Economic Geology News
Is Buried Anorthosite Genetically Related to the Green Garnet Gemstones in Southern Kenya? (17 April 2015)
Recently, I was prompted to re-analyse work that I had done long ago in Kenya, geologically mapping the Mwatate Quadrangle and exploring the gemstone deposits of the area (Pohl et al. 1979).
The green garnet gemstones are grossularites coloured by Fe, Cr, Mn, Ti and V. Host rocks are calcareous, pelitic and graphitic metasediments interbedded with paragneiss and metabasalts, metamorphosed at amphibolite facies conditions up to 7 kbar and 700o C (Pohl et al. 1979), dated at 630–645 Ma (Hauzenberger et al. 2007). General dip is to the ENE but this veils polyphase folding and thrust sheets marked by small ultramafic bodies. Metamorphism and deformation occurred during the East African Orogeny that contributed to the assembly of supercontinent Gondwana. A 15 x 5 km lensoid body of charnockite protrudes from the metasediments (you can download the paper + map from my profile in ResearchGate). The charnockite was most probably tectonically emplaced into the fold-and thrust belt of the metasediments but likely was basement to the passive margin system of the Mozambiquian Ocean, upon which the sediments formed at ca. 800-600 Ma (Fritz et al. 2013) during the breakup of supercontinent Rodinia.
- Green grossularite garnet (about 25 mm across), idiomorphic, cataclastic, kelyphitic reaction rim (5 mm) of bluish tanzanite (a zoisite), epidote, clinopyroxene, spinel, quartz and scapolite. Collected at Lualenyi Mine.
Boudinage of more competent rock layers in less competent host rock is ubiquitous at Mwatate. The green gem garnets (tsavorites: Bridges & Walker 2014) are commonly enriched in boudin necks of calcsilicate boudins in graphite schist. These dilatational sites (pressure shadows) draw in the metamorphic dehydration fluids and favour coarse-grained crystallisation. This observation demonstrates a strong structural synmetamorphic and syndeformational control of garnet crystallisation. Additional control by upright folds and some faults is seen in most gemstone mines in the area (Figs. 5-10). Green grossularite deposits seem bound to a certain graphitic lithostratigraphic unit.
An aeromagnetic survey of the sheet was flown in 1977 (CIDA, Terra Surveys Ltd.); in reduced size, the map is included in my report (Fig. 12). A significant positive anomaly 9 x 2.5 km roughly coincides with the charnockite outcrop’s N-S axis but is not in accordance with shape and internal structure of the charnockite complex. The shape of the anomaly was thought to indicate considerable depth; application of the straight slope method (Milsom & Eriksen 2011) for estimating the depth of the source body produces a figure of 700 ± 200 m. At the time, we speculated that a buried mass of magnetite might be the source of the anomaly. The exposed charnockite only contains ~4 mode % of apatite, zircon and magnetite.
Considering that charnockite is a member of the anorthosite, mangerite, charnockite and rapakivi granite (AMCG) magmatic suite and related ilmenite deposits (Charlier et al. 2015) and that Proterozoic anorthosite bodies that intruded at 900-700 Ma are exposed in the nearby Eastern Granulite region, Pare Mts., Tanzania (Fritz et al. 2013, Tenczer et al. 2011), a desktop study and possibly, renewed field work concerning this remarkable magnetic anomaly appears desirable. This could be a magnetite body with Ti-V-(P) tenors that enhance the feasibility of deep mining. After all, at Kiruna in Northern Sweden, simple magnetite is profitably exploited at more than 1000 m below the surface.
Admittedly, a genetic tie-line to the green garnet gemstones is pure phantasy; but be so kind as to read my arguments: If the source body of the Mtonga magnetic anomaly is Ti-ore associated with a buried anorthosite intrusion, it is likely that a wide contact metamorphic halo and fluid expulsion were produced. Fluid passage through solidified anorthosite would be marked by the characteristic bleaching of the host anorthosite (Charlier et al. 2015). These fluids should have leached and transported the colouring metals into the cover sediments including the green garnet horizon.
Following up these speculations might result in (1) finding a new large Ti orebody, and (2) building a novel mineral (process) system for green garnet exploration. Anybody interested?
Bridges, B. & Walker, J. (2014) The discoverer of tsavorite – Campbell Bridges – and his Scorpion mine. The Journal of Gemmology 34(3), 230–241. http://dx.doi.org/10.15506/JoG.2014.34.3.230
Charlier, B., Namur, O., Bolle, O. et al. (2015) Fe-Ti-V-P ore deposits associated with Proterozoic massif-type anorthosites and related rocks. Earth-Science Reviews 141, 56–81.
Fritz, H., Abdelsalam, M., Ali, K.A., et al. (2013) Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution. J. African Earth Sciences 86, 65–106.
Hauzenberger C.A., Sommer H., Fritz H., Bauernhofer A., Kroner A., Hoinkes G., Wallbrecher E. and Thöni M. (2007) SHRIMP U-Pb zircon and Sm-Nd garnet ages from the granulite-facies basement of SE Kenya: Evidence for Neoproterozoic polycyclic assembly of the Mozambique Belt. J.Geological Society London, 164, 189–201. http://dx.doi.org/10.1144/0016-76492005-081
Milsom, J.J. & Eriksen, A. (2011) Field geophysics. 4th edn. 304 pp. The Geological Field Techniques Series, Wiley. www.wiley.com/go/milsom/geophysics4e
Pohl, W.L., Nauta, W.J. & Niedermayr, G. (1979) Geology of the Mwatate Quadrangle and the Vanadium Grossularite Deposits of the Area (with a Geological Map 1:50,000). Kenya Geol. Survey Report No. 101, 55 pp, 13 Figs., Nairobi. https://www.researchgate.net/profile/Walter_Pohl
Tenczer, V., Hauzenberger, C.A., Fritz, H., Hoinkes, G., Muhongo, S. & Klötzli, U. (2011) The P–T–X(fluid) evolution of meta-anorthosites in the Eastern Granulites, Tanzania. J. Metamorphic Geol. 29, 537–560. doi:10.1111/j.1525-1314.2011.00929.x
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The Anthropocene – a Voice of Reason (24 March 2015)
Wherever you live, you must have learnt from the media that in geological terms, our time should forthwith be called the Anthropocene (not Holocene any more) because humans so strongly impact on the Earth. Sounds like a good idea, doesn’t it? Why not? – At least that was my reaction when I first heard about the proposal some 15 years ago. And you?
Basically, all members of the geological community are professionally responsible for such a change and should be engaged. In his latest blog on Research News from the Earth Sciences , Steve Drury reports on recent developments in the (scientific) discussion. I strongly recommend to read his cool, dry and reasoned treatment.
For a taste, read the sample paragraph below, which provides the gist of Steve’s analysis:
“So the Anthropocene adds the future to the stratigraphic column, which seems more than slightly odd. As Richard Monastersky notes, it is in fact a political entity: part of some kind of agenda or manifesto; a sort of environmental agitprop from the ‘geos’. As if there were not dozens of rational reasons to change human impacts to haul society back from catastrophe, which many people outside the scientific community have good reason to see as hot air on which there is never any concrete action by ‘the great and the good’. Monastersky also notes that the present Anthropocene record in naturally deposited geological materials accounts for less than a millimetre at the top of ocean-floor sediments. How long might the proposed Epoch last? If action to halt anthropogenic environmental change does eventually work, the Anthropocene will be very short in historic terms let alone those which form the currency of geology. If it doesn’t, there will be nobody around able to document, let alone understand, the epochal events recorded in rocks. At its worst, for some alien, visiting planetary scientists, far in the future, an Anthropocene Epoch will almost certainly be far shorter than the 10 4 to 10 5 years represented by the hugely more important Palaeozoic-Mesozoic and Mesozoic-Cenozoic boundary sequences; but with no Wikipedia entry.”
If you wish to brush up your knowledge of the geological time-scale, visit the International Commission on Stratigraphy website where you can download for free the latest version (2015-1) of the International Stratigraphic Chart & Time-Scale, as yet without the Anthropocene.
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Exploring for Porphyry Copper Deposits (18 February 2015)
Admittedly, global resource figures seem not to imply a scarcity of copper, and some might conclude, that there is little need for exploration:
In 2014, world mine production of copper contained in concentrate was 18.7 Mt, with the major share provided by Chile, China, Peru, USA, DR Congo and Australia. Global mining reserves in the ground amount to 700 Mt of the metal and continue to grow (USGS 2015). Excluding reserves, large formally declared copper resources are available (Mudd et al. 2013) and undiscovered recoverable copper is substantial (Kesler & Wilkinson 2008). Land-based resources comprise mainly porphyry copper (86%, USGS 2015). Undiscovered resources hold an estimated 3500 Mt of Cu. So is there plenty of copper?
Looking at the reserves/production (R/P) ratio at 37 (years), however, shows that exploration for copper is definitely not an idle game but serious business. Replacement of presently operating mines is needed and considering the long leadtime from discovery to first production, the sooner the better.
It seems fitting that the January Newsletter of SEG (Society of Economic Geologists) opens the year with an article about porphyry copper deposits, in which Halley et al. (2015) report on some results of a three-year industry project on footprints of magmatic-hydrothermal porphyry copper systems. Note the term “system” that expresses the recent trend that not deposits are sought (which are never identical) but indications for the mineralizing process system (which is less variable).
The authors’ abstract may serve as an overview of the contents covered:
“Whole-rock lithogeochemical analyses combined with short-wave infrared (SWIR) spectroscopy provide a rapid and cost-effective method for prospecting for porphyry-type hydrothermal systems. Lithogeochemistry detects trace metals to average crustal abundance levels and allows vectoring via gradients of chalcophile and lithophile elements transported by magmatic-hydrothermal ore and external circulating fluids that are dispersed and trapped in altered rocks. Of particular use are alkalis in sericite and metals such as Mo, W, Se, Te, Bi, As, and Sb, which form stable oxides that remain in weathered rocks and soils. SWIR mapping of shifts in the 2,200-nm Al-OH absorption feature in sericite define paleofluid pH gradients useful for vectoring toward the center of the buoyant metal-bearing magmatic-hydrothermal plume”.
I particularly like their updated (compared with the Lowell-Guilbert-1970 model) version of the alteration zoning that characterizes porphyry copper deposits, in a cross section that is rich in detail. In similar sections, the distribution of trace elements along the hypogene plume is shown, from below the main ore level through highest grade and mineralization top, to the advanced argillic alteration zone with polymetallic veins.
The paper offers new geochemical approaches for the exploration geologist. Another hope for finding new clusters of porphyry copper deposits comes from global dynamics:
Known porphyry copper provinces will continue to attract exploration crews. They crowd in continental volcanic arc setttings such as the Andes and are leaders in present copper mining. Deposits in continent-arc or continent-continent collision zones and post-subduction settings are less common. Yet, the lately discovered 400 km long Miocene (17-14 Ma) Gandese porphyry copper-molybdenum belt in Tibet intruded the Himalayas that were formed by the Indian-Asian collision. The mineralised porphyry intrusions in Tibet formed long after subduction related (120-70 Ma) and syn-collisional (65-38 Ma) magmatic activity in the region. Zengqian Hou et al. (2013) suggest that the post-collisional fertile potassic magmas were derived from a juvenile, thickened lower mafic crust, not from subcontinental lithospheric mantle (SCLM) nor from old lower crust. These days, in an open access paper “hot off the press” Zengqian Hou et al. (2015) discuss this subject in more detail.
Who is the next team to make a similar discovery?
Halley, S., Dilles, J.H. & Tosdal, R.M. (2015) Footprints: Hydrothermal alteration and geochemical dispersion around porphyry copper deposits. SEG Newsletter 100, 1, 12-17.
Zengqian Hou, Zhiming Yang, Yongjun Lu, Anthony Kemp, Yuanchuan Zheng, Qiuyun Li, Juxing Tang, Zhusen Yang & Lianfeng Duan (2015) A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones. Geology published 5 February 2015, 10.1130/G36362.1 Open Access http://geology.gsapubs.org/cgi/content/abstract/G36362.1v1?papetoc
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Structural Core Logging Best Practice (06 January 2015)
If you wish to update your core logging skills, consult the paper by Sian Bright and co-authors. It is extremely useful in that it provides information on up-to-date technologies including critical comparison of hard- and software. The writing is clear and lucid. Figures and photographs are excellent.
Sections of the paper cover downhole survey tools, core orientation including methods such as marking and equipment, core preparation for logging, structural data collection, with a short but useful introduction to structural geology applied to modelling, e.g. what to measure and measurement techniques. The first author’s training as a structural geologist is revealed by advice such as the advantage of creating interpretive illustrations of important features, summarized into preliminary structural schemes while working on the core, and the early transformation of data into dip/dip directions.
Manual or software assisted plotting of data on stereonets is suggested and examples of practical application are provided. Advantages and disadvantages of two commonly used software packages(GEOrient©, Dips) are discussed. Visualization possibilities of the core data and output are succinctly presented; methods include 3D Leapfrog® and Micromine®, and 2D GIS and Google Earth.
Logging personell in the core shed, but also exploration and mine managers, regulating authorities and academic teachers will read this paper to their advantage.
Bright, S., Conner, G., Turner, A. & Vearncombe, J. (2014) Drill core, structure and digital technologies. Applied Earth Science 123, 47-68.
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