Alteration and Mineralization Types of the Laurani Porphyry Deposit in Central-Andean Metallogenic Belt, Bolivia

1 Introduction The Laurani porphyry deposit is located in the Altiplano,an extensive North-South trending structural basin that formed in Central-Andean metallogenic belt,Bolivia.The Altiplano poly-metallic province contains sub-volcanic ore

Comparison of Different-sized Chromite Mineralizations in the Yarlung-Zangbo Ophiolite Belt, Southern Tibet

Podiform chromitites are characteristically occurred in ophiolites(e.g.,Thayer,1964;Dickey,1975).However,the metallogenic processes for podiform chromitites are still unclear.Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers(Dickey,1975;Boudier and Coleman,1981),but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites(Dick,1977;Dick and Bullen,1984).Recent studies by the majority of authors have suggested that melt-rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites(Zhou and Robinson,1994;Zhou et al.,1996,2005,2014;Robinson,2008;Page and Barnes,2009;Uysal et al.,2009,2012;González-Jiménez et al.,2011,2015).Based on the occurrence of some ultrahigh pressure minerals(e.g.diamond and coesite)in chromitites,it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling(Yang et al.,2007;Yamamoto et al.,2013).Although geat progresses have been made towards understanding the genesis of podiform chromitites,some fundamental issues in remain unanswered.For examples,what are the major controls on the size of chromitites?And why some ophiolites contain large podiform chromitite bodies,whereas most ophiolitic massifs are essentially chromitite-barren? The Yarlung-Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world.It contains fresh peridotites as well as different-sided podiform chromitites.The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China.In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small-scale chromitite bodies.Such characteristics make the Yarlung-Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites. The Luobusha chromitites are large lens and enclosed in dunite.In contrast,the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes.The closely spatial association of the chromitites and dunite envelopes,together with their textural features,support a petrogenetic model that the chromitites from the Luobusha,Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite.In terms of chemical composition of chromite,there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo.Chromite from the Luobusha chromitites has high Cr#(71-82),whereas Chromite in the Dazhuqu chromitites show relatively low Cr#(16-63),and chromite in the Dongbo chromitites includes low Cr#(11-47)and high Cr#(70-81)types.For the Dongbo and Dazhuqu massifs,linear trends of Cr#with Mg O,Fe Ot,Ni,Ga,V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites,suggesting that the melt-rock reaction may provide major budget of Cr for the chromitites.The similar compositions at a given Cr#in chromite from these rocks also demonstrate that the chromitites may have been formed by in-situ crystallization of chromite under low melt/rock ratio.In contrast,the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites,implying that the formation of the chromitite bodies requires a continual replenishment of Cr-rich melts from deeper mantle.Fractionation and accumulation of chromite from a large volume of Cr-rich melt may play an important role on the formation of the Luobusha chromitites.MORB-normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr-rich boninitic melt at surpra-subduction zone(SSZ)setting.However,the Dongbo and Dazhuqu chromitites have formed originally from a MORB-affinity melt at a mid-ocean ridge(MOR)environment. In summary,the Luobusha chromitites crystallized from a Cr-rich melt in a dynamic conduit,where fractional crystallization and crystal settling play a key role in formation of the large chromitites.In contrast,the small-scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts.Podiform chromitites can be formed in MOR environment,whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.

GEOCHEMICAL REASERCH ON ORE-CONTROLLING STRUCTURE IN LIAODONG LUOQUANBEI-BAIYUN GOLD MINERAL BELT

In recent2 0 years,because of the finding ofmany large- superlargegold deposits,we re- new the theories for prospecting and gain many things and conceptions.The current geo- chemical and gold- forming theories underline the importance of the early submarine volcano- sedimentation,metamorphic differention,sedimentation of terrigenous clastics,thermal spring and it’s sedimentation,syngenesis process and other hypergene supplying the source for metallogenic materials.According to the study for source bed(rock) and depsitional for- mation of gold,we find that gold will be gradually enriched and mineralized in source bed (rock) ,because of variousgeologicprocesses,such as regional metamorphism ormigmatiza- tion,geothermal bittern,volcanism. The ore- control of deep and giant fault and ductile shear beltand tectono- flash space is emphasized,especially,we should notice the long- term, succession and multistage of the

A Preliminary Review of Metallogenic Regularity of Silver Deposits in China

China has widely distributed silver deposits,and is rich in silver resources.Although silver deposits are mainly associated with Pb-Zn deposits,a number of independent silver deposits have also been discovered in recent years.Silver deposits include different types,such as submarine volcanism and continental volcanism related type,intrusion related type,and sedimentary related type.This study summarized the metallogenic regularity of China＇s silver deposits systematically based mainly on the data from 490 silver deposits.It is shown that submarine volcanic sedimentary type,continental volcanic or sub-volcanic type,skarn type,hydrothermal type（including vein type and stratabound type）,sedimentary metamorphic type,sedimentary type and regolith type should be regarded as the most important prediction types of silver deposit.A total of 32 silver mineralization belts and 111 silver concentration areas have been delineated.The map of ＂Spatial distribution of silver mineralization belts in China＂ and other series of maps finished in this study may provide a theoretical basis for the evaluation and prognosis of silver resources potential in China.

Main deposit styles and associated tectonics of the West Junggar region,NW China

The West Junggar region, located in the loci of the Central Asian Orogenic Belt, is a highly endowed metallogenic province with 〉100 tonnes Au, 〉0.7 Mt Cu, 〉0.3 Mt Mo, and 〉2.3 Mt chromite as well as significant amounts of Be and U. The West Junggar region has three metallogenic belts distributed systematically from north to south： （1） late Paleozoic Saur Au-Cu belt; （2） early Paleozoic Xiemisitai- Sharburt Be-U-Cu-Zn belt; （3） late Paleozoic Barluk-Kelamay Au-Cu-Mo-Cr belt. These belts host a number of deposits belonging to at least eight economically important styles, including epithermal Au, granite-related Be-U, volcanogenic massive sulfide （VMS） Cu-Zn, podiform chromite, porphyry Cu, hydrothermal quartz vein Au, porphyry-greisen Mo（-W）, and orogenic Au. These deposit styles are associated with the tectonics prevalent during their formation. Five tectonic-mineralized epochs can be recognized： （1） Ordovician subduction-related VMS Cu-Zn deposit; （2） Devonian ophiolite-related podiform chromite deposit; （3） early Carboniferous subductionrelated epithermal Au and porphyry Cu deposits; （4） late Carboniferous subduction-related granite-related Be-U, porphyry Cu, and hydrothermal quartz vein Au deposits; and （5） late Carboniferous to early Permian subduction-related por- phyry-greisen Mo（-W） and orogenic Au deposits.