Oxidation and other geochemical processes in hydrothermal mineral deposits commonly remobilize elements and produce relative enrichments and depletions within specific zones of a deposit. Multivariate geostatistical techniques, particularly cross-covariance analysis, applied to geochemically homogenous data can at least partially quantify the remobilization of elements in a specific zone. The maximum positive cross-covariance between a pair of variables may deviate from the origin (earlier concentration loci) by a vector, known as the delay effect or lag vector.  This lag vector is denoted as l_xy(z), where x, y are the variables of interest in geochemically homogenous zone z, for which cross-covariance analysis is done. The l_xy(z) may indicate the direction and distance of elemental mobility from its origin. Previous applications of the lag vector to hydrogeology and atmospheric science suggest the concept can be successfully applied to hydrothermal deposits where element remobilization is evident from field and geochemical data.

 

To test this application we used assay data for exploration drill-hole samples from the Pierina Au-Ag deposit where late stage oxidation has remobilized Cu, Au and Ag. Oxidized and unoxidized zones were divided into 50m thick horizontal layers and a cross-covariance analysis was used to derive lag vectors for the pairs Au-Ag, Cu-Au and Cu-Ag in each layer.  Preliminary results of this analysis indicate three directions of relative remobilization in the oxidized zone, represented by lag vectors with azimuths and dips of 300⁰ and 60⁰ for Au-Ag 240⁰ and 30⁰ for Cu-Au, and 270⁰ and 60⁰ for Cu-Ag.  These vectors closely correspond to known structural trends in the deposit.  Descending oxidizing fluids apparently were primarily controlled by these three sets of faults or joints.  Clearly, the technique has implications for exploring oxidized deposits and discovering satellite deposits.  Future research will further test this application in well studied hydrothermal deposits and attempt to develop a method for determining absolute distances and directions of metal remobilization.