Golden Cross is a volcanic-hosted, low-sulfidation,
epithermal Au-Ag deposit in the Hauraki Goldfield. Irrespective of the rock type
(andesite versus dacite), hydrothermally altered wallrocks are enriched in K,
Rb, As and Sb around Au-Ag mineralised veins of the Empire vein system and
stockwork. Sodium and Sr are strongly depleted around these veins, whereas Mg
and Fe are locally depleted. Calcium and Si display mixed behaviour, varying
from unchanged to depleted to enriched. Other elements including Al, Ti, Mn, Zr,
Y, and Nb show little change or are immobile during hydrothermal alteration.
Element enrichment and depletion trends are directly related to the degree of
alteration and the alteration mineralogy. Addition of K and Rb and converse loss
of Na and Sr in intensely altered wallrock surrounding veins corresponds to the
replacement of igneous plagioclase by hydrothermal adularia. Sodium and Sr occur
in moderately altered rocks on the margins or in hard bars' that contain
plagioclase. Locally, elevated Si concentrations and depletion of Mg adjacent to
the Empire vein system reflects intense silicification of the wallrocks and the
replacement of mafic minerals by quartz rather than by chlorite.
The geochemical zonation patterns at Golden Cross are
very similar to those at the McLaughlin deposit California, and shallow levels of the active
Broadlands Ohaaki geothermal system. Therefore, determination of K, Na, Rb, Sr,
and Ca enrichment and depletion patterns provides insight into alteration
intensity and mineralogy that is complimentary to traditional trace element
analysis of As, Sb, Hg, Au, Ag, Zn, and Pb used in the exploration of shallow
level (< 500 m) epithermal Au-Ag deposits.