Geophysics

Results

 - Geophysics

 (last update 11/09/2009)

Introduction

Various geophysical surveys have been conducted over the Rio Grande property between 2000 and 2006 by both Teck Corporation and Antares Minerals Inc. Figures 1 to 5 show each of these geophysical data sets in relation to the completed drill holes (1 thru 77).

Chargeability

The following preliminary observations can be made regarding the chargeability data at the Rio Grande project (Figure 1). 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.

Figure 16: Drill holes overlain on the chargeability IP map.

 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 (Figure 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.

Figure 2: Drill holes overlain on the resistivity IP map.

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).

Magnetics

The ground magnetic data is presented in three formats;

1. Total Field Analytical Signal (Figure 3),
2. Reduce to Pole Analytical Signal, upward continued 40m (Figure 4), and
3. Reduce to Pole Analytical Signal, 1st Derivative (Figure 5).

Figure 3: Drill holes overlain on the total field analytical signal ground magnetics map.

Figure 4: Drill holes overlain on the reduced to pole analytical signal ground magnetics map.

Figure 5: Drill holes overlain on the reduced to pole ground magnetics map.

Images show elevated magnetics in the north of the area surveyed and relatively low magnetics in the south (Figures 3 to 5). 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.