摘要:

The competent opaque minerals in the Archaean Golden Grove deposit, pyrite and magnetite, retain pre-regional metamorphic textures despite the lower greenschist-facies grade of metamorphism. The pre-regional metamorphic textures and structures recognized include the development of pyrite and magnetite overgrowths, the replacement of pyrrhotite by pyrite, the conversion of a primary hematite–goethite mineralogy to magnetite and, as a result of thermal metamorphism, further local replacement of pyrrhotite (and sphalerite) by magnetite. Comparisons between pyrite from the Cu-rich mineralization at the base of the deposit and that from the Zn-rich mineralization in the hanging wall indicate that postdepositional modification and recrystallization were more extreme at the base of the deposit. The pre-regional metamorphic textures and structures indicate that pyrite and magnetite overgrowths developed almost immediately after primary precipitation ceased and that overgrowths continued to develop into the late hydrothermal–diagenetic stage of mineralization. A large proportion of the sulphide–Fe-oxide mineralization was formed at shallow depth within the volaniclastic host rocks, but at two horizons (the base and hanging wall) the mineralization formed at or very near the sea floor. These two periods of near sea-floor sulphide precipitation are separated by an oxide-dominated opaque-mineral assemblage, originally hematite–goethite and secondary marcasite but now converted to magnetite and secondary pyrite. The microtextural evidence supports a three-stage evolution of the ore deposit, two sulphide exhalative phases of mineralization separated by a stage of more oxidized hydrothermal activity, or, alternatively, sea-floor weathering during which hematite–goethite formed and marcasite partly replaced earlier formed sulphides.

ISSN:0008-4077    

DOI:10.1139/e85-001

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Studies of the sulphide–magnetite fabric at Golden Grove. Western Australia, metamorphosed under conditions of lower to middle greenschist facies, indicate that pyrite and magnetite deformed in a brittle manner, whereas pyrrhotite, sphalerite, chalcopyrite, and galena deformed as ductile sulphides. In the accompanying silicate assemblage, pressure-solution deformation has been a significant deformation mechanism. An optical-microscope study of etched sphalerite reveals a wide range of microstructures indicative of ductile deformation, including lattice dislocations, subgrains and subboundaries, annealing twins, variable grain-boundary geometry, and recrystallization. The microstructures are distributed randomly through the sphalerite fabric and are similar to that formed during steady-state creep of deformed metals and that reported in dynamic recovery and recrystallization of deformed quartz. It is concluded that the ductile sulphides are modified by synkinematic recovery and recrystallization and that postkinematic recovery, annealing, and recrystallization, which are commonly reported for sulphide deposits, are not present.

ISSN:0008-4077    

DOI:10.1139/e85-002

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The Poohbah Lake alkaline complex is a late synkinematic Kenoran pluton in the Quetico gneiss belt of the western Superior Province. Three units of the complex, porphyritic syenite (PS), malignite (M: a nepheline–clinopyroxene–K-feldspar rock), and hornblende syenite (HS), as well as baked Archean schists near the intrusive contact, have a predominantly reverse-polarity R magnetization with mean directionD = 198°I = −22.5° (k = 62, α95 = 5°,N = 13 sites) and a paleopole at 60°E, 50.5°N. Pyrrhotite and coarse primary magnetite are carriers of the R remanence. PS, M, and biotite pyroxenite (BP) exhibit also a predominantly normal N magnetization carried by fine-grained, probably secondary magnetite. N is systematically steeper than R: its mean direction isD = 359.5°I = +55.5° (k = 28, α95 = 8°,N = 13 sites) with a paleopole at 90°E, 77.5°N. R and N do not record an asymmetric field reversal, since reverse-polarity N vectors and normal-polarity R vectors are occasionally found. R resembles in polarity and direction the natural remanent magnetization (NRM) of the 2630 Ma Matachewan diabase. It is probably the primary NRM of the Poohbah Lake pluton, with an age of about 2650 Ma in approximate agreement with the K/Ar isochron age of 2700 ± 25 Ma. N resembles in polarity and direction NRM's from the 2580 Ma Shelley Lake granite and the late Archean Burchell Lake granite. It is probably a secondary NRM about 2550 Ma in age, as suggested by updated K/Ar mica ages. The characteristic NRM of HS samples and secondary magnetizations in other rock types have poles on the Grenville Track of the polar wander path but there is no evidence for Grenvillian-age events in the area.

ISSN:0008-4077    

DOI:10.1139/e85-003

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The lake-bottom sediments of the shelf area of Batchawana Bay are composed of sands, and the basinal sediments grade from silts to coarse-grained clay basin ward. There is a significant relationship of decreasing mean grain size to increasing water depth for the sediments of the basinal areas. Although the sediments of Batchawana Bay would appear to be formed by the variable mixing of sand and clay, hydraulic action probably removed the clay end member and ultimately deposited it in the lake proper.Grains of magnetic minerals deposited on a lake bottom tend to align in the direction of the Earth's magnetic field at the time of deposition, imparting a remanent magnetic direction to the sediments. The late Holocene sedimentary sequence of Batchawana Bay provides a record of the secular changes of the Earth's magnetic field. "Type" paleodeclination and paleoinclination logs plotted against time can be utilized to date the sedimentary sequence of Batchawana Bay. This paleomagnetic time scale indicates that glacial outwash and varve deposition continued for approximately 600 years after glacial retreat from the lake basin to about 8700 years BP.

ISSN:0008-4077    

DOI:10.1139/e85-004

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The Grenville tholeiitic dikes of Late Proterozoic or early Paleozoic age cut marbles and gneisses of the Grenville Province within and adjacent to the Ottawa rift structure. Where traversed the swarm consists of about 40 large dikes (up to 100 m wide) representing a crustal extension of 1 km. The principal minerals are olivine, augite, pigeonite, plagioclase, magnetite, ilmenite, K-feldspar, and quartz. Crystals of pigeonite around olivine and complex augite–pigeonite composite grains suggest that the reactions olivine + melt → pigeonite and pigeonite + melt → augite have taken place. Conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization, but crystal – melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180–1060 °C. Rock textures (subophitic, combined ophitic–subophitic, and equigranular) in dike centres are related to dike width and are determined principally by the influence of cooling rate on the nucleation of augite. The K2O content of the gabbro (centres of 15 dikes, 3–100 mm wide) ranges from 0.2 to 1.2% and is closely correlated with other elements, positively with Ti, Na, P, Rb, Sr, Y, Zr, and Ba, and negatively with Mg, Ca, and Cr. Relatively minor within-dike variation exists in the form of local (centimetre-scale) inhomogeneity, slight enrichment in K and Na, and depletion in Ca and Mg in the centre of one large dike and complex rhythmic variation in K, Na, Ca, and Fe across one small dike. Centre–margin comparisons in several large dikes indicate minor residence con tamination (Ca, Mg) in the margins of some dikes cutting marble. Of the various possible causes for variation in composition, those favoured at present are Fractional crystallization at depth (separation of augite and plagioclase), the partial preservation of compositional heterogeneity during intrusion, and the gravitational sinking of early formed crystals (with the upward displacement of K-enriched melt) in the central regions of large dikes.

ISSN:0008-4077    

DOI:10.1139/e85-005

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The 2.7 Ga old Helen Iron Formation (HIF) with its uniquely large siderite–Pyrite orebody conformably overlies chemically altered rhyolite–dacite pyroclastic rocks, including an ottrelite-bearing alteration pipe, the product of exhalative venting in freshly accumulated pyroclastics. HIF, in turn, is conformably overlain by pillowed mafic lava flows.HIF internal stratigraphy is conformable from lower massive siderite–sulfide through middle sulfidic, carbonaceous chert, to upper magnetitic chert. Major- and trace-element and carbon- and sulfur-isotope data demonstrate cyclic fluctuations in chemical precipitation from early chemically complex to later chemically "simple" components.HIF development is attributed to seawater-charged volcanic exhalations in a transitory subaqueous cauldron subsidence basin. Volcanic exhalations contained (1) the main chemical components of HIF; (2) nutrients that triggered intense bacterial activity, the principal low-Eh-inducing agent; and (3) acidic components (e.g., HCI, HF), the principal low-pH-inducing agent. HIF carbonates (predominantly siderite) and sulfides (mainly pyrite) are of marine carbonate and marine sulfate origin, respectively. Organic activity was a vital catalyst in carbonate–sulfide precipitation. HIF is notably deficient in base and noble metals, thereby indicating low circulation–exhalation temperatures, in sharp contrast to contemporary hot, metal-rich geothermal brines at modern spreading ridge axes. This transient subaqueous late Archean volcanic environment, then, was unique in scale but not in kind.

ISSN:0008-4077    

DOI:10.1139/e85-006

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

An attempt has been made to identify the processes that give rise to a number of depth trends in the magnetization of a 3.1 km vertical section of Icelandic-type oceanic crust and to assess the possibility that similar processes act, and depth trends occur, in typical oceanic crust. The depth trends in the Icelandic section consist of a general increase in saturation and induced magnetization to 2 km crustal depth, below which flow magnetization decreases while dike magnetization remains constant, and of large changes in flow magnetization that occur on a scale of a few hundred metres below 3 km crustal depth.Increase in saturation and induced magnetization with depth in the upper 2 km is thought to be the result of two processes: a decrease in low-temperature oxidation from the original lava surface to 700–800 m crustal depth, thence an increase in hydrothermal alteration with depth. This interpretation is based on oxide petrography and Curie temperatures, which show a weakly defined minimum in the 700–800 m interval, then an increase to ubiquitous "magnetite" values at just below 2 km crustal depth. Although the relationship between magnetic properties and oxide alteration is reasonably well known for the low-temperature oxidation process from laboratory studies and ophiolite and typical ocean-crust analogs, the change in magnetic properties during hydrothermal alteration is not generally known, nor are ophiolite or typical ocean-crust analogs presently available.Decrease in flow saturation and induced magnetization below 2 km is likely to be the result of alteration of magnetite (sensu lato) to nonmagnetic phases, either on a fine scale to hematite (s.l.) between 2 km and 3 km, or by leaching of iron, leaving anatase pseudomorphs after magnetite (s.l.) below 3 km. The relatively low porosity of the dikes is likely to have protected dike magnetite below 2 km from such oxidation and leaching processes.The study confirms that secondary magnetite in several forms is an important magnetic constituent of the flows in the lower part of the section, particularly where decomposition of primary magnetite is widespread. Secondary magnetite occurs as vermiform or bladelike masses, as rims associated with former silicates, or as fresh continuous magnetite occurring either as subhedral grains or as "reconstructed" primary grains in which relics of sphene-replaced ilmenite lamellae grids are seen.In conclusion, the possibility that the near-surface magnetization of typical ocean crust is commonly the minimum value for a layer extending downwards to the onset of an epidote-bearing facies deserves serious consideration, as does the possibility that strong, stable magnetization of secondary origin occurs in flows where dike density becomes significant.

ISSN:0008-4077    

DOI:10.1139/e85-007

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The Meguma Zone of southern Nova Scotia is a suspect terrane in the eastern Appalachians. Sm/Nd isotopic data on six samples from the thick sequence of flyschoid metasediments of the Meguma Group show that these rocks have a mean crustal residence age ofTDM = 1773 ± 95 Ma, considerably older than their Cambro-Ordovician depositional age. This information should be useful in locating the matching terrane for the Meguma and ultimately in determining whether it was derived from one or more Precambrian sources with different crustal residence times.

ISSN:0008-4077    

DOI:10.1139/e85-008

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Geophysical studies suggest that the thin crust characteristic of the Basin and Range Province extends eastward beneath the west margin of the Colorado Plateau and Rocky Mountain regions. In Arizona and Utah, zones perhaps over 100 km wide may be defined, bounded on the west by the east limit of upper crustal normal faults that account for more that 10% extension and on the east by the east limit of thinning beneath the Colorado Plateau. A discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning, assuming the "discrepant zone" crust was as thick as or thicker than the Colorado Plateau – Rocky Mountain crust prior to extensional tectonism.If various theories appealing to crustal erosion are dismissed, mass balance problems evident in the discrepant zones are most easily resolved by down-to-the-east normal simple shear of the crust, moving lower and middle crustal rocks that initially were within the zones up-and-to-the-west to where they now are locally exposed in the Basin and Range Province. West of the discrepant zones in both Arizona and Utah, east-directed extensional allochthons with large displacement are exposed. These geophysical and geological observations complement one another if it is accepted that the entire crust in both Arizona and Utah failed during extension on gently east-dipping, east-directed, low-angle normal faults and shear zones over a region several hundred kilometres wide.Large-scale, uniform-sense normal simple shear of the crust suggests the entire lithosphere may do the same. Such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear. In the case of the Arizona, Utah, and Red Sea extensional systems, and possibly the Death Valley extensional terrain, a broad topographic arch, typically 1500–2000 m higher than the extended terrain, is present, suggesting lithospheric thinning in areas predicted by the hypothesis.

ISSN:0008-4077    

DOI:10.1139/e85-009

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Seismic data collected from the Ensenada Bay earthquake swarm of late 1981 were used to calculate the spectra of ground displacement. Data from the stations of Ensenada (ENX) and Cerro Bola (CBX), at epicentral distances of 14 and 57 km, respectively, were used to evaluate source parameters. The focal depths determined for these events were less than 10 km. The focal mechanism was a strike-slip fault type, with the plane of motion striking N52°W, parallel to the Agua Blanca Fault. Seismic moments ranging from 3.44 × 1019to 5.99 × 1020 dyn∙cm (3.44 × 1014to 5.99 × 1015 N∙cm) were estimated for events with local magnitudes in the range 1.7–2.3. The source dimensions were found to be 186 ± 36 m and the stress drops between 3 and 66 bar (0.3 and 6.6 MPa), comparable to results obtained in previous studies of shallow events (depths <10 km). The Ensenada swarm could be attributed to a localized zone of high-strain energy at the intersection of two faults. Ratios ofPtoScorner frequencies were evident for only five events; they were 1.39 ± 0.38. Magnitude and seismic moment from other studies were compared with the Ensenada data in the range of magnitudes 0–3. All the data can be accommodated by logM0 = 1.5 ML + (16.9 ± 1.1). The Ensenada earthquake swarm and the Victoria earthquake swarm, which occurred in the Mexicali valley in 1978, have similar source radii and corner frequencies for the same range of seismic mome

ISSN:0008-4077    

DOI:10.1139/e85-010

出版商:NRC Research Press    

年代:1985

数据来源: NRC