Dear Filipe

You can try the link

http://etd.uovs.ac.za/ETD-db//theses/available/etd-05172013-124826/unrestricted/Kottke-LevinJ.pdf

and

Stratiform Chromite Deposit Model 2010, By Ruth F. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Seal II; Scientific Investigations Report 2010–5070–E, USGS.

and

Moghadam et al. 2013. Arc-related harzburgite–dunite–chromitite complexes in the mantle section of the Sabzevar ophiolite, Iran: A model for formation

of podiform chromitites. Gondwana Research...

cheers

Dear Mr. Costa,

maybe you can find some ideas how to tackle your chemical problems of Cr in ultrabasic rocks, e,g, dunite and harzburgite:

Chromite Ore - Characterisation through to processing Australasian Institute of Mining and Metallurgy Publication Series, 2008, Pages 339-351McCallum, D.A. | Bruckard, W.J. | Pownceby, M.I.

Preparation of Four Chromium Ore Reference Materials (Geostandards and Geoanalytical Research Volume 37, 2013, Pages 95-101 Cheng, Z. | Huang, H. | Liu, M. | Gu, T. | Yan, W. | Yan, M.

Analytical and mineralogical studies of ore and impurities from a chromite mineral using X-ray analysis, electrochemical and microscopy techniques Talanta Volume 74, 2008, Pages 1592-1597Sánchez-Ramos, S. | Doménech-Carbó, A. | Gimeno-Adelantado, J.V. | Peris-Vicente, J.

Chromitite and platinum-group-element mineralization at middle arm brook, central advocate ophiolite complex, Baie Verte Peninsula, Newfoundland, Canada Canadian Mineralogist Volume 49, 2011, Pages 1523-1547Escayola, M. | Garuti, G. | Zaccarini, F. | Proenza, J.A. | Bédard, J.H. | Vanstaal, C.

Petrology of Al- and Cr-rich ophiolitic chromitites from the Muǧla, SW Turkey: Implications from composition of chromite, solid inclusions of platinum-group mineral, silicate, and base-metal mineral, and Os-isotope geochemistry Contributions to Mineralogy and Petrology Volume 158, 2009, Pages 659-674Uysal, I. | Tarkian, M. | Sadiklar, M.B. | Zaccarini, F. | Meisel, T. | Garuti, G. | Heidrich, S.

Dunite xenoliths and olivine xenocrysts in gabbro from Taihang Mountains: Characteristics of Mesozoic lithospheric mantle in Central China Journal of Earth Science Volume 21, 2010, Pages 692-710 Xu, W. | Wang, C. | Yang, D. | Wang, F. | Pei, F.

Tibet Luobusa different types of characteristics and genetic model chromite discussion Acta Petrologica Sinica Volume 30, 2014, Pages 2137-2163 Xiong, F.H. | Yang, J.S. | Ba, D.Z. | Liu, Z. | Xu, X.Z. | Feng, G.Y. | Niu, X.L. | Xu, J.F.

Chromium in olivine occurrence: Tibet Luobusa mantle peridotite and chromite-rich olivine and implications for chromium causes of deep mantle Acta Petrologica Sinica Volume 30, 2014, Pages 2125-2136Liang, F.H. | Yang, J.S. | Xu, Z.Q. | Zhao, J.N.

Best regards

Harald G.Dill

Probably by means of major, trace and rare earth elements to observe the relavant anomalies especially in Eu and Ce. Please read my papers in several journals as a appear as first autor clicking Kasper-Zubillaga. Greeting from Mexico.

Your questions is too broad and vague. There is a vast literature out there on chromite and chromite-bearing rocks and their significance. What is it that YOU want to know? What would you do with whole-rock XRF data? YOU'VE GOT TO BE MORE SPECIFIC.  Try the following ref. to see how these authors used Cr-spinel as a petrogenetic indicator in specific cases. 

Dick, HJB & Bullen, T (1984) Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas: Contributions to Min. Pet.  Volume 86, Issue 1, pp 54-76

OK. What questions do you wish to answer with the XRF whole rock data? Have you looked at the rock samples using a transmitted and reflected light microscope? Much can be done even WITHOUT electron microprobe data on individual minerals. It all depends on what QUESTIONS you are trying to answer. Think hard and deep.