An integrated geological, geochemical, isotopic and geochronological study on the auriferous systems in the Botwood Basin and environs, central Newfoundland

The purpose of this project was to study and compare 20 gold occurrences from within the Botwood Basin and surrounding lithologies. The Botwood Basin is located within the eastern Dunnage Zone of central Newfoundland and encompasses a region comprised of Middle Paleozoic cover sequences deposited upon dominantly Ordovician rocks. The area has been subdivided into two tectonostratigraphic belts that are separated by a major fault (the Dog Bay Line), which has been defined as a major Silurian terrane boundary. The Indian Islands Belt to the southeast of the fault encompasses the deep to shallow marine Davidsville and Indian Islands groups. The Botwood Belt to the northwest of the fault includes the shallow marine to terrestrial Botwood and Badger groups. The large bimodal Mount Peyton Intrusive Suite (MPIS) intruded some of these units during the Late Silurian to Early Devonian in the central Botwood Basin. The MPIS is a composite post kinematic intrusion consisting mainly of gabbro and granite with minor tonalite, diorite and granodiorite phases. The relationship between the gabbro and granite phases, as well as the relationship of the suite to the surrounding sedimentary lithologies, is still poorly understood due to the lack of contact exposures. The auriferous occurrences occur dominantly within the Ordovician Davidsville and Silurian Indian Islands groups and also within intrusive units throughout the region. A key question to be answered was whether regional intrusive suites (granitic to gabbroic) were key components of the ore-forming systems, acting as heat sources driving ore fluids, or just as rheologically contrasting host lithologies. -- Reconnaissance mapping confirmed the presence of Indian Island Group rocks to the southeast and resulted in the discovery of new fossiliferous outcrops in the north. These latter localities, at Duder Lake and east of Ten Mile Lake, are significant to regional stratigraphy as they contain Wenlock fossils that are characteristic of the Indian Islands Group. This extends the group to the northwest of the current location of the Dog Bay Line. -- Mapping observations along the eastern margin of the MPIS define the relationship between the felsic and mafic phases of the intrusive suite, as well as the relationship of the suite to the Indian Islands Group. Field relationships suggest that the granite is younger than the gabbro; these include the gabbro's lack of chilled margins and gabbro pieces stoped by the granite suggesting the gabbro was cooled prior to the granite intrusion. Immediately west of Glenwood, a dioritic dyke, which has been correlated with the MPIS, intrudes the Indian Island Group sedimentary rocks. Along Red Rock Brook a faulted relationship is inferred between the granite phase of the MPIS and the Indian Islands Group, and thus, it is possible that the relationship is a fault-modified intrusive contact. -- There are different gabbroic intrusive suites in the region as defined by whole-rock geochemistry. Mafic intrusive dykes to the north of the Trans Canada Highway (TCH) are petrographically and somewhat geochemically similar to the mafics of the MPIS. However, these dykes contain slightly higher Ti and less SiO₂ contents. This may indicate that the dykes are fractionated equivalents of the MPIS, originated from a separate but similar magma source. The intrusive dykes at Duder Lake are petrographically and geochemically distinct from all intrusive bodies examined to the south inclusive of the dykes north of the TCH, the MPIS and the Paul's Pond intrusives. The data also indicate that there is an intermediate phase to the 'bimodal' MPIS. The sedimentary sequences display subtle differences in trace element contents between groups and these differences may be dependant on their locations in the region. -- There are wide ranges in sulphur isotope ratios for sulphide mineral separates from different occurrences and the dominant control appears to be the lithological source of the sulphur. That is, occurrences within deep marine sedimentary lithologies are negative in terms of δ³⁴S (‰). Occurrences in proximity to intrusive suites are near 0 ‰. Occurrences in which S was derived from igneous rocks have ratios that are slightly to moderately positive in terms of δ³⁴S (‰). -- Trace element compositions of pyrite suggest that different auriferous deposit types have recognizable signatures. For example, several of the pyrite grains from the Mustang Prospect contained the 'toxic suite of elements' characteristic of Carlin-type pyrite. Some pyrite from the Stog'er Tight and Hurricane Prospects contained elevated W and Te recognized in orogenic lode gold occurrences. Pyrite from the Bruce Pond Epithermal Prospect resembles that from low-sulphidation epithermal types of occurrences. -- Geochronological data indicate that magmatism in the central to northern Botwood Basin was episodic from the Middle Silurian to Early Devonian. Age data also show that the granitic phase of the MPIS is younger than the gabbroic phase as (LAM-ICP-MS) U-Pb ages of ca. 430 Ma and 410 Ma were obtained for the diorite and granite phases of the MPIS, respectively. An intrusive gabbroic dyke to the north of the MPIS and the Charles Cove granodiorite were also dated at ca. 430 Ma suggesting that at least some of the magmatism in the northern Botwood Basin corresponded with MPIS magmatism. Thus, the magma may have been more deep-seated and widespread than previously thought. With the recognition that the MPIS granite intruded in Early Devonian time, it is more feasible to recognize the MPIS as a possible heat source for mineralization, at least along its eastern margin. The 494 ± 7 Ma age for the Huxter Lane intrusive in the Botwood Basin basement reveals that auriferous mineralization in the region spans a significant geological time span (494 to 380 Ma). -- Inherited zircons were common in both the MPIS and Charles Cove Pluton granites suggesting that Botwood Basin granitoids may have been generated through crustal anatexis of lower crustal material by mantle-derived gabbroic melts. The age of inheritance in both intrusions of ca. 1850-1800 Ma suggests that the melts may have sampled either Indian Islands Group rocks, or rocks that were their detritus source, as detrital components from that Group indicate a source region of a similar (1850 Ma) age. -- There are several auriferous mineralization styles present in the Botwood Basin including epithermal, orogenic and perhaps Carlin-type. Although these occurrences display similarities in terms of mineralization characteristics, the tectonic and geological disparities throughout the region have led to differences between the occurrences. Thus, a generic model for 'Botwood Basin-type' auriferous mineralization is unrealistic.

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