Breccia Pipes Research Articles

Thermal history of the Permian formations from the Breccia Pipes area (Grand Canyon region, Arizona)

The thermal history of the Permian formations from the Breccia Pipes area in the Colorado Plateau has been assessed using different methods. Burial reconstruction indicates maximum temperatures between 100 and 140 °C for a 30 °C/km geothermal gradient. A temperature of 140±5 °C integrated over 10 to 50 Ma. has been deduced from fission track analysis. This range of temperatures is consistent with trapping temperatures obtained by fluid inclusions study (75–140 °C). Fission track and burial hypotheses have been introduced in a kinetic model for organic matter maturation. The computed Tmax average 445 and 421 °C respectively. A comparison with organic geochemistry data from the Permian Toroweap formation is carried out and demonstrates that the two above hypotheses constitute a thermal framework for the organic matter evolution. Main conclusions referring to beginning of oil window and regional uplift ages (Laramide orogeny) as well as maximum temperature ranges are deduced from this study.

Evidence for Cu-(Ag) mineralization by magmatic-meteoric fluid mixing in Keweenawan fissure veins, Mamainse Point, Ontario

Locally rich (up to 40% Cu) Cu-(Fe) sulfide mineralization with minor Ag occurs in fault-related fissure veins cutting, but probably penecontemporaneous with, Keweenawan plateau basalt lava flows at Mamainse Point, Ontario. Paragenetic studies of samples from the Coppercorp mine define four stages of mineralization: (1) early pyrite-(chalcopyrite) with strong silicification and sericitic alteration; (2) a transitional stage of chalcopyrite-hematite and bornite-hematite assemblages, with propylitic alteration; (3) main-stage chalcocite-hematite, often replacing earlier sulfides, with propylitic alteration; and (4) late-stage, low-temperature calcite deposition, with local Cu degrees , Ag degrees , Cu arsenides, and cuprite. This paragenetic sequence suggests an increase in f (sub O 2 ) , perhaps accompanied by an increase in pH and a decrease in total sulfur activity.The mineralized fissures crosscut but are coeval with regional-scale low-grade (zeolitic to local epidotic) alteration in the plateau basalt sequence; their mineralogy, particularly the early sericitic wall-rock alteration, is at variance with the regional alteration mineralogy. This contrast is supported by fluid inclusion and light stable isotope data. Temperatures of the regional alteration reached little higher than 300 degrees C at the base of the lava pile, and fluid salinities were generally below approximately 2 equiv wt percent NaCl, suggesting the dominant presence of meteoric water. However, in the fissure veins, fluids varied from or = 450 degrees C and approximately 22 equiv wt percent NaCl. Fluid inclusions representing the hot saline fluids were only found in the presence of copper mineralization, and it is therefore concluded that these fluids were responsible for metal transport. Analysis of decrepitation residues from fluid inclusions in chalcocite indicate that the fluids were dominantly NaCl-CaCl 2 -H 2 O solutions; suppression of the critical curve by the presence of Ca (super +2) permits the interpretation that these saline, high-temperature fluids were near their critical point at approximately 450 degrees C. Their high temperatures compared to the ambient fluids suggest that they were derived from below the Keweenawan lava pile and that they may have been associated with intrusive magmatic activity. Approximately positively correlated arrays of homogenization temperature vs. salinity data from fluid inclusions in ore samples suggest that mixing between the hot saline fluid and the cooler meteoric ground waters was an important mechanism for deposition of Cu-(Fe) sulfides. The combined effects of rapid cooling, dilution, neutralization, and oxidation resulting from this process would cause a significant drop in copper solubility with consequent deposition of ore; this process would also explain the observed paragenetic sequence of reduced to oxidized assemblages.The delta 13 C values of calcite from the fissure veins average -3.9 + or - 0.9 per mil (n = 12), and fluid delta 18 O ratios calculated from seven samples of quartz and epidote intergrown with sulfides range from 10.7 to 2.5 per mil (+ or -2ppm due to temperature uncertainties). These isotopic ratios overlap the ranges for magmatic carbon and water, respectively. Eight samples of late calcite from mineralized veins indicate fluid delta 18 O values ranging from 4.8 to -3.1 per mil (+ or -0.5-4.5ppm). Calcite from barren veins (n = 3) and from veinlets associated with the regional alteration (n = 4) extend this range down to -6.9 per mil; delta 13 C ratios of between -2.7 and +0.7 per mil were obtained from these last four samples. Negative fluid delta 18 O values and delta 13 C values of carbonates near zero per mil suggest the dominant presence of meteoric waters and atmospheric carbon, respectively, in the regional system and the dominance of meteoric water in barren veins in the fissures; this interpretation is in agreement with the fluid inclusion data. The range of delta 18 O from magmatic to lighter values in the mineralized veins suggests that meteoric water was also present during ore deposition and was mixed in varying proportions with a magmatic fluid. The isotopic data, therefore, support a model of meteoric ground water and magmatic (or highly exchanged) fluid interaction in the fissure veins.Comparisons are made to the nearby Keweenawan Jogran Cu-(Mo) porphyry, and the Tribag Cu-(Mo) breccia pipes. Similarities between these deposits and the Mamainse Point fissure veins in mineralogy, fluid properties, and light stable isotope compositions suggest a genetic link. It is proposed that the Coppercorp, Tribag, and Jogran deposits represent successively deeper levels of exposure in Keweenawan porphyry-related hydrothermal systems. Richest ore deposition occurred in the high-level fissure veins at Coppercorp, due to focused mixing of hot metalliferous fluids with meteoric ground waters.

Spectral Discrimination of Uranium-Mineralized Breccia Pipes in Northwestern Arizona: ABSTRACT

The price of uranium is currently the lowest in more than a decade. The only type of uranium deposit that is economically viable in the depressed uranium market is such high-grade ore as the unconformity type found in Canada and Australia. Exploration for uranium-bearing breccia pipes in northwestern Arizona by both domestic and foreign companies is currently active because of the relatively high-grade ore they contain and their tendency to be polymetallic. In the US, uranium-mineralized breccia pipes are one of the few deposits that can compete in the current market. A stepwise discriminant analysis was performed on spectral data acquired from the field, laboratory, and Landsat thematic mapper (TM). The principal objectives were (1) to investigate the fundamental differences in the spectral properties of outcrops on the surface of breccia pipes and the background, (2) to choose TM bandpasses that were statistically optimum for distinguishing between breccia pipes and the background, and (3) to compare the results of the field, laboratory, and TM digital data which were acquired by different instruments having different spatial and spectral resolutions.

Relationship Between Collapse History and Ore Distribution in Sage Breccia Pipe, Northwestern Arizona: ABSTRACT

The Sage pipe is similar to other collapse breccia pipes in northern Arizona which have their beginnings in cave systems in the Redwall Limestone. Stoping of successively younger units caused the upward propagation of the pipe and provided the pipe-filling breccia. The Sage pipe extends at least 2,500 ft (762 m) vertically; the horizontal dimensions range from 100 to 300 ft (30.5-91 m), depending on variations in the adjoining host stratigraphy. The composition and distribution of breccia facies suggest a complex collapse history and variability in the mechanics of collapse. Rock failure took place both by block stoping and by decementation of sandstone and siltstone followed by flow of unconsolidated grains. The resulting breccias range from matrix to fragment-dominated, to sand flow breccia resulting from flow of individual grains. Episodic secondary collapse or readjustment within the breccia pile complicated facies distribution. Paragenetic studies indicate multiple periods of mineralization at Sage resulting in enrichment in an extensive suite of elements. Ore-grade uranium mineralization extends vertically for nearly 700 ft (213 m). Lateral distribution of the ore is variable and is directly related to breccia facies distribution. In generally, the more permeable breccias tend to be the most highly mineralized. Fracture, intergranular,more » and interfragment permeability were important to mineral distribution. Breccia continuity or plumbing was also important to lateral and vertical mineral distribution.« less

Exploration Geology of Canyon Breccia Pipe South of Grand Canyon, Arizona: ABSTRACT

Exploration Geology of Canyon Breccia Pipe South of Grand Canyon, Arizona: ABSTRACT

Mineralization at Cananea, Sonora, and the paragenesis and zoning of breccia pipes in quartzofeldspathic rock

Correlative early and late deposit types in the Cananea district correspond to similarly classified deposit types in other porphyry copper districts. Potassic alteration, massive silicates in the Colorada orebody, and prograde skarn were produced by early, probably predominantly magmatic-hydrothermal fluids from a limited number of porphyry stocks. Later stockwork-disseminated deposits associated with sericitic alteration were contemporaneous with breccia pipes, retrograde alteration of skarn, and limestone replacement deposits. Fluids associated with the relatively late deposits were probably predominantly meteoric-hydrothermal, but geologic relations suggest that the earliest fluids in some of these deposits may have included fluid exsolved from porphyry. Mineralization in both early and late deposits precipitated in the sequence: silicates followed by sulfides, except for early molybdenite in some pipes.Many pipes in the district appear to have formed by collapse into open space at depth. Consistent paragenetic sequences, structures, and textures shared by these pipes suggest that collapse occurred into space formed at the same time relative to emplacement of the responsible porphyry phases and evolution of the associated hydrothermal systems. A possible analogue of the space into which collapse took place is abreccia at the outer contact of a large porphyry stock. Small outlying structures containing sulfide mineralization and breccia or intensely fractured rock appear to be associated with the upper terminations of pipes.Late mineralization has high overall pyrite/Cu(Fe) sulfides ratios, greater than 1. Wherever the sulfide sequence could be established, it is (1) pyrite, (2) Cu(Fe) sulfides, and (3) sphalerite + galena + tetrahedrite, except for small amounts of second-stage sulfides. Zoning of these numbered sulfide stages in breccia pipes in quartzofeldspathic rock and also in limestone replacement deposits is inferred to be 1, 1 + 2, 1 + 2 + 3, and 1 + 3 from the proximal to the distal ends of the deposits. Pipes within disseminated deposits contain the lower portion of the pattern. Textures indicative of local replacement of wall rock and early gangue minerals by later phases (e.g., sulfides) are common. The common occurrence of late paragenetic stages in a pipe indicates that the pipe remained open during precipitation of the main stage of economic sulfides.Garnet in pipes in quartzofeldspathic rock is diagnostic of limestone at depth nearby. Early adularia and quartz-adularia alteration of wall-rock fragments in the Cananea-Duluth pipe persist to much greater distances above limestone than garnet does and might prove to be similarly diagnostic.

Relationship of Sonoran tungsten mineralization to the metallogenic evolution of Mexico

The tungsten province of northwestern Mexico is the southernmost of the Cordilleran tungsten provinces of North America and the westernmost of a parallel series of Mexican metal provinces which contain Cu, Au-Ag, Pb-Zn-Ag, F, Sn, Mn, U, Fe, and other metals. On its western side, the tungsten province primarily contains skarns associated with Paleozoic limestones. The eastern side primarily contains pegmatites and quartz veins in Laramide plutons. The eastern margin of the tungsten province overlaps the porphyry copper province where a series of breccia pipes contain copper and lesser tungsten.Isotopic dating, using K-Ar, 40 Ar/ 39 Ar, and Rb-Sr methods, indicates that tungsten mineralization is Laramide with ages ranging from 47 to 65 m.y. Hornblende and biotite K-Ar and 40 Ar/ 39 Ar ages are discordant by an average of 11 m.y., and biotite and muscovite associated with mineralization record ages averaging 14 m.y. younger than related intrusive rocks. These large discordances are inferred to be the result of slow cooling in a relatively deep batholithic environment.Sr isotope initial ratios for intrusive rocks associated with tungsten deposits indicate that mineralization occurred independently of basement rock type. Deposits within terranes underlain by cratonic basement are associated with intrusive rocks having higher initial ratios than deposits within terranes underlain by accreted oceanic basement. Initial ratios range from 0.70901 at San Antonio (eratonic) to 0.70570 at San Alberto (accreted).The age of the tungsten province is similar to that of the porphyry copper province and is approximately 20 m.y. older than Ag-Au and Pb-Zn-Ag mineralization in the Sierra Madre Occidental and the central plateau. Tungsten mineralization is approximately 10 m.y. older than rhyolite-hosted U mineralization and approximately 25 m.y. older than rhyolite-hosted F, Sn, and Fe mineralization. The northwestern Mexico tungsten province is slightly younger than other Cordilleran tungsten provinces which are pre-Cenozoic in age.

Size, depth and related structures of intrusions under stratovolcanoes and associated geothermal systems

The subvolcanic structural configuration of faults, stress field, permeability controls and intrusion(s) shape, size and depth are primary factors controlling the location and quality of a geothermal system. Subvolcanic stocks under stratovolcanoes are typically 1 to 3 km in diameter and emplaced at depths of 1 to 4 km. Exposed stocks in the Western Cascades of Oregon are of similar size and are emplaced at depths of 1 to 2 km. Shallow magma chambers in general are typically 4 to 9 km deep, and several times larger than the subvolcanic stocks. Likely subvolcanic structures are radial compression fractures around the magma chamber, and above the chamber extension faulting, stockwork fractures and breccia pipes. Pre-existing or regional structures and stress field will influence the location and shape of magma bodies and associated thermal systems. This study combines data from areas of active stratovolcanoes and older eroded volcanic areas to formulate a detailed structural model of the plumbing system under a stratovolcano. The model is correlated with factors controlling associated geothermal systems. A geothermal system may occur at depths of 1–2 km under a stratocone's base. Deeper systems associated with shallow magma chambers are likely to be present.

Subsidence resulting from the dissolution of Permian gypsum in the Ripon area; its relevance to mining and water abstraction

Abstract Because gypsum dissolves rapidly in flowing water the subsurface dissolution of Permian gypsum at Ripon results in severe subsidence. The pattern of subsidence and methods of propagation are similar to those which might result from mining. Breccia pipe propagation and bulking factors for Permian rocks are discussed. The implications both of gypsum dissolution and the forceful expansion of anhydrite by hydration in flooded gypsum mines are assessed. The removal of gypsum and possible subsidence caused by large-scale water abstraction and mine dewatering are also considered.

Ironstone-Gossan discrimination: Pitfalls of a simple geochemical approach — A case study from northeast Scotland

Located in northeast Scotland, the Lecht manganiferous ironstone occurs as several minor and one principal outcrop within deeply weathered Dalradian meta-sediments. The distribution of these shows is controlled primarily by an underlying porous breccia pipe and not by Dalradian stratigraphy or faulting, as previously suggested. The deposit is composed principally of goethite and cryptomelane, with minor hematite, ramsdellite, pyrolusite, lithiophorite, chalcophanite and woodruffite. The ironstone is enriched in several target and pathfinder elements, particularly Zn and Ba which are primarily concentrated in the manganese oxides. Detailed examination of the geochemistry demonstrates that the enrichments are actually more typical of non-economic ironstones (particularly bog-ore) than gossans (a conclusion supported by field, textural and mineralogical evidence), illustrating the danger of relying upon simple geochemical surveys alone for ironstone-gossan discrimination. No relict sulphides, secondary ore minerals, native metals, gangue minerals or “boxwork” textures were observed in either hand specimen or polished section. The morphology and textures of the Lecht ironstone are typical of those observed in bog-iron ores and in weathered profiles. The Lecht ironstone is considered to have been derived from prolonged weathering of the local Dalradian meta-sediments. These are enriched in target and pathfinder elements and are regarded as a prospective sequence. Cementation of the subsequent regolith by solutions rich in iron, manganese and other elements, combined with bog-ore formation and penetration of the breccia pipe by these solutions, produced the complex and varied morphology and geochemistry seen in the deposit today. The Lecht deposit may represent the distal manganiferous expression of a goldrich zinc-lead exhalative deposit hosted by the Dalradian meta-sediments of the region.

Ironstone-gossan discrimination: Pitfalls of a simple geochemical approach — A case study from northeast Scotland

Abstract Located in northeast Scotland, the Lecht manganiferous ironstone occurs as several minor and one principal outcrop within deeply weathered Dalradian meta-sediments. The distribution of these shows is controlled primarily by an underlying porous breccia pipe and not by Dalradian stratigraphy or faulting, as previously suggested. The deposit is composed principally of goethite and cryptomelane, with minor hematite, ramsdellite, pyrolusite, lithiophorite, chalcophanite and woodruffite. The ironstone is enriched in several target and pathfinder elements, particularly Zn and Ba, which are primarily concentrated in the manganese oxides. Detailed examination of the geochemistry demonstrates that the enrichments are actually more typical of non-economic ironstones (particularly bog-ore) than gossans (a conclusion supported by field, textural and mineralogical evidence), illustrating the danger of relying upon simple geochemical surveys alone for ironstone-gossan discrimination. No relict sulphides, secondary ore minerals, native metals, gangue minerals or “boxwork” textures were observed in either hand specimen or polished section. The morphology and textures of the Lecht ironstone are typical of those observed in bog-iron ores and in weathered profiles. The Lecht ironstone is considered to have been derived from prolonged weathering of the local Dalradian meta-sediments. These are enriched in target and pathfinder elements and are regarded as a prospective sequence. Cementation of the subsequent regolith by solutions rich in iron, manganese and other elements, combined with bog-ore formation and penetration of the breccia pipe by these solutions, produced the complex and varied morphology and geochemistry seen in the deposit today. The Lecht deposit may represent the distal manganiferous expression of a gold-rich zinc-lead exhalative deposit hosted by the Dalradian meta-sediments of the region.

Contrasting evolution of fluorine- and boron-rich tin systems

Individual Sn provinces or regions within provinces are sometimes enriched in fluorine or boron, giving rise to fluorine-rich and boron-rich environments. The structural styles of mineralisation within these environments are similar except that hydrothermal intrusive breccia pipes are more common in boron-rich environments and apogranite/massive greisen systems are more common in fluorine-rich environments. The increased solubility of H2O in B-bearing magmas compared to F-bearing magmas may play a role in the structural evolution of the mineralising systems. The greater mechanical energy produced during crystallisation of B-rich magmas provides a mechanism for breccia pipe and stockwork formation, while the more passive crystallisation of F-rich magmas often results in the formation of disseminated mineralisation. The partitioning of boron toward the aqueous fluid phase and the enhanced solubility of silica in the fluid phase frequently results in tourmalinisation and silicification of the wall-rocks in B-rich environments. In contrast, feldspathic and sericitic alterations usually predominate in F-rich environments.

A fluid inclusion and sulfur isotopic study of precious and base metal mineralization spatially associated with the Patch and Gold Cup breccia pipes, Central City, Colorado

A fluid inclusion and sulfur isotopic study of precious and base metal mineralization spatially associated with the Patch and Gold Cup breccia pipes, Central City, Colorado

Formation of late Caledonian subvolcanic breccia pipes at Cruachan Cruinn, Grampian Highlands, Scotland

ABSTRACTPipe-shaped breccia bodies associated with diorite intrusions are composed mainly of angular clasts of local schists with a few transported clasts of quartzite. Plate shaped fragments are commonly oriented to define planar fabrics in the breccias. These features indicate the operation of gas fluidisation within the pipes and both entrainment and expanded bed conditions are inferred. The fabrics result from the collapse of the fluidised suspensions as the gas flow declined. Dilational fracture patterns in the country rock comparable with the stress release patterns found around mine shafts can be matched with the fractures required to produce the angular schist clasts. It is concluded that fracturing and the introduction of fragments into the fluidised breccia system was a continuous process and that the pipe diameter increased progressively with time. Microdiorite sheets and related stock like bodies of diorite cut and metamorphose the breccias. Compaction, hornfelsing and hydrothermal alteration also contributed to breccia formation.

Arizona Strip Breccia Pipe Program: Exploration, Development, and Production: ABSTRACT

Arizona Strip Breccia Pipe Program: Exploration, Development, and Production: ABSTRACT

Mantle-derived sapphirine

AbstractA xenolith from the Delegate breccia pipe (New South Wales, Australia) contains sapphirine in equilibrium with aluminous clinopyroxene, garnet, and plagioclase (An48). This unusual assemblage probably developed from a clinopyroxene (±spinel ± plagioclase) cumulate during cooling from > 1400°C to c. 1000°C at pressures near 15 kbar. The sapphirine is close to the 7:9:3 composition, suggesting that bulk composition is more important than P-T conditions in determining the stoichiometry of natural sapphirines. A similar occurrence of sapphirine has also been recorded in mantlederived xenoliths from the Stockdale kimberlite in Kansas. Re-examination of sapphirine granulites from Finero suggests that their primary assemblages and origin may have been similar to those of the Delegate xenolith. Sapphirine is clearly stable under upper-mantle conditions in Ca-Al-Mg-rich bulk compositions.

Behaviour of rare earth elements during hydrothermal alteration at the Buena Esperanza copper-silver deposit, northern Chile

A study of the REE behaviour in alteration zones of the Buena Esperanza subvolcanic Cu Ag deposit, located in the Coastal Range of northern Chile, reveals that the elements are mobile in the rocks during alteration. The ore-forming process developed in three different stages of alteration-mineralization. The hydrothermal alteration of the early and middle stages affected basaltic fragments of a breccia pipe. La and Ce were leached from the rock fragments located at the bottom and at the middle part of the breccia pipe and partly redeposited at the top during alteration. Sm, Eu and Tb were released selectively and the heavy REE behaved as relatively immobile elements. Sericite is the most important alteration mineral for fixing the REE during this stage. The incoming fluids had low REE contents. The late stage of hydrothermal alteration happened simultaneously with the intrusion of a gabbroic volcanic neck, affecting mainly these subvolcanic rocks. REE patterns of samples from the top and bottom of the alteration zone exhibit roughly parallel trends, but are systematically enriched in REE in comparison to unaltered samples. This enrichment seems to be more intensive at the top of the altered area coinciding with the enrichment of Rb, K and Cu in these rocks. Epidote plays the most important role for fixing this hydrothermal input of REE during the latest stage of alteration.

Metallogeny and tectonic development of the Tasman Fold Belt System in Queensland

The northern part of the Tasman Fold Belt System in Queensland comprises three segments, the Thomson, Hodgkinson- Broken River, and New England Fold Belts. The evolution of each fold belt can be traced through pre-cratonic (orogenic), transitional, and cratonic stages. The different timing of these stages within each fold belt indicates differing tectonic histories, although connecting links can be recognised between them from Late Devonian time onward. In general, orogenesis became younger from west to east towards the present continental margin. The most recent folding, confined to the New England Fold Belt, was of Early to mid-Cretaceous age. It is considered that this eastward migration of orogenic activity may reflect progressive continental accretion, although the total amount of accretion since the inception of the Tasman Fold Belt System in Cambrian time is uncertain. The Thomson Fold Belt is largely concealed beneath late Palaeozoic and Mesozoic intracratonic basin sediments. In addition, the age of the more highly deformed and metamorphosed rocks exposed in the northeast is unknown, being either Precambrian or early Palaeozoic. Therefore, the tectonic evolution of this fold belt must remain very speculative. In its early stages (Precambrian or early Palaeozoic), the Thomson Fold Belt was probably a rifted continental margin adjacent to the Early to Middle Proterozoic craton to the west and north. The presence of calc-alkaline volcanics of Late Cambrian Early Ordovician and Early-Middle Devonian age suggests that the fold belt evolved to a convergent Pacific-type continental margin. The tectonic setting of the pre-cratonic (orogenic) stage of the Hodgkinson—Broken River Fold Belt is also uncertain. Most of this fold belt consists of strongly deformed, flysch-type sediments of Silurian-Devonian age. Forearc, back-arc and rifted margin settings have all been proposed for these deposits. The transitional stage of the Hodgkinson—Broken River Fold Belt was characterised by eruption of extensive silicic continental volcanics, mainly ignimbrites, and intrusion of comagmatic granitoids in Late Carboniferous Early Permian time. An Andean-type continental margin model, with calc-alkaline volcanics erupted above a west-dipping subduction zone, has been suggested for this period. The tectonic history of the New England Fold Belt is believed to be relatively well understood. It was the site of extensive and repeated eruption of calc-alkaline volcanics from Late Silurian to Early Cretaceous time. The oldest rocks may have formed in a volcanic island arc. From the Late Devonian, the fold belt was a convergent continental margin above a west-dipping subduction zone. For Late Devonian- Early Carboniferous time, parallel belts representing continental margin volcanic arc, forearc basin, and subduction complex can be recognised. A great variety of mineral deposits, ranging in age from Late Cambrian-Early Ordovician and possibly even Precambrian to Early Cretaceous, is present in the exposed rocks of the Tasman Fold Belt System in Queensland. Volcanogenic massive sulphides and slate belt-type gold-bearing quartz veins are the most important deposits formed in the pre-cratonic (orogenic) stage of all three fold belts. The voicanogenic massive sulphides include classic Kuroko-type orebodies associated with silicic volcanics, such as those at Thalanga (Late Cambrian-Early Ordovician. Thomson Fold Belt) and at Mount Chalmers (Early Permian New England Fold Belt), and Kieslager or Besshi-type deposits related to submarine mafic volcanics, such as Peak Downs (Precambrian or early Palaeozoic, Thomson Fold Belt) and Dianne. OK and Mount Molloy (Silurian—Devonian, Hodgkinson Broken River Fold Belt). The major gold—copper orebody at Mount Morgan (Middle Devonian, New England Fold Belt), is considered to be of volcanic or subvolcanic origin, but is not a typical volcanogenic massive sulphide. The most numerous ore deposits are associated with calc-alkaline volcanics and granitoid intrusives of the transitional tectonic stage of the three fold belts, particularly the Late Carboniferous Early Perman of the Hodgkinson—Broken River Fold Belt and the Late Permian—Middle Triassic of the southeast Queensland part of the New England Fold Belt. In general, these deposits are small but rich. They include tin, tungsten, molybdenum and bismuth in granites and adjacent metasediments, base metals in contact meta somatic skarns, gold in volcanic breccia pipes, gold-bearing quartz veins within granitoid intrusives and in volcanic contact rocks, and low-grade disseminated porphyry-type copper and molybdenum deposits. The porphyry-type deposits occur in distinct belts related to intrusives of different ages: Devonian (Thomson Fold Belt), Late Carboniferous—Early Permian (Hodgkinson—Broken River Fold Belt). Late Permian Middle Triassic (southeast Queensland part of the New England Fold Belt), and Early Cretaceous (northern New England Fold Belt). All are too low grade to be of economic importance at present. Tertiary deep weathering events were responsible for the formation of lateritic nickel deposits on ultramafics and surficial manganese concentrations from disseminated mineralisation in cherts and jaspers.

Geochemical exploration for mineralized breccia pipes in northern Arizona, U.S.A.

Thousands of solution-collapse breccia pipe crop out in the canyons and on the plateaus of northern Arizona. Over 80 of these are known to contain U or Cu mineralized rock. The high-grade U ore associated with potentially economic concentrations of Ag, Pb, Zn, Cu, Co and Ni in some of these pipes has continued to stimulate mining and exploration activity in northern Arizona, despite periods of depressed U prices. Large expanses of northern Arizona are comprised of undissected high plateaus; recognition of pipes in these areas is particularly important because mining access to the plateaus is far better than to the canyons. The small size of the pipes, generally less than 600 ft (200 m) in diameter, and limited rock outcrop on the plateaus, compounds the recognition problem. Although the breccia pipes, which bottom in the Mississippian Redwall Limestone, are occasionally exposed on the plateaus as circular features, so are unmineralized near-surface collapse features that bottom in the Permian Kaibab and Toroweap Formations. The distinction between these two classes of circular features is critical during exploration for this unique type of U deposit. Various geochemical and geophysical exploration methods have been tested over these classes of collapse features. Because of the small size of the deposits, and the low-level geochemical signatures in the overlying rock that are rarely dispersed for distances in excess of several hundred feet, most reconnaissance geochemical surveys, such as hydrogeochemistry or stream sediment, will not delineete mineralized pipes. Several types of detailed geochemical surveys made over collapse features, located through examination of aerial photographs and later field mapping, have been successful at delineating collapse features from the surrounding host rock: (1) Rock geochemistry commonly shows low level Ag, As, Ba, Co, Cu, Ni, Pb, Se and Zn anomalies over mineralized breccia pipes; (2) Soil surveys appear to have the greatest potential for distinguishing mineralized breccia pipes from the surrounding terrane. Although the soil anomalies are only twice the background concentrations for most anomalous elements, traverses made over collapse features show consistent enrichment inside of the feature as compared to outside; (3) B. Cereus surveys over a known mineralized pipe show significantly more anomalous samples collected from within the ring fracture than from outside of the breccia pipe; (4) Helium soil-gas surveys were made over 7 collapse features with discouraging results from 5 of the 7 features. Geophysical surveys indicate that scaler audio-magnetotelluric (AMT) and E-field telluric profile data show diagnostic conductivity differences over mineralized pipes as compared to the surrounding terrane. These surveys, coupled with the geochemical surveys conducted as detailed studies over features mapped by field and aerial photograph examination, can be a significant asset in the selection of potential breccia pipes for drilling.

Breccias related to explosive volcanism

Explosive breccia pipes were formed in the phreatomagmatic eruption of Taal volcano 60 km south of Manila, Philippines in September 1976. They were photographed in eruption which consisted of a series of small explosions occurring during the peak of activity at 10 sec intervals. These four breccia pipes formed on the collapse fault marking the margin of the much more intense eruption of 28 September 1965 when a diatreme 800 m in diameter formed. Renewed eruption in October 1977 on one of the breccia pipes blasted out the sealed plug and breccia fragments in all stages of alteration were included in the tephra. They ranged from barely agglomerated to strongly cemented through argillized and pyritized to totally replaced by silica and pyrite, fragments indistinguishable from those found in many mining districts. The violence of the explosion initiating a phreatomagmatic eruptive sequence results in strong crackling of the walls of the pipe and the crakle zone may become mineralized (cf. Braden, Chile). Usually the crater collapses on ring faults after eruption, forming a marginal tectonic breccia which can be mineralized (cf. Balatoc, Philippines.) The small craters with steep inner walls and tapering outer slopes which form during phreatomagmatic eruptions, sometimes containing lakes, are called maars and they are the surficial expression of an explosive breccia pipe or diatreme. Superficially similar craters are formed over kimberlite pipes. Hydrothermal explosion craters are somewhat similar. Collapse breccia pipes can form in the same sequence with explosive pipes. When one rising plug encounters ground water and explodes, fluostatic pressure on any other cupolas rising from the same magma drops rapidly, resulting in withdrawal of magma and collapse of walls and roof (Perry 1961). “Fluidization” is not thought to be of any significance in formation of breccia pipes (Wolfe 1980) contrary to Reynolds (1954) and many authors who have cited that work. An explosive breccia pipe is like an open window resulting in rapid depression of the isotherms of a pluton. A concentrated brine front can build up in minutes after an explosion. The very rapid cooling may explain why many explosive breccia pipes are altered by silica and pyrite only, the system being cooled before metallic solutions have time to replace breccia matrix or fragments.