ALBERT

Description: Gold deposits in the Jiaodong Peninsula represent a primary gold resource in China and mostly exhibit similar ore-forming features related to sericite-quartz-pyrite alteration and other controls from (micro-)structural deformation. This study investigates the pyrite textures and trace elements in the Sizhuang gold deposit (〉100 t Au) to document the key factors impacting on the genesis of the Jiaodong-type orogenic deposits. Three main types of pyrite are identified: (1) the first generation of pyrite (Py1) occurs as disseminated euhedral to subhedral grains in K-feldspar-albite-rutile-hematite and sericite alteration (stage 1), (2) Py2 as aggregates in quartz-sericite-pyrite altered rocks or quartz-pyrite veins (stage 2) can be subdivided into Py2a as irregular cores, Py2b as a zoned overgrowth on Py2a, and Py2c as overgrowth on early pyrite, and (3) Py3 as fine-grained crystals in siderite-polymetallic veins (stage 3). Primary gold at the Sizhuang deposit is coevally or slightly later deposited with Py2b, Py2c, and Py3. Laser ablation–inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses show that the highest Co and Ni contents in Py1 and high but variable Co in Py2b favors the involvement of deep high-temperature magmatic waters at stage 1 and middle stage 2. The elevated As contents from Py2a to Py2c and depletion of trace elements (e.g., Co, Ni, As and Te) and high Au/Co, Cu/Ni, and As/Ni values in Py2a and Py3, combined with published H-O isotope data, imply a meteoric water ingress during stage 2–3. Thus, the fluid evolution at Sizhuang is a consequence of pulsed deep magmatic fluid release plus progressive meteoric fluid ingress. The rhythmic Co–As–Ni–Au bands of Py2b additionally suggest episodic changes in the composition of ore-forming fluids. Moreover, the sharp textural features (e.g., pyrite overgrowth on previously cataclastic crystals) of Py2 and As-Cu-rich and Co-poor bands in zoned Py2b probably also reflect rapid metal deposition and self-organization and subsequent mineral crystal growth due to the pressure release during phase separation in the Sizhuang deposit. Considering the significantly concentrated gold (〉1300 t) in the regional Jiaojia fault zone and Au-bearing mineral formation related to phase separation (boiling) in the Sizhuang deposit, gold mineralization in the Sizhuang deposit was interpreted to be controlled by the pressure-driver owing to the seismic activities in the Jiaojia fault system.

Description: Mafic intrusions on the NE shoulder of the Midcontinent Rift (Keweenawan LIP), including Cu–PGE mineralized gabbros within the Coldwell Complex (CC), and rift parallel or radial dykes outside the CC are correlated based on characteristic trace element patterns. In the Coldwell Complex, mafic rocks are subdivided into four groups: (1) early metabasalt; (2) Marathon Series; (3) Layered Series; (4) Geordie–Wolfcamp Series. The Marathon Series are correlated with the rift radial Abitibi dykes (1140 Ma), and the Geordie–Wolfcamp Series with the rift parallel Pukaskwa and Copper Island dykes. U–Pb ages determined for five gabbros from the Layered and Marathon Series are between 1107·7 and 1106·0 Ma. Radiogenic isotope ratios show near chondritic (CHUR) εNd(1106 Ma) and 87Sr/86Sri values that range from –0·38 to +1·13 and 0·702537 to 0·703944, respectively. Distinctive geochemical properties of the Marathon Series and Abitibi dykes, such as Ba/La (14–37), Th/Nb (0·06–0·12), La/Sm (3·8–7·7), Sr/Nd (21–96) and Zr/Sm (9–19), are very different from those of the Geordie–Wolfcamp Series and a subset of Copper Island and Pukaskwa dykes with Ba/La (8·7–11), Th/Nb (0·12–0·13), La/Sm (6·7–7·9), Sr/Nd (5–7·8) and Zr/Sm (18–24). Each unit exhibits covariation between incompatible element ratios such as Zr/Sm and Nb/La or Gd/Yb, Sr/Nd and Ba/La, and Nb/Y and Zr/Y, which are consistent with mixing relationship between two or more mantle domains. These characteristics are unlike those of intrusions on the NW shoulder of the MCR, but resemble those of mafic rocks occurring in the East Kenya Rift. The results imply that an unusual and long-lived mantle source was present in the NE MCR for at least 34 Myr (spanning the 1140 Ma Abitibi dykes and the 1106 Ma Marathon series) and indicate potential for Cu–PGE mineralization in an area much larger than was previously recognized.