Low-sulfidation Au Research Articles

Intermediate sulfidation epithermal Pb - Zn (± Ag ± Cu ± In) and low sulfidation Au (± Pb ± Ag ± Zn) mineralization styles in the Gonzalito polymetallic mining district, North Patagonian Massif

The Gonzalito polymetallic mining district is located within the northeast tip of the 600 km-long NE-trending polymetallic belt of the North Patagonian Massif, in Patagonia, Argentina. It comprises Paleozoic igneous-metamorphic rocks of the Mina Gonzalito complex, which are intruded by Middle Triassic trachytic-andesitic dykes of the Monasa Fm. through a complex system of high-angle fractures. Rhyolitic dykes crosscut the pre-Jurassic rocks. Numerous semi-parallel vein-like mineral deposits, whose classification and age have been subject of debate, sharply crosscut the igneous-metamorphic, trachytic-andesitic and rhyolitic units. Mineralization shows a strong structural control through a complex system of semi-parallel NW-N-NE oriented fracture-hosted mineral deposits. Extensional jogs and open dilational fractures allow multiband vein infills, vein-like hydrothermal and fault-tectonic breccias, stockwork veins and veinlets with primary growth open-space and boiling-derived textures, indicating a brittle geological setting that is proper of the epithermal environment. Based on the structural setting, sulfide-gangue-alteration mineral assemblages and textures, sulfide mineral chemistry and whole-rock geochemical analyses, two different styles of hydrothermal deposits can be distinguished: massive-sulfide intermediate sulfidation epithermal Pb – Zn ± Ag ± Cu ± In and disseminated-sulfide low sulfidation Au ± Pb ± Ag ± Zn. The first has a main-stage pyrite – In-rich sphalerite – chalcopyrite – galena ore assemblage in scarce quartz - carbonates gangue. The second shows disseminated pyrite ± sphalerite ± chalcopyrite ± galena with economic grades of Au in abundant quartz – adularia ± fluorite gangue. SHRIMP U–Pb zircon data of rhyolites reveal Early Jurassic ages of 193 ± 2 and 191 ± 2 Ma and, along with their bulk-rock geochemical features, they can be included within the Chon Aike magmatic province, locally known as Marifil volcanic complex. Crosscutting relationships between the Lower Jurassic rhyolites and the epithermal veins in the Gonzalito district, and the presence of Oligocene basalts partially covering the epithermal deposits towards the western area, constrain the mineralization event between the Lower Jurassic and the Oligocene. However, the crosscutting relationships of this district along with the already proven genetic, temporal and spatial links between epithermal ore and fluorite-rich deposits and the Jurassic magmatism-rifting of Patagonia suggest that the studied epithermal deposits might be yielded closer or during the Lower Jurassic.

Mineralogy and microthermometry of the Escondida, Gabriela and Margarita mineralized structures from the Cerro Moro epithermal deposit, Deseado Massif, Argentina

Cerro Moro deposit is a low sulfidation Au–Ag epithermal deposit located in Santa Cruz province, southern Patagonia Argentina. It is hosted by numerous structurally controlled quartz veins developed in close spatial and temporal relationship with the Jurassic extension and magmatism. The main area in Cerro Moro comprises the Escondida horst block, limited by NW-SE listric faults which provided the channels for the hydrothermal system. The Escondida (including its high grade ore shoot Zoe), Gabriela and Margarita prospects are representative areas documenting the dynamics of the major fault systems. Four stages of mineralization were defined in Cerro Moro. Gabriela and Margarita prospects, at the center and NE of the deposit, respectively, show good development of bladed quartz replacement textures with adularia crystals and scarce translucent/opaque minerals from the stage 1 (pyrite-sphalerite > chalcopyrite ≫ galena-gold). The Escondida prospect, representing the southwest boundary of the Escondida horst block, is the most significantly mineralized structure hosting the base metal-rich and high grade Au–Ag mineralization. In this vein and in the Zoe high grade ore shoot, the translucent/opaque minerals are abundant (sphalerite, galena, chalcopyrite, lesser pyrite, marcasite, gold, silver and different quantities of Ag-bearing minerals) and mainly related to the dark grey, finely-grained quartz-muscovite veins from the stage 3 and to a lesser degree, from the stage 2. The presence of porous massive, crustiform, moss and colloform textures are interpreted to be formed by a sudden pressure release, where hydrofracturing and repetitive boiling occurred and lead to mineral deposition. Whitish quartz-adularia veinlets from the stage 4 may crosscut the previous sequences. The transparent, translucent and opaque minerals in Cerro Moro express a typical low sulfidation system, where the vein system is the result of the infilling and deposition of low temperature (174–265 °C) and salinity (0.4–0.7 wt% NaCl(eq)) hydrothermal fluids.

Gold-telluride-sulfide association in the Jinqu Au deposit, Xiaoqinling region, central China: Implications for ore-forming conditions and processes

The Jinqu Au deposit, located in central Xiaoqinling gold field along the North China Craton’s southern margin, has total reserve of 21 t of Au with 4.23 g/t Au on average. It consists cccmainly of ore-bearing auriferous quartz veins in amphibolite facies metamorphic rocks of the Archean Taihua Group. The deposit’s mineral paragenesis and ore-forming processes have never been studied. This paper reports its mineral paragenesis, which includes sulfides (pyrite, galena, chalcopyrite, pyrrhotite, sphalerite, molybdenite and bornite), oxides (rutile, scheelite, hematite, magnetite), tellurides (sylvanite [(Au, Ag)2Te4], hessite [Ag2Te], petzite [Ag3AuTe2], krennerite [Au3AgTe8], calaverite [AuTe2], stützite [Ag5−xTe3], empressite [AgTe], rucklidgeite [(Pb, Bi)3Te4], altaite [PbTe], volynskite [AgBiTe2], tetradymite [Bi2Te2S], lingbaoite (AgTe3) and melonite [NiTe2]) and native elements (tellurium and gold). Based on our data, the associated ore-forming processes can be deduced as: iron oxides → (CO2(g) and H2S(g) loss, ƒTe2/ƒS2 ratio increase) sulfides, Au-Ag-tellurides → (condensation of H2Te vapor) Pb-Bi-tellurides → (mixing with meteoric water) carbonate. It has been indicated by thermodynamic modelling that formation of Au-Ag-Te-bearing minerals occurred under physico-chemical conditions of logƒTe2 values from −7.28 to −3.21, logƒO2 values from −26.67 to −26.44, logƒH2S values from −1.34 to 0.34, logƒS2 values from −9.53 to −5.64, logαAu+(aq)/αAg+(aq) values from −4.72 to −4.55 and pH values from 3.33 to 4.20 at 365 ℃ and logƒTe2 values from −10.53 to −6.62, logƒO2 values from −31.99 to −30.98, logƒH2S values from −0.65 to −0.36, logƒS2 values from −13.62 to −7.20, logαAu+(aq)/αAg+(aq) values from −6.42 to −6.21 at 335 ℃. The changes in physico-chemical conditions and in precipitation of Au-Ag-Te-bearing minerals were due to boiling or to condensation of Te-rich magmatic gas into ore-forming fluids. These reflect evolution from an early oxidizing to a later relatively reducing mineralization environment. Therefore, compared with the Dahu Au-Mo deposit and with the Yangzhaiyu Au deposit, which exist in the southern and northern parts of the Xiaoqinling region, respectively, the ore paragenesis and ore-forming physico-chemical conditions of the Jinqu Au deposit were different such that its characteristics are similar to low-sulfidation Au deposits.

Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji

The Navilawa caldera is the remnant of a shoshonitic volcano on Viti Levu, Fiji, and sits adjacent to the low-sulfidation Tuvatu epithermal Au–Te deposit. The caldera occurs along the Viti Levu lineament, approximately 50 km SW of the Tavua caldera, which hosts the giant low-sulfidation Emperor epithermal Au–Te deposit. Both calderas host alkaline rocks of nearly identical age (~5.4–4.6 Ma) and mineralization that occurred in multiple stages. The gold mineralization in these locations is spatially and genetically related to monzonite intrusions and low-grade porphyry Cu-style mineralization. Potassic, propylitic, phyllic, and argillic alteration extends from the Tuvatu Au–Te deposit towards the central, northern, and eastern parts of the Navilawa caldera where it is spatially associated with low-grade porphyry Cu–Au mineralization at the Kingston prospect and various epithermal Au–(Te) vein systems, including the Banana Creek and Tuvatu North prospects. Chalcopyrite, and minor bornite, occurs in quartz–calcite–(adularia) veins in the Kingston deposit associated with weak propylitic and phyllic alteration, whereas NE-trending epithermal gold veins at the Banana Creek and Tuvatu North prospects are associated with weak potassic alteration that is overprinted by propylitic and phyllic alteration. Gold is accompanied by chalcopyrite, galena, and sphalerite in quartz–pyrite veins that also have a Ag–As–Hg–Te signature. The temperature range for phyllosilicates in the phyllic alteration (chlorite ± smectite ± corrensite ± illite) is in good agreement with temperatures recorded from previous fluid inclusion studies of quartz at the Banana Creek Au prospect (~260 °C) and the nearby Tuvatu Au–Te deposit (205 to 382 °C). Sulfur isotope compositions of pyrite (−6.2 to +0.4‰) from the Banana Creek prospect indicate a likely magmatic source of sulfur. Oxidation of the ore fluids or a direct addition of volatiles to the hydrothermal fluids may account for the lighter isotopic values. The similarities of the igneous rock types and compositions, transition from porphyry- to epithermal-style mineralization, alteration assemblages, paragenetic relationships, and stable isotope data suggest a common origin for the porphyry- and epithermal-style mineralization within the Navilawa and between the Navilawa and Tavua calderas.

Multiple mineralization events in the Zacatecas Ag-Pb-Zn-Cu-Au district, and their relationship to the tectonomagmatic evolution of the Mesa Central, Mexico

Mineralization in the Zacatecas district is polymetallic (Ag, Zn, Pb, Cu, and Au) and occurs as skarn-type and epithermal veins formed in different metallogenetic stages. The oldest mineralization in the district is skarn-type, Cu-rich with lesser Zn-Pb-Ag, and is considered to be close in age to felsic dikes and plugs dated at ∼51 Ma. Epithermal mineralization occurs in both low- and intermediate-sulfidation styles. Intermediate-sulfidation veins (the Veta Grande, Mala Noche, El Bote, and La Cantera veins) are polymetallic, Ag-rich, hosted in ESE- to SE-striking structures, and were formed at ∼42 Ma (adularia 40Ar/39Ar isochron age from Veta Grande of 42.36 ± 0.18 Ma; 2σ, MSWD = 0.76). Low-sulfidation Au-(Ag) mineralization occurs in the N–S-trending El Orito vein system, which yielded an adularia 40Ar/39Ar isochron age of 29.19 ± 0.20 Ma (2σ, MSWD = 1.8). These ages and the differences in structural orientation indicate that the two styles of epithermal mineralization are temporally and tectonically unrelated.The mineral paragenesis of the Mala Noche deposit consists of early skarn-type Cu mineralization overprinted by later epithermal Pb-Zn-Ag veins. Skarn-type minerals include relicts of prograde silicate minerals (hedenbergite and garnets), retrograde silicate minerals (ilvaite, grunerite, stilpnomelane, epidote, clinochlore), and ore minerals (chalcopyrite, pyrite, sphalerite, galena, magnetite, wolframite, and minor bismuthinite). Epithermal mineralization is characterized by layered to vuggy quartz veins and breccias, with phyllic wallrock alteration (quartz, sericite-illite). The veins consist of quartz, calcite, dolomite, and ankerite with variable amounts of base metal sulfides (sphalerite, galena, pyrite, minor chalcopyrite, and rare acanthite and stromeyerite).The Veta Grande epithermal mineralization was emplaced in two main stages of Ag-rich quartz veining, with narrow selvedges of phyllic (quartz-sericite) wallrock alteration. Stage I consist of quartz, calcite, and minor adularia intergrown with pyrite, followed by sphalerite, galena, and lesser chalcopyrite, acanthite, pyrargyrite, and jamesonite. Stage II mineral paragenesis is similar to stage I but is characterized by amethystine quartz and contains less abundant sulfide minerals.The ore mineral paragenesis of the El Compas vein, within the El Orito System, consists of quartz, adularia, calcite, and chalcedony with minor pyrite, followed by rare aguilarite, naumannite, electrum, and native gold.The skarn-type and intermediate-sulfidation mineralization is coeval with Eocene subduction-related magmatism in the Zacatecas area, which is constrained by zircon U-Pb ages for igneous rocks between 51 and 42 Ma. The emplacement of these magmas was controlled by the same regional-scale, ESE- to SE-trending, transtensional structures that controlled the skarn-type and intermediate-sulfidation deposits. This mineralization is thus interpreted to be related to the last stages of subduction volcanism in central Mexico, under transtensional stress conditions. In contrast, no nearby magmatism is clearly related to the Oligocene low-sulfidation system. However, its age and structural orientation (N–S), combined with a regional change in magma composition from Eocene calc-alkaline to Oligocene bimodal volcanism in central Mexico, suggest that the low-sulfidation mineralization is related to post-subduction continental extension processes, reflecting the beginning of Basin and Range tectonics.

Mineralogy of tin-sulfides in the Zijinshan porphyry–epithermal system, Fujian Province, China

The Zijinshan orefield is a Cu–Au–Mo–Ag porphyry–epithermal mineralization system of Cretaceous age in southeastern China, comprising the Zijinshan high-sulfidation (HS) Au–Cu deposit, Luoboling porphyry Cu–Mo deposits, Yueyang low-sulfidation Au–Ag–Cu deposit, and Wuziqilong and Longjiangting intermediate-sulfidation (IS) Cu deposit. A number of Sn-(W)-sulfides occur in the Zijinshan orefield, including kiddcreekite, hemusite, colusite, vinciennite, stannoidite, mawsonite and kësterite. The occurrence represents the first report of hemusite, colusite and vinciennite in China. The relative abundance and diversity of Sn-(W)-sulfide-bearing assemblages make the Zijinshan orefield possibly unique in the world, and carries implications for the evolution of the mineralizing environment. Detailed electron probe studies of the Sn-minerals reveal that kiddcreekite, hemusite, mawsonite, stannoidite and kësterite have compositions close to ideal formulae. In contrast, colusite displays grain-scale zoning expressed by W, Sn and As. Vinciennite displays compositional variation depending on mineralization style: an average composition of Cu10.74Fe3.88Sn(As0.85,Sb0.08)S15.4 in the Zijinshan HS deposit, Cu10.71Fe3.9SnAs0.85S15.34 in the Wuziqilong Cu deposit and Cu10.76Fe3.7Sn1.02 (As0.5,Sb0.45)S15.54 with higher Sb contents (3.32%) in the adjacent Longjiangting Cu deposit.In the Zijinshan HS Au–Cu deposit, the main Cu-mineral assemblage varies from bornite-rich to digenite- to covellite-rich with increasing depth. Correspondingly, tin mineralogy varies from kësterite at upper levels to stannoidite-dominant at mid-to-upper levels of the copper orebody, to vinciennite- and mawsonite-dominant at depth, implying an increase in sulfidation and oxidation state downwards. Moreover, kiddcreekite is always partially replaced by wolframite and (hypogene) covellite at depth, also indicating overprinting by high-fO2 and -fS2 fluids in the covellite stage of mineralization. Kiddcreekite, along with a primary chalcopyrite–pyrite mineral association, occurs widely in the Zijinshan orefield (Zijinshan, Yueyang, Luoboling, Wuziqilong, Longjiangting and Dayanli), and is likely indicative of the early IS stage of epithermal system. Analogous with published data, kiddcreekite most commonly occurs in porphyry-style mineralization, implying the presence of the mineral has potential as an indicator of porphyry systems. Vinciennite-bearing Cu–Sn–As mineral associations occur in a specific position in the Zijinshan HS Au–Cu, Wuziqilong and Longjiangting Cu deposits, indicative of a transitional zone between porphyry and HS epithermal-style mineralization. The occurrence of the identified Sn-(W)-sulfide-bearing assemblages has considerable potential implications for understanding the architecture of a complex porphyry–epithermal environment and for exploration for underlying porphyry copper deposits. There is potential for discovery of a subjacent porphyry Cu deposit within the Zijinshan orefield.

Fluid Inclusion and Stable Isotope Studies at Don Sixto, a Precious Metal Low Sulfidation Deposit in Mendoza Province, Argentina

AbstractThe Don Sixto mining area in Mendoza province, central‐western Argentina, contains an epithermal low sulfidation Au–Ag deposit. It is a small deposit (∼4 km2), with a gold resource of 36 t. In Don Sixto, ore minerals are disseminated in the hydrothermal quartz veins and hydrothermally altered volcanic‐pyroclastic rock units of Permian–Triassic age. On the basis of the texture, ore mineral paragenesis and cross cutting relationship of gangue minerals, seven stages of mineralization were recognized and described. The first six stages are characterized by quartz veins with minor amounts of base metal minerals and the last stage is represented by fluorite veins with minimal quantities of base metal minerals; the precious metal mineralization is mainly related to the fourth stage. The hydrothermal veins exhibit mainly massive, crustiform and comb infilling textures; the presence of bladed quartz replacement textures and quartz veins with adularia crystals are indicative of boiling processes in the system. Fluid inclusion and complementary stable isotope studies were performed in quartz, fluorite, and pyrite samples from the vein systems. The microthermometric data were obtained from primary, biphasic (liquid‐vapor) fluid inclusion assemblages in quartz and fluorite. The maximum values for salinity and homogenization temperature (Th) came from the stage IV where quartz with petrographic evidence of boiling has average values of 4.96 wt% NaClequiv. and 286.9°C respectively. The lower values are related to the last stage of mineralization, where the fluid inclusions in fluorite have average salinities of 1.05 wt% NaClequiv. and average homogenization temperatures of 173.1°C. The oxygen and sulfur isotopic fractionation was analyzed in quartz and pyrite. The calculated isotopic fractionation for oxygen in the hydrothermal fluid is in the range of δ18OH2O = −6.92 up to −3.08‰, which indicates dominance of a meteoric source for the water, while sulfur reaches δ34SH2S = 1.09‰, which could be reflecting a possible magmatic, or even a mixed source.

The Ojolali region, Sumatra, Indonesia: Epithermal gold–silver mineralisation within the Sunda Arc

The Ojolali region in Sumatra, Indonesia contains two main epithermal gold–silver deposits: (i) Tambang, a Ag–Au intermediate-sulfidation deposit with base-metal sulfides and Mn-carbonate in the ore assemblage; and (ii) Bukit Jambi, a high-level, low-sulfidation Au–Ag deposit related to a zone of near-surface steam-heated acid sulfate alteration, overprinted by supergene oxidation, with most of the ore hosted in the oxide zone. Alteration and mineralisation are hosted in a window of Miocene intermediate to mafic volcanics, related to NNE-trending silicified structures identifiable in regional resistivity and ground magnetic maps. All units are from the same fractionation sequence. The Tambang footwall was determined to be more mafic in composition than other units across the region, including the Bukit Jambi footwall. In comparing the intermediate-sulfidation state, base-metal and carbonate-rich assemblage at Tambang against the high-level, low-sulfidation / steam-heated alteration assemblage at Bukit Jambi, it is inferred that Tambang was more proximal to the magmatic fluid source, with lateral fluid movement and equilibration with host rock occurring between Tambang and Bukit Jambi. Although the basin is floored by basalt, the geochemistry of this and the overlying andesitic units suggests that they are comagmatic and of calc-alkaline affinity. Similar settings should occur elsewhere within the Sunda Arc and these should also be highly prospective for epithermal gold–silver deposits.

Age and sources of gold mineralization in the Marmato mining district, NW Colombia: A Miocene–Pliocene epizonal gold deposit

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum. Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6 ± 0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6 ± 0.4 Ma), which bounds the Calima Terrain. A porphyry andesite–dacite (6.7 ± 0.1 Ma), hosting the Au–Ag veins, shows a measured 87Sr/ 86Sr between 0.70440 and 0.70460, ε Nd between + 2.2 and + 3.2 and 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The 87Sr/ 86Sr and ε Nd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and − 12.91 to + 10.0, respectively, whereas the corresponding Pb isotopic ratios ( 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. 87Sr/ 86Sr and ε Nd values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from − 1.11 to + 2.40. In the deepest veins, ε Nd values lie between + 1.25 and + 3.28 and the 87Sr/ 86Sr of calcite and pyrite fall between 0.70444 and 0.70930. The 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average 87Sr/ 86Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen.

Some notes on quinine and its associated alkaloids

The Ojolali region in Sumatra, Indonesia contains two main epithermal gold–silver deposits: (i) Tambang, a Ag–Au intermediate-sulfidation deposit with base-metal sulfides and Mn-carbonate in the ore assemblage; and (ii) Bukit Jambi, a high-level, low-sulfidation Au–Ag deposit related to a zone of near-surface steam-heated acid sulfate alteration, overprinted by supergene oxidation, with most of the ore hosted in the oxide zone. Alteration and mineralisation are hosted in a window of Miocene intermediate to mafic volcanics, related to NNE-trending silicified structures identifiable in regional resistivity and ground magnetic maps. All units are from the same fractionation sequence. The Tambang footwall was determined to be more mafic in composition than other units across the region, including the Bukit Jambi footwall. In comparing the intermediate-sulfidation state, base-metal and carbonate-rich assemblage at Tambang against the high-level, low-sulfidation / steam-heated alteration assemblage at Bukit Jambi, it is inferred that Tambang was more proximal to the magmatic fluid source, with lateral fluid movement and equilibration with host rock occurring between Tambang and Bukit Jambi. Although the basin is floored by basalt, the geochemistry of this and the overlying andesitic units suggests that they are comagmatic and of calc-alkaline affinity. Similar settings should occur elsewhere within the Sunda Arc and these should also be highly prospective for epithermal gold–silver deposits.