Canadian Minerologist Vol. 23, pp. 619634 (1985) MINERALOGY AND PETROLOGY OF THE HAM KIMBERLITE, SOMERSET ISLAND, NORTHWEST TERRITORIES BRUCE C. JAGO AND ROGER H. MITCHELL Deportment of Geologlt, Lakehead University, Thunder Bay, Ontario p7B 5E1 AsstRAcr The Ham diatreme and dyke form separate fracture- controlled post-Silurian intrusive bodies in uorth-central Somerset Island, Northwest Territories, and consist of hypabyssalmacrocrystalline serpentine-calcite kimberlite and kimberlite breccia.Alteration of previously consoli- dated kimberlite by fluids that deposited serpentine+ cal- cite resulted in the development of modified kimberlites whose magnetic siguature differentiates them from the unaltered varieties.Compositional data areprovided for olivine, garnet and phlogopite macrocrystals. The spinel ranges in composition from aluminousmagnesian chromite to magnesian ulvdspinel-magnetite. Serpentine exhibits extensivecompositional variation; lizardite - I T is deve- loped in serpophitic segregations and retrograde olivine replacements,whereaslizardite - 6ll occurs in prograde deutericreplacement and recrystallization.Ruby xenocrysts have been observed. Keywords: lomberlite, magneticexpression, garnet, spinel, serp€ntine, ruby, Somerset Island, Northwest Terri- tories. SoMMAIRE Le diatrbme et le filon de Ham, dans la partie centre- nord de l'lle de Somerset (Territoires du Nord-Ouest), onr 6t€ mis en place le long d,un systbme de fissures post- silurien. Ce sont des massifs hypabyssaux de kimberlite macrocristalline i serpentineet calcite, et de brdche kim- berlitique. La kimberlite ddjd consolid€e,modifi6e 9a et ld par desfluides satures etrserpentine et calcite, possdde une signaturemagl€tique qui la distingue de la roche saine.On 6tablit la composition chimique de macrocristaux d'olivine, {e grenat et de phlogopite. Le spinelle passe, en composi- tion, d'une chromite alumineuse et magndsienne I un ulvdspinelle-magndtite maFdsien. La serpentinemontre une grande variation dans sa composition; la lizardite-l !n estpr6sente dansles s€grdgations serpophitiques et lespseu- domorphoses rdtrogrades d,olivine, tandis que la lizardite-61/ est caract6ristiquede la serpentine de rempla- cement deutdrique prograde et de recristallisation. On trouve aussi du rubis en xdnocristaux. ...i (Traduit par la R6daction) Mots-clds: kimberlite, expressibn magndtique,grenat, sp! nelle,serpentine, rubis, ile de Somerset, Territoiresdu Nord-Ouest. INTRoDUCTION The Ham diatreme and dyke, the most northerly known kimberlites of the Somerset Island kimber- Iite province Mitchell 1976), are exposedas frost- heavedregolith within the headwaters of the Cun- ningham River in north-central Somerset Island, NorthwestTerritories(Fig. l). The intrusivebodies occur in Ordovician limestone and are believed to be post-late Silurian in age, but to predate Cenozoic volcanic activity associated with Eurekan rifting (Mitchell 1975,Mitchell & Platt 1984). Frcrn REI-etroNs Figures 2 and 3 illustrate the geologyand mapetic expression of the Ham diatreme and dyke. The flanks of the diatreme consist of highly magnetic (maximum 6800 -y abovebackground)Type-lA kim- berlite and enclosea weakly magnetic core (maxi- mum 600 ? above background) of Type-lB and Type-2kimberlite. The highermagnetic response of Type-lA kimberlile may be attributed to its relatively unalterednature. In addition, Type-lA kimberlite contain$ a largerproportion of both primary, mag- netic, iron-bearing oxidesand secondary magnetite produced by serpentinizationthan doesType-2 kim- berlite. A roughly lenticular (75 x l0 m) magnetic anomaly (maximum 400 7 above background) is associated with frost-heaved regolith abovethe Ham dyke. The symmetricaland uncomplicatednature of the anomaly, together with petrographic and field evidence,indicates that the Ham dyke is a vertical, uniformly magnetized, single-phase, unalteredintru- sive body that is similar to the Ham diatreme of Type-lA kimberlite. Geophysical reconnaissance between the Ham diatreme and dyke does not indi- cate a near-srufacelink betweenthe two bodies at the present levelof erosion;therefore,agerelations cannot be established. Structural control of the diatreme and dyke in rela- tion to two intersectingfractures can be inferred from Figure 2. The intersection of these two frac- tures (50" and 125") coincides with the bulk ofthe Ham diatreme and a zone of intense alteration. 6t9