Potassic Alteration Zones Research Articles

Lithogeochemistry, C-O isotopes and Stratabound Alteration Halos to Stratiform Copper Mineralisation, Zambian Copperbelt

Summary A major lithogeochemical study of the basin sediments surrounding the stratiform copper deposits of the Zambian Copper Belt has revealed extensive stratabound zones of both potassic and sodic alteration. The syn-ore potassic alteration is represented by enrichment of biotite and K-feldspar in the enclosing sediments, with whole rock K 2 O values varying from 3.5 to 10 wt%. The sodic alteration (albite-calcite), post-dates the copper mineralisation, and occurs in the Upper Roan siltstone-carbonate sequence overlying the potassic alteration zone. Na 2 O values vary from 1.5 to 6.3 wt%, corresponding to a maximum of 60 wt% albite in the altered rock. A comprehensive study of carbon and oxygen isotopes of various carbonate types throughout the sediments of the Zambian Copper Belt has shown a systematic relationship to alteration and copper mineralisation that has significant application in mineral exploration. Mineralised and potassically altered zones show strong coupled depletion in both carbon and oxygen isotopes, with δ 13 C = -6 to ‐26 per mil and δ 18 O = 6 to 20 per mil. The strongly negative δ 13 C values of carbonate in the ore shale and potassic footwall clastics indicates the significant role of oxidation of organic carbon in the mineralising process. The fact that the relatively clean altered and mineralised footwall clastics also show this negative carbon isotope shift suggests that organic reaction products migrated out of the shale into the permeable footwall clastics prior to mixing with the ore fluid, leading to redox reactions causing coupled oxidation of organic matter and reduction of sulfate to H 2 S resulting in copper sulfide precipitation.

Mass Changes during Hydrothermal Alteration/Mineralization at the Sar-Cheshmeh Porphyry Copper Deposit, Southeastern Iran

The Sar-Cheshmeh porphyry copper deposit is associated with a Miocene dioritic to granodioritic pluton that intruded Eocene volcano-sedimentary and related rocks. Copper mineralization was accompanied mainly by phyllic, and to a lesser extent potassic, alteration. These hydrothermal alteration episodes were studied in terms of mass transfer and element mobility. Isocon plots illustrate these changes quantitatively. Al, Ti, and Nd were relatively immobile, and mass was essentially conserved during alteration. At all stages in the evolution of the hydrothermal system, the volume change was inappreciable. In the potassic alteration zone, enrichment of K and depletion of Mg, Ca, and Fe took place. These changes attended replacement of plagioclase and amphibole by Na- and/or K-feldspar and biotite, respectively. Potassic alteration was associated with a major addition of Cu, as evident from the occurrence of disseminated chalcopyrite and bornite in this zone. Phyllic alteration was accompanied by depletion of Na, K, Mg, Rb, and Ba and enrichment of Si and Cu. Losses of Na, K, and Fe reflect sericitization of alkali feldspar and destruction of ferromagnesian minerals. The addition of Si is consistent with widespread silicification, which is a major feature of phyllic alteration, as well as the addition of Cu mobilized from the potassic zone.

Mineralogical and chemical evolution of the Ernest Henry Fe oxide–Cu–Au ore system, Cloncurry district, northwest Queensland, Australia

The Ernest Henry Cu–Au deposit was formed within a zoned, post-peak metamorphic hydrothermal system that overprinted metamorphosed dacite, andesite and diorite (ca 1740–1660 Ma). The Ernest Henry hydrothermal system was formed by two cycles of sodic and potassic alteration where biotite–magnetite alteration produced in the first cycle formed ca 1514±24 Ma, whereas paragenetically later Na–Ca veining formed ca 1529 +11/−8 Ma. These new U–Pbtitanite age dates support textural evidence for incursion of hydrothermal fluids after the metamorphic peak, and overlap with earlier estimates for the timing of Cu–Au mineralization (ca 1540–1500 Ma). A distal to proximal potassic alteration zone correlates with a large (up to 1.5 km) K–Fe–Mn–Ba enriched alteration zone that overprints earlier sodic alteration. Mass balance analysis indicates that K–Fe–Mn–Ba alteration—largely produced during pre-ore biotite- and magnetite-rich alteration—is associated with K–Rb–Cl–Ba–Fe–Mn and As enrichment and Na, Ca and Sr depletion. The aforementioned chemical exchange almost precisely counterbalances the mass changes associated with regional Na–Ca alteration. This initial transition from sodic to potassic alteration may have been formed during the evolution of a single fluid that evolved via alkali exchange during progressive fluid-rock interaction. Cu–Au ore, dominated by co-precipitated magnetite, minor specular hematite, and chalcopyrite as breccia matrix, forms a pipe-like body at the core of a proximal alteration zone dominated by K-feldspar alteration. Both the core and K-feldspar alteration overprint Na–Ca alteration and biotite–magnetite (K–Fe) alteration. Ore was associated with the concentration of a diverse range of elements (e.g. Cu, Au, Fe, Mo, U, Sb, W, Sn, Bi, Ag, F, REE, K, S, As, Co, Ba and Ca). Mineralization also involved the deposition of significant barite, K(–Ba)–feldspar, calcite, fluorite and complexly zoned pyrite. The complexly zoned pyrite and variable K–(Ba)–feldspar versus barite associations are interpreted to indicate fluctuating sulphur and/or barium supply. Together with the alteration zonation geochemistry and overprinting criteria, these data are interpreted to indicate that Cu–Au mineralization occurred as a result of fluid mixing during dilation and brecciation, in the location of the most intense initial potassic alteration. A link between early alteration (Na–Ca and K–Fe) and the later K-feldspathization and the Cu–Au ore is possible. However, the ore-related enrichments in particular elements (especially Ba, Mn, As, Mo, Ag, U, Sb and Bi) are so extreme compared with earlier alteration that another fluid, possibly magmatic in origin, contributed the diverse element suite geochemically independently of the earlier stages. Structural focussing of successive stages produced the distinctive alteration zoning, providing a basis both for exploration for similar deposits, and for an understanding of ore genesis.

The El Galeno and Michiquillay porphyry Cu–Au–Mo deposits: geological descriptions and comparison of Miocene porphyry systems in the Cajamarca district, northern Peru

El Galeno and Michiquillay are early to middle Miocene Cu–Au–Mo porphyry-related deposits located in the auriferous Cajamarca district of northern Peru. The El Galeno deposit (486 Mt at 0.57% Cu, 0.14 g/t Au and 150 ppm Mo) is associated with multiple dioritic intrusions hosted within Lower Cretaceous quartzites and shales. Emplacement of the porphyry stocks (17.5–16.5 Ma) in a hanging wall anticline was structurally controlled by oblique faults superimposed on early WNW-trending fold-thrust structures. Early K-feldspar–biotite–magnetite (potassic) alteration was associated with pyrite and chalcopyrite mineralisation. A quartz–magnetite assemblage that occurs at depth has completely replaced potassically altered rocks. Late- and post-mineralisation stocks are spatially and temporally related to weak quartz–muscovite (phyllic) alteration. High Au grades are associated with early intrusive phases located near the centre of the deposit. Highest Cu grades (~0.9% Cu) are mostly associated with a supergene enrichment blanket, whilst high Mo grades are restricted to contacts with the metasedimentary rocks. The Michiquillay Cu–Au–Mo deposit (631 Mt at 0.69% Cu, 0.15 g/t Au, 100–200 ppm Mo) is associated with a Miocene (20.0–19.8 Ma) dioritic complex that was emplaced within the hanging wall of a back thrust fault. The intrusive complex is hosted in quartzites and limestones. The NE-trending deposit is crosscut by NNW-trending prospect-scale faults that influenced both alteration and metal distribution. In the SW and NE of the deposit, potassic alteration zones contain moderate hypogene grades (0.14 g/t Au and 0.8% Cu) and are characterised by chalcopyrite and pyrite mineralisation. The core of the deposit is defined by a lower grade (0.08 g/t Au and 0.57% Cu) phyllic alteration that overprinted early potassic alteration. Michiquillay contains a supergene enrichment blanket of 45–80 m thickness with an average Cu grade of 1.15%, which is overlain by a deep leached cap (up to 150 m). Cu–Au–Mo (El Galeno-Michiquillay) and Au-rich (Minas Conga) deposits in the Cajamarca region are of similar age (early–middle Miocene) and intrusive rock type (dioritic) associations. Despite these geochronological and geochemical similarities, findings from this study suggest variation in metal grade between the hybrid-type and Au-rich deposits result from a combination of physio-chemical factors. These include variations in temperature and oxygen fugacity conditions during hypogene mineralisation resulting in varied sulphide assemblages, host rock type, precipitation of ubiquitous hydrothermal magnetite, and late hydrothermal fluid flow resulting in a well-developed phyllic alteration zone.

Bajo de la Alumbrera Copper-Gold Deposit: Stable Isotope Evidence for a Porphyry-Related Hydrothermal System Dominated by Magmatic Aqueous Fluids

Alteration zones at the gold-rich Bajo de la Alumbrera porphyry copper deposit in northwestern Argentina are centered on several porphyritic intrusions. They are zoned from a central copper-iron sulfide and gold-mineralized potassic (biotite-K-feldspar +/- quartz) core outward to propylitic (chlorite-illite-epidote-calcite) assemblages. A mineralized intermediate argillic alteration assemblage (chlorite-illite +/- pyrite) has overprinted the potassic alteration zone across the top and sides of the deposit and is itself zoned outward into phyllic (quartzinuscovite-illite +/- pyrite) alteration. This study contributes new data to previously reported delta(18)O and delta D compositions of fluids responsible for the alteration at Bajo de la Alumbrera, and the data are used to infer likely ore-forming processes. Measured and calculated delta(18)O and delta D values of fluids (+8.3 to +10.2 and -33 to -81 parts per thousand, respectively) confirm a primary magmatic origin for the earliest potassic alteration phase. Lower temperature potassic alteration formed from magmatic fluids with lower delta D values (down to -123 parts per thousand). These depleted compositions are distinct from meteoric water and consistent with degassing and volatile exsolution of magmatic fluids derived from an underlying magma. Variability in the calculated composition of fluid associated with potassic alteration is explained in terms of phase separation (or boiling). if copper-iron sulfide deposition occurred during cooling (as proposed elsewhere), this cooling was largely a result of phase separation. Magmatic water was directly involved in the formation of overprinting intermediate argillic alteration assemblages at Bajo de la Alumbrera. Calculated delta(18)O and delta D values of fluids associated with this alteration range from +4.8 to +8.1 and -31 to -71 per mil, respectively Compositions determined for fluids associated with phyllic alteration (-0.8 to +10.2 and -31 to -119 parts per thousand) overlap with the values determined for the intermediate argillic alteration. We infer that phyllic alteration assemblages developed during two stages; the first was a high-temperature (400 degrees-300 degrees C) stage with D-depleted water (delta D = -66 to -119 parts per thousand). This compositional range may have resulted from magma degassing and/or the injection of new magmatic water into a compositionally evolved hydrothermal system. The isotopic variations also can be explained by increased fluid-rock interaction. The second stage of phyllic alteration occurred at a lower temperature (similar to 200 degrees C), and variations in the modeled isotopic compositions imply mixing of magmatic and meteoric waters. Ore deposition that occurred late in the evolution of the hydrothermal system was probably associated with further cooling of the magmatic fluid, in part caused by fluid-rock interaction and phase separation. Changing pH and/or oxygen fuoracity may have caused additional ore deposition. The ingress of meteoric water appears to postdate the bulk of mineralization and occurred as the system at Bajo de la Alumbrera waned.

The Use of Sub-Audio Magnetics (SAM) in Gold Exploration – Examples from the Yilgarn Craton, WA

The results of Sub-Audio Magnetic (SAM) surveys and the relationship to other geophysical datasets and geological information are presented for three different styles of gold mineralisation within the Archaean Yilgarn Craton, W.A.The Jericho prospect is located to the north of the Butcher Well deposit in the Laverton area and is hosted within a mafic unit consisting of predominately high-magnesium basalt. The source of a broad SAM total field magnetometric resistivity (TFMMR) response initially could not be identified as it did not appear related to the depth of cover or weathering from shallow drilling. Further analysis of the SAM result, following additional drilling and a dipole-dipole induced polarization (IP) survey, indicated that the source of the SAM TFMMR response in the southern portion of the survey area is due to a combination of variation in depth of the regolith and a number of narrow potassic alteration zones within the basaltic unit.The objective of the SAM survey at the Bronco prospect in the Southern Cross region was to map perturbations within the massive pyrrhotite-pyrite horizon that hosts gold mineralisation. In order to maximise current flow within the sulphide horizon, the electrodes were placed within the sulphide units at the base of two open pits located along strike. The TFMMR data shows a response having a width that significantly narrows to both the north and south of the survey area and correlates with the width of the primary sulphide. In addition, a number of flexures and offsets have been delineated. High-intensity magnetic responses, due to maghemite in the regolith, have resulted in noise being introduced in the TFMMR data.Gold mineralisation at the Triple ‘O’ lode within theCornishman Deposit is associated with quartz-pyrrhotite-pyrite alteration of banded-iron formation that is located below a thrust in a ‘blind’ location. The sulphide alteration can be semi-massive and very conductive (up to 3000 S/m), but the use of traditional electromagnetic techniques at similar deposits in the region has not proven particularly successful because of the small spatial footprint of the sulphidic alteration. Hence, the objective of the SAM surveying was to determine whether the technique could successfully map the narrow, sub-horizontal sulphidic alteration zones. The TFMMR data highlighted a broad conductive zone that is comprised of three discrete conductors, two of which have been confirmed by drilling to result from sulphide alteration of the banded-iron formation and to have associated gold mineralisation. The third anomaly is thought to be due to the preferential weathering of a non-mineralised shear zone.

Structural controls on hypozonal orogenic gold mineralization in the La Ronge Domain, Trans-Hudson Orogen, Saskatchewan

The La Ronge Domain is a granite–greenstone belt in the Saskatchewan segment of the ca. 1.9–1.8 Ga Trans-Hudson Orogen. The La Ronge volcanic arc was accreted to the Archean Hearne craton from ca. 1.87 to 1.86 Ga. Subduction of oceanic lithosphere beneath the accreted La Ronge – Hearne margin produced a voluminous suite of continental-arc intrusions. In the Waddy Lake area, the 1852.6 ± 1.5 Ma Corner Lake stock and 1859 ± 4 Ma and 1861 ± 2 Ma feldspar porphyry dykes crystallized from magmas generated from melting of the subducted oceanic slab. During the ca. 1.83–1.80 Trans-Hudson collision of the Hearne craton with the Archean Sask and Superior cratons, a penetrative regional foliation and a steeply plunging lineation formed within the La Ronge Domain. During further contraction across the domain, the deformation became localized in dextral and oblique-slip shear zones that generally follow contacts between more competent and less competent rock units. Orogenic gold mineralization is associated with quartz veins that are surrounded by hypozonal potassic and sulfidic alteration zones. The Komis gold deposit, the only past-producing gold mine in the Waddy Lake area, formed in the strain shadow of the Round Lake stock during the development of the regional foliation and lineation. Mineralization is associated with quartz veins that cut through tonalite dykes that behaved more brittlely than the surrounding metavolcanic rocks. The Golden Heart and Corner Lake gold deposits are hosted by south-side-up oblique-slip shear zones, which belong to a regional system of structures that extend from Saskatchewan to Manitoba.

The Kwyjibo Cu-REE-U-Au-Mo-F Property, Quebec: A Mesoproterozoic Polymetallic Iron Oxide Deposit in the Northeastern Grenville Province

The Kwyjibo deposit is the most significant of a number of examples of the iron oxide-copper-gold class of deposits to have been found in the Grenville. It is a Mesoproterozoic Cu-REE-Mo-F-U-Au iron oxide occurrence located approximately 120 km northeast of Sept-Iles, Quebec. The deposit, which comprises different textural facies of magnetite and hematite mineralization, occurs along the southeast margin of the Canatiche granitic complex. Porphyroidal leucogranite and volcanic equivalents of the Canatiche Complex are the main hosts of the mineralization. Calc-silicate rock and hornblende-biotite gneiss, possibly belonging to the supracrustal Manitou Complex, flank the deposit to the southeast and south. Magnetite is ubiquitous in the Canatiche granitoids and generally occurs as disseminations and fine agglomerations. Near the deposit, magnetite becomes progressively more abundant, forming veinlets, veins, stockworks, and pseudobreccias, culminating in locally brecciated massive magnetite, the dominant type of iron oxide concentration. Specular hematite occurs locally as disseminations and veinlets. Mineralization at Kwyjibo is complex and multiphase and is divided into early granophile, magnetite, main polymetallic, and specular hematite stages. The main polymetallic mineralization, comprising chalcopyrite, pyrite, molybdenite, fluorite, and REE-bearing minerals, is superimposed on preexisting magnetite-rich rocks and shows little evidence of deformation. The Kwyjibo deposit has a number of features in common with other polymetallic iron oxide deposits around the world, including granitic host rocks of within-plate affinity, various textural facies of iron oxide mineralization styles, and sodic and potassic alteration zones. A close spatial association with a major deep seated structure is also postulated. The Grenville province hosts a major anorthosite belt and is a world-class titaniferous metallogenic province. The Proterozoic era, and especially its anorogenic periods, are the main metallogenic epochs both for iron oxide-copper-gold deposits and anorthosite-hosted Ti-rich iron oxide deposits. Empirical observations suggest a possible petrogenetic relationship between these two important ore deposit types.

Porphyry-Style Alteration and Mineralization of the Middle Eocene to Early Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru

Originally known for its Fe-Cu skarn mineralization, the Andahuaylas-Yauri belt of southeastern Peru is rapidly emerging as an important porphyry copper province. Field work by the authors confirms that mineralization in the belt is spatially and temporally associated with the middle Eocene to early Oligocene (~48–32 Ma), calc-alkaline Andahuaylas-Yauri batholith, a composite body with an areal extent of ~300 × 130 km emplaced into clastic and carbonate strata (e.g., Yura Group and Ferrobamba Formation) of Jurassic to Cretaceous age. Batholith emplacement included early-stage, mafic, cumulate gabbro and diorite between ~48 and 43 Ma, followed by pulses of granodiorite and quartz monzodiorite at ~40 to 32 Ma. Coeval volcanic rocks make up the middle Eocene to early Oligocene Anta Formation, a sequence of >1,000 m of andesite lava flows and dacite pyroclastic flows with interbedded volcaniclastic conglomerate. Sedimentary rocks include the red beds of the Eocene to early Oligocene San Jeronimo Group and the postmineralization late Oligocene to Miocene Punacancha and Paruro formations. Eocene and Oligocene volcanic and sedimentary rocks are interpreted to have accumulated largely in both transtensional and contractional synorogenic basins. New and previously published K-Ar and Re-Os ages show that much of the porphyry-style alteration and mineralization along the belt took place during the middle Eocene to early Oligocene (~42–30 Ma). Thus, batholithic magma emplacement, volcanism, and sedimentation are inferred to have accompanied a period of intense deformation, crustal shortening, and regional surface uplift broadly synchronous with the Incaic orogeny. Supergene mineralization is inferred to have been active since the Pliocene on the basis of geomorphologic evidence and a single K-Ar determination (3.3 ± 0.2 Ma) on supergene alunite. The belt is defined by 31 systems with porphyry-style alteration and mineralization, including 19 systems grouped in 5 main clusters plus 12 separate centers, and by hundreds of occurrences of magnetite-rich, skarn-type Fe-Cu mineralization. Porphyry copper stocks are dominated by calc-alkaline, biotite- and amphibole-bearing intrusions of granodioritic composition, but monzogranitic, monzonitic, quartz-monzonitic, and monzodioritic stocks occur locally. Hydrothermal alteration includes sericite-clay-chlorite, and potassic, quartz-sericitic, and propylitic assemblages. Calcic-potassic and advanced argillic alteration associations are locally represented, and calc-silicate assemblages with skarn-type mineralization occur where carbonate country rocks predominate. Porphyry copper deposits and prospects of the belt range from gold-rich, molybdenum-poor examples (Cotabambas), through deposits carrying both gold and molybdenum (Tintaya, Los Chancas), to relatively molybdenum-rich, gold-poor end members (Lahuani). Gold-only porphyry systems are also represented (Morosayhuas). Gold-rich porphyry copper systems are rich in hydrothermal magnetite and display a positive correlation between Cu and Au in potassic alteration. The bulk of the hypogene Cu (-Au, -Mo) mineralization occurs in the form of chalcopyrite and bornite, in intimate association with early-stage potassic alteration which, in many deposits and prospects, is variably overprinted by copper-depleting sericite-clay-chlorite alteration. Most porphyry copper systems of the belt lack economically significant zones of supergene chalcocite enrichment. This is due primarily to their relatively low pyrite contents, the restricted development of quartz-sericitic alteration, and the high neutralization capacities of both potassic alteration zones and carbonate country rocks as well as geomorphologic factors. Leached cappings are irregular, typically goethitic, and contain copper oxide minerals developed by in situ oxidation of low-pyrite, chalcopyrite (-bornite) mineralization. Porphyry copper-bearing stocks emplaced in the clastic strata of the Yura Group and certain phases of the Andahuaylas-Yauri batholith may develop appreciable supergene chalcocite enrichment in structurally and lithologically favorable zones. A model for the region suggests that the calc-alkaline magmas of the Andahuaylas-Yauri batholith and subsequent porphyry-style mineralization were generated during an event of subduction flattening which triggered the crustal shortening, tectonism, and uplift assigned to the Incaic orogeny. Shortening of the upper crust would have impeded rapid magma ascent favoring storage of fluid in large chambers which, at the appropriate depth in the uppermost crust, would have promoted large-scale porphyry copper emplacement. Geodynamic reconstructions of the late Eocene to early Oligocene period of flat subduction in the central Andes suggest that emplacement of the Andahuaylas-Yauri batholith took place at an inflection corridor in the subduction zone broadly coincident with the position of the present-day Abancay deflection. Similarly, evidence from southeastern Peru suggests that the Andahuaylas-Yauri belt may be continuous with the late Eocene to early Oligocene porphyry copper belt of northern Chile and that the process of subduction flattening in southern Peru also may have taken place in northern Chile between ~45 and 35 Ma.

REE composition of primary and altered feldspar from the mineralized alteration zone of alkaline intrusive rocks, western Yunnan Province, China

The Yao'an gold deposit is situated northwest of Kunming in Yunnan Province, China, is associated spatially with Yao'an alkaline intrusion. The Yao'an gold mineralization may be divided into two ore stages: the stage I (sulfide stage) fine-grained pyrite-chalcopyrite-galena stringer veins and the stage II (oxide plus sulfide) tectonic breccias and banded ore. The stage I ores occur in the zones of pervasive potassic alteration. The laser ablation ICP-MS technique was used to analyse REE in primary perthites and altered K-feldspars of potassic alteration zone, related to mineralization in the Yao'an gold deposit. The primary perthites from Yao'an syenite porphyry all have similar REE features with light REE enrichment, large positive Eu anomalies and small but significant negative Ce anomalies. The REE patterns of altered K-feldspars have broadly similar features, but compared to the primary perthite have much higher REE concentrations and smaller positive Eu anomalies. These characteristics show that the hydrothermal fluids responsible for alteration were REE-rich, due to high levels of complex-forming anions, but had large negative Eu anomalies and other features best ascribed to magmatic crystal fractionation. These fluids were derived essentially from the magmatic system.

Porphyry-Epithermal Transition: Maricunga Belt, Northern Chile

The Refugio, Aldebaran, and La Pepa districts in the Maricunga belt of northern Chile contain advanced argillic alteration zones that locally host high-sulfidation epithermal gold deposits in proximity to porphyry gold (± copper) deposits. The spatial association suggests a genetic link. Mineralized zones are characterized by four main vein types that formed at different times and have specific zonal relationships. A-veinlets are the earliest and deepest vein type. They are restricted to potassic alteration zones in intrusive rocks. A-veinlets contain variable amounts of quartz, magnetite, biotite, and chalcopyrite and locally have K feldspar halos. They have nonmatching, irregular vein walls and lack internal symmetry. Hypersaline liquid-rich inclusions coexisting with vapor-rich inclusions in A-veinlets indicate temperatures as high as nearly 700°C and pressures between 200 and 400 bars. Assuming a lithostatic load, depths of 0.8 to 1.6 km are inferred. Zones of abundant A-veinlets contain mostly <1 ppm gold and 0.1 to 0.4 percent hypogene copper. Banded quartz veinlets occur mostly above A-veinlets and cut A-veinlets where they overlap. Dark gray bands, the color resulting from a high density of vapor-rich fluid inclusions and micron-sized grains of magnetite, commonly occur as symmetric pairs near the vein walls. Vein walls are parallel and slightly wavy, vuggy vein centers are common, and alteration envelopes are absent. Data from rare liquid-rich inclusions in banded quartz veinlets indicate temperatures <350°C at pressures between 20 and 150 bars. Assuming a hydrostatic load, depths of 0.2 to 1.5 km are inferred. Zones of abundant banded quartz veinlets generally contain 0.5 to 2 ppm gold and <0.1 percent hypogene copper. D-veins are pyrite veins with quartz-sericite-pyrite halos. They are widespread and crosscut A-veinlets and banded quartz veinlets. The brittle nature of D-veins and limited fluid inclusion data suggest temperatures <400°C. D-veins serve as important time lines. They are nowhere truncated or crosscut by intrusions, A-veinlets, or banded quartz veinlets. Quartz-alunite replacement veins, referred to as ledges in this paper, are typical of the high-sulfidation epithermal environment. They are mostly limited to overlying volcanic rocks. They contain local core zones of vuggy residual quartz that can contain enargite or, at higher elevations, barite. Of the three districts studied only La Pepa has mineable quartz-alunite ledges, which contain an average gold grade of about 20 ppm. A spectrum of porphyry-style deposits exists. Cerro Casale at Aldebaran shares many characteristics of porphyry copper deposits worldwide, whereas Verde at Refugio is a true porphyry gold deposit. Potassic alteration zones and A-veinlets are strongly developed at Cerro Casale, whereas they are absent at Verde. Banded quartz veinlets predominate at Verde, whereas they occur only at the upper levels of Cerro Casale. The Pancho deposit at Refugio and the Cavancha deposit at La Pepa are telescoped systems in which banded quartz veinlets overprint potassic alteration zones and A-veinlets. A-veinlets and banded quartz veinlets cut and are cut by intrusions, indicating multiple cycles of intrusion→potassic alteration→A-veinlets→banded quartz veinlets during formation of porphyry-style mineralization. Banded quartz veinlets are thought to have formed by flashing of magmatic fluids during episodic transitions from lithostatic to hydrostatic pressure. Loss of sulfur to the vapor phase during flashing inhibited formation of copper-sulfides in banded quartz veinlets and, therefore, resulted in high gold/copper ratios. Where rising magmatic vapors condensed into overlying meteoric water along faults, barren quartz-alunite ledges formed. This conclusion is supported by equivalent 40Ar/39Ar dates on hydrothermal biotite associated with porphyry-style ore and alunite from barren ledges at Aldebaran. 40Ar/39Ar dates at La Pepa indicate alunite formed at least 140,000 years to as long as 900,000 years after hydrothermal biotite. Within the high-sulfidation epithermal environment, the development of ore depends on the ability of late, moderate-salinity magmatic fluids to reach the surface without condensing a brine upon ascent. Cooling and boiling of the moderate-salinity fluid below its critical temperature results in the formation of sericite at depth and alunite near the surface that is essentially synchronous with high-sulfidation ore formation. The timing of the switch from lithostatic pressures to brittle hydrostatic conditions, relative to the life of the hydrothermal system, might determine how much porphyry-style ore forms relative to high-sulfidation epithermal ore.

The distribution of rare earth elements between monzogranitic melt and the aqueous volatile phase in experimental investigations at 800 °C and 200 MPa

The partitioning of the rare earth elements between a peraluminous monzogranitic melt and a chloride-bearing, sulfur- and carbon dioxide-free, aqueous volatile phase was examined experimentally as a function of chloride and major element concentrations at 800 °C and 200 MPa. The light rare earth elements (e.g. La, Ce) partition into the aqueous volatile phase to a greater extent than the heavy rare earth elements (e.g. Yb, Lu). Distribution of the rare earth elements and the major elements H, Na, K, Ca, and Al between the melt phase (mp) and aqueous volatile phase (aq) is a function of the chlorine concentration in the system, and our data are consistent with the rare earth and major elements occurring as chloride complexes in the aqueous volatile phase. Apparent equilibrium constants for experiments at 800 °C and 200 MPa, K ′ REE,Na aq/mp , expressed as the ratio of the concentration of a given rare earth element in the aqueous volatile phase to the concentration of the same element in the melt phase, divided by the cubed ratio of sodium in the aqueous volatile phase to the concentration of sodium in the melt phase, decrease systematically with increasing atomic number from K ′ La,Na aq/mp = 0.41(±0.03) to K ′ Lu,Na aq/mp =0.11(±0.01), except for Eu. These experimentally derived apparent equilibrium constants for the rare earth elements can be used in a numerical simulation of magmatic volatile exsolution. The simulation gave results consistent with the elemental distribution in the potassic alteration zone of a deep porphyry copper deposit, but higher concentrations of heavy rare earth elements are released into the magmatic aqueous solution than are captured in the secondary mineralization.

Chemical composition of biotite from the Casino porphyry Cu–Au–Mo mineralization, Yukon, Canada: evaluation of magmatic and hydrothermal fluid chemistry

The chemical composition of biotite has been determined for the Dawson Range batholith and the Casino, porphyry-style Cu–Au–Mo occurrence, Yukon Territory, Canada. Biotite from the propylitic, phyllic and potassic alteration zones of the Casino occurrence possesses higher X Mg, Al 2O 3, SiO 2, F and Cl, and lower TiO 2, BaO and MnO concentrations than biotite outside these alteration zones. This phenomenon is similar to that recorded for biotite of other porphyry Cu deposits. However, the absolute concentrations of the oxides and halogens vary significantly among deposits. Calculated log ( fH 2O)/( fHF), ( fH 2O)/( fHCl), ( fHCl)/( fHF) ratios, determined from biotite microprobe data, indicate that hydrothermal fluids associated with potassic alteration at Casino were similar to those associated with phyllic alteration. In contrast, fluids responsible for propylitic alteration had higher log ( fH 2O)/( fHF) and less negative log ( fHF)/( fHCl) values. The log ( fH 2O)/( fHCl) and ( fHF)/( fHCl) ratios established for Casino are similar to those for other porphyry Cu deposits. However, log ( fH 2O)/( fHF) values vary greatly among porphyry deposits. This variation in log ( fH 2O)/( fHF) values may be related to the source of the magmas and/or magmatic processes (assimilation/fractional crystallization) associated with melt on its ascent through the crust prior to the exsolution of magmatic fluids that are responsible for porphyry mineralization. The biotite compositions from the Dawson Range batholith are either not consistent with the Fe–F and Mg–Cl avoidance principles or record changes in the F/Cl ratio of the fluid during crystallization of the granitic body.

The San Jorge porphyry copper deposit, Mendoza, Argentina: a combination of orthomagmatic and hydrothermal mineralization

The San Jorge porphyry copper deposit (SJPCD) is hosted by Carboniferous clastic sedimentary rocks and Permian intrusions located within the Permo-Triassic belt of Chile and Argentina. Its hypogene mineralization and alteration are products of superposed orthomagmatic and hydrothermal events that were strongly fault controlled. Copper related to orthomagmatic processes includes disseminated chalcopyrite in the matrix of porphyritic granodiorite and andesite, and chalcopyrite with tourmaline and quartz in breccias, both of which have accompanying potassic alteration. Soon thereafter, disseminated chalcopyrite is associated with a structurally controlled silicification of the sedimentary sequence. Finally, multiple episodes of hydrofracturing, probably driven by a deep-seated intrusion, deposited sulfide minerals in veinlets throughout the sedimentary sequence; the centers of these systems are characterized by potassic alteration. Total sulfides, which include chalcopyrite, pyrite, arsenopyrite, and pyrrhotite, and pyrite:chalcopyrite form a linear NNE trend, parallel to the main faults. Quartz–sericite is the dominant alteration and is ubiquitous. Zones of potassic alteration can be delineated even though phyllic alteration can be superposed. Much of the system evolved under reducing conditions. Despite uplift along a reverse fault during the Tertiary, and subsequent erosion, the system is preserved at high levels. Supergene processes redistributed copper in secondary oxides and sulfides. These processes were more effective where the deposit is covered by unconsolidated alluvial sediments. The unique history of the San Jorge deposit renders it an important variation of porphyry copper-style mineralization.

Early magnetite-amphibole-plagioclase alteration-mineralization in the Island copper porphyry copper-gold-molybdenum deposit, British Columbia

The initial hydrothermal event in the evolution of the ca. 377 Mt, 0.41 percent Cu, Island Copper porphyry Cu-Au-Mo deposit, northern Vancouver Island, was the formation of an extensive alteration-mineralization facies dominated by magnetite, calcic amphibole, and intermediate to sodic plagioclase. Early-stage quasipervasive magnetite-rich alteration and associated magnetite-rich veinlets in this Middle Jurassic, island arc-hosted, hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata. The magnetite-rich alteration zone, 500 to 700 m wide and originally extending over at least 450 m vertically, exhibits an outward zonation from quartz-magnetite-albite (An (sub < or =6) )-amphibole (trace) - apatite (trace), through quartz-magnetite-amphibole-albite (An (sub 6-10) ) or oligoclase-andesine (An (sub 15-36) )-apatite (trace) + or - scapolite (trace), to amphibole-magnetite-oligoclase or sodic andesinc (An (sub 15-39) ) + or - quartz + or - apatite (trace) assemblages. Definition of early-stage mineral assemblages, impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration, was facilitated by the use of novel petrographic techniques, including incident-light Nomarski differential interference contrast microscopy. Early-stage mineralization comprises several well-defined vein-veinlet types which were demonstrably emplaced prior to the main-stage chalcopyrite-pyrite stockwork and associated K silicate alteration. Minor chalco-pyrite and pyrite occur in some early-stage veins, and magnetite is a constituent of many later main-stage sulfide-rich veins, but the distinction between these two major vein systems is clear.Mass-exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks, with an Fe enrichment approaching 450 percent (20 g/100 cc) in the dacite dike, and lesser and more variable Na enrichment, with both Ti and Al behaving as mobile constituents. Paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite-rich alteration-mineralization and imply that this event was initiated above 650 degrees C and at pressures of at least 1 kbar and was generated by FeCl 2 -rich brines, at first single phase and moderately saline (avg 15 wt % NaCl equiv), but latterly boiling at ca. 560 degrees to 645 degrees C and < or = ca. 0.55 kbars. The main-stage potassic alteration zone has the configuration of an annulus, ca. 100 to 150 m in width, which was entirely superimposed on the more extensive early alteration-mineralization zone at ca. 430 degrees to 575 degrees C and below 450 bars. Gold, although correlated overall with Cu and potassic alteration, was probably extensively introduced in the early stage.It is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids, in which SO (super -) 2 exceeded both H 2 S and SO (super 2-) 4 . The deposition of abundant magnetite and the widespread and intense Fe metasomatism which define the early-stage record the high solubility of Fe as FeCl 2 in high-temperature brines in equilibrium with magnetite-bearing quartzofeldspathic rocks, as well as the retrograde solubility of magnetite between ca. 550 degrees and 750 degrees C. Whereas the high SO 2 /H 2 O + SO 4 ratios of the initial hydrothermal fluids at Island Copper, and in other similar deposits, may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas, they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating, S-rich, mafic melts. The mineral assemblages of the early-stage veins and associated quasi-pervasive alteration cannot be assigned to the accepted alteration-mineralization facies of porphyry systems, such as A veins and potassic alteration, and record specific and distinct conditions of fluid-rock interaction.

Genesis of the Giant Late Miocene to Pliocene Copper Deposits of Central Chile in the Context of Andean Magmatic and Tectonic Evolution

Multiple large mineralized breccia pipes (Cu grades up to >10%; individual pipes with >10 × 106 metric tons of Cu) are prominent, if not dominant, features in the three giant Andean Cu deposits of Los Pelambres, Los Bronces-Rio Blanco, and El Teniente of central Chile. At Los Bronces-Rio Blanco, over 90% of the >50x 106 metric tons of hypogene Cu occurs within the matrix of breccias and/or clasts and wall rock altered in association with the formation of these breccias, while at the other two deposits a lesser but still significant amount of Cu ore also is directly related to breccias. At both Los Pelambres and Los Bronces-Rio Blanco, high-grade (>0.5%) Cu occurs in zones of potassic alteration characterized by stockwork biotite veining and intense biotitization associated spatially, temporally, and genetically with biotite breccias. At Los Bronces-Rio Blanco, high-grade ore also occurs in younger tourmaline breccia pipes, emplaced both within and around the older central biotite breccia complex and potassic alteration zone after a period of uplift and erosion. Potassic alteration, sericitization, silicification, and mineralization of clasts in these tourmaline breccias occurred during their formation. At El Teniente, a significant amount of high-grade Cu ore also occurs in different tourmaline-rich breccias, including the marginal portion of the Braden breccia pipe and a related zone of quartz-sericite alteration that surrounds this pipe. Small, shallow, weakly mineralized or barren silicic porphyry intrusions occur in each of these three deposits, but their main role has been to redistribute rather than emplace mineralization. The mineralized breccia pipes in each deposit were emplaced into early and middle Miocene volcanic and plutonic rocks during the late Miocene and Pliocene by the expansion of boiling aqueous fluids. Fluid-inclusion and stable-isotope data indicate that the high-temperature, saline, metalrich fluids that produced the brecciation, precipitated the Cu ore in the matrix of the breccias, and generated the associated alteration and mineralization in clasts and wall rock were magmatic in origin. These magmatic fluids were not derived from the early and middle Miocene host plutons, which already were solidified at the time of breccia emplacement. Sr- and Nd-isotopic compositions of breccia-matrix minerals indicate that breccia-forming fluids were exsolved from magmas that were isotopically transitional between older volcanic and plutonic host rocks and younger silicic porphyry stocks, dikes, and extrusives. The fact that the roots of the breccias have not yet been encountered implies that these magmas cooled at depths >3 km to form plutons not yet exposed at the surface. The generation of the multiple mineralized breccias at each deposit occurred over a relatively short (but still significant) time period of 1 to 3 million years, during the final stages of existence of the long-lived (7gt;15 m.y.) Miocene magmatic belt in central Chile. The decline of magmatic activity in this belt was tectonically triggered, as subduction angle decreased in association with the subduction of the Juan Fernandez Ridge. This caused a decrease in the sub-arc magma supply and subsequently eastward migration of the magmatic arc, as well as crustal thickening, uplift, and erosion, which led to the superposition of younger and shallower alteration and mineralization events on older and deeper events in each deposit. The giant Cu deposits of central Chile cannot be explained by a static model in which their size is a function of the mass of a single pluton or the longevity of a single hydrothermal convection system. These deposits are giant because they were produced by multistage processes involving the formation, over a period of 1 to 3 million years, of multiple superimposed mineralized breccias and associated alteration zones resulting from the exsolution of metalrich magmatic fluids from independent magma batches cooling at depths >3 km. Neither an unusually large magma supply nor Andean magmas of unusually high Cu content is required to produce the sequence of multiple mineralization

A metamorphosed, potassic alteration zone associated with the Bleikvassli ZnPbCu orebody, Northern Norway

The Bleikvassli ZnPbCu deposit is located in amphibolite facies schists and gneisses in the Rødingsfjellet Nappe Complex, North Norwegian Caledonides. A microcline gneiss occurs as a lens-shaped body, up to 250 m thick, in the footwall and as small, irregular bodies in the hanging wall. The microcline gneiss contains 40–75% microcline, and is characterized by a spotty texture. The footwall microcline gneiss is chemically zoned, with highest K 2O content (10–12 wt.%) and highest alkali-to-alumina ratio (0.76) in the central part. The mica and plagioclase contents increase towards the margins of the gneiss, which grades laterally into the surrounding kyanite-mica schist. The field relations and the chemical composition of the gneiss indicate that the potassium-rich protolith formed by addition of alkalies, SiO 2 and heat to a pelitic sediment. The microcline gneiss has thus been interpreted as the central part of a hydrothermal alteration zone associated with the Bleikvassli orebody.

On a proposed plutonic porphyry gold deposit model

A plutonic porphyry gold deposit model is proposed that is imilar to the plutonic porphyry copper deposit model. However, unlike the plutonic porphyry copper deposit model, the proposed model is deficient in copper and contains less than 1 percent total sulfides. In the proposed model, gold is accompanied by scheelite, molybdenite, arsenopyrite, a variety of bismuth sulfides, tellurides, and native bismuth. The host rock varies from granite to granodiorite stock. Most of the ore is in the pluton. Deposits cited as examples of the proposed model are the Mokrsko deposit in Czechoslovakia, the Fort Knox deposit in the United States, and the Dublin Gulch deposit in Canada. In each of these deposits, pervasive potassic or phyllic alteration zones accompany the gold ore, which is disseminated in quartz-rich stockworks, veinlet swarms, and veins. Tonnages of gold-bearing material are large, but grades are low in the cited deposits. The proposed model is distinct from other gold deposit models because of the low Cu to Au ratio and the association of Au, Bi, W, and Mo.

The sulphur springs geothermal field, St. Lucia, lesser Antilles: Hydrothermal mineralogy of wells SL-1 and SL-2

Two wells have been drilled to depths of 1413 and 2213 meters in the geothermal field of Sulphur Springs, St. Lucia, and reveal a complex volcanic sequence characterized by collapse episodes followed by the emplacement of dacite domes. The geothermal reservoir consists of fractured volcanic rocks and produces superheated steam. Well-bottom temperatures are around 270–290°C. The hydrothermal alteration found in both the productive SL-2 well and the non-productive SL-1 is strongly reminiscent of that of porphyry copper deposits, with (1) an inner, high-temperature potassic zone characterized by the occurrence of dravitic tourmaline, quartz, and biotite, (2) an outer propylitic alteration zone that is partly superimposed on (3) a potassic alteration zone. The alteration mineral assemblages indicate that the hydrothermal system has cooled at the levels sampled.

Rare-earth element and heavy-metal behaviour associated with the epithermal gold deposit on Lihir Island, Papua New Guinea

A thick sequence of alkaline intrusions and volcanic rocks underlies a Quaternary caldera on Lihir Island. The sequence is host to a still-active subaerial hydrothermal system and associated epithermal gold mineralization. Chondrite-normalized (La/Lu) cn and (La/Sm) cn ratios progressively increase up the alteration sequence from the potassic alteration zone, to the argillic zone to the advanced argillic zone. (Tb/Lu) cn ratios only significantly increase in argillic and advanced argillic assemblages. Surface oxide alteration lithologies and acid sulphate water precipitates possess distinctly lower (La/Lu) cn and (Tb/Lu) cn ratios than the underlying subsurface alteration units. The changes in the REE, LREE and HREE fractionation trends from subsurface to surface alteration zones reflect the transition from a magmatic-hydrothermal, neutral chloride fluid regime at depth to acid sulphate meteoric waters in the upper portion of the alteration profile. Boiling of the LREE-Eu-enriched magmatic fluids occurred at a depth of at least 750 m. It is proposed that pronounced differential flow rates of the vapour and liquid phases and solution chemistry changes approximately above 300–350 m caused the incorporation of LREE and Eu into anhydrite-calcite veins and the deposition of LREE and HREE into wallrocks of upper parts of the potassic alteration unit. Condensation of the vapour phases into meteoric waters gave rise to low-temperature acid fluids that deposited large amounts of LREE within argillic and advanced argillic alteration units. This was also accompanied by HREE mobility due to large fluid volumes, acid fluid conditions and abundant sulphate complexes within the solutions. The HREE were either lost from the hydrothermal system or deposited in oxide assemblages and acid sulphate water precipitates.