Metamorphic history of the Hemlo gold deposit from Al₂SiO₅ mineral assemblages with implications for the timing of mineralization 

Powell, W.G., Pattison, D.R.M., and Johnston, P.

1999, Canadian Journal of Earth Sciences, v. 36, pp. 33-46.

The Hemlo deposit is associated with an envelope of feldspathic alteration and coincident fracture-controlled aluminous alteration, now characterized by the common occurrence of aluminosilicate minerals. The mineral assemblages associated with this alteration are zoned from Ky-Sil-St-Gt at the margins to only Ky-Sil in more intensely altered rock.

All three aluminosilicate polymorphs are present at Hemlo. Fibrolitic sillimanite is the youngest form, and clearly overprints both kyanite and andalusite. Prismatic kyanite occurs in folded quartz veinlets, along with staurolite and garnet, and also occurs in the alteration selvages of andalusite-bearing quartz veins. These boudinaged quartz veins contain deformed andalusite that has been extensively replaced by muscovite, and commonly contains inclusions of corroded coarse-grained kyanite occurs. Clearly the andalusite post-dates the kyanite inclusions. However, the andalusite is highly altered and deformed relative to kyanite found in the vein selvages. These textures indicate a complex history of aluminosilicate growth within the deposit. From youngest to oldest these aluminosilicate events are: 1) growth of kyanite in quartz veins, 2) overgrowth of kyanite by andalusite in quartz veins, 3) a second growth of kyanite, this time associated with biotite and staurolite or garnet, and 4) growth of fibrolitic sillimanite.

These minerals developed through polymetamorphism of the deposit. Early kyanite and andalusite-bearing assemblages must have developed during low-pressure contact metamorphism (approximately 3 kbar), possibly associated with the regional suite of granodiorite plutons (ca. 2688 Ma). Kyanite-staurolite-bearing assemblages developed during peak regional metamorphism. During isothermal uplift, localized fluid flow resulted in the late development of fibrolitic sillimanite.

The fact that aluminous alteration associated with the Hemlo deposit has experienced peak regional metamorphic conditions indicates that mineralization must predate, or be coeval with, regional metamorphism and deformation. Furthermore, the presence of pre-regional metamorphic andalusite within the altered rocks of the Hemlo deposit adds supports to models proposing pre-metamorphic mineralization.