VMS DEPOSITS OF THE ORDOVICIAN ROBERT'S ARM GROUP, NEWFOUNDLAND APPALACHIANS : DIFFERENT INTERPRETATIONS AND NEW PROBLEMS

The northern part of the ca. 470 Ma Robert's Arm Group consists of five basalt-dominated tectonic terranes (s.l.), separated by steepened, south to southeast-dipping, north-directed thrust faults. The structurally lowermost Boot Harbour terrane also contains well-preserved submarine felsic volcanic rocks, possibly erupted under deep marine conditions. The structurally overlying Pilley's Island terrane contains distinct felsic rocks associated with significant VMS mineralization and alteration. Structurally overlying Mud Pond terrane basalts have a regional hematite (± epidote) alteration signature. These are in turn overlain by the Triton terrane, comprising fresh basalts and mafic intrusive rocks. The mutual relationships of these four calcalkaline terranes, and to the tholeiitic rocks of the adjacent Crescent terrane, are uncertain, but the Robert's Arm Group may be the disrupted, lower limb of a northward-overturned anticlinal nappe structure. The Silurian Springdale Group rests unconformably upon three different Robert's Arm Group terranes, suggesting some pre-430 Ma deformation and imbrication, and earliest deformational events took place ca. 470 Ma, soon after deposition. Silurian-age deformation was also regionally important.

VMS deposits in the Pilley's Island terrane have been extensively disrupted by thrusting and the "original" facies relationships proposed in earlier studies are largely illusory. Mineralization is dominated by Cu-rich pyritic bodies, but there are indications of high-grade polymetallic "transported" ores akin to those of the Buchans camp. Exploration has now identified blind, low-grade VMS deposits of considerable size. The complex structure results in repetition of ore- horizon rocks at depth, and these are targets for continued exploration; however, many outstanding stratigraphic and structural problems still remain to be addressed.