Modern transient electromagnetic systems are able to take measurements in the transmitter on‐time. Integrating measurements taken during the transmitter switch‐off and those collected in the transmitter off‐time yield an estimate of the primary field plus the secondary inductive‐limit response. If the transmitter loop position is known and the position and orientation of the receiver dipoles are known, it is possible to calculate the primary field. When the theoretical primary field is subtracted from the measured inductive‐limit‐plus‐primary response, the inductive‐limit response can be isolated. An anomalous inductive‐limit response is a diagnostic feature of highly conductive ore bodies. On‐ and off‐time PROTEM data collected in a drill hole proximal to the Reid Brook Zone (one of the Voisey’s Bay deposits in Labrador, Canada) shows a strong inductive‐limit anomaly corresponding to an off‐hole conductor. A drill hole targeted to test this conductor intersected 20.4 m of mineralization, including 8.25 m of massive sulfide.

Trace element concentrations in igneous rocks are frequently lognormally distributed, and Shaw (1961) suggested that minor minerals in igneous rocks might also be lognormally distributed. There is accumulating evidence to show that bulk magnetic susceptibility (BMS) often closely follows a lognormal distribution in fresh igneous rocks but, because BMS is dependent upon oxide-grain size and mineralogy, it is not obvious why this should be so. We have adopted Shaw's simple theoretical model as an argument to account for the lognormal distribution of BMS. The obvious prevailing conditions must be that the original melt was uniform, that pressure, temperature and oxygen fugacity were constant and that the minerals were formed at equilibrium. In addition, because BMS is also a function of grain size and mineralogy, the distribution of these latter parameters must be insensitive to changes in P , T , etc. at equilibrium. It appears that there must be a limit to the lognormal law in trying to apply it to minerals of greater concentration. However, BMS proxies for oxide content, and because it does closely follow the lognormal distribution it thus appears that, in crystalline igneous rocks, the lognormal law is obeyed by minerals of concentrations at least up to a few percent. Knowledge of the mean and variance of such distributions can be useful in a variety of petrological applications, especially in drillcore logging; a simple example is presented.

The McConnell nickel deposit is an elliptical amphibolite-biotite quartz diorite pod within the Sudbury Metabreccia that surrounds the main mass of the Sudbury Igneous Complex. The geometry of the deposit is tabular with a strike length of approximately 152 m and depth of 610 m. Various magnetic surveys were conducted in this study area. A regional aeromagnetic survey was conducted by the Ontario Geological Survey. A more detailed ground magnetics survey and borehole magnetic survey are used in the interpretation of the character of the causative body. Five boreholes which intersect the deposit at 40 m, 105 m, 135 m, 210 m, and 250 m provide good lithological control of the deposit with depth. Eight other boreholes used in this study did not have logged lithology, but all 13 holes were logged with gamma ray, density, spectral gamma-gamma, IP, resistivity, SP, magnetic susceptibility, temperature, and three-component magnetic field probes. Only the measurements of magnetic susceptibility and three-component magnetic field measurements and their interpretation will be discussed in this paper. Three-component magnetic vector surveys show different signatures for boreholes that pass through a magnetic source body, and for holes that are adjacent to the source body. Using inverse models of the surface magnetic data, values of magnetic susceptibility and magnetic remanence direction and intensity for the various lithological units are estimated. Modelling of the subsurface data reveals fine structure within the ore body and may be used to estimate the direction and distance to the magnetic bodies.

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.

The Voisey's Bay area, in northern Labrador, presents unusual problems in the interpretation of audiofrequency magnetotelluric data. The survey area is located between two east-oriented fjords; the sea-water filling these is assumed to have a resistivity of about 0.3 ohm.m. This is in strong contrast toi the resistive Proterozoic metamorphics underlying the survey area, which have resistivities in the range 10,000 to 100,000 ohm.m. Thus, the sea-water interface can be expected to have a major impact on measurements. In addition, recent sediments occur in minor pods along drainages and near lakes, and these again cause significant AMT responses. Previous work (e.g. Mackie and Watts, 1999) has modeled the gross effect of the sea, and Balch et al. (1998) presented a comparison between AMT and AEM surveys over the same area. In this paper, we show how the use of AEM data to quantify the resistivity and extent of shallow conductors improves the numerical interpretation of AMT data.