Re–Os dating of chalcopyrite from selected mineral deposits in the Kalatag district in the eastern Tianshan Orogen, China

The Baishan porphyry Mo deposit (0.72 Mt; 0.06 % Mo) is located in the interior of the eastern Tianshan orogenic belt in Xinjiang, NW China. The deposit comprises 15 orebodies that are associated with monzogranite and granite porphyry stocks and are structurally controlled by roughly EW-trending faults. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating of the monzogranite and granite porphyry yielded the Middle Triassic age (228 ± 2 to 227 ± 2 Ma), which coincide with the molybdenite Re–Os model ages ranging from 226 ± 3 to 228 ± 3 Ma. The Triassic monzogranite and granite porphyry belong to high-K calc-alkaline series and are characterized by high SiO2 and Al2O3 and low MgO, TiO2, and P2O5 concentrations, with negative Eu anomalies (δEu = 0.55–0.91). The least-altered monzogranite and granite porphyry yield uniform e
 Nd(t) values from +1.6 to +3.6, and wide (87Sr/86Sr)
 i
 ratios ranging between 0.7035 and 0.7071, indicating that they were derived from the lower crust. In situ O–Hf isotopic analyses on zircon using SIMS and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) indicate that the δ18O and e
 Hf(t) values of zircon from a monzogranite sample vary from 6.1 to 7.3 ‰ and +8.0 to +11.7, respectively, whereas zircon from a granite porphyry sample vary from 6.2 to 6.9 ‰ and +7.3 to +11.2, respectively. The geochemical and isotopic data imply that the primary magmas of the Baishan granite were likely derived from partial melts from the lower crust involving some mantle components. The Baishan Mo deposit and granitic emplacement were proposed to be most likely related to post-orogenic lithospheric extension and magmatic underplating. An extensional event coupled with the rising of hot mantle-derived melts triggered partial melting of the lower crust, as well as provided metals (Mo).

Geological, geochronological, geochemical, and Sr–Nd–O–Hf isotopic constraints on origins of intrusions associated with the Baishan porphyry Mo deposit in eastern Tianshan, NW China

Rhenium–osmium (Re–Os) dating of sulfides has transformed economic geology. Critical steps that turned this technology into a routine tool in ore geology are summarized. How we assign relative and absolute time to metal-moving processes is combined with presentation of radiometric clocks important to dating ore minerals. The assumptions inherent in single mineral model ages and isochrons are illustrated using the Re–Os system. Critical points for dating in the ore environment are made by teasing apart a real Re–Os data set for molybdenites from the Los Pelambres porphyry Cu–Mo deposit in Chile. A second example from the Aitik Cu–Ag–Au deposit in Sweden uses molybdenite dating to retrieve a larger metallogenic–orogenic record in a complex terrane. The isotopic mechanics behind arsenopyrite and pyrite for dating Au mineralization are presented. Discussion of metals and hydrocarbons using the Carlin Au district in western United States concludes that geochronology and its associated isotopic tracers (initial ratios) will be increasingly in demand. Connecting radiometric ages with their field context provides a critical check on both radiometric accuracy and ore deposit models. This chapter never loses sight of the fact that geologic media are characterized by natural chaos – and the impact of this on geochronology is continually revisited.

Dating and Tracing the History of Ore Formation

The Eastern Tianshan Orogenic Belt of the Central Asian Orogenic Belt and the Beishan terrane of the Tarim Block, NW China, host numerous Fe deposits. The Cihai Fe deposit (>90 Mt at 45.6 % Fe) in the Beishan terrane is diabase-hosted and consists of the Cihai, Cinan, and Cixi ore clusters. Ore minerals are dominantly magnetite, pyrite, and pyrrhotite, with minor chalcopyrite, galena, and sphalerite. Gangue minerals include pyroxene, garnet, hornblende and minor plagioclase, biotite, chlorite, epidotite, quartz, and calcite. Pyrite from the Cihai and Cixi ore clusters has similar Re–Os isotope compositions, with ∼14 to 62 ppb Re and ≤10 ppt common Os. Pyrrhotite has ∼5 to 39 ppb Re and ∼0.6 ppb common Os. Pyrite has a mean Re–Os model age of 262.3 ± 5.6 Ma (n = 13), in agreement with the isochron regression of 187Os vs. 187Re. The Re–Os age (∼262 Ma) for the Cihai Fe deposit is within uncertainty in agreement with a previously reported Rb–Sr age (268 ± 25 Ma) of the hosting diabase, indicating a genetic relationship between magmatism and mineralization. Magnetite from the Cihai deposit has Mg, Al, Ti, V, Cr, Co, Ni, Mn, Zn, Ga, and Sn more elevated than that of typical skarn deposits, but both V and Ti contents lower than that of magmatic Fe–Ti–V deposits. Magnetite from these two ore clusters at Cihai has slightly different trace element concentrations. Magnetite from the Cihai ore cluster has relatively constant trace element compositions. Some magnetite grains from the Cixi ore cluster have higher V, Ti, and Cr than those from the Cihai ore cluster. The compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures. Combined with regional tectonic evolution of the Beishan terrane, the Re–Os age of pyrite and the composition of magnetite indicate that the Cihai Fe deposit may have derived from magmatic–hydrothermal fluids related to mafic magmatism, probably in an extensional rift environment.

Re–Os isotopic ages of pyrite and chemical composition of magnetite from the Cihai magmatic–hydrothermal Fe deposit, NW China

http://www.igg.cas.cn/xwzx/yjcg/201208/W020120802379135834014.pdf

The role of crustal contamination in the formation of Ni–Cu sulfide deposits in Eastern Tianshan, Xinjiang, Northwest China: Evidence from trace element geochemistry, Re–Os, Sr–Nd, zircon Hf–O, and sulfur isotopes

New Re-Os data for chalcopyrite, bornite, and pyrite from the carbonate-hosted Cu deposit at Ruby Creek (Bornite), Alaska, show extremely high Re abundances (hundreds of ppb, low ppm) and contain essentially no common Os. The Re-Os data provide the first absolute ages of ore formation for the carbonate-hosted Ruby Creek Cu-(Co) deposit and demonstrate that the Re-Os systematics of pyrite, chalcopyrite, and bornite are unaffected by greenschist metamorphism. The Re-Os data show that the main phase of Cu mineralization predominantly occurred at 384 ± 4.2 Ma, with an earlier phase possibly at ~400 Ma. The Re-Os data are consistent with the observed paragenetic sequence and coincide with zircon U-Pb ages from igneous rocks within the Ambler metallogenic belt, some of which are spatially and genetically associated with regional volcanogenic massive sulfide deposits. The latter may suggest a temporal link between regional magmatism and hydrothermal mineralization in the Ambler district. The utility of bornite and chalcopyrite, in addition to pyrite, contributes to a new understanding of Re-Os geochronology and permits a refinement of the genetic model for the Ruby Creek deposit.

Re-Os SULFIDE (BORNITE, CHALCOPYRITE, AND PYRITE) SYSTEMATICS OF THE CARBONATE-HOSTED COPPER DEPOSITS AT RUBY CREEK, SOUTHERN BROOKS RANGE, ALASKA

The northern margin of the Helena Embayment contains extensive syngenetic to diagenetic massive pyrite horizons that extend over 25 km along the Volcano Valley-Buttress fault zone and extend up to 8 km basinward (south) within the Mesoproterozoic Newland Formation. The Sheep Creek Cu-Co deposit occurs within a structural block along a bend in the fault system, where replacement-style chalcopyrite mineralization is spatially associated mostly with the two stratigraphically lowest massive pyrite zones. These mineralized pyritic horizons are intercalated with debris flows derived from synsedimentary movement along the Volcano Valley-Buttress fault zone. Cominco American Inc. delineated a geologic resource of 4.5 Mt at 2.5% Cu and 0.1% Co in the upper sulfide zone and 4 Mt at 4% Cu within the lower sulfide zone. More recently, Tintina Resources Inc. has delineated an inferred resource of 8.48 Mt at 2.96% Cu, 0.12% Co, and 16.4 g/t Ag in the upper sulfide zone. The more intact upper sulfide zone displays significant thickness variations along strike thought to represent formation in at least three separate subbasins. The largest accumulation of mineralized sulfide in the upper zone occurs as an N-S–trending body that thickens southward from the generally E trending Volcano Valley Fault and probably occupies a paleograben controlled by normal faults in the hanging wall of the Volcano Valley Fault. Early microcrystalline to framboidal pyrite was accompanied by abundant and local barite deposition in the upper and lower sulfide zones, respectively. The sulfide bodies underwent intense (lower sulfide zone) to localized (upper sulfide zone) recrystallization and overprinting by coarser-grained pyrite and minor marcasite that is intergrown with and replaces dolomite. Silicification and paragenetically late chalcopyrite, along with minor tennantite in the upper sulfide zone, replaces fine-grained pyrite, barite, and carbonate. The restriction of chalcopyrite to inferred synsedimentary E- and northerly trending faults and absence of definitive zonation with respect to the Laramide Volcano Valley Fault in the lower sulfide zone suggest a diagenetic age related to basin development for the Sheep Creek Cu-Co-Ag deposit.

Geologic Setting, Sedimentary Architecture, and Paragenesis of the Mesoproterozoic Sediment-Hosted Sheep Creek Cu-Co-Ag Deposit, Helena Embayment, Montana

The ca. 1,500 to 1,325 Ma Mesoproterozoic Belt-Purcell Basin is an exceptionally preserved archive of Mesoproterozoic Earth and its paleoenvironmental conditions. The Belt-Purcell Basin is also host to world-class base metal sediment-hosted mineralization produced in a variety of settings from the rift stage of basin evolution through the subsequent influence of East Kootenay and Grenvillian orogenies. The mineral potential of this basin has not been fully realized yet. New rhenium-osmium (Re-Os) data presented here for chalcopyrite, pyrite, and black shale contribute to refine a robust genetic model for the origin of the Black Butte copper±cobalt±silver (⁠Cu±Co±Ag⁠) deposit hosted by the ca. >1,475 Ma Newland Formation in the Helena Embayment of the Belt-Purcell Basin in Montana, USA. Chalcopyrite Re-Os data yield an isochron age (⁠1,488±34 Ma⁠, unradiogenic initial 187Os/188Os composition Osi‐chalcopyrite=0.13±0.11⁠) that overlaps with the geological age of the Newland Formation. Further, the Re-Os data of synsedimentary to diagenetic massive pyrite yield evidence of resetting with an isochron age (⁠1,358±42 Ma⁠) coincident with the timing of the East Kootenay orogeny. The unradiogenic Osi-chalcopyrite at ca. 1,488 Ma (⁠0.13±0.11⁠) argues for derivation of Os from a magmatic source with a 187Os/188Os isotopic composition inherited from the upper mantle in the Mesoproterozoic (⁠Osmantle 1,475 Ma=0.12±0.02⁠). The unradiogenic Osi-chalcopyrite also suggests limited contamination from a continental crustal source. This source of Os and our new sulfur isotopic signatures of chalcopyrite (–4.1 to +2.1‰-VCDT) implies a dominantly magmatic source for metals. We integrate our new results and previously published geological and geochemical evidence to conceptualize a genetic model in which Cu and metals were largely contributed by moderate-temperature, reduced magmatic-hydrothermal fluids carrying reduced sulfur species with a magmatic origin and flowing as highly metalliferous fluids within the shale sequence. A subsidiary derivation of metals during thermally forced shale diagenesis is possible. Chalcopyrite mineralization replaced locally massive synsedimentary to diagenetic pyrite units close to the sediment-water interface, i.e., an ideal locus where magmatic-hydrothermal fluids could cool and the solubility of chalcopyrite would fall. We suggest that Cu mineralization was coeval with the timing of an enhanced thermal gradient in the Helena Embayment triggered until ca. 1,455 Ma by tholeiitic dike swarm that intruded into Archean basement rocks and intersected the NE-SW-trending Great Falls Tectonic Zone.

/pdf/synsedimentary-to-diagenetic-cu-co-mineralization-in-4xjyd7vtze.pdf

Synsedimentary to diagenetic Cu±Co mineralization in Mesoproterozoic pyritic shale driven by magmatic-hydrothermal activity on the edge of the Great Falls Tectonic Zone–Black Butte, Helena Embayment, Belt-Purcell Basin, USA: evidence from sulfide Re-Os isotope geochemistry

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Re-Os SULFIDE (BORNITE, CHALCOPYRITE, AND PYRITE) SYSTEMATICS OF THE CARBONATE-HOSTED COPPER DEPOSITS AT RUBY CREEK, SOUTHERN BROOKS RANGE, ALASKA

Geologic Setting, Sedimentary Architecture, and Paragenesis of the Mesoproterozoic Sediment-Hosted Sheep Creek Cu-Co-Ag Deposit, Helena Embayment, Montana

Synsedimentary to diagenetic Cu±Co mineralization in Mesoproterozoic pyritic shale driven by magmatic-hydrothermal activity on the edge of the Great Falls Tectonic Zone–Black Butte, Helena Embayment, Belt-Purcell Basin, USA: evidence from sulfide Re-Os isotope geochemistry