Footwall Alteration Zones Research Articles

Recognizing and quantifying metamorphosed alteration zones through amphibolite facies metamorphic overprint at the Key Anacon Zn–Pb–Cu–Ag deposits, Bathurst Mining Camp, New Brunswick, Canada

The Key Anacon deposits, Bathurst Mining Camp, New Brunswick, are hosted in upper greenschist- to amphibolite-facies felsic volcanic rocks. The occurrence of cordierite–biotite and garnet–biotite–muscovite assemblages parallel to the regional tectonic fabric in the metamorphosed hydrothermal alteration zones point to a pre-metamorphic mineralization event that was synchronous with sub-aqueous volcanism. Modeling the altered felsic volcanic rocks in the system K2O–Fe2O3–MgO–Al2O3–SiO2–H2O–TiO2 (KFMASHT) and comparing the observed peak metamorphic assemblages with those produced in a petrogenetic grid allows us to interpret the style of pre-metamorphic hydrothermal alteration related to deposit formation. The compositional change in the stratigraphic footwall (structural hanging wall) is characterized by mass gains of 0.1 to 4.0 wt.% Fe2O3 (Total), 0.7 to 22.2 wt.% MgO, and 0.5 to 55.2 wt.% CaO, and mass losses of 25.1 to 56.7 wt.% SiO2, 0.2 to 2.0 wt.% Na2O, and 0.3 to 3.8 wt.% K2O (the values of the mass changes cited here are in absolute terms).Variable gains and losses of Zn, Pb, and Cu are characteristic of the footwall alteration zones with Zn displaying gains proximal to the massive sulfide lens, and losses distal to the sulfide lens. The alteration indices (AI) values increase as the massive sulfide lens are approached from either the footwall or hanging wall, whereas the Ghandi index (GI) discriminates the intensely chlorite-altered rocks proximal to mineralization from the sericitic altered rock in more distal areas. Overall, there is an increase of the GI from the weakly to moderately altered zone (GI=1.3 to 6.0) to the more intensely altered zone (GI=6.1 to 60). These physical and geochemical observations are consistent with early feldspar-destructive alteration followed by chloritization proximal to the sulfide lens and accompanied by sericitization alteration distal prior to sulfidation and oxidation during prograde metamorphism.

Hydrothermal Alteration Associated with Copper-Gold Mineralisation at Dhani Basri, Dausa District, Rajasthan

Abstract: The shear controlled, epigenetic Cu-Au mineralization at Dhani Basri area in Dausa district of Rajasthan is hosted within silicified zones traversing Mangalwar complex of rocks, of Archaean age. The mineralisation here is distinctly associated with the hydrothermal alteration manifested in the form of phyllic, propyllic, potassic and silicification alteration domains. The field and microscopic evidences are in conformity with the geochemical variation in the parameters within the mineralized zone, in the hanging wall and footwall alteration zones. Besides using the conventional method of presentation of the alteration data, the author on an experimental basis has used Alteration Box plot (Large et al. 2001) and the Alteration Index (AI) (Ishikawa, 1969) to depict the hydrothermal alteration in the environment other than the volcanic hosted massive sulphide depositional environment, for which these ratios and diagrams have been recommended by Ishikawa (1969) and Large et al. (2001). In the present case the author has been able to demonstrate the alteration very effectively with the help of these ratios and indices and has observed that these could very well be utilized in the environments other than the one, for which these diagrams are prescribed. The different trends described by Large et al. (2001) in their alteration box plot very clearly discriminate between the hydrothermal trends and the diagenetic trends of alteration, hence even if there is slight contamination by the meteoric fluids in the environments other then the volcanic hosted massive sulphide that can be separated out in the diagram. The author in his effort has observed that using the Al can very well bring out the trends of various alteration types and CCPI (carbonate-chlorite-pyrite index) in combination with the alteration mineralogy. A case study of hydrothermal alteration at Dhani Basri copper gold mineralization has been presented here.

Oxygen Isotope Characteristics of the Footwall Alteration Zones in the Hongtoushan Volcanogenic Massive Sulfide Deposit, Liaoning Province, China and Restoration of Their Formation Temperatures

Abstract:The Hongtoushan Volcanogenic Massive Sulphide Deposit (VMSD) occurs in the Hunbei granite‐greenstone terrane, Liaoning Province, NE China. Rocks in the mining area have been metamorphosed around 3.0–2.8 Ga to upper amphibolite facies at temperatures between 600°C and 650°C. Cordierite‐anthophyllite gneiss (CAG) in the Hongtoushan mining area, which occurs hundreds of meters below the ore horizon, corresponds to the metamorphosed semi‐conformable alteration zone of the VMSD hydrothermal system, whereas the one immediately below the main ore layer represents the metamorphosed pipe‐like alteration zone. Whole‐rock oxygen isotope signatures were well preserved in both types of CAGs, although the mineral components have been entirely changed during regional metamorphism. Therefore, whole‐rock oxygen isotopes can be used to estimate the formation temperature of both types of alteration zone. Calculations show that the semi‐conformable and pipelike alteration zones for the Hongtoushan submarine hydrothermal system were formed at 290–360°C and 285–320°C, respectively, whereas estimates for the former were slightly higher than that of the latter, indicating that the semi‐conformable alteration zone represents the deep part of the Hongtoushan seafloor hydrothermal system, while the pipe‐like alteration zone represents the discharge conduits for metal‐rich fluids, which is closer to the seafloor.

Geology and Geochemistry of Highly Metamorphosed Footwall Alteration Zones in the Hongtoushan Volcanogenic Massive Sulfide Deposit, Liaoning Province, China

AbstractThe Hongtoushan Archean Cu–Zn volcanogenic massive sulfide (VMS) deposit, which was metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650°C, occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. Stratiform cordierite–anthophyllite gneiss (CAG) that occurs hundreds of meters below the ore horizon in the Hongtoushan district corresponds to the metamorphosed semi‐conformable alteration zone of the VMS hydrothermal system, whereas the CAG that contains abundant deformed sulfide‐bearing quartz veins immediately below the main ore layer represents the metamorphosed discordant alteration zone. Whole‐rock geochemistry indicates that stratiform CAG was derived ultimately from five lithologies (basalt, basaltic andesite, andesite, dacite, and rhyolite), while discordant CAG derived from a single lithology (rhyolite). Amphibolite and biotite‐rich gneiss are identified as a metamorphosed least‐altered precursor for these CAGs. Mass change calculation indicates that, compared to the least‐altered rocks, stratiform CAG is enriched in Fe and Mg, and depleted in Na, K, Ca, Cu, Pb and Zn, while discordant CAG is enriched in Fe, Mg, Si, Na, Pb, Cu and Zn, and depleted in K. HREE and HFSE (Zr, Ti, Nb and Ta) behaved inertly during submarine alteration, whereas Rb, Sr, Ba and LREE (especially Eu) were leached off.Both stratiform and discordant CAGs are depleted in 18O, with values up to 7‰ lower than their corresponding least‐altered precursors. Addition of Fe, Mg, and depletion of Ca, K, Sr, and 18O, indicate that hydrothermal alteration for both types of CAGs was characterized by chloritization prior to metamorphism. Stratiform CAG could be used to evaluate the mineralization potential of VMS in metamorphic terranes, while discordant CAG containing sulfide‐bearing quartz veins could be a good indication for overlying stratiform massive sulfide ores as well as an exploration target itself.

Applying the box plot to the recognition of footwall alteration zones related to VMS deposits in a high-grade metamorphic terrain, South Africa, a lithogeochemical exploration application

Alteration zones (more commonly foot wall alteration zones) are related to volcanic-hosted massive sulfide (VMS) deposits and represent unique features that may be targeted during exploration. Of these, the chloritic foot wall alteration pipe is the most extensive and characteristic of VMS deposits. This feature is geochemically identified by a strong relative enrichment in aluminium and magnesium and a coupled depletion in calcium and sodium, giving rise to chloritic rocks in the primary environment of formation. During high grade regional metamorphism such chloritic precursor rock types are replaced by an unusual mineral paragenesis, typically containing magnesium rich cordierite, phlogopite, orthoamphiboles or orthopyroxenes and aluminium rich minerals such as sillimanite and corundum. This suggests that the unusual geochemical features of the alteration zone, retained during the deformation and metamorphism, should be recognisable in lithogeochemical exploration. The massive sulfide deposit in the eastern part of the metamorphic Namaqua Province, South Africa, at Areachap, Kantienpan and the defunct Prieska Cu–Zn Mine are hosted by a Mid-Proterozoic volcano sedimentary succession known as the Areachap Group. These deposits were affected by a complex deformation and metamorphic history and represent examples of upper amphibolite to granulite grade metamorphosed VMS deposits. The application of the known lithogeochemical methods is especially complicated where the geology is not well understood, due to the poor rock exposure of complexly deformed and metamorphosed areas, such as in the eastern part of the Namaqua Province. The box plot presents a more readily applicable lithogeochemical method to characterize and identify the alteration process, but it was designed for relatively un-metamorphosed environments. It is demonstrated here that the box plot may also be applied to high-grade metamorphic terrains and that the mineral phases used in defining the boxplot in low grade metamorphic environments may be replaced by their equivalents in high grade metamorphic terrains. The compositional trends of the metamorphic minerals themselves may be used in defining the boxplot for high grade metamorphic terrains. These include the transition of: annite to phlogopite; grossular to almandine or pyrope, augite to enstatite or clinoenstatite, and hornblende to gedrite or cummingtonite. Close to the ore zone, the relative Mg content of pyroxene, cordierite and biotite are higher than further away from this zone. It could be demonstrated that the changes in the mineral compositions are gradational when comparing unaffected rocks with progressively more altered wall rocks. Conclusions based on an application of the isocon method demonstrate that primary footwall alteration zones in the Areachap Group's VMS deposits are characterized by elemental depletion of Na 2O, CaO, Sr, Ni, V and La and enrichment of MgO, Fe 2O 3(total), S, Zn, SiO 2, Co and F. It is shown that the whole rock compositions of rocks that were independently identified as the metamorphic equivalents of altered rocks, using the isocon method, plot in the correct place in the box plot for high grade regionally metamorphosed terrains. This establishes the box plot as an effective and practical tool for lithogeochemical exploration for VMS deposits in complexly deformed high grade metamorphosed terrains.

Sulfur isotope characteristics of metamorphosed Zn–Cu volcanogenic massive sulfides in the Areachap Group, Northern Cape Province, South Africa

Zn- and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the ∼1.0–1.2 Ga Namaquan Orogeny. Sulfur isotope data (δ34S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group. Although sulfides from each site have distinct sulfur isotope values, all δ34S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ34S value of 18.5‰, very different from that of associated sulfides. At one of the studied sites (Kantienpan), a distinct increase in δ34S of sulfides is observed from the massive sulfide lens into the disseminated sulfides associated with a distinct footwall alteration zone. Sulfide–sulfide and sulfide–barite mineral pairs which recrystallized together during amphibolite- and lower granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of the Zn–Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution associated with high-grade regional metamorphism of the massive sulfide ores of the Areachap Group did not result in extensive sulfur isotopic homogenization. This is similar to observations in other metamorphosed VMS deposit districts and confirms that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism.

Indicator minerals as guides to base metal sulphide mineralisation in Betul Belt, central India

Zn-bearing minerals that act as indicator minerals for base metal sulphide mineralization from the Proterozoic Betul Belt, central India with special emphasis on their genetic significance have been discussed. Sulphide mineralisation is hosted by the felsic volcanic rocks and has similarities with volcanic-hosted massive sulphide deposits in other parts of the world. Synvolcanic hydrothermal alteration is crudely zoned with an inner high Mg-Ca core and an outer wider envelop of Al-Fe rich mineral assemblage. Most of the prospects have strata bound, moderately to steeply dipping, multiple, sub-parallel sheet like ore bodies composed of disseminated and semi-massive to massive ores. Zn-bearing spinel, staurolite, biotite and ilmenite typically occur within the foot-wall alteration zones in close proximity to the sulphide mineralization. Zincian spinel is ubiquitous irrespective of the nature of alteration zone. Zincian staurolite is nearly absent in Mg-Ca alteration zones but commonly present in Al-Fe alteration zone along with zincian ilmenite. Zn-bearing biotite in intimate association with zincian spinel is generally found in Mg-Ca alteration zone and in the transition to Al-Fe alteration zone. Most of these indicator minerals can be considered as products of desulphidation of sphalerite during metamorphism. Mechanisms other than desulphidation like formation of gahnite by overstepping of the zinc saturation limit of biotite during retrogression to chlorite and formation of zincian staurolite at the expense of gahnite is also recorded. Field presence of these minerals has immense significance in exploration in Betul Belt as they occur in close spatial relationship with the sulphide rich zones and therefore act as direct vectors to ore.

Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden

The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphide-quartz vein system with high gold grades was mined in the second half of the 1990′s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.

Alteration of Felsic Volcanics Hosting the Thalanga Massive Sulfide Deposit (Northern Queensland, Australia) and Geochemical Proximity Indicators to Ore

Thalanga is an Early Ordovician, stratiform Zn-Pb-Cu-Ag-rich massive sulfide deposit in northern Queensland (Australia) with a total resource of 6.6 Mt of ore. The host-rock succession consists of variably altered rhyolite in the footwall and a dacite-dominated volcano-sedimentary sequence in the hanging wall. The sulfide deposit and the enclosing volcanic sequence were metamorphosed under upper greenschist conditions. A laterally continuous footwall alteration zone extends beneath the entire deposit (~3,000 m) and to a stratigraphic depth of at least 300 m below the ore lenses. The bulk of this zone is occupied by feldspar-destructive, muscovite-biotite-chlorite-rich, mottled alteration facies with disseminated pyrite, representing the metamorphic equivalent of phyllosilicate-dominated hydrothermal alteration. Discordant zones of intense quartz-pyrite alteration represent the principal fluid pathways during mineralizing hydrothermal activity. Locally, quartz-K feldspar alteration facies exist on the fringes of the system, and calcareous alteration and chlorite-pyrite alteration facies exist in the upper part of footwall rhyolite, proximal to sulfide lenses. Quartz-feldspar-porphyritic rhyolite laterally surrounding the footwall alteration zone and feldspar-porphyritic dacite in the hanging wall are unaltered or weakly altered. The compositional diversity of altered footwall rhyolite implies that hydrothermal alteration at Thalanga was a complex processes. This paper presents a model for the evolution of the footwall alteration zone. Calcareous alteration probably represents the initial phase of hydrothermal activity. This was followed by diffuse upwelling of acidic, seawater-dominated fluids causing destruction of primary feldspar, precipitation of pyrite, and formation of hydrothermal sericite, chlorite, and clay minerals. Subsequent, intense quartz-pyrite alteration was directly associated with mineralization. The Thalanga footwall alteration zone has several geochemical characteristics that show systematic changes with increasing proximity to ore. These include Na depletion and elevated Mg, S, alteration index (AI), chlorite-carbonate-pyrite index (CCPI), Mo, Bi, and As. Furthermore, the X Mg of chlorite and biotite increases systematically from values of 0.45 to 0.5 in least altered rhyolite to values >0.9 immediately below ore. These geochemical features can be used in exploration for massive sulfide deposits as vectors to ore, at the prospect scale, and in discrimination of prospective hydrothermal from unprospective diagenetic alteration systems, at the regional scale.

Geology, Genesis, and Exploration Implications of the Footwall and Hanging-Wall Alteration Associated with the Hellyer Volcanic-Hosted Massive Sulfide Deposit, Tasmania, Australia

Hellyer is a large (16.2 million metric tons), high-grade (13.9% Zn, 7.1% Pb, 0.4% Cu, 168 g/t Ag, 2.5 g/t Au), sea-floor, mound-style, polymetallic volcanic-hosted massive sulfide (VHMS) deposit located in the Mount Read Volcanics of western Tasmania. The deposit is hosted by the Que-Hellyer Volcanics, a sequence of late Middle Cambrian mafic to felsic coherent volcanics and polymict volcaniclastics. Hydrothermal alteration occurs in the regional footwall, immediate footwall, and hanging wall. Alteration in the regional footwall is confined to patchy quartz, albite, and chlorite, with minor sericite, epidote, and hematite. Underlying Hellyer is a zoned alteration pipe with a central siliceous core (quartz-sericite), which passes into zones of chlorite, chlorite-carbonate, sericite-chlorite, and finally sericite-quartz (stringer envelope zone) on the margin. Overlying the central part of the deposit, within the hanging-wall basalt, is a distinctive and zoned alteration plume. Five alteration zones have been identified: fuchsite, chlorite, carbonate, quartzalbite, and sericite. Fuchsite-dominated alteration occupies the central portion of the hanging-wall alteration plume. Chlorite and carbonate alteration surrounds the fuchsite zone with carbonate zones forming near to the ore deposit and chlorite zones extending above and lateral to the carbonate. Outward is quartz-ablite alteration, which extends laterally into distal sericite alteration. Mass-change calculations for the footwall and hanging wall indicate that, in general, the footwall alteration zones are depleted in CaO, Na2O, La, Sr, Ni, Cr, and V but have enrichments of Fe2O3, MnO, MgO, K2O, S, and most metals. Compared to the host basalt, the hanging-wall alteration has gained CaO, K2O, Na2O, CO2, S, in Rb, Ba, Ag, As, Mo, Sb, Cs, and Tl, while Fe2O3, MnO, MgO, P2O5, La, Sr, Pb, Zn, Th, U, Cd, and Nd are depleted. CaO, Na2O, Cr, V, and Ni are depleted in the footwall andesite but enriched in the hanging-wall alteration plume. This relationship suggests that these elements were sourced from the breakdown of feldspars, pyroxenes, and andesitic groundmass of the footwall lithologies and transported in the hydrothermal fluid into the overlying basalt and precipitated as albite, calcite, and white micas in the hanging-wall alteration. The development of alteration associated with the Hellyer VHMS deposit occurred in three stages. Stage 1 regional footwall alteration was formed by unfocused hydrothermal convection of seawater down into the recently deposited volcanic pile at temperatures between approximately 250 degrees celsius and 200 degrees celsius and at low to moderate water/rock ratios. Stage 2 alteration formed by structurally controlled fluid flow from a deep intensifying hydrothermal convection system and created the footwall alteration pipe. Decreasing water/rock ratios and temperatures, over a range of 350 degrees celsius to less than 200 degrees celsius, led to the development of the concentric alteration pipe mineral zones. Based on modeling of whole-rock d18Ofr values, geochemical modeling, and mineral assemblages, the siliceous core is interpreted to have formed at temperatures near 350 degrees celsius, the chlorite-rich alteration zone at 300 degrees celsius to 250 degrees celsius, and the outer sericite-rich alteration at temperatures of 250 degrees celsius to 150 degrees celsius. The final, synmineralization fluid-flow event, stage 3, created the hanging-wall alteration plume. After rapid burial of the deposit by basalt, continuation of upward hydrothermal fluid flow created the zoned hanging-wall alteration. Distribution of hanging-wall alteration assemblages suggests a temperature gradient from approximately 250 degrees celsius for the fuchsite zone to lower temperatures (150 degrees celsius) for the distal quartz-albite and sericite alteration zones. Alteration mineralogy, mineral chemistry, lithogeochemistry, and stable isotope characteristics of the footwall and hanging-wall alteration have been combined into a comprehensive set of vectors, which can be used in exploration for VHMS deposits in similar geologic settings.

Short Wavelength Infrared (SWIR) Spectral Analysis ofHydrothermal Alteration Zones Associated with Base Metal Sulfide Depositsat Rosebery and Western Tharsis, Tasmania, and Highway-Reward, Queensland

Spectral analysis of rock samples by short wavelength infrared (SWIR) analysis, using the portable infrared mineral analyzer (PIMA) spectrometer, is a relatively new field method with applications in mineral exploration. The technique allows rapid field-based identification of hydrothermal alteration minerals and delineation of alteration zones. Variations in mineral composition, crystallinity, and relative abundance may also be determined. Phyllosilicates, including white mica and chlorite, are particularly amenable to short wavelength infrared analysis. These are prominent alteration minerals in the felsic volcanic rocks in the Mount Read Volcanics, Tasmania, and the Mount Windsor subprovince, North Queensland. They have regional distributions as diagenetic and metamorphic products and exist locally in alteration zones associated with volcanic-hosted sulfide deposits. A geochemical and spectral investigation of white micas in the least altered rocks in the Mount Read Volcanics has shown considerable background compositional variations. The white micas range from phengite to moderately sodic muscovite and SWIR spectral measurements provide reliable estimates of their compositions. Chlorites also have a broad compositional range in these rocks. However, SWIR spectral analysis has proven to be unreliable in determining chlorite composition in samples where chlorite exists in low proportions relative to white mica or in mixed assemblages with epidote. White micas in footwall alteration zones of the Rosebery stratiform Zn + Pb deposit (Tasmania) are variably phengitic, similar to those in regional background volcanics. A unit of relatively unaltered volcaniclastic sandstone in the Rosebery hanging-wall sequence contains anomalous sodic muscovite in proximity to ore lenses. White micas associated with the disseminated to massive pyritic Cu + Au deposits at Western Tharsis (Tasmania) and Highway-Reward (North Queensland) range in composition from phengite to slightly sodic muscovite. In these symmetrically zoned alteration systems, white micas grade in composition from phengite, in peripheral zones, to slightly sodic muscovite, in proximal zones near ore. In all three cases, the white mica compositional variation is a vector to ore that is effectively measured by SWIR spectrometry, with particular application in prospect-scale exploration. Chlorite compositions at Rosebery are not strictly spatially related to ore but tend to be most magnesian in zones of high chlorite abundance. In the outer alteration zone at Western Tharsis, there is a faint proximal trend to iron-rich chlorites. However, it does not represent a well-defined vector to ore in either case. Attempts at quantitative estimation of the relative abundance of white mica and chlorite in mixed samples, by spectral analysis, have produced results that are imprecise in comparison to geochemically derived estimates. The data scatter relates to the low proportions of chlorite in the majority of the samples analyzed, resulting in weak chlorite absorptions. Better results may be obtainable, in specific alteration or lithostratigraphic domains, by use of customized spectral unmixing software. This investigation has shown that PIMA-based SWIR spectrometry is a potent field method of mapping compositional variations in white micas that represent empirical vectors to at least some types of volcanic-hosted sulfide deposits. It has also highlighted its effectiveness in field identification of megascopically difficult or indeterminate alteration minerals, such as pyrophyllite, zunyite, and topaz, which may have genetic implications and exploration significance.

The Spectrum of Ore Deposit Types, Volcanic Environments, Alteration Halos, and Related Exploration Vectors in Submarine Volcanic Successions: Some Examples from Australia

Variations in shape, metal content, alteration mineralogy, and volcanic host rocks of the ore deposits in the two major volcanic-hosted massive sulfide (VHMS) districts of eastern Australia, the Cambrian Mount Read Volcanics and the Cambro-Ordovician Mount Windsor subprovince, strongly reflect their volcanic environment, conditions of ore formation, and hydrothermal alteration processes. Lens and sheet-style polymetallic zinc-rich deposits such as Rosebery, Hellyer, Que River, and Thalanga are considered to have formed in moderate to relatively deep water environments (500-1,000+m). These deposits probably formed either on the sea floor (e.g., Hellyer, Que River) or by replacement of porous volcaniclastic units directly below the sea floor (e.g., Rosebery). The footwall alteration associated with these polymetallic VHMS deposits was controlled by host-rock permeability and porosity, which are in turn related to volcanic facies type, degree of fracturing, and synvolcanic structural architecture. Focusing of hydrothermal fluids along synvolcanic structures has resulted in well-zoned chlorite-sericite footwall alteration pipes within footwall lavas at Hellyer. On the other hand, diffuse fluid flow through very thick pumice breccia at Rosebery and Hercules has resulted in strata-bound, sericite-dominated footwall alteration zones parallel to the paleosea floor and the ore lenses. Massive and disseminated, pyritic Cu-Au deposits, such as those in the Mount Lyell field and at Highway-Reward, formed by subsea-floor replacement and are associated with only minor zinc-lead massive sulfide ore. These deposits formed from higher temperature fluids (>300 degrees C), in which copper transport is enhanced, and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions (e.g., Highway-Reward). The Cu-Au ore lenses may be strata-bound (e.g., Mount Lyell) or crosscutting pipes (e.g., Highway-Reward) depending on the structure and permeability characteristics of the felsic volcanic host rocks. The presence of high-sulfidation alteration minerals (e.g., pyrophyllite, zunyite) in some of the Cu-Au deposits (e.g., Mount Lyell field) indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system. Alteration zonation associated with the Cu-Au VHMS deposits is more symmetrical than that of the Zn-rich deposits, with sericite-rich alteration extending into the hanging wall, in keeping with the subsurface replacement origin of these deposits. Synvolcanic gold-rich deposits, with high gold/base metal ratios are less common than the Cu-Au and Zn-rich VHMS ore types. The gold-rich ores (e.g., Henty, South Hercules) are strata bound in nature, have low sulfide contents, and are associated with central zones of intense silicification, surrounded by envelopes of sericite-pyrite and carbonate alteration. Volcanological and geochemical studies at Henty indicate the gold-rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies, lavas, and limestones. This spectrum of Cu-Au, Zn-rich, and Au-only deposits in the Mount Read Volcanics and the Mount Windsor subprovince is interpreted to represent a continuum from classic sea-floor VHMS ores toward those with features more akin to porphyry Cu-Au and epithermal Au-Ag deposits. This spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows: (1) proportions of volcaniclastic, lava, and subvolcanic intrusive facies; (2) depth of seawater; (3) permeability and porosity of volcanic host rocks; (4) balance between magmatic components and seawater components in the ore fluid; and (5) temperature and acidity of the ore fluid. Mineralogical, lithogeochemical, and isotopic studies have revealed a range of alteration vectors useful in exploration for both the Zn-rich and Cu-Au VHMS deposits. Carbonate and white mica compositional variations are highlighted as important mineralogical vectors; thallium and antimony halos may be useful trace element vectors; and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system.

Geochemical exploration using acid insoluble residues of rocks for volcanic-hosted massive sulphide deposits, Rosebery area, western Tasmania

The Rosebery deposit is one of the major massive sulphide deposits in the Mount Read Volcanic belt of the western Tasmania. The 200×20 km belt of the Mount Read Volcanics, which is one of the world's richest provinces of volcanic-hosted massive sulphide (VHMS) deposits, hosts more than 40 mineral occurrences including major massive sulphide deposits at Mount Lyell, Hercules, Rosebery, Que River and Hellyer. In this study, hot HNO 3 insoluble residues of rocks were used to define the geochemical halos associated with VHMS deposits, and to identify alteration related to VHMS mineralisation. The residues after the acid treatment are composed mainly of quartz, mica and feldspar (plagioclase and K-feldspar) with traces of chlorite and kaolinite in some samples. Quartz, sericite, chlorite, carbonate, etc., are the principal alteration products and relate to both VHMS mineralisation and to metamorphism and deformation in the region. There are no significant differences between the VHMS related and unrelated mineralogical alteration. There are two types of geochemical halos in the Rosebery area. The type-1 halo trends NE–SW, is related to wall-rock alteration, and is defined by enrichment in Cl, and possibly K and Rb, and depletion in Al, Ca, Na, Ti and Sr. It passes discordantly from the west to east across the White Spur Formation, the altered footwall, the host rock of the Rosebery deposit, the hanging wall and the Mount Black Volcanics. The type-2 halo, trending N–S (largely stratabound), is related to massive sulphide mineralisation and is characterised by enrichment in Fe, Mn, Ba, Zn, Pb and possibly K, Rb and F. It outlines the mineralised host rock and footwall alteration zone, and also extends toward the south into the unaltered footwall and hanging wall rocks. Both halos intersect at Rosebery in zones of mineralisation and associated footwall alteration. Recognition of geochemical halos has led to formulation of geochemical indices for identification of alteration related to VHMS mineralisation. Binary relations between (Mn × Ba × F) and (Ca × Na × Sr), and (Mn × Ba) and (Na × Sr), and their ratios (Mn × Ba × F)/(Ca × Na × Sr) and (Mn × Ba)/(Na × Sr) identify the alteration related to massive sulphide mineralisation and distinguish it from alteration unrelated to the mineralisation in the Rosebery area. These geochemical indices are also used for the other deposits which all display similar geochemical alteration signatures. The alteration related to VHMS mineralisation can be distinguished from alteration unrelated to mineralisation by higher values of the geochemical indices of (K × Mn × Ba × Rb), (Mn × Ba × F) and (Mn × Ba).

Electron paramagnetic resonance (EPR) spectroscopy in massive sulphide exploration, Rosebery mine area, western Tasmania, Australia

Electron paramagnetic resonance (EPR) spectroscopy of hot HNO3 insoluble residues of rock powders is used as a new exploration technique for the volcanic-hosted massive sulphide (VHMS) deposit in the Rosebery mine area. The EPR signal intensities measured in 326.5±5 mT sweeps are strong in the altered rocks, and show a negative correlation with Ca, Na and Sr, and a positive correlation with K/Na, Rb/Sr and (K × Rb)/(Ca × Na × Sr). The EPR intensities measured in 326.5±100 mT sweeps show high values in the footwall pyroclastics, host rocks and hanging wall pyroclastics near and around the Rosebery deposit, and correlate positively with K, Fe, Mn, Ba, F, Rb, Zn, Pb and Zr. The Rosebery deposit and associated footwall alteration zone are located at the intersection of two elongated paramagnetic halos. The first is characterized by strong intensities of [AlO4]° signals measured at magnetic flux density sweeps over 326.5±5 mT, trends NE–SW, and passes discordantly from the west to the east the White Spur Formation, altered footwall (footwall alteration zone), host rock of the Rosebery deposit, hanging wall and Mount Black Volcanics. The second, largely stratabound, halo is defined by strong intensities of Mn2+ sextets observed at magnetic flux density sweeps over 326.5±100 mT, runs N–S following the stratigraphic trend, and outlines the mineralized host rock and footwall alteration zone. It also extends toward the south into the unaltered footwall and hanging wall rocks. The first type of halo is considered to be related to wall rock alteration due to the VHMS mineralization processes as well to later Devonian metamorphism, and the second is thought to be related to massive sulphide mineralization alone.

Zincian staurolite in the Dry River South volcanic-hosted massive sulfide deposit, northern Queensland, Australia: an assessment of its usefulness in exploration

Staurolite from the Dry River South volcanic-hosted massive sulfide deposit in northern Queensland, Australia is enriched in ZnO (2.5–6.8%) only within the massive sulfide lens or in highly pyritic biotite-chlorite schist just below the massive sulfide lens. In the footwall alteration zone, staurolite ZnO levels vary between 0.4 and 1.3%, whereas in hanging wall metagraywackes and metapelites, the ZnO content is mainly below 1.0%. As staurolite from metapelitec rocks contains up to 7.9% ZnO, high ZnO levels in staurolite do not necessarily indicate a relationship to Zn-rich massive sulfide. Staurolite grains from the Dry River South amd other massive sulfide lenses have low TiO 2 concentrations (mainly <0.4%) relative to staurolite grains from metasediments and alteration zones (mainly >0.4%). The low concentration of TiO 2 in staurolite from the massive sulfide lens results from the low initial Ti concentration in exhalative ores. High ZnO and low TiO 2 values are indicative of staurolite associated with Zn-rich massive sulfide. Zincian staurolite is a potential exploration indicator at both prospect and reconnaissance scales. At the prospect scale, zinc levels have potential in distinguishing true from false gossans, distinguishing low grade portions of massive sulfide lenses from barren massive pyrite bodies, and characterizing alteration zones. At the reconnaissance scale, staurolite can be collected in the heavy mineral fraction of stream sediments, and multiple grains can be analyzed rapidly using modern electron microprobes.

Waterloo and Agincourt prospects, northern Queensland: Contrasting styles of mineralization within the same volcanogenic hydrothermal system

The Waterloo and Agincourt prospects are two small volcanic‐hosted massive sulphide deposits that occur close to the stratigraphic top of a large alteration zone within the Trooper Creek Formation of the Cambro‐Ordovician Seventy Mile Range Group. Although only 1 km apart, the two deposits show differences in the style and mineralogy of mineralized zones in terms of metal contents, ratios and correlations, and sulphur isotope systematics. The Waterloo deposit is a small, high‐grade zinc‐copper‐rich massive sulphide lens, whereas the Agincourt deposit consists of low‐ to moderate‐grade disseminated zinc and lead through much of the alteration zone with only minor baritic sulphide lenses. These differences resulted from higher temperature fluids in the Waterloo deposit, and the swamping of the Agincourt hydrothermal system by felsic volcaniclastic flows. Major ore minerals present at both deposits include pyrite, sphalerite, galena, chalcopyrite and barite. Waterloo has significant quantities of tellurides and bornite, minerals not observed at Agincourt. The Waterloo deposit is characterized by high copper ratios (100Cu/[Cu+Zn+Pb]) and high zinc ratios (lOOZn/ [Zn+Pb]). Agincourt has low copper ratios and more typical zinc ratios. At Waterloo gold correlates with copper, whereas at Agincourt gold correlates with zinc. Alteration of footwall andesitic units at Waterloo is characterized by two principal mineralogical assemblages: (i) weak carbonate‐pyrite‐sericite; and (ii) a strong sericite‐quartz‐pyrite+carbonate alteration. Major and trace element data for relatively unaltered and variably altered andesitic volcanic units indicate that Al, Ti, P, Zr, Nb and the rare earth elements remained essentially immobile during hydrothermal alteration, whereas the most intense sericite‐quartz‐pyrite alteration resulted in strong relative enrichment in K, Rb and Bi, and mared relative depletion in Ca, Na, Sr, Mn and Cu. δ34S values of sulphide minerals from Waterloo and Agincourt are lower than other Cambro‐Ordovician volcanic‐hosted massive sulphide deposits, and the values increase from the footwall alteration zone, into the massive sulphide/barite lenses. These characteristics are most likely caused by sulphide deposition from oxidized hydrothermal fluids.

Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

AbstractTransmission electron microscopy (TEM) was utilized to determine the origins of berthierine and chlorite in the core of the footwall alteration zone of the Kidd Creek massive sulfide deposit, Ontario. TEM images show lamellar intergrowths of packets of berthierine, mixed-layer chlorite/berthierine, Fe-Mg chlorite, and relatively Fe-rich chlorite that contain dislocations, stacking faults, kink bands, and gliding along (001). Interstratification of packets of berthierine and chlorite with one to several tens of layers commonly is associated with terminations of a layer of chlorite by two layers of berthierine. Layers in adjacent domains of berthierine and chlorite are continuous across interfaces that transect their common {001} planes. High-strain zones that cut across cleavage planes, consisting of distorted layers and lens-shaped pores, are associated with stacking faults and gliding along cleavage planes in chlorite crystals. Similar features separate interstratified chlorite/berthierine of different structures and textures, implying development of such composite grains after deformation of chlorite. Electron diffraction patterns show that the chlorite is an ordered one- or two-layer polytype or a one-layer polytype with semi-random stacking, and that the berthierine is a one-layer polytype with semi-random stacking epitaxially intergrown with chlorite.Coexisting chlorite and berthierine have nearly identical ranges of compositions, containing Si ≅ 5, Al ≅ 6, and Fe ≅ 6.5–8.5 pfu, and minor, variable Mg and Mn contents, in formulae normalized on the basis of 20 total cations. This implies polymorphism among Fe,Al-rich members of the serpentine and chlorite groups. In one of the samples, berthierine and mixed-layer chlorite/berthierine coexist with chlorite having two compositional ranges, including Fe-rich chlorite with a relatively wide range of Fe-Mg contents, and a dominant Fe-Mg chlorite. In another sample, compositionally homogeneous Fe-rich chlorite is associated with berthierine and mixed-layer chlorite/berthierine; Fe-Mg chlorite was not detected.The microstructural relations and the presence of coexisting polymorphs, complex mixed layering, heterogeneous polytypism, and wide ranges of mineral compositions are consistent with replacement of chlorite by berthierine under non-equilibrium retrograde conditions, in contrast to the generally assumed prograde origin for other berthierine occurrences.

Geologic and geochemical controls on gold-rich stringer mineralization in the Que River Deposit, Tasmania

The Que River mine in western Tasmania was developed on a small (3.3 million metric tons) but high-grade (>20 wt % Zn + Pb) polymetallic massive sulfide deposit hosted by Cambrian volcanics and volcaniclastic sediments of the Mount Read Volcanics. The massive ore is surrounded by an extensive zone of low-grade stringer vein and disseminated mineralization. This stringer zone formed during hydrothermal alteration of andesitic lavas and related volcaniclastics in the stratigraphic footwall. The hydrothermal alteration mineral assemblage (sericite-silica-pyrite + or - base metal sulfides + or - potash feldspar) survived subsequent lower greenschist-grade regional metamorphism.Part of the stringer zone has elevated Au (up to about 5 ppm), Sb (up to 1,000 ppm), and Ba (up to 2,000 ppm) values. Much of the gold-enriched stringer zone occurs in a coarse polymictic volcaniclastic containing distinctive millimeter-size patches of cream-white alteration. In thin section the cream-white patches are seen to comprise intergrowths of coarse white mica and base metal sulfide. The gold-enriched zone is also characterized by the presence of solid inclusion-rich potash feldspar not observed elsewhere in the stringer zone.The entire stringer zone is crudely zoned with a Cu-bearing core beneath the central part of the massive orebody (Large et al., 1988) surrounded by a Pb-Zn-bearing stringer mineralization which merges with a distal Pb-Zn-Ag-Au-As-Sb-Ba stringer mineralization in the gold-enriched zone. Sulfur isotope signatures show a progression from lightest values in mineral separates from massive ore (delta 34 S pyrite ca. 8ppm) to heaviest values in gold-enriched zone samples (delta 34 S pyrite up to 17ppm).The gold-enriched stringer zone is interpreted to be the more distal part of the footwall alteration zone developed during formation of the main Que River massive sulfide orebody. Best development of this distal gold-bearing stringer mineralization occurred in the more permeable coarse volcaniclastics. It is interpreted that the Au was transported as a bisulfide complex and precipitated in the outer portions of the stringer when fluids were oxidized and cooled during mixing with seawater

Genesis of kaolin minerals and pyrophyllite in Kuroko deposits of Japan: Implications for the origins of the hydrothermal fluids from mineralogical and stable isotope data

Hydrogen and oxygen isotope and X-ray analyses were performed on kaolin minerals (kaolinite, dickite, and nacrite) and pyrophyllite from the Minamishiraoi and Inarizawa deposits of northern Japan. Dickite, nacrite, and mica-rich mica smectite mixed-layer minerals occur as envelopes around barite-sulfide orebodies in the footwall alteration zones of the Minamishiraoi and Inarizawa deposits, while kaolinite and smectite-rich mica smectite mixed-layer minerals occur in the outer alteration zones away from the orebodies. The hydrogen and oxygen isotope ratios of kaolin minerals from the Minamishiraoi and Inarizawa deposits fall in a narrow range of −24 to −32%. for hydrogen and 7.8 to 9.9%. for oxygen. These isotope ratios indicate that the hydrothermal fluid responsible for kaolin formation was the same as that which caused mineralization. The δ 18O values of kaolin minerals of the Minamishiraoi deposit increase in order of nacrite < dickite < kaolinite. This trend indicates that nacrite formed at the highest temperature among the three kaolin polytypes. The oxygen isotope ratio of monoclinic pyrophyllite ( δ 18 O = 5.2%.) from the Inarizawa deposit is lower than that of nacrite ( δ 18 O = 7.5%.). The difference of δ 18O values between the pyrophyllite and nacrite may be explained by a higher temperature of formation for the pyrophyllite. The mineralogie and isotopic data of the footwall alteration halos are useful as indicators of the size and metal contents of Kuroko orebodies. The footwall alteration halo of the Minamishiraoi deposit (small size and barite-rich deposit) contains kaolin and mica smectite mixed-layer minerals, while those of the larger polymetallic Kuroko deposits contain seriate and chlorite. Thermodynamic data of these clay minerals indicate that the Minamishiraoi deposit formed at relatively lower temperature and farther from the heat source than larger polymetallic deposits. The chemical data for hydrothermal solutions from the 13°N East Pacific Rise hydrothermal site, as presented in the literature, plot in the kaolinite stability field at 150°C or less. Therefore, active submarine hydrothermal systems such as 13°N East Pacific Rise should form kaolin minerals in the alteration halo, as long as the temperature of the hydrothermal fluid is below sericite and chlorite stability.

Lithogeochemistry of wainaleka Cu-Zn volcanogenic deposit, Viti Levu, Fiji, and possible applications for exploration in tropical terrains

Abstract A small Kuroko-type Cu-Zn deposit exhibiting metal zoning and alteration assemblages comparable with documented proximal volcanogenic deposits, occurs at the top of a felsic fragmental pile, mantling a large sodic rhyolite domal complex. The domal complex occurs within predominantly mafic to intermediate lavas and volcaniclastic rocks with low-potash island-arc tholeiitic affinities, representing the basal section of the early Tertiary (Eocene to middle Miocene) Wainimala Group near the southern coast of Viti Levu, Fiji. Lithogeochemical trends identified in analyses of rock chip samples from traverses across the domal complex reflect alteration zoning. Sodium, Ca and Sr are strongly depleted within the quartz-sericite foot-wall alteration zone (Zone I) 200 m below mineralization. Potassium, Rb and weaker Mn, Zn and Co depletion and Cu, Pb and Mg enrichment define clay-sericite (Zone II) and clay carbonate (Zone III) footwall alteration 600 to 1200 m below mineralization. Hanging-wall albite-chlorite-calcite-zeolite alteration (Zone IV) is accompanied by enhanced Zn, Pb, Co, Mn, Sr and Na values. Significant mine-scale lithogeochemical trends obtained from systematic sampling of a mineralized borehole section include K and Rb enrichment in the zone of strongest quartz-sericite alteration associated with mineralization and broad depletion of Mn, Na, Ca and Sr within altered footwall fragmental rocks. Minor Cu, Pb, Zn and Ag enrichment has accompanied low-grade propylitic alteration of hanging-wall rocks up to 50 m above mineralization. Analysis of weathered bedrock samples from traverses above the mineralized borehole section indicates that primary geochemical trends occur in the weathered zone. Outcropping gossan has strongly anomalous Cu (535 ppm-21.5%), Zn (3300 ppm-6.15%), Pb (420–8200 ppm), As (200–7000 ppm) and Hg (33–670 ppm) values. Application of lithogeochemistry as a follow-up exploration method in a tropical area such as Wainaleka was investigated as a possible replacement for ridge, spur and base-of-slope soil sampling techniques. Ridge-top auger samples and creek outcrop samples were collected at approximately 100 m intervals and a density of 70/km2. Elements (including Cu, Pb, Zn, Mn, Rb, Sr, Na, K, Ca and Mg) were selected for analysis because of specific associations with mineralization and alteration, and low analytical costs. Single- and multi-element dispersions effectively outline mineralization and attendant alteration.