terrace–kitimat area, british columbia , nts 103i · recent regional mapping by the british...

1
VOLCANIC FACIES, DEFORMATION AND ECONOMIC MINERALIZATION IN PALEOZOIC STRATAOF THE TERRACE–KITIMAT AREA, BRITISH COLUMBIA , NTS 103I Pignotta, Geoffrey S., Hardel, Bryan G., Meyers, Jessica L. and Mahoney, J.B. [email protected] Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI, 54702 Recent regional mapping by the British Columbia Geological Survey has indicated that a package of Paleozoic volcanogenic strata, the Mt. Attree volcanics, host potential Volcanic-Hosted Massive Sulphide (VHMS) mineralization (Nelson et al., 2008a; McKeown et al, 2008). These Paleozoic strata are situated along the westernmost margin of Stikinia and intruded by Jurassic through Eocene plutons of the easternmost Coast Plutonic Complex (CPC). Along strike to the north, Paleozoic and Mesozoic volcanic rocks similar to the Mt. Attree volcanic package host significant VHMS deposits, like the Tulsequah Chief, Foremore, Eskay, Granduc and Anyox (McKeown et al., 2008). The recognition of this economic potential combined with updated regional mapping, geophysical data from the recently completed QUEST-West survey and newly reanalyzed and released RGS stream sediment data from the Terrace-Kitimat area provide a solid foundation from which additional studies of economic potential can be based. The objective of this work is to complete structural analysis, geochemical characterization and economic mineral assessment of six targeted exposures of potential Paleozoic volcanogenic and marine sedimentary strata in the Terrace-Kitimat area. During the summer of 2009, detailed mapping and sample collection was completed in the six target areas outlined below. Data compilation, petrographic, structural, geochemical and geochronologic analyses are in progress. These analyses will constrain the nature and timing of economic mineralization and deformation, establish a geochemical framework for volcanic and plutonic rocks in the area and assess the regional economic potential of these Paleozoic strata. Introduction 103I/09 103I/10 103I/08 103I/01 103I/02 103I/07 Terrace Kitimat Legend Area mapped by Nelson et al., 2008c at 1:50,000 Paleozoic Zymoetz Group Jurassic & Cretaceous volcanics Jurassic, Cretaceous & Eocene intrusions 5 km Copper River area Mt. Clague area Nash Ridge area Mt. Attree area Mt. Bolton area Gazelle area Geology of the Terrace-Kitimat area derived from BC MapPlace (BC Geological Survey, 2008). New geological mapping by Nelson et al. (2008c) falls within the box with the heavy black outline; boxes with white outline highlight targeted exposures of Paleozoic Mount Attree volcanic rocks. Abbreviations: Jkp, Jurassic Kleanza pluton; Ewc, Eocene Williams Creek pluton. 337 +/-13 Ma Terrane map of British Columbia, showing location of major volcanic- hosted massive sulphide (VHMS) deposits of Stikinia. Location of study area (Target Area outline) and overlap of project with QUEST-West project (area outlined in red). Economic mineralization in the Terrace-Kitimat area is concentrated along the intrusive boundary between the eastern margin of the Coast Plutonic Complex (CPC) and supracrustal rocks of Stikinia. Devonian to Permian arc volcanics and platform carbonates form the basement to Stikinia in the Terrace-Kitimat area and are overlain by Triassic and Lower Jurassic marine sedimentary and volcanic arc rocks. These supracrustal assemblages are intruded by Jurassic, Late Cretaceous and Eocene plutonic rocks of the CPC. Map modified from McKeown et al., (2008). QUEST-West Project Area Paleozoic Zymoetz Group Peri-Laurentian Terranes STIKINIA Paleozoic rocks; Stikinia Small terranes, Southern Stikinia Quesnellia Paleozoic Quesnellia Yukon-Tanana Slide Mtn. North America (Laurentia) Cache Creek Coast Plutonic Complex Target Area Major VHMS deposits of Stikinia Paleozoic Mesozoic References McKeown, M., Nelson, J. L. and Friedman, R. (2008): Newly discovered volcanic-hosted massive sulphide potential within Paleozoic volcanic rocks of the Stikine assemblage, Terrace area, northwestern British Columbia; in Geological Fieldwork 2007, BC Ministry of Energy, Mines and Petroleum Resources, Paper 2008-1, p. 103–116. MINFILE (2008): MINFILE BC mineral deposits database; BC Ministry of Energy, Mines and Petroleum Resources, URL <http://www.em.gov.bc.ca/Mining/Geolsurv/Minfile>, [September 2008]. Nelson, J. L., Kyba, J., McKeown, M. and Angen, J. (2008a): Geology of the Chist Creek Map Area (NTS 103I/08); BC Ministry of Energy, Mines and Petroleum Resources, 1:50 000 scale. Nelson, J. L., Kyba, J., McKeown, M. andAngen, J. (2008b): Terrace Regional Mapping Project Year 3: contributions to stratigraphic, structural and exploration concepts, Zymoetz River to Kitimat River, (NTS 103I/08); in Geological Fieldwork 2007, BC Ministry of Energy, Mines and Petroleum Resources, Paper 2008-1, p. 159–174. Nelson, J. L., McKeown, M., Cui, Y., Desjardins, P., Nakanishi, T. and Shirvani, F. (2008c): Terrace preliminary geodata release (103I east half); BC Ministry of Energy, Mines and Petroleum Resources, GeoFile 2008-11. Regional Location Terrace Mapping and Target Locations Nash Ridge Area Copper River Area Mt. Attree Area Gazelle Area Mt. Bolton Area Mt. Clague Area Target Area Summaries · · · · · · · · · There are observable lithofacies trends within the Mount Attree volcanic packages including: 1) Distinct differences between volcanic lithofacies in the areas west of Terrace compared to the eastern areas. Western areas are more felsic, contain more intrusive material, are generally higher metamorphic grade and have ubiquitous lavender quartz eyes, which are completely absent in eastern Mount Attree exposures. 2) Lithofacies trends have also been noted within the individual targeted exposures and are being examined further through petrographic and geochemical analyses. 120 geochemical samples collected, processed and analyzed; data analysis and interpretation pending. 25 assay samples collected, processed and analyzed; Data analysis and interpretation pending. 12 samples collected and processed for geochronology; analyses pending. Suite of samples prepared for thin section analysis. Several detailed maps completed from target areas. The Gazelle area has shown the most economic-mineral potential. Ongoing petrographic, geochemical and geochronological work will help constrain the ages of the volcanic packages, their genesis and the timing of mineralization with respect to deposition and deformation. Summary University of Wisconsin- Eau Claire undergraduate student Jessica Meyers measuring foliation in Mt. Attree volcanics near the Gazelle property. Exposures of Mount Attree volcanics in the Copper River are strongly dominated by medium- to coarse-grained nonwelded tuff and upright, generally northwest-striking and moderately north-dipping epiclastic volcanic rocks. There is a general trend of increasing clast size in the volcaniclastic rocks from south to north. In the upper part of the northern section, channels cut epiclastic volcanic rocks. Notably little economic mineralization observed in this section. that consist of fining-upward sequences of polymictic pebble conglomerate to feldspathic litharenite, Assay Sample locations h Foliation o Dike Dike trend Fault trace 77 54 64 86 79 58 80 75 515980E 6030510N 517870E 6030450N Legend Andesitic lapilli tuff Breccia Lavender quartz eyes Gabbro Rhyolitic lapilli tuff Biotite schist 0 250 125 Meters Preliminary Strip Map of White Creek Road Distinctly different volcanic lithofacies are observed in the Nash Ridge area (and Mount Clague area described below), which is located approximately 10 km due west of type exposures of Mount Attree volcanic rocks. The area consists of a southern region dominated by mixed volcanic units and a northern region (north of the Skeena River) that consists primarily of deformed plutonic rocks. Unique to volcanic units found in the Nash Ridge and Mount Clague areas are spectacular lavender quartz eyes, which distinguish them from all other units to the east. Detailed mapping was completed where necessary in target areas. The preliminary strip map above was completed primarily by UWEC undergraduate student Bryan Hardel. Geochemical and assay samples have been processed and analyzed. Data evaluation and interpretation is pending. The southern region consists of rhyolitic to dacitic crystal-lithic tuff, porphyritic dacite flows, aphanitic and porphyritic (plagioclase) andesite flows and/or sills, dacitic to andesitic flows containing rhyolite clasts and medium- to coarse-grained nonwelded andesitic fragmental tuff. Rhyolitic to dacitic units typically contain abundant lavender quartz eyes, but all rock types and most exposures contained some lavender quartz eyes (See photo below). Intercalated within the volcanic rocks are lenses of coarse-grained, strongly foliated biotite schist. The entire package of volcanic rocks is metamorphosed to upper-greenschist facies. Foliation strikes consistently ENE-WSW and dips steeply. Rhyolite tuff with distinctive lavender quartz eyes characteristic of many units found in the Nash Ridge area and some units in the Mount Clague area located further south near Kitimat. Of note is that in this area the typical Mount Attree fragmental andesites observed both in eastern and western exposures contain lavender quartz eyes. Student Bryan Hardel points out a cross-cutting andesite dike in the White Creek Road map area. Aphanitic andesite dikes (variably porphyritic with plagioclase and hornblende phenocrysts) are present throughout the volcanic package and are typically undeformed. Coarse-grained epiclastic volcanogenic rocks from the Copper River area. Clasts are predominantly porphyritic andesite and dacite. Gossan rich in disseminated pyrite, chalcopyrite, arsenopyrite and small undeformed veinlets or stringers of chalcocite from the central region of the Gazelle area. Magnetite skarn found along contact between Mount Attree volcanic rocks and an Eocene (?) intrusion found along the western edge of the Gazelle exposures. Mount Attree volcanics in the southern Copper River area interbedded with calc-silicate. The Gazelle area is a primary target because of previously recognized potential for economic mineralization (e.g., MINFILE occurrence 103I 185; MINFILE, 2009) and the interpretation of VHMS-style mineralization in the area (McKeown et al., 2008). Numerous geochemical, assay and geochronological samples were collected from this area to help assess mineral potential and determine genetic and timing relationships. Most mineralization in the Gazelle area occurs in its central region, where the predominantly felsic volcanic rocks are more intensely deformed and metamorphosed, and contain more gossans than surrounding volcanic rocks in the Gazelle area and all exposures of Mount Attree volcanic rocks to the east of Terrace. The exposures of Mount Attree volcanic rocks in the Mount Clague area are somewhat similar in character to those found in the Nash Ridge area. Particularly interesting is the presence of lavender quartz eyes in both volcanic and plutonic units found in this area. Volcanic rocks exposed are nearly completely surrounded by a heterogeneous intermediate to mafic intrusive complex. Dominant volcanic rock types include quartz-phyric rhyolitic to dacitic tuff, where quartz phenocrysts are both of the lavender and clear variety. Minor occurrences of lithic dacitic tuff and andesitic nonwelded tuff with lavender quartz eyes (xenocrysts?) also noted in the matrix. Most of the area consists of tonalite, diorite and microdiorite intrusions with minor occurrences of granite and gabbro. Felsic plutonic units, in particular granitic intrusions, often contain lavender quartz phenocrysts/xenocrysts (?). The Mount Bolton area was not mapped during the most recent BCGS regional mapping campaign in the Terrace–Kitimat area. It was selected as a target because it is along and across strike of other mapped exposures of Mount Attree volcanic rocks and contains proximal RGS Cu, Mo and Au anomalies. This north-trending, elongate body of volcanic rock was previously mapped as possible lower Jurassic Telkwa Formation by Woodsworth et al. (1985). The northern half of the target area was mapped and sampled for petrographic, geochemical and geochronological analyses.The volcanic units in this area likely belong to the Jurassic Telkwa Formation based on their lower grade of metamorphism and lack of alteration. A sample of quartz-phyric rhyolitic welded tuff was collected for dating. Mineralization is lacking in this area, with one exception consisting of minor disseminated pyrite and vein pyrite observed in one rhyolite tuff. Intruding the package of volcanic rocks in the west is a medium-grained, foliated hornblende-biotite granodiorite, orignally interpreted by Nelson et al. (2008a, b) to be part of the Jurassic Kleanza pluton. However, a sample collected for geochronology yielded a Mississippian age of 337 13 (See starred location on Terrace regional map). This intrusive relationship requires that the Mt. Atree volcanic complex is Mississippian or older. ± Photo of eastern Mount Attree area with Ambition Formation limestone exposed in saddle and small gossans further east along slope. The Mount Attree area is the type locality for the Mount Attree volcanic complex. Volcanic rocks in this area include a western unit that is dominated by aphanitic andesitic flows, sills and small stocks and crosscutting porphyritic andesite dikes. This transitions abruptly to an eastern unit characterized by fine- to medium-grained fragmental volcaniclastic andesite with minor aphanitic andesite flows and sills, and porphyritic andesite dikes. A structural contact between Mount Attree volcanic units and Ambition Formation limestone is exposed in several localities in the Mt. Attree area. 50 150 250 350 450 0.00 0.02 0.04 0.06 0.08 0.0 0.2 0.4 0.6 207 206 Pb Pb /235 /238 U U Intercepts at 92±250 & 337±13 [±14] Ma MSWD = 1.3

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lavender

quartz

eyes,w

hic

hare

com

ple

tely

absentin

easte

rnM

ountA

ttree

exposure

s.

2)

Lith

ofa

cie

stre

nds

have

als

obeen

note

dw

ithin

the

indiv

idualta

rgete

dexposure

sand

are

bein

gexam

ined

furth

er

thro

ugh

petro

gra

phic

and

geochem

icalanaly

ses.

120

geochem

icalsam

ple

scolle

cte

d,pro

cessed

and

analy

zed;data

analy

sis

and

inte

rpre

tatio

npendin

g.

25

assay

sam

ple

scolle

cte

d,pro

cessed

and

analy

zed;D

ata

analy

sis

and

inte

rpre

tatio

npendin

g.

12

sam

ple

scolle

cte

dand

pro

cessed

for

geochro

nolo

gy;analy

ses

pendin

g.

Suite

ofsam

ple

spre

pare

dfo

rth

insectio

nanaly

sis

.S

evera

ldeta

iled

maps

com

ple

ted

from

targ

etare

as.

The

Gazelle

are

ahas

show

nth

em

osteconom

ic-m

inera

lpote

ntia

l.O

ngoin

gpetro

gra

phic

,geochem

icaland

geochro

nolo

gic

alw

ork

will

help

constra

inth

eages

ofth

evolc

anic

packages,th

eir

genesis

and

the

timin

gofm

inera

lizatio

nw

ithre

spectto

depositio

nand

defo

rmatio

n.

Su

mm

ary

Univ

ers

ityofW

isconsin

-E

au

Cla

ireunderg

raduate

stu

dent

Jessic

aM

eyers

measurin

gfo

liatio

nin

Mt.

Attre

evolc

anic

snear

the

Gazelle

pro

perty.

Exposure

sofM

ountA

ttree

volc

anic

sin

the

Copper

Riv

er

are

stro

ngly

dom

inate

dby

mediu

m-

tocoars

e-g

rain

ed

nonw

eld

ed

tuff

and

uprig

ht,

genera

llynorth

west-s

trikin

gand

modera

tely

north

-dip

pin

gepic

lastic

volc

anic

rocks.

There

isa

genera

ltre

nd

ofin

cre

asin

gcla

stsiz

ein

the

volc

anic

lastic

rocks

from

south

tonorth

.In

the

upper

part

of

the

north

ern

sectio

n,channels

cutepic

lastic

volc

anic

rocks.N

ota

bly

littleeconom

icm

inera

lizatio

nobserv

ed

inth

issectio

n.

thatconsis

toffin

ing-u

pw

ard

sequences

ofpoly

mic

ticpebble

conglo

mera

teto

feld

spath

iclith

are

nite

,

Assay

Sam

ple

locatio

ns

h

Folia

tion

o

Dik

e

Dik

etre

nd

Fault

trace

77

54

64

86

79

58

80

75

515980E

6030510N

517870E

6030450N

Leg

en

d

Andesitic

lapilli

tuff

Bre

ccia

La

ve

nd

er

qu

artz

eye

s

Gabbro

Rhyolitic

lapilli

tuff

Bio

titeschis

t

02

50

12

5

Mete

rs

Pre

limin

ary

Strip

Map

of

Wh

iteC

reek

Ro

ad

Dis

tinctly

diffe

rentvolc

anic

lithofa

cie

sare

observ

ed

inth

eN

ash

Rid

ge

are

a(a

nd

MountC

lague

are

adescrib

ed

belo

w),

whic

his

locate

dappro

xim

ate

ly10

km

due

westofty

pe

exposure

sofM

ountA

ttree

volc

anic

rocks.T

he

are

aconsis

tsofa

south

ern

regio

ndom

inate

dby

mix

ed

volc

anic

units

and

anorth

ern

regio

n(n

orth

ofth

eS

keena

Riv

er)

thatconsis

tsprim

arily

ofdefo

rmed

plu

tonic

rocks.U

niq

ue

tovolc

anic

units

found

inth

eN

ash

Rid

ge

and

Mount

Cla

gue

are

as

are

specta

cula

rla

vender

quartz

eyes,w

hic

hdis

tinguis

hth

em

from

all

oth

er

units

toth

eeast.

Deta

iled

mappin

gw

as

com

ple

ted

where

necessary

inta

rgetare

as.T

he

pre

limin

ary

strip

map

above

was

com

ple

ted

prim

arily

by

UW

EC

underg

raduate

stu

dentB

ryan

Hard

el.

Geochem

icaland

assay

sam

ple

shave

been

pro

cessed

and

analy

zed.D

ata

evalu

atio

nand

inte

rpre

tatio

nis

pendin

g.T

he

south

ern

regio

nconsis

tsofrh

yolitic

todacitic

cry

sta

l-lithic

tuff,

porp

hyritic

dacite

flow

s,aphanitic

and

porp

hyritic

(pla

gio

cla

se)

andesite

flow

sand/o

rsills

,dacitic

toandesitic

flow

sconta

inin

grh

yolite

cla

sts

and

mediu

m-

tocoars

e-g

rain

ed

nonw

eld

ed

andesitic

fragm

enta

ltu

ff.R

hyolitic

todacitic

units

typic

ally

conta

inabundantla

vender

quartz

eyes,butall

rock

types

and

most

exposure

sconta

ined

som

ela

vender

quartz

eyes

(See

photo

belo

w).

Inte

rcala

ted

with

inth

evolc

anic

rocks

are

lenses

ofcoars

e-g

rain

ed,stro

ngly

folia

ted

bio

titeschis

t.T

he

entire

package

ofvolc

anic

rocks

ism

eta

morp

hosed

toupper-g

reenschis

tfa

cie

s.F

olia

tion

strik

es

consis

tently

EN

E-W

SW

and

dip

sste

eply.

Rhyolite

tuff

with

dis

tinctiv

ela

vender

quartz

eyes

chara

cte

ristic

ofm

any

units

found

inth

eN

ash

Rid

ge

are

aand

som

eunits

inth

eM

ountC

lague

are

alo

cate

dfu

rther

south

near

Kitim

at.

Ofnote

isth

atin

this

are

ath

ety

pic

alM

ountA

ttree

fragm

enta

landesite

sobserv

ed

both

ineaste

rnand

weste

rnexposure

sconta

inla

vender

quartz

eyes.

Stu

dentB

ryan

Hard

elpoin

tsouta

cro

ss-c

uttin

gandesite

dik

ein

the

White

Cre

ek

Road

map

are

a.

Aphanitic

andesite

dik

es

(varia

bly

porp

hyritic

with

pla

gio

cla

se

and

horn

ble

nde

phenocry

sts

)are

pre

sentth

roughoutth

evolc

anic

package

and

are

typic

ally

undefo

rmed.

Coars

e-g

rain

ed

epic

lastic

volc

anogenic

rocks

from

the

Copper

Riv

er

are

a.C

lasts

are

pre

dom

inantly

porp

hyritic

andesite

and

dacite

.

Gossan

rich

indis

sem

inate

dpyrite

,chalc

opyrite

,ars

enopyrite

and

sm

all

undefo

rmed

vein

lets

or

strin

gers

ofchalc

ocite

from

the

centra

lre

gio

nofth

eG

azelle

are

a.

Magnetite

skarn

found

alo

ng

conta

ctbetw

een

MountA

ttree

volc

anic

rocks

and

an

Eocene

(?)

intru

sio

nfo

und

alo

ng

the

weste

rnedge

ofth

eG

azelle

exposure

s.

MountA

ttree

volc

anic

sin

the

south

ern

Copper

Riv

er

are

ain

terb

edded

with

calc

-silic

ate

.

The

Gazelle

are

ais

aprim

ary

targ

etbecause

ofpre

vio

usly

recogniz

ed

pote

ntia

lfo

reconom

icm

inera

lizatio

n(e

.g.,

MIN

FIL

Eoccurre

nce

103I

185;M

INF

ILE

,2009)

and

the

inte

rpre

tatio

nofV

HM

S-s

tyle

min

era

lizatio

nin

the

are

a(M

cK

eow

netal.,

2008).

Num

ero

us

geochem

ical,

assay

and

geochro

nolo

gic

alsam

ple

sw

ere

colle

cte

dfro

mth

isare

ato

help

assess

min

era

lpote

ntia

land

dete

rmin

egenetic

and

timin

gre

latio

nship

s.M

ostm

inera

lizatio

nin

the

Gazelle

are

aoccurs

inits

centra

lre

gio

n,w

here

the

pre

dom

inantly

fels

icvolc

anic

rocks

are

more

inte

nsely

defo

rmed

and

meta

morp

hosed,and

conta

inm

ore

gossans

than

surro

undin

gvolc

anic

rocks

inth

eG

azelle

are

aand

all

exposure

sofM

ountA

ttree

volc

anic

rocks

toth

eeastofTerra

ce.

The

exposure

sofM

ountA

ttree

volc

anic

rocks

inth

eM

ountC

lague

are

aare

som

ew

hatsim

ilar

inchara

cte

rto

those

found

inth

eN

ash

Rid

ge

are

a.P

artic

ula

rlyin

tere

stin

gis

the

pre

sence

ofla

vender

quartz

eyes

inboth

volc

anic

and

plu

tonic

units

found

inth

isare

a.V

olc

anic

rocks

exposed

are

nearly

com

ple

tely

surro

unded

by

ahete

rogeneous

inte

rmedia

teto

mafic

intru

siv

ecom

ple

x.D

om

inantvolc

anic

rock

types

inclu

de

quartz

-phyric

rhyolitic

todacitic

tuff,

where

quartz

phenocry

sts

are

both

ofth

ela

vender

and

cle

ar

varie

ty.M

inor

occurre

nces

oflith

icdacitic

tuff

and

andesitic

nonw

eld

ed

tuff

with

lavender

quartz

eyes

(xenocry

sts

?)

als

onote

din

the

matrix

.M

ostofth

eare

aconsis

tsofto

nalite

,dio

riteand

mic

rodio

ritein

trusio

ns

with

min

or

occurre

nces

ofgra

nite

and

gabbro

.F

els

icplu

tonic

units

,in

partic

ula

rgra

nitic

intru

sio

ns,ofte

nconta

inla

vender

quartz

phenocry

sts

/xenocry

sts

(?).

The

MountB

olto

nare

aw

as

notm

apped

durin

gth

em

ostre

centB

CG

Sre

gio

nalm

appin

gcam

paig

nin

the

Terra

ce–K

itimatare

a.It

was

sele

cte

das

ata

rgetbecause

itis

alo

ng

and

acro

ss

strik

eofoth

er

mapped

exposure

sofM

ountA

ttree

volc

anic

rocks

and

conta

ins

pro

xim

alR

GS

Cu,M

oand

Au

anom

alie

s.T

his

north

-trendin

g,elo

ngate

body

ofvolc

anic

rock

was

pre

vio

usly

mapped

as

possib

lelo

wer

Jura

ssic

Telk

wa

Form

atio

nby

Woodsw

orth

etal.

(1985).

The

north

ern

half

ofth

eta

rgetare

aw

as

mapped

and

sam

ple

dfo

rpetro

gra

phic

,geochem

icaland

geochro

nolo

gic

alanaly

ses.T

he

volc

anic

units

inth

isare

alik

ely

belo

ng

toth

eJura

ssic

Telk

wa

Form

atio

nbased

on

their

low

er

gra

de

ofm

eta

morp

his

mand

lack

ofalte

ratio

n.A

sam

ple

ofquartz

-phyric

rhyolitic

weld

ed

tuff

was

colle

cte

dfo

rdatin

g.M

inera

lizatio

nis

lackin

gin

this

are

a,w

ithone

exceptio

nconsis

ting

ofm

inor

dis

sem

inate

dpyrite

and

vein

pyrite

observ

ed

inone

rhyolite

tuff.

Intru

din

gth

epackage

ofvolc

anic

rocks

inth

ew

estis

am

ediu

m-g

rain

ed,

folia

ted

horn

ble

nde-b

iotite

gra

nodio

rite,orig

nally

inte

rpre

ted

by

Nels

on

etal.

(2008a,b)

tobe

part

ofth

eJura

ssic

Kle

anza

plu

ton.H

ow

ever,

asam

ple

colle

cte

dfo

rgeochro

nolo

gy

yie

lded

aM

issis

sip

pia

nage

of337

13

(See

sta

rred

locatio

non

Terra

ce

regio

nalm

ap).

This

intru

siv

ere

latio

nship

require

sth

atth

eM

t.A

tree

volc

anic

com

ple

xis

Mis

sis

sip

pia

nor

old

er.

Photo

ofeaste

rnM

ountA

ttree

are

aw

ithA

mbitio

nF

orm

atio

nlim

esto

ne

exposed

insaddle

and

sm

all

gossans

furth

er

east

alo

ng

slo

pe.

The

MountA

ttree

are

ais

the

type

locality

for

the

MountA

ttree

volc

anic

com

ple

x.V

olc

anic

rocks

inth

isare

ain

clu

de

aw

este

rnunit

thatis

dom

inate

dby

aphanitic

andesitic

flow

s,sills

and

sm

all

sto

cks

and

cro

sscuttin

gporp

hyritic

andesite

dik

es.T

his

transitio

ns

abru

ptly

toan

easte

rnunit

chara

cte

rized

by

fine-

tom

ediu

m-g

rain

ed

fragm

enta

lvolc

anic

lastic

andesite

with

min

or

aphanitic

andesite

flow

sand

sills

,and

porp

hyritic

andesite

dik

es.A

stru

ctu

ral

conta

ctbetw

een

MountA

ttree

volc

anic

units

and

Am

bitio

nF

orm

atio

nlim

esto

ne

isexposed

insevera

llo

calitie

sin

the

Mt.

Attre

eare

a.

INS

ER

TG

EO

CH

RO

ND

ATA

50

150

250

350

450

0.0

0

0.0

2

0.0

4

0.0

6

0.0

8

0.0

0.2

0.4

0.6

20

7

206

Pb

Pb

/23

5

/238

U

U

Inte

rcepts

at

92±250

&337±13

[±14]

Ma

MS

WD

=1.3