118 PROCEEDINGS OF THE ROYAL SOCIETY OP QUEENSLAND. 



In North Queensland ultra-basic rocks are abundant from 

 Almaden to Charleston and Georgetown. As in the Territory, 

 they are older than the granites. 



Chlorific Rocks and Schists. — Throughout the Northern 

 Territory and North Queensland tin areas chloritic scliists and 

 chloritic rocks are of very frequent occurrence, and are 

 extremely favourable host rocks for tin deposition. They 

 are frequently topazised in the vicinity of the lodes. In the 

 Territory we have these rocks at Hidden Valley, Horseshoe 

 Creek, Mary River tin field, Mount Todd, &c. We have at 

 Maranboy a closely allied rock, but so intensely silicified as to 

 be akin to homfels. In Queensland they are the dominant 

 rocks in the Koorboora and Irvinebank districts. In the 

 Territory the '" chlorite " areas are of late pre-Cambrian age, 

 metamorphosed by the early Cambrian granites. In North 

 Queensland they are usually regarded as coeval with the Silurian 

 Cliillagoe series*. The chlorite schists are unfossiliferous and 

 possess the textural characteristics of metamorphosed volcanic 

 tuffs. 



The writer has observed that the period of granite intru- 

 sion, which was pre-Cambrian in the Darwin district, was 

 continued into the Cambrian in the McArthur district (Yah 

 Yah Creek) ; and possibly that would also be its age in the 

 Mount Oxide region (Burke). It is quite possible that the 

 volcanic x^eriod that produced the tuffs was an early manifes- 

 tation of the igneous activity which later led to the " Older 

 Porphyry " intrusions, and occurred progressively later as it 

 made east. Thus the Koorboora-Irvinebank series may 

 easily be as late as Ordovician, and the granites here 

 Carboniferous . 



The formation of chlorite points to tuffs having been 

 covered with some thousands of feet of sediment and depressed 

 into the Upper-Middle zone of the earth's crust prior to the 

 igneous and hydato-igneous activity. The same rock types 

 more intensely metamorphosed in a lower zone, where minerals 

 of an anhydrous nature would form, become magnetite- 

 hematite-quartz homfels. This is a frequent type at Maranboy 

 (Northern Territory). 



* I have, however, come to the conclusion that the North Queens- 

 land chlorite schists are older than the Chillagoe series and probably 

 of Orvodician age. 



