Hello Valuepro
If I may, by way of a bit of data I will augment Will’s reply.
The following is the Brodeur Penninsula geology into which the JI kimberlites were emplaced:
GEOLOGY AND PHYSIOGRAPHY
Flat-lying Ordovician and Silurian carbonates are exposed along the steep coastline of the Brodeur Peninsula and in the deeply incised river gorges. Between these river gorges, the land surface is an undulating plateau with elevations in the 250 to 500 m range. Except at the crests of some hills, a thick blanket of glacial till was deposited by a small ice cap centred on the peninsula during the last glaciation and beyond the northern limit of the continental glacier (Laurentide Ice Sheet) which covered much of Canada.
See Geological Survey of Canada, Terrain Sciences Division Internet Site
sts.gsc.nrcan.gc.ca
Although isolated gneissic floats and erratics provide evidence of an earlier much more extensive glaciation, the till consists mainly of carbonate blocks in a matrix of pulverized carbonate. It is grey or light brown, contains few nutrients and supports very sparse, if any, vegetation. Annual rainfall is low. The landscape in summer has the appearance of a brown, undulating and barren desert.
The following site provides some description of climatic and geological conditions existing at the time of the deposit of these limestones.
ucmp.berkeley.edu
To my knowledge, the emplacement ages of JI kimberlites have not as yet been published, but pipes on Sommersett Island 100 km to the northwest and other pipes on the Brodeur Peninsula have been dated to around 400-410ma (Silurian-Devonian). If JI pipes were intruded at the same time, it is possible that they were emplaced while under water.
The following TWG News Release references and drill logs, indicate that at a minimum, the upper 100 metres of the Freightrain pipe is at least in part crater facies meaning kimberlite was blasted out of the crater and subsequently fell back and washed back into the crater. That reconsolidation, presumably subject to earth tremors and possibly wave action, would naturally result in some preferential redistribution and settlement according to mass. In short, to some degree, larger heavier diamonds would have tended to slide down any scree slopes and settle within the loose pyroclastic sediment until compaction and reconsolidation limited such sifting action.
October 31, 2001
The kimberlite logged at Freightrain is consistent with previous petrographic studies by Lakefield Research Limited ("Lakefield") which report both crater and diatreme facies textures.
Large blocks of xenoliths are not unusual in large pipes and have been reported e.g. in the Premier kimberlite mined by DeBeers in South Africa (Diamond Geology - published on www.debeersgroup.com).
December 20, 2000
Lakefield also investigated the kimberlite by microscope examination of thin sections. A pyroclastic texture suggests crater facies material and related mineral assemblage. These factors suggest shallow erosion, the possibility of pipes linked below surface and a large potential tonnage within the 10 - 15 ha cluster.
An 18.41 tonne mini-bulk surface sample taken from Freightrain last spring produced an implied grade of 0.198 c/t but as you will note from the NR excerpt below, previous smaller grab samples of 560kg and 21kg produced implied grades of 1.87 c/t and 1.11 c/t respectively.
May 10, 2001
Under the supervision of Twin Mining’s Director of Diamond Mining - Dallas Davis a 15 tonne sample is being recovered from sample site #1 and a 3 tonne sample is planned to be taken from sample site #2. (See press release of December 20, 2000, there designated as pipe #2 and pipe #5). On these sites previous samples of kimberlite of 560kg and 21kg respectively have yielded calculated grades of 1.87 and 1.11 carats per tonne (cpt) and diamond indicator mineral values for diamond inclusion chromite and G10 garnets were 68.5% and 47.3% as well as 59.0% and 76.8% respectively.
On a large pipe like Freightrain, all of these samples, including the 18 tonne sample, are too small to draw any meaningful conclusions as to pipe grade but what they do demonstrate is that grade is variable, especially at surface where erosion appears to have been limited and may vary depending on whether crater or diatreme facies was sampled, whether that area of crater facies was subject to preferential settlement of diamonds after emplacement, whether the area sampled is one phase of a multi-emplacement diatreme and whether there has been simple asymmetrical distribution of mantle xenocrysts within the kimberlite matrix.
As you can see from the following TWG drill logs (Oct 31, 2001 NR) and summary, there is significant variation in the nature of the logged kimberlite and the sizes of xenocrysts and xenoliths within the kimberlite matrix. The usual enrichment pattern is for eclogitic diamonds to be typically found within mantle xenocrysts while peridotitic diamonds are typically found in the kimberlite matrix. Emplacement sorting can vary how and where these might be concentrated.
summary_kimberlite_logs_311001.htm
October 31, 2001
It contains an important volume of autoliths, varying in size of up to 25mm. Many holes recovered mantle xenoliths up to 70mm in diameter, containing pyroxene and olivine assemblages characteristic of lherzolite and hartzburgite. Key indicator minerals such as pyrope garnets, diopside, phlogopite and spinel are common, olivine macrocrysts (up to 70mm) and serpentine nodules (up to 120mm) were noted to occur throughout the Freightrain kimberlite drill core.
Finally, many of the later samples were taken using NQ (47mm diameter) core holes which are very small. This could have been responsible for shattering some diamonds, it could have through it narrow bite and shear chance, completely missed diamonds and would certainly be less likely to capture larger diamonds intact than would a larger RC drill.
So yes, as Will indicated, diamond grade could improve with depth (but also with larger sampling) at least up to the diatreme cap/face and possibly below. This is more likely to be the case in kimberlites emplaced under shallow seas, multi-phase kimberlites, in crater facies and larger less homogenous pipes such as Freightrain than smaller ones which are more likely to have been emplaced as single not-multi-phase eruptions.
Finally, if any one has not seen the plan view of Freightrain, a magnetic plan of the pipe can be viewed through a link at the bottom of the Oct 21, 2001 NR. PlanFreightrain311001.htm
You can see two distinct lobes along a diagonal axis, which may be multiple phases of the intrusion, or simply the only perimeter outcrops of a much larger diatreme not screened by the limestone rafting.
Regards
Vaughn |
