Abstract
Glacial drift exploration methods are well established and widely used by mineral industry exploring for blind deposit in northern territories, and rely on the dispersion of mineral or chemical signal in sediments derived from an eroded mineralized source. Gold grains themselves are the prime indicator minerals to be used for the detection of blind gold deposits. Surprisingly, very little attention has been dedicated to the information that size and shape of gold grain can provide, other than a simple shape classification based on modification affecting the grains that are induced in the course of sediment transport. With the advent of automated scanning electron microscope (SEM)-based gold grain detection, high magnification backscattered electron images of each grain are routinely acquired, which can be used for accurate size measurement and shape analysis. A library with 88,613 gold grain images has been accumulated from various glacial sediment surveys on the Canadian Shield and used to detect trends in grains size and shape. A series of conclusions are drawn: (1) grain size distribution is consistent among various surveys and areas, (2) there is no measurable fine-grained gold loss due to natural elutriation in ablation or reworked till, or during the course of reverse circulation drilling, (3) there is no grain size sorting during glacial transport, severing small grains from large ones, (4) shape modification induced by transport is highly dependent on grain size and original shapes, and (5) the use of grain shape inherited from neighboring minerals in the source rocks is a useful feature when assessing deposit types and developing exploration strategies.
Highlights
Accepted: 30 March 2021Gold grain dispersion in secondary environment is an extensively used exploration method for tracing blind gold deposit, especially in glaciated terrain [1,2,3,4,5]
Gold grain size distributions obtained obtained from from various various surveys surveys are are surprisingly surprisinglysimilar, similar, which phenomenon might be related to the log-normal distribution of size graininsize in which phenomenon might be related to the log-normal distribution of grain source source rocks
Considering how andthe disjointed the the surveys are, the oftight grain size suggests that such a distribution is ana intrinsic feature of orogenicoverlap ofdistributions the grain size distributions suggests that such distribution is an intrinsic feature of orogenic-related gold deposits
Summary
Gold grain dispersion in secondary environment is an extensively used exploration method for tracing blind gold deposit, especially in glaciated terrain [1,2,3,4,5]. Scheelite is common in orogenic gold deposit [16], tourmaline is suggestive of intrusion-related deposit [17], grossular or andradite are associated with porphyries [21], etc These approaches are typically limited by the cost of conducting elaborate indicator mineral studies (2–3 × the cost of extracting gold grains), which need to be conducted on a larger (250–1000 μm) size fraction of the sample. Arsenopyrite or chalcopyrite are commonly associated with gold mineralization, but since arsenic or copper are not partitioned into common mineral, but almost exclusively in sulfides or sulfosalts, their abundance can more be detected by simple aqua-regia induced plasma (ICP) spectrometry [15,29]) Studying these indicator minerals is of obvious interest with regards to characterizing the signature of a deposit but of limited applicability in exploration. Such research remains hampered by the lack of systematic information on gold grain characteristics and the variability among types of deposit
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