- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
NARROW
GeoRef Subject
-
all geography including DSDP/ODP Sites and Legs
-
South America
-
Peru (3)
-
-
United States
-
Colorado
-
Colorado mineral belt (1)
-
-
Colorado Plateau (1)
-
Idaho (2)
-
Montana (1)
-
Utah (2)
-
-
-
commodities
-
brines (1)
-
metal ores
-
base metals (2)
-
cobalt ores (2)
-
copper ores (2)
-
gold ores (1)
-
polymetallic ores (2)
-
silver ores (1)
-
tungsten ores (2)
-
uranium ores (2)
-
vanadium ores (2)
-
-
mineral deposits, genesis (7)
-
petroleum
-
natural gas (1)
-
-
-
elements, isotopes
-
carbon
-
C-13/C-12 (3)
-
-
hydrogen
-
deuterium (1)
-
-
isotope ratios (2)
-
isotopes
-
stable isotopes
-
C-13/C-12 (3)
-
deuterium (1)
-
O-18/O-16 (3)
-
S-34/S-32 (3)
-
-
-
metals
-
rare earths (2)
-
-
noble gases (2)
-
oxygen
-
O-18/O-16 (3)
-
-
sulfur
-
S-34/S-32 (3)
-
-
-
geochronology methods
-
K/Ar (1)
-
-
geologic age
-
Cenozoic
-
Tertiary
-
Neogene (1)
-
-
-
Mesozoic
-
Jurassic
-
Upper Jurassic
-
Brushy Basin Member (1)
-
Morrison Formation (1)
-
-
-
-
Precambrian
-
upper Precambrian
-
Proterozoic
-
Mesoproterozoic
-
Revett Quartzite (1)
-
-
-
-
-
-
igneous rocks
-
igneous rocks
-
plutonic rocks
-
granites
-
S-type granites (1)
-
-
-
volcanic rocks
-
pyroclastics
-
ash-flow tuff (1)
-
-
-
-
-
minerals
-
minerals (1)
-
silicates
-
framework silicates
-
feldspar group
-
alkali feldspar
-
sanidine (1)
-
-
-
-
orthosilicates
-
nesosilicates
-
andalusite (1)
-
sillimanite (1)
-
-
-
sheet silicates
-
clay minerals (1)
-
mica group
-
muscovite (1)
-
-
-
-
sulfides (1)
-
-
Primary terms
-
absolute age (1)
-
brines (1)
-
carbon
-
C-13/C-12 (3)
-
-
Cenozoic
-
Tertiary
-
Neogene (1)
-
-
-
diagenesis (1)
-
economic geology (4)
-
geochemistry (2)
-
hydrogen
-
deuterium (1)
-
-
igneous rocks
-
plutonic rocks
-
granites
-
S-type granites (1)
-
-
-
volcanic rocks
-
pyroclastics
-
ash-flow tuff (1)
-
-
-
-
inclusions
-
fluid inclusions (4)
-
-
isotopes
-
stable isotopes
-
C-13/C-12 (3)
-
deuterium (1)
-
O-18/O-16 (3)
-
S-34/S-32 (3)
-
-
-
magmas (1)
-
Mesozoic
-
Jurassic
-
Upper Jurassic
-
Brushy Basin Member (1)
-
Morrison Formation (1)
-
-
-
-
metal ores
-
base metals (2)
-
cobalt ores (2)
-
copper ores (2)
-
gold ores (1)
-
polymetallic ores (2)
-
silver ores (1)
-
tungsten ores (2)
-
uranium ores (2)
-
vanadium ores (2)
-
-
metals
-
rare earths (2)
-
-
metasomatism (1)
-
mineral deposits, genesis (7)
-
minerals (1)
-
noble gases (2)
-
oxygen
-
O-18/O-16 (3)
-
-
petroleum
-
natural gas (1)
-
-
pollution (1)
-
Precambrian
-
upper Precambrian
-
Proterozoic
-
Mesoproterozoic
-
Revett Quartzite (1)
-
-
-
-
-
sedimentary rocks
-
carbonate rocks (1)
-
clastic rocks
-
red beds (1)
-
-
-
South America
-
Peru (3)
-
-
sulfur
-
S-34/S-32 (3)
-
-
United States
-
Colorado
-
Colorado mineral belt (1)
-
-
Colorado Plateau (1)
-
Idaho (2)
-
Montana (1)
-
Utah (2)
-
-
-
sedimentary rocks
-
sedimentary rocks
-
carbonate rocks (1)
-
clastic rocks
-
red beds (1)
-
-
-
ORE GENESIS CONSTRAINTS ON THE IDAHO COBALT BELT FROM FLUID INCLUSION GAS, NOBLE GAS ISOTOPE, AND ION RATIO ANALYSES—A REPLY
The Spar Lake Strata-Bound Cu-Ag Deposit Formed Across a Mixing Zone Between Trapped Natural Gas and Metals-Bearing Brine
Ore Genesis Constraints on the Idaho Cobalt Belt from Fluid Inclusion Gas, Noble Gas Isotope, and Ion Ratio Analyses
Genesis of the tabular-type vanadium-uranium deposits of the Henry Basin, Utah; reply
Genesis of the tabular-type vanadium-uranium deposits of the Henry Basin, Utah
Front Matter
Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt: Introduction, General Discussion, and Summary
Abstract THE carbonate-hosted ore deposits at Leadville, Gil-man, Red Cliff, Aspen, Alma, Tincup, Kokomo, and Mount Sherman have enjoyed a long and storied production history. These orebodies, as well as dozens of smaller deposits, are all located in the central Colorado mineral belt and together constitute an important metallogenic province (Figs. 1 and 2). Production Recorded metal production of the major districts in this province to date has consisted of 1,630,000 metric tons of zinc, 1,500,000 metric tons of lead, 145,000 metric tons of copper, 15,600,000 kg of silver, and 110,000 kg of gold (Table 1). For several reasons these figures represent only a portion of the metal concentrated by nature in these deposits: 1. Early production records are probably incomplete. 2. Inefficient methods were used to process much of the ore mined during the 1800s, particnlarly for zinc and copper. 3. The ores in the principal mining districts were partially removed by erosion prior to mining. 4. Significant reserves remain in the Leadville district.
Regional Geologic and Tectonic Setting of the Central Colorado Mineral Belt
Abstract The central Colorado mineral belt has an impressive wealth of mineral deposits, including the world-class deposits at Leadville, Gihnan, and Climax. The geologic history of the area spans more than 1.8 Ga, commencing with early Proterozoic accretion of volcanic-arc and back-arc complexes to the southern margin of the Archean Wyoming craton. These rocks were complexly deformed and intruded by large early and middle Proterozoic batholiths. During Paleozoic and Mesozoic time, the Proterozoic basement complex was buried beneath several kilometers of marine and continental sediments and was partially exhumed during Pennsylvanian orogenic uplift. Subduction-related alkalic to calc-alkalic magmatism and uplift affected the region during the Late Cretaceous-early Tertiary Laramide orogeny. Postsub-duction Oligocene and younger extension generated the north-trending Rio Grande rift zone, which was accompanied by bimodal magmatic activity. Most of the mineral deposits in the central Colorado mineral belt are associated with Oligocene subduction-related magmatism or to later rift-related activity. With the exception of the carbonate-hosted ores at Aspen, Laramide deposits are comparatively small. A few carbonate-hosted deposits (“Sherman-type”) maybe of Paleozoic age.
Paleozoic Stratigraphy, Tectonism, Thermal History, and Basin Evolution of Central Colorado
Abstract The Cambrian through Mississippian stratigraphic sequence of central Colorado comprises a thin seqnence (300-400 m thick) of interbedded quartz sandstones, carbonates, and minor shales separated by several unconformities. Extensive Paleozoic dolomitization of the carbonate units has occurred, increasing porosity and permeability in the Mississippian Leadville Formation. Karst solntion features, integrated cave systems with collection sinkholes and cutters, subhorizontal channels, and outlet chimneys, developed in the Leadville Formation and, to a lesser extent, in the Devonian Gilman and Dyer Formations beneath a regionally extensive karst erosion surface of Late Mississippian age. Although the sediments were deposited on a relatively stable cratonic shelf, several large-scale tectonic features, the Gore Range and ancestral Front Range-Wet Mountain uplift areas, were active along their bounding faults, as were the Homestake shear zone and other intrabasinal faults. These structures influenced sedimentary patterns and facies throughont the early and middle Paleozoic. Pennsylvanian and Permian strata of central Colorado record a history of active basin subsidence along nnmerous faults and contemporaneous sedimentation of as much as 5,000 m of noumarine to marine strata. Central Colorado was broken into a mosaic of fault blocks, on which recurrent tectonic activity produced the sedimentary and structural basin of the northern Central Colorado trough, including many adjacent mountainous erosional areas, the ancestral Uncompahgre and Front Range uplifts, the basin-center ancestral Sawatch uplift, and many other uplifts within the basin. Abrupt thickness variations and facies changes, onlap and overlap relationships, and occasionally, angular discordances occur adjacent to faults and uplift blocks. Local volcanism apparently occurred in the Pennsylvanian using the intersection area of the Gore and Mosquito faults as a conduit. Organic-rich source rocks occur abundantly in the Lower and Middle Pennsylvanian strata of the Belden, Gothic, Minturn, and Eagle Valley Formations. Thermal modeling of source-rock maturation data shows that the geothermal gradient locally within the rift basins in central Colorado during the Pennsylvanian and Early Permian was anomalously high (40°-48°C/km). Oil generation from Belden source rocks commenced in the Pennsylvanian and Early Permian and, in the local anhydrite-salt basin grabens of thickest late Paleozoic sediments, oil generation probably terminated in the late Paleozoic. Organic-rich mudstones, shales, and gypsum of the Minturn and Eagle Valley Formations were heated to the point of significant methane generation in the late Paleozoic. The maximum thermal maturity of Pennsylvanian strata was achieved during deepest burial near the end of the Cretaceous, prior to the uplift and erosion caused by Laramide and Tertiary tectonic activity and intrusion of the Pando Porphyry 72 Ma. Major porosity reduction and water expulsion from the Pennsylvanian fine-grained sediments, source rocks, and gypsum and the associated development of impermeability occurred during the first 1,000 to 2,000 m of burial in the Pennsylvanian. With continued sedimentation and burial in the late Paleozoic, large volumes of organic acids, oil, and gas were generated from the solid kerogen of the organic-rich source rocks. It is likely that the significant volume expansion created overpressured hydraulic conditions. Fracturing of the impermeable source rocks probably provided hydrologic continuity with intrabasinal and basin margin faults and fluid migration along the faults to underlying and overlying permeable strata. The confining nature of the thick sequence of generally impermeable overlying strata and the excellent permeability of the underlying Leadville Formation with its karst solution features suggest that large volumes of basinal fluids, hydrothermal organic acid-rich waters and hydrocarbons, may have moved from the overpressured source rocks into and through the underlying Leadville strata.
Igneous Rocks and Carbonate-Hosted Ore Deposits of the Central Colorado Mineral Belt
Abstract Many carbonate-hosted snlfide deposits of the central Colorado mineral belt are interpreted as products of hydrothermal systems associated with Late Cretaceons to Oligocene magmatism, Cordilleran magmatism migrated rapidly northeastward and inland during the Late Cretaceous-early Tertiary Laramide compressional orogeny. In Colorado, Laramide magmatism advanced northeastward, from La Pata to Jamestown, along en echelon segments of the Colorado mineral belt between about 75 and 65 Ma. Magmatism of the monzonite association generally preceded magmatism of the granodiorite association. Monzonitic magmatism was predominant in the northeastern Colorado mineral belt, but granodioritic magmatism was predominant in the central Colorado mineral belt. Some monzonitic intrnsive suites are associated with zoned polymetallic hydrothermal systems, characterized by distal and/or late gold-silver telluride veins. Some granodioritic intrusive suites are associated with zoned polymetallic hydrothermal systems, with internal copper-molybdenum occurrences, proximal gold-bearing pyritic veins, and/or distal silver-lead-zinc deposits. Between about 45 and 35 Ma, monzonitic and granodioritic magmatism retreated to the southwest, toward the Cordilleran trench. Laramide compression waned and post-Laramide plutonism surged, giving rise to many large plutons of the monzogranite association in the central Colorado mineral belt. Silver-lead-zinc deposits, with by-product copper and gold and minor but characteristic antimony and bismuth, accompany many intrusions of the monzogranite association. Some monzogranitic suites are transitional to quartz monzonite and/or granodiorite, and their accompanying hydrothermal assemblages tend to be relatively enriched in gold and zinc (as at Breckenridge, 45-40 Ma). Other monzogranitic suites are transitional to granite, and their accompanying hydrothermal assemblages tend to be relatively enriched in silver, lead, and molybdenum (as at Montezuma, 40-36 Ma). Fissure veins tended to form in silicate rocks, as at Montezuma-Argentine, whereas skarns formed locally in proximal carbonate rocks, as at Breckenridge and Leadville. Replacement veins and disseminations tended to form in fractured carbonate rocks, lateral and distal to monzogranitic intrusions, as in the Monarch, Tincup, and Tennessee Pass districts. Massive sulfide mantos tended to form in carbonate rocks above inferred monzogranitic intrusions (and beneath preore sills), as at Leadville (about 43-33 Ma), Tennessee Pass-Buckeye Gulch (42-40 Ma), and’ Gilman (34.5 Ma). Composite volcanic suites were erupted from calderas along the axis of the Sawatch uplift between about 36 and 34 Ma. Minor hydrothermal deposits, with diverse characteristics, are spatially associated with these calderas, and/or with related, pre- and postcaldera intrusions. Beginning at about 35 Ma, intrusions of the bimodal alkali-feldspar granite-lamprophyre association were emplaced in the central Colorado mineral belt, near its intersection with the then-incipient Rio Grande rift system. Porphyry molybdenum deposits formed above some cupolas of composite stocks of alkali-feldspar granite porphyry, as at Climax (33-24 Ma). After about 28 Ma, basaltic volcanism became increasingly common and widespread in the expanding Rio Grande rift system. Basalts generally are unmineralized at present levels of exposure but locally are associated with sulfur-rich hot springs. Fluorine-rich hot springs and fluorspar deposits also occur locally along normal faults of the Rio Grande rift system.
Abstract Geochemical studies of Leadville Formation dolomites provide information about the solutions from which they formed. Seven dolomite types can be grouped into six geochemical types on the basis of trace element contents and oxygen isotopes. Medium and coarse crystalline dolomite are indistinguishable on the basis of either trace element or oxygen isotope contents. Low trace element abundances and slightly negative δ 18 O PDB values are snggestive of diagenesis in solutions containing a significant meteoric component. Fine crystalline dolomite formed penecontemporaneously with deposition and contains heavier oxygen than either medium or coarse crystalline dolomite or undolomitized Leadville Limestone. The oxygen isotope data supports dolomitization of fine crystalline dolomite by evaporatively concentrated sea water followed by stabilization by meteoric water. Two varieties of zebra spar dolomite precipitated into cavities formed during karst dissolution. Trace element abundances and 18 O composition of earlier formed “cloudy zebra spar” are similar to those of medium and coarse crystalline dolomite and suggest that this dolomite formed as cement during karst solution erosion. Later formed “clear zebra spar” contains lighter oxygen and higher Fe and other trace elements, and fluid inclusions in it indicate precipitation above 130°C from evolved brines. This dolomite is interpreted to have formed from sedimeutary brine fluids after burial by Pennsylvanian sediments. Baroque dolomite occurring as cement in karst breccia bodies contains very light oxygen (δ 18 O PDB = —20%) and is high in Fe, Mn, and base metals. This dolomite is intimately associated with sphalerite and galena in some ore deposits and formed by recrystallization of detrital dolomite sand in karst breccias during ore deposition. Paleomagnetic studies reveal a well-defined late Paleozoic remanence in most dolomite types spatially removed from Laramide-Tertiary igneous activity. The remanences are carried in magnetite of possible diagenetic origin. Preservation of these remanences suggests that these rocks have not undergone significant aquatic alteration since late Paleozoic time.
Dolomitization and Diagenesis of the Leadville Limestone (Mississippian), Central Colorado
Abstract The Leadville Limestone (Kinderhookian-Osagean) was deposited on a shallow marine shelf impinging on an exposed landmass in the area of the present Front Range and Wet Mountains. The upper Leadville Formation is truncated by an unconformity of regional extent, and karst features related to this unconformity are common within Leadville strata. Along the eastern margin of the Leadville depositional area, the Leadville Formation has been altered to massive dolostone. Seven types of dolomite were recognized. Fine crystalline dolomite is widespread throughout central Colorado and is associated with supratidal facies. Textures indicate pe-necontemporaneous dolomitization in a sabkha setting. Medium and coarse crystalline dolomite make up the bulk of the Leadville Dolomite and are not associated with any specific depositional facies. Dolomitization was largely restricted to the edge of the Mississippian depositional area and was accompauied by the formation of chert nodules. This period of regional dolomitization preceded karst dissolution and brecciation and the creation of caverns. Dolomitization occurred in a sea water-fresh water mixing zone. Zebra dolomite consists of bands of fine, medium, or coarse dolomite alternating with coarse, white dolomite spar. Zebra dolomite is best developed adjacent to karst channelways, and the zebra spar precipitated into voids created by karst dissolution. Two generations of zebra spar are present: early cloudy zebra spar, which precipitated as isopachous crusts during later stages of karst erosion, and later clear zebra spar, which precipitated from hot brines after burial by Pennsylvanian sediments. Baroque dolomite formed as breccia matrix in karst caverns. Textures indicate that this dolomite formed by replacement of internal dolomitic cave sediment. Baroqne dolomite precipitation was accompanied by precipitation of base metal sulfides. These features predate stylolite formation and indicate precipitation prior to deep burial. A second episode of base metal deposition occurred after stylolite formation had ceased. This episode was accompanied by replacement of dolomite by calcite and pyrite. The latest dolomite is a cement which occurs in surface exposures and is most likely related to Recent ground-water movement.
The Development of Paleokarst and Other Solution Features in the Mississippian Leadville Dolomite, Central Colorado
Abstract The Mississippian Leadville Dolomite in central Colorado hosts a large number of paleokarst and other solution features ranging in age from the Mississippian to the present. In many cases, these features provided pathways of high permeability for subsequent fluid movement. Four main episodes of dissolution have been identified. In the early Osagean, prior to deposition of the Castle Butte Member, subaerial exposure and minor karstification of the Red Cliff Member of the Leadville Dolomite formed breccias at the surface and karst solution features in the shallow subsurface. In the Late Mississippian, following the retreat of the sea from central Colorado, extensive karst features developed on and within the Leadville Dolomite. Following burial of the Leadville Dolomite in the Pennsylvanian, hydrothermal solution features of various ages were created. Pennsylvanian age, migrating basinal brines and/or convecting hydrothermal fluids associated with igneous intrusive activity in the Laramide and/or mid-Tertiary may have caused the dissolution. Finally, in the late Tertiary or early Quaternary, karstifieation was initiated as erosion exposed the Leadville Dolomite to meteoric water.
Abstract To constrain speculation about the character of the host rock at the site of manto ore deposition, we have studied the Leadville Dolomite peripheral to three similar systems of high-temperature, pyritic mantos in central Colorado. The deposits studied are at Leadville, Buckeye Gulch, and Gilman. Comparison of unmineralized sections shows that the Leadville Dolomite has systematic internal stratigraphy, consisting of 15 named beds (from base to top: 6G-8G, 1-12). Many of the mantos are located in one of four stratigraphic positions: the uppermost Leadville, beds 7-8, bed 3, and bed 1. Three of these mineralized beds (containing the largest orebodies) share the characteristics of relatively coarse grain size and porous texture. The fourth mineralized bed (7-8) is the locus of most of the paleocaves in the Leadville Dolomite. In addition, some beds in the Leadville (6G, 4) are selectively unmineralized even in regions of extensive replacement, and these beds are distinguished by their fine grain size and low porosity. Limited data indicate that the fine-grained beds are six orders of magnitude less permeable than the coarse-grained zebra-textured beds, which in turn are as much as two orders of magnitude less permeable than the manto orebodies. This suggests that the principal control on manto formation was permeability contrast in the dolostone beds of the Leadville Dolomite and that paleocaves were only locally important. The permeability of initial fluid conduits was almost certainly increased by the ore-forming fluid, thereby focusing a higher fraction of the flow into the initial channel. This positive feedback mechanism would result in runaway permeability enhancement and the focusing of essentially all of the flow into narrow conduits. The characteristic shape of the mantos in central Colorado may therefore be an inevitable consequence of the mechanics of fluid flow within carbonate rocks. The general lack of pa-leokarst cave control for the high-temperature pyritic manto deposits is confirmed by comparison of orebody morphology to solution features of hydrothermal and paleokarst origin.
Abstract The Leadville district has yielded gold, silver, and base metals for more than 125 years, principally from dolostone-hosted massive sulfide replacement bodies (mantos). The carbonate host rocks were deposited on a Paleozoic shallow marine shelf overlying a Proterozoic (1.4 Ga) granitic basement, and subsequent diagenetic processes converted the shelf rocks to do-lostones. Multiple stages of dissolution with infilling generated paleokarst units within the uppermost dolostone, the Leadville Dolomite. The ore host rocks lie in the Mosquito Range on the eastern flank of the Laramide Sawatch uplift. Southwest-directed, east-dipping, low-augle thrust faults offset by high-angle reverse faults characterized the compressional environment in the Leadville district during the Laramide orogeny. The first Phanerozoic igneous eveut in the area, emplacement of the Pando Porphyry, occurred contemporaneously with low-angle thrusting. The Pando Porphyry was emplaced along thrust planes and along an unconformity at the top of the Leadville Dolomite. Six igneous events resulting in sills, dikes, and a small stock followed during Tertiary time, culminating with the emplacement of the Breece Hill stock in the eastern part of the Leadville district. All of the preore igneous rocks are latite to quartz monzonite in composition: Four genetic types of ore deposits are present within the Leadville district: (1) contact-related magnetite-serpeutine bodies, (2) dolostone-hosted barite-silver-minor base metal replacement and open-space filliugs termed “Sherman-type ore,” (3) dolostone-hosted zinc-lead-silver-gold massive sulfide replacement bodies with associated veins, referred tg as “Leadville-type ore,” and (4) placer gold. The contact-related magnetite-serpentine bodies replace dolostone aud formed during emplacement of the Breece Hill stock as well as locally around the margin of au older stock southeast of Breece Hill. These ores were used sporadically as smelting flux. The main Leadville district ore deposits are principally massive sulfide replacement bodies in dolostone, but there are related quartz-pyrite-gold veins, disseminated quartz-pyrite-gold in porphyry, and quartz-base metal veins. These are zoned spatially about the Breece Hill stock with central-most quartz-pyrite-gold veins and local disseminations surrounded by quartz-base metal veins and mautos. Mantos closer to Breece Hill have higher gold coutent (Ag/Au = 20), as do those localized along an east-dipping low-angle thrust fault west of Breece Hill; with increasing distance from Breece Hill silver/gold ratios of ores within mantos increase to>75. Manganese in gangue and alteration products increases away from the stock as well. Leadville orebodies in dolostone are dominated by pyrite with lesser marmatite, galena, chalcopyrite, tetrahedrite, pyrrhotite, marcasite, eleetrum, and late-stage veinlets with elevated silver, gold, bismuth, and tellurium contents. Late golden barite, dolomite, and rare fluorite are found in veinlets or vugs within ore. The sulfide coutent of ores in dolostone is>65 vol perceut. The ores are massive and commonly exhibit banding due to textural, size, or min-eralogical differences between bands. The bands parallel sedimentary layering or fracture ‘ planes cutting the dolostones. Diffusion-controlled replacement resulted in ores that locally exhibit concentric banding about unreplaced dolostone bodies, reflecting the stratigraphic and structural controls on ore fluid movement. Local sulfide replacement of paleokarst results in complete preservation of finely laminated cave fill with soft sediment impact features due to rock falls in the paleocaves; however, Leadville ores replaced principally massive dolostone with paleokarst localizing only miuor amouuts of ore. Hydrothermal alteration of the Breece Hill stock is extensive with both sericitic and argillic products common. Adjacent to veins and mantos more intense sericitic alteration is friuged by argillic and propylitic assemblages, extending as much as 100 m from ore. All assemblages contain abundant siderite or manganosiderite. The carbonate host rocks exhibit limited visible alteration exteuding uo more than 5 m from ore. Thermal effects on dolostone insoluble residue phyllosilicates is more extensive. Potassium-argon dates on sericite in Johnson Gulch Porphyry yield an average age on the Leadville ore event of 39.6 ± 1.7 Ma or early Oligocene. Fission-track dates on apatite and zircon from altered preore Johnson Gulch Porphyry indicate that cooling of the hydrothermal system occurred over a 6-m.y. period. The Leadville orebodies are cut by rhyolite porphyry dikes and plugs; these, in turn, have been broken by fragmental porphyry, a fluidized breccia system of tabular to irregular bodies around the perimeter of the Breece Hill stock. Late-stage quartz-base metal sulfide veins cut the fragmental porphyry and yield similar fluid inclusion data as found in the main Leadville-type orebodies. The rhyolite and fragmental porphyries were apparently derived from an intrusive system centered beneath the Breece Hill stock. As the fragmental porphyry bodies were emplaced, the downdropped block, a large triangular-shaped graben in the southeastern part of the district, subsided. Postmineral normal faulting (east side up) associated with development of tbe Rio Grande-Upper Arkansas rift exposed the ores in several blocks up the western flank of the Mosquito Range, leading to oxidation and supergene enrichment of the Leadville-type orebodies and the development of placer gold in California Gnlch, the site of initial discovery.
Abstract The Leadville district vein and manto ores are thermally and isotopically zoned around the Breece Hill intrusive center. Fluid inclusions in quartz, sphalerite, dolomite, rhodochrosite, and barite are simple two-phase water liquid-vapor systems. The central-most quartz-pyrite-gold veins yield filling temperatures between 320° and 379°C compared to the fringing quartz-base metal veins which have values between 288° and 349°C. The mantos yield main-stage temperatures between 270° and 328°C. The late quartz, dolomite, rhodochrosite, and golden barite yield temperatures ranging between 270° and 180°C. Fluid salinities in all stages range between 0.4 and 7.0 equiv wt percent NaCl. The ore-forming system formed under a total pressure of 1,2 kbar based on sphalerite geobarometry; an average salinity of 3.3 equiv wt percent NaCl requires a correction of 90°C to the filling temperatures. Oxygen isotope data for altered igneous whole rocks show δ 18 O values are highest in the uppermostpart of the central alteration zone at Breece Hill (whole rock δ 18 O = 10.4-12.2‰) and along the periphery of the Breece Hill stock on the Yak tunnel level (10.4-12.2‰); the fringing manto ores along the western and northwestern part of the district are isotopically heavier (11.6-13.9‰). Within the Black Cloud mine on the southeastern margin of the Breece Hill stock but 760 m below the surface, the igneous rocks have isotopically lower δ 18 O values ranging from 8.7to 10.9permil. This vertical zoning of oxygen isotope compositions indicates the influx of hot high 18 O (δ 18 O = 5-8%) flnid from beneath the Breece Hill stock at a large water-rock ratio. Whole-rock δ D values display a reversed, districtwide zoning; the lowest δ D value for igneous whole rocks (—115.7‰) is from the uppermost altered central zone, and with increasing depth the δD value increases (—105.2 to —82.9‰). Main ore-stage vein and manto quartz exhibits a δ 18 O value of 10.1 to 14.2 per mil that yields a calculated fluid δ 18 O range of 5.2 to 9,2 per mil. Late vein and vug quartz is progressively lighter. Carbonate host rocks show 18 0 depletion on a very restricted scale from normal regional values of δ 18 O = 22 to 24 per mil; pearly dolostone and bird’s-eye dolomite show the strongest 18 O depletion (10.8-15,5‰). Dolomite and siderite within massive manto ore have δ 18 O values of 9.8 to 13.1 per mil. Late vug-filling dolomite ranges from 9 to 12.5 per mil. Similar depletions in carbonate δ 13 C values from a regional background of 0.2 to 1.0 per mil range between —0.5 and —2.2 per mil. Sulfur isotope analyses of pyrite (δ 34 S = 1.30-3.20% 0 ), sphalerite (-0.50 to +2.20%»), and galena (—2.40 to +0.70‰) yield a calculated δ 34 Sσs = 1.8 per mil. Pyrite-sphalerite and sphalerite-galena pairs yield calculated temperatures with means of 345° and 408°C, respectively, in close agreement with pressure-corrected fluid inclusion filling temperatures. Lead isotope analyses of early to late galena and preore K feldspar from associated Tertiary igneous rock show that galena lead is notably thorogenic and of the same composition as the Tertiary K feldspar. A two-stage model of lead generation yields a secondary isochron that intersects the Stacey and Kramer curve at about 1.70 Ga, the general age of the basement rocks in central Colorado. The location of the Leadville district on the northeastern margin of the largest gravity low in Colorado suggests that a batholith may have released magmatic fluids into the structurally complex Leadville district. The isotopic data all suggest magmatic derivation of water, sulfur, and lead.
Breccia Bodies in the Leadville District, with Emphasis on Occurrences in the Black Cloud Mine, Lake County, Colorado
Abstract Within the Leadville district and, in particular, the Black Cloud mine, heterolithic breccia bodies cut sulfide replacement bodies. The breccia bodies form dikes and large pipes, generally focnsed along faults around the perimeter of the centrally located Breece Hill stock. The breccia bodies have been informally termed “fragmental porphyry” because of the presence of pseudophenocrysts which are actually crystal fragments. Postore but prebreecia rhyolite porphyry dikes and plugs were intruded around the perimeter of the Breece Hill stock from a deep stock, The rhyolite porphyry was subsequently incorporated as fragments in the later fragmental porphyry. The breccia bodies are composed of hydrothermally altered and unaltered rock fragments. Four different types of breccia represent separate developmental stages. Stage 1 breccias are most prevalent and are composed of subangular fragments of every rock type found in the district, set in a dense rock flour matrix. Stage 2 is composed of crosseutting dikes of intrusive fine-grained breccia. Stage 3 breccia forms flat-dipping tabular bodies near the top of the Leadville Dolomite where black shale and/or a Late Cretaceous igneous sill is present above sulfide mantos. The distinctive feature of stage 3 breccia is its black matrix derived from black shale. The stage 4 breccia consists of narrow dikes with well-rounded fragments. Fragments of massive sulfide manto ore are found within all breccias, and veins of similar sulfide minerals crosscut all stages of the breccia. The fragmental porphyry breccia bodies lack trace metal enrichment except where fragments of sulfide orebodies have been entrained. The presence of local late golden barite cementing breccia fragments indicates the breccia bodies formed later than the main-ore stage. Postore rhyolite porphyry dikes and plugs were intruded around the perimeter of the Breece Hill stock from a deep stock. The breccias are believed to have formed much like diatremes by fluidization of rock along hydrothermal fluid conduits as the ore-forming magmatic hydro-thermal system collapsed and meteoric water encroached. The meteoric fluids were volatilized by residual magma from the deep stock, entraining some of the postore rhyolite porphyry. Some breccia pipes may have vented onto the paleosurface; several increase in diameter near the present surface. The diatreme formation rapidly dissipated heat and fluids, causing the final collapse of the hydrothermal system. Subsidence of a graben, the downdropped block, occurred with the release of hydrothermal fluids and fluidized, comminuted rock material. The close spatial and temporal relationship of fragmental porphyry breccias with orebodies and the crosseutting late veinlets of ore minerals within the breccias indicate proximity of the breccia bodies to ore fluid conduits; as such, they are excellent guides to blind mineral deposits within the Leadville district.