Ore-forming Stages Research Articles

Paleomagnetic Constraints on Ages of Mineralization in theKalahari Manganese Field, South Africa

We report paleomagnetic data from samples spanning several grades of enrichment in the Kalahari manganese field, South Africa, in order to assess mineralogical aspects of the ore-forming stages, and also to date these stages through comparison to previously existing, well dated paleomagnetic results from the Kaapvaal-Kalahari craton. Our paleomagnetic study confirms a multistage evolution for the orebodies, with three distinct, ancient remanent directions preserved. An early diagenetic remanence direction (MAM-1), associated with dusty hematite inclusions (1–10 μm) that are omnipresent in the microcrystalline matrix of low-grade, Mamatwan-type ore, yields a tilt-corrected paleomagnetic pole (–08.2° N, 111.1° E, dp = 5.6°, dm = 11.1°; n = 6 specimens) that is similar to previous results from the immediately underlying Ongeluk lavas. A late diagenetic or weak metamorphic overprint (MAM-2), carried by recrystallized hematite (20–250 μm), within both Mamatwan- and Wessels-type ore, generates a paleomagnetic pole (present coordinates 12.1° N, 321.8° E, dp = 3.4°, dm = 6.0°; tilt-corrected 16.1° N, 317.8° E, dp = 3.4°, dm = 6.4°; n = 14 specimens) that resembles those from the ca. 1900 Ma Hartley lavas and Mashonaland sills. The MAM-2 overprint may be related to Kheis thrusting at 1750 to 1800 Ma as previously proposed or to magma-driven fluid migration during rifting as the Hartley-Mashonaland igneous event perforated the Kalahari craton. The third magnetic component observed in our sample suite (WESS) is restricted to high-grade Wessels-type ore, rich in high Fe hausmannite and coarser hematite (0.1–1.0 mm), in the immediate vicinity of north-trending normal faults. It yields a pole (54.4° N, 033.7°E, dp = 4.7°, dm = 9.1°; n = 7 specimens) that is similar to both the ca. 1250 and 1100 Ma portions of the Kalahari craton’s apparent polar wander path. Either of these ages would be in accordance with previous multigenetic models for the Wessels event and its regional crosscutting relationships. Our WESS paleomagnetic pole, combined with previous paleomagnetic results from the Sishen-Postmasburg region, temporally links Kalahari manganese field hydrothermal upgrading with east vergent thrusting in the Griqualand West foreland, during the early or medial stages of the late Mesoproterozoic Namaqua orogeny.

Evolution and paragenetic context of low δD hydrothermal fluids from the Panasqueira W-Sn deposit, Portugal: new evidence from microthermometric, stable isotope, noble gas and halogen analyses of primary fluid inclusions

The evolution, paragenetic context and origin of remarkably low δD hydrothermal fluids from the Hercynian Panasqueira W-Sn-Cu(Ag) deposit have been investigated through a combined microthermometric, stable isotope, halogen and noble gas fluid inclusion study. Large variations in δD between −60 ‰ and −134‰ have been observed in primary fluid inclusions from growth zones in a quartz crystal (Pa66) paragenetically constrained to the main sulfide stage (MSS) of ore formation. The same fluids exhibited relatively constant fluid inclusion homogenisation temperatures (254 to 260°C), salinities (7.4 to 8.7 wt.% NaCl equivalent) and calculated fluid δ 18O (3.8 to 4.4‰). All the fluids exhibited 40Ar excesses. Mean molar Br/Cl and I/Cl ratios varied from 2.3 × 10 −3–4.2 × 10 −3 and 8 × 10 −4–10 × 10 −4 respectively, with the more Br-rich fluids being associated with the more deuterium-depleted fluids. The low palaeolatitude of Panasqueira throughout the main stages of ore formation and the overwhelming predominance of meteoric fluids in the main, late and post-ore mineralising fluids with δD of −40 to −65‰ lends little support for any models involving low δD palaeometeoric water. The limited range of 40Ar∗/Cl ratios (1.1 × 10 −5–1.6 × 10 −5) also precludes boiling during the MSS or mixing of meteoric water with highly fractionated deuterium-depleted magmatic waters as viable mechanisms for producing the low δD fluids. The high Br/Cl and I/Cl of the MSS fluids indicate significant fluid interactions with organic rich sediments or metasediments, in particular regionally abundant Carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85‰ compared to palaeometeoric water. The large range of δD observed in the MSS fluids may be explained by isotopic exchange of palaeometeoric water with these coals at varying water/rock (wt./wt.) ratios between 0.02 and 0.002 with fluid oxygen isotopic compositions controlled by exchange with the Beira Schists. Such a model is consistent with the requirements that any satisfactory model must take into account viz. (i) the relative constancy of temperature, salinity, 40Ar∗ and fluid δ 18O during excursions to low δD; (ii) the high I/Cl and Br/Cl ratios observed; and (iii) the coincidence of these low δD fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions. The model indicates that (meta)sedimentary sources of ore forming components became increasingly important during the evolution of the Panasqueira Sn-W deposit and accounts in part for the observed progression from oxide-dominated to base-metal sulfide-dominated parageneses.

Physicochemical Environment during the Formation of the Mozumi Skarn‐type Pb–Zn–Ag Deposit at the Kamioka Mine, Central Japan: A Thermochemical Study

Abstract: The physicochemical environment during the ore formation of the Mozumi skarn‐type Pb–Zn–Ag deposit, Kamioka mine, Central Japan is discussed with silicate phase equilibria using calculated phase diagrams. The mineral assemblages, mineral chemistry, and fluid inclusion data are coupled with thermodynamic calculation to estimate the stability fO2–XCO2condition of the mineral assemblages at each ore formation stage. The skarn was approximated by the model system CaO–A12O3–FeO–SiO2–O2–CO2–H2O including grossular‐andradite garnet and clinozoisite‐epidote solid solutions. The solid solutions are combined into the calculation using “pseudocompound approximation”, and real boundary of the mineral assemblages other than simple activity corrected diagram was provided. The diagrams also show isoplethal contour for garnet and epidote.

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.

Massive copper–zinc sulphide deposits in the Urals

The Urals is one of the most important regions in the world for the occurrence of volcanogenic massive sulphide deposits. Here are found 90 deposits of this type: 32 of them are worked out already, 16 are being worked now, 42 are explored but are not yet being worked. Some of the sulphide deposits, such as Gaiskoye, Uchalinskoye and Degtyarskoye are very large: their individual tonnages exceed 100 million. The volcanogenic massive sulphide deposits are the main sources supplying the copper- and zinc-productive plants of the region. Three types of the economic massive sulphide deposits: Cyprus, Urals, and Baimak types, are distinguished in the region. The main weight of the sulphide deposits is located within Tagil and Magnitogorsk troughs, while several others are situated in the Urals–Tobol uplift. The copper-dominant deposits of Cyprus type are hosted by tholeiitic basalt, which occurs in the basal parts of the Paleozoic volcanic sequences. They were formed near zones of ocean floor spreading. The copper–zinc sulphide deposits of Urals type are widespread in the region, being hosted by bimodal rhyolite–basalt assemblages. Deposits of this type were generated within back-arc and intra-arc basins along the paleovolcanic belts. The zinc–copper–barite deposits of Baimak type are connected with andesite–dacite–granite volcanic–plutonic complexes, which formed under island-arc conditions. A geological–geochemical model has been developed for the formation of the sulphide deposits of Urals type. According to this model, the deposits were formed during two stages: the first one, sedimentary-exhalative, took place within submarine volcanic depressions and the second one, hydrothermal-metasomatic, manifested itself after the sulphides of the first stage formation had been overlain by tuffs and lavas. The physical–chemical parameters of the first stage of ore formation have been determined as: temperature 250–300°C, pressure 200–400 bar, pH=3–6, Eh=−100 mV. For the second stage these parameters were: T=70–390°C, P=630–710 bar, pH=7.0–9.8, Eh=from +200 to 365 mV. The zonations of the ore bodies and haloes of wall-rock alteration were formed under the influence of the temperature and acidity–alkalinity fields which existed in the hydrothermal-metasomatic stage. The zones of rich ores were formed at this stage, due to leaching of the more mobile elements (Cu, Zn, Cd, Au, Ag) from parts of the ore bodies and their deposition in other parts. On the basis of the estimation of the addition and/or loss of elements in different zones of wall-rock alteration and disseminated ore, it can be shown that some elements were extracted from the host rocks (Fe, Cu, Zn) while others were supplied by endogenous fluids (K, Si, S). Modification of sulphide ores took place in several sulphide deposits of the Urals. These stages are: (1) pyrrhotite (330–520°C); (2) bornite (190–270°C); (3) regional metamorphism under variable facies conditions; (4) contact metamorphism (500–620°C). Nevertheless, some of the deposits have a very well-preserved character, they non-deformed and non-metamorphosed. The Urals may serve as a standard for comparative studies of the volcanogenic massive sulphide deposits in other orogenic belts.

Zonation in primary halos and geochemical prospecting pattern for the Guilaizhuang gold deposit, eastern China

The Guilaizhuang gold deposit, with an average grade of 8.10 g/t Au and reserves of over 30 mt, is a subvolcanic epithermal deposit. The deposit is hosted in Paleozoic carbonate rocks in the western Shandong metallogenic terrane of the littorine Pacific metallogenic domain, eastern China, and is associated spatially with an early Mesozoic subvolcanic alkalic intrusive complex (188–190 Ma). The orebody was discovered at the end of the 1980s based on anomalies of Au in stream sediment samples at a map scale of 1∶200,000. The ore is rich in Au, Ag, Te, V, F, As, Sb, Tl, W, and Mo but poor in Cu, Pb, and Zn. The ore is similar in its trace elements to Carlin-type Au deposits. The transverse element association zonation of the deposit is as follows: (on the hanging wall) F⟸ W−Mo−As−Tl⟸Se−Sb−Bi⟸Au−Ag−Te (orebody) ⟹ Se−Sb−Bi (on the foot wall). The axial zonation is as follows: Au⟹Ag⟹Sb⟹V⟹Zn ⟹W⟹F⟹Mo⟹Tl⟹As. Indexes such as (Au+Sb)d/ (As+Tl)d and (Au+ Ag)d/(As+Tl)d decrease with depth but dramatically increase at the level where the orebody pinches out, which indicates another orebody might exist at depth. Multivariate statistical analysis suggests that the ore (halo)-forming process can be divided into two stages: alteration and mineralization. The former includes: potash feldsparization, albitization, silicification, and fluoritization. The latter includes the following substages: arsenopyritization and scheelitization; pyritization, chalcopyritization, and sphaleritization; and native gold, electrum, and calaveritization. The last substage is considered to be the main ore-forming stage in the formation of the deposit.

The Machangqing copper-molybdenum deposits, Yunnan, China ? an example of Himalayan porphyry-hosted Cu-Mo mineralization

The Machangqing copper-molybdenum deposits occur in the Shanjian fold belt, Yunnan Province, China. Two types of ores are distinguished: (1) Cu-Mo quartz veinlets in magnetite-series granite porphyry; and (2) Cu-Mo skarns occurring at the contact between the Ordovician sedimentary sequence and the granite porphyry. With decreasing temperature and\(a_{K^ + } /a_{H^ + } \) of hydrothermal fluids initially in equilibrium with K-feldspar, the following alteration patterns developed within the porphyry, from the center outwards: silicification, K-silicate, phyllic and argillic alteration. The paragenetic sequence of alteration minerals observed in the Cu-Mo skarns resulted from decreasing temperature and/or increasing\(a_{Ca^{2 + } } /a_{H^ + }^2 \) of the hydrothermal fluids initially in equilibrium with grandite garnet. Fluid inclusions in quartz suggest boiling during the mineralization. The mineralization temperatures based on filling temperatures and salinities of quartz are in the following ranges: about 265° to 400 °C and 5.0 to 14.6 wt.% NaCl eq. for the Cu-Mo veinlets; and 200° to 500 °C and 10.2 to 42.0 wt.% NaCl eq. for the Cu-Mo skarns. As is evident from log fo2-pH diagrams, ores of the early stage of mineralization in the Cu-Mo skarns, characterized by the assemblage magnetite + pyrite + rare pyrrhotite + K-feldspar + quartz, were deposited from highly alkaline and high temperature fluids. With decreasing temperature and fo2, the pH of the ore fluids was shifted towards slightly alkaline to neutral, with the resultant formation of the main stage ores, characterized by the assemblage chalcopyrite + pyrite + molybdenite + sphalerite + K-feldspar +sericite (muscovite) + epidote + uartz. Very minor amounts of ore minerals, including matildite, bismuthinite and electrum, are associated with a late stage of ore formation.

Mineral chemistry and oxygen isotopic analyses of uraninite, pitchblende and uranium alteration minerals from the Cigar Lake deposit, Saskatchewan, Canada

The Cigar Lake unconformity-type U deposit is one of the largest and highest grade U deposits in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada. Cigar Lake has recently been the focus of an international, 3-a, collaborative program in which this U deposit was studied as a natural analogue for a spent nuclear fuel repository. The deposit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults and the unconformity between Athabasca group sandstones and Aphebian metasediments. Three stages of U mineralization have been identified based on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably altered to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coffinite. U Pb chemical ages of the 3 stages of U mineralization from Cigar Lake coincide with the 3 major fluid events that precipitated clay and silicate minerals at 1500 Ma, 950 Ma, and 300 Ma, throughout the entire Athabasca Basin. Stage 1 and 2 uraninite and pitchblende have the lowest δ 18O values that range from −30.1 ‰ to −15.2‰; whereas, stage 3 uraninite has δ 18O values ranging from −10.0‰ to −3.4‰. Uranyl oxide hydrate minerals have δ 18O values that range from −11.3‰ to −8.2‰; whereas, uranyl minerals have much higher δ 18O values. Based on U Pb chemical ages, δ 18O values, and petrographic relationships of U alteration minerals associated with primary U mineralization, the Cigar Lake U ore is similar to U ore from other unconformity-type U deposits in the Athabasca Basin. Therefore, the Cigar Lake ore deposit, although surrounded by clay and sandstone barriers, has been effected by the same fluid events that have altered other unconformity-type U deposits in the Athabasca Basin. The 3 stages of ore formation and associated alteration minerals permit the detailed study of fluids responsible for U deposition and alteration. This information provides the necessary context for the evaluation of the Cigar Lake deposit as a “natural analogue” for the disposal of spent nuclear fuel in underground vaults in rocks of the Canadian Shield.

Fluid chemistry and processes at the Porgera gold deposit, Papua New Guinea

The Porgera gold deposit in Papua New Guinea is a world-class example of an alkalic-type epithermal gold system (stage II), which overprints a precursor stage of magmatic-hydrothermal gold mineralization (stage I). Gas and ion chromatographic analyses of fluid inclusions contained in vein minerals from both mineralization stages have been carried out in order to constrain the compositions of the fluids involved in, and the processes attending, ore deposition. These data indicate the presence of three end-member liquids, the most dilute of which was present throughout the mineralization history and is interpreted to represent evolved groundwater of meteoric origin. Its composition is estimated to have been approximately 500 mM Na+, 10 mM K+, 5 mM Li+, 250 mM Cl−, 0.15 mM Br−, and 0.01 mM I−, plus significant concentrations of dissolved gases. More saline liquids were also present during the two main stages of ore formation, and although their compositions differ, both are interpreted to have been derived at least in part from magmatic fluids, and to have been the media by which gold was introduced into the system. Stage I minerals contain fluid inclusions which decrease in salinity towards this dilute end-member composition through the vein paragenesis, reflecting progressive dilution at depth of the magmatic fluid source by groundwaters. Ore deposition is thought to have been caused largely by simple cooling and/or wallrock reactions, although limited in situ fluid mixing may also have occurred. The most saline fluids, present in early quartz and pyrite, contain at least 810 mM Na+, 530 mM Ca2+, 130 mM K+, 12 mM Li+, 87 mM SO4 2−, 960 mM Cl−, 1.1 mM Br−, and 0.05 mM I−, plus significant but variable concentrations of dissolved gases. Fluid inclusions from stage II hydraulic breccia veins reveal the presence of two distinct liquids with contrasting salinities, which were present at different times during vein formation. A higher salinity liquid appears to have predominated during mineralization, whereas lower salinity groundwaters filled the structures during intervening periods. The ore-forming fluid may have been forcibly injected into the veins from depth during fracturing and depressurization events, displacing the resident groundwaters in the process. The original composition of this fluid is estimated to have been at least 1770 mM Na+, 59 mM K+, 180 mM Li+, 210 mM SO4 2−, 680 mM Cl−, 1.4 mM Br−, and 0.09 mM I−, plus 1.5 mol% CO2, 0.19 mol% CH4, and 0.04 mol% N2. Gas chromatographic analyses of fluid inclusions from stage II samples show a decrease in total gas content between early unmineralized veins and post-mineralization vuggy quartz (suitable samples could not be obtained from the ore stage itself). Post-mineralization samples plot along an experimental gas-saturation curve in the CO2-CH4-H2O-NaCl system, obtained at conditions similar to those attending stage II ore deposition at Porgera (200–300 bar, ˜165 °C). These results are interpreted to indicate a period of depressurization-induced phase separation during hydraulic fracturing, which resulted in rich ore deposition. Volatile gases such as CH4 and N2, in addition to CO2 in solution, are shown to have a significant negative effect on total gas solubility. This effect may be of critical importance in lowering the temperature and increasing the depth (pressure) at which phase separation can occur in epithermal systems.

A three-stage metallogenic model for gold deposits in metamorphosed microclastic rocks

Gold deposits occurring in metamorphosed microclastic rocks are distributed extensively at home and abroad. Some deposits of this type are of superlarge tonnage. The formation of gold deposits in metamorphosed microclastic rocks involves three stages : the sedimentary stage, the regionally metamorphic stage, and the ore-forming stage. At the first stage, microclastic sedimentary source rocks were developed in a relatively semi-enclosed reducing sea basin and were enriched in carbon, sulfur and gold. At the second stage, the gold adsorbed on organic matter and clay minerals was released and poorly concentrated during the destruction of organic matter and the depletion of clay minerals by regional metamorphism with increase temperature and pressure. At the third stage, a tectono-hydrothermal event took place. As a result, gold was leached from metamorphosed microclastic rocks, transported to ore depositional locus and/or mixed with gold of other sources in the course of migration, and finally precipitated as ores. Gold deposits of this type were eventually formed at the third stage, and they also can be classified as the orogenic belt type and the activation zone type. The gold deposits occurring in metamorphosed microclastic rocks are the products of reworking processes and the influence of magmatism should be taken into consideration in some cases.

Hydrothermal input into sediments of the Mid-Atlantic Ridge

Abstract The analysis of geochemical profiles in metalliferous sediment cores obtained from the Mid-Atlantic Ridge rift valley at the TAG, MARK and 15°N hydrothermal vent fields and near 27°10′N allowed a study of the history of hydrothermal processes. Of the three hydrothermal zones of the TAG field, the North hydrothermal zone reached its maximum activity between 6000 and 25 000 years ago, the MIR Mound 10 000 years ago and the Active mound 15 000 years ago. Evidence of intensive hydrothermal sedimentation is not seen near the MARK field. The MARK sulphide mounds were probably formed in the course of a single hydrothermal episode that began 4000 years ago. Most of the sulphides of the newly found hydrothermal field near 15°N were formed in the course of an event traced by a Fe-enriched layer 70–90 cm below the seafloor. At present, this field is only slightly hydrothermally active. Three Fe-enriched layers correspond to the possible stages of ore formation at a site of presumed hydrothermal activity near 27°10′N. Their relative intensity is decreasing with time.

An ore genetic model for the Lubin—Sieroszowice mining district, Poland

The Lubin-Sieroszowice mining district is a world-class copper-silver, stratabound ore deposit that lies near the Lower-Upper Permian boundary. It transgresses the Werra dolomite, the Kupferschiefer organicrich shale and the Weissliegendes sandstone, which overlie barren Rotliegendes sandstone. On the basis of underground and microscope observations and light stable isotope data, and thermodynamic calculations, a new ore genesis model is proposed whereby ore minerals were deposited in the following stages: Stage 0 was synsedimentary or earliest diagenetic and contains 100s ppm of base metals trapped by clay minerals, and minor sulphides. Stage I was early diagenetic and contains 1000s ppm base metals. It is characterized by bornite and overlying chalcopyrite + pyrite that lie a short distance above the Rotliegendes/Weissliegendes contact. The sulphides were deposited near the interface between an overlying, buffered, reducing fluid (1), largely derived from the Kupferschiefer, and an oxidizing fluid (2) in the Rotliegendes. Stage II is the main ore-forming stage. This stage is late diagenetic, peneconcordant, lies near the Kupferschiefer/Weissliegendes contact, and contains several percent base metals.It is associated with the hematite-bearing Rote Faule facies and is characterized by vertical zonation. A central chalcocite zone is flanked above and below by bornite and chalcopyrite. Silver occurs with all the above sulphides. Galena and sphalerite occur mainly just above copper zone, whereas pyrite is usually present in the upper part of the copper zone and together with galena and sphalerite. Metals were transported in a copper-rich oxidizing fluid (3), which probably originated deep in the Permian basin, reacted with organic matter in the Kupferschiefer, and mixed with reducing fluid (1) in the Weissliegendes, resulting in the observed mineral zonation. Stage III is late diagenetic, discordant and is represented by massive and dispersed chalcocite ore present on the peripheries and below anhydrite-cemented Weissliegendes sandstone. It resulted from redistribution of earlier copper and silver minerals by descending, reduced, sulphur-rich fluids (4). Stage IV consists of rare polymetallic veins of no economic importance that cut the stratigraphy and are probably related to Alpine tectonism. The richest and thickest ore is in the Weissliegendes, 10–15 km east of the Rote Faule facies (Fig. 1). It probably occupies structures that trapped fluid (1) which was the main precipitant of metals in the sandstone.

Fluid inclusion studies of zoning in the Dachang tin-polymetallic ore field, People's Republic of China

The Dachang tin-polymetallic ore field, Guangxi, People9s Republic of China, is one of the largest in the world and contains the Changpo, Bali, and Longtaoshan tin-base metal deposits, the Dalfulou black shale-hosted cassiterite-sulfide deposit, and the Lamo Cu-Pb-Zn proximal skarn deposit. These are hosted by Devonian carbonate-rich sediments metamorphosed to hornblende hornfels facies near the underlying Yanshanian Longxianggei granite (91 Ma). Plutonic intrusion and relevant mineralization are controlled by the north-northwest-trending Longxianggei and Changpo anticlines. The conduit for distal skarn solutions was a reverse fault (F1). Tin mineralization at Changpo is genetically related to the S-type Longxianggei granite with proximal skarns precipitated near its contact and distal ores 300 to 500 m vertically away from the contact.Hydrothermal alteration is grossly zoned outward from the granite contact as follows: (a) proximal ore skarn zone (garnet and pyroxene), (b) tourmaline-silicified zone, (c) sericite (+ or - cassiterite) zone, and (d) carbonate(-sulfosalt) vein zone. Zones b to d do not occur above the Lamo deposit whereas zone a is missing directly below the large Changpo deposit but occurs lateral to it. Nearest to the granite outcrop, the Lamo Cu-Pb-Zn proximal skarn is developed with a temporal sequence of stages: (A) garnet-vesuvianite-wollastonite + or - diopside, (B) Ca amphibole-epidote-fiuorite-quartz, (C) biotite-sericite-sulfides, and (D) sulfides-carbonate-chlorite. Distal skarns have the following temporally and spatially distributed stages: (1) tourmaline-quartz-cassiterite, (2) arsenopyrite-cassiterite (+ or - other sulfides), and (3) sulfosalts-carbonate (most distal).There is a zonal pattern of both temperatures and ore fluid compositions deduced from fluid inclusions as follows: proximal skarn zone: 450 degrees to 600 degrees C, CO 2 /CH 4 35 wt percent NaCl equiv; tourmaline-silica zone: 350 degrees to 450 degrees C, CO 2 /CH 4 = 0.10 to 10.00, salinity = 12 to 30 wt percent NaCl equiv; sericite zone: 250 degrees to 350 degrees C, CO 2 /CH 4 >10, salinity 2 dominated, no CH 4 , salinity = 6 to 8 wt percent NaCl equiv.Daughter crystals include antarcticite (CaCl 2 . 6H 2 O), halite, sylvite, carbonate, MgCl 2 . 6H 2 O), and fluorite. Thermodynamic calculations based on fluid inclusion composition and measured temperature show that f (sub O 2 ) values of the ore-forming stages were very low, well below the FMQ buffer and increased from early to later stages. Tin precipitation was affected by a low oxygen fugacity of ore fluids partly resulting from the occurrence of black shale, and a decrease in temperature.

Isotopic systematics (O, H, C, Sr, Nd) of superimposed barren and U-bearing hydrothermal systems in a Hercynian granite, Massif Central, France

The Hercynian (325 Ma) granitic complex of Saint-Sylvestre experienced two hydrothermal episodes: the first one leading to the formation of a typical alteration facies called “episyenite”, mainly characterized by the dissolution of granitic quartz; the second one leading to the deposition (at ∼280 Ma) of a major U deposit. The present study documents the isotopic characteristics of these hydrothermal events. 18 O 16 O , D/H and 13 C 12 C systematics demonstrate that barren episyenites were generated during hydrothermal interaction of large amounts of mainly meteoric water with the granite; the ore-depositing fluid equilibrated in a sedimentary formation before entering the granite. Episyenitization was dated at 301 ± 2 Ma, which is 25 Ma later than the granite emplacement. The Rb-Sr isochron relationship implies isotopic equilibrium within the fluid phase and between the fluid and the host rocks, which confirms the high water/rock ratio inferred from O-H isotope systematics. A coupled Nd and Sr isotope investigation demonstrates that at least Nd was reworked from the granite in the episyenite genesis, but Sr and possibly Nd had an exotic origin consistent with the inferred sedimentary origin during the ore-forming stage. The present study emphasizes the fact that spatially superimposed hydrothermal events in a granitic environment can be: (1) disconnected from the magmatic history of the granite itself; and (2) mutually disconnected from chronological and chemical points of view.

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km 2 which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins. Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S 2 accompanied by variations in the activity of Te 2 during the Gold stage. Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO 2 vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization. Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.

Thermochronology of economic mineral deposits; dating the stages of mineralization at Panasqueira, Portugal, by high-precision 40 / 39 Ar age spectrum techniques on muscovite

40 Ar/ 39 Ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history (thermochronology) of the Panasqueira, Portugal, tin-tungsten deposit, a deposit spatially associated with a belt of Hercynian plutons. Muscovite samples with an age difference as small as 2.2 m.y. (0.7% of the age) are statistically distinct. Statistics are even better for comparison of multiple samples from separate events; that is, a difference of 0.9 m.y. (0.3%) can be resolved in this approximately 300-m.y.-old deposit. The major tin and tungsten ore-forming stages, which are the oxide-silicate stage, the main sulfide stage, and greisenization, occurred between 296.3 + or - 0.8 (1 Sigma ) and 291.6 + or - 0.8 m.y. (1 Sigma ). The first substage of the oxide-silicate stage was a short-lived thermal pulse at 296.3 + or - 0.6 m.y.; the fluids responsible may have emanated from the known granite cupola. The main sulfide stage was active at 294.5 + or - 0.9 m.y. as a slightly longer lived pulse with oldest evidence for this stage (295.8 + or - 0.6 m.y.) coming from areas farthest away from the known cupola and youngest evidence (293.5 + or - 0.8 m.y.) closest to the cupola. A second substage of the oxide-silicate stage occurred as a short-lived thermal pulse at 292.9 + or - 0.7 m.y., synchronous with greisenization of the cupola and alteration of the silica cap at 292.1 + or - 0.4 m.y. The duration of activity of the oxide-silicate stage, the main sulfide stage, greisenization, and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4.2 + or - 0.5 m.y. (1 Sigma ). Minor argon loss from all dated muscovites occurred during later reheating, probably during the longer lived pyrrhotite alteration stage. A single center, the known cupola, had a prolonged role and was the source for main sulfide stage, oxide-silicate stage II, greisenization, and alteration of the silica cap and possibly oxide-silicate stage I and the pyrrhotite alteration stage; however, a separate source for these latter two stages cannot be ruled out.This study is an example of a new and powerful application of 40 Ar/ 39 Ar age spectrum dating of muscovite. Because of the high precision demonstrated in this study, it is now possible to establish time constraints necessary for solving some of the long-standing problems in economic geology. Beyond this, the unique geologic situation of Panasqueira has allowed us to quantify the thermal characteristics of muscovite. Published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees C during rapid cooling or short reheating and a temperature of approximately 270 degrees C during slow cooling or extended reheating. Argon-loss patterns displayed by all dated muscovites resulted from reheating after original closure; the mechanism for this argon loss appears to have been argon transport by volume diffusion. Thus, 40 Ar/ 39 Ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age, duration, and number of episodes of mineralization.

Geological features and genesis of the Jinduicheng porphyry molybdenum deposit, Shaanxi Province, China

Situated in the Henan-Shaanxi fault-uplift area on the southern margin of the Sino-Korean Paraplatform, the Jinduicheng porphyry molybdenum deposit is the most important molybdenum producer in China. During Yenshanian the Jinduicheng granite porphyry was emplaced in metaspilite of the Proterozoic Xionger Group, controlled by a NW-trending fault. Mineral compositions are mainly quartz (25–40%), microcline and microcline-perthite (27–40%) and plagioclase (An 8–14, 14–32%), associated with minor biotite and muscovite, and phenocrysts are made up of K-feldspar, quartz and plagioclase. Accessory minerals include magmatite, apatite and zircon. The porphyry contains SiO2 73.83% and K2O + Na2O 8.06% (with K2O/Na2O ratio being 1.82), beloning to the calc-alkaline series. Mineralization occurs in the porphyry body and biotitized and hornfelsized spilite within the exocontact zone. The maximum depth of mineralization reaches 1000 meters below the surface. According to mineral assemblage, ore veins are classified into five types: (1) pyrite-quartz; (2) pyrite-K-feldspar-quartz; (3) pyrite-molybdenite-quartz; (4) pyrite-molybdenite-K-feldspar-quartz; and (5) muscovitefluorite-pyrite-molybdenite-quartz. As the most important economic molybdenum mineral, molybdenite occurs in various forms. Wall-rock alterations show a gross zonation of K-feldspathization-greisenization → silicification → propylitization from the porphyry outwards, of which silicification is most intense and has close genetic relationship with Mo mineralization. Fluid inclusion studies yield homogeneous temperatures ranging from 250 to 240°C, with the main stage between 400 and 300°C. Gas inclusions frequently coexist with multiphase inclusions containing such daughter minerals as halite, sylvite, molybdenite and K-feldspar. Under moderate-high temperatures, lowerfo2, highfs2 and weak acidity conditions, boiling of ore-forming fluids is a prerequisite for the precipitation of molybdenite. Sulfur, oxygen and carbon isotopic compositions suggest that at the high temperature stage (450°C) magmatic water is dominant and at the main ore-forming stage a mixture of magmatic water and meteoric water is expected. At the late stage, the mixture is predominated by meteoric water. Sulfur and molybdenum are mostly of magmatic origin.

Lacustrine-Humate Model for Primary Uranium Ore Deposits, Grants Uranium Region, New Mexico

Two generations of uranium ore, primary and redistributed, occur in fluvial sandstones of the Upper Jurassic Morrison Formation in the San Juan basin; the two stages of ore formation can be related to the hydrologic history of the basin. Primary ore formed soon after Morrison deposition, in the Late Jurassic to Early Cretaceous, and a model, the lacustrine-humate model, is offered that views primary mineralization as a diagenetic event related to early pore fluid evolution. The basic premise is that the humate, a pore-filling organic material closely associated with primary ore, originated as humic acids dissolved in pore waters of greenish-gray lacustrine mudstones deposited in the mud-flat facies of the Brushy Basin Member and similar K shale beds in the Westwater Can on Member. During compaction associated with early burial, formation water expelled from lacustrine mudstone units carried these humic acids into adjacent sandstone beds where the organics precipitated, forming the humate deposits that concentrated uranium. During the Tertiary, much later in the hydrologic history of the basin, when Jurassic sediments were largely compacted, oxygenated ground water flowed basinward from uplifted basin margins. This invasion of Morrison sandstone beds by oxidizing ground waters redistributed uranium from primary ores along redox boundaries, forming ore deposits that resemble roll-front-type uranium ores.

The significance of microbiological processes in lakustrine manganese ore formation

SummaryThe role of microorganisms in the formation of lakustrine manganese ore is manifested both in reducing processes of Mn4+ into Mn2+ and in oxidizing the latter into Mn4+.Oxidizing processes with participation of microorganisms, mainly Metallogenium, take place under physico-chemical conditions corresponding to manganese stability against chemical oxidation. The use of methods of selective culture technique elective media, quantitative determination on membrane filters and electron microscopic analysis of the ore from Punnus-Järvi Lake have shown direct participation of manganese oxidizing microorganisms in the formation of concretions. Peculiar morphology of these organisms contributes to retention and binding of manganese oxides on the surface of concretions. Data on ore-forming role have been experimentally confirmed: with the use of Metallogenium concretions of manganese oxides were obtained in laboratory which had a size of 2–5 mm. In tests with chemical reaction with a solution in the absence of microorganisms no formation of concretions took place.The data considered show that ore-forming activity of microorganisms is substantially manifested at principal geochemical stages of ore-formation, namely, in sedimentation and diagenesis.

Uranium Deposits of Grants Region: ABSTRACT

Uranium of the Grants region is present predominantly in continental sandstone of the upper part of the Jurassic Morrison Formation, but significant, though lesser, deposits are in limestone of the Jurassic Todilto Formation and in black shale of the Cretaceous Dakota Formation. The deposits are disseminations that form runs ranging in volume from a few hundred to several million tons. The ore consists predominantly of uraninite, uraniferous carbonaceous material, coffinite, and such secondary oxidized minerals as tyuyamunite, carnotite, and uranophane. The runs are localized in the sandstone in conjunction with associated mudstone, interstitial carbonaceous material, and primary sandstone-channel trends. In limestone localization is related to folds and fracture zone . Most production has come from the Ambrosia Lake mines in the Grants district and the Jackpile and Paguate mines in the Laguna district. The ores, which generally have been 0.20-0.30% U308, are treated at mills operated by Kerr-McGee Corporation, Homestake-Sapin Partners, and The Anaconda Company, all in the Grants district. In the 17 years since discovery in 1950, the deposits have yielded more than 30 million tons of concentrates valued in excess of 1 billion dollars. Since deposition of the Jurassic host rocks, a geologic history involving tilting, faulting, erosion, changing groundwater environments, and oxidation has influenced considerably the existence and character of the deposits. It is evident that their origin is not simple. However, a pre-Dakota age is generally agreed on for the original ore runs, and at Ambrosia Lake two principal stages of ore formation, separated by a period of Laramide faulting, are recognized. All older ores have been modified by Quaternary oxidation, solution, and enrichment during which time much secondary tyuyamunite, metatyuyamunite, and carnotite formed. End_of_Article - Last_Page 212------------