Late Dikes Research Articles

Paleomagnetism of the Contact Aureole and Late Dikes of the Otto Stock, Ontario, and its Application to Early Proterozoic Apparent Polar Wandering

The Otto Stock consists mainly of syenite. Except for some mafic phases the syenites are magnetically unstable, but late-stage lamprophyre dikes and basic rocks in the contact aureole contain abundant stable material. Their mean direction of magnetization is 330°, +71 °(18 sites, 84 samples, α95 = 3°) with a corresponding pole at 69°N, 133°W (A95 = 5°). A subordinate later magnetization also occurs in three lamprophyre dikes. Using these and previous results a path of apparent polar wander for Laurentia for the interval 2200 to 1800 m.y. of simple form is constructed. The fact that a single simple polar path can be constructed, which is consistent with data from localities spread widely over the Laurentian Shield, indicates that the Hudsoni orogeny may not have been accompanied by very large relative motions of the bordering Archean cratons.

Geothermometry from the Thermoluminescence of Contact-Metamorphosed Limestone

The amount and distribution of residual thermoluminescence in limestone contact zones is theoretically calculated by numerically integrating the appropriate thermal-decay equations over the time and temperature conditions predicted by heat-conduction theory. The resultant thermoluminescence pattern is best characterized by the relative width of the zone of complete thermoluminescence annealing. Based on this feature, a method of determining magma temperature is developed and analyzed. Effective magma temperatures of 950° C. are inferred from the natural thermoluminescence in limestone wall rock in association with late Pleistocene basaltic flows and dikes in northern Arizona.

Granitic Formations in the East-Central Sierra Nevada Near Bishop, California

This report establishes lithologic units among the granitic rocks of the east-central Sierra Nevada near Bishop, California. In this area the Sierra Nevada batholith is composed chiefly of quartz-bearing plutonic rocks ranging in composition from quartz diorite to alaskite but includes scattered small masses of darker and older plutonic rocks and remnants of metamorphosed sedimentary and volcanic rocks. The granitic rocks are in discrete plutons, either in sharp contact with one another or separated by thin septa of metamorphic or mafic igneous rock or by late aplitic dikes. The granitic rocks are grouped into lithologic units on the basis of composition, texture, and intrusive relations. The units include six new formations, three informal units made up of the rocks in several plutons, and four informal units that include the rocks in single plutons. The new formations are the Inconsolable Granodiorite, Tinemaha Granodiorite, Wheeler Crest Quartz Monzonite, Round Valley Peak Granodiorite, Lamarck Granodiorite, and Tungsten Hills Quartz Monzonite.

The discovery of a copper-zinc deposit at Garon Lake, Quebec

An exploration venture that led to the discovery of a small ore-grade Cu deposit in the very active Mattagami Lake exploration area of northwestern Quebec is described. The Garon Lake property was staked to cover a small, isolated E.M. anomaly that was recorded during an airborne survey of a folded post-Keewatin basic intrusion. A ground geophysical survey confirmed the presence of a narrow but good conductor and of a coincident, positive magnetic anomaly. Diamond drilling has indicated the presence of a body of massive sulfides containing a little over 2% Cu. The sulfide deposit is in a group of metamorphosed Keewatin lavas, tuffs, and sediments intruded by older gabbro and by systems of late unmetamorphosed dikes, of which lamprophyre, olivine diabase, and albite granite, are examples. The strike and dip of the mineralization are essentially parallel to the strike and dip of the country rock. The shoot of massive sulfides plunges at about 60 degrees . Pyrrhotite, pyrite, chalcopyrite, and sphalerite occur in varying proportions and show textural evidence of having replaced the pre-existing rock. The mineralization is near the contact between amphibolite (derived from lavas) and quartz-rich schist (derived from sediments). More particularly, the sulfide body is closely associated with an alteration aureole of cordierite-anthophyllite hornfels. Examples of base metal sulfide deposits that are structurally and genetically related to the formation of Mg-rich cordierite-bearing metasomatic zones have been described from other parts of the world. Geographically the closest described example is the Amulet ore body in the Noranda district. The discovery of this type of deposit on the Garon Lake property sheds new light on exploration in the Mattagami Lake area.

CRITERIA FOR THE MODE OF EMPLACEMENT OF THE ALKALINE STOCK AT MOUNT MONADNOCK, VERMONT

The alkaline stock at Mount Monadnock, Vermont, described briefly by Wolff (1929), has been restudied in detail. Its petrography and structure are discussed here and conclusions are drawn as to its mode of emplacement. The stock consists of plutonic and hypabyssal rocks which intrude folded Ordovician (?) schist and quartzite. The longer axis, trending north-northwest across the strike of the country rock, is 3 miles long, and the shorter one about 2½ miles. Essentially the stock consists of quartz syenite, but it encloses a long arcuate mass of older essexite and transition rock, and along its eastern edge are later intrusions of granite. Late dikes of various compositions cut the plutonics and surrounding metamorphic rocks. The following facts have been established: (1) The igneous rocks are typical representatives of the White Mountain magma series (Mississippian?), quite lacking in foliation and lineation. (2) The stock is discordant and has an elliptical ground plan. (3) In detail the boundary is irregular and characterized by abundant dikes and xenoliths. (4) The igneous rocks make sharp contacts with the metamorphosed country rock. (5) The arcuate mass of older essexite is undoubtedly a screen. (6) The small bodies of late granite resemble ring dikes. (7) The stock is cut by prominent sets of steeply dipping radial and tangential joints. (8) The late dikes show radial and tangential patterns. (9) Along the northern and southern margins of the stock the country rock shows strikes and dips which differ from the regional ones. From these criteria it is concluded that the plutonic rocks have invaded the crust by cauldron subsidence accompanied by the stoping of large arcuate slabs and smaller blocks from the walls of the magma reservoir

The origin of the tungsten ores of Boulder County, Colorado

A narrow zone of sericitic alteration encases the Boulder County ferberite veins, but gives way abruptly to a wide outer envelope of argillized rock. The mineralogy indicates sustained attack by early acid solutions that finally changed to neutral and alkaline solutions. Sericitic alteration is conspicuous next to the gold telluride veins and the argillic envelope is rare. The mineralogy of the tungsten and telluride vein filling indicates a progressive change from acid to alkaline solutions and harmonizes with the evidence given by the wall rock alteration. Experiments and the paragenesis suggest that ferberite was precipitated in slightly acid solutions. Galena, tetrahedrite and the gold tellurides are later and presumably precipitated in alkaline solutions. Relatively late pre-ore dikes of biotite latite and latitic explosion breccia are associated with the tungsten and gold-teluride veins. Their composition and field relations suggest that a biotite latite magma heavily charged with volatiles was the source of the mineralizing solutions. It is believed that acid emanations from this magma rose through hot porous explosion breccia with little change in character,, but soon dropped their load above the upward termination of the breccia in the overlying fissures where they reacted with the wall rock and became neutralized and finally alkaline through the acquisition of bases from the quartz monzonite that makes up the country rock of the deposit.