Sericitic Alteration Research Articles

Evaluation of the role of Cambrian granites in the genesis of world class VHMS deposits in Tasmania

An analysis of the distribution, composition and alteration zonation of Cambrian granites which intrude the Mt Read Volcanics of western Tasmania provides evidence that there may have been a direct input of magmatic fluids containing Fe, Cu, Au and P to form the copper-gold volcanic-hosted massive sulphide (VHMS) mineralisation in the Mt Lyell district. Interpretation of regional gravity and magnetic data indicates that a narrow discontinuous body of Cambrian granite (2–4 km wide) extends along the eastern margin of the Mt Read Volcanic belt for over 60 km. The Cambrian granites are altered magnetite series types which show enrichment in barium and potassium, and contrast markedly with the fractionated ilmenite series Devonian granites related to tin mineralisation elsewhere in the Dundas Trough. Copper mineralisation occurs in a linear zone above the apex of the buried Cambrian granite body at the southern end of the belt, from Mt Darwin to the Mt Lyell district over a strike length of 25 km. Gold and zinc mineralisation are concentrated higher in the volcanic stratigraphy more distant from the granite. Overlapping zones of alteration extend from the granite into the surrounding volcanic rocks. An inner zone of K-feldspar alteration is overprinted by chlorite alteration, which passes outwards into sericite alteration. Magnetite ± pyrite ± chalcopyrite ± apatite mineralisation is concentrated in the chlorite alteration zone as veins and low grade disseminations. The Mt Lyell copper-gold stringer and disseminated mineralisation is hosted in felsic volcanic rocks 1 to 2 km west of the interpreted buried granite position. Magnetite-apatite ± pyrite veins in the Prince Lyell deposit at Mt Lyell are very similar to the veins in the halo of the granite, further south, and provide evidence for magmatic fluid input during the formation of the copper-gold VHMS deposits. A model involving deeply penetrating convective seawater, mixing with a magmatic fluid released from the Cambrian granites, best explains the features of VHMS mineralisation in the Mt Lyell district.

Neodymium and strontium isotope study of the Blue Tier Batholith, NE Tasmania, and its bearing on the origin of tin-bearing alkali feldspar granites

The middle to late Devonian plutons of the Blue Tier and Eddystone Batholiths of NE Tasmania intrude quartz-wacke and mudstone of the Ordovician-Devonian Mathinna Beds. Four granitoid types are recognized: hornblende-biotite granodiorite, biotite granite, biotite-cordierite-garnet granite and alkali feldspar granite (AFG). Minor diorite bodies and dolerite dykes are also present. Tin (+ W) mineralisation is genetically associated with the AFG. Major granitoid types (other than AFG) of the Blue Tier and Eddystone Batholiths all have similar 143Nd/ 144Nd initial ratios (ϵNd = − 5.0 to − 6.5) at Ma. More fractionated felsic samples within individual plutons have lower ϵNd values, suggesting progressive involvement (up to about 5%) of the sedimentary country rocks which have ϵNd of about − 11 at 370 Ma. The source-rock model age for all granitoids (except AFG), based on a depleted mantle evolution model (T DM), is about 1.6 Ga. On the basis of geochemical data, 87Sr/ 86Sr initial ratios (0.707-0.713), oxygen isotope ratios (δ 180 = + 9 to + 12), and Rb/Sr and K/Ar isotopic age dates (368 to 388 Ma), the major plutons were interpreted to have been generated independently from heterogeneous crustal sources. Geochemical variation within each major granitoid type, with the possible exception of the AFG, is due to fractional crystallisation accompanied by some country rock assimilation. Integrated Nd, Sr and O isotopic systematics of the Blue Tier Batholith of the Bassian Terrain are different from granite batholiths in the mainland of the Lachlan Fold Belt. Similarity of isotopic systematics of the Blue Tier Batholith to those of Devonian granites in central Victoria reinforces the geological correlation between western Tasmania and South Australia in the early Paleozoic. Two competing hypotheses for the generation of SnW mineralized AFG have been proposed on the basis of geological relationships, chemical and mineralogical trends and stable and radiogenic isotopes. The first hypothesis (Groves et al., 1977; Higgins et al., 1985; Higgins, 1990) links the AFG with the country rock, more mafic Poimena Granite, through processes of fractional crystallisation and subsequent metasomatic modification. The second hypothesis suggests that these AFG are unrelated to the Poimena Granite; they are specialised granites, derived from a different parent with distinctive chemical-isotopic systematics (Sun et al., 1986; Mackenzie, 1986; Mackenzie et al., 1988, 1990). According to the first hypothesis, the AFG exhibit a range in ϵNd initial values from − 5.9 to − 2.2. However, there is no known isotopic mechanism which can produce this range within the geological time frame of intrusion and solidification. At this time the authors favour the second hypothesis because of chemical and isotopic correlations. On the basis of a distinct discontinuity between Poimena and AFG on a Y versus Ga/Al plot there is also a clear grouping of initial ϵNd for the Poimena Granite (around −6), and the AFG (− 2.4). It is argued that those few “AFG” having much lower ϵNd than − 2.4 are either samples of altered Poimena Granite (two samples with ϵNd = − 6.8 and − 9.1) or a result of extensive metasomatic modification (one sample, ϵNd= − 5.9) involving circulation of fluids in equilibrium with the country rocks (Poimena Granite initial ϵNd = − 6 and Mathinna Beds = − 11). Hydrothermal minerals from the Anchor tin deposit show a range in initial ϵNd and 87Sr/ 86Sr correlated with paragenesis: apatite formed during albitisation of the Lottah Granite has Nd and Sr isotopic compositions intermediate between those of the Lottah and Poimena Granites. Greisen fluorites have initial ϵNd between − 4.0 and − 4.6, closer to the Lottah average composition, while initial 87Sr/ 86Sr approaches that of the metasedimentary country rock (0.729). Fluorites formed during sericitic alteration have lower ϵNd compositions (< −6.1) and high 87Sr/ 86Sr (> 0.721). These data support an origin of tin mineralisation through mixing of an AFG-derived magmatic fluid with a externally-derived meteoric-groundwater dominated fluid.

U-Pb and Rb-Sr isotopic systematics of fluids associated with mineralization of the Dartmoor granite, southwest England

Quartz veins from early hydrothermal mineralization within the 280 Ma Dartmoor granite contain abundant fluid inclusions, which were sampled using crush-leach techniques and analyzed for Rb, Sr, U, and Pb content and isotopic ratios. The present-day lead isotopic signature of some of the crushleach fluids is similar to that of granitic alkali feldspars from Dartmoor and other localities in southwest England, although some fluid samples contain a more radiogenic Pb component. The more radiogenic fluids fell into, or near, the granitic K-feldspar field upon correction of the present-day fluid Pb ratios for in situ U decay over ca. 280 Ma, but the lead isotope ratios of the less-radiogenic fluids became significantly lower than that of the granitic K-feldspars. The apparent overcorrection may be real, or may reflect either fluid mixing due to leaching of secondary fluid inclusions, differential leaching of U and Pb, the presence of an undetermined U-rich included phase (or fracture-filling) within the quartz, or a combination of the above. The variation of fluid U/Pb ratios also suggests some complications regarding either the selective retention of U and Pb on quartz surfaces, or the presence of submicroscopic U-rich inclusions. Therefore, we suggest that in situ decay corrections to lead isotopic data obtained using crush-leach techniques be applied with great caution. The fluid 87Rb/ 86Sr values vary little (1.40–1.74), as do their present-day strontium isotope signatures ( 87Rb/ 86Sr = 0.71814–0.71968). When corrected for in situ 87Rb decay over 280 Ma, the fluids contain significant excess radiogenic Sr (Sr i = 0.7118–0.7141), relative to the Dartmoor granite S i (=0.7101). If the Sr in the fluids is derived exclusively from the granite, or some granite-related source, and closedsystem evolution of the Rb-Sr isotopic system is assumed, the amount of excess 87Sr is a function of the time difference between Sr closure in the granite and Sr closure in the vein. Age estimates from the intersection of granite and fluid inclusion Sr evolution curves are in agreement with published 40Ar- 39Ar ages of late dikes and sericitic alteration at Birch Tor-Vitifer. Our results further suggest that the fluids related to quartz-tourmaline-cassiterite mineralization within the Dartmoor granite are themselves related to granite-derived, late-stage aplitic melts.

Structure, vein paragenesis, and alteration in the Al Wajh gold district, Saudi Arabia

The Al Wajh gold district contains small deposits of gold-bearing quartz veins located in sheared and altered Neoproterozoic mafic lavas and volcaniclastic sandstone and siltstone. The veins formed during multiple episodes of deformation and have structural and mineralogic features characteristic of mesothermal, low sulfide, gold-bearing quartz veins. Three early deformation episodes (D 1 , D 2 , D 3 ) are interpreted as progressive phases of a major deformation event that culminated about 660 Ma; episode D 4 makes up part of a later event about 620 Ma. The bulk of the gold-bearing veins in the district are located in D 2 and D 3 structures. D 1 veins consist of thin quartz veins in phyllite. D 2 veins are located in throughgoing, steeply dipping sinistral shear zones and in the crests of folds along the shears. Da veins, typically associated with carbonate alteration, are concentrated in small thrust faults that in places modify earlier D 2 shears. Umm al Qurayyat, the largest deposit in the district (155,634 metric tons averaging 3.8 g/t gold), comprises shallowly to moderately steeply dipping sheeted quartz veins in the hanging wall of a D 3 thrust in basaltic andesite and dacite. It is centered on an inner sericite alteration zone and an outer orange-weathering carbonate alteration zone. Structural evidence indicates that sericite alteration was contemporaneous with D 2 deformation, predating the main quartz veining event, whereas carbonate alteration was contemporary with D 3 thrusting and emplacement of the deposit's principal gold-bearing quartz veins. Quartz at Umm al Qurayyat contains an average of 3.1 g/t gold and sericite-altered wall rock contains an average of 3.9 g/t. Basaltic country rock outside the deposit contains as much as 1 g/t gold, the higher grades occurring adjacent to the prospect.

Mount Rawdon, Southeast Queensland, Australia; a diatreme-hosted gold-silver deposit

A high tonnage-low grade gold deposit at Mount Rawdon in southeastern Queensland is hosted by a diatreme (1.5 x 1.3 km in dimension) emplaced into metasediments of the Carboniferous Curtis Island Group and bodies of Briggs Creek Granodiorite immediately north of an east-northeast-trending fault. Andesites, trachytes and rhyolites of the Late Triassic Arabanga Volcanic Group outcrop less than 500 m west of the diatreme. Swarms of trachyte and rhyolite dikes crosscut the diatreme, one forming a ring dike along the contact of the diatreme and host metasediments. The diatreme is filled by polymict, moderately to poorly sorted, matrix-supported fragmental units dominated- by clasts of variably altered dacite porphyry, basement metasediments, and granodiorite clasts. The fragmental units are gradational to marginal breccia which forms a steeply dipping pipelike structure generated by emplacement of a body of dacite porphyry. The diatreme is intruded by a central dacitic plug and numerous smaller dacite plugs and dikes. Premineralization alteration within the diatreme is associated with intrusions and consists of phyllic and propylitic assemblages generated by dacite porphyries, crosscut by trachyandesites with propylitic alteration and trachytes with weak illite-smectite alteration. Sericitic alteration is centered on a sparsely porphyritic dacite plug forming an apophysis to the central dacite and overprints propylitic alteration in it and in the fragmental units. Sericitic alteration is overprinted by silica-pyrite and chlorite alterations. The chloritic alteration is intimately associated with mineralization. Mineralization postdates dacitic and trachyandesitic intrusions and occurs as disseminations in the matrix of fragmental units, as replacement patches and stringers. Three stages of mineralization are recognized: stage 1, which is volumetrically the most significant, contains pyrite, arsenopyrite, sphalerite, galena, native...

Borosilicate alteration associated with U-Mo-Zn and Ag-Au-Zn deposits in volcanic rocks

Borosilicate alteration is developed in many uranium-molybdenum-zinc deposits in volcanic piles such as at Novazza (Italy), Novoveska Huta (Slovakia), Ben Lomond (Queensland, Australia), the Saar-Nahe basin (Germany), the Shengyuan district (China), and at many deposits in Russia. These uranium-molybdenum-zinc deposits share similar styles of alteration with silver-gold-zinc deposits in volcanic rocks, like those of the Humboldt Range (Nevada, USA), the Basin district (Montana, USA) and Equity Mine (British Columbia, Canada). In thick volcanic piles, fluid-rock interaction commonly develops under the influence of convective hydrothermal systems. In borosilicate-bearing systems the alteration is characterized by changes in borosilicate mineralogy and chemistry under mesothermal conditions. Zonal distribution of different compositions of tourmaline can be observed including regular increases of alkali deficiency in the X(3a) site from peripheral (chlorite-epidote-prehnite) to internal (sericite and/or pyrophyllite) alteration zones. In the central part of these systems tourmaline is absent and the alteration assemblage is characterized by an andalusitedumortierite-quartz association. Such changes in mineral assemblage are related to an increase in temperature from 250 to 350–400 °C and to decreases in pH and Eh, from the outer toward the inner parts of the hydrothermal system. The zonation of the borosilicate-bearing alteration systems is similar to that of aluminosilicate-bearing hydrothermal systems (muscovite, pyrophyllite, andalusite) developed in volcanic rocks. A kaolinite-alunitejasperoid assemblage is generally associated with the borosilicate alteration, and seems related to a late hydrothermal phase mainly located in graben structures.

The nature and origin of a granitoid-related gold deposit at Dargue's Reef, Major's Creek, New South Wales

Dargue9s Reef is a mesothermal, gold sulfide deposit developed in altered granitoid rocks of the Braidwood Granodiorite in southeastern New South Wales, Australia. Two main lodes consist of narrow zones (0.6-9.5 m wide) of intense sericitic alteration and pyritization (15-30% pyrite), enclosed in areas of propylitic alteration. Deposition of barren euhedral-subhedral pyrite accompanied early-stage alteration and was followed by deposition of irregular pyrite containing numerous small inclusions of silicates, calcite, chalcopyrite, Bi sulfo- salts, galena, gold, trace tellurides, native bismuth, and pyrrhotite. Separate aggregates of chalcopyrite, Bi sulfosalts, and tetrahedrite are intergrown with the silicate alteration minerals. Fluid inclusion data indicate that gold-bearing hydrothermal fluids were CO 2 bearing, of low to moderate salinity, and medium to low temperature ( 34 S, -0.4 to--3.4%) and disseminated pyrite in the host granodiorite (delta 34 S, 1..4-2.5%) are consistent with a magmatic sulfur source. Carbon isotope data for calcites from the mineralization indicate delta 13 C values for the fluid close to 0 per mil. These are not inconsistent with a magmatic C source, but they are also consistent with derivation of CO 2 from minor limestones in the country rocks. Textural and fluid inclusion evidence suggest that CO 2 activity in the fluids increased with time, which could be explained by late-stage introduction of country-rock fluids or possibly influx of fluid from a later intrusion at depth. Oxygen isotope data for calcites from the ores (6.5-10.9%) and late-stage veinlets (6.5%) implicate fluids similar to those in porphyry-mineralizing systems...

A least-squares approach to mass transport calculations using the isocon method

A statistically rigorous approach based on the isocon method (Grant, 1986) to evaluate mass transport is presented. Chemical analyses of multiple samples of unaltered (parent) and altered rock are used to calculate the average oxide or element concentration and its standard deviation for each rock population. In a typical application, the uncertainties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty. An efficient algorithm is presented to select the immobile elements by identifying the maximum number of elements that are, within their uncertainties, compatible with the same isocon. The actual isocon is constructed by forcing a line through the origin and optimizing its slope using a weighted least-squares procedure for the selected immobile elements. Thus, individual uncertainties for each immobile element are taken into account. As an illustration, the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the Bingham porphyry copper system. The surprising result indicates that the apparent increase of potassic alteration phases is a consequence of acid leaching of calcium and sodium. No mass transport of potassium is indicated. Silica, water, and sulfur were added during alteration. The best fit isocon requires a small mass increase during each alteration process which, however, is within the uncertainty obtained for the slope of the isocon. A software package is available from the authors in the form of a FORTRAN 77 source code.

Geophysical Exploration for Gold in Deeply Weathered Terrains; Two Tropical Cases

Basic geophysical techniques were used to characterize two gold deposits that lie within tropical-to-subtropical climates. These two deposits are the Posse deposit, which lies within the Mara Rosa Greenstone Belt of central Brazil and the Syama Mine which lies within the Bago, Greenstone Belt of southeastern Mali. Both deposits were discovered by geo-chemical techniques but subsequent magnetic and induced polarization (IP) surveys were used to identify rock types and basic structures and contacts while VLF-EM was used to define more detailed structures associated with both ore deposits. The radiometric method was used at Syama to map the surface expression of sericitic alteration. The success of geophysics in both areas led to an extension of those surveys along strike. The results were used to assist in the ranking of geochemical anomalies.The desire for additional oxide reserves at Syama and the success of the ground geophysical surveys led to an airborne magnetic and radiometric survey over the mine. The results confirmed the usefulness of the two methods for mapping geology and structure in greenstone belts of West Africa. Additionally, the results were used to identify and rank prospective oxide reserve targets. Initial follow-up soil auguring was successful in identifying significant gold reserves but the success rate declined as the lesser-ranked targets were tested. Potassium concentrations unrelated to the subsurface geology in areas of transported laterite limited the success of the venture.Although both deposits are covered by a thick mantle of laterite, in neither case did this impede the ability of basic geophysical techniques to provide very useful information relating to lithologies, structure and alteration.

Calculated stability of aqueous tellurium species, calaverite, and hessite at elevated temperatures

Thermodynamic calculations of the systems Te-O-H, Au-Te-Cl-S-O-H, and Ag-Te-Cl-S-O-H up to 300 degrees C, using the isocoulombic principle, show that the nature of aqueous tellurium species in equilibrium with native tellurium and the nature of aqueous chloride and sulfide species in equilibrium with calaverite (AuTe 2 ) and hessite (Ag 2 Te) are strongly dependent upon physicochemical conditions.Te 2 (super 2-) is the most important aqueous tellurium species in equilibrium with native tellurium in epithermal or mesothermal deposits in which sericitic alteration is dominant, and f (sub O 2 ) is near the hematite-magnetite buffer. HTeO 3 (super -) is the dominant aqueous species at f (sub O 2 ) conditions more oxidizing than the hematite-magnetite buffer, whereas HTe (super -) is dominant under reduced conditions. For Au and Ag contents of between 0.1 and 1 ppb, Sigma S = 0.01 m, m (sub Cl (super -) ) = 1 (conditions typical of epithermal or mesothermal gold-silver ore-forming fluids), and a Sigma Te = 1 ppb, calaverite is precipitated generally from solutions enriched in Au(HS) 2 (super -) and HAu(HS) 2 , whereas hessite is precipitated from solutions enriched in Ag(HS) 2 (super -) at near-neutral to slightly alkaline conditions, and from AgCl 2 (super -) at f (sub O 2 ) conditions higher than the hematite-magnetite buffer. The stabilities of binary silver tellurides and ternary gold-silver tellurides cannot be calculated due to a lack of thermodynamic data.The stability of calaverite can be used, in conjunction with silicate-sulfide-oxide assemblages, to determine the physicochemical conditions of gold-silver telluride formation. For example, the mutual stability of calaverite, quartz, pyrite, hematite, chalcopyrite, bornite, and sericite at 200 degrees C for conditions of Sigma S = 0.001 m, m Ca (super 2+) = 0.019, m Na (super +) = 0.25, m Mg (super 2+) = 0.002, and m K (super +) = 0.02 suggests that stage 1b mineralization in the upper portions of the Golden Sunlight deposit, Montana, precipitated from a fluid with an f (sub O 2 ) between 0.10 (super -40) and 10 (super -37) and a pH of 4.3 to 5.7. The stability of hessite covers most conditions of ore formation (oxidizing to reducing, and acid to alkaline) and is a less useful indicator of ore-forming conditions.

Au-Cu-Ag skarn and replacement mineralization in the McLaren Deposit, New World District, Park County, Montana

The McLaren deposit is one of five sediment-hosted Au-Cu-Ag skarn and replacement deposits that lie within the New World district near Cooke City, Park County, in south-central Montana. The deposit is hosted by gently dipping micrite and dolomitic and calcareous shale of the Cambrian Meagher Formation and occurs along the southwestern contact zone of the Tertiary, dacitic Fisher Mountain intrusive complex.Hydrothermal alteration in the McLaren deposit is concentrically zoned relative to the Fisher Mountain intrusive complex and was controlled by sills, faults, and lithologic contacts. The earliest alteration is biotite hornfels in shale and recrystallization of limestone. This was followed by proximal potassic alteration and distal propylitic alteration of intrusive rocks and early epidote alteration of sedimentary rocks. Early epidote alteration consists largely of epidote (Ps (sub 24-33) ) and K feldspar with lesser amounts of amphibole, andraditic garnet (Ad (sub 32-99) 99), and diopsidic pyroxene (Hd (sub 10-30) ). Early epidote alteration occurs predominantly in the Park Formation where it formed an impermeable barrier to later Au-Cu-Ag-bearing hydrothermal fluids that replaced the underlying Meagher Formation.Pervasive sericitic alteration of intrusive rocks, genetically related to late epidote alteration, quartz-pyrite alteration, and magnetite-rich replacement assemblages in adjacent sedimentary rocks, postdates the early alteration assemblages. Late epidote alteration consists predominantly of epidote, amphibole, pyrite, magnetite, carbonate, and chalcopyrite. Quartz-pyrite alteration postdates late epidote alteration and is associated with chalcopyrite and phyllosilicates. Quartz-pyrite rock occurs as extensive replacements in the Meagher dolomitic limestone (with biotite, chlorite, and talc) and veins and disseminations in the underlying Wolsey shale (with muscovite).Gold mineralization of >1.37 ppm occurs with quartz-pyrite alteration, with lesser amounts related to late epidote alteration and magnetite-rich replacements. Native Au and electrum correlate with abundant pyrite (>30%), abundant chalcopyrite (>0.10%), and to a lesser extent, acicular magnetite. Although Au mineralization may occur both proximal and distal to the contact of the Fisher Mountain intrusive complex, magnetite/pyrite ratios, metal contents, and magnetite textures are systematically zoned within the Meagher Formation with respect to the intrusive complex in the northern part of the McLaren deposit.Fluid inclusions in epidote, garnet, and quartz indicate that early epidote alteration formed at temperatures >600 degrees C and from fluids with high salinities (>26 wt % NaCl equiv), whereas late epidote, quartz-pyrite, and magnetite-rich replacements, and the bulk of sulfide and gold mineralization, formed at lower temperatures between 240 degrees and 415 degrees C (avg 328 degrees C) from fluids with lower salinities (generally between 3.6-11.8 wt % NaCl equiv). The preponderance of pyrite and magnetite in the mineralized rocks suggests that the main stage of Au deposition was characterized by high sulfur and oxygen fugacities.

Geochemistry of late (~ 1.1 Ga) fluid inclusions in rocks of the Kapuskasing Archean crustal section

Three outcrops, well constrained by geochronological and structural studies, and representing a traverse running from tonalite-dominated outcrops in the eastern Wawa gneiss terrane to high-grade granulites of the Kapuskasing structural zone, were mapped and sampled in detail in order to study the trapped fluids. All fluid inclusions in quartz are secondary and consist mostly of CO2-dominated (type II) and saline aqueous (type IIIa) fluids usually occurring on separate healed fractures but also coexisting on some fractures. Healed fractures in quartz contain fluid inclusions but are associated with carbonate–sericite alteration where they pass into adjacent mineral grains. Homogeneous H2O–CO2–salt fluid inclusions (type Ia) in carbonate-rich veins of probable Keweenawan (~ 1.1 Ga) age were trapped at 400–550 °C and ambient pressures of 1.5–2 kbar (1 kbar = 100 MPa). As these fluids cooled on penetration into cool (~ 200 °C) country rocks along fractures they underwent open-system H2O-CO2 phase separation from ~ 350 °C down to ~ 190 °C, producing a range of fluid compositions, including physically segregated CO2-rich (type II) and H2O–salt–rich (type IIIa). Combined gas and ion chromatographic bulk fluid inclusion analyses show that fluid types II and IIIa are not related to shield brines. Br−/Cl− ratios of samples containing phase-separated fluids are similar to the Br−/Cl− ratio of fluids in the carbonate-rich vein. The results of this study show that Keweenawan alkalic magmatism caused widespread carbonate alteration throughout the Kapuskasing structural zone and Wawa gneiss domain. The CO2 component of the fluids is probably magmatic in origin, whereas the aqueous part could also be magmatic or, alternatively, formation waters activated by Keweenawan magmatism.

U-Pb dating of minerals in alteration halos of Superior Province massive sulfide deposits: syngenesis versus metamorphism

U-Pb geochronology of igneous zircon from rhyolitic host rocks to the Archean Kidd Creek, Geco and Winston Lake massive sulfide deposits, in the Superior Province of Ontario, shows that volcanism, which accompanied mineralization, occupied a narrow time span (2717±2 Ma, 2720±2 Ma and 2723±2 Ma, respectively). Precise ages of hydrothermal monazite, allanite and rutile from alteration zones surrounding the above deposits indicate that these minerals crystallized 40–70 million years after volcanism. Monazite from Kidd Creek mine is 2659±3 Ma old, in agreement with spatially associated 2664±25 Ma old rutile. Monazite from a biotite schist at Geoco mine gives a similar age of 2661±1 Ma. However, monazite from a sericite schist, which hosts the ore at Geco mine, is 2675±2 Ma old. Abraded large monazite grains from three units in the Winston Lake deposit are coeval with biotite crystallization and record an age of 2677±2 Ma, approximately the same as monazite in the sericite schist at Geco. Data points from allanite fractions from both the Winston Lake and Geco deposits fall on a Pb-Pb isochron that gives an age of 2672±5 Ma. Rutile from Winston Lake gives a younger age of 2651±6/-2 Ma and may date retrograde alteration of biotite to chlorite. The ca. 2676 Ma age of monazite from Winston Lake and in the sericite schist at Geco mine probably dates a regional metamorphic event that affected most of the southern Superior Province. The ca. 2660 Ma old monazite in the biotite schist at Geco mine and in the chlorite-sericite alteration at Kidd Creek may date later K-metasomatism caused by metamorphically derived fluids that were focussed along old fault structures. Such fluids were also responsible for local sulfide remobilization. Monazite and rutile are spatially associated with chlorite and sericite alterations at Kidd Creek. Their young ages indicate that these originally syngenetic mineral assemblages may have been significantly affected by regional metamorphism. Formation of monazite at all three deposits studied was a result of significant REE remobilization during metamorphism. The discrete character of syn-metamorphic hydrothermal activity in different units of the same deposit, as well as its synchroneity among different, widely separated deposits, requires a mechanism for episodic injection of heat and fluid into the crust on a regional scale. These activities are broadly coeval with, and probably related to, plutonism within adjacent metasedimentary subprovinces and middle to lower crustal metamorphism in the Superior Province.

Cretaceous epithermal Au-Ag mineralization in the Muju-Yeongam district, Republic of Korea: Muju mineralized area.

The gold-silver vein deposits of the Muju area, about 180km SSE of Seoul, are examples of the epithermal gold-silver deposits of the Republic of Korea. The deposits are composed of quartz and calcite veins that crosscut Precambrian gneiss and Cretaceous volcanic rocks (dacite porphyry of ≈ 130 Ma, and rhyodacitic crystal tuff) and intrusive rocks (porphyritic granite of ≈ 102 Ma). Mineralization occurred in four stages (I, II, III, and IV) which filled preexisting fault fracture zones. Hydrothermal wall-rock alteration up to 10m wide is characterized by sericitic, silicic, chloritic and pyritic assemblages. Pipe-like vuggy silica alteration characteristically occurs at the innermost and surface zones of dacite porphyry-hosted quartz veins. K-Ar dating of alteration sericite (≈ 103 Ma) indicates a late Cretaceous age for ore mineralization, which is probably associated with the intrusion of the porphyritic granite. Fluid inclusion and stable isotope data show that electrum, argentite, sphalerite and galena were deposited at temperatures between 260° and 180°, from fluids with salinities between 3.2 and 0.5 wt. % NaCl equiv.. Fluid inclusion evidence of boiling suggests pressures of ≈ 40 bars during late portions of stage II mineralization, corresponding to depths between 150 and 400m, assuming lithostatic and hydrostatic loads. Equilibrium thermodynamic interpretation of mineral assemblages indicates that the gold-silver-depositing fluids had a pH of 4.4 to 5.4 and log activities of S2 and 02 of -12 to -16 atm and -37 to -38.7 atm, respectively. Gold-silver deposition was a combined result of the cooling of ore fluids through mixing with less-evolved meteoric waters and the decrease of sulfur activity due to sulfide precipitation. Sulfur isotope compositons of stage II sulfide minerals range from 0.5 to 5.9 per mil, yielding a δ34Σs value near 4 per mil which indicates a local igneous source for sulfur. The oxygen and hydrogen isotope values of hydrothermal fluids (δ18O=-6.6 to -1.3 per mil, δD=-83 to -73 per mil) are consistent with meteoric water dominance approaching unexchanged meteoric water values.

Mineralogic, fluid inclusion, and stable isotope evidence for the genesis of carbonate-hosted Pb-Zn(-Ag) orebodies of the Taebaek Deposit, Republic of Korea

The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, the largest Pb-Zn producer in Korea, occurs as chimney-shaped carbonate-sulfide vein orebodies developed in Cambro-Ordovician limestones and shales. Compared with other deposits (the Bonsan and Dongjeom) of the Yeonhwa I mine, the Taebaek deposit is characteristically skarn free and more rich in lead and silver. These characteristics suggest that the Taebaek deposit formed farther from a deep, hidden, associated igneous body. A K-Ar date of alteration sericite indicates that the mineralization at Taebaek took place during Late Cretaceous times (75 Ma). Four stages of ore deposition have definite sulfide assemblages: stage I, hexagonal pyrrhotite + arsenopyrite + native bismuth; stage II, pyrite + sphalerite + arsenopyrite; stage III, monoclinic pyrrhotite + sphalerite + galena + electrum + Ag minerals; and stage IV, barren calcite. Later stages of mineralization occur at higher (shallower) portions of each orebody, indicating that the locus of mineralization migrated toward higher levels with increasing paragenetic time.Fluid inclusion data indicate that temperatures and salinities of ore fluids progressively decreased with time (380 degrees -250 degrees C and up to 8 wt % NaCl equiv during stage II, and 305 degrees - 180 degrees C and down to 0 wt % NaCl equiv during stage III), mainly due to progressive mixing with cooler and more dilute meteoric waters. Fluid inclusion data and geologic arguments indicate that pressures during the mineralization were in the range of 210 to 420 bars. Thermochemical considerations indicate the activity of H (super +) decreased when fluids became equilibrated with host limestones, which resulted in a pH change (from about 5 to 7) and decreases in f (sub S 2 ) and f (sub O 2 ) . The pH increase, combined with a decrease in temperature, was responsible for the spatial and temporal variations of iron sulfides and ore precipitation.The delta 34 S values of ore sulfides have a narrow range between 1.6 and 5.3 per mil. The calculated delta 34 S (sub Sigma S) values of ore fluids are constant at 3.5 to 4 per mil, indicating a deep-seated igneous source of sulfur. The delta 13 C values of carbonates in ores range between -7.2 and -5.5 per mil and are considerably more negative than those (-1.9 to +0.9ppm) of the host lime-stones. It is likely that carbon in the ore fluids was a mixture of deep-seated magmatic carbon and limestone-derived carbon. Oxygen and hydrogen isotope studies revealed that the delta 18 O and delta D values of waters in ore fluids decreased gradually with time from +9.8 and -68 per mil (stage II) to +1.9 and -95 per mil (stage IV), respectively. This indicates that magmatic water was important during the early zinc mineralization but was progressively replaced by meteoric water during the later lead and silver mineralization.

A post-deformational, post-peak metamorphic timing for mineralization at the Archaean Big Bell gold deposit, Western Australia

Previous workers were unable to find any suitable criteria to time gold mineralization at Big Bell mine — an enigmatic, low-grade, large-tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the Murchison Province of the Archaean Yilgarn Craton, Western Australia. Earlier studies suggested that the gold mineralization occurred by hydrothermal alteration, either synchronous with volcanism or prior to peak metamorphism ∗. The following conclusions have been drawn from a detailed meso- and microstructural analysis of the mine sequence and greenstone belt hosting the orebody: 1. (1) Prior to mineralizaton, the Big Bell area underwent five phases of deformation and M1 regional metamorphism to the staurolite-andalusite zone of the amphibolite facies for metasediments. 2. (2) Granite emplacement post-dated D5 folding causing local, M2, thermal metamorphism. This peaked at the Ms+Qtz breakdown reaction for pelites, on or close to the And-Sil transition, but below the onset of partial melting, characterized by the narrow non-degenerate divariant field involving seven KFMASH phases: Qtz+Ms+Bt+Crd+Kfs+Als+H 2O This hornfelsing consumed, cross-cut and post-dated S5 cleavage. Prograde Qtz+And+Sil veins replaced this hornfels assemblage (and S5 cleavage) and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post-deformational hornfelsing. 3. (3) Retrograde M3 assemblages and two stages of gold-related mineralization, cross-cut and replaced all previous structures. The retrograde gold-bearing assemblages are both underformed. Stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations: Py-Po-Ccp-Ilm-Mag-Au-Tur-Sch-Rt-Spn-Zrn-Ms(Bt)-Ep-Czo-Qtz The second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre-existing mineralogies, including quartz, are totally destroyed within sericitic domains. The stage 2 mineral associations are: Apy-Sbt-Sb-Mo-Sp-Au-Py-Po-Ilm-Mag-Ccp-sericite-Chl-carbonate (Cal)-Ep-Czo-Qtz 4. (4) Gold mineralization at Big Bell was post-deformational and post-peak metamorphic. Unlike many Archaean gold deposits there are no obvious shear zones controlling the formation of the Big Bell gold deposit at the structural level exposed either on the surface or in drill core. Big Bell Mine was in a low-strain domain during the formation of late-stage sinistral sub-vertical D6 shear zones, that are typically concentrated at the granitoid-greenstone contacts. The retrograde character of the mineralization has been deduced from detailed meso- and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the Murchison Province.

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Geological characteristics and tectonic setting of proterozoic iron oxide (CuUAuREE) deposits

Recent work on the Olympic Dam CuUAuAg deposit, South Australia, the Wernecke Mountain breccias, Yukon, the Kiruna iron ore district, Sweden, and the southeast Missouri iron ore district, and a review of literature on other iron-rich mineral deposits in Proterozoic rocks, suggest that these occurrences constitute a distinct class of ore deposits characterized by low-titanium, iron-rich rocks formed in extensional tectonic environments. Other examples of this class may include the mineral deposits of the Great Bear magmatic zone of northwest Canada, the Bayan Obo district of China, and perhaps the Redbank breccia pipes of the Northern Territory, Australia. We designate this class of deposits as Proterozoic iron oxide (CuUAuREE) deposits, and propose that the ore deposits generally referred to as ‘Kiruna-type’ should be considered a subset of this larger class. Salient characteristics of this class of deposits are as follows: 1. (1) Age. The majority of known deposits, particularly the larger examples, are found within Early to mid-Proterozoic host rocks (1.1–1.8 Ga). 2. (2) Tectonic setting. The deposits are located in areas that were cratonic or continental margin environments during the late Lower to Middle Proterozoic, and in many cases there is a definite spatial and temporal association with extensional tectonics. Most of the districts occur along major structural zones, and many of the deposits are elongated parallel to regional or local structural trends. The host rocks may be igneous or sedimentary; many of the deposits occur within silicic to intermediate igneous rocks of anorogenic type. However, mineralization in many deposits is not easily related to igneous activity at the structural level of mineralization. 3. (3) Mineralogy. The ores are generally dominated by iron oxides, either magnetite or hematite. Magnetite is found at deeper levels than hematite. CO3, Ba, P, or F minerals are common and often abundant. The deposits contain anomalous to potentially economic concentrations of REEs, either in apatite, or in distinct REE mineral phases. 4. (4) Alteration. The host rocks are generally intensely altered. The exact alteration mineralogy depends on host lithology and depth of formation, but there is a general trend from sodic alteration at deep levels, to potassic alteration at intermediate to shallow levels, to sericitic alteration and silicification at very shallow levels. In addition, the host rocks are locally intensely Fe-metasomatized. In spite of these similarities, many variations occur between and within individual districts, particularly in deposit morphology. Individual deposits occur as strongly discordant veins and breccias to massive concordant bodies. Both the morphology and the extent of alteration and mineralization appear to be largely controlled by permeability along faults, shear zones and intrusive contacts, or by permeable horizons such as poorly welded tuffs. Thus, the variations of morphology are explicable in terms of local wall-rock and structural controls. Similarly, local variations in mineralogy and geochemistry may be largely attributable to wall-rock composition, and to P, T, and fo2 controls related to depth of formation. We believe that these deposits formed primarily in shallow crustal environments (<4–6 km), and that they are expressions of deeper-seated, volatile-rich igneous-hydrothermal systems, tapped by deep crustal structures. The global occurrence of this type of deposit at approximately 1.8 to 1.4 Ga suggests a relation to a global rifting events effecting continental crust, possibly the break-up of a Proterozoic supercontinent. Secular cooling of the Earth insured that subsequent rifting and mineralizing events might generate deposits similar in kind but smaller in magnitude.

Regional setting, structure, and descriptive geology of the middle Proterozoic Syama gold deposit, Mali, West Africa

The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold mineralization is hosted by a thin (0.5-2.0 km) but regionally extensive sequence of basalt and andesite, interbedded graywacke and argillite, and andesitic-lamprophyric intrusions. This sequence is bounded on the east by andesitic conglomerate and graywacke and on the west by interbedded graywacke and argillite. In the Syama mine area, this ore-bearing sequence is structurally layered, overturned, and east vergent. Massive competent basalt and conglomerate bound the hanging wall and footwall, respectively, of the mineralized interval.Lateritic weathering developed an oxide orebody that overlies sulfide mineralization. The oxide orebody extends to 35 to 40 m below the surface; sulfide mineralization extends to a depth of at least 500 m. Mineable oxide reserves are 3.0 million tons with an average grade of 3.20 g/ton. Geologic sulfide reserves from the top of sulfides to 320 m below the surface are 21 million tons with an average grade of 4.02 g/ton.The oxide deposit was expressed on the surface as a series of 3- to 15-m-deep ancient mine workings, which extended 800 m along the strike of the deposit, and sparse outcrops of silicified rock. Oxide ore occurs as two deposits: a surficial deposit consisting of ancient tailings and dump material which are 0.1 to 5 m thick and extend up to 200 m away from the ancient workings; and an in situ lode or saprolite deposit which lies immediately below the ancient workings and extends downward into sulfide mineralization.The sulfide deposit consists of lenticular bodies of intense ankerite-quartz veinlet stockworks, zones of sheeted ankerite-quartz veinlets, and breccia bodies. Pyrite is the principal gold-bearing mineral and occurs disseminated in veinlet halos and breccia clasts. Green chloritic basalt is the preferred but nonexclusive host of the mineralization; it is commonly bleached by ankerite, albite, and sericite alteration in the pyritic gold-bearing zones. Silica, Na 2 O, Au, CO 2 , S, and C have been added to mineralized zones hosted by basalt; Fe 2 O 3 , Al 2 O 3 , and trace metals have been removed. Andesitic-lamprophyric intrusions and graywacke-argillite layers also host disseminated pyrite mineralization. Faulted beds of graphitic graywacke-argillite and zones of silicification bound most individual orebodies.The volcanic rocks at Syama were probably originally deposited in a narrow rift or graben in an intra-arc setting. Gold mineralization overlapped with compressional deformation and intrusive activity. The mineralized horizons at Syama lie within a sequence of rocks which has been either stacked or attenuated by layer-parallel reverse faults. These faults coincide with abrupt regional- and mine-scale contrasts in rock competency. Intrusive rocks form a volumetrically significant part of the mineralized sequence and may have aided hydrothermal alteration as a source of heat. Both the graywacke-argillite layers and the altered basalt and intrusions may have been a source for some of the elements and fluids responsible for the mineralization and alteration.The mineralization at Syama is similar to other middle Proterozoic gold deposits in the Birimian Shield of west Africa and to Archean gold deposits hosted by mafic volcanic rocks in terms of tectonic setting, wall-rock alteration, form of mineralization, and associated structures.

Déformation coaxiale en bordure de la discontinuité structurale de Lyndhurst, sous-province de l'Abitibi

The Lyndhurst discontinuity is a major east–west structure located some 40 km north of Rouyn–Noranda. It separates the rhyolitic and sedimentary units of the Hunter Mine Group to the north from the basalts of the Kinojévis Group to the south. Evidence of deformation is observed only in the rhyolites and sediments along the south edge of the Hunter Mine Group. The deformation zone is approximately 1 km wide and is continuous for about 30 km. The Kinojévis Group rocks are not deformed. Deformed rhyolites show a strong sericite and chlorite alteration of hydrothermal origin. The competency of the rhyolites is significantly reduced by the presence of these phyllosilicates, which results in the deformation being preferentially localized in the more altered rocks. Competency contrasts observed on a mesoscopic scale are also valid on the microscopic and megascopic scales. The structural analysis of the deformation zone reveals different arrays that characterize three distinct sectors. These arrays reflect competency contrasts of the lithology and a crenulation cleavage. The stretching lineation is generally steeply plunging. Although the deformation seems significant in a zone contiguous to the Lyndhurst discontinuity, the poor development of the stretching lineation, the preservation of the original crystalline shapes of phenocrysts in the rhyolites, and the constant symmetry of the pressure shadows suggest a global coaxial deformation. This deformation regime is difficult to reconcile with a compressive fault such as a thrust fault.