Exploration companies commonly rely on geochemistry to identify alteration of
distinctive geochemical and mineralogical character, surrounding metal sulphide
deposits that were precipitated from hydrothermal fluids. However, examination of
raw analytical data is prone to error due to closure effects and the difficulty in
removing the effects of background variation in unaltered rocks from the variations
imposed by later hydrothermal alteration. Closure can be avoided by using ratios, or
by utilising mass balance approaches based on fixing volume, mass or concentration
changes between samples of parent and daughter lithologies. Using a parent-daughter
approach is limiting, because only pairs of samples can be compared at any one time
and because an unaltered equivalent must be produced for each sample examined in
this way. Pearce Element Ratio analysis and General Element Ratio analysis (PER
and GER) are not restricted in this fashion, and are more amenable to interrogation of
large data sets. PER and GER are also capable of decoupling background variation
from that variation due to hydrothermal alteration. Furthermore, these ratio methods
are readily applied to commercially derived lithogeochemical assays.
In this study, various analytical methods and interpretive techniques (including PER
and GER) have been applied to identify alteration in rocks around the Century and
Elura Zn-Pb-Ag deposits, and to assess whether primary ore-related alteration effects
can still be identified once altered rocks have been subjected to weathering.
Ratios of trace elements over a conserved element have been used to generate a suite
of pathfinder elements for each deposit. Elements enriched in host rocks around both
deposits include the economic metals Zn, Pb and Ag, along with Rb and Tl. Sodium is
ubiquitously depleted in altered rocks. Other elements in the pathfinder suites are
distinctive to each deposit type, and include a number of major and trace elements
that are added or removed from the rocks around the mineralised zones. For example,
Sb and As are enriched in rocks around Elura mineralisation while Ge and Cd are
enriched in samples around Century deposit.
Iron carbonate development accompanied by potassic alteration, the destruction of
albite and the absence of chlorite are the dominant mineral alteration effects at both
deposits. PER and GER diagrams have been used to quantify the intensity of this
alteration and allow lithogeochemistry to be used to vector towards high intensity
alteration, which is adjacent to Century and Elura mineralisation. These ratio methods
are applied to both visibly and cryptically altered rocks at both deposits, and have a
very high degree of success in classifying alteration in unweathered rocks.
The following simple PER ratios indicate proximity to Elura mineralisation:
Ca/C, K/Al for shales, K/(Al-Na) for siltstones/sandstones.
The following simple PER ratios indicate proximity to Century mineralisation:
Mn/Ti, Mg/Ti and Fe/Ti vs C/Ti, K/Ti vs Al/Ti, K/Ti vs (Al-Na)/Ti.
Pathfinder elements can be overlain onto PER and GER diagrams to aid in ranking the
prospectivity of samples, and to assess mineral hosts for individual pathfinder
elements.
Weathering destroys most indicators of alteration in the Elura area, while alteration
signatures are better preserved in host rocks around the Century deposit.
| Identifer | oai:union.ndltd.org:ADTP/218689 |
| Date | January 2004 |
| Creators | Whitbread, Michael Andrew Ian, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Michael Andrew Ian Whitbread |
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