Geology, ore deposits and applied rock geochemistry of the Buchans Group

Thurlow, J. Geoffrey (1981) Geology, ore deposits and applied rock geochemistry of the Buchans Group. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Buchans Group is a regionally extensive, submarine, calc-alkaline volcanic suite erupted at a point in time near the Ordovician-Silurian boundary. It is subdivided into Lower and Upper Subgroups, each of which consists of interbedded mafic to felsic flows, breccias, pyroclastics and related sediments. The Lower Subgroup, which contains the economic ores, consists of four volcanic formations which have been repeated structurally by thrust faults, forming seven major volcanic sequences. The economically barren Upper Subgroup is subdivided into three major sequences. -- Plutonic rocks associated with the Buchans Group include the Feeder Granodiorite, Hungry Mountain Complex and the Topsails Granite. The earliest of these, the Feeder Granodiorite, is considered to be a time-equivalent plutonic facies of the Buchans Group. Deformed plutonic facies of the Hungry Mountain Complex were tectonically emplaced upon the Buchans Group during a major episode of southeast-directed Silurian thrusting. The Siluro-Devonian Topsails Granite intrudes all older formations and, in the Buchans area, consists of two main phases: a high level alkali-feldspar phase and a coarser grained equigranular peralkaline phase. Gently dipping alkali amphibole-bearing ignimbrite forms roof pendants within the Topsails Granite. These are interpreted as extrusive equivalents of the Granite and may be outliers of the Springdale Group -- The Buchans ores are high grade polymetallic massive sulphide deposits associated with submarine felsic volcanic rocks near the top of the Lower Buchans Subgroup. Three genetically related ore types are recognized; stock-work ore, in situ ore, and mechanically transported ore. Stockwork ore consists of networks of veinlet and disseminated base metal sulphides within highly silicified and locally chloritized host rocks. In situ ore lies stratigraphically above stockwork mineralization and is composed of banded to streaky yellow ore, black ore and barite. Transported ore consists of discrete sulphide and barite fragments in breccias which flank in situ ores and form substantial distinct orebodies. Their origin is interpreted as a result of brecciation at source, transport down-slope in paleotopographic channels by gravity-driven submarine debris flows, and deposition in depressions. The Buchans ores and their environment are similar to those of the Japanese Kuroko deposits in almost all respects. -- Whole-rock chemical analyses have shown that Buchans Group volcanic rocks are similar in chemical abundances and variations to other calc-alkaline volcanic suites. There was apparently little chemical evolution in Buchans Group flows through the stratigraphic column, and flows within the ore horizon sequences are essentially similar to their counterparts elsewhere in the Buchans Group. Pyroclastic rocks of the ore horizon are chemically similar to those at other stratigraphic horizons and show no anomaly related to mineralization with the exception of high Ba content. -- Study of the unaltered pyroclastic rocks of the ore horizon has shown that there are no regional variations in element concentrations, element ratios, discriminant scores or factor scores, either along or across stratigraphy, which bear relationship to mineralization. Erratic base metal concentrations are present in host pyroclastic rocks only within 50 m of ore. Visibly altered footwall rocks have undergone chemical changes typical of rocks in most massive sulphide terranes. -- Anomalously high concentrations of Ba in ore horizon pyroclastics is a regional feature which provides a means of distinguishing ore horizon pyroclastics from similar lithologies at other stratigraphic horizons. The Ba anomaly is caused by microscopic barite which was widely distributed by subaqueous pyroclastic flows which transported most ore horizon pyroclastics.

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