Breccia-hosted copper-molybdenum mineralisation at Rio Blanco, Chile
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Abstract:
The Rio Blanco-Los Bronces ore deposit is located at 33°l5'S in the Principal Cordillera of central
Chile. It lies midway between the Los Pelambres and El Teniente porphyry copper deposits, which
together define one of Chile's most economically significant metallogenic belts. Rio Batholith and
surrounding Tertiary volcanic and volcaniclastic rocks at approximately 5-4 Ma. The bulk of the
>50 million metric tonne copper resource is hosted by the Rio Blanco Magmatic Breccia and the
Sur-Sur Tourmaline Breccia.
The geometry of the Rio Blanco-Los Bronces system is asymmetrical, and appears to have been
controlled by three principal fracture orientations that trend N, NW and NE. Two N-trending
faults tenninate at the eastern and western boundaries of the Rio Blanco-Los Bronces deposit and
have acted as pull-apart structures that allowed hydrothermal fluids and felsic magmas to be
emplaced within the ore deposit. The N-trending faults are the largest and most deeply eroded
fault orientations in the district and at Sur-Sur are occupied by Tounnaline Breccia. The phyllic
altered Tourmaline Breccia is marginal to the potassic altered Rio Blanco Magmatic Breccia,
which constitutes the core zone between the two N-trending fault terminations. Although the NWand
NE-trending faults crosscut the Tounnaline Breccia, their fracture intensities are highest where
strongly mineralised zones occur along the strike length of the breccia body, indicating that these
fracture trends were active during brecciation and helped to localise fluid flow.
Paragenetic studies indicate nine main stages of veins, breccia and porphyry emplacement in the
Rio Blanco and Sur-Sur sectors of the ore deposit. The stages are: ( 1) magnetite-actinolite alteration;
(2) potassic stockwork veins; (3) Rio Blanco Magmatic Breccia and Sur-Sur Tourmaline
Breccia; ( 4) Feldspar Porphyry; ( 5) potassic stockwork veins; ( 6) Quartz Monzonite Porphyry and
Don Luis Porphyry; (7) molybdenite stockwork veins; (8) chalcopyrite stockwork veins; and (9) D
veins. Barren dacite and rhyolite intrusions cross-cut the ore deposit complex.
A change from potassic to phyllic alteration defines the contact between the Rio Blanco Magmatic
Breccia and Sur-Sur Tounnaline Breccia. The Rio Blanco Magmatic Breccia occupies a large
volume of rock within the Rio Blanco, La Union, Don Luis and Sur-Sur sectors, and extends over
a vertical interval of ~2 km. The Tourmaline Breccia lies transitionally outward from the Magmatic
Breccia mainly in the Sur-Sur sector. There is a 100m thick gradational contact between deeplevel
biotite breccia and shallow-level tourmaline breccia at Sur-Sur. Paragenetic studies ofbreccia
cement minerals in the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia reveal a
similar cement infill sequence involving initial biotite and/or tounnaline precipitation at clast
margins, followed by sulfate (anhydrite) and oxide (specularite) mineral phases, and in tum by
chalcopyrite and magnetite. Spatial zonation of breccia cement minerals occurs in the Rio Blanco and Sur-Sur breccias. Zonation
of biotite and tourmaline coincides with zonation of potassic and phyllic alteration, respectively.
Chalcopyrite is spatially associated with stage 3 Magmatic Breccia and biotite alteration in
the Rio Blanco to Don Luis sectors, and with stage 3 Tounnaline Breccia and quartz-sericite
alteration in the Sur-Sur sector. Outward from the potassic and phyllic altered zones; a propylitic
assemblage occurs that is defined by chlorite alteration and pyrite-specularite breccia cement minerals.
New 40 Arf39 Ar geochronology of hydrothermal biotite from the base of the Sur-Sur Tourmaline
Breccia and whole rock sericite from the top of the Sur-Sur Tounnaline Breccia yielded ages of
4.78 ± 0.04 Ma and 5.42 ± 0.09 Ma, respectively. Both 87Sr/86Sr and aNd analyses for tounnaline
and anhydrite from the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia range
between 0.7040 and 0.7044, and +1.70 and +2.53, respectively. 206Pbf2°4Pb values for anhydrite
cement in the Rio Blanco Magmatic Breccia and the Sur-Sur Tourmaline Breccia ranged between
17.558 and 18.479, 207Pbf204pb values ranged between 15.534 and 15.623, and 208Pb/204Pb values
ranged between 37.341 and 38.412. The early-fonned anhydrite cement has Pb isotopic compositions
that are less radiogenic than the sulfide ores and host rocks, and also has elevated initial Sr
ratios compared to the host rocks. Pb and Sr in anhydrite are interpreted to have been sourced
from rocks and/or waters external to the main magmatic-hydrothermal system.
Oxygen/deuterium isotopes for tourmaline breccia cement minerals have 'magmatic' values, however
recalculated values of propylitic-altered samples from previous workers indicate a meteoric
water component of up to 25%. Zonation of sulfur isotope compositions occurs in the mineralised
breccias, particularly at Sur-Sur. The Rio Blanco sector is characterised by sulfides with 834S
values between -3.94 and +3.34 and sulfates between +10.07 and + 17.86 values. The Sur-Sur
sector is characterised by sulfides with 834S values between -4.12 and +2.65 and sulfates between
+11.15 and +13.39. These values are consistent with a magmatic-hydrothermal sulfur source. At
Sur-Sur, the most negative ()34S compositions (834S < -3 per mil) are spatially associated with the
highest copper grades and specularite cement.
The Rio Blanco Magmatic Breccia and Sur-Sur Tourmaline Breccia contain co-existing low salinity
liquid-rich and vapour-rich fluid inclusions and localised zones containing co-existing vapour-rich
and hypersaline fluid inclusions. Homogenisation temperatures from >600 to 131 oc have been
measured, but most are between 450° and 300°C. Complete salinity arrays from 0-69 wt.% NaCl
equivalent were observed, and eutectic temperatures are commonly below -35°C. Minimum pressure
estimates from fluid inclusions are between 48 and 368 bars. An average lithostatic formation
depth of 200 m and a hydrostatic formation depth of 2300 m below the palaeo-surface have been
calculated, indicating that up to approximately 1 km of erosion has occurred since breccia formation. The mineralised breccias in the Rio Blanco and Sur-Sur sectors are magmatic-hydrothermal in
origin. They formed when magmatic-hydrothermal fluids (brine and gas) exsolved from deepseated
magma and potentially mixed with an external water. Hydrostatic pressures catastrophically
exceeded lithostatic load plus the tensile strength of the confining rocks leading to brecciation.
At Sur-Sur, fault rupture along the Rio Blanco Fault may have been a trigger for magmatic-hydrothennal
brecciation. Phase separation of magmatically-derived aqueous fluid occurred at the onset
ofbrecciation, with a low density gas phase (carrying H20, S02, HCl and B20 3) separating physically
from the dense copper-bearing brine. The gas phase fluxed through the breccia column first,
where it condensed into exotic groundwaters of uncertain derivation, resulting in the deposition of
oxide-stage cements (anhydrite, specularite, tourmaline) from a hybrid low salinity water. Subsequent
upwelling of the magmatic-hydrothermal brine resulted in main stage sulfide deposition, possibly
in part due to fluid mixing with the hybrid water.Keywords:
Breccia
Stockwork
Batholith
Dike
Felsic
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Detailed geologic mapping has been used to show that Agua Rica is a porphyry-style Cu-Mo-Au deposit that was first overprinted by polystage brecciation associated with a high sulfidation epithermal event and then by a barren surface-venting phreatomagmatic diatreme, prior to a final stage of supergene enrichment. It was emplaced in the Miocene (~8–5 Ma) as an outlier of the Farallon Negro Volcanic Complex in northwestern Argentina.
The Agua Rica deposit lies next to the contact between Precambrian or lower Paleozoic metasedimentary rocks and coarse-grained Ordovician granites. In a first pulse of Miocene magmatism, equigranular to porphyritic intrusions were emplaced, with minor potassic alteration and weak Cu-Mo mineralization. Subsequent intrusion of feldspar porphyries was associated with intense porphyry-style stockwork veining, potassic and propylitic alteration, and disseminated Cu-Mo-Au mineralization (molybdenite, chalcopyrite ± bornite ± pyrite). The present alteration and mineralization pattern is dominated by an almost pervasive overprint of high sulfidation epithermal assemblages (phyllic and advanced argillic alteration and Cu-Au-Ag-As-Pb-Zn mineralization) in breccia cements and as void fillings. Covellite is the dominant copper mineral in the ore and seems to have partly or completely replaced chalcopyrite and bornite of the earlier porphyry events. The high sulfidation epithermal assemblages are closely related to the emplacement of a largely clast-supported hydrothermal breccia. Three major bodies of this breccia have been mapped on the basis of clast lithology, clast shape and size, degree of alteration, and composition of breccia matrix. Igneous breccia with a fine-grained porphyritic matrix is intimately associated and interfingers with the base of the hydrothermal breccia columns. A final phase of magmatic hydrothermal activity formed a matrix-supported and commonly bedded crater infill breccia. It formed by a surface-venting phreatomagmatic eruption, as shown by a continuous downward transition from bedded breccias to clast-supported breccias with sandy or pumiceous matrix to a solid igneous breccia with a fine-grained porphyritic matrix in the lower core of the conical crater infill breccia body. Graded, matrix-rich epiclastic sediments subsequently filled the crater. Magmatic activity was terminated by a dike of unmineralized biotite porphyry, which intruded the crater infill breccia. Talus breccia was shed into the crater from the rim. Supergene leaching and enrichment, which replaced covellite, pyrite, chalcopyrite, and bornite by chalcocite and secondary covellite, formed an enrichment blanket that was dissected by the present-day, steeply incised topography.
The distinctive feature of the Agua Rica hydrothermal system is the occurrence of early, weakly mineralized intrusions, later feldspar porphyries with stockwork-hosted chalcopyrite-bornite-molybdenite mineralization, hydrothermal breccias with an epithermal pyrite-covellite overprint, and barren surface-venting breccias—all exposed at one location within 1,000 m of vertical exposure. Reconstruction of the time sequence of these geologic elements indicates that Agua Rica is the result of a protracted history of magmatic hydrothermal activity with superposition of several intrusion events that probably extended over several million years during progressive regional uplift, erosion, and explosive unroofing.
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Copper-bearing breccia pipes in the Redbank area intersect an interbedded sequence of igneous and dolomitic sedimentary rocks which have undergone various degrees of metasomatism.The steeply inclined breccia pipes are of small size and cylindrical form and typically show in situ brecciation. The breccia matrix and associated veins consist essentially of various proportions of microbreccia, dolomite, quartz, chlorite, celadonite, hematite, K-feldspar, apatite, and chalcopyrite, with minor barite, rutile, galena, and pyrobitumen. K-metasomatism is most intense in the vicinity of the breccia pipes and associated veining, and there is mineralogical and textural evidence indicating that fluids enriched in K, Cl, P, Mg, Ce, La, CO 2 , and H 2 O were introduced at the time of breccia-pipe formation. Carbonate and sulfide minerals from brecciated and metasomatized rocks at lower stratigraphic levels have isotopic compositions consistent with magmatic hydrothermal derivation. However, the delta 13 C values of the great bulk of the dolomite in the breccia pipes indicate remobilization of sedimentary carbonate. Furthermore, the sulfur isotope ratios of the main sulfide mineralization, which occurs near the top of the brecciated sequence, are variable and generally enriched in 34 S relative to the minor amounts of sulfide at lower levels.It is concluded that the breccia pipes formed by explosive release of fluids following the buildup of significant over-pressure in a postulated carbonated, K-rich trachytic magma at depths of roughly 2 to 3 km beneath the surface. This was accompanied by intense metasomatism and precipitation of some carbonate and sulfide minerals which partly infilled the open spaces. The thermal gradients and fracturing caused extensive circulation of connate brine (and possibly descending sea water). The brine remobilized sedimentary and magmatic hydrothermal components in and around the pipes and copper mineralization in the Redbank area mainly precipitated from this brine.
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Greisen
Cassiterite
Breccia
Argillic alteration
Wolframite
Stockwork
Topaz
Porphyritic
Molybdenite
Arsenopyrite
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The early Proterozoic Crandon massive sulfide deposit occurs in a 280-km-long, east-trending greenstone belt along the southern margin of the Southern province of the Canadian Shield. The deposit is conformably contained within a sequence of subaqueous andesitic to dacitic pyroclastics, flows, and associated chemical sedimentary rocks which strike N 85 degrees W and dip 70 degrees to 90 degrees N. Regional metamorphism in the area achieved lower greenschist facies. Regional dip of the rocks is near vertical, but smaller scale or penetrative deformation features are rare.The footwall of the deposit consists of a thick sequence of andesitic tuff overlain by a series of volcanic breccias. Some of the breccias resulted from local fumarolic activity, whereas other breccias represent a complex history of phreatic and phreatomagmatic volcanism and silicification some distance from the deposit. Examination of the breccia types, distribution, and thicknesses suggests at least two source areas for the breccias. The breccias served as permeable conduits for the ore fluids, directing them laterally toward a syndepositional graben where the fluids were vented into a topographic depression on the ocean floor during a hiatus in local volcanic activity. Up to 100 m of massive sulfide consisting of laminae of pyrite and sphalerite with minor chalcopyrite, galena, quartz, chlorite, sericite, and dolomite and minor interbedded tuff, chert, argillite, sandy tuff, and dolomite were deposited.Following chemical sedimentation, hydrothermal venting continued producing crosscutting vein mineralization. Vein development is intimately linked to the distribution of breccia in the footwall. Ascending fluids continued to migrate laterally through the permeable breccia and deposited vein mineralization which plugged the original vent areas with fine-grained precipitates of silica and sulfide. Secondary porosity was created by refracturing of tuff and breccia. Vein mineralization, localized in the footwall of the massive sulfides, exhibits a systematic compositional variation with time and space from west to east. Beneath the west end of the deposit the earliest veins consist of quartz and grade eastward into quartz-chalcopyrite-pyrite, quartz-pyrite-sphalerite-chalcopyrite, pyrite-sphalerite, and finally pyrite veins. The distribution and degree of veining suggests two source areas which correspond to the two source areas for the lithic breccias.Recoverable reserves in the deposit are estimated at 61 million metric tons averaging 1.1 percent Cu, 5.6 percent Zn, 0.5 percent Pb, 37 g/metric ton Ag, and 1.0 g/metric ton Au.Alteration of the footwall rocks at Crandon consists primarily of silicification, sericitization, pyritization, and minor chloritization. Interaction of ore fluid with wall rock has resulted in enrichment in SiO 2 , Fe, K, F, S, Cu, Zn, As, Sb, Ba, Au, Hg, Pb, Bi, Se, and Cd and depletion in Al, Mg, Ca, Na, V, and Sr. The morphology of the alteration zone at Crandon is tabular, due to control by primary and secondary porosity.
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The Coalstoun porphyry copper prospect in southeast Queensland lies adjacent to the north-northwest-trending Perry fault zone near the intersection with an east-northeast-trending structural lineament. Late Permian intrusions at the prospect are dominantly porphyritic biotite + or - hornblende microtonalite and quartz microdiorite emplaced into Devonian ?-Carboniferous quartz-rich fine-grained metasediments. Three intrusive centers, each containing several distinct phases, are recognized in an area of 6 X 2.5 km. The intrusions appear to have attained shallow crustal levels as evidenced by porphyritic texture, abundance of metasedimentary xenoliths, rafts and screens, moderately developed fracturing, and the presence of numerous surrounding breccia pipes.A core of hydrothermal biotite alteration containing copper grades of 2,000 to 4,000 ppm and up to 150 ppm Mo is present in the central intrusion. The biotite zone possibly overlies a deep, relatively unaltered core and grades outward into a chlorite-carbonate alteration zone which extends out into surrounding metasediments. Small pockets of quartz-sericite-pyrite and hypogene sericite-clay alteration occur surrounding, and within, the biotite zone, and the complete hypogene alteration system is partially blanketed by supergene clay (-seriate) alteration containing enriched copper grades. Associated breccia pipes are dominantly composed of angular metasedimentary fragments and exhibit quartz-sericite-pyrite and sericite-clay alteration with sporadic tourmaline. Copper grades decrease outward from the biotite zone, whereas pyrite: chalcopyriteratios increase.Alteration in the biotite and chlorite-carbonate zones is nearly isochemical, with a minor decrease in Na 2 O/K 2 O, an increase in Rb/Sr, and increases in Cu and K 2 O in the former. Volumetrically minor quartz-sericite-pyrite and sericite-clay zones generally show decreases in MgO, CaO, Na 2 O, Ba, Cr, Ga, Sr, V, Y, Zn, and Na 2 O/K 2 O and increases in H 2 O, S, K 2 O, K/Rb, and Rb/Sr.Hydrothermal biotites commonly pseudomorph magmatic biotite and hornblende pheno-crysts, are finer grained, and are pale brown to greenish-brown compared to magmatic biotites which are orange-brown. Hydrothermal biotites contain lower TiO 2 , total Fe and FeO, Fe/Fe + Mg + Mn, and Na 2 O, and higher (A1) VI and possibly higher Fe (super 3+) /Fe (super 2+) + Fe (super 3+) than the magmatic biotites. The differences between the two biotite types are interpreted as indicating a decrease in pressure and/or temperature and increases in oxygen and sulfur fugacities in the formation of hydrothermal biotite.Initial emplacement of porphyry intrusions (possibly generated in a continental margin tectonic environment) to high crustal levels was controlled by regional fault intersections. Columns of shattered rock above the intrusions were mobilized as breccia plugs ahead of the rising stocks. Crystallization and cooling of the stocks was accompanied by retrograde boiling, fluid expulsion through the breccias, additional collapse structures, and stockworking. A high-temperature biotite zone formed with pervasive and fracture-controlled mineral deposition from high-salinity, possibly oxidizing, magmatically derived hydrothermal fluids, which introduced Cu, Mo, S, and K 2 O and initially extended toward the margins of the intrusives.Expulsion of magmatic fluids and interaction with convecting meteoric waters possibly caused deposition of quartz-sericite-pyrite and hypogene sericite-clay alteration assemblages peripheral to, and within, the biotite zone. Gradual cooling of the system initiated an inward collapse of the meteoric fluid convective cells, forming pervasive retrograde chlorite-carbonate alteration in the metasediments and outer portions of the stocks.Possibly from the Tertiary to the present, topographically controlled supergene clay (-sericite) alteration has occurred, with significant secondary copper sulfide development.
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The copper deposits of Perú consist of porphyry Cu±Mo, Au, Ag, breccia pipe Cu-Mo, enargite vein and replacement Cu±Au, Ag, Zn, Pb, calcic skarn Cu±Fe, Au, Zn, amphibolitic skarn Cu±Fe, volcanogenic massive sulfide Cu-Zn, vein and manto Cu±Ag, Pb, Zn, Sn, W, and sandstone ("red bed") Cu types. The vast majority of these deposits formed during the Andean Orogeny and are geographically and chronologically distributed in well-defined metallogenic domains. These domains correlate with geochemically distinct magmatic episodes.The magmatic and metallogenic domains appear to be controlled in part by transverse growth-faults in the Mesozoic and older basement rocks underlying the intensely folded and thrust-faulted Mesozoic and Tertiary rocks of the higher structural levels of the Cordillera. During the Andean Orogeny the extent of magmatism and the corresponding metallogenic provinces were influenced by subducted plate segmentation and by continental margin basement tectonics. In addition, the lithologic nature of the host rocks played an important role in determining the types of copper deposits formed.Porphyry Cu, breccia pipe Cu-Mo and calcic skarn Cu deposits are related to the Pomahuaca, Coastal and Caldera batholiths, as well as to felsic Cordilleran volcanism between 8° and 12°S. However, the largest and richest porphyry Cu deposits are related to the Caldera batholith. The Cobriza Cu-bearing skarn is the only significant copper deposit of pre-Mesozoic age.Perú has many ore deposits associated with the Miocene felsic extrusive and intrusive rocks along the Cordillera, forming veins and disseminations in igneous rocks and noncarbonate sedimentary rocks, and replacement mantos, pipes and veins in limestones. Several are large and high-grade enargite-type deposits containing mainly Cu, Ag, Au, Pb and Zn, accompanied by significant amounts of Cd, Te, Se, In, Bi and Tl. Others are veins and mantos containing Cu±Ag, Pb, Zn, Sn, W.The Mesozoic volcanosedimentary sequences along the coast host volcanogenic massive sulfide Cu-Zn and vein/manto-type amphibolitic skarn Cu±Fe deposits.Red bed Cu deposits are relatively unimportant in Perú.The following information on the history of copper mining in Perú has been condensed largely from Samame (1979), Petersen et al.(1990) and Benavides (1990).In Perú, gold and silver were apparently used before copper. The latter was first mined and processed by the pre-Inca Chimú culture along the northern coast and by the Tiahuanaco civilization in the Lake Titicaca region.Copper became an important metal during the Inca period,
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The Cripple Creek district has yielded nearly 21 million troy ounces of gold since its discovery in 1891. The orebodies occur as narrow veins within Precambrian and Tertiary rocks and as bulk tonnage deposits within tectonic and hydrothermal breccias.The district is localized within and adjacent to a 27.9- to 29.3 + or - 0.7-m.y.-old nested diatreme-intrusive complex. Two magmas, phonolitic and alkali basaltic in composition, generated volcanic flows, subvolcanic intrusions, and phreatomagmatic breccias. Magma mixing is suggested by intermediate composition latite-phonolite and syenite. Subsidence of the diatreme complex rocks is indicated by (1) a thick fluvial-lacustrine sedimentary sequence in the eastern subbasin, (2) the presence of carbonaceous debris, ripple-laminated rocks, and dessication cracks in sedimentary rocks at depths more than 300 m below the present surface, (3) by the fracture systems near the diatreme subbasin margins that reflect basement rock influence, and (4) by flat-dipping veins near intrusive bodies or small breccia bodies (e.g., the Cresson diatreme).The vein deposits as exemplified by those of the Ajax mine cut Precambrian crystalline rocks and Tertiary rocks of the diatreme complex. Within the Precambrian rocks the veins are radial to the margins of the diatreme system and are sheeted zones with rock dissolution and open-space fillings. Where the veins cut the Cripple Creek breccia, they are an irregular anastomosing fracture zone. The major veins exhibit remarkable vertical continuity, extending to more than 1,000 m below the present surface. Vein-related hydrothermal alteration occurs in a narrow selvage that extends outward no more than five times the vein width. Secondary K-feldspar, dolomite, roscoelite, and pyrite occur within an inner zone adjacent to the veins, whereas an outer zone contains sericite, montmorillonite, magnetite, minor secondary K-feldspar, and pyrite. There is no expansion of the alteration zones in the upper level mine exposures.Five stages of minerals are recognized in the Ajax mine veins: (1) quartz-fiuorite-adularia-pyrite-(dolomite-marcasite), (2) base metals-quartz-pyrite, (3) quartz-fluorite-pyrite-hematite-rutile, (4) quartz-pyrite-rutile-calaverite-acanthite, and (5) quartz-fluorite-dolomite. The proportions of each stage vary within and between veins, but the ore mineralogy is consistent throughout the vertical extent of the developed vein systems. Horizontally, gold values ranged between 0.5 and 1.0 oz Au per short ton.Fluid inclusion analyses have documented the presence of early stage 1 saline fluids (33- >40 equiv. wt % NaCl) with the higher salinities found in the upper 300 m of the Ajax mine levels; the fluids were boiling and contained CO 2 . Stage 2 and 3 fluid inclusions exhibit progressively lower homogenization temperatures, and salinities are markedly lower (0-8.3 equiv. wt % NaCl). The telluride ore was deposited from weakly boiling, dilute fluids (1.4-3.5 equiv. wt % NaCl) with temperatures below 150 degrees C.The bulk tonnage deposits, as exemplified by the Globe Hill area, consist of mineralized tectonic and hydrothermal breccias cutting pyroxene-bearing alkali trachyte. Four structural events occurred at Globe Hill: (I) emplacement of hydrothermal breccia bodies along a north-west-trending 1,800- by 700-m zone; (II) intersecting tectonic adjustments along steep variable-strike zones on the western margin of the stage 1 breccias; (III) intrusive breccia emplacement at the major stage II fault intersection; and (IV) hydrothermal brecciation centered to the immediate east of the Globe Hill pit and characterized by a matrix consisting of anhydrite, carbonate, fluorite, pyrite, and base metal sulfides.Two hydrothermal events generated gold-silver mineralization and associated wall-rock alteration in the bulk tonnage deposits. The precious and base metals occur with alteration products in breccia clasts or in matrix minerals within the hydrothermal and tectonic breccias. The fluids responsible for alteration-mineralization were boiling as indicated by wide ranges of filling temperatures in fluid inclusions of the same mineral grain, extensive development of explosion texture in quartz and celestite, and large variations of liquid/vapor ratios in fluid inclusions within individual crystal growth zones. Temperatures were below 200 degrees C as indicated by minimum filling values. Capping of boiling shallow hydrothermal fluids appears to have been enhanced by the alkali trachyte porphyry intrusion at Globe Hill, which acted as a permeability barrier to upward-migrating fluids. Vapor-dominated fluids developed over-pressuring, leading to hydrothermal brecciation and low-grade gold deposits. On the other hand, the vein systems in the Cripple Creek district formed along structures open to the surface; hence, hydrothermal brecciation did not occur.
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Hypogene
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The Raul-Condestable is a >32 Mt copper-gold deposit located on the Peruvian coast, 90 km south of Lima. Early studies include Ripley et Ohmoto (1977 and 1979), Cardozo (1983), Injoque (1985), Atkin et al. (1985), and Vidal et al. (1990). Recently, the deposit has been interpreted to belong to the iron oxide copper-gold class (IOCG), based on its alteration and ore mineralogy (Barton and Johnson, 1996 and 2000; de Haller, 2000 and 2006; de Haller et al., 2001, 2002, and 2006; Injoque, 2002; Sillitoe, 2003; Williams et al., 2005). The results and interpretations presented hereafter can be found in more detail in de Haller (2006) and de Haller et al. (2006). The ore consists of chalcopyrite, pyrite, pyrrhotite, and magnetite, and is found as dissemination, pore infill, replacement, and veins in amphibolitized rocks which are part of a basaltic to dacitic volcanosedimentary sequence. The geology of the studied area comprises a series of superposed volcanic edifices of Late Jurassic to Early Cretaceous age, which are part of a larger volcanic/seashore island to continental arc system. Particularly good exposures of the tilted host sequence allow the mapping of the Raul-Condestable IOCG deposit in a nearly complete oblique cross section, from its associated volcanic edifice down to a paleodepth of about 6 km (Fig. 1). Two hydrothermal events are defined, corresponding to a Main ore stage (the IOCG mineralization itself) and Late stage calcite-sulfide veins (with no economic significance). A regional dolerite dyke swarm cuts all the geologic units present in the area, including the Main ore stage copper mineralization. Late stage calcite-sulfide veins are later and cut the dolerite dykes. U-Pb zircon ages indicate that in the deposit area felsic magmatic activity took place between 116.7 ±0.4 and 114.5 ±1 Ma, defining a new Raul-Condestable superunit, the oldest so far, of the Peruvian Coastal Batholith (Atherton and Sanderson, 1985; de Haller et al., 2006; Mukasa, 1986a; Mukasa and Tilton, 1985a; Pitcher, 1985). This superunit is located west of the main part of the Batholith and includes a dacite-andesite volcanic dome and a subvolcanic quartz-diorite porphyry sill-dike complex that were emplaced at 116.7 ±0.4 Ma and 116.4 ±0.3 Ma, respectively, followed by tonalite stocks and dikes emplaced between 115.1 ±0.4 and 114.5 ±1 Ma. All these rocks contain hornblende and/or biotite but no pyroxene and correspond to silica- and water-rich magmas following a calcic differentiation trend. Hf isotopic data on zircons (!Hf(115 Ma) = 5.2 to 7.5) and Pb isotopic data on whole rock, combined with lithogeochemical results, suggest that magmas were generated by partial melting of the upper mantle, enriched through hydrous metasomatism and/or melting of subducted pelagic sediments. The lack of zircon inheritance suggests that there was no direct involvement of continental crust. These findings are in agreement with previous regional works (Couch et al., 1981; Jones, 1981; Beckingsale et al., 1985; Mukasa and Tilton, 1985a and 1985b; Wilson, 1985; Mukasa, 1986a and 1986b; Macfarlane et al., 1990). The Raul-Condestable IOCG deposit is connected in space and time with the magmatism of the Raul-Condestable super-unit. The mineralization was emplaced in the core of the daciteandesite volcanic dome at an estimated paleodepth of 2 to 3 km, surrounding two tonalitic intrusions emplaced at 115.1 ±0.4 and 114.8 ±0.4 Ma (Fig. 1). The U-Pb age of hydrothermal titanite from IOCG (Main ore stage) veins at 115.2 ±0.3 Ma indicates that the mineralization was coeval with (or more probably just followed) the emplacement of the tonalites. Copper ore is associated with a zoned alteration pattern, which surrounds the tonalite intrusions (Fig. 1). It consists of a core of biotite alteration and quartz stockwork, grading outward to actinolite (±magnetite, ±chlorite, ±titanite, ±scapolite, ±albite, ±epidote) and upwards to sericite + Fe-chlorite alteration. An upper distal alteration halo consisting of hematite-chlorite surrounds the sericite + Fe-chlorite and actinolite alterations laterally. Most of the ore is spatially associated with the actinolite alteration and, to a lesser extent, with the sericite + Fe-chlorite alteration. The Main ore stage ore paragenesis is characterized by two end-members, corresponding to an oxidized and a mineral associations. The oxidized mineral association consists of the sequence hematite-magnetite-pyrite-chalcopyrite, and the mineral association to the sequence pyrrhotite-pyrite-chalcopyrite. In the oxidized mineral association, early bladed hematite is almost completely transformed to magnetite. The oxidized mineral association is zoned from proximal (feeder veins) to distal from chalcopyrite-pyrite-(magnetite ±hematite), to pyrite-magnetite, and then to magnetite.,. In contrast, in the mineral association, zoning consists of chalcopyrite-pyrite, pyrite-pyrrhotite, and then pyrrhotite with, locally, intermediate product (pyrite-marcasite). Four types of fluid inclusions are found in quartz from stockwork and IOCG veins. They consist of 4 phase (liquid-vapor-halite-iron chloride), 3 phase (liquid-vapor-halite), and 2 phase (liquid-vapor) inclusions, of which most are liquid-dominated. Vapor homogenization temperatures are similar for the three liquid-dominated inclusion types, and range from 137.2 to 231.7 °C (n = 34) with a mean at 172 °C. For most hypersaline inclusions (probably up to >50-60 wt. % NaCl equ.), final homogenization would occur through daughter salts dissolution at temperatures higher than 300°C, where fluid inclusions start to leak. Salinity of 2 phase liquid-dominated inclusions range from 11.7 to 19.0 wt. % NaCl equ. (n = 4), with eutectic between -34.6 and -58 °C indicating complex polysaline fluids. The Main ore stage sulfides yield 34S values of +1.0 to +26.3 ‰, with a median at 6.6 ‰ (n = 51), consistent with data by previous authors (-9.3 to +23.3 ‰, n = 198; Ripley and Ohmoto, 1977). The 34S values of Main ore stage sulfides are dependent on the stratigraphic position, with deep seated vein samples corresponding to 34S of 1.0 to 6.3 ‰ (average around 3.5 ‰, n = 13), and shallower samples to 34S of +2.7 to +26.3 ‰ (median around +7.5 ‰, n = 39). Increased dispersion of the data at shallower levels coincides with an abrupt increase of the permeability of the volcanosedimentary sequence, related to the presence of volcanic breccias and tuffs. Similar patterns are observed for pyrrhotite, pyrite, and chalcopyrite. The positive 34S values are best explained through thermochemical Rayleigh fractionation of seawater sulfate. Sulfides found in Late stage calcite-sulfide veins show strongly negative 34S values ranging between -32.7 and -22.9 ‰ (n = 6), which contrast with the positive values obtained for Main ore stage sulfides, and might indicate a different source, probably biogenic. All the results for the Main ore stage point to the participation of two different fluids: (1) a deep-sourced, oxidized, hot, saline, and metal-rich magmatic fluid channeled through feeder veins; and (2) a seawater-derived, reduced, cooler, saline, and metal-poor fluid, circulating in a predominantly basaltic to basaltic-andesitic aquifer. Biotite alteration and quartz stockwork are interpreted to occur in the magmatic dominated core of the system, with the low Th(v) recorded in stockwork and Main ore stage vein quartz being compatible with boiling magmatic fluids originally trapped in amorphous silica (Fournier, 1985 et 1999). The upper distal hematite-chlorite alteration is thought to represent a seawater recharge zone. Actinolite and sericite + Fe-chlorite alterations would represent a mixing zone, where most of the ore precipitated. The oxidized mineral association can be explained by precipitation from magmatic fluids following the SO2-H2S gas buffer line (Einaudi, 2003; Giggenbach, 1987 and 1997) at high temperature (>350 °C) and water/rock ratio, then progressively reduced and cooled through interaction with the wall rock, with or without fluid mixing with the reduced seawater derived fluid. The mineral association (pyrrhotite-pyrite-chalcopyrite) is explained through the mixing of deep magmatic fluids, already partially or totally reduced through reaction with wall rock at medium to low water/rock ratio, with evolved (reduced) seawater that was heated at about 350 oC at the margin of Tonalite 1. Our findings support previous suggestions that the presence of external seawater-derived, evaporitic, or metamorphic brines is not a prerequisite to the formation of IOCG deposits, and that the primary contributor consists of magmatic fluids (Pollard, 2000 and 2001; Sillitoe, 2003). This is consistent with the widespread occurrence of the oxidized mineral association (hematitemagnetite- pyrite-chalcopyrite±bornite) observed in Andean IOCG deposits (Sillitoe, 2003; and references therein). The Raul-Condestable deposit appears to have no genetic link with the later dolerite dykes swarm or any other mafic magmatic event, and the hydrated magmatism of the Raul- Condestable super-unit, and in particular Tonalite 1, is considered as the most probable mineralizing magmatic fluid source.
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The Productora Cu-Au-Mo deposit is hosted by a hydrothermal breccia complex in the Coastal Cordillara of Region III, northern Chile. Mineralisation at Productora extends discontinuously over 8 km in a northeast-oriented corridor. The current resource, which includes the neighbouring Alice porphyry Cu-Mo deposit, is estimated at 236.6 Mt grading 0.48 % Cu, 0.10 g/t Au and 135 ppm Mo.
The district is characterised by hydrothermal mineralisation associated with Mesozoic arc magmatism along the convergent South American plate margin. The Andean tectonic cycle led to the formation of orogen-parallel metallogenic belts, each representing discrete epochs of mineralisation characterised by distinctive deposit styles and a stepped eastward younging of the deposits. During the Mesozoic, a range of deposit styles formed under extensional conditions in central and northern Chile, including small porphyry Cu, large iron oxide-Cu-Au (IOCG), magnetite-apatite, manto-Cu and precious metal epithermal deposits.
The Mesozoic mineral systems in Northern Chile formed within a convergent tectonic regime where evolving subduction style, magmatic arcs and associated back-arc basin stratigraphy, plutonic complexes and the crustal scale syn-arc strike-slip Atacama Fault Zone (AFZ) evolved in response to changes in the prevailing geodynamic conditions. The Productora deposit is hosted within a thick sequence of broadly coeval rhyolite to rhyodacite lappili tuffs (`128.67 ± 1.29 Ma; U-Pb_(zircon)`) and breccias between two major intrusions; the Cachiyuyito tonalite (129.8 ± 0.1 Ma) and Ruta Cinco granodiorite batholith (`92.05 ± 1 Ma; U-Pb_(zircon)`). Several phases of intermediate- and mafic-dykes cross cut the deposit. The deposit is hosted in a domain of structural complexity defined by major northwest-striking faults (normal obliqueslip with dextral strike-slip and northeast side up dip-slip) that crosscut major north to northeast-striking faults (normal oblique-slip with sinistral strike-slip, and east-side up dip-slip movement).
Hydrothermal breccias, tectonic breccias, veins and alteration assemblages observed on two cross sections at Productora have been separated into five paragenetic stages. Stage 1 produced quartz - pyritecemented hydrothermal breccia with muscovite alteration. Stage 2 formed a chaotic matrix-supported tectonic breccia with kaolinite - muscovite – pyrite alteration. Stage 3 tourmaline – pyrite – chalcopyrite ± magnetite ± biotite-cemented hydrothermal breccias are associated with K-feldspar ± albite alteration. Stage 4 veins contain chalcopyrite ± pyrite ± sericite, illite, epidote and chlorite, and stage 5 veins contain calcite. The stage 3 breccias have been further subdivided into five facies based on their mineralogy. The Productora hydrothermal system crosscuts earlier-formed sodic-calcic alteration and magnetite-apatite mineralisation associated with the Cachiyuyito stock.
The breccia complex formed as a result of at least two stages of hydraulic fragmentation (stage 1 and stage 3) and one tectonic breccia event (stage 2), which shows evidence of multiple episodes of reactivation. Alteration minerals are consistent with moderate temperature ( 300°C) alkaline fluids during stage 3. Lower temperature (<300°C), weakly acidic fluids prevailed during stages 4B and 4C, and alkaline fluids predominated during stages 4A, 4D and 5.
Main stage mineralisation is associated with the stage 3 hydrothermal breccia (average grade 0.34–0.62 % Cu, 0.8–0.14 g/t Au, 66–128 ppm Mo) and stage 2 tectonic breccia (average grade 0.8 % Cu, 0.21 g/t Au, 141 ppm Mo). Chalcopyrite is the dominant hypogene Cu-sulphide mineral and occurs predominantly as stage 3 breccia cement and syn-breccia veins with pyrite in equal proportion. Chalcopyrite and pyrite are disseminated in the stage 2 breccias. Pyrite is elevated in the south compared to the north, and chalcopyrite:pyrite ratios are ~1.00 and <0.25 respectively. Gold and Cu are strongly associated spatially, and synchrotron XRF and LA-ICP-MS analyses indicate that Au occurs as micron to sub-micron grains on pyrite and chalcopyrite grain boundaries. In this study, mineralisation at Productora was dated using Re-Os on stage 3 molybdenite at 130.1 ± 0.6 Ma.
The Alice Cu-Mo porphyry deposit is situated 400 m to the west of Productora. Mineralisation occurs as disseminated chalcopyrite and quartz – pyrite – chalcopyrite ± molybdenite vein stockwork hosted by a granodiorite porphyry stock (121.1 ± 2.1 Ma). Potassic alteration (biotite ± actinolite replacing hornblende) is associated with quartz – sulphide veins. Mineralisation was dated by Re-Os on molybdenite at 124.1 ± 0.6 Ma (within error of the porphyry stock). The margins and deeper parts of the system are overprinted by albite ± epidote ± sericite alteration, which locally caused destruction of biotite and chalcopyrite. The Alice porphyry is spatially associated with the Silica Ridge lithocap, which is characterised by massive textureless quartz-altered rock above domains of alunite, pyrophyllite and dickite.
At Productora, `δ^(34)`\(S_{sulphide}\) values range between -8.5 and +2.2 ‰. This is consistent with a magmatic sulphur source and fluids evolving under oxidising conditions with no significant input from evaporateor seawater-sourced fluids. Stage 3 tourmalines (n = 8) have average initial Sr of 0.70397, consistent with Cretaceous intrusive rocks and mantle-derived Sr. One stage 4 epidote sample returned a more radiogenic initial Sr value of 0.70525, indicating fluid mixing or fluid- wall rock interaction. An eNd value of +5.2 (n = 1, tourmaline) is consistent with the Cretaceous and Jurassic igneous rocks of the region. LA-ICP-MS analysis on stage 3 pyrite (n = 8) for Pb isotopes and trace elements indicate low Pb content ( 530 ppm).
Supergene mineralisation in the north of Productora (chrysocolla, malahcite and Cu-wad) is indicative of a geochemically mature weathering environment developed under near neutral to alkaline pH. In the south, the supergene assemblage is less mature (chalcocite/digenite) and indicates in-situ weathering of chalcopyrite.
Quantitative and predictive geometallurgical models have been developed to integrate geological findings with geometallurgical data in order to advance ore body knowledge at Productora. To improve the understanding of mineralogical variability across the deposit, whole rock geochemistry (33-element ICP-MS) has been converted to mineral proportions through calculated mineralogy using linear programming for each assay interval in the deposit. Calculated mineralogy results for major minerals, including quartz, K-feldspar, albite, pyrite, iron oxides, chalcopyrite and molybdenite, are excellent (`R^2 > 0.8`) when compared with the measured mineralogy by quantitative X-ray diffraction. A new sample classification scheme for dominant Cu-species (oxide, transitional-oxide, transitional-sulphide, sulphide or insoluble) was also developed based on sequential leach data and S wt%. The new scheme enabled domains of weak acid insoluble Cu-wad to be identified. Machine learning algorithms were used to predict Cu-species class using a series of nine proxies (sample depth in drill hole, Ca %, Cu %, Fe %, K %, Mn ppm, S %, Ln(Cu/S) and the logged regolith term). The optimum algorithm was Bagging-REPTree with five iterations. Applied to the training set with ten-fold cross validation, the model is 67.6 % accurate. The model is most successful at recognising sulphide, transitional-sulphide and insoluble samples with accuracy of 70.2 %, 87.5 % and 67.8 % respectively against the training set.
Based on textural, mineralogical, stable and radiogenic isotope data, the Productora breccia complex is inferred to be a magmatic-hydrothermal breccia complex formed as a result of explosive volatile fluid release at depths causing brecciation and alteration of the overlying rock mass. Metal-bearing fluids were of magmatic affinity and evolved under oxidising conditions. Despite sharing many similarities with the Andean IOCG clan (strong structural control, regional sodic-calcic alteration, local U), fluid evolution at the Productora Cu-Au-Mo deposit is consistent with that of a porphyry magmatic hydrothermal breccia (sulphur-rich, acid alteration assemblages and relatively low magnetite, <5 wt%). The Productora camp provides an excellent example of the close spatial association of Mesozoic magnetite-apatite, porphyry (Alice) and a magmatic-hydrothermal breccia mineralisation styles, a relationship seen throughout the Coastal Cordillera of northern Chile.
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