Minor Ore Research Articles

Paleoproterozoic East Pana Layered Intrusion (Kola Peninsula, Russia): Geological Structure, Petrography, Geochemistry and Cu-Ni-PGE Mineralization

The East Pana intrusion is a part of the Paleoproterozoic Fedorova–Pana complex (FPC), which belongs to the group of Fennoscandian layered mafic–ultramafic massifs. This article discusses the magmatic stratification of the East Pana intrusion, as well as Cu-Ni and platinum-group elements (PGE) mineralization (PGE zones A, B and C) in its various parts with a total length of more than 20 km, including the East Chuarvy PGE deposit. Based on the whole-rock data on the distribution of major, trace, and ore-forming elements, it is assumed that PGE zone A belongs to the main ore–magmatic system of the FPC, while PGE zones B and C belong to the minor ore–magmatic systems. At the same time, additional magmatic injection played an important role in the formation of economic Cu-Ni-PGE mineralization (PGE zone B), characterized by high PGE concentrations and moderate palladium enrichment. On the normalized distribution spectra of trace elements, the crystallization products of this injection (Gabbronorite Zone 2) have a positive Zr-Hf anomaly, which distinguishes it from host rocks with an anomaly of the opposite sign (Gabbronorite Zone 1, Gabbro Zone). It is assumed that this portion of magma was intruded as a sill of crystal mush, the fractionation of which at depth led to its enrichment with residual liquid.

Environmental impact of potentially toxic elements on soils, sediments, waters, and air nearby an abandoned Hg-rich fahlore mine (Mt. Avanza, Carnic Alps, NE Italy)

The decommissioned fahlore Cu-Sb(-Ag) mine at Mt. Avanza (Carnic Alps, Italy) is a rare example of exploited ore deposits, as the tetrahedrite (Cu6[Cu4(Fe,Zn)2]Sb4S13) is the main ore mineral found. This multi-compartmental geochemical characterisation approach provides one of the first case studies regarding the geochemical behaviour and fate of Hg, Sb, As, Cu, and other elements in solid and water matrices and of Hg in the atmosphere in an environment affected by the mining activity of a fahlore ore deposit. Elevated concentrations of the elements (Cu, Sb, As, Pb, Zn, Hg) associated with both (Zn-Hg)-tetrahedrite and to other minor ore minerals in mine wastes, soils, and stream sediments were observed. Concentrations in waters and stream sediments greatly decreased with increasing distance from the mining area and the Igeo index values testify the highest levels of sediment contamination inside the mine area. Thallium and Ge were associated with the “lithogenic component” and not to sulfosalt/sulphide minerals. Although mine drainage water often slightly exceeded the national regulatory limits for Sb and As, with Sb being more mobile than As, the relatively low dissolved concentrations indicate a moderate stability of the tetrahedrite. The fate of Hg at the investigated fahlore mining district appeared similar to cinnabar mining sites around the world. Weak solubility but the potential evasion of gaseous elemental mercury (GEM) into the atmosphere also appear to be characteristics of Hg in fahlore ores. Although GEM concentrations are such that they do not present a pressing concern, real-time field surveys allowed for the easy identification of Hg sources, proving to be an effective, suitable high-resolution indirect approach for optimising soil sampling surveys and detecting mine wastes and mine adits.

From Fossil to Active Hydrothermal Outflow in the Back‐Arc of the Central Apennines (Zannone Island, Italy)

AbstractPost‐orogenic back‐arc magmatism is accompanied by hydrothermal ore deposits and mineralizations derived from mantle and crustal sources. We investigate Zannone Island (ZI), back‐arc Tyrrhenian basin, Italy, to define the source(s) of mineralizing hydrothermal fluids and their relationships with the regional petrological‐tectonic setting. On ZI, early Miocene thrusting was overprinted by late Miocene post‐orogenic extension and related hydrothermal alteration. Since active submarine hydrothermal outflow is reported close to the island, Zannone provides an ideal site to determine the P‐T‐X evolution of the long‐lived hydrothermal system. We combined field work with microstructural analyses on syn‐tectonic quartz veins and carbonate mineralizations, X‐ray diffraction analysis, microthermometry and element mapping of fluid inclusions (FIs), C, O, and clumped isotopes, and analyses of noble gases (He‐Ne‐Ar) and CO2 content in FIs. Our results document the evolution of a fluid system of magmatic origin with increasing mixing of meteoric fluids. Magmatic fluids were responsible for quartz veins precipitation at ∼125 to 150 MPa and ∼300°C–350°C. With the onset of extensional faulting, magmatic fluids progressively interacted with carbonate rocks and mixed with meteoric fluids, leading to (a) host rock alteration with associated carbonate and minor ore mineral precipitation, (b) progressive fluid neutralization, (c) cooling of the hydrothermal system (from ∼320°C to ∼86°C), and (d) embrittlement and fracturing of the host rocks. Both quartz and carbonate mineralizations show noble gases values lower than those from the adjacent active volcanic areas and submarine hydrothermal systems, indicating that the fossil‐to‐active hydrothermal history is associated with the emplacement of multiple magmatic intrusions.

Age and Origin of the Dongping Au-Te Deposit in the North China Craton Revisited: Evidence from Paragenesis, Geochemistry, and In Situ U-Pb Geochronology of Garnet

Abstract Dongping is the largest Au-Te vein deposit (~120 t Au) in the North China craton, but its age, origin, and setting remain unsolved. Here, we integrate paragenesis, geochemistry, and in situ U-Pb geochronology of garnet to constrain the timing and possible origin of the Dongping Au-Te deposit. Gold mineralization at Dongping is hosted in the Shuiquangou alkaline complex (ca. 401–390 Ma) and dominated by quartz-sulfide veins with minor ores in adjacent alteration envelopes. Andradite to grossular garnets are recognized in pre-, syn-, and post-ore quartz veins as well as mineralized alteration envelopes and are closely associated with a variety of ore and gangue minerals, mainly including K-feldspar, quartz, specularite, magnetite, pyrite, tellurides, epidote, and calcite. The paragenetic, textural, fluid inclusion, and compositional data suggest that garnets precipitated directly from a low-salinity fluid at 302° to 383°C and 90 to 330 bar. Garnets from various veins and alteration envelopes have similar U contents ranging from 0.80 to 13.89 mg/kg and yield reproducible U-Pb dates of 142 ± 5 to 139 ± 6 Ma (1σ) by laser ablation-inductively coupled plasmamass spectrometry. The dating results suggest that gold mineralization at Dongping occurred in the Early Cretaceous and thus preclude a genetic link between Au-Te mineralization and the ore-hosting alkaline intrusion as commonly suggested. When combined with independent geologic, geochemical, and geochronological studies, the new garnet U-Pb dates allow us to classify the Dongping Au-Te deposit as an oxidized intrusion-related gold deposit, with the causative magma likely derived from melting of an ancient enriched lithospheric mantle source due to destruction of the subcontinental lithospheric keel beneath the North China craton—a catastrophic event induced by the westward subduction of the Paleo-Pacific plate. This study highlights garnet U-Pb dating as a potential robust geochronometer for gold vein deposits elsewhere.

Genesis and prospecting perspective of Xianglushan iron deposit, Sichuan Province, China

More than 30 ore bodies have currently been discovered in the Xianglushan iron deposit in the central part of the Huili–Dongchuan Aulacogen. The main orebody is stratiform or stratoid within the volcanic sedimentary metamorphic rock of the Paleo–Mesoproterozoic Yinmin formation. Minor ores intrude as veins in the country rock and the mineral components of the iron ore are mainly magnetite and hematite. The ore evolved through the Paleo– Mesoproterozoic submarine volcanic eruption deposition period and the Neoproterozoic iron slurry superimposed transformation, and is identified as a sedimentation–transformation-type deposit. A study is conducted on the regional metallogenic background and geological characteristics of the ore deposit; results show that the ore-forming setting of the Xianglushan iron ore is excellent and that the potential of finding large-scale rich iron deposit is considerable.

The Serra Pelada Au-Pd-Pt Deposit, Carajas, Brazil: Geochemistry, Mineralogy, and Zoning of Hydrothermal Alteration

The Serra Pelada Au-Pd-Pt deposit is located within the Carajas mineral province, which also hosts the world-class iron ore and iron oxide-hosted copper-gold deposits of the Amazon craton in Brazil. The unusual low-temperature hydrothermal mineralization at Serra Pelada is epigenetic, hosted by metasedimentary rocks of the Aguas Claras Formation and structurally localized in the Serra Pelada overturned syncline. The orebodies are controlled by the intersection of subvertical NE-trending fault zones with metasiltstones, mainly at the syncline’s hinge, with minor ore occurrences at the upper and lower limb. Intense tropical weathering over the last 70 m.y. has completely overprinted the shallow ore in and near the flooded open pit, but primary hydrothermal features are preserved in deeper drill core delineating the remaining resource. Gold, platinum, and palladium mineralization is associated with intense argillic alteration, hematite breccias, and silicification, with the highest grade ore hosted by brecciated metasiltstones that are highly enriched in amorphous carbon. Distal alteration zones comprise a reducing and oxidizing alteration front. The hydrothermal mineral paragenesis comprises kaolinite, quartz, sericite, amesite (Mg-rich Al-silicate), amorphous carbon, hematite, monazite, rutile, pyrite, and a complex assemblage of Bi-, Ag-, Pb-, Cu-, Co-, Ni-, Pt-, Pd-, and Au-bearing, chalcogenide (S, Se), and arsenide (As, Sb) minerals. Major element changes during hydrothermal alteration include C and Mg addition, K depletion, localized silica loss, and silicification with notable introduction of trace elements including light rare earth elements (LREE), Bi, Pb, U, V, Cu, Co, Ni, and As. The hydrothermal alteration and element association of the Serra Pelada deposit show geochemical similarities with unconformity-related uranium deposits, which may also be enriched in Au, Pd, and Pt and were formed by mixing of fluids that interacted with a highly oxidized cover sequence and highly reduced rock packages in structures of brittle-ductile strain.

Mineralogical, IR-spectral and geochemical monitoring of hydrothermal alteration in a deformed and metamorphosed Jurassic VMS deposit at Arroyo Rojo, Tierra del Fuego, Argentina

Fil: Biel, C.. Universidad de Zaragoza; Espana. Productos Minerales para la Industria S.A.; Espana

Bi-melt formation and gold scavenging from hydrothermal fluids: An experimental study

The role of polymetallic melts in scavenging ore components has recently been highlighted in the context of fluid-poor metamorphosed ore deposits. In contrast, the role of polymetallic melts in systems dominated by hydrothermal fluids remains poorly understood. Using a simple Au–Bi model system, we explored experimentally whether such polymetallic melts can precipitate directly from a hydrothermal fluid, and investigated the ability of these melts to scavenge Au from the solution. The experiments were conducted in custom-built flow-through reactors, designed to reproduce a hydrothermal system where melt components are dissolved at one stage along the flow path (e.g., Bi was dissolved by placing Bi-minerals along the fluid path), whereas melt precipitation was caused further along the flow path by fluid–rock interaction. Bi-rich melts were readily obtained by reaction with pyrrhotite, graphite or amorphous FeS. When Au was added to the system, Bi–Au melts with compositions consistent with the Au–Bi phase diagram were obtained. In the case of fluid reaction with pyrrhotite, epitaxial replacement of pyrrhotite by magnetite was observed, with textures consistent with an interface-coupled dissolution–reprecipitation reaction (ICDRR). In this case, the metallic melt precipitated as blebs that were localized at the replacement front or within the porous magnetite. Direct fractionation of Bi–Au melts from a hydrothermal fluid, or precipitation of a Bi-melt followed by partitioning of Au from ambient fluid, offer new pathways to the enrichment of minor ore components such as Au, without requiring fluid saturation with respect to a Au mineral. This mechanism can explain the strong geochemical affinity recognized between Au and low-melting point chalcophile elements such as Bi in many gold deposits. Examples of deposits where such a model may be applicable include orogenic gold deposits and gold skarns. Contrary to models involving migration of polymetallic melts to explain element remobilization, only small quantities (ppm) of polymetallic melts are required to affect the Au endowment of a deposit via interaction with a hydrothermal fluid. The experiments also show that micro-environments can play a critical role in controlling melt occurrences. For example, reaction fronts developing via ICDR reactions can promote melt formation as observed during the replacement of pyrrhotite by magnetite. The associated transient porosity creates space for the melt and promotes melt-fluid exchanges whereas the reaction front provides local geochemical conditions favorable to melt precipitation (e.g., reduced, low aH 2S(aq), and catalytic surface).

Geologic Evolution of the Caylloma Epithermal Vein District, Southern Peru

The silver- and base metal-rich Caylloma epithermal district in the Tertiary volcanic belt of southern Peru has been worked intermittently since the Incaic period and has produced over 100 million ounces (Moz) of Ag. Intermediate-sulfidation mineralization is present in veins hosted by Miocene andesitic volcanic and volcaniclastic rocks, with minor ore in underlying folded Jurassic sedimentary basement. New 40 Ar/ 39 Ar dates give a host rock age of 20.30 ± 0.11 Ma (andesitic volcanic matrix), a hydrothermal alteration age of 18.35 ± 0.17 Ma (adularia in vein wall rock), and postmineralization ages of 11.8 ± 0.8 and 12.25 ± 0.07 Ma (sanidine and biotite from a rhyolite dome). Gangue minerals include quartz, calcite, rhodonite, rhodochrosite, pyrite and minor adularia, illite, barite, and helvite. Ore minerals include sphalerite, galena, chalcopyrite, and tetrahedrite. Hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks. Hydrothermal alteration types include silicification (quartz-adularia and quartz-illite) and propylitization (chlorite + calcite ± illite). Banded veins show four stages of mineral precipitation: (1) early sugary quartz, chalcedony, pyrite; (2) manganese minerals; (3) quartz + sulfides; and (4) late calcite + quartz. Cyclic bands in the manganese stage (early sulfides, coarse- to medium-grained quartz, late rhodonite + calcite + chalcedony) are a few millimeters to 5 cm thick and form ore bands up to 1 m thick. Veins occupy dextral normal faults (020°–050°, 45°–70° SE) and extension fractures (060°–090°, 70°–90° SE) and are 1 to 25 m wide and as much as several kilometers long. The veins display complex and multi-episodic filling with textures characteristic of open-space precipitation such as crustiform banding, symmetric banding, vugs, breccias, and cockade and comb textures. We present a structural model in which the principal veins formed in subparallel northeast-striking dextral-normal faults and related extension fractures within a northwest-striking structural corridor bounded by sinistral regional faults; second- and third-order veins formed by bookshelf sliding related to movement on higher-order, dominantly strike-slip faults. Slickenlines have low rakes (20°–50° SW), and fault-kinematic analysis suggests the extension axis was oriented 4°/330° and the shortening axis 55°/234°, in agreement with the strain field commonly proposed for the early-middle Miocene of southern Peru. Ore grade is discontinuous, with high-grade ore zones having strike lengths of tens to hundreds of meters and extending about 300 m downdip. Ore shoots in fault-hosted veins are narrow subvertical zones a few tens of meters wide (measured in the plane of the vein), separated by lower-grade ore zones. Ore shoots in extension fractures are shorter downdip and horizontally more continuous. Some ore shoots occur as half-cymoids that form along the main vein only in the hanging wall of synmineral transverse faults. Mineral composition and metal content are zoned horizontally and vertically; silver, base metals, and calcite and manganese minerals increase to the northeast and downward, and sulfide content increases gradually with depth. Shallower levels of the hydrothermal system are exposed to the west and southwest in the district. Evidence from sulfur isotopes and high-salinity fluid inclusions is consistent with a model in which magmatic fluids played a role in the hydrothermal system, introduced through channels of enhanced structural permeability. The cyclic nature of mineral precipitation with abrupt changes in mineralogy, textures, and fluid composition leads us to postulate that cyclic injections of magma-derived hydrothermal fluids occurred. Economic mineralization is estimated to have formed at a depth of about 650 m below the paleowater table, with a temperature of formation of ~270°C.

Geological and Geochemical Characteristics of the Zhulazhaga Gold Deposit in Inner Mongolia, China

Abstract Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first large‐scale gold deposit that was found in the middle‐upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. 1 Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold, the first lithological unit of the MZF became the source bed for the late‐stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitive ore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation.A total of 100 two‐phase H2O‐rich and 7 three‐phase daughter crystal‐bearing inclusions were measured in seven gold‐bearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two‐phase H2O‐rich inclusions range from 155 to 401°C, with an average temperature of 284°C and bimodal distributions from 240 to 260°C and 300 to 320°C respectively. The salinities of the two‐phase H2O‐rich inclusions vary from 9.22wt% to 24.30wt% NaCl equiv, with a mode between 23wt% and 24wt% NaCl equiv. Comparatively, the homogenization temperatures of the three‐phase daughter crystal‐bearing inclusions vary from 210 to 435°C and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore‐forming fluid is meso‐hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold‐bearing fluid.The δ18O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated δ18O values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore‐forming fluid may be the product of mixing of meteoric water and magmatic water. Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre‐concentrate in the strata. The extensive and intensive Hercynian tectono‐magmatic activity not only brought along a large number of ore‐forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore‐controlling structure, wall‐rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.

Enargite by XPS

X-ray photoelectron spectroscopy was used for characterizing the enargite surface. Freshly cleaved samples were analyzed at liquid nitrogen temperature. Enargite is a copper arsenic sulfide of formula Cu3AsS4; it is used as a minor ore of copper. Enargite is a potential source of arsenic and may create environmental problems through the release of toxic elements upon oxidation.

An evaluation of the inorganic and organic geochemistry of the San Vicente mississippi valley-type zinc-lead district, central Peru; implications for ore fluid composition, mixing processes, and sulfate reduction

Mississippi Valley-type zinc-lead deposits and ore occurrences in the San Vicente belt are hosted in dolostones of the eastern Upper Triassic to Lower Jurassic Pucara basin, central Peru. Combined inorganic and organic geochemical data from 22 sites, including the main San Vicente deposit, minor ore occurrences, and barren localities, provide better understanding of fluid pathways and composition, ore precipitation mechanisms, Eh-pH changes during mineralization, and relationships between organic matter and ore formation. Ore-stage dark replacement dolomite and white sparry dolomite are Fe and rare earth element (REE) depleted, and Mn enriched, compared to the host dolomite. In the main deposit, they display significant negative Ce and probably Eu anomalies. Mixing of an incoming hot, slightly oxidizing, acidic brine (H 2CO 3 being the dominant dissolved carbon species), probably poor in REE and Fe, with local intraformational, alkaline, reducing waters explains the overall carbon and oxygen isotope variation and the distributions of REE and other trace elements in the different hydrothermal carbonate generations. The incoming ore fluid flowed through major aquifers, probably basal basin detrital units, with limited interaction with the carbonate host rocks. The hydrothermal carbonates show a strong regional chemical homogeneity, indicating access of the ore fluids by interconnected channelways near the ore occurrences. Negative Ce anomalies in the main deposit, that are absent at the district scale, indicate local ore-fluid chemical differences. Oxidation of both migrated and indigenous hydrocarbons by the incoming fluid provided the local reducing conditions necessary for sulfate reduction to H 2S, pyrobitumen precipitation, and reduction of Eu 3+ to Eu 2+. Fe-Mn covariations, combined with the REE contents of the hydrothermal carbonates, are consistent with the mineralizing system shifting from reducing/rock-dominated to oxidizing/fluid-dominated conditions following ore deposition. Sulfate and sulfide sulfur isotopes support sulfide origin from evaporite-derived sulfate by thermochemical organic reduction; further evidence includes the presence of 13C-depleted calcite cements (~-12‰ δ 13C) as sulfate pseudomorphs, elemental sulfur, altered organic matter in the host dolomite, and isotopically heavier, late, solid bitumen. Significant alteration of the indigenous and extrinsic hydrocarbons, with absent bacterial membrane biomarkers (hopanes) is observed. The light δ 34S of sulfides from small mines and occurrences compared to the main deposit reflect a local contribution of isotopically light sulfur, evidence of local differences in the ore-fluid chemistry.

Determination of refractory gold distribution by microanalysis, diagnostic leaching and image analysis

Samples of drill core, ore and concentrates from the Proterozoic Suurikuusikko Au deposit, Kittila, Central Lapland were studied by optical microscopy, image analysis, Secondary Ion Mass Spectrometer (SIMS), trace gold analysis by electron microprobe, and diagnostic leaching techniques to characterize the mode of occurrence of the gold. The main ore minerals are pyrite, arsenopyrite and gersdorffite, and the minor ore minerals include chalcopyrite, sphalerite, tetrahedrite, galena, bournonite and rutile. Graphite is found in all samples as microcrystalline grains. Pyrite, arsenopyrite and occasionally gersdorffite occur as disseminated intimately intergrown grains and as large grain aggregates. Diagnostic leaching tests show that an average of only 4.1% of the gold is in cyanide leachable form i.e. free gold, whereas on average 57% of the gold is bound to pyrite and arsenopyrite as inclusions or as lattice gold. The weight percentage of arsenopyrite and pyrite in the concentrate determined with image analysis on backscattered electron images is 65.2 and 34.7, respectively. Trace Au analyses done with EPMA using the Australian CSIRO-TRACE program from the drill core samples and concentrates show that the average gold content in pyrite is 46 ppm (192 analyses) and in arsenopyrite 279 ppm (136 analyses). The CSIRO-TRACE microprobe analyses correspond favourably to SIMS analyses, e.g. 69 ppm for pyrite (16 analyses) and 217 ppm for arsenopyrite (22 analyses). The distribution of gold in concentrates was calculated as free gold 4.1%, gold in pyrite 22.7%, and gold in arsenopyrite 73.2%. Both arsenopyrite and pyrite show strong zoning when treated for 1–2 min with KMnO4 dissolved in sulphuric acid, and trace Au microprobe analyses show that the zonal bands reflect different concentrations of gold in the minerals. Free gold was not found by optical microscopy, but a rare mineral search technique using TURBO-SCAN runs on the drill core samples and concentrates located 111 gold grains. The grains have a large compositional variation from silver-bearing gold to electrum and Au-Ag-amalgam.

The Siljan Ring impact structure: possible connections with minor ores in its neighborhood: F. E. Wickman, GFF, 116(3), 1994, pp 145–146

The Siljan Ring impact structure: possible connections with minor ores in its neighborhood: F. E. Wickman, GFF, 116(3), 1994, pp 145–146

The Siljan ring impact structure: Possible connections with minor ores in its neighborhood

Abstract Martanberg and Slattberg are two old, small mines situated outside, but near the Siljan Ring structure and possibly both are influenced by the impact. The bedrock formed in the main c. 1700 Ma ago. Martanberg is known as a small abandoned copper mine with chalcopyrite and pyrite. Its ore seems to have been formed during two widely separated periods. Slattberg is an old nickel mine which has been worked intermittently, most recently during the two world wars. The host rock is an amphibolite dike which possibly has changed its mineralogical composition during a later event. Both these ores may be explained as a result of the Siljan impact about 360 Ma ago. Wickman, F.E., 1994: The Siljan Ring impact structure: possible connections with minor ores in its neighborhood. GFF, Vol. 116 (Pt. 3, September), pp. 145–146. Stockholm. ISSN 1103–5897.

Late to post-Hercynian hydrothermal activity and mineralization in Southwest Sardinia (Italy)

Several kinds of base metal deposits occur in the lower Paleozoic of southwest Sardinia (Iglesiente-Sulcis mineral district). This paper deals with those deposits which are generally referred to as Permo-Triassic, because they accompany and postdate the Hercynian orogeny and are related to magmatic activity.A large number of previously published geochemical data, integrated with additional new data (Sr, Pb, O, C, and S isotopes), are reviewed and discussed in the frame of the late to post-Hercynian geologic evolution of southwest Sardinia. According to geological and mineralogieal characteristics, three types of deposits can be distinguished: (1) skarn ores related to late Hercynian leucogranitic intrusions, (2) high-temperature veins, and (3) low-temperature veins and karst filling. Pervasive epigenetic dolomitization phenomena are geochemically related to the low-temperature deposits.Sr and Pb isotopes of the first and second types (0.7097-0.7140 87 Sr/ 86 Sr; 17.97-18.29 206 Pb/ 204 Pb; 38.11-38.45 208 Pb/ 204 Pb) are distinctly more radiogenic than those of the third type (0.7094-0.7115 87 Sr/ 86 Sr; 17.86-18.05 206 Pb/ 204 Pb; 37.95-38.19 208 Pb/ 204 Pb) which, in turn, are closer to Paleozoic ores and carbonates. Fluid inclusion data indicate that the fluids responsible for mineralization of the first and second types of deposits were hot and dilute (T h = 370 degrees -140 degrees C; 20 wt % NaCl equiv) were responsible for the third type of mineralization, as well for epigenetic dolomitization of the Cambrian host rocks. O isotopes measured in minerals from the first two types (delta 18 O SMOW = 12.8-18.9ppm) are 18 O depleted with respect to the third type (delta 18 O SMOW = 15.9-22.1ppm). These data, coupled with fluid inclusion formation temperatures, indicate that the fluids responsible for the first two types of mineralization were 18 O enriched with respect to those of the third type and related hydrothermal phenomena. The delta 34 S CDT in sulfides of the first two types vary between 3.7 and 10.73 per mil, whereas the values of the third type range from 12.0 to 17.9 per mil.Late to post-Hercynian mineralization is thus explained as the result of three distinct, though partly superimposed, hydrothermal systems. System 1 developed closer to the late Hercynian leucogranitic intrusions and led to the formation of the first and subsequently the second type of mineralization. The relatively hot and diluted fluids had a heated meteoric, or even partly magmatic, origin. Metals were leached from an external, radiogenic source, represented either by Hercynian leucogranites or by Paleozoic metasediments. Sulfur had a partly magmatic signature. System 2 was characterized by very saline, colder fluids which promoted dolomitization, silicification, and vein and karst mineralization. These fluids share the typical characteristics of formation waters, even though their origins remain highly speculative. The hydrothermal system was mainly rock dominated, with only a minor participation of the external radiogenic source of metals. Sulfur was derived by recirculation of pre-Hercynian strata-bound ores. System 3 records the invasion of fresh and cold meteoric waters which precipitated only minor ore and calcite gangue. It may represent the further evolution of system 2, possibly spanning a time well after the Permo-Triassic.The timing of all these phenomena is still questionable, due to the poor geologic record of the Permo-Triassic in southwest Sardinia. Nevertheless, the hypothesized scenario bears many similarities with hydrothermal processes documented throughout the Hercynian in Europe and spanning the same time interval. A comparison with the latter mineralization and hydrothermal activities leads to the hypothesis that the first two types of mineralization are linked to late Hercynian magmatic activity, whereas the third type may be related to either strike-slip or tensional tectonics which, throughout Europe, mark the transition from the Hercynian orogeny to the Alpine cycle.

栃洞鉱山におけるトラックレス・マイニングを活用した操業の改善, 合理化

In 1968, the Trackless Mining System was introduced to Tochibora Mine for the first time in Japan.Since then, the mechanization and the rationalization of each underground work have been carried out with the practical application of this system and we have succeeded in saving operating cost to a large extent.Moreover, the ratio of minor ore deposits occupying for percent of the minable ore reserve has been increasing with the advance of mining large-sized ore deposits. Under the above aggregating mining conditions, waste dilution has been improved without lowering mining efficiency and the grade of crude ore has been raised by the improvement of mining method or by the expanding application of the Mechanized Cut and Fill method.As the result, in spite of the reduction from 4, 800t/day to 3, 200t/day due to the economical depression in 1979, we have reduced the operating costs under the unfavorable conditions of raising labor and other costs. And we have come to be able to carry on the stable mining management, overcoming the unfavorable conditions of low ore grade mining (Ag 19 et, Pb 0.3%, Zn 4.2%), while many of unprofitable metal mines are going to be closed or suspended.

The geology of the Kambalda nickel field, Western Australia

The Kambalda nickel field contains the major concentration of volcanic peridotitc-associated nickel ores in the Archean of Australia. It occupies an area of 400 km 2 underlain by two sequences of ultramafic, mafic, and felsic volcanics, and sedimentary rocks, which can be broadly correlated with previously erected regional stratigraphic sections. Most ores occur in the lower (Kambalda) sequence at the base of the Kambalda ultramafic rocks; the ultramafic rocks of the upper (Bluebush) sequence contain only minor mineralization. Four phases of deformation occurred over an interval of more than 300 m.y., resulting in a dominant north-northwest structural trend. Faulting was active during original volcanism and continued intermittently until after mafic-felsic dike intrusion. The ultramafic rocks have been serpentinized, and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism.The Kambalda ultramafic rocks are of variable thickness and in places exceed 1,000 m. The lower member of the ultramafic rocks, occupying approximately one-third of the sequence, comprises thicker high magnesium flows (i.e., generally > 36% volatile-free MgO) commonly separated by sulfidic quartz-albite sediments. Thinner, lower magnesium flow units characterize much of the upper member. The ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few, grouped, ribbonlike original orebodies. Eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt-ultramafic contact; these troughs are thought to be original volcanic depressions which focused later deformation. The remainder is hanging-wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow. Hanging-wall orebodies occasionally grade into stratigraphically equivalent interflow sediments.The ores contain varying proportions of massive, matrix, and disseminated sulfides. The hypogene mineral assemblage is pyrrhotite-pentlandite-pyrite-(chalcopyrite), but millerite is locally significant. The massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position. Metamorphism has caused recrystallization of the ores and some redistribution of elements. There is commonly marked variation of mineralogy and S/Ni ratios between contact and overlying hanging-wall ores, and within and between original ore surfaces within shoots, but the relative contributions of primary composition and metamorphism to this variation are uncertain.In ore environments, the footwall basalt-ultramafic contact is more deformed than in non-ore environments, the basal ultramafic flows are thicker and richer in magnesium, the trend toward lower magnesium flows upward through the ultramafic formation is less well developed, and there is better textural development and compositional differentiation within flows. Non-ore environments usually contain interflow sediments, whereas there is generally a sediment-free prism of ultramafic rocks above ore environments.Most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor. Volcanic exhalations probably contributed to minor ore in sulfidic sediments.