GEOCHEMISTRY OF HYDROTHERMAL ALTERATION AT THE DERI MASSIVE SULFIDE DEPOSIT, SIROHI DISTRICT, RAJASTHAN, NW INDIA

The 1.1 Ga volcanogenic massive sulphide deposit at Deri in the Sirohi district of south Rajasthan occurs within a bimodal volcanic suite of tholeiites and rhyolites, with minor amounts of andesites and tourmal ine-bearing chert, interlayered with arkosic sediments. The ores and t he enclosing rocks have undergone superposed deformation and polymetam orphism initially under amphibolite facies conditions and later under hornblende hornfels facies conditions. Metamorphism, however, has not affected the bulk composition of the rocks to any significant degree. Three distinct semiconformable alteration facies, characterized by the ir conspicuous magnesian mineralogy, are recognized in the host rocks: (1) hornblende-biotite-plagioclase-quartz schist (AMV); (2) cordierit e-anthophyllite-chlorite hornfels (AFV); and (3) biotite-chlorite(-ser icite) schist/hornfels (BCS). The first is derived from the mafic volc anics, whereas the other two represent progressive alteration of felsi c volcanic protoliths. Fe, Mg and water were added and Na was removed from all the alteration facies in varying amounts. The maximum enrichm ent is noted in BCS for Mg and Fe, whereas the maximum depletion is se en in this facies for Si, an element which is also depleted significan tly in AFV. AMV on the other hand, shows enrichment of Si, Ca and to s ome extent, in Al. Alumina is also enriched considerably in BCS, proba bly due to clayey alteration and extreme leaching of silica. Amongst t he trace elements, Rb, Ba, Nb and Y are gained in most of the facies, except in BCS, where Ba and Y show distinct depletion. The LREE, from La to Sm, were enriched about 1.5- to 3.0-fold in all the facies with a maximum in AFV where the flux took place at constant inter-REE propo rtions: 1.0 La, 0.79 Ce, 0.48 Nd and 0.35 Sm. Eu was depleted from bot h felsic facies, 7-fold in BCS to 4-fold in AFV, during alteration. Th e HREE (Er to Lu) remained immobile in all the altered facies. The che mical and mineralogical zonation in the alteration facies are interpre ted to be due to the progressive reaction of an evolving sea-water hyd rothermal fluid with the bimodal volcanic protoliths during convective circulation. Fluid-rock interaction, guided by vertical and lateral t hermal gradients, produced a sericite-quartz assemblage in the felsic volcanics at the expense of feldspar during the initial stages (175 de grees C) which formed a sericite-chlorite zone upon rising temperature (200-250 degrees C) by base-fixing reactions. A further temperature i ncrease (to similar to 300 degrees C) and deeper circulation in the ma fic pile introduced more Fe and Mg, thereby transforming the previousl y formed assemblage to a nearly pure chloritic zone and the most inten sely altered biotite-chlorite(-sericite) facies.