Large Dikes Research Articles

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.

Petrology of the Grenville swarm of gabbro dikes, Canadian Precambrian Shield

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.

Neptunian Dikes and Sills in Devonian Reef Complexes of Canning Basin, Western Australia: ABSTRACT

Neptunian dikes and sills are conspicuous features of early-cemented limestones in Devonian reef complexes of the Canning basin. They are sedimentary fillings of fissures formed by fracturing of the limestones soon after deposition, and are most abundant in the reef subfacies, but also occur in some back-reef and marginal-slope deposits. The dikes and sills were filled with contemporary sediment (including oolite, peloid grainstone, biomicrite, and calcareous sandstone), encrusting organisms (Renalcis and stromatolites), and early radiaxial calcite cement. Some of the sediment is dolomitized or mineralized. Some dikes contain concentrically coated balls of radiaxial calcite up to 0.3 m (1 ft) across. Sediment in the dikes and sills was cemented rapidly, and repeated refracturing and infilling occurred during brief intervals. The largest dikes are up to 6 km (3.5 mi) long, 20 m (65 ft) wide, and originally extended more than 80 m (260 ft) below the surface. However, most are less than 100 m (330 ft) long and 5 cm (2 in.) wide. Neptunian sills are up to 0.8 m (2.5 ft) thick. The principal strike directions of major dike approximately parallel the original reef fronts of underlying basement ridges. Lateral extension through the development of fissures amounted to as much as 6.5%, and averaged about 3% in measured reef sections. The fissures allowed circulation of large volumes of seawater to considerable depth within the reef complexes, and must have played a significant role in early diagenesis. Fissuring probably resulted from earthquake shaking, differential compaction of underlying sediments, and slippage along bedding planes, especially in the steeply dipping marginal-slope deposits. End_of_Article - Last_Page 517------------

Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario

Proterozoic diabase and gabbro dikes (~2120 Ma old) form a major northwest-oriented swarm extending about 300 km from the Mesabi iron range in Minnesota to the vicinity of Kenora, Ontario. The dikes were emplaced into Archean crust at about the time that the Early Proterozoic basin of the Lake Superior region was opening by rifting, and the swarm and the basin may therefore be tectonically related. The dikes are overlain unconformably by the Animikie Group, the upper sedimentary sequence in the Proterozoic basin in Minnesota, but may be approximately coeval with mafic volcanic rocks in the pre-Animikie Mille Lacs Group. A two-stage tectonic model involving (1) regional right-lateral crustal shear in the late Archean and (2) hot-spot rifting in the Early Proterozoic is proposed to account for the swarm.The dikes are iron-rich quartz tholeiites that are differentiated toward dioritic compositions. Late alteration to hydrous phases, including blue–green amphibole, chlorite, and sericite, together with lesser amounts of prehnite and epidote, is ubiquitous but variable in intensity, and is regarded as a deuteric phenomenon. The interior portions of some large dikes are compositionally layered parallel to contacts; the layers differ from each other in the proportions of primary hornblende, clinopyroxene, and plagioclase and thus range in composition across the gabbro–diorite boundary.Chilled margins of the dikes contain flow-aligned phenocrysts of plagioclase, clinopyroxene, titanomagnetite, and ilmenite. The clinopyroxenes occur as three morphotypes that have distinct compositions, indicating a complex intratelluric history. The dike magma was emplaced into cool Archean crust at an inferred temperature of about 1085 °C and was quenched in a matter of minutes at the dike walls. Complete solidification at the centers of dikes wider than 100 m appears to have taken more than 40 years.

Role of Neptunian Dikes in Structural Evolution of Reefs: ABSTRACT

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The radiometric age of a Proterozoic hyperite diabase in Värmland, western Sweden

Abstract Across Varmland County there stretches in N-S direction a broad belt of black diabases known as hyperites. To the east it borders on a diffuse tectonic zone separating the gneiss complex of southwestern Sweden from the Svecokarelian orogenic belt. To the west, the diabases are partly involved in a late, middle Proterozoic thrust zone, the Mylonite Zone. The hyperite diabases appear as variably boudinated sills and dikes which generally follow the northwest strike of the older gneissic rocks. A pegmatoid variety from the interior of a large dike was collected for radiometric dating. Separated zircons plot on a regression line with an upper intercept with the Concordia at 1550 Ma, which is interpreted to correspond to the crystallization of the hyperite diabase. The lower intercept is at 880 Ma, which coincides with a Rb-Sr total rock-mineral isochron age of 975 Ma within the limits of error. It is concluded that a metamorphic event at 900–1000 Ma has caused Sr isotopic homogenization within the an...

The Form and Growth of Fingered Sheet Intrusions

The form of several sheet intrusions in a sandstone-shale host rock is regular, but much more complex than the term “sheet intrusion” implies. The periphery of a sill emanating from the Shonkin Sag laccolith in Montana is composed of fingers of igneous rock with lengths of about 100 m, widths of 3 to 5 m, and thicknesses which vary from about 1.2 m near their points of coalescence to about 0.6 m near their terminations. At finger terminations, the host rock is wedged aside and compacted, and along the sides of fingers, buckling and shearing of strata are common. Energy expended by magma to dilate and flow through a sheet is small compared to that for fingers. Initiation of fingers is attributed to instability of the advancing interface between a viscous magma and a more viscous host rock. Fingers coalesce into a sheet such that cusp-shaped grooves filled with deformed host rock remain in the contact. Systematic offsets of sheet-intrusion contacts form where offset fingers coalesce. Several large dikes (5 to 20 m thick) near La Veta, Colorado, exhibit contact offsets of as much as 3 m in magnitude, which give the dike the appearance of a clapboard wall. Small dikelets, horns , often project from the outer edge of offsets. If fingers grow in a surface perpendicular to the least principal compressive stress, offsets may develop as the intrusion propagates through a region where this stress changes orientation. Grooves and offsets form parallel to finger lengths and thus indicate the propagation direction of the intrusion. Flow of magma subsequent to finger coalescence may not be parallel to the propagation direction.

The Geochemistry of the Granitic Rocks of Halifax County, Nova Scotia

Studies of part of a large batholith in southwest Nova Scotia show that granitic rocks of two different ages penetrate the Lower Palaeozoic metasediments of the Meguma Group. They are distinguished by their field relationships, mineralogy, and petrology from an older series, varying from biotite granodiorite through quartz monzonite to muscovite biotite granite, which is penetrated by younger large dikes, up to 1.5 km wide, of alaskite. Rb–Sr whole rock isochron ages show that the older series was emplaced approximately 415 m.y. ago and that the younger alaskite dikes were emplaced about 350 m.y. ago. Major and trace element chemistry shows that the older series of granitic rocks were formed by differentiation of a calc-alkali magma in situ. The younger dikes are all highly silicic, show limited major element variation, and have been affected by hydrothermal metasomatism.

New Virtual and Paleomagnetic Pole Positions from Isotopically Dated Precambrian Rocks in Wyoming, Montana, and Arizona: Their Significance in Establishing a North American Apparent Polar Wandering Path

Paleomagnetic directions have been determined from well-dated Precambrian rocks, in large part mafic dikes, in Wyoming, Montana, and Arizona. Ages, determined radiometrically, range from 700 to 2,550 m.y. When these data, together with the compilation of acceptable well-dated paleomagnetic poles from North America, are treated by a running-mean method, a generalized belt of apparent polar wandering is suggested. This belt is simple in form, and the rate of polar wandering is nearly uniform. The existence of large-scale loops of relatively rapid wander velocity is largely unsupported.

Crystal growth and nucleation in two large diabase dikes

The total number of nuclei, Z, of a single phase formed in a unit volume of a crystallizing magma at the distance y from the contact of a semi-infinite intrusion is[Formula: see text]where n is a numerical constant determined by growth controlling process during the nucleation period and the value of m in the expression assumed for the nucleation rate, J[Formula: see text]The observed variation of Z for clinopyroxene and plagioclase from the contact towards the center of two large dikes appears to follow this relation. In the 106 m wide Kigaviarluk olivine tholeiite dike the value of n is significantly greater for the clinopyroxene (−2.2) than the plagioclase (−0.84). In the 60 m Grenville dike, a quartz tholeiite, the n value (−1.5) is the same for both minerals. The growth-controlling mechanisms of plagioclase and clinopyroxene appears to be sensitive to the bulk composition of the magma from which it is crystallizing.

Cohesion and Flow Phenomena in Clastic Intrusions: ABSTRACT

Physical differences between water-saturated cohesionless sand and cohesive fine mud account for many sedimentary structures. Relative bulk cohesion is controlled by relations of intergranular adhesion caused by electromagnetic or Van der Waal's forces versus the disturbing effect of forcible migration of pore fluid or gas. Where expulsion of fluid is slow, mud retains greater cohesion than sand. If sand is loosely packed, movement of fluid, especially if sudden, may cause spontaneous liquefaction and conversion to a viscous slurry with negligible strength. Empirical evidence of major differences of cohesion comes from abundance of shale pebbles, load structures, current sole marks, contorted stratification, and clastic intrusions. These are most characteristic of alternating sand-mud sequences which possessed many inherent-strength discontinuities. All degrees of liquefaction, flow, intrusion, and stopping are frozen in rocks--from local in-place liquefaction of sand strata to immense dike complexes and sand volcanoes. Clastic dikes are of special interest. Typically they contain fine sand, but in a few places may have coarse gravel. Wall rock is generally mud, and stoped cohesive mud xenoliths are common; some dikes split and even rejoin along strike. Internal lamination and some grain alignment may occur, and these suggest chiefly l minar flow of viscous suspensions. A few dikes display excellent groove and flute marks on their sides, evidencing scouring of cohesive wall rock during intrusion and indicating direction of intrusion. Some large dikes completely evacuated their source stratum with concomitant subsidence of all overburden. Important relative age criteria are provided. Most dikes are straight, suggesting that compaction was completed largely before intrusion. In fact, compaction End_Page 610------------------------------ of mud probably provided most of the pore water which liquefied intercalated sand. The exceptional wavy dikes apparently were intruded earlier and compacted later with their wall rock. Sill-like masses are more problematical; some evidence clearly indicates liquefaction after burial, but others of these masses could have formed at the depositional interface. Certain strata were liquefied in place after burial and are not true intrusions, although they strongly resemble sills. Some synsedimentary folds can be distinguished from tectonic ones where sandstone dikes cut through them and prove an early, soft-sediment origin. Dikes along slaty cleavage have been cited as evidence of early formation of such cleavage. Some intrusions are useful in determining dates of migration and deposition of ores or fluids. Dikes may show close relations with regional structures, but many do not. Though more common in tectonically mobile regions, they also occur in stable ones. They most probably originated by shocks from earthquakes or from sudden loading. They are more common than is generally realized and their usefulness to the geologist has not been appreciated fully. End_of_Article - Last_Page 611------------

Relationship of ore to prophyry in the Basin and Range Province, U.S.A

Data obtained during recent detailed mapping of intrusive areas in the eastern Great Basin combined with an examination and review of 107 mining districts in the Basin and Range Province of the United States containing hydrothermal deposits of Au, Ag, Pb, Zn, Cu, Mo, W, Kg, and U has led to the recognition of significant differences between intrusives, (those associated with hydrothermal mineralization) and intrusives, (without significant mineralization). For 86 percent of the deposits of Cretaceous and younger age there is a significant amount of associated intrusive porphyry or aphanitic rock present as large dikes, sills, and breccias with aphanitic matrix or more commonly as stocks or pipes occurring with or without granitoid textured rock. These rock types with few exceptions, are exposed within 2 miles of the ore deposit. The great majority of barren intrusives are of granitoid rocks only, except for the presence of smaller porphyry dikes and sills. Deposits of tungsten show an association preference with granitoid intrusives. These statistics indicate that in finding or developing a new ore deposit of the above metals other than tungsten, it is necessary to have a large intrusive aphanite porphyry existing within 2 miles of the exploration area in order reasonably to expect (for 86% of the cases) the existence of a significant ore body. Because barren intrusive porphyries do exist in the Basin and Range Province, further investigation of the differences and similarities between productive and barren porphyry plutons is contemplated.

Evaluation of Criteria for the Forcible Intrusion of Magma

Small Eocene rhyolite dikes in the Black Hills were forcibly intruded into steep-dipping pre-Cambrian schists and nearly flat Paleozoic rocks. Structural criteria for forcible intrusion are compelling in both rocks, but the full amount of deformation is measured only in the nearly flat strata. Larger dikes and stocks deformed the nearly flat Paleozoic strata, but deformational effects are difficult to detect in the steep-dipping schists, which suggests that criteria for forcible intrusion of magma into schistose rocks are poor, and negative evidence must be used with caution. Structural criteria for the forcible intrusion of a body as large as the Sierra Nevada batholith are still more unsatisfactory. However, there are indications that this batholith was built up by multiple intrusions of a number of small intrusive units, each showing about as much deformation of the schist wall rocks as do the smaller intrusive bodies of the Black Hills. An appraisal of evidence pertaining to forcible intrusion of magm...

Granitization near Killarney, Ontario

Near Killarney, Ontario, the Killarnean granitic intrusions are associated with recognizable Huronian formations and with a “stony porphyry”1 of questionable origin. The writer previously has described this rock as an ultra-metamorphosed quartzite, but later writers have declared that it must be of intrusive magmatic origin. Unmistakable transitions along the strike from this porphyry into parts of the Gowganda formation have been discovered on George Lake, about 6 miles from Killarney along the general strike of rocks of disputed origin. The contact between Huronian rocks of familiar type and those in the Huronian-Killarnean complex strikes nearly straight from Killarney toward the southeastern comer of Wanapitei Lake. The contact zone is complicated by intense shearing of intrusions within the Huronian sedimentary formations older than the main displacement, followed by widespread granitization of Huronian rocks, by igneous intrusion of very coarse porphyritic granite in large dikes within the displacement zone, and by the injection of dikes into the Huronian rocks west of the line of major disturbance. All these intrusive and transitional rocks must be distinguished from one another in relation to the various recognizable Huronian formations. The Killarney locality is important because it shows extensive granitization and very significant rock transitions within a relatively small area.

Borabora lavas

Borabora, one of the Society Islands, represents three eroded remnants of a once more extensive volcano. Quaquaversal dips of the lava‐flows and hundreds of radially striking, basaltic dikes locate the crater in the submerged lagoon between the large eastern island and the down‐faulted remnant of the western crater‐rim. Ellipsoidal structures of subaerial origin in the lavas extend from sea‐level to nearly 2500 feet and are believed due to rapid vesiculation and consequent billowing of the lavas. The lavas and dikes show a limited range of composition ranging from oceanites to olivine‐poor basalts. The only significant differentiation, other than concentration of olivine, is in a large dike in which the center shows a concentration of alkalies. The limited variation in composition is in contrast to the occurrence of trachytic and syenitic types on Tahiti and Moorea.

An Analysis of Hunting Records for the Prairie Farm Project, Saginaw County, Michigan, 1937

central part of Saginaw County, Michigan (Towns 10-11 North, and Ranges 3-4 East, Fig. 1). It consists of 8,956 acres of flat land isolated by the Flint and Shiawassee Rivers and by a system of large canals and dikes.' The canals, which enclose all except 555 acres of the project, are several feet deep and 20 to 30 feet wide. The large dikes, approximately 12 feet high and 60 feet wide, prevent the inundation of the area when the two rivers are at flood stage. Extensions of the canals and dikes con-

Magmatic Differentiation in Mount Girnar

The Girnar laccolith was intruded into a thick covering of basalt of late Cretaceous or early Eocene age in Kathiawar, India, which was lifted up in the form of a dome. Slow cooling of the magma gave rise to olivine-gabbro, diorite, monzonite, syenite, nepheline-syenite, and granophyre. This paper deals principally with the mechanism of intrusion and differentiation. Three phases of movement are represented by olivine-gabbro, diorite-monzonite, and granophyre, respectively. Diorite and monzonite occur in the center and are surrounded by olivine-gabbro. Granophyre is intruded in the marginal hills of basalt in the form of large dikes. This structural feature is explained by a circular system of fractures in basalt due to arching. The differentiation is shown to be the result of fractional crystallization; sediment-assimilation or immiscibility in the liquid state is regarded as improbable. The physical conditions which prevailed indicate quiet crystallization and absence of convection currents and crystal settling. Field evidence has been noticed supporting the origin of nephelite-syenite by the dissociation of the polysilicate feldspar.

On the Mechanics of Dike Intrusion

This paper considers the mechanics of intrusion of large dikes as exemplified by the Medford Dike in eastern Massachusetts. Associated with the large dike are many older cross-dikes belonging to at least seven systems. A careful mapping of these dikes shows clearly that they have not been offset by the intrusion of the large dike. This suggests that the Medford Dike did not intrude the country rock by pushing aside the walls of a fracture, but rather that the country rock formerly occupying the dike chamber was removed by some process of stoping. Evidence is set forth to show that a theory of piecemeal stoping is not as satisfactory an explanation as a hypothesis of large-block stoping-i.e., that the whole block formerly occupying the dike chamber sank downward in the upwelling basaltic magma.