PROJECTS - Don Sixto (La Cabeza)
Geophysics
Comprehensive geophysical surveying has been undertaken by AMD on the Don Sixto Project. A number of ground geophysical exploration surveys have been conducted with varying success. The most useful method proved to be radiometrics, with thorium "lows" coinciding with zones of mineralisation and detailed gradient array IP surveys that outlined structures and zones of prospective host rocks. Other geophysical methods used to survey the area include ground magnetics and IP dipole-dipole resistivity and chargeability surveys.
All ground surveys were conducted by AMD personnel using company-owned equipment.
Exeter is currently re-processing the majority of the existing geophysical datasets as a preliminary to evaluating the district for additional Don Sixto-style gold mineralisation discoveries.
Induced Polarization / Resistivity Surveys
Several phases of IP surveying have been conducted over the Don Sixto project area during 1997-1998. These include 100m x 200m and 50m x 50m gradient array surveys over selected portions of the project area, plus a number of dipole-dipole lines. Out of all these surveys, the 50m x 50m gradient array survey has provided by far the most useful data for the interpretation of structural and lithological elements. Contrary to expectations, the technique has not, to date, delineated alteration and/or mineralisation zones although a number of chargeability/resistivity anomalies remain unexplained. In typical epithermal districts, where steep-dipping quartz veins are associated with clay-pyrite alteration selvedges, gradient array resistivity has proved to be an effective tool in exploration. At Don Sixto however, there is no significant resistivity or chargeability contrast between the quartz veins and the silicified rhyolite wall rock. It has also been shown that the vein resistivity responses are not enhanced by the presence of clay-pyrite alteration envelopes as observed in drill cores.
Induced Polarization - conductivity
(i) 100m x 200m Gradient Array Survey
This survey was completed over the entire Don Sixto Grid, covering a total of 107.2 line kilometres. Measurements were taken over east-west gridlines with station spacings of 100m and line intervals of 200m. The objective of this work was the search for concealed extensions of exposed mineralisation at the various Don Sixto prospects. The results of the survey are as follows:
- Broad zones of coincident resistivity and medium level chargeability sub-parallel to the La Mandibula Ridge.
- A broad zone of high (>100msec) chargeability lie northeast of the La Luna prospect, although probably is related to hornfelsed graphitic shales underlying the ignimbrites.
- The survey could detect areas of ignimbrite (resistivity highs, relative chargeability lows) but could not differentiate between silicified and unsilicified ignimbrites with the exception of the Mandibula Ridge.
- This survey failed to detect the La Luna quartz-veined and silicified ignimbrites probably due to wide line spacing.
Induced Polarization - resistivity
(ii) Dipole-Dipole Line Survey (50m; n=6)
This object of this survey was to determine what an IP response would show over known areas of mineralisation and to explore for similar responses under the colluvium.
A co-incident chargeability and resistivity anomaly occurs directly over the Mandibula breccia zone. The resistivity anomaly appears to be shallow and is probably related to silicification. The chargeability anomaly appears deeper and may be related to sulphides rather than underlying carbonaceous shale.
A weak shallow resistivity response occurs over the La Luna prospect indicating that the prospective felsic volcanic lithologies and silicification may not persist downward.
(iii) Dipole-Dipole Line Survey (100m, n=4)
Although this survey failed to accomplish its intended purpose, that of exploring for feeder systems in the area, it nevertheless identified anomalous zones warranting further evaluation. These zones include:
- A moderate resistivity anomaly over the Madonna ridge immediately NW of the Mandibula prospect. This anomaly is well defined in the gradient array surveys.
- A strong resistivity and moderate chargeability anomaly on the northwest strike extension of the Mandibula. Rock chip sampling across this zone returned anomalous gold values from 100-200 ppb.
- An elevated resistivity response 100-200m to the west and north west of El Ojo prospect. The anomaly is largely underlain by sand but includes two outcrop areas of altered ignimbrite with no visible mineralisation.
- A moderate resistivity anomaly forming a northwesterly ridge extending from the vicinity of the Oreja showings over 800m to the vicinity of the El Ojo prospect. This anomaly is mostly under alluvial cover.
- A strong resistivity anomaly is centred on the El Cachete prospect. Hole LCP-48 (6m @5.7gpt Au, 9m @ 6.1 gpt Au) would have tested the upper levels of this anomaly.
(iv) 50m x 50m Gradient Array Survey
This survey covered a wide area and totaled 99.5 line kilometres. With the primary objective of locating drill targets under alluvium the survey provided excellent definition of structures, distribution of lithologies and silicified zones resulting in the delineation of several drill targets.
(v) 50m x 50m Gradient Array Survey- (Extension)
This survey was completed as an extension to the 1997 50m x 50m gradient array survey. A total of three IP blocks were completed tracing the La Mandibula trend a further 2km to the northwest. A total of 25.2 line kilometres were surveyed covering an area of 119.5Ha bringing the total area covered by the 50 m x 50m survey to 695 Ha.
The Don Sixto IP programme resulted in the following observations:
Resistivity highs (>1500 Ohm/m) are related to areas of:
- Volcanics >75m thick overlying siltstone/quartzite but not necessarily implying silicification. Felsic volcanics and porphyries at El Labio, El Ojo and La Luna do not register as resistivity highs. Drilling at these prospects has shown the volcanics to be thin "veneers" (< 75m thick) overlying the siltstones. These areas of thin volcanics tend to attenuate the chargeability anomalies associated with the underlying carbonaceous siltstones.
- Massive fresh to propylitically altered rhyolitic volcanics (e.g., SE and south-central sectors of the Don Sixto Grid).
- Silicified rhyolitic volcanics (e.g., El Cachete, La Mandibula Ridge, La Madonna, & El Cuello). Note that an intense resistivity anomaly at the La Madonna prospect (9600E, 11200N, immediately north of the La Mandibula prospect) is coincident with a ridge of finely brecciated, silicified and pyritic ignimbrites. The zone was partially tested by drill hole LCP-34 without any significant results. The source of this resistivity anomaly has not been satisfactorily explained.
Chargeability highs (>25 msec) are related to:
- areas of carbonaceous siltstone underlying the ignimbrite sequence. The chargeability anomalies tend to be stronger in areas where ignimbrite is absent. Most areas covered with soil have higher chargeability.
- Strong chargeabilities are associated with a siltstone-sandstone sequence in the northeast portion of the grid. Two attempts to drill this anomaly were abandoned due to caving after reaching graphitic siltstone.
- LCP-42 testing a coincident chargeability and resistivity anomaly encountered unmineralised and hornfelsed siltstone and sandstone.
The areas of lower chargeability values are associated with unaltered, to propylitically altered rhyolitic volcanics in areas fringing the silica-sericite-pyrite alteration zone (Landsat TM anomaly) and areas of thicker altered volcanics overlying carbonaceous shales. In some covered locations, relative chargeability "lows" are interpreted to reflect (thin) zones of unexposed volcanics overlying the carbonaceous shale. Hole LCP-91 intersected ignimbrite with anomalous gold.
In general, better zones of mineralisation appear associated with coincident chargeability and resistivity responses of moderate magnitude.
Ground Magnetometer Surveys
A detailed 25m x 10m ground magnetic survey was completed over the main alteration zone to determine if more subtle features associated with structures, alteration or mineralisation could be identified.
The surveys were conducted by AMD personnel using four portable proton precession GSM-19 magnetometers. One unit was utilized as a base station. Two surveys were carried out, one with the sensor 1.8m above ground and the other 5m above ground.
Data from both surveys was processed by Quantec Geophysics of Mendoza and plotted in total field format.
Major observations resulting from the ground magnetometer surveys include the following:
- The propylitic alteration is associated with lower magnetic gradients due to the destruction of magnetie.
- Remnants of Quaternary basalt lava flows in palaeo-drainages show higher magnetic gradients.
- Northwesterly granophyre and basic dykes with high responses were delineated in the El Labio Shear, between El Cachete and La Mandibula prospects, and at El Cachete.
- Magnetic highs define paleo channels containing magnetite sand. Several of these old channels align with known faults from surface mapping.
Ground Magnetometer Survey Data Re-Processing
Exeter contracted Vector Research Pty Ltd of Australia to re-process the ground magnetic survey data that employed a sensor height of 5m above the ground. A "2-dimensional pattern recognition algorithm" developed by Vector Research and termed an Overburden Filter™, was utilised to enhance the resolution of the magnetic data. As stated by Vector Research "it can resolve detail in survey data and 'see' the sub-surface geology and its structures through the masking effects of overburden and cultural interference." This is particularly crucial at the Don Sixto Project where the extensive sands contain sufficient quantities of magnetite to mask both geology and structures about the known prospects.
This new large database of filtered images is presently being evaluated to assist with detailed geological and structural interpretation, at both the prospect and semi-regional scale.
Ground Radiometric Survey
A ground radiometrics survey was conducted by AMD personnel over portion of the Don Sixto Grid using an Exploranium GR-256 portable gamma-ray spectrometer and Exploranium GPS-21 radiation detection system. Data were processed by Quantec Geophysic Ltd of Mendoza. Main findings and interpretations from this work are summarized as follows:
Thorium (Th)
Thorium primarily defines the areas of bedrock outcrop, showing little contrast between areas of propylitic and silica-sericite altered volcanics. The sand covered areas consistently show as lows and negative features. Most importantly however, is that most mineralised outcrops are associated with lower values compared to the non-mineralised felsic volcanics equivalent.To date, this has been the only geophysical method which effectively differentiates mineralised zones. This association with mineralisation can be seen at La Luna East, El Ojo, La Mandibula, and El Labio West. Several other anomalous thorium responses are coincident with Au, Ag, As, Sb and Mo rockchip geochemistry anomalies and warrant future drill testing.
Potassium (K)
The potassium data effectively defines an "outcrop map", with the greatest values detected over the silica-sericite/illite-pyrite alteration zone, and (slightly) lower values detected on the propylitic alteration areas. All sand covered areas register as potassium lows. Like thorium, (some) mineralised zones coincide with weaker potassium responses.
K/Th Ratio
This ratio has been shown to the most effective discriminator between silica-sericite-pyrite alteration (high values) and propylitic alteration (low values). The lowest values are over areas of sand cover. At El Cachete, within the silica-sericite alteration zone, there is a NE-trending corridor of anomalously low K/Th values. The significance of this feature has not been resolved to date, but it is notable that the better intersections in drill holes LCP-46 and LCP-48 are close to the "corridor".
Uranium (U)
Uranium primarily defines areas of outcrop (and possibly sub-crop). It does not contrast between silica-sericite-pyrite and propylitic alteration assemblages. Like thorium and potassium, uranium has anomalously low values over mineralisation at La Luna and El Ojo.
Ground radiometrics has been the only geophysical method which, in most cases, has successfully differentiated between mineralised altered volcanics and the non-mineralised equivalents.
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