Blackridge Project, QLD


Impact’s Blackridge project covers 150 square kilometres and comprises three 100% owned Exploration Permits (EPM26806, EPM27410 and EPM27571) and one granted Mining Lease ML2386 (Figure 1) which lies in the centre of the project area (ASX Release August 31st 2018).

Figure 1. Location and regional geology of the Blackridge Project.

The project covers about 90% of the southern part of the Blackridge-Miclere gold field located near Clermont in central Queensland (Figure 1).

The gold produced at Blackridge was mostly hosted in basal conglomerates of Permian-aged sedimentary basins which include the mined coal measures that unconformably overlie the Anakie metamorphic rocks of Middle Ordovician age and older (Figures 1 and 2).

The unconformity is present at surface over about 1,500 metres of trend at Blackridge. Much of the lease is covered by loose gravel with only a few outcrops of conglomerate and schist in places. This cover, within which small gold nuggets have been found by prospectors over many years, has hindered previous exploration and there has been no recent systematic exploration in the area.

Figure 2. Tenure and geology of the Blackridge Project.

In 2019 Impact completed a major bulk sampling programme at Blackridge with a view to trial mining of its mining lease in conjunction with four other mining leases over which Impact held an option to purchase a 95% interest.  The option lapsed in November 2019 because commercial terms could not be negotiated with the holder of the mining lease.

Since that time Impact has consolidated its ground holding in the area and is now considering its options to progress the project.

The increased ground position at Blackridge was warranted by the results of the bulk sampling programme which confirmed the potential for significant volumes of free digging oxide material with exceptional gold recoveries of at least 95% and probably as high as +98% using simple wet gravity processing (ASX Release September 18th 2019).

Following successful results from a Phase 1 sample program in 2018, comprising samples weighing up to 1 tonne, a second-hand mobile water processing plant capable of processing up to 50 tonnes of material per day was purchased and commissioned by Impact for a second phase of bulk sampling (ASX Releases October 23rd 2018 and June 18th 2019).

For each sample, a panned concentrate was collected and sent for gravimetric fire assay at Intertek Laboratories in Perth. In addition the tailings from the panned samples (“panned tailings”) were sent for cyanide leaching by the Leachwell method at ALS Laboratories in Townsville to check for potential losses of gold by Impact’s processing methodology to the fine tailings. In addition the trenches and samples were detected for gold nuggets using a handheld metal detector.

The results shown in Figure 3 and Figure 4 show examples of the panned concentrates and also the weights and sizes of the nuggets found in the trenches.

It is appropriate given the nature of the material sampled to report the results in grams per loose cubic metre as was done previously (ASX Release October 23rd 2018).

Figure 3. Examples of the panned concentrates from Foxes Lead on ML2386 and picture of nuggets from all trenches. All gold is the property of Impact Minerals Limited.

Figure 4. Gold results, previous gold production, geology and mining lease locations for the central Blackridge project area. All results are presented in grams per loose cubic metre. The second phase sample results are highlighted in the yellow call out boxes.


Twelve trenches were dug at varying intervals of between 50 metres and a few hundred metres over a strike length of 1,000 metres (Figure 4).

All samples were free digging indicating there are potentially large volumes of easily mineable oxide material in at least the top 4 to 5 metres of the profile at Blackridge (Figure 5). The depth of the oxide material is unknown but previous drilling in the area indicates relatively easily processed material may be present down to at least 25 metres or more in many places (ASX Release May 29th 2018).

In addition, every trench returned gold with values ranging from 0.07 g/m3 to 2.17 g/m3 at an average of 0.36 g/m3.  The values were calculated from the panned concentrate, the Leachwell assays and the weight of nuggets.

It is remarkable that these concentrates represent back calculated head grades of less than 10 parts per billion and attest to the unique processing characteristics of the oxide-hosted gold.

Eight of the 12 trenches returned gold nuggets and the grades are mostly influenced by the number of nuggets in the sample. In particular Trench BRC015 returned 11.5 grams of nuggets from 12 tonnes of rock to give a grade of 2.17 g/m3 (Figures 3  and 4).

Trench BRC015 lies close to the start of “Foxes Lead”, one of the high-grade “runs” mined in the late 1800’s and early 1900’s (Figure 27 and ASX Release October 23rd 2018).  Previous production data from this era indicates that the number of nuggets is likely to significantly increase as the high-grade runs are approached with reported grades commonly of more than one ounce per tonne (Figure 4,
ASX Release October 23rd 2018).

These leads are quite robust and extend continuously down dip for at least 1,500 metres in places (Figure 4). Numerous leads may be present along the unconformity within the area of the mining leases (Figure 4).

These new results suggest that the basal unconformity could be weakly to moderately mineralised over the entire 1,000 metres of trend sampled and that it is potentially mineralised for a further 500 metres to the south, past the Hard Hill prospect (Figure 4).

In addition Impact considers it possible that further closer spaced sampling could return more nuggets along the entire unconformity and in particular close to the high grade leads, thus potentially increasing the average grade. If this were the case, then there would be clear potential at Blackridge for a large bulk mining operation.

Figure 5. Photographs showing the free digging nature of the oxide material in the top 4 to 5 metres and also the target unconformity. Samples were taken approximately 1.5 metres above to 0.5 metres below the unconformity. There are large volumes of free-digging material across the Blackridge project.


The bulk sampling programme has demonstrated the potential for large volumes of easily mined and easily processed material from surface to a depth of at least 5 metres with exceptional liberation characteristics of the contained gold.

Further sampling is required to establish the grade distribution in the profile above the target unconformity horizon, as this will have a significant influence on the strip ratio of any potential open pit mine. Previous drilling has demonstrated that gold is present sporadically throughout the entire 100 metre thick sedimentary package at Blackridge (ASX Release May 29th 2018).

In addition further sampling at closer spaced intervals is required along the unconformity from north to south to determine if zones of better grade are present (Figure 4).

The next most practical step forward for Impact would be to commence much larger sampling programmes on its granted mining lease. Impact is considering its options for doing this work which will include discussions with various speciality contractors.