Contribution of the magnetic fabric to the kinematics of the Jarque Fault (Iberian Range)
REVISTA DE LA SOCIEDAD GEOLOGICA DE ESPANA
Authors: Sierra Campos, Pablo; Roman Berdiel, Teresa; Casas Sainz, Antonio M.; Marcen, Marcos
Abstract
In this paper the Anisotropy of the Magnetic Susceptibility (AMS) is applied together with the structural analysis along the Jarque Fault (Iberian Range) in its north-western sector. The Jarque Fault has a NW-SE direction, and extends longitudinally along at least 50 km, being one of the main structures that compartmentalize the Aragonese Branch of the Iberian Range. The S-C structures observed in the Jarque outcrop show a dextral-reverse component on planes with NW-SE trend and strong dip to W consistent with the subhorizontal striae measured in fault planes of the Oseja outcrop. The dominant magnetic foliation coincides with the intermediate orientation between the S and C planes, and the magnetic lineation shows two maxima. The main maximum has a subvertical plunge being parallel to the direction of maximum slope of the planes of foliation and faults and also approximates the intersection lineation between S and C planes. The second maximum has a subhorizontal plunge that approximates the transport direction inferred from striae and S-C structures. The geological map shows structures oblique to the trace of the main fault, such as minor faults with WNW-SES trend and folds with E-W direction compatible with the dextral movement of the Jarque Fault. The results obtained indicate a structure resulting from the reactivation of an ancient (late-Variscan to Triassic) fault during the Alpine Orogeny associated with the NNE-SSW compression.
The geology and mineralization of the Long Chieng Track (LCT) subvolcanic Au-Ag-Cu-Pb-Zn deposit, Lao PDR
ORE GEOLOGY REVIEWS
Authors: Leaman, Peter; Manaka, Takayuki; Jarical, Kristalyn; Villar, Michael; Libao, Jose B.
Abstract
The Long Chieng Track deposit contains a mineral resource of 32 Mt @ 0.77 g/t Au + 4.9 g/t Ag + 0.12% Cu. It is described here as a subvolcanic polymetallic Au-Ag-Cu-Pb-Zn deposit occurring within a folded and faulted Permian age volcanosedimentary formation in the Truong Son Fold Belt of north-central Laos. The geological sequence is interpreted to have formed in a subduction related arc environment. Mineralization is associated with dacitic porphyritic dykes and small stocks which were intruded into a sequence of volcaniclastics, carbonaceous sediments, sedimentary breccias, and limestones. Age dating by U-Pb geochronology of the dacitic porphyry unit yielded an intrusion age of 279 +/- 4 Ma. Whole rock chemical analysis demonstrates that the rock suites are classified as I-type talc-alkaline series with elevated levels of alkali elements. Mineralization at LCT occurs predominantly as stockwork veins which are spatially associated with the dacitic intrusions. The veins may be classified into main four types including undifferentiated (VNN), base metals (BMV), massive pyrite (MPY) and buck (BUK) veins. All vein types are mineralized except the BUK veins. Cross-cutting and overprinting relationships indicate that the VNN and BMV veins were formed earlier than the MPY veins. The dominant sets include steep NNE dipping (667010) and steep NNW dipping (70 degrees/340 degrees) veins, and shallow to moderate SSE dipping veins (21 /172). The majority of high gold grades are hosted within the VNN veins which predominantly dip to the SSE. In contrast, lead and zinc are dominant in the BMV veins, while the highest copper grades are mainly hosted in steep NNE and NNW dipping MPY veins. Ore minerals include pyrite, galena, sphalerite, chalcopyrite, electrum and native gold with minor arsenopyrite and molybdenite. Gold occurs either as electrum or native gold grains or as inclusions in pyrite. Five distinct alteration phases are recognized: (1) quartz-adularia-sulfide; (2) sericite-quartz-pyrite-carbonate +/- chlorite (3) Fe carbonate; (4) epidote-carbonatechlorite; (5) kaolin-halloysite +/- alunite. Phases 1 to 4 are generally progressively zoned from the deposit centre to the periphery. Phase 5 is associated with late steam-heated hot spring alteration and weathering and oxidation. The field geology relationships suggests that the depth of formation can be estimated to be approximately 1-3 km below the paleo-surface. The style of mineralization and geological relationships of the LCT deposit are similar to those described in other deposits formed in the transitional zone between the porphyry and epithermal environments. Therefore, we consider that sub volcanic style is the most appropriate environment to characterize the LCT deposit.