Latite Porphyry Research Articles

Nature and Origin of Zoned Polymetallic (Pb-Zn-Cu-Ag-Au) Veins from the Bingham Canyon Porphyry Cu-Au-Mo Deposit, Utah

Abstract Polymetallic veins (Pb-Zn-Cu-Ag-Au) at the world-class Bingham Canyon, Utah, porphyry Cu-Au-Mo deposit have long been recognized, but poorly understood. They are laterally zoned outward from the center of the porphyry deposit transitioning from Fe-Cu to Pb-Zn-Cu-Ag-Au mineralization. Physical and chemical characterization of these polymetallic veins provide insight into the origin, timing, and controls of ore deposition. These sheared, sulfide-rich, NE/SW- trending veins are dominated by pyrite and multiple generations of quartz, with lesser amounts of other sulfide and gangue minerals. Gold (0.27–4.61 ppm) provides the most value to the ore, though the veins contain substantial Cu and Ag as well. Host rocks include Eocene monzonite and Paleozoic limestone and quartzite—all of which can contain economic ore lodes. Associated alteration is predominantly sericitic and argillic, with mineralization in wall rocks restricted to 1.5 m from the vein margins. Mineral assemblages vary with distance from the center of the main porphyry Cu-Au-Mo deposit and the modal abundances are dependent on the host rock. The appearance of both galena and sphalerite (and tennantite to an extent) occur along a boundary that creates a halo around the center of the associated porphyry deposit. This is accompanied by a shift in metal ratios and an increased concentration of chalcophile trace elements in sulfides from the polymetallic veins as determined by electron microprobe analyses (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Significant hosts of Ag include galena and tennantite, and Cu is hosted primarily in chalcopyrite, tennantite, and sphalerite. The main host of Au could not be determined, but Au could be focused along fractures or hosted in inclusions found in pyrite. The δ34S values of vein pyrite have a narrow range (2.3–3.4‰) suggestive of a magmatic source, whereas δ18O of quartz is more variable (11.5–14.0‰). These values are similar to several other polymetallic vein deposits associated with porphyry Cu deposits. This can be explained by fractionation of magmatic fluids at lower temperatures (350°–250°C) and/or mixing with exchanged 18O-rich meteoric water. Ore grades (Cu, Ag, Au) improve with distance from the center of the porphyry deposit; however, this is accompanied by higher concentrations of deleterious elements (e.g., Pb, As, Bi) for downstream processing. These polymetallic veins were created sequentially throughout the formation of the deposit. Initial joints in the sedimentary rocks probably formed as a result of emplacement of a barren equigranular monzonite intrusion, with continued dilation and propagation in all host rocks with each subsequent intrusion. The northeast orientation of the joints was controlled by the regional stress field, which is more apparent distal to the center of the Bingham deposit. Vein mineralization appears to postdate all intrusions and the porphyry Cu-Au mineralization; however, it may be related to the late fluids responsible for Mo mineralization in the main porphyry orebody that followed intrusion of the quartz latite porphyry. Quartz-sericite-pyrite mineralization associated with the veins precedes galena-sphalerite-pyrite mineralization. This was followed by late precipitation of chalcopyrite and tennantite and late normal faulting.

Mineralogy and sulfur isotope geochemistry of polymetallic, porphyry-epithermal mineralization peripheral to the Golden Sunlight gold mine, Montana

The Golden Sunlight mine, located 50 km east of the famous Butte porphyry/lode deposit, is the largest gold mine in Montana, and has produced over 3.6 M oz of gold in its 36-year history. Most of this gold has come from the Mineral Hill breccia pipe (MHBP), a west plunging, cylindrical body of brecciated latite porphyry and siliciclastic sedimentary rocks of the Mesoproterozoic Belt Supergroup. This paper focuses on four areas of mineralization outside the MHBP termed the Apex, Bonanza, North Pit, and 102/Sunlight zones. The Apex and 102/Sunlight areas consist of auriferous, pyrite-rich veins cutting Precambrian sediments and the Cambrian Flathead Quartzite, whereas the Bonanza and North Pit areas are polymetallic Mo-Cu-Ag-Au prospects hosted in latite porphyry and Belt sediments. Recent dating (40Ar/39Ar of igneous biotite) shows the porphyry at Bonanza was emplaced at 78 ± 1 Ma, which is several m.y. younger than the porphyry associated with the MHBP (81.9 ± 1.9 Ma, zircon U/Pb), and several m.y. older than the main phase of the Boulder Batholith that hosts the Butte deposit.Overall, the ore and gangue mineral assemblages in the peripheral deposits are similar to those described previously for the MHBP deposit. Pyrite, locally As-rich, is abundant in all settings, with varying amounts of galena, sphalerite, chalcopyrite, bornite, molybdenite (at Bonanza), tetrahedrite-tennantite, and traces of pearceite, tetradymite, aikinite, calaverite, petzite, goldfieldite, buckhornite, gold and electrum. Gangue minerals include quartz, carbonates (Fe-dolomite, siderite, magnesite), barite, anhydrite, adularia, and alumino-phosphate sulfate (APS) minerals. Spectral analysis indicates that some of the adularia is ammonium rich, i.e., buddingtonite.Values of δ34S for sulfide minerals from the peripheral deposits overlap with values obtained by previous workers from the MHBP. The bulk of the analyses (including all samples from the porphyry-hosted Bonanza deposit) have δ34S values between −12 and −4‰, with a tail to heavier values >+5‰. It is postulated that most of the lighter S was introduced into the district by late Cretaceous magmatic/hydrothermal fluids, whereas the heavier S is attributed to sedimentary pyrite in the Precambrian Greyson and LaHood formations. A weak negative correlation exists between gold grade and the δ34S of associated pyrite: all samples in this study that contain >0.1 oz/ton Au have δ34S < −5.9‰. Two possible explanations for the isotopically light S at Golden Sunlight are explored: 1) assimilation of biogenic pyrite from underlying sedimentary units; and 2) fractionation of S isotopes between SO4 and H2S in a system where oxidized S > reduced S.

Separation of Molybdenum and Copper in Porphyry Deposits: The Roles of Sulfur, Redox, and pH in Ore Mineral Deposition at Bingham Canyon

The giant Bingham Canyon porphyry Cu-Mo-Au deposit (Utah) is associated with Eocene subvolcanic intrusions. It shows a distinct metal zonation above a barren core, with dominantly shallow Cu-Au mineralization (Cu stage) following the early quartz monzonite porphyry (QMP) intrusion, and spatially deeper Mo mineralization (Mo stage) occurring in a separate vein set exclusively after a late quartz latite porphyry (QLP) intrusion that truncates earlier Cu-Au veins. To understand this metal separation and the geochemical process of molybdenite mineralization, we investigated fluid inclusions by microthermometry, Raman spectroscopy, and laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) microanalysis in low- and high-grade quartz veins of both mineralization stages. In deep, low-grade quartz veins interpreted to represent the root zone of the Cu stage we found high concentrations of Cu, S, and Mo in the fluid inclusions, whereas in low-grade Mo-stage veins, we found lower Cu, but similar concentrations of S and Mo, compared to the inferred input fluids to the Cu stage. Sulfur and copper concentrations were similar in intermediate-density-type fluid inclusions in deep low-grade Cu-stage samples, whereas intermediate-density-type inclusions in low-grade Mo-stage veins have S contents that exceed their Cu contents. In high-grade Mo-stage vein, we found large variations of Mo concentrations in coexisting brine and vapor inclusions. Compared to the P-T conditions of the Cu precipitation stage (90–260 bars and 320°–430°C), the Mo-precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360° to 580°C. Mass-balance calculation based on the compositions of intermediate-density inclusions and brine + vapor assemblages, interpreted to be derived by phase separation during decompression of the ascending single-phase intermediate-density fluid, indicate that the mass of vapor phase exceeded that of brine by about 9:1 in both mineralization stages. Combining this mass balance with the analyzed vapor/brine partitioning data indicates that more than 70% of Mo and S (by mass) in the deposit were deposited from the vapor phase. Earlier Cu-Au deposition was similarly dominated by vapor, but recently published data about postentrapment Cu diffusion in and out of fluid inclusions cast doubt on previous quantifications, suggesting that almost none of the copper was deposited by brine. Mo is less likely to be modified by selective diffusion, and high Mo contents (max 0.0054 Mo/Na in intermediate density; 380 μ g/g Mo in brine) in the hydrothermal fluids were maintained from the early Cu stage to the late Mo stage. This indicates that Mo concentration was not the decisive factor for separate precipitation of late Mo ore at Bingham Canyon. Instead, the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids, i.e., a large subvolcanic magma reservoir. This is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions, a tentative difference in the Fe/Mn ratio in fluids of both veining stages, incipient muscovite alteration along high-temperature molybdenite veins, and an increasing tendency for Mo to fractionate from brine to vapor. We suggest that the early Cu-stage fluids were slightly more oxidized and neutral, allowing Cu-Fe sulfides to saturate first, while molybdenite saturation was suppressed and Mo was lost from the early ore stage. By contrast during the later Mo stage, the fluids were more reduced and acidic, thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two-phase fluid, consistent with textural observations. This interpretation may imply more generally that small differences in redox potential and acid/base balance of the magmatic source of porphyry-mineralizing systems may be decisive in the temporal and spatial separation of the two metals.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. I. Sequence of Intrusions, Vein Formation, and Sulfide Deposition

The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz mon-zonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast–trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast– and northwest-striking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber.

CHEMICAL SUBSTITUTIONS IN OXIDIZED TOURMALINE IN GRANITE-RELATED MINERALIZED HYDROTHERMAL SYSTEMS, WESTERN TURKEY

Tourmaline is the most common borosilicate mineral in granite-related magmatic-hydrothermal ore deposits in the Aegean region of western Turkey. Hydrothermal deposits there include the mesothermal argentiferous Pb–Zn–Cu deposit at Kadikalesi (Bodrum–Mugla) and the porphyry Au deposit at Kisladag (Esme–Usak), with related potassic igneous rocks and accompanying Tertiary strata of western Anatolia. At Kadikalesi, mesothermal vein-type mineralization is linked to monzodioritic intrusions in a subvolcanic suite, which is overlain by volcanic rocks ranging in composition from calc-alkaline to potassic. In the non-brecciated hydrothermal systems, tourmaline occurs as subhedral to anhedral crystals in monzodiorite or as an essential mineral in veins and their reaction zones. At Kisladag, magmatic-hydrothermal brecciation associated with multiphase intrusions of latite porphyry is related to the Kisladag caldera. Tourmaline is an abundant and widespread alteration-induced mineral throughout the Kisladag porphyry system. Electron-microprobe analyses of the tourmaline crystals collected from both hydrothermal deposits show significant chemical variation within the range of buergerite compositions. The main substitution mechanism involves the CaFe 2+ [Na −1 (Al,Fe 3+ ) −1 ] exchange vector. Compositions deviate from ideal schorl–dravite along a trend that closely approximates the uvite exchange-vector. Mossbauer spectroscopy shows the presence of significant Fe 3+ in all the tourmaline crystals, suggesting a high oxidation state for the ore-forming fluids, and for related hydrothermal systems associated with emplacement of subvolcanic intrusions.

Hydrothermal alteration and mass exchange in the hornblende latite porphyry, Rico, Colorado

The Rico paleothermal anomaly, southwestern Colorado, records the effects of a large hydrothermal system that was active at 4 Ma. This hydrothermal system produced the deep Silver Creek stockwork Mo deposit, which formed above the anomaly's heat source, and shallower base and precious-metal vein and replacement deposits. A 65 Ma hornblende latite porphyry is present as widespread sills throughout the area and provided a homogenous material that recorded the effects of the hydrothermal system up to 8 km from the center. Hydrothermal alteration in the latite can be divided into a proximal facies which consists of two assemblages, quartz-illite-calcite and chlorite-epidote, and a distal facies which consists of a distinct propylitic assemblage. Temperatures were gradational vertically and laterally in the anomaly, and decreased away from the centra heat source. A convective hydrothermal plume, 3 km wide and at least 2 km high, was present above the stock-work molybdenum deposit and consisted of upwelling, high-temperature fluids that produced the proximal alteration facies. Distal facies alteration was produced by shallower cooler fluids. The most important shallow base and precious-metal vein deposits in the Rico district are at or close to the boundary of the thermal plume. Latite within the plume had a large loss of Na2O, large addition of CaO, and variable SiO2 exchante. Distal propylitized latite samples lost small amounts of Na2O and CaO and exchanged minor variable amounts of SiO2. The edge of the plume is marked by steep Na2O exchange gradients. Na2O exchange throughout the paleothermal anomaly was controlled by the reaction of the albite components in primary plagioclase and alkali feldspars. Initial feldspar alteration in the distal facies was dominated by reaction of the plagioclase, and the initial molar ratio of reactants (alkali feldspar albite component to plagioclase albite component) was 0.35. This ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0.92 as the reaction progressed. Much of the alkali feldspar albite component in the proximal facies reacted while the, primary plagioclase was still unreacted, but the ratio for these assemblages increased to 1.51 when the plagioclase entered the reaction paragenesis. Plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of Na2O. CaO is lost in the distal facies as hornblende reacts to chlorite, although some calcium may be fixed in calcite. CaO is added to the proximal facies as the quantity of chlorite replacing hornblende increases and epidote and calcite are produced.

Sierra de Santa Maria, Velardena mining district, Durango

The Sierra de Santa Maria is located in the Velardena mining district in northeastern Durango in the Republic of Mexico. The Pb-Zn-Ag ore deposits have been exploited since the sixteenth century. Production from the Santa Maria dome is, at the time of writing, 800 metric tons per day from two principal mines, the Los Azules mine and the Santa Maria mine, both located within the northeastern flank of the sierra.The deposits are located along a Mesozoic basin-uplift margin at the junction between the Sierra Madre Oriental, the Sierra Madre Occidental, and the Mesa Central. Host sediments are the Lower Cretaceous Aurora and Cuesta del Cura limestones and the Upper Cretaceous Indidura and/or Caracol Formations. Mineralization is spatially and temporally associated with a 33-m.y.-old quartz latite porphyry stock and associated quartz latite porphyry and rhyolite dikes.Major sulfide minerals existent within the dome include pyrite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, and galena. Minor sulfides are boulangerite and freibergite. Traces of geocronite, argentite, polybasite, proustite, and Bi sulfosalts are found locally. Sulfide mineralization occurs as skarn-replacement, carbonate-replacement, disseminated, breccia, and epithermal fissure vein ore. The ore zones are divided and named on the basis of the dominant sulfide peculiar to their respective assemblages: the pyrrhotite ore zone (skarn-replacement ore), the arsenopyrite ore zone (disseminated ore in quartz latite porphyry), and the pyrite ore zone (carbonate-replacement ore). The other two ore zones in the sierra are the epithermal ore zone (fissure vein ore) and the breccia ore zone (quartz latite porphyry breccia ore).A variety of alteration assemblages are recognized within the dome. These are, in order of occurrence, presulfide-stage prograde skarn, potassic and propylitic alteration; sulfide-stage retrograde skarn; late sulfide-stage phyllic alteration; and postsulfide-stage argillic alteration. Prograde and retrograde skarn, potassic, propylitic, and phyllic alteration are all attributed to hypogene hydrothermal activity while argillic alteration is probably a supergene effect.Fluid inclusion analyses reveal that two distinct fluids caused prograde skarn alteration (550 degrees C, 0.75 kb, 10 equiv wt % NaCl) and sulfide mineralization (375 degrees C, 0.12 kb, 40-60 equiv wt % NaCl). A fracturing and related boiling event after skarn formation but prior to sulfide mineralization is proposed to explain the pressure and compositional variations between fluids.Sulfur isotope analyses of the sulfides (0ppm vs. CDT) and host Mesozoic sedimentary rocks (-7.3-3ppm vs. CDT) suggest that the sulfur was derived from a juvenile (i.e., magmatic) source. No delta 31 S compositional variations were observed paragenetically or spatially.Lead/zinc ratios in the Santa Maria mine indicate that fracture intersections with the Santa Maria dike were the major hydrothermal fluid pathways. This is further supported by higher Bi/Sb ratios in galenas within the central portions of the inferred hydrothermal fluid channels than along the distal fringes. Finally, the variation in sulfide mineralogy between ore zones is attributed to zonation, apparently controlled by increasing sulfur and oxygen fugacities as the hydrothermal fluid migrated away from the stock. This effect was probably due to increasing small-scale meteoric ground-water interaction with magmatically derived hydrothermal fluids distally from the stock.

Petrologic and stable isotope study of the gold-bearing breccia pipe at the Golden Sunlight Deposit, Montana

The Golden Sunlight, a gold-silver deposit in the Whitehall mining district, Jefferson County, Montana, is centered on a silica-cemented breccia pipe with fragments of both Belt Supergroup sedimentary rocks and a latite porphyry. Native gold and gold tellurides occur finely disseminated within gangue minerals that have filled open spaces in the breccia pipe.Alteration in and around ( approximately 0.8 km radius) the pipe is principally quartz-sericite. Within the pipe, breccia fragments are also characterized by a highly silicified outer rind. Openspace filling is divided into four periods as follows according to the presence of the following minerals: (I) quartz and pyrite, with minor hematite, chalcopyrite, bornitc, and gold minerals; (II) chalcopyrite, with lesser amounts of galena, sphalerite, and martasite; (III) additional gold-silver tellurides and tennantite; and (IV) barite, quartz, and sericite with minor carbonates and pyrite. Isotopic study shows delta 34 S values of sulfides between -12.2 and +3.1 per mil, whereas barite values range from 1.2 to 5.9 per mil. Oxygen isotope values of whole-rock breccia samples range from 6.5 to 13.8 per mil, quartz from 10.5 to 14.3 per mil, sericite from 9.7 to 15.5 per mil, and unaltered country rock from 12.3 to 16.6 per mil. Fluid inclusion homogenization studies indicate a temperature of deposition for period I mineralization of approximately 200 degrees C. The coexistence of sylvanitc and petzite in period III suggests a temperature less than 170 degrees C. Values of delta 18 O (sub H 2 O) computed from quartz and temperature data and sericite delta 18 O values range from -2.4 to +4.0 per mil. Hydrogen isotope values of fluid inclusions in period I quartz and water in equilibrium with late sericite fall betweem -56 and -33 per mil. Fluid inclusion freezing study of one sample of period I quartz suggests a fluid salinity of <1 equiv. wt percent NaCl.Isotopic and geologic evidence is consistent with tbrmation of a breccia pipe caused by exsolution of a vapor from a crystallizing magma. Oxygen and hydrogen isotope data suggest that extensive isotopic exchange between a magmatic fluid and igneous rocks occurred prior to open-space filling. Initial brecciation was accompanied by an isoenthalpic temperature decrease, which coupled with conductive cooling led to the precipitation of large amounts of silica. Textural features indicate that brecciation and fracturing, followed by fluid infiltration, occurred episodically. Sulfur isotope data is also consistent with derivation of sulfur from a magmatic source characterized by a delta 34 S (sub Sigma s) value between 0 and 5 per mil. Evidence from mineral assemblages and sulfur isotopes suggests that gold was most likely transported as a chloride complex and was deposited due to a pH increase accompanied by decreases in f (sub O 2 ) and possibly temperature.

Porphyry copper and tourmaline breccias at Los Bronces-Rio Blanco, Chile

The Los Bronces-Rio Blanco deposit is located on the west side of the Andes in central Chile about 69 km from Santiago. Los Bronces is a breccia complex superimposed on the west side of an earlier major porphyry copper system. The Rio Blanco mine currently exploits the north-central part of this porphyry deposit and has started operating a large copper-bearing tourmaline breccia, Sur-Sur, about 2 km south of the present mine.The Los Bronces-Rio Blanco deposit was formed on the east side of the San Francisco batholith. This intrusion is strongly peraluminous and has a calc-alkaline composition with an alkali-calcic affinity. The batholith took a minimum of 11.5 m.y. to form from the early Miocene (20.1 m.y.) to the late Miocene (8.6 m.y.). The porphyry copper mineralization, alteration, and copper tourmaline breccias were formed over a period of at least 2.5 m.y. between 7.4and 4.9 m.y. ago.A postmineral volcanic neck or diatreme at La Copa erupted within, and removed a large segment of, the northern part of the porphyry copper system, thus marking the last evidence of magmatic activity in the area. K-Ar age determinations of biotites indicate the diatreme erupted in the early Pliocene between 4.9 and 3.9 m.y. ago.The prebreccia porphyry system exhibits propylitic, sericitic, silicic, and potassic alteration. A unique alteration feature of this system is the replacement of mafic minerals by specularitc and/or tourmaline within the propylitic zone. The porphyry system contains disseminated and stockwork copper-iron-molybdenum sulfide mineralization within an area of about 12 km 2 .Los Bronces is composed of at least seven different copper-bearing tourmaline breccias that form one large contiguous kidney-shaped body about 2 km long and 0.7 km wide, at the present erosion surface. The breccia body crops out at elevations between 4,150 and 3,450 m. The various breccias are characterized by their locations, matrices, clasts, shapes, types, and degrees of mineralization and alteration. The breccias are usually monolithic but in some cases are bilithic or heterolithic with most clasts consisting of quartz monzonite or andesitc with locally minor amounts of quartz latite porphyry, monzodiorite, and vein quartz. The breccia matrices consist of variable amounts of quartz, tourmaline, specularitc, anhydrite, pyrite, chalcopyrite, bornitc, molybdenite, sericite, chlorite, and rock flour.The seven different breccias types are identified from oldest to youngest as Ghost, Central, Western, Infiernillo, Anhydrite, Fine Gray, and Donoso. The breccia complex has sharp contacts with the surrounding intrusive rocks and andesites. Internally, the breccia contacts are locally well defined, but elsewhere they coalesce, interfinger, or display gradational contacts. Breccias at Los Bronces are interpreted as being emplaced explosively, followed by collapse after pressure release of hydrothermal fluids.The primary mineral distribution is best known in the Donoso breccia which has been the center of mining activity since its discovery in 1864. In spite of the coarse and irregular nature of the sulfides in the matrix, chalcopyrite, pyrite, and specularitc at the 3,670-m, open-pit operating level show a tendency to be distributed in irregular shells in which one of the three minerals predominates in any one shell. The transitions between shells are rapid. Several semiellipsoidal shells of alternating high and low copper grades are also apparent from the copper distribution of underground level 3640 and from various cross sections. The shells are approximately vertical, subparallel to the Donoso breccia contacts, which dip inward.Secondary enrichment enhanced the primary grade in the southern two-thirds of the Los Bronces breccia complex and in much of the surrounding porphyry copper system. The degree and depth of enrichment are a function of breccia and fracture permeability and extend to a depth of more than 500 m in certain favorable sectors. The shape and depth of the enrichment blanket and overlying leached capping suggest-that the enrichment process is related to the present ground-water regime and is still active.

A diatreme-hosted gold deposit at Montana Tunnels, Montana

A bulk-minable, gold-silver deposit at Montana Tunnels, southwestern Montana, is located in the central part of a diatreme. This diatreme was emplaced along the faulted contact between andesitic volcaniclastic rocks, assigned to the Late Cretaceous Elkhorn Mountains Volcanics, and a sequence of quartz latitic ignimbrites, which are part of the Lowland Creek Volcanics of middle Eocene age. A north-northeast-striking swarm of quartz latite porphyry dikes was emplaced during the waning stages of Lowland Creek volcanism. The two largest dikes, dated at 45 to 50 m.y. (middle Eocene), were intruded into the diatreme prior to the cessation of brecciation and mineralization.The diatreme underlies a 1-km 2 area and is known from drilling to extend steeply downward for at least 310 m. The principal rock type in the diatreme is a matrix-rich breccia. It is characterized by a sand-size tuffaceous matrix of quartz latitic composition containing fragments of contiguous volcanic wall rocks and intrusive rocks derived from the Late Cretaceous Boulder batholith. The batholith is inferred to underlie the Montana Tunnels area. Foundered blocks of volcanic wall rocks lie at various attitudes against the walls of the diatreme and within it. Those composed of Elkhorn Mountains Volcanics underwent hydraulic brecciation after they sank into the diatreme. Blocks interpreted to be composed of pyroclastic base surge deposits from a subaerial tuff ring and a piece of carbonized wood subsided into the diatreme from the palcosurface. The diatreme is separated into two unequal parts by a west-northwest-striking oblique slip fault.Sulfide minerals in the diatreme occur as disseminations in the breccia matrix and, in sub-ordinate amounts, as widely spaced, multidirectional veinlets and as the matrix to brecciated blocks of Elkhorn Mountains Volcanics. All forms of mineralization consist of pyrite, sphalerite, galena, minor chalcopyrite, and rare electrum accompanied by a gangue of manganocalcite, siderite, and minor quartz. Clastic grains and fragments of the sulfides and gangue minerals are common and attest to multiple alternating episodes of brecciation and mineralization. In the ore zone the introduction of abundant manganocalcite and siderite was accompanied by pervasive sericitic alteration but only weak kaolinization and silicification. Sericitization grades outward beyond the ore zone to chlorite-montmorillonite-carbonate alteration, an assemblage that also characterizes the interiors of the late mineral dikes in the diatreme. Gold occurs in electrum, with a fineness of about 550, as inclusions of less than 200 mu m in pyrite and sphalerite. Inclusions of electrum and galena were precipitated together. Silver is present mainly in solid solution in galena.The Montana Tunnels gold-silver deposit is characterized geochemically by anomalously high concentrations of zinc, lead, and manganese and, in comparison to many volcanic-hosted precious metal deposits, is low in arsenic, antimony, and mercury. This metal association and the low fineness of the gold are features shared by the Wau gold deposit in Papua New Guinea. It is suggested that the mineralized diatreme observable at Montana Tunnels was emplaced several hundred meters beneath the palcosurface, upon which a gold-bearing maar volcano, like that partly preserved at Wau, was constructed.

Relationships between mineralization and silicic volcanism in the Central Andes

Studies of late Tertiary silicic volcanic centres in the Western and Eastern Cordilleras of the Central Andes show that three volcanic environments are appropriate sites for mineralization: (1) ring-fracture extrusions post-dating large calderas; (2) similar extrusions within ignimbrite shields; and (3) isolated, small silicic volcanoes. Subvolcanic tin mineralization in the Eastern Cordillera is located in silicic stocks and associated breccias of Miocene age. The Cerro Rico stock, Potosi, Bolivia, contains tin and silver mineralization and has an intrusion age apparently millions of years younger than that of the associated Kari Kari caldera. Similar age relationships between mineralization and caldera formation have been described from the San Juan province, Colorado. The vein deposits of Chocaya, southern Bolivia, were emplaced in the lower part of an ignimbrite shield, a type of volcanic edifice as yet unrecognized in comparable areas of silicic volcanism. The El Salvador porphyry copper deposit, Chile, is related to silicic stocks which may have been intruded along a caldera ring fracture. Cerro Bonete, Chile, provides a modern example of the volcanic superstructure which may have overlain isolated mineralized stocks and breccia pipes such as that of Salvadora at Llallagua, Bolivia. Existing models for the genesis of porphyry copper deposits suggest that they formed in granodioritic stocks located in the infrastructure of andesitic stratovolcanoes. Sites of porphyry-type subvolcanic tin mineralization in the Eastern Cordillera of Bolivia are distinguished by the absence of such andesitic structures. The surface expression of a typical subvolcanic porphyry tin deposit was probably an extrusive dome of quartz latite porphyry, sometimes related to a larger caldera structure. Evidence from the El Salvador porphyry copper deposit in the Eocene magmatic belt in Chile suggests that it too may be more closely related to a silicic volcanic structure than to an andesitic stratovolcano. The dome of La Soufriere, Guadeloupe is proposed as a modern analog for the surface expression of subvolcanic mineralization processes, the phreatic eruptions there suggesting the formation of hydrothermal breccia bodies in depth. Occurrence of mineralized porphyries, millions of years after caldera formation, does not necessarily indicate that intrusions and mineralization are not genetically related to the sub-caldera pluton, but may be a consequence of the long thermal histories (1–10 million years) of the lowermost parts of large plutons. Caldera formation can only inhibit mineralization by dispersal of ore metals when these are of magmatic origin, and ignimbrites should not be taken as being unlikely to be associated with porphyry mineralization. Whether ore metals are of wall rock or magmatic origin, the key to understanding the relationships between silicic volcanism and mineralization lies in the fractionation of trace elements within large zoned magma chambers during their igneous history, and their subsequent hydrothermal migration. Small, highly mineralized intrusions formed late in a caldera cycle (such as the Cerro Rico) may be due to the introduction of fresh supplies of mafic magma into the lower parts of the main pluton.

Geochronology of igneous intrusions and porphyry copper mineralization at Bingham, Utah

Previously published and newly determined K-Ar dates of biotite, hornblende, and augite in the Bingham area are reinterpreted according to their relative sequence of geological events. All K-Ar dates from altered and unaltered igneous rocks in and around the Bingham mine fall within a time span of 3.2 m.y., between 39.8 + or - 0.4 m.y. and 36.6 + or - 0.3 m.y., and suggest a close genetic relationship between igneous and hydrothermal activity.The oldest igneous rocks in the area are represented by the equigranular monzonite of the Last Chance stock and the equigranular monzonite phase of the Bingham stock. The monzonites form part of one intrusive phase, dated at 39.8 + or - 0.4 m.y. Younger ages occur in the same rock type and are probably the result of argon degassing close to the younger quartz monzonite porphyry intrusion. This porphyry was the hydrothermal heat source around which the different alteration and sulfide assemblages are zoned. The time of emplacement of the quartz monzonite porphyry cannot be ascertained because no unaltered quartz monzonite porphyry exists at the surface nor in drill holes.Plugs of quartz latite porphyry in the central pit have few fractures and are weakly altered and sparsely mineralized, suggesting that they intrude mainly after sulfide mineralization and alteration. The plugs cut strongly mineralized, altered, and fractured monzonite and quartz monzonite porphyry; they appear to be the youngest intrusive rocks in the mine. The average age of four K-Ar determinations of biotites from the plugs is 38.8 + or - 0.3 m.y.Porphyry copper mineralization most likely took place during several pulses of hydrothermal activity that started shortly after the intrusion of equigranular monzonite (39.8 m.y.) and probably culminated prior to the emplacement of quartz latite porphyry plugs in the central pit (38.8 m.y.). The youngest K-Ar age obtained from the mine area was determined from a selected sample of biotites with a large proportion of hydrothermally altered. Mg-rich biotite and probably represents one of the last pulses (36.6 + or - 0.3 m.y.).

Chemistry of biotite and apatite from a vesicular quartz latite porphyry plug at Bingham, Utah

Biotite and apatite occur with copper and iron sulfides, quartz, and traces of other minerals in a vesicular quartz latite dike which intrudes the quartz monzonite stock in the Bingham mine, Utah. Within each of three distinct phases of the dike, biotite in the rock adjacent to vesicles, and in secondary aggregates, is richer in Mg and F and depleted in Ba, Fe, Ti, and Cl relative to magmatic biotite. Both magmatic biotite and hydrothermal biotite increase in Mg and F content from the border phase of the dike to a central phase to a late aplitic phase. Apatite compositions show closely analogous variations involving F enrichment and Fe and Cl depletion. The trends in biotite composition within this single small intrusion are markedly similar to those documented by more extensive studies of the potassic zones at Bingham and at Santa Rita.The hydrothermal solutions that formed or exchanged with the secondary biotite were probably not markedly fluorine rich. Log f (sub H 2 O) /f HF during crystallization of the vesicle filling was about 5. Fluorine distribution between adjacent biotite and apatite grains does not represent high temperature equilibrium partitioning and cannot be used to determine the temperature of vesicle filling.Biotite compositions within the different phases of the dike and of the Bingham stock appear to indicate separate evolution from magmatic to hydrothermal conditions within the intrusive phases of the dike. However, the observed compositional variation could be due to different degrees of exchange with a common, chemically evolving hydrothermal fluid.

Ore fluid-magma relationships in a vesicular quartz latite porphyry dike at Bingham, Utah

An isolated 20-m-long, quartz latite porphyry dike with unusual sulfide-bearing vesicles is described. The dike occurs within the Bingham porphyry copper orebody and contains xenoliths of previously veined, altered, fractured, and mineralized monzonite. The dike is massive, highly porous but impermeable. It contains 0.5 percent Cu as chalcopyrite in vesicles which range in size from microscopic to 15 cm in diameter. The dike is zoned, with the brown, highly vesicular core zone and a late aplitic phase containing more K 2 O, MgO, Cu, S, and porosity and less Fe 2 O 3 , CaO, and Na 2 O than the green rock which forms the border of the dike. The dike is composed of quartz, altered plagioclase, biotite, and very large orthoclase phenocrysts in a groundmass of orthoclase, quartz, biotite, and vesicles. Crystallization of the groundmass is believed to have produced a strong permeability contrast with the highly fractured monzonite host rocks and resulted in a closed chemical system in the center of the dike. The proportions of minerals and void space in the vesicles and fluid inclusion data are used to calculate the composition of the fluid which filled the vesicles. The composition is:CuFeS 2 9% by weightSiO 2 8NaCl 21KCl 11(Na, K)F 11H 2 O 40These constituents comprised about 15 percent of the magma just before the groundmass froze. Fluid inclusion studies indicate the quartz in the vesicles crystallized from a boiling fluid at 480 degrees C. Other authors calculate the confining pressure to be less than 800 bars. The magma which formed this dike was probably subaluminous, rich in halogens, alkalis, and water and probably remained magmatic in its behavior to temperatures well below 600 degrees C. The consistency of these conclusions with published experimental results is discussed.

Geology and geochronology of the Yandera porphyry copper deposit, Papua New Guinea

Porphyry copper mineralization at Yandera is located within the Bismarck intrusive complex, a synorogenic batholith emplaced during folding and faulting accompanying collision of the Australian plate and an island-arc zone in mid-Miocene time. Regional stresses during solidification of the pluton produced fracture systems along which younger quartz diorite and dacite stocks, plugs, and dikes were emplaced, an event which produced further complex fracturing and intrusion breccias about 12 to 14 m.y. ago. Still younger, relatively small plugs and dikes of fine-grained 6.5 m.y. aplitic quartz monzonite porphyry and quartz latite porphyry are centrally located within a broad, diffuse zone of disseminated copper mineralization and associated biotite-orthoclase alteration. K-Ar age dates suggest that copper mineralization is genetically linked to emplacement of these intrusions, with better overall hypogene copper grades clearly controlled by spacing and orientation of pre-existing fractures. The potential ore zone is a broad, diffuse area with relatively high fracture density peripheral to the earlier quartz diorite intrusions.Telescoped and overlapping apparent sequences of alteration and sulfide assemblages are evident: an early quartz-orthoclase-biotite alteration with weak, low-sulfide, disseminated and veinlet molybdenite-chalcopyrite-pyrite-bornite mineralization is pervasively developed. Overprinted is strong vein and veinlet chalcopyrite-pyrite-bornite mineralization with associated sericite-chlorite-magnetite-epidote alteration. These are, in turn, overprinted with late pyritic veins with associated strong quartz-clay-sericite alteration.Yandera does not exhibit marked concentric zoning of alteration and mineralization about a centrally located hypabyssal stock as in most porphyry copper deposits of the world, but diffuse, poorly developed zoning is suggested by:a. restriction of bornite to the center of the deposit;b. potassium-silicate alteration and chalcopyrite mineralization in the central area;c. development of pyrite-clay-sericite fracture-controlled mineralization and alteration peripheral to the central area; andd. lead-zinc soil geochemical anomalies peripheral to copper mineralization.The Yandera deposit is similar to some porphyry copper deposits in British Columbia and the Philippines with its location near an internal contact within an area of complex, multiple intrusion in a major batholith, dominance of fracture control of sulfide mineralization, and lack of significant lead-zinc-silver deposits fringing the copper mineralization.

Emplacement of a Dike Swarm in the Buffalo Mountain Pluton, Nevada

Research Article| November 01, 1973 Emplacement of a Dike Swarm in the Buffalo Mountain Pluton, Nevada THOMAS R. NEFF THOMAS R. NEFF 1Department of Geology and Geography, Weber State College, Ogden, Utah 84403 Search for other works by this author on: GSW Google Scholar Author and Article Information THOMAS R. NEFF 1Department of Geology and Geography, Weber State College, Ogden, Utah 84403 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (11): 3689–3696. https://doi.org/10.1130/0016-7606(1973)84<3689:EOADSI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation THOMAS R. NEFF; Emplacement of a Dike Swarm in the Buffalo Mountain Pluton, Nevada. GSA Bulletin 1973;; 84 (11): 3689–3696. doi: https://doi.org/10.1130/0016-7606(1973)84<3689:EOADSI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Buffalo Mountain pluton in southern Humboldt County, Nevada, is cut by an extensive dike swarm. Dikes occur mainly within the plutonic rocks, although a few extend as much as 3.5 km into the surrounding sedimentary rocks. The dikes are composed of rhyolite porphyry and quartz latite porphyry or their altered equivalents.Dikes vary in width from a few centimeters to several hundred meters. In plan, individual dikes are sigmoidal or curving, and they branch and intersect intricately to form a very complex pattern. A statistical analysis of dike orientations shows a strong north trend and a weaker northwest trend.The amount of dilation produced by dike injection is approximately 1,160 m in an east-west direction. Space to accommodate dilation probably was the result of a regional east-west crustal relaxation. The dike swarm apparently formed during three overlapping stages. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

K-Ar Dating of the 'Cork' (Burwash Creek) Cu-Mo Prospect, Burwash Landing Area, Yukon Territory

A 26 ± 0.3 m.y. mean age was determined for quartz latite porphyry near Burwash Creek, Yukon Territory. This apparent age for mineralized porphyry represents the youngest documented porphyry prospect in the Canadian Cordillera. Cretaceous ages determined for Kluane Range intrusions agree with geologic evidence, and early Tertiary ages from the Ruby Range batholith agree with a previous radiometric determination.

GRANITIZATION AND HYDROTHERMAL ALTERATION AT BINGHAM, UTAH

The rocks exposed in the disseminated copper deposit at Bingham, Utah, consist of folded Pennsylvanian quartzites, limestones, and dolomites within which have been formed granite, actinolite syenite, granite porphyry with associated aplites, and biotite-quartz latite porphyry. Field and petrographic evidence shows that the granite porphyry and biotite-quartz latite porphyry are magmatic. That the granite and actinolite syenite may have been formed by granitization is strongly suggested by: (1) the presence of a microgranitic rock in the granite, recognized as being granitized quartzite, and (2) the replacement of quartzite by feldspar, observable only with the aid of the microscope, that occurs along the contact of the granite. Other chemical, textural, compositional, and other evidences support this hypothesis. The hydrothermal activity attending the ore deposition took place in seven stages following the formation of the igneous-appearing rocks. Except for the first two, which can be separated on good microscopic evidence, all stages can be differentiated by structural field relations. Stage I saw the formation of kaolinite and illite. During Stage II hydrothermal biotite and sericite were widely developed. In Stage III, chlorite and hydrothermal biotite were formed, but in restricted areas. In Stage IV, quartz and sericite formed over wide areas, while during Stare V quartz was deposited in open fissures without accompanying alteration. Stage VI is the sulfide stage, and Stage VII saw the development of sericite and allophane in open fissures.

Tin-silver veins of Oruro, Bolivia, Part I

The tin-silver ores of Oruro, Bolivia, are classed as xenothermal deposits. They occur at relatively shallow depths in complex veins in and adjacent to small stocks of quartz latite porphyry. Fracturing was closely associated with the process of mineralization. Erosion and deep oxidation developed a gradational capping of oxidized or 'paco' ores, composed of cassiterite, limonite, jarosite, and quartz at the outcrops of the primary sulphide veins. Tin is still mined from the oxidized zone, but the bonanza silver ores were mined out many years ago.

Geology of the northern Quitman Mountains Trans-Pecos, Texas

The northern Quitman Mountains lie within the Sierra Madre Oriental structural province, where it projects northward from Mexico into Hudspeth County, Texas. The Permian Briggs formation with a maximum exposed thickness of about 200 feet unconformably underlies the Jurassic Malone formation, which, although absent in places, attains a maximum thickness of 100 feet. The Lower Cretaceous represented by 10,000 feet of sedimentary rocks, consists of the following formations, the maximum exposed thicknesses of which are given in parentheses: Torcer (300 feet), Yucca (5500 feet), Bluff (about 1800 feet), Cox (700 feet), Finlay (100 feet), Espy (1280 feet), and Etholen (500 feet). Unconsolidated basin deposits of late Tertiary and Quaternary age fill the valleys. The sedimentary rocks are intensely folded and faulted in the northern Quitman area, although no one fold can be followed very far. Three thrusts faults—the Devil Ridge, Red Hills, and Quitman—have a combined net slip of approximately 8 miles. Normal faults generally have a net slip less than 200 feet, though one exceeds 1000 feet. The main orogenic disturbance is probably late Cretaceous. Comagmatic and mildly alkalic volcanic and intrusive rocks are probably early Tertiary. Lavas and pyroclastic rocks, for which the name Square Peak volcanic series is here proposed, aggregate approximately 3500 feet and constitute a single unit in the center of the mountains. Flows range from rhyolite and trachyte to basalt; rhyolites are most abundant. Later basining, probably due to magmatic subsidence below the volcanics, has dropped the central portion of the volcanic rocks approximately 4500 feet. Centripetal dips of flows along the outer contacts are as high as 90°; average inward dips range from 20° to 40°. The Quitman pluton, a discontinuous elliptical ring of intrusives around the volcanic rocks, is interpreted as a ring-dike with an accessory stock at the northern end. Earliest intrusion in the area was a diorite; the main intrusion averages quartz monzonite. Plugs and dikes of aplite, granite porphyry, rhyolite porphyry, and quartz latite porphyry are youngest. Scattered mineral deposits are found in the irregular contact-metamorphic aureole around the pluton.