Ultramafic Dykes Research Articles

Postcratonization mafic and ultramafic dykes of the Yilgarn Block

Postcratonization intrusions in the Yilgarn Block of Western Australia are predominantly dykes with high length/width ratios and sharp contacts with minimal thermal metamorphism of country rock. Dyke frequency and number of relative age relationships increase towards the exposed margins of the Yilgarn Block. Dykes in the northern, southern and western margins of the Yilgarn Block are mafic, ranging from gabbro to magnetite‐rich leucodolerite, and have apparently been intruded over a long time interval in response to periodic reactivation of the tectonically active craton margins. Dykes in the central Yilgarn Block range from porphyritic olivine picrite to magnetite‐rich quartz dolerite and display a spectrum of chemical compositions with an overall trend of tholeiitic iron‐enrichment. The concentration of both Archaean and Proterozoic rocks of high‐Mg nature in the central Yilgarn Block is suggestive of a fundamental control, perhaps in the mantle source area, and also indicates that ultramafic magmas wer...

Magnesite abundance as a guide to gold mineralization associated with ultramafic flows, Timmins area

Altered komatiitic flows of the Tisdale Group locally contain secondary calcite, dolomite or magnesite, where alteration intensity and bulk-rock composition are favourable. Talc is present only in the least carbonatized samples and does not coexist with quartz and magnesite. Magnesite is present only in carbonatized ultramafic komatiitic flows (> 20 wt.% MgO) containing at least 18 wt.% loss on ignition (> 14 wt.% CO 2). Magnesite abundance increases with increasing loss on ignition and MgO (MgO + CaO) ) whole-rock ratio. Altered flows having magnesite to dolomite weight precent ratios exceeding 70 occur in proximity to mineable concentrations of gold and define surface-exploration targets 500 m in diameter. The spatial association of gold with the more magnesian ultramafic komatiitic flows arises in part because of the reactive nature of olivine-rich rocks in the presence of CO 2-bearing aqueous hydrothermal fluids, and possibly because the most magnesian ultramafic flows occur in proximity to an eruptive vent area where the likelihood of rock-fluid interaction is greatest because of the local structural regime. Talc-magnesite—quartz assemblages are present in some ultramafic dykes which cut the older Deloro Group volcanic rocks; the apparent absence of gold in these rocks may be attributed to the higher formation temperature of the talc-bearing, relative to the talc-free, magnesite-quartz assemblages.

Ultramafic dykes on the northern Skagerrak coast of Sweden

Abstract Ultramafic dykes associated with dolerites and rhomb porphyries along the northern Skagerrak coast of Sweden seem closely related with the igneous activity in the Oslo Rift zone. On the basis of field evidence, petrography and geochemistry, a cumulus origin of the ultramafic dykes is suggested.

Discussion

J. E. M. W hite said that with reference to the eastern contact zone, a photograph was presented showing inclusions of hornfelsed gabbro that are apparently undisplaced, in a structural sense, in the ultramafic host; indeed, they literally seem to be suspended within the latter. Could this anomaly be conceivably resolved through recourse to a model involving either selective assimilation or fluidization? Regarding the fine grained late ultramafic dykes cutting both the intrusion and the eastern contact aureole, you observed that an amphibole is present and as these veins are widespread, despite their generally aphanitic texture, could it be possible that a rather high water content would have greatly enhanced the fluidity of this particular magma? Moreover, there may be some interrelation between the formation of the hornblendite facies in the comparable ultrabasic plutons of SE Alaska and the development of the amphibole in the late dykes of the Reinfjord complex. Finally, within this context of the dyke magmas, can you account for the increasing basicity of the main intrusion as a whole, a trend which ultimately culminated in the emplacement of the central core of dunite? Could a progressive melting of the upper mantle be involved here as has been envisaged for the se Alaskan analogues? Dr B. E. Leake supported the author’s interpretation. The sequence of intrusions, commencing with gabbro and culminating in a dunite core was extremely difficult to explain by crystallization from one magma unless the ultramafic rocks were emplaced in their present position as already crystallized

The geology of two small layered hornblende peridotite (picrite) plutons in South Greenland

Two ultramafic dykes, which pinch and swell both vertically and horizontally and have a form similar to some kimberlite bodies, were intruded 1700 to 1335 m.y. ago during the 2nd episode of Ketilidian plutonism. Contact metamorphism of the adjacent granite was slight -albite-epidote hornfels facies. Petrological, mineralogical and chemical evidence indicates that the parent magma was picritic in composition, that it underwent flowage differentiation as a result of rapid intrusion, and then magmatic differentiation with gravity settling. The internal picritic (chemical classification) layered group crystallised first and includes hornblende peridotites, hornblende-hypersthene peridotites and rare harzburgite. The border group which crystallised later is formed of hypersthene-olivine hornblendites that chemically are picrites, and mica hornblendites that chemically are monzonites with affinities to kentallenite. Hornblende gabbro is a minor late differentiate. Steeply dipping banding cuts the earlier rhythmic layering and is thought to have formed by diffusion of material into cooling cracks developed prior to complete solidification. There are local concentrations of sulphides and platinoids. Three complete silicate analyses and a number of analyses for the platinoids, Au, Cr, Co, Cu, Ni, Ag, Pb and Zn are presented.