Gold in Leadhills
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(This is a technical summary. Please talk to your geological adviser for an explanation.)
A great deal can be deduced about the source of alluvial gold from the overall chemistry, the nature of inclusions and, in some cases, the type of internal chemical heterogeneity. Though useful information can be gleaned from a single grain, a sufficient number of grains is required to characterize a population from any source. In general terms, gold inherits characteristics both from the general geological environment in which the mineralisation is present and also from the mineralising solutions themselves.
Within the Exploration Area there are four main types of gold recognizable on the basis of composition and inclusion assemblage, each of which indicate a different type of provenance or host mineralisation. The commonest type - Type 1 ca. 70% contains around 8.4% to 13.1% Ag and inclusions, mostly of arsenopyrite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite and cobaltite, characteristics typical of mesothermal gold from shear-zones within a turbidite environment, suggesting sources within the local Lower Palaeozoic sedimentary rocks. The second type - Type 2 is richer in Ag with levels around 15.8% to 31.3% with a basic igneous signature indicated from inclusions of Sb-rich gersdorffite, pyrite, Ni arsenide and other Ni-rich minerals, and shows some spatial association with discordant Tertiary basaltic dykes which cut the sedimentary rocks. The third type - Type 3, which is mostly found in an area mantled by glacial drift, is poor in Ag at around 1.2% but frequently Cu-bearing and with inclusions of platinoids and Cu oxide. Associated with this type of gold are grains of the intermetallic compound Au3Cu and platino ids such as iso ferroplatinum and a complex Ir-rich phase. Its source is problematical and could partly reflect detrital material associated with ophiolitic debris, which is common in the northernmost turbidites, and partly a local source of lode gold. The fourth gold type is also Ag-poor up to 3.2%), but frequently contains Pd, up to 6.3%, while the inclusions comprise selenides of Pb, Hg and Cu
Exploration Targets
Our research indicates that some gold is probably derived from the direct erosion of mineralised rock, particularly in the higher gradient tributary streams, but the majority is derived from alluvial terrace deposits that are being actively eroded at the present time. In the lower part of the Snar valley the gold is derived from glacial till. The distribution of heavy mineral grains within drainage sediment is highly uneven and the occurrence of gold grains is particularly sporadic.
About 10 years ago a 50-metre length of the Shortcleugh Water was excavated extensively from bank to bank by gold prospectors because of the relative abundance of coarse sub 3 mm grains of gold and therefore much information about the detailed distribution of gold is available. This coarse gold, in grains up to 2 g in weight, occurs almost exclusively in cracks within bedrock, which mostly comprises Portpatrick Formation wacke.
Where hard greywacke is exposed, gold occurs in cracks, but in softer rock gold is present in cracks only when the rock is mantled by at least 0.8 m of gravel. Finer grains of gold are also relatively common in gravel in the same general part of the river. The shape of some of the gold grains extracted from cracks suggests that vertical movement of gold within the crack has taken place under conditions of spate. However, once trapped in the crack the gold is effectively immobile. Coarse gold of similar size has not been found in any of the tributaries of the Shortcleugh Water.
A great deal can be deduced about the source of alluvial gold from the overall chemistry, the nature of inclusions and, in some cases, the type of internal chemical heterogeneity. Though useful information can be gleaned from a single grain, a sufficient number of grains is required to characterize a population from any source. In general terms, gold inherits characteristics both from the general geological environment in which the mineralisation is present and also from the mineralising solutions themselves.
Within the Exploration Area there are four main types of gold recognizable on the basis of composition and inclusion assemblage, each of which indicate a different type of provenance or host mineralisation. The commonest type - Type 1 ca. 70% contains around 8.4% to 13.1% Ag and inclusions, mostly of arsenopyrite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite and cobaltite, characteristics typical of mesothermal gold from shear-zones within a turbidite environment, suggesting sources within the local Lower Palaeozoic sedimentary rocks. The second type - Type 2 is richer in Ag with levels around 15.8% to 31.3% with a basic igneous signature indicated from inclusions of Sb-rich gersdorffite, pyrite, Ni arsenide and other Ni-rich minerals, and shows some spatial association with discordant Tertiary basaltic dykes which cut the sedimentary rocks. The third type - Type 3, which is mostly found in an area mantled by glacial drift, is poor in Ag at around 1.2% but frequently Cu-bearing and with inclusions of platinoids and Cu oxide. Associated with this type of gold are grains of the intermetallic compound Au3Cu and platino ids such as iso ferroplatinum and a complex Ir-rich phase. Its source is problematical and could partly reflect detrital material associated with ophiolitic debris, which is common in the northernmost turbidites, and partly a local source of lode gold. The fourth gold type is also Ag-poor up to 3.2%), but frequently contains Pd, up to 6.3%, while the inclusions comprise selenides of Pb, Hg and Cu
Exploration Targets
Our research indicates that some gold is probably derived from the direct erosion of mineralised rock, particularly in the higher gradient tributary streams, but the majority is derived from alluvial terrace deposits that are being actively eroded at the present time. In the lower part of the Snar valley the gold is derived from glacial till. The distribution of heavy mineral grains within drainage sediment is highly uneven and the occurrence of gold grains is particularly sporadic.
About 10 years ago a 50-metre length of the Shortcleugh Water was excavated extensively from bank to bank by gold prospectors because of the relative abundance of coarse sub 3 mm grains of gold and therefore much information about the detailed distribution of gold is available. This coarse gold, in grains up to 2 g in weight, occurs almost exclusively in cracks within bedrock, which mostly comprises Portpatrick Formation wacke.
Where hard greywacke is exposed, gold occurs in cracks, but in softer rock gold is present in cracks only when the rock is mantled by at least 0.8 m of gravel. Finer grains of gold are also relatively common in gravel in the same general part of the river. The shape of some of the gold grains extracted from cracks suggests that vertical movement of gold within the crack has taken place under conditions of spate. However, once trapped in the crack the gold is effectively immobile. Coarse gold of similar size has not been found in any of the tributaries of the Shortcleugh Water.
