A comparison of magnetic character and alteration in three granite drill cores from eastern Canada

Abstract

Bulk magnetic susceptibility (BMS) measurements have been made on granite drill cores from the St. George batholith (New Brunswick), the South Mountain batholith (Nova Scotia), and the Wedgeport pluton (Nova Scotia). The primary magnetite concentrations of the two Nova Scotia cores are statistically indistinguishable, thus lending support to the hypothesis that the Wedgeport pluton, despite being 50 Ma younger, is a satellite of the South Mountain batholith.The St. George core has a primary magnetite concentration over 30 times greater than the Nova Scotia cores, but low-temperature alteration (attributable to subsurface weathering) has greatly reduced its magnetite content. The two Nova Scotia S-type granites are shown to fall into the ilmenite-series category, whereas the St. George granite, which is either S- or A-type, is transitional between the magnetite and ilmenite series.The general observation of intergranular hematite and reduced BMS in the outcrops of some granites is suggested to have important consequences for primary oxidation studies and aeromagnetic interpretation.

How Can Deep Geothermal Projects Provide Information on the Temperature Distribution in the Upper Rhine Graben? The Example of the Soultz-Sous-Forêts-Enhanced Geothermal System

“…Magnetic susceptibility (MS) was evaluated with a Kappabridge KLY-4 Kappameter on a determined volume of cutting (1 cm 3 ). Indeed, [50] and earlier studies [51][52][53] show that alteration strongly modifies the magnetic susceptibility of plutonic rocks, because of a lowered magnetite content. In particular, [53] worked on the Soultz granite.…”

Gravity and magnetic signatures of the Ackley Granite Suite, southeastern Newfoundland: implications for magma emplacement

“…The magnetic anomaly can be reproduced by the "stoped" model ( Fig. 6) but cannot be reproduced using the granite shape determined from the gravity inversion and the measured granite susceptibility, or any variant of it that allows for normal increases in susceptibility with depth such as detected by Harding et al (1988).…”

On the lognormal distribution of oxides in igneous rocks, using magnetic susceptibility as a proxy for oxide mineral concentration

“…The chief advantage of the recognition of lognormal BMS populations, especially from drillcore data, lies in being able to use their means and variances for the characterization of the rock body as a whole. Various applications of these statistics then become possible such as; detection of alteration ; aid in the classification or evolution of granites (Ishihara 1977;Takahashi, Aramola & Ishihara 1980); comparison of local or regional magmatic phases (Harding et al 1988; and our present work on granite-granodiorite phases in the Eye-Dashwa Lakes pluton, northwestern Ontario). It is worth mentioning that the degree of reduction in BMS around some deep (>1000m) fracture zones of the Eye-Dashwa Lakes pluton is almost as severe as in the upper ==loom weathered zones (Latham et al 1987), thus indicating that such fracture zones have experienced a degree of alteration comparable to the upper weathered zones.…”

“…Using a large data base, acquired under the Canadian Nuclear Fuel Waste Management Program, Lapointe, Morris & Harding (1986) gave examples of how lognormal or approximately lognormal populations of bulk magnetic susceptibility (BMS), from drillcores of granitic rocks, may be recognised as straight line segments in cumulative frequency (CF) plots, and how the original populations are affected by alteration of the granite. Harding et al (1988) gave examples of how the statistical parameters of lognormal distributions of BMS data from drillcores may be used to throw light on a number of petrological and geophysical questions.…”