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Basic Intrusions at Yudanamutana. 



Turning now to the north-western margin of the igneous and metamorphic 

 complex forming the Mount Painter belt, there is, flanking it, in the neighbour- 

 hood of Yudanamutana, a massive quartzite formation extending far on either 

 hand as the dominant feature in the physiographic relief. It passes just south 

 and east of the township of Yudanamutana, and was noted to extend west to 

 the vicinity of the Wheel Ellen Mine and east to the high land block known 

 as Freeling Heights. 



A splendid section is exposed in the gorge cut through it by the stream 

 which leads from the township southward. In its broader features, this quartzite 

 formation resembles the great quartzite of the Bolla-Bollana Gorge on the 

 opposite side of the Mount Painter belt, but there are yet many points that need 

 reconciling before their identity can be definitely asserted. The evidence is, 

 however, strongly in favour of their identity, and, further, indicates that this 

 quartzite and boulder bed (for in places beds of massed boulders appear in the 

 formation) is the basal stage of a sedimentation cycle postdating the highly 

 metamorphic intervening belt. 



Along the outcrop to the south-west, in the vicinity of the Wheel Ellen 

 Mine, a section of the beds passes down from tillite and certain other associated 

 boulder beds in which practically all the boulders are quartzite, a formation which 

 outcrops at Red Hill, through slaty beds which are in part calcareous and 

 dolomitic to the quartzite and grit beds of the ridge at the Wheel Ellen Mine 

 itself. This formation and some associated softer bands beneath it rest upon 

 acid schistose and gneissic rocks, amongst which is a typical augen gneiss similar 

 in general features to that well-known example near the south-west extremity 

 of the main outcrop of the Broken Hill lode. In this area, below the quartzite, 

 many quartzose reefs occur crossing the formations ; these always contain much 

 micaceous iron and ilmenite. The overlying beds have been considerably meta- 

 morphosed, for example, actinolite is noticeably developed in the quartzite and in 

 calcareous beds higher up. 



At Yudanamutana there is clear evidence of basic igneous activity post- 

 dating the great quartzite formation to which reference has just been made. 

 Slaty and calcareous rocks overlying the quartzite are intruded by doleritic 

 basic rocks with the accompaniment of considerable metamorphism producing 

 hornstones and spotted schists, and causing the development of tremolite and 

 actinolite in the more calcareous and dolomitic bands, which also are the matrices 

 for the main depositions of copper ores formerly mined in this locality. It is 

 interesting to note that some fine specimens of massive scheelite were got in 

 association with the copper ores in actinolite rock at the smelter site. The 

 copper was obviously introduced in association with the basic igneous rock. 



The ore depositions are as irregular patches both in the intruded rocks and 

 in phases of the igneous rock itself. The principal primary minerals thus repre- 

 sented are iron and copper pyrites and micaceous hematite. A pinnacled out- 

 crop to the north of the township known as the Cockscomb is constituted of 

 several large, parallel ferriferous reefs of this kind. There they are principally 

 micaceous iron and jasperoid rock with some vesicular gossan, the latter 

 indicating a weathered and removed pyritic content. 



A specimen of an altered diabase from an intrusion at Yudanamutana has 

 the following characters : — The hard specimen appears as a dense faintly 

 greenish, dark-grey rock in which no porphyritic constituents are visible. Under 

 the microscope a definite doleritic intersertal structure is preserved by the 

 plagioclases which are still clear, but carry abundant inclusions of actinolite 

 rods, minute biotite flakes, and grains of magnetite. The section is loaded with 

 bundles of secondary diablastic actinolite. 



