Volcanic-associated Massive Sulfide Research Articles

The subsea-floor replacement origin of the Ordovician Highway-Reward volcanic-associated massive sulfide deposit, Mount Windsor Subprovince, Australia

The Highway-Reward massive sulfide deposit is hosted by a submarine (below storm-wave base), silicic, synsedimentary intrusion-dominated volcanic succession. The succession includes at least 13 porphyritic units in a volume of 1 X 1 X 0.5 km. Peperitic upper margins suggest that most of the rhyolites, rhyodacites, and dacites were emplaced as small ( 300 degrees C), H 2 S-dominant fluids by replacement of rhyolite, rhyodacite, and peperite. Lower temperature (<300 degrees C) fluids that diffused from the margins of the pipes deposited a halo of sphalerite-rich ore.

Geochemistry of the Dundonald Komatiite-Basalt Suite and genesis of Dundeal Ni deposit, Abitibi Subprovince, Canada

The genesis of konatiites, basalts, and associated Ninineralization in the Dundonald township area, On- tario, is critical to understanding the metallogenic evolution of the Kidd-Munro assemblage, one of the most primitive volcanic assemblages in the world. The 2.5-kin-thick stratigraphic succession has a basal calc-alkalic basalt-dacite-rhyolite sequence (2716.8 ___ 2.1 Ma, U-Pb zircon) cut by the Dundonald peridotite-gabbro sill (27077 Ma, U-Pb zircon), overlain by komatiitic and basaltic flows containing magmatic Ni sulfide mineral- ization, which are, in turn, overlain by chemically distinctive low Ti basalt flows. The Dundonald komatiites correlate with komatiites in the footwall of the giant Kidd Creek volcanic-associated massive sulfide deposit 40 km to the west, but are slightly older than the komatiites of Munro township which are 40 km to the east. The Dundonald komatiites comprise a Munro-type, Al-undepleted komatiite suite, with liquid compositions having A120:fFiO,o = 18 to 22 and MgO up to 30.3 vet percent (anhydrous, n = 24). Most samples in the ko- matiite suite have slightly depleted light BEE contents, with LaN/SmN = 0.6 to 1.1 and LaN/YbN = 0.4 to 0.9 (n -- 21), and negligible Ti/Ti* and Zr/Zr* anomalies. Their major and trace element geochemistry is consistent with a derivation by sigfificant partial melting of a chemically primitive mantle with little or no influence of majoritc garnet. Flows that are host to the Dundeal and Alexo Ni deposits have distinctively high ThpM/NbpM ratios (>1.7, n = 3), suggesting crustal contamination, and peperite textures that are consistent with contami- nation from nearby graphitic argillite. The chemically distinctive lmv Ti basalts have 49 to 53 percent SiOs, 4.5 to 9.5 percent MgO, 18.8 to 21.5 percent A1203, 0.30 to 0.37 percent TiOs, moderately depleted light REE con- tents, with LaN/SmN = 0.9 to 1.2 and La.N/YbN = 0.4 to 0.6, high Ni, Cr, and Ba contents (570-6,800, 390-630, 90-320 ppm, respectively; n = 6). Their high AI203 contents place them in the calc-alkalic field on a Jensen cation plot, although their trace element ratios (Zr/Y, La/Yb) are more consistent with a tholeiitic affinity. They are compositionally identical to, and they correlate with, low Ti basalts in the Kidd Creek Volcanic Complex. Their geochemistry is consistent with a derivation by partial melting of a refractory harzburgitic mantle that has undergone a previous melt extraction, possibly influenced by a large ion lithophile-enriched hydrous phase. The Empire flow is the thickest and most primitive komatiite flow and is host to the Dundeal Ni deposit (re- source of -0.4 Mt at 2.0% Ni). The Empire flow contains an elongate dunite-peridotite basal unit interpreted as the core of a flow that thermally eroded into footwall heterolithic breccias and pillowed andesites. The main Dundeal Ni horizon overlies part of the basal dunite-peridotite unit within the flow channel and extends over the cutbank wall. The Dundeal deposit and the nearby Dundonald South Ni deposit have lo;v total sulfide con- tents and high Ni/S ratios (0.9 and 1.0, respectively) in comparison to most komatiite-hosted magmatic sulfide deposits. The high Ni tenor in sulfide is believed to be due to assimilation of reducing carbonaceous sediment which causes greater Ni partitioning into the magmatic sulfide phase.

Discussion: Sulphur isotope composition of the Brunswick No. 12 massive sulphide deposit, Bathurst Mining Camp, New Brunswick: implications for ambient environment, sulphur source, and ore genesis

The authors (W.D. Goodfellow and J.M. Peter) report an impressive amount of chemical data on the Brunswick No. 12 orebody, one of the world’s largest volcanic-associated massive sulphide deposits. They have proposed that most of the sulphur in the massive sulphide ore and the pyrites in adjoining sediments of the Patrick Brook Formation was probably derived from an overlying, anoxic seawater column in which sulphate reduction had taken place, the resulting H2S having mixed with venting metal-rich fluids. The principal lines of evidence brought in to support this idea are (1) the sediments of the Patrick Brook Formation underlying the footwall sedimentary and volcanic rocks of the Nepisiguit Falls Formation, which hosts the orebody, have S/C values and Mn and Fe contents that indicate anoxic conditions in the seawater during sedimentation; (2) the δ34S values of the sulphides in the ore types and the underlying feeder zone are similar to those of pyrites in the Patrick Brook Formation and to coeval pyrites in the Selwyn Basin; and (3) there is no alternative source of reduced sulphur because there would be inadequate sulphate remaining in deep circulating seawater to provide sufficient reduced sulphur for ore formation, and there is little evidence for the presence of evaporite sulphate in the deep footwall sequence. The significance of their major lines of evidence, and also those of Lentz et al. (1996), is discussed below.

The deep structure of a sea-floor hydrothermal deposit

Hydrothermal circulation at the crests of mid-ocean ridges plays an important role in transferring heat from the interior of the Earth1,2,3. A consequence of this hydrothermal circulation is the formation of metallic ore bodies known as volcanic-associated massive sulphide deposits. Such deposits, preserved on land, were important sources of copper for ancient civilizations and continue to provide a significant source of base metals (for example, copper and zinc)4,5,6. Here we present results from Ocean Drilling Program Leg 169, which drilled through a massive sulphide deposit on the northern Juan de Fuca spreading centre and penetrated the hydrothermal feeder zone through which the metal-rich fluids reached the sea floor. We found that the style of feeder-zone mineralization changes with depth in response to changes in the pore pressure of the hydrothermal fluids and discovered a stratified zone of high-grade copper-rich replacement mineralization below the massive sulphide deposit. This copper-rich zone represents a type of mineralization not previously observed below sea-floor deposits, and may provide new targets for land-based mineral exploration.

Eastern Victoria: a New Exploration Frontier?

High-quality airborne magnetic and radiometric data provide the cornerstone for the geological understanding of the relatively unexplored eastern highlands of Victoria. The Geological Survey of Victoria has obtained over 180 000 line kilometres of new airborne data in eastern Victoria. This data collection is a major component of the Victorian Initiative for Minerals and Petroleum (VIMP).Eastern Victoria has an excellent geological section across the eastern Lachlan Fold Belt. The Early Silurian Benambran event deformed Ordovician turbidites. Two cycles of rifting during the Silurian and Early Devonian resulted in transtensional rift-like grabens. Silicic volcanic units and marine sediments filled these rifts. The Late Silurian Bindian deformation resulted in major northwest and northeast fault trends which are apparent in the geophysical data. Granites with varied magnetic and radiometric responses have intruded the area, with several reaching the surface to produce calderas.The area is rich in potential mineral targets. These include porphyry style copper-gold mineralisation, dyke-associated tin-gold mineralisation, volcanic-associated massive sulphide mineralisation, epithermal gold mineralisation, and carbonate-hosted Irish style lead-zinc mineralisation.The new geophysical data allow improved mapping of the geological units of eastern Victoria, interpretation of the regional structural features, and identification of exploration targets. The regional geophysical datasets provide an excellent basis for mineral exploration in the under-explored eastern Victoria.

Facies analysis of a 1.9 Ga, continental margin, back-arc, felsic caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen region, Sweden

The Bergslagen mining district in south-central Sweden has been a major metal producer for more than 1,000 years. In geologic terms, Bergslagen is the intensely mineralized part of a large, Early Proterozoic (mainly 1.90-1.87 Ga), felsic magmatic region of mainly medium to high metamorphic grade in the Baltic Shield. The district contains a diverse range of ore deposit types, including banded iron-formation, magnetitecalc-silicate skarn, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and strata-bound Zn-Pb-Ag-(Cu-Au) sulfide ores, and W skarn. Most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns.This study is a field-oriented, regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration, deformation, and metamorphism. The region is interpreted as an extensional, probably back-arc, active continental margin magmatic region. It evolved through stages of intense magmatism, thermal doming, and crustal extension, followed by waning extension, waning volcanism, thermal subsidence, reversal from extension to compressional deformation, regional metamorphism, and structural inversion. The volcanic successions comprise interfingering proximal (near vent), medial (volcano flanks), and distal (volcano margin) facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite-rhyolite complexes. Nonwelded to poorly welded pyroclastic flow and fallout units, and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies. Subvolcanic porphyritic intrusions and cryptodomes are also abundant. The pyroclastic debris was shed from mainly shallow-water and subaerial proximal areas to shallow- and moderately deep-water (above and below storm wave base) distal areas.Most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash-siltstone, limestone, and vitric crystal sandstone and breccia. These mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base, but locally above the storm wave base. Water depths are interpreted to have been mainly 10 to 500 m, with local deeper areas. Although the ore deposits occur in medial to distal facies associations, many occur above, or laterally adjacent to, subsided volcanic vent complexes. This suggests a spatial, temporal, and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers. Except for the apatite-bearing iron deposits, it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes. This is possible, especially for the base metal sulfide deposits, which have intense localized footwall alteration in felsic volcanic rocks. However, an important role of the volcanic centers may have been to provide strong positive pertubations (+ or - magmatic hydrothermal input) to the regional convective geothermal system.The main Zn-Pb-Ag-(Cu-Au) ore deposits span a range between two end-member types: stratiform ash-siltstone-hosted sea-floor deposits that have similarities to some stratiform sediment-hosted Zn-Pb-Ag deposits, especially the Broken Hill-type deposits; and strata-bound volcanic-associated limestone-skarn subsea-floor replacement deposits that more closely resemble felsic volcanic-associated massive sulfide deposits. Facies models are presented for both. The banded iron-formations are interpreted as chemical suspension deposits, rhythmically interbedded with rhyolitic ash-siltstone. The skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements. Apatite-bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite-rhyolite volcanic center.

U-Pb zircon geochronology of the Selbaie Cu-Zn-Ag-Au mine, Abitibi Subprovince, Canada

The Selbaie mine is a large tonnage, low-grade Cu-Zn-Ag-Au deposit with both stratiform and epithermal characteristics. Mineralization comprises lower grade, disseminated Zn-Ag ore and pyritic siliceous sinter of the A1 zone at the surface; and higher grade Cu-Zn-Ag-Au quartz sulfide lenses of the A2 zone below and transitional with the A1 zone, and of the B zone  1 km to the west. The copper-rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins, vein arrays, and breccia. They have irregular, locally gradational boundaries and variable attitudes, with dips from 25 o to 90os The mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows, which are cut by mafic and felsic intrusions and felsite dikes. The felsic volcanic rocks and a tonalitic marginal phase from the adjacent Brouillan pluton are calc-alkalic and have geochemical signatures that support a cogenetic relationship. The mineralization and mine stratigraphic succession are truncated by the Brouillan faults, a series of subparallel, west-directed, predominantly brittle thrusts with the Brouillan pluton on the upper plate above the mine stratigraphic succession. Galenas from stratiform and vein ore have uniform lead isotope compositions of 2o6pb/2o4Pb = 13.184 to 13.211, '2o7pb/2o4pb = 14s351 to 14.369, and '2ospb/2o4pb = 33s039 to 33.100 , with model/ values of 8.1-8.2, and model K values of 4.1 to 4.2 (n = 5); among the most primitive of Abitibi volcanic-associated massive sulfide depositss They have model ages that range from 2.71 to 2.68 Ga. A U-Pb zircon age of 2729_+ Ma for a sericite-altered, calc-alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type. A U-Pb zircon age for a felsite dike that cuts postore east-west foliation is 2726 _+ 3 Ma, slightly younger but overlapping at the 20- level with the age for the quartz feldspar porphyry flow, indicating that the copper-rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization. A sample of sheared, chloritic granodiorite taken from one of the Brouillan faults near the margin of the Brouillan pluton has a complex zircon opulation Six concordant anal ses of abraded, multigrain fractions, individual grains, or grain tips 20, P, 206 '. s

Underwater chemistry creates massive seafloor mineral deposits

Cores retrieved from a mound at the bottom of the sea where sulfide deposits are actively forming indicate that these modern-day accumulations are comparable in size with valuable ancient deposits on land. The cores also provide important new clues about how volcanic-associated massive sulfide deposits are initially formed. A multinational team of scientists brought these insights back from a fall 1994 cruise of the Ocean Drilling Program (ODP), an international partnership for geological oceanography funded principally by the U.S. National Science Foundation, and reported its work in Nature [ 377 , 713 (1995)]. The results of this expedition have been eagerly awaited, according to Randolph A. Koski, a geologist with the U.S. Geological Survey in Menlo Park, Calif. The study provides, for the first time, a glimpse of the processes operating within such a deposit, he commented in Nature . Overcoming the engineering challenges of drilling into an active hydrothermal system, the team drilled 17 holes int...

Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada

Research Article| February 01, 1995 Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada C. Tucker Barrie C. Tucker Barrie 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (2): 169–172. https://doi.org/10.1130/0091-7613(1995)023<0169:ZTOHTR>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation C. Tucker Barrie; Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada. Geology 1995;; 23 (2): 169–172. doi: https://doi.org/10.1130/0091-7613(1995)023<0169:ZTOHTR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract High-temperature, high-silica rhyolites are commonly found near Cu-Zn volcanic-associated massive sulfide (VMS) deposits in the Archean Abitibi subprovince. They are characterized by anhydrous phenocrysts (feldspar, quartz) in concentrations of generally <15 modal percent and have >73% SiO(2), high incompatible element contents, Rb/Sr > 1, Zr/Y < 5, La(N)/Yb(N) < 3.5, and negative Eu anomalies. They have zircon saturation temperatures of 840–940 ° C, as calculated by using the zircon solubility model, which are higher than most other Abitibi rhyolites. Considering their high zircon saturation temperatures, their composition, and their spatial and temporal associations with tholeiitic mafic and ultramafic rocks, Abitibi high-temperature rhyolites are similar to silicic volcanic rocks found in hotspot-related oceanic settings with anomalously high heat flow, such as in eastern Iceland. The association of high-temperature rhyolites with VMS deposits may provide a useful exploration guide where zircon inheritance is minimal, such as in many of the primitive Archean and Early Proterozoic greenstone belts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Fluid inclusion studies on a multiply deformed, metamorphosed volcanic-associated massive sulfide deposit, Joma Mine, Norway

Meso- and microscale structural analysis and the spatial distribution, morphology, and microthermometry of fluid inclusions reveal part of the fluid evolution of a multiply deformed, metamorphosed volcanic-associated massive sulfide deposit in the Scandinavian Caledonides, Norway. The ore and host rocks have suffered greenschist facies metamorphism and four deformation events. Metamorphism peaked during D 2 to D 3 and waned to subgreenschist facies by D 4 . Of the four distinct fluid inclusion types from D 2 to D 4 structural settings, primary inclusions occurring in crack-seal growth zones (type I) in D 2 vein quartz are the only preserved inclusions related to D 1 to D 3 ore remobilization and vein growth. All subsequent inclusions are secondary within healed microfracture space. Type II secondary inclusions exist in vein quartz regardless of structural setting; their formation cannot therefore predate D 4 . Both type I and type II inclusions underwent a pervasively destructive decrepitation event syn- to (more probably) post-D 4 . The leakage and/or decrepitation is attributed to internal overpressures accompanying rapid decompressive regional uplift after the D 4 event. Two populations of postdecrepitation inclusions (types III and IV) have consistent vapor to liquid ratios and yield low temperatures (T h = 137 degrees -297 degrees C) and low salinities (0-6.7 wt % NaCl equiv). They possibly record flushing by meteoric water late in the uplift history.

Silicification and metal leaching in semiconformable alteration beneath the Chisel Lake massive sulfide deposit, Snow Lake, Manitoba

Regionally extensive semiconformable zones of silicified, Fe-Mg metasomatized and epidotized, dominantly mafic, volcaniclastic strata and lava flows are exposed 1 to 2 km stratigraphically beneath the Chisel Lake Zn-Cu massive sulfide deposit, Snow Lake district, Manitoba. The alteration zones occur within a Lower Proterozoic medium-grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments, and lie between subvolcanic tonalite sills and the massive sulfide deposit.Semiconformable alteration occurs at two main stratigraphic positions. The lower zone of silicification in pillowed and massive mafic lavas is inferred to have occurred close to the sea floor and probably was not related directly to formation of the Chisel Lake massive sulfide deposit. The stratigraphically higher zone, which may be spatially and temporally associated with massive sulfide deposition, occurs in a approximately 300-m-thick heterolithic mafic volcanic breccia and wacke unit. This volcaniclastic-hosted alteration is zoned laterally from dominantly silicification and epidotization to mainly Fe-Mg metasomatized, garnet-chlorite + or - biotite + or - staurolite rocks nearer the Chisel Lake sulfide deposit. A discordant footwall Fe-Mg alteration zone directly beneath the sulfide deposit extends toward, and may meet, the semiconformable Fe-Mg metasomatized zone.Silicification contributed to partial to complete replacement of volcanic clasts, beds in the volcaniclastic unit, rocks adjacent to some felsic dikes and pillow interiors by quartz and sodic plagioclase. Mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds, dikes, and pillows indicate that SiO 2 increased by up to 50 percent of its initial value, and Na 2 O by up to 30 percent. Up to 80 percent of the FeO, MgO, CaO, and Zn was removed during silicification. Elemental fluxes during Fe-Mg metasomatism are generally opposite those characterizing silicification and are of comparable magnitude. Epidotization resulted in depletion of Na, total Fe (but increased the Fe (super 2+) /Fe (super 3+) ratio), Mg, Mn, K, Zn, and Ba, and enrichment in Ca and Sr relative to least altered rocks. Almost constant interelement ratios of Ti, Zr, and Al in altered and less altered rocks indicate that these elements were essentially immobile during metasomatism and subsequent medium-grade regional metamorphisrm. Limited data suggest that the heavy REE were also immobile during silicification.The subconcordant silicification in the mafic volcaniclastic unit is interpreted to have formed at subsea-floor depths of 1 to 2 km where a felsic dike swarm and subvolcanic tonalite sills heated Si-rich evolved seawater above the temperature of the silica solubility maximum ( approximately 340 degrees -450 degrees C at pressures below 900 bars) causing silica deposition and metal leaching. A portion of the Fe, Mg, and possibly Zn that was leached from the subconcordant silicified zone may have been transported laterally away from this environment, thereby producing the semiconformable Fe-Mg metasomatized zones. Cross stratal structures similar to the hydrothermally altered synvolcanic faults that are known to cut the semiconformable alteration may represent fluid flow paths from a subconcordant metal reservoir in the volcaniclastic unit to the sea floor, where massive sulfides were deposited.Semiconformable pervasively silicified zones, particularly those associated with Fe-Mg metasomatized and/or epidotized rocks, are significantly larger exploration targets than areas of proximal alteration, and indicate large-scale hydrothermal mass transfer. Zonation of silicification to Fe-Mg metasomatism laterally within the alteration may provide a vector toward the discordant Fe-Mg-enriched alteration zones that commonly underlie volcanic-associated massive sulfide deposits.

Gold distribution in the Mobrun volcanic-associated massive sulfide deposit, Noranda, Quebec; a preliminary evaluation of the role of metamorphic remobilization

The Mobrun Zn-Cu-Au-Ag orebodies show metal zonation typical of volcanic-associated massive sulfide deposits, although some metamorphic remobilization of metals has occurred. The nature and distribution of primary facies are the result of synvolcanic deposition and reworking of sulfides by hydrothermal fluids. Secondary facies resulting from metamorphism and deformation locally overprint primary facies and include transgressive veins which contain remobilized Au as electrum. The distribution of secondary facies was controlled by the mechanical properties of the mineralization as well as of the surrounding rocks; the mechanical response of different facies to deformation influenced the development and distribution of zones containing easily recoverable gold. Preservation of primary textures is best in the Main and Satellite lenses. The 1100 lens also shows significant preservation of primary facies; however, development of secondary facies is pronounced. Textural, structural, and chemical evidence indicates that the 1100 lens was subjected to more extreme metamorphic recrystallization and deformation than the other lenses. Metamorphic remobilization was important in releasing refractory Au from sulfides and locally concentrating it in a recoverable form (electrum).

Application of lithogeochemistry to exploration for deep VMS deposits in high grade metamorphic rocks, Snow Lake, Manitoba

Volcanic-associated massive sulphide deposits in the Snow Lake area of Manitoba are related to mineralogically and chemically distinct alteration zones. It is generally accepted that these zones represent crosscutting, subconformable or conformable synvolcanic alteration zones, which were coeval with and have been metamorphosed with the massive sulphides. Metamorphism ranges from upper greenschist facies to middle amphibolite facies. Surface lithogeochemical anomalies led to the discovery of small massive sulphide lenses at a vertical depth of 250 m in the Raindrop Lake area, southwest of Snow Lake, Manitoba. Variations in mineral assemblages of middle amphibolite facies alteration zones and analysis of variations in major and trace element chemistry were used to guide deep drilling at Raindrop Lake. The massive sulphide lenses are stratigraphically underlain by a low angle crosscutting “pipe” and a conformable footwall “apron” alteration. The alteration zones are composed of assemblages of garnet, staurolite and chlorite, and, less significantly, biotite, muscovite and kyanite. They are characterized by loss of Na and Ca, and addition of Fe, Mg, Cu and Zn. Mapping the alteration is aided by the application of the metamorphic AFM phase diagram for the appropriate metamorphic facies. Increasing intensity of alteration can be identified by the first appearance of new mineral phases, which are represented on the AFM diagram. These mineral trends coincide with loss of Na and Ca relative to Al, and increased Mg and Fe. Chemical alteration indices ACNK (molecular proportion Al 2O 3/(CaO + Na 2O + K 2O) and AI = 100 × [(MgO + K 2O)/(MgO + K 2O + CaO + Na 2O)] combined with Cu and Zn variation helped to quantify the intensity of alteration, despite being insensitive to Fe. The crosscutting pipe is dominantly Fe enriched, with a Cu-enriched core, Zn enriched margins and widespread Na and Ca depletion. Mineralogically it is identified by garnet, staurolite and chlorite and follows an iron and aluminum enrichment trend on the AFM diagram. The conformable alteration zone is characterized by local strong Mg enrichment, extensive Na and Ca depletion and variable Cu and Zn. Mineralogically it is characterized by the presence of chlorite and kyanite and follows a magnesium and aluminum enrichment trend on the AFM diagram.

Reconstruction of the tectonic, volcanic, and sedimentary setting of strongly deformed Zn-Cu massive sulfide deposits at Benambra, Victoria

The Wilga and Currawong massive sulfide deposits at Benambra are hosted by a strongly deformed, Upper Silurian rhyolite-dacite-andesite-basalt-sediment succession, dominated by silicic volcanics and sedimentary rocks. This host succession developed in an ensialic back-arc or intra-arc basin of limited extensional origin on the active Gondwana continental margin. Strong deformation is related to compressional and transcurrent crustal movements during closing of the depositional basin at the end of the Silurian and to subsequent steep faulting.The massive sulfide deposits are essentially synvolcanic because (1) they have the same three generations of tectonic structures and the same intensity of deformation as their host rocks, suggesting that the ores predate the earliest compressional deformation, (2) the ores are constrained to a Late Silurian age similar to their host rocks because both the host rocks and this earliest deformation are Late Silurian in age, (3) there is a tight stratigraphic control on the location of the massive sulfides, and (4) the stratigraphy, depositional setting, and alteration are similar to many other less-deformed volcanic-associated massive sulfide districts.Deformation and alteration inhibit detailed stratigraphic correlation and modify rock textures, but do not destroy facies relationships, which allow sedimentological and volcanological interpretation. Sedimentary facies associations indicate that mineralization occurred in the basin center after fault-controlled subsidence from a mixed subaerial and subaqueous environment with active rhyolitic volcanism, through a marine shelf environment with mixed limestone-volcaniclastic sedimentation, to a moderate- to deep-water environment with mudstone and turbidite sedimentation and rhyolitic to basaltic volcanism.Juvenile elastic and nonelastic volcanic rocks are closely associated, subequal in volume, and comprise 50 to 70 percent of the volcano-sedimentary sequence. Evaluation of fragmentation and emplacement processes indicates that the juvenile volcaniclastics are mainly in situ, resedimented and intrusive hyaloclastites, rather than pyroelastics. Classification of primary contact relationships of the volcanics into passive, slightly disruptive and highly disruptive types, and the internal facies organization of the volcanic units, indicate that the volcanics form extrusive domes, extrusive tabular flows, partially emergent cryptodomes and sills, and entirely intrusive sills. Shallow sills and cryptodomes emplaced into wet subsea-floor sediments are texturally similar to, but more abundant than, lavas in the environment of mineralization. These shallow intrusions are distinguished by disruptive upper contacts including sediment-veined and sediment-matrix hyaloclastite. Their recognition is critical because the mineralized stratigraphic interval can only be correlated using volcanics and sediments emplaced on the sea floor. Based on extrusive units, the massive sulfides occur within 100 m above the last major extrusive rhyolites and within 50 m below the first major basalt lava.Integrating the tectonic, structural, sedimentological, and volcanological data shows that the massive sulfides formed in siltstone, in the proximal (near-vent) facies association of a low profile, mainly nonexplosive, moderate to deep-water submarine volcano composed of turbiditic sediments interleaved with numerous rhyolitic to basaltic sills, lavas, and associated hyaloclastites. The ores can be related in space and time to the advanced stage of extension and subsidence in a continental margin incipient rift, when relatively deep-marine conditions were attained, generation of voluminous rhyolitic crustal melts waned, and basaltic magma started to penetrate up deep extensional faults to the sea floor. The spatial association of the last rhyolite extrusions, the massive sulfides, and the first basalt extrusions, suggests that these rocks are genetically related and that all were fed via the same extensional fault system. This setting of silicic volcanic-associated massive sulfides at Benambra provides a model for the many deposits related to mainly nonexplosive volcanism and an alternative to the recently popular submarine pyroclastic caldera model.

“Pseudostratigraphy” and thrusting in relation to the structural evolution of the Joma ore-body, North Trøndelag, Norway

Joma Mine exploits a stratiform CuZn volcanic-associated massive sulphide deposit within the Norwegian Caledonides. The ore layer overlies an inferred feeder system hosted by mafic metavolcanites within the grossly inverted stratigraphy of the Leipik Nappe. Mine-scale inversion is supported by orebody zonation and the relative position of ore layer and feeder system. Mesoscale structural data from regionally distributed quartz-rich phyllites require two major periods (D 2 and D 3) and one minor period (D 4) of deformation. D 2 generated a transposition foliation containing an elongation lineation essentially co-linear with F 2 hinge lines. D 3 formed a NW -dipping crenulation foliation as hinge surface to SW -plunging F 3 folds. Before F 3 , D 2 linear data probably trended NW in the shallowly W -dipping transposition foliation. D 4 formed minor kinkbands. Mesoscale data from the ore layer affirm the deformation sequence, but also suggest a D 1 thrusting and folding event. F 2 vergence and facing at regional and mine scales further support D 1. The ore-layer sequence varies between and, less commonly, within layers, and lacks consistent polarity. This variable sequence is ascribed to D 1 and D 2 thrusting, folding and sliding, and is herein termed “pseudostratigraphy”. The existence of level-scale pseudostratigraphy contrasts with preservation of gross relationships at mine scale, and places limitations on the dimensions and consequences of D 1 and D 2 structures. At the end of D 1, pseudostratigraphy of the thrust-disrupted ore system occupied the lower limb of an E-closing regionally developed antiformal anticline. Folding and sliding during D 2 resulted in the further disrupted ore system being inverted and occupying the lower limb of a large-scale, E-closing, F 2 antiform. In D 3, the inverted ore layer occupied and was slightly S of the broad hinge zone of the regionally developed NE-closing Joma Synform; its present arcuate geometry reflects this. D 4 had little influence on the structural evolution of the orebody.

Proximal Cu + Zn exhalites in the Argyll Group Dalradian, Creag Bhocan, Perthshire

Synopsis Stratabound Cu + Zn mineralization at Creag Bhocan, near Tyndrum, represents the proximal equivalent of previously reported distal exhalites in the Ben Challum Quartzite of the Argyll Group Dalradian. The Creag Bhocan mineralization is characterised by pyrite, chalcopyrite and sphalerite hosted by chloritic schists and albitic quartzites. Mineral inclusions within syngenetic pyrite suggest ore formation at temperatures between 250°C and 350°C from reducing, acidic (pH3–5) hydrothermal fluids, in which H 2 S was the dominant aqueous sulphur species. Affinities with the volcanic-associated massive sulphide (VMS) group of deposits are suggested for the Creag Bhocan mineralization. High heat flow and minor intrusives associated with crustal extension are thought to have driven the circulation system, and the mineralization within the Ben Challum Quartzite as a whole has many characteristics similar to those of Besshi-type deposits.

Two-fold classification of Archean volcanic-associated massive sulfide deposits

Two-fold classification of Archean volcanic-associated massive sulfide deposits