New Gravity Survey Delivers Significant Targets
St George Mining Limited (ASX: SGQ) has identified multiple new targets for nickel-copper sulphides identified by the latest geophysical surveys at its flagship high-grade Mt Alexander Project in the north-eastern Goldfields of Western Australia.
An area of particular interest is the West End Prospect, which encompasses the western extension of the Cathedrals Belt as it approaches the interpreted major crustal Ida Fault in the western section of the Mt Alexander tenement package.
The intrusive unit in the Belt that hosts high-grade nickel-copper sulphides is interpreted to dip towards the north-west and extend into the West End area. This was confirmed by drill hole MAD197 which intersected the intrusive unit at 493.9m downhole.
Ground-based EM surveys have been completed along the Cathedrals Belt and successfully identified shallow EM conductors that resulted in the nickel-copper sulphide discoveries at Stricklands, Investigators, Cathedrals and Radar. However, the ground-based EM surveys were of limited effectiveness at West End because of the conductive cover to the west of Investigators.
A high-resolution ground gravity survey has now been completed over West End and Investigators, and provided new data that supports the prospectivity of the area for nickel-copper sulphides.
A gravity survey measures the density of rocks and the density contrast between different types of rocks in the Earth’s crust. Due to the relatively high density contrast between the host intrusives that contain massive nickel-copper sulphides and surrounding rocks, gravity surveys can identify dense bodies that may contain massive sulphides within the less dense terrain.
Executive Chairman, John Prineas, said high density massive sulphides and their host rocks will typically present as gravity highs. Blue related colours show areas of less dense material or cover.
A number of gravity features have been identified along the mineralised trend of Investigators and at West End. These areas have never been drilled. Of particular interest are a string of large gravity highs, the largest of which appear at West End.
Significantly, several of these features are located down-dip along the interpreted plane of the ultramafic intrusives and seen in the gravity data as discrete ‘bulls-eyes’ within areas of lower density. These features are ranked as high priority.
The gravity survey was completed at West End and Investigators on very close spacing of 25m between stations. This spacing provides the highest resolution data possible for imaging discrete bodies as well as disjointed bodies and bodies with complex geometries.
Mr Prineas said that as gravity surveys can be affected by geological changes in the cover sequence, the gravity survey was also completed over a section of Investigators where nickel-copper sulphides are already confirmed by drilling to provide a comparison to the untested West End area.
At Investigators, the known intrusive hosted nickel-copper sulphides and numerous untested DHEM conductors correlate to a trend of moderate gravity anomalism with higher density trends, providing evidence that the gravity data is potentially mapping intrusive hosted sulphide mineralisation.
MAD199 intersected a thick 10.96m interval of nickel-copper sulphides from 333.6m downhole, which is the deepest occurrence of massive sulphides drilled in the Cathedrals Belt to date.
Mr Prineas said this discovery is an important milestone at the Cathedrals Belt as it confirms that massive nickel-copper sulphides are present at depth, beyond the interpreted detection limit of surface EM surveys.
He said the results in MAD199 create added confidence that the extensive intrusive mineral system – already identified over an east-west strike of 6.5km – has potential to host significant mineralisation at depth.
The DHEM survey in MAD199 has recorded a strong on-hole EM response between 345m to 350m downhole with three new EM plates modelled. The DHEM survey in MAD195 also resulted in three EM plates being modelled, with one of these plates (MAD195_p1) intersected by MAD199. All EM plates have varying strike lengths, dip directions and conductivities, indicating a complex geometry to the massive sulphide source – and the likelihood that there is more than one source.
Step-out drilling has been designed to test the extent of the DHEM conductors and the down-plunge continuity of the MAD199 mineralisation. The first follow-up drill hole will target approximately 40m to the north-west of MAD199 – an area down plunge of the intrusive host unit and interpreted mineralised trend. This new drill hole – MAD201 – will commence once MAD200 is completed.
MAD200, approximately 500m west of the discovery made by MAD199, is planned to a depth of 600m to test a broad SQUID MLEM anomaly. Drilling has advanced to approximately 433m downhole.
MAD197 was completed at West End as a deep exploratory hole and successfully intersected an 18.7m thick intrusive unit from 493.9m downhole.
The DHEM survey in MAD197 recorded several high frequency responses that increase towards the end of the hole. These responses were seen in the mid to late delay times at downhole depths of 490m, 505m, 545m, 555m, 585m and 600m. Data from the responses was insufficient to allow reliable modelling of the source. Potentially, the source is a distant conductive body (more than 50m from MAD197) or a non- mineralised geological feature.
Further drilling is planned along strike from MAD197 to continue the search for nickel-copper sulphides in this trend.
MAD198 was completed at Stricklands to a downhole depth of 415.1m to test down-dip from the shallow high-grade nickel-copper sulphide deposit at Stricklands.
The DHEM survey in MAD198 recorded a weak anomalous response at 380m to 390m downhole. The data was inadequately constrained to allow modelling of an EM plate for drill targeting. Potentially, the source is weak sulphide mineralisation (e.g. stringer sulphides) proximal to the hole, a distant conductive body (more than 50m from MAD198) or a non-mineralised geological feature.
Additional drilling is planned to further test for mineralisation below the shallow Stricklands deposit.
“The results of the gravity survey are very exciting and could provide a great advance in the targeting of nickel-copper sulphides in the Cathedrals Belt,” Mr Prineas said.
“A number of the gravity features are compelling drill targets for massive sulphides. These gravity highs are located within the interpreted trend of the mineralised intrusive host unit, an ideal location for dense massive sulphides.
“The concurrent use of downhole EM and drilling is continuing to provide a vector to mineralisation at depth. The multiple EM plates modelled around the new high-grade discovery in MAD199 support the potential for further high-grade mineralisation in this area.
“Drilling continues 24/7 and we look forward to reporting more drill results.”
An additional drill programme is being planned to test the new gravity features. The targets are in areas with no previous drilling, which requires earthworks and access routes to be prepared prior to drilling.
