The Late Cretaceous diamondiferous pyroclastic kimberlites from the Fort à la Corne (FALC) field, Saskatchewan craton, Canada: Petrology, geochemistry and genesis

February 27th, 2017  Posted by jmwarnsloh

The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to − 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12–15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6–0.7 Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3–2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.

Keywords: Fort à la Corne; Kimberlite; Laurentia; Rodinia; Saskatchewan craton

DOI:dx.doi.org/10.1016/j.gr.2016.12.013

Petrography

February 17th, 2016  Posted by jmwarnsloh

Petrogaphy is the detailed systematic description and classification of rocks. Petrography is largely a descriptive science. Identification of minerals, structures and textures is typically done in hand specimen and in thin section using a magnifier glass, stereo microscope and polarizing microscope.

When Petrography is applied to natural stones and other rocks used for building materials, it allows identification of the components and can determine the presence of inadequate phases which can affect the quality, e.g. pyrite and clay minerals.

J.M.W. Geo-Consulting performs petrographic examinations of ores and natural stones following the European standard EN 12407 and En 12670. Used methods are stereo microscopy, polarization microscopy and ore microscopy.

Good contacts to university facilities provide the access to perform x-ray powder diffraction analyses of rocks and other mineral-phases as well as the phase quantification using the Rietveld-method.

Natural Stone Analysis

February 17th, 2016  Posted by jmwarnsloh

We are specialists in the mineralogical and petrographic investigation of natural stones, rocks and mineral resources by microscopic analysis. Looking for an independent, reliable third-party, analysing natural stones, rocks and minerals? – J.M.W. Geo-Consulting performs petrographic analysis to evaluate the mineral composition, microstructure and inter-component relationships of natural stones and other building materials, e.g. slates

Principal services:

  • Mineralogical analysis and reporting on ores and industrial minerals
  • Petrographic analysis and reporting on natural stones and rocks
  • Classification and naming of natural stones
  • Petrographic examination according to EN 12407 and EN 12440
  • Additional XRD analysis in cooperation with our partners, including determination of clay minerals and mineral phase quantification using the Rietveld refinement technique

TriAngle 1.2 for LibreOffice is ready!

December 5th, 2014  Posted by jmwarnsloh

The LibreOffice version of TriAngle is now ready for download at the TriAngle sub page!
It offers the same functionality like its Excel counterpart and is the ideal template for people who prefer to use free software on their PCs.

Have fun!

TriAngle paper will be printed soon!

November 21st, 2014  Posted by jmwarnsloh

Warnsloh, J.M. (2015) TriAngle: A Microsoft Excelâ„¢ spreadsheet template for the generation of triangular plots. N. Jahrb. Miner./J. Min. Geochem., 192, 1, 101-105. DOI: 10.1127/0077-7757/2014/0267

will be available in a print version beginning of next year. For the abstract you can have a look at ingentaconnect.

TriAngle 1.2 is ready!

June 2nd, 2014  Posted by jmwarnsloh

A new version of TriAngle is now ready for download at the TriAngle sub page.

New feature:

  • Changes in layout and line formats
  • Feldspar diagram looks now better due to smoothed lines
  • The manual to create individual masks got a revision and more figures

In comparison to other Excel spreadsheets for triangular plots, TriAngle offers the user the opportunity to design individual masks by simply inserting the required plotting coordinates in the pre-defined fields in the ‘Template’ worksheet. TriAngle calculates the resulting X-Y-coordinates and draws the mask.
Labels can be inserted in the same way, too.
A manual how to create individual masks and labels is shown in the worksheet ‘Manual’.

TriAngle Version 1.1 is ready for download now!

March 20th, 2014  Posted by jmwarnsloh

A new version of TriAngle is now ready for download at the TriAngle sub page.

New feature:
In comparison to other Excel spreadsheets for triangular plots, TriAngle offers the user the opportunity to design individual masks by simply inserting the required plotting coordinates in the pre-defined fields in the ‘Template’ worksheet. TriAngle calculates the resulting X-Y-coordinates and draws the mask.
Labels can be inserted in the same way, too.
A manual how to create individual masks and labels is shown in the worksheet ‘Manual’.

TriAngle: A Spreadsheet template for the Generation of Trilinear Plots

September 24th, 2013  Posted by jmwarnsloh

TriAngle is a Microsoft ExcelTM spreadsheet template to create triangular plots in an easy way.

Features:

  1. Simple to handle
  2. Automatic normalisation to 100% of inserted data
  3. No use of VBA; to adapt easily the file to other desktop and mobile spreadsheet programs which do not support VBA
  4. Automatic display of apexes titles
  5. Automatic display of a legend (can be deleted if not desired)
  6. Selection of typical masks used in the daily business of geoscientists – the user has the chance to add own masks
  7. High quality of the resulting chart for direct use in documents without reformatting with a drawing software
  8. Added data are shown immediately after insertion in the table
  9. Further masks and grids can be designed and inserted by the user

A Literature review on the Geology and Mineral Industry of Sierra Leone

September 24th, 2013  Posted by jmwarnsloh

Geologic Country Report with Emphasis on Diamonds, Gold, and Titanium (July 2011)
The Republic of Sierra Leone is located on the West Coast of Africa, between latitudes 7 and 10 north and longitudes 10.5 and 13 west.
The mining sector is a significant contributor to Sierra Leone’s economy; it accounted for about 90 % of export revenues, mainly from diamond exports, and 20 % of the gross domestic product (GDP). Sierra Leone was the world’s 10th ranked producer of diamond by volume in 2009 and the world’s third ranked producer of rutile.

Showing geographic data with spreadsheet software

March 27th, 2012  Posted by jmwarnsloh

Despite the growing amount of free GIS software packages (examples are listed at the end of this little article) most of the people still think about more or less expensive software solutions, provided by a few companies (some of them you will find at the end of this article as well…) and refrain from showing their geo-spatial data. Others may simply not be in the mood or lack the time to learn the handling of new software.