Cu-Zn Volcanogenic Massive Sulfide Research Articles

Insights into the indium enrichment of the Ashele VMS Cu-Zn deposit, Altay, NW China

Indium (In) is a critical metal used in the photovoltaic and semiconductor industries, which have exhibited extraordinary growth in demand. Indium is produced as a by-product of mining from different ore deposits (e.g., epithermal, sediment-hosted, and skarn). Volcanogenic massive sulfide (VMS) deposits are an important source of In; however, the mechanism of In enrichment is not fully understood. Here, we combine mineralogy with in situ trace element and S-Pb isotope geochemistry to reveal the enrichment of indium in the Ashele VMS Cu-Zn deposit (1.08 Mt. Cu, 0.43 Mt. Zn) located in Altay, NW China. The Ashele deposit is hosted in the metamorphosed Devonian felsic-bimodal volcanic rocks. This deposit consists of seafloor hydrothermal, metamorphic hydrothermal, and supergene stages. The seafloor hydrothermal stage comprises macro-scale Cu-rich bands (chalcopyrite, pyrite, and minor sphalerite) and Zn-rich bands (sphalerite, pyrite, and minor chalcopyrite). Indium is mainly hosted by chalcopyrite (mean 178 ppm) and sphalerite (mean 214 ppm) and occurs in the lattice. Mineral assemblages and trace element geochemistry suggest that the Cu-rich bands were deposited under high temperatures (> 300–350 °C) and sulfur fugacity (−7.2 to −4.9), whereas the Zn-rich bands were formed under lower temperatures (180–220 °C) and sulfur fugacity (−15.7 to −11.5). The interlayered Cu-rich and Zn-rich bands may reflect the oscillating temperature and sulfur fugacity variations. In situ S isotopic compositions of sulfides cluster within two ranges: 1–3 ‰ and 3–6 ‰, suggesting two endmembers: volcanic origin and reduced seawater sulfate. Pb isotopic ratios are similar to those of the host volcanic rocks, indicating that the metals may be derived from the felsic volcanic system. During metamorphism, the indium may be retained, but Cu contents of sphalerite become more homogeneous. Most In-rich VMS deposits worldwide are hosted by the felsic-dominant system in island arc and back-arc settings. These tectonic settings are conducive to the production of felsic volcanic systems, which are more likely to contain In mineralization. This study highlights the enrichment mechanism of indium in VMS deposits and suggests that the South Altay could become an important source of In.

Copper for the early oxhide ingots traced to the South Urals

Oxhide ingots are flat, rectangular slabs of metal, most commonly of copper, with protruding corners of varying styles. These Late Bronze Age artifacts were widespread across the Mediterranean for approximately half a millennium between the 16th and 11th centuries BC. Although they are generally known as essentially a Cypriot “commodity brand” by the late 15th century BCE, the origin of this ingot’s distinct shape has remained unknown. With the expansion of the lead-isotope database of Eurasian copper ores, the origin of the oldest chemically characterized oxide ingots may now be defined. The bimodal lead isotope compositions of 15 early, pillow-shaped oxhide ingots excavated in Crete are consistent with those of copper ores from the Southern Urals, which are known to have been actively mined in the Bronze Age. Together, Pb isotope and trace element compositions indicate that the majority of the ingots were cast from copper derived from Ural-type Cu-Zn volcanogenic massive sulfide deposits. However, two ingots enriched in Ni, As, and Co match Main Ural Fault-type ores that are hosted by ultramafic rocks.The characteristic design of the pillow-shaped ingot was likely related to the particulars of overland transport, facilitating their long-distance conveyance on pack animals. There is further support of a non-Cypriot origin of this form in the possible association between the words for “ingot”, “copper” and “hide” across languages of Western Asia and the Mediterranean. Thus, an eastern origin of the oxhide form for ingots is likely.

The Zircon U-Pb Age, Hf Isotopes, and Lithogeochemistry of Ore-Bearing Rocks from the Archean Hongtoushan Volcanogenic Massive Sulfide Deposit in the North China Craton: Implications for Tectonic Setting

Volcanogenic massive sulfide (VMS) deposits are globally significant sources of metals. The Hongtoushan VMS deposit is the only large Archean Cu-Zn VMS deposit in the North China Craton, carrying substantial economic value. Significant deformation and metamorphism have made the tectonic setting of the Hongtoushan VMS deposit the subject of extensive debate. This study investigates the petrogenesis and chronology of the ore-bearing host rocks from the Hongtoushan Cu-Zn VMS deposit in the North China Craton. By utilizing whole-rock geochemical analyses and zircon dating, this research sheds light on the origin and evolution of the ore-bearing rocks within the deposit. The whole-rock geochemical analysis data indicate that the Hongtoushan ore-bearing rock series is mainly composed of amphibole plagioclase gneiss (basalt protolith) and biotite plagioclase gneiss (andesite and rhyolite protolith), suggesting a complete volcanic cycle from basic to medium-acidic volcanic rocks. The amphibole plagioclase gneiss has slight LREE enrichment patterns with unremarkable depletions of Nb, Ta, and Ti and belongs to contaminated ocean plateau basalt (OPB) in terms of composition, which is generally interpreted as being generated from the mantle plume head. Meanwhile, the biotite plagioclase gneiss has relatively steep LREE enrichment distribution patterns with remarkable negative Ta, Nb, and Ti anomalies and a wide range of Zr/Y ratios, indicating their classification as FI- and FII-type felsic rocks; they were likely formed through the fractional crystallization of basic magma combined with crustal assimilation. When combined with the zircon dating results, the ore-bearing host rocks of the Hongtoushan VMS deposit were generated via a continuous magmatic evolution process. The zircon dating of the host rocks indicates a formation age of between 2609 and 2503 Ma, with metamorphic events between 2540 and 2466 Ma, which is consistent with the 2.5 Ga-related global mantle plume event. Further research shows that the ore-bearing host rocks are more likely to have been formed in a mantle plume-related stretching environment, possibly a margin rift.

SIMS oxygen isotopes indicate Phanerozoic fluids permeated a Precambrian gold deposit

The Reef Deposit is an anomalous AuCu occurrence in the Paleoproterozoic terranes of northern Wisconsin, better known as host to significant CuZn volcanogenic massive sulfide (VMS) deposits. Previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a VMS deposit during the Penokean orogeny (~1.9–1.8 Ga), with the most recent mineralization/remobilization activity associated with late Paleozoic Mississippi Valley-type (MVT) fluid flow (Haroldson et al., 2018a, 2018b). Here we use the oxygen isotope history of the Reef Deposit to verify and further examine the deposit's protracted development.Laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in δ18O from 6.8 to 10.0‰ (VSMOW), and a trend is observed of increasing δ18O values of parallel vein zones along a 400-m traverse from northwest to southeast, likely from a temperature gradient during initial formation. Temperature estimates for a VMS deposit setting (230 to 400 °C) are consistent with a hydrothermally shifted formation fluid, using seawater δ18O estimates during the Penokean Orogeny and evolving to higher δ18O by incorporation of magmatic fluids or interaction with local crust.In situ oxygen isotope measurement by Secondary Ion Mass Spectrometer (SIMS) of cross-cutting quartz and carbonate range in δ18O from 19.4 to 28.4‰ for quartz, 25.3 to 28.1‰ for dolomite, and 9.6 to 29.3‰ (VSMOW) for calcite. High δ18O values (>19‰) are measured in a crustiform-textured quartz stockwork microveinlet, in dolomite observed in late carbonate microveinlets, and calcite in settings associated with the late quartz and dolomite and a separate calcite-only setting directly linked with gold mineralization/remobilization.

Geochemistry and chronology of the early Paleozoic diorites and granites in the Huangtupo volcanogenic massive sulfide (VMS) deposit, Eastern Tianshan, NW China: Implications for petrogenesis and geodynamic setting

The Eastern Tianshan orogen contains many late Paleozoic porphyry Cu and magmatic Cu-Ni deposits. Recent studies demonstrate that several early Paleozoic volcanogenic massive sulfide (VMS) Cu-polymetallic and porphyry Cu deposits were discovered in the northern part of Eastern Tianshan. This study presents zircon U-Pb, whole-rock geochemical, and Sr-Nd isotopic data for granites and diorites from the Huangtupo VMS Cu-Zn deposit, northern part of the Eastern Tianshan. Our results can provide constraints on the genesis of intermediate and felsic intrusions as well as early Paleozoic geodynamic setting of the northern part of Eastern Tianshan. LA-ICP-MS zircon U-Pb analyses suggest that the granites and diorites were formed at 435 ± 2 Ma and 440 ± 2 Ma, respectively. Geochemical characteristics suggest that the Huangtupo granites and diorites are metaluminous rocks, exhibiting typical subduction-related features such as enrichment in LILE and LREE and depletion in HFSE. The diorites have moderate Mg#, positive εNd(t) values (+6.4 to +7.3), and young Nd model ages, indicative of a depleted mantle origin. The granites exhibit mineral assemblages and geochemical characteristics of I-type granites, and they have positive εNd(t) values (+6.7 to +10.2) and young Nd model ages, suggesting a juvenile crust origin. The early Paleozoic VMS Cu-polymetallic and porphyry Cu deposits in the northern part of Eastern Tianshan were genetically related. The formation of the early Paleozoic magmatic rocks as well as VMS and porphyry Cu deposits in the northern part of Eastern Tianshan was due to a southward subduction of the Junggar oceanic plate.

Mineralogy, microchemistry and fluid inclusion studies of the Besshi-type Nudeh Cu-Zn VMS deposit, Iran

The southwestern Sabzevar basin is the north of Central Iranian Microcontinent hosts abundant mineral deposits, including exhalative Mn mineralization and Cu-Zn volcanogenic massive sulfide (VMS) deposits. Amongst them, the Nudeh Besshi-type Cu–Zn volcanogenic massive sulfide (VMS) deposit is hosted within the lower part of a Late Cretaceous volcano-sedimentary sequence composed of alkali olivine basalt flows and tuffaceous silty sandstone. Based on investigations into the ore geometry, mineralogy, and texture, we recognized three different ore facies: (1) a stockwork of sulfide-bearing quartz veins cutting across the footwall volcano-sedimentary rocks and representing the stringer zone; (2) a massive ore type, displaying replacement texture with pyrite, chalcopyrite, sphalerite, friedrichite, and minor magnetite; and (3) a bedded ore type, with laminated to disseminated pyrite and chalcopyrite. EPMA studies indicate a distinctive minor element distribution between the different ore types of the Nudeh deposit. The Fe content in the sphalerite ranges from 0.65–1.80 wt.%, indicating the Fe-poor nature of the sphalerite. However, the Cd content in sphalerite ranged between 0.164–0.278 wt.%. According to the mineral compositions, Zn, Se, and Ag are found in bornite as minor elements. In the bedded ore facies, the pyrite contains higher levels of Se (up to 0.35 wt.%). The Zn content in the friedrichite in all of the ore samples is low. The Co/Ni ratios in pyrite from the Nudeh ore are lower than those of most magmatic deposits, but are similar to those from volcanogenic deposits, and hence support the proposed hydrothermal origin of the deposit. Two generations of quartz, Q1 and Q2 in the stockwork veins, contain primary fluid inclusions and these contain two phases (liquid and vapor). The lack of vapor-rich inclusions or variable liquid/vapor ratios indicate that the fluids did not boil at the site of trapping. Salinity for both Q1 and Q2 fluid inclusions ranges between 2.2–6.8 wt.% eq. NaCl. Homogenization temperatures for inclusions in the Q1 and Q2 veins average at about 296 °C and are similar to the temperatures of hydrothermal fluids discharged through vents in many modern seafloor VMS deposit. The Nudeh Besshi-type VMS deposit appears to have formed on the seafloor and based on the salinity and temperature constraints from the underlying stockwork, a buoyancy plume model is proposed as a mechanism for precipitation.

Geological and geochemical constraints on the Cheshmeh-Frezi volcanogenic stratiform manganese deposit, southwest Sabzevar basin, Iran

The Cheshmeh-Frezi Mn deposit belongs to the southwest Sabzevar basin to the north of the Central Iranian microcontinent. This basin, which hosts abundant mineral deposits including Mn exhalative and Besshi-type Cu-Zn volcanogenic massive sulfide deposits, followed an evolution closely related to the subduction of the Neo-Tethys oceanic crust beneath the Central Iranian microcontinent. Two major sedimentary sequences are recorded within this basin: (I) the Lower Late Cretaceous volcano-sedimentary sequence (LLCVSS) and (II) the Upper Late Cretaceous sedimentary dominated sequence (ULCSS). The Cheshmeh-Frezi Mn deposit is hosted within red tuff with interbeds of green tuffaceous sandstone of the LLCVSS. Mineralization occurs as stratiform blanket-like and tabular orebodies. Psilomelane, pyrolusite and braunite are the main minerals of the ore, which display a variety of textures. Such as layered, laminated, disseminated, massive, replacement or open space fillings. The footwall and hanging-wall volcanic rocks are predominantly andesite and trachyandesite rocks. Footwall and hangingwall volcanic rocks at Cheshmeh-Frezi are enriched in light rare earth elements (LREEs) compared to chondrite, have steep REE patterns, and generally show Ta and Nb depletions relative to chondrite which are characteristic of back-arc environments. The significant geochemical characteristics of ore such as high Mn content (12.41–33.14 wt%; average 19.41 wt%), low concentration of Fe (0.64–2.27 wt%; average 1.63 wt%), high Ba (49.7–9901ppm, average 2728.67ppm), LREE>HREE, and negative Ce and Eu anomalies reveal a primary distal hydrothermal-exhalative source for mineralization. Cheshmeh-Frezi deposit, in comparison with different types of volcanogenic manganese deposits shows broad similarities with the Cuban-type Mn deposits such as tectonic, host and associated rock types, geometry, textures, structures, mineralogy and lithogeochemistry.

Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China

The Ashele volcanogenic massive sulfide (VMS) Cu-Zn deposit is placed in the Devonian volcanic-sedimentary basin of the Chinese Altay Orogen, NW China. It occurs in the form of well-developed lensoid orebodies within the Devonian Ashele Formation. The orebodies display analogous deformational characteristics with respect to their host rocks. Two periods of ore formation are distinguishable in the Ashele VMS Cu-Zn deposit: the first one is related to the genesis of banded and massive ores, whereas the second one is characterized by the formation of earlier Cu-dominated- and later Pb-Zn dominated-polymetallic quartz veins. In most cases, the primary banded textures of ores are still preserved. However, some veins cross cut the earlier generated ores with brecciated, durchbewegung, recrystallization and pressure-solution textures produced, indicating that substantial remobilization of metals has occurred due to intense reworking of the VMS. Four types of fluid inclusions were recognized in Cu-dominated quartz veins encompassing CO2-H2O (C-type), aqueous water (W-type), daughter mineral-bearing (S-type) and carbonic (PC-type) fluid inclusions. The homogenization temperatures of fluid inclusions mainly vary between 220 and 280°C, whereas their salinities focus on 4 to 8wt.% NaCl equiv. Gases in fluid inclusions identified using Raman spectroscopy consist of CO2, CH4 and N2. Characteristics such as mesothermal temperature, low salinity and elevated CO2 content indicate that the ore-forming fluid might have been of metamorphic origin. Based on the major C-type FIs, the trapping pressures are greater than 8.5MPa, corresponding to minimum ore-forming depths are deeper than 3.1km. Taking into account the regional geodynamic evolution, it is concluded that the early formed ore layers were generated in an arc-related geotectonic setting; while deformation and metamorphism that were responsible for metal remobilization in the ore system happened in a post-subduction regime. Based on the micro-textural observation coupled with fluid inclusion data, we conclude that the Ashele Cu-Zn deposit is a typical remobilized VMS deposit intensely overprinted by subsequent deformation and metamorphism.

Variations in Large Synvolcanic Alteration Zones at Snow Lake, Manitoba, Canada, with Proximity to Associated Volcanogenic Massive Sulfide Deposits

Juvenile 1.89 Ga oceanic arc volcanic rocks of the Flin Flon volcanic belt at Snow Lake are characterized by extensive zones with anomalous 1.82 Ga metamorphic mineral assemblages, including porphyroblasts of garnet, staurolite, amphibole, biotite, gahnite, and/or kyanite. They were produced from altered rocks created during premetamorphic, 1.89 Ga, synvolcanic, hydrothermal fluid-rock interaction. Three separate episodes of hydrothermal alteration are recognized that span evolution of the host volcanic rocks from a primitive (nascent arc?) to mature arc geotectonic setting. The geologic, geochemical, mineralogical, and isotopic attributes of the zones indicate that they include volcanogenic massive sulfide (VMS)-related alteration, and they were produced at high and low temperatures and formed in seafloor/near seafloor and subseafloor (intrastratal) environments. Large-scale alteration zones at Snow Lake are up to 20 km in strike length and 0.8 km wide. Their large exploration “footprint,” which is 36 times greater in areal extent compared to associated “pipe-like” alteration zones, means that such zones provide a useful target to “vector-in” exploration to VMS depositional settings within volcanic belts. The VMS-related large-scale alteration zones at Snow Lake display diagnostic variations in intensity and style of alteration along strike toward VMS deposits, are stratigraphically underlain by altered portions of synvolcanic intrusions, are crossed by discordant zones of more intensely altered rocks, and can be demonstrated to have formed by interaction with high-temperature(>350°C) hydrothermal fluids. Cu-Zn–rich VMS deposits at Snow Lake formed in flow-dominated sequences, are hosted by large rhyolite flow complexes, and comprise lensoid orebodies. In contrast, Zn-Cu–rich VMS deposits, although also rhyolite associated, formed in volcaniclastic-dominated sequences, are spatially related to a district aquifer, and comprise stratiform, laterally continuous orebodies. This suggests that exploration for Cu-Zn VMS deposits could selectively target flow-dominated sequences and focus on rhyolite flow complexes that display significant alteration. Similarly, exploration for Zn-Cu VMS deposits could focus on volcaniclastic-dominated sequences in which a potential aquifer has been identified.

Geology, ore facies and sulfur isotopes geochemistry of the Nudeh Besshi-type volcanogenic massive sulfide deposit, southwest Sabzevar basin, Iran

The southwest Sabzevar basin is placed in the southwestern part of a crustal domain known as the Sabzevar zone, at the north of Central Iranian microcontinent. This basin hosts abundant mineral deposits; particularly of the Mn exhalative and Cu-Zn volcanogenic massive sulfide (VMS) types. The evolution of this basin is governed by the Neo-tethys oceanic crust subduction beneath the Central Iranian microcontinent and by the resulting continental arc (Sanandaj-Sirjan) and back-arc (Sabzevar-Naien). This evolution followed two major sequences: (I) Lower Late Cretaceous Volcano-Sedimentary Sequence (LLCVSS), which is indicated by fine-grained siliciclastic sediments, gray basic coarse-grained different pyroclastic rocks and bimodal volcanism. During this stage, tuff-hosted stratiform, exhalative Mn deposits (Nudeh, Benesbourd, Ferizy and Goft), oxide Cu deposits (Garab and Ferizy) and Cu-Zn VMS (Nudeh, Chun and Lala) deposits formed. (II) Upper Late Cretaceous Sedimentary Dominated Sequence (ULCSS), including pelagic limestone, marly tuff, silty limestone and marl with minor andesitic tuff rocks. The economically most important Mn (Zakeri and Cheshmeh-sefid) deposits of Sabzevar zone occur within the marly tuff of this sequence. The Nudeh Cu–Zn volcanogenic massive sulfide (VMS) deposit is situated in the LLCVSS. The host-rock of deposits consists of alkali olivine basalt flow and tuffaceous silty sandstone. Mineralization occurs as stratiform blanket-like and tabular orebodies. Based on ore body structure, mineralogy, and ore fabric, we recognize three different ore facies in the Nudeh deposit: (1) a stringer zone, consisting of a discordant mineralization of sulfides forming a stockwork of sulfide-bearing quartz veins cutting the footwall volcano-sedimentary rocks; (2) a massive ore, consisting of massive replacement pyrite, chalcopyrite, sphalerite and Friedrichite with magnetite; (3) bedded ore, with laminated to disseminated pyrite, and chalcopyrite. Chloritization, silicification, sericitization and epidotization are the main wall-rock alterations; alteration intensity increases towards the stringer zone. The δ34S composition of the sulfides ranges from −1.5‰ to +3.69‰ with a general increase of δ34S ratios of massive ore facies to stockwork zone. The heavier values indicate that some of the sulfur was derived from seawater sulfate that was ultimately thermochemically reduced in deep hydrothermal reaction zones. Sulfur isotopes, along with sedimentological, textural, petrological, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a Besshi-type VMS ore deposit.

The Glacier Creek Cu-Zn VMS Deposit, Southeast Alaska: An Addition to the Alexander Triassic Metallogenic Belt

The Glacier Creek volcanogenic massive sulfide (VMS) deposit, Alaska, is hosted within Late Triassic, oceanic back-arc or intraarc, rift-related bimodal volcanic rocks of the allochthonous Alexander terrane, known as the Alexander Triassic metallogenic belt. The Alexander Triassic metallogenic belt is host to the world-class Greens Creek Zn-Pb-Ag VMS deposit near Juneau in the south and the giant Windy Craggy Cu-Co VMS deposit in British Columbia, about 250 km to the north. The Glacier Creek deposit, located ~80 km southeast of Windy Craggy, consists of four tabular massive sulfide lenses within a bimodal mafic volcaniclastic and rhyolitic sequence. The mineralization-hosting stratigraphy is folded by a deposit-scale anticline and offset by a thrust fault near the axial surface of the fold. A resource of 8.13 Mt has been inferred from drilling, with grades of 1.41% Cu, 5.25% Zn, 0.15% Pb, 0.32 g/t Au, and 31.7 g/t Ag. Six main mineralization types are recognized, dominated by massive barite-sphalerite-pyrite, which is replaced at the base and center of the main lenses by massive and semimassive chalcopyrite-pyrite-quartz. The flanks and tops of the lenses are carbonate rich and consist of interbedded calcite-dolomite, barite and sulfide, resedimented massive barite-sulfide, and mineralized massive carbonate rocks. Tuffaceous hydrothermal sediment, with a distinct positive Eu anomaly, overlies the massive sulfide. Pyrrhotite and chalcopyrite in stringers constitute the main “feeder zone.” Stringer-style sphalerite-pyrite mineralization occurs above and below the lenses. Fe-poor sphalerite is dominant throughout the lenses, whereas Fe-rich sphalerite occurs at the stratigraphic top and bottom of the lenses in pyrrhotite-rich zones. Galena, tennantite-tetrahedrite, and arsenopyrite are the most important trace minerals within massive barite-sphalerite-pyrite mineralization, which is generally enriched in Sb, Hg, and Tl. Mineralization-related gangue minerals include barite, quartz, barian muscovite, calcite, dolomite, albite, chlorite, hyalophane, and celsian. Four types of alteration are recognized in the dominantly basaltic host rocks: pervasive muscovite-rich alteration, quartz-pyrite alteration associated with sulfide stringers, stratabound carbonate-bearing alteration, and background epidote-bearing alteration. Mass balance calculations indicate gains of S, Fe, Si, and K with coincident losses of Ca, Na, and Mg in all of the alteration types. Trace elements, Tl, Sb, Hg, Ba, Zn, Cu, and As were added to the rocks, whereas Sr was lost. Short wavelength infrared (SWIR) spectroscopy shows an increase in the wavelength of the AlOH absorption feature toward mineralization at a scale of 30 to 50 m, coincident with a general decrease in the Na, K, and Al and increase in the Fe, Mg, and Ba content of muscovite. The Glacier Creek deposit is transitional in character between Greens Creek, which is more Zn, Pb, and precious metal rich, and the Windy Craggy deposit, which is more Cu and Co rich, reflecting differences in the basement rocks and depositional settings within the Alexander Triassic metallogenic belt. Mineral-chemical studies and sulfur isotope data suggest that the Glacier Creek deposit formed under initially oxidized and sulfate-rich conditions that evolved to more reduced conditions in the latest stages of mineralization. The abundant argillite and presence of hyalophane rather than barite in the immediate hanging wall of the deposit may be an indication of a deepening basin and development of local anoxia, similar to Greens Creek.

Main deposit styles and associated tectonics of the West Junggar region, NW China

The West Junggar region, located in the loci of the Central Asian Orogenic Belt, is a highly endowed metallogenic province with >100 tonnes Au, >0.7 Mt Cu, >0.3 Mt Mo, and >2.3 Mt chromite as well as significant amounts of Be and U. The West Junggar region has three metallogenic belts distributed systematically from north to south: (1) late Paleozoic Saur Au-Cu belt; (2) early Paleozoic Xiemisitai-Sharburt Be-U-Cu-Zn belt; (3) late Paleozoic Barluk-Kelamay Au-Cu-Mo-Cr belt. These belts host a number of deposits belonging to at least eight economically important styles, including epithermal Au, granite-related Be-U, volcanogenic massive sulfide (VMS) Cu-Zn, podiform chromite, porphyry Cu, hydrothermal quartz vein Au, porphyry-greisen Mo(-W), and orogenic Au. These deposit styles are associated with the tectonics prevalent during their formation. Five tectonic-mineralized epochs can be recognized: (1) Ordovician subduction-related VMS Cu-Zn deposit; (2) Devonian ophiolite-related podiform chromite deposit; (3) early Carboniferous subduction-related epithermal Au and porphyry Cu deposits; (4) late Carboniferous subduction-related granite-related Be-U, porphyry Cu, and hydrothermal quartz vein Au deposits; and (5) late Carboniferous to early Permian subduction-related porphyry-greisen Mo(-W) and orogenic Au deposits.

Shallow Three-Dimensional Structure from Two-Dimensional Crooked Line Seismic Reflection Data over the Sturgeon Lake Volcanic Complex

In principle, partial three-dimensional structural information can be obtained from two-dimensional crooked line seismic reflection data by processing the traces as swath three-dimensional data, but this potential has rarely, if ever, been fully exploited. Here, we apply procedures newly designed for this purpose to mining exploration data from a high-resolution, two-dimensional crooked line survey over a part of the Sturgeon Lake Archean greenstone belt in northwest Ontario, Canada. We then examine the results in the light of geologic data based on about 20 years of mining activity in the immediate area. The area appears highly reflective, even when using standard two-dimensional data-processing methods. But because geologic dips in the section are typically steep and the majority of viewable contact surfaces do not lie directly below the processing and acquisition lines, special data-processing procedures have been essential. Despite appreciable background noise in the constructed images owing to the limitations of the crooked line data set, a large variety of reflection events are quite clearly positioned in three dimensions. The acquisition and processing lines run northeast-southwest, and most of the observed reflection events strike approximately east-west and dip northward. In the upper 1.5 km, they correlate well with expected layering of the mafic-felsic volcanic rocks, and in the vicinity of the Cu-Zn volcanogenic massive sulfide deposit at Lyon Lake, they agree well with information from drilling. The three-dimensional processing has successfully imaged an ore-controlling thrust fault at the Lyon Lake deposit, contacts within the volcanic pile (including the top of the Beidelman Bay subvolcanic intrusive complex), and fold structures in the volcanic stratigraphy. The results suggest that simple three-dimensional seismic surveys of limited offset and azimuth range may be sufficient for imaging structures in this type of geologic environment.

The alteration geochemistry and petrology of the Lar Cu-Zn deposit, Lynn Lake area, Manitoba, Canada

The Lar deposit is a stratiform Cu-Zn volcanogenic massive sulfide deposit lying within a bimodal but mainly basaltic sequence of lower Proterozoic volcanic rocks located about 60 km southwest of Lynn Lake, Manitoba, within the southern arm of the Lynn Lake greenstone belt. The alteration zone associated with the deposit consists of a semiconformable zone of intense Mg enrichment typified by a coarse-grained, hornfelsic-textured anthophyllite-cordierite + or - garnet assemblage, enveloped by a diffuse zone of K enrichment marked by the presence of biotite. Less prominent alteration facies occur locally throughout the volcanic pile, both stratigraphically above and below the ore horizon.The zone of intense Mg enrichment lies stratigraphically directly below the massive sulfide mineralization. The rocks of this zone are characterized by strong additions of Mg, B, S, and the chalcophile elements in addition to strong depletions of Na, Ca, Sr, and Eu. Within the altered envelope of the ore lenses the Cu/Zn ratio decreases upward and outward. Mass balance calculations indicate mass change ratios ranging from 0.77 (23% mass loss) to 1.70 (70% mass increase) were produced in the rocks during alteration. Spatial distributions of alteration effects corrected for mass change are illustrated on three-dimensional mesh diagrams.Applications of the study to mineral exploration are evaluated. Outer limits of alteration can be detected more sensitively by their geochemical signatures than by their mineralogical expression, as can alteration zone boundaries. The value of mass balance-corrected flux data for these purposes is demonstrated by the wide range of mass change ratios observed in these rocks which would be represented as noise in raw data.

Ability of the fry method to characterize pressure-solution deformation—reply

The Ashele volcanogenic massive sulfide (VMS) Cu-Zn deposit is placed in the Devonian volcanic-sedimentary basin of the Chinese Altay Orogen, NW China. It occurs in the form of well-developed lensoid orebodies within the Devonian Ashele Formation. The orebodies display analogous deformational characteristics with respect to their host rocks. Two periods of ore formation are distinguishable in the Ashele VMS Cu-Zn deposit: the first one is related to the genesis of banded and massive ores, whereas the second one is characterized by the formation of earlier Cu-dominated- and later Pb-Zn dominated-polymetallic quartz veins. In most cases, the primary banded textures of ores are still preserved. However, some veins cross cut the earlier generated ores with brecciated, durchbewegung, recrystallization and pressure-solution textures produced, indicating that substantial remobilization of metals has occurred due to intense reworking of the VMS. Four types of fluid inclusions were recognized in Cu-dominated quartz veins encompassing CO2-H2O (C-type), aqueous water (W-type), daughter mineral-bearing (S-type) and carbonic (PC-type) fluid inclusions. The homogenization temperatures of fluid inclusions mainly vary between 220 and 280 °C, whereas their salinities focus on 4 to 8 wt.% NaCl equiv. Gases in fluid inclusions identified using Raman spectroscopy consist of CO2, CH4 and N2. Characteristics such as mesothermal temperature, low salinity and elevated CO2 content indicate that the ore-forming fluid might have been of metamorphic origin. Based on the major C-type FIs, the trapping pressures are greater than 8.5 MPa, corresponding to minimum ore-forming depths are deeper than 3.1 km. Taking into account the regional geodynamic evolution, it is concluded that the early formed ore layers were generated in an arc-related geotectonic setting; while deformation and metamorphism that were responsible for metal remobilization in the ore system happened in a post-subduction regime. Based on the micro-textural observation coupled with fluid inclusion data, we conclude that the Ashele Cu-Zn deposit is a typical remobilized VMS deposit intensely overprinted by subsequent deformation and metamorphism.

Tin in volcanogenic massive sulfide deposits; an example from the Geco Mine, Manitouwadge District, Ontario, Canada

Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Geco Cu-Zn volcanogenic massive sulfide deposit as well as in some of the smaller massive sulfides in the Manitouwadge district. Cassiterite and minor Sn-bearing andradite, epidote, rutile, and phlogopite occur within the massive sulfide ores. Nigerite-24R of composition (Sn, Ti) x (Zn, Mg, Fe, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 2, and mixed polytypes of hoegbomite with the general formula (Ti, Sn) x (Fe, Mg, Zn, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 1, occur in the footwall rocks which comprise the feeder zone. Hoegbomite is the primary tin-bearing mineral in the districtwide cordierite-gedrite gneisses which represent footwall conformable alteration zones.A relatively high f (sub O 2 ) during metamorphism and also during the formation of the ores accounts for the presence of cassiterite rather than stannite or the simple tin sulfides in the ores. Nigerite and hoegbomite occur only in Ca-Na-deficient Mg-Fe-rich aluminous rocks and are associated with Zn staurolite, corundum, cordierite, sillimanite, (Zn, Fe) spinels, and Fe-Ti oxides.Bulk-rock tin assays systematically increase as the Geco orebody is approached: districtwide cordierite-gedrite gneisses (10-30 ppm), feeder zone (50-330 ppm), and orebody (100-3,000 ppm). Hanging-wall felsic gneisses and nearby mafic gneisses and granites contain less than 3 ppm tin. Small nearby pegmatites contain 2 to 10 ppm tin.From textural evidence, phase equilibria calculations, and the lithologic distribution of tin, a model for the Geco deposit is developed in which the districtwide cordierite-gedrite gneisses represent one segment of the fossil hydrothermal system and in which Cu-Zn and Sn mineralizations are closely associated. Nigerite, hoegbomite, and spinel from Geco occupy distinct and characteristic (Zn, Fe, Mg, Mn) solid solution fields. Hence, these species, which are known to occur as detrital heavy minerals may be useful pathfinders in geochemical exploration for eroded volcanogenic massive sulfide deposits.

Geochemistry of the alteration pipe at the Bruce Cu-Zn volcanogenic massive sulfide deposit, Arizona

The Bruce volcanogenic massive sulfide deposit is a conformable lens which lies at an andesite-rhyolite contact within the 1.7- to 1.0-b.y.-old Yavapai Series exposed south of Bagdad, Arizona. The upper part of a funnel-shaped alteration pipe replaces andesite within the stratigraphic footwall of the deposit. This pipe can be divided into three zones based on characteristic mineralogy: a chlorite zone consisting of chlorite (40-90 wt %) and quartz completely replaces the andesite in the central part of the pipe. A sericite zone containing subequal amounts of chlorite, sericite, and quartz is peripheral to the chlorite zone in the upper part of the pipe. The modal abundances of alteration minerals decrease outward through a gradational zone to unaltered andesite. Amphibole and biotite occur as metamorphic minerals; they are most abundant in the gradational zone. Some biotite is texturally and chemically related to the chlorite and sericite and is also probably hydrothermal in origin. All of the rocks within the pipe have been depleted in 18 O with whole-rock delta 18 O values ranging from 2.4 to 7.6 per mil. A linear relationship exists between the oxygen isotope composition and the quantity of hydrothermal chlorite in the alteration pipe. Both the amount of chlorite and the 18 O depletion are functions of the extent to which the rock has reacted with the hydrothermal fluid. The bulk chemistry of the altered rocks, when measured as a function of increasing alteration progress (decreasing delta 18 O), shows strong enrichments in MgO and FeO and depletions in SiO 2 , K 2 O, NA 2 O, and CaO. The most altered and most 18 O-depleted samples are nearly completely altered to chlorite; their bulk chemistry approaches that of the chlorite in the sample. All of the ferromagnesian minerals become more Mg-rich toward the center of the pipe. The Fe/(Fe + Mg) ratio of chlorite ranges from 0.8 in the least altered sample to 0.3 in the center of the chlorite zone. Extrapolating the linear relationship between the amount of chlorite in a sample and its delta 18 O value to 100 percent chlorite gives a value of 2.1 for the pure chlorite. At 250 degrees and 300 degrees C water in equilibrium with this chlorite would be 1.1 and 2.05 per mil, respectively. These data are comparable to delta 18 O values of water issuing from hot springs on the East Pacific Rise and are consistent with a slightly 18 O-shifted seawater as a source for the ore-forming solutions. Prior to entering the alteration pipe, the seawater reacted with volcanic rocks at depth, lowering the pH of the fluid such that chlorite was stable when the fluid entered the pipe. Subsequent formation of chlorite continued to lower the pH and decreased the activity of Mg in the solution, such that the fluid moved into the sericite stability field.