Shear Zone-hosted Gold Deposit Research Articles

Hydrothermal alteration and mineralization in the Faina greenstone belt: evidence from the Cascavel and Sertão orogenic gold deposits

The Faina greenstone belt is located in central Brazil and contains the Cascavel and Sertão gold deposits (500,000 oz Au total resource and past production). The bulk of the gold is hosted in quartzite and biotite schist at Cascavel, and carbonaceous schist and banded iron formation at Sertão. The metamorphic grade of the host rocks is greenschist facies.Cascavel is a vein-hosted gold deposit with orebodies mainly controlled by F2 fold hinges, with minor mineralization in D3 shear zones. Hydrothermal alteration in quartzite is characterized by distal quartz-K-feldspar ± muscovite, intermediate quartz-fuchsitic mica-biotite-pyrite, and proximal quartz-white mica-pyrite ± chalcopyrite. The biotite schist is characterized by distal quartz-chlorite-calcite and proximal white mica-biotite-tourmaline-siderite-pyrite. Native gold is located within intensely deformed quartz veins, and locally as free gold disseminated in proximal alteration zones.Sertão is a shear zone-hosted gold deposit with orebodies controlled by D3 shear zones including gold-bearing shear veins and surrounding hydrothermally altered wallrocks. Hydrothermal alteration is characterized by distal quartz-ankerite-chlorite-pyrite ± chalcopyrite and proximal quartz-white mica-siderite-pyrite-arsenopyrite ± chalcopyrite. Gold is in equilibrium with arsenopyrite and within arsenopyrite disseminated in the proximal alteration zone.Chlorite geothermometry suggests temperatures of 335–407 °C and 328–397 °C for the gold-bearing hydrothermal fluids in the Sertão and Cascavel deposits, respectively. Arsenopyrite geothermometry on arsenopyrite within gold-bearing veins in the Sertão deposit yields temperatures between 310 and 480°C.Structures controlling hydrothermal alteration and gold mineralization in both deposits are developed during a regional D2 event and subsequently deformed during a D3 event. Hydrothermal alteration minerals are consistently replacing metamorphic minerals. The alteration assemblages in both deposits are similar, albeit their modal mineralogy is controlled by the host rock composition. Therefore, both the Cascavel and Sertão deposits are interpreted as orogenic, mesozonal gold systems comparable with other orogenic gold systems in Brazil and worldwide.

Gold concentrations in metamorphic fluids: A LA-ICPMS study of fluid inclusions from the Alpine orogenic belt

Vein and shear-zone hosted gold deposits in orogenic terrains of Archean to Phanerozoic age are formed from dominantly metamorphic fluids. It is debated, however, whether normal crustal source rocks are adequate to generate economic deposits, or whether selectively gold-enriched sources such as mafic to ultramafic lavas, black shales or an input of magmatic fluid make a decisive difference between the formation of high-grade deposits and barren vein systems. As an essential baseline study in this debate, we have analyzed the metal and sulfur content of fluid inclusions in barren veins across a prograde sequence from greenschist to amphibolite facies in the Central Alpine metamorphic belt. Comparison of the analyzed fluid compositions with thermodynamic solubility calculations shows that the fluids record metamorphic dehydration, decarbonation and desulfidation of the continental crust. However, gold with concentrations of 0.003 to 0.03ppm is increasingly undersaturated in the highest-temperature aqueous–carbonic fluids, which otherwise resemble those forming major gold deposits elsewhere. Our results show that the regional-metamorphic fluids could carry 10–1000 times higher gold concentrations, implying that pre-enriched source rocks or hydrous magmas may play an essential role in generating highly gold-endowed orogenic belts.

Earth's Oldest Rocks.: MARTIN J. VAN KRANENDONK, R. HUGH SMITHIES, and VICKIE C. BENNETT, Editors. Pp. 1307. 2007. Developments in Precambrian Geology. 15. El-sevier. Amsterdam. ISBN-13: 978-0-444-52810-0, ISBN-10: 0-444-52810-5. Price: USD150.

I fondly remember a graduate class in the early 1980s on Archean geology at Queen’s University in Kingston, Canada, run by Herb Helmsteadt, Dugald Carmichael, Ed Ferrar, and John Haynes, with most other members of staff chipping in with talks and discussions. It was a very holistic, integrated way of teaching students about greenstone belts and shear zone-hosted gold deposits, komatiites, stromatolites, and so forth. This cross-disciplinary teaching was a “trademark” of the earth sciences at Queen’s, then way ahead of its time. The class had interesting and enthusiastic students, many eager to map and analyze Archean rocks of the Canadian Shield and, in particular, the Slave craton and the Superior province. Some are now leaders in Archean studies (and authors of chapters in this book; the principal editor of this book, too, completed his Ph.D. in this department). The most interesting aspect of the geology graduates from Queen’s University (and from others throughout Canada) is the sustained collegiate manner in which they set about their tasks to study the ancient real estate of Canada, including the Acasta gneisses, now recognized as the oldest preserved rocks on our planet (described in chapter 3.1). Their systematic collaboration has yielded the most comprehensive “open source” inventory of ancient lithosphere available anywhere in the world. Field-mapping has remained a prime objective, well supported today by a number of excellent geochronology laboratories that have sprung up all over the place, rooted in the superb high-precision work at the Ontario Museum and the mentorship of Tom Krogh. In the mid-1980s, the geologists also joined forces with an emerging geophysics consortium, culminating in the Lithoprobe program that systematically paved geotransects across Canada, producing seismic images of its crust (www.lithoprobe.ca). Other geophysical methods such as teleseismic and magnetotelluric probing joined in to look deeper into the lithosphere …

Decrepitation Thermometry and Compositions of Fluid Inclusions of the Damoqujia Gold Deposit, Jiaodong Gold Province, China: Implications for Metallogeny and Exploration

The recently discovered Damoqujia gold deposit is a large shear zone-hosted gold deposit of disseminated sulphides located in the north of the Zhaoping fault zone, Jiaodong gold province, China. In order to distinguish the temperature range of cluster inclusions from different mineralization stages and measure their compositions, 16 fluid inclusions and 5 isotopic geochemistry samples were collected for this study. Corresponding to different mineralization stages, the multirange peaks of quartz decrepitation temperature (250–270, 310–360 and 380–430 °C) indicate that the activity of ore-forming fluids is characterized by multistage. The ore-forming fluids were predominantly of high-temperature fluid system (HTFS) by CO 2-rich, and SO 4 2--K + type magmatic fluid during the early stage of mineralization and were subsequently affected by low-temperature fluid system (LTFS) of CH 4-rich, and Cl −-Na +/Ca 2+ type meteoric fluid during the late stage of mineralization. Gold is transferred by Au-HS − complex in the HTFS, and Au-Cl − complex can be more important in the LTFS. The transition of fluids from deeper to shallow environments results in mixing between the HTFS and LTFS, which might be one of the most key reasons for gold precipitation and large-scale mineralization. The ore-forming fluids are characterized by high-temperature, strong-activity, and superimposed mineralization, so that there is a great probability of forming large and rich ore deposit in the Damoqujia gold deposit. The main bodies are preserved and extend toward deeper parts, thereby suggesting a great potential in future.

The Tianger (Bingdaban) shear zone hosted gold deposit, west Tianshan, NW China: Petrographic and geochemical characteristics

The Wangfeng–Tianger–Saridala ore district consists of over 20 orebodies with total proven gold reserves exceeding 15 tonnes. The district is located on the EW-striking Tianger (Bingdaban) shear zone in west Tianshan Mountains, northwest China. The lensoid orebodies are distributed along the shear zone in a region about 25 km in length and 1 km in width. Enhanced fluid flow and fracturing controlled location and orientation of mineralized zones during deformation along narrow shear bands oriented oblique to the overall shear zone margins. Fabrics in fractures and strain shadow overgrowths generally plunge steeply, parallel to the lineation in the surrounding cataclasite matrix consisting of new grains of quartz and minor amounts of micas. Native gold occurs in mylonitized granites or mylonitized sulfide–mica–quartz veins. Gold grade is closely related with the occurrence of sulfide–quartz veins. Pyrite and pyrrhotite in gold-bearing sulfide–quartz veins contain inclusions of native gold. Chondrite-normalized rare earth element (REE) distribution patterns for pyrite from the orebodies show light REE enrichments and negative Eu anomalies. Relationships between gold contents and Th/U, Zr/Hf and (La/Yb) N values for pyrites imply that gold-rich pyrites are different from gold-poor pyrites, thus indicate that the pyrites in the orebodies have a different origin compared with those in wall rocks. A Rb–Sr isochron for the mylonitized Au-bearing quartz veins indicates an age of 224 ± 14 Ma with an initial 87Sr/ 86Sr ratio of 0.7294 ± 0.0089 (MSWD = 1.1). This age is conformed by 40Ar/ 39Ar dating on muscovites (220.9 to 222.5 Ma), which is interpreted as the age of ore-formation in the Tianger deposit. High initial 87Sr/ 86Sr ratios (0.7294), high δ 34S values (+ 11.2 to + 16.5‰), low δ 18O H2O values (1.67 to 3.07‰) and low δD values (− 84 to − 104‰) indicate that the ore-forming fluid was unrelated to any magmatic process.

Gold in the Yellowstone Greenstone Belt, Northwest Territories: Results of the EXTECH III Multidisciplinary Research Project.: C. D. ANGLIN, H. FALCK, D. F. WRIGHT, AND E. J. AMBROSE, Editors. Pp. 448. Geological Association of Canada, Mineral Deposits Division, Special Publication no. 3 (DVD included). 2006. ISBN 1-897095-09-0. Price Can$76.00, Member Price Can$57.00.

The Yellowknife greenstone belt in the Archean Slave province is one of Canada’s major gold mining districts, and one of the best preserved and exposed Archean volcanic belts worldwide. Declining reserves, following more than 60 years of continuous gold production, stimulated the initiation of the Yellowknife EXploration science and TECHnology project in 1998. The resulting EXTECH III project, the third of a series of projects initiated by the Geological Survey of Canada in collaboration with various university and industry partners, had the objective of defining new exploration targets for gold mineralization in the Yellowknife greenstone belt using new technology and exploration concepts accomplished through multidisciplinary studies and ran from 1999 to 2003. The EXTECH project built on a comprehensive geological and geochronological database developed over 60 years of government and industry mapping programs. Progress on the geological understanding of the Yellowknife greenstone belt had been incremental, with significant contributions from Jolliffe, J.F. Henderson, Brown, J.B. Henderson, and, more recently, Helmstaedt, Bleeker, and coworkers. The recent studies challenged some of the long-held views for the gold-bearing zones throughout the Yellowknife district, which since Boyle (1961) had been viewed as type examples of shear zone-hosted gold deposits. An improved understanding of the structural and metamorphic evolution of the region also warranted a new look at the controls on gold mineralization. Willingness from government, industry, and universities provided the impetus to develop the cooperative and multidisciplinary approach undertaken by the EXTECH program. The resulting volume contains 25 papers that are organized into five parts: historical background; regional geology; mineral deposits and regional metallogenic studies; exploration technology; and data integration. A mix of new data, integration, and interpretation of the principal deposits in the Yellowknife belt are brought together with chapters on the history and regional geology of the belt. Unfortunately, color figures and …

Geology, fluid inclusions, isotope geochemistry, and geochronology of the Paishanlou shear zone-hosted Gold Deposit, North China Craton

The Paishanlou gold deposit lies along the northern margin of North China Craton gold province, the third largest Au province in China. The deposit is mainly hosted in Archean metamorphic rocks of the Jianping Group and is structurally controlled by two sets of ductile shear zones. Gold mineralization is closely associated with intense hydrothermal alteration along the ductile shear zones, with a typical greenschist facies alteration assemblage of sericite + chlorite + calcite + biotite + quartz and a distinct alteration zoning of inner pyrite-sericite zone, middle carbonate zone, and outer epidote-biotite-chlorite zone from orebody to wall rock. Fluid inclusion petrography and microthermometric results suggest that four types of fluid inclusions are present at the deposit. Type 1 CO 2-rich inclusions have homogenization temperatures of 22.4–27.9 °C. Type 2 H 2O–CO 2 inclusions show salinities of 4.6–12.3 wt.% NaCl equivalent and Th-tot of 242 to 337 °C. Type 3a primary aqueous inclusions have salinities of 5.9–11.2 wt.% NaCl equivalent and Th-tot of 187–289 °C, while Type 3b secondary aqueous inclusions have salinities of 3.1–8.3 wt.% NaCl equivalent and Th-tot of 128–239 °C. Type 4 multiphase inclusions have salinities ranging from 31.4 to 35.2 wt.% NaCl equivalent and Th-tot of 370–447 °C. The close association of CO 2-rich inclusions and H 2O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. Estimated P– T values range from 1.4 to 1.9 kbar at 322–417 °C. The δ 18O values of the ore-forming fluids are calculated to range from 3.8‰ to 7.4‰ and the δD values from − 87.3 to − 116.2‰, indicating a mixed source of deep-seated magmatic water and shallower meteoric water. The δ 34S values (0.3–7.5‰) of sulfide ore minerals are similar to those of both pyrite from metamorphic country rocks and whole rock sulfur of metamorphic country rocks. The 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb of sulfide ores range within 16.40–17.0, 15.21–15.37, and 36.69–37.38, respectively, with the model ages of 397–727 Ma, consistent with those of Late Mesozoic granites and lying between Archean metamorphic rock lead and the Proterozoic dolomitic marble lead. These sulfur and lead features reflect the fact that the ore-forming materials originated mainly from multiple wall-rock sources. The well-defined Ar/Ar plateau age (126.6 ± 1.1 Ma) of biotite from the NNE ductile shear zone provides a possible temporal link between Au mineralization, magmatism and deformation at the Paishanlou deposit. The concordance of the biotite age with other well-constrained mineralization ages for the deposit, and the intrusion age of the granite, suggests that Au mineralization was essentially contemporaneous with late Mesozoic granitic magmatism and the second stage of mylonitization. In terms of its genetic linkage with the subduction process at the leading northern edge of the North China Craton, the Paishanlou deposit can be favored as an orogenic gold deposit. Its formation benefited from the combined action of protracted activity of ductile fault zones, hydrothermal circulation associated with granitoid intrusion and preferred structural setting favoring gold deposition. The major geodynamic switch towards generalized extensional tectonics that occurred during uplift, and thinning of the upper crust after the collisional stages in the northern edge of the North China Craton, may be the key factors controlling Au concentration and enrichment in the Paishanlou deposit.

Mineralogical characterisation and process mineralogy of goldbearing rocks from the Nalunaq gold deposit, Greenland

The Nalunaq deposit is a shear-zone hosted gold deposit, characterised by high grades (up to 5240 g/t Au over 0˙8 m). The host rocks are Palaeoproterozoic metamorphosed mafic rocks in the Nanortalik district in South Greenland. As a generic type, the Nalunaq gold mineralisation is a mesothermal vein-type gold deposit, hosted in amphibolitefacies metamorphosed mafic rocks. The gold is associated with sheeted quartz veins, hosted in a large-scale shear structure, which appears to relate to regional thrusts. They vary in width between 0˙05 m and 1˙8 m and form a relatively continuous structure. However, possibly due to extensive post-mineralisation deformation, there is no simple relationship between gold grade and thickness of the quartz veins. Eight samples were examined from amphibolites (host rock), quartz veins and amphibolites cut by quartz veins. The mineralisation is a low-S system, and consists mainly of pyrrhotite, löllingite, pyrite, arsenopyrite, chalcopyrite, gold and electrum, scarce Bi-sulphosalts, covellite and galena, with gangue minerals dominated mainly by amphibole, plagioclase feldspars, quartz and clinopyroxene. Most of the gold grains occur predominantly as inclusions in silicate minerals (72–99˙9%), interstitial to silicates (0–27˙5%), inclusions in sulphides that are locked in silicates (0–6˙7%), and inclusions in sulphides (0–2˙9%). Preliminary upgrading of gold by heavy media mineral separation in one quartz vein sample revealed that gold primarily occurs in free form (97%). Mineralogical information on gold provides significant information on gold-bearing grains, recognition of the different phases, grain size and associations, important in both exploration and gold recovery.

The impact of the linkage between grade distribution and petrofabric on the understanding of structurally controlled mineral deposits: Ouro Fino Gold Mine, Brazil

Ore deposit structural analysis, using a combination of structural geology and geostatistics, has direct application in the mining industry. Its main goal is to integrate structural measurements and assay data to create a method in which structurally controlled deposits are modeled numerically. This provides guidance to grade control and pit optimization during mining, improves prediction of orebody geometry and orientation, and provides more effective exploration strategies for surrounding areas. The method leads to a better understanding of how mineralized fluids percolated and were focused at the Ouro Fino Mine, a shear zone-hosted gold deposit in Minas Gerais State, Brazil. In this mine, gold is distributed along permeability pathways within rock fabrics that were produced or modified during the Brasiliano orogeny, when the Espinhaço–Araçuaı́ sequence was inverted towards the São Francisco craton during a basement-involved fold-and-thrust regime. The resulting permeable zones are conformable with the C surface, within which two other clusters of fabric elements control the large-scale features of the mineralization: (1) a cluster of fabric elements (mineral, stretching and intersection lineations) that plunges SE; and (2) a sub-horizontal cluster along folds and intersection lineations and the strike of the shear zone.

Study on the physical and chemical conditions of ore formation of Hetai ductile shear zone-hosted gold deposit and discovery of melt inclusions

The Hetai ductile shear zone-hosted gold deposit occurs in the deep-seated fault mylonite zone of the Sinian-Silurian metamorphic rock series. In this study there have been discovered melt inclusions, fluid-melt inclusions and organic inclusions in ore-bearing quartz veins of the ore deposit and mylonite for the first time. The homogenization temperatures of the various types of inclusions are 160°C, 180 – 350°C, 530°C and 870°C for organic inclusions, liquid inclusions, two-phase immiscible liquid inclusions and melt inclusions, respectively. Ore fluid is categorized as the neutral to basic K+-Ca2+-Mg2+ -Na+ − SO 4 2− -HCO 3 − -Cl− system. The contents of trace gases follow a descending order of H2O> CO2> CH4> (or CO> C2H2 > C2H6> O2>N2. The concentrations of K+, Ca2+, SO 4 2− , HCO 3 − ,. Cl−, H2O and C2H2 in fluid inclusions are related to the contents of gold and the Au/Ag ratios in ores from different levels of the gold deposit. This is significant for deep ore prospecting in the region. Daughter minerals in melt inclusions were analyzed using SEM. Quartz, orthoclase, wollastonite and other silicate minerals were identified. They were formed in different mineral assemblages. This analysis further proves the existence of melt inclusions in ore veins. Sedimentary metamorphic rocks could form silicate melts during metamorphic anatexis and dynamic metamorphism, which possess melt-solution characteristics. Ore formation is related to the multi-stage forming process of silicate melt and fluid.

Fluid Characteristics of Granitoid-Hosted Gold Deposits in the Birimian Terrane of Ghana: A Fluid Inclusion Microthermometric and Raman Spectroscopic Study

Fluid inclusions in vein quartz from 10 granitoid-hosted gold deposits and prospects in the Birimian terrane of Ghana, as well as from the Sansu mine (Ashanti shear zone type) and quartz veins in the nonmineralized Princess Town granodiorite, were studied by microthermometry and Raman microspectrometry. Fluid inclusions from the granitoid-hosted gold deposits are dominated by aqueous H2O-CO2-NaCl type 1 and liquid CO2-N2 ± CH4 type 2, with minor (<10%) aqueous H2O-NaCl type 3. Type 1 inclusions show large variations in CO2 phase volume proportions (10–90%) at 25°C and have salinities commonly between 0 and 6 wt percent NaCl equiv. Their bulk densities fall in a major range from 0.62 to 1.11 g/cm3. Type 2 inclusions show no visible H2O phase at room temperature and have bulk densities between 0.30 and 0.92 g/cm3. Type 3 inclusions, containing 10 to 20 vol percent H2O vapor, have salinities of 1 to 8 wt percent NaCl equiv and bulk densities of 0.77 to 1.03 g/cm3. In most cases, the three types of fluid inclusions coexist as groups in individual quartz grains of the samples studied. These fluid inclusions are interpreted to be trapped during phase separation of an originally homogeneous H2O-CO2 fluid, with low salinity (<6 wt % NaCl equiv) and moderate to high density (0.65–0.95 g/cm3). The type 1 fluid inclusions are suggested to be heterogeneous mixtures of the two end members of type 2 and 3 inclusions. Fluid immiscibility is documented by petrographic characteristics and microthermometric results of the three types of inclusions. Trapping temperatures and pressures, estimated from microthermometry of type 3 inclusions, equation of state, and the P-T-X nature of the H2O-CO2-NaCl system, are mainly between 200° and 350°Cand 1 and 3 kbars for the gold deposits. By contrast, fluid inclusions in the Sansu mine at Ashanti mostly comprise the liquid CO2-N2 ± CH4 type 2. The inclusions are considered to represent postentrapment modifications of trapped fluids. In addition, fluid inclusions in barren vein quartz from the Princess Town granodiorite comprise the low-salinity (commonly <6 wt % NaCl equiv) H2O-NaCl type 3. Raman microspectrometry shows that gaseous compositions of fluid inclusions from both granitoid- and shear zone-hosted gold deposits are mainly composed of CO2 (80–95 mol %), with significant amounts of N2 (2–20 mol %) and CH4 (0–10 mol %). The distinct low-salinity H2O-CO2-rich fluids from the granitoid-hosted gold deposits are comparable in composition to those from the major Ashanti and Tarkwaian types of gold deposits in the Birimian terrane of Ghana. These fluids are most likely to be metamorphic in origin and associated with the waning stages of the regional Birimian orogeny. Gold deposition within the Birimian granitoids was related to fluid phase separation and sulfidization of host rocks during hydrothermal alteration and mineralization. The present study, together with previous publications, suggests that fluid inclusions are characterized by H2O-CO2-NaCl and/or CO2-N2 ± CH4 types in mineralized areas, whereas H2O-NaCl fluid compositions dominate in barren areas. Fluid inclusion characteristics may, therefore, be a useful tool for regional gold exploration in the Birimian terrane of Ghana.

Round Hill Shear Zone-Hosted Gold Deposit, Macraes Flat, Otago,New Zealand: Evidence of a Magmatic Ore Fluid

The 0.8-Moz Round Hill shear zone-hosted gold deposit is one of several located on, or near, the Hyde-Macraes shear zone, Macraes Flat, New Zealand. Field relationships show stockwork veins are the oldest part of the hydrothermal system, followed by flat and ramp veins. Hanging-wall shear veins formed early but have been modified by late deformation. Fluid inclusion microthermometric results for dominant type I inclusions show that flat and ramp veins have the same average wt percent NaCl equiv values (2.2± 0.6%) and were precipitated from the same fluid. Salinity data for these veins overlap with those from hanging-wall veins (average 1.3 ± 0.3%) and stockwork veins (1.6 ± 0.6% for vein margins; 0.9 ± 0.3% for vein centers), although both of the latter trend toward lower overall wt percent NaCl equiv values, suggesting mixing between relatively dilute and more saline end members. All of the above veins have similar temperatures of homogenization (T h ), ranging between 110° and 188°C. Fluid inclusion trapping temperatures are considered to be ~300°C, equating to a ~10-km depth of formation. Carbon dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities. Stockwork and hanging-wall veins thus have true salinities of <1 wt percent NaCl equiv (<0.17 molal) and flat and ramp veins have true salinities of Fluid inclusion volatiles from Round Hill and other Macraes deposits are dominated by H 2 O (99.02–99.69 mol %), with lesser CO 2 (0.14–0.76 mol %), N 2 (0.03–0.32 mol %), and CH 4 (0.06–0.17 mol %). Noticeable concentrations of hydrocarbons occur in many of the samples. Fluid inclusion cation and anion data show few components of relatively low concentration are dissolved in the hydrothermal fluid, consistent with CO 2 -corrected microthermometric measurements. A range in fluid inclusion δ D H 2 O values obtained for the Round Hill deposit is coincident with the different vein types and also indicates mixing between two fluids. One fluid is considered to be O isotope shifted meteoric water, as trapped by the early stockwork veins, and has the lowest δ D H 2 O values of –80 per mil. The other fluid is represented by flat and/or ramp and hanging-wall veins that have δ D H 2 O values up to –40 per mil. The Macraes δ D H 2 O data show a trend toward the magmatic water end member for geothermal systems associated with convergent plate boundaries and are consistent with a 25 to 50 percent magmatic component in the Macraes hydrothermal system.

Round Hill Shear Zone-Hosted Gold Deposit, Macraes Flat, Otago, New Zealand: Evidence of a Magmatic Ore Fluid

Round Hill Shear Zone-Hosted Gold Deposit, Macraes Flat, Otago, New Zealand: Evidence of a Magmatic Ore Fluid

Fluid regimes related to the formation of lode-gold deposits in the Rio Itapicuru greenstone belt, Bahia: a fluid inclusion review

The shear zone-hosted gold deposits of the Rio Itapicuru Greenstone Belt, northeastern Brazil, are located in its southern and northern sectors, the former including Fazenda Brasileiro Mine and Fazenda Canto deposit and the latter Fazenda Maria Preta, Mari and Ambrósio deposits. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy in mineralized quartz veins of these deposits reveal two main populations of inclusions: primary, CO 2 - (± CH 4 ± N 2 ) inclusions (type I), and low salinity (&lt; 6 wt% NaCleq.) H 2 O - CO 2 - (± CH 4 ± N 2 ) inclusions (type II), representing fluid regimes, which were active during the gold mineralizing events. Both types of ore fluids are interpreted as part of a deep hydrothermal system, in which the fluid components may have been derived by devolatilization reactions during regional metamorphism and/or from mantle-magmatic sources (e.g. CO 2 , H 2 S). The nature of the CO 2 -rich fluid regime, atypical in mesothermal lode-gold deposits, still remains an open question. Gold deposition from bisulfide complexes took place at different crustal levels, at temperatures varying from 280 up to 500°C, and pressures from 1 up to 4.4 kb, probably closely linked with and dependent upon redox changes that accompanied the interaction of the fluids with mafic and carbonaceous host rocks.

Fluid regimes related to the formation of Lode-Gold Deposits the Rio Itapicuru Greenstone Belt, Bahia: a fluid inclusion review

The shear zone-hosted gold deposits of the Rio Itapicuru Greenstone Belt, northeastern Brazil, are located in its southern and northern sectors, the former including Fazenda Brasileiro Mine and Fazenda Canto deposit and the latter Fazenda Maria Preta, Mari and Ambrosio deposits. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy in mineralized quartz veins of these deposits reveal two main populations of inclusions: primary, CO 2 - (± CH ± N 2 ) inclusions (type I), and low salinity (< 6 wt% NaCleq.) H 2 0 - CO 2 - (± CH 4 ± N ) inclusions (type II), representing fluid regimes, which were active during the gold mineralizing events. Both types of ore fluids are interpreted as part of a deep hydrothermal system, in which the fluid components may have been derived by devolatilization reactions during regional metamorphism and/or from mantle-magmatic sources (e.g. CO 2 H 2 S). The nature of the CO 2 -rich fluid regime, atypical in mesothermal lode-gold deposits, still remains an open question. Gold deposition from bisulfide complexes took place at different crustal levels, at temperatures varying from 280 up to 500°C, and pressures from 1 up to 4.4 kb, probably closely linked with and dependent upon redox changes that accompanied the interaction of the fluids with mafic and carbonaceous host rocks.

Geology and genesis of Nalunaq Palaeoproterozoic shear zone-hosted gold deposit, South Greenland

The Nalunaq deposit is a shear zone-hosted gold deposit characterized by very high grades (up to 5240 g Au/t over 0.8 m). The host rocks are Palaeoproterozoic metabasic rocks of the Nanortalik Nappe, which is part of the Ketilidian Mobile Belt. The Nanortalik Nappe consists of bedded massive pyrrhotite/graphitic chert horizons overlain by fine-grained metabasalts with pillow structures and metadolerites followed by metatuffs and meta-agglomerate. This sequence was intruded by a porphyritic biotite granite (ca 1755-1725 m.y.) and subsequently by rapakivi granites.The gold mineralization occurs in a 0.5-2.0 m wide shear zone consisting of sheeted quartz veins with associated calc-silicate alteration. The mineralized shear zone is exposed for 800 m on the east face and 1200 m on the north face of Nalunaq Mountain. Surface mapping, diamond drilling (78 holes totalling 11 452 m) and adit development (288 m) have shown that the shear zone that hosts the gold-bearing Main Vein is a continuous structure. This structure was subsequently displaced by slightly rotating dextral strike-slip faults.Gold is present mainly as native gold and as minor maldonite. Free gold appears mainly as inclusions in quartz, diopside, plagioclase and as minor inclusions in löllingite and arsenopyrite. Gold also occurs in fractures and at grain boundaries of quartz, löllingite and arsenopyrite. These occurrences of gold indicate that several influxes of gold and/or stages of remobilization took place. The gold grains vary in size from 1 µm to 4 mm and their distribution is irregular, which makes it difficult to estimate the grade of the deposit from 1.5-to 5-kg samples.Fluid inclusions associated with gold in quartz and diopside contain three types of fluids: aqueous, carbonic and mixed carbonic-aqueous. Moderate to highly saline (14-26 wt% NaCl+CaCl2 equivalent) aqueous fluids with the inferred composition H2O-NaCl-CaCl/MgCl±LiCl predominate. The average homogenization temperature is 201°C. The carbonic fluid contains less than 5 mol% CH4 and has densities in the range 45-60 cm3/mole.The proximal alteration minerals (e.g. diopside, Ca-rich amphibole and Ca-rich plagioclase) and opaque assemblages (e.g., löllingite, arsenopyrite and pyrrhotite) show that the deposit was formed under upper greenschist- to amphibolite-facies conditions. The texture of löllingite rimmed by arsenopyrite suggests that the deposit was formed at high temperatures and the gold-mineralized structure was subsequently also active at lower temperatures.The shear zone was formed under ductile conditions and the major emplacement of quartz veins occurred in the subsequent brittle-ductile regime during multiple influxes of fluids. The shear zone clearly postdates the amphibolite-facies regional metamorphism as it is discordant to the metamorphic foliation.This mineralization, together with the similar hypothermal shear zone-hosted gold mineralization at Xutseq on the southeast coast and stream sediments that are generally highly anomalous in gold, indicates that a widespread gold-mineralizing system may have been active in South Greenland in the late stages of evolution of the Ketilidian Mobile Belt.

Geochemistry of main types of gold deposits in shear zones, China

Shear zone-hosted gold deposits in China can be divided into four types: ductile, brittle-ductile, ductile-brittle and brittle, of which the ductile and brittle types are the basic ones. All these types of gold deposits have their own geochemical characteristics. The Hetai gold deposit in Guangdong Province, for example, is a mylonite-type gold deposit in a ductile shear zone. With increasing mylonitization, obvious changes took place in trace elements in minerals and rocks, enriching gold and mineralizing elements. The S and Pb isotope data indicated that the ore-forming materials were derived from the strata. Hydrogen and oxygen isotopic and fluid inclusion studies also implied that the ore-forming fluid was much closer to meteoric water from the early to the late ore-forming stage. The Linglong gold deposit, Shangdong Province, is a quartz-type gold deposit in a brittle shear zone. Changes in rocks, minerals and trace elements occurred in the process of formation of gold quartz veins, and the analytical results of S, Pb, H and O isotopes showed that ore deposition is connected not only with the Jiaodong Group, but also with anatexic granites.

Re-Os, Sm-Nd, U-Pb, and stepwise lead leaching isotope systematics in shear-zone hosted gold mineralization: genetic tracing and age constraints of crustal hydrothermal activity

A combined Re-Os, Sm-Nd, U-Pb, and stepwise Pb leaching (PbSL) isotope study of hydrothermal (Mo-W)-bearing minerals and base metal sulfides from two adjacent shear zone hosted gold deposits (RAN, Kimberley) in the Harare-Shamva greenstone belt (Zimbabwe) constrain the timing of the mineralizing events to two periods. During an initial Late Archean event (2.60 Ga) a first molybdenite-scheelite bearing paragenesis was deposited in both shear zone systems, followed by a local reactivation of the shear systems during an Early Proterozoic (1.96 Ga) tectono-thermal overprint, during which base metal sulfides and most of the gold was (re-)deposited. While PbSL has revealed an open-system behavior of the U-Pb systematics in molybdenite and wolframite from the RAN mine, initial Archean Re-Os ages are still preserved implying that this system in these minerals was more resistant to the overprint. A similar retentivity could be shown for the Sm-Nd system in scheelite and powellite associated with the above ore minerals. Re-Os isotopic data from the Proterozoic mineralization in the Kimberley mine point to a recent gain of Re, most pronouncedly affecting Fe-rich sulfides such as pyrrhotite. A significant Re-loss in powellitic scheelite (an alteration phase of molybdenite-bearing scheelite), coupled with a marked loss of U in W-Mo ore minerals, complements the observation of a major Re uptake in Fe-sulfides during oxidizing conditions in a weathering environment. Pyrrhotite under these conditions behaves as an efficient Re-sink. Lead isotope signatures from PbSL residues of molybdenite, powellite, and quartz indicate a continental crustal source and/or contamination for the mineralizing fluid by interaction of the fluids with older sedimentary material as represented by the direct host country rocks. Our investigation reveals the potential of the Re-Os isotopic system applied to crustal hydrothermal ore minerals for genetic tracing and dating purposes. The simplified chemical separation of Re and Os from geological material used in this study, together with improvements of chemical yields, will enable high precision data to be collected rapidly on crustal material with low Os concentrations in the future.

Geochemical characteristics of two types of ores from Jinshan shear zone-hosted gold deposit, Jiangxi, with a discussion on the mechanism of two-stage mineralization

Two types of gold ores, siliceous mylonite and quartz vein, formed at the first and second stages of mineralization respectively, can be clearly recognized in the shear zone-hosted gold deposit at Jinshan, Jiangxi. Similarity in REE and trace elements between the siliceous mylonite and the country rocks indicates that the ore metals were supplied by the surrounding strata during the first stage of mineralization. On the other hand, as indicated by fluid inclusion data, the ore-forming fluid at the second stage was of meteoric origin and the precipitation of gold was caused by phase separation.