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Altered rhyolites that host the Archean Kidd Creek volcanogenic massive sulfide deposit have constant Hf-Th-Ta ratios throughout the entire deposit. Such constant ratios suggest that footwall and immediate hanging-wall rhyolites represent a single batch of high silica rhyolitic magma and that fractionation of accessory minerals was suppressed during crystallization.

Although extensive alteration has destroyed most primary minerals in the rhyolites and disturbed their geochemistry, high field strength elements such as Th, Ta, and Hf remained immobile during alteration and may be used to identify the protoliths and to determine the original trace element geochemistry of even the most altered rocks. Quantitative estimation of rare earth element (REE) mobility in the rhyo-lites demonstrates significant mobility, particularly in the highly altered stringer zone, in the bornite zone, and in the southernmost part of the deposit in rhyolite fragmentals. Nearly quantitative removal of light REE occurred in highly silicified rhyolite immediately underneath the massive sulfides. Furthermore, REE mobility was identified up to a distance of 2 km from the deposit.

Mafic rocks at Kidd Creek, excluding footwall komatiite flows and plagioclase porphyritic pillow basalts in the uppermost hanging wall, can be separated into four groups. This classification is consistent with stratigraphic position and general field appearances of the mafic rocks. Group I consists of primitive low TiO2 basaltic flows in the footwall of the distal Kidd Creek horizon and is characterized by concave-upward chondrite-normalized REE patterns. Group II consists of relatively low TiO2 pillow basalts and associated gabbro sills in the immediate hanging wall. This group includes the “spotted gabbro sills” at the mine and is characterized by midocean ridge basaltlike, slightly light REE-depleted patterns. Group III consists of high TiO2 gabbro sills that intrude both the hanging wall and footwall of the deposit and have slightly light REE-enriched patterns. Group IV represents more evolved gabbro to diorite sills and could be related to group III. The groups have distinct and relatively undisturbed REE patterns. The REE sys-tematics and incompatible element ratios such as Th/Yb versus Ta/Yb indicate that groups I, II, and III are unlikely to be related by fractional crystallization. Different parental magmas are implicated.

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