Concepts and Revised Models for Phanerozoic Orogenic Gold Deposits

Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: 1) crustal scale fault associated (CSF) subtype, 2) sediment-hosted orogenic gold (SHOG) subtype, 3) forearc (FA) subtype, and 4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.

U-Pb and Sm-Nd Evidence for Episodic Orogenic Gold Mineralization in the Kalgoorlie Gold Camp, Yilgarn Craton, Western Australia

Different genetic and timing models for gold mineralization in the Kalgoorlie gold camp (Yilgarn craton, Western Australia) suggest either broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma or a punctuated mineralization history from ca. 2675 to 2640 Ma with the involvement of early magmatic-hydrothermal systems (represented by the Fimiston, Hidden Secret, and Oroya gold-telluride lodes) and late metamorphic fluids (represented by the Mt. Charlotte gold stockwork veins). The results of U-Pb and Sm-Nd geochronological studies of zircon, apatite, and titanite from pre-ore dikes and syn-ore dikes constrain the absolute timing of mineralization and provide new evidence to this timing controversy. Emplacement ages constrained by U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon data are interpreted to be similar for both the pre-ore dikes (n = 10) and syn-ore dikes (n = 7) at ca. 2675 Ma. An inferred emplacement age of ca. 2675 Ma for the syn-ore dikes is supported by a Sm-Nd isochron age from apatite (laser ablation-inductively coupled plasma-mass spectrometry; LA-ICP-MS) of 2678 ± 15 Ma and by a U-Pb titanite age (LA-ICP-MS) of 2679 ± 6 Ma. The results of chemical abrasion-isotope dilution-thermal ionization mass spectrometry U-Pb zircon analysis from the pre- and syn-ore dikes are complicated by multistage Pb loss, reverse discordance, and potential inheritance. However, the data are compatible with the emplacement of Fimiston/Hidden Secret gold mineralization at ca. 2675 Ma and suggest a younger age for Oroya mineralization at ca. 2665 Ma. These results contrast with models for orogenic gold deposits that invoke broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma. The bulk of the Kalgoorlie gold camp’s estimated 2,300 t Au endowment was emplaced at ca. 2675 Ma as Fimiston/Hidden Secret Au mineralization. This early Au mineralization was deformed and overprinted twice by subordinate Au mineralization at ca. 2665 (Oroya mineralization) and ca. 2640 Ma (Mt. Charlotte mineralization). Gold mineralization in the Kalgoorlie gold camp was protracted in nature from ca. 2675 to 2640 Ma and reflects the interplay of early magmatic (Fimiston, Hidden Secret, Oroya) and late metamorphic (Mt. Charlotte) hydrothermal fluid systems in the formation of hybrid intrusion-related and metamorphic orebodies.

Origin and Characteristics of the Shwetagun Deposit, Modi Taung-Nankwe Gold District and the Kunzeik and Zibyaung Deposits, Kyaikhto Gold District in Mergui Belt, Myanmar: Implications for Fluid Source and Orogenic Gold Mineralization

The Mergui Belt of Myanmar is endowed with several important orogenic gold deposits, which have economic significance and exploration potential. The present research is focused on two gold districts, Modi Taung-Nankwe and Kyaikhto in the Mergui Belt comparing their geological setting, ore and alteration mineralogy, fluid inclusion characteristics, and ore-forming processes. Both of the gold districts show similarities in nature and characteristics of gold-bearing quartz veins occurring as sheeted veins, massive veins, stockworks to spider veinlets. These gold deposits are mainly hosted by the mudstone, slaty mudstone, greywacke sandstone, slate, and slaty phyllite of Mergui Group (dominantly of Carboniferous age). The gold-bearing quartz veins generally trend from NNE to N-S, whereas some veins strike NW-SE in all deposits. The gold-bearing quartz veins are mainly occurred within the faults and shear zones throughout the two gold districts. Wall-rock alterations at Shwetagun are mainly silicification, chloritization, and sericitization, whereas in Kyaikhto, silicification, carbonation, as well as chloritization, and sericitization are common. At Shwetagun, the gold occurred as electrum grains in fractures within the veins and sulfides. In Kyaikhto, the quartz-carbonate-sulfide and quartz-sulfide veins appeared to have formed from multiple episodes of gold formation categorizing mainly as free native gold grains in fractures within the veins or invisible native gold and electrum within sulfides. At Shwetagun, the ore minerals in the auriferous quartz veins include pyrite, galena, and sphalerite, with a lesser amount of electrum, chalcopyrite, arsenopyrite, chlorite, and sericite. In Kyaikhto, the common mineralogy associated with gold mineralization is pyrite, chalcopyrite, sphalerite, galena, pyrrhotite, arsenopyrite, marcasite, magnetite, hematite, ankerite, calcite, chlorite, epidote, albite, and sericite. At Shwetagun, the mineralization occurred at a varying temperature from 250 to 335°C, with a salinity range from 0.2 to 4.6 wt% NaCl equivalent. The Kyaikhto gold district was formed from aqueous–carbonic ore fluids of temperatures between 242 and 376°C, low to medium salinity (<11.8 wt% NaCl equivalent), and low CO2 content. The ore-forming processes of the Shwetagun deposit in the Modi Taung-Nankwe gold district and the Kyaikhto gold district are remarkably comparable to those of the mesozonal orogenic gold systems.

Texture and Trace Element Composition of Rutile in Orogenic Gold Deposits

Rutile from a wide range of orogenic gold deposits and districts, including representative world-class deposits, was investigated for its texture and trace element composition using scanning electron microscopy, electron probe microanalysis, and laser ablation-inductively coupled plasma-mass spectrometry. Deposits are hosted in various country rocks including felsic to ultramafic igneous rocks and sedimentary rocks, which were metamorphosed from lower greenschist to middle amphibolite facies and with ages of mineralization that range from Archean to Phanerozoic. Rutile presents a wide range of size, texture, and chemical zoning. Rutile is the dominant TiO2 polymorph in orogenic gold mineralization. Elemental plots and partial least square-discriminant analysis suggest that the composition of the country rocks exerts a strong control on concentrations of V, Nb, Ta, and Cr in rutile, whereas the metamorphic facies of the country rocks controls concentrations of V, Zr, Sc, U, rare earth elements, Y, Ca, Th, and Ba in rutile. The trace element composition of rutile in orogenic gold deposits can be distinguished from rutile in other deposit types and geologic settings. Elemental ratios Nb/V, Nb/Sb, and Sn/V differentiate the rutile trace element composition of orogenic gold deposits compared with those from other geologic settings and environments. A binary plot of Nb/V vs. W enables distinction of rutile in metamorphic-hydrothermal and hydrothermal deposits from rutile in magmatic-hydrothermal deposits and magmatic environments. The binary plot Nb/Sb vs. Sn/V distinguishes rutile in orogenic gold deposits from other geologic settings and environments. Results are used to establish geochemical criteria to constrain the source of rutile for indicator mineral surveys and potentially guide mineral exploration.

Deformation Mechanisms in Orogenic Gold Systems During Aseismic Periods: Microstructural Evidence from the Central Victorian Gold Deposits, Southeast Australia

In many orogenic gold deposits, gold is located in quartz veins. Understanding vein development at the microstructural scale may therefore provide insights into processes influencing the distribution of gold, its morphology, and its relationship to faulting. We present evidence that deformation processes during aseismic periods produce characteristic quartz microstructures and crystallographic preferred orientations, which are observed across multiple deposits and orogenic events. Quartz veins comprise a matrix of coarse, subidiomorphic, and columnar grains overprinted by finer-grained quartz seams subparallel to the fault trace, which suggests an initial stage of cataclastic deformation. The fine-grained quartz domains are characterized by well-oriented quartz c-axis clusters and girdles oriented parallel to the maximum extension direction, which reveals that fluid-enhanced pressure solution occurred subsequent to grain refinement. Coarser anhedral gold is associated with primary quartz, whereas fine-grained, “dusty” gold trails are found within the fine-grained quartz seams, revealing a link between aseismic deformation and gold morphology. These distinct quartz and gold morphologies, observed at both micro- and macroscale, suggest that both seismic fault-valving and aseismic deformation processes are both important controls on gold distribution.