Geophysics

Results

 - Geophysics

Chargeability

The following preliminary observations can be made regarding the chargeability data at the Rio Grande project (Figures 1 and 2). The principal area of high chargeability is contained within an approximately circular feature (Figure 1), which coincides reasonable well with the main area of known alteration and mineralization at Rio Grande. The areas of very high chargeability (i.e., red to magenta colours in Figures 1) are likely due to high pyrite content; which has been confirmed by drill-holes RGT_01_05 and RGA_05_13. This circular distribution of high chargeability is interpreted to be part of a marginal pyritic halo on a porphyry system.

The largest zone of high-chargeability, located on the southwest corner of the circular (Figure 1), is coincident with the lowest topographic part of the system and where it appears the depth of oxidation is less than is encountered in most other parts of the system. Hence in this area, primary sulphides are likely preserved at shallower depths, as observed in drill-hole RGT_01_05. Contrarily, the northeast half of the system occurs in a high topographic position, where the oxidation is strong, and extends to greater than 200 metres depth. Here the chargeability appears slightly weaker which is interpreted to be due to the increased depth before primary sulphide minerals are encountered, and hence, a weaker signal. Here again there is field and drill-hole evidence indicating the presence of a strongly supergene altered pyritic halo marginal to the main system.

Areas of moderate chargeability, such as those cut by drill-holes RGA_05_14 and RGA_05_15, appear to coincide with areas where pyrite and chalcopyrite, both as fracture fillings and disseminations were found. Therefore, it would seem that areas of intermediate or moderate chargeability are potentially of interest, versus the high chargeability features which appear to in most cases to be pyrite, likely related to the marginal pyritic halo. Additionally, areas of particular interest may be where intermediate chargeability features occur adjacent to high chargeability features.

Finally, the chargeability clearly defines several important structural orientations; west-northwest east-southeast (WNW-ESE), east-northeast west-southwest (ENE-WSW), and north-northeast south-southwest (NNE-SSW). All of these orientations are known regional orientations, as well as important orientations observed in the detailed fracture data from the mineralized zones.

Resistivity

The following preliminary observations can be made regarding the resistivity data at the Rio Grande project (Figures 2). A pronounced area of lower resistivity, roughly circular in shape, is coincident with the main Rio Grande intrusive complex centre and the principal area of hydrothermal alteration and mineralization within that. Inside this circular feature there are at least two annular structures which enclose zones of the most highly fractured rocks, within which are found the principal mineralized zones. In addition, there are a number of smaller areas of higher conductivity which may be related to mineralized zones at depth.

There several areas of higher resistivity such as southwest of the principal zone of alteration and two smaller centres located to the north and southeast, which may represent weakly fractured, subvolcanic intrusions, similar to the main host rock at Rio Grande, the Bi-modal Feldspar Porphyry Intrusion (unit BFPi). Finally, there is a well developed pattern of linear features that can be seen in the data and these are interpreted to be faults and corridors of high fractured rocks. Four principal orientations can be deduced; northwest-southeast (NW-SE), west-northwest east-southeast (WNW-ESE), east- west (E-W), and northeast-southwest (NE-SW).

Real Section IP Lines

The following is a preliminary assessment and interpretation of the 2004-2005 Gradient Array Induced Polarization (GAIP) survey results for Rio Grande by geophysical consultant Webb (2005). Specifically an evaluation of the Real Section IP data lines (Figures 3 to 12).

Five lines of Real Section IP data from Antares and seven lines of historic pole-dipole IP data from Teck were modeled and resultant section images imported to FracSIS. Data was not available for a further two lines of historic pole-dipole IP. Data quality appears to be quite good for both datasets. Peter Rowston’s modified version of the UBC 2D IP inversion software was used for the inversions. Both Real Section and pole-dipole inversions show good correlation with gradient array plan images. There is some ‘along line’ displacement of the sources at depth where the strike of the source is not orthogonal to the Real Section / pole-dipole line. This is not unexpected and may, in some cases, indicate (horizontal) direction toward the stronger part of the source. As would be expected, the greater density of Real Section field data results in significantly improved resolution, versus that of the pole-dipole inversion sections. There is a general, but not consistent correlation of elevated chargeability with lower resistivity; with this combination commonly peripheral to magnetic sources.  Resistivity generally appears to give the better definition of edges.

The complex structural setting for the project is particularly evident in resistivity sections, especially within, and near, the “circle of prospects’ where structure density appears to increase markedly. Many of these structures are also (at least weakly) chargeable. Apparent north dips toward the southern end of sections and apparent south dips toward the northern end of sections imply a structural focus and probably an intrusive complex at depth below the “circle of prospects”. Resistivity sections show a network of lower resistivity zones with some linear, some arcuate, and some exhibiting a ‘paisley’ pattern. These patterns reflect structure with the ‘paisley’ and arcuate patterns being due to 3D effects on the 2D inversion (i.e., structural complexity, especially structures sub parallel to the line). ‘Streaking’ at depth (usually 150 to 200m below surface) and approximately parallel to surface of the modeled resistivity and, to a lesser extent, the chargeability, is likely a function of the “depth of investigation” limit for the configurations used (not discounting possible coupling). However, given the setting of the project, the structural complexity and the fact that the ‘streaking’ occurs at essentially the same depths for both configurations, this ‘streaking’ may represent depth of complete oxidation or possibly water table. Occasional, less obvious ‘pseudo-horizontal’ features in the resistivity sections may indicate the presence of sub-horizontal faulting (e.g., 613200E below the crest at ~7231000N resistivity).

Some boundaries are quite clear and assumed to be lithological. Good examples are observed toward the northern and southern ends of both resistivity and chargeability inversion sections for Line 614400E. Similar, although not so well defined, boundaries can be interpreted toward the northern end of lines 613200E and 614000E. High chargeability / low resistivity are not exclusively related to low magnetic susceptibility.

Magnetics

The ground magnetic data is presented in three formats;

1. Total Field (Figure 13),
2. Reduce to Pole, upward continued 40m (Figure 14), and
3. Reduce to Pole, 1st Derivative (Figure 15).

Images show elevated magnetics in the north of the area surveyed and relatively low magnetics in the south (Figures 13 to 15). There is an apparent gradient across the survey with levels increasing from southeast to northwest. This gradient appears to be reflecting changing lithology rather than a regional effect. Several prominent linear features are apparent and are best observed in shaded images of RTP. Full and part circular features, which may be reflecting intrusive rocks at depth, can also be interpreted. The magnetics in the vicinity of the current prospect areas is quite disturbed with strong north-south and northeast linear features dominating the zone inside the circle defined by the supplied prospect outlines (“circle of prospects”). A prominent northwest trending magnetic ridge extends from the gap between the North and #7 prospects to the northwest corner of the survey area. A less prominent northeast trending magnetic ridge, which coincides with a topographic ridge, passes from the gap between the North and Sofia prospects through the northeast corner of the survey area. These features may be the local (magnetic) representation of regional lineaments. 

Images were assessed for breaks and linear features that may represent significant structures. “Circular features” were also noted as they may represent deeper intrusive rocks. The most prominent linear features are oriented northeast. These northeast oriented lineaments cross cut and occasionally terminate a set of less prominent east northeast lineaments. Other orientations (north-south, east-west and northwest) are also present across the survey area. These are almost all short strike length features and often give the impression that the northeast and east northeast features have disrupted them. Of these, the north-south oriented features within the “circle of prospects” were considered important as they are represented by prominent magnetic highs. The implication being that north-south structures within the “circle of prospects” have opened up and allowed the emplacement of intrusive rocks and/ or passage of mineralizing fluids. A small set of short strike length north northeast oriented lineaments is peculiar to the northwest corner of the survey area. 

The northeast direction is prominent in model results with interpreted intrusive rocks within the “circle of prospects” showing that orientation. The model indicates that northwest oriented structures are also important; disrupting the northeast trending interpreted intrusive rocks. Modelling indicates that the bulk of the intrusive rocks in the survey area are non-magnetic. This, combined with the modeled short depth extent of their sources (see discussion of individual prospects), implies that the source of the north-south magnetic highs within the “circle of prospects” is secondary magnetite associated with alteration.

Several circular features are evident in the magnetics. The most prominent of these is approximately 1500m in diameter and is located to the northwest of, and slightly overlapping, the “circle of prospects”. A strong northwest oriented linear disrupts the elevated magnetics within this feature. The “circle of prospects” itself does not generate a circular feature in the magnetics however; it does lie at the north-western end of a somewhat weakly defined northwest oriented “elliptical circular”. Other circular and arcuate features are also present. Smaller features, such as that seen centred on ~614660E, 7231990N, may represent the response from individual intrusive plugs and their associated alteration. 

Circular features are shown to be polygonal rather than circular, a common feature in these types of system. This is particularly the case for the prominent circular to the northwest of the “circle of prospects” whose geometry is suggestive of an intrusive source. The feature mentioned above, centred on ~614660E, 7231990N, shows in the model as having short depth extent above a larger body of non-magnetic material with susceptibilities increasing to ~3600RL before decreasing into the larger body.

Previous Teck (2000-2001) Surveys

The following discussion of the previous 2000-2001 geophysical work were taken and modified from Ambrust et al. (2005).

Induced Polarization (IP)

An induced polarity (IP) and resistivity survey was performed for Teck in 2000-2001 by Quantec Geophysics, using personnel from the Quantec Argentina office in Mendoza Argentina, and the Quantec Chile office in Antofagasta, Chile. The purpose of the survey was to attempt to detect areas of disseminated sulphide and/or magnetite mineralization at depth (most surface sulphides are at least partially oxidized). The survey was conducted in the time domain using a pole-dipole array and 50-meter dipole spacing. After several lines were surveyed and the results reviewed, the dipole spacing was lengthened to 100 meters for the rest of the survey in order to image what appeared to be a more deep-seated chargeability anomaly. In all, 23.9 line-kilometres on nine north-south lines spaced between 200 and 400 meters apart were surveyed. Pseudo-sections produced from this survey suggested the presence of a large chargeability feature in the southern part of the survey area, as well as two elongated chargeability bodies; one coincident with the Discovery Zone, and the other a north-northwest trending body located west of the main zone affected by SDM alteration.

IP and resistivity surveys were inverted using a 2-D inversion program (DCIP2D) developed at University of British Columbia and a plan map at the 4,050-meter elevation, and six sections through the inversion model showing chargeability and resistivity. The "depth reliability" of these inversion sections is estimated to be about 250 meters for the 50-rn dipoles and 500 meters for the 100-rn dipoles. This estimate is arbitrary and based only on the appearance of the pseudo-sections. Because the model is not constrained at depth, sections produced from surveys with 100-meter dipole-spacing produce more pronounced anomalies than that for lines with 50-meter dipole spacing.

The inverted IP data produced a large chargeability anomaly that underlies much of the southern and western parts of the grid area. The IP anomaly is presumed by Quantec to result from the presence of moderate amounts of disseminated pyrite and/or chalcopyrite, or the presence of a large amount of disseminated magnetite, or both. The anomaly generally starts at a depth of approximately 200 meters, although it rises to near surface on line 613600E. The chargeability anomaly appears to be open-ended to the east, and perhaps to the north. A large part of this anomaly was not drill tested by Teck, particularly to the west and north.

The resistivity model shows mottled highs in near surface areas, with generally lower anomalies at depth. This effect may be a function of the depth to the water table, with higher resistivity recorded in unsaturated, near surface rocks. It may also reflect the presence of K-feldspar or strong Ca-Na alteration in near surface areas. A pronounced low anomaly is present in the core of the chargeability high on line 613600E.

An independent review of the previous magnetic and induced polarization work completed by Quantec, was conducted by CAM consultants as part of Antares 43-101 technical review. CAM concluded that although the results do not have a unique interpretation, the models selected by Quantec are reasonable approximations based on the available survey data. Furthermore, they suggested that modifications to this interpretation will undoubtedly be made as more geological data are obtained from additional drilling and new surface exposures.

Magnetics

A surface magnetic survey was performed for Teck in 2000 by Quantec Geophysics, using personnel from the Quantec Argentina office in Mendoza Argentina (Unger and Rideout, 2001). The data was processed by personnel from the Quantec Argentina office. The magnetic survey was run over the periods from October 4-15 and November 24-28, 2000. The total magnetic field was measured using a GSM-19 magnetometer and a base station for diurnal corrections. Readings were taken at 25-meter intervals on north-south trending lines spaced 200 meters apart in the center of the grid and 400 meters apart on the ends of the grid. The survey covered approximately 66 line-kilometres or roughly 12 square kilometres.

Data were reduced to pole and contoured using Geosoft mapping software. According to Quantec, prominent magnetic deflections from near surface sources, together with large line separations produced a map with an irregular appearance, suggesting that additional measurements on lines between the existing lines would improve the interpretation. In particular, the Rio Grande core zone of alteration is distinguished from surrounding areas on the total field magnetic map by a roughly east-west trending zone of pronounced magnetic highs with flanking magnetic lows. On a scale of several tens of meters, there is not an obvious correlation between surface rocks with abundant magnetite and the total field magnetic map. In general, however, the zone of irregular highs and lows does correspond to the area with abundant magnetite in the rocks, and the east-west-trending anomaly in general does correspond in part to a zone of intense scapolite-diopside-magnetite mineralization that extends through the middle of the core alteration zone.

The ground magnetic data were inverted using the UBC Geophysical Department’s MAG3D software. It is important to understand that this type of data manipulation does not produce a unique solution; that is, there can be a multitude of different models that fit the data equally well. The magnetic inversion model indicated the presence of strongly magnetic material at depth, with magnetic susceptibilities ranging from 0.15 to over 0.4. The general shape of the magnetic material in the area of the main alteration zone is that of a relatively flat-lying to hemi-cylindrical body that is up to 300 meters thick. At the north end of the grid, the magnetic material appears to dip sharply to the north where it coalesces with a deeper magnetic body. Smaller scale features, such as a possible south-dipping fault zone at approximately 7231300N on line 613600E, may be significant as well.