Mp. Simpson et al., Hydrothermal alteration and hydrologic evolution of the Golden Cross epithermal Au-Ag deposit, New Zealand, ECON GEOL B, 96(4), 2001, pp. 773-796
Citations number
72
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
Golden Cross, located in the Coromandel peninsula, New Zealand, is a classi
c example of a volcanic rock-hosted, low-sulfidation epithermal gold-silver
deposit. Gold and silver ore is confined to the Empire vein system and sha
llow-level stockwork. The veins are hosted by Miocene to early Pliocene and
esites and dacites of the Waipupu Formation and the Waiharakeke Dacite that
are unconformably overlain by the postore Whakamoehau Andesite.
Hydrothermal alteration minerals display distinct spatial and temporal zona
tion around veins, as defined along three cross sections (1,000 m long x 45
0 m deep), located 200 m apart, which transect the Empire vein system. Alon
g these sections replacement quartz, chlorite, and pyrite are ubiquitous wi
th the abundance of quartz veinlets increasing toward major veins. Replacem
ent adularia envelops the Empire vein system and shallow stockwork in each
section. It is coextensive with, and is variably replaced by, illite that p
rogressively grades upward and laterally into a zone of interstratified ill
ite-smectite that mantles the deposit. Replacement calcite and minor sideri
te formed contemporaneously with, and also overprint, the above minerals, w
hereas late barren calcite veins crosscut mineralized quartz veins. Kaolini
te +/- pyrite veinlets, together with rare, very local alunite, formed duri
ng late-stage hydrothermal activity.
Clay mineral zonation is well developed. Illite occurs at depth and close t
o the veins, grading outward and upward into illite-smectite, with minor sm
ectite occurring similar to 600 m east of the Empire vein system. This over
all zonation reflects paleothermal gradients of similar to 150 degreesC on
the periphery to > 220 degreesC near the veins, consistent with the observe
d TI, range of 150 degrees to 240 degreesC for fluid inclusions in quartz,
platy calcite, and late barren calcite veins. Final ice-melting temperature
s for inclusions mostly range from 0.0 degrees to -1.4 degreesC, correspond
ing to apparent salinities of less than 2.4 wt percent NaCl equiv. Ice-melt
ing temperatures combined with vapor bubble expansion on crushing indicate
the presence of dissolved CO, in some platy calcite and late-stage barren c
alcite. The CO? content is estimated to range from 0.35 to 3.5 wt percent,
with the lower limit set by fluid inclusion vapor expansion during crushing
and the upper limit by the absence of any observable clathrates. Depth est
imates based on inclusions in platy calcite suggest that the shallow-level
stockwork zone formed about 100 m below the paleowater table under hydrosta
tic conditions.
Veins and alteration minerals at Golden Cross formed in the shallow part (<
400 m) of a hydrothermal system analogous to geothermal systems in the nea
rby Taupo Volcanic Zone. The assemblage of quartz, adularia, chlorite, illi
te, calcite, and pyrite reflects the upflow of near-neutral pH to weakly al
kaline chloride waters. Con temporaneously, steam-heated CO2-rich waters fo
rmed on the margins and reacted with the wall rock to form the assemblage o
f calcite, siderite, smectite, illite-smectite, and kaolinite. Eventual the
rmal collapse and invasion of these peripheral CO2-rich waters into the for
mer upflow zone produced late-stage barren calcite veins and an overprint o
f clay-carbonate alteration minerals. At the same time, localized steam-hea
ted acid-sulfate waters, generated above the water table, descended to form
the late overprint of kaolinite, pyrite, and alunite.