The use of the “immobile” elements Zr and Ti in lithogeochemical exploration for massive sulphide deposits in the precambrian pecos greenstone belt of northern New Mexico

Abstract

Mineralogical and geochemical data of surface rocks of the Mid-Proterozoic Pecos greenstone belt in Northern New Mexico and results of drill core data of the Jones Hill deposit are presented and discussed within the geological framework derived from detailed geological mapping. Over 800 rock samples were analyzed for 25 elements (all major and several trace elements) and the lithologies were mineralogically and petrographically determined. The results show that the stratigraphic sequence of the Jones Hill deposit is composed of felsic pyroclastics, exhalites and pyroclastics of mafic and mixed felsic-mafic origin, upon which are superimposed certain alteration effects. In volcanic rocks of felsic origin Mg, Fe, Mn, Cu, Pb, Zn, As are enriched and Na, Ca, Sr are clearly depleted when compared with their unaltered source rocks (Table I). Zirconium and Ti are found to be relatively immobile during alteration and subsequent intrusive, deformational and metamorphic events. Alteration trends, best defined by the Na 2 O/MgO ratios and systematic metal zoning, confirm that the stratigraphic sequence is overturned. Lithogeochemical anomalies outline a target which is bigger (over 300 m) and more precisely defined than do other methods, mapping of alteration assemblages or soil geochemistry. New important aspects have evolved from this study, which are applicable to rock geochemical exploration and the recognition of alteration/mineralization anomalies in general. Ratios of Zr/TiO 2 can be shown to reflect the primary pre-alteration-premetamorphic composition of metavolcanic rocks (including shallow intrusives, flows, tuffs, and volcaniclastics). This ratio also enables the determination of the original composition of variably altered rocks of identical parentage (source rock). This determination is a fundamental prerequisite for the recognition and quantification of alteration trends. Direct applications are, for example, that apparently similar chlorite schists derived through hydrothermal alteration, metamorphism, and deformation Zr/TiO 2 ratio of 0.04 (and greater); however chlorite schists derived from mafic volcanic sources and metamorphosed without much hydrothermal alteration have a much lower Zr/TiO 2 ratio of 0.0075 (and less). The altered rock related to massive sulphide alteration can thus be recognized and distinguished from similar rocks of different origin. In addition, the relative concentrations of the immobile elements Zr and Ti are useful in making stratigraphic correlations in structurally complex terrain such as the Pecos greenstone belt. The assumption of a relative immobility of these elements in samples from Jones Hill is supported by the following observation: 1. (1) The TiO 2 and Zr values of Jones Hill rocks fall in narrow clusters (Fig. 1) as opposed to a random scatter plot. 2. (2) The element abundances (ratios) are very similar to: (a) granite (Type A), fresh felsic volcanics (Type A) of the same area and same age (Table I) and basalt (Type B) of the same area; and (b) fresh basalts and rhyolites reported from young volcanic island arcs. 3. (3) Highly altered chlorite schists directly underlying the main ore horizon and within the ore horizon, display Zr/TiO 2 ratios identical to less altered felsic rocks in the foot wall. These rock geochemical methods can be demonstrated to be applicable at all scales ranging from regional reconnaissance through local prospect evaluation down to detailed drill core interpretations during deposit delineation stages.