The Relationship Between Two Types Of Paleo-oil Reservoirs And Mineralization In The Lannigou Carlin-type Gold Deposit In The Nanpanjiang Basin

The Nanpanjiang Basin hosts more than 200 gold deposits,it is the second-largest Carlin-type gold district in the world.In addition to gold deposits,the basin also develops a large number of paleo-oil reservoirs,which are spatially associated with gold deposits or occurrences.During the last two decades,many studies have been carried out on the gold deposits and paleo-oil reservoirs and made significant progress.However,it remains highly debated about the metallogenic age,material sources and genetic relationship between these gold deposits and widely developed paleo-oil reservoirs.To solve these problems,we conducted systematic interdisciplinary studies of ore deposit geology and petroleum geology on the superlarge Lannigou gold deposit and the nearby Laizishan and Banjie paleo-oil reservoirs.By means of Sm-Nd dating,in-situ trace elements,and C-O-Sr isotopes of calcite,in-situ trace elements and in-situ S isotope of pyrites,as well as bitumen Re-Os dating,trace elements and organic C isotopic analysis,this study mainly draw the following conclusion:(1)This study identified two types of paleo-oil reservoirs in the Nanpanjiang basin.The reef-type paleo-oil reservoirs are distributed in the pores,fractures,and caves of Permian reef limestone.Bitumen Re-Os dating indicates that this type of paleo-oil reservoir was formed at the early stage of the Late Triassic(234.5 ± 9.3 Ma,MSWD=2.5).Trace elements,rare earth elements,and organic carbon isotopes further reveal that this type of bitumen is mainly derived from Devonian source rocks.The fault-type paleo-oil reservoir mainly occurs as film or thin-coating and co-existed with calcites in the structural fractures in the gold deposit and cut through the gold ore bodies.Bitumen Re-Os(172.7±6 Ma,MSWD=1.8)and calcite Sm-Nd dating(188 ± 14 Ma,MSWD = 0.34)indicate that this type of hydrocarbon accumulation event occurred in the Early to Middle Jurassic.They mainly originated from the Permian source rocks,which are different from the reef-type paleo-oil reservoir.(2)In combination of calcite Sm-Nd and bitumen Re-Os dating,this study indicates that the Lannigou gold deposit was formed at the Late Triassic(~212 Ma).(3)Based on the analysis of in-situ trace elements and C-O-Sr isotopes of three types of calcites,the study suggests that the ore-forming materials were mainly derived from regional basement rocks and country rocks.(4)Pyrites from the Laizishan and Banjie paleo-oil reservoirs have similar in-situ trace elements and sulfur isotopic compositions to those of pyrites from the Carlin-type gold deposits,implying that paleo-oil reservoirs provided numerous ore-forming materials(e.g.,metal and sulfur)needed for sulfides precipitation.(5)This study established a genetical link between basin evolution,gold mineralization,and hydrocarbon accumulation in the Nanpanjiang Basin.At the EarlyMiddle Triassic,the South China Block collided with the Indochina Block and deposited huge(>6000 m)sedimentary rocks in the Nanpanjiang Basin,which led to the further burial and maturation of Devonian and Permian hydrocarbon source rocks.As a result,the Late Carboniferous and Permian generated crude oil,which was sourced from Devonian source rocks,started thermal cracking and forming abundant solid bitumen and dry gas(mainly CH4)in the Nanpanjiang basin.At the same time,oil cracking has resulted in large metals(e.g.,Au)and sulfur released into ore-forming fluid,which contributed to the formation of large-scale Carlin-type gold mineralization in the Nanpanjiang Basin.