Tintina Gold Province Research Articles

Hydrothermal monazite and xenotime chemistry as genetic discriminators for intrusion-related and orogenic gold deposits: implications for an orogenic origin of the Pogo gold deposit, Alaska

Attempts to geochemically distinguish between metamorphic-hydrothermal systems that form orogenic gold deposits and both reduced and oxidized magmatic-hydrothermal systems using isotopes or metal associations have proven ambiguous, particularly for orogenic gold and reduced intrusion-related gold systems. The absence of conclusive geochemical discriminators and the overlap in geologic characteristics have led to gold deposit models being potentially incorrectly applied, which in turn negatively affect regional mineral exploration and mine planning. In this study, in situ electron microprobe geochemical analyses of hydrothermal monazite and xenotime crystals associated with different types of gold-bearing deposits are shown to be effective geochemical discriminators. There are notable differences in mineral chemistry such as rare earth element (REE) profiles, total light REE, Dy, Er, Pr, Y, Nd/Sm, and La/Sm that distinguish monazite precipitated from metamorphic-hydrothermal fluids that form orogenic gold deposits and those precipitated from magmatic-hydrothermal fluids that form both porphyry Cu-Mo-Au and reduced intrusion-related gold deposits. Notable differences in overall xenotime abundances and concentrations of heavy REEs, Ca, and Sc are distinctive between the different deposit classes for xenotime. The origin of the controversially classified Pogo gold deposit, Tintina gold province, Alaska, which has been characterized as both a reduced intrusion-related and an orogenic gold deposit, is tested based upon the noted chemical differences associated with these hydrothermal phosphates. The findings of this study have implications for exploration and mine development in the Tintina gold province and other areas that contain deposits that are controversially classified as either orogenic or as magmatic-hydrothermal gold deposits.

Origin of the intrusion-related Lang Vai gold-antimony district (Northeastern Vietnam): Constraints from fluid inclusions study and C[sbnd]O[sbnd]S[sbnd]Pb isotope systematics

The Lang Vai ore district, located in the central part of the Lo Gam domain in the Northeast Vietnam, contains at least three deposits: Khoun Puc, Lang Vai, and Lung Luong. The general paragenetic sequences for these deposits may be represented by the following mineral associations (from early to late): (i) arsenopyrite, pyrite, native gold, sphalerite; (ii) Bi-rich Pb-Sb sulfosalts; (iii) Bi-poor Pb-Sb sulfosalts; (iv) Sb sulfosalts; (v) Sb-rich minerals, native antimony; and (vi) Pb-rich Sb sulfosalts (as a distal mineral association). Three major types were recognized within the primary fluid inclusions based on the phases present at room temperature, including: V-type, vapor-rich inclusions; L-type, liquid-rich inclusions; and C-type, CO2-rich inclusions. The early Au-As stage quartz crystals contain the V- and L-type fluid inclusions. The results indicate that the initial ore-forming fluid belonged to a (CO2-CH4)–H2O–NaCl system characterized by high temperatures (290–352 °C) and variable to moderate salinities (3.16–13.89 wt% NaCl), which are analogous to reduced intrusion-related ore-forming fluids that exhibit high homogenization temperatures and moderate salinities. The quartz crystals from the Bi-Pb-Sb mineral paragenesis (occurred only at the Khoun Puc deposit) contain both V- and L-type fluid inclusions and characterized by high temperatures (mainly 304–340 °C) and moderate salinities (6.28–8.22 wt% NaCl equiv.). The quartz crystals from the middle hydrothermal stage contain the V- and L-type fluid inclusions that yield moderate homogenization temperatures (240–290 °C) and variable salinities (3.3–8.2 wt% NaCl equiv.). These features indicate that the fluid evolved into an (CO2 ± CH4)–H2O–NaCl system at moderate temperatures, lower CH4 concentrations, and variable salinities, thus indicating changing physicochemical conditions. In the last Sb stage (the Lang Vai deposit) and Pb-Sb stage (the Lung Luong deposit), the ore-forming fluid belongs to CO2–H2O–NaCl system, which is characterized by low temperature (180–240 °C), enrichment of CO2, and medium-low salinity (1.12–9.5 wt% NaCl equiv.), that corresponded to typical fluids which formed the Sb-, and distal AgPbZn veins of the IRGS deposits in the Tintina Gold Province of Alaska USA and Yukon Canada. The fluid inclusion studies and laser Raman spectroscopy suggest that the early fluids were dominated by CH4, whereas the late fluids, by CO2. Our study suggests that the fluid boiling and accompanying decrease in the solubility of gold-bisulfide complex played an important role in gold deposition at the Khoun Puc and Lang Vai deposits. The trapping pressures estimated from the boiling fluid inclusions were 123–343 MPa during the ore-forming process. This suggests an alternating lithostatic–hydrostatic fluid system controlled by a fault-valve activity at a depth of 12.9–15.2 km. The characteristics of the S isotopes and fluid inclusions suggest that the ore-forming fluids were of magmatic origin. During mineralization process, the fluid–rock interaction between the fluids and host rocks resulted in the increase of δ34S values of the fluids as reflected in the range of sulfur isotope from the different deposits. On a plumbotectonic diagram, the sulfides show correlations between 206Pb/204Pb and both 207Pb/204Pb and 208Pb/204Pb, which suggest a two-component mixing between a high radiogenic end-member and another low radiogenic end-member. The δ13CPDB vs. δ18OSMOW study clear show that C and O in the Khoun Puc ore-forming fluids were most likely to be derived from the magma with limited contribution from the carbonate host rocks (predominantly magmatic origin of hydrothermal fluids). Wherein, the calcite separates from the Lang Vai and Lung Luong deposits have δ18OSMOW values higher than those of the Khoun Puc, suggesting that the carbonate host rocks may have provided more C and O for ore-forming fluids in these deposits. The results, combined with existing data on the regional geology, ore geology, ore geochemistry, age of mineralization, and tectonic setting, indicate that the Lang Vai ore district is an intrusion-related gold system.

Petrogenesis of Postcollisional Magmatism at Scheelite Dome, Yukon, Canada: Evidence for a Lithospheric Mantle Source for Magmas Associated with Intrusion-Related Gold Systems

The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits. Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87 Sr/ 86 Sr from 0.711-0.714) and Pb isotope ratios ( 206 Pb/ 204 Pb from 19.2-19.7), and negative initial ɛ Nd values (−8.06 to −11.26). Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.

Chemical Compositions of Fluid Inclusions in Intrusion-Related Gold Systems, Alaska and Yukon, Using PIXE Microanalysis

Proton-induced X-ray emission (PIXE) has been used to characterize the multielement chemistry of the diverse fluid inclusions found in intrusion-related gold systems in the Tintina gold province, Yukon and Alaska. The studied samples are from shallow-level examples that contain coexisting brine (type 3) and carbon dioxide-bearing vapor (type 4) inclusions (e.g., Shotgun, Donlin Creek Dome area, Mike Lake, and Brewery Creek) and deeper level deposits (e.g., Pogo, Dublin Gulch, and Emerald Lake) that contain low-salinity carbon dioxide- (type 1) and/or methane-rich (type 5) inclusions, which locally are overprinted by late secondary type 3 inclusions (e.g., Pogo and Emerald Lake). Major element ratios, K/Ca and Mn/Fe, of both synore high-salinity (type 3) and low-salinity (types 1, 4, and 5) inclusions are >1 and 0.24), and at Pogo low K/Ca ratios (<0.2). Chlorine and bromine data have been used to trace the source of salinity. Two distinct groups of Br/Cl mol ratio are recognized. Group 1 includes type 3 inclusions from the Pogo region, Mike Lake, Brewery Creek, and Emerald Lake, which have Br/Cl mol ratios consistent with typical magmatic values, mostly above 0.5 x 10–3 and below 1.54 x 10–3 (seawater). Group 2 comprises type 3 inclusions from Donlin Creek and Shotgun in southwestern Alaska, which have Br/Cl mol ratios from 2.34 x 10–3 to 6.37 x 10–3, potentially reflecting a halogen contribution from the local sedimentary crust (the Kuskokwim basin) considered to be the primary source of the granite melts. The data also provide insights into important metal contents of the fluid inclusions, including copper, zinc, lead, tungsten, and arsenic; however, gold, bismuth and antimony were all below the detection limits for these elements by the PIXE technique. The results explain some of the distinct metal associations of shallow and deep intrusion-related gold systems. Fluid inclusions in deposits emplaced at shallow crustal levels are characterized by higher iron, manganese, zinc, and lead contents due to the greater abundance of chlorine. Tungsten is more elevated in the low-salinity, carbon dioxide-bearing fluid inclusions in deposits at deeper levels, consistent with high tungsten in the deposits and likely due to the formation of tungstate rather than chloride complexes. Copper and arsenic have similar concentrations in both low- and high-salinity inclusions, also suggesting that ligands other than chlorine were important for these elements. Experimental and microanalytical studies have shown that copper, arsenic, and gold can complex with sulfur and do not require chlorine, exclusively, for metal transport. This may explain why deposits at both shallow and deep levels contain gold despite the wide variation in salinity and different fluid types present.

Geochemical Constraints on the Genesis of the Scheelite Dome Intrusion-Related Gold Deposit, Tombstone Gold Belt, Yukon, Canada

The Scheelite dome intrusion-related gold deposit, western Selwyn basin, Yukon, is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid-Cretaceous Tombstone-Tungsten magmatic belt, Tintina gold province, Alaska and Yukon. A variety of mineralization styles occur throughout a 10- × 3-km east-trending corridor and include reduced Au- and W-rich skarns, Au-, W- and Ag-Pb-Zn-Sb–rich quartz tension-vein arrays, and multiphase fault veins and isolated zones of Au-rich sericite-carbonate altered rock. Integrated U-Pb SHRIMP data for magmatic zircon and Ar-Ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m.y. of magma emplacement. Fluid inclusion, oxygen isotope, and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from 550°C. High-temperature Au-rich skarns formed at >400°C, whereas vein-hosted mineralization formed at 280° to 380°C. In skarns, Au is strongly associated with enrichments of Bi, Te, W, and As, whereas a variety of Au-rich veins occur, with As-rich (type 1), and Te- and W-rich (type 2) end members. Silver-Pb-Zn-Sb veins are typically Au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis. The fluid inclusion data indicate that all mineralization styles were formed from low-salinity (≤4 wt % NaCl equiv) aqueous-carbonic fluids, consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks. However, the nonaqueous fluid was predominantly CH 4 in skarn, CO 2 in Au-Te and Au-W veins, and a fluid with roughly equal amounts of CO 2 , CH 4 , and N 2 in Au-As and Ag-Pb-Zn-Sb veins. Oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive δ 18 O values during interaction with 18 O-enriched metasedimentary strata. Sulfur isotope data suggest two possible sources of sulfur, a magmatic source characterized by δ 34 S values of approximately −5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative δ 34 S values of approximately −15 to −10 per mil. Collectively the data indicate that gold at Scheelite Dome was deposited from a magmatic-hydrothermal system. Interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids, higher CH 4 /CO 2 ratios, and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels. Progressive reduction and cooling of hydrothermal fluids, in addition to phase separation in vein-hosted mineralization, were the mechanisms for gold deposition. Compared to other intrusion-related gold deposits associated with the Tombstone-Tungsten magmatic belt magmatism, exposed mineralization at Scheelite Dome is predominantly hosted by hornfelsed metasedimentary rocks. This results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data.

Geology and geochemistry of the Clear Creek intrusion-related gold occurrences, Tintina Gold Province, Yukon, Canada

The Clear Creek gold occurrences lie within deformed lower greenschist-facies rocks of the western Selwyn basin. They consist of auriferous, sheeted quartz veins that cut six Cretaceous stocks and their hornfels. The veins contain 1–2% combined pyrite and arsenopyrite, with lesser pyrrhotite, bismuthinite, and scheelite, as well as 2–5 g/t Au. New 40Ar/39Ar geochronology of hydrothermal micas indicates that the veins formed within 1–2 million years of granitoid emplacement. Fluid inclusion microthermometry defines a parent ore fluid of ~81 mol.% H2O, 14 mol.% CO2, 4 mol.% NaCl ± KCl, and 1 mol.% CH4, which unmixed into a low- and high-salinity immiscible pair. This suggests a more saline parent fluid and a greater degree of fluid unmixing relative to the other occurrences in the eastern Tintina Gold Province. Inclusions trapped in As- and Bi-rich, high-gold grade veins have homogenization temperatures of 300–350°C, whereas inclusions found in more Ag- and Pb-rich veins are characterized by temperatures of 250–300°C. Fluid inclusion geobarometry suggests hydro-fracturing and gold deposition at 5–7 km depth. The δ18O values of quartz samples range from 13–16‰ (per mil) and δ34S for sulfides are also consistent between –3.0‰ and 0‰, with the exception of some outliers from the Contact Zone of the Pukelman stock that indicate a lower temperature second phase of mineralization. It remains uncertain as to whether the Clear Creek ore fluids were exsolved from magmas at depth or from devolatilization reactions within the contact metamorphic aureoles of the intrusions.