Morphology and chemistry of placer gold in the Bagrote and Dainter streams, northern Pakistan: Implications for provenance and exploration

Abstract

The Bagrote and Dainter valleys in Gilgit Baltistan Pakistan, belonging to the Kohistan island arc and southwestern domain of the Karakorum metamorphic complex, respectively, are a location of regional placer gold mining. However, no in situ hydrothermal gold deposits have been discovered in this region. Here, we report the textural, morphological, and geochemical characteristics of placer gold from the stream sediments of the Bagrote and Dainter valleys with a view to trace the provenance. The morphological features of the studied gold grains indicate proximal source mineralization. The gold grains display a microtexture typical of mechanical abrasion rather than chemical activity. Their chemical composition of most of the grains from each stream falls in a restricted range (minor alloying elements mainly Cu) with high fineness (1,000 Au/(Au + Ag)), suggesting derivation from a homogeneous source or limited number of mineralization types. The high Cu content of the gold grains from the Bagrote stream suggests that the gold here might be associated with previously undiscovered porphyry gold mineralization, whereas the Au–Ag–Hg alloys from the Dainter stream suggest a genetic linkage with undiscovered fault‐controlled auriferous veins of epithermal origin in the southwestern part of the Karakorum metamorphic complex. The chemical composition of both streams placer gold in triangular diagram in terms of Au–Ag–Cu shows all the gold grains from Dainter stream were derived from epithermal type of mineralization, and most of the plots of Bagrote stream fall on the domain of porphyry epithermal type of deposits which are also in consistent with a continental arc setting in the study area. The mechanically developed microtexture, homogeneous chemical composition in individual grains from core to rim, and absence of high fineness rims of the gold grains from both streams, suggest the absence of any supergene regrowth and direct transfer from the primary source through glacier to the fluvial system. The Gilgit Baltistan province, dominated by dry temperate climate and glacial environment having steep geomorphology, precluding any significant supergene processes such as silver leaching and addition of authigenic gold on detrital gold grains.

Typomorphic Features of Placer Gold from the Billyakh Tectonic Melange Zone of the Anabar Shield and Its Potential Ore Sources (Northeastern Siberian Platform)

“…The analytical lines were Au-Mα, Ag-α, and other elements-Kα. The standards were gold 850% -Au, Ag; FeS 2 (pirite)-Fe, S; CuFeS 2 (chalcopyrite)-Cu; FeAsS (arsenopyrite)-As; ZnS (sphalerite)-Zn; Ca 5…”

“…When the roundness ratio increases, the primary tubercular and dimpled surface is lost in the placers, as well as its angles. Resulting from its long-term presence in the hydrodynamic environment, the morphology of gold undergoes major changes by crushing, rolling, and abrasion [1,5]. Native gold fineness in deposits of different depths and formation types has large variations of data, resulting from the impact of multiple factors within ore deposits.…”

Characteristics and Potential of Placer Gold Deposit in Lakan Bilem Block, West Kutai District, East Kalimantan, Indonesia

“…The grain size of gold is typically ranging from 0.1 to 1.2 mm (Figure 6). Based on the gold morphologies including platy, angular and coarse surface, it may indicate that the process of gold deposition is predominantly controlled by hydrodynamic transportation at a relatively moderate regime (Alam et al, 2019;Girard et al, 2021;McLachlan et al, 2018;Wierchowiec, 2002).…”

Detrital gold as an indicator mineral

“…This approach has recently found application in placer districts where knowledge of the regional metallogeny may be incomplete, e.g. Cameroon (Omang et al 2015;Dongmo et al 2018), Pakistan (Alam et al 2018) and/or specific placer-lode relationships unclear, e.g. Russia (Lalomov et al 2017;Nevolko et al 2019).…”

“…The majority of studies which have focussed on placer-lode relationships have been undertaken in the context of regional gold exploration (e.g. Lalomov et al 2016;Alam et al 2018;Nevolko et al 2019). Such studies are a refinement of the classic 'panning back to the source' approach historically adopted by prospectors, because the variation between compositional signatures of sample populations provides a discriminant for different sources.…”