DATE: Thursday 3rd August 1978 at 4.30pm
VENUE: 2nd floor, Science Centre,
35 Clarence Street, Sydney
SPEAKER: Murray Rogers, Supervising Geologist (Tasmania),
TOPIC: “King Island Scheelite”
Notes by M. W. (Mike) Brown – Kennecott Explorations
Murray quoted only available reserve data i.e. 7 million tonnes at 0.8% WO3.
Current production is 350,000 tonnes at 0.8% WO3 from two ore bodies viz. Dolphin (60%) and Bold Head (40%).
Presentation was ‘slide based’ and covered geology of the deposit as well as mentioning some aspects of the mining method and current rock mechanics studies.
Scheelite mineralization is disseminated (ave. 0.05mm – 0.2mm) within andradite (Ca2 Fe2 Si2O12) garnet skarns which occur within the metamorphosed Grassy Group (i.e. “Mine Series”). Murray recognises two metamorphic (metasomatic) events:
(i) Formation of marble by outward movement of Ca and introduction of Fe, Si and Al and;
(ii) Intrusion of solutions into marble, formation of andradite skarn and deposition of scheelite.
Mineralised fluids are believed to have originated from a granitic (adamellite) intrusive stock. Major faulting provided channelways for ore-forming solutions. The tungsten content of the sphene in the adamellite is 150 ppm which is almost ten times greater than average.
Mineralising solutions were probably alkaline and travelled some distance away from the granite to incur low pH conditions necessary for deposition. The pressure, temperature, salinity and chemical aspects of the deposit have been and are the subject of studies and many questions are not yet resolved.
Average Mo content of the deposit is 0.04% – mainly in powellite (Ca Mo WO4) In response to a question Murray said that the Kara Deposit (Tasminex) was similar to King Island. At Kara a granitoid has intruded a sandstone with interbedded ‘limey’ units.
Murray mentioned soil geochem over the deposits and at the end of the talk I asked him about the grid size and anomalous values. He indicated a spacing of 100 x 25 metres and said that W values >30 ppm were considered ‘good but up to 300 ppm was recorded over the orebodies. Murray was not keen to discuss the use of pathfinders, but mentioned Mn as one which may prove useful in future work.
I also asked about optimal distance from the ‘granite’ for deposition to which he replied “50 to 200 metres”.
..R Cox………..JF Gilfillan………GG Lowder………..MJ Turbott..