Reprocessing of airborne gravity gradiometry data reveals prospective trends near Fosterville gold mine.
The Fosterville gold mine in Central Victoria, Australia, is just a two-hour drive from Melbourne, where I am in COVID-19 lockdown for the third week running. The mine currently has reserves of 2.72 million oz. at 31 grams per tonne (g/t), and it is one of the world’s highest-grade, most profitable gold mines. Recent spectacular grade results at Fosterville has sparked a resurgence in gold exploration in Victoria, and airborne gravity gradiometry may well be the game changer as exploration moves further under cover north of Fosterville.
Geological Setting
The gold mineralisation at Fosterville Mine is contained in Lower Ordovician (~485-470Ma) turbidite sequences of interbedded sandstones, siltstones and shales. The sequence is metamorphosed to sub-greenschist facies.
The turbidite sequence has been folded into a repeating set of north-northwest striking, upright chevron folds and occasional open folds. During folding, vertical axial planar (in finer sediments) and radial cleavages (in sandstones) developed and are best observed in the fold hinges.
The gold mineralization is associated with the intrusion of nearby Upper Devonian (~380-370Ma) granite plutons resulting in the emplacement of late gold-in-veins at the Fosterville Gold Mine. Within the Fosterville Mine License the north-northwest trending Fosterville Fault is strike extensive and dips steeply west.
Gold mineralisation correlates with gravity anomalies
Geophysics, and in particular ground gravity surveying, has proven to be a valuable tool for exploring for additional gold mineralisation around Fosterville, as a number of anomalously high gravity responses correspond with known areas of gold mineralization, including the Fosterville and O’Dwyer’s Fault Systems. Just outside the 28.5 km2 Fosterville Mining License (MIN5404) the smaller Hallanan’s (23Koz Au) and Goornong South (33Koz Au) gold deposits are also associated with local gravity highs. Drill follow-up in 2017 on ground gravity highs on the O’Donnell’s and Goornong trends revealed discrete structural zones hosting anomalous gold mineralization.
The cause for the observed relationship between gold mineralisation and gravity highs is yet to be determined. One possibility could be due to zones of increased fluid flow introducing denser minerals into the mineralised area, or possibly the folding and stacking of the shale sequences creating the local increase in density.
A regional airborne gravity gradiometer survey becomes public domain
In early 2019, looking for gravity highs/gold mineralisation further afield, the Fosterville mine owner, Kirkland Lake Gold (KLG), had the good sense to fly a 10-day fixed-wing airborne gravity gradiometer survey over, amongst other tenements, their 579km2 exploration tenement (EL3539) surrounding the Fosterville Mining License.
Airborne gravity gradiometer surveys can map variations in the earth’s gravitational field with much higher spatial resolution (~200m) and accuracy (~0.2 mGal) than conventional airborne gravity surveys. The survey was flown using a FALCON airborne gravity gradiometer system.
Unfortunately for KLG the exploration tenement EL3539 lapsed in late February 2019 and returned to the state of Victoria along with the acquired gravity gradiometer data. The tenement was put out for general tender in late 2019 in a larger land package, and the gravity gradiometer survey became public domain (with the Fosterville Mining License excised).
Data enhancement maps mineralised trends at Fosterville Mine
I thought it would be worth having a look at the released data and see if re-processing and data-enhancement by Nordic Geoscience could improve the imaging, and sure enough:
Figure (a) shows a map of Fosterville Mine Licence and surroundings with mineralisation trends, historic mines and active pits. The map was originally released by KLG in May 2018 for the announcement of the Robin’s Hill discovery on the O’Dwyer’s trend.
Figure (b) shows the vertical gravity gradient data, GDD, as provided by the airborne survey company, CGG. At first glance the data does not look that great with little lateral continuity and limited amplitude range, but that is primarily due to the relatively small gravity gradient responses from the metamorphosed shale folds and the mineralized zones almost drowning out in the background noise.
Figures (c) and (d) show the result of Nordic Geoscience reprocessing and enhancement of the original airborne gravity gradiometry data. The enhanced imaging clearly reveals and maps the continuation of each of the named mineralized trends beyond the Fosterville Mine Licence. Both Hallanan’s and Goornong South gold deposits are associated with positive density anomalies. (Disclaimer: the imaging has been degraded prior to publication; a higher resolution product of the entire AGG survey is available from Nordic Geoscience).
Discussion
Future explorers should take note that the prospective Fosterville trend actually appears to veer towards the north-west beyond the Fosterville Mine Licence. Also the O’Dwyer’s trend looks prospective towards the south-east, provided it continues across the Redesdale Fault, whilst the Sugarloaf trend looks deeper and less compelling.
The state of Victoria is yet to decide which companies will be awarded the tendered North Central Victorian goldfields tenements, but I would urge the lucky winners to take another look at the airborne gravity gradiometer data, because the prospective trends established at Fosterville can be mapped a long way north under increasing cover. With the right data processing and enhancement airborne gravity gradiometry could be a game changer in gold exploration under cover in Victoria.
How do you think airborne gravity gradiometry could advance your exploration project?
You are welcome to contact Nordic Geoscience to discuss how you can extract maximum value from airborne gravity gradiometry data.
Thanks for reading, stay at home and stay safe!
-Asbjorn
Asbjorn Norlund Christensen is a consulting geophysicist at Nordic Geoscience, a bespoke geoscience consultancy in exploration geophysics and data science – www.nordicgeoscience.com