalcopyrite veinlets (Py2) yield a Re–Os isochron age of 102.0 ± 3.4 Ma (MSWD = 0.2). Pyrite separated from quartz–pyrite–galena–sphalerite veins (Py3) yield a Re–Os isochron age of 100.9 ± 3.1 Ma (MSWD = 0.019). Re–Os isotopic analyses of the three types of auriferous pyrite suggest that gold mineralization in the Jinchang Deposit occurred at 105.6–97.8 Ma (includes uncertainty). The initial 187Os/188Os values of the pyrites range between 0.04 and 0.60, suggesting that Os in the pyrite crystals was derived from both crust and mantle sources.


2021 ◽  
Author(s):  
Wei Gao ◽  
Ruizhong Hu ◽  
Albert H. Hofstra ◽  
Qiuli Li ◽  
Jingjing Zhu ◽  
...  

Abstract The Youjiang basin on the southwestern margin of the Yangtze block in southwestern China is the world’s second largest Carlin-type gold province after Nevada, USA. The lack of precise age determinations on gold deposits in this province has hindered understanding of their genesis and relation to the geodynamic setting. Although most Carlin-type gold deposits in the basin are hosted in calcareous sedimentary rocks, ~70% of the ore in the Badu Carlin-type gold deposit is hosted by altered and sulfidized dolerite. Although in most respects Badu is similar to other Carlin-type gold deposits in the province, alteration of the unusual dolerite host produced hydrothermal rutile and monazite that can be dated. Field observations show that gold mineralization is spatially associated with, but temporally later than, dolerite. In situ secondary ion mass spectrometry (SIMS) U-Pb dating on magmatic zircon from the least altered dolerite yielded a robust emplacement age of 212.2 ± 1.9 Ma (2σ, mean square of weighted deviates [MSWD] = 0.55), providing a maximum age constraint on gold mineralization. The U-Th/He ages of detrital zircons from hydrothermally mineralized sedimentary host rocks at Badu and four other Carlin-type gold deposits yielded consistent weighted mean ages of 146 to 130 Ma that record cooling from a temperature over 180° to 200°C and place a lower limit on the age of gold mineralization in the basin. Hydrothermal rutile and monazite that are coeval with gold mineralization have been identified in the mineralized dolerite. Rutile is closely associated with hydrothermal ankerite, sericite, and gold-bearing pyrite. It has high concentrations of W, Fe, V, Cr, and Nb, as well as growth zones that are variably enriched in W, Fe, Nb, and U. Monazite contains primary two-phase fluid inclusions and is intergrown with gold-bearing pyrite and hydrothermal minerals. In situ SIMS U-Pb dating of rutile yielded a Tera-Wasserburg lower intercept age of 141.7 ± 5.8 Ma (2σ, MSWD = 1.04) that is within error of the in situ SIMS Th-Pb age of 143.5 ± 1.4 Ma (2σ, MSWD = 1.5) on monazite. These ages are ~70 m.y. younger than magmatic zircons in the host dolerite and are similar to the aforementioned U-Th/He cooling ages on detrital zircons from hydrothermally mineralized sedimentary host rocks. We, therefore, conclude that the Badu Carlin-type gold deposit formed at ca. 144 Ma. The agreement of the rutile and monazite ages with the U-Th-He cooling ages of Badu and four other Carlin-type gold deposits in the Youjiang basin suggests that ca. 144 Ma is representative of a regional Early Cretaceous Carlin-type hydrothermal event formed during back-arc extension.


2021 ◽  
Vol 21 (1) ◽  
pp. 246-261
Author(s):  
Hongye Feng ◽  
Yiwen Ju ◽  
Bo Chen ◽  
Weixuan Fang ◽  
Hongjian Zhu ◽  
...  

The mineralogical and compositional characteristics of gold-bearing minerals and the occurrence of gold are not only of great significance to exploring the sources of ore-forming materials and their formation mechanisms but also helpful for designing reasonable beneficiations and smelting schemes and achieving remarkable economic benefits. This paper presents an integrated study on the crystal characteristics, elemental composition and distribution of pyrite (the main gold-bearing minerals), on the basis of electron probe microanalysis (EPMA), scanning electron microscopy (SEM), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and nano-secondary ion mass spectrometry (NanoSIMS). The occurrence of gold in the Shuiyindong gold deposit and Ashawayi gold deposit has been studied by means of microscopy, SEM, and EPMA images, elemental correlations, S–Fe–As ternary diagrams, logAs–logAu diagrams and Au/As ratios. The gold in pyrite of the Shuiyindong deposit is in the form of nano gold inclusions and lattice gold. The gold in pyrite of the Ashawayi deposit dominantly exists in the form of nano gold inclusions or is present as micro-nano gold particles in the cracks or edges of pyrite, some of which can exist as lattice gold. The ore-forming hydrothermal solution of the Shuiyindong gold deposit is mainly underground hot brine, but it may be reformed by a deep magmatic hydrothermal solution or volcanic-subvolcanic hydrothermal solution. The ore-forming hydrothermal solution of the Ashawayi gold deposit is mainly derived from the metamorphic hydrothermal solution formed during the orogenic process, and the ore-forming process or post-mineralization process may be reformed by the leaching of underground hot brine. Finally, the characteristics of ore-forming fluids and evolution of the two types of deposits are determined via pyrite element surface scanning. This paper shows that micro-nanoscale study of gold-bearing pyrite is of great significance to understanding the gold mineralization process and is worth further study.


2017 ◽  
Vol 54 (5) ◽  
pp. 494-511 ◽  
Author(s):  
Xi-hui Cheng ◽  
Jiu-hua Xu ◽  
Jian-xiong Wang ◽  
Qing-po Xue ◽  
Hui Zhang

The Hamadi gold deposit is located in North Sudan, and occurs in the Neoproterozoic metamorphic strata of the Arabian–Nubian Shield. Two types of gold mineralization can be discerned: gold-bearing quartz veins and altered rock ores near ductile shear zones. The gold-bearing quartz veins are composed of white to gray quartz associated with small amounts of pyrite and other polymetallic sulfide minerals. Wall-rock alterations include mainly beresitization, epidotization, chloritization, and carbonatization. CO2-rich inclusions are commonly seen in gold-bearing quartz veins and quartz veinlets from gold-bearing altered rocks; these include mainly one-phase carbonic (CO2 ± CH4 ± N2) inclusions and CO2–H2O inclusions with CO2/H2O volumetric ratios of 30% to ∼80%. Laser Raman analysis does not show the H2O peak in carbonic inclusions. In quartz veins, the melting temperature of solid CO2 (Tm,CO2) of carbonic inclusions has a narrow range of −59.6 to −56.8 °C. Carbonic inclusions also have CO2 partial homogenization temperatures (Th,CO2) of −28.3 to +23.7 °C, with most of the values clustering between +4.0 and +20 °C; all of these inclusions are homogenized into the liquid CO2 state. The densities range from 0.73 to 1.03 g/cm3. XCH4 of carbonic fluid inclusions ranges from 0.004 to 0.14, with most XCH4 around 0.05. In CO2–H2O fluid inclusions, Tm,CO2 values are recorded mostly at around −57.5 °C. The melting temperature of clathrate is 3.8–8.9 °C. It is suggested that the lowest trapping pressures of CO2 fluids would be 100 to ∼400 MPa, on the basis of the Th,CO2 of CO2-bearing one-phase (LCO2) inclusions and the total homogenization temperatures (Th,tot) of paragenetic CO2-bearing two-phase (LCO2–LH2O) inclusions. For altered rocks, the Tm,CO2 of the carbonic inclusions has a narrow range of −58.4 to ∼−57.0 °C, whereas the Th,CO2 varies widely (−19 to ∼+29 °C). Most carbonic inclusions and the carbonic phases in the CO2–H2O inclusions are homogenized to liquid CO2 phases, which correspond to densities of 0.70 to ∼1.00 g/cm3. Fluid inclusions in a single fluid inclusion assemblage (FIA) have narrow Tm,CO2 and Th,CO2 values, but they vary widely in different FIAs and non-FIAs, which indicates that there was a wide range of trapping pressure and temperature (P–T) conditions during the ore-forming process in late retrograde metamorphism after the metamorphism peak period. The carbonic inclusions in the Hamadi gold deposit are interpreted to have resulted from unmixing of an originally homogeneous aqueous–carbonic mixture during retrogress metamorphism caused by decreasing P–T conditions. CO2 contributed to gold mineralization by buffering the pH range and increasing the gold concentration in the fluids.


Minerals ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 888
Author(s):  
Jung Hun Seo ◽  
Yevgeniya Kim ◽  
Tongha Lee ◽  
Marcel Guillong

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.


2021 ◽  
Vol 30 (1) ◽  
pp. 53-64
Author(s):  
Nazim A. Imamverdiyev ◽  
Vasif M. Baba-zadeh ◽  
Samir S. Mursalov ◽  
Anar A. Valiyev ◽  
Mamoy I. Mansurov ◽  
...  

The article describes Reza gold deposit of Ugur exploration area located in Geda- bek Ore District of the Lesser Caucasus in NW of Azerbaijan. It is established that main mineralization in the Reza gold deposit consists of hematite-barite-quartz-kaoline veins- veinlets and breccia, pyrite stock-stockverk and quartz-sulfide veins. On the main orebody surface center secondary quartzites with vein-veinlets barite-hematite mineralization have occured over which remain accumulations of hydrous ferric oxides cementing breccias of quartz and secondary quartzites. “Reddish mass” is also observed in erosion parts, being an oxidation product of stock and stockverk limonite-hematite ores. Representing typical gossans, these accumulations by the data of trenches for thickness about 5-10 m contain gold 0.3-3.5 ppm and silver 1.0-45.0 ppm. There are three zones of gold mineralization within the Reza gold deposit: oxide mineralization; transition zone mineralization; sulfide mineralization. The oxide gold mineraliza- tion consists of clay-gravel weathering crust of kaolinite type. The gold-bearing mineralization has been oxidized to a depth of ap- proximately 50-100 meters. Typically, the gold mineralization is coarser and a minor increase in gold grade occurs within the oxides compared to the original rocks. The nugget effect increase in the gold grade of the oxides does not exceed approximately 10%. Deposit alteration signature has characteristics which suggest the current outcrop level may be near the top of a mineralized, gold-bearing high sulfidation epithermal (HSE) system. The gold mineralization at the deposit is interpreted as forming in shallow high sulfidation epith- ermal systems. The mineralization has been noted in well-confined hydrothermal breccia and associated with pyrite stock-stockwork. The majority of the deposit material and current estimates are formed within the barite-hematite-quartz-kaoline mineralization in the secondary quartzite rocks. The main brecciation and stockwork are hosted within secondary quartzite, sometime massive silicified andesite porphyritic rocks.


Minerals ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 399 ◽  
Author(s):  
Zheng ◽  
Yang ◽  
Gao ◽  
Chen ◽  
Liu ◽  
...  

The Nibao gold deposit, which includes both fault-controlled and strata-bound gold orebodies, constitutes an important part of the Yunnan–Guizhou–Guangxi “Golden Triangle” region. Defining the mineralization age of these gold orebodies may provide additional evidence for constraining the formation ages of low-temperature orebodies and their metallogenic distribution in South China. Petrographic studies of gold-bearing pyrites and ore-related quartz veins indicate that these pyrites coexist with quartz or filled in vein-like quartz, which suggests a possible genetic relationship between the two from Nibao gold deposit. Minerals chemistry shows that Rb and Sr are usually hosted in fluid inclusions in quartz ranging from 0.0786 to 2.0760 ppm and 0.1703 to 2.1820 ppm, respectively. The Rb–Sr isotopic composition of gold-bearing quartz-hosted fluid inclusions from the Nibao gold deposit were found to have Rb–Sr isochron ages of 142 ± 3 and 141 ± 2 Ma for both fault-controlled and strata-bound orebodies, respectively, adding more evidence to previous studies and thus revealing a regional gold mineralization age of 148–134 Ma. These results also confirm the Middle-Late Yanshanian mineralizing events of Carlin-type gold deposits in Yunnan, Guizhou, and Guangxi Provinces of Southwest China. In addition, previous studies indicated that antimony deposits in the region which were formed at ca. 148–126 Ma have a close affinity with gold deposits. This illustrates that the regional low-temperature hydrothermal gold mineralization is related in space and time to the Yanshanian (ca. 146–115 Ma) magmatic activity. Specifically, the large-scale gold and antimony mineralization are considered to be inherently related to mantle-derived mafic and ultramafic magmatic rocks associated with an extensional tectonic environment. Based on the initial 87Sr/86Sr ratios of 0.70844 ± 0.00022 (2σ) and 0.70862 ± 0.00020 (2σ) for gold-bearing quartz veins from fault-controlled and strata-bound gold orebodies, respectively, at the Nibao gold deposit, as well as the C, H, O, and S isotopic characteristics of gold deposits located in the Golden Triangle region, we suggest that the mantle-derived material can be involved in the formation of the Nibao gold deposit and that the ore-forming fluid can be derived from a mixed crust–mantle source.


2021 ◽  
pp. 28-35
Author(s):  
Sergei MILYAEV

The formation conditions and characteristics of secondary dispersion haloes in the gold-bearing weathering crust of a gold-sulfide-quartz deposit are considered. The fraction < 0,1mm is demonstrated to be preferable for lithochemical prospecting for the Vasilkovskoe type gold mineralization. The secondary dispersion haloes of gold, identified based on analyses of this fraction, are characterized by the maximum size and productivity.


Minerals ◽  
2019 ◽  
Vol 9 (4) ◽  
pp. 235 ◽  
Author(s):  
Hinyuen Tsang ◽  
Jingya Cao ◽  
Xiaoyong Yang

The Chaoyangzhai gold deposit is one of the newly discovered medium to large scale turbidite-hosted gold deposits in Southeast Guizhou, South China. In this study, laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) zircon U–Pb dating on the tuffaceous- and sandy-slates of Qingshuijiang Formation, Xiajiang Group, and gold-bearing quartz vein yielded similar age distributions, indicating that zircon grains in gold-bearing quartz vein originated from the surrounding tuffaceous- and sandy-slates. In addition, the youngest weighted mean ages of the zircon grains from the tuffaceous- and sandy-slates were 775 ± 13 Ma and 777 ± 16 Ma, respectively, displaying that the tuffaceous- and sandy-slates of the Qingshuijiang Formation were likely deposited in Neoproterozoic. Based on their major and trace element compositions, the tuffaceous- and sandy-slates were sourced from a felsic igneous provenance. The sandy slates have higher contents of Au (mostly ranging from 0.019 to 0.252 ppm), than those of the tuffaceous slates (mostly lower than 0.005 ppm). The δ34SV-CDT values of pyrite and arsenopyrite of the gold-bearing samples range from +8.12‰ to +9.99‰ and from +9.78 to +10.78‰, respectively, indicating that the sulfur source was from the metamorphic rocks. Together with the evidence of similar geochemical patterns between the tuffaceous- and sandy-slates and gold-bearing quartz, it is proposed that the gold might be mainly sourced from sandy slates. The metamorphic devolatilization, which was caused by the Caledonian orogeny (Xuefeng Orogenic Event), resulted in the formation of the ore-forming fluid. Gold was likely deposited in the fractures due to changes of the physico-chemical conditions, leading to the formation of the Chaoyangzhai gold deposit, and the large-scale gold mineralization in Southeast Guizhou.


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